"Rock of the Month # 292, posted for October 2025" ---

The Largest Goldfield in History,

The Witwatersrand of South Africa was for decades the world's unchallenged, pre-eminent supplier of gold. I am no expert on the "Wits", but the fact that the MINLIB bibliography has 462 records on the region, 1903 onwards, is a testament to the economic might and geologic significance of the region. In recent years the Wits mines, some of which reached record depths, had lost some of their shine, and as of 2024 the South African national gold mine output had fallen from first to 12th amongst the countries of the world (World Gold Council ranking).

The Geology of the Witwatersrand

The classic map of the Witwatersrand goldfields in the Johannesburg region shows a region extending from Droogefontein in the east to Elandsfontein in the west, including the Langlaagte farm where the key historic gold discovery was made (Mellor, 1916). The Witwatersrand gold fields are storied mines, described in some classic works (e.g., Rickard, 1944, pp.366-402; Cloos, 1953, pp.54-61) and detailed reviews (Antrobus, 1986; McCarthy, 1990; Handley, 2004; Frimmel and Nwaila, 2020). The wider region includes a number of individual gold fields: the East, Central, West and South Rand, plus the Klerksdorp field, West Wits Line, Orange Free State (Welkom) and Evander fields. The "Wits" (or, the Rand) is notable in several regards:

1. Prodigious cumulative gold production
2. Great depths of some of the mines in their heyday
3. Prolonged debate over the origins of the gold deposits.

1. Production

There was unmatched gold production from a small region within the larger Wits sedimentary basin (the "golden arc" with all the mines is only 8,000 km² in area), and production peaked at about 1,000 tonnes/year in 1970 (Handley, 2004). The incomparable riches of the Wits are such that from 1886 until the end of 1987 the area produced a total of 41,700 T Au and 146,000 T U₃O₈ from mineralized quartz-pebble conglomerates, quartzites and thin layers of kerogen (Pretorius, 1991). By 2020, the Wits had produced some 53,000 tonnes of Au metal, close to one-third of the gold mined throughout history, since the initial finds in 1884-1886. The Vredefort dome, the largest or second-largest verified major terrestrial impact structure, is essentially in the centre of the Wits sedimentary basin, which extends circa 350x200 km in area, oriented SW-NE (Frimmel and Nwaila, 2020). Much may remain, but the increasing depths, harsh underground conditions and evolving socioeconomic order may render it uneconomic to recover, despite unprecedented recent (2025) surges in the bullion price.

2. Depths

Rickard (1914) noted that a range of metallic ore deposits (silver, gold, copper) may extend and be mined for thousands of feet below surface. At that time the deepest mine on the Rand was the Jupiter, at 5,040 feet. Deep underground mines, say more than 1 mile (1609 m) deep, have been with us for more than a century, whether seeking copper in northern Michigan or gold or base metals in parts of Canada and elsewhere. For a long time the gold mines of the Kolar greenstone belt in south India were amongst the deepest, attaining a depth of >11,000 feet. By the 1990s, the deepest mine on the world was Western Deep Levels in the Wits. This mine produced 1.32 million ounces of Au in 1991, reaching a depth of 12,730 feet (3,880 metres, almost 2 1/2 miles below surface --- where the rock temperature is 131°F, or 55°C: Anon, 1992).

3. Geology and Origins

The historic view of the Wits gold deposits ascribed to them a paleoplacer origin, that is, that these sediment-hosted, quartz-rich and pyritic gold deposits had formed by deposition of placer (fluvially transported) gold and associated heavy minerals (uraninite, zircon, magnetite...), washed down from some distant, primary source, and that these gold-rich gravels were then buried and preserved. In the 1980s in particular, some argued for a different genesis, involving hydrothermal alteration and metamorphism, perhaps with remobilization and redeposition of syngenetic gold, a "modified paleoplacer" option.

