Zircon U-Pb age, trace element and Hf isotope composition of Sepon Au-Cu deposit, Laos: tectonic and metallogenic implications

Xin-yu Wang, Dian-hua Cao, Zong-qi Wang, An-jian Wang, Yu-dong Wu, 2018. Zircon U-Pb age, trace element and Hf isotope composition of Sepon Au-Cu deposit, Laos: tectonic and metallogenic implications, China Geology, 1, 36-48. doi: 10.31035/cg2018006

Citation:

Zircon U-Pb age, trace element and Hf isotope composition of Sepon Au-Cu deposit, Laos: tectonic and metallogenic implications

a.
School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China
b.
MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China

More Information

Corresponding author: Cao Dianhua email address: dianhuacao@gmail.com

Received Date: 28 November 2017

Revised Date: 28 January 2018

Accepted Date: 02 February 2018

Available Online: 27 February 2018

Publish Date: 25 March 2018

Abstract

Abstract

The Truong Son Fold Belt, located at the northeastern margin of the Indochina Block, is considered to be tectonically linked to the subduction of the Paleotethys Ocean and subsequent collision. Sepon is one of the most important super large deposits of the Truong Son Fold Belt. Our LA-ICP-MS zircon U-Pb dating results show that granodiorite porphyry samples from the Sepon deposit have ages of 302.1±2.9 Ma, which is a crucial phase for magmatic-tectonical activities from the Late Carboniferous to Early Permian and has avital influence on the mineralization of copper and gold. Zircon from granodiorite porphyry yields ε_Hf (t) values of 4.32 to 9.64, and T_DM2 has an average age of 914 Ma, suggesting that the source of the granodiorite porphyry in the region were mainly mantle components but underwent mixing and contamination of crust materials. The Ce⁴⁺/Ce³⁺ value of zircon in the granodiorite porphyry varys greatly from 2.4 to 1438.29, which shows magma mixing might occur. Considering the characteristics of trace elements in the zircon and the whole rock geochemical characteristics of intrusion rocks as well as the characteristics of regional volcanic-sedimentary association, it is indicated that the tectonic setting may be the continental arc environment. The Sepon Au-Cu deposit is derived from emplacement of calc-alkaline intermediate-acid magma with coming from deep sources in the subduction process of the Paleotethys Ocean, forming porphyry Mo-Cu, skarn Cu-Au mineralization and a hydrothermal sedimentary-hosted Au mineralization in the wall rocks.
- granodiorite porphyry /
- Zircon U-Pb ages /
- Hf isotope /
- Trace elements /
- Sepon /
- Laos

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1. Introduction

The Sepon Au-Cu deposit is the largest polymetal deposit ever discovered in Laos, of which the resources of Gold and Copper is 102 t (Au: 1.6 g/t) and 196×10⁶ t (T Cu: 2%) (Zhu HP et al., 2013; Shi MF et al., 2017) respectively. It has been investigated as a typical deposit, which will be of importance in understanding the regional metallogeny in the Paleotethys stage. At least three broad mineralization styles are recongised in the Sepon porphyry Au-Cu deposit: sedimentary rock-hosted Au, Cu-Au skarn and porphyry Mo-Cu. There have been few researches done in the deposit, and only Cromie PW, (2010)has done more in-depth research, such as the zircon U-Pb ages of the granodiorite porphyry, molybdenite Re–Os ages, the occurrence of Au in pyrites with different forms and structures, trace element investigations of sulphides, sources and characteristics of ore-bearing fluids, and sources of ore materials.

In the Sepon deposit, the outcropping rocks are heavily weathered and whole-rock geochemical analysis data is not suitable for petrogenesis research. Zircon is high in enclosure temperature, strongly resistant to weathering and alteration, contains massive information on magma origin and evolution. Its topography, precise U-Pb geochronology and Hf isotope compositions can be used to define the timing and sources of the magmatism forming the magma of the porphyry deposits and can analyze information about the ore-bearing magmatism (Peytcheva I et al., 2009). Composition of trace elements in the zircon may reflect component evolution of crystallization solutions (Bea F et al., 1994; Sano Y. et al., 2002; Thomas JB et al., 2002; Nardi LVS et al., 2013), origin of trace elements (Hoskin PWO and Ireland TR, 2000; Belousova E et al., 2002) and tectonic settings (Breiter K et al., 2016).

The Sepon deposit is also the largest Au-Cu polymetal deposit ever discovered in the Truong Son Fold Belt. In this paper, the metallogenic geologic setting and deposit characteristics of the Sepon deposit has been summarized. The Sepon ore-bearing porphyry was tested for zircon U-Pb-Hf isotopes and trace elements and the ore-bearing magma was investigated for the emplacement age, origin and tectonic environment.

2. Geologic Setting

The tectonic framework in Laos is shown in Fig. 1. During the orogeny period of the Paleozoic and the Early Mesozoic, the Indochina block was formed from deformed crust around the Phanerozoic metamorphic rocks (Workman DR, 1975; Fontaine and Workman, 1978; Stokes RB and Smith S, 1990; Lepvrier C et al., 1997; Maluski H et al., 2005) and it underwent at least three main orogenic folding periods: the Hercynian period (middle Carboniferous), the early Indochina period (Permian-early Triassic) and the late IndoChina period (late Triassic). Orogenic folds might have existed before the Paleozoic but no evidence is found in Laos (Stokes RB et al., 1990; Cromie PW, 2010).

Figure 1. Tectonic location map of the Sepon deposit, Laos (modified from Cromie, 2010).

