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RussiaaGeologyond GeophtsicsVol. 47, No. 3,pp.3 15-341, 2N6' Geoloeila- i GeofrzitaUDC553.,U1(571.53)

THE GIANT SUKHOI LOG GOLDDEPOSIT.SIBERIAB.L. Woodl and NP. Popov*

StarTechnology SystemsN.L, Moscow, Russi4PO Box 6325 Univenity of NSfi Sy&rey, NSW 1466, Australio+Lenzolato Open foint Stock Compoty,Bodaibo, Russia

The Sukhoi Log gold deposit is cenhally located in the L,nagoldfield region, approximately850 lcn IrlE fmm the city of Irkutsk, aad is hoshd in Upper Proterozoic marine sandstone,carbonaceous slate and phyuita, metemorphosdto low greensddst facis in an ouflyingpart of the major Akitkan Foldbelt. Ihe disseminatedpyritic tabular orebody has nooutcrop, is defined solely by assay grades and is located in the axial zone of a large,near-isoclinal, reclining anticline" Highest ore grades occur in pyritic black shale bds,especially where they cross the axial zone and include two elongate highr grade ('1-9ppngold) cylindrical zones, ermd ore pillars, along the gently plunging anticlinal crest Theanticline is exposedE-W over a length of 3 km and plungesat approximately 10' degreesNW. The uial plane and orebody dip 15" N, and the tatter is opento depth beyond 4fi) m.Three phasesof syn- and post-metamorphic rnesoscopic olding developd eharacteristicstructures. The first two (Fr and F2) are congruent with the anticline and localizedquarE-pyrite-gold veinlet mineralization in Fr axial plane deavages St), in narro% spacedaxial zonesof small non-penehative folds (Fz), in irregular ilisseminatedzones n shale,andin small inegular clusters (stringers) of q[arb-csrbonatc veins. A third phase generatedscattered kink-fold banats F3) and iregular crumpled zoneswithout minralization. A laterepisodeof transgrssivmsothermalquartz-vein mineralization developedmany low-gradeauriferous veins that have been the nain sources of the extensivealuvial gold deposits.The mineralization assemblages consisten0yquartz-pyrite.gold ctrbonate with minor baseand platinum group metals. Pyrite is widely dishibutcd in black shale throughout thedeposig at between 2 and 5 percnt, and in lesser amounts in a disseminated envelopearound the orebody. In the outer parts of the orebody and in the endosing mineralizationenvelope t contains gold of bigher frnenss 90F920), whereasdustered and veinlet pyriteis more common in the interior productive zong with gold of Iower fineness(840-880).Thedeposit was intensively explored from October 1971 to De&mber LW. The orebody at acutoff grade of I ppn gold, is an elongate rregular planoconvexshetup to 140 m thick,dipping NNE at 15" to 30', with a length of 22 h, a down dip width of more than 500 n'and is probably open to depth. It contains 384 m;llion tonnes with an averagegrade of 2.5to 2.7 grams per tonne. Adilitional rsources nclude 165 million tonnet at 2.0 ta 2.3 ppmin a low grade, possiblepit extension,atrd 205 miUion tonnesat 0.8ppm in the mineralizationenvelope.Sequences f pyrite-gold paragen*is, of textural and structural chargs,and limited isotopedata indicata that four distinct stagesof mineralization occurred during MOR subductionand Riphean closure of the major Proteromic Akitkatr FoldbelLDisseminateil,pyritit, slnmetamorphic,greenschist,goA-PGM, Iznq Patom,Bai*al' Riphean'Neoproterozoic, ate Precambrfun, dge-subiluainn, Akitkata Obkit

1 Conespondingauthocbrycelwood@hotsnail.com@2006UIGGM. SIBEHIAN BRANCH OF THE RAS

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RussitaGeologyand Geophtsics Vol.47,No.

Fig. 1.,4 - Lccation map. B - Sukhoi Log hill, view west The orebody s an extensive lat sheet hatunderlies the ridge (Y) behind the higher summit in centue and dips norti to the rigbt under thesnowfreld,Two cleared ines on left are for a road andpowerlineto a fuhrre plant site; the valley in leftcentre with alluvial workings is namedSukhoi Log, translated asDry Gulch.

INTRODUCTIONThe SukhoiLog gold deposit n Siberia@ig. 1, A) was discovered n 1961.andntensively explored n the19?0s.Many detailed ntemal reports were written and a few paperspublishedn Russian,but litde is loown ofthe deposit outsideof Russiabecauseof former ofEcial secrecy estrictions.The following account s basedonmuchdetailed nforrnation n Depaftnental aJrdnstitutearchives hatwereavailableo the authorsduringa lengthyfeasibility study from 1993 to 1996,but many are not yet accssibleor general eferenceoutside Russia. naddition to the following geological account, he origin of the disseminated ediment-hosted ineralization sexplained n somedetail as the result of the collision of a MOR spreading onewith a Late Proterczoicsubduction

zotl.ITTSTORYOF DISCOVERY

The exploration for and discovery of the Sukhoi Log gold deposlt was the culminationof many yearsofstudyard developmentof the Lena goldfieldsby severalgenerations f geologistsand miners. The deposit sentirely "blind', that is, it has no surfaceoutcrop, and it was discoverednot by pmspectorsusing traditionalmethods,but by geologiss using new concepts of ore deposition, state-of-the-artmethods of geochemicalprospecting, ndultimately diamond drilling.The history of goldfields in the I.ena region datesback to 1846 when the first alluvial gold depositswerediscoveredn the valley of the Khomolkho River 30 km NE of SukhoiLog. In the 1860s ich alluvial gold depositswere ound in many widespreadocalities, ncludingthe small valleys on lhe north and south sidesof Sukhoi Loghill, namedRadosmy,Zorinski and Sukhoi -og (or Dry Gulch) (Frg. 1, B). More than 30 t of gold was extractedfrom these hree alluvial depositsbetween1863 and 1900.The crest of the watershedhere exposedmany argegold-bearing uartzvein outqops on which themine Sergievwasstartedon top of the hill in 1886,and themineUtesisty n 1894.The main targets or primary gold during 1899-1904were theseand other quartz veins, ftomwhich about 1 t of gold was mined duringprospecting, ut gold recoverypmvedditfrcult andthe work wasoftennot profitable.In the 1960sgeoiogis V.A. Buryakproposed n the basisof knowngeochemically nomalous old in SukhoiLog rock sampleshat sulfide-goldore may be presentand hat this shouldbe a majorexplontion objective 1, 2].The first drillholes that followed in the springand summerof 1961revealedgold-bearing ulfide mineralizationat depth,and verihedBuryak's proposition.Furtherdrilling andfeasibility studieswereundertaken p to 1971byIrgircdmet n hkutsk, and very intensive explorationstartedn the autum of 1971and was completedat the endof 1977.The work included 209.6 kn of diamondcore drilling in 8ul6drillholes, 11.7km of underground rives(Nos. 1 and 2), 61 raises, 1,546 m of which were in ore, 110.3km of tenches, 13,000channel samples,hree

3 1 8

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RussionGeologyand Geophysics VoL 47, No. 3bulk samplesof i50 t, 800 t, and980 t, many tensof thousaads f assays or gold, and mary analysesor otherpurposes,. he data a.rcpreservedas voluminousarchives n Russianat Bodaibo,Moscow, and various nstituteselsewhere, ndprovide a very detailed database f the deposit.In addition to the Sukhoi tog deposit, severalessergold depositsof the same ype were discoveredn theregion with considerably maller reserves, ut nevertheless ould collectivelysupportmajor mining andproduction-These ncludeVerninsky, Vysochaishy,Nevsky, and wo small,physically separateensoidorebodies n the flanksand lower levels of the Sukhoi Log deposit,namedCentralRadostnyafi Z,apadnyRadostny.

