The Haymiliyah Sulphide Ores (Haylayn Massif, Oman Ophiolite): In-situ Segregation of PGE-poor Magmatic Sulphides in a Fossil Oceanic Magma Chamber


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Abstract

The Semail ophiolite in Oman is one of the few ophiolitic complexes that may display locally magmatic sulphide ores in layer 3 of the crustal section. The ores found in the wadi Haymiliyah plutonic sequence, are composed of low Ni-pyrrhotite, chalcopyrite, pyrite and pentlandite. They are located at the bottom of a thick two-pyroxene and noritic gabbro unit, the Main Laminated Noritic Gabbro Unit (MLNGU) that crystallized from evolved tholeiites according to a calc-alkaline liquid line of descent. The MLNGU rests on coarse-grained layered gabbros of the Main Layered Gabbro Unit (MLGU) crystallized from more primitive MORB-like magmas. A detailed mineralogical study coupled with analyses of S, Se, chalcophile transition metals (Cu, Ni, Platinum-group elements, PGE) and Au allows two stages to be distinguished in the precipitation of the sulphides. Sulphide modal abundances start to increase at the top of the MLGU (up to 4%) where orthopyroxene becomes a major phase. Sulphides in the MLGU are only intercumulus and Cu-rich, indicating a S-undersaturated regime. The fine-grained two-pyroxene gabbros at the bottom of the MLNGU contain up to 16% sulphides (monoclinic pyrrhotite, pyrite, chalcopyrite). However, their precious metal contents are very low and far from economic grade (Σ PGE+ Au <15 ppb except one concentration at 230 ppb in the richest layer). Sulphide droplets in cumulus silicates indicate that sulphides precipitated in the fine-grained gabbros from a S-saturated magma. Sulphide liquid immiscibility was likely triggered by a set of unusual circumstances, such as a progressive S enrichment in high fO2 environments resulting from the closure of the Haymiliyah magma subchamber, the lack of a strong Fe-enrichment trend, and possibly a sudden temperature drop. The abundance of disseminated ores suggests that gravity segregation was inefficient; sulphide droplets probably nucleated at the crystallization front of the silicates. This hypothesis (of in-situ crystallization of sulphides) would also account for the very low precious metal contents. All the S present in the magma chamber was mobilized into the ores since the underlying MLNGU and isotropic gabbros are almost devoid of sulphides.

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