Abstract

Stratabound mineral deposits, particularly of gold and associated sulfides, are widely distributed within beds of banded iron-formation in the Archean greenstone belts of Rhodesia. The occurrence of gold at the Vubachikwe gold mine has been studied together with samples of mineralized iron-formation from eight other deposits. These are the Beehive, Camperdown, Connemara, Empress, Marvel, Nelly 404, Pickstone, and Sherwood Starr mines.At the Vubachikwe mine the orebodies occur in several thin beds of banded iron-formation which are interlayered with mafic and felsic aquagene tuffs. These rocks form part of a mafic-ultramafic volcanic assemblage characteristic of the Sebakwian Group and are overlain by basaltic and andesitic lavas and tuffs characteristic of the Bulawayan Group. The orebodies are stratiform and are confined to beds of sulfide and mixed sulfide-carbonate facies banded iron-formation, which consist of layers alternately rich in quartz (cherty), arsenopyrite, pyrrhotite, and ankerite. The gold is occluded within arsenopyrite grains and the average ore grade is 11 ppm. The petrographic and structural evidence indicates that the gold and sulfides are premetamorphic and predeformation in age.It is proposed that, for all the deposits studied, the gold, the sulfides, and the iron-formation are consanguineous. Experimental evidence on gold solubility and data from active fumarolic-stage geothermal areas support the view that thermal brines are capable of transporting significant quantities of gold and other metals to the surface as soluble complex ions. A volcanogenic model for the origin of banded iron-formation, the sulfides, and the gold is favored. Such deposits are considered to represent submarine chemical precipitates deposited from solutions extruded from subaqueous (and possibly subaerial) active fumaroles.There is a regional stratigraphic control to the distribution of these Stratabound mineral deposits, most of which occur in iron-formation associated with the lower mafic-ultramafic volcanic assemblage (Sebakwian Group).

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