Aerial imaging helps mine exploration

Advances in aerial imaging that better define Nevada's proven mineral trends are helping the state's hard rock mining companies narrow down their search for new gold deposits - and providing no shortage of exploration work for the state's fleet of drilling contractors.

Companies such as Barrick Gold and Newmont Mining Corp. have long used standard aerial photography in their quest to discover new mineralization zones. However, the advent of newer techniques, such as spectral imaging and gravity mapping, are giving Nevada geologists a clearer picture of what to look for in their exploration efforts.

Spectral imaging measures the amount of sunlight being reflected from the Earth's surface. Various minerals have differing reflective wavelengths and other characteristics that can be measured with instrumentation in aircraft flyover, and those characteristics give exploration geologists a better understanding of the mineralogy of a particular area. Companies seek areas with "signatures" similar to Nevada's many gold-rich trends.

The technique works best in areas such as Nevada with highly exposed rock formations and little vegetative cover, says Ed Cope, vice president of exploration for Barrick Gold's North American operations.

"It is just another tool, not a silver bullet in any way, shape, or form," Cope says. "When gold deposits are forming, the fluids that bring gold into the rocks impart a change or signature called an alteration, and spectral techniques are used to detect that alteration."

Gravity mapping uses gravitometers to measure the density of materials in basins covered by gravel and other materials that are masking gold deposits. The measurements are used to determine the depth of the cover, as well as variations or contrasts in the density of rocks that can help determine mineral-rich faults. The technique, which is still being refined, enables geologists to explore large areas of land but is rather costly.

Because exploration geologists in Nevada know what gold deposits look like, they can use that understanding to find similar spectral signatures and gravitational measurements when exploring in places such as New Guinea or Pakistan.

Both techniques are more applicable and beneficial to unexplored new frontier areas as opposed to Nevada, which has been fairly well scoured by the boots of exploration geologists, Barrick's Cope says.

"The ranchers and hunters have a pretty good network of roads (in Nevada) and we can get pretty much anywhere we need to go," he says. "But Nevada is an exceptionally well-endowed area. We will employ these techniques to key in on a new area, or to collect detailed information on one of these trends where we are doing some work.

Aerial work typically is followed by boots on the ground, Cope notes, because geologists still need to go into the field and verify aerial findings. Aerial exploration also helps companies pursue new acquisitions and stake claims on areas that show promise.

"We wouldn't plan a drilling program on aerial information, but they provide us with a couple of different tools to help us key in on areas where the geology may be obscured," Cope says. "The objective of all of it is to get us to that next gold mine discovery."

Newmont has pioneered development of geophysical techniques that include gravity and magnetic imaging, which aid the company in exploration for new and covered deposits, says spokeswoman Mary Korpi.

Gravity mapping and spectral imaging typical are outsourced to Canadian companies. However, the work has led to unprecedented growth for northeastern Nevada drilling companies and roughneck crews, many of which are located in Elko.

Barrick Gold will spend nearly 40 percent of its global exploration budget in Nevada this year, says company spokesman Lou Schack. Barrick employs a staff of about 85 geoscientists and technicians, as well as scores of additional consultants.