Gemstone Exploration Techniques

The techniques for finding and recovering gemstones have changed little since the first human was dazzled by a precious colored stone. Today, almost all of the world’s gemstone mining is done on a low-tech, artisanal level – miners digging by hand or with light equipment, seeking pay dirt.

In most places, exploration is a hit-and-miss process, with deposits typically being small and widely dispersed. Miners dig, and either they get lucky or they don’t.

Slowly, however, some high-tech applications are making inroads into this ancient industry. They’re brought in mainly by the larger companies who evaluate deposits to determine if they are worth mining, while mapping the most likely places to dig. The corporate experts often come from backgrounds in metal or diamond mining, where this type of exploration is standard and adequately funded by investors.

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Ground Penetrating Radar (GPR)

The most widely-used gemstone exploration techniques today are ground-penetrating radar, known as GPR; trace-element analysis, which involves seeking signature elements as clues to where gems may lie; and use of a device called a “terra thumper,” which identifies differences in the structure of the host rock through seismic analysis.

GPR has proven useful in providing subsurface mapping of potential gem-bearing pockets, or “vugs,” but the readings can be confused by moisture in the ground, and they can’t separate gem-bearing pockets from the non-gem-bearing ones.

Experts disagree on whether using devices like GPR can ever be cost-effective in exploration for gemstones, which normally occur in small lodes and do not fetch as much as diamonds or gold in the marketplace.

Experiments in mapping gem deposits with GPR have already been tried, with variable success. For example, in 1994 GPR was used in the Old Himalaya tourmaline mine in California. At first, ground moisture produced indistinct imagery, but further experimentation yielded enough information that searchers were able to discern subterranean pockets. However, the question of whether recognizable vugs contained gems still had to be answered by digging.

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According to Bill Larson of Pala International, owner of the Himalaya Mine, “The imagery was good, but you couldn’t tell if it was a pocket or a crack. So you mine and find a clay seam, You couldn’t discern an actual gem pocket at that time. But they did find pockets. They were running at that time around 30 precent, and they may have jacked it up to 40 precent, but we were shooting six false anomalies for every four that we got. We never found a big one.”

Despite results like these, some believe that GPR is the wave of the future for gemstone exploration. Geophysicist Jan Francke of Associated Mining Consultants Ltd. is regarded as a leading expert on the use of GPR in gemstone applications. “I wouldn’t hesitate to say it is the most ideal and best-suited technology to image the high resolution needed to see very small features within pegmatite,” he told Colored Stone.

The main drawback with GPR, he said, is the penetration depth. “The higher resolution we need, the less penetration we get. The general rule is that anything beyond three meters [9.8 feet] is unrealistic. But the resolution within that three meters is down to the centimeter scale. So we are seeing very, very small items 300 centimeters into a rock base or a dyke.”

Francke said the images produced by GPR are virtually threedimensional, allowing prospectors to examine the size and shape of vugs. “Historically, we’d just go back and forth with a GPR system to develop a three-dimensional picture. But within the past six months or so, we’ve developed a multiple-channel GPR, so you complete one sweep with the system and it takes multiple cuts and immediately displays it in the third dimension. So not only do we see where the anomaly is, for example a vug, but we see the depth and its shape all in one pass.”

The devices themselves are small and portable and can be passed over target areas in a wheeled cart, dragged by a bicycle, or carried by hand. The machine consists of a laptop computer, a control box that fits in a backpack, and a 12-volt battery. Francke says smaller units sell for as little as $20,000, making them economically viable for working small claims.

GPR only works on ground level and is not applicable to airborne exploration. Making GPR airborne-friendly involves some complicated physics which presently lead to diminishing returns, Franke said. He added that GPR works well in most alluvial environments, but in sites containing conductive clays the technique is not appropriate as the conductivity renders confused imagery.

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“If GPR is not appropriate, the next option is refraction seismic methodology,” says Francke. “It’s simply going to give us the base of the channel, in other words, show us where the bedrock is. It may show you where pockets of gravel are. but it’s a much lower-resolution technique than GPR.”

