Sulfide Oxidation and Production of Gossans,Ashanti Mine,Ghana

At the Justice mine, in the Ashanti goldfields of southwestern Ghana, chemical weathering of gold- bearing sulfide-rich lodes has produced a series of characteristic mineralogical and geochemical features that are diagnostic. In this type of gold mineralization, the most abundant sulfides are arsenopyrite, pyrite, pyrrhotite, and chalcopyrite with minor bornite and sphalerite. Gold occurs predominantly as native gold, spatially associated and chemically bound with arsenopyrite. Elsewhere gold-silver tellurides are present in quartz veins. During sulfide oxidation, arsenopyrite is replaced by amorphous and crystalline Fe-Mn arsenates, goethite, hematite, and arsenolite in box- and ladderwork textures. In the extremely weathered gossans exposed at surface or in exploration pits, goethite, hematite, and scorodite are present as pseudomorphs of oxidized arsenopyrite, which can be used as a visual pathfinder for gold-arsenic mineralization. As with arsenopyrite, pyrite and pyrrhotite alteration produces boxwork and ladderwork textures with the sulfide replaced by goethite, hematite, and a complex limonite. Copper sulfides and goethite replace bornite and chalcopyrite in ladder-type textures. With more intensive weathering, this assemblage is replaced by cuprite, goethite, and hematite. Gold mineralogy in the gossan is complex, with evidence of in situ precipitation of supergene gold as well as alteration of hypogene native gold. The concentration of pathfinder elements decreases in the gossan as a result of supergene leaching. Mass- balance calculations confirm that gossan production largely is in situ and, consequently, the hypogene geochemical dispersion patterns are preserved even though the proportion of many elements decreases as intensity of weathering increases.

The problem remains of discriminating between auriferous and non-auriferous gossans, or those produced by pedological concentration of iron. Although mineral textures such as box-and ladderwork replacement and mineral pseudomorphs are useful field criteria, the most reliable guide for evaluation still is trace-element geochemistry. By use of multi-element discriminant analysis, gossans of different origins can be distinguished (along with their surface expression) from ironstones and barren lateritic soils. In regional reconnaissance studies, the evaluation of trace-element geochemistry as a discriminant along with field mapping may indicate gold potential of even extremely altered products of mineralization and, in so doing, provide a basis for the classification of weathered samples.