Diamonds may be more than pipedreams

Droves of exploration companies have rushed to Yukon Territory in recent years to hunt for sizable deposits of gold, silver, copper, rare earth elements and base metals. Employing the most advanced geophysical and geochemical techniques available, along with their best hunches, these explorers, like others around the globe, are pulling out all the stops to find commercial quantities of the minerals currently riding the winds of strong demand and high prices.

But noticeably absent from the list of lucrative commodities being sought by the mob that invaded the Yukon this field season is one mineral resource that is firing the imagination and enthusiasm of miners exploring neighboring Northwest Territories and Nunavut - diamonds.

The reason: Diamond deposits are unlikely to be found in the Yukon because the rocks are too young. Or so goes conventional wisdom.

Yet anecdotal and proximal evidence, some of it unsubstantiated, suggests that miners combing the hills, valleys and streams of the territory these days may want to keep at least one eye open to the possibility of finding diamonds.

Weak geological evidence

Diamond prospectors had been looking for the glittery stones in North America for 450 years, but Canada had no diamond mines before the late 1990s. The Ekati mine is about 200 miles northeast of Yellowknife, Northwest Territories and is the site of Canada's initial diamond discovery 20 years ago. Today the country's handful of diamond mines, most of which are located in Northwest Territories, rank third in world production. Though current production is declining, more diamond deposits are being discovered and developed across northern Canada.

While the diamond-rich Northwest Territories lies to the east, the rocks underlying most of Yukon Territory are generally considered to be unlikely geological settings for diamond deposits.

Most economic diamond deposits in the world have been identified on Archean cratons, the oldest and most stable parts of the continents. At the base of these continental nucleuses, in a high-pressure region (about 200 kilometers (124 miles) deep), temperatures are lower than in the younger mobile belts and this creates an environment allowing diamonds to be stable. Kimberlites and other deep-seated magmas which originate in the mantle ascend to the Earth's surface and provide transportation for diamonds and other deep mantle minerals (such as diamond-indicator minerals). The mantle outside of the cratonic areas is believed to be too hot to contain diamonds.

"The Slave Province rocks (in Northwest Territories) are 4 billion years old, and our oldest rocks are something like 1.8 billion years old," Yukon Geological Survey economic geologist Lara Lewis told Mining News. "The diamonds won't remain in the upper mantle in this younger mobile belt."

Yukon Territory occupies the northern portion of a large geologic (and physiographic) province known as the Cordillera. This province is composed of relatively young mountain belts that range from Alaska to Mexico. Like most of the Cordillera, the Yukon has a diverse array of rock types that record more than a billion years of geological history, according to the YGS.

Most of the rocks have been affected by folding, faulting, metamorphism and uplift during various deformation events over at least the past 190 million years. This deformation has resulted in a complex arrangement of rock units and mountainous terrain.

There are two main geological components in the Yukon largely separated by a major, northwest-trending fault, the gold-rich Tintina Trench that extends well into Alaska to the west and dips down into northern British Columbia to the southeast.

Rocks northeast of the Tintina Trench are old (>1000 to 300 million years), mainly sedimentary and represent the Ancient North American margin. Rocks southwest of the Tintina Trench are mostly young (350 to 20 million years old), mainly igneous and metamorphic, and represent numerous crustal fragments called accreted terranes whose place of origin is uncertain.

During most of the Yukon's geological history, the terranes were not attached to North America, but were accreted to the western margin of ancient North America between 190 and 120 million years ago. Rocks in the zone between the accreted terranes and ancient North America have been extensively deformed and form a belt known as the Teslin Suture Zone. This belt has subsequently been cut by the Tintina Fault which has caused some complexity in this region. The territory's southwestern region is composed of a younger, complex mosaic of varying rock types that amalgamated and accreted to the stable sedimentary package.

Geologically, the Yukon is located outside of the North American craton in a mobile belt with Paleozoic and Mesozoic tectonic activity, according to YGS. It is, therefore, not in a conventional setting for diamond deposits, in contrast to the Northwest Territories. Glacial transportation of diamonds from the North American craton into Yukon is generally not supported, as there were no glaciations in many of the areas of Yukon with reported diamond occurrences.

1000 to 300 million years), mainly sedimentary and represent the Ancient North American margin. Rocks southwest of the Tintina Trench are mostly young (350 to 20 million years old), mainly igneous and metamorphic, and represent numerous crustal fragments called accreted terranes whose place of origin is uncertain.

During most of the Yukon's geological history, the terranes were not attached to North America, but were accreted to the western margin of ancient North America between 190 and 120 million years ago. Rocks in the zone between the accreted terranes and ancient North America have been extensively deformed and form a belt known as the Teslin Suture Zone. This belt has subsequently been cut by the Tintina Fault which has caused some complexity in this region. The territory's southwestern region is composed of a younger, complex mosaic of varying rock types that amalgamated and accreted to the stable sedimentary package.