The two major underground rivesQ,{os. and 2) of the 1970sprogramwere re-openedn 1995,andprovideddirect access o and observationof a largepart of the interior of the orebody or purposesof the feasibility studythat was conducted n 1993-96 by Star Mining Corporation,Sydney,Australia.LOCATION. TERRAIN AND ACCESS

The Iflzoloto project area ies between57 and 60 degreesNorth latitude, approximately900 km south ofthe Arctic Circle (Fig. 1, A) and is 105,000kmz in area(Fig. 2). The climate s strongly continentalwith averagetemperatures f -21 qCin Januaryand +18 t in July and with a maximum-minimum ange of +30 t to -50 9C.The winter months from October o May aregenerallydry andwindless with heavy snow and ce formation,andall waterwaysare ftozen. The summersare warmand mostly dry.The region comprisesa brcad, subcircular opographicdome approKimately 50 lm in diameter, ermed hePatomHighland, that rises to altitudes of 1650m in the centreand to morc than 1800 m in the SE. It is paruyencircledby the Vitim and Lena Rivers, and dissected y several ributary river systemshat exhibit well-definedconcentricand radially divergent trends(Fig. 2). The terrain s one of extensivo'high ounded ridges with somestepslopesalong scarpsof sedimentaryocks(Fig. l, B). The major high level landformsdeveloped nderseveralepisodesof Pleistocene ce-sheterosion, a late phaseof trunk valley glaciation, engthy periods of periglacialerosion,and severalphasesof interglacial warm climate chernicalerosion.The depalluvial and colluvial depositsin most channelsand river valleys result from long periodsof gendedrainageand glacial erosion.Siberian orest (taiga) consistingof birch, maple,pine, cedar, spruceand larch coversrnountainslopesandsomevalleys up to about1,200 m above sea evel. Upland areasandbroad crestscarry a sparse over of subarcticshrubs,or morecommonly consist of extensive ockfields occasionallywith periglacialpatternedgound a.nd ocksEearns.Regular airline servicesoperatebetween he city of kkutsk and the lrna goldfrelds ownshipof Bodaibo,(pop. ^40,000),from which road access s available o all outlying villages and operations.

REGIONAL GEOLOGY. The regionalgeologicalsetting of SukhoiLog and he Lenagoldfleld is that of a complex olded sedimentaryiequence at the exposednortheastend of the Akitka:r Foldbelt and the included Olokit Zone, partsof both ofwhich extend nto the western side of the goldfield area as shown n Fig. 3. The 250lon southwesterly xtensionof the Olokit 7nne, and the 800 km of the Akitkan Foldbelt are exposed n tle mountainousPrimorskyRangealong he westernshoreoflake Baikal. Gmphysicalevidence uggestshat he total length of the Foldbelt ncludingthat which is concealed o the northeastbeneathPhanerozoic latfonn covet may be more than 1,500km, and tswidth is between50 and 250 km (Frg. 3). It is thus a majorPrcterozoicorogenicbelt within the Siberial craton,betwen he Aldan Shield to the east and the MaganProvince o the west. Thick formationalunits in the OlokitZone of the Akitkan Foldbelt are exposedalong the westernside of the PatomHighlands between he AbchadandVilyui Faults n the Mama township district (seeFig. 3). In contrast,a muchthinner condensed atomsequence(see Figs. 3 and 4) overlies Archean high-gradegneiss and Charametasediments f the Aldan Shield on thesoutheastemideof the Highlands.Stratigraphy. The PatomHighland exposes trongly oldedrocks mainly of Pmterczoicage,and s sunoundedon three sides by flat-lying Paleozoic and younger cover of the Siberian Plaform @g. 3). The stratigraphicsequences f the Upper Proterozoicare well developed nd are represented y depositsof marineshelf, slope andeuxinic basinal acies [3].In summary, he Precambrian equence f the region (Figs. 3 and 4) consistsof the following:l. Archeanhigh-grade ocks (Chamunits) of the Aldan Shield,exposed ocally in the SE; gneiss, nigmatite,slate,quartzite.2. A minor Early Proterozoic sequenceunconformableon the Aldan, of slatesand quartzites(Nerukan

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Ru.ssianGeologyatd Geophysics Vol. 47, No,3

!+Bulbukhb

f4t @z f;la l-Jlrl=-le l-:-ls lE-lro [EFla[-;-]s l-;lo Iolzf-rz IVB Au

Fig.2. Area of oFrations., - dredging,2 - mechanicalexcavating 3-pardtrg,4- mhes. Primary depodits:1-Sukhoi og, 2-Pervenets-Vernensky,3-Vysochaishy.lvlain alluvial deposib:4 -Shusman,5 - Vasilievsky,6-Aftemovsky,7 - Ust'-Dzhegilakar, 8 - Khoalokan,9 - ClhertovoKoryto, 10 - Bulbukhta, 11 - Mama-Vitim,12- Dalnyaya Taiga" 7- dis&ict canters,S- mining rlistricts and prospectbass,9 villages,10 - hyrlroelechi-calplant, .l,l - main airyort, l? - dirt roadg 13- rivers, 14- adminisfratiye boundary.

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Russian Geologyand Geophtsics

II*-FI F-ffi2 FIa F'rl'lu []11.ls I-]lofTllz [T-la Fls Hrc nn :112Fig.3. Geologicalnap ofthe Lena.Bodaibo goklfieklsarea(modified n part from [612,24. Ph- Phanerozoic platfomcover); UP - Upper Proterozoic (PatomGroup): t - fire carbonacous lasticsard limestotres'Uppr Patom(Bodaibounits), uP3; 2 - marble, carbonac@us sanrlstoneand sLatq Mirtlle Patom (Kadalik units), UPt 3 - conqlomeratc,quartzite, datc, lower Patom (Batangalakh units), uPf; 4 - MP - Mesoproteromic' MPM - MedYezh'yaFormation- vokanic and sedimentary rocls; LP - Lower Proterozoic, LPN - Nrnkatr urit - quarEit slate; A - Atchean(ChaE utrits) - hghride gneiss,quarbite, slae. tntrusive rocks: 5 - Prleozoic gratriles,pyroxene granit s, grano-diodtc, G3 - Uppr Proterozoic to Eocmbrian. d - uilifferentiated Lower and Miiltlle Proterozoic grsnit' gneiss'atiabase,migmatite, local volcadc rocls, G2. 7 --regional itike belt, with basic and laEpmphyric dikes shown.8 -primary dissemina&d golil ileposits: 1- s;khoi Lo& 2 - Pervenets-vernenshr,3 - vysochaishy.9 - primary veingolddeposits:4-Shusman,5-VasilieYsky,6-Artemevsky,T-UstlDzhegdakar,S-Khodokan,9-ChertovoKoryto. 10 - Bulbukhta, 11 - Mama-Vitim, 12 - Dalnyaya Taiga. 10 - roarls; Ir - rivers; 12 - adminisaativeboundary.

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Russiaa Geologyand Geophlsics VoL 47, No.3

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Russian Ceoloqyand CeophysicsFormation),and a few l-ower and Middle Proterozoic spilites,keratoPhyres,(Medvuh'ya Formaton) in the westernHighlands.

-.VoL47,No.3chlorite^slates and basic volcanics

3. The Upper ProterozoicPatom Group, s divided into three main subgroups.- the lower BallaganakhSubgroup,with an importantcomponentof conglomerate ndgritstone;- the middle Kadalikan Subgroup,dominatedby ca.rbonateocks containingalgal remains; his unit hoststhe main quartz vein gold deposits of the region and the disseminatedpyrite-gold deposit of Sukhoi I-og incarbonaceoushales;- the upper Bodaibo Subgroup,dominatedby fine clastic,calcareousand dark carbonaceousocks whichalso containgold depositsof disseminated yritic type, altematingwith rhyolitic molasse-tyPe onglomerates.4. Numerous ntrusive complexes nclude many varieties of granite, and also a regional NE-SWtrcndingbasicdyke belt [3] that lies 20 km east. f Sukhoi Log, Vysochaishyand Pervenets-VerninskyFig. 3), in whichlamprophyre as been dated by the Sm-Nd methodat 31359 Ma [4]. Someof the granitesare Early andMiddleProterozoic@aikalian), others are Paleozoic and are dated at 354 Ma to 322 Ma (Hercynian)[5]. The nearestganite to Sukhoi Log is the small.Konstantinovsky tock, 6 km to the SW, which is dated at 290120Ma [5].ID the Patom Highland the Bodaibo Subgroup is Epiproterozoic [6], ard lies without any apparentunconformityon the NeoproterozoicKadalikanSubgroup Fig.4). The Kadalikan Subgroupncreases onsiderablyin thickness orthward rom the Bodaibo area o the lower BolshoiPatomRiver, and n the Sukhoi og-Kropotkinarea ncludesa sequenceof sandstone, arbonaceoushale andphyllite, named he I(homolkho Formationas alocal unit of the 1500 m thick Valyukhta Fomation (Fig. 4). This unit hosts the pyritic gold mineralizationatSukhoiLog andVysochaishy.The uppermostormation of the Kadalikan, he Imnyakh(500-?00 m), consistsof calcareous late,siltstone,quartz and mica-quarz sandstone nd lesserbandsof limestone.The overlying .qunakit Formation 20G-1200n)of the BodaiboSubgroupconsistsof polymict sandstone,ericiteauartzshale,andcarbonaceoushalewhich hoststhe sulfide-gold nineralizationof the Pervenets-Veminskyeposits.Above the Aunakit lie thee formationsof fineto coa$e clasticswhich aggregateup to 3700 m in thickness.Structure- The Patom Highland has a distinctive regionalstructuralpattern marked by folds il the UpperProterozoicmetasedimentary nits, the axes of which vary systematicallyn tend from NE in the wst to SE inthe east. n plan view this forms a semi-circulararangementwith the Marakan-Tungus rough to the north andthe BortaiboSynclinorium to the south, n the laner of which are the main vein-gold orebodies.SukhoiLog liesin theanticlinorium etweenhem.The Patom Group rocks were strongly folded in a major deformationphaseaccompanied y low-graderegionalgreenschistaciesmetamorphismwhich to the south s locally overprintedby mtamorphicaureolesoftht Paleozoicgranites.The fold tracesare convex to the north, towards the SiberianPlatform @ig. 3). In the

