河南洛宁沙沟Ag-Pb-Zn矿床银的赋存状态及成矿机理

位于河南洛宁境内的沙沟热液脉型Ag-Pb-Zn矿床是熊耳山地区近年来新发现的大型矿床.野外观察和矿相学研究表明成矿过程包含4个阶段, 分别为石英-菱铁矿阶段(Ⅰ)、石英-闪锌矿阶段(Ⅱ)、石英-银矿物-方铅矿阶段(Ⅲ)和石英-碳酸盐阶段(Ⅳ), 其中Ⅱ、Ⅲ阶段为主成矿阶段.扫描电子显微镜-能谱分析(SEM-EDS)和电子显微探针微区成分分析(EMP)结果显示, 沙沟矿床中的银以不可见银和可见银两种形式存在, 但以可见银为主.不可见银主要以次显微包体(< 1 μm)的形式被包裹在黄铜矿和闪锌矿等硫化物中, 而可见银通常以各种银的独立矿物形式交代方铅矿和黄铜矿等硫化物或充填在硫化物和石英的显微裂隙内.结合本文研究和前人对沙沟矿床流体包裹体的研究认为, 银和铅、锌等金属离子在成矿早期高温阶段以氯络合物的形式搬运, 随着成矿热液温度和氧逸度的降低以及pH值的升高, 氯络合物因稳定性降低而解体, 硫氢络合物成为银、铅、锌的主要迁移形式.随着成矿热液温度的继续降低, 铅、锌等金属硫氢络合物开始分解, 方铅矿、黄铜矿和闪锌矿等硫化物得以沉淀, 此时部分银以显微和次显微包体银的形式被包裹于这些硫化物中.铅锌硫化物的大量沉淀引起成矿热液组成和性质的显著变化, 最终导致银从硫氢络合物中彻底解体, 并与Cu+、Sb3+等离子结合形成大量独立银矿物(如含银黝铜矿、硫锑铜银矿和辉铜银矿等), 而溶液中过饱和的银则以自然银的形式沉淀. 

Occurrence of Silver in the Shagou Ag-Pb-Zn Deposit,Luoning County,Henan Province:Implications for Mechanism of Silver Enrichment

The Shagou Ag-Pb-Zn vein-type deposit, located in the Luoning county of Henan Province, is hosted in amphibolite facies metamorphic rocks of the Archean Taihua Group and consists of four stages of mineralization: the quartz-siderite stage (Ⅰ), the quartz-sphalerite stage (Ⅱ), the quartz-silver minerals-galena stage (Ⅲ), and the quartz-carbonate stage (Ⅳ), with stages Ⅱ and Ⅲ being the main mineralization stages. Scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) and electron microprobe (EMP) analysis suggests that both visible and invisible Ag are present in the ores. Invisible Ag occurs mainly as sub-micron inclusions (< 1 μm) within chalcopyrite, sphalerite and other sulphide minerals, whereas visible Ag occurs as silver minerals that replaced galena, chalcopyrite and other sulphides, or fills microfractures in sulphides and quartz. Combined with existing fluid inclusion data, our results indicate that Ag+, Pb+, Zn+ and other metal ions were transported as chloride complexes at the early high-temperature stage. When temperature and oxygen fugacity of the fluids descended, coupled with increasing pH, chloride complexes started to decompose to liberate Ag+, Pb+, Zn+ to form HS- complexes. Continuous drop of temperature and likely degassing of ore fluids, led to separation of Pb+ and Zn+ from HS- to form galena and sphalerite, into which some amounts of Ag+ were incorporated, forming sub-microscopic to microscopic silver inclusions. With the changes of composition and property of ore-forming fluids caused by precipitation of voluminous sulfide, Ag+ was released from HS- complex to combine with Cu+ and Sb3+, forming silver minerals (e.g. polybasite, jalpaite and native silver).