Supergene mobilization and redistribution of platinum-group elements in the Merensky Reef, eastern Bushveld Complex, South Africa

Abstract

Near-surface supergene ores of the Merensky Reef in the Bushveld Complex, South Africa, contain economic grades of platinum-group elements, however, these are currently uneconomic due to low recovery rates. This is the first study that investigates the variation in platinum-group elements in pristine and supergene samples of the Merensky Reef from five drill cores from the eastern Bushveld. The samples from the Richmond and Twickenham farms show different degrees of weathering. The whole-rock platinum-group element distribution was studied by inductively coupled plasma-mass spectrometry and the platinum-group minerals were investigated by reflected-light microscopy, scanning electron microscopy, and electron microprobe analysis. In pristine (“fresh”) Merensky Reef samples, platinum-group elements occur mainly as discrete platinum-group minerals, such as platinum-group element-sulfides (cooperite–braggite) and laurite as well as subordinate platinum-group element-bismuthotellurides and platinum-group element-arsenides, and also in solid solution in sulfides (especially Pd in pentlandite). During weathering, Pd and S were removed, resulting in a platinum-group mineral mineralogy in the supergene Merensky Reef that mainly consists of relict platinum-group minerals, Pt-Fe alloys, and Pt-oxides/hydroxides. Additional proportions of platinum-group elements are hosted by Fe-hydroxides and secondary hydrosilicates (e.g., serpentine group minerals and chlorite). In supergene ores, only low recovery rates (ca. 40%) are achieved due to the polymodal and complex platinum-group element distribution. To achieve higher recovery rates for the platinum-group elements, hydrometallurgical or pyrometallurgical processing of the bulk ore would be required, which is not economically viable with existing technology.

Mineralogy and mineral chemistry of detrital platinum-group minerals and gold particles from the Elbe, Germany

“…In general, the PGMs are of primary origin and formed during hightemperature processes as shown by other studies (Cabri et al, 2022a). The secondary processes caused compositional modifications of PGMs under supergene, low-temperature conditions, as similarly observed in laterites and near-surface PGE ore bodies (Aiglsperger et al, 2017;Bowles, 1986;Bowles et al, 2017;Junge et al, 2019;Korges et al, 2021;Locmelis et al, 2010;Oberthür, 2018;Oberthür et al, 2013b;Oberthür and Melcher, 2005;Zaccarini et al, 2013). The different mobility of metals results in a distinct compositional pattern and zoning of PGM grains.…”

Metal Mobility in Embryonic-to-Proto-Ni-Laterite Profiles from Non-Tropical Climates

“…The parent rocks from the two studied profiles both had an UMIA index of 4. This index reached values between 11 and 18 in Los Reales, whilst it was much higher in the lower (53)(54)(55)(56)(57)(58)(59)(60) and upper (73-83) oxide zones of Camán, respectively (Figure 17b; Table S1). The low UMIA values in the Los Reales profile are characteristic of saprolite originating during early laterization.…”

“…In these conditions, Pt may form aqueous hydroxide species but is less mobile than Pd in surface environments due to the higher oxidation potential to enter into solution [51][52][53]. The relatively soluble behavior of Pd (and to a lesser extent Pt) in near-surface conditions suggests that Pd may be liberated during the initial stages of weathering from host minerals (PGM, Fe-Ni sulfides, and native Au) to be incorporated into porewater as, e.g., Pd 2+ (OH) 2(aq) and then partially mobilized throughout the profile [54][55][56][57][58]. This may result in Pd (and Pt) being incorporated or trapped onto clay and oxy-hydroxide-rich horizons or being reduced by Mn or organic matter compounds, due to their high surface area and highly negatively charged interfaces [2,[59][60][61][62][63].…”