In licence areas with sufficient rivers and streams, sampling stream sediments may be favoured for early-stage mineral exploration over soil sampling, or even done in conjunction with soils to better develop an understanding of the geology. The principle behind this method is that the sediments represent eroded material from the higher ground and so looking at this, the geology of the catchment area can be inferred. The sampling programme must be carefully planned so that the correct sediment size is chosen, depending on what minerals the geologist is looking for.
Similar to soil sampling, this technique is low-tech and inexpensive, consisting of simply a combination of a pan and a series of metal sieves. The geologist will collect several kg of sediment from the stream bed, firstly placing the top 10-20cm back into the stream as this upper layer is naturally contaminated with highly mobile metals such as iron and manganese. Particularly large pieces of rock are similarly removed, and then the rest is fed through a series of increasingly finely-meshed sieves until the desired size of sediment particles has been isolated for bagging, to be sent for lab chemical analysis. In mineral exploration, the size will be planned beforehand by the geologist, depending on what metals they are searching for. Generally, coarser sediment is sampled when looking for gold, tungsten and titanium, whereas when looking for copper, lead and zinc, finer particulates (<0.06mm) would be the target. However, when conducting stream sampling for inferring the change in rock types, as opposed to exploring for specific minerals, it is important to pick a consistent particle size to avoid bias, e.g. the nation-wide Tellus sampling collect samples between 0.15-2mm.
Sites for sampling must be carefully selected so that they are not affected by contamination from upstream roads and other man-made structures. Additionally, smaller-scale tributaries are chosen over the main river itself, so the results can be more confidently allocated to a specific area which feeds that stream. When using this method to survey the general geology, several points are chosen across the stream bed to get a representative sample. However, mineral exploration focuses on sampling natural traps for the denser sediments such as on the stream margins.
Geologists taking a stream sediment sample.
Panning is very similar to stream sediment sampling; however it goes one step further in that the final layer of sediment in the pan is then partly-submerged in the water and shaken at a low angle to gently separate the lighter material – which floats away – from the denser material which remains. Because of the very high density of gold, this simple technique has been used around the world for millennia. In fact, this was one of the chiefly used methods to collect nuggets by miners in the 19th century Californian and Klondike gold rushes. This is not considered a viable form of mining in Ireland, and so is more of a reconnaissance activity to check if any dense metals are picked up in an area, or even done for recreation purposes. However, in the developing world, it still remains as an economic practice on an artisanal scale.
Similar to soil sampling, this method is a cheap and effective way of kick-starting a mineral exploration programme over an area with sufficient stream coverage. If the results prove interesting, the decision may be made to undertake soil sampling to back-up the stream sediment findings and develop a more confident and complete picture of the surrounding geology. For areas deemed as potentially hosting ore deposits, drilling may then be decided at specific locations of interest, providing the permission has been granted by the legal authorities and the land owner.