CRC LEME OPEN FILE REPORT 190
INTERPRETATION OF TWO GEOCHEMICAL TRAVERSES ACROSS BASE METAL MINERALISATION AT THE ARIPUANA PROSPECT - MATO GROSSO, BRAZIL
M Cornelius and C G Porto
April 2005
ABSTRACT
A small geochemical and biogeochemical survey was undertaken along two traverses at Anglo American Brazil Ltda's Aripuana prospect in the Mato Grosso province of Brazil . The study involved the collection of regolith samples (humic soil (A-horizon); colluvium (B-horizon) and saprolite) along Traverse 1 that intersects an approximately 130 m long gossan body near the top of a ridge. The Ahorizon and a more clay-rich and reddish soil (B-horizon) were sampled along a shorter traverse (Traverse 2) in dominantly depositional terrain. Traverse 2 intersects 10-20 m wide base metal mineralization that is concealed by approximately 5-10 m of barren regolith. Gossan chip samples and samples of drill core intersecting the gossan body along Traverse 1 were also taken for comparison. In addition, mull and bark samples were collected along Traverse 1, and mull samples along Traverse 2. The mull comprised a thin (<50 mm) layer of decomposing leaves and fine roots with varying amounts of soil particles embedded in it.
Soil samples were split into <75 µm and <250 µm fractions and analysed following multi acid, aqua regia or fusion digests. Mull and imbauba tree (Moraceae family, genus Cecropia) bark samples were prepared by dissolving dried matter and ashed matter (controlled ignition) in aqua regia, followed by an ICP-MS finish.
Target element concentrations along both traverses are on average two times greater in the <75 µm fraction than in the <250 µm fraction. However, anomaly to background ratios in the <250 µm fraction along Traverse 1 are one to two times greater than in the <75 µm fraction. Along Traverse 2, anomaly to background ratios are similar in both size fractions. High anomaly to background ratios in the coarse fraction along Traverse 1 may be due to coarse gossan detritus, derived from the large gossan body uphill and along the traverse.
The results of this survey suggest that fine-grained samples are preferable for regional work, whereas coarser material is more suitable for surveys in the prospect area due to the widespread occurrence of gossan outcrops. A-horizon humic soil samples show the greatest anomaly to background ratios of all sample media. Mull samples (ashed) show similar ratios to those in B-horizon samples and provide adequate information for delineation of the gossan zones along both traverses. Bark samples show very weak signatures of target elements but appear to show concealed mineralization in one sample.
The element suite based on anomaly to background ratios, that best identifies the base metal - precious metal bedrock mineralization in soil at Aripuanã, comprises: Ag, As, Au, Cd, Co, Cu, Fe, In, Mn, Mo, Pb, Sb, Se, Tl and Zn.
Further work is warranted to investigate the use of mull, bark and other organic sample media, to assess their value for routine exploration.
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