Abstract 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.

Abstract Appreciable concentrations of titaniferous magnetite occur in several of the ultramafic complexes that are exposed along a linear belt 350 miles in length in southeastern Alaska. Four of the larger bodies, at Klukwan, Snettisham, Union Bay, and Duke Island, are of economic interest. Large magnetite concentrations (15–20 percent of the rock by volume) of very uniform grade occur in the hornblende pyroxenite units of these complexes. In several of the complexes (including Union Bay and Duke Island) a consistent zoning of ultramafic rock types occurs, with dunite, peridotite, olivine pyroxenite, and hornblende pyroxenite occurring in sequence from the center outward to the margin. Only the hornblende pyroxenite zone is exposed at Klukwan and Snettisham, but the mineralogy of this zone is identical in all the complexes. The minerals are diopsidic augite (Di 85 Hed 15 to Di 70 Hed 30 ), hornblende (abnormally high in A1 2 O 3 and low in SiO 2 relative to most igneous hornblendes), magnetite, and accessory ilmenite and hercynitic spinel. Accessory amounts of olivine may occur in the interior parts of the magnetite pyroxenite zones of the complexes that contain olivine pyroxenite and/or dunite, but the magnetite concentrations drop abruptly as the amount of olivine increases; magnetite is largely absent from the olivine pyroxenite zones. The many common features of these ultramafic bodies indicate that they formed by a single mechanism, namely by crystallization of ultramafic magmas. Evidence for this includes: (1) graded layering in rhythmically-bedded olivine pyroxenite and magnetite pyroxenite; (2) significant contact metasomatism and metamorphism at the margins of the bodies; and (3) good correspondence between observed rock types and experimentally studied liquids in the system diopside-olivine-iron oxide. It is suggested that the complexes formed by a combination of fractional crystallization and multiple intrusion of ultramafic magmas, and that the magnetite-hornblende pyroxenite zones in particular crystallized from a liquid roughly corresponding to their present chemical composition. This liquid must have had a total Fe content of about 15–18 percent by weight.