陕西旬阳地区小河金矿硫铅同位素组成及地质意义

doi:10.19657/j.geoscience.1000-8527.2021.115

摘要/Abstract

摘要：

小河金矿是近年来在南秦岭中带发现的中型金矿床,矿石类型为微细浸染型,矿床受地层和构造双重控制。在野外工作基础上,根据矿物组合及穿插关系划分了4个成矿阶段:Ⅰ,成矿早期少硫化物石英脉成矿阶段;Ⅱ,石英脉、黄铁矿、毒砂成矿主阶段;Ⅲ,石英脉-多金属硫化物成矿主阶段;Ⅳ,方解石、石英脉成矿晚阶段。其中Ⅱ、Ⅲ阶段是主要金矿化阶段。不同阶段样品的原位硫同位素结果显示:成矿早阶段石英脉期的黄铁矿δ³⁴S值为20.80‰~25.77‰,均值为23.59‰;主成矿期II阶段中黄铁矿、毒砂δ³⁴S值为15.46‰~19.12‰,均值为17.5‰;主成矿期Ⅲ阶段中方铅矿、闪锌矿δ³⁴S值为11.35‰~16.78‰,均值为13.88‰。硫同位素特征指示硫以沉积硫为主,成矿过程可能存在低δ³⁴S值热液的持续加入。金属硫化物Pb同位素测试结果显示²⁰⁶Pb/²⁰⁴Pb为17.882 1~18.367 4,²⁰⁷Pb/²⁰⁴Pb为15.614 0~15.674 1,²⁰⁸Pb/²⁰⁴Pb为38.016 3~38.934 2,指示小河金矿铅主要源于地壳,同时伴随幔源铅的混入。综合矿床地质特征及硫、铅同位素地球化学特征,认为小河金矿成矿过程可能存在流体混合作用。

关键词: 南秦岭, 小河金矿, 成矿阶段, 硫同位素, 铅同位素

Abstract:

The Xiaohe is a medium-sized gold deposit discovered in the central belt of the South Qinling Orogen in recent years. The ores are fine-disseminated, and the orebodies are controlled by both strata and structures. Based on field work, four metallogenic stages are divided according to mineral assemblages and crosscutting relationships: Ⅰ. Quartz veins with little sulfide in early-ore-stage; Ⅱ. Quartz veins with main-ore-stage pyrite and arsenopyrite in main mineralization stage; Ⅲ. Quartz veins with main-ore-stage polymetallic sulfides; Ⅳ. Late-ore-stage calcite and quartz veins. Stage Ⅱ and Ⅲ are the main gold mineralization stages. In situ sulfur isotope analysis show that the δ³⁴S value of stage Ⅰ pyrite=20.80‰-25.77‰ (avg.23.59‰), stage Ⅱ pyrite and arsenopyrite=15.46‰-19.12‰ (avg.17.5‰), stage Ⅲ galena and sphalerite=11.35‰-16.78‰ (avg.13.88‰). This indicates that the sulfur is mainly sedimentary-sourced, and there is continuous addition of low-δ³⁴S hydrothermal fluid during the ore-forming process. The sulfide Pb isotope analysis yielded ²⁰⁶Pb/²⁰⁴Pb=17.882,1-18.367,4, ²⁰⁷Pb/²⁰⁴Pb=15.614,0-15.674,1, and ²⁰⁸Pb/²⁰⁴Pb=38.016,3-38.934,2, indicating that the lead was derived mainly from the crust with certain mantle input. Based on the deposit geological and S and Pb isotope characteristics, we suggest that fluid mixing occurred in the Xiaohe gold mineralization.

Key words: Xiaohe gold deposit, metallogenic stage, sulfur isotope, lead isotope, south Qinling Orogen

中图分类号:

P618.51

孟五一, 刘家军, 魏立勇, 张振, 吴欢欢, 范堡程, 潘元, 李国英, 贾彬. 陕西旬阳地区小河金矿硫铅同位素组成及地质意义[J]. 现代地质, 2021, 35(06): 1587-1596.

