Genesis of ilmenite-series I-type granitoids at the Baogutu reduced porphyry Cu deposit, western Junggar, NW-China

Abstract

The Baogutu porphyry Cu deposit is a typical reduced porphyry Cu deposit, likely related to ilmenite-series I-type granitoids. However, the nature of the granitoids (ilmenite-series or magnetite-series) and the genesis of the Baogutu deposit are still under debate. In order to resolve these issues, whole-rock magnetic susceptibility, geochemistry and Sr–Nd–Pb isotopic, zircon U–Pb dating and Hf–O isotopic compositions were carried out. Three different intrusive phases are recognized within the deposit, from oldest to youngest, they are diorite with trace gabbro, diorite-granodiorite porphyry, and hornblende diorite porphyry, all of which were emplaced in the Late Carboniferous (320–306Ma) and show a metaluminous, calc-alkaline I-type granitoid character with typical supra-subduction zone geochemical affinities. The intrusions are characterized by widespread primary pyrrhotite without anhydrite and hematite, dominant ilmenite over magnetite, low whole rock magnetic susceptibility (<1×10−4 emu g−1 oe−1 or <3×10−3 SI unit) and low whole rock Fe2O3/FeO ratios (<0.4), indicating that the granitoids are ilmenite- rather than magnetite-series I-type granitoids. Whole rock Sr–Nd–Pb isotopic compositions show limited variation but slightly enriched characteristics with (87Sr/86Sr)i values of 0.70357–0.70404, εNd (t) of+6.3 to+7.8, 206Pb/204Pb of 18.20–19.54 and 208Pb/204Pb of 37.97–39.55. Zircon Hf–O isotopic compositions show εHf (t) values of+10.7 to+15.8 and δ18O of 5.3–7.4‰. Zircon and apatite saturation thermometries yield temperatures of 720 to 920°C with relatively higher temperatures for the porphyries than for the diorite. Limited variations in Sr–Nd–Pb–Hf–O isotopic compositions and extremely young whole rock T2DM (Nd) (430 to 570Ma) and zircon TDMC (Hf) (310 to 640Ma) do not indicate significant crustal contamination during magma ascent or emplacement. Rather the Baogutu ilmenite-series I-type granitoids were probably formed by mixing between mantle-derived mafic magma and juvenile lower crust-derived felsic magma. Their relatively enriched characteristics and low oxidation state were probably inherited from magma sources (mantle wedge and juvenile lower crust) that had been previously contaminated with less than 8% of subducted oceanic sediments and reduced by CH4-bearing volatiles released from the asthenospheric mantle through a slab window.