northemnear-platformmargin of the PatomHighlan4 the sediment$ isplay great thickness,gentle olding withslight vergence owafd the platrorm and a very low gradeof grenschistmetamorphism.Fold zonesoccur farthersouth n the BodaiboSynclinorium n which the thicknessof thestrata s greatlyreducedand he rocksare ntenselyfolded and metamofphosed t lower to middle gradesof the greenschist acies. The folds are assymmetrical,reclining, near-isoclinal,with vergence facing" southward o the Mama-Vitim zone.Within the latter zone,thetwo lower subgroupsacquire a grcat thicknessagain,displayfrequentrhythmic altemationsof Boumaflyschoidtype and extendout of the l,ena region as a part of the majorAkitkan Foldbelt that extendssouth o Lake Baikal(Fig. 3). At regional scale, he sequence f deformationaleventsappearso be simple,with fust-phase enetratiYefleiurat-slip folding (F1), generating arge first-, second-,and third-order folds, axial plane foliation, schistosityandcleavage.A secondphaseof lesserdeformation s alsowidespread ut is usuallyevident as locally separatezonesof smallflexural folds (F2)that rotate he earlier F1foliation structures. 2axialplanesareusuallysubvertical,ard axial structures " commonlyvery inegular and nontpnetrative.Both the F1 a'J.d 2 generations reevidentin the SukhoiLog deposit andhave exerted significant ocal structuralcontrol over the mineralization.A third phaseof deformationdevelopeda range of nonpenetrative tructuEs (Fa) such as kink-folds, foldbands,and fault-fota combinations.This phaseprobably involved severaluNelated ocal episodes, t leastone ofwhich included the emplacementof the widespreadbut subeconomicquartz vein-gold deposits of the Lenagoldfields, ncluding thobe n the Sukhoi Log area(Fig. 5, A).

GEOLOGY OF TIIE DEPOSITThe following account of the Sukhoi Log geology is basedon references iled, on earlier extensive ilereportsco-authoredby N.P. Popov, and on personalobservations f the authors'Earliest reports nclude an unpublishedgeological accountand map at 1:10,000by Buryak and Kochetkov

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RussianGeoloqyand Geophtsict . Vol.47, No. J

f--1rr

5

Fig. 5. Schenatic plan (A) and crosssection(8), Sukhoi Log; [9], with ore blocks added. -calcareouscarbonaceous iltstoneand slate; 2 - carbonaceous iltstone,phyllit; 3 - carbo-naceousphyllit, slate, siltstonei 4 - faultsi 5 - disseminatedpyrite; 6 - normal facingbedding; 7 - overhrrned bedding; 8 - ore blocks defrned by drilling and undergrounddeyelopment;9 - mineralization mne; 10- zoneof economicore; II - drill holc

(1959),anddetailed ublishedeports n 1964Fl, and n 1969 8]. on oresandrocks of SukhoiLog. A briefaccountand map of the geologr and the vein gold depositsof Sukhoi Log was published n 1971 3] asPartofa wider studyof th Lenagold-bearingegion.A generalizrddescriptionof the sulfide-goldmineralization,ogetherwith a schematicmap and section, was published n 1974[9], and this indicated or the first time the unique

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Russia,.Geologyand Geophysics Vol.47,No.3qualitiesof the deposit. A more detailed account n English waspresented y Buryak in 1986 10], andprovidesthe basis or muchof the fotlowing. A further report n 1987discussedhe origin of the gold mineralization 1].Stratigraphy, The stratigraphicsequence f the host rocks of the deposit area consistsof two conformablesuites of upper Prcterozoic age named Khomolkho [hrn] and Imnyakh [im]. The Khomolkho Formation issubdivided ithologically into three Subformations, he Lower (hm1) - carbonaceous hale and siltstone withinterlayersof carbonaceousimestone 10G-150m); the Middle (hm2)grey limestonewith interlayersof siltstoneand carbonaceoushale 110-200m); the Upper(hm3) carbonaceoushale,phyllite and siltstone 150-180m).

The overlying Imnyakh Formation s subdivided ithologically in two Subfomations: the Lower (im1) -mainly calcareous andstone,haleand siltstone, esscommonly irnestone; rd theUpper(im2) mainly imestonewith interlayersof shale and dotomite.The ore-bearing one occurs n the sediments f theUpperKhomolkho Subformationhm3)within the hingezone and imbs of the SukhoiLog Anticline (seeFig. 5).Furtherdetailedstratigraphic ubdivisionof the hm3ore-hosting equence f theUpper Subfonnations basedon lithology (type and rhythm of 'lamination,physical propenies,cleavage).Five units havebern identified n dresequence asedon thesecriteria and are illustrated n detail by Buryak et al. [10].Intrusiye rocks. The small granitoid Konstantinovsky tock is located 6 km to the SW of Sukhoi Log angis a memberof theKonkuder-Mamakan omplex. t appea.rst the surfaceas wo adjacent utcropareas f 0.5 km',within a smallnegativegravity slemaly. Geophysical ata ndicate hat the body has a stepsouthern ontactanda gentlyslopingnoftheastemone oward SukhoiLog. The rock is a biotite porphyrite n the centre,s fire-grainedir the contact zone, and is Permian,dated at 29Oj20 Ma [12]. A 10G-250m wide contactmetamorphic ureolesurroundshe outcrop,with greaterwidth to lhe norththan he south. Several atitudiDaldikes of graaiteporphyryandquartz porphyry are locatednearby-Other more distant intrusives include large PaleozoicgraniG domes to the south, ages of which areCarboniferous rd range rom 354112Ma to 33Gt10Ma [4]. A NE trendingzone of basic and amprophyricdikeshave beendatedat 313+59Ma (alsoCarboniferous) y the Sm-Ndmethod; hey ar.e ut by late stage290120MaPermiangranite porphyry dikes [4]. All these ntrusives epresenta major cycle of Hercynianmagrnaticactivityfollowing long aftertlte convergence nd suturingof the Akitkan Foldbelt. However here s no evidence o indicatethat therewre any significant effectson the SukhoiLog or lesserdisseminated old deposits uchasVysochaishy.Host rock details.The host ocks of thegoldmineralization onsistof a monotonous equencef KhomolkhoFormation hin-bedded lack shaleand siltstone hat are consideredo havebeendepositedn oneor more restrictedanoxic basins.Bedding s evident,at scalesof a few centimetres, y altemations f grey siltstoneand black shale,and at finer scale by lessevident thin layers of fine dark silt with slate. Many black shalebeds are essentiallyhomogeneous,lthoughunder he microscope,a laminationby segregation f fine sericite lakes can be discerned.This layering at millimeter scale s metamorphicbut undoubtedlyeflectspre-metamorphic ompositionalayeringof sediment.n somepartsof the orebody small areaswith low carboncontentandof slightlybleached ppearanceconsist of more micaceousphyllite, or chloritic phyllite (terrned istwaeniteor bercsite n Russian erminology)but appear o be the result of local, late stagehy&otherrnal activity with little evident relationship o goldmineralization.Bottom structures,suchas oad casts have not been ound or reporte{ nor havecross-bedding,ipple driftor other nternal structures.All varieties of sedimentary ocks are metamorphosedo a minor variable degree,dependingon the pdorlithology. The mineral composition of the phyllites is monotonousand includes quartz (in large paft clastic),sericite. siderite and carbonaceousmaterial. Isser minerals include ankerite, albite, chlorite, biotite, rutile,tourmaline and rarely clinochlore.With the exceptionof siderite all the metamorphicminerals are fine to veryfine grained(to a few microns); siderite typically occurs as distinctly larger porphyroblastsup to 4-5 mm inlength) n the finer groundmass. he sizes of sideriteporphyroblasts re more or less constantwithin individualthin carbonaceoushale beds but may rrary markedly rom one bed to the next. Furthermorehe individual bedsare comrnonlyseparated y wider intewals of non-sideritic ca.rbonaceoushale, ndicating their premetamorphic.carbonate omposition.Three main yarietiesof host rock are recognized:1. Quartz-carbonate-sericitearbonaceous hale,witr 12-207oqu.afiz,40-70% carbonatesmainly siderite-ankedtewith lessercalcite, and 10-20Vo ericite.Carbonaceous aterial occulsas separatelakes uP to 0.05mm,usually from 0.1 to 2-1Eobut in some hin layers p to 5Eo.XRD analyses ailed to detectgraphite,while othertests ailed to detect ittle more than a trace of bituminous compounds.A concentrate ampleextracted rom alarge volume of carbonaceoushyllite was found to be similar to callnel coal [7]. The material s presumedo bemainty amorphous arbon, l spite of the low-gradegreenschistacies metamorphism.However, ecent ntensive

