Ore depositional process in the Navan Zn-Pb deposit, Ireland

ECONOMIC CEOLOCY AND BULLETIN VOL.93 OF THE SOCIETY THE OF ECONOMIC GEOLOCISTS August1998 Ore Depositional Processes in the NavanZn-PbDeposit,Ireland I. K. ANDERSON,* Department of AppliedGeology, University of Strathclyde, Glasgow G1 1XJ,Scotland J. H. ASHTON, Outokumpu TaraMinesLtd.,Knockumber, Navan,CountyMeath,Ireland A.J.BOYCE, • A. E. FALLICK, Scottish Universities Research andReactorCentre,EastKilbride,Glasgow G75OQF,Scotland ANDM. J. RUSSELL Department of Geology andAppliedGeology, University of Glasgow, Glasgow G128QQ,Scotland Abstract TheNavanZn-Pbdeposit in Irelandishosted bya lowerCarboniferous, carbonate-dominated sedimentary sequence. In excess of 97 percentof theoreis hostedby a Courceyan, shallow-water succession termedthe PaleBeds.Theoreoccurs ascomplex strata-bound, tabularlenses oftendislocated byfaulting andtruncated by a submarine erosional slidestructure. Abovethissurface(unconformity) liesthe economically minor, but genetically importantConglomerate Groupore hostedby a complex debrisflowtermedthe Boulder Conglomerate. Timingconstraints on the mineralization canbe definedby the relationship to the erosion surface andstyleof mineralization, suggesting a lateChadian or earlyArundian age(•345 Ma).Thebulkof the ore formed below the erosion surface in the semilithified to lithified Pale Beds ore. The mineralogy of theeconomic oresis relatively simple,beingdominated by sphalerite andgalenain a 5/1 ratio.Pyriteandmarcasite arepresentin subordinate amounts in themajority of thePaleBedsorebut dominatethe Conglomerate Groupore andthe PaleBedsore immediately belowthe erosionsurface, particularly lens2-1. Gangueminerals aredominated by calcite,dolomite, andbarite. Themorphology of themineralization reflects subsea-floor emplacement. Theorebody occurs asnumerous strata-bound horizons, ranging fromintense, high-grade massive sulfides tolowergradesulfide layers separated by barrenlimestone. In all cases,thereis strongevidence for tectonicdisruption duringore deposition. Sulfides weredominantly deposited asopen-space growthandreplacement of hostrock.Open-space textures includedendritic-skeletal, stalactitic, internalsediment, geopetal, andcoarse-bladed forms.The generalabsenceof coarse, euhedral, cavityliningtextures or substantial collapse brecciation suggests thatlargecavities werenotpresent priortomineralization. Replacement textures rangefromdelicate pseudomorphs ofbiodasts to moredestructive granularstyles. Sulfide sulfurisotope analyses exhibittwodominant groupings of 6a4sfromaround-23 to -5 and0 to 15 per mil, respectively, anda thirdminorgrouping around-32 to -28 per mil. At leasttwosources of sulfurare invoked.Valuesaround-23 to -15 per mil are considered to be the resultof bacteriogenic reductionof lower Carboniferousseawatersulfate(•20%o), with a characteristicfractionationaround35 to 45 permilawayfromsulfate; thisisthemostcommon rangeof 634S in thedeposit. Values around 8 to 15 per mil areinterpreted to be hydrothermal sulfidetransported withthemetal-bearing fluid.Mixingof these twocomponents is inferredfromtheisotopic dataandtrendsin themineralparagenesis. Baritehasa mean of6a4S = 21 _+2 permil,whichfallswithintherangegenerally recognized forlowerCarboniferous seawater sulfate. We consider thatcontemporaneous seawater sulfatewasthedominant source of sulfurin the barite at Navan,implyingthatseawater accessed the PaleBedsat the timeof mineralization. Presentaddress: NavanResources PLC, KennedyRoad,Navan,CountyMeath,Ireland. Corresponding author: emafi, [email protected]. ac.uk 0361-0128/98/1984/535-2956.00 535 NO.5 536 ANDERSON ET AL. 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NAVAN GROUP COURCEYAN MAJOR UNCONFORMITY 0 •. 8 KM SILURIAN UNOIFFERENTIATED VOLCANICS ORDDVICIAN• •_• INTRUOEO BY LATE SEDIMENTS CALEDONIAN •SYENITES FIG. 1. Location andgeologic settingof the Navandeposit(afterAshton,1995). Introduction complex submarine-slide structure knownasthe"erosion surface" in the southeastern section of the orebody (Boyce et THEN^V^NZn-Pbdeposit, located30 kmnorthwest of Dubal., 1983a;Andrew and Ashton,1985; Philcox,1989, Ashton lin (Figs.i and2), isa strata-bound orebody hostedbylower bya debris-flow BoulderConglomCarboniferous carbonates. The depositwas discovered in et al.,1992).It iscovered erate which hosts the remainder of the ore in pyriticmassive 1970by Tara Prospecting, 2 km northwest of the townof knownasthe Conglomerate Groupore. Navan (O'Brien and Romer, 1971), with initial reservestotal- sulfides Thispaperoutlines thestratigraphy, structure, andaspects ing 69.9 millionmetrictons(Mt) grading10.1percentZn of the deposit.Closeattentionis paid and2.6 percentPb. Due to actions of thirdparties,oreon of the mineralization ore texturesand highlighting the correlation the northeast sideof the deposit becameunavailable to Tara to describing between texture and sulfur isotope composition. Prospecting, leavingsome60.9 Mt grading10.1percentZn and2.7 percentPb available for mining.Production comStratigraphy mencedin 1977andupto theendof 1996amounted to some The Navanorebodyoccurswithina sequence of lower 44 Mt. Currentore reserves total 17.1Mt at 8.2 percentZn Carboniferous shallow-water carbonates lying unconformably and2.5 percentPb,withmineralresources totaling18.6Mt outcropof the Longfordat 7.2 percentZn and2.4 percentPb (Outokumpu Annual on andjustwestof the southwest inlier(Fig.1).A stratigraphic section Report,1996).Currently, Outokumpu TaraMinesLtd.pro- DownLowerPaleozoic in Figure3 anddiscussed in the following secducesaround2.6Mt oforeperannumgrading 8.0percentZn is presented tion. and2.0percentPbfromanunderground, highlymechanized, trackless operation. Lower Paleozoic basement Of the69.9Mt reserves initiallydelineated northandsouth LowerPaleozoic rocks belowtheregional basalCarboniferof the Blackwater River,in excess of 97 percentis hostedby comprise a structurally complex andvaried Courceyan shallow-water carbonate lithologies termedthe ousunconformity andSilurian lithologies. ThebasicstrucPaleBeds.The ore occursin complexstrata-bound tabular seriesof Ordovician lenses,oftendislocated by faulting,and is truncatedby a ture consists of a fault-bounded belt of Ordovician meta- NAVAN ZN-PBDEPOSIT,IRELAND: ORE DEPOSITIONALPROCESSES 537 ß ß' ZONE ß / ß ß ß ß ß ß ß ß ß 500 METRES • - CONGLOMERATE GROUP ORE PLAN SHOWING LOCATION OF MAIN ORE I• - FAULT AT BASE OF 5LENS LENSES, MAJOR FAULTS AND /4"/ - FAULT ATBASE OF5 LENS • - OUTLINE OF4,3 C 1 LENSES FACIES CHANGES INTHE • - WRENCH COMPONENT PALE BEDS (REVERSE THROW)ß ßß - OUTLINEOF 5 LENSORE (NORMAL THROW) FIG.2. Ore lenses, mainfaults,andfacieschanges in the PaleBeds(afterAshton,1995), sediments andmeta-volcanics up to 5 kmwiderunningin a Limestone isthefirstcarbonate horizon ofsignificance. These northeast direction enclosed to the northwest and southeast argillaceous, crinoidal limestones arethoughttobeof shallow bySilurian sediments (Vaughan, 1991).TheOrdovician rocks marinelagoonal origin,butto thewestof themine,limestone comprise shales andsandstones, with significant tuffaceousconglomerates occuras channeldeposits andcut into the material,tuffs,lavas,andvolcanic conglomerates (Romano, underlying Laminated Bedssequence (Anderson, 1990;Rizzi, 1980;Vaughan, 1991).The Silurianrockscomprise varied 1993;Fig.3). AbovetheMuddyLimestone lie thePaleBeds graywackes, sandstones, siltstones, and mud rockswhich comprising a variedsequence of shallow-water carbonates, Vaughan(1991) hassubdivided into severalfault-bounded dolomitichorizons,and subordinatesandstonesand shales. terranes. ThebasalunitofthePaleBedsisa distinctive fine-grained, usually pale gray micritic limestone, containing oncholites LowerCarboniferous andbird'seyetextures, termedtheMicriteunit.Several pale TheLowerPaleozoic metamorphic pileis unconformablydolomitichorizonsoccurwithin the Micrite unit, one of which overlainby lowerCarboniferous (Courceyan) impurered formsa prominent bedsome10m thickandis animportant sandstones andpolymict pebbleconglomerates, whichcon- controlto orelocalization in thewestern partof the deposit. rain local caliche horizons and are termed the Red Beds. TheMicriteunitrepresents deposition in a dominantly interIntense hematization oftheLowerPaleozoic rocks commonlytidalenvironment (Strogen et al.,1990;McNestry andRees, occurs immediately beneath theunconformity. Theysharply 1992). passupwardinto the LaminatedBeds,a variedsuitedomiThe Micriteunitpasses upwardintoa variedsequence of natedbythinlybedded,oftenbioturbated, darkargillaceouspaletomedium gray,frequently oolitieandbioelastie ealearesiltstones andmudstones. A horizon ofchalcedonic silicarep- nites(grainstones). Distinctivemarkerhorizonshavebeen resents a replaced anhydrite horizon.Thesebedsrepresent definedwhichareusedto subdivide thedeposit intovarious the onsetof marinetransgression (Strogenet al., 1990; ore lensesandact asvaluableguidesto detailedstructural McNestry andRees,1992;Rizzi,1993).Theoverlying Muddy interpretation of the orebody(Philcox,1984,unpub.data; 538 ANDERSON ET AL. SE AGE GROUP • FI NGAL LOCAL NOMENCLATURE UPPER DARK L litESTONES •{• GROUP (CALP ) .... TCBER EROSION COLLEEN MUDSTONE. SURFACE CHAD I AN WAULSORTIAN ABL LIMESTONES GROUP ARGILLACEOUS BIOCLASTIC LIMESTONES i SHALE'%' PALE L IMESTONES NAVAN UPPER SHALEY PALES i MIDDLE SHALEY PALES LOWER SHALEY PALES UPPER OARK NARKER z PALE NOOULAR MARKER BEDS i,i GROUP BEDDED / UN[' LONER SANDSTONE NARKER LOWER DARK CHANNEL MICRITE MUDDY LIMESTONE MIXED BEDS MAJOR UNCONFORMIT¾ LAMINATED BEDS RED BEDS CHANNELING ,C• ,• ..... IN NARKER NIC•ITE UNIT MUDDY LIMESTONE LAMINATED BEDS RED BEDS ....... -V -V- V--V- -V-V--V- -V-VIV-V- ICa 50 METF•ES LOWER PALAEOZOIC SILURIAN] ORDOVI CIAN Fig. 3. Stratigraphic sequence in the Navanminearea(afterAshton,1995). considered to haveformedin Figs.3-5). The markerhorizons aregenerally a fewmeters ism).The latteris generally (Strogen et al., 1990). thick,characteristically exhibitgreatersand,silt,orshalecon- relativelydeepwaterenvironments tentthanthe enclosing limestones, andare oftendolomitic. In thevicinityofthemineanunconformity, locallytermed There is substantial lateral variation in both the nature of the the "erosion surface," of probable lateChadJan age,hasremarkerhorizons andthe enclosing PaleBeds.At leasttwo movedsubstantial partsof the Courceyan succession, downsignificant erosional episodes havebeenidentified withinthe cuttingin a southerly direction andtruncating LowerPaleoPaleBeds,definedby distinctive linearfaciesbeltswhich zoicrockssouthof Navan(Philcox,1989).The erosionsurface trend in a north to northwestdirection (Andrew and Ashton, isconsidered to be theresultof anepisode of complex, com1985;Anderson, 1990;Rizzi,1993).Theseincludea major posite,low-angle gravitysliding,resulting frommajorextenchannel-likefeature, which has removed most of the Micrite sionandgrowthfaulting in the NavanareaduringtheChadunit in the easternpart of the deposit,anda laterseriesof ian,andwasprobably accompanied byscouring of contempolimestone microconglomerate channeldeposits, whichoccur raneousfault scarps(Boyceet al., 1983a;Philcox,1989; between