Chromite deposits in the northern Oman ophiolite: Mineralogical constraints

Chromite deposits in the northern Oman ophiolitic complex occur in three structural contexts, i.e., (1) at the base of the cumulate series, (2) in the top kilometer of the mantle sequence, and (3) in the deeper parts of the mantle. Types 1 and 2 are characterized by the diversity of interstitial silicates where in decreasing order of abundance olivine, clinopyroxene, orthopyroxene, plagioclase, and amphibole occur, as opposed to type 3 which contains only olivine. They differ however in ore texture. Similar silicates also occur as euhedral inclusions in chromite crystals, but their proportions are reversed. The composition of the interstitial silicates is comparable to that found in early cumulates. Type-1 and type-2 chromite deposits crystallized from a magma similar to that from which the basal cumulates formed (Al₂O₃, 15.1–16.1 wt%; FeO/MgO, 0.55–0.60). The type-3 chromites were derived from a magma of much lower Al₂O₃ content (12.5 wt%). It is considered that they belong to an older episode in the magmatic evolution of the complex.