Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China

Abstract

The Ashele volcanogenic massive sulfide (VMS) Cu-Zn deposit is placed in the Devonian volcanic-sedimentary basin of the Chinese Altay Orogen, NW China. It occurs in the form of well-developed lensoid orebodies within the Devonian Ashele Formation. The orebodies display analogous deformational characteristics with respect to their host rocks. Two periods of ore formation are distinguishable in the Ashele VMS Cu-Zn deposit: the first one is related to the genesis of banded and massive ores, whereas the second one is characterized by the formation of earlier Cu-dominated- and later Pb-Zn dominated-polymetallic quartz veins. In most cases, the primary banded textures of ores are still preserved. However, some veins cross cut the earlier generated ores with brecciated, durchbewegung, recrystallization and pressure-solution textures produced, indicating that substantial remobilization of metals has occurred due to intense reworking of the VMS. Four types of fluid inclusions were recognized in Cu-dominated quartz veins encompassing CO2-H2O (C-type), aqueous water (W-type), daughter mineral-bearing (S-type) and carbonic (PC-type) fluid inclusions. The homogenization temperatures of fluid inclusions mainly vary between 220 and 280°C, whereas their salinities focus on 4 to 8wt.% NaCl equiv. Gases in fluid inclusions identified using Raman spectroscopy consist of CO2, CH4 and N2. Characteristics such as mesothermal temperature, low salinity and elevated CO2 content indicate that the ore-forming fluid might have been of metamorphic origin. Based on the major C-type FIs, the trapping pressures are greater than 8.5MPa, corresponding to minimum ore-forming depths are deeper than 3.1km. Taking into account the regional geodynamic evolution, it is concluded that the early formed ore layers were generated in an arc-related geotectonic setting; while deformation and metamorphism that were responsible for metal remobilization in the ore system happened in a post-subduction regime. Based on the micro-textural observation coupled with fluid inclusion data, we conclude that the Ashele Cu-Zn deposit is a typical remobilized VMS deposit intensely overprinted by subsequent deformation and metamorphism.