Paleomagnetism of the premier mine kimberlite

Abstract

Sixteen samples of kimberlite have been collected from deep underground in the Premier Mine at Cullinan in the Transvaal. The stability of the natural remanent magnetization of the samples has been well tested by alternating-field demagnetization and confirmed by thermal demagnetization. The magnetization is shown to be associated with magnetite, some of which is of primary origin and some of secondary origin. The magnetite of secondary origin was formed contemporaneously with or shortly after the emplacement of the kimberlite as a result of extensive hydrothermal alteration of primary minerals. It has not been possible thus far to decide between a thermoremanent and a chemical remanent origin for the observed magnetization. Previous paleomagnetic evidence has demonstrated that the kimberlite was emplaced in Precambrian times rather than in the Cretaceous, as was previously supposed. This evidence is supported by recently published geochronological work, which suggests an age of about 1750 m.y. The paleomagnetic pole at the time of emplacement of the kimberlite is located at 51.1°N, 37.5°E with A95 = 7.5°. This position is significantly different from, and lies approximately 17.5° away from, that obtained in a previous study of the post-Waterberg diabase sills, one of which cuts across the pipe. The age difference between the kimberlite emplacement and the sill's intrusion is not thought to be very great.