Thursday, May 31, 2012

Gold Deposition in Dry Placers (Part 3)

(Shallow or exposed bedrock should always be sampled in dry placer environments.)


"Hit or Miss" Gold Distribution

In my previous post, I used the term erratic to describe dry placer gold deposition. This "hit or miss" distribution of gold particles in dry washes and arroyos is, once again, the result of sudden or intermittent high water events like thunderstorm downpours and resultant flash flooding.

In these infrequent high-water surges, placer gold is moved and mixed over, under, and through angular gravels that can easily entrain or entrap gold fines, flakes, and even nuggets. At the same time, deeper gold can be exposed near the surface or pushed downward. In the strictest sense, there's no real rhyme or reason to how gold gets deposited in dry placers (in most instances).

The Exception Not the Rule

One thing to understand here, however, is that dry placer locations that were formed from ancient running streams probably have their best gold values (nuggets, pockets, extensive paystreaks, etc.) just above or on bedrock itself. That's why in some dry placer districts you'll come across vertical shafts that the old timers dug straight down through dry stream gravels to reach the good stuff.

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That said, in my own experience working numerous dry placers in New Mexico, Arizona, and southeastern California, these sorts of ancient stream situations are the exception rather than the rule. I suspect that most of the dry placer locations you'll come across out there will, once again, contain placer gold in the topmost layers of gravel rather than the opposite. Still, if you come across shallow or exposed bedrock in a wash or arroyo, make sure you sample both it and the overburden sitting above it. Common sense strategy, right?

(Old timers working a shallow dry placer in Nevada back in the day.)

In terms of paystreak distribution, dry placer environments tend to be very "spotty," for lack of a better descriptive term. Don't count on finding extensive paystreaks or significant pockets of placer gold spread over large areas like you can find in certain wet stream situations. At the same time, you may find small or isolated paystreaks or pockets in key wash or arroyo locations that are quite rich for their size. Usually, these spots will be sitting just below the surface of existing gravels or behind the downstream side of larger obstructions.

Gold Panning Kits
Mining Equipment

Here's a tip for you in conjunction with this idea: it often happens that good gold can be found in high-pressure areas in arroyos and dry washes just as easily as it can be found in low-pressure spots. Now this is something that would never work for you in a running stream environment since the hydraulic principles governing gold deposition remain constant. In dry placer environments just the opposite is true, however.

Once flash flooding halts abruptly (which it invariably does in dry placers) all the gold being carried downstream by that high water in a dry wash or arroyo will suddenly "drop and stop" right where it was just being transported a millisecond before. Put a visual image of this in your mind's eye...huge volumes of water rushing downstream, carrying rocks, huge boulders, debris, and gold with it. Then, a sudden slowing or even abrupt stopping of this water flow...where's the gold going to be deposited?  Answer...potentially anywhere near the surface.

That's it for this round. Good luck out there.

If you liked this post, you may want to read: "Hard Lessons: What I've Learned in 30+ Years as a Small-Scale Gold Miner (Part 1)"

(c)  Jim Rocha (J.R.)  2012

Questions? E-mail me at jr872vt90@yahoo.com





2 comments:

  1. love those pictures of old timers. I think I see the word "Tonopah" hand written on the edge of that one. I've been to Tonopah many times and my dad did a great deal of electrical work in a molybdenum mine (a lot of gold came out of that moly mine) when I was a kid. fun stuff. Thanks J.R.!

    -Bo

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  2. Yep Bo, that was near Tonopah. Nevada had some great gold and silver strikes...Searchlight, Bullfrog, the Comstock...and many smaller too numerous to count. I appreciate your support and comments. best to you. J.R.

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