The use of GPR is not meant to replace trenching or drilling. but merely to find the most likely places to dig About 99 percent of GPR usage worldwide is for civil engineering applications such as the locating of buried pipes and other structures. Whether GPR manufacturers will work toward improving mining applications will depend on the extent to which miners embrace the technology.

Trace Element Analysis

Another popular technique for prospecting is trace element analysis of areas suspected of containing gemstones. An extensive study has been made of an alluvial sapphire deposit in Montana by researchers from the University of Toronto.

The scientists were able to catalog several trace elements that naturally occur in sapphire-rich areas. This data can now be used as clues in other areas where sapphires or other colored stones are believed to exist.

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A major application of the trace element analysis technique took place at the Seahawk emerald mine in the Piteiras region of Brazil. Seahawk President Louis A. Lepry explained that his company had been focused on gold exploration in Latin America and in 1998, with -old prices down, the publicly-traded company broadened its mandate to include colored stones.

“We approached that properly like we would a gold property and conducted systematic grid sampling of soil,” he said. “There was no real outcrop on this particular property, just weathered soil, extending anywhere from two to 50 meters [6.6 to 164 feet] in thickness. We did a multi-element soil geochemistry survey on a grid across what we knew to be the highest potential, and that came from the fact that we had regional geophysical surveys in hand. We gridded about a two-square-kilometer [0.8-square-mile] area and conducted those surveys, and then followed on with an auger sampling program. That body of information was then condensed into defining drillable targets. All the emeralds in the region had come from a very distinct horizon, two to five meters [6.6 to 16.4 feet] thick.”

Seahawk drilled a total of 45 holes with an average depth of about 180 meters [590.4 feet] and found emerald crystals in some of the cores. The exploration team conducted multielement geochemistry testing of several thousand samples, looking for 30 different elements. Lepry said the operation mined roughly 800 tons in 2001 and is now geared to produce 50 to 106 tons a day.

Higher Technology Application

Not all experts agree that higer technology has an important role to play in the world of colored stones, however. Mining consultant Gordon Austin conceded that high technology applicable to diamond exploration. where the indicators are well known, and the economic returns justify expenditure, but he frowns at suggestions of local subsistence miners and prospectors being replaced by Space Age scanners.

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“You can use ground thumpers – seismic survey – to map bedrock alluvial contours, which can be used in exploration of alluvial deposits,” he said. He noted, however, that most gemstone deposits are discovered by artisanal miners based on local folklore and surface exposures and recovered simply by mining a likely spot.

“Because of the very nature of gemstone deposits,” he contends, “they’re extremely difficult to do exploration for.” He points to several instances where large sums of money spent on exploration could have been better invested in the actual mining. Aside from that, large sums of money generally mean larger companies, and not many large concerns are involved in gemstone mining at the present time.

Social and Economic Impact

Another issue is the social and economic impact of automating exploration. Local miners have supported themselves for generations by low-tech mining, and some goverments are taking a hard stand against proposals by foreign companies to bring improved exploration and recovery methods to historical gemstone mines.

Sri Lanka has banned the use of most high-tech methods, says Austin, “because they don’t want mass mining to come in and tear the country up, and possibly deplete a resource that has supported maybe 25 percent of the country’s population for several hundred years. Operating as it is right now, [mining will] continue to support a large part of the population for another couple hundred years.”

Despite the drawbacks and uncertainties of high-tech gemstone exploration, some progress is inevitable. What lies in the future? According to Francke, some technology that has been developed for military and law enforcement usage may well be applicable to mining and exploration in the future. One promising technology involves extremely high X-ray frequencies, a technique presently being used by customs inspectors to see through the walls of shipping containers. This and other devices may become the ultimate companions of or heirs to – the pan, the pick, and the shovel.

By Martin Stone
© Colored Stone - 2002
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