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Although there is no stratigraphic and geochronological evidence directly implying that the Precambrian crystalline basement is present, a small amount of high-grade metamorphic rock series has been found in northwest Laos (Fig. 1). This has been deemed as the most ancient stratum in Laos, formed mainly in the Proterozoic, and predominantly composed of migmatite, gneiss, quartz-mica schist, graphite schist and quartzite etc. Early Paleozoic formations are mainly distributed in northeast Laos (Fig. 1), with the outcropped rocks primarily epimetamorphic limestone, shale, sandstone (quartzite) and conglomerate, and a small amount of marine-facies limestone, sandstone (quartzite) and conglomerate. The mid-late Paleozoic formations are mainly marine-facies limestone, sandstone and argillite. The Mesozoic comprises overlying Triassic marine-facies sedimentation mainly distributed in the northwest (Fig. 1) and mainly consists of detrital sediments and detrital-sandwiched limestone, generally containing intermediate-acid volcanic rocks, which are products of strong denudation of the uplifted region by upthrowing of folds in most regions in the late Triassic (covered by Triassic-Cretaceous terrestrial-source and paralic-facies sandstone and conglomerate). Ever since the Jurassic period, especially in the late Cretaceous, massive red argillaceous siltstone and fine sandstone have been formed, and evaporite series have occurred.

The outcropped formations in the Sepon deposit are mainly Paleozoic detrital rocks and carbonatite (Fig. 2a), which form a set of semi-graben-basin shore-shallow-abysmal facies and continental facies river sediments, the lower part of which is Ordovician sandstone, conglomerate and calcareous shale as well as turbiditic sandstone, the middle part of which is Silurian calcareous shale, laminated shale, conglomerate and volcanic rock, and the upper part of which is Devonian bioclastics-sandwiched dolomite, limestone, nodular calcareous shale and siltstone, with a formation thickness up to 2000 m. Mineralized gold is present mainly in calcareous shale of the Devonian Discovery Formation, then in bioclastic dolomite of the Devonian Nalou Formation, the calcareous shale of Silurian-Devonian Kengkeuk Formation, clay and siltstone of the Ordovician-Silurian Nampa Formation in descending order.

Figure 2. Geological sketch map and a schematic section model showing the mineralization styles in the Sepon deposit, Laos ( after Fig. 2a provided by geological department of Sepon mine；Fig. 2b modified from Manini AJ. et al., 2003).

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Fracture structures have been developed in the deposit, and these can be divided mainly into two groups: NW-strike fractures parallel to Truong Son fracture system, which is in the direction of the tectonic line of the Truong Son Fold Belt; EW-strike fractures parallel to the basin’s basement fracture, which is parallel with the border of the basin. Both groups of fractures intersect at the place where the products of the granodiorite porphyry intrusions and some small composite plutons of intermediate-acid dykes concentrate, which is also the most favorable place for mineralization.

In the deposit, the magma activities feature developed granodiorite porphyry and a few veins. The outcropping granodiorite porphyry is mostly weathered, the residual coarse phenocrysts are quartz and plagioclase, and the matrix is white clay mineral. During exploration work, relatively fresh porphyritic texture is found in the deep drill holes. The phenocrysts are composed of quartz, plagioclase, orthoclase and few subhedral green hornblende. Quartz (15%-30%) is round, elongated, in bay contour, about 5 mm wide, 10 mm long. Plagioclase (20%-40%) is euhedral or subrounded, about 5 mm wide and 10 mm long. K-feldspar (5%-20%) is subhedral and less than 5 mm in diameter. Only a small amount of amphibole is 2 mm in diameter.

Mineralization in the deposit is closely associated with the late Carboniferous activities of the intermediate-acid magma and therefore it is a porphyry-skarn deposit (Fig. 2b). There are three kinds of mineralization developed in the deposit: porphyry Mo-Cu mineralization, skarn Cu-Au mineralization near the porphyry bodies and sedimentary-hosted Au mineralization at the far end. The central porphyry Mo-Cu ore bodies are mainly distributed in Thengkham South and Padan, the neighboring skarn Cu-Au ore bodies mainly in Khanong, Discovery East, Thengkham South and Phavat, etc., and the sedimentary-hosted Au ore bodies mainly in Nalou and Discovery. Geometries of the sedimentary-hosted Au ore bodies mainly include: continually-outspread belt-like ore bodies, bedded ore bodies inclined at a small angle and nearly without contact with the fault, and the fault-controlled bedded ore bodies. The main ore minerals found in the deposit are: pyrite, galena, sphalerite, chalcopyrite, tetrahedrite, malachite and azurite, and the alteration types mainly include: carbonatization, siliconization(iasperoidization), argillization, dolomitization, sericitization, skarnization (to form garnet, chlorite, epidote), and hornstonization in the non-calcareous sediment).

3. Method

Rock samples were taken from the granodiorite porphyry intrusion in the Nalou ore district of the Sepon Au-Cu deposit, at the positions shown in Fig. 2a. Samples are weathered and weakly altered, the tectonic fissure is undeveloped and no late vein is found.

Zircon crystals with higher purity are separated by conventionally crushing and gravity-concentrating samples and then put under binoculars to manually select zircon samples with purity above 99%. Preparation of zircon targets and cathode-luminescence image analysis were done at at Beijing Ion Microprobe Centre, Beijing, China. Zircon U-Pb and trace elements were determined at the Experimental Center of Geosciences (Beijing), and Hf isotope was analyzed at the MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing.

In-situ U-Pb zircon dating and trace elements in zircon were done with a laser plasma mass spectrometer composed of a laser ablation system UP193SS (New Wave Research, USA) and a quadrupole plasma mass spectrometer Agilent 7500 (AGLENT, USA) (Liu JH et al., 2014). Laser at frequency 10 Hz, at the wavelength 193 nm and in the spot-beam diameter of 36 μm was used in analysis, and, during age computation, the standard zircon 91500 was used as an external standard to calibrate isotope ratio and TEM as blind sample for control; the international standard sample NIST612 was used as external standard and Si as internal standard to compute the content of elements. Andersen’s method used for the common lead correction was the same as the one for Andersen (Andersen T, 2002) and data were processed with the program Glitter 4.

