Isotope and trace-element patterns below the Merensky Reef, Bushveld Complex, South Africa; evidence for fluids?

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doi: 10.1016/0009-2541(93)90171-E
Authors:Reid, D. L.; Cawthorn, R. G.; Kruger, F. J.; Tredoux, M.
Author Affiliations:Primary:
University of Cape Town, Department of Geochemistry, Rondebosch, South Africa
Other:
University of the Witwatersrand, South Africa
Volume Title:Chemical Geology
Source:Chemical Geology, 106(1-2), p.171-186. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:1993
Note:In English. 43 refs.; illus. incl. 6 tables, sect., geol. sketch map
Summary:Mineralisation in the Merensky Reef of the Bushveld Complex has recently been attributed to the ingress of fluids, possibly via channels beneath depressions in the footwall of the Reef referred to as potholes. In this study samples from immediately below three potholes and from the same horizon (a norite) under normal Reef have been analysed for oxygen and strontium isotopes, chlorine and noble metals to investigate this possibility. Results from one pothole are interpreted as retaining the primary magmatic oxygen isotope signature, with average δ18O of plagioclase and orthopyroxene being +7.1±0.1per mil and +6.6±0.1per mil, respectively, and imply a δ18Omelt of +6.8per mil. Interaction with an early magmatic fluid cannot be detected from the oxygen isotope patterns and therefore the importance of this process cannot be assessed. Results from this oxygen isotope study of the western lobe of the Bushveld Complex are practically identical to those obtained for the laterally equivalent horizons in the eastern lobe. A second pothole shows more variability, with the norite under the pothole Reef showing secondary alteration of plagioclase to paragonite and epidote, and orthopyroxene to serpentine and chlorite. Fresh plagioclase from these altered samples is only slightly depleted in 18O, compared with normal Reef. However, measured whole-rock δ18O-values are up to 1per mil lighter than those predicted from mineral modes, and suggest that the secondary assemblage contains a component depleted in 18O, which could have been the result of exchange with a low-δ18O fluid. This fluid interaction was probably a late-stage, relatively low-temperature event. Secondary alteration of the footwall was accompanied by introduction of chlorine. Au contents also appear to be slightly enriched in the footwall under pothole Reef.
Subjects:Alkali metals; Alkaline earth metals; Geochemistry; Isotope ratios; Isotopes; Lithogeochemistry; Metal ores; Metals; Mineral deposits, genesis; Mineralization; O-18/O-16; Ore-forming fluids; Oxygen; Radioactive isotopes; Rb-87/Sr-86; Rubidium; Sr-87/Sr-86; Stable isotopes; Strontium; Trace elements; Africa; Bushveld Complex; Merensky Reef; South Africa; Southern Africa; Transvaal region; Transvaal South Africa
Abstract Numbers:93M/4451
Record ID:1993010346
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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