LA-ICP-MS trace element geochemistry of sphalerite: Metallogenic constraints on the Qingshuitang Pb–Zn deposit in the Qinhang Ore Belt, South China

Abstract

Many hydrothermal vein-type Pb–Zn deposits were discovered in the Qinhang Ore Belt (South China), but their origin remains enigmatic due to the lack of constraints on the ore-forming process and ore-fluid physicochemical conditions. The Qingshuitang (0.426 Mt @ 3.65% Zn and 1.94% Pb) is a representative Pb–Zn deposit in the central Qinhang Ore Belt, and its mineralization comprises four stages, namely quartz-pyrite-arsenopyrite (I), quartz-galena-sphalerite (II), barite-galena-sphalerite (III), and calcite-galena-sphalerite (Ⅳ). Three generations of sphalerite were identified, namely Sp1 (Stage II), Sp2 (Stage III), and Sp3 (Stage IV).The mineralization temperature at Qingshuitang, as determined by the GGIMFis sphalerite geothermometer, is 82–204 °C (Sp1), 66–104 °C (Sp2), and 72–172 °C (Sp3), consistent with their high Ga/In (avg. 3461) but low In/Ge (<3.97) and Zn/Cd (avg. 199) ratio. These suggest that the Qingshuitang ore-forming fluid was low-temperature, with its average sulfur fugacity (fS2) evolving from Sp1(10–22.6 atm) to Sp2 (10–27.1 atm) and then to Sp3 (10–23.5 atm). Moreover, the decreasing median Mn content from Sp1 (4.86 ppm) to Sp2 (1.71 ppm) indicates an elevated oxygen fugacity (fO2) of the ore-forming fluids from Stage II to III, while the increasing median Mn content to Sp3 (2.78 ppm) reveals a subsequent fO2 drop. The low Fe–Mn–In–Co contents, low Zn/Cd (avg. 199), high Ge–Cd–Ga contents, and high Cd/Fe (avg. 1.39), Cd/Mn (721–7741), Ga/In (avg. 3469) and Ge/In (avg. 605) ratios in the Qingshuitang sphalerite resemble Mississippi Valley-type (MVT) sphalerite, but are markedly different from those in magmatic-related deposits (i.e., epithermal-, skarn-, and volcanogenic massive sulfide (VMS)-type). The Qingshuitang Pb–Zn deposit has distinct epigenetic characteristics, and similar low-temperature ore-fluids to those of MVT deposits. Therefore, we suggest that the Qingshuitang is a MVT deposit, and the physicochemical shift between Stage III and IV is probably associated with basinal brine incursion into the ore-fluids.