Abstract

The Great Dyke of Zimbabwe is a major resource of platinum-group elements (PGE) and represents the world's second largest deposit, after the Bushveld Complex. The Darwendale subchamber contains more than 75 percent of all the PGE mineralization in the Great Dyke and is the subject of this investigation. The mineralization is closely associated with sulfide-bearing zones located in pyroxenites toward the top of the ultramafic sequence. This environment contrasts with many other mineralized layered intrusions in that substantial plagioclase crystallization did not occur prior to the formation of the PGE-enriched sulfide layers. Several sulfide layers occur within the bronzitites of the topmost cyclic unit rather than at the base, suggesting that a major mixing event of residual magma with a new influx of primitive liquid was not the primary control of the mineralization.The lower sulfide zones are disseminated over 10 to 20 m and contrast with the uppermost sulfide zone (S1), which is highly mineralized and generally less than 1.5 m in thickness. In the S1 zone, values for Pt are as high as 8 ppm over a narrow interval with Pt/Pd close to 2. The maximum enrichment of PGE occurs at the first appearance of sulfide as a result of the high partitioning of these elements into sulfide liquid. The order of enrichment in the sulfide is Ir > Pd > Pt [raquo] Au > Cu = Ni, and as the abundance of sulfide increases, this phase becomes highly depleted in these elements. The enrichment pattern is consistent with fractional segregation of sulfide. Repetition of the mineralization results from periodic disturbance of the magma system, which is related to relatively small pulses of new magma and overturns within the stratified magma column. The stratified magma was sustained by the repeated influxes of new magma entering as fountains.A series of peaks for Pt, Pd, and the base metals occurs in the S1 zone. Contrasting patterns in the vertical profile for Pt and Pd are attributed to the differences in the effective partition coefficients of these two elements in an environment of rapidly changing sulfide proportions.Marked lateral variations in the degree and style of mineralization are observed from axis to margin of the subchamber. Whole-rock concentrations of Pt are highest nearest to the margin in the very narrow and highly mineralized sulfide zone S1, but these fall to low values in the axis where the zone is more disseminated. In any given vertical section the maximum concentration of PGE in sulfide is similar in each of the sulfide zones. The total amount of Pt for the entire mineralized zone and the concentration of Pt in sulfide are, however, significantly higher in the axial facies compared to the margin. There are also major lateral variations in maximum values of Cu + Ni, and Ni/(Cu + Ni). These lateral variations are attributed to a difference in the thermal regimes arising from the flared shape of the Great Dyke in cross section and its influence on the fluid dynamics of the magma chamber. Extensive borehole drilling has permitted detailed two-dimensional contour patterns to be deduced for the distribution of Pt and Pd and the base metals in the S1 suboutcrop over a large area of the Darwendale subchamber.

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