Abstract

A geochemical survey was undertaken along two traverses at the Aripuanã base metal deposit in the Mato Grosso province of Brazil. Traverse 1 has residual and colluvial soils and intersects a 130-m-long gossan body near the top of a ridge. Traverse 2 intersects 10–20-m-wide base metal mineralization concealed by c. 5–10 m of transported cover. Humic (A-horizon) and clay-rich and reddish (B-horizon) samples were collected on both traverses; saprolite was sampled on Traverse 1. The compositions of these regolith samples were compared with those of litter and bark samples from Traverse 1, and litter samples from Traverse 2. The soil samples were split into <75-μm and 75-250-μm fractions and, with saprolite samples, analysed for 68 elements following multi-acid, aqua regia or fusion digests. Samples of litter and bark from the imbauba tree (Moraceae family, Cecropiagenus) were prepared by digesting dry tissue and ash in mixed acids.

Humic A-horizon soil samples exhibit the greatest anomaly contrast of all sample media. Fine-grained samples (<75 μm) are preferable for regional exploration, whereas coarser material (75–250 μm) is more suitable for prospect-scale surveys seeking gossan outcrops. Litter samples have similar contrasts to those in B-horizon samples, and delineate the gossan zones along both traverses. Bark samples from trees over the gossan are all anomalous and may indicate mineralization beneath barren regolith. The element suite that best identifies mineralization in soil at Aripuanã comprises the target elements Cu, Pb, Zn and Au, and associated pathfinder elements In, Mo and Hg. In dry litter, the best anomaly contrast is exhibited by Cu, Pb, Zn, As, Mo, Sb and In. In dry bark, the best indicators are Pb, Mo and Sb.

In depositional terrain, such as Traverse 2, both soil and litter appear to identify the location of mineralization but the number of samples is insufficient to exclude the possibility (at a probability >95%) of obtaining the same results by chance.

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