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Platinum Group Elements (PGE) and Platinum Group MineraIs (PGM) Distribution and petrological evolution of southern Kohistan Arc, Northern Pakistan.

Abstract : The intra-oceanic arc crust in the southern Kohistan is represented by the JijalSpat-Complexes, Kamila Amphibolite and Chilas Complex. The Jijal Complex is a layered intrusion having dunite, olivine-clinopyroxenite, websterite, garnet websterite and garnet gabbro. Geochemistry indicates that this formed by fractional crystallization from a high-Mg tholeiitic melt in an arc environment with pressures between 10 and 12 kbar, equilibrated at temperatures of 800° to 1 100°C. The rocks of Thak Gah (Chilas complex) crystallized under lower pressure (5 to 6 kbar) and equilibrated at temperature between 800° and 950°C. They represent a shallow magma chamber in the same arc. Plutonic rocks from Kamila and Chilas are also geochemically similar to modern intra-oceanic arc. However, fine-grained Kamila metavolcanics, are more akin to the N-type MORB, suggesting fragments of pre-existing oceanic crust. In the Jijal Complex, BMS, PGM and PGE bearing phases are present in the form of inclusion in or on the edges of the silicate assemblages. The main PGM' s are temagamite, moncheite, merenskyite and sperrylite. In dunites, Ir, Ru, Ni and Cr enrichments indicate that both Ir and Ru are partitioned with chromites and can be clearly correlated to high f02. Ir and Ru enrichments in chromite-rich dunite of Thak Gah and Spat Complexes have the same origin. In Jijal Gamet-websterite, Ir, Ru, Pt, Pd and Au correspond to a prominent peak of Ni and Cu which is due to the precipitation of sulphide under lower f02. In upper garnet gabbro, Pt-Pd-Cu enrichments correspond to late removal of PPGE by magmatic fluids. Such mineralizations present important variations with Bushveld-type. They indicate that sulphide saturation due to simple crystallization can give rise to deposits such as can be seen in the Ga-websterite. Such mineralization characterizes arc related PGE's deposits.
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Allah Bakhsh Kausar. Platinum Group Elements (PGE) and Platinum Group MineraIs (PGM) Distribution and petrological evolution of southern Kohistan Arc, Northern Pakistan.. Mineralogy. Université Joseph-Fourier - Grenoble I, 1998. English. ⟨tel-00717476⟩

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