A Mineralogical and petrological investigation of the Platreef on Drenthe 778 LR, northwest of Potgietersrus

Abstract

The Platreef on Drenthe 778 LR lies at the base of the Rustenburg- Layered Suite of the Potgietersrus limb of the Bushveld Complex. Archaean granites form the floor rocks of this reef. It is 250 metre thick and consists of lower 50 metre thick feldspathic pyroxenite layer, overlain by 170 metre of norites. A 30 metre thick feldspathic pyroxenite layer overlies the norites and the top of the Platreef is taken to be where inverted pigeonite first appears in the sequence. Orthopyroxene is a cumulus mineral throughout the Platreef and the most magnesian orthopyroxene has a composition of En75 . At the bottom of the Platreef is a sharp reverse trend in the composition of the orthopyroxene where it becomes more Mg-rich, from En60 to En70 . A sharp compositional change from En75 to En60 at the top of the Platreef, concomitant with the entry of inverted pigeonite, is taken to indicate that a new pulse of magma may have occurred at that level in the intrusion. However, plagioclase which has a composition of An70 in the Platreef, does not show such a change. Metasedimentary inclusions of Transvaal Sequence dolomite abound in the Platreef on Drenthe. High temperature metamorphic minerals such as monticellite, forsterite and diopside occur in these xenoliths. The equilibrium temperature in the xenoliths is estimated to be in the order of 950°C at 3 and 4 Kbar. Many of the xenoliths have, however, been hydrated and their original metamorphic mineralogy has been obscured. A large discontinuous zone of hydration, loosely termed a "serpentinite" zone, occurs near the top of the Platreef. This unit consists of hydrated igneous rocks, small hydrated xenoliths and totally altered rock, the origin of which is obscure. Serpentine, brucite, hydrogrossular and chlorite may be the major mineral in any part of this unit. This unit is considered to have formed due to the dissociation of a Sio2 -poor dolomite. Major sulphide mineralization is associated with this "serpentinite" zone. Platinum-group element bearing Ni-Cu sulphides make the Platreef of economic interest. The sulphides occur disseminated in the igneous rocks and in the "serpentinite" zone. Ni:Cu ratios decrease with increasing height in the Platreef and two distinct sulphide assemblages occur: pyrite, pyrrhotite, pentlandite and chalcopyrite in the lower 150 metres and pyrrhotite (with or without troilite), pentlandite and chalcopyrite (with or without cubanite) in the upper 100 metres. Monoclinic pyrrhotite occurs mainly in the lower 150 metres and hexagonal and stoichiometric pyrrhotite in the upper 100 metres. This indicates that the sulphides are relatively enriched in metals in the upper part of the Platreef. The sulphides of the "serpentinite" zone correspond to the sulphides in the fresh igneous rocks of equivalent stratigraphic position in the Platreef. Alteration features such as magnetite which replaces sulphides however, occur in the "serpentinite" zone. The sulphides in the lower part of the Platreef are considered to have formed from an inuniscible sulphide liquid which separated from the magma due to the lowering of the temperature in the magma because of the proximity of the country rocks. Contamination of the magma due to the dissociation of a Sio2 -poor dolomite changed critical physicochemical parameters such as f02 in the magma which resulted in the 0 formation of more immiscible sulphide liquid in the upper part of the Platreef, especially in the "serpentinite" zone. Comparison of the Platreef on Drenthe with the Merensky Reef shows that, in mineral composition, the Platreef is slightly less basic than the Merensky Reef. The Platreef is also more leucocratic.