Geology, Fluid Inclusions, and Oxygen Isotope Geochemistry of the Baiyinchang Pipe-Style Volcanic-Hosted Massive Sulfide Cu Deposit in Gansu Province, Northwestern China
Geology, Fluid Inclusions, and Oxygen Isotope Geochemistry of the Baiyinchang Pipe-Style Volcanic-Hosted Massive Sulfide Cu Deposit in Gansu Province, Northwestern China
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DOI：
10.2113/gsecongeo.103.1.269
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发表时间：
2008
期刊：
影响因子：
5.8
通讯作者：
Hou Zengqian;K. Zaw;P. Rona;Li Yinqing;Qu Xiaoming;Song Shuhe;Peng Ligui;Huang Jianjun
中科院分区：
文献类型：
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作者：
Hou Zengqian;K. Zaw;P. Rona;Li Yinqing;Qu Xiaoming;Song Shuhe;Peng Ligui;Huang Jianjun
白银厂块状硫化物铜存款矿床（折腰山和火焰山矿）产于北祁连造山带晚元古代-早古生代海底火山岩带中的一个与上覆玄武岩流有关的喷出隐穹隆内的早寒武世海底长英质火山岩序列中。该存款由两个矿化带组成：一个30 cm厚的层控富锌硫化物透镜，与赤铁矿铁锰硅质岩有关;另一个下伏的不协调块状矿石为主的硫化物带，被热液蚀变管包裹，从中心的石英岩到边缘的石英绢云母。不整合硫化物带占折腰山铜矿铜储量的90%。矿石类型主要有四种：（1）管状磁黄铁矿-黄铁矿±黄铜矿矿石，（2）块状硫化物矿石，（3）浸染状矿石晕，（4）下盘细脉矿石。磁黄铁矿-黄铁矿±黄铜矿管道平面呈椭圆形，直径30 × 50 m。管道部分取代了上覆的块状黄铁矿透镜，并向下延伸至少150 m，逐渐被富含黄铜矿的细脉和含黄铜矿的石英脉所取代，这些细脉和石英脉被不协调的热液蚀变包络所包围。块状黄铜矿-黄铁矿透镜体与火山层理不协调，含有长英质火山寄主岩的残余斑块，通常被绿泥石化火山单元内的浸染状硫化物晕所包围。这些特征表明，折腰山矿床是一个主要由火山容矿岩经海底交代而成的管状存款。流体包裹体的研究表明，它们可分为四种类型：（1）I型两相流体包裹体，（2）II型含子矿物多相流体包裹体，（3）III型富CO2流体包裹体，（4）IV型富CH 4流体包裹体。II型包裹体均一温度高（Th），为320° ~ 430°C，含高盐度流体（31 - 38重量% NaCl当量），并与富含蒸汽、高Th（最高487°C），中等盐度不整合硫化物带和相关蚀变岩中的（10 - 16重量% NaCl当量）包裹体，这表明岩浆流体对热液系统的可能贡献。富蒸汽、高Th（>300°C）和含水、低Th（<220°C）的I型流体包裹体在细脉带中共存，表明加热的海水在补给带中与岩浆流体（气体）混合。块状黄铜矿-黄铁矿体和强烈绿泥石化管道中的大多数I型流体包裹体具有低Th（62°-225°C）和高盐度（15.0 - 23.0重量% NaCl当量），表明海底裂缝中形成了致密的盐水带，其中硫化物通过开放空间填充和主火山岩的置换而积累。来自上覆不整合硫化物带的11个石英样品的δ 18 O值在每密耳8.8至11.1之间的有限范围内，由此我们计算出相应的热液流体在160°至278°C的温度范围内的δ 18 O值在每密耳-5.3至+3.1之间。全岩δ 18 O值
The Baiyinchang massive sulfide Cu deposit (Zheyaoshan and Huoyanshan mines) is hosted by an early Cambrian, submarine, felsic volcanic succession within an extrusive cryptodome associated with an overlying basaltic flow, in a Late Proterozoic-early Paleozoic submarine volcanic belt in the north Qilian orogen, northwestern China. The deposit is comprised of two mineralized zones: a 30-cm-thick, strata-bound Zn-rich sulfide lens associated with hematitic Fe-Mn cherts, and an underlying, discordant massive ore-dominated sulfide zone enveloped by a hydrothermal alteration pipe that is zoned from chlorite in the center to quartz-sericite at the margin. The discordant sulfide zone accounts for 90 percent of the Cu reserves of the Zheyaoshan mine. It consists of four main ore types: (1) pipelike pyrrhotite-pyrite ± chalcopyrite ore, (2) massive sulfide ore, (3) a disseminated ore halo, and (4) footwall stringer ore. The pyrrhotite-pyrite ± chalcopyrite pipe has an elliptical shape in plan and is 30 × 50 m across. The pipe partially replaces the overlying massive pyrite lens and extends downward at least 150 m, to be gradually replaced by chalcopyrite-rich stringer veins and chalcopyritebearing quartz veins surrounded by a discordant hydrothermal alteration envelope. Massive chalcopyrite-pyrite lenses discordant to volcanic bedding, containing relict patches of felsic volcanic host rocks, are commonly enveloped by a disseminated sulfide halo within a chloritized volcanic unit. These features suggest that Zheyaoshan is a pipe-style deposit that formed mainly by subsea-floor replacement of volcanic host rocks. Studies of fluid inclusions indicate that there are four types: (1) type I two-phase, aqueous fluid inclusions, (2) type II daughter mineral-bearing, multiphase fluid inclusions, (3) type III CO2-rich fluid inclusions, and (4) type IV CH4-rich fluid inclusions. Type II inclusions have high homogenization temperatures (Th) ranging from 320° to 430°C, contain high salinity fluids (31‐38 wt % NaCl equiv), and coexist with CO2-rich fluids found in vapor-rich, high-Th (up to 487°C), moderate salinity (10‐16 wt % NaCl equiv) inclusions in the discordant sulfide zone and associated altered rocks, suggesting a possible contribution of a magmatic fluid to the hydrothermal system. The coexistence of vapor-rich, high-Th (>300°C) and aqueous, low-Th (<220°C) type I fluid inclusions in the stringer zone suggests that heated seawater mixed with magmatic fluid (gas) in the feeder zone. Most type I fluid inclusions in the massive chalcopyrite-pyrite body and in the strongly chloritized pipe have a low Th (62°‐225°C) and high salinities (15.0‐23.0 wt % NaCl equiv), suggesting that a dense brine zone developed in fractures in the subsea floor where sulfides accumulated by open-space filling and replacement of host volcanic rocks. Eleven quartz samples from the overlying discordant sulfide zone yielded a restricted range of δ18O values between 8.8 and 11.1 per mil, from which we calculate that the corresponding hydrothermal fluids had δ18O values ranging from ‐5.3 to +3.1 per mil over a temperature range of 160° to 278°C. Whole-rock δ18O values