A PETROGENETIC MODEL FOR THE KOPERBERG SUITE - EVIDENCE FROM THE JUBILEE MINE, NAMAQUALAND, SOUTH-AFRICA

Detailed mapping and petrological studies of the anorthosite, mica dio rite, pyroxene leucodiorite, and pyroxene diorite intrusions in the Ju bilee ore body indicate that they were sequentially emplaced and that they are discrete magmatic events, not merely arbitrary subdivisions w ithin a continuous magmatic lineage. Despite the small size of each in trusion, the rocks represent cumulates involving variable proportions of plagioclase, orthopyroxene, mica, quartz, and immiscible sulphide. The steep structures into which they were emplaced during granulite fa cies metamorphism contained anatectic melts of the country rock gneiss es which led to extensive hybridization of basic and acid magma. The m afic minerals have a Mg/(Mg+Fe) ratio far higher than the An content o f plagioclase, which is atypical of basic intrusions and is a conseque nce of this mixing process. The initial Sr-87/Sr-86 ratios of the diff erent intrusions increase with decreasing An content of the plagioclas e, substantiating this interpretation. The sulphide mineralization was triggered by this assimilation process that caused the separation of an immiscible sulphide liquid. However, subsequent oxidation has destr oyed the original magmatic sulphide assemblage at Jubilee to produce b ornite and chalcopyrite.