Magmatic-Hydrothermal Activity in the Shuteen Area, South Mongolia

Abstract

The Shuteen area is located in the Gurvansaikhan island arc terrane of South Mongolia. It contains a large domain of intensely developed silicic and advanced argillic alteration that has strongly affected the adakitic volcanoplutonic Shuteen Complex. The local occurrence of low-grade copper mineralization within porphyritic intrusive rocks is indicative of porphyry-style mineralization. Hydrothermal quartz veins host gold mineralization locally. The results of scanning electron microscope-cathodoluminescence imaging and fluid inclusion microthermometry indicate that the veins and altered rocks at Shuteen were produced by multiple hydrothermal events. The occurrence of CO2 inclusions in quartz from granitoids and the δ 34S values obtained for sulfide and sulfate within the hydrothermal breccias provide evidence of the involvement of magmatically derived fluids in the early stages of the Shuteen magmatic-hydrothermal system, inferred to be sourced from a porphyry-style intrusive center. Late-stage fluids produced abundant barren quartz veins and clay alteration assemblages. The weak mineralization and widespread alteration in the Shuteen Complex are typical of lithocaps found in high-level porphyry copper-(gold-molybdenum) and high sulfidation epithermal districts. The presentday erosion level at Shuteen is relatively shallow, implying that economic porphyry-style mineralization may exist at depth within or beneath the Shuteen lithocap.