Monday, August 15, 2011

Main U.S. Gold Deposit Types (Part 1)

In the broadest sense, understanding certain aspects of gold geology and mineralization characteristics is fundamental to success as a gold prospector or miner. Although there are a few anomalies out there, most highly mineralized, gold-bearing deposits in the U.S. fall into major deposit zones or types.
Here are some of these major deposit types :

1) Carlin: Named after the famed Carlin, Nevada open-pit mines, these sorts of deposits are formed primarily from micron-sized gold particles embedded in carboniferous limestone and dolomite. Carlin-type deposits rarely produce placer gold but can be unbelievably rich (if difficult and expensive to mine). Other minerals associated with these deposits are pyrites, jasper, calcite, and carbon-related minerals. Major geological faults and fractures are typical of the underlying geological structure of Carlin-type deposits.


(Here's how you recover micron gold from a Carlin-type deposit.)

2) Creede: These types of gold deposits should be most familiar to Colorado miners, although they occur in other parts of the West as well. Most Creede-type deposits are from the Tertiary Period and are often associated with ancient volcanic calderas. Associated minerals for these deposits include galena, sphalerite, silver sulphides, and free-milling gold in quartz and calcite. Placer gold is often found in quantity with these deposits.

Where to Find Gold

3) Cripple Creek: Named after another very famous gold district in Colorado, Cripple Creek deposits are formed from gold tellurides in veins and along along ancient breccia "pipes." In addition to gold, you'll find minerals like calcite, barite, monzonite, and syenite in Cripple Creek-type gold deposits. Once again, placer gold is rare in these types of deposits because much of the gold here will be in chemical and not free-milling form.

(Gold King Mine near Cripple Creek, Colorado back in the day.)

4) Cyprus: Why this name was chosen for these deposits I have no idea, but they can be very rich and fairly easy to work. Cyprus-type deposits are typically highly pyritic and contain chalcopyrite in large quantities as well. They too tend to be associated with ancient (Tertiary) volcanics that have "morphed" over time. You can find basalt, diabase, dunite, and gabbro with Cyprus deposits but don't expect large amounts of vein or placer gold...these deposits are minor gold producers for the most part.

Gold Pans
Gold Prospecting Books

5) Goldfield: These types of gold deposits are named after the area (and town) in Nevada where they were first discovered. They can be extremely rich in lode gold associated with epithermal quartz and alunite veins and, at times, breccia pipes from the Tertiary Period. Other rocks and minerals associated with Goldfield-type deposits are latite and rhyolite. Minor placers may exist downslope from these deposits.

That's it for this round. There are more deposits to come, so stay tuned.

(c)  Jim Rocha (J.R.)  2011

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


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