Exposures of the Eldorado Conglomerate which occurs in the late Archaean (c. 2.7 Ga) Chinhoyi Greenstone Belt. 

 

The Eldorado Conglomerate is a deformed polymictic diamictite, up to about 50m thick, containing cobbles and boulders (up to 2m in length) of granitoids, greenstones, porphyries, chert and BIF, which occurs within a pyroclastic sequence of ash-fall, lapilli and lithic tuffs and agglomerates, which in turn overlies pillowed mafic greenstones and banded iron-formation. The Eldorado diamictite is regarded as a possible lahar, with rounded clasts of different lithologies ripped up from the wallrocks in a violent volcanic eruption, mixed with huge volcanic bombs that are flattened.  It was the host rock for the Eldorado Gold Mine which in the first decade of this century was the largest gold mine in the then Southern Rhodesia (now Zimbabwe). The Eldorado Mine declared a total production of 492 211 ounces of gold between 1906-1944, at an average grade of 10.44 dwt/ton. The mineralized zone was about 400' wide, and 2500' deep, and there were two parallel "reefs" or ore shoots, between 70" and 140" thick. The gold was free milling, with very little sulphide. Considerable interest and geological debate was sparked off after its discovery in 1894, when it was initially thought to be a Witwatersrand-type reef or "banket" (Mennell, 1905; Gregory, 1906). Stagman (1961) gives the following account of the debate:

 

"The conglomerate was the subject of much controversy. When the ore body was found it was at first believed to be a fossil placer analogous to those on the Witwatersrand. It was named "the banket", which led to great excitement as it was concluded that the mineralization would extend for miles on strike and down to thousands of feet in depth. The rock was traced for many miles to the south-west and north-east, and some 20 000 claims were pegged, which represent 2000 average blocks, banked five to ten deep, covering 50 miles of strike. This is, indeed, the picture presented by the old claims plans, when all the country was pegged from south of the Angwa river, near the Maggiemac mine, round in a curve to the Great Dyke, beyond the Muriel Mine."

 

"It soon became apparent that, away from the Eldorado mine, very little of the conglomerate was auriferous. The suggestion was then advanced that the rock at the mine was not a sediment, but a rolled breccia of some sort."