Ore genesis of the Duotoushan Fe-Cu deposit, Eastern Tianshan, NW China: Constraints from ore geology, mineral geochemistry, fluid inclusion and stable isotopes

Abstract

The Duotoushan Fe-Cu deposit is located in the western segment of the Aqishan-Yamansu metallogenic belt in Eastern Tianshan, Xinjiang (China). Magnetite orebodies are mainly developed in the Upper Carboniferous volcano-sedimentary rocks and are mostly stratabound. Pyrite Re-Os dating has yielded an isochron age of 312±24Ma (MSWD=1.5). Five hydrothermal and one supergene alteration/mineralization stages have been recognized from the spatial distribution of alteration zoning, mineral assemblages and textural relationships. The earliest alteration of albite (Ab84-98Or2-16)±amphibole was related to the nearby albitization granite porphyry. Prograde alteration was developed on the footwall of the intermediate-mafic volcanic rocks, and formed a garnet (Ad60-95Gr5-40)-clinopyroxene (Hd0Di93Jo7 to Hd10Di83Jo7) assemblage. Alteration associated with magnetite can be further divided into three sub-stages, consisting of magnetite-amphibole±titanite (stage III-A), magnetite-amphibole-epidote (stage III-B) and epidote-magnetite-quartz±amphibole±titanite±pyrite (stage III-C) mineral assemblages. The secondary amphibole is dominantly actinolite. Pyrite, with minor chlorite-chalcopyrite veins, cut the magnetite ore, followed by later quartz/calcite/hematite veinlets.Based on fluid inclusions (FIs) and stable isotopes studies, the pre-ore stage (stage II) was formed at high temperature (>500°C) and originated from magmatic-hydrothermal fluids (δ18OH2O=+7.1‰, δDH2O=−85‰). Two garnet samples have similar δ18OH2O values (+5.6‰ and +6.4‰) but considerably lower δDH2O values (−134‰ and −139‰), indicating the overprinting by late meteoric water. Iron ore and hydrous minerals assemblages and the associated fluids had overprinted the early anhydrous minerals. Early magnetite mineralization stage (stage III-A) shows clear magmatic water input, as constrained from the δ18OH2O (+6.9‰ to +7.8‰) and δDH2O (−85‰ to −72‰) values. Later magnetite stage was magmatic-hydrothermal fluid-dominated, but with mixing of heavy δDH2O, as indicated by stable isotope compositions (δ18OH2O=+6.1‰ to +7.8‰, δDH2O=−47‰ to −32‰). Sulfur isotope values of the stage IV pyrite give a δ34S interval of +8.8‰ to +12.1‰, suggesting significant addition of external basin fluid/seawater or evaporite sulfur source to magmatic fluid. The low temperatures (83‒213°C, average=135°C), generally medium-high but variable salinities (9.9‒27.2wt.% NaCl equiv., average=21.3wt.%) and Ca-Mg-rich components of stage V fluids show clear features of involvement of basinal brines/seawater and minor meteoric water in the late hydrothermal stages.We infer that the temperature drop of the hydrothermal fluids may have been the main cause of massive magnetite precipitation at the Duotoushan deposit, whereas increasing pH values, decreasing temperature and fluid mixing maybe responsible for Cu precipitation. Ore deposit geology, mineral geochemistry and fluid evolution confirm that the Duotoushan Fe-Cu deposit is different from typical skarns and may represent a distinct group of Fe (-Cu) deposits formed during the Aqishan-Yamansu back (intra-) arc basin inversion, which show many similarities with the Central Andean IOCG deposits.