Superficial geochemical dispersion around sulphide deposits: Some examples in France |
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| Authors: | J Barbier E Wilhelm |
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| Institution: | Bureau de Recherches Géologiques et Minières, Service Géologique National, B.P. 6009, 45018 Orléans Cédex France |
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| Abstract: | Examination of geochemical exploration data accumulated in France over a period of almost 20 years has led to detailed consideration of the relationship between anomalies and mineral deposits.Geochemical anomalies associated with sulphide mineralizations seem to follow several general rules i.e. Ag and Pb are almost systematically present, As is very frequent, while Cu and Sb may also be present. The behaviour of Zn seems to be controlled by the lithological character of the host rock with anomalies in carbonate media even for occurrences without economic interest and an absence of anomalies in non-carbonate media, even for Zn deposits of considerable size. Moreover, the contents of this element are greatly amplified by contamination from old workings. The abundance (or scarcity) of supergene minerals in the occurrences directly influences the nature of the anomalies. Lead anomalies frequently correspond to the presence of Pb oxides such as pyromorphite while Zn anomalies derive from Zn oxides which are much more common in carbonate media (smithsonite-hydrozincite) than in schistose host rock. High Cu and As contents are most frequently linked to the presence of “ferruginous fragments” (gossans) derived from sulphides. As a general rule, the various geochemical peculiarities or associations encountered in soils are very much similar to those of the oxidized part of the deposits.We are therefore led to assume that, contrary to the usual conceptions, superficial dispersion is basically mechanical and that the anomalies detected near mineralizations are due to the presence in the soils of supergene minerals, gossan microfragments and more or less mineralised rock debris, etc. Mechanical migration of this kind can be explained by the existence in France during the Pleistocene of cold climates of the tundra type with associated permafrost. The thawing in summer of the superficial part of the frozen overburden resulted in massive downslope solifluction favoring mechanical migration over distances in the order of 100 m and contributing to the surface dispersal of the mineralizations. These phenomena are extremely important for geochemical exploration because they allow and justify the use of large grids (200 m × 200 m or even 400 m × 400 m) during soil surveys at least at the initial stage of prospecting on areas of several square kilometers.These conclusions are valid not only for France but for all regions with the same paleoclimate. These form a belt bounded to the north by glacial formations linked to ancient inland ice and to the south by the maximum extension of Pleistocene permafrost, including, in particular, the north of the United States and Central Europe. This paleo-climate belt characterized by traces of solifluction constitutes a well-defined unit in which geochemical prospecting has proved itself extremely effective. |
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