| Citation: |
Jun Zhong, Chao-nan Hu, Hong-hai Fan, Yu-qi Cai, Qing Chen, Jin-yong Chen, Yan-ning Meng, 2019. A new type U-Th-REE-Nb mineralization related to albitite: A case study from the Chachaxiangka deposit in the northeastern Qaidam Basin of China, China Geology, 2, 422-438. doi: 10.31035/cg2018133
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A new type U-Th-REE-Nb mineralization related to albitite: A case study from the Chachaxiangka deposit in the northeastern Qaidam Basin of China
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Abstract
The U-Th-REE-Nb (Ta)-polymetallic mineralization is generally related to either the silica-undersaturated syenites, the silica-oversaturated alkaline/peralkaline granites or igneous carbonatites. In this study, the authors report a new mineralization type, which is related to the magmatic-hydrothermal albitite (with mineral assemblage predominated by albite with volume content > 90%), as exemplified by the Chachaxiangka deposit in Qinghai Province of China. The Chachaxiangka deposit is the first albitite-related U-Th-REE-Nb deposit recognized in China and the mineralization can be divided into 3 types: the vein-type, the disseminated veinlet type and breccia type, of which the former 2 are predominant. Three mineralization stages can be identified according to the detailed mineralogical analyses, including the magmatic stage, main hydrothermal mineralization stage and post-ore stage. By comprehensive analyses of the mineralogical, major and trace element compositions, the authors suggest that the albitite vein is magmatic-hydrothermal in origin and both the magmatic evolution and overprint of the hydrothermal fluids play important roles in the formation of the albitite and related polymetallic mineralization. Phase separation between the silicate melt and carbonate/phosphate melt might take place in the magmatic stage, yet the immiscibility between the silicate melt and chloride-dominated fluids is the most important mechanism for the REE mineralization and also causes the Nb-Th re-mobilization and enrichment. The red color of the albitite aplite vein is an eye-catching prospecting mark in the field and more mineralization can be expected at depth and in the surrounding areas. The discovery of the new albitite type U-Th-REE-Nb mineralization give rise to new ideas during future U-Th-REE-Nb exploration, not only in the Qaidam-Altun belt, but also other areas across China.
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References
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Jun Zhong, Chao-nan Hu, Hong-hai Fan, Yu-qi Cai, Qing Chen, Jin-yong Chen, Yan-ning Meng, 2019. A new type U-Th-REE-Nb mineralization related to albitite: A case study from the Chachaxiangka deposit in the northeastern Qaidam Basin of China, China Geology, 2, 422-438. doi: 10.31035/cg2018133
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Figure 1.
Schematic map showing tectonic subunits of the Qilian-Qaidam orogenic belt (after Song SG et al., 2014).
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Figure 2.
Simplified geological map (a) and exploration cross-section profile No. 6 (b) of the Chachaxiangka deposit (after Zhong J et al., 2018).
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Figure 4.
Typical handspecimen photo of the host schist and microscopic photos of the schist and albitite vein ores. a−Representative green epidote-chlorite schist; b−microphotograph showing the major mineral assemblage of epidote-chlorite-albite-quartz in the host schist; c−representative microphotograph of albitite ore; d−ore minerals occur within veinlet in representative albitite ore. Ab−albite; Cal−calcite; Ep−epidote; Ms−muscovite; Q−quartz.
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Figure 3.
Field photos and hand specimen photos of the Chachaxiangka deposit. a−A 20 cm-wide albitite ore vein parallel to the schistosity of the Tanjianshan Group schist; b−typical vein-type red-brown ore, note the dark host rocks at the margin in the upper part; c−veinlet type ores, ore veinlets occur along the schistosity of the host schist; d−typical disseminated veinlet type ore; e−disseminated ore veinlets along microfractures, cut by the post-ore white calcite stockworks; f−typical breccia type ore with hydrothermal mineral cements (mostly composed of chlorite, clay minerals, pyrite).
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Figure 5.
BSE images of the ore minerals from both the magmatic stage and main hydrothermal mineralization stage. a−“Drop-like” vigezzite grains occurring as melt inclusions in albite; b−subhedral-anhedral thorite grains hosted in zircon, which is intergrown with euhedral pyrite grains; c−subhedral-anhedral uraninite-bearing pyrochlore intergrown with monzonite; d−vigezzite grains intergrown by unknown Ca-Ti-Si-Nb mineral (probably Nb-bearing titanite); e−ore minerals occur in a vein, with the occurrence of a nearby apatite-calcite veinlet; f−magnification of Fig. 5e, showing ore mineral assemblage of monzonite, allanite, intergrown with apatite; g−fine-grained thorite intergrown with zircon in the void of the albitite ore; h−bastneasite coexists with allanite along the schistosity in the host schist (marked by the epidote layer); i−euhedral-anhedral hydrothermal zircons with small uranite and pyrchlore inclusions coexist with monzonite. Ab−albite; Aln−allanite; Ap−apatite; Bst−bastnaesite; Cal−calcite; Ep−epidote; Mon−monazite; Nb-Ttn−Nb-bearing titanite; Py−pyrite; Pyr−pyrochlore; Th−thorite; Ur−uraninite; Vig−vigezzite; Zr−zircon.
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Figure 6.
Phase patched mineral mapping using AMICS for the albitite vein-type ore sample AA-7 in the Chachaxiangka deposit. a−Whole view of the thin section of sample AA-7, showing that the ore minerals occur as micro-veinlets, further cut by post-ore calcite stringers; b−magnification in Fig. 6a, showing the ore mineral assemblage of allanite+apatite+Nb-titanite+vigezzite+calcite+chlorite; c−magnification of Fig. 6a, showing the vigezzite+allanite+Nb-titanite+chlorite mineral assemblage, cut by post-ore calcite stringer. Ab−albite; Aln−allanite; Ap−apatite; Cal−calcite; Chl−chlorite; Mon−monazite; Nb-Ttn−Nb-bearing titanite; Vig−vigezzite.
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Figure 7.
Mineral paragenesis of the Chachaxiangka deposit.
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Figure 8.
The total alkali vs. silica (TAS) diagram (a) and R1-R2 diagram (after De la Roche H et al., 1980) (b) for the ores and host rocks in the Chachaxiangka deposit.
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Figure 9.
Trace element and rare earth element compositions of the ores and host schists in the Chachaxiangka deposit (Primitive mantle and chondrite data are from Sun SS and McDonough WF, 1989). a−Chondrite-normalized REE distribution pattern of the vein-type ores; b−primitive mantle-normalized spider diagram showing the trace elements of the vein-type ores; c−chondrite-normalized REE distribution pattern of the disseminated veinlet-type ores; d−primitive mantle-normalized spider diagram showing the trace elements of the disseminated veinlet-type ores.