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Keywords:

  • Qiaxia Cu deposit;
  • fluid inclusion;
  • CO2-rich fluid;
  • orogenic lode deposit;
  • Altay orogenic belt;
  • China

The Qiaxia Cu deposit occurs as veins controlled by NW-extending structures in the Devonian volcano-sedimentary Kelan Basin of the Altay orogenic belt, Xinjiang, China. Igneous and sedimentary strata exposed in the mining area have been subjected to greenschist-facies metamorphism represented by an assemblage of chlorite–epidote–biotite ± garnet ± amphibole. The hydrothermal ore-forming process can be divided into the early, middle and late stages, represented by banded magnetite-quartz, Cu-polymetallic-quartz veins and carbonate-quartz ± pyrite veinlets, respectively. The main ore minerals including chalcopyrite and other polymetallic sulphides are revealed to have mainly formed in the middle stage. Four types of fluid inclusions (FIs), including aqueous (W-type), carbonic-aqueous (C-type), purely carbonic (PC-type) and daughter mineral-bearing (S-type), have been identified at the Qiaxia copper deposit. The early-stage quartz captures the C- and W-type primary FIs completely homogenized at temperatures of 335–388 °C with low salinities of 5.51–8.66 wt.% NaCl equiv.; whilst the late-stage quartz or calcite contains only the W-type FIs with homogenization temperatures of 122–239 °C, and salinities of 0.18–7.86 wt.% NaCl equiv. This indicates that the metallogenic system evolved from CO2-rich, metamorphic to CO2-poor, with the addition of late meteoric fluid, and that a significant CO2-escape may have occurred during the evolution. The coexistence of all four types of FIs can only be observed in the middle-stage minerals, even in the microscopic domain of a crystal, representing an association trapped from a boiling fluid system. These FIs are homogenized at temperatures ranging mainly from 288 to 329 °C, with two salinity clusters of 3.39–11.75 wt.% NaCl equiv. and 38.93–46.37 wt.% NaCl equiv., respectively. The unique CO2-rich FIs including C- and PC-types in the middle stage implies the metal precipitation results from fluid boiling, CO2-escape and transient oversaturation primarily from metamorphic devolatilization. Hence, the Qiaxia Cu deposit can be taken as an example of an orogenic lode Cu system resulting from metamorphic devolatilization in Triassic continent-continental or intercontinental collision; and a deposit-scale metallogenic model has been proposed to interpret the mechanism of formation of the Qiaxia Cu deposit. Copyright © 2013 John Wiley & Sons, Ltd.