Thursday, February 4, 2010

Prehistoric Rivers of Gold (Part 4)

(A small section of Motherlode Tertiary gravels exposed along a hillside.)

In my previous two posts on the subject of prehistoric rivers of gold I’ve quoted from geologist Waldemar Lindgren’s famous treatise, “Tertiary Channels of the Sierra Nevada of California.” Now I want to elaborate on a few specific pieces of information culled from Lindgren’s work.

Tertiary Channel Geography

Lindgren correctly identified the key geographic locations of Northern California’s prehistoric auriferous rivers as residing within the boundaries of Butte, Sierra, and Yuba counties. Remember, however that it’s been over one hundred years since Lindgren wrote “Tertiary Gravels of the Sierra Nevada of California” and much more has been learned about Tertiary Channels since his ground-breaking work.

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Other California counties (including Nevada County) also contain gold-bearing Tertiaries. Interestingly enough, it’s been postulated that some of these prehistoric or ancient rivers of gold extend as far north as Alaska and as far south as Central and South America. It should be noted here, however, that the geologists are not all in agreement on this particular point.

There are other locations both in the United States and elsewhere that contain Tertiary gravels but not all of these are true “rivers of gold.” In other words, not all of these Tertiary Channels contain gold in any significant quantity….like modern rivers, some are non-auriferous.

The Importance of Quartz and Metamorphic Rocks

A significant finding in Lindgren’s “Tertiary Gravels of the Sierra Nevada of California” is the fact that some the best gold values in these prehistoric rivers were associated with areas where metamorphic rocks and quartz were combined in abundance. Quartz is likely known to all of us, but if you’re not sure what metamorphics are here is a simple definition:

Metamorphics are rocks that typically undergo drastic structural changes over time, with the primary “change” drivers being intense heat and very high pressure. Examples of metamorphics include gneiss, schist, marble, quartzite, and slate.

Many of the gold-bearing regions of the American West and Southwest contain metamorphic rocks, including most of placer gold areas I visit or work here in New Mexico. Typically these metamorphics were associated with ancient volcanic activity.

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Moreover, Lindgren states that the richest Tertiary gold zones in California’s Northern Motherlode region are found where channels of metamorphics and quartz gravels cross or “bisect” existing auriferous vein material. These so-called Tertiary “contact zones,” if located and worked effectively, can produce astonishing amounts of placer gold (as proven in the 19th and early 20th Centuries).

In my next post on this subject I’ll discuss Lindgren’s findings concerning the typical distribution of gold values in Tertiary Channels as well as the amount and size of that gold. Until then take good care and best of luck to one and all.

If you liked this post, you may want to read: “Prehistoric Rivers of Gold (Part 3)”

http://goldbedrockgold.blogspot.com/2010/01/prehistoric-rivers-of-gold-part-3.html

© J.R. 2010

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

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