Geology and mineralization of the Daheishan supergiant porphyry molybdenum deposit (1.65 Bt), Jilin, China: A review

3.1   Strata

The eastern and southeastern portions of the mining area encompass the Lower Paleozoic Toudaogou Formation, acting as the immediate surrounding rocks for the Daheishan deposit’s ore-forming plutons. The upper part of this formation consists of metasandstones, phyllitic slates, and plagioclase actinolites interspersed with marble lenses. The middle part also consists of plagioclase actinolites interbedded with marble lenses, while the lower part comprises plagioclase actinolites mingled with metasandstones, metamorphic intermediate to intermediate-acid tuffs, volcanic breccias, rhyolites, dacites, and andesites (Fig. 2). The near-ore lithology of the Toudaogou Formation features gray to grayish-green plagioclase actinolites, biotite hornfels, grayish-white diopside hornfels, and black slates interbedded with sucrosic marble lenses. Previous measurements of trace elements in the Toudaogou Formation show molybdenum content exceeding the average value of corresponding rocks in the crust, with molybdenum concentrations of 30×10⁻⁶ and 65×10⁻⁶‒75×10⁻⁶ in lavas and tuffaceous rocks, respectively. The Mesozoic Upper Triassic Nanloushan Formation is exposed in the eastern and southern sections of the mining area, predominantly composed of gray rhyolitic breccia-bearing crystal tuffs, rhyolitic crystal tuffs, and dacitic crystal tuffs intermingled with thinly laminated black tuffaceous slates.

Figure 2.  Geological map of the Daheishan mining area (after Hou XG, 2017).

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3.2   Structures

The ore-forming plutons of the Daheishan deposit are situated in the EW-trending uplift-fault belt and at the core of the Qiancuoluo overturned anticline comprised of Lower Paleozoic strata. It’s also located at the margin of the Mesozoic volcanic faulted basin and the intersection of the EW-trending basement fault zone and NE-trending faults. The structures within the study area can be classified into three groups: (1) EW-directed structures, predominantly composed of compressive faults; (2) an overturned syncline, with crumpled structures and bedding faults developing inside; (3) NE-trending compressive faults and NW-trending torsional faults. The ore-forming porphyry plutons are found in the composite area of these structures.

3.2.1   Fold structures

The regional basement structure within the study area is composed of Caledonian and Late Hercynian to Early Indosinian structural layers, which collectively form the main body of the Jizhong synclinorium. The Hercynian tectonic activity gave rise to compact, linear, and overturned folds. These folds are generally arcuate along the axis, with the arch apex extending westward. Associated with these folds are strike-thrust faults, as well as NE- and NW-trending torsional faults, exhibiting a dense zonation pattern.

3.2.2   Fault structures

The predominant fault structures are the result of Mesozoic tectonic activity (Fig. 3). In addition to the NE-directed volcanic-magmatic tectonic belt, numerous NNE-trending faults, such as the Kouqian-Liuhe fault zone, and associated NW-trending faults were formed. During this period, relatively older faults, like the NE-trending Yitong-Shulan and Dunhua-Mishan faults, experienced inherited torsion. Concurrently, the EW-trending faults, including the Daheishan-Cuoluotun-Tudaogou paleo-uplift zone, experienced renewed uplift. This formed an uplift-fault belt that spanned approximately 20 km from north to south and about 50 km from east to west. This uplift-fault belt played a critical role in shaping the molybdenum ore field. Tectonic movements triggered magmatic intrusion. Where NE- and EW-trending basement faults intersected with NW-trending faults, the eruption of intermediate-acidic magmas ensued, followed by the intrusion of mafic-intermediate-acidic magmas. This led to the formation of the Jizhong volcanic-intrusive complex. As a result, a joint rock- and ore-controlling tectonic framework was established. The joint fissures corresponding to large-scale NE- and EW-trending faults provide favorable conditions for molybdenum ore enrichment.

Figure 3.  Structural outline map of the Daheishan mining area (after Lu ZQ, 2017).

1‒Anticlinal axis; 2‒overturned anticlinal axis; 3‒synclinal axis; 4‒deep fault zone; 5‒fault zone inferred; 6‒crustal fault; 7‒Mesozoic volcanic faulted area; 8‒E-W-trending upfaulted zone; 9‒crater; 10‒molybdenum deposit (ore occurrence); 11‒copper deposit (ore occurrence); 12‒polymetallic deposit (ore occurrence)

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3.2.3   Ore-controlling breccia pipes

Breccia pipes are significantly developed in the Daheishan deposit. The creation of ore-bearing breccias may be attributed to the expansion caused by the escape of volatile constituents from magmas during the ascent and emplacement of granodiorite porphyries. As magma crystallization progressed and anhydrous minerals formed, magmas became increasingly volatile, which increased the vapor pressure. With falling temperatures and escalating vapor pressure, the magmas degraded and boiled, resulting in the significant escape of gas and violent expansion. Brecciation occurred swiftly and extensively when the vapor pressure brought about by expansion surpassed the tensile strength of the external intrusive rocks. This study posits that the breccia pipes and fractures, which were formed due to hydraulic fracturing in the brecciation process, are the primary ore-bearing structures. This contrasts with the previous conclusion that molybdenum mineralization is unassociated with these structures.

3.3   Intrusive rocks

The primary intrusive rocks within the mining area encompass medium- to fine-grained granites, K-feldspar granite porphyries, granodiorites, inequigranular granodiorite porphyries, molybdenum-bearing granodiorite porphyries, diorites, and quartz diorites. These rocks occur as batholiths or stocks. Mafic-ultramafic rocks, porphyrites, and granite porphyries also exist, taking the form of walls or veins. The ore-bearing plutons are complex plutons, taking an elliptical form in the planar view and extending 8 km in length and 3.5 km in width in the northeast direction. Recent research from the Second Geological Survey Institute of the Geology and Mineral Exploration and Development Bureau of Jilin Province identifies four phases of intrusive rocks, as presented in Table 4. Regarding regional rock-controlling characteristics, the emplacement of the ore-bearing complex plutons was regulated by EW-trending faults. In contrast, the morphologies of the plutons were controlled by intersections of NE- and EW-trending faults, in addition to overturned anticlines and their turning parts. The formation of the ore-bearing complex plutons of the Daheishan deposit coincided with several cryptoexplosions, leading to the local formation of cryptoexplosive breccias.

Table 4.  Statistics of intrusive rocks in the Daheishan mining area (after Lu ZQ, 2017).

Pluton	Changgangling biotite granodiorite	Qiancuoluo inequigranular granodiorite	Qiancuoluo inequigranular granodiorite porphyry	Qiancuoluo felsitic granodiorite porphyry
Morphology	Elliptical	Elliptical	Irregular	Crescent
Scale	Length: 8 km, width: 3.5 km, area: 28 km2	Length: 2.25 km, width: 1.65 km, area: 3.7 km2	Length: 0.9 km, width: 0.35 km, area: 0.46 km2	Length: 0.275 km, width: 0.14 km, area: 0.038 km2
Attitude	NE-trending long axis, pitch direction: southeast	NE-trending long axis, dip direction: NW, dip direction: 80°	W-E-trending long axis, dip direction: south, 70°‒80°	W-E-trending long axis, dip direction: south, 70°‒80°
Textures and structures	Hypidiomorphic granular textures, with porphyroid or porphyritic textures locally; lumpy structures	Hypidiomorphic granular textures and inequigranular porphyroid textures; lumpy structures	Porphyritic textures primarily, with porphyroid or porphyroclastic textures partially; matrix showing hypidiomorphic granular textures and microscopic anhedral granular textures or felsic textures; lumpy and brecciated-mottled structures	Dominated by porphyritic textures, with porphyroclastic textures locally; matrix showing felsic textures; lumpy structures
Mineral assemblages	Lithogenetic minerals: quartz, plagioclase, alkali feldspar, and biotite; accessory minerals: sphene, zircon, apatite, rutile, magnetite, etc.	Lithogenetic minerals: plagioclase, quartz, alkali feldspar, and a small amount of biotite; accessory minerals: zircon, apatite, rutile, magnetite, etc.	The speckles: plagioclase and quartz primarily, with a small amount of alkali feldspar and biotite; accessory minerals: zircon, sphene, apatite, magnetite, etc.	Phenocryst minerals: plagioclase, quartz, alkali feldspar, and a small amount of biotite; accessory minerals: zircon, apatite, rutile, pyrite, etc.
Isotopic age/Ma	1784±0.9	169.9±2.3	169.9±0.9
Contact relationship	Intrusive contact with the Lower Paleozoic Toudaogou Formation and the Late Triassic Nanloushan Formation	Intrusion into the Toudaogou Formation in the north and east and the Changgangling granodiorite plutons in the south and west	Intrusion into inequigranular biotite granodiorites	Intrusion into inequigranular biotite granodiorites and granodiorite porphyries

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The Changgangling biotite granodiorite pluton of the first phase is in intrusive contact with the Lower Paleozoic Doudaogou Formation and the Late Triassic Nanloushan Formation. As revealed by trenching in the Beishan Mountain of the Daomuhetun area, this granodiorite pluton intruded into the dacitic volcanic rocks of the Nanloushan Formation. This pluton on the side of the contact zone shows significantly small mineral particles, forming a cooling edge with a width of about 1 m. Many xenoliths of surrounding rocks occur in the pluton, with sizes varying in the range of 1‒40 cm. The volcanic rocks in the surrounding rocks show significant mineral recrystallization and a baking edge with a width of about 5‒10 cm, which is brown due to weathering. The Beishan granodiorite pluton in the Houcuoluotun area was invaded by the Early Jurassic Dadingzi granite pluton, which forms a cooling edge with a width of 10 cm near the contact zone.

As for the Qiancuoluo inequigranular biotite granodiorite pluton of the second phase, its north and east sides are in intrusive contact with the Toudaogou Formation, and its south and west sides intruded into the marginal fine-grained granodiorites of the Changgangling granodiorite pluton. The clear contact boundary between the two plutons and the absence of cooling and baking edges indicate a short time interval between the formation of both plutons. The xenoliths of surrounding rocks are widely distributed in the biotite granodiorite pluton, especially in its eastern half. The xenoliths are dominated by the metamorphic intermediate-mafic volcanic rocks of the Toudaogou Formation, followed by metasandstones and minor quantities of marbles and ultramafic rocks. At the western boundary of the pluton is the residual cover of the marginal fine-grained biotite granodiorites of the Changgangling pluton.

The Qiancuoluo granodiorite porphyries of the third phase intruded into inequigranular granodiorites. Quartz veins are found to be filled along the contact zone between their contact zone at the surface and depth. The upper part of the porphyries hosts various breccias as surrounding rocks. Besides a few strata and ultramafic breccias, these breccias are dominated by inequigranular granodiorite breccias.

The Qiancuoluo felsitic granodiorite porphyries, in the form of apophyses or veins, intruded into inequigranular granodiorites the granodiorite porphyries. These apophyses and veins exhibit significantly different alteration characteristics from their surrounding rocks. Specifically, they lack the quartz-sericite alteration widespread in surrounding rocks and host weak molybdenum mineralization. These phenomena suggest that the intrusion of the felsitic granodiorite porphyries occurred after the dominant mineralization period.

Overall, the Daheishan complex plutons in the area are distributed in the Changgangling–Qiancuoluo area, extending in the NNE direction. They take an elliptical shape planarly, with a length of 8 km, a width of 4 km, and an area of about 32 km². Their morphologies and distributions vary with geological structures.

4.1   Ore body

The main ore body of the Daheishan deposit is a large, single ore body simply shaped ore pipe with a large upper part and a small lower part, measuring approximately 1700 m in length and width, covering an area of 2.3 km², and extending vertically for around 500 m (Fig. 4). It appears irregularly round in a planar view, with its top eroded away. The high-grade ores of the deposit are partially centered and partially suspended in the middle-upper part of the ore body, which has a NE strike and extends in only one direction. However, due to its large cross-section and approximately round appearance on the ground surface, the main ore body of the Daheishan deposit appears as a large-scale ore pipe. Exploration data were used to delineate three annular contours within ore-bearing plutons, corresponding to molybdenum grades of 0.08%, 0.04%, and 0.02% sequentially outwards. The inner annular contour displays a dumbbell shape with a west-east length of about 160 m and a north-south width of 140‒320 m. The middle annular contour shows a pear shape with a west-east length of about 800 m and a north-south width of about 700 m, while the outer annular contour displays a round shape with a diameter of about 1000 m. Furthermore, the main ore body of the Daheishan deposit appears columnar in the profile. Accordingly, it resembles a pot that gradually tapers downward.

Figure 4.  Profiles of ore bodies along lines 14 and 16 in the Daheishan molybdenum deposit (after Hou XG, 2017).