MENG Wuyi, LIU Jiajun, WEI Liyong, ZHANG Zhen, WU Huanhuan, FAN Baocheng, PAN Yuan, LI Guoying, JIA Bin. Sulfur and Lead Isotope Compositions of the Xiaohe Gold Deposit at Xunyang, Shaanxi Province, and Its Geological Significance[J]. Geoscience, 2021, 35(06): 1587-1596.

图/表 11

图1 区域地质简图(据文献[8]修改)

Fig.1 Schematic regional geological map (modified after ref.[8])

图2 小河金矿矿床地质图

Fig.2 Geological map of the Xiaohe gold deposit

图3 小河金矿矿石特征

Fig.3 Drill-core and hand-specimen photos of the Xiaohe gold ore

图4 小河金矿矿石镜下矿物组构 (a)Ⅲ阶段的磁黄铁矿交代前期形成的黄铁矿;(b)Ⅲ阶段晚期黄铁矿呈细脉状交代早期形成的闪锌矿; (c)热液期黄铁矿交代成岩期的草莓状黄铁矿,热液期黄铁矿自形程度较高,以立方体为主;(d)Ⅱ阶段的毒砂、黄铁矿呈星点状分布于蚀变地层;(e)Ⅲ阶段晚期的黝铜矿交代前期的黄铜矿;(f)Ⅲ阶段晚期的方铅矿交代早期闪锌矿,闪锌矿与黄铜矿为固溶体分离。Py.黄铁矿;Po.磁黄铁矿;Sp.闪锌矿;Apy.毒砂;Ccp.黄铜矿;Thr.黝铜矿;Gn.方铅矿;Qz.石英

Fig.4 Reflected light microphotographs of the Xiaohe gold ore

图5 小河金矿矿物生成顺序

Fig.5 Mineral paragenesis of the Xiaohe gold deposit

图6 小河金矿矿石硫同位素点位与测试结果 (a)(b) 早阶段黄铁矿硫同位素点位与测试结果;(c)—(f) 晚阶段黄铁矿硫同位素点位与测试结果;(g) 毒砂硫同位素点位与测试结果;(h)(i) 方铅矿、闪锌矿硫同位素点位与测试结果。测试结果单位为Ma

Fig.6 Analysis spots and results of in-situ sulfur isotope analysis of the Xiaohe gold ore

表1 小河金矿矿石金属硫化物原位S同位素组成

Table 1 In-situ S isotope composition of metal sulfides for ores from the Xiaohe gold deposit

样品编号	矿石类型	成矿阶段	δ³⁴S/‰
y20-py1	黄铁矿	Ⅰ	25.77±0.14
y20-py2		Ⅰ	25.25±0.19
y20-py3		Ⅰ	23.42±0.19
y20-py4		Ⅰ	22.71±0.17
y20-py5		Ⅰ	20.80±0.35
y20-py6		Ⅱ	19.86±0.19
y20-py7		Ⅱ	19.38±0.18
y19-py1		Ⅱ	19.12±0.17
y19-py2		Ⅱ	17.94±0.17
y19-py3		Ⅱ	16.29±0.51
y19-apy1	毒砂	Ⅱ	15.50±0.45
y19-apy2		Ⅱ	15.46±0.46
y19-apy3		Ⅱ	16.43±0.47
y21-sp1	闪锌矿	Ⅲ	16.39±2.47
y21-sp2		Ⅲ	16.31±2.36
y22-sp3		Ⅲ	15.73±0.17
y22-sp4		Ⅲ	16.78±0.18
y22-gal1	方铅矿	Ⅲ	11.71±0.37
y21-gal2		Ⅲ	11.44±0.25
y22-gal3		Ⅲ	11.35±0.32
y21-gal4		Ⅲ	11.29±0.28

表2 小河沟金矿金属硫化物样品Pb同位素组成及相关参数

Table 2 Lead isotope composition and parameters of sulfide samples from the Xiaohe gold deposit

