L363

BUTLETIN DE I'ASSOCIATION MIilERATOGIOUE TIU CANADA

rHE CANADIANnlrNEnArooFrJOURNAT OF THE MINERALOGICAT ASSOCIATION OF CANADA

Volume 35

The C anadian M ine ralo g is tVol.35, pp. 1363-1378 (1997)

December 1997 Part 6

SHUNGITES: THE C-RICH ROCKS OF KARELIA, RUSSIA

PETERR. BUSECK1

Departntents of Geology and Chenistry, Arizona Snte University, Tem.pe, Arizorw 85287-1404, U.S.A.

LIIDMILA P. GALDOBINA, WADIMIR V. KOVALEVSKI eNo NA|IALIA N. ROZHKOVA

Institute of Geolagy, Karelian Science Centre, Russion Academy of Science, Petrozavod.sl<, Karelia. Russia

JOHNW. VALLEY

Depamnent of Geology and Geophysics, University of Wisconsin, Madison, Wiscowin 53706, U.S.A.

ALEXANDER Z. ZAIDENBERG

Institute of Geology, Karelian Science Centre, Russian Academy of Science, Petrozavodslg Karelia, Russia

ABSTRA T

The Lower hoterozoic rocks Q.O to 2.L Ga) of the Shunga districl near Lake Onega, Karelia, Russia contain large amountsof elemental carbon - about 25 x 10to tonnes in an area of roughly 9,000 square kilometers. The rocks occur in a horst-grabentransition zone between the Baltic Shield and the Russian Pladorm. Biogenic, metasomatic, and volcanogenic origins havevariously been proposed for the carbon in these rocks. Most rocks in a 1200- to 2000-m stratigraphic sequence contain at leastseveral weightpercent carbon, and localized areas contain up ta98 wIVo glassy carbon, a most unusual natural form. The elassycarbon is deep black, has a pronounced conchoidal fracture, and a Mohs hardness of 3.5. Its high luster makes it look almostmetallic; it has a low density (1.9-2,0 Elctr) and high electrical conductivity (about 1@ S/cm). The glassy bodies are relativelysmall (tens of meters in extent) and extremely brittle. The carbon shows diffirse X-ray spectra; high-resolution transmissionelectron microscopy imagas indicate that limit€d stucture exists, primadly in the form of poorly orgenized $aphite-like layersin roughly munded units, but there is considerable heterogeneity. Analysis of carbon isotope ratios of samples from threelocalities yields 6tcc values between -26.4 and,-37.6Voo PDB, Values correlate to locality rather than to rock type, suggestingthat the glassy carbon was locally remobilized from the surrounding counEy rocks. In one samplg clasts of almost pure glassycarbon have a valrure of. -37 .SVoo and occur in a matix containing roughly 3O wt%o carbon with a composition of -37 .6%o. lnsamples from another locality, vein material of almost pure carbon (-26.7%o) cross-cuts rock of the same isotopic composition(-26.57u),but also with only roughly 30 wtTo carbon. The authors differ regarding the implications of the carbon-isotopic dataJWV and PRB interpret them as indicating a biogenic origin, either in ritu or remobilized during low-grade metamorphism,whereas LPG interprets the field and isotopic data as indicating an abiogenic, volcanic origin.

Keywords;shungite, glassy carbon, Proterozoic, carbon isotopes, Shunga, Karelia l,ake Oneg4 Russia.

r E-mnil address: pbuseck@asu.edu

13@ TTIE CANADIAN MINERALOGIST

Somuarns

Les roches d'ige prot6rozolque inf6rieur (de 2.0 d2.L Ga) du district de Shungq prbs du lac Oneg4 Kar6lie' en Russie,contiennent des quantit6s importanGs de carbone i l'6tat 6l6mental, environ 25 x l0ro tonnes dans une rdgion d'environ 9,000kilomBtres carr6s, Les roches se trouvent dans une zone de transition avec horsts et grabens entre le bouclier baltique et laplate-forme russe. Des processus biog6nique, m6tasomatique, et volcanogenique ont 6t6 propos6s pour expliquer Ia formationdu carbone dans ces roches. La plupart des roches dans une s6quence stratigraphique de 1200 i 2000 m contiennent au moinsquelques pour cents (poids) de carbone, et i certains endroits, il y ajusqu'i 987o de carbone d 6clat vitreux, aspect tres rare. trs'agit d'un matdriau noir fonc6, ayant un cassure conchoidale, et une duret6 de Mohs de 3.5. Son 6clat frappant fait presquepenser qu'il s'agit d'un m6tal. Ce mat6riau, appel6 couramment shungite, a une faible densit6 (1.9-2.0) et une conductivit66lectrique 6lev6e (environ 100 S/cm). Les masses i aspect vitreux sont de dimensions assez restreintes (dizaines de mdtres)et tres cassantes. Le carbone montre un spectre flou en difFraction X; en microscopie 6lectronique i trammission e rdsolution61ev6e, il semble y avoir une structure cristalline sur une 6chelle locale, surtout sous forme de domaines de graphiteimparfaitement agencds plus ou moins arrondis, mais avec une h6t6rogdn6it6 assez importante. Une analyse des rapportsisotopiques du carbone provenant d'dchantillons de tois endroits a donn6 des valeurs de 6r3C entre -26.4 et -37 .6Voo pn rappofid l'6talon PDB. Ces valeurs montrent une corrdlation avec le site de pr6ldvement plutdt qu'avec le type de roche, ce qui faitpenser que le carbone'aitreux" a 6t6 remobilis6 localement d partir des roches encaissantes. Dans un 6cha:rtillon, des fragmentsde carbone presque pur donnent une valeur -37.5%o, et se trouvent dans une matrice contenant environ307o de carbone parpoids ayaat une composition isotopiqne de -37 .6%o. Dans un autre endroit, un 6chantillon de carbone presque pur pr61ev6 d'unevetne (-26.7Tu) recoupe une roche hdte dont le carbone a sensiblement la m6me composition isotopique (-26.5%0)' quoique lateneur en carbone est seulement environ 307o. Nous ne pr6sentons pas une interpr6tation unanime de ces donn6es isotopiques.Selon JWV et PRB, il s'agirait de carbone d'origine biog6nique, soit iz si/z ou bien remobilis6 au cours d'une recristallisationm6tamorphique de faible intensit6. Selon LPG, les observations de terrain et les donn6es isotopiques indiqueraient plut6t uneorigine abiogdnique, voire volcanique.