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The main ore body occurs primarily within granodiorite porphyry plutons and their surrounding inequigranular granodiorite plutons, with the majority of the high-grade ores located in the ore-bearing granodiorite porphyries in the middle-upper part of the porphyry plutons (Fig. 5). However, the high-grade ores do not spatially or originally correlate with cryptoexplosive breccia pipes. The southeastern portion of the main ore body extends narrowly into the metamorphic intermediate-mafic volcanic rocks of the Lower Paleozoic Toudaogou Formation. The intensity of mineralization gradually diminishes from the center of the ore body to its periphery, and no distinct boundaries exist between the ore body and its surrounding rocks or between low-grade and high-grade ores. The mineralization shows concentric zonation, exhibiting molybdenite, chalcopyrite, and sphalerite horizontally from the center outward and molybdenite, azurite, and pyrite vertically from top to bottom. The main ore body shows elemental zones of Mo, Cu, Pb, and Zn from the center outward. In a planar view, molybdenum grade is high in the center and decreases outward. In the vertical direction, the sulfur grade increases at depths greater than 200 m, and the gold grade correlates positively with the pyrite grade. The main ore body’s top, especially the top of high-grade ores, frequently hosts inequigranular granodiorites and minor amounts of breccias or xenoliths of surrounding rocks of the Toudaogou Formation. This suggests that mineralization and enrichment have extended beyond the top of the porphyry plutons and into surrounding rocks. Beneficial associated elements like copper, lead, zinc, and silver are relatively enriched at the edges of the main ore body. The quartz-K-feldspar alteration, widely exposed on the ground surface, is distributed concentrically from the center of the ore body outward together with quartz-sericite and propylite alteration zones. These findings suggest that the Daheishan deposit may have undergone moderate denudation.

Figure 5.  3D profile of the main ore body of the Daheishan molybdenum deposit (after Lu ZQ, 2017).

1‒Changgangling granodiorite; 2‒Qiancuoluo inequigranular granodiorite; 3‒Qiancuoluo granodiorite porphyry; 4‒felsitic granodiorite porphyry; 5‒low-grade ore body (Mo grade: 0.02% ‒0.08%); 6‒high-grade ore body (Mo grade: ＞ 0.08%)

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The upper part of the Daheishan deposit contains oxidation zones and mixed mineralized belts, with the oxidation transition zones typically found at a depth of 10‒35 m. The North China climate makes it difficult for secondary enrichment zones to form. Copper mineralization associated with granodiorites in the Duobaoshan ore block primarily occurs in the inner and outer contact zones between the plutons and the surrounding rocks. The main ore body is largely located in the outer contact zones, extending downwards into the plutons. Thicker and larger ore blocks mostly occur near the top strata, that is, near the edges of the inner contact zones. Existing knowledge of the ore body reveals that when the outer contact zones have a mineralization range with a width of 200 m and a thickness of 50 m, the copper ore bodies in the lower inner contact zones will expand correspondingly, forming thicker and larger ore bodies. The porphyry copper mineralization associated with granodiorite porphyry intrusions mainly formed in the outer contact zones of the porphyry plutons, with the main ore body distributed primarily in the upper part of the periphery of the granodiorite porphyry plutons. The mineralization is more prominent in the hanging wall (southwest) of the porphyry plutons than in its foot wall and is at a higher degree in the pitching part (northwest) than in the tilted part.

In terms of spatial distribution, the ore bodies are all distributed in a circular pattern around porphyry plutons. The major ore body is largely distributed in the upper part around the major porphyry plutons, generally 0‒500 m away from the latter. The most intense mineralization is observed at a distance of 50‒150 m away from the porphyry plutons, where the ore grades are high and the mineralization is uniform. The mineralization gradually weakens towards either side. The porphyry plutons at burial depths exceeding 500 m frequently correspond to ore bodies with large burial depths as well. In instances where the porphyry pluton has undergone substantial denudation, only a few ore body branches with small extensions remain on the hanging wall and foot wall of the porphyry plutons. Some dykes tend to occur in the upper part of the porphyry plutons, and the mineralization alteration and mineralization around are generally weak (Ma HW, 1990; Luo MJ et al., 1991; Pei RF, 1995; Rui ZY et al., 2004).

4.2   Ore minerals

The molybdenite in the Daheishan deposit is filled in the fractures or joints of rocks in the form of veinlets or occurs on both sides of quartz veinlets. Some of it is disseminated, distributed uniformly in granodiorites. Besides, some molybdenite scatters in slightly altered sections and at the edges of ore bodies in the form of star-shaped coarse crystals.

4.2.1   Composition of ore minerals

In the Daheishan deposit, major ore minerals include pyrite and molybdenite, while minor minerals include sphalerite, chalcopyrite, tetrahedrite, and scheelite The major characteristics of these ore minerals are as follows (Fig. 6).

Figure 6.  Hand specimens and photomicrographs of ores from the Daheishan molybdenum deposit (after Xing K, 2021). (a)‒Granite porphyry; (b)‒biotite monzogranite; (c)‒granite porphyry bearing quartz molybdenite veins and quartz pyrite veins; (d)‒granite porphyry; (e)‒biotite monzogranite, with magmatic biotite becoming epidote due to hydrothermal alteration; (f)‒quartz molybdenite veins with sericite-quartz alteration under reflected light.

Note: Bi‒biotite; Chl‒epidote; Kfs‒feldspar; Mol‒molybdenite; Pl‒plagioclase; Py‒pyrite; Qtz‒quartz; Ser‒sericite-quartz alteration

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Molybdenite: molybdenite in the Daheishan deposit is lead gray, with a metallic luster and gray-black streaks. It mostly occurs as semi-idiomorphic or anhedral crystals, while a small amount of coarse-grained molybdenite appears as idiomorphic crystals. It reflects a grayish-white color and exhibits significantly varying reflection pleochroism. Its crystals are frequently in the shape of curved flakes or leaves, displaying wavy extinction and even a phenomenon similar to kinks in deformed rocks. Cleavages are very common in the molybdenite. Additionally, the molybdenite exhibits bireflection, non-significant internal reflection colors, and high heterogeneity.

Pyrite: pyrite in the Daheishan deposit is yellow to light brassy yellow, with a metallic luster, high hardness, and greenish-black streaks. Iridescence is common on its surface. The pyrite occurs as hypidiomorphic crystals mostly and idiomorphic crystals partially. It reflects a light-yellow color (yellowish-white), without pleochroism. Regarding morphologies, it occurs as homogeneous cubic, pentagonal dodecahedral, and octahedral idiomorphic crystals. It has greatly varying grain sizes of 0.02‒2 mm dominated by 0.5‒1 mm. It frequently presents dissolution textures after being metasomatized by chalcopyrite, sphalerite, quartz, and calcite or cataclastic textures under the action of force. In addition, the pyrite shows an eroded surface and local cataclastic textures due to its fragility.

Sphalerite: sphalerite in the Daheishan deposit is yellow or light yellow, with a light-green or light-blue tone and vitreous-adamantine lusters. It is translucent, brittle, and anhedral granular. It presents distinct triangular striations, with grain sizes of mostly about 0.1 mm. It, together with chalcopyrite, frequently forms exsolution textures. Some of it contains chalcopyrite in the form of droplet-shaped inclusions, with veinlet-like sphalerite observed to penetrate pyrite. The sphalerite is homogeneous and shows a yellow internal reflection color, without pleochroism and bireflection.

Chalcopyrite: chalcopyrite in the Daheishan deposit is brassy yellow and hypidiomorphic or anhedral granular, with a strong metallic luster and blackish-green streaks. It shows conchoidal fractures, without cleavages. Blue-violet iridescence is frequently visible on its grain surfaces. It has low hardness and grain sizes of 0.07‒0.2 mm dominated by about 0.1 mm. It reflects a brassy yellow and is weak heterogeneity, without idiomorphic crystals, pleochroism, or internal reflection color.

Tetrahedrite: tetrahedrite in the Daheishan deposit is closely associated with chalcopyrite, showing similar metallogenic stages and spatial distribution except that the former has a lower tetrahedrite content. The tetrahedrite is iron-black, with an asphalt-like or submetallic luster, pink to brown streaks, and low hardness. It has a smooth surface, flat fractures, anhedral granular textures, and grain sizes of 0.07‒0.2 mm dominated by about 0.1 mm. It is usually associated with chalcopyrite, rarely occurring alone. In addition, it primarily occurs in quartz veins bearing molybdenite and pyrite.

Scheelite: scheelite in the Daheishan deposit is white or light yellow and translucent, with a strong silky luster and colorless streaks. It is extremely brittle and hypidiomorphic or anhedral granular, with grain sizes of 0.07‒0.15 mm dominated by around 0.1 mm. It primarily formed during the high-temperature gas-liquid quartz-K-feldspar alteration, occurring in quartz-K-feldspar veins and quartz veinlets.

Besides, the Daheishan deposit contains traces of gold, bismuthinite, pyrrhotite, magnetite, ilmenite, arsenopyrite, lillianite, marcasite, and clausthalite. Its secondary minerals include limonite, malachite, cerusite, wulfenite, and molybdite. It also has non-metallic minerals such as quartz, feldspar, biotite, actinolite, and diopside, as well as altered minerals such as sericite and chlorite. Regarding the metal sulfide assemblages, their contents are in the order of pyrite > molybdenite > sphalerite > chalcopyrite > tetrahedrite, consistent with the metal sulfide assemblages and the content ranking of some typical porphyry copper (molybdenum) deposits at home and abroad.

4.2.2   Ore textures

Ore textures primarily include crystalline, metasomatic, exsolution, and compressed textures. Among them, the crystalline and metasomatic textures are the main ore-forming types. The main characteristics of these textures are as follows.

Idiomorphic granular textures: textures of this type are primarily present in swiftly growing pyrite, which predominantly occurs as cubic crystals, followed by pentagonal dodecahedral crystals, with streaks on the crystal faces. Disseminated pyrite frequently displays this type of textures.

Hypidiomorphic granular textures: textures of this type are also primarily present in pyrite, followed by ilmenite and marcasite. Disseminated and veined pyrite mainly shows hypidiomorphic or idiomorphic granular textures.

Anhedral granular textures: textures of this type develop in many minerals, such as magnetite, pyrrhotite, chalcopyrite, tetrahedrite, and sphalerite, usually occurring as aggregates.

Foliaceous or scaly textures: textures of this type are unique to molybdenite, usually exhibiting a directional arrangement. Large molybdenite shows foliaceous textures, while small molybdenite presents scaly textures, mostly occurring as aggregates.

Tabular texture: textures of this type develop in some molybdenite and accessory minerals formed by magma crystallization, such as ilmenite. These minerals, exhibiting anhedral to hypidiomorphic tabular textures, mostly occur as aggregates.

Chrysanthemum petal-like textures: textures of this type are uncommon, also developing exclusively in molybdenite. Foliaceous or scaly molybdenite often forms radial aggregates, presenting textures similar to chrysanthemum petals. These textures, with diameters of 0.5‒5 mm, are frequently visible in wide quartz veins and pegmatoid veins formed in the early stage.

Poikilitic textures: textures of this type are common, primarily manifested as pyrite wrapping chalcopyrite, pyrrhotite, or ilmenite. These textures result from the minerals crystallized early being trapped by those formed late.

Metasomatic relict textures: textures of this type are common, primarily forming by the metasomatic replacement of ilmenite, chalcopyrite, sphalerite, and pyrrhotite by pyrite; the metasomatic replacement of pyrite by tetrahedrite and chalcopyrite, and the metasomatic replacement of ilmenite by molybdenite. The incomplete metasomatism leads to the formation of metasomatic relict textures.

Skeleton textures: textures of this type are very rare, manifested mainly as pyrite metasomatizing ilmenite and appearing as the tabular pseudomorphs of ilmenite.

Exsolution textures: textures of this type are also very rare, usually in a droplet shape. In other words, droplet-shaped chalcopyrite is wrapped in sphalerite, or sphalerite is wrapped in chalcopyrite grains, indicating that they were miscible at high temperatures and then the solid solution phase unmixed as the temperature dropped, forming immiscible paragenetic structures. When the temperature dropped swiftly, the solid solution phase was unmixed quickly, and accordingly, the precipitates formed droplet-shaped textures before migrating and aggregating.

Cataclastic textures: early-crystallized pyrite became cataclastic under the action of late dynamic effects, thus forming cataclastic textures. Occasionally, relative displacement occurs along the crushed fissures, and the resultant interstices are frequently filled with other metallic minerals.

Crumpled textures: foliaceous molybdenite is subjected to bending deformation rather than crushing under the action of dynamic effects, thus forming crumpled textures, which tend to be characterized by wavy extinction.

4.2.3   Ore structures

The ore structures primarily comprise veined and veinlet-disseminated structures, followed by disseminated structures, as well as brecciated and lumpy structures.

Disseminated structures: the structures of this type, which are widespread and dominated by sparse disseminated structures, are unevenly distributed. In these structures, metallic minerals such as molybdenite, pyrite, ilmenite, and chalcopyrite occur as monocrystals or aggregates. These minerals have grain sizes generally less than 0.5 mm, except for some pyrite, which has a grain size of 2 mm. Generally, the structures of this type indicate the ore-forming fluids with high temperatures and concentrations and ores with high effective porosities and permeabilities, which are conducive to infiltration metasomatism.

