Fluid mixing leads to main-stage cassiterite precipitation at the Xiling Sn polymetallic deposit, SE China: evidence from fluid inclusions and multiple stable isotopes (H–O–S)

Abstract

The Xiling Sn deposit in eastern Guangdong Province comprises the Fengdishan Sn and the Saozhoudi Sn–Pb–Zn ore blocks and has long been regarded as a volcanic–subvolcanic system related to Sn polymetallic mineralization. Here, we present fluid inclusion microthermometric data from different ore stages and H–O–S isotope data of hydrothermal minerals to constrain the genesis of the Xiling deposit. Fluid inclusions from stage I have Th values from ~ 340 to 420 °C and salinities from ~ 15 to 17 wt% NaCl equivalent, while homogenization temperatures of fluid inclusions from stages II to V range from ~ 150 to 320 °C, and salinities range between ~ 1 and 6 wt% equivalent. The oxygen and hydrogen isotopic composition of quartz and cassiterite (δDfluid − 65‰; δ18Ofluid 3.6 to 6.3‰) suggest that the ore-forming fluids from stage I have a distinct magmatic signature, whereas those from stage II through stage IV (δDfluid from − 80 to − 49‰; δ18Ofluid from − 3.7 to 2.5‰) show characteristics of mixing between meteoric and magmatic fluids. Moreover, δ34S values for sulfides from the Fengdishan ore block have a narrow range of 0.6 to 2.5‰ with a mean close to 0‰, consistent with a magmatic sulfur source. By contrast, δ34S values for ore minerals from the Saozhoudi ore block range from 3.4 to 11.5‰, suggesting involvement of a sedimentary sulfur source. In addition, a previous geochronological study has shown that the volcanic–subvolcanic host rocks have an age of 160–170 Ma, while the Sn polymetallic mineralization has an age of about 145 Ma. Our data support a model of mixing of magmatic brine from a hidden granitic intrusion with meteoric water. The S isotope data and the observed temperature gradient of the fluid system suggest that the Sn mineralization is developed in the central part of the ore system, while the Sn–Pb–Zn and Pb–Zn mineralization occurs in the distal part. This finding might have important implications for exploration in the region.