Stewart R. Wallace — 1919–2009 - Society of Economic Geologists

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16 SEG NEWSLETTER No 78 JULY 2009 ... from 15 base-metal sulfides in deeper ore (Wagner, 1929). Some areas were trial mined, but low metal prices in 1929 resulted in closure of most operations in the Bushveld, including those on the pipes. The one exception was Kroondal, where mining of the thin yet extraordinarily high grade Merensky reef (ore was hand sorted prior to milling) proved to be viable (Wagner, 1929). THE PGE DEPOSITS Discovery and Geology of the PGE Deposits of the Bushveld Complex, South Africa (Continued) The UG2 reef The UG2 reef is a textbook example of a stratiform deposit: mineralization is constrained to a layer of chromitite, albeit locally with one or more barren partings of feldspathic orthopyroxenite, which may vary from a few centimeters to several meters thick (Fig. 5A). The UG2 is persistent in some sectors of the eastern and western limbs for tens of kilometers along strike and to depths of at least several kilometers. The footwall and hanging-wall rock units reveal considerable lateral variation, but, typically, the chromitite occurs in association with feldspathic orthopyroxenite and noriteanorthosite. Mining of a narrow reef, particularly in high tonnage operations, is a specialized business and in the case of the UG2 is exacerbated by problems caused by potholes (Lomberg et al., 1999), areas from a few meters to several hundreds of meters in diameter where part of the footwall sequence is absent. Dilution resulting from hanging-wall collapse due to planes of weakness associated with thin chromitite stringers (“leader seams”) is an additional problem. Ore reserve calculations, excluding losses due to potholes and dilution, are, however, relatively simplistic, despite evidence of a pronounced nugget effect. The double peak of mineralization (bottom and either middle or top loaded, depending on layer thickness) is persistent throughout the intrusion, and is defined by total PGE and interelement ratios (Hiemstra, 1986). The antipathetic relationship between grade and thickness has been widely remarked upon (e.g., Viljoen and Schurmann, 1998). The footwall lithology of the UG2 varies from anorthosite, leuconorite, or orthopyroxenite to pegmatoidal feldspathic pyroxenite. Minor amounts of PGE are located in the footwall, specifically if it is pegmatoidal, but are typically directly associated with disseminations or stringers of chromitite. The irregular basal contact of the chromitite FIGURE 5. Vertical profiles of some typical reefs: (A) UG2 reef from the Marula mine, Winnaarshoek; (B) Merensky “thin-reef” from the Brakspruit section, Rustenburg Platinum mine (after Viljoen and Hieber, 1986); (C) Merensky “wide-reef” from the Marula mine, Winnaarshoek. when the footwall is pegmatoidal contrasts sharply with the planar upper contact. The chromitite layer may be thinner in areas where the pegmatoid contains abundant chromitite, an observation the authors have made at the Crocodile River mine, where large areas of the UG2 are underlain by leuconorite. The grade of the UG2 is typically 4 to 8 g/t PGE+Au. The content of base-metal sulfides is typically very low (avg
JULY 2009 No 78 SEG NEWSLETTER 17 may be preserved (>50 g/t). In some areas, PGE are associated with interstitial base-metal sulfides that extend into the footwall (Cawthorn, 1999). In some parts of the eastern limb, cryptic layering occurs on a scale of centimeters in a wide-reef facies (Fig. 5C), as described in the Winnaarshoek locality by Mitchell and Scoon (2007). In the wide-reef facies, two chromitite stringers constrain the width of the mineralized reef zone, as is typically the case, but they are separated by a layer of feldspathic orthopyroxenite that is relatively thick (1.8 m on average) and includes a barren middling. Moreover, the lithology between the chromite stringers throughout most of the western limb is a pegmatoidal orthopyroxenite, an unusual assemblage the origin of which has been much debated (e.g., Cawthorn and Boerst, 2006), whereas in the eastern limb the principal layer of pegmatoid occurs below the mineralized zone. The average grade of the Merensky reef is similar to the UG2, al though it is far more variable. Typically, a much larger number of PGM species occur (Kinloch and Peyerl, 1990) and lateral variation of the PGM contrasts with the regularity of the primary layering. PGM are spatially associated with base-metal sulfides (1–2 %), constituting approximately equal abundances of pyrrhotite, pentlandite, and chalcopyrite; the relative paucity of pyrrhotite is unusual (Liebenberg, 1970). Base-metal sulfides are an important by-product from the Merensky and assist with downstream smelting operations, whereas smelting of sulfide-poor UG2 concentrates is problematic. The Platreef The Platreef is restricted to a relatively small area of the northern limb, and is typically located a few tens of meters above the base of the intrusion, where the floor ranges from Archean granite to sedimentary rocks of