Journal of Conference Abstracts

Gold Accumulation in Tropical Plants,
Cuiabá-Mata Grosso, Brazil: The Possibilities for a
Reliable Cross Checked Geobotanical Gold Prospect

Wilson Andrade & James A. Sturdevant

Instituto de Quimica, Universidade Federal Fluminense, Niteroi, Rio de Janiero, Brazil.

Gold mobilization in nature has been the subject of many investigators for over a century. There is no doubt that secondary gold mineralisation has been formed by migration of gold released from primary gold bearing rocks. Several hypotheses have been proposed to explain gold mineralisation in natural environments. In the laterite near Cuiabá-Mata Grosso, primary gold has migrated from a quartz vein, has dispersed over a saprolite zone and has been concentrated in laterite essentially free of organic material. When gold is concentrated at the bottom of a soil profile, mechanical processes traditionally have been invoked. In tropical environments, gold commonly is found at the top of the soil profile, even at the level of plant roots. The dissolved gold (3 ng g^-1) found in all water samples is the background range for natural waters. These low levels of dissolved gold are not sufficient to account for gold enrichment in laterite unless another process is active. Therefore, it is suspected that a sort of chemical process is active.

In the course of this research, it was discovered that two species of plants ­ Curatella americana and Xylopia aromatica ­ are effective collectors and transporters of trace amounts of gold. The ashes of these species were found to contain up to 320 ng g^-1 of gold in Curatella and 139 ng g^-1 of gold in Xylopia, against nondetectable gold found in a background sample. The discovery of these two species as transporters and concentrators of gold is reasonable evidence of a bioactive process. The two above mentioned species showed the same trend regarding gold concentration in their leaves. Unlike most tropical plants, these species undergo a dormant stage with leaf renewal every seasonal cycle and are abundant over the whole region. These observations suggest that Curatella and Xylopia could simultaneously be used in a biogeobotanical prospect technique for gold with a high confidence level. This innovative procedure would avoid errors due to nugget effects in soil sampling, plant sickness in conventional leaf sampling and long-term gold accumulation in bark. Siegel and Paguaga have found 1300 ng g^-1 of gold in ashes of Curatella americana in Costa Rica.

References

Siegel, F.R. & Paguaga, A.S. J. Geochem. Expl. 41, 169-180 (1991).