Chemical dispersal processes play a dominant role in creating geochemical anomalies over blind orebodies, a fundamental difference from the detrital origin of anomalies derived from near-surface deposits. Secondary geochemical dispersion can only be achieved through a sequential process involving weathering and leaching, transport in solution, and final immobilization, possible at some distance, by processes such as precipitation or adsorption on some specific mineral or organic component. The present study compares the geochemistry of the various fractions of stream-sediment samples collected in Brittany (France) near deep-seated Paleozoic polymetallic mineralized zones, to samples obtained from background areas. The work consisted of separating the different grain-size fractions, with emphasis on ultra-fine fractions (minus 0.5 microns) enriched in adsorbing complexes, and analyzing these fractions for base metals. Simultaneously, the hydrochemical dispersion around the same zone of mineralization was studied by analyzing groundwater samples. Background samples have base-metal contents that are low and evenly distributed in the different size fractions. Slight enrichment in the minus 0.5 micron fraction in several elements including Zn was mainly related to a higher proportion of phyllosilicates. For samples collected near mineralized zones, the metal contents are very low in the whole sample, as well as in any individual coarse, intermediate and fine fraction. On the contrary, in the ultra-fine fraction, highly anomalous Zn contents were obtained (800-2000 ppm), corresponding to an average anomaly/background ratio of 5. Such anomalous values are most likely related to organic matter, sulphides, and/or Fe/Mn hydroxides. Hydrogeochemical studies confirm that such anomalies can be considered as resulting from hydromorphic dispersal around mineralized areas. Although the segregation of the ultra-fine size fraction and its analysis are technically feasible, it was found that the results showed insufficient reproducibility to be used as a routine exploration tool. © 1989.
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