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Nickel contents of olivine have been widely used as petrogenetic indicators and as fertility indicators for magmatic sulfide potential of mafic-ultramafic intrusions, on the assumption that olivines crystallized from magmas that had equilibrated with sulfide liquid should be relatively depleted in Ni compared with a sulfide-free baseline. This has given rise to a large accumulation of data that is brought together here, along with data on volcanic olivines, to critically evaluate the effectiveness of the approach. We identify multiple sources of variance in Ni content of olivine at a given Fo content, including variability in mantle melt composition due to depth, water content (and possibly source), subsequent fractional crystallization with and without sulfide, recharge and magma mixing, batch equilibration between olivine and sulfide at variable silicate-sulfide ratio (R), and olivine/liquid ratio; and subsequent equilibration during trapped liquid crystallization in orthocumulates. Baselines for Ni in olivine in relation to Fo content are somewhat lower in orogenic belt settings relative to intrusions in continental large igneous provinces (LIPs). This is probably related to differences in initial parent magma compositions, with plume magmas generally forming deeper and at higher temperatures. No clear, universal discrimination is evident in Ni in olivine between ore-bearing, weakly mineralized, and barren intrusions, even when tectonic setting is taken into account. However, sulfide-related signals can be identified at the intrusion scale in many cases. Low-R factor and low-tenor sulfides are associated with low-Ni olivines in several examples, and these cases stand out clearly. Anomalously high-Ni olivines are a feature of some mineralized intrusions, in part due to trapped liquid reaction effects. However, in some cases, this mechanism cannot account for the magnitude of enrichment. In these cases, enrichment may be due to re-entrainment of "primitive" Ni-rich sulfide by a more evolved Fe-rich magma, driving the olivine to become Ni-enriched due to Fe-Ni exchange reaction between sulfide and olivine during transport. An extreme case of this process may account for ultra-Ni enriched olivine at Kevitsa (Finland), but more subtle signals elsewhere could be positive indicators. A lack of clear mineralized/barren distinction in specific groups of related intrusions, e.g., the deposits of NW China or the Kotalahti Belt in Finland, may well be due to "false negatives" where undiscovered mineralization exists in specific intrusions or in their feeder systems, or may also be due to a multiplicity of confounding factors. Wide variability of both Fo and Ni between related intrusions at regional scale may be a useful regional prospectivity indicator, more than an intrusion-scale discriminant, and is certainly informative as a petrogenetic indicator. In general, the use of Ni-olivine as a fertility tool is more likely to generate false negatives than false positives, but both are possible, and the technique should be used as part of a broader weight-of-evidence approach. [ABSTRACT FROM AUTHOR]

High-altitude Tajiks (HA-Tajiks), Tibetans and Sherpas are three groups of high-altitude native people in China. The differences in the mtDNA genome between the three populations and the role of the mtDNA genome in the high-altitude adaptation of HA-Tajiks were seldom investigated. In this study, 80 HA-Tajiks were enrolled, and their whole mtDNA genomes were sequenced. The haplogroup of each subject was determined by comparison to the revised Cambridge Reference Sequence (rCRS). Ten additional populations from East Asia and Central Asia, including Tibetans and Sherpas, were selected as references. The top haplogroup was U, followed by H, T and J. Principle component analysis and genetic distance analysis indicated that HA-Tajiks showed a close relationship with Wakhi Tajiks, Pamiri Tajiks and Sarikoli Tajiks, indicating that they should be considered one nation scattered around the Pamirs. The difference in the mtDNA genome between HA-Tajiks and Sherpas was significantly greater than that between HA-Tajiks and Tibetans. Among the 13 genes related to the OXPHOS pathway encoded by the mtDNA genome, HA-Tajiks showed more significant differences in ND3 and CYTB compared to Tibetans. Compared to Sherpas, HA-Tajiks showed more significant differences in ND1, ND2, COX1, ATP8, ATP6, ND3, ND4L, ND4, ND5 and CYTB. The associated functional changes and underlying molecular mechanisms should be explored by molecular and biochemical investigations in further studies. [ABSTRACT FROM AUTHOR]

