I'm a geology student, and recently I've been thinking about the methods used by the Romans to mine their gold and other metals. So far I've found that Pliny the Elder discusses it briefly in his Natural Histories, describing hydraulic methods and some deep vein mining. He also refers to something called "segutilim", which he calls "earth that gives indication of the presence of gold".
I can't imagine what this was, but it's a very interesting geological and historical question! Any other info or theories would be great.
Source: http://www.perseus.tufts.edu/hopper/text?doc=Perseus:abo:phi,0978,001:33
Sorry if this is vague, but in the TV documentary series "What the Romans did for us", there was an examination of a Roman gold mine in the UK, which used a very interesting technique: a spring on top of a gold rich hill was dammed and used to fill up a large catchment pond. When full, the water was released all at once, pouring down the hill and flushing the gold out of the rock and soil. Colecting it was simply a case of workers sifting through the silty remains for gold nuggets with their hands.