Andrew P. Gize

The role of fulvic acid in the supergene migration of gold in tropical rain forest soils

Abstract In tropical terrains, gold is released during lateritic weathering from the breakdown of hypogene ore. Soil organic matter provides potential ligands for gold complexing, of which fulvic acid (FA) is the dominant mobile component. At the Ashanti mine, southwest Ghana, a strong correlation between gold and soil organic matter exists. Dissolution studies show that Au, Au-Ag alloy, calaverite, and KAuCl 4 are dissolved by soil fulvic acid. Fulvic acid dissolution increases with pH with a 25 mgL −1 fulvic acid solution leaching 156 μgL −1 Au from KAuCl 4 over twenty-eight days at pH 3 and 477 μgL −1 Au at pH 12. Fulvic acid composition is shown to influence the potential to complex gold. High-sulphur fulvic acid (4.2%S, 2.7%N, 41.1%0) solubilizes more gold than high-nitrogen fulvic acid (0.9%S, 4.9%N, 46.7%O), and more than high-oxygen, low-sulphur fulvic acid (0.7%S, 4.4%N, 53.9%O) at the same pH. From Fourier transform-infrared (FTIR) spectroscopy and titration studies, gold is suggested to be incorporated into fulvic acid in a pseudocrystalline form, probably as a colloid at low pHs. The binding mechanism involves initial formation of a gold complex, possibly a hydroxy complex, by an ion-exchange mechanism. This intermediate complex is then slowly reduced by fulvic acid to a gold-fulvate colloid. In alkaline solutions, gold is fixed by reduction of an intermediate species and chelation with gold bound to S-ligands. Given the low ratio of gold in natural soil waters at Ashanti (0.01−84.4 μg L −1 ) to fulvic acid (15 mgL −1 ) the greatest control on gold-fulvate complexing is the availability of gold and not thermodynamic considerations of the gold-complex stability.

The Role of Organic Matter in Ore Transport Processes

Organic matter is a common constituent of low-temperature hydrothermal mineral deposits hosted by sedimentary rocks which are themselves organic-rich or part of a maturing sedimentary basin. Hydrothermal mineral deposits in basement rocks have also been reported to contain hydrocarbons, which may have migrated into their present sites from adjacent sedimentary basins. Of particular interest is the well-known association between organic matter and Pb-Zn mineralization of Mississippi Valley type (e.g., Barton, 1967; Macqueen and Powell, 1983; Marikos et al., 1986; Gize and Barnes, 1987), where bitumens and petroleum liquids are reported to occur. In addition, a close relationship between organic matter and mineralization is known for Carlin type Au deposits (Hausen and Park, 1986) and for disseminated sandstonehosted U mineralization (Vine, 1962; Leventhal, 1986). Organic matter may also be involved in the formation of sandstone-hosted Pb and Cu(-V-Co) deposits, which characteristically show many diagenetic features (Bjorlykke and Sangster, 1981; Rickard et al., 1979; Sverjensky, 1987).

Generation of compositionally atypical hydrocarbons in CO2-rich geologic environments

Bitumen seepages from a trachyte flow on the Suswa volcano (East African Rift) are anomalously enriched in O and N heteroatomic organic compounds and depleted in aliphatics. Compositional and geologic data suggest that the biogenically derived bitumens were extracted and/or transported by liquid or supercritical CO2, rather than H2O, possibly from caldera-lake sediments. Compositionally atypical hydrocarbons and reduced carbon associated with other alkaline-peralkaline igneous complexes, as well as Hg and Au deposits, may also reflect CO2, fluids. Hydrocarbon extraction and migration in CO2-rich fluids are considered realistic mechanisms in certain geologic environments.

Aspects of the Organic Geochemistry and Petrology of Metalliferous Ores

Reduced organic matter occurs in intimate association with many types of metalliferous ore deposits. The Kupferschiefer, an extensive Middle Permian Cu-Pb-Zn-S horizon in Europe, is a bituminous shale. Bitumens and liquid petroleum frequently occur in Mississippi Valley-type Pb-Zn-S deposits, or in the geologically similar Illinois Fluorite District. In the South African Witswatersrand, gold and uraninite (UO2) occur within the “Carbon Leader.” Pyrobitumens are present in hydrothermal vein systems, such as the Ag-Ni-Co deposits at Kongsberg, Norway. “Hydrocarbon fronts” are considered by some exploration geologists to be an essential feature of Carlin-type, “no see-um,” disseminated gold deposits.