(Iraduit par la R6daction)

Mots-cl6s: shungite, carbone'aifieux", prot€rozo'ique, isotopes de carbone, Shunga, Kar6lie, lac Onega, Russie.

INTRoDUcnoN

The rocks of the Shunga district in the northwesternregion of Lake Onega, Karelia in Russia have longattracted interest because of tleir abundant carbon.It occurs throughout much of tle Lower Proterozoicsequence, with contents locally ranging up to 98 wt%oand consisting of almost pure glassy carbon. Thecarbon-bearing rocks are in an area of approximately9,000 square kilometers that contains about 25 x 1010tonnes ofelemental carbon (Galdobina 1993).

In addition to their geological inlerest, the carbon-richrocks of the Shunga region have properties that makethem usefirl to industry. pe1 e;ample, by heating in airbetween 1080 and 1100"C, samples containing only Iwt%o carbon have been converted to highly porous,pumice-like rocks that have been ussd sqmmerciallyfor insulating fillers (Sokolov & IGlinin 1975). Samplescontaining 25 to 35 wt%o carbon have been added toconcrete to make it an electrical conductor, resulting ina product that is useful for resistive heating as well asproviding shielding against electromagnetic radiationin a frequency range of more than 10 MHz at a level ofnot less than 100 dB (Solovov et a\.7990). Additionof powdered rocks to plastics and rubber improvestheir mechanical properties, particularly their thermalexpansion and resistance, and high-frequency conduc-tivity. Shungite rocks have been mined for their use asantifriction agents when placed into composites; theyare also effective sorbents forremoval ofboth organic andinorganic substances from water (Sokolov et al.1984).

There is an extensive literature, most of which isin Russian, about the caxbon-rich rocks of tle Shungaregion. [nostrantsev (1879) called tlem "shungite";he concluded ttrat shungite is an extreme sample ofnoncrystalline carbon (following the prevailingliterature on shungite, we here use "caxbon" to refer.to elemental or organis carbon, as opposed to thatpresent in carbonate). Despite the strong opinionsof many of the people who have studied the area,there is no consensus in the literature regarding theorigin of tlese intriguing carbon deposits. Manyhypotheses have been advanced; these includebiogenic @orisov 1956, Volkova & Bogdanova 1986),metasomatic (Ivankin et aI. L987, Kalinin 1990,Lobzova & Galdobina 1987, Galdobina et al. 1995),metamorphic (Timofeyev 1,92-tI), and volcanogenicmodes of origin (Fersman 1922, Artamonov &Kekkonen 1935, Sokolov & Kalinin 1975, Golubev& Akhmedov 1995). Generalov (1995) studied ttreopaque minerals in shungrtes, and suggested that theserocks have a similar origin to deposits in black shales,such as those of the Kola Peninsula. Khavari-Khorasani& Murchison (1979) and Volkova & Bogdanova (1986)proposed that the two types of shungite richest incarbon (types I and II) are meta-antbracite coal ofbiological origin. Although only mentioned in passing,it appears to be acceptedby some peffoleum geologiststhat shungite (of unspecified Epe, but presumablytype I) is a highly evolved bitumen (Cornelius 1987,Meyerhoff & Meyer 1987, Meyer & De Witt 1990,Parnell et al. L994\.

SHUNGITES: THE C-RICH ROCKS OF KARELIA. RUSSIA 1365

For consistency with what appears to be prevailingusage, we will continue to use the term shungite todesignate reduced-carbon-bearing rocks from the LakeOnega region. Following the terminology of Borisov(1956), they are grouped into various types on the basisoftheir carbon contents (Table 1). Tlpes III to V aremore abundant and widespread than fypes I and tr.

We know of no direct measurements of the ages ofthe shungite rocks. They mainly belong to theLudicovian series (Fig. 2), and all age estimates arerelative to the underlying Yatulian (also spelledJatulian) and overlying Suisarian rocks. The U-Pb agesof zircon in the carbonates and diabase of the upperYatulian horizon are, respectively,2.l5 and 2.30 Ga(Levchenkov et al. 1.990). Sm-Nd measurements ofSuisarian picrite basalts indicate an age of 1.98 Ga(Puhtel et al. 1992), which coincides well witl theLower Proterozoic stratigraphy of the Baltic Shield(Levchenkov et al. 1990). From these data, we estimatethe age of the shungite rocks as between 2.0 and2.1 Ga. On the basis of its widely disseminated,fine-grained nature and field relations, Galdobina(1987) assumed that the carbon is endogenic and ofcomparable age to the host rocks.

TABLE I. TYPES OF SHUNGITE BASED ON CARBON CONTENT'

Typs Ca6on Physiel@qtrntf sppearrne

Abundam ldajorbose-roc.ks

Ftc. 1. Location map for the greater Shunga region. Thecircled cross norfh of Petrozavodsk marks the position ofthe Shunga deposits.

In this paper, we summarize some of the salientfeatures as well as report on the general geologicalsetting of shungite. An added feature of interest is thatthe fust report of the natural occlurence of fullereneswas from type-I shungite rocks (Buseck et aI. 1,992),similar to those described below.

Typrs op Srrulqcnn aNn rrun Acs

The literature is confusing as to the msaning of theterm shungite. It has been used to describe all carbon-bearing rocks ofthe extensive Lake Onega region (Fig.L), with the different types of shungite distinguished bytheir carbon contents. The term has also been used todescribe the structural state oftheir carbon (Kovalevski1994), so that shungite has been applied to both therocks and their elemental carbon.

Some authors use shungite as an adjective, as ino'shungite slate" and ooshungite diabase" (e.g.,Sudovikov 1937), whereas others refer to shungiterocks and then specify types (e.9., Sokolov et al. L984).A third procedure is to use botl terms, e.g., o'lydite(type-V shungite)" (e.9., Sokolov & Kalinin 1975).Galdobina (1993) referred to ooshungite-bearing"

followed by the rock descripfion (carbon content andpetrography), so that in this case shungite appears torefer to the carbon within the rock (Kalinin 1990.Parfen'yeva et al. L994,Zaidenberg et a\.1"995).

