Ore genesis of the Saridala gold deposit, Western Tianshan, NW China: Constraints from fluid inclusion, S-Pb isotopes and Ar-40/Ar-39 dating

摘要

The Saridala gold deposit is located in the Western Tianshan (Xinjiang, NW China), southwestern part of the Central Asian Orogenic belt (CAOB). The deposit is mainly hosted in Precambrian metamorphic rocks and Early Paleozoic granites, and is structurally controlled by the Shenglidaban ductile shear zone. The gold orebodies consist of gold-bearing quartz veins and altered mylonite. The mineralization comprises three stages: quartz-pyrite veins (early stage), sulfide-quartz veins (middle stage), and quartz-carbonate veins/veinlets (late stage). Ore minerals and native gold were mainly formed in the middle stage. Four fluid inclusion types in quartz veins were identified: pure CO2 (PC-type), CO2-H2O (C-type), aqueous (W-type) and daughter mineral-bearing (S-type) inclusions. Hydrothermal fluids related to the mineralization are composed of CO2 -H2O-NaCl +/- N2 fluids (115-307 MPa) with T-h(total) values of 256-475 degrees C (peak at 320-380 degrees C) and salinities similar to 11 wt% NaCl equiv. (mostly 4-7 wt% NaCl equiv.). Ore minerals were likely deposited via fluid immiscibility and the subsequent fluid mixing. Except for the two negative outliners (-4.5 and -4.8 parts per thousand), delta S-34 parts per thousand values of the ore pyrite range from 11.0 to 17.9%0, suggesting that the ore-forming fluids contained heavier sulfur from marine sulfate-bearing rocks. Lead isotope compositions of the ore sulfides are characterized by being highly radiogenic and their relatively wide ranges: 18.01-19.62 (Pb-206/Pb-204), 15.58-15.78 ((207)/(204) Pb) and 37.95-39.08 (Pb-208/Pb-204). These data indicate that the lead in the ores may have been derived from an orogenic reservoir with some crustal input. Ar-40/Ar-39 age dating on the sericite from the syn-mineralized quartz-sericite-sulfide alteration yielded a plateau age of 337.6 +/- 1.7 Ma, an age coeval with the subduction-collision tectonic transition related to the closure of the North Tianshan Ocean. We concluded that the Saridala gold deposit shares many similarities with typical orogenic gold deposits, and was likely formed in an orogenic environment. (C) 2017 Elsevier B.V. All rights reserved.