Infrared microthermometric and noble gas isotope study of fluid inclusions in ore minerals at the Woxi orogenic Au–Sb–W deposit, western Hunan, South China

Abstract

The Woxi Au–Sb–W deposit, hosted by the Neoproterozoic low-grade metamorphic clastic rocks, is located in a brittle-ductile shear zone within the Xuefengshan Range, South China. Orebodies are predominantly banded quartz veins, which are strictly controlled by bedding faults and display significant vertical extents (up to 2km) without obvious vertical metal zoning. Fluid inclusions hosted in quartz, scheelite, and stibnite from quartz–scheelite and quartz–sulfide–gold veins have been studied using conventional and infrared microscopy, respectively. Four types of fluid inclusions were identified based on petrography, including type I (two-phase, liquid-rich aqueous inclusions), type II (two- or three-phase, CO2-rich inclusions), type III (two-phase, vapour-rich aqueous inclusions), and type IV (single-phase aqueous inclusions). The fluid inclusions in ore minerals (scheelite and stibnite) and their coexisting quartz largely share similar characteristics in terms of their types, homogenization temperatures and salinities. This is consistent with the fact that these ore minerals are always intergrown with quartz. Microthermometric and laser Raman data indicate a low-to-moderate temperature (140–240°C), low salinity (<7.0wt.% NaCl equiv.), CO2-rich, N2-bearing aqueous ore-forming fluid. Such fluid is further identified as a deeply non-magmatic crustal fluid rather than a mantle-source fluid by the significantly low 3He/4He ratios (0.002–0.281Ra), and a small amount of meteoric water or host-rock-buffered fluid could be involved. W ore precipitation was probably associated with mixing between a deeply-originated crustal fluid and host-rock-buffered fluid based on the fluid inclusion features in scheelite and quartz-I. However, Au and Sb ore deposition probably resulted from boiling which was caused by the marked pressure drop. Geological features (such as banded structure and crack-sealing structure) also indicate that fluid pressure fluctuation induced by fault-valve mechanism occurred during ore precipitation. These characteristics of the ore-forming fluids in the Woxi deposit are in good agreement with the definition of orogenic gold deposits and the Woxi Au–Sb–W deposit is probably an atypical orogenic gold deposit for its unique ore-forming element association.