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Vol.47,No.3RussianGeologya"d Geophysicsmicrofossil research on samples from Dalnyaya Taiga (locality 12 in Fig. 3) and other localities ir the Highlardsstongly suggest a significant carbon input of biogenic origin (see later).2. Siderite shales are charactedzed by plentiful ard widespread siderite porphltoblasts at 4G{07o of the rockvolume, and they impart a coarser granular texture to the rock and a hrotty irregularity to schistosity or cleavagesurfaces. The preferential shatigraphic distribution of the carbonate porphyroblasts in shale beds and their absencefrom interveniug psammitic strata strongly suggestsorighal sedimentary-diagenetic control of carbonatization ratherrhan a pervasive hydrothermal metasomatism.3. Quartzitic siltstone or slatg consists of more or less equigranular quartz grains about 0-1 mm with anadmixture of sericite to around 7O/o, and ankerite to 5-109o. Siderite porphyroblasts are also usually present, butin small amounts.Numerous quartz-carbonate hclusions, striagen, a.ndveinlets arc commou in phylite and shale, and frequentlycontain pyrite and other sulfrdes. They arc internally impoverished in carbonaceous matter, which is in the formof fine flakes (0.005-{.007 mm) markedly concentated along boundary zones. The inclusions aad veinlets havea thictrness from 1.5 mm to 6-7 mm io cross-section and consist of quartz and car-bonatecrystals oJ 0.2-{.5 mm,with pyrite aggregates of cuhic and pentagonaldodecahed{al crystals.A large number of whole rock analyses of phyllites within and outside the ore zone irdicates considerableuniforrnity of chemical and mineral composition [7]. Semiquantitative spectral alalyses of phyllite ardquartz-carbonate shale revealed a consistent range of milor and trace elements, such as bariurn, chromium, zilc,nnnganese, strontium, lead nickel, cobalt, copper, vanadium, scandium arrd beryllium, with arsenic and bismuthsignificantly absent (but present at low levels in ores).

Structure. The dominaat structure of the locality is the regionally thid-order Sukhoi Log Anticline, whichencloses the deposit- ft lies on the southern side of the regionally second-order Marakan-Tunguska Trough(synclinorium) with which it is congruent.The Sukhoi Log Anticline is a large, reclining, nearly isoclinal flexural-slip fold with planar limbs and ar. acutely inflected hhge zone (Fig. 5). It developed synchronously with progressive low-grade greenschist faciesmetamorphism and is a typical fust phase fold structue, designated Fl- The fold and the deposit it hosts constitutea GICSIAC (Gold in Carbonaceous Shale in Anticlinal Crcst) in the terminology of Shields [13] but differs frommost other anticlinal types in that the mineralization is essentially disseminated. Saddle reefs in intrastratal crstsare almost entircly absent, a.od he penetrative axial schistosity and cleavage appear o have been the mail conduitsatrd locators of pervasive fluid movement.The fold hinge zone is exposedfor 3.3 lcr along the crest of Suldoi Log hi Gigs. l, B and 5) and thevarious structural elements change odentation to a small degrge along strike. Near the west end the axial plarestrikes at 295" and dips 25'NNE and the axis plunges 1G-13'NW; in the cenFal part the axial plane strikes 278';its dip increases to 35" N and the axis plunges at around 8'WNW; neal the east end, the fold is less acute, theaxial plane strikes at around 270' aJld dips 38' N, aad the axis plunges at 3-5" WNW.Dissemfurated pyrite is evident in phy[ite outffops throughout the length of the anticli.ue, but subsurfaceeconomic mineralization is developed or y in the cntral part, with tLigher gold values mainly wherc the axialplunge is low Fl. The black shales throughout the fold have a penetrative axial plane cleavage, defined by orientedmica flakes. Also present at many places is a fine, close-spaced axially parallel lineation consisting ofmicro-crenulatedmica.Parasitic F' flexural-slip folds of second or third order to the main anticline are visible in undergrculd diivewalls, more especially in weU-bedded strata (Fig. 6, A, B). They all have similar sinuous profiles and are congruentin each limb to the main anticline and to the main planar foliation and cleavage. Many include planar veinlets ofhigh-grade quartz-pyrite-gold in their axial cleavages, and at some locations the fold limbs are offset or partiallytuansposed aqoss several close-spaced veinlets, In a few small Fl fold-crests, quartz-pyrite-gold veinlets alsooccupy bedding pla:re openings in the manner of saddle-reef structures.Other folds of small dimensions and more angular asymmetric profile are present but are less comrton. Inthese the main ,S1 oliation and the associated schistose fabric are rotated by the shorter lirnbs. Also, au irregularaxial plane fracture system may be developed especially where the hinges are more acutely inflected. Theseoyerprint the earlier generation of F1 folcis, and so occurred later in a different temperature-pressure regime as aseparatephase of deformation (F2). Many are htersected by or associated with small, high-grade quartz--cartton-ate-pyrite-gotd vehs of irregular, miniature saddle reef form and size (Fig. 6, A, B). They apparently developedunder post-metamorphic conditions, but in a temperature range at which mineralizing fluids still remained mobile.

A later generation of small folds is also present as widely separated na$ow zones of kink-bancls (Ff inwhich the bedding and foliation of phyllites are acutely inflected and fractured without developmetrt of quartzveiniug. The kint-bands (or short fold limbs) are generally subvertical with a northwestern trend, and ale bounded5ZO

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RussianGeologyand Geophysics Vol. 47,Na i

Fig 6. A - Compositestructural scheme, howing small sealeF1 folds in bedding (S0)with axial planeschistosity1Sr1and sheetedveinlets(Sv);F2 folds in schistosityand bedding with contorted veinlets(Cv);F3 kink-folils ia bands of crumpling; late quartz-gold tensionalvein (trv). I - Photographof Drive No. 1*-uU, ho*iog F, fold in bedding with sheeted einlets(Sy),broad f2 fold in beddingand schistosity' srgepy'rite nodule connectingwith contorted veinlets(Cr), and zoneof mall F3 kink'folils.

on each side by a distinct iregular axiat plane fracture. This generationmay be related to a seriesof smallleft-laterai subvertical aults shown n Fig. 5, or possibly o two setsof quartz vein depositsstriking at 320' mrd285-290. reported by Kazakevich[3]. F3 folds have tittle apparenteffect on the widesPreadmineralizationbutmay be brittle precursorso thefracturecontrolled atequafiz-veindeposits,whichrcpresenthe ast ow temperaturesage of hydrothermaldewatering. Faults. No major faults ate known within the SukhoiLog deposit,but one local fault underlies he alluvialdepositsmmediately along the southside of the Sukhoi Log ridge (Ftg. 1, B). The fault does not separatemajorrock formationsand occursonly within Patom Group stmts-Fifteenor more smallparallel leftJateral faults are spaced longthe ength of the SukhoiI-og anticline.Theystrikeat 340-350" @g. 6) and

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RussianGeolagyard Geophrsics Vol. 47, No.

Fig.7. A - Disseninated prrite (Pyrite f). B - Shee&d veinlets'pyrite + quartz (Pyrite If). C - Porphyroblastic pyrite (Pyrite III).

