Geological characteristics and genesis of the Tuwu porphyry copper deposit, Hami, Xinjiang, Central Asia

摘要

The Tuwu copper deposit, located in the Dananhu-Tousuquan island arc between the Kazakhstan-Junggar and Tarim plates, is one of the largest porphyry copper deposits in western China. Ore-hosting rocks consist of Early Carboniferous (333 + 2 Ma) granodiorite- and plagiogranite-porphyries. Both the porphyries are elongate. The orebodies, consisting of veinlet-disseminated mineralization within the poiphyries, grade range from 0.30 percent to 1.50 percent Cu. Mineralization can be assigned to six stages. Wall-rock alteration associated with the mineralization include silicification, chloritization, epidotization, sericitization and carbonatization. Three alteration zones have been recognized: phyllic zone, chlorite-biotite zone and propylitic zone. Re-Os dating indicates a 322.7 + - 3 Ma age for the deposit. Delta~(34)S values of the ore sulfides range from - 0.9 per thousand to +1.3 per thousand, with a mean of 0.1 l per thousand, reflecting a deep sulfur source. Delta~(18)O values of quartz in the ores range from 7.70 per thousand to 9.70 per thousand, and the delta~(18)O value of chlorite in the ores is 6.62 per thousand. Calculated delta~(18)O_(water) values of quartz and chlorite range from - 5.37 per thousand to +6.62 per thousand and the delta D values range from - 48 per thousand to -63 per thousand, suggesting that the ore fluids were mainly magma-derived in the early stage of mineralization. However, values for the late mineralization stage decrease, suggesting the addition of meteoric waters during the mineralization. Fluid inclusions within quartz grains are generally 3 to 30 um in size, with gas proportions ranging from 5 percent to 60 percent, mostly from 10 percent to 15 percent. Homogenization temperatures of fluid inclusions are relatively low, bracketed between 101 and 409 deg C, mostly between 100 and 200 deg C. Salinities exhibit a wide range from 0.35 to 14.61 wt. percent NaCl equiv., with a mean of 7.72 wt. percent NaCl equiv., suggesting that oreiluids have low to moderate salinity. The ore-bearing plagiogranite poiphyries show a strong depletion in heavy rare-earth elements, with average values of Yb and Y of 0.65 and 5,90 ppm, respectively, this being a characteristic geochemical signature for adakite. The geochemical signature of the adakites within the granitoid rocks represents a characteristic guide for further exploration for copper porphyry-type ore deposit in Eastern Tianshan.