Xiongcun, Tibet: A telescoped system of veinlet-disseminated Cu (Au) mineralization and late vein-style Au (Ag)-polymetallic mineralization in a continental collision zone

Abstract

The Xiongcun Cu–Au deposit occurs in the western part of the Gangdese porphyry and skarn Cu–Au mineralization belt in the Himalayan–Tibetan Orogen and was formed in an intracontinental orogenic environment related to the India–Asia collision. Sericite 40Ar/ 39Ar dating of the Xiongcun deposit indicates that ore formation occurred around 38.11 ± 0.90 Ma, much earlier than the main ore-forming ages (20 to 14 Ma) of the Gangdese Cu–Au metallogenic belt. The potassium feldspar K–Ar age of the sulfide-bearing pegmatoid dyke in the Xiongcun district is 47.62 ± 0.70 Ma. These ages indicate that the mineralization in Xiongcun is related to relaxation (ca. 52 to 40 Ma) and subsequent uplift (40 to 38 Ma) of the orogen. In the Xiongcun deposit, mineralization, hosted by Jurassic pyroclastic rocks and controlled by NW-striking faults, can be divided into two styles, i.e. veinlet-disseminated Cu ± Au mineralization and vein-style Au ± Ag-polymetallic mineralization. The former is associated with local remnant K-silicate alteration, extensive phyllic alteration and peripheral widespread propylitization. The latter, overprinting the former, is associated with strong silicification, chloritization and kaolinization. The early veinlet-disseminated Cu ± Au mineralization is similar to porphyry-type mineralization and has a mineral assemblage of quartz–sericite–pyrite–chalcopyrite–bornite–magnetite–pyrrhotite–sphalerite ± rutile ± anhydrite. The late vein-style Au ± Ag-polymetallic mineralization, characterized by a mineral assemblage of quartz–pyrite–pyrrhotite–sphalerite (rich in Fe)–chalcopyrite–galena–native gold–electrum–chlorite–kaolinite ± barite ± calcite ± siderite ± hematite, belongs to a sulfide-rich low-sulfidation-type epithermal system. The H, O and S isotopic compositions and high salinities of fluid inclusions indicate a magmatic contribution and increasing contents of meteoric water in the mineralized system from early to late stage. The wide range of homogenization temperatures (380 to 120 °C), homogenization pressures (45.92 to 1.94 bar) and salinities (36.61 to 1.23 wt.% NaCl equiv.) suggest rapid uplift and are consistent with the geological setting. Because of uplift, the hydrothermal system of Xiongcun evolved from a porphyry system to an epithermal system, resulting in an immature porphyry system and the telescoping of late vein-style Au ± Ag-polymetallic mineralization onto the early veinlet-disseminated Cu ± Au mineralization.