Gold enrichment of the Guilaizhuang Au-Te deposit, the southeastern margin of the North China Craton: Constraints from petrography, composition and in-situ sulfur isotope of pyrite

The Guilaizhuang Au-Te deposit is the most representative alkaline igneous-rock-related epithermal Au deposit on the southeastern margin of the North China Craton (NCC). Key aspects regarding the occurrence states and enrichment mechanisms of Au remain unclear. Pyrite, as a principal gold-hosting mineral, plays a crucial role in deciphering the ore-forming process; however, detailed data on its textural characterization and in-situ geochemistry are scarce. In this study, the ore-forming process is divided into three stages, (1) quartz-pyrite (stage I), (2) Au-polymetallic sulfide-telluride (stage II), and (3) carbonate (stage III). Two types of pyrite are identified, including euhedral disseminated pyrite (Py1) from the early stage (stage I) and coarse-grained anhedral pyrite (Py2) from the main mineralization stage (stage II). In addition, LA(-MC)-ICP-MS is employed for the first time to conduct in-situ trace element spot/mapping and S isotope analyses on these two types of pyrite. Through comprehensive petrography, EPMA, and LA-ICP-MS analyses, four main occurrence states of Au are determined, including auriferous telluride, native Au, “invisible gold” and electrum. “Invisible gold” occurs as both solid solution (Au⁺) within the crystal lattice of pyrite and micro-scale mineral inclusions enclosed within pyrite. Py2 shows higher concentrations of As, Au, Ag, and Te compared to Py1. In As-pyrite, As substitutes for S⁻ as As⁻ rather than for Fe²⁺ as As^2+/3+ and is incorporated into the pyrite. The coupled distribution and positive correlation between Au and As indicate that As facilitates the incorporation of Au into the pyrite lattice. Tellurium mainly occurs as solid solution within pyrite, whereas Pb exists as galena and minor altaite inclusions within pyrite. Py1 exhibits δ³⁴S values ranging from −2.60 to +3.1 ‰ (mean = −1.1 ‰), whereas Py2 yields values from −10.6 to +2.0 ‰ (mean = −4.5 ‰), indicating that the S in pyrite is of magmatic origin. The lower δ³⁴S values in Py2 result from isotopic fractionation during fluid boiling, where ³⁴S is preferentially partitioned into sulfate while ³²S enriches in sulfide phases. Furthermore, the concentric zoning of Co-Ni and As-Au-Te within the pyrite provides additional evidence for fluid boiling. Based on the comprehensive analyses above, this study proposes that the gold enrichment at Guilaizhuang is not only influenced by the changes of physicochemical conditions (e.g., temperature and pH) but also facilitated by Te-rich metallic melts that enhance Au scavenging and deposition during the ore-forming process. These findings are of considerable significance for understanding trace element incorporation into pyrite, and for revealing the mechanism of Au enrichment and hydrothermal evolution processes in alkaline igneous-rock-related epithermal Au deposit.