Dear Dr. Campos,

your question is closely linked to the scope of your project. Each grain size fraction has its special meaning. I can only speculate on your aim. For chemical analysis, I used to take the grain size interval < 63 µm, for placer and heavy mineral investigation it is wise to take the fraction between 63 and 300 µm. In this case you should know which part of the drainage system you are going to sample. Sometimes, a maximum size of 300 µm can be too low. I realized this when I was looking for cassiterite in the upper reaches of fluvial drainage system. For provenance analysis or stream sediment analysis focusing on target selection it is often also prudent to combine sand-sized material with pebble analysis. I want to make you aware of the geological and geomorphologic setting which both play also a significant part when it comes to constrain the size interval.

I renounce to refer to any paper, because each of the studies behind those publications should have gone through some "brain-storming" with regard to this issue prior to any sampling.

Good luck !

Harald G. Dill

Prof. Dill is right, the choice strongly depends on the question you want to answer. If your study includes determining whatever composition of sampled sediments then selecting the proper grain size is of critical importance. The dependency of composition from grain-size is a well known problem, no matter if you're doing petrography, geochemistry, heavy mineral analysis, geochronology or thermochronology. For instance, recent published papers proved the preferential distribution of detrital zircon ages or garnet end-members in specific grain-sizes. This can be either inherited from original parent rock grain size (garnet from granulite is bigger than a garnet from a granite) or recycling processes (due to comminution during transport). In this case I'd use the grain-size fraction 40-350 micron.

Splitting your sample in different grain-sizes could be useful, although time consuming. There we go again... what's the scope of your study?

Best of luck,

Luca Caracciolo

From practical experience , silt size has worked  quiet consistently for Au , but one must make sure cobbles and other bigger constituents are sieved out .

An aqua regia or bottle roll digest has usually gone ahead to give some good follow up zones . Hope this helps .  

Dear Prof. Dill, Luca & Kwabena

Thank you for your comments.

In sum, sampling active stream sediments is a project driven activity.

From the exploration point of view, for an area that has the potential for the occurrence of a variety of commodities (not just gold) and that has been not studied in detail, and considering that sampling active stream sediments is a time consuming operation, seems that it is,OK to focus on collecting samples on first and second order streams, and to carry out a more efficient process, using a sieve equivalent to fine sand grain size (e.g., about 1.4 to 1.6 mm). Then in the lab (if need), different subsamples of different grain sizes can be separated to focus on specific finer sizes, according to the need, and based on preliminary studies of selected samples.

What do you think?

Dear Nelson,

clearly the size of the grains is related to the type of research that you are carring out. If you are working to recovery heavy minerals, or to study the solid transportation of a River, it is nevertheless necessary to have a detailed framework about all grains, of all dimensions , that you can capture with different mechanical system. Only after when you pass to the analytical part you need of a detailed dimension. For example you need of micron for X-Ray diffractometry, XRF or IPC mass, you need of a grain of < 1 mm to concentrate heavy minerals in Vertical Spiral concentrator or in Giggs. 

I am working on the same subject on the Zambesi River in Mozambique. We have found very interesting data either in to the present bottom trasportation, or in the old alluvial deposits. In both cases we have found, Mineral of Iron, Titanium, Rare Earth, gold, silver and platinum, also in to the sands with granulometry > 2 mm.

For that we are usiging a dredge and a crusher to reduce the dimension of grains after their capture. Many South African mining companies, excluding grain size> 2 mm, but we have found in those very good quantities of minerals having  market.

Francesco

Thank you for your information.

As mentioned above by Dr. Dill, it is important to consider the usefulness of Splitting your sample in different grain-sizes...

Nelson

Hi Nelson,

your strategy is reasonable. Check the links below if you want more details on the grain-size control on sediment composition. 

Good luck!

Luca

https://www.researchgate.net/publication/235991154_Mineralogical_and_chemical_variability_of_fluvial_sediments_1_Bedload_sand_Ganga-Brahmaputra_Bangladesh

https://www.researchgate.net/publication/230725036_Influence_of_granitoid_textural_parameters_on_sediment_composition_Implications_for_sediment_generation

 https://www.researchgate.net/publication/283470032_Sediment_Generation_in_humid_Mediterranean_setting_Grain-size_and_Source-rock_Control_on_Sediment_Geochemistry_and_Mineralogy_Sila_Massif_Calabria

Data Mineralogical and chemical variability of fluvial sediments ...

Article Influence of granitoid textural parameters on sediment compo...

Article Sediment Generation in humid Mediterranean setting: Grain-si...