Skarn Mineralization Of The Anqing Copper-iron Deposit In Anhui Province

The Anqing copper-iron deposit is a typical skarn-type deposit in the Anqing-Guichi ore concentration area along the Middle and Lower Yangtze River metallogenic belt. Detailed field investigation and indoor petrgraphic, mineralogical and elemental geochemical study have been carried out, with a focus on analysing of its alteration and mineralization zonation and seting up of a model for its skarn mineralization.The alteration and mineralization zoning of the Anqing copper-iron deposit is significant, from wall rock to the pluton occuring in order:limestone, marble, skarnized marble, diopside garnet skarn, diopside skarnized diorite, and diorite. Skarn near outer contact zone enriches diopside, mainly with iron and copper mineralization, while skarn near internal contact zone are rich in garnet, mainly with copper mineralization. Petrographic studies show that the skarn mineralization process can be divided into skarn period and hydrothermal alteration period. Skarn period includes early skarn, magnetite, and late skarn stages, while hydrothermal alteration period consists of quartz-sulfide and quartz-carbonate stages. Mineralogical studies suggest garnets are all andradite, and the garnets formed in the late stage of skarn are rich in Fe and lack in Al, different from those formed in the early stage of skarn. The pyroxene is diopside in the early skarn stage, while hedenbergite in the late skarn stage. Major element geochemical profile indicates a typical mutual infiltrative-metasomatism genesis of the deposit, with gradual decreasing of a total amount of rare earth elements from pluton through skarn to wall rock.The Anqing copper-iron deposit should be formed by magmatic hydrothermal mutual infiltrative-metasomatism. After emplacement of magma, Na, K, and some Fe and Mg from the magma went into the silicate minerals eventually constituting the diorite. The ore-forming elements of Fe, Cu, S, Mg were separated from the magma and existed in the form of halide/oxide, forming a post-magmatic hydrothermal solution. The post-magmatic hydrothermal solution infiltrated and migrated through the lithological transition zone between pluton and carbonate, causing mutual infiltrative-metasomatism activities responsible for the formation of skarn. During the mutual infiltrative-metasomatism process, Mg mainly migrated from exocontact zone to internal contact zone, so that skarn near exocontact zone is rich in diopside. In contrast, Fe mainly migrated from internal contact zone to exocontact zone and gradually enriched at the exocontact zone to precipitate in the form of magnetite at magnetite stage. As the forming of chalcopyrite at quartz-sulfide stage, iron-copper mineralization occurs mainly in the exocontact skarn zone, while copper mineralization in the internal skarn zone.