Given the insolubility of Au in water direct determination of Au in ground or surface water is unlikely to be very useful - of your three media soil would be the best choice for the direct determination of Au. Alternatively if you identified other elements which are strongly associated with Au in epithermal Au mineralization, depending upon their geochemistry and mobility, they might serve as 'pathfinder' elements. On occasion As is used as a pathfinder for Au mineralization, for example.

In the late 1970's and early 1980's the British Geological Survey (BGS) carried out stream sediment and water sampling and analyses for much of of Britain and published geochemical maps, etc. I don't know the specific references but if you searched for Dr. Jane Plant (who was at the BGS at that time) you'd no doubt find links to the work.

Hope this is of some help.

Dear Nelson,

The soil geochemistry in prospecting for ore deposits uses very effectively for decades in Russia, Bulgaria and other EastEuropean contries. When the outcrops are widespead then the bed rocks are sampled. In soil geochemistry the choice of sampling framework is very important and responsoble. The rectangular frameworks are recommended when the direction of the ore bodies is clear and the lines are projected perpendicular to the ore strikes. Otherwise, the square grid is recommended.

For example see Hawkes. H.E., J.S. Webb. Geochemistry in mineral exploration, New York, 1964.

Au is present at low but definitely detectable concentrations in water in mineralised areas... see the linked article. We're preparing a dataset for publication right now using DGT to determine Au and associated elements in groundwater.

There are also some studies showing that vegetation can provide a signal over mineralisation (see Anand, R.R., Cornelius, M., Phang, C., 2007. Use of vegetation and soil in mineral exploration in areas of transported overburden, Yilgarn Craton, Western Australia; a contribution towards understanding metal transportation processes. Geochemistry: Exploration, Environment, Analysis, 7, 267-288.)

Article Development of the Diffusive Gradients in Thin Films Techniq...

Thanks Andrew, quite useful papers. I've found another recent paper on vegetation in Au exploration.

Melvyn Lintern, Ravi Anand1, Chris Ryan & David Paterson 2013. Natural gold particles in Eucalyptus leaves and their relevance to exploration for buried gold deposits. NATURE COMMUNICATIONS | 4:2614 | DOI: 10.1038/ncomms3614

The choice of the sampling media will also depend of the surface area that you have to cover, the climatic zone, geomorphology of the area....In order to obtain a representative data set for future geochemical interpretation , the density of the sampling is also highly important : stream sediments may be sampled at a low density compared to soil samples.