Carlin-type gold deposits -- discovered just 50 years ago and found only in Nevada -- have turned the Silver State into one of the world's top gold producers. In the short time since the rare gold deposits were unearthed, scientists have been stymied by questions of how they came to be.
Until now.
Geologists from 51³Ô¹ÏºÚÁÏ and UNR developed what's being touted as the most comprehensive model to date to explain how the unique gold deposits formed. It's a case of perfect conditions, they say. The team contends that magmatic activity in the region roughly 40 million years ago combined with geologic processes already under way, to produce the deposits.
"There is no other place we know of at this time on the planet that contains this form of gold in this concentration," says 51³Ô¹ÏºÚÁÏ geoscience professor Jean Cline, who has spent the better part of 20 years examining Carlin-type gold deposits. "This tells us that whatever was involved in forming these deposits here in Nevada was unusual and didn't happen in the geologic past in many, if any, other parts of the world."
Since mining for Carlin-type gold began in Nevada in 1961, more gold has been extracted from the deposits -- more than $225 billion worth at today's gold prices -- than was ever mined during the California gold rush of the 1800s.
Named for the Nevada town near where it was first discovered, Carlin-type gold is invisible to the naked eye. It appears only at the atomic scale embedded within the mineral pyrite, better known as fool's gold. Unable to see the gold, early prospectors missed the vast Carlin-type deposits. That invisibility is also what makes Carlin-type gold so hard to find and even more difficult to analyze.
"Prior to the discovery of Carlin deposits, most major ore types were already described in textbooks, giving geologists a framework for how metal ore deposits form," says Adam Simon, 51³Ô¹ÏºÚÁÏ geosciences assistant professor and member of the research team. "Then came the Carlin deposits and the tendency was to fit them into existing types. But this doesn't work, which led to controversy among geologists that still exists."
Back to the Future
So what makes Nevada so unique, geologically speaking?
Over the region's geologic history that spans hundreds of millions of years, mountains have been built, ocean levels have risen and fallen and -- here's the important part for our story -- Nevada's rocky crust has been pulled apart slowly to uncover the terrain of the Great Basin.
"The formation of Carlin-type gold deposits in Nevada seems to be related to common geologic processes that uncommonly overprint one another to a remarkable degree only in Nevada," says Simon.
The formation model suggests that Carlin-type gold deposits formed when high-temperature fluids rose from deep within the earth's surface, carrying with them gold and other metals.
The source of the fluid has been debated, though the research team presents new evidence that links the age of the deposits to magma released during intense tectonic activity.
"The deposits formed at temperatures similar to gold deposits associated with volcanoes, but Carlin deposits are hosted not in volcanoes but in sedimentary rocks," says Simon. "Plus, the chemistry of the ore fluid is similar to that found in ore fluids formed in volcanic environments -- not sedimentary rocks -- which makes them more unique."
And harder to figure out.
A combination of low pressure and high temperatures as well as large geologic faults channeled the fluid toward the earths' surface. The fluid cooled and formed the ore fluid essential for depositing gold-rich pyrite into surrounding rocks.
"Carlin-type deposits represent a perfect storm of Nevada's ideal geology -- a tectonic trigger and magmatic processes, resulting in extremely efficient transport and deposition of gold," says John Muntean, a research economic geologist with the Nevada Bureau of Mines and Geology at the University of Nevada, Reno. "Understanding how these deposits formed is important because most of the deposits that cropped out at the surface have likely been found."
A New Gold Rush?
Geologic models are the blueprints that exploration geologists use to scour the globe in search of certain combinations of rocks, structures, and tectonic history that could lead to the next big discovery.
"Which model a geologist believes will determine where they spend their exploration dollars," says Cline. "What we can see in surface rocks or drill core is usually only the uppermost part of the ore system; the source of the fluid and the processes that generated them are miles below the surface, leading to ambiguous results and heightening the need for reliable models."
The newly published model may not directly result in new discoveries, researchers say, but formation models can lessen the risks associated with ongoing, costly exploration efforts. They also can lead both academia and industry to re-examine current exploration techniques and work together to turn new ideas into practice.
"Our model won't be the final word on Carlin-type deposits," Muntean says. "We hope it spurs new research in Nevada, especially by people who may not necessarily be ore deposit geologists."
The model won't be the final word from the research team, either. They have targeted areas of Northern Nevada that may help unearth further details on the formation of Carlin-type gold deposits. The team also will evaluate other parts of the world, namely China, with a similar geologic history to see if similar gold deposits have formed.
"To finally add some of the pieces of the puzzle that I've been working on for more than 20 years to those that other geologists have discovered -- and to construct a coherent and plausible story -- is very exciting," says Cline.
The formation model was published in the February issue of the journal Nature Geoscience. The work was funded by grants from the National Science Foundation, the United States Geological Survey, Placer Dome Exploration, and Barrick Gold.
