(Photo I took looking straight down at a clay layer in the N. Calif. Motherlode. Gold was only in the gravel and rock atop this layer, not inside it or deeper down.)
One very good question that seems to surface again and again in my e-mail inbox as well as the post comments sections is whether or not placer gold can be found below clay layers as well as on top of them? Well, let me digress from my bench gold posts monetarily and take a stab here at answering that query.
Barriers and Seals or "Traps"
I'm shooting you straight when I tell you I've studied the mining history of the American West at length, including every gold rush (big or small) that occurred there (and here in the Southwest where I live). I've also researched numerous gold rushes in other parts of the world such as Australia, Canada, South America, and Asia. In all that study I've come across quite a few references to clay layers and desert cements (caliches) and how they "interacted" with gold. Additionally, with nearly 36 years of small-scale gold prospecting and mining experience behind me now, I think I can speak with a measure of certainty and confidence about the nature of clay layers and their relationship to placer gold.
Whether found in dry (desert) or wet placer contexts, clay layers or caliche typically act as impermeable layers to the passage of placer gold in it's downward gravitational journey. "False" bedrocks or barriers, if you will. But what many small-scale prospectors and miners fail to realize is that caliche or clay layers can also act as seals or "traps" beneath more recent clay layers closer to the surface. In some placer locations, multiple clay layers exist at various depths from the surface right on down to true bedrock and reaching these deeper layers is typically not an easy task because miners like you and I are dependent on picks, shovels, and small-scale gear combined with lots of elbow grease.
The Question at Hand
I've read many personal accounts and diaries from the California Gold Rush where the presence of clay layers is mentioned time and time again. In nearly every case the authors of these accounts and diaries were very clear in stating that they found placer gold both atop clay layers and beneath them...sometimes sitting on top of each existing clay layer or in the gravels just above them. What these astute old timers talked much less frequently about was placer gold being mixed together inside the clay or caliche layers themselves.
(My son working the same section of clay layer...small nuggets were recovered here atop, not below, the clay.)
So what does this tell you? Well, it answers the question at hand from the historical perspective pretty succinctly, don't you think? Yes, gold can be found beneath existing clay layers. A sharp-eyed and experienced geologist or mining engineer could probably back this premise up and provide all the theoretical reasons why this is so. Without being one of these "experts," I can only surmise that since the bottoms of gold-bearing streams or washes are formed in successive layers of dirt, gravel, sand, clay, and rock over time then the gold they carry will end up (in greater or lesser amounts) in paystreaks or pockets on those layers or trapped beneath them.
Now, after all this explanation and justification hoopla about gold and clay layers I'm gonna throw a great big monkey wrench into the works. I've been at this mining thing of ours for nearly 36 years and you know what? I have never found much in the way of placer gold after digging (or dredging) down through the top-most or highest clay layer. That doesn't mean there isn't any gold below the layer I worked, it just means that in my personal experience I've always found my best gold mixed in with the rock and gravel sitting in the top 6-12 inches right over an upper clay layer (including the surface of the clay itself.) Ditto for most caliches.
Yep, I hear you loud and clear...maybe if I had dug down deeper like a 3rd World coolie another 5 or 10 or 30 feet through successive layers of rock, gravel, and clay I may have struck it rich. Sorry, but as a solitary miner who usually goes it alone I'm just not going to do that amount of work on an assumption. A good sample drilling rig might answer the clay layer question best of all, but that's out of the question for most of us.
So what's my best clay layer gold answer to you as an individual prospector or miner? Why, it's right here in this post for all to see...but you'll have to make that choice yourselves!
Best to you all.
(c) Jim Rocha (J.R.) 2014