The Merensky Cyclic Unit and its impact on footwall cumulates below Normal and Regional Pothole reef types in the Western Bushveld Complex |
| |
Authors: | M D Roberts D L Reid J A Miller I J Basson M Roberts D Smith |
| |
Institution: | (1) Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa;(2) Department of Geology, University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa;(3) 140 North Wilson Avenue, no. 7, Pasadena, CA, USA;(4) Northam Platinum Mine, P.O. Box 441, Thabazimbi, 1380, South Africa;(5) Present address: Tect Geological Consulting, 15 Rue le Havre, Monte Sereno, Somerset West, 7130, South Africa |
| |
Abstract: | The Merensky Reef of the Bushveld Complex occurs in its highest stratigraphic position as a heterogeneous, pegmatitic, feldspathic
melanorite bounded by two narrow chromitite stringers at the base of the Merensky Cyclic Unit (MCU). In the Swartklip Facies
of the Rustenburg Layered Suite, the occurrence of widespread thermal and mechanical erosion termed “potholing” has led to
the subdivision of the Merensky Reef into Normal Reef and Regional Pothole Reef sub-facies. The transition between the two
sub-facies occurs where the MCU transgresses the lower chromitite stringer of the Normal Merensky Reef and cuts down into
the underlying cumulate lithologies. In the Regional Pothole Reef at the Northam Platinum Mine, several economic reef types
are identified, where the Merensky Reef becomes conformable to cumulate layering, in particular, to the footwall marker (NP2
reef type) and the upper pseudoReef (P2 reef type). The Normal Merensky Reef, as well as the P2 and NP2 Reefs, contains economic
platinum group element (PGE) grades and includes the lower portion of the MCU melanorite and the Merensky Chromitite. Whole
rock geochemistry indicates that this package is compositionally identical in Normal, P2, and NP2 Reefs, suggesting that the
base of the MCU is a relatively homogeneous drape over both Normal and Regional Pothole Reef regions. However, the lower sections
of the three Reefs are variables depending on the depth of transgression of the MCU. In the Normal and P2 reef types, transgression
by the MCU was arrested within harzburgites, melanorites, and norites, resulting in coarse, pegmatitic textures in the immediate
footwall units. For the NP2 Reef, transgression by the MCU was arrested within leucocratic rocks and resulted in the formation
of troctolites below the Merensky Chromitite. These troctolites are characterised by a coupled relationship between olivine
and sulphides and by changes in major element chemistry and PGE contents relative to equivalent units in the footwall of the
Normal Reef. Along with micro-textural relationships, these features suggest that troctolization of leucocratic cumulates
in the NP2 Reef beneath the Merensky chromitite was a result of a reactive infiltration of a chromite-saturated melt and an
immiscible sulphide liquid from the overlying MCU, rather than a significant fluid flux from below. In all reef types, the
concentration of S defines symmetrical peaks centred on the Merensky Chromitite (and chromitites from pre-existing cyclic
units in Normal and P2 Reefs), whereas PGE concentrations define asymmetrical peaks with higher PGE contents in reconstituted
footwall rocks relative to the MCU melanorite. This signature is attributable to a magmatic model of PGE collection followed
by deposition towards the base of the MCU and within reconstituted footwall rocks. The continuity of the asymmetrical magmatic
PGE signature between the Normal Reef and Regional Pothole Reef sub-facies indicates that PGE mineralization inherent to the
Merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted
footwall. |
| |
Keywords: | Merensky Reef Regional Pothole Platinum group element Mineralization Troctolite |
本文献已被 SpringerLink 等数据库收录! |
|