Monday, July 20, 2009

Working Clay Layers (Part 1)

 (Gold-bearing clay layer, N. Yuba River, California)

I am sure most of you out there are familiar with the fact that clay layers often act as a type of false or "artificial" bedrock. But what some of you may not know is that there is a wide variability in clay layer characteristics, including their ability to produce placer gold. However I have distilled these characteristics into 4 main areas:

Location

In over 30 years of small-scale placer gold mining I have found clay layers in just about every (but not each) location I've prospected or mined. This includes dry desert placers, semi-wet placers containing only small amounts of water, and full-blown wet placers with large volumes of running water available.

Gold Panning Kits
Gold Concentrators
Metal Detectors

The main variation to be aware of here is that the clay layers in many desert or dry placer locations are in most cases composed of caliche ("cah-lee-chay"), the notorious "desert cement." Caliche forms from the hot sun and the calcium-rich or alkali soils one often finds in desert regions. (If you've ever drywashed a desert claim you know only too well how difficult caliche can be to process.)

Colorization

I have come across clay layers that were red, pinkish red, reddish orange, yellow, yellow-orange, blue, bluish gray and every variation in between (this list is by no means complete, by the way). I suspect the color of a given clay layer is more dependent upon the surrounding geology than it is on its capacity for producing good gold values. (The main exception to this rule can be found in the Motherlode Region's "Great Blue Lead" where tertiary gravels and distinctive bluish-gray clay layers can produce exceptional gold values.)

If you take a look at my photo (taken while working a private claim along the N. Yuba River last summer) at the beginning of this post, you can see that the clay layer takes on a slightly reddish-orange hue. But in some spots this same clay layer became very pinkish red in tone. So the point I am making here is that even within one small area a clay layer can make subtle color changes.

Composition

Most clay layers I've worked were composed of the clay itself combined with lightly to moderately cemented rock and gravel. Again, the main exception comes in the form of caliche in dry placer areas.
Although caliche provides the extreme end of the "hardness" scale for clay layers, the remainder usually run the gamut from very wet and "gooey" to moderately gummy, to only slightly damp (with every variation in between). The key point here is that each type of clay layer provides a "stopping point" for any new gold being deposited upon it.

Disposition

The actual disposition of a clay layer has much to do with its ability to be a gold producer. Clay layers situated outside of normal gold deposition points (i.e., inside bends, low-pressure areas, etc.) typically are not good producers of placer gold.

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Conversely, those clay layers laying either in or adjacent to good gold deposition points can be excellent gold producers. Mining knowledge, experience, and a thorough sampling strategy are your best allies in determining whether a given clay layer will prove fruitful or not.

There's more to come on this topic in my next post. Until then, be safe and keep smiling.

If you liked this post, you may want to read: "Making a Living at Gold Mining: Steve Hagar's Story (Part 2)"


(c) J.R. 2009

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

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