Wilson Javier Vargas Andrade - Profile on Academia.edu (original) (raw)
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Papers by Wilson Javier Vargas Andrade
Chemical Geology, 1992
Peat-derived humic and fulvic acids, isolated using widely accepted techniques, were investigated... more Peat-derived humic and fulvic acids, isolated using widely accepted techniques, were investigated for their ability to reduce gold (III) chloride and dissolve elemental gold. Au (III)-chloride (1-20 mg 1-t) was reduced to elemental colloidal Au by both humic and fulvic acids (1-20 mg 1-1) under all conditions studied. The growth rate and mean size of the colloidal Au formed was monitored using UV-visible spectroscopy. Larger colloids were formed in the presence of humic (~ 60-nm mean diameter) than fulvic acid (~ 20-nm mean diameter). Otherwise, colloidal growth rates were similar for humic and fulvic acids. Colloidal Au formation was completed in ~ 8 days at pH 4 and slowed to > 14 days at pH > 7. Cupric ion (2.10-5-2.10-3 M) accelerated colloidal Au development from 2 to 20 times while equal concentrations of Ca 2+ had no effect. About 10 mg 1-1 humic and fulvic acids were necessary to completely reduce 10 mg 1-1 Au(III)chloride which is equivalent to a reduction capacity of ~ 15 meq e-/g humic substance. IR spectroscopy and acid titration results suggest that humic substance oxidation included the addition of phenolic, alcohol and ketone groups which helps to account for the appreciable reduction capacity observed. Ten mg 1-1 solutions of these same humic substances did not dissolve elemental Au to levels > 1/~g 1-1 over periods up to 150 days. Consequently, it is concluded that humic and fulvic acids function primarily as reductants of oxidized Au species rather than as dissolution and complexation agents for elemental Au. This implies that other components of natural organic matter are primarily responsible for the dissolution and complexation of elemental Au in surficial environments.
Journal of Geochemical Exploration, 1991
Increasing evidence indicates gold is mobile in the surficial environment. In arid regions this i... more Increasing evidence indicates gold is mobile in the surficial environment. In arid regions this is attributable to complexing by chloride and near oxidizing sulfides to thiosulfate or related ligands. In this exploratory study, the extent and mechanism of gold mobility in a deeply weathered tropical rainforest environment has been investigated by analysis of waters, vegetation and stream sediment at the Salobo Cu-Au and Bahia Au-Cu deposits in the Carajas Mineral Province, Para Province, Brazil. Waters from drill holes, an adit and a stream draining the Salobo deposit contain 11 to 73 ng/L dissolved Au, compared to background levels of 2 to 3 ng/L. Elevated concentrations of dissolved Au are tentatively attributed to complexing by thiosulfate generated by accelerated weathering of sulfides beneath the steep slopes at the deposit. At the Bahia deposit, which occurs beneath an ancient deeply weathered plateau surface, the highest level of dissolved Au in small streams draining the deposit is 3 ng/L. Vegetation (multiple species) over ore at both deposits contains elevated concentrations of gold (65-400 ng/g of ash). The data suggest that vegetation may be a useful medium for gold exploration, but that in view of the results at Bahia, waters are of questionable value in deeply weathered low-relief tropical rainforest areas. The mobilization of gold by vegetation on ancient surfaces over 106-107 year time periods appears adequate to explain near-surface enrichment of Au and its lateral dispersion, thereby contributing to lateritic gold deposits.
Chemical Geology, 1992
Peat-derived humic and fulvic acids, isolated using widely accepted techniques, were investigated... more Peat-derived humic and fulvic acids, isolated using widely accepted techniques, were investigated for their ability to reduce gold (III) chloride and dissolve elemental gold. Au (III)-chloride (1-20 mg 1-t) was reduced to elemental colloidal Au by both humic and fulvic acids (1-20 mg 1-1) under all conditions studied. The growth rate and mean size of the colloidal Au formed was monitored using UV-visible spectroscopy. Larger colloids were formed in the presence of humic (~ 60-nm mean diameter) than fulvic acid (~ 20-nm mean diameter). Otherwise, colloidal growth rates were similar for humic and fulvic acids. Colloidal Au formation was completed in ~ 8 days at pH 4 and slowed to > 14 days at pH > 7. Cupric ion (2.10-5-2.10-3 M) accelerated colloidal Au development from 2 to 20 times while equal concentrations of Ca 2+ had no effect. About 10 mg 1-1 humic and fulvic acids were necessary to completely reduce 10 mg 1-1 Au(III)chloride which is equivalent to a reduction capacity of ~ 15 meq e-/g humic substance. IR spectroscopy and acid titration results suggest that humic substance oxidation included the addition of phenolic, alcohol and ketone groups which helps to account for the appreciable reduction capacity observed. Ten mg 1-1 solutions of these same humic substances did not dissolve elemental Au to levels > 1/~g 1-1 over periods up to 150 days. Consequently, it is concluded that humic and fulvic acids function primarily as reductants of oxidized Au species rather than as dissolution and complexation agents for elemental Au. This implies that other components of natural organic matter are primarily responsible for the dissolution and complexation of elemental Au in surficial environments.
Journal of Geochemical Exploration, 1991
Increasing evidence indicates gold is mobile in the surficial environment. In arid regions this i... more Increasing evidence indicates gold is mobile in the surficial environment. In arid regions this is attributable to complexing by chloride and near oxidizing sulfides to thiosulfate or related ligands. In this exploratory study, the extent and mechanism of gold mobility in a deeply weathered tropical rainforest environment has been investigated by analysis of waters, vegetation and stream sediment at the Salobo Cu-Au and Bahia Au-Cu deposits in the Carajas Mineral Province, Para Province, Brazil. Waters from drill holes, an adit and a stream draining the Salobo deposit contain 11 to 73 ng/L dissolved Au, compared to background levels of 2 to 3 ng/L. Elevated concentrations of dissolved Au are tentatively attributed to complexing by thiosulfate generated by accelerated weathering of sulfides beneath the steep slopes at the deposit. At the Bahia deposit, which occurs beneath an ancient deeply weathered plateau surface, the highest level of dissolved Au in small streams draining the deposit is 3 ng/L. Vegetation (multiple species) over ore at both deposits contains elevated concentrations of gold (65-400 ng/g of ash). The data suggest that vegetation may be a useful medium for gold exploration, but that in view of the results at Bahia, waters are of questionable value in deeply weathered low-relief tropical rainforest areas. The mobilization of gold by vegetation on ancient surfaces over 106-107 year time periods appears adequate to explain near-surface enrichment of Au and its lateral dispersion, thereby contributing to lateritic gold deposits.