Veined structures: the structures of this type, which are the most important structures of ores, were formed by metal sulfide minerals and gangue minerals, such as quartz, in various micro-faults and fractures. Generally, these structures exhibit straight vein walls and clear boundaries. They roughly reflect that ore-bearing rocks host fractures, where the filled ore-forming fluids exhibited a decreased temperature. The metallic minerals in veins primarily include molybdenite and pyrite, followed by ilmenite, chalcopyrite, sphalerite, and galena. There are three mineralization patterns, and the details are as follows: (1) extremely fine-grained molybdenite is evenly distributed in quartz veins in dust form, exhibiting gray veins. This mineralization pattern is termed vaporific molybdenite mineralization. (2) Slightly larger-grained molybdenite tends to be directionally distributed on vein walls or within quartz veins, forming roughly parallel intermittent thin strips. These thin strips are relatively developed at veins edges and gradually diminish towards the central part. (3) Pure sulfide mineral veinlets consist primarily of molybdenite and pyrite. Vein structures can be divided into broad-vein, veinlet, and micro-vein structures with vein widths of > 10 mm, 1‒10 mm, and < 1 mm, respectively. Most of them are veinlet structures. Molybdenum mineralization in broad veins tends to appear as aggregates or lumps, while micro-veins primarily form by the penetration of relatively pure molybdenite along small fractures. As a result, molybdenite fills are commonly visible on fracture surfaces. Occasionally, molybdenite or pyrite veinlets penetrate into molybdenum-bearing quartz veins, forming complex veined structures. When multiple sets of fractures develop and ore-bearing veins intersect with each other, a stockwork structure is formed.

Veinlet-disseminated structures: when multiple sets of ore-bearing veins intersect with each other, some disseminated mineralization tends to occur nearby, thus forming veinlet-disseminated structures. The structures of this type reflect the diffusive metasomatism of the surrounding rocks by ore-bearing hydrothermal fluids as the fluids penetrated and filled into fractures. In particular, disseminated molybdenum mineralization is more prone to form in intergranular pores of sericite near veins. In addition, veinlet-disseminated structures can also be formed by the superimposition of veined and disseminated structures. As a main structural type of ores, veinlet-disseminated structures are widely distributed, especially in high-grade ores.

Brecciated structures: the structures of this type are visible in structurally fractured zones and cryptoexplosive breccia pipes. They can be divided into three types according to the compositions of breccias and cements: (1) Ores and ore-bearing veins are fractured into breccias, and cements primarily include hydrothermal alteration materials such as lithic fragments, hydromuscovite, quartz, and carbonate; (2) Rocks are fractured into breccias, and cements consist mainly of quartz, molybdenite, pyrite, and sericite. These breccias are mostly cut through by molybdenum-bearing quartz veins; (3) Breccias are produced from the fracture of both rocks and molybdenum-bearing quartz veins, and their cements contain both metallic sulfides, such as molybdenite and pyrite, and altered minerals, such as quartz, hydromuscovite, and hydromica. Brecciated structures are of great significance for ore origin, indicating that the Daheishan deposit underwent multiple tectonic activities during its metallogenic process and was subjected to cryptoexplosion after the dominant mineralization period.

Lumpy structures: pyrite in the form of lumpy aggregates is distributed in quartz veins or rocks, forming lumpy structures. These structures are rare, especially lumpy molybdenite.

4.3   Alteration and zoning of surrounding rocks

The ore-bearing plutons in the Daheishan deposit generally underwent hydrothermal alteration, which can be divided into two periods. The first period, associated with granodiorite porphyries, is the most closely related to mineralization. The second period, associated with felsitic granodiorite porphyries, represents the superimposed alteration following the dominant mineralization period. The alteration types primarily include K-feldspar alteration, biotite alteration, silicification, sericite alteration, muscovite alteration, hydromuscovite alteration, kaolin alteration, zeolite alteration, and carbonation. The two periods of alterations are superimposed and can be roughly divided into five alteration zones, as follows (Fig. 7).

Figure 7.  Sketch map of alteration zones in the Daheishan molybdenum deposit (after Lu ZQ, 2017). Chl-Ep: propylitization; Q-Ser: quartz-sericitization; Q-Pe: potassium feldspathization; Py-Q-Ser: ferrosericitization; Q-ρe: quartz core-quartz veins.

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4.3.1   Quartz-K-feldspar alteration zone

The quartz-K-feldspar alteration zone, which is the earliest alteration zone in the mining area, was formed by the superimposition of various altered rocks with three alternation types, namely K-feldspar, quartz-K-feldspar, and biotite-quartz-K-feldspar alterations. The altered rocks in this alteration zone are distributed in granodiorite porphyry plutons and inequigranular granodiorite plutons in outer contact zones. In a planar view, this alteration zone exhibits a subcircular distribution pattern with granodiorite porphyries as the center. Vertically, it gradually contracts from the alteration zone’s edge near the surface to the deeper part of the center of the granodiorite porphyries. This alteration zone exhibits uneven alteration intensity, which is high in the upper and interior parts and low in the lower and edge parts. The altered mineral assemblages in the strongly altered sections of this alteration zone primarily show an areal distribution pattern, while the weakly altered sections predominantly present linear alterations distributed as sparse veins. Occasionally, alternating strong and weak alterations are distributed in some sections. Due to the superposition of later alterations, this alteration zone transitions into beresite and quartz-sericite alteration zones inward and outward, respectively. In the deeper part, the exterior of this alteration zone transitions directly into the propylite alteration zone.

4.3.2   Quartz core-stockwork zone

The quartz core-stockwork zone is composed primarily of granodiorite porphyries subjected to strong quartz alteration. This zone is characterized by dense quartz stockworks and several large-scale quartz-pegmatoid veins with lengths of 150‒300 m and widths of 20‒50 m, which form the nearly-WE-directed quartz core. These wide veins are surrounded by granodiorite porphyry breccias, indicating that they were formed by filling and metasomatism along tectonic weak zones. Late alteration superposition led to extensive sericite alteration on the joint and fracture surfaces of the veins. Furthermore, the pegmatoid K-feldspar and surrounding rock breccias within the veins were all metasomatized by late quartz and sericite. This zone, deviating from the center of the granodiorite porphyry plutons, is positioned in the middle-upper part of the southern branch of the granodiorite porphyry plutons distributed in the southeast of the Daheishan deposit. With a small outcrop area, this zone transitions to the quartz-sericite alteration zone outward. The large quartz-pegmatoid veins, with originally slight molybdenum mineralization, exhibit moderately to weakly molybdenum mineralization due to the superposition of the quartz-sericite alteration and ore-bearing quartz veins.

4.3.3   Quartz-sericite alteration zone

The quartz-sericite alteration zone is distributed in a ringlike pattern around the quartz-K-feldspar alteration zone and the quartz core-stockwork zone. This alteration zone, exhibiting stronger alternation in the eastern part of the mining area than in the western part of the area, extends into the metamorphic intermediate-mafic volcanic rocks of the Toudaogou Formation. It extends shallowly downward, with the extension not exceeding the elevation of 0 m. This alteration zone is formed by the metasomatism of different protoliths by veined or zonal alteration veins of sericite hydromuscovite and quartz-sericite hydromuscovite. An alteration zone with 10% or more of the above-mentioned veins can be classified as a quartz-sericite alteration zone. The altered protoliths predominantly comprise inequigranular granodiorites, followed by granodiorite porphyries, metamorphic intermediate-mafic volcanic rocks, and metasandstones. The exterior of this alteration zone transitions into the propylite alteration zone.

4.3.4   Beresite alteration zone

The beresite alteration zone is mainly housed in the middle-upper part of the granodiorite porphyry plutons. In a planar view, this alteration zone displays a nearly-W-E-directed zonal distribution pattern in a narrow range. Similar to the outcrop range of molybdenum-rich ore bodies, this alternation zone is located in the center of horizontal alteration zones. The south and north sides of this alteration zone transition to the quartz-K-feldspar alteration zone, and the east and west sides transition to the quartz-sericite alteration zone. In the center of the upper part of this alteration zone, sericite, hydromuscovite, and pyrite, which are paragenetic, metasomatized granodiorite porphyries planarly, gradually presenting a zonal or veinlet distribution towards the surrounding beresite. As the alteration intensity weakens, the altered veins show gradually decreased occurrence frequency and vein widths. Generally, an alteration zone with more than 10% of altered veins can be classified as a beresite alteration zone. This alteration zone, which is superimposed on the quartz-K-feldspar alteration zone, exhibits some residual quartz-K-feldspar altered rocks formed early between beresite bands. This alteration zone has a vertical depth of over 300 m and transitions to the quartz-K-feldspar alteration zone.

4.3.5   Propylite alteration zone

The propylite alteration zone, which is the outermost alteration zone of the Daheishan deposit, is distributed around granodiorite porphyries. This alteration zone is composed of altered rocks subjected to propylite alteration, such as inequigranular granodiorites, granodiorite porphyries, metamorphic intermediate-mafic volcanic rocks, and metasandstones. It covers a wide distribution range, reaching 1‒2 km away from the center of the Daheishan deposit. Propylite alteration within plutons is evidenced by varying degrees of chlorite alteration and weak epidote alteration of primary biotite, as well as the weak clay alteration and carbonation of plagioclase, with some plagioclase showing decalcification-induced oligoclase-albite alteration of clear rims. The propylite alteration of the metamorphic intermediate-mafic volcanic rocks and metasandstones in the Toudaogou Formation is mainly manifested as follows: the irregular veinlets of chlorite and a minor amount of epidote are filled and metasomatized along rock joints and fractures or distributed along both sides of quartz veinlets. Compared to the typical propylite alteration, the propylite alteration in the mining area of the Daheishan deposit is relatively weak.

The developmental degrees and spatial distribution of various alteration zones in the mining area are closely related to granodiorite porphyries, resulting from the superposition of gas-liquid and hydrothermal alteration stages after the mineralization period of granodiorite porphyries. In combination with the alteration mechanisms of ore-bearing hydrothermal fluids, this study proposes that the alteration zoning in the Daheishan deposit has the following characteristics: with granodiorite porphyries as the center, the alteration zones consist of beresite, quartz-K-feldspar, quartz-sericite, and propylite alteration zones outward. The quartz-K-feldspar alteration zone, formed in the gas-liquid alteration stage, and the quartz-sericite alteration zone, formed in the early hydrothermal alteration stage, exhibit a gradual transition pattern, constituting the main alteration model in the Daheishan deposit. In the early stage, the quartz-K-feldspar alteration was extremely developed and widespread, especially in the upper-central part of the quartz-K-feldspar alteration zone, on which the beresite alteration zone under the control of W-E-oriented fracture zones is superimposed. A quartz core deviating from the mineralization center exists in the upper part of the granodiorite porphyries. In the late stage, epithermal alteration, such as carbonation and zeolite alteration, formed and was superimposed on various alteration zones. However, epithermal alteration is intense in the moderately deep part of the quartz-K-feldspar alteration zone and in the quartz-sericite alteration zone but weak in the beresite alteration zone.

Based on the alteration of the Kalamazoo deposit and data on 27 porphyry copper/molybdenum deposits in America, Shan WL et al. (1991) proposed the alteration model of porphyry copper/molybdenum deposits, which is consistent with the alteration model of the Daheishan deposit.

4.4   Mineralization periods and stages

The Daheishan deposit is the result of ore-bearing hydrothermal fluids acting on rocks. The entire alteration and mineralization process witnessed the evolution of ore-bearing hydrothermal fluids. Based on this, the Daheishan deposit has two main mineralization periods: the hydrothermal mineralization period and the supergene mineralization period (Table 5).

Table 5.  Mineralization stages and mineral generation sequence of the Daheishan molybdenum deposit (after Wang ZG, 2012).

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4.4.1   Hydrothermal mineralization period

During this period, the ascending ore-bearing hydrothermal fluids began to mix with groundwater. Based on the changes in the physical and chemical conditions of ore-bearing hydrothermal fluids, as well as the paragenetic and combination relationships of hydrothermal minerals, this hydrothermal mineralization period can be divided into four mineralization stages:

Quartz-magnetite stage: with the emplacement of porphyry magmas, the ore-bearing hydrothermal fluids rose and then reacted with rocks, forming potassic-alteration rocks composed of altered minerals such as biotite, K-feldspar, and quartz in the early stage. In the process of potassic metasomatism, H₂S appeared in the ore-bearing hydrothermal fluids as the temperature continuously decreased. The combination of H₂S with metal elements led to the formation of metal mineralization, such as early disseminated magnetite and pyrrhotite, which are scattered in rocks. The alteration at this stage was dominated by veined penetration, forming extensive quartz-K-feldspar veinlets and pegmatoid veins. These veins penetrated the early-formed rocks subjected to biotite-quartz-K-feldspar alteration. Furthermore, disseminated molybdenite, pyrite, and chalcopyrite were formed along with the alteration.