>75-98 blaok,gh$ybst€q vsylwonc'hoidal ftactre

>35-75 ble.l., sligldygldsy lowlust€fl: promin€otrutangular jotnnng;lays€d

>20-35 blsck, dullurt€r; m€dimmryiw

>10 - 20 blac,k, dul lud3q hesvily hiebjohte4 lely Aiable

< 10 gsyishtode€pblaclqfin€ hChgraircd (-10 pn dimetq);nasgive

Shgitstyp6-trand-m

dolmits,lydite

IinoSdgdolmit8,rtrts

sandston€,dolomite

smd$o1ls,dolonite

I s$er Borirov 0956), with clca of Inosbds 0879) 8nd Artmmov &Kekkom 1935). I expwed itr wt 7r.

Geot-ocrcAr- SBTTING AND Srnucnrns

The Karelian shungite rocks are prominent in tlearea around the northwestern part of Lake Onega.Galdobina (1987 , L993) and Ivarkin et al. (1.987) bavesummarized the Proterozoic geology of this region(called "the Onega structure"), and the followingdiscussion is based largely on the information given inthose papers.

t366 TTIE CANADIAN MINERALOGIST

l0 20 40 6O SUwt%oC

Ftc. 2. Stratigraphy of rocks of the Onega structure, around the northwestem part of Lake Onega (Galdobina 1987). Carbonconcentrations: 1,3 fron drill holes,2Zaghogqno and Maksovo deposits, and 4 Shunga deposit. L> indicates the locationof dikes or sills.

' | , * * * ' " '

The Onega structure is located in a formerlytectonically active transition zone between tle BalticShield and the Russian Plaform. The zone is bent in aflexure-likemanner and is cutby many deep NE-SW-and shallower NW-SE-trending faults (Fig. 3). On thebasis of aerial photography and confirming drill-holes,wide grabens alternate with narrow horsts.

The Onega structure is filled with LowerProterozoic rocks @ig. 2). From the top down tleyare: (a) sandstones of Vespian age, preserved in ttrecores of small synclinal structures, @) 200 to 600 m ofcarbonaceous rocks ofKalevian age, (c) 1200 to 2000m ofcarbon-bearing rocks ofthe Ludicovian sequencewhich, depending on locality, include betrreen four andten sills of diabase, with total thictrcnesses up to 12fi) m,

and (d) 400 to 600 m of carbonate rocks ofYatulianage. The Onega structure is surrounded by Archeanrocks, mainly granite gneisses, although UpperArchean greenstones and Lower Proterozoic Sumianand Sariolian formations occur in its northwesterncorner.

The Ludicovian sequence is of special interest. Itconsists of the Suisarian and carbon-bearingZaonezhsky suites (Fig. 2). The bottom 180 to 2C0 m oftle Zaonezhsky suite contains siltstones, dolomites,and carbonate - quartz - mica and quartz - mica rockswith dispersed sulfides. Carbon is rare in this bottomsection. The upper Zaonezhsky suite includes tuffs,dolomite, basalt and lydite (a compact, fine-grained,blackish variety of volcanogenic jasper); essentially

E)(PLANATION

ryF7;q

Wli:-::l.1l-j i .:.1' l

l L " l|]'ffil i . ' ' l r ; Il , ' r ' i : : i ; l

l. -= Ouartz - sandstones

-

ffiF:

mIs:-V=zJ

b:"r"dr:':-:-l

Siltstones

Tuffs

Tuffites

CarbonappoUs plagio-quartz-chlonE sclusui

Tuff conglomerates

Basalt flows

Dolomites

Lydites

Carbonaceous rocks (C <2OVo)

Carbonaceous rocks (C> 2OVo)

Carbonatequar.tz-mica andquarE-mrca rocKs

Sandstones

Locations of eabbro-diabasesills

Granite-gneisses

SHLJNGIIES: TllE C-RICH ROCKS OF KARELLA. RUSSIA

YATUI.IANLOWERPROTEROZOICf]]ltuI : .. :l vePsian

KALEVIAN

Fl Nigozero horizon (suite):lE siltstones free of carbon

///ifi Carbon-bearingquartz-plagioclase-albiteV'//'///tl sclists and tuffite

LUDICOVIANr7.7..7,7)

V. i,{/:/. / suisann horizon (suite)

fiImHffiffi Picritic basalt intrusions

ffi Zaonezhskv horizon (suite): lower subsuiteffi 6s1.mites is welt as quarri-biotite and- quartz-biotite-carbondte rocks with scarce carbon

E Taonezhsky horizon (suite): upper subsuite|.....:E carbon-bearingrocks

Carbonate rocks

Terrigenous rocks

Post-Yatulian intrusions

Sariolian

Sumian

Fault traces

/ Obsewed

,i urerrea

[,E unr"tat"tt*nt*L

lllll o*"t," gneisses

l--r i--lf=ffi

mF:.:l

Effiffi

ARCHEAN

Frc. 3. Geological map ofthe area around the northwestem part ofLake Onega (Galdobina 1987).

1368

all contain free carbon, typically I to 25 wtVo. Latehydrothermal solutions resulted in metasomatizeddolomitic rocks and sulfide ores. Diabase sills, each upto 50 to 100 m thick, were emplaced during the laststage ofvolcanism and appear to be associated with thecarbon-bearing rocks. The degree of structural orderingof the carbon increases with proximity to tle intrusions,suggesting that the carbon preceded emplacement.Pillow basalts within the sedimentary pile show thatvolcanism occurred during sedimentation.

The most unusual and characteristic features of theupper part of the Zaonezhsky section (600 to 650 mthick) are varied-sized bodies of carbon-rich rocks (25to 75 wtVo carbon). They occur at four stratigraphiclevels @ig. 2). In places, there are veins with carboncontents up to 98 wtVo.In contrast, the Suisarian suiteis essentially free of carbon and consists of basic andultrabasic lava flows and tuffs (Fig. 2). All shungiterocks are greenschist facies (Volodichev 1987).Metasomatized dolomitic rocks lie above and belowthe carbon-rich horizons.