the Lower Sandstone and Nodular Markers someAshtonet al., 1992).The erosionsurfaceis overlainby a whatfartherto thewest(Fig.3). debris-flow breccia conglomerate knownastheBoulderConTheoverlying ShaleyPalesconsist of thinlybeddedealear- glomerate, varying from<1 to >50 m in thickness andcompolymict, limestone-dominated clasts which gilliresandealearenites with localbryozoan-rieh andthinly prisingchaotic, beddedcalcareous sandstones consideredto representa displayconsiderable variationin size,shape,and packing. in thickness, texture,andcomposition are deeperwateroffshore depositional equivalent ofthesandand Rapidvariations debris oolitelithologies of the Pale Beds(Philcox,1984, 1989). commonandthe unit is clearlya typeof submarine AbovetheShaley PalesliestheArgillaeeous Bioelastie Lime- flow(Boyceet al., 1983a;CookandMullins,1983;Binney, arefrequently subangular to angular in outline, stoneconsisting of dark,well-bedded, andstrongly erinoidal 1987).Clasts ofupto8 m,andareusually composed ofPale argillaeeous limestones. Towardthetoptheybecome increas- havediameters Limestone lithologies, inglyerinoidal andpalerastheypassintotheoverlying Waul- Beds,ShaleyPales,andWaulsortian in a darkargillaceous matrix(Fig. 6B). Clastsof sortianLimestone (probably with significant localdiaehron- enclosed NAVAN ZN-PB DEPOSIT, IRELAND: ORE DEPOSITIONAL PROCESSES 539 NN SECTION B17NN SE SURFACE •' I I I I I ] I I I I I IIIIlllllli iIiIiIiIi1111• tlllllllllllll i11111111111II •r, CONGLOMERATE F• ZONE 2 I ARUNDIAN•-•UPPER DARKLIMESTONES r•BOULOER CONGLOMERATE NN-SE STRIKE SECTION CHADIAN ,•EROSION ACROSS NAVAN OREBODY •SHALEY PALES i iUSM- UPPER SANDSTONE UDMUPPER DARK •BOULOER •PALE BEDS- WITHMARKER HORIZON•NODNODUL•R COURCEYAN r"•-]MUDDY LIMESTONE I LSM LOWER LOWER SANOSTONE --LDM OARK I•ILAMINITED CONGLOMERATEGROUP ORE (>5% ZN+PB) MOSTLY SURFACE BEDS MAJOR •LONER 50 UNCONFO•M•TY PALAEOZOICS [] MASSIVE SULPHIOES CONGLOMERATE PALEBEDSORE(>5%ZN+PB) I•IREO BEDS v•w/ PYRITIC 100 150 200 250 ß METRES TERTIARYDOLERITEDYKE FID.4. Typicalnorthwest-southeast cross section through the Navandeposit (afterAshton,1995). LowerPaleozoic lithologies occurintheBoulder Conglomer-per Dark Limestones, rangingfromArundianto Asbianin ateto thesouthof theorebody wheretheerosion surface has ageandcomprising thin darkmudstones with well-bedded removed virtually theentireChadJan andCourceyan section ealearenites andealeirudites exhibiting turbiditieaffiliations (>500 m thick).TheBoulder Conglomerate isoverlain bya (Rees,1987;Strogenet al., 1990).The natureof the basal thicksequence ofwell-bedded limestones knownastheUp- contactwiththe BoulderConglomerate is variable: in some •"•CONGLOMERATEGROUPORE (>5% ZN+P8) MOSTLY PYRITIC MASSIVE CONGLOMERATE USM - UPPER S•NDSTONE [•SHALEY PALES UOH- UPPERDARK E•PALE SULPHIOES COURCEYAN r•BOULOER •'6•UPPER DARK LIMESTONES •80ULOER CONGLOMERATE ,vv, EROSION SURFACE BEDS - WITH MARKERHORIZONS NOD - NODULAR •HUOOYLIHESTDNE F•]LAHINATED BEDS F•-lRED eEOS PALEBEDSORE(>5%ZN+PB) • MAJO• LSH - LOWER SANDSTDNE LDH - LOWER DARK -,-'•'•--TOPOFHICRITE UNIT UNCONFO•MIT¾ •C•LDWER PALAEOZOICS •TERTIA•Y 00LERITE DYKE FIG.5. Typical east-northeast-west-southwest longitudinal section through theNavandeposit ('after Ashton, 1995), 540 ANDERSON ET AL. Fic. 6. A. Micriteunitwithpale,stylolitic, birdseyes micritesovedainby a massive dolomitic horizon.Contactbelow thecoin.B. BoulderConglomerate comprising poorlysorted,subangular clasts of paleWaulsortian Limestone anddark PaleBeds.C. Massive, high-grade mineralization in lens2-5,withpalemassive sulfide toward thetoprightandbrecciation ofthedarkerhostcalcarenites in thebottom center.D. Irregular mineralization (palegray)in lens1-5in theMicriteunit. locations it isquitesharp,butin othersit isgradational, with the erosionsurfaceanddoesnot penetrate the UpperDark alternating mudstone andthin pebbleconglomerate layers Limestones in the main mine area. The T fanlt exhibits evidence succeeded by a thinlybedded,usually pyriticmudstone up of several stages of movement beforeandduringtheformation to 20 m thick, termed the Thin Bedded unit. oftheBoulder Conglomerate andparflyduringlaterHercynian activity. It displaces the erosion surface locallyby morethan Structure 50 m andextends intothe UpperDarkLimestones (Fig..4). Conglomerate thickens noticeably onthehangingThe localstructure is dominated by a complex, fanlted, TheBonlder thepresence of a broadly antiformal structure whichplunges gentlysouthwestwall(south)sideof theT fanltandindicates faultscarp duringthedeposition of thisdebrisfromthewestern apexoftheLongford-Down inlierjusteastof syndepositional the mine(Fig.1). Theorebody is cutby a series of fanits:an flowunit. Boththe B andT fanitscutthe PaleBedsore (Andrew earlyeast-northeast-trending anda laternortheast-trending set, and Ashton,1985;Ashtonet al., 1986). tectonism resulted in thelater,north-east trendwhichareregarded asproducts of reactivation of majorbase- Hercynian faults(Fig.2). The A mentfaultzones during lateCourceyan toChadian andHercyn- ingA, C, D, Castle,andRandalstown theoreandtheUpperDarkLimestones (Fig. iantimes,respectively. Theearliest structures to affecttheCar- fanltdislocates boniferoussuccession, the B and T fanits,and smallersimfiar 4) with a dextralstrikeslipon the orderof 180 m anda component of up to 80 m. The C faultzoneis more structures (e.g.,F1, F2, Fzs,F26,Fs7,etc.;Fig. 4) strikeeast- reverse thantheA faultandpossibly represents reactivation northeast andusually dipat moderate angles to thesoutheast.complex Several of thesefaultsexhibita listticprofileanddownthrowalongan eadierstructure; it displays an apparentnormal southward--the B faultbyabout70 m andtheT faultbyup throw (<100 to >150 m) but with local reversemovement to 200 m. Both the B and T faults are considered to be late andis alsoconsidered to havea component of dextraldisCourceyan to Chadian in age,since theB fanltistruncated by placement (--•200m). The D and Randalstown faultsare NAVANZN-PBDEPOSIT,IRELAND:ORE DEPOSITIONALPROCESSES largerwrench-style structures with reversecomponents of movement exceeding 100m. North-trending, oftentight,en echelon foldsoccurwithintheUpperDarkLimestones and theseincrease in intensity toward theA andC faults.Finally, several transgressive doleritesillswereintrudedduringthe Tertiary,striking in a north-south direction anddipping westwardat angles from20øto 65ø (Turneret at.,1972). Carbonate Diegenesis andDolomitization Petrographie studiesutilizingeathodolumineseenee and staining techniques indicate thatseveral stages of eelciteeementationanddolomitization are presentwithinthe Pale Bedscarbonates. Theseareillustrated in Figure7. 541 summarized asan earlydarkluminescent fringingcement, followedby well-zoned, dull to brightyellowluminescent crystalovergrowths, andsubsequent mediumyellow,blocky cement.Thissequence is similarto thatdescribed by Miller (1986)asrepresenting precipitation frommarineporewaters in a shallowburial environment.In some limestones,such calcitecements arepostdated by well-developed rhombsof nonluminescent, ferroandolomite witha brightredluminescentzoneprecipitated asa cementin remnant porosity. Dolomites Dolomitielithologies in the PaleBedsgenerally occuras 0.5to 4.0-m-thick bedding-parallel horizons, such as the Limestones LowerSandstone Marker.Animportant characteristic feature Under cathodoluminescence, the limestonesin the Pale is thepresence of a higherdetritalcontent(siltto finesand) Bedsgenerally exhibitthreestages of calciteeementation,relativeto typicallimestones in thePaleBeds.In eathodoluCATHODOLUMINESCENCE TRANSMITTED LIGHT/STAINING stage 4 dolomite cement (essentially non-luminescent with a bright red zone in places) .-• late-stage ferroan dolomite ";'• "':"•'• • W •"e=' blocky celclte becoming more ferroan in later sieges • early bladed calcite cement :• ,,,:.,,carbonate allochems stage 3 blocky calcite cement (medium luminescent) stage 2 zoned calcite cement (dull-brightluminescent) ,stage I calcite rimmingcement (dark to non-luminescent) carbonate allochems FIG.7. Diagram illustrating carbonate cements (stained) in typical PaleBedscalcarenites undertransmitted lightand eathodolumineseenee. 542 ANDERSON ET AL. minescence, threestages of dolomitization canbe observed. Bedsore(1-3% Fe) butincrease substantially in the2-1lens The earlieststageis a fine-grained (<50 •m), dull brown belowtheConglomerate Groupore.Gangue minerals include luminescent, mosaic-type replacement of thecarbonate allo- barite, calcite, and dolomite. ehems,formingup to 75 percentof the rock,with dolomite The PaleBedsmineralization exhibits a complex varietyof rhombsfrequently displaying corroded margins. Thisis fol- stylesandrelationships with the hostrockswhichare now lowedby a darkto nonluminescent cementoccurring both described withreference to well-exposed underground headin veinsandasovergrowths ontheearliercorroded rhombs. ings.The dominantsulfidedepositional mechanisms are Sphalerite islocallypresentin veinscontaining thisdolomite open-space precipitation, veining, andreplacement. Theyare stage. Thethirdstageofdolomitization occurs ascoarse, well- represented bya complex rangeof oretextures andmorpholdeveloped nonluminescent rhombswith a brightred zone, ogies, whichexhibitconsiderable variation dependent onthe of mineralization. The degreeof preferentially deposited in veinsandsmallpockets andvugs localhostrockandintensity as a cement,and appearsto correlatewith the late-stage development of thesemechanisms cangivethe orea signifidolomitein the limestones. The laststageof eementation in cantlyvariableappearance, throughstrata-bound, massive, vein,breccia,anddisseminated morpholotheserocksis a brightyellow,blotehyluminescent calcite bedding-parallel, of the mineralization andhostcement,usuallyoccurring in veinsandvugscontaining the gies.The grossmorphology third-stage dolomitecement. rockrelationships is described first,beforewe consider the Thefine-grained replaeive natureoftheearliest stagedolo- complex texturalassemblages withinthe ores. mite is consistent with formation in a shallow burial environ- Geometry of mineralization andhost-rock relationships in ment (Bumsand Baker,1987;Hird et al., 1987).It is pro- the Pale Beds posedthat the detritalelasticcomponentenhancedthe permeability of certainhorizons, whichwereselectively doloOnthebasisof gross geometry andrelationships withhost mitized.Latereoarser graineddolomitecements wereintro- rocks,these can be subdividedas follows. ducedin veinssynchronous with,andpostdating themineral- Massive, high-grade mineralization (5 lens;Fig.6C):This ization. styleof mineralization occurswithincalcarenites andlesser in the 5 lensin the easternpart of the deposit, A moreverticallyextensive zoneof dolomitization in the dolomites horizonare westernmineareahasa pittedor vuggyappearance in drill wheremicritesmoretypicalof thisstratigraphic coreandisthesubject of recentlycompleted research (Rizzi, removed bychanneling. Thismineralization isbestdescribed 1993). asmassive conformable lenses, formingsomeof the highest grade ore in the mine, with vertical intersections of up to Mineralization and Ore Textures 15 m >40 percentZn + Pb. Due to the intensityof