Transport of gold and silver in oxygen‐saturated fluids and the formation of high‐fineness gold in saprolitic supergene environments

This paper presents the results of thermodynamic calculations on the solubility of gold and silver in low‐temperature, moderately saline, oxygen‐saturated fluids. Based on the solubilities of gold and silver it is argued that the quantity of gold transported by the fluids depends on the concentration of silver in the primary ores. In ores where the silver/gold ratio is high (1 to > 10), the fluids become saturated in silver and can not dissolve geologically significant concentrations of gold. In ores where the silver/gold ratio is low (< 1), the fluids remain undersaturated with respect to silver and are able to dissolve geologically realistic concentrations of gold and silver. The oxidized fluids start depositing gold and silver as they move downwards and are reduced by the Fe⁺²‐bearing minerals of the primary ores. The occurrence of gold in lateritic profiles can be explained by a prolonged process of interaction between the fluid and primary ores, during which gold and silver precipitate and redissolve selectively at the gradually advancing oxidation‐reduction interface.     

Rounded cobbles that have not travelled far: incorporation of corestones from saprolites in the South Mountain area of southern Nova Scotia, Canada

Saprolitic palaeosurfaces occur at several localities on the granitoid rocks of the South Mountain Batholith of Nova Scotia. There are three ages of saprolites within the study area: pre-Pleistocene, pre-Triassic and pre-Carboniferous. Within these ‘in-place’ weathered horizons, there are remnant ellipsoidal blocks of unweathered granitoid referred to as corestones. These corestones are isolated rounded pods of relatively unweathered material surrounded by rotted granitoid saprolitic material. The weathered material which surrounds these corestones is poorly consolidated and easily eroded. The erosion of these horizons produces a lag deposit that contains many rounded corestones which can be incorporated into subsequent sedimentary units. The rounded boulders, cobbles and pebbles of granite within many of the Pleistocene glacial deposits in southern Nova Scotia are probably related to the incorporation of these saprolite related structures, given the locally derived (within 400 m of the source) nature of the tills. The presence of saprolites at unconformities of various ages on the South Mountain granitoid rocks suggests that incorporation of saprolitic material probably occurred along a number of palaeosurfaces in the past. The recognition of this process has implications for the interpretation of rounded granite-clast conglomerates and quartz-rich sandstones of various ages within the stratigraphic record of eastern Canada. Similar palaeosurfaces elsewhere in the world also have related saprolite derived sedimentary rocks associated with them. In summary, well-rounded spherical pebbles, cobbles and boulders of granitoid material incorporated in sedimentary strata need not have travelled far from source nor are they necessarily recycled from older conglomerates.