Magnetomineralogy of Botswana kimberlites

Abstract

The magnetic properties and mineralogy of kimberlites from four pipes (BK53, BK56, AK08, and AK06) in Botswana are studied. It is shown that magnetic characteristics of the kimberlites as well as chemical and phase compositions of the oxides from kimberlite groundmass reflect the difference in evolution of kimberlite melts and formation conditions of these bodies and are associated with the grade of these diamoniferous bodies. The relatively low-grade (0.22 carats per ton) kimberlites from the southern lobe of the AK06 pipe contain picrochromites with high chrome and low titanium contents (up to 49.4 wt % Cr2O3 with 3.9 wt % TiO2), which occur in the paramagnetic state at a temperature of T > −180°C. Their magnetic properties above room temperature are determined by titanomagnetites with the Curie point at +370 to + 500°C, which were formed at the later stages of crystallization as a result of the gradual decline in the PT-parameters. Most spinelides in the Botswana kimberlites from the low-grade pipes (below 0.07 carats per ton) were crystallized at higher PT-parameters than in the southern lobe of pipe AK06, in an environment with highly active titanium. The chromspinelides that were segregated previously turned out to be unstable, which led to the formation of magnesian-chrome and magnesian ulvospinels (15.9–29.7 wt % TiO2; 12.8–22.8 wt % MgO; 0–11.3 wt % Cr2O3; and 1.2–12.4 wt % Al2O3) with Curie points TC = −50 to −80°C. These conditions were unfavorable for preservation of chrome-spinelides and diamonds. At lower PT-parameters, a small amount of finely dispersed magnetite with a single-domain and pseudo-single domain structure was formed.