Geomorphological evolution and supergene gold ore at Posse, Goias State, Brazil

Abstract

At Posse, central Brazil, primary gold mineralization occurs near the base of a sequence of metamorphosed volcanic and volcaniclastic rocks. During the Velhas geomorphological cycle (Oligocene to Lower Pleistocene) the region experienced extensive lateritization. Subsequent incision of the Velhas surface under predominantly humid conditions led to degradation of the lateritic profile and pedogenesis. The present weathering profile above the mineralized bedrock at Posse is 15–18 m thick. An investigation of gold distribution from bedrock through the weathering profile reveals that: (a) the updip continuation into saprolite of a section of highly mineralized felsite is slighlty enriched in gold and shows greater gold grade homogeneity compared to the parent material; and (b) gold enrichment in the overlying soil is particularly pronounced in a Stone Line horizon. The observed gold distribution in the weathering profile has arisen through saprolitization during the Velhas cycle, then leaching, compaction and downward (and limited lateral) transport of gold during pedogenesis. The climatic and geomorphological history of central Brazil has led to the formation of a low grade supergene gold orebody in the weathering profile at Posse.