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Mining News: Alaska diamonds enigma persists New geological models speculate on potential source of Alaska’s off-craton carboniferous gems; Tintina Fault aids, complicates search Shane Lasley Mining News Editor’s Note: This article is a reprint from Fortune Hunt Alaska, a magazine investigating the oil, gas and mineral potential in the “Last Frontier.” Fortune Hunt was published with the May 7, 2011 edition of Petroleum News and is available online at www.petroleumnews.com and www.mininnewsnorth.com.
North of 60 Mining News dedicates this article to the memory of Don Lasley 09/30/1944 – 05/17/2011. Lasley discovered the first independently verified macro-diamond found in Alaska. Setting aside the textbooks and adopting a new geological model may be the first requirement for the adventurous explorer seeking the source of alluvial diamonds found in Alaska.
The precious gems have been found in the workings of placer gold mines in Interior Alaska and rumors of similar discoveries swirl in other regions of the state and neighboring Yukon Territory. But neither of these Far North jurisdictions host the geological conditions required to produce the glimmering stones, according to the conventional model.
Three diamonds discovered along a two-mile, or three-kilometer, stretch of Crooked Creek in the Circle Mining District during the 1980s sparked the curiosity of geologists hoping to discover the source of the gems.
The first of the Crooked Creek diamonds, dubbed Arctic Ice, was a one-third carat stone discovered by Don Lasley during clean-up operations at Jim Regan’s gold placer-mining operations on the Diamond No. 1 claim about seven miles, or 11 kilometers, west of the mining town of Central.
The carboniferous gem was first shown to John Sims, former director of the Alaska Department of Commerce and Economic Development’s Office of Mineral Development, who forwarded the stone to a DeBeers subsidiary which authenticated the diamond.
Following Lasley’s discovery, two additional finds were made at Crooked Creek. The Warren Diamond, a 1.4 carat gem was discovered by Mary Warren about 2.5 miles, or 4 kilometers, downstream from the Regan claims. And, just downstream from the Warren Diamond, Paul Manual found a third 0.84 carat gem in the stream. All three stones were independently verified.
Though the State of Alaska, De Beers and other explorers have scoured the gently rolling hills surrounding the site of these discoveries, their investigations failed to turn up kimberlites or lamporites that host such gems in the major diamond-producing regions of the world. This lack of geological evidence caused experts to suspect the Crooked Creek diamonds had traveled far from their source, and exploration interest in the source of the stones waned.
New models A clue to the Crooked Creek diamond riddle may be found in Australia, where a similar and even more compelling enigma lies.
According to the generally accepted geological model, only the thickest portions of ancient crustal regions, or cratons, provide the unique conditions for diamonds to form. This thick continental crust – extending deep enough into the mantle to create a sweet spot known as the diamond stability field – does not exist in eastern Australia where more than 500,000 carats of placer diamonds have been recovered. Nor does it exist in Interior Alaska.
As a result, geologists theorize that some other geological event must have occurred to cause the conditions needed for diamond formation. One such idea is the “subduction” model.
Geologists suggest that as one plate dives under another, it could create diamond-forming conditions. If a subducting plate is thick enough, according to scientists, it would remain much cooler than the surrounding mantle it is plummeting into. This could create a situation that mimics the diamond-forming conditions of cratons.
It is suggested that under optimal conditions, this subducting slab carrying a carbon-rich payload could continually create diamonds that would be carried deeper and ultimately be destroyed by the diving plate. But at the end of subduction, according to the theoretical model, an opportunity arises for the glittery stones to be transported to the surface in a fashion similar to classic diamond-forming eruptions.
The deep crustal structure model, another postulate with principles similar to the subduction zone model, may better fit the conditions surrounding the Crooked Creek diamonds and other reported discoveries in Alaska, Yukon Territory and British Columbia. This theory deduces that massive structures such as large-scale fault systems could create the diamond stability field.
The confirmed diamond finds on Crooked Creek and most of the unverified diamond discoveries in neighboring Yukon Territory were located along the Kaltag-Tintina Fault, and several other rumored finds have been made along the Farewell-Denali Fault. These are the major intra-continental strike-slip faults that form the boundaries of the Tintina Gold Belt.
“You have to have a deep crustal feature that could tap deep into the earth,” said Dave Szumigala, senior minerals geologist for the Alaska Department of Natural Resources Division of Geological & Geophysical Surveys. “The best postulation now for what is working for the diamonds found in Central is that you have the Tintina Fault, and that is a deep regional structure, and maybe that has provided a conduit for tapping things below the crustal level.”
Daunting feat Although these geological models provide explorers with new ideas for what to seek when exploring for diamonds in Alaska, the search for the source of the few gems that have been discovered may still be a daunting feat.
Classic diamond-bearing kimberlite, or lamporite, pipes are very small, typically ranging from 100 meters to 1,000 meters across. The small surface size coupled with the lack of glaciation and large permafrost-bound low-lying areas of Interior Alaska compromise the effectiveness of airborne geophysical and remote-sensing prospecting techniques in this region.
Movement along the Tintina Fault – which passes within about a mile, or 1.6 kilometers, of the Crooked Creek diamond discoveries – adds to the complexity of finding the source of these placer diamonds. Evidence shows that the region south of this strike-slip fault has traveled some 300 miles, or 490 kilometers, to the west – greatly extending the prospective area.
Old fashioned geological investigating coupled with a mind open to new ideas may be what it takes to solve the riddle and discover the source of Alaska’s placer diamonds.
Shane Lasley, author of this report, is Don Lasley’s son.
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