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 multi-collector 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 (Py1, pre-ore 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 (∼2,480–58,100 ppm, ∼55.4–1,610 ppm, ∼29.1–232 ppm, and ∼24.1–376 ppm, respectively), while Py4 has the highest Au, As, Cu, and Se contents (∼70 ppm, ∼4,200, ∼1,630 ppm, and ∼38.3 ppm, respectively). The δ³⁴S 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‰, indicating that they were most likely generated by bacterial reduction from marine sulfate during sedimentation/diagenesis. Meanwhile, the δ34S values of ore pyrites have relative narrow δ³⁴ values, mostly varying from -5 to 5‰, 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 (∼77–99 Ma) are clearly younger than the mineralization age of the Nibao gold deposit (∼141 Ma), a magmatic source is unlikely in Nibao. All pyrites in this study show a positive correlation (R²=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.