SHUNGITES - THE C-RICH ROCKS OF KARELIA, RUSSIA

The Lower Proterozoic rocks (2.0 to 2.1 Ga) of the Shunga district nea r Lake Onega, Karelia. Russia contain large amounts of elemental carbo n - about 25 x 10(10) tonnes in an area of roughly 9,000 square kilome ters. The rocks occur in a horst-graben transition zone between the Ba ltic Shield and the Russian Platform. Biogenic, metasomatic, and volca nogenic origins have variously been proposed for the carbon in these r ocks. Most rocks in a 1200- to 2000-m stratigraphic sequence contain a t least several weight percent carbon, and localized areas contain up to 98 wt% glassy carbon, a most unusual natural form. The glassy carbo n is deep black, has a pronounced conchoidal fracture, and a Mohs hard ness of 3.5. Its high luster makes it look almost metallic; it has a l ow density (1.9-2.0 g/cm(3)) and high electrical conductivity (about 1 00 S/cm). The glassy bodies are relatively small (tens of meters in ex tent) and extremely brittle. The carbon shows diffuse X-ray spectra: h igh-resolution transmission electron microscopy images indicate that l imited structure exists, primarily in the form of poorly organized gra phite-like layers in roughly rounded units, but there is considerable heterogeneity. Analysis of carbon isotope ratios of samples from three localities yields delta(13)C values between -26.4 and -37.6 parts per thousand PDB. Values correlate to locality rather than to rock type, suggesting that the glassy carbon was locally remobilized from the sur rounding country rocks. In one sample, clasts of almost pure glassy ca rbon have a value of -37.5 parts per thousand and occur in a matrix co ntaining roughly 30 wt% carbon with a composition of -37.6 parts per t housand. In samples from another locality, vein material of almost pur e carbon (-26.7 parts per thousand) cross-cuts rock of the same isotop ic composition (-26.5 parts per thousand), but also with only roughly 30 wt% carbon. The authors differ regarding the implications of the ca rbon-isotopic data. JWV and PRE interpret them as indicating a biogeni c origin, either in situ or remobilized during low-grade metamorphism, whereas LPG interprets the field and isotopic data as indicating an a biogenic, volcanic origin.