Quartz-pyrite-molybdenite stage: at this stage, the ore-bearing hydrothermal fluids, still dominated by deep-source ore-bearing gas and liquids, were mixed with some groundwater and thus gradually became mixed ore-forming liquids. Large amounts of hydromuscovite and sericite were formed by the hydrolyzation of plagioclase and K-feldspar. As a major feature of this stage, the oversaturation of silica in the hydrothermal fluids led to strong quartz-sericite alteration, as well as beresite alteration characterized by the combination of pyrite, hydromuscovite, sericite, and quartz. Subsequently, H₂S was hydrolyzed by SO₂ as the temperature decreased. Then, H₂S was combined with metal elements in the hydrothermal fluids, leading to the precipitation of large quantities of metal sulfide minerals such as molybdenite, pyrite, chalcopyrite, and sphalerite. These metal sulfide minerals filled fractures as molybdenum-bearing quartz veins, pyrite-molybdenite-quartz veins, and pyrite-quartz veins mostly, causing strong molybdenum mineralization. In the late stage of the molybdenum mineralization, molybdenite or pyrite veinlets with few gangue minerals frequently penetrated along fractures owing to the further concentration of sulfur and metal ions in the ore-forming fluids, as well as appropriate physical and chemical conditions such as temperature, pressure, and pH. This mineralization stage, witnessing the formation of large-scale molybdenum ore bodies, is the most significant mineralization stage of the mining area. It is noteworthy that the mineralization at this stage involved multiple mineralization processes, which were superimposed on the middle-upper parts of ore bodies, leading to the formation of high-grade ore blocks. The fractures remained open, providing conditions for multiple filling of ore-forming fluids.

Overall, the molybdenum-bearing quartz veins widened and the grain sizes of molybdenite increased from early to late at this stage. In addition, the sequence of alteration and mineralization at this stage can be verified by the cross-cutting relationships of various veins.

Quartz-polymetallic-sulfide stage: at this stage, a large amount of groundwater entered and dominated the ore-bearing hydrothermal fluids, making the fluids neutral to slightly alkaline. This stage witnessed the formation of pyrite, chalcopyrite, bornite, sphalerite, galena, molybdenite, and quartz veins. Among them, quartz veins showed an uneven ore distribution, with pyrite and molybdenite veins concentrated in local sections.

Quartz-carbonate stage: at this stage, the ore-bearing fluids had already evolved into low-temperature (below 200°C) hydrothermal fluids dominated by groundwater, with very low concentrations of metal ions and only a small amount of crystallized pyrite. This stage witnessed the formation of calcite-fluorite, calcite-zeolite, and quartz veins, which were very clean and bore rare metal sulfides. After this stage, the whole hydrothermal activity ended.

4.4.2   Supergene mineralization period

The Daheishan deposit exhibited very weak supergene mineralization and the absence of secondary enrichment, with small amounts of secondary oxides, such as molybdite, malachite, and limonite, only visible near the surface. Therefore, the supergene mineralization period of the Daheishan deposit has no industrial significance.

4.5   Resources and ore chemical composition of the Daheishan deposit

In the Daheishan deposit, the primary mineral—molybdenum—has an average grade of 0.081%, and the associated minerals copper, sulfur, gallium, and rhenium have average grades of 0.033%, 1.67%, 0.001%, and 0.0012%, respectively. To date, the remained mineral resources of the deposit within the mineral right are as follows: for molybdenum, remained ore content: 234933 kt, metal content: 189849 t, average grade: 0.081%; for sulfur, ore content: 234933 kt, sulfur content: 3923381 t, average grade: 1.67%; for copper, ore content: 234933 kt, metal content: 77258 t, average grade: 0.033%; for gallium, ore content: 234933 kt, metal content: 2349 t, average grade: 0.001%; for rhenium, ore content: 189849 t, metal content: 2 t, average grade: 0.0012% (Shi ZY et al., 2008; Wang ZG, 2012). The cumulative proven mineral resources are as follows: for molybdenum, ore content: 264899 kt, metal content: 215510 t, average grade: 0.081%; for sulfur, ore content: 264899 kt, sulfur content: 4423813, average grade: 1.67%; for copper: ore content: 250260 kt, metal content: 81469 t, average grade: 0.033%; for gallium, ore content: 264899 kt, metal content: 2649 t, average grade: 0.001%; for rhenium, ore content: 215510 t, metal content: 3 t, average grade: 0.0012% (Wang ZG, 2012).

Wang L (2012) analyzed the chemical composition of ore samples, concluding that the significantly varying contents of major oxides in the Daheishan deposit are related to the different types and intensities of hydrothermal alteration (Table 6). In sections with strong quartz-K-feldspar and quartz-sericite alterations, the K₂O, SiO₂, and H₂O contents increase to varying degrees, while the FeO, Fe₂O₃, MgO, Al₂O₃, and TiO₂ contents decrease. The industrial ore bodies of the deposit are located in the superimposed position of K-feldspar, quartz-sericite, and beresite alterations. Accordingly, they exhibit strong alteration, with average SiO₂ and K₂O contents being 14% and 16.26% higher than those of low-grade ores, respectively. The ores have a single useful component—molybdenum. The main associated useful component is sulfur, with contents varying in the range of 0.24%‒2.70% and an average grade of 2.00%, meeting the standards for comprehensive utilization. Other beneficial elements, such as Cu, Ga, Re, Au, and Ag, fall below the standards for comprehensive utilization. However, Cu, with an average grade of 0.033%, was recycled and utilized due to the improved beneficiation process and was included in the estimation of resources and reserves.

Table 6.  Contents of major chemical elements in ores of the Daheishan molybdenum deposit (after Wang L, 2012).

Element	Content/%		Element	Content/%		Element	Content/%
	Average	Range		Average	Range		Average	Range
O	48.41	40.08‒60.84	H	0.144	0.051‒0.517	Pb	0.0044	0.0003‒0.35
Si	32.26	28.33‒34.36	Cl	0.12	0.01‒0.28	W	0.0042	0.00031‒0.06
Al	7.79	6.77‒11.47	P	0.059	0.022‒0.11	As	0.00179	0.0002‒0.02
K	4.17	1.33‒7.47	Mo	0.049		Ga	0.00156	0.0005‒0.003
Fe	2.23	0.58‒5.50	Ba	0.044	0.003‒0.25	Ni	0.00088	0.0005‒0.0024
Na	1.98	0.26‒3.29	Sr	0.038	0.0025‒0.1395	Co	0.000654	0.0002‒0.001
Ca	1.33	0.20‒2.19	Cr	0.0304	0.001‒0.08	Sn	0.000595	0.0002‒0.0025
S	1.67	0.24‒2.70	Cu	0.029		Ag	0.000074	0.000005‒0.002
Mg	0.46	0.05‒0.90	Mn	0.023	0.008‒0.054	Re	0.00006	0‒0.00014
Ti	0.27	0.19‒0.42	Zn	0.0159	0.001‒0.9	Ru	0.0000001	0.0000001
C	0.197	0.08‒0.44	Rb	0.0144	0.0007‒0.0324	Au	0.01g/t	0.00000002‒ 0.00000018
F-	0.16	0.04‒0.40	V	0.006	0.002‒0.015

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5.1   Mineralization epoch

The age of mineralization for the Daheishan deposit has been a point of study and contention for several researchers. Some researchers determined the K-Ar isotopic age of biotite in the Daheishan plutons at 354 Ma, which, however, is deemed unreliable due to the influences of hydrothermal fluids and late-stage alterations. Several other studies have produced different age estimates (Table 7). For example, Ge WC et al. (2007) determined the zircon U-Pb ages of ore-free monzogranites and ore-bearing granodiorite porphyries in the deposit to be 178±3 Ma and 170±3 Ma, respectively. Zhou et al. (2014) determined the ages of early Changgangling biotite granites to be 177.9±2.3 Ma and the ages of late Qiancuoluo biotite granodiorites and granodiorite porphyries to be 169.9±3.2 Ma and 167.6±4.0 Ma, respectively. These two sets of ages roughly align with the findings of Ge WC et al. (2007). In addition, through Re-Os isotopic dating, Wang CH et al. (2009) determined the ages of 10 molybdenite samples with different attitudes collected from the Daheishan mining area, obtaining their model ages of 168.1‒169.1 Ma and Re-Os isochron age of 168.2±3.2 Ma. Zhang et al. (2015) found the Re-Os age of a single molybdenite sample to be 167.2 Ma, which, combined with the age data obtained by Wang CH et al. (2009), constituted an isochron age of 168.7±3.1 Ma. The isochron age of molybdenite obtained by Zhou et al. (2014) is also the Middle Jurassic (171±8 Ma), which is consistent with the two groups of ages mentioned above within the error range. The formation age of ore-bearing granites aligns with the crystallization age of molybdenite within the error range, indicating that the Daheishan deposit was formed during the Middle Jurassic (170‒168 Ma). Zhou LL et al. (2014) determined the Ar-Ar age of muscovite, obtaining a plateau age of 163.6±0.9 Ma, which is believed to represent the end time of magmatic activity or late hydrothermal activity, or in other words, the age of the final mineralization stage (Chen JS et al., 2015). According to Lu ZQ (2017), monzogranite samples had zircon U-Pb isotopic ages of 180±2‒177±2 Ma, with a weighted average of 178.4±0.9 Ma [mean square weighted deviation (MSWD)=0.20], representing the formation age (Early Jurassic) of monzogranites in the Daheishan deposit. Furthermore, granodiorite porphyry samples had zircon U-Pb isotopic ages of 172±3‒167±2 Ma, with a weighted average of 169.9±0.9 Ma (MSWD=0.41), representing the formation age (Middle Jurassic) of granodiorite porphyries in the Daheishan deposit (Lu ZQ, 2017). All these results lead to the conclusion that the formation and mineralization ages of the Daheishan deposit should be earlier than 164 Ma, corroborating that the Daheishan deposit was formed during the Middle Jurassic and experienced the Early Yanshanian mineralization (Hou XG, 2017).

Table 7.  Statistics of diagenetic and mineralization ages of the Daheishan molybdenum deposit (after Hou XG, 2017).

Test subject	Isotopic age/Ma	Dating method	Data source
Ore-free monzogranite	178±3	SHRIMP zircon U-Pb dating	Ge WC et al., 2007
Changgangling biotite granite	177.9±2.3	LA-ICP-MS zircon U-Pb dating	Zhou LL et al., 2014
Qiancuoluo biotite granodiorite	169.9±2.3	LA-ICP-MS zircon U-Pb dating	Zhou LL et al., 2014
Qiancuoluo granodiorite porphyry	167.6±4.0	LA-ICP-MS zircon U-Pb dating	Zhou LL et al., 2014
Ore-bearing granodiorite porphyry	170±3	SHRIMP zircon U-Pb dating	Ge WC et al., 2007
Molybdenite	168.2±2.3	Molybdenite Re-Os dating	Zhang Y et al., 2013
Molybdenite	168.7±3.1	Molybdenite Re-Os dating	Zhou LL et al., 2014
Molybdenite	171±8	Molybdenite Re-Os dating	Zhou LL et al., 2014
Muscovite	163.6±0.9	Ar-Ar isotopic dating of molybdenite	Zhou LL et al., 2014

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Many researchers are interested in the diagenetic and metallogenic epochs of molybdenum deposits, including the Daheishan deposit in central Jilin. They have obtained many new insights and abundant data through geochronological research (Sun SS et al., 1989; Sun FY et al., 2000; Ge WC et al., 2007; Wang CH, 2009; Ju N et al., 2012; Zhang Y, 2013). The following summary presents the research results about the diagenetic and metallogenic epochs of representative porphyry molybdenum deposits in this area (Table 8).

Table 8.  Statistics of the diagenetic and mineralization ages of porphyry molybdenum deposits in central Jilin (after Ju N, 2020).

Number	Deposit	Sample/dating method	Age/Ma	Sample quantity	Data source
1	Daheishan	Molybdenite Re-Os dating	168.2±3.2	10	Wang CH, 2009
		Molybdenite Re-Os dating	168.7±3.1		Zhang Y, 2013
		Zircon U-Pb dating	170±3		Ge WC et al., 2007
		Molybdenite Re-Os dating	169.8±2.4		Zhou LL et al., 2014
2	Fua’npu	Molybdenite Re-Os dating	166.9±6.7	5	Li LX et al., 2009
		Molybdenite Re-Os dating	171±3	8
		Zircon U-Pb dating	179±2
		Zircon U-Pb dating	167.05±0.81		Zhang Y, 2013
		Zircon U-Pb dating	170.42±0.91		Zhang Y, 2013
3	Jidetun	Zircon U-Pb dating	175.7-168.2		Zhang Y, 2013
		Molybdenite Re-Os dating	168±2.5	4	Zhang Y, 2013
		Molybdenite Re-Os dating	169.1±1.8
4	Dashihe	Molybdenite Re-Os dating	186.7±5.0	5	Ju N et al., 2012
		Molybdenite Re-Os dating	185.6±2.7		Zhou LL et al., 2014
5	Chang’anpu	Zircon U-Pb dating	182.1±1.2		Ju N et al., 2012
		Molybdenite Re-Os dating	168±6.2	5
		Zircon U-Pb dating	170.2±1.6

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Li BL et al. (2009) conducted Re-Os isotopic dating of five molybdenite samples from the Fu’anpu molybdenum deposit. The results showed that molybdenite samples had Re content of 9.94×10^-6‒15.12×10⁻⁶, model ages of 165.3±2.4‒167.0±2.3 Ma (average: 166±1.0 Ma), and an isochron age of 166.9±6.7 Ma, with an MSWD of 0.60. Accordingly, they concluded that the Fu’anpu deposit formed during the Early Yanshanian. Yu XQ et al. (2008) also conducted Re-Os isotopic dating of eight molybdenite samples from the Fu’anpu deposit, obtaining model ages of 166.9‒169.9 Ma, a weighted average of 168.22±0.87 Ma, and an isochron age of 171±3 Ma. Therefore, they drew the same conclusion about the formation age of the Fu’anpu deposit. Through zircon U-Pb isotopic dating, Liu B (2001) determined that the porphyroid monzogranites (ore-hosting surrounding rocks) of the deposit have a weighted average age of 179±2 M. Similarly, Zhang Y (2013) conducted the zircon U-Pb isotopic dating of porphyroid monzogranites and migmatitic granites in the Fu’anpu deposit, obtaining weighted average ages of 167.05±0.81 Ma and 170.42±0.91 Ma, respectively. All these results indicate that the Fu’anpu molybdenum deposit is the product of the Early Yanshanian metallogenic event.