Zircon Hf isotope was detected with the multi-collection plasma mass spectrometer Neptune and the UV laser ablation system New Wave UP213 (LA-MC-ICP-MS), zircon Hf isotope was analyzed at the in-situ of U-Pb age analysis point, and in the experimental process, He (an element) was used as gas to carry ablated substance. The laser ablation beam was 55 μm in diameter and the time for laser ablation was about 27 s. In determination, the zircon international standard sample GJ-1 was used as a reference material. The relevant instrument operational conditions and the detailed analysis process were the same as those used by Hou KJ et al., (2007). During the analysis, the tested ¹⁷⁶Hf/¹⁷⁷Hf weighted average of zircon samples GJ-1 was 0.282012±17 (2SD, n=24), which was totally consistent with those reported in literatures (Elhlou S et al., 2006; Hou KJ et al., 2007) within the range of error. In computing ε_Hf(t), the ¹⁷⁶Hf/¹⁷⁷Hf ratio and the ¹⁷⁶Lu/¹⁷⁷Hf ratio of the chondrite was 0.282772 and 0.0332, respectively. In computing the single-stage Hf model age (T_DM1), the ¹⁷⁶Hf/¹⁷⁷Hf ratio and the ¹⁷⁶Lu/¹⁷⁷Hf ratio were 0.28325 and 0.0384 respectively for the depleted mantle. In computing the two-stage Hf model age (T_DM2) , the f_Lu/Hf ratios were –0.34, –0.55 and 0.16 for the lower crust, the average crust and the depleted mantle, respectively. The decay constant of ¹⁷⁶Lu was 1.867×10^-11 a^-1(Söderlund S et al., 2004). For related computation formulas, refer to those used by Wu et al., ( 2007).

4. Results

4.1 Zircon LA-ICPMS U-Pb data

Most zircon grains are subhedral to anhedral. They are prismatic crystals and range from 80-250 μm in length and 60-120 μm in width. Their CL images commonly show oscillatory zoning (Fig. 3a). The LA-ICPMS zircon U-Pb analysis results of samples are presented in Table 1.

Figure 3. Cathodoluminescence (CL) images of represenitative zircons and concordia plots for zircon from the grandiorite porphyry of Sepon deposit.

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Table 1. Zircon LA-ICPMS dating of the granodiorite porphyry in the Sepon deposit.

Points	Element/10^-6		Th/U	Isotopic ratios						Age/Ma
Points	Th	U	Th/U	²⁰⁷Pb/²⁰⁶Pb	1σ	²⁰⁷Pb/²³⁵U	1σ	²⁰⁶Pb/²³⁸U	1σ	²⁰⁷Pb/²⁰⁶Pb	1σ	²⁰⁷Pb/²³⁵U	1σ	²⁰⁶Pb/²³⁸U	1σ
1.1	197	1092	0.2	0.0603	0.0019	0.4017	0.0126	0.0483	0.0007	505	88	328	10	303	4
1.2	52	494	0.1	0.0518	0.0018	0.3286	0.0117	0.046	0.0007	275	54	288	9	290	4
1.3	192	692	0.3	0.0537	0.0018	0.3479	0.0117	0.047	0.0007	359	49	303	9	296	4
1.6	68	554	0.1	0.0531	0.0018	0.3489	0.012	0.0477	0.0007	333	51	304	9	300	4
1.7	113	966	0.1	0.0591	0.0019	0.3896	0.0127	0.0478	0.0007	434	88	316	10	300	4
1.8	114	1192	0.1	0.0531	0.0017	0.3394	0.0109	0.0463	0.0007	333	46	297	8	292	4
1.9	79	744	0.1	0.0526	0.0017	0.3627	0.012	0.05	0.0008	311	48	314	9	315	5
1.11	75	673	0.1	0.0524	0.0018	0.347	0.0119	0.048	0.0007	303	51	302	9	302	4
1.12	109	767	0.1	0.0507	0.0017	0.3383	0.0115	0.0484	0.0007	227	50	296	9	305	4
1.13	134	805	0.2	0.0533	0.0019	0.3648	0.0132	0.0496	0.0008	341	54	316	10	312	5
1.14	174	1009	0.2	0.0524	0.0017	0.337	0.0112	0.0467	0.0007	302	48	295	9	294	4
1.15	291	966	0.3	0.0568	0.0019	0.3753	0.0128	0.0479	0.0007	483	49	324	9	302	4
1.16	153	810	0.2	0.052	0.0018	0.3512	0.0121	0.049	0.0007	287	51	306	9	308	5
1.17	90	730	0.1	0.0527	0.0018	0.3562	0.0125	0.0491	0.0008	314	52	309	9	309	5
1.18	171	1180	0.1	0.0543	0.0018	0.3633	0.0123	0.0485	0.0007	384	49	315	9	305	5
1.19	153	904	0.2	0.0541	0.0019	0.36	0.0128	0.0482	0.0007	377	53	312	10	304	5
1.20	152	951	0.2	0.0519	0.0018	0.3535	0.0123	0.0494	0.0008	281	52	307	9	311	5
1.22	122	756	0.2	0.0517	0.0018	0.349	0.0125	0.0489	0.0008	274	53	304	9	308	5
1.24	98	871	0.1	0.0528	0.0019	0.3539	0.0126	0.0487	0.0008	318	53	308	9	306	5
1.25	67	523	0.1	0.0518	0.002	0.3367	0.0129	0.0472	0.0007	274	59	295	10	297	5
1.26	94	745	0.1	0.052	0.0019	0.3488	0.0127	0.0487	0.0008	285	55	304	10	306	5
1.27	98	588	0.2	0.0527	0.002	0.3365	0.0128	0.0463	0.0007	317	58	295	10	292	4
1.28	232	1080	0.2	0.0516	0.0018	0.3474	0.0124	0.0488	0.0008	269	53	303	9	307	5
1.30	68	753	0.1	0.0511	0.0019	0.3492	0.0132	0.0496	0.0008	244	58	304	10	312	5

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The U and Th contents of zircons show a wide range, from 52×10^-6 to 291×10^-6 for U and from 494×10^-6 to 1192×10^-6 for Th, with Th/U values ranging from 0.1 to 0.3. In general, the high Th/U values ( generally > 0.1) for most of the zircons suggest that they are of magmatic origin; the low Th/U values (generally < 0.1) for most of the zircons suggest that they are of metamorphic origin ( Siebel W et al., 2005; Wu YB and Zheng YF, 2004). The high Th/U ratios (> 0.1) suggest their magmatic origin. Twenty-four zircons analyses in the samples give consistent²⁰⁶Pb/²³⁸U ages (290-315 Ma) that plot on the U-Pb concordia (Fig. 3b). The weighted mean age ²⁰⁶Pb/²³⁸U age is 302.1 ± 2.9 Ma (n=24, MSWD = 2.4), and represents the age of crystallization of the granodiorite porphyry.

