辽北地区金矿床铅同位素地质研究

辽北太古宙变质地体上发育有三类金矿:太古宙锌、铜块状硫化物矿床伴生金矿、太古宙同韧性剪切带变生(质)热液金矿和显生宙岩浆热液金矿。块状硫化物伴生金矿中方铅矿模式年龄平均为2610Ma,其产状反映为活化年龄;其黄铁矿含较多放射性成因铅,存在铅同位素后期增长效应,模式年龄不具计时意义。变生热液金矿中黄铁矿铅同位素组成特点为~(206)Pb变化很大,~(207)Pb变化很小,~(208)Pb也变化很小,结合黄铁矿产状说明存在铅同位素短期生长效应,而铅源自太古宙绿岩。显生宙岩浆热液金矿中铅同位素均不具计时意义,但各自组成等铅域,在铅构造图解上处于地幔-下地壳线附近。本区金矿铅同位素研究说明,尽管铅同位素在大多数情况下不具计时意义,但结合矿床地质研究尚可标定成矿物质来源和成矿环境。

Lead Isotopic Studies Of Gold Deposits In Northern Liaoning

There occur three types of gold deposits in northern Liaoning Archean metamorphic terrain, i.e., gold deposits associated with Archean zinc-copper massive sulfides, Archean metamorphosed gold deposits in synchronous ductile shear zones and phanerozoic magmatic-hydrothermal gold deposits. Galena from gold-bearing zinc-copper massive sulfide ores contain least radiogenic lead, and lead isotopes are distributed on the whole along isochrons. The average stacey’s two-stage model age is 2610 Ma, and the mode of occurrence of galena indicates the age of remobilizaticm of sulfides, namely, the age of reenrichment of gold and silver. Pyrite, which crystallized earlier than galena, contains relatively more radiogenic lead and displays much younger model age (681-1838 Ma). The insignificant variation in Th/U ratio suggests that radiogenic lead must have been produced as a result of in situ U and Th decay of pyrite.Four pyrite samples from metamorphosed hydrothermal gold ore show great variation in ²⁰⁶Pb/²⁰⁴Pb ratio but comparatively neglible variation in ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios from earlier to late generation, accompanied by the decrease in Th/U ratio. Such variation or short-span growth effect resulted from the U decay in the process of crystallization of pyrite from a fluid with high U/Pb ratio, whose geochemistry proves it to be a metamorphic hydrothermal solution derived from greenstone in the area.The lead isotope model ages of Mesozoic magmatic-hydrothermal gold deposits Oil the southern and northern side of the Hunhe river(1491 Ma and 692 Ma respectively)are both significantly larger than the mineralization age. The lead isotopes form two separate isoplumb fields in the Pb-Pb diagram and are close to the mantle-lower crust evolution line in the plumbtectonic model. Moreover, the ores are similar to their related granites in lead isotope composition. All these data point to a magmatic-hydrothermal origin of lead. The hydrothermal solution, in turn, must have come from the magna formed through the remelting of greenstone and/or supercrust rocks originally derived from the mantle. It can therefore be concluded that although in most cases the lead isotope model ages are older or younger than the age of gold mineralization, the lead isotope composition and distribution do reflect the origin of lead and the metallogenic environment. Nevertheless, the interpretation of lead isotopes should be invariably based Oil geological and geochemical studies of gold deposits.