Geologically, the Yukon is located outside of the North American craton in a mobile belt with Paleozoic and Mesozoic tectonic activity, according to YGS. It is, therefore, not in a conventional setting for diamond deposits, in contrast to the Northwest Territories. Glacial transportation of diamonds from the North American craton into Yukon is generally not supported, as there were no glaciations in many of the areas of Yukon with reported diamond occurrences.

Neighboring discoveries

Geologists say the possibility of a large diamond find in Yukon Territory is remote, but there is still the potential for unconventional diamond deposits in the territory.

Lewis said the fact that diamonds have been found near the Yukon border in western Northwest Territory, in at least two locations to the west in Interior Alaska and to the south in northern British Columbia supports the idea of diamonds in the Yukon.

The Mountain Diatreme a 600-meter-diameter kimberlite pipe was discovered in 1973 by Welcome North Mines Ltd. but subsequent exploration yielded only a few microdiamonds.

In 2002, Golconda Resources Ltd. and Shear Minerals Ltd. announced the discovery of 15 micro-diamonds and one macro-diamond from 9.97 kilograms of drill core on their Shulin Lake property in Interior Alaska.

Another half-dozen or so diamonds have been found by placer miners at various locations in Interior Alaska, particularly near the Yukon border. In fact, the diamond cartel De Beers has made several exploration forays into the Alaska bush, following up on reports of these discoveries.

In northern British Columbia, a small diamond also was reportedly discovered on the Xeno Property located at the northern end of Dall Lake,175 kilometers (108 miles) south of Watson Lake, Yukon, in the Kechika Ranges of the Cassiar Mountains. Exploration on the property in 2002 identified a lamprophyre dyke that varies in width from a few meters to over 50 meters and is exposed intermittently along strike for 2.5 kilometers (1.5 miles). A 32-kilogram surface sample of this dyke returned one transparent, green, cuboid micro-diamond measuring 0.38x 0.30 x 0.25 millimeters.

In addition, placer miner Marvin Sherman reported discovering a single diamond from his operation at Wilson Creek, B.C. in the early 1990s. Sherman said a University of British Columbia professor positively identified the stone.

Tantalizing reports

In Yukon Territory, reports of diamond discoveries date back to the 1900s. But most of the occurrences on record were not confirmed or corroborated by reliable witnesses.

Longtime Yukon prospector Bill Harris and seasoned geologist Scott Casselman compiled a report for the Yukon government in 2002 of diamond occurrences reported in the territory.

Following the discovery of diamonds in the Northwest Territories, a few diamond exploration companies quietly explored selected localities in the Yukon, according to the report.

However, the Yukon has seen no documented discovery of a diamondiferous kimberlite, lamproite or ultramafic rock in the territory.

Still, rumors and unconfirmed diamond discoveries abound. In the Klondike and 60 Mile River areas, there have been reports of at least 28 diamonds being found, primarily by placer miners who claimed they showed up in their concentrates, according to Casselman and Harris.

One tantalizing report was published in the Whitehorse Weekly Star in October 1906: "Henry Hebb, a Kluane miner arrived in town a few days ago with a pocket full of stones that were pronounced by all who saw them to be diamonds of purest (words faded). The stones were found on Dixie creek in the Kluane district imbedded in a blue clay, the same as the world's most precious stones are found in South Africa. Hebb left for the outside where he will have the stones tested by experienced lapidaries."

Casselman and Harris also cited a report by Dawson-area prospector, Tom Morgan, who said two diamonds were recovered by Jim Moravec, a private gold buyer in the Dawson area. The diamonds were recovered from concentrates that Moravec purchased from placer mining operations on Upper Dominion Creek.

Also recovered were garnet and chrome-diopside from Indian River area concentrates and garnets from Dominion Creek area concentrates. Numerous garnets also were recovered from placer concentrates on Lower Dominion Creek by Tara, Jim and Dagmar Christie at their operation, as well, according to the report.

"This high proportion of discovery relative to the rest of the territory may, in part, be due to the amount of exploration and placer mining activity conducted in this area. Elsewhere, in placer mining camps in the Kluane and Atlin areas and in Alaska, there have been discoveries, but not to the same degree. Other parts of the territory that have not seen the same level of activity may be equally prospective," they added.

Despite these unconfirmed reports of diamond occurrences, most geologists remain skeptical about the Yukon's prospectivity for diamonds. Add to this the work of a young geologist, Yana Fedortchouk conducted a research project in 2006 in which she visited the sites of the unconfirmed reports of diamond discoveries looking for indicator minerals in stream sediment. Fedortchouk found no such indicator materials.

She concluded that Yukon diamonds, if they exist, will have unconventional sources and characteristics and that unconventional techniques likely will be needed to find them.

"Prospecting for placer diamonds of ancient origin requires different methodology than prospecting for placers formed by the erosion of the conventional diamond sources, and requires a good understanding of the local geology," she added.