Morphology of Witwatersrand gold grains from the Basal Reef; evidence for their detrital origin

Abstract

Thousands of rounded particles of fine-grained gold, released from a crossbedded gold specimen of the Basal reef placer in the Welkom gold field after acid digestion of the host quartz arenite, have revealed morphological features and abrasion textures that are characteristic of detrital gold. The size range, which is between 0.038 and 0.473 mm, has a lognormal distribution with a mean of 2.88 phi units (0.136 mm) and a standard deviation of 0.48. This is a characteristic of well-sorted sediment. The shapes are predominantly disclike with toroidal forms being most common. A Corey shape factor of 0.08 indicates extreme flattening. The preponderance of these shapes is interpreted to be the result of selective sorting by entrainment of particular shapes during extensive transport.A knobby surface texture similar to placer grains recovered from both recent and Devonian placers, and produced during laboratory-controlled tumbling with sand and cobbles in water, is considered distinctive of sediment abrasion. Secondary gold, comprising 25 percent of the total gold content, is finer grained, has an interstitial jagged form, and displays unabraded smooth to crystalline surfaces.Study of this remarkable specimen has produced unequivocal evidence of the detrital origin of gold in the extensive Basal reef placer and led to research concerning its metamorphic overprints.