Polygenetic Compound Metallogenesis Of The Cishan Gold Deposit,Tongling,Anhui Province,China

There are three types of gold orebodies in the Cishan gold deposit: porphyry type,skarn type and transitional type.Porphyry gold deposits occur in the sericitized zone in the upper and middle part of pyroxene diorite body,and mainly contain disseminated and veinlet disseminated gold ores.Skarn-type gold deposits occur in skarns in the contact zone between pyroxene diorite and lower Triassic carbonate strata,and mainly occur porphyry and vein-like gold ores.Transitional gold ores occur in skarn pyroxene diorites in the contact zone between pyroxene diorite and lower Triassic carbonate rocks,and mainly contain veinlet disseminated and porphyritic gold ores.The mineralogical analysis of pyroxene and amphibole phenocrysts and accumulative pyroxene and amphibole in pyroxene diorite indicates that the initial magma may originate from mantle-derived underplating basalt magma generated in post-collision tectonic environment.The basalt magma undergoes crystallization differentiation during deep magma chamber and shallow emplacement,resulting in copper and iron loss and gold enrichment in the evolved magma.This is the key controlling factor for the formation of the Cishan gold deposit and even most of the gold deposits in Shizishan ore-field.Mineralogical and trace element(REE)analysis results show that,from magmatic rocks to transitional zones then to skarns,the oxygen fugacity of magma-fluid system increases gradually,and the formation temperature of magnetite decreases in turn.The study of mineral fluid inclusions shows that from porphyry type to transitional type to skarn type mineralization,the overall oxidation and CO2 content of fluids increase,indicating that the farther away from rock mass,the influence of magma on fluids decreases,and the influence of strata on fluids increases.Transitional mineralization has a large homogenization temperature range in each mineralization stage,especially in pyrite silicide stage.Fluid composition is quite complex,and fluid boiling and mixing may occur.The accumulation of metallic minerals is caused by the process of pressure change(i.e.fluid overpressure-fluid boiling and fracture generation-decompression and drainage)in the magma-fluid metallogenic system from the magmatic stage.The process may undergo cycles until the whole metallogenic system is completely open and mixed with atmospheric water.The gold deposits in the Shizishan ore field in Tongling were formed in the following processes including the Cishan gold deposit.Au is enriched by the crystallization and differentiation of basaltic magma in deep magma chamber and shallow magma chamber.The evolved gold-rich pyroxene dioritic magma intruded into the lower Triassic carbonate rock surrounding strata.Magma produces magmatic fluids by cooling and crystallizing.The magma reacts with the carbonate surrounding rocks to produce skarn fluids.Magmatic fluids mix with skarn fluids to produce mixed fluids.Then the gold deposits are formed by metasomatic reaction that magmatic fluids,skarn fluids and magma-skarn mixed fluids metasomatizing pyroxene diorite,skarn and skarn fossil pyroxene diorite...