Multiple fluids involved in granite-related W-Sn deposits from the world-class Jiangxi province (China)

Abstract

This paper aims at providing new insights into W-Sn ore-forming processes within one of the largest granitic provinces in the world (Nanling Range, South China), which was emplaced during the Jurassic-Cretaceous period. The origin, composition and pressure-temperature conditions of fluids involved in the W-Sn ore-forming processes have been investigated by microthermometry, Raman spectroscopy, LA-ICPMS, hydrogen isotope analyses of fluid inclusions and oxygen isotope analyses of minerals from the Maoping and Piaotang W-Sn deposits. For each deposit, pre- (quartz), syn- (wolframite and cassiterite) and post-ore (quartz, topaz and fluorite) minerals were studied.In both deposits, the vast majority of fluid inclusions are aqueous with salinities between 0.0 and 12.6 wt% equiv. NaCl and homogenization temperatures between 136 and 349 °C. A minor proportion (~5%) of inclusions observed in the ore-stage quartz from Maoping have aquo‑carbonic compositions. For both deposits, four compositional groups are defined. Early quartz fluid inclusions are characterized by salinities between 0.4 and 9.0 wt% equiv. NaCl, trapping temperatures between 150 and 350 °C, and pressures between 20 and 150 MPa. LA-ICPMS analyses of these fluid inclusions reveal a wide range of Na, K and Li concentrations, as well as relatively low metal contents (W < 40 ppm). Values of δ18O in quartz range from −3.6 to 5.3‰ VSMOW while δD values of the fluid inclusions range from −59 to −51‰ VSMOW. The salinity of fluid inclusions in wolframite, cassiterite, topaz and fluorite is between 2.4 and 11.2 wt% equiv. NaCl, trapping temperatures are between 200 and 600 °C, and pressures range from 20 to 250 MPa. LA-ICPMS analyses of these fluid inclusions reveal higher concentrations of Na, K and Li as well as Cs and metals (e.g. between 10 and 220 ppm W). Values of δ18O in wolframite, cassiterite, topaz and fluorite crystals range from −3.0 to 3.3‰ VSMOW while δD values of fluid inclusions in these mineral phases range from −78 to −72‰ VSMOW.At Piaotang, fluid inclusions in wolframite as well as in post-ore quartz and fluorite have salinities of 5.6 to 12.6 wt% eq. NaCl, trapping temperatures between 150 and 400 °C and pressures of 20 to 150 MPa. LA-ICPMS analyses of these fluid inclusions reveal similar compositions to early quartz fluid inclusions. Values of δ18O in wolframite and quartz crystals range from −0.8 to 5.2‰ VSMOW, while the δD values of fluid inclusions range from −66 to −62‰ VSMOW.Collectively, the data suggest the involvement of four aqueous fluid end-members, mixed episodically in the mineralization process: (A) a low-salinity, low-temperature, metal-poor, low-δ18O, low-δD fluid derived from meteoric water; (B) a high-salinity, high-temperature, metal-poor, high-δ18O, low-δD fluid derived from a differentiated peraluminous granitic magma; (C) a high-salinity, high-temperature, metal-rich, lower δD magmatic fluid derived from a more differentiated peraluminous granitic magma; and (D) a high-salinity, high-temperature, metal-rich, high-δ18O, low-δD magmatic fluid.This study shows that multiple fluids with distinct magmatic and meteoric origins were involved in the formation of these W-Sn deposits and that the dilution of metal-bearing magmatic fluids by meteoric fluids was probably the main driver for ore deposition. The common fluid history of the two deposits studied, as well as similarities with other deposits in the Jiangxi province, points toward common ore-forming processes at the regional scale.