THE ORIGIN OF THE XENOLITHS IN THE SELUKWE SUBCHAMBER, GREAT DYKE, ZIMBABWE

The unique Selukwe Subchamber of Great Dyke is bounded by the Shurugwi greenstone belt on the west side for approximately 25km and granitoids on the east side, as compared to other subchambers that are bounded on both sides by granitoids. It is also the narrowest section of the entire Great Dyke. The extensive xenolith suite is found on the western flank and the central zone of the subchamber. Whole rock and trace element analysis of 159 borehole samples and 200 surface samples using XRF and ICP-MS, as well as mineral analysis using an electron microprobe and petrographic studies have been done on thin sections and polished sections. The xenoliths fall into three categories (1) Metasedimentary quartzite and banded iron formation from Shurugwi greenstone belt, (2) Wall or roof-rock differentiates which are representative of the fine grained primitive Border Group which is poorly exposed or has been thermally eroded during the formation of the layer series, and (3) Primitive differentiates that were crystallized in the conduit or feeder zone to the Great Dyke at depth and were transported during the emplacement of the layered series. Characteristic minerals are enstatite, forsterite (Fo₉₀), Cr spinels, and rare clinopyroxene. The enstatite presents a high Mg #, (Mg/Mg+Fe²⁺ 0.78-0.80) ratios. Rocks comprising high olivine also have high Cr (>3000 ppm) and high Ni (>1000 ppm) and are associated with abundant interstitial chromite and pyroxene. This study is important as the xenolith suite has an impact on the mining operations of the platinum zone in the P1 pyroxenite layer at the Unki Mine.