Ore-forming model of the Dapingzhang Cu-polymetallic deposit in the Sanjiang Region, Southwest China

The Dapingzhang deposit in the Sanjiang region of Southwest China is a representative volcanogenic massive sulfide system. In this paper, we conducted a detailed field mapping combined with petrography, ore microscopy and in-situ LA-MC-ICP-MS sulfur isotope analysis to resolve the uncertainties on its spatial architecture, metal zoning and ore-forming processes. The deposit is composed of an overlying stratiform V1 formed by seafloor exhalation and an underlying V2 dominated by subseafloor replacement-filling type and feeder conduit infilling. In the cross section, it shows a clear upward zinc-rich and downward copper-rich zonation: V1 hosts zinc-rich massive ores, while the upper and lower parts of V2 host clast-supported to disseminated copper mineralization and copper-rich vein and stockwork ores, respectively. Permeable volcaniclastic media of the lithologic unit Ⅲ provided the main pathways and massive replacement sites. Sulfur isotopes span -0.99‰ to 6.50‰ and define a vertical progression from low and tightly clustered values near 0‰ in V1, to a higher and dispersed values in the upper V2, and finally to convergent values of 4‰~6‰ in the lower V2, recording a transition from open exhalative conditions to semi-closed replacement and finally to mixing and homogenization within feeder conduits. Microtextural mineralization evidence of the deposit includes concentric ring botryoidal pyrite, zoned pyrite and sphalerite, comb-textured quartz, phreatic to implosion breccias, and pervasive chalcopyrite replacement of pyrite and sphalerite with abundant "chalcopyrite disease" in sphalerite, documenting a long-lived replacement and "zone refining". Building on previous work, we establish a semi-permeable, semi-closed ore-forming system for the deposit governed by the superposition of exhalative sedimentation, subseafloor replacement, and deep feeder-conduit infilling, which coherently explains the "Zn-up-Cu-down" vertical zoning together with the overall Cu-rich and relatively Pb-Zn-poor character. We further quantitatively link the vertical δ34S sequence-low-value clustering, mid-to-high dispersion, and mid-to-high convergence to the evolutionary model of three mineralized units and, accordingly, propose some exploration guidance as follows: (1) In the lower part of V2, prioritize belt-like zones should lie in cryptic explosive brecciation overprinted by intense silicification where δ34S converges to delineate Cu-rich conduits; (2) In the V1 and at horizons overlain by a siliceous-jasper cap, we should use low δ34S as a criterion to select Zn-rich targets.