MINERALOGY$rite is distributed n three main moctes,l) disseminated, i; lamellar or veined,and (3) porphyroblastict5l. The fust is widespreadwithin and outside he ore zone and ranges rom midoscoPicto 1 cm crystals ncarbonaceoushyllite @g. 7, A). Lamllar fuegular veinsgenerallyof small sizeare alsowidespreadbroughoutthe orebody,while sheeted einlets(Fig. 7, d) are locally developed. herephyllitic slatebeds cross he hinge ofF, anticlines, and are regularly spaced n concordancewith the snongly penetative axial-Planecleavage.Srilfide-quartz heetedveinletsarc typically 0.3 mm to 0.8 mm thick with separationntervalsof 1-4 cm, overseveralmeters.Porphyroblasticpyrite occursas large separate rystals,or as subidiomorphic lustersn specificlayen or bedsof black shale Fig.?, C). Their dimensionsmay rcach2-3 cm, and heirgoldcontents considerablyless han the other varieties.A fourth much rarer modecomprisesnodularcrystallinepyrite as clustersof 1-2 cm 'tyes" having elongatedquartzpressue shadowsorming the "comers"of the eyes,or even morE arely as ensoidaggregatesp to 5 cmin length,comprisingcores of crystallinepydte surrounded y 1-2 mm framboidal spheroidsn concentricayers.The texture s consistentwith a low-temperature yndiagenetic ulfideproductof bacterialactiviry Gig. 8, A).Additional ore minerals associaGdwilh pyrite but amounting o less than 27oof all sulfides nclude lhe.following: pynhotite, chalcopyrite,scutterudite-chloanthite,phalerite,galena,gersdorffite,milledte, pendardite,violarite, rutile, magnetite.Sphalerites the mostcommon, ocally to 3%. Tracesoccurof anenopyrite,argentite,monazite,apatite and zircon.Gotd is distributed as separate ains up-to 200 microns, n fractures n pydte, at pyrite grain boundaries,and at pyrite{uartz boundaries.Most of thegold is associatedwith the Pyriteand eastwith quartz.Sumde textures. Apart fiom the rare nodularpyrite of syndiagenetic acterialorigin mentionedabove, hethrcemain modesof disseminated ydte areas follows (Fig. 7).Pyrite I occun as very finely disseminateddiomorphiccubic crystalsand aggregatesn phyllitic black shale

qr scate

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RussianGalogltandGeophysics

cn scateFig. 8. A - Large zonednodule of crystalline pyrite rimmed by framboidal pyrite. E - Zoned noduleof".y.taUio" pyntC with qnarb-pyrite-carbonate vein. C - Zond nodule showing partial hanspositionwith shale nto layers.

of the ore zone ald in the surmundingenvelope.The sizeof gra.ins'variesrom 0.1 mm to 2-3 mm. The gold inpyrite content n the ore zone is 40 to 57 ppm. This pyfite is also widespreadbut of lower grade n the outeraureoleof the ore envelope n the fourth and fifth Khomolkhosubformations.t containshigher Ni and Co andlower As thanothervarieties. t also haswidely dispened#S values.from 6%o o *?-OVoquggestive f a marineorigin by bacterial sulfate reduction in a seafloordepositionalenvirorunent 15]. Under the microscopea fineequignnular texturc is evident,with somecrystalsenclosinga sphericalpyrite core filled with fine disseminatedcarb6nparticles surrounded y an outerpyrite zonedevoid of carbon nclusionsand with a well{efined crystallineoutline.This is also suggestiveof a diageneticorigin for at least someof the pyrite, with the core correspondingto a bacterialspheruleof initially amorphous ulfide and carbon,crystallizeddiagenetically,and then overgrownby later idiomorphic pyrite.pyrite tr fiils cieruagesas sheetedor lamellar veinlets,and fractureswherc it forms small irregular veins(stringers).pyrite tr containsdisseminations f the samematerialsasryrite I but their size is m]rch smaller.Theaueole lota'content is 53 ppm (range rom 27 to 203ppm), and he silver contentvaries rom 5 to 23 ppm.Thispyrite containshigherAs, and hasmore homogen"oord4S values rom +6%oo +11%o,ndicatinga homogenizedsource 15], suchas low greenschistaciesmetamorphicluids.pyrite Itr is pre"ent n the form of singleporphyroblasticwell-formed crystalsof up 1-2 cm in diameteroras clustersof large intergrown crystals(Fig. 7, A. The gold content s low from 0.1 to 5 Ppm. t formed laterthan pyrite I ana Pyrite II as shown by enclosedpre-existing structurcs, ower temperatures f formation and"o*porition of intemal inclusions.Otherelementsn Pyrite Itr includehigberAs and esserNi than n the others'with a total absence f Cu.Rare tectonizednodulesof pyrite occur as boudins n zonesof high competeocycontrastwith interbeddedsandstone ndphyllite, Pyritic boudinshavehighergotd grades han adjacentore, from 22 ppm to 113ppm.

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RussianGeologyand GeophJsics . VoL47, No.3Pynhotite occurs in tlp shales, in quartz-pyrite veins and in Bnite I and tr. The form is isometric, irregular,droplike. An associationwith gold is rare.Chalcopyrite forms small aggregates n quartz veinlets. In shales and Pyrite I and tr it is found together withthe pyrrhotite as crystals from hundredths to tenths of a nrillimetre. Semiquantitative spectral analysis indicatesthat it contains silver. Sphalerite is in separategrains with galena and chalcopydte in quartz veins andquartz-sulfideveins.Galena is rarely present, in the form of single grains up to 0.2 mrn in pyrite, chalcopyrite and sphalerite, orwith gold and quartz.Arsenopyrite forms scarce idiomorphic grains 0.1 to 0.3 rnm, in quartz and pyrite. Other sulfides are rareand are in the form of small single grains. Argentite is very rare and is associated with chalcopyrite.

MODE OF OCCURRENCE OF PRECIOUS METAISGold. Gold is presentonly in pyrite or quaxtz-pyrite-carbonateeins. It rcacheshighestvalues n sheetedveinles (5G-200ppm in pyri@, andhasoverall ow yaluesn Pyritem (arcund5 ppm n pydte).Gold of economicor geochemicallyanomalous oncentrationss absent rom black shale hat lacks pyrite.Two geneticallydifferent types of gold arc recognized n the depositand differ in purity. The puer gold(900-920 fine) is in minor dropshapedparticles nside pyrite crystals associatedwith other sulfides or quartz.Suchgold is thought to be syngeneticwithin metamorphogenicyrite, and to have formed primarily by dropletenlargement f original very fine dispersed old within formerly syndiagenetic yrite [15], as n theca.rbonaceous

spheruliticcores of somePyrite I crystals.The lesspuregold (840-880fine) was deposited ater as coarsergrainsin veitrsandsheeted einlets,with Pyrite I and r andquaftz,andhas undergoneimited hydrothermal ranslocationand contaminationby other metals. The relative abundances f the three modesof typical gold occurrences reshown n Fig. 9, A-C and n Table 1.

Fig. 9. A - Coarsegold ncluded n pyrite. B - Gold in quartzadjacent to pyrite. C- Coarcegoldat plrite-quartz boundary.33 0

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RussianGeologyand Geophlsics VoL 47, No.3Table 1

Modes of Occurrencc of Gold

Association*Sample

M2 M3 M8 Average Gravityconcentrate Flotationconcenfatelocked in pyriteI-ocked n gaagueGrain boundariesLiberated

95.74. 3

87.8t2-2

66.02.43 1 . 6

83.20.816.19. 414.4

12.09.478.6

Gold Particle Size Distibutions (vol.%)Grainsize,pm Locked in ppite Locked in gangue Interstitial between pyritegrains

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RussianGeoIoByand Geophtrics VoI. 47, No. 3

Fig. 10. Forrn of gold grains.A - Liberated gold in flotationconcentiate.B - Coarsegold particles n graYity concentrate.

andpurity of gold within first generationpydte implies that the metal must have been available n disseminatedform in the black shalematrix from the onsetof pyrite crystallization 21], as does he occurrence f other nativeaccessory lementssuch as PGM, Cu, Fe, Cr, W, Ti 122)'This phaseof Pyrile I developmentwas ttrus syn- tolate-metamorphic,y which time deformationwaswell advanced,with an axialplaneschistosiry ontainingPyrite rlamellae beginning to develop in the Sukhoi I-og anticline.Possibly somegrowth of sheeted einlets n axialcleavages lso occurredat the same ime.As this processcontinued,Pyrite tr with lesserquartz and the carbonates, rikeriteand dolomite,began odominate he veinlet assemblage, nd to developclustredand isolatedthin transgessivequarE-gold-carbonateand quartz-goldveins (Fig. 8,8). Sideritporphyroblasts recEt transgrcssively y Pyrite tr Yeinlets, ndicatingthe youngerage of the tatter. During this stage he minor accessorybase metal and platinum group sulfidesdeveloped nder conditionsof increasingsulfur activity [22], and someof the elementswere ncorporatedn gold,resulting n the lower finenessof $rite tr gold.The se4uence f sulfide mineralization erminatedwith crystallizationof Pyrite Itr, mainly as single largeporphyroblasts n black shale or less commonlyas crystal aggregatesFrg. 7, C) with litde or no quaru ordeforrnation eatwes. Thesetransgressivelyntersectall other pyrltic structuresand related quartz veins. Alsonoteworthy s thefuconsistentlyower gradeof containedgold (10 ppm for PyritI and r),the higher As contentand the absence f Cu.It shouldbe noted here thatPyrite Itr of this account s a later generation han that of Korobeinikovet aI.t18, p. 437-4381which is a clustered-stringerype containingPyrite tr of this account.

