Nature and evolution of fluid inclusions in the Cenozoic Beiya gold deposit, SW China

Abstract

The Beiya Au deposit in the Sanjiang metallogenic belt, SW China, is a porphyry-skarn deposit that formed in an intracontinental setting. Three stages of hydrothermal veins, namely quartz veins (Stage 1), quartz-pyrite veins (Stage 2), and Au-rich sulfide-quartz veins (Stage 3) were identified on the basis of field relationship and petrography under scanning electron microscopy cathodoluminescence (SEM-CL). Intermediate density aqueous fluid inclusions (ID1, ID2), high density brine inclusions (B1, B2), and low density vapor inclusions (V1, V2) have been identified in both Stage 1 and Stage 2 veins. Only liquid-rich fluid inclusions (L3) were found in Stage 3 veins. Raman spectroscopy revealed that all the inclusions contain abundant CO2. H2S was identified in ID2 and V2 inclusions. Salinities and homogenization temperatures of the intermediate density aqueous fluid inclusions (ID1, ID2), high density brine inclusions (B1, B2), and low density vapor inclusions (V1, V2) are 7.4 ± 1.1–11.9 ± 2.2, 33.6 ± 0.5–39.2 ± 1, and 1.6 ± 0.7–5.8 ± 0.8 (wt% NaCl equiv.) and 400 ± 35–440 ± 37, 395 ± 20–414 ± 6, and 386 ± 5–416 ± 4 °C, respectively. Salinities and homogenization temperatures of the L3 inclusions range from 4.9 ± 1.6 to 9.8 ± 0.7 (wt% NaCl equiv) and from 301 ± 28 to 398 ± 4 °C, respectively. Initial intermediate density aqueous fluid inclusions contain 2000–10,000 ppm Fe, 2000–4000 ppm Cu, 200–600 ppm Zn, 200–500 ppm Pb, 17–89 ppm Mo, 1–8 ppm Ag, and 0.9–5.9 ppm Au, comparable to that of intermediate density inclusions in subduction-related porphyry deposits. Brine inclusions are rich in metals and have 9000–24600 ppm Fe, 3400–11,000 ppm Cu, 1200–5200 ppm Pb, 500–3000 ppm Zn, hundreds of ppm Mo, and tens of ppm Ag. The corresponding vapor inclusions contain lower metal concentrations: 400–6000 ppm Fe, 1700–4700 ppm Cu, 50–270 ppm Pb, 100–500 ppm Zn, and minor Mo and Ag. Low to intermediate density fluids are abundant and are inferred to have been the main metal transporting medium. A temperature decrease to approximately 350 °C during Stage 3 triggered metal precipitation.