Zhang Y (2013) tested four granodiorite samples from the ore-forming plutons of the Jidetun deposit, obtaining single-grain zircon U-Pb ages of 175.7‒168.2 Ma, a weighted average Re-Os isotopic age of 165.9±1.2 Ma, and an isochron age of 168±2.5 Ma. Shao JL et al. (1990) found that the molybdenite samples from the Jidetun molybdenum deposit had Re-Os isotopic model ages of 168.79±0.42‒169.91±0.47 Ma, a weighted average age of 169.31 Ma, and an isochron age of 169.1±1.8 Ma (MSWD=7). These similar mineralization ages indicate that the Jidetun molybdenum deposit was formed during the Early Yanshanian.

Wang CH et al. (2009) tested 10 molybdenite samples from the Daheishan deposit, obtaining Re-Os isotopic model ages of 166.9‒169.6 Ma and an isochron age of 168.2±3.2 Ma. For granodiorite porphyries closely related to mineralization and granodiorites unrelated to mineralization in the Daheishan deposit, Ge WC et al. (2007) obtained the sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages of 170±3 Ma and 178±3 Ma, respectively. Zhang Y (2013) determined the Re-Os isotopic model age of molybdenite in the deposit to be 168.7±3.1 Ma. Ju N (2020) determined that biotite monzogranites in the Chang’anpu copper/molybdenum deposit have an average zircon U-Pb age of 182.10±1.20 Ma. Therefore, it can be inferred that the Daheishan deposit was formed during the Early Yanshanian.

Ju N et al. (2012) dated the Re-Os isotopic ages of five molybdenite samples from shallow ore bodies of the Dashihe deposit, obtaining model ages of 182.1±2.7‒191.9±2.6 Ma, a weighted average age of 186.7±5.0 Ma, and an MSWD of 11.8. Sun DY et al. (2011) determined that the molybdenites in the Dashihe deposit have a Re-Os isotopic model age of 185.6±2.7 Ma. The two groups of data are roughly consistent, indicating that the Dashihe molybdenum deposit formed during the Early Yanshanian.

As revealed by the above metallogenic chronological research, the metallogenic events of molybdenum deposits represented by Daheishan and Chang’anpu in central Jilin primarily occurred during the Middle Jurassic. These metallogenic processes occurred almost simultaneously, indicating that the metallogenic process in the late stage of the Early Jurassic or the early stage of the Middle Jurassic occurred in the same geodynamic setting. It is noteworthy that for most molybdenum deposits such as Daheishan, Fu’anpu, and Chang’anpu, the U-Pb ages of ore-bearing porphyry plutons are nearly consistent with the Re-Os ages of molybdenite in ores, indicating that porphyry plutons are more likely to be ore-forming plutons. However, for some molybdenum deposits such as Jidetun and Dashihe, the U-Pb ages of ore-bearing porphyry plutons deviate from the Re-Os ages of molybdenite by more than 5‒10 Ma, indicating that the mineralization of molybdenum was later than the diagenetic age of ore-bearing porphyry plutons, and thus the porphyry plutons are not ore-forming plutons.

5.2   Sources of ore-forming fluids and materials

5.2.1   Properties of ore-forming fluids

Many previous researchers have conducted studies on the ore-forming fluids of the Daheishan deposit. This deposit contains various fluid inclusions, primarily including mono-phase vapor inclusions (V), mono-phase liquid inclusions (L), two-phase vapor-liquid inclusions (L+V), and daughter mineral-bearing three-phase inclusions (L+V+S; Table 9). CO₂ is the main component of fluid inclusions, and the daughter minerals consist of halite, Fe₂CO₃, and Fe₂O₃. The ore-forming fluids belong to a CO₂-H₂O-NaCl system (Zhang Y, 2013). As indicated by the thermometry of fluid inclusions, the Daheishan deposit has high homogenization temperatures of 160°C‒417.6°C, salinities of 4.32%‒41.05% NaCleqv, and fluid densities of 0.62‒1.03 g/cm³, signifying medium- to high-temperature and medium- to high-salinity fluids (Tang RL et al., 1995; Tu GZ et al., 1988; Zhang Y, 2013). Zhang Y et al. (2013) estimated that the Daheishan deposit had a mineralization depth range of 1.63‒4.61 km, which is nearly the same as the results derived by, and Wang Q et al. (2005) and is similar to the mineralization depths of typical porphyry deposits (Fig. 8).

Table 9.  Characteristics of inclusions in the Daheishan molybdenum deposit (after Lu ZQ, 2017).

Inclusion type	Size/μm	Homogenization temperature/°C	Salinity/ %NaCleqv	Metallogenic pressure /MPa, calculation method	Depth/km	Data source
V+L	6‒12	171‒398	1.6‒19.8	30‒100 Isovolumic cross plot	4	Wang Q et al., 2007
S+V+L	8‒16	313‒443	37.9‒49.8
V+L	5‒25	232‒397	1‒17	8‒22 Isovolumic cross plot	0.83‒0.94	Wang ZG et al., 2011
S+V+L	6‒10	243‒369	34‒44
V+L	8‒12	196.5‒401.3	1.7‒7.1	14.5‒35.4 Empirical equation	1.5‒3.5	Wang L et al., 2012
S+V+L	8‒16	313.2‒440.5	38.71‒49.92
V+L	3‒16	160‒417.6	4.48‒12.55	16.3‒46.1 Empirical equation	1.63‒4.61
S+V+L	4‒10	320‒405	34.43‒41.05

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Figure 8.  Histogram and curves of homogenization temperature vs. salinity of inclusions in the Daheishan molybdenum deposit (after Zhang Y, 2013).

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The ore-forming fluid system of the Daheishan deposit exhibited a certain changing pattern from the early and middle mineralization stages to the late mineralization stage, with homogenization temperatures gradually decreasing from 300°C‒460°C to 196.5°C‒300°C, and salinities shifting from 1.7%‒49.92% NaCleqv to 1.7%‒7.1% NaCleqv (Table 10). In the early and middle mineralization stages, the two-phase vapor-liquid aqueous inclusions displayed high temperatures and low salinities. In contrast, the daughter mineral-bearing three-phase inclusions exhibited high temperatures and high salinities, similar to the fluid inclusions of magmatic-hydrothermal fluids. The coexistence of the two types of inclusions reveals the occurrence of a boiling process. Accordingly, the changes in the physical and chemical conditions of fluids led to large-scale mineral precipitation. In the late mineralization stage, the involvement of meteoric water significantly reduced the temperature and salinity of fluids, forming numerous two-phase vapor-liquid aqueous inclusions with low temperatures and salinities. The ore-forming fluids of the Daheishan deposit, which were high-temperature and high-salinity magmatic fluids in the early stage evolved into the coexisting high-temperature, high-salinity inclusions and high-temperature, low-salinity inclusions in the middle stage under the effects of decompressional boiling or immiscibility and finally into medium- to low-temperature and low-salinity fluids dominated by meteoric water in the late stage (Fig. 9).

Table 10.  Characteristics and parameters of inclusions in the Daheishan molybdenum deposit (after Zhang Y, 2013).

Mineralization stage	Inclusion type	Size/μm	Gas-liquid ratio/%	Measurement quantity	Tm (ice)/°C	Th/°C	Salinity /%NaCleqv	Density /g.cm−3	Hydrostatic pressure /MPa	Depth/km
(Ⅰ)	L+V	4‒8	10-20	2	−6.5‒−8.3	400‒417.6	9.8‒12	0.65‒0.66	41.1‒46.1	4.1‒4.6
	L+V+S	4‒10	20‒25	2	‒	403‒405	39.85‒41.05	0.94	39.9‒41.1	4‒4.1
(Ⅱ)	L+V	3‒12	15‒25	2	−3.5‒−4.7	359‒368	4.48‒6.58	0.62‒0.67	29.9‒32.3	3‒3.2
	L+V+S	6‒10	15‒20	5	‒	340‒378	35‒38.16	0.97‒1.02	35.8‒38.2	3.6‒3.8
(Ⅲ)	L+V	5‒12	10‒20	57	−2.9‒9.6	230‒340	4.94‒13.55	0.73‒0.89	20.4‒36	2.1‒3.6
	L+V+S	6‒8	5‒15	2	‒	320‒340	34.43‒35.78	1.02‒1.03	34.4	3.4
(Ⅳ)	L+V	5‒8	5‒15	1	−6.5	218	7.43	0.89	20.39	2.04
(Ⅴ)	L+V	4‒6	5‒15	2	−8.3‒12.5	160‒185	7.8‒9.5	0.93‒0.97	16.3‒17.6	1.6‒1.8

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Figure 9.  Diagrams of T-W-ρ (a) and salinity - homogenization temperature - pressure (b) of inclusions in the Daheishan molybdenum deposit (after Zhang Y, 2013).

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5.2.2   Sources of ore-forming fluids

Wang L (2012) tested the hydrogen and oxygen isotopes in quartz samples from the Daheishan deposit, obtaining slightly varying δ¹⁸O values of 7.9‰‒11.1‰ and significantly varying δD values of −46.7‰‒71.7‰. The δ¹⁸O values of ore-forming fluids from the Daheishan deposit were calculated using the equation for equilibrium fractionation between minerals (e.g., quartz and calcite) and water, expressed as δ¹⁸OQ-δ¹⁸O_H2O=3.38×10⁶/T2-3.4 (Woodcock et al., 1978; Wei JY,1987; Wu G et al., 2005; Wu et al., 2007). The δD values were directly measured from fluid inclusions in quartz and calcite (Fig. 10). The obtained δ¹⁸O and δD values all fell between the magmatic water and meteoric water lines, closer to the magmatic water line. It is generally recognized that the ore-forming fluids of porphyry copper/molybdenum deposits consist primarily of magmatic water and meteoric water (Rui ZY et al., 1984; Zhu BQ et al., 2001) and that the isotope exchange between magmatic water and ore-forming fluids is the primary reason for the variation in δ¹⁸O values of ore-forming fluids. Therefore, the ore-forming fluids of the Daheishan deposit belong to magmatic water, possibly originating from a deep or even mantle source. Zhou LL et al. (2014) analyzed the hydrogen and oxygen isotopes of quartz veins from various mineralization stages of the Daheishan deposit. The results showed that ore-bearing quartz vein samples had δ¹⁸O_H2O values of 8.5‰‒10.3‰ and δD values from −72‰ to −51‰. These data primarily fell below the magmatic water line in the δD-δ¹⁸O diagram, far from the meteoric water line. However, the samples from molybdenum-bearing quartz veins in the dominant mineralization of the Daheishan deposit had δ¹⁸O_H2O values of 1.71‰‒5.51‰ and δD values ranging from −71.7‰ to −46.74‰, which fell significantly closer to the left meteoric water line in the δD-δ¹⁸O diagram (Wang ZG et al., 2012; Zhang Y, 2013). These results reveal that the ore-forming fluids of the Daheishan deposit primarily comprised magmatic water, mixed with a minor amount of meteoric water in the late mineralization stage. Wang YD et al. (1986) tested the oxygen isotopes of different minerals from various mineralization stages of the Daheishan deposit. They found that ore-forming fluids from the quartz-K-feldspar alteration stage, the beresite alteration stage, and the late carbonation stage had average δ¹⁸Ow values of +5.5‰, +2.69‰, and −4.85‰, respectively, indicating the inclusion of meteoric water in the mineralization process. Yu XQ et al. (2008) tested fluid inclusions in ore-bearing quartz veins from the early, main, and late mineralization stages, obtaining homogenization temperatures of 208°C‒443°C, 197°C‒398°C, and 171°C‒301°C and salinities of 2.9%‒49.8%, 1.6%‒43.9%, and 1.6%‒19.8% NaCleqv, respectively. Li T et al. (1963) found that the Daheishan deposit in the main mineralization stage had mineralization temperatures, pressures, and depths of 300°C‒390°C, 5‒25 MPa, and 0.83‒0.94 km, respectively. Determined that the fluid inclusions in the Daheishan deposit had homogenization temperatures of 160°C‒417.6°C, salinities of 4.48%‒41.05% NaCleqv, δ¹⁸O values of 7.3‰‒10.5‰, and δD values of −62‰‒−64‰. They concluded that the ore-forming fluids were primarily magmatic water, with the contribution of meteoric water in the later stage.

Figure 10.  Projection of hydrogen and oxygen isotopes of the Daheishan molybdenum deposit (after Hou XG, 2017).

