Tuesday, May 29, 2012

Gold Deposition in Dry Placers (Part 2)

 (The best piece of mining gear for use in dry placers, the dry washer.)

Think Outside the Box

Remember, the key factor to bear in mind when sampling or mining dry placers is the overall erratic nature of gold deposition in those desert or dry gold locations. Once again, one of the best tips I can give you is to learn to think outside the box when it comes to your dry placer gold mining efforts. OK, back to the task at hand.

Most dry or desert placers will contain gold that is rough or coarse, with very little rounding or wear caused by...yep, you're correct...water. Quite often, desert placer gold will still contain particles of its host rock attached. Why is this, you ask? Once again, the lack of consistent hydraulic (water) flow and nearness to source veins or lodes are the two most common reasons.

Contributing Factors

The lack of consistent water flow in dry placer locations means that gold particles of all sizes and shapes (this includes fine gold as well as nuggets) tend NOT to move their way down through the gravels of washes and arroyos as easily or as quickly as gold does in wet placer environments. Another contributing factor to this fact is that the gravels found in dry placer areas are typically angular or sub-angular, which means they tend to "interlock" and act as a barrier to downward movement more than the rounded rocks and gravels typical of wet placers.

Mining Equipment
Gold Concentrates
 
The next point to understand here is that placer gold may be entrained at just about any depth in a given dry wash or arroyo but it's been my experience that most desert or dry placer gold is contained in the top 2 feet  of gravel, with much of that mixed into the first 6-8 inches. Sure, there are exceptions to this rule of thumb...I'm just speaking here from my personal perspective.

Almost a Complete Reversal

Each time a flash flood or rare high-water event occurs the topmost layer of gold (typically contained in the first 6-12 inches of gravel) will move or mix slightly downward as new gold is laid down above it. Then again, I've seen numerous instances where an existing dry placer gold layer has been exposed on or near the surface by the force of the flash-flooding. (See how confusing dry placers can be?)

 (Old dry placer tunnel headframe.)

Eventually however, dry placer gold will (like its wet placer cousin) work its way downward toward bedrock or any false bedrock layers like clay or caliche (desert cement). Interestingly enough, in dry placers the finer gold particles will move downward more quickly and easily than the larger heavier pieces like nuggets. This is almost a complete reversal from gold's downward deposition movement in running streams and is something you need to bear in mind when out in the field.

Hang tough out there and the best of luck to you.

If you liked this post, you may want to read: "One Miner's Claim Nightmare"

(c)  Jim Rocha (J.R.)  2012

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

2 comments:

  1. This explains alot. Recently, a fellow desert rat and i conducted a little experiment. We dry washed 3 spots on an arroyo bank. The 3 sites were within several feet of each other. Basically above and below a 3-4 ft thick caliche line. All 3 holes produced gold, BUT, the area above the caliche line produced the most. The top 3 feet of material. Before reading this series, i was confused, now i am more educated. Thanks.

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    1. Glad that this series is a help to you. There's more to come so stay tuned. Thanks for commenting...J.R.

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