Geology and geochemistry of the Shangmanggang red clay-type gold deposit in West Yunnan

Abstract

The Shangmanggang red clay-type gold deposit was discovered in West Yunnan in the early 1990s, which is strictly controlled by the Shangmanggang fault, belonging to a new type of gold deposits. Its red clay profile, about 20 m thick, is divided into six zones: topsoil zone, slope-wash zone, travertine zone, eluvium zone, saprolite zone, and unweathered bedrocks. The degree of laterization is relatively high in the eluvium zone where gold is very rich. Ore minerals mainly consist of quartz, goethite, and clay minerals. Sand and silt fractions are mainly composed of quartz and goethite with minor feldspar; clay fractions comprise illite, kaolinite, quartz, a small quantity of goethite, feldspar, and hydrargillite. The variation of gold content in the red clays is similar to that of sand fractions. Gold is probably closely associated with iron nodules developed in the red clays. At the same time, gold is well correlated with clay minerals and Fe, Mn oxides. Gold exists mostly in the form of dissociated gold and microfine gold, which is adsorbed on the surface or in the cracks of quartz, clay minerals, and goethite. Geochemistry characteristics of the major elements, trace elements, and REEs in the red clays of the Shangmanggang red clay-type gold deposit in West Yunnan were studied in detail. The results show that Al 2O 3, Fe 2O 3, and K 2O were migrated and precipitated together in the red clay profile, but the case is not true for SiO 2. As a whole, SiO 2 is high in content, desilication is incomplete, and the degree of laterization is low in the red clays, except for a high degree of laterization in the eluvium zone. In contrast with the sedimentary rocks and altered rocks, the red clays come from the Mengga Formation mudstone with laterization. Au, Hg, As, Sb, Cu, Pb, Zn, and Mo are coextensive trace elements in the red clay profile and are significantly enriched in the eluvium zone where laterization is relatively complete. The crust-normalized large-ion-lithophile element patterns in the red clays are similar to those of both altered rocks and the Mengga Formation. The chondrite-normalized REEs patterns in the red clays are also similar to those of both altered rocks and the Mengga Formation. The LREEs are enriched, and gold is concentrated in the location where REE fractionation is strong. Our work reveals that gold comes mainly from the altered rocks, and the red clays are mainly derived from the Mengga Formation and altered rocks.