Relationship Between Pge and PGM in the Bushveld Complex

Abstract

The abundances of the platinum-group elements (PGE), and their relative proportions, are summarized for all the mines exploiting the Merensky Reef on both western and eastern limbs of the Bushveld Complex, in South Africa. Grades for the mined reef interval are uniform over 100 km of strike length, at 5-7 g/t, except for Union mine, where it is currently 8 g/t. The relative proportions of the various PGE are extremely constant over this same distance. In contrast, the platinum-group minerals (PGM) vary enormously. On a regional scale around the western limb, the mineralogy ranges from alloy-dominated in the northwest to sulfide-or telluride-arsenide-dominant in the southeast. Local features that are important in terms of mining are potholes and discordant ultramafic bodies. Comparative studies of the PGE from close to and within such structures again show constant absolute and relative abundances. Limited data on the PGE from PGE-mineralized pipes (Onverwacht, Mooihoek and Driekop) suggest that they are Pt-dominated, and have a totally different relative pattern of distribution of the PGE than that in the stratiform mineralization. The PGM in the vicinity of these pipes also tends to be dominated by alloys, and in potholes by alloys and tellurides and arsenides. Hence, on both a regional scale and very local scale associated with disturbances in the reef, there is a dichotomy between the constancy of the PGE and the variability of the PGM. These observations suggest that the primary mechanism concentrating the PGE is a first-order process, and that the evolution of the PGM is the result of secondary processes related to cooling, local changes in f(S 2 ) in the crystallization environment, and subsolidus re-equilibration. The debate as to whether the PGE are derived by fluid scavenging of a footwall succession or have accumulated from supernatant magma is reexamined, and unresolved problems pertaining to both processes are highlighted.Keywords: Bushveld Complex, platinum-group elements, platinum-group mineralogy, Merensky Reef, potholes, pipes, mineralization, South Africa. SOMMAIRELe niveau de concentration des éléments du groupe du platine (EGP), ainsi que leurs proportions relatives, sont examinés pour toutes les mines exploitant le banc de Merensky, dans les secteurs ouest et est du complexe de Bushveld, en Afrique du Sud. Les teneurs des sections exploitées de ce banc sont uniformes, entre 5 et 7 g/t, sur une distance de 100 km, sauf à la mine Union, où la teneur est présentement 8 g/t. Les proportions relatives des EGP sont d'une constance frappante sur cette distance. En revanche, les assemblages de minéraux du groupe du platine varient énormément. Sur une échelle régionale autour du secteur ouest, ces minéraux vont d'alliages dans le nord-ouest aux minéraux sulfurés ou bien aux tellurures et arséniures vers le sud-est. Les "nidsde-poule" et les venues ultramafiques discordantes sont des accidents locaux qui exercent une influence sur les activités minières. Des études comparatives des EGP près de ...

Platinum-Group Element Deposits in Layered Intrusions: Recent Advances in the Understanding of the Ore Forming Processes

“…These studies complemented the extensive literature available (e.g. Alapieti and Lahtinen 1986;Cawthorn et al 2002;Kingston and El-Dosuky 1982;Kinloch 1982;Kinloch and Peyerl 1990;Lee 1996;McLaren and De Villiers 1982;Prichard et al 2004). A wide range of PGM has been reported in various deposits and localities around the world (see Cabri (2002) and references therein for details) and included notably PGE-rich sulfides, tellurides and bismuthotellurides, arsenides, antimonides or alloys.…”

Variations in the Nature of the Platinum-Group Minerals in a Cross-Section Through the Merensky Reef at Impala Platinum: Implications for the Mode of Formation of the Reef

“…There have been many studies of platinum-group minerals (PGM) in the Bushveld complex, e.g., Cawthorn et al (2002b), Merkle & McKenzie (2002). The PGM assemblage is variable along strike of the Merensky Reef.…”

“…Euhedral crystals of PGM constitute the more common texture. Cawthorn et al (2002b) observed that although the PGE content of the Merensky Reef is relatively constant, the minerals that host the PGE vary considerably. In their opinion, this pattern indicates a uniform process of collection and a secondary evolution of the PGM, which varies according to conditions locally in the reef.…”

PGE Production in Southern Africa, Part I: Production and Market Trends

“…The Merensky reef consists of pegmatoidalmelanorite (70%-90% orthopyroxene) that contains base-metal sulfides and up to four chromite seams confining the pegmatoid layer [45,63,64]. The melanorite is about 10 cm to 30 cm thick, whereas the chromite seams measure 1 cm to 3 cm [45].…”

“…The occurrence of PGE elements is closely linked to the presence of Fe-Ni-Cu sulfides [44]. The PGE-bearing minerals occur enclosed in base-metal sulfides, along grain boundaries and the contact zones of sulfides and silicates or oxides [44,63]: in 38% to 97% of occurrences, PGE-bearing minerals were found to be enclosed in or attached to base metal sulfides; the remainder are enclosed in silicates [64,66]. The most commonly occurring base metal sulfides are pyrrhotite, pentlandite and chalcopyrite, with~40%,~30% and~15%, respectively [65,66].…”