the relationships withthehostlithologies areless The Navanorebodytakesthe form of numerous strata- mineralization, thanin the otherstylesof mineralization. boundorehorizons occurring in anareaof some3 km2 and well constrained of trendingin a northeast to east-northeast direction, approxi- However,sulfideveiningof the hostrockat the margins sulfideis observed, andzonesof high-grade ore matelyparallel tothemajorfaulting. Themainfaultsnaturally the massive dividethe orebodyinto threezones:northof the B fault arefrequently connected byloxver grade,high-angle veining. (zone 1), betweenthe B and the A and T faults(zone2), and Strata-bound mineralization in the micrites(5 lens;Fig. the massive high-grade mineralizasouthof theA fault(zone3; Fig.4). Smallersatellite deposits 6D): Movingwestward, are known to the northwestat Lisearton,Tatestown-Seallans- tiontendsto breakup intoseveral, smallertabularto lensold town (Andrewand Poustie,1986),and Clogherboy to the layers, wherethe5 lenshostrocksaredominated bymicrites. apsoutheast (Fig.1). Mineralization bothin themainorebody Theselensesfrequentlyextendoververticaldistances 20 m andcomprise the mostlaterallyextensive andin the surrounding satellites is mostlaterallypersistent proaching dose to the base of the Pale Beds in the basal Mierite unit. mineralization atNavan.A prominent dolomite horizon forms concordant hanging walltooneofthelargest ofthese Mineralization in the stratigraphically highergrainstone li- a sharp, sharp,discordant, often thologies in the PaleBedsis largelyconfined to the central lenses.The ore typicallydisplays irregularcontacts withthe hostmicrite,witha areaoftheorebody, withtheexception of orein the 1-4lens, high-angle, thickening andthinningof the mineralization. lyingnorthof theB faultin zone1, andrecently delineated pronounced headings ananastomosing appearance mineralization in theupperpartofthePaleBeds,lyingsouth- In manyunderground is evident,withsulfides occurring asan westof the maindeposit,termed"lensU" (Ashtonet al., to the mineralization Sulfides are typically 1995;Fig. 2). Similarly,Conglomerate Groupore hosted irregularnetworkwithinthe micrites. oftenoccurring asa complex melange ofclasts and withintheBoulderConglomerate is restricted to thecentral disrupted, layers.Darkquartz+ feldspar-dominated argillite andsoutheastern partof theorebody wheretheerosion sur- disrupted commonly occurs withinthe sulfides, anda discontinuous 1facecutsdeeplyintothe PaleBedssuccession (Fig.4). seamis oftenpresent at theconThe mineralogy of the PaleBedshostedore is simple, to 3-cm,blackargillaceous the economic minerals consisting of sphalerite andgalenain tactswith the micrite(Fig.8). Bedding-parallel, ore horizons (1 to 4 lenses; Figs.9Aoverallproportions of around5/1, with smallquantities of highersulfidehorizons silver(ca.10-15 g/tin millfeed).Freibergite andpyrargyrite D and 10,):Thesestratigraphically several relatedstyles of stratiform sulfideandvary are the principalsilverminerals; boumonite andsemseyitecomprise Sulfidesare preferentially havealsobeenreported(G. Steed,unpub.data).Pyriteand from 0.3 to 4.0 m in thickness. atthecontact between a ealearenite andanoverlymamasiteare presentin subordinate amountsin the Pale developed NAVANZN-PBDEPOSIT,IRELAND:ORE DEPOSITIONALPROCESSES 543 t pO'ltmmerahzalian• NWtrending calciteflllldlamt /sharp, cross-cutt•ng contacts ! j between massivelUll)hide m•cr•te I horizon and the micrite m.r,t. , •. •• ::2: .... '"'""': F•G,8. Drawingof anunderground heading in the Micriteunit,rite1-5lens,illustrating thethickening andthinning of themineralization, high-angle contacts in places betwveen sulfide andmierite,anddarkargillaeeous material commonly occurring at thesecontacts. ingsilty,dolomitized lithology. Thehanging-wall andfootwall veinsoftenextendup intothe NodularMarker,andfunnelcontacts withinan individual horizonare sharpandconsist shaped features areobserved in the footwall(Fig.11B). of an essentially planarcontact withthe overlying dolomite Crosscutting sulfide veins(Fig.11C):Veinsranging froma to 0.5 m thickoccurthroughout thedeposit, butanirregular, undulating contact withtheunderlying lime- fewcentimeters sphalerite, galena,ironsulfides, andbarite.The stone,generally resultingin a pronounced thickening and containing veiningoccurs asa largeeast-northeast-trendthinningof anorehorizon(Fig.9A).Diffuse,narrow,sulfide mostextensive however,thisstyleof stringers aredeveloped in dolomites overlying ore,andeco- ing swarmin the 9.-5and9.-4lenses; nomic mineralization can extend into the dolomite as sulfidevein mineralization is quantitatively a minorcomponent of cementedbrecciazones.In someareas,dark argillaceousthe ore. Sulfidetexturesin the veinsare usuallysimilarto sediment is locallypresentin the basalsections of the hori- those observed in the massive sulfide horizons which are in the following section. zons,preferentially developed withindepressions in thefoot- described wall contact. Breccias (Fig. 11D): Angular,breeeiamineralization is developed in the siltydolomitie lithologies. In Sections of moremassive andcomplex mineralization pass preferentially verticalto subvertical breeeiazonescomprislaterally intoseveral narrow(5-30 cm)bedding-parallel lay- someexamples, ing angular elasts of host rock cemented by sulfides connect ers,separated by unmineralized hostrock(Figs.9B-D and narrow,bedding-parallel sulfidehorizons. 10).Theselayerstakethe formof sharp,bedding-parallel Disseminated sulfides: Thisstylecommonly occurs aslowandanastomosing sulfide veinsandbandsofsulfide withmore in calcareous sandstone horizons, dolodiffuseandirregularhostrockcontacts. More diffuseand grademineralization mites, and local bioelastie bands (Shaley Pales) where sphalerirregular sulfidebandscomprise laterally discontinuous and itc replaces bioelastie components andfragments. It isparticdisrupted, sphalerite-rich layers(Figs.9C-D and10). in dolomites associated withthe5 lens Bedding-parallel, podiform, high-grade orehorizons (e.g., ularlywelldeveloped and dolomites of the U lens 2-2 lens;Fig. 11A):In the centralportionof the deposit, a ore andin sandyealearenites (Ashton et al., 1995). particularly distinct, massive orehorizonoccurs immediately beneath theNodularMarker.Although thesemassive sulfides are broadlybeddingparallel,theyexhibithighlyirregular Sulfidetextures boundaries andareoftendiscontinuously developed. TheupThe sulfidemorphologies previously described exhibita perandlowercontacts withthehostrockaresharp; however, complex varietyof intimately associated oretextures. Varia- 544 ANDERSONET AL. c !) FIG.9. A.Bedding-parallel, massive sulfide inthe2-1lens.Notetheundulating footwall andtheplanar hanging wall. B. Bedding-parallel, sphalerite layering (palegray)in the2-1lens.C. Bedding-parallel layering of sulfide (palegray)in the2-1lens.D. Bedding-parallel layering of sulfide (palegray)in the2-3 lens.Sulfide brecciation occurs to thebottom left of pen. tionin theproportion anddistribution of thediffering tex- gradeandmassive sulfides istheoccurrence ofsphalerite geoturescommonly reflects thedegreeandintensity of mineral- petaltextures. Thesecomprise basal,finelylaminated, pale ization, aswellastheprincipal oredepositional processes and brown,microcrystalline sphalerite layersandcoeval, rhythmithenatureof thehostrock.Thefollowing distinct styles are cally banded orcolloform sphalerite onthecorresponding upper the majorcomponents. surface (brown-orange). Galenacommonly formssmall,upReplacement of carbonate by sphalerite: Oneof the sim- wardly growing, bushlike skeletal tocubicgrowths nucleated on plesttextures occurs inbedding-parallel layering inthecalcar- thelaminated sphalerite (Fig.12C).Occasional laminae comenites,wherethinsection petrography indicates thatmuch prisingsfit-sized quartzgrainsareobserved interbedded with of the sphalerite comprises a fineamorphous, palebrown, thesphalerite, withauthigenic quartz overgrowths onthedetrital granular texture, oftenafteroolitesandbioclasts (Fig.12A grains. Although individual geopetal structures are of limited andB). Geopetal sphalerite sediments andcoeval rhythmi- extent, theyfrequently occurascomplex, composite sequences, callybandedtexturesoftenoccurwithinnarrow,horizontal whereconsiderable disruption isevident contemporaneous with veinsassociated withthisreplacement. mineralization (Fig.12C). Geopetal sphalerite: A characteristic featureof bothlower Layeredsphalerite sediment: Largerscale,palebrown NAVANZN-PBDEPOSIT,IRELAND:OREDEPOSITIONAL PROCESSES '""' $phalerltedisruptedIntoclaili '• 545 /-- /.. FIG. 10. Sketchof bedding-parallel mineralization in the 2-3 lens. (Fig.13C).The bestexamples comprise sphalerite sediments are evidentin bedding-parallel, high- ent-daystalactites bandedpyritecorewithaeieular mareasite gradehorizons, interbedded with subordinate, quartz-domi- a eoneentrieally andsubsequent overgrowths ofgalena andcolornatedargillitein places(Fig. 1œC-F).In someeasesthe overgrowths sphalerite. Geopetal sulfide sediments dominated by sediment layers canbetracedlaterally fora fewtensofcenti- banded frequently occurbeneath thestalaetitie growths. meters,andthinningof indixddual layersat theirlateralmar- sphalerite galena:Coarse-grained galenabands(Fig. ginsislocallyobserved. Gradedbeddingwithinthesediment Coarse-bladed occurwithinmassive sulfidehorizons impliescurrentactionduringdeposition, a process supported 13D) andaggregates veinsandarebestdeveloped in high-grade by the localpresence of asymmetrical growthfolds.Other andhigh-angle Galenabandscanbe fiat lyingandlayered sedimentary structures includeslumping anddraping features mineralization. crosscutting (e.g.,2-5 (Fig.iœEandF). In thinsection, the sphalerite comprises a (e.g.,2-2 lens)throughto high-angle, disrupted andbreeeiated. Individual microcrystalline to granularform,with sporadic porphyro- lens)andarecommonly galenabandsvaryfrom a few millimeters up to 5 em in blastiegalenagrowths withinthe sphalerite. Dendritic-skeletal galenaand colloformor rhythmically thickness and often exhibit a bladed texture which coarsens in ofcrystal growth. In podiform, bedding-parallel bandedsphalerite:This texture featuresa complexin- thedirection galenacommonly occurs asenerustations withina tergrowth of galenaandsphalerite, whichis bestdevelopedhorizons, of smallirregular cavities or pods,exhibiting cocksin bedding-parallel, laterallyextensive high-grade horizons network and in the strata-bound mineralization in the micrites. The combtextures. In the vertically extensive mineralization in galenainvariably occursearlyin the paragenetic sequencethe central2-5 lens,similarlytexturedgalenaoccursasan breeeiated, high-angle vein network.Crustiform andformsdendritic-skeletal andcubiccrystals, commonly as intensely overgrowths commonly postdate thebladed galena. a crystalmesh(Fig. 13A).Thesecrystals formnucleithat sphalerite Similartextured,coarse-bladed mareasite is locallydevelarecoatedandovergrown by concentric, rhythmically colorbandedsphaleritecomprising microscopic bandsvarying opedin the footwallof the T faultin the 2-5 lens,occurring veins(Fig. 11C). frompaleorangethroughto darkbrown-red in color(Fig. asan intensenetworkof crosscutting granular, coarse-zoned sphalerite: Thisisfre13B).The geometry o.fthe bandedsphalerite is essentially Fine-grained, associated withthecoarse galena, bothreplacing the controlled bythe shapeof thegalenanucleus, andadjacent quently hostsurrounding the galenabandsand forming nucleifrequentlycoalesce duringdeposition of the later carbonate selvages predating(below)the galena.In hand sphalerite. In thiswayan interconnected grapelike textureis well-zoned the sphalerite hasa variable brown-orange colordeveloped. In iron-rich partsofthe2-1lens,eolloform pyrite specimen ationandis veryfinegrained.In thin section, granular to occurs withinthistexturalassemblage. are commonandfrequently intergrown with Stalactitic growths: Rodliketextures aretermedstalactitic zonedcrystals sulfidedueto theoverallmorphological resemblance to pres- subordinate barite laths and rosettes. 