河南内乡县银洞沟银矿地质和流体包裹体特征及成因类型

CHARACTERISTICS OF ORE GEOLOGY AND FLUID INCLUSION OF THE YINDONGGOU SILVER DEPOSIT,NEIXIANG COUNTY, HENAN PROVINCE: IMPLICATION FOR METALLOGENIC TYPE

河南内乡县银洞沟银(多金属)矿床位于东秦岭北部的二郎坪地体,属于断控脉状矿床。矿石有石英脉型和蚀变岩型,围岩蚀变以硅化、绢云母化、碳酸盐化、绿泥石化为主,主要银矿物为银黝铜矿、辉银矿、自然银等。成矿过程可以划分为早、中、晚3个阶段,分别形成含黄铁矿和/或毒砂的石英脉、多金属硫化物网脉和石英-碳酸盐细脉。脉石矿物石英、碳酸盐等含大量流体包裹体,包括富/纯CO2、含CO2水溶液和水溶液3种组分类型的包裹体。矿床形成于中温(270℃～370℃)、低盐度(<11.4%)、低密度流体系统。成矿早阶段以深源富碳流体为主,中阶段流体发生沸腾,晚阶段为盐水溶液。从早到晚,包裹体捕获压力由280MPa～320MPa降为90MPa～92MPa,成矿深度略有变浅,流体系统静岩压力变为静水压力体系,趋于开放,指示区域应力场由挤压向伸展转变。银洞沟银矿的地质特征和成矿流体系统与造山型金矿一致,应属造山型银矿;结合区域构造背景,认为银洞沟银矿形成于陆陆碰撞体制,并可以借助CMF模式解释成因。

The Yindonggou silver (poly-metals) deposit, a fault-controlled lode deposit in Neixiang County, Henan province, occurs in the Erlangping Terrane,northeast Qinling Orogen. The ore is classified into quartz-vein and alteration rock types. The wall-rock alterations are dominated by silicification, sericitization, carbonation, and chloritization as well as pyritization and polymetallic sulfidization. The main silver minerals are freibergite, argentite, silver, etc. The hydrothermal ore-forming process includes: Early,Middle and Late stages, characterized by pyrite and/or arsenopyrite-bearing quartz-vein, polymetallic sulfides stockworks and carbonate-quartz veinlets, respectively. Quartz and carbonate contain lots of fluid inclusions that are recognized as three compositional types, i.e. CO_2-rich or pure CO_2, CO_2-bearing solution and solution. The deposit was formed by a mesothermal (270℃~370℃), low-density and dilute (<11.4%) fluid-system conditions. The fluid-system was rich in CO_2 in the early stage, boiled in middle stage, and characteristic of water solution in late stage.The capture pressure of fluid inclusions decreased from 280 MPa^320 MPa in early stage to 90 MPa^92 MPa in late stage,the metallogenic depths became slightly shallower and the fluid-pressure changed from lithostatic to hydrostatic as the system turned open. This indicates that metallogenesis occurred in a tectonic setting changed from compression to extension. The notations above show that the ore-forming fluid-system and geological characteristics of the Yindonggou deposit are similar to those of the orogenic-type gold deposits as summarized by previous studies,which suggests that it is a typical orogenic-type silver deposit. Considering the tectonic evolution of the host terrane, we suggest the deposit was formed in continental collision regime and the CMF model can be used to interpret its origin.