鄂东南鸡笼山矽卡岩型金铜矿床金、银、碲、铋的赋存状态及其对成矿条件的制约

鸡笼山矽卡岩型金铜矿床是长江中下游成矿带典型的矽卡岩矿床,矿体主要赋存于下三叠统大冶组碳酸盐岩与花岗闪长斑岩接触带内。根据野外观察和镜下鉴定,将成矿过程划分为进矽卡岩阶段、退化蚀变阶段、石英-硫化物阶段和碳酸盐阶段,其中石英-硫化物阶段为金和铜的主要成矿阶段。鸡笼山金铜矿床中不同类型矿石的矿相学观察和电子探针微区成分分析(EPMA)表明,金、银主要以自然金、银金矿、碲银矿、硫银铋矿等形式产出,主要载金矿物为黄铜矿和黄铁矿;同时发现鸡笼山金铜矿床中发育大量碲-铋矿物(如辉碲铋矿、针硫铋铅矿等)。成矿流体物理化学性质研究表明,鸡笼山金铜矿床中金银元素在高温热液中主要以氯络合物的形式运移,随着温度降低和流体进一步的演化,金银元素转变为以硫络合物、碲铋化物熔体等形式运移。在石英-硫化物阶段,由于硫化作用与流体的沸腾作用,流体中硫逸度降低,碲逸度升高;当流体处于黄铁矿-磁黄铁矿氧逸度范围、酸碱性呈中性-弱碱性、碲逸度(logf_(Te2))为-10.7~-8.4、硫逸度(logf_(S_2))为-11.4~-10.6时,金、银、铜元素近于同时沉淀,碲、铋和砷元素对金和银元素运移和富集起到了重要作用,最终形成了鸡笼山矽卡岩型金铜矿床。

Modes of occurrence of Au, Ag, Te and Bi elements and their constraint on the ore-forming physicochemical conditions of the Jilongshan skarn Au-Cu deposit, southeast Hubei Province

The Jilongshan skarn Au-Cu deposit is a representative skarn deposit in southeastern Hubei Province. Its orebodies are mainly hosted in the contact zone between the carbonate of the Lower Triassic Daye Group and the granite diorite porphyry. The ore-forming process can be divided into four stages, namely prograde skarn stage, retrograde skarn stage, quartz-sulfides stage and carbonate stage, and the precipitation of Au and Cu occurred at the quartz-sulfides stage. Microscopic observation and electron microprobe analysis suggest that Au and Ag occur mainly as visible Au minerals(native gold, electrum) and visible Ag minerals(hessite, matildite), respectively, and Au minerals occur mainly as inclusions in chalcopyrite and tetrahedrite or micro-fracture fillings in pyrite. Abundant Te-Bi minerals, such as tetradymite and aikinite, were observed in the Jilongshan Au-Cu deposit. In high temperature hydrothermal fluids, Au and Ag elements were probaboy transported as chloride complexes. With the decreasing temperature of the fluid and the variation of compositions, Au and Ag elements might have been transported as sulfide complexes and bismuth-telluride melts or complexes. At the quartz-sulfides stage, fluid boiling and sulfurization resulted in the decrease of f_S2 and the increase of f_Te2. When fluids lay in the oxygen fugacity range of pyrite-pyrrhotite buffer, pH values assumed neutral-to-alkaline nature, -10.7 < logf_Te2 < -8.4 and -11.4 < logf_S2 < -10.6, there existed the transfer and enrichment of Au by Te, Bi and As elements, which resulted in the formation of the Jilongshan skarn Au-Cu deposit.