原始样号	样品名	矿石类型	²⁰⁶Pb/²⁰⁴Pb	²⁰⁷Pb/²⁰⁴Pb	²⁰⁸Pb/²⁰⁴Pb	μ	ω	Th/U	Δα	Δβ	Δγ
y01	石英脉矿石	毒砂	17.882 1	15.617 4	38.284 5	9.56	39.09	3.96	74.61	21.31	46.67
y02	粉砂质板岩	黄铁矿	18.128 2	15.631 7	38.468 2	9.56	38.54	3.90	76.15	21.30	44.53
y03	粉砂质板岩	黄铁矿	18.290 7	15.668 0	38.630 3	9.61	38.64	3.89	79.88	23.28	45.69
y04	石英方解石脉	黄铁矿	18.183 7	15.652 6	38.524 7	9.59	38.66	3.90	78.29	22.59	45.44
y05	粉砂质千枚岩	黄铁矿	18.302 5	15.674 1	38.627 6	9.62	38.62	3.89	80.50	23.68	45.57
y06	黄铁矿化石英脉	黄铁矿	18.276 2	15.661 1	38.464 6	9.60	37.95	3.83	79.19	22.84	41.29
y07	黄铁矿化石英脉	黄铁矿	17.957 0	15.614 0	38.016 3	9.55	37.42	3.79	74.36	20.73	36.80
y08	黄铁矿化石英脉	黄铁矿	18.278 4	15.661 4	38.458 8	9.60	37.92	3.82	79.24	22.86	41.09
y09	黄铁矿化石英脉	黄铁矿	18.274 3	15.661 1	38.463 2	9.60	37.96	3.83	79.24	22.85	41.34
y10	黄铁矿化石英脉	黄铁矿	18.258 4	15.655 9	38.438 6	9.59	37.90	3.82	78.71	22.54	40.90
y11	铅锌矿化石英脉	方铅矿	18.304 3	15.672 8	38.636 3	9.62	38.63	3.89	80.36	23.57	45.67
y12	铅锌矿化石英脉	方铅矿	18.341 5	15.672 3	38.603 3	9.61	38.27	3.85	80.37	23.40	43.57
y13	铅锌矿化石英脉	方铅矿	18.302 3	15.671 5	38.636 0	9.62	38.63	3.89	80.24	23.50	45.66
y14	铅锌矿化石英脉	方铅矿	18.308 3	15.671 6	38.629 0	9.62	38.56	3.88	80.27	23.48	45.30
y15	黄铁矿化砂岩	黄铁矿	18.304 8	15.663 8	38.934 2	9.60	39.79	4.01	79.50	22.93	53.24
y16	黄铁矿化砂岩	黄铁矿	18.367 4	15.662 6	38.624 7	9.59	38.12	3.85	79.48	22.63	42.81

图7 小河金矿不同成矿阶段的δ34S值 Py1. Ⅰ阶段黄铁矿;Py2. Ⅱ阶段黄铁矿;Apy. 毒砂;Gn. 方铅矿;Sp. 闪锌矿

Fig.7 Sulfide δ34S values of different metallogenic stages at the Xiaohe gold deposit

图8 小河金矿铅同位素构造模式图(底图据Zartman等[30])

Fig.8 Lead isotope tectonic discrimination diagrams for sulfides from the Xiaohe gold deposit (base map after Zartman et al.[30])

图9 小河金矿矿石铅同位素Δβ-Δγ成因分类图解(底图据朱炳泉等[31]) 1.地幔源铅;2.上地壳铅;3.上地壳与上地幔混合的俯冲带铅,3a示岩浆作用,3b示沉积作用;4.化学沉积型铅;5.海底热水作用铅;6.中深变质作用铅;7.深变质下地壳铅;8.造山带铅;9.古老页岩上地壳铅;10.深变质下地壳铅

Fig.9 Δβ-Δγ genetic diagram of Pb isotopes of the Xiaohe gold ore (base map after Zhu et al. [31])

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