The Kalevian sequence consists of a mixture ofvolcanic and sedimentary rocks: tuffs, siltstones,plagioclase - qlrarlz - chlorite schists containing upto 3 wt%o carbon, and tuff-sandstones witl carboncontents between 1 and 2.5 tlt%o.T\e carbonaceousrocks are up to 200 m fhick, but they are absent in somecross-sections. Lenses up to 20 cm in diameter and2 to 3 cm thick of glassy type-I shungite (Table 1)are sparse but occur locally at Nigozero withincarbonaceous plagioclase - qvar1z - chlorite schists(Fig. 4). The Kalevian rocks are overlain by theVespian formations.

THE CANADIAN MINERALOGIST

Flc. 4. Conformable lens of type-I shungite from Nigozero. The boundaries are sharpbetween it and the host rock, a carbonaceous plagioclase - quartz - chlorite schist.

gummarizing, tle regional geology was affected bya period of continental rifting during which significantrmounts of diabase dikes and sills were emplaced intoa shelf sequence of strata. Tuffs and flows of basaltwere interlayered with shallow marine sediments. Theigneous activity provided a major source of heat thatwas presumably adequate for shungite formation, inaddition to providing metasomatic fluids during riftingand associated magmatism.

Pnvsrcer CsenacrERrsrrcs oF THE CARBoNaxn CenroN-RrcH RocKs

. 1

The rocks 'r:that

are richest in carbon (type-Ishungite) have'.a striking macroscopic appearance.They are deep black, almost glassy, with a pronouncedconchoidal fracture (Frg. 5a). Their high luster makesthem look almost metallic; they have a relativelylow specific gravity (1.9--2.0 glcm3), high electricconductivity (about 100 S/cm), and some types haveunusually high specific surface areas Q101, 440 dlg),as measured by the (BET) adsorption of gaseousnifrogen (Sokolov et al. 1984, Kovalevski et al. 1994,Zaidenberg et al. L996). For comparison, commercialgraphite powders have specific surface areas ofapproximately lO rfrlg (Kinoshita 1988), whereasactivated carbons are in the range of450 to >1000 Dflg(Kinoshita 1988, Parra et aL.7995). The brittleness oftype-I shungite makes it difficult to extract pieceslarger than a few cm in diameter from outcrops.

Although other natural carbon-rich species such asalbertite, gilsonite, uintahite, and wurzilite can alsodisplay a conchoidal frachre, none have the semimetallic

SHUNGITES: THE C-RICH ROCKS OF KARELIA, RUSSIA 1369

Ftc. 5. Macroscopic view of hand specimens of (a) type-Ishungite, and (b) type-tr shungite from the Shunga mineopening. The prominent cleat-like joints are coated witljarosite (whitish color).

to metallic luster of shungite; most appear far more likehardened paving asphalt than like the shungite. High-rank anthracite is the closest in appeamnce, but it stillappears distinctly less glassy and metallic.

Khavari-Khorasani & Murchison (1979) usedoptical microscopy, X-ray diffraction, and heatingexpedments to study threepiece.s (two'"bright''and one"dull"; pers. commun., 1997) of shungite. The p'ropertiesof shungite remained constant during heating to 850oC,and even at 2900oC the samples that were presumablytype-I shungite did not graphitize; in the terminologyof Franklin (1951), they consist of non-graphitizingcarbon.

Many studies have been made of the structuralcharacteristics of the carbon in the Shunga district.Tlpe-I shungite has been reported as X-ray amorphousby Usenbaev et al. (7977) and poorly ordered byGrew (1974). High-resolution transmission electronmicroscopy (HRIEM) indicates tlat some structureexists, primarily in the form of slightly curled

graphite-like layers @useck & Huang 1985). Jehlicka& Rouzaud (L993) commented on the inhomogeneity,as did Buseck & Huang. There are small regions(a few tentls of a nanometer across) of material thatshows no sfructure, even when imaged using IIRTEM.Kovalevski (1994) and Yushkin (1994) reportedthat the most characteristic feature of the carbon is aglobular character, with units sized up to 10 nm across.

Exalpms oF OccuRRENcEs oF IIrcH-CARBoNCoNceNTRAfloNs

Rocks from several areas have been mined for theircarbon. Some, like the deposit at Shunga that containsshungite oftypes I and tr, have been valued forthe highgrade of their carbon. However, the proven reserves oftype-I and -tr rocks are too low for current commercialpulposes. In addition, this deposit is legally protectedagainst exploitation because of its geological unique-ness. Other deposits, such ,rs those at Zazhogino andMaksovo (shungite type-Itr), have been worked torecover rocks containing rou ghly 30 wtclo carbon. Theyconsist largely of carbon and quartz and have usefulindustrial properties. We will consider these areas insequence as examples of the features of this district.

Shunga

The oldest and best known mine is at Shunga. It islocated in tie northern part of the Zaonezhsky Penin-sula between lakes Putkozero and Valgmozero, toosmall to show on Figure 3. It lies in tle northeasternpart of the Onega structure, within the upper part of thegreenschist-facies Ludicovian rocks.

The major rock fires are massive carbonaceousdolomite and lydite. The latter contains about95 wt%ofinely crystalline quartz, 2 to 4 wtVo catbon, and aboutt.5 wt%o sericitic mica and metamorphic biotite. Thelydite (shungite type-V; Table 1) is dense, has aconchoidal fracture, Mohs hardness of 7, and a densityof 2.65 g/cm3. According to Galdobina (1987), itis in amassive hydrothermal structure that cuts the dolomiteand contains dolomite xenoliths. The lydite containsfractures filled with quartz, calcite, and "gumbellite"(secondary, fibrous sericite). The dolomite is interbeddedwith lydite and type-tr material. The dolomite is dark$ay to black, finely crystalline, and contains 2 to 7wtTo reduced carbon as well as minor mic4 quafiz, atdsulfides (Sokolov & Kalinin 1975).

Various types of shungite rocks are exposed near theentrance of an adit at Shunga (Fig. 6). The most strikingfeature is a narrow, heavily jointed, seam-like layer oftype-I shungite. The layer pinches and swells fromroughly 8 to 50 cm and is continuous across approxi-mately 30 m of exposure. It appears conformable,much like a typical metamorphosed sediment.

