Geology and C-O-S-Pb isotopes of the Fangyangshan Cu-Pb-Zn deposit in the Baoshan block (SW China): Implications for metal source and ore genesis

Abstract

The Fangyangshan deposit is the first Cu-rich Pb-Zn deposit discovered in the Baoshan block (SW China), the northern continuation of the Sibumasu terrane in mainland SE Asia. The stratabound veined/lensoidal orebodies are hosted in the carbonates and calcareous slates of the Upper Cambrian Shahechang Formation. Based on mineral paragenesis and crosscutting vein relationships, three main mineralization stages (pre-ore (I), syn-ore (II-1 to II-3), and post-ore (III)) were identified at Fangyangshan. Metallic minerals include mainly pyrite, chalcopyrite, sphalerite, and galena, whilst non-metallic minerals include mainly quartz and carbonates (calcite and dolomite). The carbonate δ13CV-PDB and δ18OV-SMOW values vary from the early syn-ore (II-1: −3.67 to −3.57‰, 8.18–8.92‰), middle syn-ore (II-2: −5.88 to −5.26‰, 8.18–10.79‰), late syn-ore (II-3: −2.79 to −2.37‰, 9.97–11.42‰), to post-ore (−3.75‰, 12.79‰) stage. These C-O isotope data suggest that the early ore-forming fluids were predominantly derived from a magmatic source, and the fluids have increasing sedimentary input via wallrock carbonate dissolution. The stages I to II sulfides have consistently high δ34S values (8.24–14.91‰, avg. 11.51‰), which lie between the magmatic- and evaporite-derived δ34S fields, resembling some typical magmatic-related skarn deposits (e.g., Luziyuan, Jinchanghe, Hetaoping and Heiniuwa) in the Baoshan block. This indicates that the Fangyangshan ore sulfur may have been derived from mixture of magmatic-hydrothermal fluids and seawater sulfates (e.g., gypsum) from the local carbonate-evaporite sequences via thermochemical sulfate reduction (TSR). The Fangyangshan sulfides show a clearly linear positive correlation and an upper crustal affinity for their 206Pb/204Pb (18.2984–19.0819), 207Pb/204Pb (15.7731–15.8107), and 208Pb/204Pb (38.6550–39.3092) ratios, which is again similar to many magmatic-related Pb-Zn deposits within the Baoshan block and near the Sino-Burmese border. This also implies a common mixed source of deeper magmatic fluid and upper continental crust for the lead (and also probably other ore metals) in the regional Pb-Zn ore formation. Considering the regional tectono-metallogenic setting, we propose that the Fangyangshan Cu-Pb-Zn mineralization most probably shared a single magmatic-hydrothermal system with the nearby giant Luziyuan distal skarn deposit. These deposits have many similarities with the high-temperature carbonate replacement deposits (CRDs) elsewhere.