The mineralogy of the Wits sequence is in volumetric terms dominated by a few common minerals - but the list is long, both heavy minerals (including ore minerals) and silicates. The great Paul Ramdohr studied ore minerals of both gold and uranium, notably concentrated in the Wits strata (mine tailing dumps near Johannesburg are pale yellow because of oxidized secondary minerals of uranium). Ramdohr (1958) examined 140 polished sections from across the camp: he noted that U minerals, when ill-characterized, may be termed uranpecherz (including both colloidal, low-temperature pitchblende and crystalline, high-temperature uraninite). Carbonaceous matter, pyrite and native gold are of course critical components, and besides there is a diversity of other sulphides, sulphosalts, oxides, traces of platinum group minerals such as iridosmine and native Pt, as well as zircon and unusual silicates such as chloritoid (p.41). Oelsner (1961), in his review of ore minerals, included pyrite, pitchblende, gold and "anthraconite" (hydrocarbon) of the Wits. The pyrite exhibits a range of crystal habits and clearly occurs as more than one generation of sulphide.

In the classic, synsedimentary, paleoplacer model, the Witwatersrand is seen as a fluvial fan developed at the mouth of a major river flowing from a northwesterly source area, depositing sediment into a "shallow-water, intermontane, intercratonic lake" (Pretorius, 1975). Pretorius (1991) noted that the Wits strata were deposited between 3100 and 2750 Ma "on braid deltas and braid plains marginal to a northeasterly trending shallow-water lake": arguments around the genesis of the mineralization centre on the origins of four minerals: native gold and uraninite, pyrite and kerogen, while "another 70 ore minerals also contribute to the controversy but rarely advance to the center of the stage". Syngenetic paleoplacer arguments are based largely on sedimentological evidence, whereas epigeneticists envision movement of ore fluids through the more permeable clastic strata, pointing to evidence of regional metamorphism and hydrothermal fluid migration (Phillips and Law, 1994, 1997; Law and Phillips, 2005).

There are some clear similarities with the early Proterozoic, Huronian Supergroup of central Ontario, Canada, which hosts the low-grade uranium deposits of the Elliot Lake camp in Algoma district, and traces of other metals such as copper and gold. Within the Huronian, the evolution of an oxygenated atmosphere is dated at 2288 Ma in the upper Gowganda Formation. Prior to this event, available Au was deposited under reducing conditions, perhaps with transport of the finer material as organic-protected colloids, as suggested for the Wits; after the event, Au deposition will have been subject to secondary enrichment, as in modern placers. The enrichment of Au in kerogenous marker seams in the Wit basin reflects the action of prokaryotic organisms. Modern prokaryotes are capable of precipitating or flocculating Au, and the metal is also enriched in kerogen or "thucholite" in lower Huronian metasediments, hydrocarbon similar to material from the Vaal reef of the Wits (Mossman and Harron, 1983; Mossman and Dyer, 1985).

Native gold is relatively late in the paragenesis, that is, the time sequence of deposition of the various minerals described from the Wits by Ramdohr and others. That does not necessarily imply a distant, hydrothermal origin for the Au itself, as the observations may simply imply a short-range mobilization of in situ detrital gold (a modified paleoplacer model; Frimmel et al., 2005). A recent synthesis of the abundant evidence (Frimmel and Nwaila, 2020) sees the unique gold endowment as primarily syngenetic, the result of a late Archean mineralizing event circa 2900 Ma, followed by erosion of the Archean land surface, and the rise of photosynthetic microbes that could trap fluvial and perhaps shallow marine gold, leading to paleoplacer Au deposition in microbial mats. The Wits gold was preserved by exceptionally good armouring of continental sediments by flood basalt and a late impact melt sheet (the 2023 Ma Vredefort event), with little tectonism in the subsequent 2 billion years.

Figs. 3-4: Here (left) is a third hand specimen of conglomerate, sample 1738, which displays pyritic QPC, with striking nodular pyrite. Sample from Randfontein Estates, a polished slab donated by Johannesburg Consolidated Investment Ltd. In this slab, the QPC is composed largely of a large (>7 cm) rounded quartz pebble, the margins of which are surrounded by striking cm-sized nodular pyrite masses. The latter, sometimes fractured, are up to at least 20x16 mm in cross section. On the right: sample 1713, from the Western Deep Levels mine. In this small (35x25x8 mm) sample of dull black thucholite hydrocarbon, tiny flecks (<<1 mm) of native gold are visible on the thin margins, in hydrocarbon fibres running normal to flat face of sample. Silky lustre on one flat face, rougher on opposing face.
Sample 1738, a polished slice, a gift to visitors from the firm, 15 November 1995. Thucholite sample 1713 from David New, 18 September 1995.