4.2 Rare earth element pattern in zircon

The zircon U-Pb isotope results of granodiorite prophyry are presented in Table 2. For REEs in zircon, the total quantity (∑REE) ranges from 702.44×10^-6 to 2144.17×10^-6, the LREE is 4.78×10^-6-46.26×10^-6, the HREE is 693.36×10^-6-2129.62×10^-6 and the LREE/HREE ratio is 0.01-0.04. As shown in Fig. 4a, the zircon REE assemblage plot shows relative enrichment for HREE and relative depletion for LREE, a strong positive Ce anomaly and a feeble negative Eu anomaly, meaning it is typical magma zircon (Hoskin PWO and Ireland YR, 2000).

Table 2. Zircon trace elements data of the granodiorite porphyry in the Sepon deposit.

Points	Elements/10^-6																		LREE/ HREE	δEu	δCe
Points	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y	ΣREE	LREE	HREE	LREE/ HREE	δEu	δCe
1.1	0.66	14.27	1.34	12.18	11.56	6.25	20.67	6.43	71.15	26.79	134.5	40.89	620.96	146.24	864.83	1113.89	46.26	1067.63	0.04	1.22	2.76
1.2	0.01	5.19	0.01	0.29	0.74	0.54	4.73	2.14	33.73	16.73	99.7	33.55	535.37	133.49	581.97	866.21	6.77	859.44	0.01	0.67	115.02
1.3	0.01	12.52	0.01	0.64	2.23	1.46	14.7	6.53	93.41	41.2	204.21	60.74	841.90	183.86	1235.30	1463.42	16.87	1446.55	0.01	0.59	277.47
1.6	0.05	6.28	0.05	0.23	0.89	0.56	4.85	2.09	31.92	15.61	88.1	30.03	459.15	113.93	524.89	753.74	8.07	745.68	0.01	0.66	26.91
1.7	0.6	8.7	0.64	5.59	4.94	2.83	14.6	5.53	83.15	37.61	212.11	65.33	965.44	213.62	1254.49	1620.68	23.30	1597.39	0.01	0.94	3.07
1.8	0.11	8.62	0.11	0.88	1.56	1.09	9.07	3.92	58.66	27.38	155.47	51.77	804.35	189.20	918.81	1312.18	12.37	1299.82	0.01	0.69	17.06
1.9	0.01	5.54	0.01	0.31	0.98	0.83	8.61	3.81	57.89	27.08	148.84	46.47	689.63	155.85	869.67	1145.86	7.68	1138.18	0.01	0.59	122.78
1.11	0.01	5.5	0.01	0.01	0.59	0.51	5.64	2.41	38.32	17.54	103.96	33.96	540.96	128.33	618.04	877.74	6.63	871.12	0.01	0.56	121.89
1.12	0.19	9.25	0.23	1.66	2.04	1.63	10.28	3.77	55.27	26.44	150.43	48.67	747.91	185.82	925.49	1243.59	15.00	1228.59	0.01	0.89	9.33
1.13	0.01	7.31	0.11	1.2	2.19	1.05	8.24	3.6	50.25	23.29	130.66	40.89	651.06	159.75	767.15	1079.61	11.87	1067.74	0.01	0.67	19.73
1.14	0.13	9.15	0.23	2.65	3.2	1.98	10.01	3.82	55.21	24.74	137.57	43.9	661.82	160.44	833.82	1114.85	17.34	1097.51	0.02	0.98	10.21
1.15	0.28	14.2	0.75	7.13	7.29	4.26	17.39	6.31	79.13	33.2	178.68	55.05	786.27	193.49	1079.57	1383.42	33.90	1349.52	0.03	1.11	5.15
1.16	0.01	7.24	0.01	0.35	0.83	0.64	5.95	2.49	36.18	15.94	88.28	27.4	416.98	100.14	527.49	702.44	9.08	693.36	0.01	0.64	160.45
1.17	0.01	10.01	0.01	0.33	1.21	0.77	8.33	3.72	59.65	29.83	170	56.24	877.85	211.64	998.17	1429.60	12.34	1417.26	0.01	0.55	221.84
1.18	0.36	11.7	0.73	6.13	6.35	2.94	15.77	5.55	71.42	31.74	177.56	57.24	873.22	208.67	1080.74	1469.37	28.20	1441.17	0.02	0.86	4.18
1.19	0.34	8.41	0.65	5.73	5.44	3.24	12.08	4.7	58.85	24.87	133.46	41.34	611.07	149.03	827.03	1059.20	23.81	1035.40	0.02	1.18	3.33
1.20	0.08	9.04	0.08	0.93	2.99	1.45	22.2	9.99	148.18	64.29	327.41	93.06	1208.27	256.22	1875.42	2144.17	14.56	2129.62	0.01	0.39	25.73
1.22	0.06	6.83	0.06	0.78	1.43	1.05	8.63	3.72	53.02	25.02	138.79	44.95	679.68	163.13	846.89	1127.15	10.21	1116.94	0.01	0.71	25.47
1.24	0.01	9.03	0.01	0.37	1.11	0.76	6.86	3.39	51.21	24.79	141.81	46.69	737.80	177.75	847.57	1201.58	11.28	1190.30	0.01	0.64	200.12
1.25	0.01	6.44	0.01	0.01	0.58	0.39	5.3	2.52	39.52	17.86	97.9	29.87	425.30	93.56	599.35	719.26	7.44	711.83	0.01	0.45	142.72
1.26	0.06	8.89	0.1	0.6	1.43	0.89	7.36	3.38	52.11	24.92	143.19	47.89	736.91	177.60	841.13	1205.32	11.96	1193.36	0.01	0.68	22.36
1.27	0.09	8.3	0.22	2.05	2.25	1.65	9.21	3.53	54.42	24.95	139.12	44.16	678.80	162.62	834.99	1131.37	14.56	1116.81	0.01	0.96	10.05
1.28	0.01	11.48	0.04	0.38	1.33	0.97	8.29	3.17	43.57	19.76	105.41	32.85	495.84	120.59	644.66	843.69	14.21	829.48	0.02	0.68	80.99
1.30	0.01	3.29	0.01	0.23	0.82	0.42	5.45	2.82	46.98	22.86	126.67	39.02	558.23	124.76	739.50	931.56	4.78	926.79	0.01	0.45	72.91

