Multistage pyrites in the Nibao disseminated gold deposit, southwestern Guizhou Province, China: Insights into the origin of Au from textures, in situ trace elements, and sulfur isotope analyses

Nibao is a unique thrust fault-controlled and strata-bound disseminated gold deposit in southwestern Guizhou Province, China. In Nibao, pyrite is the major sulfide mineral and Au is structurally bound (Au+) within the pyrite lattice. In this study, we conducted detailed analyses of the pyrite chemistry and S isotope composition in Nibao using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), respectively. Through petrographic and pyrite chemical studies, four pyrite generations (Py1-Py4) were distinguished: framboidal pyrite (Pyl, preore sedimentary stage), clean pyrite overgrowing framboidal pyrite (Py2, pre-ore diagenetic stage), "spongy" pyrite (Py3, early ore stage), and overgrowth of narrow pyrite rims surrounding Py2/Py3 and disseminated pyrite associated with arsenopyrite (Py4, main ore stage). Among these, Py2 and Py4 are the most abundant. The trace element content in Py2 is characterized by a wide range of As, Cu, Sb, and Pb concentrations (similar to 2480-58100 ppm, similar to 55.4-1610 ppm, similar to 29.1-232 ppm, and similar to 24.1-376 ppm, respectively), while Py4 has the highest Au, As, Cu, and Se contents (similar to 70 ppm, similar to 4200, similar to 1630 ppm, and similar to 38.3 ppm, respectively). The delta S-34 values of pre-ore pyrites measured by LA-MC-ICP-MS in this study and the available data in the literature range broadly from -53.3 to 114.8%o, indicating that they were most likely generated by bacterial reduction from marine sulfate during sedimentation/diagenesis. Meanwhile, the 8 34 5 values of ore pyrites have relative narrow 8345 values, mostly varying from -5 to 5%o, and indicating that the S was derived either from the average of sedimentary rocks or from a magmatic source. Since igneous rocks are scarce in the region and the exposed (similar to 77-99 Ma) are clearly younger than the mineralization age of the Nibao gold deposit (similar to 141 Ma), a magmatic source is unlikely in Nibao. All pyrites in this study show a positive correlation (R-2 = 0.71) between Co and Ni, and the Co/Ni and Zn/Ni ratios of main stage Py4 are close to or within the range defined for a sedimentary-diagenetic origin, suggesting a sedimentary source is more likely in Nibao.