The distribution and main geologic features of discordant bodies of iron-rich ultramafic pegmatite in the Bushveld Complex

Abstract

Transgressive bodies of iron-rich ultramafic pegmatite, composed predominantly of varying proportions of hortonolite, clinopyroxene, ilmenite, and Ti magnetite are abundantly distributed throughout the layered mafic sequence of the Bushveld Complex. New information from field and underground studies, together with interpretations of detailed magnetic surveys, is presented along with an overview of the available literature. Some whole-rock and mineralogical-chemical data are also included.Within the Bushveld Complex a number of distinct but possibly genetically related groups of postcumulus ultramafic rock are recognized. These include, besides the main group of iron-rich ultramafic pegmatite, bodies of magnesian dunite and the well-known platiniferous ultramafic (or dunitic) pipes. Transgressive, often pipelike bodies composed almost entirely of Fe-Ti oxides occur in the main and upper zones of the complex which, although part of the iron-rich ultramafic pegmatite suite, are described separately. Composite bodies in which massive Fe-Ti oxides occur as a core within a peripheral shell of silicate-rich pegmatite are also recognized. The major occurrences of iron-rich ultramafic pegmatite are documented and depicted in a series of maps. Their distribution is subject to two major stratigraphic controls. Iron-rich ultramafic pegmatite occurs largely above the ultramafic cumulates in the lower and lower critical zones. It is abundant in the upper critical and main zones where plagioclase is a major cumulus phase but is rarely observed above the highest cumulus Ti magnetite layer in the upper zone. It is also evident that these rocks become increasingly iron-rich toward the top of the layered sequence, so that in the upper zone, Fe-Ti oxide pegmatite is the dominant variety. Many bodies, particularly the large ones, are structurally controlled and occur in disturbed areas characterized by faulting and post-Bushveld dikes.Stratigraphic magnetic anomalies in the complex may be related to anorthositic layers containing small specks of iron-rich ferromagnesian silicates and Fe-Ti oxides. This iron-rich material is considered to represent the accumulated and trapped, late-stage, residual and intercumulus fluid resulting from fractional crystallization within a cyclic unit. Lateral migration and channeling of these pegmatitic fluids into areas of structural weakness resulted in the formation of large, pipelike bodies of iron-rich ultramafic pegmatite. These bodies may have resulted from metasomatic replacement of preexisting cumulates or they may have crystallized directly from the pegmatitic fluids. The authors believe that the pegmatitic fluids have been derived from within the crystallizing cumulate pile and it is concluded that the formation of iron-rich ultramafic pegmatite is an integral feature of the layered sequence.