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5.2.3   Sources of ore-forming materials

S isotopes: Wang L (2012) and Zhou et al. (2014) conducted S isotopic analysis of the molybdenite and pyrite in the Daheishan deposit, respectively, obtaining δ³⁴S values of 0.8‰‒2.88‰ (Table 11). Specifically, molybdenite samples had δ³⁴S values of 1.1‰‒2.61‰ (average: 1.94‰), and pyrite samples had δ³⁴S values of 0.8‰‒2.88‰ (average: 1.87‰). All the S isotope values deviated slightly from those of sulfur in meteorites. Moreover, the δ³⁴S values varied within a narrow range and exhibited a normal distribution roughly. All these indicate highly homogeneous S isotopes and the characteristics of magmatic sulfur. Zhou LL et al. (2014) found that no significant fractionation of S isotopes occurred in various mineralization stages, implying that S isotopes in the Daheishan deposit almost maintain in equilibrium between molybdenite and pyrite. As shown by the analytical results of six ore samples from the Fu’anpu molybdenum deposit, these samples had δ³⁴S values of 1.5‰‒4.1‰, indicating a relatively concentrated distribution of S isotopes; the δ³⁴S values (1.5‰‒1.6‰) of molybdenite were much lower than those (2.6‰‒4.1‰) of pyrite, suggesting that the fractionation of S isotopes between molybdenite and pyrite has not yet reached equilibrium (Hou XG, 2017; Fig. 11). Wang ZG (2012) tested the S isotopes of pyrite and molybdenite in ores from the Daheishan deposit. The results demonstrated that the S isotopes in the mining area had high δ³⁴S values of +0.8‰‒+2.5‰ (average: +1.3875), suggesting a slight positive deviation from sulfur in meteorites. The results also showed that the δ³⁴S values varied slightly and exhibited a prominent tower-like distribution pattern, indicating that S isotopes are highly homogeneous and mainly of mantle origin. These results suggest that ore-forming materials primarily originated from the upper mantle or deep crust (Wang ZG, 2012; Table 12).

Table 11.  S isotopic composition of sulfides in ores of typical molybdenum deposits in central Jilin (after Hou XG, 2017).

Deposit name	Sample no.	Tested mineral	δ34SV-CDT/‰	Data source
	LM01-N12	Pyrite	5.7
	LM01-12	Molybdenite	4.5
	LM11-B5	Pyrite	5.1
	LM11-B14	Pyrite	5.2
	LM-11-W1	Pyrite	4.5
	LM-11-W1	Molybdenite	5
Luming	LM01-B33	Molybdenite	4.5
	LM-F-1	Molybdenite	5.63	Xia B, 2000
	LM-F-2	Molybdenite	5.54
	LM-1	Pyrite	6.71
	LM-20	Molybdenite	5.37
	LM-21	Molybdenite	5.52
	LM-30	Molybdenite	5.68
Huojihe	DKH11	Pyrite	1.2	Xiao QH et al., 2002
	DKH12	Pyrite	1.8
	DKH43	Pyrite	4.2
	DKH45	Pyrite	4.4
	DHSTJ4-01-B54	Molybdenite	2.5	Wang L, 2012
	3DHSTJ02-02-B13	Molybdenite	1.9
	DHSTJ02-03-B3	Molybdenite	1.1
	DHSTJ4-01-B58	Molybdenite	1.3
	DHSTJ4-01-B12	Pyrite	1
	DHSTJ4-01-B38	Pyrite	0.8
	1DHSTJ02-05-B1	Pyrite	1.3
	DHSTJ4-01-B17	Pyrite	1.2
Daheishan	DH-271	Pyrite	1.83	Zhou LL et al., 2014
	DH-258	Pyrite	2.35
	DH-259	Pyrite	2.52
	DH-270	Pyrite	2.06
	DH-263	Pyrite	2.57
	DH-305	Pyrite	2.07
	DH-78	Pyrite	2.88
	DH-301	Molybdenite	2.17
	DH-254	Molybdenite	1.96
	DH-255	Molybdenite	2.61
	DH-261	Molybdenite	2.04
	DH-272	Molybdenite	1.84
Fu’anpu	FAP-08	Pyrite	2.6	Xing SW et al., 2005
	FAP-10	Pyrite	4.1
	FAP-11	Pyrite	2.7
	FAP-29	Molybdenite	1.6
	FAP-30	Molybdenite	1.5
	FAP-31	Molybdenite	3.2

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Figure 11.  Pb isotopic characteristics of ore minerals in typical molybdenum deposits in the eastern Jilin-Heilongjiang area (after Hou XG, 2017).

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Table 12.  Pb isotopic characteristics of intrusive rocks and minerals in porphyry molybdenum deposits in the eastern Jilin-Heilongjiang area (after Hou XG, 2017).

Deposit name	Sample no.	Tested sample	206Pb/204Pb	207Pb/204Pb	208Pb/204Pb	Data source
Huojihe	110723-1	Biotite monzogranite	18.734	15.554	38.267
	110723-5	Biotite monzogranite	19.401	15.606	38.696
	090817-13	Granitic aplite	19.242	15.589	38.629
	090817-14	Granitic aplite	19.041	15.592	38.592
	090817-11	Granitic aplite	19.202	15.612	38.656
	090817-12	Granitic aplite	19.159	15.596	38.592
Daheishan	DH-271	Pyrite	18.364	15.576	38.231	Zhou LL et al., 2014
	DH-258	Pyrite	18.336	15.607	38.326
	DH-259	Pyrite	18.316	15.564	38.186
	DH-270	Pyrite	18.306	15.586	38.242
	DH-263	Pyrite	18.297	15.565	38.177
	DH-305	Pyrite	18.023	15.595	38.309
	DH-78	Pyrite	18.064	15.583	38.204
	DH-301	Molybdenite	18.172	15.553	38.106
	DH-254	Molybdenite	18.293	15.530	38.063
	DH-255	Molybdenite	18.368	15.540	38.103
	DH-261	Molybdenite	18.321	15.546	38.124
	DH-272	Molybdenite	18.427	15.581	38.234
	DH-192	Qianzuluo granite porphyry	18.697	15.583	38.502
	DH-193	Qianzuluo granite porphyry	18.708	15.580	38.494
	DH-194	Qianzuluo granite porphyry	18.705	15.590	38.518
	DH-195	Qianzuluo granite porphyry	18.657	15.548	38.379
	DH-96	Qianzuoluo granite diorite	18.731	15.600	38.583
	DH-97	Qianzuoluo granite diorite	18.671	15.563	38.409
	DH-292	Qianzuoluo granite diorite	18.711	15.555	38.494
	DH-293	Qianzuoluo granite diorite	18.720	15.552	38.476
	DH-296	Changgangling granodiorite	18.488	15.536	38.298
	DH-297	Changgangling granodiorite	18.437	15.535	38.224
	Dhs1-1	Granodiorite porphyry	18.590	15.571	38.338
	Dhs1-2	Granodiorite porphyry	19.196	15.602	38.295
	Dhs5-1	Granodiorite porphyry	18.458	15.563	38.275
	Dhs5-2	Granodiorite porphyry	18.473	15.562	38.343
		Pyrite	18.5454	15.574	38.2499
		Pyrite	18.346	15.5637	38.2157
		Pyrite	18.4554	15.5749	38.2634
Jidetun	JJd1-1	Granodiorite	18.8185	15.5771	38.6824	Zhang Y, 2013
	JJd1-2	Granodiorite	18.7938	15.5769	38.6612
	JJd2-1	Diorite-porphyrite	19.0123	15.5847	38.3524
	JJd2-2	Diorite-porphyrite	18.969	15.5862	38.3677
Fu’anpu	JFa1-1	Adamellite	18.7322	15.5684	38.446
	JFa1-2	Adamellite	19.1255	15.5878	38.4606
Xinhualong	Xhl1-1	Diorite-porphyrite	18.8479	15.5967	38.5863
	Xhl1-2	Diorite-porphyrite	18.7611	15.5903	38.5108
	Xhl2-1	Granodiorite porphyry	19.2028	15.6144	38.8775
	Xhl2-2	Granodiorite porphyry	19.2007	15.6131	38.8874

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Re-Os isotopes: Han CM et al. (2014), Zhou LL et al. (2014), and Wang CH et al. (2009) analyzed the Re-Os isotopes of molybdenite in the mining area, obtaining Re contents of 17×10⁻⁶‒43.57×10⁻⁶ (Fig. 12). This result indicates that the ore-forming materials originated from a mixed crust-mantle source. The molybdenum deposits in the study area, namely Daheishan, Fu’anpu, Xinhualong, Liushengdian, Dashihe, Jidetun, Houdaomu, Sifangdianzi, and Tianbaoshan (Dongfeng), have average Re contents of 30×10⁻⁶, 11.74×10⁻⁶, 61.63×10⁻⁶, 15.82×10⁻⁶, 6.62×10⁻⁶, 0.47×10⁻⁶, 48.81×10⁻⁶, 4.75×10⁻⁶, and 5.01×10⁻⁶, respectively (Table 13). These values are generally consistent with the characteristics of molybdenum-dominant porphyry and hydrothermal deposits in foreign countries and areas in China such as the North China Platform and the Qinling Mountains. Moreover, for the molybdenum deposits in the study area, their ore-forming materials originated from a mixed crust-mantle source or crust source, and the Re content in molybdenite of these deposits is not significantly correlated with their mineralization ages. Different from the molybdenite in the Dexing copper deposit and in the Cordillera and Andes metallogenic belts on the eastern coast of the Pacific Ocean, the molybdenite in the study area has a low Re content, which is similar to the molybdenite formed by partial melting of the thickened lower crust due to crust source transformation or delamination.

Figure 12.  Re content in molybdenite of typical molybdenum deposits in the eastern Jilin-Heilongjiang area (after Hou XG, 2017).

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Table 13.  Re content in molybdenite of typical molybdenum deposits in the eastern Jilin-Heilongjiang area (after Hou XG, 2017).

Deposit name	Sample no.	Re content/10-6	Data source		Deposit name	Sample no.	Re content/10-6	Data source
Luming	HLM3	12.68	Xu WL et al., 2002		Fu’anpu	FAP-1a	9.94	Li LX et al., 2009
	HLM-4	15.31				FAP-16	11.46
	HLM-5	15.71				FAP-1c	15.13
	HLM-5	15.71				FAP-1f	10.52
	HLM-6	11.90				FAP-1g	11.68
	06001-1	48.61	Xu WL et al., 2000			FAP-04	15.09	Yao FL et al., 2006
	06001-2	31.58				FAP-07	21.04
	06001-3	30.40				FAP-08	15.33
	06401-1	30.87				FAP-09	12.11
	06401-2	33.54				FAP-12	14.18
	06401-3	24.60				FAP-23	15.34
	06401-6	27.09				FAP-29	10.14
	TWL1-1	33.18				FAP-30	9.38
	TWL1-2	32.61			Jidetun	JD001	22.04	Yao JJ et al., 2002
	TWL1-3	49.64				JD002	10.39
	TWL1-4	34.60				JD003	9.32
	LMD90816-3	57.11	Xu ZG et al., 2008			JD004	12.90
	LMD90816-1	30.87				JD005	8.42
	LMD90S16-2	22.66			Dashihe	YB061-1	6.04	Ju N et al., 2012
	LMD90S16-8	30.48				YB061-3	6.94
	LMD90S16-13	12.01				YB061-4	5.65
	JDC	17.11				YB061-5	6.09
	JDC	17.41				YB061-5	6.18
	LM10	45.24	Yang JH et al., 2003			YB061-6	6.43
	LM12	48.63			Houdaomu	JHD04-1	50.07	Zhang Y et al,. 2013
	LM17	35.39				JHD04-2	47.12
	LM11	30.18				JHD04-3	43.82
	LM14	29.57				JHD04-4	54.23
	LM13	30.73			Jiapigou	YB010-1	78.43	Zeng PS et al., 2006
	Lm-1	37.28	Yang YC et al., 2010			YB010-2	80.76
	Lm-2	61.76				YB010-3	63.89
	Lm-3	30.20				YB010-4	57.06
	Lm-6	34.73				YB010-5	55.33
	Lm-15	21.00				YB010-6	66.50
	Lm-16	39.69			Baoshan	YB064-4	23.09	Zhang D et al., 2017
	Lm-17	38.40				YB064-6	27.83
Huojihe	TWH1-1	19.80	Zhang H et al., 2007			YB064-8	16.22
	TWH1-2	21.72				YB064-9	41.61
	TWH1-4	17.33				YB064-10	36.86	Zhang Y et al., 2013
	TWH1-5	13.19				Jxlun-010	62.30
	TWH1-3	20.09				Jxlun-014	11.31
	090817-15	13.69				Jxlun-015	73.24
	090817-15	12.90				Jxlun-016	99.66
	090S17-3	12.04			Liushengdian	YB065-1	15.75	Zhao HG et al., 2005
	090817-5	30.92				YB065-2	15.71
	13HJH11	27.58	Zhang Y et al., 2016			YB065-3	16.80
	13HJH13	18.91				YB065-4	16.19
	13HH20	17.92				YB065-5	18.08
	13HJH22	36.50				YB065-6	17.22
	13HH25	31.05				JIsm-004	11.88	Zhang Y et al., 2013
Daheishan	DHS-1a	24.15	Wang CH et al., 2009			JIsm-005	18.43
	DHS-1d	27.85				JIsm-006	18.34
	DHS-1e	27.15				JIsm-006	17.67
	DHS-1h	39.57				JIsm-007	14.81
	DHS-1i	39.36				JIsm-008	13.76
	DHS-3a	43.57			Shimadong	B7-1	24.67	Zhu BQ et al., 2001
	DHS-2c	31.03				B7-2	24.24
	DHS-2e	42.81				B8-1	31.92
	DHS-3c	29.02				B8-1	32.57
	DHS-3d	32.68				B10-1	28.91
	DH-252	27.14	Zhou LL et al., 2014			B10-2	29.06
	DH-301	24.88				B11-1	22.45
	DH-255	24.27				B11-2	19.94
	DH-261	23.95				B11-3	20.46
	DH-272	28.22
	DH273	23.38
	Jdhm-029	25.78	Zhang et al., 2013

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Mo isotopes: The δ^98/95Mo values of molybdenite in the study area generally fall within the typical Mo isotope range of molybdenite. However, molybdenite of different structures exhibits significantly different Mo fractionation. Specifically, veined molybdenite shows high δ^98/95Mo values, while disseminated molybdenite displays low δ^98/95Mo values. Regarding the difference in δ^98/95Mo values between veined and disseminated molybdenite in molybdenum deposits in the study area, the Daheishan deposit shows the highest difference of 0.79‰, the Luming deposit shows a difference of 0.56‰, and the Fu’anpu and Baoshan deposits show small differences of 0.38‰‒0.40‰ (Fig. 13). The large-scale Daheishan and Luming deposits have ore-forming fluids with medium-to-high temperatures and medium-to-high salinities (Fig. 14). In contrast, the small and medium-sized Fu’anpu and Baoshan deposits have ore-forming fluids with medium-to-low temperatures and medium-to-low salinities. Therefore, the temperature and salinity of ore-forming fluids may be the primary factors influencing molybdenite mineralization.

