Origin of the Triassic Qilinchang Pb-Zn deposit in the western Yangtze block, SW China: Insights from in-situ trace elemental compositions of base metal sulphides

Abstract

The Qilinchang Pb-Zn deposit (̴ 5Mt of sulphide ore at mean grades of 2.3–9.2% Pb and 2.7–22.5% Zn) is hosted in Early Carboniferous carbonate rocks and mainly controlled by NE-, NS-, and NW- trending structural belts between the Xiaojiang and Zhaotong-Qujing buried faults. Ore minerals are sphalerite, galena, pyrite, chalcopyrite, and marmatite, whereas gangue minerals are dolomite, calcite, and quartz. Three generations of pyrite are recognized; these include fine-grained anhedral pyrite (Py-I), medium-grained anhedral to subhedral hydrothermal pyrite (Py-II), and coarse-grained subhedral to euhedral hydrothermal pyrite (Py-III), while the sphalerite occurs in two forms as fine-grained intergrown with galena and associated with Py-I and Py-II.Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis was performed to evaluate trace element compositions of pyrite and sphalerite from different generations of ores. Py-I has relatively higher concentrations of Mg, Mn, Co, Ni, and Tl. Py-II in Cu, As, Se, Mo, and Sb relative to pyrite of other generations. The contents of trace elements in Py-III are variable but significantly enriched in Ti, V, Bi, Ag, and Pb compared to Py-I and Py-II. This variation indicates that these elements occur as micro mineral inclusions of possible Ag-Pb-bearing phases. Sphalerite of stage I has higher concentrations of Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ag, Cd, In, Sb, Hg, and Pb than stage II. These elements are homogeneously distributed in all samples analyzed. The incorporation of these elements is primarily through simple substitution mechanisms (e.g., 2Zn2+ ↔ Cu+ + In3+) as indicated from binary plots. Principal component analyses (PCA) applied to LA-ICP-MS dataset for pyrite reveal two main clusters; Py-I enriched in Ni, Co and As, Py-II and Py-III high in with V, Mn, Cu, Zn, Se, Mo, Ag, Sb, Au, and Bi. Likewise, the PCA also confirms two clusters of elements for sphalerite; Ge, Cu, As, Sb, Ag, Fe, and Cd corresponding to Sph-I and In, Sn, and Ga corresponding to Sph-II.Py-I has higher Co/Ni ratio ranging from 0.04 to 1.6 compared to Py-II and Py-III having Co/Ni ratios from 0 to 0.706 and 0 0.696, respectively. We report that the pyrites from the Qilinchang deposit have Co/Ni ratios slightly lower than typical Mississippi Valley-Type (MVT) deposits (0.2–7.2) and different from iron oxide copper-gold (IOCG) and porphyry Cu deposits. Considering these geochemical signatures, it can be suggested that the Qilinchang deposit was not related to magmatic activities. We thus propose that the ore-forming fluids responsible for the formation of the deposit were generated from a low-temperature environment, similar to typical MVT deposits.