Abstract

Contrasting with earlier usage, modern terminology considers the currently mined economic layer as constituting the Merensky Reef. On average it consists of a cumulate orhopyroxene-chromite pegmatoid containing postcumulus feldspar and clinopyroxene, although local olivine-rich or quartz-bearing facies occur. The orthopyroxene is bimodal; separate coarse and fine fractions are present. Enigmas concerning element sources for postcumulus growth and a variety of complex intergrowths pervade the occurrence and relationships of the oxide minerals, thereby highlighting the need for experimental data on these phases. The base metal sulfides occur as a temporal sequence typical of magmatic assemblages (pyrite, pyrrhotite, pentlandite, and chalcopyrite) with a host of rare minor minerals.The precious minerals are predominantly associated with the base metal sulfides. Idiomorphic braggite, cooperite, and laurite, with minor sperrylite, are the major discrete platinoid minerals. Another main platinoid constituent, Pt-Fe alloy, most commonly occurs as complex intergrowths with other minerals, principally base metal sulfides. The scarce, commonly mobile, late fraction consists of electrum, the platinoid bismuthotellurides, and other complex minerals. A significant "invisible" portion of the platinoids occurs within pyrrhotite, pentlandite, or pyrite, either as a true or as a "colloidal" solid solution.Studies of relationships of the precious minerals with the silicate-oxide gangue indicate a clear preference for their occurrence at the contact of the base metal sulfides with the gangue. Precious mineral grains are significantly smaller in chromite-rich ore compared with silicate-rich ore, while early minerals are demonstrably coarser than late-formed minerals.

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