Mechanisms for invisible gold enrichment in the Liaodong Peninsula, NE China: In situ evidence from the Xiaotongjiapuzi deposit

Abstract

Mechanisms for invisible gold enrichment hosted within Paleoproterozoic Fe-rich metamorphic rocks (marble, biotite schist, and quartz-feldspar schist) in the Liaodong Peninsula, NE China are still unclear. In this study, we choose the Xiaotongjiapuzi deposit as a case study, and present a detailed ore deposit geology and dataset of in situ chemical compositions of Fe-sulfides to address this issue. Fe-sulfides have the following paragenesis: metamorphic pyrite (Py0), first-ore stage pyrite (Py1, Py2), and a second-ore stage comprising arsenopyrite (Apy1, Apy2) and two phases of marcasite (Mas(a-b)). Early metamorphic pyrite (Py0) has relatively low concentrations of Au (<0.06–0.17 ppm). The significantly higher concentrations of Au in Py2 (1.55–423 ppm) and Apy2 (105–341 ppm) than those of Py1 (0.068–0.123 ppm) and Apy1 (2.90–7.70 ppm) suggest that gold prefers to incorporate into non-stoichiometric hydrothermal Fe-sulfides. Substitution of As1- and As3+ in Fe-sulfides may account for the coupled geochemical behavior between As and Au, Ag, Pb, Zn, Cu. Replacement led to fractured Py1 becoming enriched in Au, As, Ag, Cu, Sb and Pb, and Py0 being replaced by Au, As, Ag, Cu, Sb and Pb-rich marcasite. In situ sulfur analysis shows that Py0 has highly positive δ34S values (+13.62 to +18.55‰), suggesting it is associated with metasediments (+14 to +20‰). Py1, Py2, Apy1, and Apy2 are isotopically homogeneous and have δ34S values of +3.67 to +7.50‰, indicative of a magmatic phase. Both generations of marcasite, which replaced Py0, have yielded mixed sulfur isotopic signatures (δ34S values of +8.66 to +13.18‰). The gold mineralization in the Xiaotongjiapuzi gold deposit was the result of magmatic-hydrothermal processes. We propose two key factors: i) abundant defects in non-stoichiometric Fe-sulfides, and ii) replacement processes can cause invisible gold enrichment in Fe-sulfides in the Liaodong Peninsula. This provides a new perspective for invisible gold mineralization in other similar regions worldwide.