Element mobility in the weathering environment and surface vectors to mineralisation—A case study from the Mashitu South Cu-Co deposit, Katanga, Democratic Republic of Congo

Abstract

The Cu-Co deposits of the Katangan Copperbelt are affected by a deep chemical weathering regime. Meteoric fluids leached mobile elements from the near-surface environment and have redistributed them further down, forming remobilised supergene Cu deposits at a depth of ~30–50m which are highly economic. The same process has caused residual enrichment of less mobile elements such as Co, forming ‘cobalt caps’ in the near-surface above such supergene Cu deposits. Elements which are associated with hypogene Cu-Co mineralisation, but which are relatively immobile during supergene alteration, can make effective surface vectors to buried supergene Cu deposits; especially where these deposits have experienced a high degree of leaching and where high regional background Cu concentrations obscure surface Cu anomalies.This study uses an average-normalisation technique, applied to a large whole-rock-geochemistry database, to assess the variation in geochemistry between different depth intervals at the Mashitu South Cu-Co deposit. Geochemical variation by depth acts as an empirical proxy for element mobility during weathering, with the average concentrations of more mobile elements being depleted in the near surface profile.Of the major and trace elements which are known to be associated with hypogene mineralisation at Mashitu South, Cu, Ni, S and Zn are depleted in the near-surface environment, whereas Co, Bi, V, Mo, As and Fe experienced residual enrichment due to supergene alteration. Indeed, Bi and V exhibit an even greater degree of residual enrichment than Co, which is known as a surface pathfinder to Cu deposits in Katanga. It is suggested that Bi, V and Co would make effective surface vectors to Katangan Cu-Co deposits that have experienced a high degree of leaching; deposits which may evade detection by exploration solely for Cu surface anomalies. The average-normalisation technique utilised by this study can be used to identify immobile surface vectors to any polymetallic deposit which has been significantly affected by chemical weathering.