Diamonds

Impact diamonds are the result of shock transformation of graphite or coal that have been shocked to shock pressures in excess of 30 GPa (Masaitis 1993; Koeberl et al. 1997b; Gilmour 1998). Impact diamonds may appear as paramorphs of precursor crystals, often showing the crystallographic habits of these. Cubic and hexagonal impact diamond and lonsdaleite may occur as individual, nanometer to micrometer crystallites, but they can also occur in the form of large aggregates of up to 10 mm in size.

Such occurrences of impact diamond are known from a large number of impact structures (e.g., review by Gilmour 1998). They were probably first discovered when small diamonds, in association with lonsdaleite (a hexagonal high-pressure polymorph of carbon), were observed in the 1960s and 1970s in placer deposits, for example in the Ukraine; but at that time, their origin was unknown (e.g., Sokhor et al. 1973). For many decades enigmatic diamond finds in placer deposits in the Yakutia region in northeast Siberia had been known, but the source area for these so-called "yakutites" remained unknown. Only in the 1970s were diamonds

Fig. 8. Schematic geology of the Sudbury impact structure, indicating the occurrence of Sudbury Breccia and outer limit of shatter cone occurrence (after Spray and Thompson 1995). The diagram at the bottom (after a diagram by D.H. Rousell and co-workers) shows the distribution of major ore deposits in and around the Sudbury Structure, as well as the maximum limit of PDF and shatter cone development in the basement rocks to the structure. Lake Wanapitei is the site of a much younger (ca. 35 Ma) and 7.5 km diameter impact crater.

Fig. 8. Schematic geology of the Sudbury impact structure, indicating the occurrence of Sudbury Breccia and outer limit of shatter cone occurrence (after Spray and Thompson 1995). The diagram at the bottom (after a diagram by D.H. Rousell and co-workers) shows the distribution of major ore deposits in and around the Sudbury Structure, as well as the maximum limit of PDF and shatter cone development in the basement rocks to the structure. Lake Wanapitei is the site of a much younger (ca. 35 Ma) and 7.5 km diameter impact crater.

discovered in impactite of the Popigai Structure (e.g., Masaitis and Selivanovskaya 1972; Masaitis 1998), when the origin of the so-called yakutites was traced, through surface exploration, back to the Popigai area. Impact diamonds have since been discovered in a host of other impact structures, including Kara, Puchezh-Katunki, Ries, Ternovka, Zapadnaya, Sudbury, Chicxulub, and others (e.g., Masaitis 1993; Masaitis et al. 1999; Montanari and Koeberl 2000). Diamonds of impact (shock) origin have also been observed in a large number of meteorites (e.g., Rubin and Scott 1996).

Impact diamonds have been documented from a variety of impact breccias (Masaitis et al. 1999, and references therein; Montanari and Koeberl 2000), including impact melt breccia and suevite. At Zapadnaya, an impact structure of 4 km diameter and 115 Ma age in Ukraine, for example, impact diamonds have been observed in dikes of impact melt rock injected into the central uplift as well as in suevite from the trough surrounding the uplifted core. According to Gurov et al. (1985), the Zapadnaya crater was formed in a graphite-bearing Proterozoic granite. At Popigai, a stratigraphic section through the fill of the crater basin around the central uplift comprises allochthonous breccia that is overlain by diamond-bearing suevite and impact melt rock. In the case of this large impact structure, the source of carbon from which diamond was formed is Archean graphite-bearing gneiss. At the Kara impact structure in northern Russia, diamonds also occur in impact melt rock. This impact structure is deeply eroded; values for its diameter that have been cited range from 65 to 120 km; its age is 70.3 ± 2.2 Ma (Trieloff et al. 1998). Kara is located in Permian, coal-bearing sediments (Ezerskii 1982).

Diamonds generally occur in very small amounts in impact melt rocks. Grieve and Masaitis (1994) estimated an average concentration of about 10 ppb. Upon slow cooling of large melt bodies, diamond may become metastable and then revert back to the graphite structure. Masaitis (1993) discussed that the distribution of diamond within a particular impact structure, as well as within individual melt rock and suevite breccia bodies, may be very irregular. This author also stated that diamond occurrences can occur concentrated in rays or zones emanating from areas that had comprised prominent amounts of carbon-bearing lithologies. Near the center of an impact structure, high post-shock temperatures may cause rather rapid oxidation. In contrast, farther from the central area, shock pressures would be insufficient to allow the phase transformation to diamond to occur. Thus, the extent of diamond-bearing zones may have a finite radial extent.

To date, no impact diamonds have been exploited commercially. The vast resources of these microcrystalline diamonds at the Popigai structure are non-economic, due to the complete lack of infrastructure in this remote region of northeastern Siberia that would permit commercial mining of this industrial-grade diamond deposit.

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