Carlin type gold deposits are an attractive target for explorationists, as they boast large tonnages of ore with relatively inexpensive production costs.
And in 2020, production from these deposits were the largest than from any other complex in the world.
An amazing feat, considering a majority of the current Carlin production comes from a small geographic extent – about the size of Ireland.
And the discovery of Carlin-type gold systems didn’t occur until relatively recently. As gold prospectors tramped westward in the mid 1800s to California’s rich placer gold deposits, they unwittingly walked over one of the richest gold districts in the world.
The size of Carlin systems in the Great Basin likely has to do with structures that were present well before gold was introduced. Large fault systems were formed during the onset of rifting, about 650 million years ago. As the rifting progressed, sedimentation of shallow water limestones and deeper water siltstones occurred on what is called a passive margin. Reactivation of these faults has occurred numerous times over geologic history through compressional thrusting events, intrusive events, and most recently during extension. In Nevada, it was at the onset of this last phase of extension that geologists think gold was introduced.
Hot fluids travel up these old fault pathways and find their way into the silty limestones, also called “dirty limestones”, where they dissolve the calcite and turn other minerals to clay and iron. This leaves behind a porous, sponge-like rock. Continued upwelling of hot fluids introduces bisulfide, which reacts with residual iron to form pyrite.
As more fluid is introduced, concentric bands form around early pyrite grains. Zoning around pyrite tells the story of how ore fluids evolved over time, with some bands composed of simple iron-pyrite, while other bands are distinctly arsenic-rich. These arsenic-rich bands host micron-sized grains of gold. The gold cannot be seen with the naked eye, which is why Carlin deposits are often described as hosting “invisible gold”. And although the gold is microscopic, in Carlin deposits it is abundant!
Carlin country encompasses approximately 65,000 square kilometers in northern Nevada. Of that only about 15% has been explored with the rest being obscured by upper plate rocks, young volcanics, and alluvium.
Future giant deposits are waiting to be discovered, but will require smart exploration through covered terrain – including identification of prospective land through mapping of nearby outcrops, use of geophysics to identify structures, and multiple campaigns of drilling to zero in on the mineralized system.
To learn more about Carlin systems, check out the Diamond Point, Ghost Ranch, Ecru, Callaghan, and Raven projects at orogenroyalties.com.
Music from bensound.com
Map of Ireland from vemaps.com
Crafford, A.E.J., 2007, Geologic Map of Nevada: U.S. Geologic Survey Data Series 249.
Basemaps provided by ESRI, HERE, Garmin, OpenStreetMap, and the GIS user community
Muntean, J.L., 2020, Carlin-Type Gold Deposits in Nevada: Geologic Characteristics, Critical Processes, and Exploration, Society of Economic Geologists Special Publication, no. 23, pp. 775-795.
Ridley, J., 2013, Ore Deposit Geology, Cambridge University Press, ISBN 1107022223
Forbes, The World’s Top Gold Mines, https://www.forbes.com/sites/niallmccarthy/2020/09/29/the-worlds-top-gold-mines-infographic/?sh=3526cfc65dfb