Working Bench Gravels, Part 2
In closing my first post on working bench gravels, I mentioned that I would cover 3 major topics in this second post. So here you go:
In unworked or lightly worked bench gravels paystreaks are often distributed throughout the bench laterally and vertically. The reason for this is simple. The auriferous gravels in the bench were deposited at various times during sequential or periodic natural occurrences such as annual flooding or flash flooding events (in the case of dry placers).
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Alternately, a bench's paystreaks were deposited long ago when its gravels were actually part of a river or streambed. At some point geologic forces such as shifting, earthquakes, uplift or a natural diversion of the stream's flow resulted in the old stream gravels being deposited high and dry (again, California's Tertiary Channels are a great example of this).
The paystreaks in wet placer bench gravels where consistent running water was the primary physical agent of deposition tend to be laid down in a fairly consistent fashion from layer to layer. Conversely, gold values in benches in dry placer areas where flash flooding or geologic forces were the primary causal agents tend to be much more "erratic" in terms of overall deposition.
Where's the Gold?
This is the single most problematic question for placer gold miners whether they're working benches or any other part of a stream or wash. Where's the gold? You already should have a basic grasp of deposition physics and stream hydraulics by this point. If you don't, then you need to get up to speed on that issue quickly if you want to be a successful placer miner.
In wet area bench gravels you will find that the gold has been deposited in layers or thin paystreaks from the very top of the bench right on down to where it rests on bedrock. These gold values typically don't extend very far vertically (with one major exception we'll discuss in a moment), often less than 6-8 inches. They can extend laterally or horizontally for some distance however, before they "pinch out."
In dry benches, the paystreaks are often found in what I call a "pocket" form, with little real consistency in terms of lateral or vertical deposition. (Granted, there are exceptions to this rule and consider yourself fortunate indeed if you find a dry placer bench whose gold values are evenly distributed.) Good gold pockets or small paystreaks in dry benches, like their wet placer cousins, can be found from the top layer of gravel right down to (yes, you guessed it) bedrock.
However, it has been my experience over the past 30 years that the very best gold values you will retrieve from most bench gravels, wet or dry, are in the top 2 feet of gravel sitting just above/on bedrock (or false bedrock, in some instances). Remember this point well, my friends. Remember it well....
Sampling, Mining, and Processing Bench Gravels
You must employ your very best sampling techniques to locate bench gravel paystreaks or pockets. And you must be patient and thorough while doing so. The easiest way to sample bench gravels is with a gold pan, your primary sampling tool. Get a small pick, rock hammer, and/or shovel and take consistent samples vertically and horizontally.
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DO NOT set up or run any sort of mining equipment until you've established that consistent gold values (fines, flakes, coarse pieces, nuggets, etc.) can be recovered from that bench. Otherwise, you are just stabbing in the dark and wasting your precious mining time. Sample, sample, sample!
Once you've determined that a particular bench (wet or dry) is ready to be run, your best bets in terms of mining equipment for processing those bench gravels are going to be one of these three options:
Drywasher ("puffer" or electrostatic, motorized or hand operated)
Highbanker (if it has a small garden-type "hydraulic" nozzle, even better)
Sluice Box (home-made or store bought, doesn't matter)
Make sure your equipment is set up and operating at peak efficiency. The rest is up you. How much material can you move in a day? The more you can move, the more gold you'll recover from that bench.
Good luck out there!
If you liked this post, you may want to read: "Working Bench Gravels, Part 1"
(c) J.R. 2009
Questions? E-mail me at firstname.lastname@example.org