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Figure 4. Chondrite normalized REE patterns and ε_Hf(t) values vs. age for zircon from the granodiorite porphyry in the Sepon deposit (normalization values after Sun and Mc Donough (after Sun SS, et al., 1989)).

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We employ a Ce⁴⁺ and Ce³⁺ calculation method proposed by Ballard JR et al., (2002), by which Ce⁴⁺ and Ce³⁺ ratios of zircon are calculated on the basis of a lattice-strain model for mineral-melt partitioning of Ce⁴⁺ and Ce³⁺ cations. The results are present in Table 3, which suggest that the zircon Ce⁴⁺/Ce³⁺ratio from the granodiorite porphyry varies to a great degree, ranging from 2.41 to 1438.29 (mean value 236.37), zircon Ce/Ce* ratios from 3.46 to 306.97 (mean value 82.59) and the Eu/Eu* ratios from 0.54 to 1.24 (mean value 0.90). According to the data collected by Lu YJ, (2016), compared to those barren, for the ore-bearing magma, the Eu/Eu* ratio is greater than 0.3, 1000*(Eu/Eu*)/Y greater than 1, (Ce/Nd)/Y greater than 0.01, Dy/Yb less than 0.3, and based on the tested data in this paper, Zircon Eu/Eu* ratios are from 0.54 to 1.24, 1000*(Eu/Eu*)/Y from 2.24 to 2622.10, (Ce/Nd)/Y ratio from 0.50 to 768.05 and Dy/Yb from 0.04 to 0.08.

Table 3. Partition coefficients and ratios of Ce³⁺and Ce⁴⁺ of zircons from the granodiorite porphyry in the Sepon deposit.

Points	D_Ce³⁺	D_Ce⁴⁺	Ce⁴⁺/Ce³⁺	Eu/Eu^*	Ce/Ce^*
1.1	0.13620	532.90	2.41	1.24	5.97
1.2	0.00064	323.84	263.44	0.88	127.25
1.3	0.00114	446.08	354.75	0.78	306.97
1.6	0.00163	361.20	124.58	0.83	29.90
1.7	0.03829	484.14	6.39	1.02	3.46
1.8	0.00464	532.71	59.41	0.88	19.14
1.9	0.00069	413.28	261.18	0.87	135.83
1.11	0.00014	403.98	1255.55	0.85	134.85
1.12	0.01049	454.58	27.66	1.09	10.83
1.13	0.00599	466.52	38.64	0.76	53.79
1.14	0.01519	538.23	18.58	1.07	13.09
1.15	0.05793	608.17	6.97	1.16	7.67
1.16	0.00081	489.93	290.68	0.87	177.51
1.17	0.00067	411.99	485.49	0.74	245.42
1.18	0.04811	555.57	6.90	0.90	5.64
1.19	0.04803	488.90	4.69	1.22	4.41
1.20	0.00389	512.84	74.55	0.54	28.79
1.22	0.00311	424.69	70.44	0.91	27.70
1.24	0.00073	440.91	403.13	0.84	221.40
1.25	0.00015	362.85	1438.29	0.68	157.90
1.26	0.00355	409.22	80.45	0.84	28.98
1.27	0.01179	400.20	21.87	1.11	14.26
1.28	0.00203	553.85	182.90	0.89	140.73
1.30	0.00055	407.69	194.04	0.60	80.66
Min	0.00014	323.84	2.41	0.54	3.46
Max	0.13620	608.17	1438.29	1.24	306.97
Ave	0.01652	459.34	236.37	0.90	82.59
Notes: D_Ce³⁺、D_Ce⁴⁺ and Ce⁴⁺/Ce³⁺ in zircon were calculated based on the method described in Ballard et al.(2002); The data of the granodiorite porphyry is from (2002), Eu/Eu^= Eu^N/ (Sm^N×Gd^N)^1/2; Ce/Ce^= Ce^N/ (La^N×Pr^N)^1/2.

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4.3 Zircon Hf isotope

21 zircon samples from the granodiorite porphyry were analyzed for Hf isotopic composition. The results of zircon in-situ Lu-Hf isotope analysis are shown in Table 4 and Fig. 4b. ¹⁷⁶Lu/¹⁷⁷Hf ratios acquired from zircon grains are very low (0.001252 to 0.00233), which indicates the accumulation of low-radioactive Hf after zircon crystallization (Wu FY, 2007). The f_Lu/Hf is –0.96 to –0.93, obviously less than f_Lu/Hf (–0.34) of mafic crust (Amelin Y et al., 2000) and f_Lu/Hf (–0.72) of sialic crust(Vervoort JD et al., 1996), and therefore the two-stage Hf model age T_DM2 can more truly reflect the period extracting materials in sources from the depleted mantle. ε_Hf(t) values range from 4.32 to 9.64, with the mean of 6.28. The single-stage Hf model age (T_DM1) ranges from 569 to 769 Ma, with a mean of 696 Ma and the two stage model age (T_DM2) range from 710 to 1031 Ma, with a mean 914 Ma.

Table 4. Zircon Hf isotopic data of the granodiorite porphyry in the Sepon deposit.

