The "Mg number" [Mg/(Mg + Fe) atomic ratio] of bronzite in each unit at the top of the critical zone of the Bushveld Complex decreases upward in a manner consistent with fractional crystallization. The base of each unit is marked by an increase in the Mg number, suggesting that crystallization of fresh magma is responsible for the initiation of the unit. Bulk trace and major element concentrations in pegmatoids of the Merensky reef, which itself forms the basal part of one of a series of these cyclic units, are similar to those in medium-grained feldspathic bronzitite.The base of each unit is also marked by pockets or a layer of sulfide with a higher Ni/Cu and much higher Pt/Cu ratio than the sulfides of the remainder of the unit. Thus the Pt tenor alternates from high to low three times upward through the three units. These data indicate strongly that the Pt tenor was established with the stratigraphy of the rocks and are inconsistent with models calling upon upward migration of hydrothermal or magmatic fluids to enrich the sulfides in platinum-group elements. There are no features in the rocks to indicate that fluids could have been channeled to flow laterally for the tens to hundreds of kilometers occupied by continuous Merensky reef. A model, based on that of Campbell et al. (1983), in which small amounts of sulfides form during injection of fresh magma as a turbulent plume and then settle after becoming thoroughly mixed with a large mass of magma, accounts well for all of our observations and many other features of the upper part of the critical zone.

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