Alaskan-type complexes commonly contain primary platinum-group element (PGE) alloys and lack base-metal sulfides in their dunite and chromite-bearing rocks. They could therefore host PGE deposits with rare sulfide mineralization. A detailed scanning electron microscope investigation on dunites from the Xiadong Alaskan-type complex in the southern Central Asian Orogenic Belt revealed: various occurrences of platinum-group minerals (PGMs) that are dominated by inclusions in chromite grains containing abundant Ru, Os, S and a small amount of Pd and Te, indicating that they mainly formed prior to or simultaneously with the crystallization of the host minerals; A few Os–Ir–Rurich phases with iridium/platinum-group element (IPGE) alloy, anduoite (Ru,Ir,Ni)(As,S)2−x and irarsite (IrAsS) were observed in chromite fractures, and as laurite (RuS2) in clinopyroxene, which was likely related to late-stage hydrothermal alteration. The rocks in the Xiadong complex display large PGE variations with ∑PGE of 0.38–112 ppb. The dunite has the highest PGE concentrations (8.69–112 ppb), which is consistent with the presence of PGMs. Hornblende clinopyroxenite, hornblendite and hornblende gabbro were all depleted in PGEs, indicating that PGMs were likely already present at an early phase of magma and were mostly collected afterward in dunites during magma differentiation. Compared with the regional mafic–ultramafic intrusions in Eastern Tianshan, the Xiadong complex show overall higher average PGE concentration. This is consistent with the positive PGE anomalies revealed by regional geochemical surveys. The Xiadong complex, therefore, has potential for PGE exploration.
When and how sulfide saturation occurred in the Jinchuan magmatic Ni-Cu sulfide deposit are poorly constrained. To address this issue, we investigated compositional variations and crystallization temperatures of olivine in various ores and rocks in orebody 1 of the Jinchuan deposit. The forsterite (Fo) values and Mn contents in olivine vary respectively from 79.6 to 85.2 and 1270 to 2463 ppm in net-textured ores, 76.7 to 84.2 and 1425 to 2865 ppm in patchy net textured ores, 78.7 to 84.1 and 1440 to 2649 ppm in disseminated ores, and 69.7 to 81.8 and 1495 to 3191 ppm in sulfide-barren lherzolites. The estimated crystallization temperatures of olivine in these ores/rocks range from 803 to 1163 ℃, 871 to 1314 ℃, 821 to 1507 ℃, and 929 to 1652 ℃, respectively. These data revealed two distinct groupings in both olivine Fo-Mn correlations and crystallization temperatures cluster splitting at a Fo of 79.5 and a temperature of 1100 ℃. The wider range of Mn content with a given Fo value higher than 79.5 compared to those with Fo values lower than 79.5 indicates the onset of in situ sulfide saturation at a Fo of 79.5. The termination of sulfide saturation is epitomized by olivine in disseminated ores with a peak Fo content of 84. The abrupt temperature drops from ca. 1400–1500 °C to ca. 800–1100 °C in the parental magma likely occurred when the magma flowed through the westernmost portion of this segment, which resembles a de Laval nozzle. This geometry could have induced rapid cooling and decompression of the magma, consequently promoting degassing through decompression. These reductions in temperature and H2O content of the magma would collectively result in a decrease in the sulfur concentration at sulfide saturation (SCSS), which would exceed any SCSS increase due to decompression. Our findings emphasize the critical role of magma conduit geometry in physicochemical changes of the parental magma for sulfide saturation and subsequent mineralization in the Jinchuan deposit, which may also be applicable to other magmatic deposit worldwide.
Although the involvement of hydrous fluids has been frequently invoked in the formation of stratiform chromitites in layered intrusions, there is a lack of natural evidence to signify their presence and mechanism. Here, Fourier-transform infrared spectroscopy (FTIR) of H2O in silicate minerals in the lowermost layer and G chromitite layer of the Stillwater complex, Montana, USA, shows that olivine grains have 20−55 ppm H2O, orthopyroxene has 30−45 ppm H2O, and clinopyroxene has 144−489 ppm H2O. The jointly increasing H2O contents of olivine and orthopyroxene in silicate cumulates along with magma differentiation record a negative correlation in chromitites. On the basis of poikilitic clinopyroxene, we calculated that the interstitial melts had averages of 1.3 wt% and 2.3 wt% H2O in dunite and chromitite, respectively, showing significant differences between chromitites and silicate cumulates. More than 10% of the chromite grains contained polymineralic inclusions up to 100 μm in size that were composed mainly of orthopyroxene, hornblende, plagioclase, and phlogopite. Most of these minerals were characterized by higher MgO and fluid-mobile element contents, such as Na and K, than minerals in associated silicates. Based on the mineral modes of the hydrous phases and their compositions, the trapped fluids contained ∼2.6 wt% H2O, consistent with the FTIR estimates, indicating the inclusion compositions represent interstitial melts instead of parental magmas. These observations indicate that the chromite microlites collected fluids during early crystallization, leading to a heterogeneous fluid redistribution in the melt. The fluids were collected on the surface of chromite grains during crystallization and then dissolved into poikilitic pyroxene. Chromite grains could also be efficiently floated by these fluids, causing them to migrate away from the silicate minerals in the magma channel and leading to the formation of nearly monomineralic chromitite seams. This process serves as a kinetic model indicating that chromite could be completely separated from silicates during mechanical sorting in layered intrusions.
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