Geochemical characteristics of fluid inclusions and H-O isotopes of the Ashawayi gold deposit in the Southwest Tianshan Orogen and its orogenic-type gold deposit

This paper is the result of mineral exploration engineering.

Objective The Ashawayi gold deposit, located at the central and southern margin of the "Asian Gold Belt", is a newly discovered medium-sized gold deposit in southwestern Tianshan Orogen, NW China.

Methods Through field geological survey, indoor research on fluid inclusion petrology, fluid inclusion composition analysis, fluid inclusion temperature measurement, H-O isotope and so on is carried out.

Results The deposits are hosted in Kalazierjiao Group, Late Carboniferous, and ore bodies are produced in secondary faults of the NEE-trending thrust nappe system. According to the relationship between vein cutting and mineral metastasis, the mineralization process can be divided into three stages. Microscopic observation analyses of inclusions in quartz showed that there were three compositional types of fluid inclusions, i.e. pure CO₂, CO₂-H₂O and NaCl-H₂O in the early and middle stage, but only pure H₂O inclusions in the latest. Measurement of inclusions trapped in quartz revealed that the total homogenization temperatures were 236-386℃ in the early stage, 225-301℃ in middle and 139-222℃ in last, respectively, with corresponding salinities of 1.6%-9.7%, 1.4%-12.5%, and 1.4%-7.3% NaCl eqv.. The laser Raman spectrum of fluid inclusions showed that the liquid was composed of H₂O and CO₂, a few of CH₄ and N₂, indicating that the ore-forming fluid was a kind of medium-low temperature, rich of CO₂, bearing few of CH₄ and N₂, and low-salinity metamorphic fluids. The estimated pressures based on CO₂-H₂O type inclusions are 140-180 MPa in the early stage and 130-160 MPa in the middle, corresponding a depth of 5-7 km and 5-6 km, respectively, under the static rock pressure. The content of H-O isotopes of quartz showed ore-forming fluids changing from a kind of deep-source metamorphic fluid in the early stage, to a metaphase and atmospheric precipitation mixed fluid in the middle, and to a hydrothermal fluid of late-seasonal precipitation in the last.

Conclusions Comprehensive analysis suggested that the fluid system stress reduction, immiscible or boiling, the escape of CO₂-CH₄ and other gases, and the development of vulcanization and carbonate, should result to the rapid precipitation of the gold and the deposit is a typical medium-shallow orogenic-type gold deposit.