The use of sulfide mineral chemistry to understand PGE mineralization processes in the Northern Limb of the Bushveld Igneous Complex

Abstract

The Platreef is a complex lithological unit defining the base of the Northern Limb of the Bushveld Igneous Complex (BIC) and typically characterized by complex magma-footwall interaction. It is of interest primarily because it contains significant PGE-Ni-Cu mineralization and there is no consensus regarding the mineralization processes, particularly on the role of footwall assimilation. One puzzling aspect is the apparent lack of correlation between sulfide abundance and PGE-Ni-Cu contents. Another aspect is the possible relationship between PGE-Ni-Cu mineralization in the Northern Limb and PGE-Ni-Cu mineralization elsewhere in the BIC (e.g. the Merensky Reef). This study focuses on the mineral chemistry (major and trace elements) of major sulfides: pyrrhotite (Po), pentlandite (Pn), and chalcopyrite (Ccp) in Platreef samples intersected by Ivanplats during deep drilling in the Turfspruit area. A drill core (UMT094), for which Sr and S isotope stratigraphy was collected previously, was used as reference because the isotopic data helped define the degree of footwall assimilation. Whole rock geochemistry (Cu, Ni, S, PGE) showed the existence of at least two distinct sample populations in UMT094: one defined by relatively low S/Ni (with no evidence of footwall assimilation and stratigraphically associated with PGE mineralization), the other defined by relatively high S/Ni (and stratigraphically associated with footwall assimilation and with low PGE grades). The project had three objectives. The primary goal was to assess whether the differences identified in whole rock geochemistry were also expressed in the composition of major sulfides (pyrrhotite, pentlandite, chalcopyrite) in UMT094. Having established those parameters, the second goal was to use the same approach (and assess the degree of footwall assimilation) on samples (from drill cores UMT231 and UMT233) considered representative of the main PGE-Ni-Cu mineralization (i.e. the Flatreef) in the Turfspruit area. Finally, the data obtained in this study were compared to similar data from the Merensky Reef (from published peer-reviewed literature) to evaluate possible matches between sulfide chemistry in assemblages associated with PGE mineralization in the Northern Limb and the Western Limb of the Bushveld Igneous Complex (to assess a possible common origin). After careful petrographic observations and documentation, major and trace element contents in sulfides were acquired using electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The results showed that there are no significant differences in major element chemistry (e.g. Fe/Ni in pentlandite, Fe/S in pyrrhotite) between the two populations identified. However, the trace element data validate the hypothesis tested and show that in UMT094: (a) Selenium contents in all sulfides associated with PGE-Ni-Cu mineralization are higher than in sulfides associated with footwall assimilation; and (b) Other trace elements (such as Co and Ru in pyrrhotite; Co, Pd and Ru in pentlandite; Cd and Sn in chalcopyrite) are also useful in distinguishing between sulfides in intervals with no evidence of footwall assimilation and sulfides (lower in the stratigraphy) associated with footwall assimilation. When applied to the Flatreef samples (UMT231 and UMT233) the results show that, in general, trace element content in sulfides from the Flatreef also varies with stratigraphy: sulfides from Flatreef intervals with high PGE grades (and intervals above) have trace element contents that are closer match to sulfides from the unaltered (magmatic) intervals in UMT094. Finally, when data from this study is compared to similar data from the Merensky Reef the results show significant overlap. This is interpreted as compelling evidence to conclude that the mechanisms that operated in the formation of PGE-Ni-Cu mineralization in the Flatreef were similar to those involved in the formation of the Merensky Reef (for which local footwall assimilation is not considered a significant factor). Thus, footwall assimilation was not the primary process in PGE-NiCu mineralization. These results show that trace element data in major sulfides can be an additional and useful tool to distinguish among different sulfide mineralization events