The kimeerlite occurrences from the vicinity of Rayton, South of premier mine, and from East Lynne, East of Pretoria

Abstract

The mineralogy, petrography, and petrology of five kimberlite occurrences were investigated. Four of the occurrences i.e., Rietfontein, Kaalfontein, and Montrose No. 1 pipes, and the Puntlyf fissure are south of Premier Mine. The East Lynne occurrence is just east of Pretoria. The mineralogy and petrography of the groundmass, cognate inclusions, and foreign inclusions are compared and six stages can be identified in the evolution of the kimberlites. Stage I represents the crystallization of the first generation minerals such as olivine I, phlogopite I and ilmenite in the kimberlite magma. Magnesium enrichment and possible differentiation of the potassium and water rich phase to form the Puntlyf type of micaceous kimberlite, is represented in the second stage. Stage III starts with the crystallization of second generation minerals such as olivine II. The magnesium enrichment is especially portrayed by the increase in geikielite (MgTi03) content of the ilmenite towards the outer rim, and zoning in olivine I. Crystallization of most of the groundmass minerals are completed in this stage where the chromium-rich spinels are supplanted by titanomagnetite followed by the crystallization of perovskite from the residual titanium and calcium-rich liquid. The oxygen fugacity and iron content of the older intrusions in the Kaalfontein pipe and in the Montrose No. 1 pipe increased slightly resulting in the crystallization of more manganese-rich phases (pyrophanite) and magnetite and hematite. Serpentinization of the kimberlite occurs in stage IV but depending on localized compositional differences, phlogopite II and biotite form the last sequel to the crystallization of the serpentine matrix. Silica activity results in the alteration of perovskite to titanite during this stage. Stage V represents mainly the metamorphism of serpentine to tremolite and actinolite owing to a rise in temperature in the kimberlite. Crystallization of zeolites and hydrogrossularite constitutes the hypergene alteration and crystallization in stage VI. The history of the kimberlites can be visualized from the mineralogical differences, stages of evolution and field relations. The Rietfontein pipe contained four different types of kimberlite that were emplaced in two major cycles. Five major kimberlite types were recognized in the Kaalfontein pipe. The youngest group resembling that of the Rietfontein pipe and the oldest that of the Montrose pipe where only one kimberlite is present. The micaceous kimberlite of Puntlyf represents the early stages of kimberlite formation and emplacement in fissures in to the earth's crust. Kimberlite constitutes only a small proportion of the carbonatite of the East Lynne occurrence. It is exceptionally rich in biotite and carbonate and is most probably a residual phase of the carbonatite intrusion. Thus it is not related to the same kimberlite activity as the other occurrences.