地学前缘 ›› 2020, Vol. 27 ›› Issue (4): 49-65.DOI: 10.13745/j.esf.sf.2020.4.23

• "第四届全国青年地质大会论文"专栏 • 上一篇    下一篇

冈底斯西段罗布真浅成低温热液型金银矿的成矿流体演化:来自流体包裹体、H-O同位素的证据

刘洪1(), 张林奎1, 黄瀚霄1,*(), 李光明1, 欧阳渊1, 余槐2, 梁维1, 张洪铭2, 陈小平2   

  1. 1.中国地质调查局 成都地质调查中心(西南地质科技创新中心), 四川 成都 610081
    2.四川省核工业地质局 二八二大队, 四川 德阳 618001
  • 收稿日期:2019-11-09 修回日期:2020-04-20 出版日期:2020-07-25 发布日期:2020-07-25
  • 通讯作者: 黄瀚霄
  • 作者简介:刘 洪(1987—),男,硕士,工程师,主要从事矿床学、生态地质学研究。E-mail: liuh@mail.cgs.gov.cn
  • 基金资助:
    国家重点研发计划项目(2016YFC0600308);国家重点研发计划项目(2018YFC0604103);国家自然科学基金项目(91955208);中国地质调查项目(DD20190147);中国地质调查项目(DD20190542);中国地质调查项目(DD20160015);中国地质调查局成都地质调查中心青藏高原国际大学科计划项目

Evolution of ore-forming fluids in the Luobuzhen epithermal gold-silver deposit in western Gangdisi: fluid inclusion and H-O isotope evidence

LIU Hong1(), ZHANG Linkui1, HUANG Hanxiao1,*(), LI Guangming1, OUYANG Yuan1, YU Huai2, LIANG Wei1, ZHANG Hongming2, CHEN Xiaoping2   

  1. 1. Chengdu Center, China Geological Survey, Chengdu 610081, China
    2. No.282 Brigade, Sichuan Nuclear Geology, Deyang 618001, China
  • Received:2019-11-09 Revised:2020-04-20 Online:2020-07-25 Published:2020-07-25
  • Contact: HUANG Hanxiao

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摘要：

西藏昂仁县罗布真金银矿位于冈底斯成矿带西段,其大地构造位置属于南拉萨微陆块,矿体受北西西的断裂构造控制,呈脉状、透镜状产于始新世帕那组火山岩中。按照矿石工业类型分类,矿石类型可分为角砾岩型、石英脉型和蚀变岩型等三类,主要金银矿石矿物为自然金和碲银矿等。矿区广泛繁育不同特征的热液脉体,通过系统的野外观测以及全面的岩相学研究,依据矿物共生组合、脉体切穿关系及蚀变特征,将热液脉体从早到晚划分为石英-黄铁矿阶段(S1)的石英-黄铁矿大脉、玉髓华石英-金-多金属硫化物阶段(S2)的石英-金属硫化物网脉、石英-碳酸盐矿物阶段(S3)的石英-方解石细脉。罗布真金银矿床热液脉体主要发育气液两相流体包裹体(富液两相包裹体、富气两相包裹体)和含子矿物(碳酸盐矿物)三相流体包裹体。本文在野外地质调查的基础上,对不同成矿阶段的石英脉进行了流体包裹体的岩相学观测、显微测温、成分分析以及H-O同位素测试。S1阶段流体包裹体的形成温度集中在310~330 ℃,盐度(w(NaCleq))集中在5.0%~10.1%,密度介于0.60~0.80 g/cm3;S2阶段流体包裹体的形成温度集中在240~280 ℃,盐度介于3.0%~7.0%,密度介于0.70~0.90 g/cm3;S3阶段流体包裹体的形成温度集中在121~215 ℃,盐度集中在1.0%~5.0%,密度集中在0.85~1.00 g/cm3。拉曼分析表明,罗布真金银矿的流体包裹体成分以H2O为主,并含有少量的CO2、N2、CH4等气体及方解石子晶。各热液脉体石英中流体包裹体的δ${{\text{D}}_{{{\text{H}}_{2}}\text{O},\text{V-SMOW}}}$值的变化范围为-106.1‰~-97.5‰,δ18${{\text{O}}_{{{\text{H}}_{2}}\text{O},\text{V-SMOW}}}$值的变化范围为-7.33‰~-7.13‰,展示其成矿流体主要源自火山岩围岩中的循环地下水,在早阶段还有少量岩浆水的加入。成矿流体在岩浆作用驱动下,沿着断裂从深部封闭体系运移到浅部的开放体系,迅速突破临界状态减压沸腾并产生相分离导致金属硫化物沉淀,形成矿化。随着含矿热液成矿物质及金属硫化物的大量析出,流体温度、盐度迅速降低,金属矿物成矿作用随之结束。罗布真金银矿床的成矿流体为中低温、低盐度、中低密度并含有少量CO2、N2、CH4等气体的流体,具有典型的浅成低温热液矿床成矿流体的特征。

