ISOTOPE AND TRACE-ELEMENT PATTERNS BELOW THE MERENSKY REEF, BUSHVELD COMPLEX, SOUTH-AFRICA - EVIDENCE FOR FLUIDS

Mineralisation in the Merensky Reef of the Bushveld Complex has recent ly been attributed to the ingress of fluids, possibly via channels ben eath depressions in the footwall of the Reef referred to as potholes. In this study samples from immediately below three potholes and from t he same horizon (a norite) under normal Reef have been analysed for ox ygen and strontium isotopes, chlorine and noble metals to investigate this possibility. Results from one pothole are interpreted as retainin g the primary magmatic oxygen isotope signature, with average deltaO-1 8 of plagioclase and orthopyroxene being + 7.1 +/- 0.1 parts per thous and and + 6.6 +/- 0.1 parts per thousand, respectively, and imply a de ltaO-18(melt) of + 6.8 parts per thousand. Interaction with an early m agmatic fluid cannot be detected from the oxygen isotope patterns and therefore the importance of this process cannot be assessed. Results f rom this oxygen isotope study of the western lobe of the Bushveld Comp lex are practically identical to those obtained for the laterally equi valent horizons in the eastern lobe. A second pothole shows more varia bility, with the norite under the pothole Reef showing secondary alter ation of plagioclase to paragonite and epidote, and orthopyroxene to s erpentine and chlorite. Fresh plagioclase from these altered samples i s only slightly depleted in O-18, compared with normal Reef. However, measured whole-rock deltaO-18-values are up to 1 parts per thousand li ghter than those predicted from mineral modes, and suggest that the se condary assemblage contains a component depleted in O-18, which could have been the result of exchange with a low-deltaO-18 fluid. This flui d interaction was probably a late-stage, relatively low-temperature ev ent. Secondary alteration of the footwall was accompanied by introduct ion of chlorine. Au contents also appear to be slightly enriched in th e footwall under pothole Reef.