Syn- to post-intrusive deformation in the chromitiferous harzburgite unit of the Uitkomst Complex, Nkomati Mine, Mpumalanga Province
Joubert, Pieter Lourens
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The mafic to ultra-mafic, intrusive Uitkomst Complex, situated some 20km northeast from the town of Badplaas in the Mpumalanga province of South Africa, contains a Chromitiferous Harzburgite unit that host a layer of massive chromitites at the top. The massive chromitites crop out, as three hills on the farm Uitkomst 541 JT and is exploited by South Africa’s only primary Nickel mine, the Nkomati Mine. The main research criteria of this project are the involvement of syn- and post-magmatic deformation in the petrogenesis of the Massive Chromitite Layer of the Uitkomst Complex. On chromitite hill no.1 (open pit no.3) the layer of massive chromitites consist of five smaller chromitite sub-layers with an average, individual thickness of between 1 to 3 meters. These sub-layers and lenses are stacked on top of each other and separated by layers of talc-carbonate schist, to form a massive chromitite zone with a combined thickness of up to 30m. The project consists of four interlinked investigations: 1. A geological mapping exercise in open pit three of the Nkomati Mine. This investigation forms the basis of the project and focuses on the general geological and geometrical properties of the chromitite layers. The investigation suggests that chromitite lenses MC4 and MC5 that occurs at the base of the Massive Chromitite Layer, in the study area, were deposited as a result of continuous crystallization as they are inter grown through a large gradational contact. The fractured nature and movement indicators along the bottom contacts of chromitite lenses MC1, MC2 and MC3 higher up in the layer suggest post-intrusive duplication through thrust faulting. 2. An investigation of the geological structures in the study area once again emphasized the similarities and continuity in chromitite lenses MC4 to MC5 and supports a continuous depositional event. The jointing in chromitite lenses MC1, MC2 and MC3 are not affected by the movement along the bottom contacts of the chromitite layers but get terminated at the gradational top contacts. This suggests that the fracturing along the bottom contacts might represent syn-intrusive deformation and that these three chromitite layers were rather deposited individually through magmatic processes and rules out the possibility of structural duplication. 3. A petrographic investigation revealed that each chromitite lens experienced a unique degree of crystal deformation, this syn-intrusive deformation suggests that the chromitite was already in the solid phase during transportation. Furthermore each chromitite lens has a unique chromite to matrix ratio that suggests unique physical conditions during the crystallization phase of each chromitite lens. This indicates that the chromitite lenses were deposited as rafts of semi-solid chromitite and further suggests that the rafts were initiated by various pulses of magma replenishment into the primary magma chamber, supplying the conduit system. 4. The mineral chemical investigations of the chromite crystals reveal a normal fractionating trend for chromitite lenses MC4 and MC5. This further supports that they were deposited during a continuous crystallization event. An irregular variation pattern in the Cr number and Mg number in relation to the depositional sequence of the three upper chromitite lenses support that their crystallization were initiated by various pulses of magma replenishment. The former two investigations mainly suggest the involvement of post-magmatic deformational processes, whereas the latter two investigations reveal that the geomorphology of the Massive Chromitite Layer is mostly influenced by syn-magmatic processes that occurred in both the primary magma chamber further down from the study area and the conduit system in the vicinity of the study area.