FIRST DATA ON THE DIAMONDIFEROUS MANTLE OF THE KASAI SHIELD (CONGO CRATON) FROM DIAMOND MINERAL INCLUSIONS

Chemical compositions of syngenetic mineral inclusions encapsulated within diamonds provide insight into the physiochemical conditions of their hosts’ formation. In contrast to the well studied sub-continental lithospheric mantle (SCLM) of other cratons (e.g., Kaapvaal, Slave, Siberian cratons), the SCLM of the Congo craton in Southern Africa remains enigmatic. Here we report the first electron probe microanalysis (EPMA) data from mineral inclusions in diamonds of the Kasai shield of the Congo craton, as sourced from the Kasai River, Katanga Province, DRC. Their inferred primary sources are Early Cretaceous (~118 Ma) kimberlites of the Lucapa corridor in Northeastern Angola.

A suite of 111 placer diamonds from the Kasai River have been characterized for their size, colour, and morphology. Samples range in size from 0.2 – 1 ct and are colorless (45 stones), brown (31), yellow (16), pink (9), green (8), and grey (2). Most are unresorbed or lightly resorbed octahedrons (45), followed by dodecahedrons (32), aggregates (12), macles (9), cubes or cubo-octahedrons (8) and unknown shapes (5). The nitrogen content, aggregation and cathodoluminescence of Kasai diamonds are also reported.

The diamonds host over 120 protogenetic and/or syngenetic mineral inclusions 10 - 150 um in size. The mineral inclusions have been polished in situ and their compositions have been obtained from EPMA. Thus far, 10 eclogitic clinopyroxenes with 32-55% jadeite component, 10 eclogitic garnets (average Gros27Alm33Pyr40), 2 pyrrhotite, 1 Fe-Ni sulfide (36 wt% Ni), 33 olivine (Fo = 92-94) and 9 orthopyroxene (En = 93) have been analyzed via EPMA. Based on the number of harzburgitic (22) and eclogitic (6) diamonds identified, the Lucapa kimberlites carry a significant (~20%) proportion of eclogitic diamonds. Geothermobarometry will determine the depth distribution of these parageneses in the Congo SCLM.