Thursday, August 18, 2011

Main U.S. Gold Deposit Types (Part 2)

(Beautiful specimen of Homestake-type deposit gold ore from South Dakota.)


Here are more of the main gold-bearing deposit types in the Unites States:

6) Homestake: This gold deposit type is named after the Homestake Mine in South Dakota, one of the biggest and longest-lasting gold producing hard-rock mines in the entire U.S. Homestake-type deposits are noted for their heavily pyritic strata containing very rich auriferous quartz/calcite veins in greenstone and iron formations. Geologists believe that Homestake deposits were formed from ancient hot springs that were once deep under water. Most Homestake-type deposits produce extensive placer gold deposits as well a free-milling lode gold.

Gold Concentrates
Gold Pans
Gold Concentrators

7) Kuroko: This type of gold deposit shares similarities with the Homestake type in that Kuroko gold deposits are also heavily pyritic and born of ancient, submerged hot springs. Other minerals associated with Kurokos include sphalerite, chalcopyrite, and of course, pyrites. The host rock or country rock for Kuroko-type deposits is typically rhyolite or dacite, although some gold-bearing Kurokos have actually been found in black shales (a highly unusual host for gold, by the way.). Placer gold is often found in conjunction with Kuroko deposits.

(Map of known and potential Kuroku-type gold deposits in the U.S.)

8) McLaughlin: Usually associated with volcanics that have been "morphed" into siliceous (containing silica) rock, McLaughlin-type gold deposits will contain quite a range of minerals, including pyrite, sphalerite, cinnabar (mercury ore), stibnite, realgar, and sulfur. McLaughlin deposits contain free-milling gold found within opaline quartz veins. Small placer gold deposits can be found with or near McLaughlins.

(Cinnabar or mercury ore is often found with McLaughlin-type gold deposits.)

9) Motherlode: Unquestionably the most famous of all U.S. gold deposit types, Motherlode deposits are some of the richest ever found on earth, anywhere, any time. Usually formed from mesothermal, granitic batholiths, Motherlode deposits typically contain massive gold veins locked into a range of host rocks, including greenstone, serpentine, and slate (among others). Moreover, Motherlode-type deposits contain the most extensive and abundant gold placers ever found or worked. For example, the main part of California's Motherlode gold belt is approximately 150 miles long and 50-75 miles wide. Gold can be found in veins and veinlets as well as in placer form in nearly every river, stream, creek, or wash in the region.
I'll complete this series in my next post. Until then I wish all of you out there the very best of luck.

If you liked this post, you may want to read: "California's Motherlode: Where Did All That Gold Come From?"
(c)  Jim Rocha (J.R.)  2011

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


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