滇中小水井金矿床成矿流体地球化学及成因类型探讨

小水井金矿床赋存于哀牢山造山带红河断裂东缘韧-脆性剪切构造破碎带中,容矿岩石为砂泥岩、灰岩之角砾岩、碎裂岩。硫、碳同位素研究表明,流体中碳、硫来自深部或地幔;氢、氧同位素组成则显示成矿热液主要为天水下渗及地下水循环从流经岩石获得物质而形成的混合热液流体。矿物中流体包裹体类型以气-液相为主,少量气相出现。矿石中的石英流体包裹体液相成分阳离子以Na+、K+为主,Na+/K+比值为3.056~4.940;阴离子以Cl-、SO24-为主,且Cl->SO24->F;气相成分以H2O、CO2为主,间有CH4、CO出现,属H2O-CO2-NaCl体系。主要成矿阶段流体包裹体均一温度集中于180~260℃之间,成矿深度约为1.0 km,流体密度0.65~0.9 g/cm3,流体盐度w(NaCleq)1.74%~9.08%,平均5.33%。小水井金矿床属于浅成条件下,由中低温、低盐度、低密度的混合热液流体在韧-脆性剪切构造带中形成的金矿床,其地质-地球化学特征与造山型金矿相似,成因类型应归属于浅成造山型金矿床,工业类型为构造蚀变岩型金矿。矿床的形成经历了金元素初始富集形成矿源层、成岩期后剪切-逆冲推覆构造活动过程中的构造-热液作用富集成矿、中酸性岩浆沿剪切构造带及裂隙系统侵入活动形成的含矿热液叠加富集、表生氧化-淋滤再富集时期等成矿过程。

A tentative discussion on ore-forming fluid geochemistry and genetic type of Xiaoshuijing gold deposit in central Yunnan

The Xiaoshuijing gold deposit occurs in a ductile-brittle shear zone,with the host rocks being fractured breccia and cataclasite made up of sandstone-mudstone and limestone.Studies of sulfur and carbon isotopic data indicate that the sulfur and carbon in the fluids were mainly derived from the mantle or a deep-seated place.Oxygen-hydrogen stable isotopic data imply that the ore-forming fluids mainly came from the meteoric water through infiltration,circulation and derivation of heat energy from the structural and magmatic activity.Fluid inclusions in minerals are mostly liquid-vapor(aqueous two-phase) inclusions,with minor vapor inclusions.The cations of liquid phase of inclusions in quartz are mainly Na+ and K+,with Na+/K+ ratio being 3.056~4.940,while the anions are chiefly Cl-and SO2-4,characterized by Cl->SO2-4>F-.The vapor-phase components of inclusions in quartz are composed mainly of H2O and CO2,with minor CH4 and CO.The ore-forming fluids belong to the H2O-CO2-NaCl system.The homogenization temperatures of fluid conclusions at the main metallogenetic stage vary from 180℃ to 260℃.Metallogenic depth is 1.0 km.Ore fluid density ranges from 0.65 g/cm3 to 0.9 g/cm3 and the salinity w(NaCleq) ranges from 1.74% to 9.08%,averagely 5.33%.The ore deposit belongs to the structure-controlled alteration rock-type gold deposit.The ore deposit was formed in a ductile-brittle shear zone and resulted from a mixture of low salinity,low density,mesothermal and epithermal hydrothermal fluids cycling at the shallow part of the shear zone.The ore deposit is similar to the orogenic gold deposit in geological and geochemical characteristics,its genetic type can therefore be assigned to the epizonal orogenic subtype defined by Groves,and its industrial type can be assigned to the altered cataclastic-rock type.The formation of the Xiaoshuijing gold deposit experienced four stages as follows: ① the formation of ore source bed by original concentration of Au element;② the enrichment metallogenesis of the structure-hydrothermal process of shear-thrust napping at the post-diagenetic stage;③ the superimposition enrichment metallogenesis of ore-bearing thermal fluids of intermediate-acid magmatic intrusion along the shear zone and the fissure system;and ④ the re-enrichment metallogenesis of epigenetic oxidation-leaching.