Friday, February 7, 2014

Gold-Bearing Bench Gravels (Part 1)


 (More than two troy ounces of California placer gold.)

I've talked about gold-bearing bench gravels peripherally in the past, but in this series of posts I'll be addressing benches directly. Want to learn more about bench gravels? Then read on, my friend.

"High and Dry"

I suspect there are very few of you out there who don't know what bench gravels are. Moreover, a great many of you have worked benches at some point or another in your small-scale mining careers with greater or lesser success (hopefully the former and not the latter).

For those gold prospecting and mining "newbies" or novices out there who may not know, bench gravels are gold-bearing gravels that are typically found along the sides or banks of existing stream beds or dry washes, or that have been left "high and dry" some distance away from current stream courses. Bench gravels typically become available (i.e., "workable") for any number of reasons, including:
  • Large-scale geologic events such as earthquakes, vulcanism, uplifts, fractures, etc.
  • Low water conditions, heavy water usage, or damming upstream.
  • Mud and rock slides. 
  • Streams or washes changing direction, shape, configuration, size, or depth.
Take it to the Bank

I've found gold-bearing bench gravels in both wet and dry placer areas and have worked them hard with mixed success. Some were excellent gold producers and some were not (and I'll theorize on this issue a bit later), but when benches are good they are damn good. I'm talking coarse gold, small nuggets, "chunkers," fine gold, and flakes...and lots of them. Right off the bat I can think of three different bench gravel locations (two in California and one here in New Mexico) that I'd be working like a maniac this very minute if I could easily or legally.

(Note the bench gravels to the left in this photo.)

What this means is that it pays (literally) to get your nose out of that wash or stream bottom at times and take a closer look at the gravel banks to your sides or above and beyond you. That stream or wash you're prospecting or mining currently wasn't always right where it is now. In fact, very few gold bearing streams, creeks, rivers, gullies, dry washes, or arroyos follow the same path they did 100, 1,000, 10,000, 100,000, or 1,000,000 years ago. You can take that tiny piece of mining knowledge to the bank brothers and sisters.




Doesn't it follow suit then that bench gravels represent not only exposed portions of a current stream or wash, but even old or ancient sections of it? That should give you pause to think from, a gold recovery standpoint. If it doesn't, then I don't know what to tell you.
There's more to come on this topic, so stay tuned.

(c) Jim Rocha (J.R.) 2014

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

10 comments:

  1. JR, Ya know......I never really thought about this much in my area. It's been easier to dig in the creek itself. The one big problem I've had though is that as the hole gets deeper, so does the water. Once it gets over my hip boots I've been moving a few feet upstream digging a ditch of sorts as I go.. After reading this, I can picture several spots where I could get much deeper that might be pretty good. Dang Jim, you might have just put me onto something really good here......!! I'll let you know, snow will need to go first though! Just go back from checking wolf traps, empty again...... Thanks! Gary

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  2. If there are bench gravels there Gary, you might do well to check them out. You'll have to run lots of material...try a portable sluice box. Best, J.R.

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  3. Good morning JR, When you say "lots of material", do you mean more than if I was digging in the creek? I use a small sluice mostly anyway. I shovel directly into it with a screen over most of the riffles so it is self classifying with the baseball /golf ball size rocks rolling off the screen after being washed clean.Digging dry, should I classify it first, or run it through as is? The bigger rocks would still have dirt on them doing it that way. One of the spots I've been trying is a steep sided deep canyon. There is a fair amount of water there,and some gold. I can see round river rock, as well as signs of past water high up on the banks in places (20-30 feet above the creek). Most of the gold I have found here has been downstream from that spot, with really nothing above there. I have though the gold must be coming out of the rock between those places, but maybe it is coming from that old gravel with the spring thaw. I would need to pack the dirt in a bucket to the sluice about 50 feet or so, maybe a bit more. Spring snow melt shows flowing water across this spot. Because the mountain is so tall,there is a lot of water there until around June or July. Would you work it the same as if it were wet? Dig behind boulders and such, or is there a better way? I find a lot of lead bird shot in this spot, there must have been someone shooting clay pidgins or something here once. I'm talking about an ounce or so of lead in the sluice at times.....at least I know my sluice is working! Thanks,Gary

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  4. Like tailings, the whole idea behind working benches is moving material...the more the better typically. You may run into occasional pockets and paystreaks, but it's essentially a volume game as opposed to sniping, crevicing, etc. Best, J.R.

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  5. Hi JR, now this is a subject I look forward to reading. I try very hard to try and emagine how the landscape and rivers were over a few hundred years. Water run-off and new gullies and creeks are very interesting because I know they have moved or changed shape but I often confuse myself when trying to picture the landscape of old.
    Sometimes I find in my area that what was once up is now down if that makes any sence to you?
    I llok at some creeks / gullies and it looks to me that they have moved but then have gone as far as they can because they end up against a hard wall or rock face. I assume that this is the case yet I still have trouble visualising where the original path was. I believe the gold once free of its host rock will take the easy path down hill
    There is much talk around the camp fires here in Australia and I would like your thoughts on the matter. We seem to have two schools of thought on this subject! One mob believe that the bigger gold will be futher away from where it was born, the other mob believe the bigger gold will be closer to the source. I believe the later. I know there are many veriables with hight degree of fall and so on but for this exercize lets asume the hill is of medium slope and leads to a gully gently gently making it way down the foot hills. Whats your thoughts?

    GS

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  6. I agree with you Mate. I tend to believe the latter about larger gold being closer to the source, whatever that source may have been. However, there is new scientific evidence that states some large gold nuggets were "born" without the benefit of eroding ledges or veins. Aussie scientists were the first on this, by the way. But I'm still an old school miner using the lessons taught me early on about gold deposition by old timers who knew their stuff. Hope all is well with you! J.R.

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  7. Now that I thought a little bit Muskrat {where you're digging and getting lead} I would suggest moving upstream some and try going a little deeper. My thinking is the gold was there before the lead. Over time, I'd think the entire round rock, lead, sand and gold matrix would slowly over time work it's way down stream. Gold being a little heavier than lead presents a possibility of falling out before the lead settled if they were present together at the same time. If you can find a covered ledge with cracks up on the bench I'd try some crevicing with at least a half bucket of water to classify , just to make sure the big rocks get cleaned too. If it's 50 ft. or so to the sluice that wouldn't be that far of a walk/climb.

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  8. Anonymous, Thank you for your advice. I'm looking forward to going back there this summer. I hadn't thought of the lead not sinking as deep,...you could be right. Looking on Google maps at the satellite picture, there looks like a few spots up the creek much farther that could be good too. I quit going up farther when I quit finding gold, but looking at these pictures, I think that part of the creek might not be where it used to be. I guess that's what makes this fun.......Thanks again, Gary

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  9. hmm just a quick question was looking at a guy's 20 ac federal claim and was wondering if i might file off his
    gps corners to locate a new claim?? found some signs of placer at Hope Alaska

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  10. That's an interesting questions and to be honest, I don't know the answer. You could check with the BLM on details or one of the readers here might respond with a good answer. Bes tof luck! J.R.

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