Reduced skarn gold deposits:Basic characteristics,current understanding,and perspectives in China
Reduced skarn gold deposits(RSGD)occur at calcareous sedimentary rock and have been famous for inde-pendent deposits with high gold grade(5～15 g/t).Now,the studies of RSGD are poor.Some important questions lack enough understanding,e.g.,metallogenic background,source and genesis of reduced magma,and efficient gold enrichment mechanism.This paper systematically reviews the geological characteristics,spatiotemporal dis-tribution,metallogenic mechanism,and prospecting indicator,discusses research difficulties and metallogenic po-tential and prospecting prospect in China.The RSGD shows some characteristics:① unique Au-Bi-Te-As±Co element group,lacking base metals such as Cu;②The skarn consists of hedenbergite with minor garnet;③The ore-related intrusive rock belongs to the reduced ilmenite series,including gabbro diorite,diorite,and granodio-rite;④ Representative metal minerals with low sulfur fugacity,have lollingite,maldonite,and S-poor Te-Bi minerals.The key issues or poor research mainly include:①the reduced magmatic fluid shows many attributes,e.g.,temperature,salinity,oxygen fugacity,sulfur fugacity,water content,type and content of reduced component,which attribute plays key effect during RSGD formation?② Au could migrate in various ways,e.g.,chloride complexes,sulfur complexes,Bi-Te melt,and nanometer-flocs.Which mechanism is the most efficient for gold migration and enrichment in reduced magmatic fluid?③The oxidation-reduced properties of reduced ore-form-ing magma have been controversial.Some scholars believe that the primary magma is always reduced,but others indicate that the primary magma is oxidized and later mixed with reduced materials;④There is a phenomenon of metal element zoning,showing W-Mo mineralization in deep and Au-Sb-Bi mineralization at shallow.Which factor controls the zoning pattern?In conclusion,the RSGD shows favorable mineralization potential and pros-pecting prospects in China.The superposition area of W/Sn and Au mineralization regions(belts)in the eastern China,as well as the distribution region of reduced intrusive rocks in the western China,could be favorable ex-ploration area for RSGD.
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