Sedimentology and gravel bar morphology in an Archaean braided river sequence: the Witpan Conglomerate Member (Witwatersrand Supergroup) in the Welkom Goldfield, South Africa

Abstract

Abstract The Witpan conglomerate member is the lower, thicker, more channelized of the two auriferous conglomerates that comprise the Archaean ‘A’ Reefs (Aandenk Formation, Central Rand Group, Witwatersrand Supergroup) mined in the southern part of the Welkom Goldfield, Republic of South Africa. It is interpreted as the deposit of a degrading, gravelly, braided river system with well developed topographical differentiation. Three morphological elements have been identified in the Witpan Conglomerate member, namely Major Channels, Minor Channels and Terrace areas which are comparable to Levels 1, 2 and 3 of the modern Donjek River. Nine facies are present in the Witpan Conglomerate member combined into eight facies assemblages which show differing three dimensional architecture and relationship to channels. The four conglomeratic bar facies assemblages (FA1 to FA4) are interpreted as longitudinal, diagonal, transverse and side bars respectively and are combined to form compound bars. The three quartzitic channel fill facies assemblages (FA5 to FA7) represent active, slough and abandoned channel fills. The eighth facies assemblage (FA8) forms the interchannel terrace areas. The distribution of gold mineralization in the Witpan Conglomerate member is related to sedimentological factors including the presence of massive gravels, the accessibility of these gravels to flood water and the formation of areas of confined, convergent flow. Hence the bar facies assemblages are the richest followed by channel fill facies assemblages with the interchannel facies assemblage having generally low gold concentrations. Of the bars, diagonal bars are the most favourable for gold mineralization followed by longitudinal bars then side bars with transverse bars being the lowest.