Characteristics And Genesis Of The Wushan Copper Deposit, Jiangxi Province

Mineralization of skarns with multiple origins has been one of hot topics in the field of economic geology. The Middle- Lower Yangtze River Valley(MLYRV) is the most important metallogenic belt of skarn copper(gold) deposits in Eastern China. The Wushan deposit is a typical skarn compound copper deposit of the MLYRV. A detailed geological, mineralogical, geochronological and geochemical study has been carried out on the Wushan copper deposit, Jiangxi Province, with focus on an analysis of skarn evolution and ore genesis so as to set up a model for Mineralization of skarns with multiple origins.Geological investigation and petrographic observation indicate the occurrence of stratabound sulfide, stratabound skarn, and contact metasomatic skarn orebodies and the existence of three corresponding types of ores. The three types of ores are obviously different in mineral assemblage, texture and structure, and mineral typomorphy, and display signatures of syn-sedimentary, syn-sedimentary with magmatic hydrothermal, and magmatic hydrothermal activities, respectively. Correspondingly, there are three types of wall-rock alteration associations. The stratabound sulfides orebodies are corresponded to a low-temperature hydrothermal alteration association characterized by silicification and carbonatization with sericitization, the stratabound skarn orebodies to a middle-temperature hydrothermal alteration association characterized by serpentinization and tremolization with silicification and carbonatization, and the contact metasomatic skarn orebodies to a high-temperature hydrothermal alteration association characterized by garnetization, diopsidization, actinolitization, and epidotization with silicification and carbonatization. The ore vein cross-cutting and mineral replacement relationships are consistent with the existence of two epochs of mineralization, i.e., exhalationsedimentary and magmatic hydrothermal ones. The latter includes skarn, magnetite, quartz-molybdenite, quartz-pyrite-chalcopyrite and carbonate stages.Using the LA-ICP-MS zircon U-Pb, garnet Rb-Sr and Sm-Nd isochrone, and pyrite and chalcopyrite Rb-Sr isochrone methods, the ages of the Wushan granodiorite, skarn and orebodies are defined in the ranges of(144.4±1.1) Ma,(143.4±6.8) to(136±16) Ma, and(136.2±6.8) Ma, respectively, indicating the formation of the Wushan deposit in the Yanshanian period of the Late Mesozoic Era and the duration of the ore-forming process for 8 Ma. The Wushan granodiorite belongs to metaluminous, high-K calc-alkaline series with mixing material cource of crust with mantle, and the skarn is a product of infiltration metasomatism. The colloidal pyrite is characterized by primary sedimentary signatures, and the grain pyrite, chalcopyrite, and magnetite by magmatic hydrothermal metasomatic signatures. Isotopic geochemistry studies support the derivation of sulfur in ores mainly form deep mantle, and partially form strata, and Pb from mantle and lower crust. Results of the H-O isotopes show that ore-forming fluids were mainly magmatic water in the prograde metasomatic stage, and the meteoric water was gradually increased in the retrograde stage. Results of C-O isotopes suggest that carbon was generally form marine sedimentary carbonate. From the early skarn stage to the carbonate stage, temperature and salinity of ore-forming fluids continuously decreased from 378~518°C and 17.3~45.1%Na Cleq to 113~250°C and 3.4~11.9%Na Cleq, respectively. Fluid boiling and mixing of magmatic hydrothermal fluid with meteoric water occurred during this evolution process. The hydrothermal mixing in the skarn stage was related to the formation of the skarn. The hydrothermal boiling and mixing in the quartz-sulfide stage may be a significant mechanism for the formation of the copper deposit.As mentioned above, the Wushan deposit is a typical overprinting compound copper deposit, which experienced two periods, three types of mineralization. The first period of mineralization occurred in the Late Carboniferous, when massive sulfide deposits or protore with ores mainly consisting of sulfur and iron were formed by exhalative sedimentary. The second period of mineralization took place in the Early Cretaceous. On the one hand, skarnization in contact zone resulted from magmatic hydrothermal fluids filling and metasomatism lead to the formation of the contact metasomatic skarn orebodies. On the other hand, overprinting and reworking of the massive sulfides or protore by magmatic hydrothermal fluid moving along interlayer detachments between the Devonian siliceous siltstones and the Carboniferous carbonate rocks contributed to the enrichment of Cu and formation of the stratabound skarn orebodies near the Wushan granodiorite and stratabound sulfide orebodies far from the Wushan granodiorite.