Money, lots of money.. here in montanas there has been all sorts of mining done. Here is a short video about some of the mines. The town of helena was built with gold from a placer creek where the gold was surface found.
ideo.search.yahoo.com/search/video?fr=mcafee&p=what+is+the+gold+mine+by+silver+city+montana&type=E210US714G0#id=1&vid=0499ea043835f2bcf2a33b1eaf0b1d85&action=click
A lot depends on how deep underground you are looking. Montana tech in Butte Montana has a drone degree and they mention underground detection of minerals... Another buisness my wife has was aided by a guy with satilite access for seeing what is several miles under ground... very expensive.
And if you are looking for near surface gold this rig says it goes down 30 meters. https://www.ebay.com/itm/1563324614...0914bf&gclid=b978c0b54db71451388ca71fd60914bf
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By your question, I assume you want coarse macroscopic gold which is quite rare. I good metal detector will help IF you don't have metallic ground support or metallic utilities (pipes, electric wire, etc.). A lower cost option is to carefully observe the rock and look for small pieces in fractures, in vugs, in quartz and with sulfides.
Most mining companies are looking for microscopic gold.
