65% of America's gold production is from the famous Carlin-type gold deposits of northern Nevada.  That's about 6.5% of the world's gold production.  About 200 million ounces of gold have been produced from a 200 x 300 km 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.  As examples, two Carlin-type gold ores from adjacent mines in northern Nevada are shown below.  Gold mineralization in this particular area occurred 39 million years ago, during the late Middle Eocene.



Twin Creeks Gold Ore (bedding plane view; 9.9 cm across) - this Carlin-type gold ore is from the Twin Creeks Mine.  The lithology is an auriferous, silicified-decalcified siltstone/mudstone deriving from the Comus Formation (Lower Ordovician).  The gold is finely disseminated and is too small to be visible to the naked eye.

The rock comes from the Twin Creeks Gold Deposit, mainly located in the nose of a fold.  This ore grades to about 0.20 to 0.25 ounces of gold per ton of rock, which is “high-grade” for this particular mine.

Locality: northern end of Twin Creeks Mine's Megapit, at about 800' below pit edge, just west of Rabbit Creek wash, ~8 km northeast of the Getchell Mine, northeast of Winnemucca, Potosi Mining District, eastern Humboldt County, northern Nevada, USA (41º 15' 08" North, 117º 10' 25" West).



Turquoise Ridge Gold Ore (cross-section view, ~5.0 cm across) - this Carlin-type gold ore is from the Turquoise Ridge Mine.  There’s no visible gold - it’s all finely disseminated.  The Turquoise Ridge Deposit is immediately adjacent to the classic Getchell Mine.  The ore grades out to about 0.35 ounces of gold per ton of rock.

Lithology: auriferous altered mudshale with highly contorted bedding (post-depositional, soft-sediment deformation).  The rock has been decalcified and weakly silicified.

Host Rock Stratigraphy: Comus Formation, Lower Ordovician

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, 117º 15' 10" West)




Most Carlin-type gold ores info. from:


Keith Wood (pers. comm.) (geologist at Twin Creeks Mine)


Tretbar et al. (2000) - Dating gold deposition in a Carlin-type gold deposit using Rb/Sr methods on the mineral galkhaite.  Geology 28(10): 947-950.


Muntean (2010) - The origin of Carlin-type gold deposits.  Oral presentation at the 2010 Geological Society of America Annual Meeting (Denver, Colorado).



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