Fractionation of chromium, nickel, cobalt and copper in a differentiated dolerite?granophyre sequence at Red Hill, Tasmania

Abstract

Fractionation of tholeiitic magma in the Red Hill intrusion produced a gradational series of rocks ranging from dolerite to granophyre (McDougall, 1962). Granophyres are enriched in Fe, Si and alkalies, and impoverished in Mg, Ca and Al. With fractionation the magma was depleted rapidly in Cr and Ni owing to their removal in early crystallizing pyroxene and iron oxides. Cobalt decreases gradually from chilled dolerite to silicic dolerite, followed by a significant maximum in the most Fe‐enriched rocks, and finally decreases markedly in the granophyres. Cobalt follows Fe²+ closely and shows no obvious relationship with Mg. Copper was progressively enriched in the magma during the main stages of fractionation until precipitation of sulphide occurred, which caused impoverishment of Cu in the final liquid. Copper also is present in the silicates, it substitutes for Na in the feldspars and Fe²+ in pyroxenes and iron oxides.