Abstract

Modal analyses of the sulfide minerals show a gradual increase in the amount of pyrrhotite upward from the base of the Upper Zone from about 55 percent to 95 percent in the olivine diorite of Subzone D and a concomitant decrease of chalcopyrite from 40 percent to 3 percent. Pentlandite also decreases from 6 percent at the base to less than 1 percent in the olivine diorite. Other sulfide minerals are cubanite, pyrite, and mackinawite. The Pt/Pd ratio of 0.8 for mineralized anorthosite at the base of the Upper Zone is considerably lower than the Pt/Pd ratio for mineralized layers lower in the layered sequence of the Bushveld Complex.The sulfides have a twofold mode of occurrence: firstly, as interstitial to cumulus minerals in the mineralized anorthosites and in most of the magnetite gabbros of Subzones A, B, and C of the Upper Zone, and secondly, as minute droplike bodies evenly distributed throughout some of the magnetite gabbros and throughout the olivine diorite of Subzone D.The presence of sulfides at the top of some anorthosite layers immediately underlying magnetite layers remains difficult to explain because it implies separation of an immiscible sulfide liquid before crystallization of the magnetite cumulate. Crystallization of Fe-rich olivine and clinopyroxene together with magnetite would reduce the solubility of sulfur in the crystallizing portion of the magma and cause the formation of the tiny sulfide droplets in the olivine diorite of Subzone D.The abundance of sulfides in the Upper Zone as compared with the Main Zones is explained as being due to (a) effective adcumulus growth during crystallization of the rocks of the Main Zone, which prevented large amounts of intercumulus liquids from being trapped and resulted in an increase of the sulfur content in the remaining magma, (b) the gradual enrichment of FeO in the remaining magma, a process that increased the solubility of sulfur and prevented the separation of an immiscible sulfide liquid during crystallization of the Main Zone, and (c) the influx of a considerable amount of fresh, undifferentiated magma at the level of the Pyroxenite Marker of the Main Zone.Some sulfide-bearing layers in the Upper Zone are of potential economic interest for copper, nickel, and precious metals. Modal, chemical, petrologic, and stratigraphic data pertinent to the mineral evaluation of the Upper Zone are given.

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