The Upper Critical Zone in the Swartklip Sector, north-western Bushveld Complex, on the farm Wilgerspruit 2JQ: II. Origin by intrusion of ultramafic sills with concomitant partial melting of host norite-anorthosite cumulates

Abstract

Abstract In the Bushveld Complex, the ultramafic (orthopyroxenite/harzburgite with chromitite) layers that host most of the PGE and chromite mineralization in the Upper Critical Zone display well-documented discordant basal contacts with their anorthositic and noritic host rocks. Whilst not so well documented, there is evidence that the upper contacts of these units are also discordant. We review the nature of the contacts between the ultramafic units and adjacent plagioclase-rich lithologies. These include contact phenomena like pegmatoidal lithologies and thin magmatic reaction chromite stringers. We conclude that most, if not all, ultramafic layers were intruded as sills into pre-existing norite/anorthosite cumulates. The sequence of norites and anorthosites that hosts the ultramafic layers was built up by a prior series of multiple tholeiitic (A-type) magma intrusions. The spectrum of lithologies from melanorite through to (mottled) anorthosite represents differing degrees of partial melting in response to these successive magma influxes. Density and competence contrasts between layers of plagioclase-rich rocks in turn provided pathways for sill propagation of subsequent ultramafic (U-type) magmas. The ultramafic magmas further modified the host norites and anorthosites by processes of partial melting and metasomatism. The ultramafic units themselves accumulated as composite sills in response to multiple magma injections. In the western Bushveld Complex, particularly including the Swartklip Sector in the north-western part of the complex, the Merensky Reef is represented by various facies that occur at different levels in the host stratigraphy. This phenomenon has been referred to by the term “regional potholing”, and has been attributed to the erosion of footwall cumulates by new influxes of magma. We suggest that a series of step-and-stair-type transitions of intruding sills to successive stratigraphic levels might more appropriately explain the various facies of the Merensky Reef.