Fluid inclusion and isotope evidence for magmatic-hydrothermal fluid evolution in the Tuwu porphyry copper deposit, Xinjiang, NW China

Abstract

The Tuwu deposit (557 Mt at 0.58% Cu) is the most typical and economically significant one of several porphyry copper deposits in the Dananhu-Tousuquan Arc Belt near the Turpan-Hami Basin, northwest China. The mineralization occurs as chalcopyrite and minor bornite in quartz-bearing veins and host porphyries (i.e. diorite porphyry and tonalite porphyry), and is closely associated with potassic (e.g. K-feldspar, magnetite, and hematite) and phyllic (e.g. sericite, pyrite, albite and chlorite) alterations. Fluid inclusions were studied in different generations of quartz, defined as QzIA+IB, QzII, QzIII, and QzIV. Four types of fluid inclusions were recognized, namely liquid-rich biphase (L0), vapor-rich biphase (V0) and triphase (V1, with one opaque mineral), as well as brine-rich (B0; with or without an opaque mineral) inclusions. Based on fluid inclusion microthermometry using FIA method, the fluids associated with QzIB (early ore stage) are characterized by relatively high temperatures (306–458 °C) and salinities (50.3–62.1 and 6.5–22.6 wt% NaCl + CaCl2 equiv. for B0 and L0 inclusions, respectively), under lithostatic pressure of 167 to 444 bars (depth of 0.6–1.6 km), and the fluids associated with QzII and QzIII (main ore stage) have progressively decreasing temperatures from 201° to 345 °C, under hydrostatic pressure of about 90 bars (depth of 0.9 km), which induced to large-scale mineralization at Tuwu. Mineralogy and physicochemical calculations suggest that the early stage occurred at alkaline and high-oxidized fluid environment (pH = 9.6 to 11.5; logfO2 = −26.4 to − 24.3), whereas the main ore stage took place at acidic and low-oxidized conditions (pH = 4.0 to 5.0; logfO2 = −30.8). The fluids associated with QzIV (late ore stage) show low homogenization temperatures (<204 °C) and salinities (0.6–4.2 wt% NaCl + CaCl2 equiv.), implying addition of meteoric water. The δ18O values of quartz and chlorite separates range from 7.7 to 12.1‰ (n = 21), with the calculated δ18Ofluid values of − 5.4 to 9.0‰, and the δD values of fluids range from −99 to −45‰. All these observations combined with previous geochronological and geochemical results reveal that the Tuwu deposit was formed by magmatic-hydrothermal fluids related to the Late Carboniferous felsic magmatism (i.e. tonalite porphyry magma) in a subduction setting. The Cu concentration in magma source is probably the key factor controlling the anomalous Cu mineralization at Tuwu, and the oxygen fugacity and temperature decreasing of the fluids resulted in abundant precipitation of Cu-riched sulfides.