Petrogenesis of contact-style PGE mineralization in the northern lobe of the Bushveld Complex: comparison of data from the farms Rooipoort,Townlands, Drenthe and Nonnenwerth |
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Authors: | W D Maier L de Klerk J Blaine T Manyeruke S-J Barnes M V A Stevens J A Mavrogenes |
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Institution: | (1) Centre for Exploration Targeting, University of Western Australia, Crawley, 6009, Australia;(2) Caledonia Mining Corporation, Mokopane, South Africa;(3) Department of Geology, University of Pretoria, Pretoria, 0001, South Africa;(4) Sciences de la Terre, Université du Quebec à Chicoutimi, Chicoutimi, G7H 2B1, Canada;(5) Research School of Earth Sciences, Australian National University, Canberra, 0200, Australia |
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Abstract: | In the present study, we document the nature of contact-style platinum-group element (PGE) mineralization along >100 km of
strike in the northern lobe of the Bushveld Complex. New data from the farm Rooipoort are compared to existing data from the
farms Townlands, Drenthe, and Nonnenwerth. The data indicate that the nature of the contact-style mineralization shows considerable
variation along strike. In the southernmost portion of the northern Bushveld, on Rooipoort and adjoining farms, the mineralized
sequence reaches a thickness of 700 m. Varied-textured gabbronorites are the most common rock type. Anorthosites and pyroxenites
are less common. Chromitite stringers and xenoliths of calcsilicate and shale are largely confined to the lower part of the
sequence. Layering is locally prominent and shows considerable lateral continuity. Disseminated sulfides may reach ca. 3 modal
% and tend to be concentrated in chromitites and melanorites. Geochemistry indicates that the rocks can be correlated with
the Upper Critical Zone. This model is supported by the fact that, in a down-dip direction, the mineralized rocks transform
into the UG2-Merensky Reef interval. Between Townlands and Drenthe, the contact-mineralized sequence is thinner (up to ca.
400 m) than in the South. Chromitite stringers occur only sporadically, but ultramafic rocks (pyroxenites, serpentinites,
and peridotites) are common. Xenoliths of calcsilicate, shale, and iron formation are abundant indicating significant assimilation
of the floor rocks. Sulfides may locally form decimeter- to meter-sized massive lenses. PGE grades tend to be higher than
elsewhere in the northern Bushveld. The compositions of the rocks show both Upper Critical Zone and Main Zone characteristics.
At Nonnenwerth, the mineralized interval is up to ca. 400 m thick. It consists largely of varied-textured gabbronorites, with
minor amounts of igneous ultramafic rocks and locally abundant and large xenoliths of calcsilicate. Layering is mostly weakly
defined and discontinuous. Disseminated sulfides (<ca. 3 modal %) occur throughout much of the sequence. Geochemistry indicates
that the rocks crystallized mainly from tholeiitic magma and thus have a Main Zone signature. The implication of our findings
is that contact-style PGE mineralization in the northern lobe of the Bushveld Complex cannot be correlated with specific stratigraphic
units or magma types, but that it formed in response to several different processes. At all localities, the magmas were contaminated
with the floor rocks. Contamination with shale led to the addition of external sulfur to the magma, whereas contamination
with dolomite may have oxidized the magma and lowered its sulfur solubility. In addition to contamination, some of the magmas,
notably those of Upper Critical Zone lineage present at the south-central localities, contained entrained sulfides, which
precipitated during cooling and crystallization. |
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Keywords: | Platinum-group elements Bushveld Complex Contact-style mineralization Platreef South Africa |
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