The mineralogy and petrology of kimberlite and its related inclusions, with special reference to Premier Mine

Abstract

The structural and petrochemical relationships of the post-Waterberg alkaline intrusions north-east of Pretoria were investigated. It was found that apart from being contemporaneous these intrusions were influenced by the same tectonic pattern. The different magmas concerned all belong to the alkaline basaltic rock series, but they appear to have formed as a consequence of separate differentiation sequences in the mantle of the earth. The alkaline intrusions simulates the normal alkaline differentiation trend, whereas the kimberlite and carbonatite intrusions formed individually disconnected from the alkaline basalts and from one another. The Premier Mine kimberlite was found to consist of at least four separate intrusions starting off with the kimberlite of group-I which was highly explosive succeeded by the kimberlites in the western and eastern portions of the mine, which were less explosive and terminating with the unexplosive basaltic kimberlite veins. The latter rock-type, which was formerly described as carbonatite, compares favourably with the massive basaltic kimberlites of Benfontein Wesselton, Dutoitspan and Jagersfontein. These rocks also show some correspondence to the melilite basalts, however, chemically they appear to be impoverished in magnesium, and enriched in calcium, phosphorous, sodium and potassium. Owing to the fact that their mineralogical constituents are not in equilibrium, the micaceous kimberlites differ from both the melilite basalts and the basaltic kimberlites, but in this respect they resemble the kimberlite breccias. The influence of the mode of emplacement on the various rock-types concerned has also been investigated. It was concluded that all the alkaline rocks originate in the eel ogi tic zone, which occurs at a depth of more than 100 km below the surface and that, depending on the extent of crystallization of enstatite and olivine, a complete sequence of undersaturated alkaline rocks may result. The comparison of the mineralogy of the constituents in eclogite, kimberlite and peridotite revealed that the minerals of kimberlite and peridotite show a remarkable correspondence, whereas eclogitic minerals are absent in kimberlite. 11 etrologically it was found that the clinopyroxene in the eclogite modules are invariably in a stage of alteration, and this alteration product consists of a fine grained clinopyroxene. Some undevitrified clinopyroxene glass was also encountered in some of the eclogi te nodules. The eel ogi te nodules often display metamorphic tentures, whereas the garnet peridotite nodules revealed the sequence of crystallization observed in the primary phenocrysts in kimberlite, and as predicted by O'Hara (1968) in the system diopside-pyrope at 30 kb to 20 kb, viz. pyrope, enstatite, clinopyroxene, olivine and spinel. Chemically the eclogite appears to be undifferentj_ated, whereas the ultramafic nodules are uniformly differentiated, corresponding to the first stages of the normal differentiation sequence for tholeiitic magmas.