Gold - anomalous catchment basins: a GIS prioritization model considering drainage sinuosity

Gold values in sediments of anomalous catchment basins are a function of several geological factors, including average composition of underlying rocks, geochemical controls, such as dispersion processes and scavenging of metal ions by oxides, and hopefully by the effects of mineralization processes. The influence of all these factors, in turn, is a reflection of sorting ('natural sampling') of minerals in drainages during sediment accumulation. Drainages with lower gradients, more sinuous paths, or both, are more likely to yield higher gold levels, as these factors restrict dispersion. To test the sinuosity hypothesis, 52 gold bearing basins (above the detection limit of 3 ppb in the <177 mu m sieve fraction), out of 196 sampled basins situated entirely in rhyolites of the Eo-Ordovician Castro Basin (Parana State, southern Brazil), were modeled in a GIS with respect to total lengths of drainage and distances from uppermost to lowermost points. The table of basin attributes was edited to include these data and sinuosity indexes were calculated by dividing total stream length by the distance between uppermost and lowermost points. The results were reclassified into three ranges for sinuosity (low, medium and high). To each of those ranges tentative empirical values of 1, 1.5 and 2 were assigned. Gold values for each basin were weighted by classes of sinuosity and re-classified, resulting in a modified gold anomaly map. Some basins otherwise not considered first priority were thus emphasized, while other basins which were originally considered anomalous, were de-emphasized. It is proposed that the resulting map reflects more accurately the factors affecting the dispersion of gold, and, as all basins are situated on a single rock type, accentuates anomalies related to mineralization. Basins selected using sinuosity-weighted gold anomalies correlate well with those selected by pathfinder associations, and the two approaches an considered complementary Both methods select the central portion of the Castro Basin as a primary candidate for follow-up work. (C) 1999 Elsevier Science B.V. All rights reserved.