Points	Age/Ma	¹⁷⁶Hf/¹⁷⁷Hf	2σ	¹⁷⁶Lu/¹⁷⁷Hf	2σ	¹⁷⁶Yb/¹⁷⁷Hf	1σ	(¹⁷⁶Hf/¹⁷⁷Hf)₀	ε_Hf(t)	T_DM1/Ma	T_DM2/Ma	f_Lu/Hf
1.1	304	0.282791	0.000019	0.001252	0.000021	0.031231	0.000517	0.282784	7.11	658	863	–0.96
1.2	290	0.282759	0.000023	0.00189	0.000041	0.045024	0.000679	0.282749	5.55	716	952	–0.94
1.3	296	0.282779	0.000018	0.001956	0.000009	0.048535	0.000194	0.282768	6.36	688	905	–0.94
1.6	304	0.282741	0.000027	0.00201	0.000018	0.044335	0.000714	0.28273	5.2	743	984	–0.94
1.7	300	0.28275	0.000022	0.00205	0.000009	0.053523	0.000578	0.282739	5.42	732	968	–0.94
1.8	292	0.282724	0.00002	0.002006	0.000011	0.051977	0.000118	0.282713	4.32	769	1031	–0.94
1.9	315	0.282859	0.00002	0.001814	0.000039	0.043229	0.000681	0.282849	9.64	569	710	–0.95
1.11	302	0.282759	0.000021	0.001343	0.000039	0.034802	0.000663	0.282752	5.93	704	937	–0.96
1.13	312	0.282801	0.000023	0.002282	0.000099	0.057341	0.002172	0.282787	7.4	662	850	–0.93
1.14	294	0.282746	0.000018	0.00164	0.000017	0.038489	0.00072	0.282737	5.21	730	976	–0.95
1.15	302	0.282749	0.000021	0.002158	0.000039	0.061937	0.000437	0.282737	5.41	735	969	–0.93
1.16	308	0.282819	0.000024	0.001939	0.000011	0.056503	0.000419	0.282808	8.06	629	806	–0.94
1.17	309	0.282758	0.000021	0.00147	0.000016	0.042549	0.00039	0.282749	6	709	938	–0.96
1.20	311	0.282812	0.000022	0.001891	0.000014	0.053641	0.000264	0.282801	7.88	638	819	–0.94
1.22	308	0.282793	0.000027	0.002185	0.000015	0.062936	0.000385	0.282781	7.09	671	868	–0.93
1.24	306	0.282719	0.000022	0.001494	0.000007	0.046236	0.000228	0.282711	4.56	765	1027	–0.95
1.25	297	0.282792	0.00002	0.00233	0.000035	0.052	0.000199	0.282779	6.78	676	879	–0.93
1.26	306	0.28274	0.000016	0.001526	0.000005	0.040493	0.000273	0.282731	5.29	736	981	–0.95
1.27	292	0.282775	0.000019	0.001548	0.000013	0.039011	0.000293	0.282767	6.24	685	909	–0.95
1.28	292	0.282779	0.000015	0.002034	0.000024	0.050976	0.000639	0.282768	6.28	689	906	–0.94
1.30	312	0.282762	0.000016	0.001856	0.000025	0.045408	0.000184	0.282751	6.12	711	932	–0.94

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5. Discussion

5.1 Timing of magmatism and related mineralization

The Truong Son Fold Belt, located at the northeast margin of the Indochina block, part of the east Paleotethys, preserved the evolution history of the Paleotethys Ocean. As the most important Fe-Au-Cu mineralized belt in the Indochina Block, the Truong Son Fold Belt is found many deposits during the Late Carboniferous to Early Permian, Sepon Au-Cu deposit, Phu Kham Cu-Au deposit, Phou Nhouan Fe deposit, Long Chieng Track Au deposit, Ban Houayxai Au-Ag deposit, Pha Lek Fe deposit, KTL-Bohr Thong-Tharkhek Cu-Au deposit and so on. The main deposit types includes skarn-related Fe deposit, porphyry-related Cu-Au deposit, porphyry-related skarn Cu-Au deposit, epithermal Au-Ag deposits, and intrusion-related Sn deposit. The published geochronological data indicates (Table 5) that main mineralization of the Truong Son Fold Belt was associated with calc-alkaline and intermediate-acid arc magmatism during Late Carboniferous to Early Permian period (Kamvong et al., 2013; Khin Z et al., 2014; Lai CK et al., 2014).

Table 5. Summary for magmatic-related ore deposits in Truong Son Belt from Late Carboniferous to Early Permian.

