Mineralization and ore genesis of the Qiaoxiahala Fe-Cu-(Au) deposit in the northern margin of East Junggar terrane, Central Asian Orogenic Belt: Constraints from fluid inclusions and stable isotopes

The Qiaoxiahala Fe-Cu-(Au) deposit (1.44 Mt ore @ 43-53% Fe, 0.55-2.21% Cu and 0.13-2.4 g/t Au) is located in the northern margin of East Junggar terrane, NW China. The deposit is hosted in the upper part of the Middle Devonian Beitashan Formation mafic and felsic volcanic rocks and sedimentary interbeds, and is spatially closely related to local fractures. Six hypogene hydrothermal stages have been identified, including (Stage I) early skarn, (Stage II) late skarn, (Stage III) magnetite mineralization, (Stage IV) magnetite-pyrite mineralization, (Stage V) chalcopyrite mineralization and (Stage VI) late veins. These six stages include three major mineralization events, as represented by the (Stage III) magnetite-epi dote-K-feldspar-quartz-calcite, (Stage IV) magnetite-pyrite-garnet-quartz-calcite and (Stage V) chalcopyrite-chlorite assemblages. The Stage III magnetite mineralization fluids were likely of high temperature (354-386 degrees C), low-medium salinity (9.9-14.7 wt% NaCl equiv.), Mg/Fe-rich and organic-rich with lithostatic pressure of 1.3-1.5 kbar and corresponding depth of 4.3-4.8 km. Oxygen and hydrogen isotopes showed that the O-18-rich (9.7-11.2 parts per thousand) and D-rich (-26.3 parts per thousand) fluids were possibly modified from the Early Carboniferous seawater interacting with the Beitashan Formation organic-rich limestone or tuff. In comparison, the Stage IV magnetite-pyrite mineralization fluids were likely of medium-high temperature (272-453 degrees C), low-medium salinity (2.9-19.5 wt% NaCl equiv.), Mg/Fe-rich, with pressure of 0.8-1.2 kbar and corresponding depth of 2.6-4.0 km. The Stage V chalcopyrite mineralization fluids were likely of low-medium temperature (186-270 degrees C), low-medium salinity (1.4-21.8 wt% NaCl equiv.), Ca- or Na-rich, with pressure of 0.4-0.6 kbar and corresponding depth of 1-2 km. Hydrogen and oxygen isotopic compositions (Stage IV: delta D-water = -141 to -92 parts per thousand; delta O-18(water) = 8.9-11.1 parts per thousand Stage V: delta D-water = -104 parts per thousand; delta O-18(water) = 7.2 parts per thousand) suggest that the ore fluids were of connate marine origin. Furthermore, Stage IV delta C-13(water) values (-0.6 to 0 parts per thousand) also indicate interactions between seawater and sedimentary wall-rocks. The negative sulfur isotopic compositions of chalcopyrite and pyrite (-4.2 to -0.2 parts per thousand) may demonstrate a biogenic sulfur source. Compared with the arc-related IOCG deposits in the central Andean orogen, the Qiaoxiahala Fe-Cu-(Au) deposit shares many common features in geology, mineralization, alteration and various sources of fluids, indicating a possible IOCG-like mineralization-style and potential to discover other Late Paleozoic arc-related IOCG deposits in East Junggar. (C) 2017 Elsevier B.V. All rights reserved.