546 ANDERSON ET AL. D FIG.11. A. Podiform, massive sulfide (palegray)in the2-2lens.B. Funnel-shaped features filledwithmassive sulfide in thefootwall ofthe2-2lens.C. High-angle marcasite veining in the2-5lens.D. Breccia-style mineralization in the2-5 lenswithpalesphalerite cementing dolomitic clasts. Complex, chaotic clasts: Thistexturecomprises complex,developed at reentrant angles in thecolloform textures. Rare matrix toclast-supported, poorly sorted assemblages ofangu- bournonite maybe associated withthecoarse barite. lartosubangular polymict fragments ranging fromafewmfilimetersto 15 cm in size,withina detritalquartz-feldsparInterpretation mud(Figs.14Aand15).Thistexturefeatures strongly in Delicate replacement ofhostcarbonates isclearly evidenced the micrite-hosted mineralization. Clastscomprise sulfidesbytheallochem pseudomorphs, withmoreintense replacement exhibiting manyof the textures described aboveaswell as generally resulting intheobliteration oftheseallochem compounmineralizedhost rock (abundantmicrite clastsin the 1-5 nents anda moreamorphous, grannlar texture. lens mineralization).Micrite clastswithin the sulfideshave Textures including dendritic-skeletal, stalactitic, geopetal, sharp, rounded to subrounded margins thatappear tobethe colloform, coarse-bladed growths, and sphalerite sediment result of dissolution.None of the micrite clastsstudied exhibit areinterpreted to represent snlfidedeposition in openspaces replacement bysulfides. Dark,discontinuous argillite layers withinthecarbonate hostrocks. Thegeneral absence ofeuhearedeveloped withthismineralization andare commonlydral,coarse-grained, cavity-lining textures orsubstantial brecdeformed bysulfide dasts(Fig.15). ciation(collapse) of theencompassing hostrocksuggest that Honeyblende sphalerite _ calcite-dolomite-barite: This is largecavities werenotpresent priorto themineralization. It commonly thelaststage of mineralization andbestdevelopedis, therefore, morelikelythatopenspacewascontinually infilling porositywithin thedendritic galena andcolloform sphal- developed duringthemineralization. Theinternalsulfide sederite(Fig.14B).It consists ofcoarse yellow honeyblende sphal- imentsprobablyformedby rapidnucleation of sphalerite eritecrystals associated withcoarse, whitebariterosettes and withintheorefluidandsubsequent deposition outofsuspenlaths andcalcite-dolomite, commonly brecciafing allpreviously sion.Textures suchasdendritic, stalactitic, andrhythmically deposited sulfides. Thecalcite also replaces portions ofthecollo- banded formscanalsoindicate rapidprecipitation of sulfides form,rhythmically banded sphalerite andisoftenpreferentially on mixingof two solutions bringingaboutsupersaturation NAVANZN-PBDEPOSIT,IRELAND:ORE DEPOSITIONALPROCESSES D E F 0 cm 5 Fie. 12. A. Bioclasts in a calcarenite are replaced by sphalerite (horizontal dimension = 2.0 ram).B. Oolitesin a calcarenite arereplaced bysphalerite (horizontal dimension = 0.5ram).C. Geopetal sphalerite (finelayering) withbushlike, skeletal galena, deformed anddisrupted. D. Layered sphalerite atthetopofthe2-4lens.E. Sphalerite sediment interbedded withquartz-rich material, illustrating soft-sediment deformation. F. Sphalerite-argillite sediment draping sulfide clasts. 547 548 ANDERSON ET AL. FIG.13. A. Dendritic galena (horizontal dimension = 10ram).B.Colloform, rhythmically banded sphalerite (horizontal dimension -- 1.0ram).C. Stalactitic pyritegrowths. D. Bedding-parallel, coarse-bladed galena. (Roedder, 1968;HonjoandSawada, 1982;Garcia-Ruiz, 1986; disruption of previously deposited sulfides,ultimatelyresultingin a complex assemblage of clasts. beenproduced experimentally by chemical gardengrowth Howdid theopenspacein whichsulfides weredeposited withrapidprecipitation bymixing ofanacidic andanalkaline form? In the micrites, discordant contactsbetween sulfide solution (Russell et al., 1984).Rapidlyprecipitated oretex- andhostrocksuggest thatfracturing mayhavecontributed turesmaybe explained by mixingof the ore-bearing fluid significantly to the iuitialgeneration of space.However, witha second fluidat thesiteof oredeposition (seebelow). sharp,solutional contacts suggest thatthesefractures localCrosscutting veinscontaining coarse-bladed galena ormar- izeddissolution andcreation ofopenspace duringmineralizacasiteare interpretedas extension fractureswhichallowed tion. Suchdissolution is supported by the presenceof theacidic metal-bearing fluids toascend tocontinually higher rounded clasts of micrite within the massive sulfides and trunlevelsin the carbonate hostrocks: thepresence of coarse- cationoflayeredbirdseye fabrics andstylolites in themicrites bladedmarcasiteindicativeof an acidsolution(Murowchick by the sulfides (Fig. 16).The micrites, beingmechanically and Barnes,1986). weakerandmoresusceptible to dissolution thancalcarenites The complex chaoticclasts areindicative of continual dis- and dolomites,formed an ideal host for mineralization. ruptionof the sulfides duringthe mineralizing event.This Openspacein the calcarenites mayhavedeveloped by couldarisefromseismic activity relatedtoongoing extensionsimfiarprocesses but commonly occurredat preferential duringmineralization. The presence of featuressuchasin stratigraphic horizons, wheredissolution wasaccompanied situdisrupted andbrecciated veinsandgeopetal sedimentsby substantial replacement, Thelocation of particularly high on the uppersurfaces of clastsis consistent with continual gradesulfides beneath dolomite horizons reflects localization passage of hydrothermal fluidsthrough, andsulfide deposi- of mineralization at a markedchangein the chemicaland tionwithin,the accumulating melange of clasts. Thiscould mechanical properties of thehostrocks. arisein a continually enlarging system of openspaces, with The originof siliciclastic argillites withthesulfides, parRussellet al., 1989). Stalactiticand rodlike structureshave NAVANZN-PBDEPOSIT, IRELAND:OREDEPOSITIONAL PROCESSES 549 14. A. Chaoticsulfideclastsin the micrite-hosted 1-5 lensmineralization. B. Coarsehoneyblende sphalerite brecciating dendritic andcolloform sulfides (horizontal dimension = 5 ram). Conglomerate, pyrite-deficient areasof ticularlyin themicrites,is unclear.Petrography of themi- baseof the Boulder andgalena occur withinthematrix andasa replacecritesindicatesonlya minorsiliciclastic component(1- sphalerite Rareclasts andnodules of silica2%). However,discreteargillitebedsare presentin the mentof theconglomerates. havebeenrecorded fromtheBoulder Conglomerate Micrite unit. Two possible explanations are considered.hematite Argillitelayerspresentin theMicriteunitcouldhavebeen close to the mineralization. Thelaminated pyriteoftenexhibits soft-sediment deformaassimilated duringthe mineralization. Alternatively, an exandisdominated byframboidal growths. These oticclastic inputmayhavebeensupplied duringtheminer- tionfeatures framboids locally coalesce toformsmallconcretions. Sphaleralizingevent. itcandgalena occurwithintheironsulfides asmassive bands, Conglomerate Groupore disseminations, andcrosscutting veins,andit is clearthata M'meralization in the BoulderConglomerate occurs almost substantial amountof theZn-Pbsulfides postdate thepyrite exclusively in the southeastern areaof theorebody occupyingandformedbyreplacement. Thisis a similarparagenesis to two areason either side of the A fault (Ashtonet al., 1992). the Fe-richpartsof the 2-1 lensimmediately beneaththe Thethickest andmostlaterally persistent intersections of Con- Conglomerate Groupore.The breccias withinthe massive glomerate Grouporearefoundin zone3 between theA and sulfides occurin a varietyofforms.Clasts dominated byironC faultsystems whereoreextends overa 40-m-vertical interval. rich(pyriteandmarcasite) massive sulfides andangular ripup It is typically a highlypyriticmassive sulfidewitha vagueto clasts of argillitearecommonly intercalated in themineralmoderately welldeveloped bedding-parallel layering, a highly izedconglomerates (Fig.17A).In places, pyriteandsphalerite variable composition withrespect to theproportion of Fe to form the matrix to clasts of Pale Beds and Waulsortian LimeZn-Pbsulfides, andsubstantial variation in the grainsizeand stone(Fig.17B).Rareclasts ofiron-deficient sulfides, simfiar texture ofthesphalerite andgalena. Several styles ofmineraliza-to PaleBedshostedore andshowing truncation of layering tioncanbe recognized. Thebulkof theorecomprises high- at dastmargins, arepresentin the BoulderConglomerate. grademassive sulfides ascomplex breccias andintergrowths of In othercases, however, it is.evident thatsphalerite haslocally themargins of PaleBedsclasts. marcasite-pyrite, sphalerite, andgalena. Layers andpatches of replaced TheConglomerate Grouporeisinterpreted tohaveformed massive andlaminated pyritecontaining onlyminorZn or Pb of theBoulder Conglomerate, occurwithor adjacent to theore.Locally, especially nearthe duringandaftertheformation 550 ANDERSON ET AL. concentration of dark orgillite at the contact between •lphides and host rock micrite org•lhte appears to be squeezed up into and along bedding in the miCrite complex• choataC clast• locally deforming the orgillite micr•te dark argillite FIG.15. Drawing ofanunderground heading in the1-5lensshowing a matrix-supported, poorlysorted, chaotic sulfide clastassemblage hosted bymicrites. Notethecommon occurrence ofdarkargillite in thesulfide horizon, inplaces deformed by sulfideclasts, andsimilarmaterialat the contacts withthe hostmicrites. withsphalerite andgalena superimposed onearlierironsulfides the purityof the separates. Although not all samples were onlyonesulfur-bearing phase. (Ashton et al.,1992).Layered framboidal pyritesuggests early pure,all contained SO2gases wereproduced fromthe separates usingstandiagenetie growth ofironsulfides. Sulfide elasts exhibiting trunprocedures (afterRobinson andKusakabe, cationof Zn-Pblayering at the elastedgesindicate thatpre- dardcombustion viously deposited sulfides wererippedupduringtheformation 1975, for sulfides;after Coleman and Moore, 1978, for sulanalyzedon an Isospee64 or VG of the BoulderConglomerate andrepresent criticalevidence fates)and subsequently forthetimingof themineralization. Thepresence of elasts of 602mass spectrometer. Reprodueibility based onfullrepeat PaleBedswith sphalerite replacement restricted to the elast analyses (including