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)

摘要

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 T-h values from 340 to 420 degrees 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 degrees C, and salinities range between 1 and 6 wt% equivalent. The oxygen and hydrogen isotopic composition of quartz and cassiterite (delta D-fluid - 65 parts per thousand; delta O-18(fluid) 3.6 to 6.3 parts per thousand) suggest that the ore-forming fluids from stage I have a distinct magmatic signature, whereas those from stage II through stage IV (delta D-fluid from - 80 to - 49 parts per thousand; delta O-18(fluid) from - 3.7 to 2.5 parts per thousand) show characteristics of mixing between meteoric and magmatic fluids. Moreover, delta S-34 values for sulfides from the Fengdishan ore block have a narrow range of 0.6 to 2.5 parts per thousand with a mean close to 0 parts per thousand, consistent with a magmatic sulfur source. By contrast, delta S-34 values for ore minerals from the Saozhoudi ore block range from 3.4 to 11.5 parts per thousand, 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.