The Ashele deposit: A recently discovered volcanogenic massive sulfide Cu-Zu deposit in Xinjiang, China

The Ashele Cu-Zn deposit is a recently discovered volcanogenic massive sulf ide deposit in Xinjiang, Northwestern China. It is the largest Cu-Zn deposi t in this type of deposits in. China, which were formed in the early period of later Palaeozoic Era. This deposit is hosted within a suit of bimodal s ubmarine volcanic rocks of the Ashele Formation of Lower-Middle Devonian Sy stem formed in an environment of paleocontinental margin rift setting. Lens oid orebodies occur between spilitic rocks developed at footwall and quartz -keratophyric tuff at hanging wall. Zonation of metal elements in the Ashel e mine is one of typical volcanic-related exhalative Cu-Zn sulfide deposits in the world. Black ores enriched in Pb, Zn and Ag occurs on the top of th e No.1 orebody in the Ashele deposit, yellow ores enriched, in Cu in the mi ddle part, and the chalcopyritization stringer below the massive sulfide or es. Zonation of ore-structure in the No.1 orebody is also apparent and corr esponds to the zoning of elements, i.e. lamellar and/or banded sulfide-sulf ate ores on the top, massive sulfide ores in the middle, and stockwork vein lets associated with altered breccia pipe on the bottom. Four epochs of min eralization in the Ashele deposit has been recognized. The first period of syngenetic-exhalative deposition of sulfides is the main epoch of mineraliz ation, and the ores deposited subsequently subjected to thermo-metamorphism at the second epoch, superimposed by hydrothermal mineralization at the th ird epoch, and weathered or oxidized at the fourth epoch. More than 100 categories of minerals have been recognized in the Ashele min e, but only pyrite, chalcopyrite, sphalerite, tetrahedrite, galena, barite, quartz, chlorite, sericite, and calcite-are dominant, making up various ty pes of ores, and alteration pipes or horizons. Studies of ore petrology sug gest that the massive ores were volcanogenic and deposited by exhalative pr ocess. Geochemical studies on the Ashele deposit not only confirm the above conclu sion, but also indicate that copper were originated from the footwall rocks of spilites through water-rock interaction, while sulfur from deep source by direct exhalation at the fumarolic stage. The vented sulfur was changed into S2- by hydrolysis and the S2- subsequently attracted Cu2+ and Fe2+ to deposit as CuFeS2. The different atomic weights of Fe, Cu, Pb and Zn may be responsible for the both vertical and horizontal zoning pattern of element s. Based on this fact, a new model for the exhalative deposits such as the Ashele deposit is suggested in this paper.