Abstract

The Merensky cyclic unit of the Bushveld Complex represents the first products to crystallize after the influx and mixing of a large new batch of magma in the chamber.Excluding the Merensky pegmatoid, the Merensky cyclic unit grades upward from an orthopyroxenite at the base, through norite to anorthosite at the top of the sequence. It is followed by the very similar Bastard cyclic unit. The Mg/(Mg + Fe) 1 ratio of the pyroxenes changes relatively smoothly from about 0.80 at the base of the pyroxenite to 0.70 at the top of the anorthosite, with no abrupt changes between rock types. There is a suggestion of an upward increase in the "average" An content of the plagioclase from midpoint values of An 73 in the norite to An 76 at the top of the anorthosite. There also appears to be a decrease in the range of An compositions (10 mole % in the norite to 4.5 mole % in the anorthosite) upward in the sequence as determined by electron microprobe.The Merensky pegmatoid situated at the base of the cyclic unit commonly, but not everywhere, contains the nickel, copper, and platinum-group minerals. The pegmatoid shows evidence of high fluid activity, such as pegmatoidal texture, high biotite content, redistribution of sulfide phases, and the presence of unusual minor phases normally associated with hydrothermal activity.Variations in the Mg/Fe ratio of the orthopyroxenes from the Merensky cyclic unit indicate that this unit formed from a limited thickness of magma (between 10 and 30 m), whereas mass balance requires that the platinum-group element, Ni, and Cu mineralization be derived from at least 4 to 5 times this volume of magma.To explain these data several processes are inferred to have occurred. Initially a new influx of slightly more siliceous magma mixed with that already in the chamber by turbulent convection. Penecontemporaneous warping of the crystal pile and some erosion and solution of the footwall crystal mush occurred, resulting in a disconformity. The mixing process also led to the formation of dense immiscible sulfide droplets which scavenged platinum-group elements, Ni, and Cu from the liquid and were then deposited on the floor of the chamber.After the mixing event, a stable diffusive regime, i.e., a double-diffusive convection system, became operative as heat was lost from the top of the magma chamber. The basal double-diffusive convection layer (about 10-30 m thick) then started bottom crystallization of orthopyroxene resulting in the upward Mg/Fe fractionation observed. Plagioclase subsequently joined orthopyroxene on the liquidus, but this phase nucleated and grew within the convecting liquid which caused convection in the layer to slow as the viscosity sharply increased. Orthopyroxene continued to crystallize from the bottom up and enclosed small plagioclase crystals in large sievelike orthopyroxene oikocrysts or mottles. The plagioclase crystals not enclosed by pyroxene started to impinge mutually and sinter to form large polygonal grains. Penecontemporaneous infiltration of material from below the Merensky unit modified the isotopic and chemical character of the crystal pile by mixing with and displacing the preexisting interstitial liquid.The Merensky pegmatoid is inferred to postdate both the initial mineralization and formation of the other units in the Merensky cyclic unit. We conclude that the pegmatoid is the result of trapping of the final late fluids expelled from the footwall rocks. This fluid reacted with the basal part of the Merensky pyroxenite and also redistributed the platinum-group elements, Ni, and Cu to form the coarse pegmatoid and its assemblage of platinum-group minerals and other sulfide minerals.

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