No	Deposit	Longitude	Latitude	Metallogenic elements	Deposit type	U−Pb Ages of ore-bearing porphyries/Ma	Alteration minerals and types	Mineral assemblages	Reference
1	KTL	103.287E	19.434N	Cu-Au	PCD-SK	285-290	Silicification, propylitic (chlorite, epidote), phyllic (sericite, pyrite)	Py+Ccp+Po+Gn+Bn+Sp+Mo+Eit	Hotson(2009), Zaw(2014)
2	Bohr Thong	103.195E	19.417N	Cu-Au	PCD-SK	282-285	Skarn prograde: garnet;skarn retrograde: epidote, chlorite	Py+Ccp+Mag+Bn+Po+Eit	Hotson(2009)
3	Tharkhek	103.238E	19.409N	Cu-Au	PCD-SK	277-280	Silicification, propylitic (chlorite, epidote), phyllic (sericite, pyrite)	Py+Ccp+Mo+Bn+Sp+Gn	Hotson(2009)
4	Phu Kham	102.908E	18.883N	Cu-Au	PCD-SK, ETL	299-306	Porphyry: potassic (K-feldspar, biotite, magnetite), phyllic (sericite, pyrite), propylitic (epidote, pyrite);skarn prograde: garnet;retrograde:chlorite, epidote, carbonate, quartz, sericite, hematite; high-sulphidation with pyrophyllite at hangingwall zone	Py+Ccp+Mag+Bn+Hem+Tet+Gn+ En+Sp+Mo+Au	Backhouse(2004), Tate(2005), Kamvong(2013), Kamvong(2014)
5	Phu He	103.256E	19.467N	Au-Ag	ETL	290	Porphyry:s potassic (K-feldspar, biotite, magnetite), phyllic (sericite, pyrite), propylitic (epidote, pyrite);skarn prograde: garnet; retrograde:chlorite, epidote, carbonate, quartz, sericite, hematite;high-sulphidation with pyrophyllite at hangingwall zone	Py+Gn+Sp+Ccp+EIt	Hotson(2009)
6	LCT	102.884E	18.937N	Au-Ag-Cu	ETL	291	Silica, adularia, sericite, chlorite, pyrite, kaolinite, halloysite	Py+Sp+Gn+Ccp+EIt	Manaka(2008)
7	Ban Houayxai	102.687E	18.927N	Au-Ag	ETL	286	Silica, adularia, sericite, chlorite, pyrite	Py+Sp+Gn+Ccp+EIt+Apy	Manaka(2008), Manaka(2014)
8	Phou Nhouan	103.45E	19.32N	Fe	SK	282		Mag+Hem+Lm	Hotson(2009), Zaw(2014), Jia(2014)
9	Pha Lek	102.94E	18.989N	Fe	SK	280-317	Skarn, hornfels, marble, chloritization, magnetite, epidote	Hem+Lm+Mag	Wang(2013), Li(2012)
10	Sepon	105.983E	16.976N	Au-Cu	PCD-SK	283-297	carbonatization, siliconization (iasperoidization), argillization, dolomitization, sericitization, skarnization (to form garnet, chlorite, epidote), and hornstonization in thenon-calcareous sediment).	Ccp+Py+Apy+Stb+Gn+Sp+Bn+Au+ Cc+Cin+Opm	Smith(2005), Cromie (2006), Cannell and Smith(2008), Cromie(2010), Boutathep(2013), Zhu Huaping(2013)
Notes: Mineral abbreviations: Py—Pyrite, Ccp—Chalcopyrite, Po—Pyrrhotite, Gn—Galena, Bn—Bornite, Sp—Sphalerite, Mo—Molybdenite, Mag—Magnetite, Hem—Hematite, EIt—Electrum, Tet—Tetrahedrite, En—Enargite, Apy—Arsenopyrite, Lm—Limonite, Stb—Stibnite, Cc—Chalcocite, Cin—Cinnabar, Opm—Orpiment. Deposit type abbreviations: SK-Skarn deposit, PCD-Porphyry deposit, ETL—Epithermal deposit.

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The zircon U-Pb analysis of the Sepon granodiorite porphyry in this study shows that the weighted mean ²⁰⁶Pb/²³⁸U age is 302.1 ± 2.9 Ma.This value indicates that the granodiorite porphyry is genetically related to the intermediate-acid magma insrusions during the Late Carboniferous to Early Permian. The Re-Os dating of molybdenite molybdenites 282.4 ± 1.6 Ma and 287.2 ± 1.0 Ma at Padan and Thenkham South of Sepon deposit respectively(Cromie PW, 2010). These results are merely from the testing molybdenite samples instead of isochron age; such results also indicate that a period slightly later than that of the diagenesis of the granodiorite porphyry. By comparing deposits formed in the same period in the region, at the KTL Cu-Au deposit, the age of ore-bearing porphyry is determined as 284.8 to 290 Ma and the Re-Os dating of molybdenite is 289.4 Ma (Hotson MD, 2009); at Phu Kham Cu-Au deposit, the age of the quartz diorite-porphyry is 299 to 306.2 Ma and the Re-Os dating of molybdenite is 304 Ma(Backhouse D, 2004; Tate NM, 2005; Kamvong et al., 2013), which means the emplacement of intrusions and mineralization occurred in roughly the same period at both deposits. Additionally, zircon trace element data of the granodiorite porphyry in the Sepon deposit shows that Eu/Eu* is 0.54 to 1.24, 1000*(Eu/Eu*)/Y is 2.24 to 2622.10, (Ce/Nd)/Y is 0.50 to 768.05 and Dy/Yb is 0.04 to 0.08, indicating that it is ore-bearing magma and revealing that the intermediate-acid magma is closely related to the Au-Cu mineralization in the deposit, which possibly provided thermal sources and minerals for mineralization.

5.2 Petrogenesis and magmatic source of the granodiorite porphyry

5.2.1 Relative oxygen fugacity (fO₂) of magma

For porphyry Cu-Au deposits, the intrusions directly associated with porphyry Cu mineralization have higher zircon Ce⁴⁺/Ce³⁺ ratios than barren intrusion. Sulfur contained in magma with high fO₂ is mainly sulfur oxide which helps metallogenic elements in the magma enrich the process of magma melting and crystallization differentiation. While zircon Ce⁴⁺/Ce³⁺ may reflect whether the magma’s fO₂ is high or low and then be used as a marker to distinguish ore-bearing porphyries from barren counterparts (Ballard JR et al., 2002). The Sepon granodiorite porphyry REE we obtained shows a consistent trend of components. It shows obvious positive Ce anomaly and weak negative Eu anomaly. The Ce⁴⁺/Ce³⁺ varys greatly from 2.4 to 1438.29，which shows magma-mixing may occur. The high value of Ce⁴⁺/Ce³⁺ may indicate the existence of high oxygen fugacity magma injecting into the lower oxygen fugacity magma.

