FORMATION OF PLATINUM-GROUP ELEMENT-ENRICHED HORIZONS AT UNKI MINE, SHURUGWI SUBCHAMBER OF THE GREAT DYKE, ZIMBABWE: DID CHROMITE PLAY ANY ROLE?

Unki Mine is situated on the Great Dyke of Zimbabwe, a mafic-ultramafic layered lopolith of intermittent mountain ridges and flat plains 550km long. At its widest part, the dyke is 12km across. The Great Dyke and its associated satellites intruded across the entire Archean Zimbabwe craton through NNE-trending fracture systems. Longitudinally, the dyke is subdivided, from north to south, into the Mavuradonha, North, and South Chambers, with further subdivisions of the North and South chambers into five subchambers. In all chambers the vertical succession is broadly comparable and consists of a lower Ultramafic Sequence and an upper Mafic Sequence. The Ultramafic Sequence in all subchambers is divided into a lower Dunite Succession of dominantly dunite and harzburgite, and an upper Bronzitite Succession of mostly harzburgites and orthopyroxenites.

The main platinum group elements (PGEs)-bearing horizon - the Main Sulphide Zone (MSZ), is a tabular stratabound layer that is hosted in pyroxenites and it is broadly similar in form throughout the length of the Great Dyke. We conducted a chromite composition study from a thin chromitite layer occurring within the MSZ at Unki Mine, in the Shurugwi Subchamber, the northern component of the South Chamber, to place some constrains on the origin of the MSZ. Chromite grains vary from 0.5-3mm and are often rimmed by, or enclose, sulfide grains.

Chromite number, Cr#, (100▪Cr/(Cr+Al)) and Mg number, Mg#, (100▪Mg/(Mg+Fe²⁺)) of 43 Unki Mine chromite analyses range from 59.9-62.8 and 37.8-46.4, respectively. Unlike chromites from most Great Dyke layers, both Unki Mine reef Cr₂O₃ and MnO contents are low, ranging from 39.39-42.58 wt% and 0.38-0.47 wt. %, respectively. The low Cr# Unki Mine chromites are interpreted to have been formed from an influx of new magma into the Great Dyke magma chamber that interacted with SiO₂-rich country rocks, triggering crystallization of both the MSZ PGEs and associated low Cr# and MnO chromites.