Tuesday, January 12, 2016

What to Look for in Bench Gravels (3)

 (Old bench/terrace gravels near Placerville, California.)

As we continue on here, I have one more example to relate to you about how good bench placer gravels can be at times so please bear with me. Then it's on to more tips on what to look for. Remember, there's ALWAYS a method to my madness, so take note.

A Solid Wall of Clay

It's not all that unusual to come across desert or dry placer benches that are composed entirely (or nearly so) of caliche but it's much rarer to stumble on bench gravels in a wet placer location that are totally composed of hard-packed clay. That said, during my days dredging along the North Yuba River in northern California I came across a few of these unusual benches when I wanted to take a break from working underwater and just do some prospecting or exploring (usually working my way upstream along feeder creeks in the area). In three separate instances I came across benches on feeder streams that were composed almost entirely of hard-packed clay and that were, at times, located a pretty good distance from the current stream flow. Now don't confuse these sorts of hard-packed benches with the caliche bound types typical of desert areas because the wet placer benches like this are typically easier to break loose and work. "Easier" may not be the best term for me to use here, but I'm speaking comparatively, OK? Anyhoo, one of these clay-bound bench gravel sections exists just slightly upstream from the North Yuba along a fairly well-known feeder stream in the area south of Downieville. How this bench was created I can't say, but my guess is that it was part of the existing creek long ago and at some point was left high and dry (and a good distance away from the modern creek's course). This bench is a solid wall of hard-packed (sun baked?) clay exhibiting all of the good characteristics mentioned thus far. The material was hard to break loose and had to be puddled in certain instances, but it was chock full of placer gold. In fact, the first bucket of material from this bench produced a nugget in my sluice box as long as one of your thumbs' finger nails and about half as wide. You know, suction dredgers can be a bit arrogant at times in terms of seeing dredging as the ONLY way to go when it comes to getting the gold. Hell, I don't judge that sort of attitude because I myself have been there too. But this bench I'm talking about produced more gold in a day sluicing than I typically recovered dredging. This may speak to my weaknesses as a dredger, or alternately it may underline again just how good some benches can be, so take note once again.

 (A "monochrome" bench in the Black Hills of South Dakota.)

Pay attention to color: No, I'm not speaking here about the flash of color in your gold pan but the color of that bench you're checking out or the colorization within it. This color thing can be a sticky wicket but with a bit of time and experience you'll no doubt get the drift. However, let me talk about this visual clue a bit. In their general or overall appearance, benches can run a gamut of color, including pale or light brown, beige, yellow, red or pinkish, blue or blue-grey, dark earth colored, and so on. At closer examination, some of these benches will include color variances within them, usually the result of layered materials dissimilar to the materials forming the bench proper. In my own experience I've found that benches exhibiting a dominant color or color variances within them tend to be the best gold producers. Remember, the colors or color variances I'm talking about here concern the overall appearance of a given bench, not the rocks or gravels contained therein...although the rocks and gravels can be "stained" by oxidation sources within the bench itself. This oxidation issue deserves further examination, so I'll talk about it separately. What colors are best? Red, red-orange, yellow, pink, yellow-orange, blue, blue-grey, and so on. WARNING! Despite what I just wrote keep your mind open in terms of benches, color or no color. The wet placer bench I described at the beginning of this post was colored an innocuous looking beige-brown with yellowish highlights. The main factor of attraction to it was that it was formed solidly from clay. But tuning into the colors of the geology around you can be important everywhere in the field, whether you're working benches or not.

(This image is useful in that it illustrates the "layered" bench concept.)

Oxidation is a dead giveaway: I can't stress this point enough to you when it comes to learning what to look for in bench gravels. Moreover, the presence of oxidation or oxidized materials is an significant visual clue in virtually everything you do as a small-scale gold miner or prospector. It's Ma Nature's way of telling you that something important is going on, that significant changes have (or are) taking place. Most importantly, oxidation can be an important visual signpost pointing you right to where the gold is. I can't stress this point enough to you, especially the greenhorns or newbies out there. So now that you realize the importance of oxidation (i.e., iron-staining, etc.) when you come across evidence of it in bench gravels it's pretty certain that the good times are about to roll. The way you'll typically spot bench oxidation will be via a reddish-orange coloration to the bench as a whole, or sections or localized areas of "rusty" looking earth or material. Pay particular attention to any layers of clay or black sands ("heavies") containing rusted metal or clumps of oxidation from nails or other metallic trash fused together. These are all signs and you should be looking for them when prospecting or working benches.

 (A heavily oxidized glacial gravel bench.)

There's more to come.

(c) Jim Rocha (J.R.) 2016

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

1 comment:

  1. Thanks for the articles dealing with desert high benches and dry washes! Great educational material.