Figure 13.  Comparison of Re content (a) and Re content vs. Re-Os model age (b) of molybdenite in typical molybdenum deposits in central and eastern Jilin (after Zhang Y, 2013).

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Figure 14.  Re-Mo content pattern of molybdenite in typical molybdenum deposits in the eastern Jilin−Heilongjiang area (after Hou XG, 2017).

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5.3   Tectonic evolution and metallogenic model

Concerning the tectonic framework, the Daheishan deposit lies within the Late Paleozoic−Early Mesozoic Paleo-Pacific tectonic zone and the Meso-Cenozoic circum-Pacific tectonic zone, located to the east of the Xing’an-Mongolian Orogenic Belt. From the Late Permian to the Early Triassic, the Heilongjiang plate, formed by medium and small blocks scattered between the North China and Siberian plates, met the North China plate in the Changchun−Yanji area, commencing the collisional orogeny phase. The Yanshanian marked a geotectonic shift in East Asia (Table 14). Within the study area, this transition period symbolizes the end of the sequential convergence of the Xing’an-Mongolian Orogenic Belt and its neighboring blocks, leading to the formation of the circum-Pacific active continental margin. The Yanshanian tectonic evolution of the Pacific tectonic domain had become the predominant factor constraining the tectonic evolution in the study area. During the Mesozoic, the Izanagi plate moved in the NW13°‒NE2° direction, leading to a small angle intersection with Eurasia (Zhu YS et al., 1995). Some scholars suggest that this small-angle oblique subduction is the primary origin of large-scale strike-slip faults, sedimentary basins, and calc-alkaline magmatic belts in eastern China (Zhu X et al., 1983). Therefore, from the Yanshanian Movement or the Jurassic onwards, the tectonic evolution of the circum-Pacific tectonic domain commenced, the study area began to experience Pacific subduction, and the eastern Jilin-Heilongjiang area was part of the circum-Pacific belt (Zhu BQ, 1998). Due to Pacific plate subduction, NNE- and NE-oriented faults were highly developed, controlling regional magmatic activity. Given this specific geotectonic location, the four-stage ore-bearing pluton of the Daheishan deposit is believed to result from hypomagma lodging at the intersections of NNE- and EW-oriented faults. As deep magmas ascended, they crystallized, differentiated, and solidified into rocks continuously. Concurrently, a significant volume of volatile constituents accumulated in the magmas, prompting the cryptoexplosion of molten lava at the magmas’ front and leading to the formation of certain-scale cryptoexplosive breccias with low ore-bearing potential. Meanwhile, the intrusion of porphyries formed numerous reticular fissures in the Qiancuoluo granodiorites, which were later filled with ore-forming fluids. During the middle-late mineralization stage, volatile constituents accumulated locally as fissures filled with various veins. As a result, internal pressure increased, leading to local cryptoexplosions and the formation of some small-scale cryptoexplosive breccia pipes (Zhang ZK, 1988). As the sole stock-type molybdenum deposit in northeast China, the Daheishan deposit is unique and distinctive, though it shares similarities with typical porphyry deposits. Its ore-forming parent rocks are calc-alkaline granitic rocks formed in a continental margin compressional tectonic setting. Conversely, the ore-forming parent rocks of the fine-stockwork molybdenum deposits in the western United States (such as the famous Climax molybdenum deposit) are primarily alkaline-calc and alkaline intrusive rocks formed in a continental back-arc extensional tectonic setting. The ore-forming plutons of the Daheishan deposit occur as stocks, differing from the fine-stockwork deposits associated with plutonic intrusions (e.g., the Huojihe and Luming deposits). This study suggests that the Daheishan deposit may be a fine-stockwork stock-type molybdenum deposit occurring in calc-alkaline ore-forming parent rocks.

Table 14.  Geological characteristics of the Daheishan deposit and its adjacent areas (after Ju N, 2020).

Deposit	Fu’anpu	Jidetun	Daheishan	Dashihe	Chang’anpu
Mineral type	Mo	Mo	Mo	Mo	Cu、Mo
Ore-controlling structure	NE- and NW-trending faults	NE- and NW-trending faults	Intersection of NE- and EW-trending faults	NE-trending dominant fault and NW-trending secondary fault	Intersection of NE- and NW- trending faults
Ore-forming plutons and mineralization age	Porphyroid monzogranite	Porphyroid monzogranite, quartz diorite	Granodiorite, granodiorite porphyry	Cryptoexplosive breccia pipe, porphyroid granodiorite	Biotite monzogranite primarily, followed by the cryptoexplosive breccia pipe
Ore body occurrence	Branched/within plutons	Ellipsoidal and stratoid/within plutons	Circular/within plutons	Turbinate /breccia pipe and within plutons	Stratiform, lensoid, veined/quartz veins, within plutons
Surrounding rock alteration	Potassic alteration, sericite alteration, silicification, chloritization, epidote alteration	K-feldspar alteration, sericite alteration, silicification, epidote alteration, kaolinization, greisenization	Silicification, K-feldspar alteration, sericite alteration, propylitic alteration	Sericitization and chloritic alteration	Silicification, potassic alteration, carbonation, chloritic alteration, epidote alteration, argillization, sericite alteration
Ore mineral	Molybdenite, pyrite	Molybdenite, pyrite, chalcopyrite, magnetite, galena, sphalerite	Molybdenite, pyrite, chalcopyrite	Molybdenite, pyrite, chalcopyrite, pyrrhotite, galena, sphalerite	Molybdenite, chalcopyrite, pyrite, magnetite, sphalerite, galena
Data source	Chen YJ et al., 1991; Zhang WH et al., 1993; Zhang Y, 2013

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It is generally believed that a porphyry deposit must be accompanied by deep fault zones nearby, which connect the upper mantle with the upper crust. The Daheishan deposit lies north of the circum-Pacific molybdenum metallogenic belt in eastern China, which evolved as follows: During the Middle and Late Mesozoic, numerous NE- or NEE-trending Mesozoic deep fault zones formed due to the influence of the Pacific tectonic movement; Accompanying these formations was the emplacement and eruption of intermediate-acid magmas. Meanwhile, the intersections of these deep fault zones with the pre-Yanshanian nearly-E-orientated paleo-structures tended to dictate the emplacement sites of intermediate-acid granitoids, thereby forming the Mesozoic tectono-magmatic belt and its associated molybdenum metallogenic belt. The metallogenic evolution of the Jilin-Heilongjiang orogenic belt is as follows: (1) during the Late Triassic, the influence of the circum-Pacific tectonic active zone led to the formation of the NE-trending Dunhua–Mishan translithospheric fault and Yitong–Shulan deep fault and dictated the distribution of regional volcanic-magmatic tectonic belts. Meanwhile, the NE-trending Jilin−Liuhe fault zone, which ran through the mining area, controlled the formation of the volcanic faulted basin in central Jilin; (2) Since the beginning of the Mesozoic, basement faults, like the Shuanghezhen−Qiancuoluo (Daheishan)−Dadingshan fault zone, displayed extensional characteristics due to the influence of the circum-Pacific tectonic active zone. As a result, an uplift-fault belt with a N-S width of 17 km and a W-E length of nearly 40 km formed, significant in controlling the formation of molybdenum ore fields (Wang CH, 2009); (3) After the Late Triassic, increased fault activity resulted in deep magma upwelling on both sides of several fault-controlled blocks. Intermediate-acid magmas erupted at the junctions of uplifts and faults, followed by the intrusions of mafic magmas and ultramafic-intermediate-acid magmas. Multiple magma intrusions formed the complex plutons of the Daheishan deposit, including Changgangling biotite granodiorites and Qiancuoluo inequigranular biotite granodiorites, granodiorite porphyries, and felsitic granodiorite porphyries in chronological order. The ore bodies in the complex plutons were primarily hosted in the granodiorite porphyry plutons and their surrounding inequigranular granodiorite plutons, and mineralization mainly occurred after magmatism (Wang CH, 2009). The deep magma differentiation during ore body formation increased the molybdenum content in the late plutons, which in turn became increasingly small. During the upward intrusion and solidification of deep magmas, a significant volume of volatile constituents accumulated in the magmas, prompting the cryptoexplosion of molten lava at the magmas’ front. As a result, certain-scale cryptoexplosive breccias were formed at the top of the predominant ore-forming granodiorite porphyries. In the late stage of magmatism, hydrothermal fluids rich in potassium, primarily in a high-temperature gaseous state, rose from the deep magma chambers. Along the intergranular pores and structural fractures of rocks, alkaline metasomatism by these fluids resulted in planar biotite-quartz-K-feldspar alteration. Consequently, the quartz content increased gradually, and the quartz filling and metasomatism along fractures led to the formation of K-feldspar or quartz-K-feldspar veins. Disseminated minerals, such as pyrite, molybdenite, chalcopyrite, tetrahedrite, and scheelite, were formed along with the K-silicate alteration in this stage. With a decrease in temperature and mixing of groundwater, ore-bearing fluids converted from a gaseous to a liquid state, and their physical and chemical properties changed accordingly. Further, the oversaturation of silica in the hydrothermal fluids led to the hydrolyzation of a large amount of feldspar, resulting in quartz-sericite alteration and beresite alteration, which were superimposed on the K-silicate alteration zone. As a result of these alterations, a large amount of molybdenite precipitated, and various ore-bearing veins, such as molybdenum-bearing quartz veins, molybdenite veinlets, quartz veins, and carbonate veins, were formed sequentially. After a long period, the mineralization process entered its middle-late stage, when volatile constituents accumulated locally as fissures filled with various veins. As a result, internal pressure increased, leading to local cryptoexplosions and the formation of some small-scale cryptoexplosive breccia pipes. In the late mineralization stage, the temperature dropped further, and groundwater predominated in and diluted the ore-forming fluids to a great extent. This dilution, along with the gradual exhaustion of ore-forming materials, led to carbonation, gypsum alteration, zeolite alteration, and weak pyritization. After this chain of events, the alteration and mineralization process ended, and a typical porphyry molybdenum deposit with concentric alteration and mineralization zoning was formed finally.

In sum, there are two metallogenic models of porphyry molybdenum deposits (Zhang Y, 2013): (1) the mixing and crystallization differentiation between intraplate-type basaltic magmas and overlying magmas from lower-crust melting; (2) the crystallization differentiation of magmas from lower-crust melting coupled with the accumulation and ore-forming processes of ore-bearing fluids. However, a contact metasomatic metallogenic model may also exist in both models. Given the formation of mafic complex-associated copper-nickel sulfide deposits in the study area during the Late Triassic (Local Chronicles Compilation Committee of Yongji County, 1985; Zhang LG., 1989), large-scale basaltic magmas might have existed in the MASH zone at least. During the lengthy cooling of the basaltic magmas, the generated heat might have been crucial for the lower-crust melting and the formation of large-scale granites in the study area. Additionally, some granites with a mixed crust-mantle source formed alongside the intraplating of basaltic magmas. Therefore, this study highlights that the diagenetic and metallogenic model should be as follows (Fig. 15): (1) the underplating of basaltic magmas induced the lower-crust melting, forming magma chambers; (2) ore-bearing fluids formed due to the intraplating of a minor portion of basaltic magmas, the mixing of these basaltic magmas with the magmas from the lower-crust melting, and the fractional crystallization of the magma chambers; (3) these fluids rose and underwent cryptoexplosion after porphyry formation, forming brecciated, veinlet-disseminated, and veined ore bodies successively; (4) these ore bodies made contact with the upper Paleozoic molybdenum-bearing strata, forming a contact metasomatic molybdenum deposit.