5.2.2 Source of Magma

In the Sepon deposit, the ¹⁷⁶Hf/¹⁷⁷Hf of the crystallized zircon from the granodiorite porphyry ranges from 0.282859 to 0.282719, indicating that the Hf isotope is homogeneous distribution in the zircon samples. The Hf isotope value indicates that there is only one single source of magma. All analysis points have positive ε_Hf(t) values (ranging from 4.32 to 9.64), indicating that more components of the mantle had participated in the diagenetic process, because it generally had high ε_Hf(t) value indicating participation of more components of mantle sources. T_DM2 age is 710-1031 Ma, with the mean value 914 Ma, reflecting the duration of extracting the materials in the sources from the depleted mantle (or the residence age of materials of the sources in the crust). In plot ε_Hf(t) values vs. age(Fig. 4b) the setting points are between the development lines of the depleted mantle (DM) and the crust, showing the characteristics of mantle-sourced Hf isotope. Therefore, the magma sources of the granodiorite porphyry in the Sepon deposit are mainly mantle source but underwent mixing and contamination of crust materials in the process of magma emplacement.

5.3 Tectonic implications

Most of intrusions in the Truong Son Fold Belt that are closely related to metallogeny of porphyry-skarn and hydrothermal deposits are calc-alkaline magma (Backhouse D, 2004; Cromie PW et al., 2006, 2010; Manaka T et al., 2008; Li YF, 2012; Kamvong T et al., 2014; Shi MF et al., 2015) . At the Phu Kham Cu-Au deposit, adakites show a strong depletion in HREE and high LREE contents (high La/Yb rations), representing the initial stage when the Ailaoshan-Song Ma Ocean branch of the Paleotethys Ocean began subducting toward the Indochina Block (Kamvong T et al ., 2014; Liu JL et al., 2012; Khin Z et al., 2014). Whole rock geochemistry of the Pha Lek diorite and granodiorite samples are metalumious calc-alkaline granites enriched of Rb, Th, U, K, Pb, lack of Ba, Ta, P, Ti and high field strength element Nb; enriched with LREEs, moderate negative Eu anomaly, which indicates island arc environment (Li YF, 2012).

Schulz JJ. et al. (Schulz et al., 2006) summarized the geochemical characteristics of trace elements contained in zircon crystallized under different tectonic settings (within the plate, volcanic arc and mid-oceanic ridge). In the Yb/Dy-Y, Lu/Hf-Y, Ce, Yb-Y and Th-Gd discrimination diagrams of zircon under different tectonic settings (Fig. 5), zircons from granodiorite porphyry in the Sepon deposit are interpreted as volcanic-arc basalt-type suit feature (VAB). Grimes CB et al., (2007)used some trace element diagrams of zircon to differentiate crystallization environments for zircon (continental or oceanic crust), and found in the U/Yb-Hf diagram of zircon under different crystallization environments (Fig. 6a), that the zircon of the granodiorite porphyry in the Sepon deposit falls within the region of continents and kimberlite (indicating the cause of mantle formation), and based on the U/Yb-Y diagram (Fig. 6b), it falls within a continental environment, indicating that the zircon originated from crystallization of magma in the continental crust. It follows that the Au-Cu ore in the Sepon deposit is closely related with arc magmatism, formed in a continental arc environment, consistent with the tectonic setting reflected by the characteristics of geologic formation, and in this area, the volcanic activity is undeveloped, and no back-arc spreading basin has been found.

Figure 5. Discrimination plots of different tectonic settings for the zircon (base map modified from Schulz, 2006)).

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Figure 6. Geochemical discriminant diagrams for zircons (base map modified from Grimes et al., 2007).

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As noted above, the south of the Truong Son Fold Belt was in a subduction environment during the late Carboniferous to early Permian, a branch of Paleotethys Ocean, Ailaoshan-Song Ma Ocean subducted toward Indochina block, causing large-scale metallogeny of arc magma thus forming the Pha Lek skarn Fe ore, KTL skarn Cu-Au ore, Phu Kham porphyry Cu-Au ore and the Ban Houayxai hydrothermal Au-Ag ore, etc. During that moment, calc-alkaline magma coming from deep sources emplaced in the Sepon deposit in the subduction process of Paleotethyan Ocean, forming porphyry Mo-Cu, skarn Cu-Au mineralization and hydrothermal sedimentary-hosted Au mineralization in the wall rocks.

6. Conclusions

In the Sepon porphyry Au-Cu deposit, LA-ICPMS zircon U-Pb dating from granodiorite porphyry is 302.1 ± 2.9 Ma and its emplacement occurred in the Late Carboniferous, suggesting an important magmatism-metallogeny that occurred in the Truong Son Fold Belt during the Late Carboniferous to Early Permian. The intermediate-acid magma intruded at the same time as the time for mineralization, thus providing thermal sources and material sources for the mineralization. The Ce⁴⁺/Ce³⁺ varys greatly from 2.4 to 1438.29，which shows magma mixing might occur. The high value of Ce⁴⁺/Ce³⁺ indicates the existence of high oxygen fugacity magma injecting into the lower oxygen fugacity magma. ¹⁷⁶Hf/¹⁷⁷Hf of zircon ranges from 0.282859 to 0.282719 with ε_Hf(t) value from 4.32 to 9.64. The mean age T_DM2 is 914 Ma, suggesting that the magma source of granodiorite porphyry in the Sepon deposit are mainly mantle source but they underwent mixing and contamination of crust materials. Considering the characteristics of trace elements in the zircon and the geochemical characteristics of intrusions as well as the characteristics of regional geologic formation, the tectonic setting might be a continental arc environment. The Sepon Au-Cu deposit is derived from emplacement of calc-alkaline intermediate-acid magma from deep sources in the subduction process of Paleotethys Ocean, forming porphyry Mo-Cu, Cu-Au mineralization and the hydrothermal sedimentary-hosted Au mineralization in wall rocks.

Acknowledgement

The author hereby extends gratitude to the colleagues in Institute of Mineral Resources, Chinese Academy of Geological Sciences, Mr. Li Yike, who provided great support during the geologic work in the field; Mr. Hou Kejun, who assisted enthusiastically the experiments described in the paper; and the reviewers who gave precious comments after the submission of the paper. This study was financially co-supported by the National Science Foundation of China (41373036, 41002027), the Geological Survey of China Geological Survey Project (121201103000150006, 121201066307).

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Zircon U-Pb age, trace element and Hf isotope composition of Sepon Au-Cu deposit, Laos: tectonic and metallogenic implications