Abstract:
The tectonic framework of the Neoarchaean - Mesoproterozoic (2.58 to 1.87 Ga) supracrustal rocks exposed in the Thabazimbi region and Kumba Fe-Mine, Transvaal Supergroup, South Africa, has been studied using remote sensing (RS) techniques (based on Landsat 8 Operational Land Imager (OLI) and in the field. This highly deformed and structurally controlled mining region boarders the northern flank of the Rustenburg Layered Suite (RLS) of the Bushveld Igneous Province (Complex), which intruded into the Transvaal Supergroup rocks and thus, played a significant role in the deformation and mineralization processes. Detailed RS mapping and structural field investigations reveals three deformational phases (D1, D2, D3) that demonstrate the tectonic history of the Thabazimbi region and its ore deposits.
A pre- to syn-Bushveld compressional deformation event (D1) caused the development of thrusts, folds and reverse faults that have an overall NW–SE to N–vergence. This compressional event resulted in the ENE–WSW and/or E–W striking Bobbejaanwater Thrust Fault (BTF) and Belt-of-Hill Thrust Fault (BHTF), which may be connected to a basal sole thrust. Thrusting of the Thabazimbi region is evident by duplication and in parts, triplication of the stratigraphy, along parallel east-west trending mountain ranges (Northern, Southern and Middle ranges). The Middle Range developed only locally between the BTF and BHTF in the Thabazimbi Thrust and Fold Belt. The BTF and BHTF are located south of the ENE-trending Thabazimbi-Murchison Lineament (TML), which they parallel in the Thabazimbi region, and are regarded as parallel thrusts that formed as a result of sporadic reactivation of the TML. A post-Bushveld extensional event (D2), which followed shortly after D1, is characterized by ENE–WSW to E–W striking, steeply S dipping normal faults in the Thabazimbi Range and E–W trending lineaments in the Rosseauspoort Range. This extensional event likely occurred during cooling and sagging of the 2.05 Ga RLS and led to steepening of the Transvaal Supergroup strata, fold axes and thrusts in the Thabazimbi Ranges, which now dip with ca. 70 degrees south.
The D3 extensional event, is defined by two major lineament trend directions, the NW–SE to NNW–SSE (D3a) and cross-cutting NE–SW orientated lineaments (D3b). These lineaments also crosscut the Rustenburg Granophyre Suite and the granitic rocks of the Bushveld Complex (≤ 2.055 Ga), and the ≤1.87 Ga Waterberg Group (upper Waterberg unconformity-bounded sequence, WUBS-II). The age of D3 is widely bracketed between post-Waterberg basin development and perhaps pre- to syn-Karoo basalts (≥ 0.18 Ga) or even later events. It is however unlikely that it has occurred in the Phanerozoic, but rather is related to the occasional deformation of unknown age visible in the Waterberg Group itself and in older rocks across the central Transvaal basin. The enrichment of the Thabazimbi BIF iron- oxides (> 15% weight percent (wt%) FeO) to high-grade haematite Fe-ores (>60 weight percent (wt%) FeO) is likely to have occurred during D1 to D2 events.
