Platinum-group mineral assemblages in the Platreef on Tweefontein, Northern Bushveld complex, South Africa
Date
2013-02-04
Authors
McCutcheon, Shauna
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Abstract
The Platreef occurs at the base of the Northern Limb of the Bushveld Complex and is variably mineralised with PGE, Cu, and Ni. The Platreef varies in thickness from a few meters to a few hundred meters and rests on progressively older sediments of the Transvaal Supergroup and Archaean granite basement northwards. Recent studies have highlighted the importance of magmatic processes, contamination of the magma by footwall rocks and syn- and post metasomatic fluid activity on the observed mineralisation.
Along the Platreef strike, the PGE grade profiles are generally top-loaded from Overysel to Tweefontein North and more variable and bottom loaded from Tweefontein Hill southwards emphasizing the importance of the change in mineralisation style at Tweefontein in relation to the whole Platreef. This study presents the first significant PGM data on the Tweefontein farm, including ten boreholes along strike, providing insight into the distinctly different PGE mineralisation styles observed. Samples were selected based on assay data, varying rock types, stratigraphic position and proximity to geological features. The selected samples were investigated using petrography, geochemistry and the automated SEM techniques of QEMSCAN and MLA. Over 9000 PGM were analysed forming one of the most comprehensive PGM studies on the Platreef to date.
The lowermost footwall intersected along the Tweefontein strike is banded ironstone of the Penge Formation. This is overlain by a metasedimentary footwall package, of variable thickness, derived from the shales and dolomites of the Duitschland Formation. Iron-rich, recrystallised, noritic sills occur at the base of the Platreef and are thought to represent sills which intruded prior to the emplacement of the Platreef. A pre- and possibly syn-Bushveld structural control resulted in irregular floor topography defined by a topographic footwall high in the central Tweefontein area and topographic depressions at Tweefontein North and Tweefontein Hill. The depression areas at Tweefontein are similar to the footwall basins at Turfspruit to the south, in which the Platreef is more lithologically complex compared to the footwall high areas. The footwall basins at Tweefontein and Turfspruit contain basal massive and submassive sulphides, which may not necessarily carry significant PGE grade.
The Platreef lithologies at Tweefontein are composed of pyroxenites and norites with minor harzburgitic lithologies and contain numerous cross-cutting granitic veins. Xenoliths/interlayers of metamorphosed Duitschland lithologies occur primarily near the base of the Platreef, but also in the middle and upper Platreef sequence reflecting roof pendants. Unlike the Platreef on the farms adjacent to Tweefontein, the Platreef and footwall lithologies are relatively unaltered, but localised serpentinisation and chloritisation occur within harzburgitic lithologies and metasedimentary interlayers.
Based on the stratigraphy and geochemical characteristics, the Platreef at Tweefontein can be subdivided into the upper and lower Platreef. The upper Platreef subdivision occurs in the top 20-40 m of the sequence and is defined by higher Mg#, Cr, Cr (ppm)/MgO and Pt/Pd values compared to the lower Platreef. In addition, the majority of the grade and base metal sulphide (BMS) content is enriched in the upper versus the lower Platreef, particularly for the northern and central parts of Tweefontein. The upper and lower Platreef may have been derived from different magma sources based on the “R Factor” concept proposed by Campbell and Naldrett in 1979 whereby the abundance of the PGE relative to the BMS content is linked to the proportion of magma with which the sulphide ore equilibrated (Naldrett, 2005b). Previous detailed geochemical studies from Tweefontein Hill southwards highlighted compositional breaks in the Platreef sequence thought to represent distinct sill-like intrusions (Hutchinson and Kinnaird, 2005; Kinnaird, 2005; Manyeruke et al., 2005; Nyama et al., 2006). They reported a more primitive sill at the top of the Platreef, which correlates to the upper Platreef at Tweefontein. The lower Platreef is therefore likely to represent a different sill intrusion. A relatively homogenous pyroxenitic package characterises the upper Platreef, although a more heterogeneous package is observed close to and at Tweefontein Hill. At Tweefontein North, the base of the upper Platreef is often marked by a chromitiferous package comprising a pegmatoidal feldspathic pyroxenite unit, up to 6 m thick, capped by a chromitite layer. Due to similar stratigraphy and high PGE grades, this distinct horizon has been compared to the Merensky Reef found elsewhere in the Bushveld Complex.
The predominant base metal sulphides (BMS) in the Platreef at Tweefontein are pyrrhotite, pentlandite, chalcopyrite with minor pyrite aligned with that found elsewhere along the Platreef strike. There is an increase in BMS content, primarily pyrrhotite, towards the base of the Platreef with massive and submassive sulphide development near the base and in the footwall, particularly at Tweefontein Hill. Sulphur isotopes and detailed mineralogical studies at Turfspruit have shown that the addition of S, As and Sb into the magma from the Duitschland footwall triggered the development of a PGE-poor sulphide liquid which was then able to mix, modify and dilute the magmatic sulphides (Hutchinson and McDonald, 2008). Due to the similarity in footwall between Turfspruit and Tweefontein, these proposed processes help to explain the increase in BMS towards the base and the development of basal massive and submassive sulphides, which are not necessarily associated with significant PGE grade.
At Tweefontein North, the processes dominating the top-loaded PGE mineralisation were primarily magmatic. The PGM assemblage, hosted by base metal sulphides and magmatic silicates, is dominated by Pt-and Pd-bismuthides and -tellurides with minor PGE-sulphides and Pt-arsenides. PGE-sulphides occur in the Platreef where the chromitiferous horizon is developed, which may indicate an environment low in volatile activity and one of the most primary mineralisation styles along the Platreef strike. The footwall high, which separates the depressions at Tweefontein North and Tweefontein Hill may have kept the Platreef at Tweefontein North relatively protected from additional processes affecting Tweefontein Hill.
In contrast, assimilation of the Duitschland footwall is thought to play a key role in the development of the variable but predominantly bottom-loaded PGE mineralisation at Tweefontein Hill. The PGM assemblage is Pd-dominant characterised by Sb-, As- and Bi-bearing PGM, reflecting the incorporation of Sb, As and Bi from the Duitschland footwall. The association of the PGE mineralisation with the extensive basal sulphide development implies that the mineralisation at Tweefontein Hill probably occurred due to the gravitational settling of a sulphide liquid containing a mix of sedimentary and PGE-hosting magmatic components.
Due to a significant PGM-BMS association in the mineralised footwall and metasedimentary interlayers/xenoliths, a downward migrating sulphide melt is believed to be the main mechanism responsible for the redistribution of PGE, predominantly Pd, into the mineralised metasedimentary lithologies.
Finally, the Platreef and footwall lithologies may be locally modified by late-stage felsic and hydrothermal fluids to form bismuthide- and arsenide-dominant PGM assemblages, primarily hosted in quartz and serpentine respectively.
This study shows the PGM and sulphide mineralisation at Tweefontein to be multifaceted, involving magmatic processes, assimilation of the Duitchland footwall into the Platreef magma and late-stage hydrothermal and felsic fluid activity. Footwall composition and irregular floor topography, resulting in depression areas at Tweefontein North and Tweefontein Hill, are believed to play a key role in what processes become significant along the Tweefontein strike. This research represents a significant contribution to the understanding of the distinctly different PGE mineralisation styles at Tweefontein and allows for a complete comparison of the Platreef PGE mineralisation from Overysel to Turfspruit.