Curatella americana L.: a biogeochemical sample medium for the Tilarán-Montes del Aguacate gold belt, Costa Rica

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Abstract

The Tilarán-Montes del Aguacate gold belt, Miramar area, San José Quadrangle. Costa Rica is a zone in active exploitation of and exploration for epithermal gold deposits. The possibility that the tree Curatella americana L. (common name “raspa”) is a gold accumulator and can therefore be used for prospecting in some districts of the gold belt, was investigated by a collection of ten samples of this tree along a traverse across Cerro Conchal, San José Quadrangle, near the Santa Clara Mine active open-pit gold operation. The traverse crossed a north-trending quartz vein. Subsamples of twigs and closed pods from the vegetation were dried and analysed by instrumental neutron activation analysis (INAA) for Au, its associates As and Sb, and other trace components (e.g. Co, Zn, Fe, Br). The analyses showed that Au in the dry plant matter is more concentrated in the twigs (average of 2.9 ppb) than in closed pods (average of 1.2 ppb). In general, Sb acts similarly with 80 ppb in twigs versus 50 ppb in closed pods. However, As is more concentrated in closed pods (130 ppb) than in twigs (90 ppb). Spatially, Au, As and Sb in both twigs and closed pods define the position of the quartz vein, with the best definition shown by the twig geochemical data. A soil sample with the highest Au concentration in the < 500 μm size fraction (1000 ppb) is closest to the quartz vein and corresponds to the tree with the highest concentration of Au in the twigs (7.2 ppb) and in the closed pods (2.4 ppb).

These initial results suggest that systematic sampling of this tree species and INAA of dried selected organs for Au, As and Sb can be an efficient and cost-effective way to find hidden quartz vein systems associated with epithermal Au deposits in areas of Costa Rica and other Central American countries where the plant ranges and has good spatial distribution.

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