Figure 15.  Schematic of tectonic evolution of typical porphyry molybdenum deposits (after Zhang Y, 2013).

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5.4   Ore prospecting models

Large-scale mineralization is the product of a certain geodynamic environment (Mao JW et al., 1999; Zhang BT et al., 2003). The study area entered a continental margin environment in which the Pacific plate subducted toward Eurasia during the Early Jurassic. The intense and complex tectono-magmatic processes led to the formation of many molybdenum, gold, and copper deposits in the study area. This study thoroughly investigated the metallogenic mechanisms of molybdenum deposits in the study area by analyzing the geological characteristics, ore-forming fluids, geochemistry, and metallogenic chronology of these molybdenum deposits (Fig. 16). Based on the metallogenic models, the metallogenic dynamic model and the integrated gravity-magnetic-magnetotelluric prospecting model have been constructed for endogenetic molybdenum deposits in the study area (Zhang DH et al., 2001; Yuan DS et al., 2012; Zhang Y, 2013).

Figure 16.  Metallogenic dynamic model of typical molybdenum deposits in central and eastern Jilin (after Zhang Y, 2013).

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5.4.1   Metallogenic dynamic model

The geodynamic model of the Daheishan deposit is as follows: During the subduction of the paleo-Pacific plate, fluids metasomatized the basaltic magmas formed by the partial melting of the Proterozoic secondary lithospheric mantle. The basaltic magmas exhibited intraplating and underplating in two mineralization periods. The underplating of the basaltic magmas during the Early Jurassic triggered the lower-crust melting, forming magma chambers, which formed ore-bearing fluids in their late evolutionary stage. Consequently, the Early Jurassic molybdenum deposits, represented by Dongfeng and Jiapigou, formed in eastern Jilin. As the Pacific plate further subducted northwestwards, the continued underplating of basaltic magma caused the lower-crust melting, forming magma chambers. A small portion of basaltic magmas mixed with the magmas from the lower-crust melting, forming ore-bearing fluids in the late evolutionary stage of the magma chambers. Consequently, the Middle Jurassic molybdenum deposits, represented by Daheishan and Fu’anpu, formed in central Jilin (Fig. 17).

Figure 17.  Metallogenic models of the Daheishan porphyry molybdenum deposit (after Zhang Y, 2013).

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5.4.2   Integrated prospecting model

Gravity anomalies: the Bouguer gravity field of the Daheishan deposit area resides within a quasi-circular gravity low area surrounded by the annular Quchaihe-Nanloushan-Wangqi-Yongji-Huangyu-Quchaihe gravity gradient belt. On the 14 km × 14 km moving average residual gravity anomaly map, this gravity low area, which showcases many local gravity lows and highs with different strengths, morphologies, strikes, and scales, is encircled by a continuous bead-like local gravity high zone. Based on the comprehensive analysis of geological data, it can be inferred that the annular Bouguer gravity gradient belt and the annular curve of zero residual gravity anomalies are caused by the annular fault structural zone formed by the intersection of NE-, NW-, EW-, and SN-trending fault belts. The local gravity lows inside are closely related to the intermediate volcanic rocks of the Late Triassic Sihetun Formation, the intermediate and intermediate-acid volcanic rocks of the Early Jurassic Nanloushan Formation, and the Jurassic-Cretaceous intermediate-acid granitoids. The local gravity highs are largely the reflection of the outcropping, semi-outcropping, and concealed Paleozoic basement strata, such as the Early Paleozoic Cambrian Toudaogou Formation, the Late Paleozoic Lower Carboniferous Luquantun Formation, and the Middle Permian Fanjiatun Formation (Table 15). Consequently, it can be inferred that the study area contains well-developed fault structures, which form the hub (center) of tectonic activity in central Jilin. Particularly, the Daheishan annular fault structure, which is jointly formed by the NE-, NW-, EW-, and SN-trending faults, controlled the multi-phase magmatic eruptions and intrusions in the study area. Moreover, this annular fault structure provided rich ore-forming materials for the formation of endogenetic metal minerals, necessary heat sources for mineralization, and favorable space for ore storage. In addition, this annular fault structure controlled the volcanic activity centers at Daheishan and Nanloushan and acted as the fundamental ore-controlling structure of the Daheishan and Nanloushan mineralization concentration areas (ore fields).

Table 15.  Comparison between the Daheishan molybdenum deposit and typical copper molybdenum deposits in the world (after Wang ZG, 2012).

	Calc-alkaline stockwork molybdenum deposit			Alkali-calcic and alkaline stockwork molybdenum deposits				Porphyry copper deposit
	Stock type	Plutonic intrusion-associated type		Climax type	Alkaline type			Island-arc type	Continental-margin type
					Porphyry	Leucogranite
Magma series	Calc-alkaline; High-K calc-alkaline			High-K calc-alkaline; alkaline-calcic	Alkaline-calcic; Alkaline			Calcic; calc-alkaline	Calcic; calc-alkaline; alkaline-calcic
K57.5 range	1.5‒2.5			25‒4.4	4.0‒6.0			0.4‒2.5	0.6 to > 5
Tectonic setting	Convergent continental plate margin			Subduction-associated continental solitary extension ring	Continental solitary extension environment	Continental back-arc discrete environment; cratonic rift zone; rifts associated with oceanic basin stretching; hotspot		Convergent plate margin	Convergent continental plate margin
Paragenetic igneous rock	Quartz monzonite; granite; alaskite; aplite	Granodiorite; quartz monzonite; granite; alaskite		Rhyolite - quartz latite: granite aplite; K2O > 5%； SiO2 > 75%	Monzonite-syenite; K2O+Na2O > 10%; SiO2 61%‒65%	Leucogranite; K2O > 5%; SiO2 73%‒76%		Diorite quartz monzonite	Granodiorite- quartz monzonite
Porphyritic texture	Present	Present or not		Present	Present or not	Present		Present or not	Present or not
Chemical composition of rocks
TiO2/%	> 0.2	0.1‒0.2		<< 0.2	> 0.5	< 0.1		> 0.3	> 0.2
Rb/10-6	150‒350	100‒350		200‒800	No available data	> 200		< 100	< 210
Sr/10-6	300‒800	100‒800		< 125	No available data	< 100		500‒1000	450‒1500
Nb/10-6	< 20	< 10		25 to > 200	> 100	15‒140		< 5	< 20
Hydrothermal system
Maximum molybdenum reserves/t	300000	1600000		2000000	Small-sized	350000		30000	1500000
MoS2 content in mineralized zone/%	0.1‒0.25	0.1‒0.20		0.2‒0.49	< 0.3	0.25‒0.30		< 0.01	0.01‒0.1
Maximum F content/%	0.1‒0.25	0.05‒0.15		0.5‒2	No available data	1 to > 5		Low	0.04‒0.4
Major element	Mo-W-Cu	Mo-Cu-(W)		Mo-F-W-Sn-Zn-Ag-Pb-Cu-U	Mo-F-Sn-W	Mo-F-Sn-W-(Bi)-(As)		Cu-Au-(Mo)	Cu-Mo-Ag-Au-Pb-Zn
HF/HC1 ratio in fluids	Low	Low		High	No available data	No available data		Low	Low
Multiple mineralized crust	None	None		Common	No available data	Yes		No	Rare
Typical deposits	Kitsault; Hall; Buckingh-am; Daheishan in Jilin	Endako; Adanac; QuartzHill? Mt. Tolman; East Kounrad; Huojihe in Heilongjiang; Luming; Fu’anpu and Jidetun in Jilin		Climax; Urad-Henderson; Mt. Emmons; Pine Grove; Questa*; Mt. Hope*; Glacier Gulch*	Three Rivers stock; Rialto stock	Cave Peak; Malmbjerg; Bordvika; Mt. Pleasant		Panguna; Atlas; Marc opper	Bingham; San Manuel

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Aeromagnetic anomalies: on the 1:50000 aeromagnetic anomaly map, the magnetic field of the ore field hosting the Daheishan deposit is dominated by quasi-circular negative anomalies—the Cuoluotun anomalies (area: about 4 km²; intensity: −100‒−200 nT) surrounded by NE-directed annular high magnetic anomalies. The northwestern part of the annular high magnetic anomalies shows a gentle zonal distribution, with the strike shifting from NE to nearly EW from south to north. This part exhibits high continuity of anomaly curves and anomaly intensity of generally 300‒500 nT. In contrast, the southeastern part of the annular high magnetic anomalies exhibits a NE-directed arc belt protruding southeastward, a steep northwestern side, a gentle southeastern side, and peaks locally, with intensity mostly exceeding 500 nT and up to maximums of 1500‒2000 nT. Furthermore, the northwestern side of this part, which presents significant negative anomalies and low-continuity anomaly curves, can be divided into several small local quasi-circular and elliptical anomalies. The annular high magnetic anomalies are primarily related to the Late Hercynian-Yanshanian magmatic activity in the Daheishan-Toudaogou area. Its northwestern part is induced by Yanshanian intrusive rocks, and its southeastern part is caused by Late Hercynian ultramafic rocks and Yanshanian intermediate-acid intrusive rocks. The Qiancuoluo negative anomalies are related to the ore-bearing complex Changgangling pluton, which, as a plagiogranite pluton, shows negative anomalies due to thermal demagnetization caused by multi-stage magmatic intrusions and hydrothermal alterations. According to the strike of aeromagnetic anomalies, the ore-bearing plutons are primarily controlled by the intersection of NE- and EW-trending faults, which are inferred to be a part of the dominant structural system of the Daheishan ore field.

Integrated gravity-magnetic-magnetotelluric prospecting model: previous researchers conducted multiple surveys around the Daheishan deposit, including 1∶10000 surface magnetic surveys, 1∶25000 spontaneous potential and high-power induced polarization sounding, and gravity surveys along profiles. Ore-bearing plutons responded at varying degrees in these surveys, including high-amplitude anomalies in apparent charging rate (Ms), spontaneous potential (U), and soil molybdenum content and low-amplitude anomalies in gravity and magnetism. Significant application results have been achieved in the Daheishan deposit using the integrated geophysical and geochemical exploration method. As indicators, the obtained three types of high-amplitude anomalies and two types of low-amplitude anomalies provide a comparable, effective model for prospecting similar deposits in the study area.

5.4.3   Prospecting favorable areas

Based on the integrated geological, geophysical, and geochemical prospecting model of the Daheishan deposit, this study performed metallogenic prediction by analyzing the metallogenic geological conditions and the integrated aeromagnetic-gravity-geochemical anomalies in the study area. As a result, three areas with negative aeromagnetic anomalies were selected as favorable sections for the prospecting of molybdenum deposits (Fig. 18).

Figure 18.  Prospecting model of the Daheishan molybdenum deposit (after Zhang Y, 2013).

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The negative aeromagnetic anomaly area in northern Miaoling Village: This anomaly area lies about 3.6 km southwest of the Daheishan deposit and about 2.9 km away from the Changgangling molybdenum ore occurrence in the west. This S-N-directed nearly-elliptical anomaly area has a length of 1.5 km, a width of 0.72 km, and edges with high gradients. This area exhibits complete traps, with the lowest anomaly intensity being −36 nT. It is surrounded by four local high-amplitude aeromagnetic anomalies in the north, east, and south and is separated from the western low-gentle positive aeromagnetic anomalies by a distinct anomaly gradient belt in the west. This anomaly area resides at the southern edge of the geochemical overlapping anomalies of molybdenum, tungsten, bismuth, and nickel of the Daheishan deposit.

The negative aeromagnetic anomaly area between Qiancuoluo and Baishi villages: This anomaly area, positioned about 3.2 km northeast of the Daheishan deposit, is an arc-shaped low-amplitude magnetic anomaly area protruding northwestward. It is composed of two splayed negative magnetic anomalies and is embedded in the high-amplitude magnetic anomalies in the southeast. It has a length of 2.6 km, a width of 0.42 km, and a lowest anomaly intensity of −50 nT. This anomaly area lies in the geochemical overlapping anomalies of molybdenum, tungsten, bismuth, and nickel in the Daheishan deposit.

The negative aeromagnetic anomaly area between Xiyang Town and Shuihugou: This anomaly area is lageniform and composed of northern moderate and southern strong negative magnetic anomalies. This anomaly area, with an N-S length of 2.3 km, a W-E width of 0.92 km, and lowest anomaly intensity of −340 nT, is located in the outer zone of the geochemical overlapping anomalies of molybdenum, tungsten, bismuth, and nickel in the Daheishan deposit.

The three negative aeromagnetic anomaly areas are all smaller than the Daheishan deposit. However, they are all situated at the edge of an N-S-directed Yanshanian tectono-magmatic zone, which is also the transition part between the N-S-directed low-amplitude gravity anomaly zone caused by the tectono-magmatic zone and the high-amplitude gravity anomaly zone in the east and southeast. These three anomaly areas share similar metallogenic geological conditions and geophysical and geochemical anomalies with the Daheishan deposit, thus showing high potential for molybdenum prospecting.