BESERVES AND RESOI]RCESThe geological imits of the envelopeof mineralizationand the boundariesof productivezonesarc entirelydependent n the ntensity of sulfide mineralization.During evaluationdrilling in the 1970s his provided hebasisfor defining and samplingore in drill cores,andwith few exceptionsproveda reliablecriterion in the light of themany systematicchecks on rcsults that were made by the various Russian authoritiesduring the progressofevaluationwork and ater.The system or estimatingRussianorereserves nd resourceswas basdon established,well definedspecificrulesand regulationswith many balancesand checks. t waswidely applied throughout he former SovietUnion.andunderwentsix major revisions since nitiation in 192'11231.In the context of the Russian systemof reserve-resourceefinition, the categories hown n Table 2 andFigure 11 are approxirnatelyequiyalent 23] to Provn/Probable eserves@), Measured/Indicatedesouces Cland C2;,and Infered ResourcesP) of the Australasian ORCCode(1996). n terms of the U.S. systemhey areequivalent o Measured/Proved esewes@), Indicated,/Probableeserves C1), nferred/Possible esourcesPartCr andPy).- In addition to SukhoiLog, Table 2 includes he two small satellite depositsRadostnyCentralandZapadny,andthreemore distantdepositsof the same ype(for locationsseeFig. 2). The Sukhoi -og to$nages rethoseof

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RussianGeoLogyand Geophysics Vol. 47, No,3

Table IModes of Occurrence of Gold

Association*Sample

MZ M3 M8 Average Gravityconcentrate FlotationconcentrateLocked n pyriteI-ocked n gangueGrain boundariesLiberated

95.74.3

87.812.2

66.02.43r-683.20.816.1

9. 414.476.2

12.09.478.6

Gold Particle Size Distributions(vol.4o)Grain size, Um I-ocked in pyrite Locked in gangue Interstitial betweenpyritegrains

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Deposit Russianmineralizationcategory Million tonnes Grade(ppm) In situ gold(million oz)

Strip ratio(wasteper t orc)Sukhoi LogIn pitSubtotal,pitPit extensionTotal SukhoiLog

Bclc252t99133

2.5-2.72.r2.7

4.24.516.0-17.310.7-11.5

3.73.7

Low grade38 420516 575 4

2.5-2.70.82.0-2.3r.9-2.1

30.9-33.35.310.6-12.246.8-50.8

3.707.03.4Rados{ny*TsentralnyRadostnyZapadnySubtotatwith SukhoiIog

C2+PlP1Pl

C2+Pl

2.08.04.65.3t4.6

o. J4.55.03.54.9

0.42l:200.740.602.4

6.0

8.0

VerninskyPervenetsYysochaishy

c:d+c2Low grade nwastec1c2

2lt0-200.815

2.90.5-1.65.3

2.00.50.11.5

3.00162.5

Total 820 53.3-57.3

RussianGeolog!and Geoplrysics VoL47, No. 3Table 2

Reservesand Resources,Primary Gold, Lena-Bodaibo Goldfield

r Mind with Snkhoi LoC.

the consultantmining andpit desig! group,VNlPlhomtechnologii (.A11-Russianesigning andResearchnstituteof Production Engineering). RGkedmet (Irtartsk Researchnstitute of PrcciousMetals) also estimatedsimilartonnagesn an earlierpit design.The infonnationon the five lesserdepositss basedon explorationarchivesheldby the Bodaibo GeotogicalExpedition,which performedmost of the exploration n previousyears.MODE AND SEQUENCE OF ORICIN

Although disseminated old-PGM mineralization n black shalehas ong beenknown at manyplaces,suchas Lyublin, Poland [20], soutieasternChina, CanadaandU.S.A. [21], many of the host rocks are thin extensiveplatform sediments f limited accessibilityor mining. More recently,anomalousevels of PGM arebeingrcPortedin increasing urnbersof gold depositsand mines n marine lyschoidcarbonaceoushale-hostedePosits f Russi4Ukaine, Kazakhstan,116-15, 24, 251, n bitumenat the BossMine, Nevada[26], and in the Magdalamine inVictoda, Australia (see ater).SukhoiLog may be one of the largestand probably the first major disseminated old-PGM dePosit o bediscoveredn rnarine carbonaceoushales,and somediscussionof its mode of origin may provide a model forotherdiscoyeriesf the samekind.As outlined earlier, the regional geological setting of the Sukboi l,og and the five lesserblack shaledisseminated epositsof the Lena goldfield is that of a complex folded sedimentarysequence t the exposednortheastern nd of the Akitkan Foldbelt and the includedOlokit Zone, Partsof both of which extend nto thewestemside of the goldfield area as shown n Fig. 3 U2, zXl.The Fotdbelt ii the only trane within the SiberianCraton hat is not underlain by Archeanbasement27,p. 4131. t is interprcted 28] to havebeena L-oweraDdMiddle Proterozoicoceanicbasinsequence1800-1600Ma)

333

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Russi4,nGealog,and Geophysics

72.2 ?2.6 73.0 73.4 73.8EFig. 11. PIan of SukhoiLog orebody,showingDrives Nos.1 and 2, ilrillhole locationsand outcropoflow-grademineralization envelope.1-3 - orebodycategories, - drill hole localities.

that was closed and underwent metamorphism n Late ltecambrian-Riphean time (65G-5?0 Ma) duringAldan-Magan collision, at the end of the Baikalial tectonG.mqgmatic ycle [5, p.8]. Recent advancesnmicrcpaleontology 29-31] and regional mappinghave enabledcorrelationof the Patom Highland stratigaphicsequencs ith those irther south n the Muya district (nea.r aksimo n Fig. 2) of oceaDic nd sland arc origirs[32-35] and confirm the variety and extensivePrccambrian nity of the Akitkan Foldbelt.In the Muya distric! the Foldbelt s a Lower to Middle Protrozoicmarginal-sea ccretionary ssemblage foceadc island volcanics, ophiolite (i.e. oceaniccrust) [36, 37] ald subduction elated ntusives and eruptives.Within the yotnger,20-25 km wide Olokit Zone along the soulheast ide of the olderaccretionary omplex,Partof the sequenceesembleshat of the Patomhighlandsbut also includesdifferentiatedvolcanicsequetrcesp to2 lcn thick. Graphitic flyschoid black shalehosts he large Kholodninskoyestratiform sulfide Pb-Zn deposit, nwhich lead isotope data ndicate an initial synsedimentary-exhalativesedex)origin of the sameage as the hostrocks(740i10 Ma), followed by metamorphicemobilization n the Riphean nterval of 600-550 Ma [12i'Several esserdepositsare also known in the Olokit Zone, ncluding barite-polymetallicypes n carbonaterocks, copper-nickelmineralization n the Doryren layered ntrusives and Avkita meta-ultramafits,nd bandedsilica-iron formations n Tyya and Medvezh'yagrenschistsFig. 3), all of which have ndicationsof rift-basinorigin in the interval 105G-700Ma [12]. As in parts of rhe Akitkan accretionaryassemblage,he rcclcs n the.Olokit Zone underwentseveralepisodesof folding, of kyanite-sillimanitezonalmetamorphism nd regression, fsuperimposedhrus* aridreverse aulting.The simitarity of the Ripheansedimentiry sequencesn the PatomIlighlands to those of the Olokit Zoneard to other parts of the Akitkan Foldbelt reinforcesdirect correlationard the demonstrable ontinuity (Fig. 3)between he gold-bearing, olded, low-gradegreenschistacies metamorphicsof the Patom lligNands with thePbZr-bearingntenselydeformed,higher-grade yanite-sillima.Dite lokit metamorphics f the accretionary kitkanFoldbelt.In this shucturalcontext the contemporaneousut less deformedmetavolcanics nd metasedimentsf the

. VoL47, No.3

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Russian Geolagtand Geophysics Vol.47,No.3Patom Highlands are interpreted o havebeen deposited n part of the Olokit marine rift basin contiguouswiththe Aki&an sea,but also partly overlappingonto an epicratonicshelf at the westem edge of the Aldan Shield,represerted n the southeastHighlands by a condensed equenceoverlying Archean Chara units (Figs. 3, 4,and 14, B). This interprctation iirther impliesthatRipheansubductionand accretionof the Akitkan-Olokit oceanicsedimentswas westerly, andoccurredeither at the Vilyui Fault (Fig. 3), or at the major geophysicallydefined butconcealed fia Fault Tnne [27].Tectonic sequence.The agesof many of the foregoingevents arc well constrainedby a variety of isotopicdata, someof which also have relevarce for the origin of the disseminated old depositsof the Sukhoi Log type.

