Quartz pebble conglomerate

from the Witwatersrand goldfields, South Africa

Figs. 1-2: In Figs. 1-3, the QPC (quartz pebble conglomerate) matrix has fine-grained granular pyrite, pale sericitic mica and white to very dark grey quartz (the latter quite possibly due to radiation damage, a result of elevated uranium content?). Pyrite also forms somewhat irregular veins up to circa 1 cm in thickness. Here are (1736, left) a large piece of QPC and (1737, right) a second sample with a prominent vein of cross-cutting granular pyrite. Both are samples of the Upper Elsburg A2 horizon, a sulphidic QPC, from the Randfontein Estates, Cooke No.3 shaft, level 106. The A2 QPC, very sulphidic, is the best-mineralized of the Elsburg reefs, with up to 40 ppm Au. In the large 1736 piece, greasy grey quartz occurs as cm-sized clasts in a dark green chloritic matrix shot though with abundant tarnished py. Bands and layers of fine-grained, recrystallized pyrite may rim the quartz pebbles. 1737 was found in a stope accessed from level 106. The stope contained QPC ore cut by barren white quartz veins 1-7 cm thick. The sample is QPC with dark grey to white quartz pebbles, some elongated, in a matrix rich in fine-grained recrystallized pyrite. The pyrite is mostly of 1 mm gs, disseminated and in bands several mm thick.
Hand specimens 1736 (in situ) and 1737 (loose) collected from the mine, 15 November 1995. A large piece equivalent to 1737 was presented to the collection of the late Ron Seavoy.

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

The Largest Goldfield in History,

IN PREP.
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 national output had fallen from first to 12th amongst the countries of the world (World Gold Council ranking).

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 qz 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.

REFERENCES

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IN PREP.

DRAFT version, Graham Wilson, 08 October 2025

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