Abstract:

Axi gold deposit, occurring in the Tulasu volcanic faultbasin in the Paleozoic activecontinentalmargin on the north side of the YiliCentral Tianshan Plate, is a lowsulfidation type of epithermal gold deposit formed in the Paleozoic era. The host rocks are andesitic and volcaniclastic rocks of the Paleozoic Dahalajunshan Formation, and the ore bodies occurred as veins in annular faults of an old caldera. Orerelated minerals consist mainly of native gold, electrum, pyrite, marcasite, arsenopyrite, hematite, limonite, and trace amounts of pyrargyrite, naumannite, polybasite, cerargyrite, etc, whereas gangue minerals consist mainly of quartz, chalcedony, sericite, calcite, siderite, adularia, etc. The main wallrock alterations were silicification, phyllic alteration, carbonatization and propylitization. The deposit is characterized by enrichments of Au, Ag, As, Sb, Bi, Hg, Se, Te and Mo relative to average crust, and low Ag／Au ratios (046111). The compositions of hydrogen, oxygen, carbon, sulfur and noble gas isotopes indicate that oreforming fluids of the Axi gold deposit consisted predominantly of circulating meteoric water. The salinities of oreforming fluid vary mainly from 07 to 31 wt％ NaCleqv, with an average of 22 wt％ NaCleqv. The homogenization temperatures are mainly of 120 to 240 ℃, with an average of 190 ℃, the fluid density is 073 to 095 g/cm3, and thus the estimated maximum mineralization depth is about 700 m. The boiling of the oreforming fluid had occurred, leading to the supersaturation of the hydrothermal solution and the deposition of ore metals. This is the main mineralization mechanism for quartzvein type ores in Axi. The oreforming fluid was buffered to a nearneutral pH in a reduced environment during mineralization. Geochemical and geological characteristics show that Axi is a typical Paleozoic lowsulfidation type of epithermal gold deposit. The preservation of this Paleozoic Axi deposit required a rapid covering by the sedimental cover of Aqialehe Formation after the formation of the deposit. Beginning in the Pliocene, the Tianshan had dominated by rapid thrusting and uplifting as a result of the collision and compression between India and Eurasia, which led to part of the deposit cover being eroded and the deposit reemerged. The discovery of Axi is therefore critical for guiding the exploration of epithermal gold deposits in preMesozoic orogenic belts.

Key words: Axi gold deposit, lowsulfidation type epithermal gold deposit, Paleozoic, oreforming fluid, boiling, preservation of ore deposit