Diversity of supergene gold expressions and implications for gold targeting in an equatorial regolith (AMG's Couriège exploration prospect, French Guiana)

Overview & Synopsis

The Couriège prospect (French Guiana) provides key examples of the diversity of gold distribution related to supergene processes. The nature of gold in two contrasted weathering profiles is examined as a function of the weathering lithofacies. The autochthonous profile hosts weathered Au-bearing quartz veins whereas the pediment profile is enriched in free gold issued from dismantled gold-bearing quartz veins. The gold distribution is controlled by: (i) the preservation of primary gold as free gold in both transported and autochthonous horizons and as electrum inclusions in detrital pyrite, and (ii) the formation of secondary gold through dissolution/precipitation processes, expressed as gold spherulites on free gold grain surface, Ag-depleted rim around transported free gold grains and Ag-depleted gold micro-inclusions hosted by oxidised autochthonous pyrite. Gold enrichment through supergene chemical processes remains limited within the truncated autochthonous profile. A new conceptual model is proposed for the area, defining the role of chemical and physical processes in gold endowment, and accounting for the geomorphological context. The overall evolution includes (i) deep weathering and peneplenation, (ii) dismantlement and transport of lateritic material, and (iii) development of a latosol cover. This study highlights that gold exploration in tropical environments must consider the residual vs transported nature and the inheritance of targeted pedogenic horizons.