CARLIN-TYPE GOLD ORES
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).