The rocks immediately above and below the seamcontain appreciably less carbon and are classified

L370 TIIE CANADIAN MINERALOGIST

F:.... Quatemryseainents ffi Lyaite

IfypoI"fr*git" ffi Smdstonsmdlim6lone

El fyp"-U.n*Sit" N sittstonc

ffi1ryp"-mru-gi" ---'Quatzfilledftacture

Ftc. 6. Schematic geological cross-section ofthe Shunga region. Tle limestone symbols show position only and not orientation.The Shunga mine entrance is marked with crosss{ lammers (after Popov 1979).

as type-tr shungite (Fig.5b,Table 1). They containpronounced, roughly orthogonal joints (much likecleats in coal) rather than being massive. The jointsurfaces are heavily coated with films of yellowjarosite as a weathering product of the accessorysulfides in the shungite. The contact betwe€n tle twotypes of layers is sharp (<1 mm) and, in most places, isdefined by what appears to be a fsdding plane(although it might be called a vein wall if it werevertical). The thickness of the type-tr material is from2 to 4 m (Sokolov & Kalinin 1975). It contains between40 and 75 *Vo carbon and the accessory mineralsq'$artz, sericite, and biotite. It is cut by veinlets ofquartz, calcite, and "gumbel1ite".

The seam of type-I shungite is the probable sourceof the sample in which Buseck et al. (L992) foundfrrllerenes. On tle basis of the results of Daly et al.(L993), it was suggested by Ebbesen et al. (1995) ardcontested by Buseck & Tsipursky (1995) that liebtninewas the probable source of the firllerenes. The type-Ishungite here is overlain by 5 to L0 m ofrocks. There isno evidenceo either within the sample or in the field, ofthe pockets of formerly molten material thal are typicalproducts oflightning stikes. In addition, tle probabilityof accidentally selecting such a sample from the massof rock seems vanishingly small, and the thin veinletcharacter of the fullerene-bearing material is in markedcontrast to the rounded to cylindrical shapes oflightning channels and fulgurites.

kzhogino

An open cut of a working type-m shungite depositoccurs at Zazhogino, roughly 20 km from Shunga. Atlarhsglas, there is a massive carbon-rich lens witldimensions -20Ox 250 x 50 m (level 2 of the crosssection, Fig. 2). The rock contains 25 ta 4O wtvocarbon, witl fine-grained muscovite, albite, quartz,

sericite, minor pyrite, and "gumbellite" in veinlets.Bulk analysis gives 50 to 65 wt%o SiO2 and between 2and 4 wt%o KrO. Drill holes show a similar underlyingbody that contains Na in place of K Qevel L of the crosssection).

Stratigraphically, the Zazhogino lens is -150 mbelow the Shunga deposit. Between them lie layers oftuffs, dolomites, bedded sulfide ores as well as basaltsills and flows (Fig. 2) and several carbonaceous lenseslike that alZazhogqno.

Maksovo

The Maksovo deposit contains tle same massivehost-rocks as does Zazhoglno, except that at Maksovothey are intensely brecciated. The deposit measuresapproximately 500 x 500 x 100 m; the southeasternpart is exposed as a result of former mining, and theremainder has been defined through drilling at25-mcenters (Fig. 7).It is located on the hanging wall adja-cent to an almost vertical normal fault with a roughly200-m throw. The breccia fragments are cemented byveinlets containing elevated levels of carbon witlinquartz. The entire area is truncated by the fault and, asshown by tle drilling, is underlain by carbonaceouslimestone. Laterally adjacent layers do not formourcrops, but they have the same mineral assemblagesas at Maksovo, except that they are free of carbon.

At Maksovo, there is an approximately verticalveinlet of glassy carbon (type-I shungite). The veinlet(Fig. 8) is located about 5 m from the fault; the two areroughly parallel. It is 3 cm wide and is exposed forabout 20 m. It has well-defined, almost planar wallsand is filled witl exfremely friable glassy carbon thatcan only be extracted as small (a few mm) flakes. TheMaksovo type-I shungite has different mechanicalproperties from that at Shunga (Sokolov et al.1984,Kovalevski 1994,Zudenberg et al. 1996). Both are

SHLINGITES: THE C-RICH RoCKS OF KARELIA, RUSSIA

Plan view

I37L

Maksovo

100 0 100 200mr---

50

0

-50

-100 -100

ffi sihstone ffi| Layer D vvvz c a rb on ate- b i ot itef

'-t7//tr metasomatite

Tutfs and carbonatemetasomatite

Albite-quartz tuff,lydite

-100

Interlayered tuffs ofvarious comPositionsMetadiabase(sills, dikes)

Approximate traceof fault

Positions of drill holes

ff i

ffi Dolomite milffi Layer E ffi[ . : . : . : .1

I

%ALayerA

ffi Layer B I:E Siltstone tutf

-t

: Layerc

Fro. 7. Schematic geological plan view and cross-sections of the Maksovo region; the Maksovo quarry is located in the centralpart of the map. Layers Ato E were mapped and apparently differentiated primarily by their carbon contents, but with a:runspecified petrography (affer Kupryakov & Mikhailov 1988).

1.372 TIIE CANADIAN MINERALOGIST

Frc. 8. Narrow vein ofglassy, type-I shungite from Maksovo.The sharp boundary is evident on the left side ofthe vein.The brittle character of the shungite combined withextensive sampling accounts for the recessed character ofthe vein.

similar on the X-ray scale, but they differ in the inten-sity distributions within the electron-diffraction rings,which indicate a stronger preferred orientation of thecarbon in the Maksovo than in the Shunga material.

Comparison ofthe high-carbon rocl<s at Shunga andMaksovo

At Shunga, the highest-carbon rock oocurs in analmost horizontal structure that resembles a seamand appears (to PRB) like a metamorphosed sediment,although the autlor (LPG) who is most familiar withthe local geology believes it to be a metasomatizedvolcanic rock. At Maksovo, carbon that macroscopicallyappears to be almost identical occurs in a narrow verticalveinlet. It also has extremely well-defined walls, but inttris instance it appears to cut across tle surroundingrocks. The immediately adjoining rockg are massive, sothat their attitudes are not known, whereas those aboveand below in the stratigraphic sequence displaybedding roughly perpendicular to the veinlet.

The Shunga and Maksovo structures tlat containtype-I shungite are similar in the sharpness of tleir

contacts and the unusual macroscopic appearance oftheir highly enriched contents of glassy carbon. Thetwo structures differ in their orientations (horizontalversas yefitcal) and parallelness of their walls, with theShunga body having a more irregular width, at leastin the relatively restricted regions where it could beobserved. The one at Shunga appears conformable,whereas ttre one at Maksova does not. It is hard toobserve these two bodies and conclude anything otherthan a remobilized origin for the carbon.

