Gold in weathering mantle of the Guiana shield (South America)

Abstract

The article describes mineral composition of laterite weathering mantle overlapping the Amamuri and Kontakt gold deposits in the greenstone rocks, located 20 km southeast of the Aurora deposit (reserves of 185 tons Au, Republic of Guyana). The major minerals of the weathering mantle include disordered kaolinite (35–90 wt. %), relict quartz and hydromica (up to 20 wt. % illite). The heavy concentrate is represented by limonite (up to 8 wt. % goethite), magnetite, hematite, ilmen-ite, anatase, ferrous rutile, psilomelane, pyrite, chalcopyrite, galena, sphalerite, covellite, zircon, tourmaline, epidote and amphibole. Native gold composes signifcant amount of heavy concentrates (up to 9 wt. %) and includes the following types: 1) lode gold in relict quartz veins and stockwork, 2) relict gold in laterite and 3) supergene gold. According to mineralogical studies, moderate late-ritization and monosialitization processes are probably due to erosion of ancient areal weathering mantle and the presence of immature laterites over the Amamuri and Kontakt deposits. Most amount of gold is relic. The processes of its redeposition and supergene alterations are weak. The dominant size of gold grains of <100 µm makes gravitational concentration of laterites more difcult, whereas weakly ordered kaolinite prevents agglomeration during cyanidation and heap leaching. Alluvial placers are probably more promising for exploration, since they provide natural fractionation of gold particles by size favorable for gravity extraction.