Occurrence and Characteristics of Chromite Deposits—Eastern Bushveld Complex

Abstract The chromite deposits of the eastern Bushveld Complex occur in the Critical Zone, a layered pile consisting mainly of pyroxenite, norite, and anorthosite, with minor gabbro, dunite, and harzburgite. With probable reserves in excess of one billion tons, the deposits are among the major mineral resources of the world. The Critical Zone is more than 75 miles long and is divisible into several sectors. The central and southern sectors are best known. Field studies indicate that they differ in rock sequences and interlayered chromite deposits. An 11,000 foot sequence of rocks in the central sector, extending from the Basal Zone upward into the Main Norite Zone, has been sampled in detail, and its evolution is being studied in terms of field data, mineralogy, chemical composition, and textures. The sequence is divisible into successive units on the basis of silicate mineralogy. The Critical Zone is also divisible into chromitic and non-chromitic intervals, according to the presence or absence of cumulate chromite. Within the chromitic intervals occur the chromitites, in which chromite is the sole cumulate. Most chromitic intervals consist of chromitic pyroxenite or anorthosite, or both, but two consist mainly of chromitic dunite and harzburgite. Broad mineralogical and compositional trends—upward change from predominant pyroxenite to predominant norite and anorthosite, upward increase in total Fe, Fe/Mg, and Fe/Cr of chromite, in Fe/Mg of orthopyroxene, and in Ab/An of plagioclase—suggest that the Critical Zone is grossly the result of progressive magmatic differentiation. Departures from these trends, however, together with major disconformities in the sequence, indicate a complex history. This is reflected in the distribution and characteristics of chromitites.