Tuesday, February 9, 2010

True Bedrock vs. False Bedrock (Part 2)




In my first post on this topic I covered some of the characteristics of true bedrock. In this post I will discuss false bedrock and how it differs in overall character from its underlying layer of support, true or real bedrock.

The Function of True or False Bedrock is the Same

Unlike true bedrock which is composed of the predominant host or country rock found in a given placer gold locale or area, false bedrock is invariably composed of various types of clays or successive clay layers. In dry desert placers these clay layers (sometimes called caliche [kah-lee-chay]) often reach the consistency of cement or hardened concrete. This latter occurs because the intense heat of desert summers and the very alkaline constituency of dry placer clays combined with assorted gravels (and a bit of moisture) react chemically to form an extremely hard and dense material (i.e., "desert cement").

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Regardless of whether false bedrocks composed of clay are in wet or dry placer areas, their function is essentially the same. They both act as impermeable layers that prevent the transition of gold to lower points in a given streambed or drywash.

Gold Moves Ever Downward

Always remember the single-most fundamental rule of placer gold (or gold in general). Gold is a very heavy and dense metal that will continue to "work" its way ever downward until it can travel no further. As long as gold DOES NOT meet an impermeable layer like true or false bedrock, it will continue this transition to depth either continually as it does in a running stream, or intermittently as it does in dry or desert placers.

False Bedrocks Can Exist in Multiple Layers

Unlike true bedrocks however, false bedrocks can sometimes be found in multiple layers at various depths in both wet and dry placers. This doesn't mean however, that each clay layer (or layer of caliche) will have gold resting atop it. Sometimes this is so and sometimes not. It all depends on when and where the false bedrock layers were laid down in time and space.

The false bedrock pictured in the two photos at the beginning of this post is a classic "pinkish" clay bound together with stream gravels and rocks. The river-borne material resting atop this section of clay layer was auriferous and produced 3 small nuggets as well as numerous flakes and fines.

Gold Atop Additional Clay Layers?

This is because the clay acted as a typical impermeable layer (false bedrock) to all the gold deposited upon it since the layer itself was laid down. However, digging into the false bedrock itself and panning or processing that material itself produced no gold whatsoever..and trust me, I dug fairly deeply down into it to establish this fact.

However, every possibility exists that in this particular location (California's North Yuba River), that other clay layers or false bedrocks exist much farther down than this topmost layer. Depending upon when those additional layers (false bedrocks) were laid down and how they were positioned depositionally to capture gold, they might contain even better and more extensive gold values.

See what I am driving at here? Unlike true bedrock which exists (no matter how far down) as a single impermeable layer, false bedrocks can actually exist in multiple impermeable layers in one location. Finally, all of these false bedrocks will be resting atop...yep, you guessed it......true bedrock.

Other Types of False Bedrocks?

Are there other types of false bedrocks out there in gold-bearing areas? Probably. I've even heard of very densely packed river or wash gravels acting as a form of false bedrock but I've never come across this in more than 30 years of small-scale mining.

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To tell you the truth, I have a very hard time believing this one. No matter how densely packed they are, stream rocks and gravels are going to have a hard time acting as a false bedrock since they cannot be truly impermeable to gold's downward movement.

Hopefully these two posts have helped explain the difference between true bedrock and false bedrock. Good luck out there!

If you liked this post, you may want to read: "True Bedrock vs. False Bedrock (Part 1)"


(c) J.R. 2010

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

8 comments:

  1. Here in New Mexico we have a mining district that has a 'gravel concretion' that acts as a false bedrock. the concretion is not a heavy clay but a very tightly packed ancient grave deposit. And I assure you, it does stop the migration of gold downward.

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  2. I also live in NM Bo. Are you speaking of "caliche?" That will definitely stop gold...but if you are speaking of tightly packed gravels themselves then that's a new one on me...thanks for providing the info. J.R.

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  3. The stuff I'm referring to is found around Placitas. it's mostly small quartz gravel with sand and heavier minerals packed in a layer so dense that it "outcrops" just like real bedrock in some places. the old timers were finding gold in and above these concretions. I'll send a picture when I get a chance. -Bo

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  4. Thanks Bo....I'd like to see that. J.R.

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  5. I placer mine a bench deposit on the Wildhorse river in the east Kootenays of British Columbia. How I started my first test hole was by locating a layer of clay based false bedrock in a cutout for the road. It was about ten feet above the road level so I dug at the road depth. Two feet down was a 8" layer of false bedrock, 3'under that layer was a 12" layer of false bedrock, 3' deeper was a 2' thick layer of false bedrock and 4' deeper we hit our forth layer of false bedrock. It took jackhammer to break thru the first three but the forth was so hard we could barely chip it away. We were already 12' down from where we started and if the road wasn't cut in then we were about 32' below the surface. We pulled a mechanical disturbance permit on this claim and will be heading out in the next month or so to dig with the excavator thru it. With the first layer visable on the bank that makes five layers of false bedrock. I am hoping to hit the bedrock this mining season along with 85 test holes planned on a five year permit. The visable layer on the bank was clay and bedrock chunks mixed together, the other layers were coarse grain sands and river gravels that were harder then concrete. The deeper the false bedrock the harder it got. Gold values were found on top and just underneath each layer. 48 tones of gold came from the wildhorse river to date. Wish me luck but has anyone ever found this many layers of false bedrock on a single hole. Email me if you have with your story. steelhead3180@yahoo.ca

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  6. I've never come across that many layers of false bedrock (clay layers) in my nearly 40 years of mining.

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    1. Only the layer sticking out of the bank was clay. The rest were round gravels with the odd piece of broken bedrock and the sands went from fine on the first layer and kept getting extremely coarse as the layers got deeper. Makes for some hard digging with my 120 excavator still. Going to buy some ultra sharp teeth for my digging bucket to see what I can do. On my road that I am building, no layers of the first 60 meters because it was material thrown over the bank from the road at the landing. The other 340 meters of road so far has been a constant layer of false bedrock on the uphill side. Been 5 weeks and 160 machine hours to do a 400 meter roadway. Mind you I am cutting down a 25 percent grade as well. If your ever in the East Kootenays on the Wildhorse mainline, I can show you. An estimated 48 tons of gold come from that river with 45 oit of the 48 tons coming from a 5 km strip, which I am at the upstream beginning of that stip on a bench. I been trying to find out more information on so many multiple layers and it's been tough. All I know is my geologist from the technical service company thinks I am on to something big. Wish me luck

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