Which methodology should i use to trace/investigate the source rock or geological structure responsible for placer gold deposit in an artisanal mining site?
Stream sediments is a composite catchment materials from the surrounding highlands and upstream. These sediments can be from the surrounding rocks, or can be transported from other sources nearby. If the rock fragments are from the surrounding rock types, so your stream sediments are from the same area, but if they contain different rock types, so your sediments are mixed with transported materials from other places. You should take care of this point.
Trace the river upstream from the placer deposit and investigate the rocks if they outcrop. You can find the gold anomaly by panning (if you are good at it) or just analyzing for gold in the heavy fraction of the river gravel.
If the river splits into subsidiary creeks upstream, need to take gravel/sand samples in all creeks and analyze the heavy portions to see if the gold anomaly is present. For a good survey, need to study the nature of the stones in gravel according to lithology, so you know whether the gold anomaly (if you find one) is associated with any particular lithology.
If there are no outcrops, you might need to sample the overburden (soil). If the area has been glaciated, need to take samples of the basal till to trace the gold anomaly.
Stream sediments is a composite catchment materials from the surrounding highlands and upstream. These sediments can be from the surrounding rocks, or can be transported from other sources nearby. If the rock fragments are from the surrounding rock types, so your stream sediments are from the same area, but if they contain different rock types, so your sediments are mixed with transported materials from other places. You should take care of this point.
Dear colleague,
The best information is obtained by sampling the stream sediments where it is particularly stored in their heavy mineral assemblages. There are different ways apart from a quantitative and qualitative HM analysis which you can follow up: Investigating the
1. morphology of Au grains
2.Au/Ag ratio of Au grains
3. minerals associated with Au grains such as PGM, As-bearing minerals, Hg-bearing minerals etc.
All these markers and proximity indicators (e.g. the Ag content increases towards the source rock, the amount of subhedral and even euhedral minerals increases in the same way) are dealt with in the papers below which can be downloaded from the RG server.
DILL, H.G., KLOSA, D. and STEYER, G. (2009) The „ Donauplatin “: Source rock analysis and origin of a distal fluvial Au-PGE (gold-platinum-group-element) placer in Central Europe.- Mineralogy and Petrology, 96: 141-161.
DILL, H.G. (2008) 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.- Journal of Geochemical Exploration, 99: 29-60.
H.G.Dill
the best way is first to carry out a stream sediment survey as all said. You can collect three types of samples.
Heavy minerals samples -- collect samples from pot holes, rapidly flowing areas, etc., do panning for concentrate the heavy minerals (better take composite samples at each locations) and analyze for the Au.
Bulk samples - take bulk samples (again composite sampling) from slightly stagnated areas and carryout analysis for pathfinder elements for Au such as As.
Moss samples -- Take moss samples in case sediments are absent due to very rapid river flow. Moss samples catches sediments
rara vez, el oro aluvial corresponde a la destrucción del oro primario. Por lo general corresponde al enriquecimiento de cavidades, lineamientos, planos de estratificación, etc, donde pueda precipitar el oro de soluciones con contenido de oro, producto de lixiviación de rocas, con un bajos background de oro, pero que han sido profundamente alteradas por procesos supergenos y percoladas por soluciones débiles (agua + contenido de ácidos que disuelven el oro), que han actuado por largos periodos. Ácidos, que por lo general, son de origen vegetal.
As said by Sir Dill you can use morphology and gold chemistry also study heavy minerals.
Dear Titus
I think that before field surveys, remote sensing studies have a high ability to identify and classifying different types of alluvial sediments and host rocks. Always altered gold bearing rocks and weathering products are detectable in processed multi-spectral satellite images. Downstream sediments from gold bearing catchment basin have high amounts of materials from weathering and degradation of related gold mineralization altered lithology. In the following figure, gold bearing alluvium are clearly distinguished.
Specifically, hypothermic deposits can precipitate tin and mercury together.
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