Wednesday, March 28, 2012

The Geology and Dynamics of Beach Gold Placers (Conclusion)

 (Sacramento River delta. Some of the Motherlode's gold was carried to sea through this maze of waterways.)

Here's the conclusion to my three-part series on this topic:

Gold Carried to the Sea

Famed Geologist Waldemar Lindgren didn't believe that all of Northern California's beach gold placers were formed from marine or Chico-type deposits of the Upper Cretaceous period. He postulated that at least some of the gold found in California's beach black sands was derived from the extremely rich deposits of gold found along the Sierra Nevada foothills in the Motherlode Region. Lindgren also stated that this gold was carried down by the various rivers and streams draining the Motherlode as well as areas much farther north such as the Trinity and Klamath regions.

Placer Gold Locations

Once entrained and moved westward, this placer gold was deposited in the sands of deltas formed where various rivers met the Pacific Ocean. However, Lindgren did concede the probability that most of this gold was very fine in terms of size and spread thinly over large areas by the time it reached the sea.

Beach Placers Along Lake Shores

Not all beach gold placers are associated with oceans. Some beach sands along lake shores in the U.S. and elsewhere have also produced  placer gold (but typically not in commercial amounts). Most of these gold-bearing lake shores are in regions where gold mineralization or transport occurs, including glacial gold such as that found in selected locations in the American Midwest and upper Midwest.

There have been instances in California's Northern Motherlode where rich pieces of gold ore and small pockets of placer gold have been found on lake shores or in very shallow water. These beach gold deposits were partly formed when dams were constructed on former gold-bearing rivers, streams, creeks, or other drainages.

Strong Hydraulic or Wave Action Needed

Most, but not all, gold bearing beach sands can be identified by the large amounts of heavy black sands they contain,as well as quartz gravels and finely divided quartz particles in those black sands. On most ocean and lake beaches, gold-bearing sands are often covered by layers of barren overburden sands and rock, sometimes to great depth.

Gold Prospecting Books
Gold Concentrators

Typically, some type of strong hydraulic or storm wave action is needed to "strip" the overburden off gold-bearing material or to expose any bedrock (including false bedrocks like clay layers) that underlies an ocean or lake beach placer. Once exposed, however, a beach placer like this can really produce good gold values as the gold-bearing material not only becomes exposed, but is also concentrated and re-concentrated. (I can attest to this fact, by the way.)

 (San Francisco area beach after a Pacific storm chewed off 30+ feet of overburden. Note the heavy black sands that people have spread their towels on...placer gold can be found in those sands.)

So there you have it.You may start looking at those ocean or lake beach sands as bit differently  now I suspect.

Good luck out there.

If you liked this post, you may want to read: "Gold Mining Questions and Answers: Part 12"

(c)  Jim Rocha (J.R.)  2012

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





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