The morphology and silver content of gold from the Upper Witwatersrand and Ventersdorp Systems of the Klerksdorp gold field, South Africa

Abstract

The morphology of the gold from the Proterozoic auriferous and uraniferous sediments of the Upper Witwatersrand and Ventersdorp Systems of the Klerksdorp gold field (South Africa) was studied by means of scanning electron microscopy. Three morphological types of gold could be distinguished; (1) detrital gold, (2) recrystallized gold, and (3) gold associated with carbonaceous material. The silver content, expressed in fineness, of the gold from various stratigraphic positions was determined by an energy dispersive X-ray microanalyzer attached to the scanning electron microscope.Initially the detrital gold was deposited after being transported not farther than 30 km from the primary source, as revealed by comparing the morphology of the detrital Witwatersrand gold with gold from modern alluvial placers. The second gold type derived from small-scale recrystallization of the detrital gold. Primitive, lichenlike plant mats partly trapped detrital gold in their filaments so that gold was incorporated into their plant structures, forming gold of type 3.The fineness of all three types of gold is identical. No selective redistribution of silver took place during the formation of gold of types 2 and 3 from the detrital gold. The silver content of the gold varied significantly according to stratigraphy. Two different facies, according to the fineness of the gold, were detected in the Vaal Reef, the main mineralized horizon of the Klerksdorp gold field.Because of the oxygen-deficient, reducing environment during Witwatersrand times, no silver was leached from the gold during its fluviatile transport. The changes in fineness of the Witwatersrand gold can be attributed to changes in the silver content of the primary gold deposit which was situated most likely in Archean greenstone-granite areas surrounding the Witwatersrand Basin.It was found that gold in economically important stratigraphic units of the Klerksdorp gold field contains relatively small amounts of silver. This tendency is already typical for primary Archean gold deposits. The changes in fineness of the gold in the Proterozoic placer deposit of the Klerksdorp gold field can be correlated with sedimentological events and to changes in the fineness of the primary gold in the probable source area.The fineness of gold in early Precambrian placers, therefore, represents an important criterion in the determination of the origin and depositional history of such deposits.