Iron oxide-copper-gold mineralization of the Devonian Laoshankou deposit (Xinjiang, NW China) in the Central Asian Orogenic Belt

Abstract

The Laoshankou Iron Oxide-Cu-Au (IOCG) deposit (41 km southwest of Qinghe City, Xinjiang, NW China) is the most important magnetite deposit with economic Cu and Au (metal reserve: 3.26 Mt Fe @ 33.5–36.4%, 9.8 kt Cu @ 0.2–0.4% and 0.14 t Au @ 0.5–1.3 g/t) in the East Junggar terrane of the Central Asian Orogenic Belt (CAOB). The deposit is hosted by volcanic rocks of the Middle Devonian Beitashan Formation. In this study, four hypogene hydrothermal alteration stages have been identified, including the Ca-silicate alteration (Stage I), amphibole-epidote-magnetite alteration/mineralization (Stage II), pyrite-chalcopyrite mineralization (Stage III) and late hydrothermal veining (Stage IV).Mineralogical, fluid inclusion (including laser Raman spectroscopy) and mineral sulfur, oxygen-hydrogen and carbon isotope analyses were carried out to elucidate the nature and origin of the hydrothermal fluids at Laoshankou: Stage I hydrothermal fluids were likely derived from a ∼590 °C magmatic fluid with δ18Owater = 9.2–9.7‰. Subsequently, the magnetite-epidote-amphibole assemblage may have precipitated from the fluids (∼530 °C) formed by the mixing of high-temperature magmatic fluids with the seawater that reacted with 18O-rich wall-rocks. The fluid-rock reactions are reflected from the δ18Owater (9.6–10.7‰) and δDwater (−25.5––23.8‰) values. The sub-economic Stage III-A (pyrite-epidote-quartz-garnet) mineralization likely occurred at ∼415 °C, and was formed by fluids with δ34Sfluid = −3.7–−0.5‰, δDwater = −110–−94‰, and δ18Owater = 7.92–8.9‰. The lower temperature and presence of organic matter of the Stage III-A fluids may have reflected the involvement of meteoric water and organic matter from the wall rocks. The economic Cu mineralization (Stage III-B) was probably formed by the mixing of a meteoric water-dominated fluid (medium temperature, low salinity and Na-dominated) and a Ca-rich formation water in equilibrium with the volcanic wall rocks (medium temperature, medium salinity and Ca-/Fe-Mg-dominated), with δ34Sfluid = −2.6‰, δ18Owater = 5.6–5.8‰, δ13Cwater = −3.3–−3.1‰, and δDwater = −92–−95‰. Towards the end of the fluid evolution, meteoric water incursion may have continued and generated the ore-barren low-temperature, low-salinity, and Na-dominated fluids. Although the Laoshankou deposit contains distinct skarn alteration, the ore mineral assemblage and ore fluid features indicate that it is not a typical skarn deposit, but share many similarities with many Central Andean IOCG deposits. We conclude that the IOCG ore-forming setting in the Devonian East Junggar was analogous to the Cretaceous Central Andes, in that they both occurred during basin inversion in a volcanic arc setting.