Geochronological, Petrological, and Geochemical Constraints on Ni-Cu Sulfide Mineralization in the Poyi Ultramafic-Troctolitic Intrusion in the Northeast Rim of the Tarim Craton, Western China

Abstract

The Poyi magmatic Ni-Cu sulfide deposit is situated in the Beishan fold belt in the northeastern rim of the Tarim craton. Many Permian magmatic Ni-Cu sulfide deposits, such as those in East Tianshan, are present in the adjacent Central Asian orogenic belt to the north. The Poyi deposit is hosted in a small dike-like ultramafic-troctolitic body that was emplaced into a much larger gabbroic intrusion. Our new zircon U-Pb isotope data reveal that these two intrusions formed ~6 Ma apart. The ultramafic-troctolitic intrusion was emplaced at 269.9 ± 1.7 Ma, whereas the gabbroic intrusion was emplaced at 276.1 ± 1.9 Ma. The results show that the Poyi deposit is the youngest among the major magmatic Ni-Cu sulfide deposits (≥0.2 Mt Ni) of Permian ages in the Beishan-Tianshan region. Sulfide mineralization in the Poyi deposit occurs as steeply dipping disseminated sulfide lenses mostly associated with wehrlites in the center of the dike. Olivine from the Poyi ultramafic rocks has Fo content up to 91 mol %, which is similar to the lower limit of mantle olivine and the most primitive within the Permian Beishan-Tianshan nickel belt. Like other magmatic sulfide deposits in this belt, olivine from the Poyi deposit is depleted in Ca (<1,000 ppm). The estimated parental magma for the Poyi most primitive ultramafic rocks contains 15 ± 2 wt % MgO. Cotectic olivine-sulfide segregation from the Poyi magma is inferred from systematic variation of Fo-Ni contents in olivine from some sulfide-barren ultramafic rock samples, and supported by the occurrence of sulfide droplets as small inclusions in olivine (Fo90 mol %) in these rocks. The involvement of multiple pulses of sulfide-charged magma with different compositions is indicated by the abrupt change of olivine Fo content with depth and the presence of olivine with similar Fo contents but dramatically different Ni abundances in the different parts of the deposit. The (87Sr/86Sr)i ratios and ɛ Nd(total) of the Poyi ultramafic rocks and troctolites range from 0.7042 to 0.7052 and from 4.9 to 6.0, respectively, which are close to the isotope compositions of depleted mantle and within the ranges of major magmatic Ni-Cu sulfide deposits of Permian ages in East Tianshan. No more than 5 wt % of bulk crustal contamination is required to explain the variations of Sr-Nd isotopes in the Poyi ultramafic-troctolitic intrusion. The abundances of incompatible trace elements in whole rocks and clinopyroxene crystals indicate very weak light REE enrichments coupled by significant negative Nb-Ta anomalies in the parental melts. Bulk sulfides in the Poyi deposit are characterized by positive correlations between any pair of platinum-group elements (PGE), indicating that PGE tenor variations in the deposit are mainly controlled by variable R-factors (magma/sulfide mass ratios). The estimated initial concentrations of PGE in the parental magma for the Poyi deposit are almost two orders of magnitude lower than the abundances of PGE in some continental picrites. Given that the parental magma for the Poyi deposit is as primitive as a primary mantle-derived magma, the depletion of PGE in the Poyi deposit is most likely due to previous sulfide segregation at depth. Based on these observations, we conclude that sulfide saturation in the Poyi PGE-depleted, Mg-rich magma was triggered by addition of crustal sulfur during magma ascent and that the Poyi deposit was a dynamic conduit used by multiple pulses of olivine- and sulfide-charged magma.