Srunan OccunnnNcrs

Within the greater Shunga region, similar rocks arefound in several areas, insluding Nigozero, Berezovets(located 80 km from Shunga but beyond the -ap area),Munozero, Chebolaksha, Tolvuya, Spasskaya Guba,and near Lake Yandomozero (Fig. 3). Outside ofKarelia we know ofno occurrences where carbon-richrocks have similar field characteristics to those of tleShunga region.

Cansox Isoropg RAfios

Analytical procedures

Selected samples were analyzed for their carbonisotopic ratios at the University of Wisconsin. Eachsample was ground to a coarse powder and treated withaqua regia to remove sulfides and carbonates. Thesamples were rinsed and dried, weighed, mixed withCuO (an amount equal to -100< the estimated weightof carbon in the semple), and sealed at vacuum in silicatubes. A -10 mm2 piece of silver foil was added toeach tube to react with any remaining sulfides.

Prior to loading tle samples, silica tubes wereheated in air at 850oC for more than l0 h to removecarbon contaminants. The sealed sample tubes werebaked at 840oC for -16 h and annealed at 550oC for8 h. After cooling, the evolved CO2 was purifiedcryogenically and analyzed with a Finnigan MAI 251mass spechometer. Blanks were measured from silicatubes containing 500 mg of CUO but no sample. Nomeasurable CO2 (<0.5 pmol) was produced in thesetests. Isotope ratios are presented in the standardper mil (%o) rotation relative to the PDB standard(Iable 2). Analyses of NBS-21 (spectroscopic graphitestandard) yielded a 6t3C value of --28.25 + 0.O7%o(Kitchen & Valley 1995).

Duplicate analyses were run of most samples, andthe results in Table 2 show excellent analyticalreproducibilities. Multiple chiFs were taken fromseveral samples (samples M2b,Z3, 26 from MaksovoandZaz,hog1no, and the Harvard sample from Shunga).The analyses of each of these samples show goodprecision, but they also show that minor but detectableheterogeneify exists within these rocks, with differencesrangrng from approximately 0.L to O.4%o.

TABLE 2. ISOTOPIC COMPCISMON AND PROPORT1ON OF CARBON,ROCXS FROM MAKSOVO, SHUNGA AND ZAZXIOGINO

SHUNGITES: TIIE C.RICH ROCKS OF KARELIA, RUSSIA

-26.9 (.94.n)-27.05 (.03.06)-27.n (26.2na7.4r (.4r.41)-26.75 (.74 .76)

(.49.s2)(.53.54)

(.72 .74)(.43.46)

-37.47 (.47.47)

-37.57

:2j..7t (.70.72)-27.@

-29.86 (.r3.88)

-26.4 (.43.4)

-26.73 (.72.74)

-26.51 (.50.52)-26.5s (.53.5O

Maksovo, Shunga, otT,azhogjno, the areas discussed inthis paper. The map (Frg. 3) shows tle localities, andbrief descriptions of rock types are given in Table 2.

The compositions are tightly grouped by depositand range betw eer -26.43 and -j7 .57 % o PDB. Sfr ikingfeatures of our results are their bimodal distribution(--26.9 L 0.5 and -37.5 X O.\%o,with only two interme-diate samples) and their consistency for each of thesethree locations, independent of the rock type. Thus, theisotopic compositions cluster by locality rather thanshungite type. Maksovo atdZazhogSno are of roughlytle same age, whereas the rocks from the Shungalocality are younger (Fie. 2).

There have been several previous measurements ofthe isotopic composition of carbon in the rocks fromtle greater Shunga region. The fust was by Rankama(1948), who investigated a sample oftype-I shungitefrom the Shunga deposit and obtained a value of42.l%oo PDB. Sokolov & Kafinin (1'975) analyzed L3shungite samples from the Maksovoo Shunga, andZazhogiro deposits and found values ranging from-2L.4 to -39.9 for types-tr, -Itr and -fV rocks and from-23.6 to -38.4%o for type-I rocks. Chukhrov et al.(1934) anafyzed nine samples from unspecifiedlocalities in the Lake Onega region and reported valuesfrom -35.9 to 41.9%a Finally, Galdobina (1987) andGaldobina et al. (1.995) studied 57 samples from thegeneral Shunga region and reported values from -17.4

to -39.5/oo. Their values also have a roughly bimodaldistribution. with values centered at-26.4 and-37.6%o.

Each of the above authors or groups interpretedtleir data regarding orign of the rocks, but there is noconsensus. Rankama (1948) and Chttkhrov et aI.(1984) interpreted tieir results as favoring a biogenicorigin, whereas Sokolov & Kalinin (1975) took nostand, and Galdobina (1987) and Galdobina et al.(1995) interpreted their data as indicating a volcanicorigin, with the more abundant higher-6rg samplesbeing earlier than ones having the lower 6t:C.

1373

6EC Prciri@. %CfDB yleld

Saqle D6siptimnmbs

MlaIvtzbttw'*uM5

S l as2b

s4s5

Zlozlb23

73'

,

7:7

Malsovo, Kardla

Glaary ca6onftonvein (typ&I dugite)Cotact ganrplc frmwi! (typel sh$gi!e)CotrtB61 smpls fronrein (typel 8bngit€)Veinedtypqltr shugiteBtsiated typsm ihrgrte

Shungp, K8rdi.

Csbon chips from'sm"; t,'peJ sh&gitoMKire typetr drungire ff'm jusbolm'm"Lydite-typev ahrSiteCado-beodng dolonitoTypol rhngite ftomllsvrq deatedbyC. hrlbrn dsing the 1935 IGCTypol ehngite Aon II€wanq coll€t€d byC. Itrlhn duiry the 1935 IGC

zu.hogho,Ihrdir

Msssive typFm sh[giteMasive typeltr ehngitsVeidet of quartz md typsl rhrgne innassiwtyp€-m shqiteVeirls! ofquartz snd type-I .hrFgtt€ inroiYg type-m lhungitsFncturu @ded with aliokmide graphite-likoluster ed €@bolite in typeltr qhungire

Fmam osted flith slickmfute graphtslt:kelusts md gunbelnsintype-m ihmgiteMassiwtype-I[ shmsite

-37.51-37.14

:37.45

8 l635 1

30

655

443.5

7t.5

63

2830

7l

25.s

30

J '

34

* Duplicde edys re m br all hrt tm cqplq the nmbds in pr€dh66indicste the phce bebind ths dsiml poids ed !d6! tt8 reprodmibtlny ofduplicde anotys.

