『Abstract
Five study areas in the Cevennes（最初のeの頭に´）,
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 modern 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 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 modern 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-flluvial 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
modern 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.
Keywords: Modern continental placer; Gold; PGE; Man-made landscape;
Central Europe』
1. Introduction
2. Geological setting and site selection
3. Methodology
4. Results
4.1. Precious metal-bearing alloys
4.1.1. Chemical composition
4.1.2. Accessory minerals
4.1.3. Grain size
4.1.4. Grain morphology
4.2. Base metal-bearing alloys
4.3. Sulfides and arsenides
4.4. Phosphates-arsenates
4.5. Carbonates
4.6. Sulfates
4.7. Iron and titanium oxides
4.8. Tin, tungsten and lead oxides
4.9. Oxide-hydroxides
4.10. Silicates
4.11. Phyllosilicates
4.12. Lithoclasts and bioclasts
4.13. Chemical composition of placer mineral assemblages
5. Discussion
5.1. Process-related subdivision of placer minerals
5.2. Controls on placer deposits in the study areas I and II
of the Cevennes（最初のeの頭に´）
5.2.1. Natural controls
5.2.2. Human impact
5.2.3. The genetic relation of placer deposits in areas I and
II
5.3. Controls on placer deposits in the Gotthard Massif
5.3.1. Natural controls
5.3.2. Human impact
5.4. Controls on placer deposits in the Erbendorf area
5.4.1. Natural controls
5.4.2. Human impact
5.5. Controls on placer deposits in the Hagendorf-Pleystein area
5.5.1. Natural controls
5.5.2. Human impact
5.6. Controls on placer deposits in Niederbayern
5.6.1. Natural controls
5.6.2. Human impact
5.7. Synopsis of placer-type deposits based on mineral-related
parameters
5.8. Synopsis of placer-type deposits based on grain-related
parameters
6. Summary and conclusions
Acknowledgments
References