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The extraordinarily well-preserved and well-exposed Semail ophiolite of northern Oman hosts several large plagiogranite intrusions in proximity to economic copper sulphide deposits of the Lasail mining district. A progression of isotopic, chemical and mineralogical transformations observed within the plagiogranites and high-level gabbros (HLG), and a comparison of these effects with those in the lowermost dykes of the immediately overlying sheeted dyke complex (SDC) tracks the evolution of hydrothermal fluids and the alteration of overlying dykes and pillow lavas during discharge of these fluids on the sea floor. The largest hydrothermal alteration aureoles, and the greatest extent of metamorphic veins and metasomatic replacement features, are found adjacent to the largest high-level plagiogranite bodies, beneath and adjacent to the major ore bodies in northern Oman. The ubiquitous presence of metamorphic actinolitic hornblende, sodic plagioclase, epidote and titanite in metabasalts within the high-temperature alteration zones points to the most likely mineralogical and structural controls on the development and evolution of the hydrothermal fluids. Depleted Cu contents of the adjacent crustal rocks and Cu enrichments above the plagiogranite intrusions demonstrate the redistribution of heavy metals adjacent to the complexes. Field relationships implicate the formation of both the epidosites and plagiogranites in the genesis of the ore deposits. An important process inferred from the field and geochemical data is the assimilation of previously hydrothermally altered basaltic and gabbroic country rocks by stoping into the magma chambers developed near the SDC-gabbro horizon in the ophiolite. We suggest that this process of combined assimilation-fractional crystallization, together with replenishment and recharge by injection and quenching of basaltic magma ‘pillows’ into these plagiogranite magma chambers (i.e. RAFC), plays a major role in the development of these composite intrusions.

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