combustion) wasbetterthan___0.2 permil resultsareexpressed in conventional margins indicates thatat leastpartof the mineralking event (l•r). All sulfurisotope asperrail(%•)deviations relativeto Cation occurred afterdeposition ofconglomerate. Thepresence ofthin delta(•5)notation, stratiform bandsof sphalerite andgalenain the Thin Bedded Diablo trollire (CDT). unitnearthebaseof theUpperDarkLimestones dearlyattest Results thatthemineralizing eventcontinued aftertheformation of the Sulfurisotope results aregivenin theAppendix andillusBoulderConglomerate. tratedin Figures18 to 24. Rapidprecipitation, open-space textures: Thesesulfide texSulfurIsotopes turesexhibit/•34S valuesfrom-32.6 to 11.3per mil (Figs. 19-21). However, themajority of theresults arelessthan0 Afterdetailed petrographie andparagenetie relationships had permil.Thereisa tendency forisotopically lightervalues to beenestablished, a sulfurisotope study wascarriedoutonsul- beassociated withparagenetieally latersulfides withina given fideandbaritesamples, representing thediffering oretextures sample(Fig.21). andstyles of mineralization. Theobjective wasto elucidate the Coarse-bladed, zonedandgranulartextures: Thisinchides origin(s) of thesulfurandoredepositional processes. bothbedding-parallel (e.g.,2-2 lens)andcrosscutting (e.g., In mosteasessulfideandsulfateminerals wereseparated 2-5 lens)styles. With the exception of oneanalysis of -15.7 frompolished handspecimens usinga fine,diamond-tippedper mil, thesesulfidetextures arecharacterized by a range from -1.1 to 14.9 per mil, dentaldrill.RoutineXRD analyses wereperformed to cheek in sulfurisotopecompositions Techniques NAVANZN-PBDEPOSIT,IRELAND:OREDEPOSITIONALPROCESSES 551 Fro. 16. Drawingof anunderground heading in the 1-5lens,showing layeredbirdseyes in theMicriteunittruncated by thefootwall of theoverlying massive sulfide. dominated byhigh•34Svalues (Figs.19,22,and23).This A raresample of celestite foundin late,crosscutting barite is exemplified bythe coarse-bladed galena, which,withthe veinsin the2-3lens,neartheB fault,gave39.1perrail.Two exception oftwoanalyses of- 1.1and-0.7 perrail,ischarac- samples of gypsum fromtheLaminated Bedsgaveresults of terizedby values>0 per rail (Fig. 22). Again,thereis a 21.0and22.5per rail. trendtowardisotopically lightervalues inparagenetically later sulfides withina givensample(Fig.23). Interpretation Bedding-parallel replacement of carbonate: Sulfurisotope All resultsare plottedin Figure18. Sulfides results fromsamples ofthismaterial exhibita rangeofvalues Summary: threemaingroupings, with the two major from-23 to -14.5 per rail (Fig. 19).The resultsfall into showessentially at -15 to -23 and8 to 12perrail,together with twogroups withsamples fromthe 1-2and1-4lenses having peakranges have lower•34Svaluesthanthosefromthe 2-2 and2-4 lenses(-23 a smallpeakaround-30 to -32 perrail.Sulfateresults a tightmeanat 21 _+2 perrail (1or). to - 19.2%ovs.- 16.6to - 14.5%o, respectively). Sulfate: Thebaritemeanwitha 634S = 21 ___ 2 perrail,falls Framboidal pyritein the Conglomerate Groupore and recognized forlowerCarboniferous colloform pyrite-marcasite in lens2-1:Onlya limitednumber withintherangegenerally sulfate: 14 to 22 perrail (Claypool et al., 1980).We ofanalyses werecarriedoutonthismineralization. Pyriteand seawater thatcontemporaneous seawater sulfate wasthedomimarcasite in the Conglomerate Grouporeandimmediatelyconsider of sulfurin thebariteat Navan.Thisinterpretation underlying a portionofthe2-1lensexhibitsimilar, extremely nantsource withstudies ofotherIrishdeposits atTynagh (Boast negative isotopic compositions, ranging from-37.3 to -28.2 isconsistent perrail(Fig.19).In bothcases, theironsulfides predate Zn- et al., 1981) and Silvermines(Cooruerand Robinson,1978; Pb mineralization. Boyceet al., 1983b;Boyce,1990).It should be notedthatthe valueat Navan issomewhat 34Senriched compared Sulfates: Baritesamples exhibit a range of•34S values from mean634S 17.7to 35.0 per rail,with the bulkof the databetween18 totheothertwodeposits (•18%oforSilvermines; Boyce, 1990; forTynagh; Boastet al.,1981),possibly reflecting and24 per rail (Figs.18 and19).Excluding threeoutlying and19.4%o heavyvalues themeanbarite•34Svalueis 21 ___ 2 perrail a distinction in ageand/orsetting between the deposits (see (1or,n = 23). below).Claypool et al. (1980)alsonotethatinterbasinal differ- 552 ANDERSON ET AL. makesit unlikelythat bacteriogenic reductionof evaporites couldhaveprovidedthe quantities of bacteriogenic sulfur invokedin the formation of the deposit. Gypsum is present in the Pale Beds some 45 to 50 km west of Navan at Keel andfartherwestat Strokestown, presenting thepossibility of a sulfate-rich brinebeingderivedfrom dissolution of such evaporites andmigrating duringdiagenesis. Thispossibility isdifficult to assess, buttherearegeological andgeochemical arguments to thecontrary. First,thebrineswouldhavebeen requiredto migrateacrossthe northeast-trending strikeof the structural grain,whichseemsunlikely.Second, thereis the problemof massand isotopic balancefor sulfur.The sulfide andsulfate 6a4S dataforNavansuggest bacteriogenic reductionin a systemopento sulfate.A sulfursourcevia transported evaporite-derived brineswould,therefore,requireunrealistic volumesof fluidto resultin the observed quantity of oresulfur.Contrast thiswiththesituation at Pine Point(Rhodes et al.,1984)whereclosed-system reduction of transported evaporite brineresults in sulfide 6a4S values directlycomparable to theoriginalevaporite value,i.e., • 20 permil.Reduction of sulfatein lowerCarboniferous seawater is favored at Navan. The replacivenatureof the PaleBedsore incorporating biogenic sulfurimpliesthatlocalcirculation musthaveeither transported reducedsulfurto the siteof ore deposition, or introduced sulfate for in situ bacterial reduction. Anderson (1983)notedthatthe amountof bacteriogenic sulfurin an economic Zn-Pbdepositistoogreatto havebeenpresentin a staticfluidpriorto introduction of the metals,andit is essential to havea continuous supply of reduced sulfurto the metalsto producethequantities of oreobserved. Thiscould be achieved by continuous in situreduction of sulfateat the Fluidinclusion temperatures at Navan, Fie. 17. A. Clasts of sulfide(palegray)in theConglomerate Groupore siteof mineralization. (lens3). B. Clasts of PaleBeds(darkgray)cemented bysulfide (palegray) whichrangefrom 150ø to 258øC(Probert,1983;Everett, in the Conglomerate Groupore(3 lens). 1997;commensurate withfluidinclusion temperatures at Silvermines andTynagh: seeSamson andRussell, 1987;Banks andRussell, 1992)suggest, however, thatthishot,hydrotherences during thelowerCarboniferous weregreater thanatother mal environment would not be conducive to in situ sulfategeologic periods. reducing bacterial activity. However, thebacteriogenic reducSulfide-bacteriogenic sulfur:Thegrouping of thenegativetionneednottakeplacein the hottestpartsof the deposi6a4S datafrom-23 to -15 permil(Fig.18)isinterpretedtionalsite.Recentworkin the Guaymas basinhasconfirmed asresulting frombacteriogenic reduction of the lowerCar- thatsulfatereducers canactively producesulfideat temperaboniferous seawater sulfate source indicated above. This com- turesbetween 80øand102øCandperhaps to temperatures as ponentisparticularly evidentin allochem replacement (Fig. greatas120øC(Elsgaard et al.,1994).Petrographic evidence 19)andtherapidprecipitation, open-space textures (Fig.20). indicates theinitiation of carbonate diagenesis, including doIt represents a bacteriogenic fractionation of some35 to 45 lomitization, priorto mineralization; open-system bacterioper mil, typicalof naturallyoccurring opensystems (e.g., genicsulfatereduction in the limitedremaining porespace Goldhaber andKaplan,1975;FisherandHudson,1987).A seemsunlikely.Sulfides in largeveinsin the 2-5 and 2-4 substantial negative 6a4S component, attributed toopen-syslensesincorporated a dominant component of bacteriogenitembacteriogenic reduction, is typicalof othermajorIrish callyderivedsulfur.The sizeof theseveins(up to 0.5 m deposits (Coomerand Robinson, 1976;Boastet al., 1981; wide)impliesmajorextensional forces,presumably allowing Boyceet al.,1983b).Theoriginof a second modeof -10 to bacteriogenic sulfide-bearing fluiddownintofractures which -6 per mil in Figures18 and19 will be discussed below. werealsoaccessed by the ascending hydrothermal fluids.In Interpreting thelow6a4S valueasbacteriogenic reductionaddition,the presenceof barite within massivesulfides of sulfate witha lowerCarboniferous seawater compositionformedby open-space infill,with an isotopic composition suggests either reductionof seawatersulfateor sulfatede- indistinguishable from contemporaneous seawatersulfate, rivedfromevaporites precipitated fromlowerCarboniferousimpliesthatseawater gainedaccess to themineralizing zone seawater. Thelackof evidence forthepresence of substantialandmayhavebeenthemediumfortransport ofbacteriogenic quantities of evaporites in thestratigraphy in theNavanarea sulfur.Subsea-floor penetration byfluidscarrying contempo- NAVAN ZN-PB DEPOSIT, IRELAND..ORE DEPOSITIONAL PROCESSES 553 Sulfides Barite FIG.18. Histogram showing the sulfurisotopes results of all sulfides andsulfates. raneous seawater sulfateis alsonotedat Silvermines andTy- the Conglomerate Groupore(Figs.18 and19).Giventheir nagh(Coomer andRobinson, 1976;Boast et at.,1981;Boyce, similarisotopic compositions, oretextures, parageneses, and 1990).It isconcluded thatthebacteriogenically derived sulfur closespatial relationship, it isreasonable to assume thatdepowastransported to thesiteof oredeposition. sitionof theseironsulfides wascontemporaneous. TheisotoThereisa significant, if minor,•534S component lowerthan picdistinction betweensuchpyrite-marcasite andsphalerite -28 per mil in pyriteandmamasite fromthe 2-1 lensand andgalenain the PaleBedsmaybe attributed to a different timingand/orsulfidedepositional environment. A similar, •34SCDT (%0) I I I I I I I I I •1 I I I I I I I I I Framboidal (Fe Sulfides,CGO) Colloform (Fe Sulfides,2-1 Lens) - ReplacedAllochems (ZnS) Laminated (ZnS) Dendritic (PbS) Honeyblende Stalactitm (Fe Sulfides) Cubic (PbS) Granular (ZnS) RhythmicallyBanded (ZnS) Zoned (ZnS) Coarse Bladed (PbS,Marcasite)Coarse Bm'ite - almost exclusively pyriticcomponent of low634S wasalso notedat Silvermines (Boyceet at., 1983b;Boyce,1990).A distinction betweenmain-stage sulfideandpyritehasalso been notedin the basemetaldepositsat Mount Isa and Rammelsberg by Eldridgeet at. (1986).A bacteriogenic sourceis favoredat Navan,althoughthe mechanism of achieving the additional fractionation is notwellunderstood (Boyceet at.,1983b;Fallicket al., 1990).It isplausible that an environment of bacteriogenic reductionin the Boulder Conglomerate andimmediately underlying Pale Bedswas moreoxidizing in proximity to the lowerCarboniferous sea, relative to thedeeperPaleBeds,resulting in additional fracrionation (through reoxidation of reducedsulfurspecies, followedby furtherbacteriogenic reduction) andpredominant deposition of ironsulfides. Notethatthe extentof bacteriogeniefractionation isgoverned bytheconditions underwhich thebacteria operated, andeventhetypeof bacteria caninfluenee thefractionation (Kaplan andRittenberg, 1964;GoldhaberandKaplan,1975);furtherspeculation asto thebacteriogenie environment anditsrelationto isotopic fractionation is,therefore, beyond thescopeof thispaper. On the basisof semiquantititative assessment of texture andisotopic compositon, wepointoutthatbacteriogenic sulfidedominates in thedeposit. Sulfide-hydrothermal sulfur:Figures18 and22 showa dis- tinctpeakin 6a4S values around 10permil,withina range FIc. 19. Summary of the ranges of sulfurisotope results fromdifferent minerals and textures. spanning •0 to 15permil.Thesevalues wereobtained largely fromcoarse-bladed growths. Theyareinterpreted to be froma separate sourceof sulfurfromthe bacteriogenic supply. The 554 ANDERSON ET AL. 10 8 i----I Rhythmically Banded • F•c. 20. Cubic • Stalactitic •1 Honeyblende • Denddtic/Skeletal I Laminated Histogram of sulfurisotope results fromsulfides interpreted to haveformedbyrapidprecipitation. twomostlikelycandidates fortheisotopically heavy componentis foundat or exceeding thevalueof thesulfate reservoir, viz. are thermochemical reduction of lower Carboniferous seawater 22 permil (of.Ohmoto andRye,1979;Ohmoto, 1986;Naylor sulfate orsulfate transported withthemetals, byorganic matter et al., 1989). Furthermore,thermochemicalsulfatereduction, orhydrocarbons atthesiteof oredeposition; anda componente.g.,byCH4,wouldhaveresulted inproduction ofbicarbonate, of hydrothermal sulfide transported withthemetals. Thepres- resulting in distinctively low613Cvaluecarbonate cements enceof significant quantities of baritein the deposit militates (Krouseet al., 1988);suchvalueshavenotbeenfoundat Navan against the transport of appreciable amounts of sulfate in the (Braithwaite andBi7.7i,1997).We advocate a hydrothermal orefluid,giventhe insolubility of bariumsulfateevenin acid component of HzStransported withthemetals, asrecognized solutions. Thereduction of seawater sulfate isiraplausible since in manyIrishdeposits (e.g.,Coomer andRobinson, 1978;Boast thedistribution ofthe634S valueisnotthatexpected ofthermo- et al., 1981; Caulfieldet al., 1986). This is consistentwith the chemical sulfate reduction. Forexample, nosulfide 634S value isotopically heavysulfurbeingdominant in paragenetically earlier stages of mineralization (seebelow).Interpretation of a component of HzStransported withthemetals implies anacid orefluid,consistent withprevious observations. Regarding the sourceof thehydrothermal sulfide, the ab2-4 Lens/massive sulphide senceof igneous activitycontemporaneous withthemineralizationand the isotopiccomposition itselfare inconsistent withanigneous origin.LowerCarboniferous diagenedc pyritehasa substantially morenegative 634S signature (Boyce, 1990);remobilization of this sulfideis unlikelyto be the sourceof the hydrothermal sulfide. Analternative originistherelease ofsulfurfromaliagenetic sulfides duringhydrothermal metamorphism (Bischoff et al., 1981)asaqueous fluidsconvect withintheunderlying Lower Paleozoic rocks(Anderson et al., 1989).Thishydrothermal sulfurthenshares an originwiththe metals(Russell, 1978, 1986;Mills et al., 1987).Were suchdiagenetic sulfides to haveprovided the inferredhydrothermal sulfurcomponent, thentheirrespective weighted meansulfurisotope compositions would be broadly similar. Diagenetic pyrite disseminaFIG. 21. Diagramillustrating morenegativesulfurisotoperesultsin paragentically latersulfides. tions,concretions, andblebsoccurring in drillcorethrough NAVANZN-PBDEPOSIT, IRELAND: OREDEPOSITIONAL PROCESSES 555 10 Coarse Bladed Zoned Granular Fic. 22. Histogram of sulfurisotope results fromcoarse-bladed andassociated textures. LowerPaleozoic mudstones beneaththe deposithavebeen sampled in detailto testthishypothesis (Boyeeet al., 1993, 1994).A strong modeof •534S values around5 to 10 permil is seen,in agreement withthehypothesis. Mixingofthetwosulfursources: A striking featureof the isotopic results is thedosecorrelation between textureand •534S values.Galenadearlyillustrates thiscorrelation, with ing of sulfidein Ordovician graywacke (sampled 100km southwest ofNavan) byseawater andbrine,at200øC contributedlittledissolved sulfidetothefluid,anda limitedsupply at 350øC(a temperature substantially higherthan150ø-258øC recordedfromthe Navandeposit;Probert,1983;Everett, 1997).Closed-system baeteriogenie reduction ofseawater sulfatecouldgivea largespread in •534S of sulfide produced andBurnie,1973);however, it wouldbe expected coarse-bladed textures exhibiting positive ornearpositive •534S(Sehwarez wouldbe toward isotopically heavier results values, compared to dendritie varieties whichhavenegative thatanytrends with time, which is the opposite of that observed in the miner•5•4S values. Similarly, alls•h•alerite alloehem replacement It is,therefore,considered unlikelythatthe spread textureshave negative•5 S values(-23 to -15%o). alization. bya singlesulfursoume. Furthermore, thereisanoverall trendtoward isotopically lighter in thedatacanbe explained fluidscouldexplain thevaria•534S values inlaterparagenetie stages ofsulfide deposition (Figs. Mixingoftwosulfur-bearing Thefirst,thehydrothermal reservoir witha 21 and23). Thisis perhaps bestillustrated in samples from tionsobserved. •534S of --• 10 per mil, is evident in paragenetieally earlier stages lens 2-5where apara•4e, netie sequence ofsulfide deposition is andtypifiedby coarse-bladed galenaand established, andthe •5 S valueshiftsfromaround7 to 11 per of mineralization Thesecond, thebacteriogenic reservoir witha(534S milin earlysphalerite andgalena to -3 permilandeventuallymarcasite. suchasdendritie growths -14 permil in subsequent color-banded sphalerite (Fig.23). of --•-20permil,isseenintextures To explain boththeranges in •5•4S andtheisotopically heavy and laminated sediments and becomes dominant in alloehem Wherebothreservoirs contribute sulfur,an values inparagenetieally earlysulfides (witha shifttoward lower replacements. isotopic composition isobserved (e.g.,the-12 values in laterstages), twopossibilities areconsidered: first, intermediate in Fig. 18). precipitation froma single sulfur-bearing fluid,andsecond, mix- to 6%0samples Tobetterassess themixing model,conventional laser-comingof twosulfur-bearing fluids. (Kelley andFalliek, 1990)isotopic profiles weremade Precipitation involving sulfurin a single hydrothermal fluid bustion ofcrystal growth, across a25-mm-thiek galena wouldrequiremajorfluctuations in eithertemperature, pH, inthedirection fo2ora combination oftheseparameters (Ohmoto, 1972;Rye band from the 2-1 lens. A distinct textural break is evident andOhmoto,1974);mostlikelya movetowardhigherfo.•. around 6 mm above the base and marks a transition from grained (upperband)galena. Importantly, it is notconsidered possible to transport suffi- finer(lowerband)to eoarser cient metal and reduced sulfur in the same solution to form an The results (Fig.24) canbe summarized asfollows: economic Zn-Pbdeposit underrealistic conditions (Anderson 1975,1983).Indeed Bisehoffet al. (1981)showedthat leach1. The rangeof valuesacross a singlegalenabandis 556 ANDERSONET AL. context, it isinteresting thatsulfides deposited asopen-space groxvths donotexhibit •4Svalues greater than0 permiland THIN SECTION TRANSMITTED UNDER d's LIGHT T r_.hythmically.banded ..aphalerite and coeval extremely negative values. Thiscouldbea result .ANGE •N0'%VALUES trendtoward IN ALL SAMPLES of mixingof a subordinate component of isotopically heavy hydrothermal sulfurtransported withthe metals, witha far moreabundant component ofbaeteriogenie sulfur; thehydrothermalsignature beingoverwhelmed by the baeteriogenie component at thesiteof oredeposition. Summary -14.48%. sediments ( -15.60 to+3.00%o) - 2.75%,3 coarse bladed ,galena , 10.21ø'oo Twosources ofsulfide sulfurareinterpreted fromtheisotopicresults, a dominant baeteriogenie component derivedby the reductionof lower Carboniferousseawatersulfate,and a ( ø7.72 to o10.97%o) subordinate hydrothermal component transported withthe metals.The latterwaspreferentially incorporated into the paragenetieally earliest sulfides. Therateofsulfursupply, and theinterplay betweenthe twosources, governs the mineral textures andthesulfurisotope signatures, thusexplaining the relationship observed between thesetwofeatures. Sulfate was derived from lower Carboniferous seawater. Discussion coarse (zoned) Iø 10.76%o (.7.10 õphalerlte to.10.76 ,%•) Controls on thelocationof mineralization Bedding-parallel mineralization isfrequently developed atthe contact between deanlimestones andoverlying detrital silt-rich dolomite (Fig.9A).Therefore, understanding thisstratigraphic control on the location of the mineralization is fundamental to the understanding of oredepositional processes. Thinsection FIG. 23. Diagramillustrating morenegative sulfurisotoperesultsin petrography indicates that the earliest stage of dolomitization paragentically latersulfides. withinthesedolomitic horizons inthePaleBedspredates sulfide deposition. Whatcontrol wasexerted bythedolomitized horizons on later sulfide deposition? The ore textures and their virtually thesameasthatobtained fromsimilar-textured garelationships with host rocks indicate that the sulfides were lenathroughout thedeposit. Thus,interpretation ofthissamdeposited asacomplex sequence ofopen-space growths inmany plemayhaveimplications forthedeposit asa whole. cases, forcing the conclusion that spaces developed preferen2. Thetextural breakduringgalenagrowthcorresponds to tiallyat thesestratigraphic levels at thetimeof mineralization. a majorshiftin theisotopic composition. The most readily envisaged processes in theformation ofopen 3. Thereis an apparent trendtowardisotopically lighter space are dilation or dissolution. values throughout thelowerbandandthetopportionof the veining wouldproduce a thickupperband.In the latter,thistrendcontinues into subse- Dilation:Bedding-parallel eningofthestratigraphic intervals hosting themineralization. quentovergrowths of rhythmically bandedsphalerite. 4. Therearemajorshiftsin galena 6s4S values overshort However,AndrewandAshton(1985)clearlydemonstrated Indeed,anantipathetic relaverticaldistances, e.g.,3.9 to 14.4per rail andbackto 6.5 thatthereisnosuchthickening. tionship between vertical thickness and ore grade is evident per mil over< 1 min. in severallenses,rendering it unlikelythat dilationwasa The resultsare interpreted in termsof variations in the majorprocess with regardto the mineralization at Navan ratioof baeteriogenie to hydrothermal sulfurcomponents.