The cyclic unit of the upper critical zone on maandagshoek 254 KT eastern Bushveld complex

Abstract

The farm Maandagshoek 254 KT is situated in the eastern lobe of the Bushveld Complex. The investigated sequence forms part of the Winterveld Norite-Anorthosite of the critical zone and consists of the UGl, UG2, UG3 and UG3A cyclic units. These cyclic units consist of a basal chromitite with or without olivine, overlain successively by melanorite, norite, and leuconorite to an anorthosite layer above. The cyclic units are punctuated by sharp contacts and a remarkable persistence along strike and dip. The En content of orthopyroxene and the An content of plagioclase decrease and increase respectively upwards through the UGl and UG2 cyclic units. Breaks in this trend occur below the chromitite layers where the En content of bronzite increases and in the anorthosite layers at the top of the cyclic units where the An content of plagioclase increases. A trace element study showed Rb and Zr to increase upward in the UGl and UG2 cyclic units. Rb and Sr display a reasonable antipathetic relationship. The Cr and Ni content of the orthopyroxenes increases upwards irregularly in the cyclic layers, but the Ni content decreases where visible sulphides are present. Cu, Ni and the platinoid elements increase in the chromitite layers, especially the UG2 chromitite layer. Clinopyroxene is not commonly cumulus and is identified as chromi~_ferous augite. The equilibrium temperature of a coexisting orthopyroxene-clinopyroxene pair was calculated according to the Wood-Banno and Wells modifications to be 1179°c and 1097°c respectively. Cumulus olivine associated with the UG2 pegmatoid and the UG3A chromitite was identified as chrysolite. The compositions of 15 chromite samples were determined and showed similar chemistry to those reported by De Waal (1975). The crystallization temperatures of coexisting olivine and chromite pairs was determined using the Jackson (1969) and Roeder et al, (1979) geothermometers. The UG2 chromitite layer contains an average of 8,5 g/t PGE and Au over a width of 0,64 metres in 22 intersections in borehole and underground samples. A study of the distribution of Pt and Pd in the weathered and unweathered zones clearly shows that both, but especially Pd, are mobile and depleted in the surface environment. The sulphide liquid in the UG2 chromitite layer was calculated to have contained 925 ppm Pt and 934 ppm Pd which is 578 and 359 times respectively the calculated levels in early immiscible sulphide liquids (Naldrett and Cabri, 1976). A model is proposed in which the UG2 chromitite layer acted as a physico-chemical barrier to the upward migrating late deuteric fluids and volatiles charged with platinoids, Au, Sb, As and Te. The platinoids were collected by earlier sulphide droplets which separated from the magma during crystallization of the UG2 chromitite layer. Structures are sub-divided into Class I and Class II features. Class I features are related to viscosity and density contrasts of layers at the time of crystallization of the magma and may be seen as bifurcating and anastomosing layers, folding and slump features, rafting of chromitite blocks, minor faults and potholes. Class II features are associated with tectonic movement which postdates the consolidation of the layering and are sub-divided into three categories; faults, dolerite dykes and pipes. The pipes are pegmatoidal in character, zoned with ultramafic cores and pyroxenite rims, have sharp contacts and cause downwarping and attenuation of the layering towards the pipes to form marginal synclines. It is postulated that intercumulus material coalesced along structural weaknesses and moved upward until the lithostatic pressure impeded upward movement at which time lateral and outward intrusions occurred. The layers of the intruded rocks bent down to accommodate the outward flowing material and gave rise to the marginal synclines. The concept of a closed cell cyclic unit is introduced to account for the inherent characteristics and origin of the cyclic units of the upper chromitite group.