Working Bench Gravels, Part 1
(Drawing courtesy of The New 49ers, http://www.goldgold.com/generalinfo.html)
What are Bench Gravels?
I can't think of one gold-bearing stream or dry wash area I've mined that did not contain bench gravels of one sort or another. What are bench gravels? Typically they are auriferous gravels left high and dry on the sides of current streambeds, or in isolated instances, some distance above those stream courses. The fabulously rich Tertiary Channels of California's Motherlode Region are prime examples (on a very large scale) of workable bench gravels.
Gold Panning Kits
Wet or Dry?
Like other types of gold placers, bench gravels can be found in both wet and dry mining locales. It's been my experience over the years that the dry type of bench is usually harder to work than those benches located adjacent to or near a stream or water source. Part of this is obvious, of course. Running a drywasher in a desert placer is always a bit more problematic and less efficient.
But the real problem that comes into play when working some dry area benches is that they are often in the form of "caliche," a type of desert cement that is extremely difficult and labor intensive to break up and process. Yes, you can find some wet area bench gravels that are loosely cemented with clays, but I've never experienced the difficulties working wet benches that I have working dry ones. I'm sure that a few of you out there already know what I'm talking about here.
"Worked Out" versus "Workable" Benches
In some instances bench gravels you encounter out in the field will have already been "worked out" or processed by other miners, and you'll be able to determine if this is the case quite easily if you perform some thorough sampling. Your gold pan will usually be filled with black sand concentrates, bits of oxidized iron and pieces of lead, but little or no gold. If this is a consistent result laterally (at distance) and vertically (at depth), forget that particular bench because it's already been worked out.
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However, in many instances just the opposite is true. And it is, of course, this second type of bench gravel you want to focus your mining efforts on. When sampling a workable bench, your gold pan will not only display the usual black sand concentrates, oxidized iron and pieces of lead, but will also consistently show "color" in the form of fines, flakes, coarser pieces ("pickers"), or even a stray nugget or two.
Keep Your Eyes Open
When I was still able to work what I termed my "Hit-or-Miss Mine" in the historic Old Placers District of New Mexico's Ortiz Mountains, I spent a great deal of time working very extensive bench gravels. These benches were very interesting from a mining standpoint because they contained gravels that had been worked out by the oldtimers and, at the same time, some gravels that had not.
The "Hit-or-Miss" bench gravels that were worked out were typified by gold pans containing large amounts of black sands, including some very large pieces of magnetite or hematite the oldtimers here called "negros," but hardly a trace of real color. On the other hand, the workable bench gravels in the same area contained isolated paystreaks of good gold, including some pretty stunning pans filled with fines, large flakes, "pickers," and a decent number of small nuggets.
In my 30 years of mining this is one of the few instances that I've come across bench gravels like this, so it's not a common occurrence out in the field. However, I wanted to bring it to your attention. So keep your eyes open because you too may run across one of these "worked out/workable" benches at some point in your placer mining career.
In my next post on working bench gravels we'll talk about bench paystreaks, where the best gold can typically be found in a bench, and the best methods for mining and processing benches.
Until then, be safe and keep smiling!
If you liked this post, you may want to read: "2 Fine Gold Recovery Systems for Black Sand Concentrates"
(c) J.R. 2009
Questions? E-mail me at email@example.com