Abstract

The sheetlike Mt. Sholl intrusion is one of several gabbro-peridotite bodies intruding a northeast-trending Archean greenstone belt near Roebourne. This study is based on approximately 6,000 m of diamond drill cores, which show that the basal margins of the intrusion consist of wedge-shaped to sheetlike extensions (30 to 150 m thick) consisting of a central zone of peridotite within a gabbro envelope with a chilled margin. Disseminated and minor massive pyrrhotite, pentlanditc, and chalcopyrite occur mainly in gabbros above the basal contacts. Locally, chilled gabbros contain sulfide droplets. Igneous textures of silicates and sulfides have survived but are extensively modified by recrystallization. Average bulk sulfide compositions show a progressive increase in Cu from massive to gabbroic to peridotitic sulfides. This may be a magmatic fractionation trend, reflecting early separation of massive sulfides and later separation of peridotitic sulfides. Some sulfide textures fit the predicted crystallization behavior when the bulk compositions are related to experimental data for the Fe-Ni-Cu-S system. However, Cu/Cu + Ni ratios vary widely with height in drill cores, and interpretation of the magmatic controls of sulfide compositions is limited by the unknown extent of metamorphic modification. The chilled gabbro has the composition of an aluminum-depleted basaltic komatiite and cannot represent the magma from which the peridotitc formed. The parent magma is inferred to be komatiitic and was emplaced as a heterogeneous body of basaltic komatiite magma containing sulfide liquid suspended in a peridotitic crystal mush. Most of the sulfide liquid settled out of the peridotitic mush into the gabbro.

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