Occurrence and distribution of invisible gold in the Shewushan supergene gold deposit, southeastern Hubei, China

The Shewushan gold deposit, eastern Hubei, China, hosted in weathered mantl e above the Shewushan thrust zone, is probably an example of a type of depo sit where all the ore-grade gold is related to surficial weathering process es of a previously uneconomic gold deposit. Electron-probe micro-analysis ( EPMA) and analytical electron microscopy (AEM), optical microscopy, and che mical dissolution were done to determine the occurrence and distribution of gold in the ore. The optical microscopy study demonstrates that gold is in visible in ordinary light. Fire assay yielded 4.34 g/t Au for clay-mineral separates. The AEM analyses show that gold associated with clay minerals oc curs as submicrometric particles at the rim of clay-mineral grains; EPMA da ta show that gold in goethite occurs as micrometric granules of native gold or as an adsorbed phase (or both). The gold content of other minerals, suc h as quartz, "chalcedony" and barite, is generally below the detection limi t of EPMA (0.05%). The characterization of invisible Au in this supergene g old deposit is beneficial to the design of a metallurgical process for Au r ecovery and to our understanding of the transport and accumulation of Au du ring the process of weathering under supergene conditions.