Geology And Geochemistry Of The Tuwaishan Gold Deposit In Western Hainan Island

The Gezhen gold belt,located on the western part of Hainan Island,is the most important gold belt in Hainan Island.A series of gold deposits such as the Tuwaishan,Baoban,Erjia and Bumo gold deposits are located along the shear zone from northeast to southwest.The Tuwaishan gold deposit is hosted by the Mesoproterozoic granitic mylonite and Neoproterozoic to Mesoproterozoic Baoban Group schists.Zircon LA-ICP-MS U-Pb dating and zircon Lu-Hf isotopic analysis from the host rock of Mesoproterozoic granitic mylonite indicate that the Mesoproterozoic granitic mylonite was formed at 1440 Ma in a continental rift setting and was sourced from juvenile crust with minor contamination of reworked crust.The Tuwaishan gold deposit is controlled by the NE trending Gezhen shear zone,and the ore mineralization type is altered granitic mylonite.The main wall rock alteration types are silicification,pyritization,sericitization,carbonation and chloritization.Based on the crosscutting relationship of ore veins,the gold mineralization of Tuwaishan gold deposit can be divided into early stage quartz-pyrite-arsenopyrite ±pyrrhotite assemblage,middle stage quartz-pyrite-polymetallic sulphide assemblage and late stage quartz-calcite assemblage.Fluid inclusion petrography and microthermometry show that the homogenization temperature of ore-forming fluid in each stage is 280?324?,211?303? and 147?259?,and the salinity is 6.20%?9.98%NaCleqv,1.74%?10.73%NaCleqv and 0.18-10.11%NaCleqv.The ore-forming fluid is characterized by medium temperature,low salinity and rich CO2.The H-O isotopic analysis of gold-bearing quartz veins show that the ore-forming hydrothermal fluid?18OSMOW,?18DSMOW and ?18OH2O are 8.1‰-13.7‰,-75.9‰-54.4 ‰ and 4.0‰-11.5‰,respectively,indicating that the ore-forming fluid originated dominantly from metamorphic water.The sulfur isotope(?34S)values of pyrite grains from the ore ranges from 4.5‰ to 7.9‰,with a narrow range of 3.4‰,indicating a source of regional metamorphism.Major and trace elements analysis on rocks from typical structure alteration mineralization profile show that water-rock reaction play an important role in gold precipitation.40Ar-39 Ar dating on hydrothermal sercite from gold-bearing quartz veins yield a 243±2.3 Ma ore-forming age for the Tuwaishan gold deposit.Based on tectonic evolution of Hainan Island during Neopaleozoic to early Mesozoic,it is inferred that the Tuwaishan gold deposit was formed during the switch from syn-collisional to post-collisional stages of collision orogeny between the Indochina block and the Southchina block.Based on deposit geology,alteration type,ore-forming fluid characteristics,ore-controlling structures,it is concluded that the Tuwaishan gold deposit belongs to orogenic gold deposits.40Ar-39Ar dating on muscovite from the Ewenling Foramtion quartz-mica schist yield a 356.8±0.9 Ma minimum age,representing the youngest possible amphibolite facies metamorphism age for the Baoban Group rocks.Based on the metamorphic devolatilization model for orogenic gold deposits,the Baoban Group rocks have already experienced amphibolite facies metamorphism at no later than 356 Ma,thus making them unable to genetate metamorphic fluid,S and Au to form orogenic gold deposits at around 243 Ma.It is inferred that and the metamorphic devolatilization of the early Paleozoic low-grade metamorphic rock series in the foot wall of the Gezhen shear zone at depth may provide the metamorphic fluid,S and Au needed for the mineralization of the Tuwaishan gold deposit.A metamorphic devolatilization metallogenic model was established for the Tuwaishan gold deposit.