- © 2012 Geological Society of South Africa
Previous development models for the exceptionally poorly-exposed Molopo Farms Complex (MFC) were based on assumed Bushveld analogies and had three key components: emplacement of magma via (a) deep feeder dyke(s), mixing of two contrasting magma-types, and independent crystallisation of the lower ultramafic and upper mafic cumulates below and above a mass of country-rocks. Re-examination of the available data, including the large drill-core archive, supports the following conclusions concerning the structure and evolution of the MFC. (1) The layered sequence evolved within a single (non-composite) magma chamber marked by height variations and roof- and wall-irregularities that limited the crystallisation of the mafic rocks in places, caused lateral variations in layer development, and led to the crystallisation of some parental and evolved residual magma as abundant sills within the roof-rocks. (2) Evolution was accompanied by repeated, small injections of only one parental magma-type (with significant country-rock contamination), sulphide exsolution and precious metals enrichment being controlled by magma cooling, FeO loss and contamination, rather than mixing of contrasting magma-types. (3) The Lower Zone of the Bushveld Complex constitutes the sole likely stratigraphic correlative of the Ultramafic Sequence of the MFC. Both successions most probably crystallised from a common (early Bushveld) B1 parental magma that, rather than entering the MFC magma chamber through a deep feeder dyke, flowed laterally from the main Bushveld magma chamber into the MFC satellite compartment via a constriction. (4) Above the lower ultramafic rocks the differing cumulate sequences of the two intrusions reflect the independent evolution of the original B1 magma in the MFC without the involvement of the (late Bushveld) B2/B3 magma.
In light of the above re-interpretation of the MFC, (1) further search for UG-2- and/or Merensky-type PGE reefs is unwarranted, (2) recently suggested potential for Voisey’s Bay-type massive nickel sulphides in postulated MFC feeder dykes may not be justified, and (3) the mineralised lower marginal zone of the US may host low grade, Ni-PGE sulphide deposits similar to those at Volspruit/Grass Valley and Sheba’s Ridge in the Bushveld Complex.