MULTIPLE SULFUR ISOTOPE STUDY SUGGESTS CRUSTAL CONTAMINATION IN THE MOLOPO FARMS COMPLEX, SOUTH AFRICA

The Molopo Farms Complex (MFC) (2044±24Ma) is a satellite intrusion of the Bushveld Complex (BC) (crystallized from 2055.91±0.26Ma to 2054.89±0.37 Ma). The BC is Earth’s largest layered mafic intrusion and is extensively mineralized with platinum group elements. The MFC is 200km west of the BC and lacks mineralization. The MFC consists of a lower serpentinized ultramafic and upper mafic layer, all under ~200m of Kalahari sand. A multiple sulfur isotope study was performed on core samples from three boreholes in the MFC. Primitive mantle δ³⁴S has a range of 0±2‰, and may be altered in igneous systems by sulfur addition from country rock or hydrothermal alteration. Non-zero Δ³³S in igneous systems is obtained from surface-derived sulfur, often from sediments exposed to an anoxic atmosphere, which is the only environment believed to fractionate Δ³³S significantly (i.e. Earth before 2.0Ga). The Δ³³S and δ³⁴S in the MFC’s mafic layer are highest at the top and decrease downward. The Δ ³³S isotope compositions range from 0.08‰ at the top of the mafic layer to 0.04‰ at the bottom, and δ³⁴S isotope compositions range from 2.0‰ at the top to 0.5‰ at the base. The ultramafic layer has negative δ³⁴S≈-5‰ and scattered Δ³³S≈0.02-0.05‰. Molopo has lower Δ³³S and δ³⁴S values and smaller ranges than all reported BC data, where mean Δ³³S is reported as 0.11±0.02‰ (range=0.09‰-0.15‰) and mean δ³⁴S is reported as 2.2±0.6‰ (range=1.1‰-3.2‰). The lower Δ³³S values indicate the unmineralized MFC has less surface-derived sulfur than the Critical Zone of the BC. Molopo’s ultramafic low δ³⁴S values are likely due to overprinting by sediment-derived fluids associated with serpentinization. MFC’s δ³⁴S values are comparable to the Uitkomst Complex, another satellite with a mineralized, serpentinized lower ultramafic (δ³⁴S≈-5%, range -3‰ to -7‰) and an unmineralized upper mafic layer (δ³⁴S range 0.3‰ to 2.7‰).