Geogene and anthropogenic controls on the mineralogy and geochemistry of modern alluvial-(fluvial) gold placer deposits in man-made landscapes in France, Switzerland and Germany

摘要

Five study areas in the Cevennes, France, the Gotthard Massif, Switzerland, and from the western edge of the Bohemian Massif, Germany, have been selected for an investigation of the natural and human impacts on modem alluvial-(fluvial) gold placer deposits. The current investigation involved the study of grain-related parameters such as morphology, grain size and accessory minerals in gold aggregates in context with a detailed analysis of heavy and light minerals as well as heavy artifacts in the mineral assemblage of the placer deposits. To provide a sound data base, all sampling sites have been selected from areas sharing the same geotectonic and lithological settings. The modern placers are located in the extra-Alpine and Alpine parts of the Moldanubian Zone of the Variscides and they are characterized by a bedrock geology made up of metapsammopelitic rocks with some basic intercalations that were intruded by late Variscan granites. The combination of grain-related and mineral-related parameters in modem gold placer deposits is an effective tool to single out the natural impact (definition of the placer-type, provenance, chemical and mechanical reworking) and human impact (plumbing residues, ceramic goods, automobile devices, Pb-Fe-Au slags and mining residues), which may be of relevance for exploration by providing pathfinder elements or of relevance for the cultural history of the study area in terms of metallurgy and manufacturing. The placer deposits cover the whole range from pure alluvial via reworked alluvial to mixed-type alluvial-fluvial placer deposits (for definition see Introduction). Zirconium is a geomorphic marker element or indicator of the slope gradient of the drainage system. In course of lateritization chemical redeposition of gold that occurred during the Cretaceous and Tertiary (Paleogene and Neogene) times (parautochthonous) does not completely eradicate the characteristics of the primary source of gold. The mechanical reworking with long distance transport in alluvial depositional environments of different flow regimes however, makes attempts to trace the gold back to its source difficult because the intergrowth of gold aggregates with its marker minerals was destroyed by this mechanical transfer. Relics of lateritization in the placer gold assemblage may be deduced from fineness vs. grain size distributions and Fe-Al oxide/hydroxides as well as phyllosilicates intimately intergrown with gold aggregates. Stolzite and gold amalgam are positive indicators for gold enrichment in the catchment area of the placer-bearing drainage system. Lead, iron, tin and antimony are suitable tracers for the human impacts on placer deposits and may assist in characterizing the metallurgical, mining and manufacturing processes through time in a certain area. The results of this study allow to evaluate the gold provenance, the paleoclimatological and palaeogeomorphological controls on modem gold placer formations, the placer-type (alluvial, reworked alluvial, alluvial-fluvial) and the anthropogenic control relevant to exploration of placers. The latter also may provide an insight into the technical history of a region.