File:Auriferous altered mudshale with contorted bedding (Carlin-type gold ore) (Turqoise Ridge Gold Ore, Nevada, USA) 3.jpg

English: Gold ore from Nevada, USA. (cross-section view)

This is an example of Carlin-type gold ore, a famous category of gold deposits in northern Nevada. Much of America's gold production is from these deposits. About 200 million ounces of gold have been produced from a 200 x 300 kilometer area in northern Nevada. Carlin-type gold deposits are hosted in sedimentary rocks and contain very finely disseminated gold. The gold is not visible (Au masses occur at the micron scale), but the gold content is high enough to make mining economic. Carlin-type gold ore rocks in northern Nevada are silicified and decalcified. The fluids responsible for this traveled along major normal faults in Nevada’s Basin and Range Province. The same fluids also introduced the gold into the rocks. Gold mineralization in this particular area occurred 39 million years ago, during the late Middle Eocene.

Seen here is a scarce example of Turquoise Ridge gold ore, from the Turguoise Ridge Mine of Nevada, which is immediately adjacent to the class Getchell Mine. This Carlin-type gold ore is an auriferous altered mudshale derived from the Lower Ordovician Comus Formation. Notice the contorted bedding - this is post-depositional, soft-sediment deformation. The rock has been decalcified and weakly silicified. There’s no visible gold - it’s all finely disseminated. The ore grades out to about 0.35 ounces of gold per ton of rock.

Locality: >2000” depth, HGB3150 area of the Turquoise Ridge Mine, eastern side of the northern end of the Osgood Mountains, northeast of Winnemucca, Potosi Mining District, eastern Humboldt County, northern Nevada, USA (41º 12’ 30” North latitude, 117º 15’ 10” West longitude)