Observations

Figure 9 shows the 6t:C values 1e1 gamFlescollected by the authors during the summer of 1995plus one collected by Dr. C. Hurlbut during the 1935International Geological Congress. All are from eitler

-26 -27 -A -n €O -31 -W €3 '34 '35 -36 47 €8

613C 7- PDB

Frc. 9. Values of 6trC for samples collected by the authors from Malsovo, Shunga, andZazhogqlLo during the summer of 1995, plus a sample of shungite collected byDr. C. Hurlbut during the 1935 Intemational Geological Congtess.

Shunga-30 €1 -p -33 -U -35 €6 '37 -S8

Maksovo-26 -27 -X) -m 30

++r Tlzlroglno-u) -27 -28 -29 €0

r374

DtscusstoN

Petroleum, coal, solid bitumens, and graphite arewell known to occur in carbon-rich districts. Thesematerials differ in essential characteristics from therocks found at Shung4 but there may be relationshi.f!$.The Shunga rocks are poor in hydrocarbons[Mishunina & Korsakova (1977) ndBondar et al.(1987) indicatedthatthey contain 0.06 to 0.17 'ut%o oforganic matter] whereas, in marked contrast, tleunmetamorphosed fossil fuels are rich in hydrocarbons.A closer relation may exist with districts that containflake graphite, sucb as those in 56 1anka, the RubyRange in Montana, IJano Uplift in Texas, New Hamp-shire, and the Adirondacks, New York (Cameron &Weiss 1960) on the one hand, or the impsonite depositsof Oklahoma and the gilsonite dikes of Utah. BothimFsonite and gilsonite are bitumens, i.e., they arc4morphous, carbon-rich solids that formed frompre-existing organic deposits.

The graphite districts mentioned above clearlyexperienced metamorphism above 500"C (e.g., Valley& O'Neil 1981, Rumble & Hoering 1986, Kitchen &Valley 1995), whereas tle impsonite and gilsoniteformed at lower temperafures, presumably below-25eC.In the giJsonite and imFsonite deposits, carbonwas presumably distilled from underlying organic-matter-rich oil shales. Temperatures were adequate tomobilize the carbon and move it to its current locations.but they were not suffrciently higb to allow crystallizationof graphite. A similar process might have contributed to

6t3c -2orl",PDBI

-50

2.5 2.O 1.5

Age (10s yr)

FIc. 10. Plot showing two "excursions" of isotopically ligbt organic carbon that occuredin the Proterozoic (after Schidlowski 1987). The carbon isotopic compositions fromthe Shunga region are in the small vertical box inset near the Francevillian"excursion." The most strongly tregative values are from the Shunga locality.

-10

-40

1.03.03.54.0

forming the graphitic areas, except that higher tempera-tures were reached. Fluids rich in CO2, CI{a, or bothcould have mobilized and uansported tle carbon fromits original sites to tlat of deposition (Rumble &Hoering 1986, Rumble et al. 1986).

The Shunga deposits presumably formed attemperatures intermediate to those at which impsoniteor gilsonite and flake graphite formed. Heating duringregional metamorphism of finely disseminatedhydrocarbons, possibly ofbiogenic origin, could haveresulted in maturation of the reduced carbon tlat isdispersed tlroughout the dolomitic and lyditic rocks(cl Dunn & Valley 1992); carbon-rich fluids, presum-ably generated by local sources of heat such as thediabase dikes and sills, concenfrated and then depositedalmost pure carbon in localized regions such as theooseam" at Shunga and the veinlet at Maksovo. Thepresence of glassy (type-D carbon in veinlets indicatesthat it was remobilized and then precipitated in thesenew locations.

The range of carbon-isotope values for samples ofshungite is similar to that of petroleum, nahrral gasocoal, and most animal and vegetable matter (Deines1980a, Hoefs 1982, Hayes et al. L983, Schidlowskiet al. 1983, Schidlowski 1987). The cause of theseuniformly low values is believed to be isotopicfractionation during photosynthesis; low 613C values inttris range are generally accepted as proofof a biogenicorigin. A possible exception is provided by a smallpercentage of diamonds that formed in the mantle andby carbon from some meteorites (Deines 1980b). These

SHUNGITES: TIIE C-RICH RoCKS OF KARELIA. RUSSIA r375

rare low-6r3C values may represent extreme distillation-induced effects or subducted carbon that was originatlyof biogenic origin. In the case of a distillationmechanism, only small amounts of carbon with thiscomposition could be formed. If this were sn importantprocess, a much larger residue with values significantlyabove --7%o would exist elsewhere, undetected" in tlemantle (Valley 1986). The carbon isotope ratios ofmeteorites form by a variefy ofprocesses, includingspallation, not duplicated on Earth. There is no sugges-tion or indication that the 25 x 1010 tonnes of carbon inKareta is meteoritic in origin.

The values of the 6rC bel ow -37%o are restricted toShunga and are lower tlan most organic matter. Atcertain discrete times in the Precambrian, such lowvalues were cornmon; values below 40%o occw insediments on several shields in tightly defined agespikes at 2.L and2.7 Ga (Strauss 1986, Schidlowski1987) (Fig. 10). The 2.1-Ga event is similar in age tothe shungites in this sfudy and is also recognized in theFrancevillian Series of Gabon.

The homogeneity in 6tgC that occurs at each depositprovides evidence linking the different types ofshungite. At Shunga, all samples but one have a 613Cvalue of -37.5I X0.05%o. Thus the type-I and type-trshungites have the same, unusually low value. Similarrelations exist at Maksovo, where the type-I andtype-Itr shungites have the same 6t3C, -n.02to.37Voo,atd atZazhoglno, where all but one sample average-26.67 t 0.20%o. These results indicate ttrat all thereduced carbon in each area has a similar origin andargue against the possibility that qross-cutting varietiesof shungite might have an origin unrelated to the ottrerforms ofreduced carbon. TWo hypotheses result fromthis observation: 1) all of the carbon in each area wasdeivedin siu from the same source, with cross-cuttingveins representing locally remobilized carbon, or 2) alIof the carbon for a given area was introduced from asingle source. A third hypothesis, fhat the reducedcarbon formed by a variety ofprocesses and was subse-quently homogenized by metamorphism, is unlikelybecause it would require that some of the carbonoriginally had an even more extreme 6rrC value, below-37.5%o, and also because graphite is highly resistantto isotopic exchange, even during granulite-faciesmetamorphism (Kitchen & Valley 1995).