the Transvaal Supergroup. The lowermost zones of the Rustenburg Layered Suite are not developed in this area. The Platreef is far more irregular than the UG2 and Merensky, is typically much thicker (up to 100 m or more), and contains PGE that are invariably associated with base-metal sulfides throughout (Viljoen and Schurmann, 1998). The Platreef is also characterized by a much lower Pt/Pd ratio than the Merensky and UG2 reefs, although the lower average grades are offset by the width of the reef. The Platreef may constitute a localized equivalent to the Merensky reef, and at the Tweefontein locality, a few kilometers to the south of the Sandsloot mine, the mineralized sequence is relatively thin and includes both chromitite stringers and a pegmatoidal pyroxenite (Viljoen and Schurmann, 1998). We find the importance of footwall contamination to the Platreef has been overstated, in part as earlier studies were restricted to an area where the floor rocks are dolomite (White, 1994). A new terminology may assist with explaining this: the sequence below the (barren) Main zone should be categorized as a Platreef unit rather than the “Platreef.” This consists of gabbronorite, feldspathic websterite, feldspathic harzburgite, and reconstituted feldspathic pyroxenite, the latter revealing a pegmatoidal or glassy texture. These rock units are in discordant intrusive relationship with one another and with the overlying Main zone. Thus the sequential stratigraphy (A, B, and C reefs of earlier workers) is inappropriate, yet despite this, a Main mineralized layer (analogous to the “B” reef?) can generally be identified. At the Akanani locality, where deep drilling has revealed a relatively thick Platreef unit downdip from the open pit Sandsloot deposit, we found PGE to be most abundant in sinuous layers of harzburgite and, to a lesser extent, reconstituted pyroxenite; the gabbronorite and feldspathic websterite are relatively weakly mineralized. The pipe deposits The pipe deposits are no longer of commercial interest and, paradoxically, have a negative affect on reef-type mines, as they disrupt the layered wall rocks. PGE were concentrated in small core zones (max diam 24 m) of coarsegrained, iron-rich dunite and wehrlite (Wagner, 1929), now mostly mined out. The bulk of each pipe (diam >300 m), however, is dominated by barren magnesian dunite. Barren outer envelopes also occur, and our unpublished mapping at Mooihoek demonstrates this component is even more extensive than the magnesian dunite. Bushveld pipes that do not reveal this zonation are typically barren (Viljoen and Scoon, 1985). LOW-GRADE PGE DEPOSITS An understanding of the economic deposits is incomplete without mention of low-grade ores in the Bushveld. These include all the chromitite layers located below the UG2 reef, as well as the Pseudoreefs that are situated between the UG2 and Merensky in the northwestern Bushveld. The Bushveld chromitites reveal regular upward trends in both their composition (e.g., decrease in the Cr/Fe ratio) and the PGE grade and tenor (Scoon and Teigler, 1994). These chromitite layers, however, report very low contents of sulfide. These data are important as they demonstrate that the UG2 and Merensky reefs do not occur in isolation but represent the culmination of a general upward increase in the PGE content of mineralized layers within the Rustenburg Layered Suite. This trend is disrupted by the low-grade, harzburgitic Pseudoreefs. The bifurcation and subsequent elimination of discrete layers of harzburgite (bounded top and bottom by stringers of chromitite) is indicative of rejuvenation of more primitive magmas that were intruded laterally and which may be interpreted as precursors to the Merensky reef (Scoon and De Klerk, 1987). GENESIS AND CONCLUDING REMARKS The holistic approach advocated by Wagner (1929) is important to our understanding of the PGE deposits in the Bushveld Complex. There is a marked spatial association of PGE with chromitite and other ultramafic rocks that have sharply defined and demonstrably discordant basal contacts, and not uncommonly sharp upper contacts. Norite-anorthosite is almost invariably barren, except in isolated cases. These relationships, in addition to the restriction of economic ores to the Upper Critical zone, are unlikely to be coincidental. Rather than the model of Camp - bell et al. (1983), in which the resident magma column provides the PGE, we believe field relationships and mass balance considerations are consistent with the “lateral mixing hypothesis” of Scoon and Eales (1989) and Scoon and Teigler (1994). Thin, hot layers of fresh, ultramafic magma, enriched in PGE (and chromite: Eales, 2000) streamed laterally into the different chambers of the intrusion. In the case of the Merensky reef, as well as some other units, the new, U-type magma was intruded into an earlier-formed crystalline substrate of to page norite-anorthosite 18 ...
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Stewart R. Wallace — 1919–2009 - Society of Economic Geologists

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