Iron-rich ultramafic pegmatite replacement bodies within the Upper Critical Zone, Rustenburg layered suite, Northam platinum mine, South Africa

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doi: 10.1180/0026461026660066
Authors:Reid, D. L.; Basson, I. J.
Author Affiliations:Primary:
University of Cape Town, Department of Geological Sciences, Rondebosch, South Africa
Volume Title:Mineralogical Magazine
Source:Mineralogical Magazine, 66(6(439)), p.895-914. Publisher: Mineralogical Society, London, United Kingdom. ISSN: 0026-461X
Publication Date:2002
Note:In English. 19 refs.; illus., incl. 2 tables, sects.
Summary:Discordant veins, pipes and occasionally subconcordant sheets of iron-rich ultramafic pegmatite disrupt the layered cumulate sequence of the Upper Critical Zone, Rustenburg Layered Suite, Bushveld Complex. These pegmatite bodies have been studied where they replace the Merensky Reef footwall at Northam Platinum Mine, situated in the Swartklip Facies of the western lobe of the Rustenburg Layered Suite. Composed chiefly of ferroaugite and fayalitic olivine, the pegmatites appear to be formed by the preferential replacement of plagioclase-rich cumulates within the layered sequence. Fe-Ti oxides, sulphide (pyrrhotite and chalcopyrite) and plagioclase also occur in variable quantities. Differentiation within the pegmatite is observed where it has spread laterally beneath the impervious Merensky chromitite layer, with the development of subparallel cm-scale layers of massive magnetitite, massive sulphide and sulphide pegmatite. While some Fe-rich mobile phase must have been responsible for the pegmatites, it is concluded that the pegmatite bulk composition does not represent the original liquid. Furthermore the mode of occurrence precludes the injection of a crystal mush. Rather it is argued, mainly on geochemical and isotopic grounds, that Fe-rich residual melts derived from the Upper Zone in the downward crosscutting gap areas migrated laterally and upwards into the adjacent Upper Critical Zone. Variable reaction with the layered cumulates produced the anastomosing pegmatite bodies.
Subsections:Igneous petrology
Subjects:Dunite; Gabbros; Geochemical methods; Geochemistry; Granites; Igneous rocks; Intrusions; Iron; Layered intrusions; Magmas; Metal ores; Metals; Mineral exploration; Norite; Pegmatite; Peridotites; Platinum ores; Plutonic rocks; Ultramafics; Africa; Bushveld Complex; Merensky Reef; South Africa; Southern Africa; Northam Mine; Rustenburg Suite
Abstract Numbers:03M/909
Record ID:2003024391
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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