These nclude (1) contentsof the main Akitkan Foldbelt, 1800-1600Ma, Lower ProterozoicPRl, [28]. (2)metaporphyries t the baseof the Olokit Group of the Foldbelt, 186315Ma t4l, (3) Pb isotopic model agesof theKholodninskoyePb-Zn sedex deposit n the middle of the Group, 740t10 Ma, Upper ProterozoicPR3[12], (4)the Dovyren layered inhusion in the middle of the Group, by three different methods, 700i20 Ma [38], (5)metaporphyries t the top of the Group, 70Oi20 Ma UpperProterozoicPR3 [39]. All these range from l,owerProtrozoic o Upper Riphean (Lste Prcambdan)n ageand all predate he final collision and closure of theAkitkan ocean.The ensuing egional metamorphismhat followed closure occurred n the rangeof 600-550 Ma,as indicated by the Rb-Sr method on Kholodninskoyehost metapelites,and by the Srn-Nd method on garnetamphibolites12].The gology and isotope data of the Kholodniaskoyea-ndseveral esser sedexpolymetallic deposits n theOtokit Group have been nterpretedby severalworkers as ndicating a rift basin envirotrmentnow preservedasthe accretedOlokit-Mamapaleorift trough [40]. Similar interpretations ut with varying emphasison metamorphic,hydrothermal,and epithennal aspectshave also beenproposedby previousobservers or the origin of the SukhoiLog and other disseminated old depositsof the Highlands,although mrn less widence. However, extensivePbisotopedata [12, Fig. 148] indicate that all the varieties of gold in the [,ena field, fiom disseminated,o vein, toplacertypes have identical Pb isotopic compositions.This may indicate ust one original gold source or all themodesof occurrence, he earliest of which at Sukhoi Log waspremetamorphic nd synsedimentary.The recentdiscoveryof platinum groupmetals n and above he Sukhoi ,og orezone has urther mplicationsof a contemporaneousmande-qTreprimary source of the metals [18, p. a37], and subsequent ynsedimentarydepositionoverlapping hat of the gold.Thus the later evolution of the Akitkan Foldbelt changed rom thatof a major couisional-subduction ystemto include a lengthy rift basin along its eastemside (Fig. l3). Wiftin theregional basin terrigenousmolasse ypeconglomerate, andstone nd limestone alterDatedwith carbonaceousedimentsas ndicated by the stratigraphy nFig. 4, in ephemersl ocally deeperanoxicbasins,analogous o the Adantis tr Deep n the presetrtRed Sea.Themain rift basin is now the accreted.Olokit-MamaZone with its sedimenury-exhalativePb-Zn mineralization,andthe epicratonicextension s dre Patom Group n the Highlandswith jts seafloorhydrothermal-exhalative u-PGMmineralization.The changeover robablybeganat about 1350Ma (mid-Proterozoic) ut was undoubtedlya lengthyPrcCeSS.Rift basin origins. Exhalative metalFecipitates are now known in a variety of geologicalenvironments nthe modern seafloor [41] and include mid-ocean idges, axis andofFaxis seamounts, ack-arc sgeading centres,and sedimented ntercratonic rifts. Examplas of the latter typ are (a) the Red Sea, n which density-statifiedmetal-richbrines overlie mineralized sedimentsn the Atlantis tr Deepwith averagevalues of 0.6 ppm gold andup to 5.6 ppm gold (b) the Gulf of California within which GuyamasBasin seafloorsedimentsare minemlizedwith low gold and up to 350 ppm Ag, north of which the spreading orc of the East Pacific Rise ntersectedandrifted the continentaledge,and(c) the Middle Valley and Escanaba rough on the northernend of the East PacificRise where sulfide sedimentsare actively forning with mainly low gold contents

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Ru"\sianCeolagtand Geophysics Vol. 47, No.3

crft tmi0.5-1.0'1.0-2.02.0-3.03.0-4.04.0

'r'0 100m

Fig. 12. Section+25 through the orebodyshowinghigher-grade gold z,onesorc pillars).

phaseof slowertrailing limb subductionwould allow emissionof crustaland mantle-sourcedomatiitic or deeperhydrothermal luids (not necessarilyvolcanic) carrying gold and PGM direc.tly nto the marine environment(Ilig. 16,B) along a considerableength of the zone andtrench, andprobably spreadingwidely acrosshe basin-A connectionwith ridge subductionandgoldmineralizationhasbe.eneported n Alaska [,14],and anotherpossibleexamplewith gold and reportdly lowJevel platinum group elementsmay occur.n the Magdalagold mine atStawell n westemVictoriq Australia [45].Ridgesubductionhasprobablyoccurredmore te4uently n the geologicalpast han hasyet been ccognizd,in that almostevery former oceanicclosure nvolvedeither shutdown death)of a spreading idge, or subduction(burial)of an active one with somedegreeof rift faulting andseafloorexhalativemineralizationprior to closure.Following SissonandPavlis lt+6,p. 9131,'the history of any oceanbasinmust at somepoint in time includethreeplate interactionssuch as subduction of oceanicridges, transformsor trienches. he resultant triple-junctioninteractionshould ead to a distinct geologicalevent n the fore arc of the plate margin. Yet consequencesf thisinteraction emain oneof the mostpoorly understood eologicphenomena."Post-rift closure and metamorphism. Convergence f the Aldan and Maganbasement locks inally closedthe ocenicOlokit rift systemand esulted n synmetamorphiceformationof sedimentsn theZoneand hePresentHighlandsduring the Riphean(600-550Ma). The metamorphism nddeformation n the Olokit Zone s intense,wiih retrogressedault-slicesof granulitic basement ncl soclinally folded kya-nite-sillimanitemetasediments12].In con8ast, he adjoiningPatomGroup sedimentswere rDetamorphosednly to low-gradegreenschistacies, andwere coherently olded under ductile conditions.This difference s attributed o tlls existenceof an underlyingresistantbasementshelf of the Aldan Shiel4 on which the Patom Group sedimentswere delnsited and latertectonicallystacked y marginal brusting southeastwarduring he ater stages f Olokit basinclosure Ftg. 14,Q.The early stages f grenscbistacies metamorphismn which clayschalge to white mica+ chlorite,generateconsiderableolumesof low-salinity fluids [47] that can mobilize nitial sulfidephasesn carbonaceousediments,suchas griegite, mackinawite,and amorphous acterialsulfide into crystallinepyrite. Go14 platinumgroupandmanyothermetals hat may initially bepresent n a varietyof organometallic ompounds20] or as hiol complexes[41] are also conyerted o inorganic crystallinephasesand native metals [17] during the clay-sericitetransition.As presswe-temperatueconditions increasemary of the initial phases ecrystallizeard change n form orcomposition,imulating laterparagenesis.. In the initially premetamorphic roto-Sukhoi -og deposit, he mineralizationwas probably spreadat lowconcentration ver a largeareaof anoxic seafloorn a locally deeperOlokit rift basin.Carbonaceous udcontainingthermochemical nd bacterialsulfur is capableof sequsteringold and other metals rom densitystratifiedbrinesafter suffrciently long residence ime, to near.economic uantities and values[41] as are certainbacteria[48].Many metals may initially form organometalliccompoundsunder such conditions, [20] so that they becomeentrainedn highly dispersedorm. At an early stageof diagenesishe compoundsmay polymerize n kerogen,crystallizeas metallocenes, r reduce o lor,ril-temperatureulfidessuch asgreigrteand mackinawite,markingtheendof the syngenetic tage.

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108'[r1r-]"lllr-az f6-lg

Fig. 13.Regional cratonic setting ofthe Patomhighlands andLena goldfields,showing theAkitkan OrogenlcBelt and the olokit zone within it [27]. The rectangle reprGents the area of the GologicalMap, Fig. 3. I -aressof major outcrop, 2 - main sutures, 3 - thicknmsof pladonn sediments conto[rs in km).

