Ya-Jing Mao

Geochemistry of the ~326 Ma Xinyuan mafic intrusion in the Eastern Junggar Terrane, Northwest China: implications for tectonic setting and magmatic Ni–Cu mineralization potential

ABSTRACT The Xinyuan intrusion of the Eastern Junggar terrane is one of the mafic–ultramafic intrusions located in the Northern Xinjiang region and is associated with the southern part of the Central Asian Orogenic Belt (CAOB). Based on the secondary ion mass spectrometry (SIMS) zircon U–Pb dating, the intrusion is 326.2 ± 1.1 Ma old. Positive zircon ɛHf(t) values (+12.4 to +15.7) and mantle-like δ18O values (5.0–5.5‰) suggest that the parental magma was derived from a depleted mantle source with a low degree of crustal contamination. Significant negative Nb–Ta anomaly and large iron lithophile element (LILE) enrichment of the Xinyuan intrusion are identical to those of the coeval basalt–andesite rocks in the study area. Temporal, spatial, and geochemical evidence suggest that the Xinyuan intrusion and coeval basalt–andesite rocks are associated with the northward subduction of the Kelameili Ocean. This, together with the ages of alkaline granites and sedimentary record in the Eastern Junggar, suggests that closure of the Kelameili Ocean was probably between 326 Ma and 310 Ma. Fractional crystallization modelling indicates that the parental magma of the Xinyuan intrusion has experienced significant fractional crystallization at depth. The Xinyuan parental magma was estimated to contain ~20 ppm Ni, which is significantly lower than the coeval basaltic magma (~200 ppm). Magma that is depleted in Ni has potentially undergone extensive fractional crystallization, during which most of the Ni has been sequestered into olivine in the early stages. The Xinyuan intrusion has Hf isotope and trace element characteristics similar to the Permian sulphide-bearing mafic–ultramafic intrusions. However, the low degree of crustal contamination apparent in δ18O data is more characteristic for the pre-Permian sulphide-poor intrusions in the area. Extensive fractional crystallization and the low degree of crustal contamination in the parental magma of Xinyuan intrusion experienced are suggestive of a low-magmatic Ni–Cu mineralization potential.

Petrogenesis and mineralization of the Hulu Ni-Cu sulphide deposit in Xinjiang, NW China: constraints from Sr-Nd isotopic and PGE compositions

The Permian Hulu intrusion is one of several sulphide-bearing Permian mafic–ultramafic intrusions in the eastern part of the eastern Tianshan located at the southern margin of the Central Asian Orogenic Belt (CAOB) in Xinjiang, NW China. The intrusion is composed of lherzolite, olivine websterite, gabbro, and gabbro-diorite. Disseminated and net-textured Ni-Cu sulphide ores are located at the bottom of the lopolith complex. Negative Zr, Hf, Nb, and Ta anomalies, whole-rock εNd(t) values of +5.7 to +8.8, and variable (Th/Nb)PM values (from 1.06 to 8.13) suggest that the source of the Hulu complexes is depleted mantle metasomatized by subducted slab-derived fluid and/or melt (~5% global subducted sediment and 15% slab fluid) that has experienced approximately 3% lower crustal and 10% upper crustal contamination. The Hulu intrusion is characterized by low PGE abundances i.e. 0.03–1.08 ppb Ir, 0.04–0.69 ppb Ru, 0.02–2.15 ppb Rh, 0.30–48.71 ppb Pt, and 0.21–344 ppb Pd. Our calculations indicate that if the Pd, Os, Ir, and Cu contents of the primary magma were 2.1 ppb, 0.03 ppb, 0.05 ppb, and 200 ppm, respectively, a variable R-factor between 200 and 1600 with residual magma that had experienced 0.01% early-sulphide segregation can explain the variation in Pd, Os, and Ir contents of sulphide-poor and disseminated sulphide samples of the Hulu deposit. Basaltic magma fractionation and assimilation and/or contamination of sulphur-bearing crustal materials might have triggered sulphur saturation to form Cu-Ni sulphide ores. Tarim basaltic PGE contents cannot be used as the mineralized parent magma for the Hulu intrusion because of the differing evolutionary trends of the Ni/Pd and Cu/Ir values. However, similar Cu/Ni and Pd/Ir values in Tarim basalts and Hulu Cu-Ni sulphide ores, as well as the same early sulphide segregation process, show that certain genetic relationships between them and magma sources are probably similar to each other.

Morphology and Particle Size Distribution of Olivines and Sulphides in the Jinchuan Ni–Cu Sulphide Deposit: Evidence for Sulphide Percolation in a Crystal Mush

The Jinchuan intrusion is the largest single magmatic nickel sulphide deposit in the world. Sulphide mineralization mainly occurs in two segments (I and II). The crystal size distributions (CSDs) of olivines in the two segments are distinctly different. For all olivines in the two segments, their grain sizes and values of the clustering index R increase with increasing olivine abundance in the cumulates, illustrating overgrowth trends for Segment II olivines and a combination of overgrowth and compaction trends for Segment I olivines. Kinked olivine CSDs and compaction trends in the olivine cumulates of Segment I suggest the presence of two olivine populations with different nucleation rates, one of which most probably grew in a deeper chamber and then became entrained in the ascending magma. In contrast, the simple log–linear CSDs of the olivine cumulates of Segment II could be the result of a single nucleation event in the current magma chamber. In both segments, olivine chadacrysts in orthopyroxene and in clinopyroxene oikocrysts show a progression in grain size from small grains in Ti-poor pyroxene cores to larger grains in Ti-rich rims. This strongly suggests simultaneous in situ nucleation and growth of olivine and both pyroxenes at the top of the crystal mush, with the poikilitic textures developing as the result of faster growth of the pyroxenes relative to olivines. Sulphides in both segments, distributed in the pore spaces between cumulus olivines, form interconnected networks on a scale of centimetres, separated by domains poor in sulphide and occupied by oikocrysts and late-crystallizing interstitial silicates. The small sulphide droplets, having equivalent sphere diameters (ESD) between 80 and 400 lm, are relatively round and show log–linear PSDs, suggesting that they may have nucleated and grown in the mush zones. The irregular morphology and the concave PSDs of the larger sulphide droplets (ESD> 400 lm) suggest that these sulphides are the result of coalescence. CSDs of olivines in sulphide-poor ( 4 vol. % disseminated sulphide) cumulates are indistinguishable from those in sulphide-rich samples ( 20 vol. % sulphide, net-textured) for both segments. This, along with other textural observations, suggests that the sulphides in sulphide-rich samples percolated through the pore space of the olivine framework and displaced the interstitial silicate liquid, after significant (40–70%) olivine crystallization. Both wetting and non-wetting textures are observed between the sulphides and surrounding olivine, which suggests the presence of two liquids in the pore space VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 1701 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2018, Vol. 59, No. 9, 1701–1730 doi: 10.1093/petrology/egy077 Advance Access Publication Date: 14 August 2018