Geologic model posits Alaska diamonds

Just because Alaska is nowhere near any of the world's kimberlite provinces, doesn't mean that geologist David Szumigala is giving up hope on finding diamonds in the state. "Thinking conventionally doesn't mean that you are right!" is Szumigala's approach to the problem. Without the presence of kimberlite, he had to draw up an alternative geological model that would explain how diamonds could theoretically be formed here.

In what Szumigala calls his eclogite model, carbon-rich sediments get subducted (sucked down a trench) 150-200 kilometers. The carbon is converted to diamonds and magma formed at these depths capture the diamonds and transports them to the surface in areas of volcanic arcs. Eclogite is a coarse-grained, deep-seated ultramafic rock, consisting essentially of garnet and pyroxene. Rutile, kyanite, and quartz are typically present.

Various diamonds and indicator minerals have been discovered on rare occasions in widely disparate areas of the state. Most of the diamonds were found accidentally during placer mining, although there is also a lode diamond exploration project under way currently at Shulin Lake in Southcentral Alaska.

Crooked Creek find in 1980s

The first three documented macrodiamonds in Alaska were found by three different operations about one to two miles apart from each other at Crooked Creek in the Circle mining district in the early 1980s, Szumigala told the Arctic International Mining Symposium in Fairbanks March 16. There would have been no reason for the three operators to collude in a scam and draw unwanted attention to their discoveries, he pointed out. One of the diamonds was a 0.33-karat white octahedron, another was a 1.4-karat pale yellow dodecahedron, and the third was a 0.8-karat twinned dodecahedron.

"We know that this was an alluvial occurrence, not sure whether it's Pleistocene or Tertiary, it's still pretty poorly understood what the gravels in that area are," Szumigala said. The diamonds were recovered during normal clean-ups in concentrates derived from Ross sluice boxes. At the time of their discovery the state concluded that the lode source was probably located to the north of the Crooked Creek location.

Goodnews Bay find in 1970s

The U.S. Geological Survey also documented microdiamonds that were found in with the platinum at Goodnews Bay in the 1970s. "Once again, these are alluvial diamonds, but they're in a totally different place in Alaska, so it'd be very difficult to say that those are both the same source," Szumigala said. Goodnews Bay is on Alaska's southwest coast, in the Bethel area. The handful of other alluvial diamonds that have been found in Alaska were also scattered around the state, and there is even a "diamond rumor map" which is about as authoritative as it sounds.

At Shulin Lake a joint venture team has been conducting a drill program for the past few years and transparent fragments of larger diamonds have been found, as well as indicator minerals like purple garnet. The lode source is still elusive, but drilling is due to resume this spring. Szumigala isn't the only person in Alaska who believes in going against the grain.