Synmetamorphic eformationof the PatomGroupgercratedan. nitially southeastacing array of fwo majorsynclinori4 Bodaibo-Vitim to the southeast ndMarakan-Tunguskao the northwest,separatd y the Kropotkinanticlinorium which includes the prcsentsoub-facing Sukhoi Log anticline. Metamorphismoccurredbeneathatectonically siackedcover of probably 7 km or rnore of Patom Group sediments, nd at a lemperature f around350 qC t491.Metamorphic fluids were derived within the brinleduqtile transition zone from the large sourtevolumes n the limbs of the anticline, and sirnilarly in the smaller anticlinal Vysochaishydeposit 35 km to thenortheast.Once iberated, he fluids scavenged isseminatedmetals ncluding gold andplatinum, and were drivenby powerfultectono-dynamic ressure radients47] generated y tectonic hinning of the fold lirnbs and hickeningoi pelitic strata n the fold hinge-Developmentof a penetmtiveaxial planecleavage S1)allowedpassage f fluidsthroughpsammitic strata, mpartedconsiderable errneability o pelitic units and ocussedgreatervolumesnto thehinge zone.The carbonaceousompositionof the pelitic strataundoubtedlyconstituteda reductantchemical rapat this stage or the metals n solution, ald sogold accumulatedn the tectonically hickenedcrestalzones o formthe two major higher-grade ore pillars" alongthe anticlinal axis (Fig, 12). The higher gold grades here alsoindicatepreferentialmobilization of gold in solutionfrom limb regionsand redeposition n the axial zones,as sreported rom the HomestakeMine t501. n addition, he overlying Imnyakh limestonesmay have functionedasa.less permeablestructural trap and so enhancedmetal accumulation n the upper levels of the anticlinal crest.Near-ciosureof the anticline by penetrative lexural slip folding (F) completed he main synmetamorphic tageof evolution of the deposit.Continuingdeformationunder decreasing tressand probably depth,generated he small F2 nonpenetratiYeflexural folds with their associated ontortedveinletsPost-metamorphic plift and cnrstalexlensionsubsquentlydeveloped he large, ensionalquartz-goldveins,whichalthoughof uniformly low grades, ourcedmost hehistodcalluvial depositsof the Lena goldfield region.In soine respects he eariy stagesof origin of the SukhoiLog deposit seemsimilar to thoseof the recently

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Fig. 14, Serial diagrammatic cross-sectiors NW-SE acrossthe Patom highlanals area depicting stagsofdevelopmentof tle Akitkan Foldbelt and Sukhoi Log gold deposit A - The Magan Provinceand AldanShieldare convergingwith subduction under the Magan Province; the Akitkan Seawith its MOR spreadingzlne and the Aldan Shield are together moving to the subductionzone-1800-1300Ma. B - Collisionof theMOR with the subduction mne is followed by rifting of the oceaniccrust of the trailing linb, rsdting inhydrothermal emission of deep-sourced rustal and litlospheric mantle fluiils through extensional aults infront of the snbduction zone. ThesegenerateexhalativeAu-PGM urineralization of seafloor sdimnts nanoxic rift basins,the nascentOlokit-Lena deposits.1300-600Ma. C - Subduction has almost dosed theAkitkan seaway.The Olokit anoxic basinal and terdgenousmolassedeposits are thrnst apinst the Aldanmargin to form the Olokit Zone and partly onto the margin to forn the sFrdefonnational, syDmetamorphieSukhoi Log and other depositsof the Lena Gotdfield (6lxl-550 Ma).

discovered edexgold depositsunderlying h Carlin field, Nevada, 51,521 in that both may havebeengeneratedby crustalrifting.The final stage of regional deformation of the Palom Highlands was probably due to transpression ysouthwarddrift of the Aldan Shieldwhich caused he regional oroclinal bending [53] of all the major structuresinto the unique semicircular gionalpatternshown n Fig. 3. This probablyoccurredduring development f theF, kink-band folds in the SukhoiLog shales,and prior to emplacement f the northeasterlyectilinear zone ofbasicand lamprophyric dikes dated at 313{9 Ma [36], that is, betweenLate Ripheanand Late Carboniferous.Although athibutedby Zohnenshain t d. [28] to southwarddrift of the Aldan Shieldalong the Zhuyastrike-slipfault during he ate Riphean, he oroclinal form may simplyhavebeen he astproduct of long-tem! highly oblique(NE-SW) closure, ather tlnn orthogonalcloswe of the Aldan Shield against he Akitkan Fold Belt. If so, thenthe supportingwesternedgeof the Aldan Shieldprobably exGnds beneath he Patom metasedimentaryover toat leastas far west as the cente of curvature.Local transpression f this magnitudenecessitatesn identifiablesource o the northeast or theorocliually translated egment. s shown n Fig. 13, here s a geophysically efinedbasin 5 km in depth now occupied by youngerplatform sedimentsying inmediately northeastof the Patom.I{ighlands.This was undoubtedly he original location of tie eastemhalf of the PatomGroupPrior o final closureof the Akitkan Foldbelt.

CONCLUSIONSFour distinct stagesof ore genesiscan be identified n the history of the Sukhoi Log gold deposit:1. Synsedimentary, yndiageneticmineralizationof marine cartonaceoussedimentsby density-statifiedmetalliferous brines, sourced by very large volumesof Au-bearing, deep crustal hydmthermal luids, and by

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RussianGeologyatd Geophysics .VoL,47,o.IPGM-bearing ithosphericmantle-sourcedluids, through rift basin extensionalaults; a "mineralizingYatg " inthe wordsof Fyfe [54,55].2. Disseminatedpyrite-gold mineralization(Pyrite I then Pyrite tr), developedduring penetrativeductile,synmetamorphic eformationand devolatilization,with pervasive luid transport of gold, PGM, andmany othometals tom depth n the fold limbs into the anticlinalhinge region.Higher gold gradesaccumulatedn tectonicallythickenedzonesof black shale units (ore pilla$) along the anticlinal hinge.3. Post metamorphicnonpenetrative mall-scale lexural folding (F2) with local segregations t hinge zonesof smallquartz-goldveins. Elsewherenon-pervasive,matl, rregular quartz-pyrite-gold-carbonateeinletsormedin clusters stringers)and scattered yrite porphyroblasts ith quartzpressureims (Pydtetr). Finally, a lateminorphaseof low-gold, coarselyporphyroblasticpyrite developedwith little or no quartz, or deformation eatures(PyriteItr).4. A widespreaddevelopmentof late stage, arge and lengthy, low-grade mesothermalquartz-goldveins,transgressivehrough he main orebody but Fobably souced ftom it.Sukhoi -og thusquatifiesas a syngenetic-rnetamorphicu-PGM depositand maypossiblybe an endmemberof a black shalehostedspectrum hat includessedimentary erived Zn-Pbsedexdeposits,mixed sedimentary ndcrustalsourcedZn-Pb-Ba.tAudeposits,and crustaland mantlesourcedAu-PGM deposits, ll of which occurwithinthe Olokit-Patom erane.During the past severalyears,similar blackshale-hosted GM havebeen discoveredat an increasingnumberof gold mines and depositsworldwide, which suggests hat this modeof mineralizationmay have occurredas anoccasional recusor stage n the development f many orogenicode-golddeposis. Thus,somePresentlydormantor extinct black shale-hosted oldfields may yet become arge, low-grade economicresourceswith combinedAu-PGM values, which separatelywould otherwisebe suberonomic.Acknowledgements.The authorswish to acknowledgehe insight, skill and dedicationof thosegeologistsandworkerswho discoveredandquantified he Sukhoi og rlepositunder SiberianconditiQns f extremedifficultyand discomfort.The membersof the geotechnical rcup wereawarded he prestigiousLenin Prize n 1980, aldincluded Vladim! AfanasievichBuryak (eader), Vladimir FeofanovichDubinin, Vladimir Andeyevich Lisiy'Nikolai PavlovichPopov, Nikolai StepanovichRomanchenko ndViktor Efimovich Ryabenko.Bryce Wood alsoacknowledgeshe very considerable upportof the late Nicolai Popov n obtainingclearanceandprcviding accessto technicaldocuments t a time when he depositwasclassed sa StateSecretand all informationwas restricted.Also acknowledgedare useful discussionsand observationswith V.V. Distler and colleaguesat the Institute ofthe Geologyof OreDeposits,Petogaphy, Mineralogyand Geochemistry,Moscow, andwith G.L. Mitrofanov andcolleaguesof the East SiberianResearch nstitute of Geotogy,Geophysicsand Mineral Resources,rkutsk. Anearliei limited version of this accountwasprsentedn 1995by B.L.Wood on behalf of Star Mining CorPoration'Sydney,at a symposiumentitled InvesEnentOpportunitisn Russid,ointly organizedby Moscow StateMiningUniversity and the Australian Institute of Mining and Meallurgy. More recentpolitical changesn Russiahaveresulted n this investsnent ppodunity rcmaining open.

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of hanl rockgold [in Russian],164pp.,NedraMoscow,1971.- 4. Neymark,L.A., E.Yu. Rytsk, and O.A. Levchenko,An eadyProteroioic-upperArchaeanage or the olokitcomplexg'i. Pre-Baikalia)using U-Pb gmchronology, n PrecambrianGeolngyand Geochronology f theSiberiancratonatd margins,206-222,Nauka,1990.5. Rundqvist,D.V., and C. Gillen, Precambriqnore depositsof the East Europeanand Sherian cratow,zM4pp.,Elseviex1997-6. Sutop, .!., Geolngical volution f the Earthduring hepre-Canbrian,439pp.,Springer-Verlag,erlin,1983.7. Stakheyev,.S., A.S. Evoilov, and A.F. Li, Chemicalanalysisof oresand enclosing ocksJromthe depositof SukihoiZog, IRGIREDMET, hkutsk, 1964." 8. eopoi, N.e., Some eaturesof geologicalcompositionand nterrelationberween ulfide mineralizationand

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