Geology, fluid inclusions and S-Pb-C-O isotopes of the Kuokuqueke Fe-Cu skarn deposit in Western Tianshan, China

Abstract

The late Paleozoic Boluokenu island arc belt, located on the northern margin of Western Tianshan, Xinjiang, hosts a series of epithermal Au, hydrothermal vein-type Pb-Zn and porphyry-skarn Fe-Cu polymetallic deposits and is one of the most important metallogenic belts in China. The Kuokuqueke skarn deposit is located in the middle of the Boluokenu metallogenic belt. Skarn and Fe-Cu orebodies crop out along the contact between the lower Silurian Nilekehe Formation and the Kenaxia-Kekeqiaoke granitic intrusions. Based on field investigations and petrographic observations, four main paragenetic stages of skarn formation and ore deposition are identified: the prograde skarn stage (I), retrograde skarn stage (II), quartz-sulfide stage (III), and quartz-calcite stage (IV). Ore minerals mainly consist of magnetite and hematite, with lesser amounts of pyrite, chalcopyrite, sphalerite and galena. Four types of fluid inclusions were identified in garnet, epidote, quartz, and calcite at different stages, i.e., bi-phase (liquid and vapor) aqueous inclusions (L-dominated homogenized to liquid phase and V-dominated homogenized to vapor phase), tri-phase (liquid, vapor and solid) aqueous inclusions (S-bearing) and liquid-only aqueous inclusions. From the early to late stages, ore-forming fluids show an evolutionary trend from high temperature and medium–high salinity to low temperature and low salinity. The fluid inclusions were trapped at pressures of 400∼600 bars and 100∼300 bars for the prograde and retrograde skarn stage, respectively, corresponding to depths of 1.6∼2.4 km (under lithostatic conditions) and 1∼2 km (under hydrostatic conditions), respectively. The δ34S values of sulfides range from −5.2 to 0.5‰, indicating a magmatic origin of the sulfur in ore-forming fluids. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of the sulfides vary from 17.893 to 18.242, 15.550 to 15.591 and 37.996 to 38.175, respectively, indicating that lead was derived from a mixed crust-mantle source of subduction; the δ13CV-PDB values and δ18OV-SMOW values of calcite range from −0.6 to 2.4 and 8.1 to 13.3, respectively, indicating that carbon in fluids was mainly sourced from the mixture of ore-related granite and host carbonate rocks. Comprehensive studies indicate that the Kuokuqueke deposit is a typical skarn deposit, in which fluid boiling occurred locally in the pre-ore stage (I) and boiling in the main ore stage (II∼III) was the main cause of ore mineral deposition, while meteoric water may have been involved in the post-ore stage (IV).