POPIGAI IMPACT DIAMONDS
By Steven B Shirey and Nikolai V Sobolev
Washington, DC, March, 2013. In late 2012, several sensation-seeking web articles in the Christian Science Monitor, the Huffington Post along with other publications described Popigai diamonds as "a vast trove of gem diamonds." Such a description is little more than excessive, overzealous mischaracterization and a dramatic overestimation of the worth of these diamonds. The claims in these sources have no scientific justification, and were based on statements from scientists with experience in kimberlitic diamond geology, not the geology of impact structures. Perhaps the interpretation of the statements was designed to raise the worth of investing in the Popigai deposits or to influence the market value of gem diamonds. But the media excitement is typical of the interest level that natural diamonds hold.
The Popigai impact structure is a circular, 100-km-diameter crater, created when a large meteorite—thought to be an ordinary chondrite—crashed onto the Anabar Shield of northern Siberia, Russia. Presently, 184 impact structures are still preserved on Earth’s surface. The largest and most famous of these impacts have led to dramatic changes such as extinction of the dinosaurs (Chixculub), turned a crust-mantle section on edge (Vredefort) and triggered deep melting leading to one of the largest magmatic ore deposits (Sudbury). The Popigai structure, estimated to be about 35.7 million years old, is the largest known impact to occur in the last 60 million years.
Diamonds formed by high transient shock pressures are known to be associated with terrestrial meteorite impacts and were discovered at the Popigai structure by Russian geologist Dr. V.L. Masaitis in 1972. This discovery initiated an extensive exploration program during the 1970s and 1980s. During this time, some 400 holes were drilled through the Popigai deposits to characterize diamond distribution and sample the impact-rocks in different parts of the buried crater. Diamonds were extracted from the shocked impact rocks and evaluated for industrial applications. Subsequent studies by additional Russian geologists including Vishnevsky, Gibsher, and Shelkov as well as western geologists, including Koeberl, Tagle, Claeys, Hough, Gilmour, Langenhorst, Pratesi, Pillinger, and others, have analyzed unique impact-generated characteristics such as diamonds (textures, stable isotopes, rare gases), silicon carbide, and fluidized melt-rock known locally as "tagamite", among other features. In summary, the Popigai structure and its diamonds have been well characterized for more than 40 years!
The Popigai diamonds were formed by high transient shock pressures and temperatures, associated with the meteorite impact, that caused the metamorphism of accessory graphite in crystalline basement gneisses. The resulting Popigai diamonds are polycrystalline aggregates composed of individual crystals several nanometers to 1 micrometer in size. The average size of these aggregates varies from 0.1 to 0.5 mm and in very rare cases reaches 1 to 5 mm. Some irregular aggregates, reaching 12 mm, are found only in local placer deposits. Diamond color varies widely from colorless, white, or yellow, to grey, dark-grey, and black. Light-colored Popigai diamonds are hard. But colored to dark Popigai diamonds have shock lamellae and/or contain impurities of lonsdaleite and graphite. These features lead to poor gem quality, variable abrasion characteristics, and low value.
Economic geology has, at times, had a sordid history of falsely raising the value of deposits known as "salting" — the Popigai diamond news blitz appears to be a prime modern example. In the words of geologist Masaitis who discovered the Popigai diamonds long ago, "This looks like a public relations campaign to get a lot of money and spend it. We do not even know how to extract [the diamonds] and how much competitive material we will get as a result. Even their mineralogical peculiarities and technical properties need to be carefully examined."