Association Between Gold Mineralization and Carbonaceous Material in Host Rocks: Constraints from C-O Isotopes and TOC–Au Relationships in the Huangjindong Deposit

The Huangjindong gold deposit, located in the central segment of the Jiangnan Orogen, is partly hosted by carbonaceous slate of the Neoproterozoic Lengjiaxi Group. However, the origin of carbonaceous material (CM) in host rocks and its relationship with gold mineralization remain poorly understood. Field investigations demonstrate that orebodies occur in both carbonaceous and CM-free slates Petrographic observations indicate that gold predominantly occurs as invisible or lattice-bound gold within sulfides, implying that sulfidation, rather than simple fluid reduction by CM, is the dominant gold precipitation mechanism. In addition, statistical analysis reveals no significan correlation between total organic carbon (TOC) values and gold grades in the mineralized carbonaceous slate (r = −0.104, p = 0.533). The δ13CV-PDB values of CM in host rocks range from −25.58‰ to −21.75‰, with an average of −23.24‰, indicating a predominantly marine sedimentary organic matter source. The hydrothermal carbonates have the δ13CV-PDB values of −6.49‰ to −3.59‰, δ18OV-PDB values of −15.38‰ to −12.20‰, and δ18OSMOW values of 15.06‰ to 18.33‰. Although the carbon isotopic compositions of these carbonates could potentially result from isotopic fractionation of CM, mass balance calculations demonstrate that the extremely low CM content (<1%) is far insufficient to form the abundant pre-ore Fe-bearing carbonates that constitute favorable chemical traps for gold mineralization. Consequently, the association between gold mineralization and CM at the deposit scale is insignificant. Based on this study, we propose that host-rock CM is not a prerequisite for gold mineralization and that future exploration should prioritize structural controls and sulfidation processes over lithological controls.