(Aussie placer nugget.)
As the title of this series of posts suggests, there is both an art and a science to finding placer gold nuggets. Adept small-scale gold miners and prospectors understand this premise well and practice it in their mining and prospecting endeavors.
POINT 5 (continued): Gold deposition physics is your best friend.
I explained what I mean by this statement in my previous post, so if you're behind the curve in that regard you may want to click here and read what I had to say. For those of you still hanging in there, let's take a look at the next deposition physics principle that factors into your ability to find nuggets.
The frequency of obstructions and obstacles and their specific locations in streams and washes is an important nugget deposition factor that should not be ignored by novice and old timer alike. Next to bedrock itself, obstructions such as large rocks, boulders, older dead falls, etc. provide excellent traps and "hiding places" for nuggets on their downstream sides as well as underneath those obstructions themselves. This latter observation should be seared into your mining and prospecting brain because when those nuggets aren't behind that obstruction you're working chances are they're underneath it. I worked one feeder creek in the California Motherlode Region where every nugget recovered was found UNDERNEATH the larger obstructions. (Got that?)
(Natural set of riffles.)
Obstructions and obstacles disrupt water flow and help create gold-trapping eddy currents that are very similar to those created by the riffles in your sluice box. Nuggets (and smaller gold as well) get pulled in and down by obstructions and their associated eddies, especially in low-pressure portions of streams and washes where gold deposition physics is clearly at work. In terms of their frequency, I would suggest to you that "more is better." One way of understanding this even clearer is to go back in time and look at those 49ers using "Long Toms" to capture placer gold. A rocker box or small sluice was good, but the Long Tom was better. Why? Its greater length meant more riffles (i.e., obstructions or obstacles). All this stated, you have a job to do. And that job is to determine which of those obstructions holds the best nugget potential.
Depth to bedrock is always a fundamental concern that needs to be addressed when searching for placer gold and especially those elusive nuggets. Bedrock covered by successive deep layers of overburden has always been the bane of gold miners...small-scale or commercial. Since many nuggets (and especially the larger ones) will be found on or close to bedrock, the depth to bedrock becomes a key element in separating the men and women from the boys and the girls out there. As a small-scale guy or gal you want to be working nugget ground with shallow or exposed bedrock. After all, who in their right mind wants to try and clear 10 or 15 feet of overburden by hand? (Count me out on that one too.)
So shallow bedrock is what you want, whether its real (country rock) or false (clay layer). Here's the problem with this nice, neat little idea though. If that shallow or exposed bedrock is easily accessed by you then as fast as ice water sells in hell, it has also been accessed by others and pretty much picked over. Gold miners can be like vultures, especially the experienced nugget hunters and bedrock gold snipers out there. Those bedrock bones can get picked clean in many, but not all, instances. This is where your own nugget or general gold recovery acumen comes into play as well as your willingness to bust butt and hit those harder to reach and harder to work spots I mentioned in an earlier post. Shallow or deep, however, bedrock is where it's at.
Oh, one more thing...many bedrock locations get "replenished" to some degree or another on a regular basis (annually, every 3 years, etc.). I know this for a fact and have experienced it directly.
Bedrock configuration is just as important as depth, although many would-be placer gold miners fail to recognize this fact clearly. Here's my basic rule for considering bedrock's nugget catching potential:
Highly fractured bedrock containing many sharp angles or traps perpendicular to the stream or wash is always better than smooth or water-worn bedrock.
I've called this sort of nugget-catching bedrock "bookshelf" in the past and that description is still a pretty good one. However, bookshelf bedrock is not always going to be present. When this happens and you find yourself working smoother, more water-worn bedrock you'll want to be looking for better drop-offs or the deepest potholes. Of course, obstructions can play a productive nugget role in both types of bedrock as do cracks, crevices, drop offs, and potholes. When working water-worn bedrock for nuggets, however, you'll want to clean out only the very deepest and most tightly packed crevices and potholes.
(Angular or "bookshelf" bedrock. Image courtesy the New 49ers.)
That's all for now. We'll talk more next time.
(c) Jim Rocha (J.R.) 2014
Questions? E-mail me at firstname.lastname@example.org