Textures and In Situ Chemical and Isotopic Analyses of Pyrite, Huijiabao Trend, Youjiang Basin, China: Implications for Paragenesis and Source of Sulfur

Many Carlin-like Au deposits occur within the late Paleozoic and Triassic Youjiang basin of southwest China. The Huijiabao trend in Guizhou Province contains over 300 metric tons (t; 10.6 Moz) of Au at an average grade of 7 to 18 g/t in a narrow corridor that is about 20 km long and 5 km wide. Petrographic and SEM studies of pyrite in barren host rocks and high-grade orebodies led to the recognition of four stages of pyrite. Py1 consists of fine-grained framboidal crystals in black mudstone. Py2 is comprised of coarser grained euhedralsubhedral clusters that are spatially related to organic matter. Py3 is coarse grained, euhedral, and occurs as overgrowths on Py1 and Py2. Py3’s porous texture, inclusion of randomly oriented detrital minerals, and association with quartz recrystallization suggest it was deformed during Late Triassic orogenesis with Py1 and Py2. Py4 generally occurs as rims on Py1 to Py3 and is intergrown with arsenopyrite. Sensitive high-resolution ion microprobe (SHRIMP) δ 34S analyses of each pyrite type and arsenopyrite show that Py1 is related to Py2 and that Py3 is related to Py4 and arsenopyrite. The S isotope compositions of Py1 (−7.5 to +5.9‰) and Py2 (−5.3 to +7.9‰) are bimodal, which suggests that H2S was generated by biogenic sulfate reduction in open marine and sulfate limited systems during sedimentation and/or diagenesis. The compositions of Py3 (−2.6 to +1.5‰), Py4 (−1.2 to +1.5‰), and arsenopyrite (−0.8 to +0.9‰) are homogeneous and have an intermediate range of values near 0‰ that suggest that H2S was derived either from average pyrite (0.2‰) in sedimentary rocks or from a concealed magmatic source. Laser ablation-inductively coupled plasma-mass spectrometer (LA–ICP–MS) trace element analyses (As, Ni, Co, Cu, Ag, Se, V) support different origins and show that Py3 and Py4 are ore related. The lower w (Co)/ w (Ni) and w (S)/ w (Se) ratios of Py1 and Py2 are consistent with formation during sedimentation or diagenesis, whereas the higher ratios of Py3, Py4, and arsenopyrite are consistent with a hydrothermal origin. The lower concentrations of Au in Py1 (0.23–2.5 ppm) and Py2 (0.06–12 ppm) show that little Au was added during sedimentation or diagenesis. The higher concentrations of Au in hydrothermal Py3 (1.1–110 ppm) and Py4 (0.34–810 ppm) indicate that most of the Au was introduced during subsequent hydrothermal fluid flow. The low Au contents of arsenopyrite (0.09–0.52 ppm) suggests they formed from Au-depleted fluids. The Au/As ratios of Py1 and Py2 are typical of diagenetic pyrite whereas Py3 and Py4 have ratios that approach those of ore-stage pyrite in Nevada Carlin-type deposits. The fracturing of Py3 and its cementation by Py4 suggests that ore fluid movement was associated with deformation. Published isochron ages on arsenopyrite (Re-Os ~200 Ma) and late calcite-realgar veinlets (Sm-Nd ~135 Ma) in the Huijiabao trend are older than mafic dikes (84 Ma) exposed ~20 km to the east. If the 200 and 135 Ma ages are valid, H2S and Au may be derived from a sedimentary source because igneous intrusions of this age have not been found. If these ages are not valid and the gold deposits are actually Late Cretaceous in age, then H2S and Au may be derived from a magmatic source. Additional geochronology and isotopic tracer studies are needed to resolve this uncertainty.