关键词: 流体包裹体, 浅成低温热液矿床, 冈底斯成矿带, 罗布真, 渐新世—中新世

Abstract:

The newly discovered Lubuzhen gold-silver deposit in Ngamring County, Tibet is situated in the southern Lhasa microterrane and belongs to the west segment of the Gangdisi polymetallic metallogenic belt. The ore bodies, controlled by near NWW fractures, occur as veins and lens in the Eocene epoch volcanics of the Pana Formation and contain mainly breccia, quartz veins and altered rocks of coexisting ores. The major ore minerals in the deposit are natural gold, silver tellurite, etc. Moreover, based on the mineral assemblages and cutting relationship and alteration features, we identified three main types of hydrothermal veins: quartz-pyrite vein of the quartz-pyrite stage (S1), quartz-polymetallic sulfides vein of the quartz-polymetallic sulfides stage (S2), and quartz-carbonatation vein of the quartz-carbonation stage (S3). On the basis of detailed geological fieldwork, we selected quartz veins of different mineralization stages for the fluid inclusion study, including petrographic characteristic observation, micro temperature measurement, compositional analysis, and hydrogen and oxygen isotope analysis. Fluid inclusions in hydrothermal veins in all three stages consist of liquid-gas (liquid- and gas-rich) biphasic and daughter mineral-bearing (carbonate minerals) multiphasic fluid inclusions, with homogenization temperature, salinity (wt%, NaCl eqv.) and density ranging in 310-330 ℃, 5.0%-10.1% and 0.60-0.80 g/cm3, respectively, in S1, in 240-280 ℃, 3.0-7.0 and 0.70-0.90 g/cm3, respectively, in S2, and in 121-215 ℃, 1.0%-5.0% and 0.85-1.00 g/cm3, respectively, in S3. According to laser Raman spectroscopic analysis, the gas phase contained small amounts of CO2, N2, and CH4, and the daughter minerals included some carbonate minerals. Hydrogen and oxygen isotopic analysis revealed that the δ${{\text{D}}_{{{\text{H}}_{2}}\text{O},\text{V-SMOW}}}$ values of fluid inclusions in quartz veins were from -106.1‰ to -97.5‰, and δ18${{\text{O}}_{{{\text{H}}_{2}}\text{O},\text{V-SMOW}}}$ values ranged from -7.33‰ to -7.13‰, which suggests that the origin of ore-forming fluids is mainly derived from circulating warm groundwater in the Pana Formation and with minor portion from magmatic activities. Our studies suggest that in the Lubuzhen Ag-Au deposit, ore-forming fluids move from the deep closed system to the shallow open system and quickly break through the critical state of decompression boiling, causing phase separation as minerals and metal sulfides precipitating from ore-bearing hydrothermal fluids. As heavy precipitation occurs, temperature and salinity of ore-bearing fluids decrease rapidly which ends the metallogenesis process for metal minerals. In brief summary, the ore-forming fluids in the Luobuzhen Ag-Au deposit contain small amounts of CO2, N2, and CH4 gases and can be characterized by low to moderate homogenization temperature, mid-range salinity and low to mid-range density, all are the characteristics of typical epithermal deposit.

Key words: fluid inclusions, epithermal deposit, gangdese polymetallic metallogenic belt, Luobuzhen, oligocene epoch

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