Sulfur isotope analysis of carbonate-hosted Zn-Pb deposits in northwestern Guizhou Province, Southwest China: Implications for the source of reduced sulfur

c To determine the source of reduced sulfur in the Zn-Pb deposits in northwestern Guizhou Province in the Sichuan-Yunan-Guizhou (SYG) Zn-Pb triangle, Southwest China, sulfur isotope analyses were carried out on the coexisting sphalerite-galena pairs of the main mineralization stage from four Zn-Pb deposits: the Shanshulin, Qingshan, Yadu, and Tianqiao deposits. These neighboring deposits are controlled by adjacent parallel NW-trending faults. The results of the sulfur isotope analysis showed delta S-34 values ranging from 11.2 parts per thousand to 20.3 parts per thousand for sphalerite, and from 8.3 parts per thousand to 17.4 parts per thousand for galena. The delta S-34 value of the sphalerite is greater than that of the galena in each coexisting sphalerite-galena pair, which indicates that sulfur isotope equilibrium was reached between the sphalerite and galena. The total delta S-34 of the ore-forming fluids was determined to be 13.0-19.0 parts per thousand, and the calculated equilibrium temperature ranged from 120 degrees C to 280 degrees C. The relatively large values of total delta S-34 combined with the relatively high temperatures indicate that the reduced sulfur most likely originated from thermochemical sulfate reduction (TSR) of seawater sulfates. The results of age-dating studies combined with the composition of ore forming fluid indicate that brine from the Youjiang Basin accounted for the mineralization of the SYG Zn-Pb triangle (Li et al., 2002; Zhang et al., 2015). Several indices indicate that the Youjiang Basin developed a thermal anomaly of 200-350 degrees C during the Middle Triassic. This thermal anomaly may have resulted in the relatively high temperature of the basin brine and oil cracking to produce methane, which further triggered TSR and reduced the aqueous sulfate in the basin brine. Expelled by the uplift of the Youjiang Basin in the Late Triassic, the basin brine carrying the reduced sulfur likely migrated through the Yadu-Ziyum NW-trending fault and supplied the reduced sulfur for Zn Pb mineralization along secondary NW-trending faults in northwestern Guizhou Province.