Gold in California's Coastal Ranges (Part 2)
("Franciscan" type formation near the San Andreas Fault.)
I mentioned in the previous post on this topic that I'd start providing you with details on gold in California's coastal mountain ranges, so here you go:
Gold Geology and Mineralization
The very first point that that needs to be made about gold in the coastal ranges is that these precious metal deposits are (or will most likely be) very limited in size, structure, and overall richness. This is especially true when California coastal range gold geology and mineralization are compared to that of the Motherlode Region. Then again, the presence of placer and lode gold the Sierra Nevada foothills is better documented, both historically and scientifically.
Where much of the Motherlode is underlain by a colossal granitic batholith hosting numerous forms of country rock intersected by Tertiary Channels, the coastal ranges are a geologic jumble containing rock from the Jurassic, Cretaceous, and Tertiary geologic epochs. What is different in the coastal ranges however, is the presence of numerous hot springs and a type of ocean-born rock formation known as "Franciscan." In some instances, these hot springs can deposit both silica and metallic minerals into veinlets and dikes of Franciscan origin.
Largest Gold Producer
One of the largest (and richest) gold producers in or near the California coastal ranges was the famous McLaughlin Mine (operated by Homestake Mining) which lies on the extreme eastern edge of the coastal ranges gold belt. Not surprisingly, the McLauglin was underlain by extreme geologic faulting and hot spring activity.
(The McLaughlin Mine before closure and reclamation.)
The McLaughlin Mine shut down operations in 1996, leaving over 200 local folks unemployed. However, before you go racing out the door with gold pan and shovel in hand, the gold values at the McLaughlin Mine were mainly in the form of microscopic particles and subparticles.
Calistoga, Wilbur Springs, and Island Mountain
Both gold and silver were discovered near Calistoga, California as far back as 1872 and various mines were worked there for over 20 years. The Palisades Mine and the Silverado were the big hard-rock producers at Calistoga, putting out over 100,000 troy ounces of gold alone. Once again, hots springs were directly related to the formation of the gold and silver ores of the Palisades and the Silverado.
North and east of Calistoga, mercury (cinnabar) and gold deposits were found at Wilbur Creek in 1865 near existing hot springs that still provide a popular attraction for many. Nearly 6,000 troy ounces of gold were
(Hot springs near Wilbur Springs, California.)
recovered from the Clyde, Cherry Hill, Manzanita Mines before work underground had to be stopped due to high heat. The area around Wilbur Springs still draws interest from commercial mining firms, although I am not aware of any new mining operations there.
Tips on Gold Prospecting
Farther north and west, a sulphide-based gold deposit containing pyrite and chalcopyrite near Island Mountain was first prospected in 1897. This entire deposit was formed by hot springs activity eons ago but is unlike others in the coastal ranges due to the presence of sulphide ores. Nearly 9,000 troy ounces of gold and 150,000 troy ounces of silver were taken out of Island Mountain, so this was a significant ore body. Economic geologists estimate that another 160,000 troy ounces remain unrecovered at Island Mountain.
That's it for this round. In my next post I'll talk about placer gold in the coastal ranges.
Until then best of luck to you all.
If you liked this post, you may want to read: "Characterisitics of Gold-Bearing Gold Veins (Part 2)"
(c) Jim Rocha (J.R.) 2011
Questions? E-mail me at email@example.com