A Study of Ore-forming Fluids in the Shimensi Tungsten Deposit, Dahutang Tungsten Polymetallic Ore Field, Jiangxi Province, China

The Dahutang tungsten polymetallic ore field is located north of the Nanling W-Sn polymetallic metallogenic belt and south of the Middle-Lower Yangtze River Valley Cu-Mo-Au-Fe porphyry-skarn belt. It is a newly discovered ore field, and probably represents the largest tungsten mineralization district in the world. The Shimensi deposit is one of the mineral deposits in the Dahutang ore field, and is associated with Yanshanian granites intruding into a Neoproterozoic granodiorite batholith. On the basis of geologic studies, this paper presents new petrographic, microthermometric, laser Raman spectroscopic and hydrogen and oxygen isotopic studies of fluid inclusions from the Shimensi deposit. The results show that there are three types of fluid inclusions in quartz from various mineralization stages: liquid-rich two-phase fluid inclusions, vapor-rich two-phase fluid inclusions, and three-phase fluid inclusions containing a solid crystal, with the vast majority being liquid-rich two-phase fluid inclusions. In addition, melt and melt-fluid inclusions were also found in quartz from pegmatoid bodies in the margin of the Yanshanian intrusion. The homogenization temperatures of liquid-rich two-phase fluid inclusions in quartz range from 162 to 363 degrees C and salinities are 0.5wt%-9.5wt% NaCl equivalent. From the early to late mineralization stages, with the decreasing of the homogenization temperature, the salinity also shows a decreasing trend. The ore-forming fluids can be approximated by a NaCl-H2O fluid system, with small amounts of volatile components including CO2, CH4 and N-2, as suggested by Laser Raman spectroscopic analyses. The hydrogen and oxygen isotope data show that D-V.SMOW values of bulk fluid inclusions in quartz from various mineralization stages vary from -63.8 parts per thousand to -108.4 parts per thousand, and the O-18(H2O) values calculated from the O-18(V-SMOW) values of quartz vary from -2.28 parts per thousand to -7.21 parts per thousand. These H-O isotopic data are interpreted to indicate that the ore-forming fluids are mainly composed of magmatic water in the early stage, and meteoric water was added and participated in mineralization in the late stage. Integrating the geological characteristics and analytical data, we propose that the ore-forming fluids of the Shimensi deposit were mainly derived from Yanshanian granitic magma, the evolution of which resulted in highly differentiated melt, as recorded by melt and melt-fluid inclusions in pegmatoid quartz, and high concentrations of metals in the fluids. Cooling of the ore-forming fluids and mixing with meteoric water may be the key factors that led to mineralization in the Dahutang tungsten polymetallic ore field.