(Andrew and Ashton,1985). A dynamicmixingregimeis plainlyindicated. Wherethe Dissolution: Dissolution atpreferential stratigraphic levels baeteriogenie component increases, systematic shiftstoward in thecarbonate sequence couldgenerate openspace. In this lower6s4S values occur, forexample, in thelowerbandand context,the dolomitic horizons appearto havea relatively thetopportionof theupperband. uniformthickness throughout thedeposit, irrespective of the The common occurrence of isotopically mixedsulfidein presence and/orabsence of underlying mineralization, indihandspecimens throughout theminehighlights theprobabil- catingthatthedolomitic horizons themselves werenotextenity thatoredeposition wasfacilitated through mixingof the sivelydissolved. Anydissolution would,therefore,haveaftwofiuids--arelatively hot,metal-rich, sulfur-poor hydro- fectedthe underlying limestones. Thisis supported by the thermalfluid with relativelycool,metal-poor, sulfur-rich highlyirregular, undulating footwallcontacts to thesesulfide modified seawater. Manyoretextures suchasstalaetitie and horizons compared totheessentially planarhanging-wall condendritie growths aresuggestive of rapidprecipitation of the tacts, and the observationthat clastsof host rock within the sulfides, whichcouldbe achieved by suchmixing.In this mineralization aredominantly limestone, withonlylocaldolo- NAVANZN-PBDEPOSIT,IRELAND..ORE DEPOSITIONALPROCESSES 557 rhythmic. .sl•halerite 25 hand-drilled traverse 20laser traverse coarse bladed galena 15 - • / o (13 10- 5- -- TEXTUiAI,. BREAK -medium.fine bladed galena Q FIG. 24. I I I I l -5 0 +5 +10 •15 Hand-drilled andlasersulfurisotope traverse across coarse-bladed galenain the2-1 lens. mite. Dissolution of the limestones could result from karstifi- metals weretransported aschloride complexes (Lydon,1986), of the sulfides wouldbe acidgenerating, eationprocesses priorto the mineralizing eventor by the then deposition hydrothermal fluidsthemselves. Rizzi(1993)hasidentified whichcouldresultin carbonatedissolution(Anderson,1983). minorkarstifieation features associated withemergent sur- The observation that high-grade horizonsare morerarely facesin the MieriteunitandPaleBedshostlithologies and developed at the contactbetweendolomiteand overlying arguesthattheyrepresent layersof highpermeability and limestone arguesagainst a purelychemical controlmodel, lowcompetence whichcouldhaveactedasa controlon the where dissolution occurs at the contact due to differences in channeling of orefluids.Karstifieation processes havebeen chemistry betweendolomiteandlimestone. It is, therefore, proposed to explaincavities associated withstrata-bound Zn- proposed thatthelithological contrast between thedolomite Pb mineralization in the AlpineMiddleandUpperTriassic andtheunderlying limestone provided a permeable pathway deposits (Klauand Mostler,1986),Nanisivik(Olson,1984), whichfacilitated the lateralflowof the hydrothermal fluids andPinePoint(Rhodes et al.,1984).In theAlpineandPine with the resultant dissolution and mineralization. It is unclear Pointdeposits, meteoric karstification ormixing zonedissolu- whatcaused theinitialpermeability atthiscontact. However, tion relatedto subaerial exposure afteremergence of the the differingcompetence andmechanical behavior of limecarbonate platformhasbeenproposed (KlauandMostler, stones anddolomitized horizons asa consequence of tilting 1983, 1986; Rhodes et al., 1984). At Nanisivik the initial andweakflexuring duringextension is considered the most karsticnetworkmayhavebeenenlarged by hydrothermallikelyexplanation. fluids (Ford, 1986). The absenceof featuresat Navan such An importantstructural controlon the mineralization is asinternalcarbonate sediments or majorcollapse breccias shownby contouredmetaldistribution plots(Andrewand andotherassociated karstic features, suggests thatlargeopen Ashton,1985).Theseindicatethatlocalized areasof higher spaces werenotpresent priorto themineralization. It ismore grademineralization arenortheast to east-northeast trending, plausible thatmajordissolution wasrelatedto themineraliz- particularly in the4 and5 lenses, withlocalswings to a more ingevent,similarto thatproposed forthe Zn-Pbdeposits in easterlytrend.Sulfideveinsin the depositgenerally strike theUpperSilesian district(Sass-Gustkiewicz et al.,1982)and northeast, and Andrew and Ashton (1985) conclude that recentlyfor PinePoint(QingandMountjoy, 1994).If the northeast fracturing and,in particular faulting, wasa primary 558 ANDERSON ET AL. NW SE .---BLOCK 15 •. ( BLOCK 14 'F$•,,,...t•. ' Lower Do•k Marker Equivalent .. !:'"""• •'5ø/u ZnePb •:: }t: _ ?:-..'•:".' ..';-.;J:"? '- :':-* '-•"•t.•J' "'"'-' '. -;'---- t-:.:/',•,-•Y.'{.t•' Llmi•t•d -.'.:':'"FW "' ' •d• FIG.25. Examples of thebuildupof sulfidein the 1-5lensin thehanging wallof normalfaults. controlonthemineralization. Twoexamples illustrating this theregional tilting(Hercynian?). However, the strongest conrelationship areshown in Figure25,wherea buildupof ore straints on the timingof mineralization canbe inferredfrom occurs ontheimmediate hanging wallofa fault.A connectionfeatures of the BoulderConglomerate andrelationships with between these accumulations of sulfide and this northeast- the immediately underlying PaleBedsmineralization (Ashton southwest faulting isinferredbutnotspecifically to theindi- et al.,1992).Layered pyfitein theBoulder Conglomerate and vidualfaultplanes whicharecommonly unmineralized (An- theThinlyBedded unitisdominated by framboids, suggesting drewandAshton,1985).It is envisaged that fracture-controlledpermeabfiity, whichlocalized mineralization, waspreferentially developed in the hanging wallof thesestructures . ::" .4' " astheyformed,givinga structural control to oredeposition. Thisissupported byalteration oftheRedBedsin theimmediatehanging wall of the B fattit,implyinga conduitfor .! ascending hydrothermal fluids(Ashton et al., 1995). -' 7. , . Timingof mineralization Detailed petrographic examination oflayered sulfides reveals thattheyformedby replacement andcavityfill ratherthan syngenetically ontheseafloor. Therefore, theonsetof mineralizationat Navanpostdated the deposition of at leastthe bulk of the PaleBedssequence. Localfeatures in the PaleBeds mineralization, suchas disrupted layersof sulfideenclosed withincalcarenites (Fig.9D), andcarbonate dikescontaining allochems cutting massive sulfides (Fig.26),suggest thatatleast someof thePaleBedslithologies werenotfullylithified atthe time of mineralization. Tfited stalactiticstructuresand associated laminated, internal sediments indicate mineralization priorto FIC. 26. Carbonate dikecontaining allochems, cuttinga sulfideband (horizontaldimension= 5 ram). NAVANZN-PBDEPOSIT,IRELAND:OREDEPOSITIONALPROCESSES 559 formation asearlydiagenetic ironsulfides. Giventheirsimfiar presence of clastsof PaleBedswithsphalerite replacement isotopic compositions anddosespatial relationship, it isreason- restricted to theclastmargins, indicating thatat leastpartof abletoassume thatthedeposition ofironsulfides intheBoulder themineralizing eventoccurred afterdeposition oftheBoulConglomerate-Conglomerate Groupore and underlying lens derConglomerate. Similarly, clasts ofPaleBedsandWaulsorare cemented by pyriteandlatersphalerite 2-1wascontemporaneous. In bothlocations, substantial sphaler- tianLimestone postdated deireandgalena postdate ironsulfides. Importantly, Ashton et al. (Fig. 17B),implyingthatsulfideprecipitation (1992)havedemonstrated thatthehighest grade Zn-Pbmineral- positionof at leastthatpart of the BoulderConglomerate. pointtothebull(ofthemineralization izationin theConglomerate Grouporecommonly occurs over Theselinesofevidence high-grade Zn-Pbmineralization in theimmediately underlyingat Navanhavingformedduringandshortly afterthe formathe2-5and3-5lenses, againsuggesting synchronous mineraliza-tionof the erosionsurfaceandBoulderConglomerate. tion. Finally,the extremely negative sulfurisotopecomposition Sulfideclasts exhibiting truncation ofZn-Pblayering atthe of pyriteandmarcasite fromthe Conglomerate Groupore clastedgesindicatethat previously deposited sulfides were andimmediately underlying PaleBedscouldbe attributed environment of bacteriogenic reduction rippedup andincorporated in the BoulderConglomerate.to a moreoxidizing Based on the sulfide textures it is difficult to assesswhether andsulfidedeposition relativeto the deeperPaleBeds(see these clasts are derived from the Pale Beds mineralization or above). Thiswouldbe consistent withproximity to thelower pyrite-deficient sphalerite-galena mineralization in the Boul- Carboniferous sea,suggesting sulfidedeposition at thetime der Conglomerate. Theydo indicatethatthe BoulderCon- of formation of the BoulderConglomerate. glomerate-forming eventwasstillongoing duringthe minerIt isenvisaged thata largeproportion oftheascending hydrofluidsentered thePaleBedsatthetimeofmajorextenalization.However,the overallscarcity of clastsof obviouslythermal mineralized PaleBedssuggests that muchof the PaleBeds sionandsubmarine debris flowformation, thusplacing themain mineralization formedduringandafterthe formation of the mineralizing eventat Chadian to earlyArundian. erosion surface anddeposition of the BoulderConglomerate Thepresence ofbaritein thedeposit witha lowerCarbonseawater sulfate isotope composition, impliesthatsea(Ashtonet al., 1999•; Ford,1996).Thisis supported by the iferous Lower Carboniferous seai containing sulfate(-21 permil) Iron-richsulfides proximalto theerosion surface,possiblyformingin a more oxidisedenviomment (-25 to -40 l•r mid Biogenicreduction of seawatersulfate to sulfide(-15 to -25 pernail), subsequently accesshag thePaleBeds Alteration of the Red Beds ha the HW of the B Fault Marcasite mineralization in the FW of the T Fault Legend • .• Upper Dark Limestones • Pale Beds (Dolomite Horizom) N Boulder Conglomerate • Rod Bc•l• /Laminated Beds Ascending hydrothermal fluids transporting metalsand limitedsulfide(-10 permil) Fig. 27. Simplified modelfor the formationof the Navandeposit. 560 ANDERSON ET AL. REFERENCES wateraccessed the mineralizing system. Thisseawater could alsoprovidea mediumfor transporting thebiogenic sulfur. Anderson, G.M.,1975,Precipitation ofMississippi Valley-type ores: ECONOMIC GEOLOC¾, v. 70, p. 937--942. --1983, Somegeological aspects ofsulfide precipitation in carbonate rocks, Themainphaseof mineralization at Navanoccurred at the in Kisvarsanyi, G., Grant,S.K.,Pratt,W.P.,andKoenig, J.W.,eds.,Internationalconference onMississippi Valley-type lead-zinc deposits. Proceedings time of formationof the BoulderConglomerate, duringa of Missouri-Rolla Press, p. 61-76. majorphaseof extension in the lateChadianto earlyArun- volume:Rolla,University I.K.,1990,Orealepositional processes intheformation oftheNavan dian (Fig. 27). 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Meath,in and an underlying limestone facilitatedthe lateralflowof Andrew,c.J.,Crowe,R.W.A.,Finlay,S., Pennell, W.M., andPyne,J.F., hydrothermal fluids.The mineralization in the Micriteunit eds.,Geology andgenesis ofmineral deposits inIreland: Dublin,IrishAssocip. 