What to Look for in Bench Gravels (Part 2)
(Bench gravels...these don't look too promising...know why?)
Before I get started on the meat of this post, I'd like to extend a hearty "thank you" to the Bedrock Dreams readers who have donated to the cause in one form or another over the years and especially those of you who show your appreciation on an ongoing basis. As you know, I no longer monetize this site so your generosity is the only concrete support I receive. So thank you all again. OK, on to the heart of the matter once again.
Clay is your best buddy (continued): I talked at length about this aspect of working placer gravel benches in my previous post so I won't belabor the point here. However, I did want to give you a couple of examples of what I'm talking about. One involves dry or desert placer benches and the other wet placer bench gravels (coming soon!), so here you go.
The first examples deal with two dry or desert benches, one in southeastern California and the other here in the high desert of northern New Mexico. During the heyday of my dry washing career early on in my small-scale mining days, I stumbled across a bench that lay close by to a small, gold-bearing wash. This particular bench was located in a public use area and was about five feet high . It extended for some length along the wash and sections of it were at what I considered to be good deposition points (we'll talk more about this bench factor later). From a visual standpoint, this bench exhibited all the signs...a mixture of rock sizes and shapes, decent looking dirt and gravel, and most importantly a clay or caliche layer at knee height that supported all this material. This bench was a hard one to attack since its gravels were hard-packed (like most desert benches) but with some earnest pick and shovel work I could knock it down and out, classify the material, and start running it through my motorized "puffer." There were decent amounts of small, coarse desert gold mixed in here and there in the upper bench gravels...in fact, most small-scale guys and gals would've been happy with the results just as I was. But once I began running the material resting in the few inches above and on the caliche, gold started showing up in greater quantity, including a few small nuggets and numerous "chunkers." In fact, this bench yielded some of the best gold I ever recovered in my dry washing days. However, before you get too excited about the prospects of a desert bench like this, you need to understand that the gold was spotty along that caliche layer...something not all that unusual in dry placer mining. But now you know what can be recovered if you use your mining street smarts in approaching desert benches. You'll end up going home a happy miner like I did. By the way, I later heard through the small-scale grapevine that some lucky SOB pulled a six gram nugget out of this same bench working the gravels adjacent to where I'd been slaving away. That nugget should've been mine but you snooze and you lose. Oh...one more thing...yes, I crushed up and panned some of the caliche itself after puddling it but it contained no gold. By the way, water is precious in desert mining so I don't recommend doing too much of this sort of thing unless you're hauling in a water trailer behind you!
(Although I've used this image before, this is still a "juicy" looking bench.)
The second example I want to give you took place here in the high desert at a very well known and historic mining area that is now totally off limits due to private property issues. I prospected and worked this area for nearly ten years until I was told in no uncertain terms that my presence was no longer welcome there. Sigh...This particular area still contains large amounts of dry placer gold that is some of the purest (around .917 fine) and prettiest desert gold I've ever recovered anywhere, any time. Anyhoo, I was prospecting and sampling a wash and not getting too much in the way of results until I came across a bench that was about shoulder height (I'm 6'3'', by the way). On my very first sample I pulled two of the largest gold flakes I've ever recovered. Each one was about a half inch long and quarter inch wide and both were perfectly flat, exhibiting none of the usual desert coarseness. Trust me, this is highly unusual in a dry or desert placer environment and how these large flakes got this way can only be due to the forces of Ma Nature or some other mechanism I still don't understand. Interspersed in this bench at about one foot intervals were dry clay layers (clay, not caliche) and I could easily see bands of coarse black sand sitting above the clay layers. An additional visual clue (which I'll discuss later on as well) was the presence of patches of heavily oxidized iron mixed in with the black sands. Unlike the previously mentioned bench in California, this bench gravel was fairly easy to knock loose and process. The result? Mixed in with those bands of heavies sitting above the clay layers was boo-coo placer gold, including coarse pieces and small nuggets. In fact, when I later brought a friend to this spot, he recovered his first nugget in those black sand bands. All in all a good find, my friends, and very good mining.
(Bench/terrace gravels sitting atop a massive band of clayish looking sediment. This spot has possibilities but there's gonna be hell to pay doing so.)
Now these examples don't mean it'll always be this way with clay in benches (wet or dry), but it does illustrate the possibilities for you. In other words, never pass a good-looking section of bench gravels by. You could end up getting more gold (and better gold) than the crap you're getting in that stream bed or dry wash. This underlines a secondary point...you should always be looking up (benches, terraces, etc.) if things aren't going well below. Trust this old timer on that point, OK?
That's it for this round. I have a wet placer clay bench gravel tale to tell you and then it's on to more bench gravel "what to look fors."
Be good to one another.
(c) Jim Rocha (J.R.) 2016
Questions? E-mail me at firstname.lastname@example.org