If all of the carbon is allochthonous and of anabiogenic origin, then the mass balance ofproducing sucha large, low-613C district requires a source of carbonunlike any that is known on Earth. Convenely, if most ofthe carbon formed biologically and is syngenetic withenclosing sediments, tlen small amounts might beremobilized by metamorphism with little orno isotopicfractionation, and all ofthe observed relations are explained.

The few samples from the Shunga region that havevalues above -2O%o Qeported by others) are probablytie result of small amounts of mixing with inorganic(carbonate-derived) carbon that has a yalue near Moo.

Dolomite adjacent to the shungite would be a potentialsource of such carbon. Carbon that is originally ofbiogenic origin could also evolve to higher 6trC valuesby maturation and devolatilization of isotopicallylighter CHa or by exchange with high 6teC carbonate(Vatley & O'Neil 1981, Kitchen & Valley 1995).

The above biogenic-metamorphic interpretation isat odds with the interpretation of the field observationsby LPG, who believes the evidence indicates anabiogenic, endogenic origin for shungite rocks.

Tin Snuqca DsPosrrs IN A Gr.onar, CoNrnrr

The period around roughly 2 Ga, approximatelywhen the Shunga deposits formed, was one of dynamicchange on Earth. It was a time of global-scale collisionof microcontinents to form modern-sized continents,resulting in the development of extensive stable plat-forms that could collect and preserve organic sediments(Des Marais 1994). The same period saw changes inthe concentrations of atmospheric O2 and CO2, anincrease in eukaryotic organisms, and profoundreconfigurations of the ocean basins @es Marus et aLL992, Des Marais 1994, 1996, Rye et al. 1995'Grotzinger 1996).

Karhu & Holland (1996) pointed out that between2.22 and2.06Ga, carbonate sediments in a wide rangeof localities worldwide experienced a sudden andunusually large positive excursion of their 6t3C compo-sitions followed by an equally steep decline. Theyinterpreted these as global effects and the result of anabnormally high rate of deposition of biogenic carbon,which, in turn, was tlle consequence of an unusuallyhigh rate of 02 production in the atmosphere. Otherevidence that the 02 content of the atmosphere under-went a substantial increase during this period includestle oxidation state of paleosols, tle geochemistry ofuranium, and the occrurence of red beds (Holland &Beukes 1990, Kasting et aI. L992, Holland 1994).Karhu (1993) estimated that the total excess 02produced during this restricted but critical period wasbetween L2 allLd 22 times that of the current inYentoryof oxygen in the atmosPhere.

Few organic-matter-rich stratigraphic units havebeen identified from this tirre period, although theyare known from both earlier and later times. It isconceivable that this gap is simply the result of acombination of limited preservation and sampling ofsuch ancient rocks. However, the Shunga rocls formedclose to this critical period, and it seems quite possiblethat they actually formed during that time, as did theextensive black shales of the Francevillian Series ofGabon (Bros et aI. 1992). Moreover, as pointed out bySchidlowski (1987) and Gauthier-Lafaye & Weber(1989), the organic carbon from the Francevillian isunusually light, as is that for the Shunga region, inapparent complementary fashion to the carbonate 6taCexcursion. These observations make precise ages for

L376 THE CANADIAN MINERALOGIST

the Shunga rocks of special interesl work is in progressto obtain such dates.

Except for a few periods of episodic increases, tleproportion of organic carbon that was buried insediments relative to carbonate has increased onlyslowly with time. However, the fraction of buriedorganic carbon peaked between 2.2 and2.0 Ga (Karhu& Holland 1996), the period when the host formationsfor the Shunga deposits formed. It is clear that thesedeposits developed during a critical nansitional periodin the history of our planet, tlus giving them ageological significance that adds to the fascination ofthe intriguing character oftheir unusual carbon.

CoNcr-ustoNs

The greater Shunga region is remarkable for thelarge amount of elemental carbon in Proterozoic (2.0 to2.1 Ga) rocks. Some rocks appear to be unique becauseof tleir glassy, amorphous form. Veinlets of glassycarbon suggest that there was extensive remobilization,perhaps under the influence of volcanism or contactmetamorphism. The carbon is, in general, isotopicallylight and ranges between -26.43 atd -37.57%oPDB.The detailed carbon isotopic compositions correlatewith local area rather tlan rock type, sugges';ng thatveins and host-rock carbon are related at each locality.This correlation indicates that all reduced carbon areach locality has tle same genesis, and that eithershungite veins are remobilized carbon derived fromadjacent layers, or all carbon in these rocks has beenintroduced from some unknown external source. Theage, local mobilization, and isotopic composition of theshungite rocks appear (to JWV and PRB) to indicateearly, abundant life in the region. On the other hand,LPG interprets the field and isotopic data as indicatingan abiogenic origin.

AcKNowI-EDGN{B.ITS

We thank Drs. A. Knoll, F. Hueber, K. Towe, and J.Gray for looking at thin sections of rocks from theShunga area for biological remains, and K. Towe forhelp in preparing the tlin sections. Nami Kitchen andMke Spicuzza helped with the carbon isotope analysis.Dr. B. Devouard helped with the X-ray identification ofjarosite, and Dr. C. Francis kindly made the Harvardmineral-collection specimen of shungite, collected byC. Hurlbut, available to us. Drs. N. Bostick, Y. Dilek,E. Grew, J. Hughes, D. Rumble, W. Schopf, R. Siever,and K. Towe provided useful cornments. Dr. A. Boucothelped with some of the Russian rock terminology.This study was supported by National ScienceFoundation (NSF) grant EAR-9219326 (to pRB),EAR-9304372 (o JWV) and by tle Foundarion forIntellectual Collaboration (Grant 44 ooshungite,, to theRussian autlors).

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Received April 21, 1997, revised rnanuscript acceptedOctober20,1997.