Oxide Mineralogy and Magnetic Properties of the Koidu Kimberlite Complex, Sierra Leone, West Africa

Abstract

A good correlation between opaque mineralogy and rock magnetic properties has been established in a study of a small number of oriented samples from four kimberlite dykes and one pipe in the Koidu complex of Sierra Leone. The pipe sample is mineralogically distinctive, in that it alone contains deuteric titanomaghaemite after titanomagnetite, and haematite after chlorite. All of the kimberlites contain xenocrystic, upper-mantle derived ilmenite that has undergone sub-solidus reduction to form Mg-Ti magnetite. Three of the dykes contain serpentinized olivine with microscopic to sub-microscopic magnetite and identifiable metal Ni-Fe alloys, and finely textured Mg titanomagnetite and magnetite as atolls surrounding complexly zoned Cr-Mg-A1 spinels. Consistent with these observations, Curie and blocking temperatures range from 520 to 590°C and high alternating field (AF) coercivities imply single domain or pseudo-single domain particles as the bulk carrier of natural remanent magnetization (NRM). In the pipe sample, however, the remanence persisting above 600°C is comparatively strong. Preliminary remanent vectors, isolated by either thermal or AF demagnetization, from four of the five samples are moderately consistent, and yield a mean (n = 4) of D = 323", Z = -17", k = 18", and ag5 = 22".