Active sediments in the channels of streams and rivers can contain low levels of metals derived from weathering of mineralized rocks within the upstream catchment (Marjoribanks, 2010). This simple fact is the basis for stream sediment sampling– one of the most widely used methods in regional geochemical prospecting (Marjoribanks, 2010).
The technique has played a major part in the discovery of many ore bodies, a good example being the discovery of the Panguna porphyry copper/gold deposit on Bougainville Island, Papua New Guinea (Marjoribanks, 2010). For the technique to work with maximum effectiveness, ideally the following conditions should be met (Marjoribanks, 2010):
- The area should be one of active erosion with an incised drainage pattern.
- The ideal sample point is on a primary drainage with a relatively small upstream catchment. Even very large anomalies are rapidly diluted in secondary or tertiary streams.
- Only the active sediment on the stream bed should be sampled. Bank material may be locally derived and not representative of the whole catchment.
- In the absence of an orientation survey to define the ideal sample size fraction, the silt fraction of the stream sediment (usually specified as–80 mesh8) should be collected. In fast flowing streams a large volume of sediment may have to be sieved in order to collect a sample of suitable weight for assay (at least 50 g but preferably 100 g is needed). Sieving therefore has to be done at site and is best done by washing a grab sample through the mesh of the sieve. The most rugged and best sieves for field use are made of stainless steel with stainless steel mesh. Sieved samples are usually stored in small Kraft paper envelopes with sealable fold-over tops.
- As much detail as possible about the sample site should be recorded. As a mini mum this will include the following information: stream width and flow, nature of the coarse float and nature of any outcrop present. This information will be invaluable when the assay results are later analysed, and potential anomalous values selected for follow-up.
- Follow-up of anomalies will usually take the form of stream sediment sampling upstream, along the anomalous drainage, to define the point of entry of the anomalous metal to the stream sediment. Further definition of the source can then be carried out by means of soil sampling on the slopes above the anomalous drainage.
Author: Roger Marjoribanks
Reference
Marjoribanks, R. (2010). Geological Methods in Mineral Exploration and Mining. Springer. https://doi.org/10.1007/978-3-540-74375-0