Platinum group element (PGE) mineralization occurs associated with mafic-ultramafic rocks in different environments. Although the PGE enrichment is primarily caused by magmatic processes, remobilization of Pd and Pt by hydrothermal fluids has likely been an important mechanism in increasing the precious metal grade in many cases. However, the timing of PGE enrichment by hydrothermal fluid processes is commonly difficult to constrain. The Jinchuan ultramafic intrusion in Northwest China is ranked the world’s third largest magmatic Ni-Cu sulfide deposit. Besides the main ore body consisting of net-textured and disseminated sulfides, there is hydrothermal mineralization associated with sheared contact zones of the intrusion, which shows elevated Cu and Pt concentrations. The unusually high Pt is hosted mainly in sperrylite within altered silicates. In this study, we applied the Pt-Os geochronometer to a Cu-Pt-rich ore body, yielding an isochron age of 436 ± 23 Ma. This age is significantly younger than the main ore formation age of ca. 825 Ma, but similar to that of the continental collision event between the Qaidam-Qilian Block and Alax Block of North China. This indicates that the intrusion may have been uplifted during the Paleozoic orogenic processes from deeper crust, resulting in the generation of the Cu-Pt-rich hydrothermal ore body. Our new data provide the first strong age constraints on the hydrothermal PGE enrichment, showing that the Pt-Os isotope system is potentially a powerful tool for dating hydrothermal overprinting on Ni-Cu-PGE sulfide deposits. [ABSTRACT FROM AUTHOR]

Abstract: The ~260Ma Abulangdang ultramafic intrusion in the Panxi region, SW China, is part of the Emeishan large igneous province (ELIP). The intrusion is composed of dunite and lherzolite with minor olivine gabbro. Olivine crystals in the dunite have Fo values from 89.0 to 86.5mol%. Chromite grains in both the dunite and lherzolite contain more than 0.5wt.% TiO2 and show a trend of increasing Fe3+ with increasing Fe2+/(Mg+Fe2+). Rocks of the intrusion have γOs(t) values from +0.1 to +1.2 and εNd(t) values from −1.9 to +2.9, similar to the high-Ti picrites, high-Ti flood basalts and Fe–Ti oxide-bearing, gabbroic intrusions of the ELIP. Modeling indicates that the Abulangdang intrusion formed by accumulation of olivine and chromite from a high-Ti picritic magma, whereas the high-Ti flood basalts and Fe–Ti oxide-bearing, gabbroic intrusions formed from derivative, evolved magmas after the early fractionation of minerals from the high-Ti picritic magma. Both the dunite and lherzolite have relatively high PGE concentrations and primitive mantle-normalized chalcophile element patterns with positive Os and Ru anomalies, in contrast to PGE-poor high-Ti flood basalts with negative Os and Ru anomalies. This is consistent with the retention of Ru–Os–Ir trace phases with olivine and chromite in the formation of the Abulangdang intrusion. Using a clinopyroxene geobarometer, the crystallization pressure of the Abulangdang intrusion is calculated to be 7.1 to 8.3kbar, equivalent to a depth of ~21 to 24km, which is deeper than that estimated for the Fe–Ti oxide-bearing, gabbroic layered intrusions of the ELIP (~5kbar). We propose that large volumes of mantle plume-derived high-Ti picritic magma underplating along the Moho boundary underwent early fractionation before eruption, and some high-Ti picritic magma ascended through a fossil conduit where the Abulangdang intrusion formed. Residual, evolved magmas either from the conduits or underplating magma chamber formed Fe–Ti oxide-bearing, gabbroic layered intrusions or high-Ti flood basalts. In this fashion, the Abulangdang intrusion resembles ultramafic portions that are missing in the Fe–Ti oxide-bearing, gabbroic intrusions of the ELIP. [Copyright &y& Elsevier]