Fluid inclusions and isotope (C, H, O, S, He, and Ar) study of the Xiaobaishitou skarn W–(Mo) deposit, East Tianshan, NW China

Abstract

The Xiaobaishitou W–(Mo) deposit (36,264 tonnes; 0.78 wt% WO3) is located in the eastern segment of the Central Tianshan Terrane in the East Tianshan, Northwest China. The deposit comprises 96 economic orebodies that developed in skarns. The orebodies are distributed along the contacts between Triassic biotite granite and Mesoproterozoic limestone. Based on crosscutting relationships and mineral assemblages, mineralization processes at Xiaobaishitou can be divided into four stages: prograde skarn (stage I), retrograde skarn (stage II), quartz–sulfides (stage III), and calcite (stage IV). Stages II and III comprised the majority of W resources, with Mo mineralization occurring mainly in stage III. Stage II also produced minor Mo mineralization characterized by garnet and/or diopside skarns with a molybdenite–chalcopyrite–pyrite assemblages.Fluid evolution during the four stages of mineralization was determined by a detailed fluid inclusion study and six types of fluid inclusion assemblages (FIAs) were recognized: L0 (liquid-rich two-phase FIAs), V0 (vapor-rich two-phase FIAs), C1 (CO2-rich two-phase FIAs), C2 (CO2-rich vapor FIAs), rare L1 (liquid-rich three-phase FIAs with the transparent mineral) and M (melt inclusions). Fluid boiling in stage II, which is caused by fluid mixing (typically the mixing of magmatic water with meteoric water) and abrupt pressure release (possibly due to local fracturing), led to the formation of inhomogeneous fluids and the precipitation of scheelite. With pressure release and abundant meteoric water mixing, fluid boiling continued in stage III and led to precipitation of more scheelite and a large volume of molybdenite.Carbon, hydrogen, oxygen, sulfide, helium, and argon isotope data from stages I to IV indicate that the ore-forming fluids had a dominantly magmatic signature (crustal granite with minor mantle composition) and were diluted by meteoric waters, which started in stage II and developed in stage III.Base on mineral geothermometers, fluid inclusion microthermometry, and multiple isotopes (C, H, O, S, He, and Ar) studies, the fluid boiling in the stages II and III at Xiaobaishitou directly led to the formation of inhomogeneous fluids and scheelite/molybdenite precipitation. However, scheelite precipitation may have resulted from an increase in fluid pH, led mainly by fluid-rock interactions. The sulfide precipitation was likely controlled by the increases in pH and fO2, caused mainly by magmatic–meteoric fluid mixing. Meanwhile, the Mantle- and crust-derived magmatic–hydrothermal fluid and shallow emplacement with low-temperature and low-density ore-forming fluid primarily led to scheelite and molybdenum symbiosis at Xiaobaishitou deposit.