281-296. isbestexplained in termsof initialdevelopment of extensive ationFor EconomicGeology, J.H.,1995,Guideto thegeology of theNavanorebody: Society of fracturingof the micrites,with subsequent enlargement of Ashton, Economic Geologists Guidebook Series, v. 21,p. 151-168. thisfracturesystemby dissolution duringthe mineralizingAshton, J.H.,Downing, D.T., andFinlay,S.,1986,Thegeology of theNavan event.Thisresulted inthecontinual development of aninter- orebody, in Andrew,c.J.,Crowe,R.W.A.,Finlay,S.,Pennell, W.M., and andgenesis ofmineral deposits in Ireland: Dublin, connected network of openspaces. Thepreferential fractur- Pyne,J.F.,eds.,Geology For Economic Geology, p. 243-280. ingof theserocksis attributed to weakerphysical properties IrishAssociation J.H., Black,A., Geraghty, J., Holdstock, M., andHyland,E., 1992, (e.g.,smallercompressive strength) compared to othercar- Ashton, Thegeological setting andmetaldistribution patterns oftheZn-Pb-Fe minerbonatelithologies in the PaleBedsstratigraphy. alization in theNavanBoulder Conglomerate, in Boxvden, A.A.,Earls,G., Twosources ofsulfide sulfurareinterpreted fromthe•5a4S O'Connor,P.G.,andPyne,J.F.,eds.,The Irishminerals industry 19801990:Dublin,IrishAssociation For Economic Geology, p. 171-210. data,a dominantbacteriogenic component derivedby the Ashton, J.H.,Black,A., Duffy,P., Geraghty, J., Holdstock, M., Hyland,E., O'Keeffe,W.G., Powell,P.J.,andFord,C., 1995,Structural setting and ordinate hydrothermal component transported in anacidso- metaldistribution patterns in the Navanorebody labs. J:IrishAssociation lutionwith the metals.Thereis a notablerelationship beforEconomic Geology Models forCarbonate-Hosted BaseMetalDeposits Conference, Killarney, Abstracts, p. 1-7. tweensulfidetextureandisotopic composition. The rateof M.J.,1992,Fluidmixingduringoredeposition at sulfursupply,andthe interplaybetweenthe two sources, Banks,D.A.,andRussell, theTynagh base-metal deposit, Ireland:European Journal of Mineralogy, reduction of lower Carboniferous seawater sulfate and a sub- governed the mineraltextures andthe sulfurisotope signa- v. 4, p. 921-931. tures,thusexplaining the relationship observed. Mixingof Binney, W.P.,1987,A sedimentologieal investigation ofMaclean channel transthe acidhydrothermal fluidwithseawater containing bacte- portedsulphide ores:Geological Survey of Canada Paper86-24,p. 107riogenic sulfidecaused oredeposition, withsulfates directly 147. Bisehoff, J.L.,Radtke, A.S.,andRosenbauer, R.J.,1981,Hydrothermal alterprecipitated usinglowerCarboniferous seawater sulfate. ationofgraywaeke bybrineandseawater: Roles ofalteration andchloride eomplexing onmetalsohibilization at200øand350øC: ECONOMIC GEOLAcknowledgments OCY,v. 76, p. 659-676. The authors acknowledge the permission andencourage-Boast,A.M.,Coleman,M.L., andHalls,C., 1981,Texturalandstableisotope forthegenesis of theTynagh basemetaldeposit, Ireland:ECOmentofOutokumpu TaraMinesLtd.tocarryoutandpublish evidence v. 76, p. 27-55. thiswork.All isotopicanalyses werecarriedout at Scottish NOMICGEOLOGY, UniversitiesResearchand ReactorCentre (SURRC), funded by the NaturalEnvironment Research Council(NERC)and the Scottish Universities. 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Lens Mineral NAV G135 NAV G007 NAV G001 NAV G002 NAV G003 NAV G004 NAV G005 NAV G006 NAV G008 NAV G009 NAV 081 NAV 082 1-2 1-2 1-2 1-3 1-3 1-3 1-3 1-3 1-3 1-4 1-5 1-5 Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Honeyblende Replaced allochems Replaced allochems Replaced allochems Laminated Replaced allochems Replaced allochems Replaced allochems Replaced allochems Replaced allochems Honeyblende Honeyblende - 13.7 -23.0 -20.9 -21.6 -22.2 -20.3 -21.4 -21.3 -22.6 -22.5 - 10.8 - 10.1 NAV G031 NAV G032 NAV G033 NAV G034 NAV G040 NAV G095 NAV G096 NAV G101 NAV G099 NAV G102 NAV G108 NAV G109 2-1 2-1 2-1 2-1 2-1 2-1 2-1 2-1 2-1 2-1 2-1 2-1 Sphalerite Sphalerite Galena Galena Galena Barite Barite Barite Barite Barite Sphalerite Sphalerite Laminated Laminated Coarsebladed Coarse bladed Cubic Coarsebarite Coarsebarite Coarsebarite Coarsebarite Coarsebarite Honeyblende Honeyblende - 15.3 -9.7 13.5 9.7 1.2 19.5 19.3 17.9 20.1 17.9 - 15.3 - 15.3 NAV 085 1-5 Sphalerite Barite Coarse barite -8.7 NAV G129 2-1 Mamasite Colloform -28.2 NAV 087 NAV 088 NAV 089 NAV 090 NAV 091 NAV 092 NAV 100 NAV 101 NAV 102 1-5 1-5 1-5 1-5 1-5 1-5 1-5 1-5 1-5 Galena Sphalerite Sphalerite Sphalerite Galena Sphalerite Galena Galena Galena Dendritic-skeletal Laminated Laminated Laminated Dendritic-skeletal Rhythmically banded Coarsebladed Coarsebladed Coarsebladed -6.9 -6.6 -10.2 -9.4 -6.1 -10.2 8.5 8.2 8.5 NAV G132 NAV G133 NAV G036 NAV G037 NAV 019 NAV 023 NAV 001 NAV 002 NAV 003 2-1 2-1 2-1 2-1 2-1 2-1 2-2 2-2 2-2 Sphalerite Mamasite Galena Sphalerite Pyrite Pyrite Sphalerite Sphalerite Sphalerite Honeyblende Colloform Coarse bladed Granular Colloform Colloform Replaced allochems Replaced allochems Replaced allochems -16.6 -30.1 7.3 7.2 -28.9 -32.9 -15.0 -14.5 -14.8 NAV 103 NAV 104 NAV 107 1-5 1-5 1-5 Galena Galena Barite Coarse bladed Coarse bladed Coarse barite 9.8 11.8 17.7 NAV 004 NAV 005 NAV 006 2-2 2-2 2-2 Galena Galena Galena Coarse bladed Coarse bladed Coarse bladed NAV 112 NAV 114 NAV 116 NAV 117 NAV 118 NAV 121 NAV 122 1-5 1-5 1-5 1-5 1-5 1-5 1-5 Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Laminated Laminated Laminated Laminated Laminated Laminated Laminated -21.1 -7.7 -8.6 -9.4 - 10.7 -6.0 -3.4 NAV007 NAV009 NAV010 NAV011 NAV012 NAV013 NAV014 2-2 2-2 2-2 2-2 2-2 2-2 2-2 Galena Sphalerite Galena Galena Barite Sphalerite Sphalerite Coarse bladed Rhythmically banded Coarse bladed Coarse bladed Coarse barite Laminated Laminated NAV 083 NAV 084 NAV 086 NAV G014 NAV G018 NAVG022 1-5 1-5 1-5 1-5 1-5 1-5 Barite Barite Barite Galena Coarse barite Coarse barite Honeyblende Coarse barite Dendritic-skeletal Sphalerite Honeyblende NAV G026 1-5 Galena Dendritic-skeletal NAV G085 NAV G088 1-5 1-5 Galena Barite Coarsebladed Coarsebarite NAVGl12 NAVG012 NAV G013 1-5 1-5 1-5 Pyrite Colloform Sphalerite Laminated NAV022 NAVG030 1-5 2-1 Pyrite Stalactitic Sphalerite Laminated Galena Laminated 21.6 23.6 24.4 27.9 -11.1 NAV Gl15 NAV G128 NAV G131 NAV 015 NAV 016 2-1 2-1 2-1 2-2 2-2 Mamasite Galena Barite Colloform Coarse bladed Coarse barite NAV017 NAV 018 2-2 Galena Coarse bladed 12.3 19.4 NAV 024 NAV 026 2-2 2-2 Sphalerite Sphalerite Laminated Replaced allochems -26.6 -12.5 NAV033 NAV034 2-2 2-2 Galena Galena Coarse bladed Coarse bladed Galena Coarse bladed Sphalerite Rhythmically banded -17.2 NAV 035 2-2 -19.3 - 18.2 NAV036 NAV037 2-2 2-2 Galena Colloform Colloform -9.3 -6.5 2-2 Marcasite Marcasite Texture Galena Coarse bladed Coarse bladed (5348 (%0) -32.3 -37.3 -3.23 1.5 7.0 -0.4 1.7 2.1 3.7 -1.1 19.5 - 19.5 - 17.5 5.8 19.3 2.1 2.6 -16.2 -20.2 6.2 0.3 9.3 10.8 11.3 NAVANZN-PBDEPOSIT, iRELAND: OREDEPOSiTiONAL PROCESSES APPENDIX Sample no. Lens Mineral NAV039 NAV041 NAV051 NAV052 NAV053 NAV055 2-2 2-2 2-2 2-2 2-2 2-2 Galena Galena Galena Galena Galena Galena NAV062 NAV063 NAV064 2-2 2-2 2-2 NAV069 NAV073 Texture (Cont.) •34S (%0) Sample no. Lens Mineral 2-4 2-4 2-4 2-4 2-4 2-4 Pyrite Sphalerite Sphalerite Sphalerite Sphalerite Sphalerite Stalactitic Laminated Laminated Laminated Laminated Laminated 2-2 SphaleriteReplaced allochems -20.0 Sphalerite Replaced allochems -22.3 Sphalerite Rhythmically banded 4.0 Barite Coarsebarite 22.5 NAVG122 NAVG123 NAVG134 NAV G072 2-4 2-4 2-4 2-4 Galena Galena Barite Dendritic-skeletal -8.2 Dendritic-skeletal -8.4 Coarse barite 21.7 2-2 2-2 Barite Galena Coarse barite Coarse bladed 21.6 9.9 NAVG073 NAVG074 2-4 2-4 Sphalerite Rhythmically banded Sphalerite Honeyblende -18.3 -18.2 NAV075 NAV076 NAV077 2-2 2-2 2-2 Galena Galena Galena Coarse bladed Coarse bladed Coarse bladed 6.5 7.6 7.5 NAVG136 NAVG142 NAVG144 2-4 2-5 2-5 Sphalerite Honeyblende Sphalerite Zoned Sphalerite Rhythmically banded -3.6 12.3 -11.6 NAV078 NAV079 2-2 2-2 Sphalerite Granular Sphalerite Granular 3.8 3.3 NAVG126 NAVG023 2-5 2-5 Sphalerite Zoned Marcasite Coarse bladed 8.8 10.9 NAV126 NAV127 2-2 2-2 Sphalerite Replaced allochems -21.1 Sphalerite Replaced allochems -14.9 Sphalerite Rhythmically banded Galena Coarse bladed -1.5 7.8 2-2 Galena NAVG049 2-5 2-5 NAV129 -0.1 NAVG047 NAVG048 NAVG050 2-5 2-2 Sphalerite Replaced allochems -19.2 NAV 074 NAV 080 NAV128 NAV130 2-2 2-2 2-2 Galena Galena Coarsebladed Coarsebladed Sphalerite Granular Coarse bladed NAVG039 2-2 Galena NAVG138 2-2 Sphalerite Honeyblende NAVG042 NAVG011 2-2 2-2 NAVG150 2-2 Bournonite Coarse latecrystals NAVG140 NAVG137 NAVG016 NAVG017 NAVG021 2-3 2-3 2-3 2-3 2-3 NAV G019 NAV G143 NAVG024 NAVG025 NAVG127 2-2 2-3 2-3 2-3 2-3 Galena Galena Coarse bladed 3.4 10.2 -11.8 2-5 Barite Galena Dendritic-skeletal Coarsebarite Coarsebladed 2-5 Sphalerite Zoned NAVG051 2-5 Sphalerite Zoned NAVG055 2-5 Galena Coarse bladed Coarsebladed NAVG053 2-5 -4.2 NAVG060 Sphalerite Honeyblende Sphalerite Zoned Sphalerite Laminated Sphalerite Laminated Sphalerite Honeyblende -11.5 3.7 -5.7 -8.9 -16.8 -12.0 -11.1 NAVG063 NAVG065 NAVG066 NAVG067 NAVG068 Sphalerite Honeyblende Sphalerite Zoned Sphalerite Rhythmically banded -19.7 22.1 10.3 8.0 7.1 8.4 -15.6 8.8 2-5 Galena 3.0 -0.9 2-5 2-5 Sphalerite Zoned Galena Coarsebladed 10.5 NAVG069 NAVG070 2-5 2-5 2-5 2-5 2-5 2-5 2-5 Sphalerite Galena Galena Sphalerite Galena Zoned Coarse bladed Coarse bladed Zoned Coarse bladed 8.8 11.0 7.7 10.8 10.2 -10.2 NAVG076 2-5 Galena Dendritic-skeletal Sphalerite Honeyblende -9.7 NAVG079 2-5 Galena Dendritic-skeletal -10.3 Barite 35.0 NAV G086 2-5 Marcasite Coarsebladed Galena Galena Galena Barite Barite Coarsebarite Cubic Cubic Cubic Coarse barite Coarse barite NAVGl10 2-3 NAV G020 2-3 2-3 Celestite Coarse barite NAV048 2-4 Galena Coarse bladed NAV054 2-4 2-4 2-4 Sphalerite Laminated Galena Galena NAV057 NAV058 2-4 2-4 Coarsebarite 20.8 -9.5 23.2 21.0 39.1 NAV G059 NAV G061 NAVG077 NAV G078 2-5 2-5 NAVG080 2-5 2.2 NAVG087 -20.3 NAVG089 Sphalerite Rhythmically banded Sphalerite Rhythmically banded Sphalerite Rhythmically banded Sphalerite Honeyblende 10.8 9.4 -2.8 -14.5 -6.8 -17.0 -14.4 Sphalerite Rhythmically banded -14.6 2-5 Sphalerite Rhythmically banded 9.3 2-5 Sphalerite Rhythmically banded 14.9 9.0 -15.6 -18.7 NAV G091 NAV G094 2-5 2-5 Galena Galena Coarsebladed Coarsebladed 0.2 8.5 2-4 2-4 Sphalerite Laminated Sphalerite Honeyblende Galena Galena -22.0 -17.4 NAVG097 NAVG098 2-5 2-5 Barite Barite Coarse barite Coarse barite 24.4 22.7 NAV061 NAV065 2-4 2-4 Sphalerite Laminated Sphalerite Rhythmically banded -19.5 7.8 NAVG106 NAVGlll 2-5 2-5 Barite Coarse barite Sphalerite Zoned 21.6 9.3 NAV 096 2-4 Galena -20.8 NAV G200 2-5 Galena NAV097 2-4 Galena Galena Stalactitic -16.8 NAV G202 NAV099 2-4 Pyrite Stalactitic -14.0 NAV 059 NAV 060 NAV095 NAV 098 2-4 2-4 Galena Laminated Laminated -24.8 -20.6 NAV G046 2-4 Galena -32.6 -12.0 -15.6 -17.5 -13.7 -23.5 Sphalerite Rhythmically banded Sphalerite Rhythmically banded 2-3 2-3 NAV 125 NAV 049 1.9 NAV G100 2-5 2-5 NAVG105 NAV G108 NAV-CEL 9.8 NAVG057 NAVG058 Barite Laminated Laminated 14.1 10.5 5.9 10.0 6.0 6.1 Texture NAVGl14 NAVGl16 NAVGl17 NAVGl18 NAVGl19 NAV G121 NAV 067 Coarse bladed Coarse bladed Coarse bladed Coarse bladed Coarse bladed Coarse bladed I 563 Cubic Cubic Dendritic-skeletal Dendritic-skeletal Stalactitic Coarse bladed -6.1 -7.4 -20.8 - 16.0 NAV 110 2-4 Galena NAV111 NAV124 NAVG145 NAV123 2-4 2-4 2-4 2-4 Sphalerite Laminated Sphalerite Laminated Sphalerite Laminated Sphalerite Laminated -11.0 -15.0 -19.3 - 16.0 NAV Gl13 2-4 Pyrite - 12.9 Stalactitic 13.3 NAV G103 NAV G104 2-5 2-5 Barite Barite Coarsebarite Coarsebarite NAVG139 2-5 NAVG201 2-5 Sphalerite Granular NAVG203 2-5 BournoniteCoarse latecrystals NAV G204 2-5 2-5 Sphalerite Zoned Galena Laminated Laminated Boumonite Coarse latecrystals NAVG045 3-5 Galena NAV020 CGO Pyrite NAV021 CGO Pyrite NAV025 CGO Pyrite Coarse bladed Framboidal Framboidal Framboidal 22.5 29.4 7.2 -19.9 - 15.7 -20.3 -14.9 -17.2 5.9 -30.2 -30.8 -32.0