Great thread, but it's hard to follow guys like Chris and Steve and still look good. I'll try, though.
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A few of you know that for about 30 years I have been collecting data on "dirt", in many forms. All the way from anecdotal reports from metal detectorists, to geological mapping, to electromagnetic analysis of many hundreds of soil and rock samples. My research has included not only iron magnetic loss angle, but also electrical conductivity, informed by an earlier career in arid lands climatology including agronomy and botany.
What's less well known is that at the same time I've been collecting data on the distribution of "gold nugget" sizes as found by metal detectors, and interpreting that data in light of "dirt data" which influences what metal detectors can actually find. FTP-Fisher probably has (by a huge margin) the best data on this kind of stuff of any company in the business.
I'm not in a position to blab everything I know. However I can, based on what I know, coming from a perspective very different from that of Chris and Steve, agree with the proposition that the economics of gold recovery follows a principle of diminishing returns and that no productive zone is ever entirely "cleaned out".
A very clear practical example of this principle is that several decades ago, there were guys swinging BFO's and TR's delighted with what they were finding in goldfields. If you were swinging one of those same units nowadays, you'd be whupped by some kid swinging a BH Junior that cost $60. (Good machine for $60, I designed it.) Swinger of old beep would however be finding nothing, unless he chanced to swing over a previously unbeeped spot wherein lurked a monster nugget close to the surface. Nearly everything in the developed world has already been beeped more than once, that's why diminishing returns.
That last comment deserves elaboration. Here in the American Southwest, there are vast areas reputed to have produced gold in the past, where you can be out there and it seems so remote that you surely must be the first human being to have set foot within a kilometer of here. Rude awakening: the next swing of the beep and you're digging iron or birdshot. You aren't the first one here, you're the last one!
I'm not saying there's no big nuggies to be found in places that have already been pounded. There are. Just damn few, it ain't virgin territory. The good news is that back then they could only find large shallow nuggets, and nowadays you can find smaller and deeper stuff. With the right knowledge and the right stuff.
--Dave J.