Conglomerate is a moderately common siliciclastic sedimentary rock. Occasionally, conglomerates have economic significance, such as the gold-bearing quartz pebble conglomerates of the Witwatersrand Supergroup in South Africa.
The quartz pebble conglomerate shown below has subrounded quartz pebbles (= grayish, glassy looking pieces), small detrital pyrite grains (barely visible as brass-colored specks between the pebbles), and detrital gold (not visible here). It represents a fluvial paleoplacer deposit.
Stratigraphy & age: Vaal Reef, near-basal Krugersdorp Formation, upper Johannesburg Subgroup, Central Rand Group, upper Witwatersrand Supergroup, Neoarchean, ~2.76-2.89 b.y.
Age of detrital pyrite grains & detrital gold grains: Mesoarchean, ~3.03 b.y.
Locality - Stilfontein Mine, ~20 km east of Klerksdorp, southeastern North West Province (= southwestern Transvaal), northeastern South Africa.
Witwatersrand quartz-pebble conglomerate (6.5 cm across) from the Neoarchean of South Africa.