mineralization Palaeozoic strata of Ireland and ...

Journal of the Geological Society Preliminary sulphur isotope data of diagenetic and vein sulphides in the Lower Palaeozoic strata of Ireland and southern Scotland: implications for Zn + Pb + Ba mineralization I. K. ANDERSON, C. J. ANDREW, J. H. ASHTON, A. J. BOYCE, J. B. D. CAULFIELD, A. E. FALLICK and M. J. RUSSELL Journal of the Geological Society 1989; v. 146; p. 715-720 doi:10.1144/gsjgs.146.4.0715

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© 1989 Geological Society of London

Journal of the Geological Society, London, Vol. 146, 1989, pp. 715-720, 3 figs, 2 tables. Printed in Northern Ireland

Preliminary sulphur isotope data of diagenetic and vein sulphides in the Lower Palaeozoic strata of Ireland and southern Scotland: implications for Zn Pb Ba mineralization

+ +

I . K . ANDERSON,13274 C . J . A N D R E W , 4 J . H . A S H T O N , 3 A . J . BOYCE’ J . B . D .C A U L F I E L D , ’A .E .F A L L I C K ’ & M . J . RUSSELL,’ ‘Isotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G 7 5 0 Q U , UK 2Department of Applied Geology, University of Strathclyde, Glasgow G1 lXJ, UK 3Tara Mines Ltd, Navan, Co. Meath, Ireland 4Navan Resources plc, 3 Railway Street, Navan, Co. Meath,

+ Pb ore deposits invoke a dual supply of sulphide. The dominant source is from the bacteriogenic reduction of Lower Carboniferous seawater sulphate, but a significant, minor supplyis derived from deep-seated sources. The 6”s range of the latter component varies among the deposits: from the lightest rangeof -15% to 0% at Keel, to the heaviest of -4% to + 14.4% in the Navan/ Tatestown area. We hypothesize that such sulphur is leached mainly from diagenetic sulphide minerals in the underlying Lower Palaeozoic sediments. 6”s of pyrite in the Silurian/Ordovician Moffat Shales (-17.1% to -0.6%), and of sphalerite and galena in Lower Palaeozoic-hosted veins at Salterstown (-8.7% to -4.5%) and Wanlockhead (-10.3% to -5.1%) are consistent with the hypothesis. Below the Navan ore deposit, Lower Palaeozoic shales containing minor diagenetic pyrite with a very wide range in 6”s have been found; 634S is typically heavy (+16%) but extreme values up to +62% are encountered. The general enrichment in ”S is in accord with the noticeably heavy isotopic composition of deep-seated sulphur in the Navan orebody and its Tatestown satellite. Our preliminary results therefore suggest that geographical variations in the 6”s range of Lower Palaeozoic diagenetic write _. may have contributed to the isotopic variationin the deep-seated sulphur among the Irish deposits. Abstract: Genetic models for many Irish Lower Carboniferous Zn

I

+

+

investigate boniferous Zn Pb f Ba deposits; and (b) to of this whether variations in the isotopic composition diagenetic pyrite could have been responsible for the geographical inhomogeneity of deep-seated sulphur 634S in the Irishdeposits.Therefore we examinedthesulphur isotopic composition of (1) typical diagenetic pyrite within Lower Palaeozoic rocks: of theSouthern the well-documentedMoffatShales Uplands of Scotland; (2) sulphides within Lower Palaeozoic-hosted veins, which may represent homogenized sulphide leached from the Lower Palaeozoic sediment pile; (3) diagenetic pyrite Lower in Palaeozoic sediments beneath the major Navan deposit. Sample locations are shown in Fig. 1.

The major Lower Carboniferous Zn Pb f Ba deposits in of styles of mineralizationfrom Irelandexhibitarange syngenetic and syndiagenetic through to epigenetic, and are predominantly hosted by a transgressive carbonate sequence & (Taylor & Andrew 1978; Boast et al.1981a;Andrew Ashton 1982,1985; Taylor 1984; Andrew et al. 1986). Russell (1968, 1978) proposed that the metals within these deposits may have been leached from the underlying Lower Palaeozoiclithologies, andPbisotopedatasupportthe derivation of the majority of the lead and presumably other metals from this source (Boast et al. 19816, 1983; Caulfield et al. 1986; O’Keeffe 1986; Mills et al. 1987; LeHuray et al. 1987).Thisoriginfor thelead raises thequestion as to whethertheLowerPalaeozoicsediments couldalso have of sulphurtothe contributedahydrothermalcomponent of hydrothermal leaching of deposits, by process a diagenetic sulphide. Sulphur isotope studies reveal two sources of sulphur in the Irish deposits (Coomer & Robinson 1976; Boast et al. et al.1986). The 1981a;Anderson et al.1986;Caulfield dominantsulphidesource was derivedfromthebacterial reduction of the Lower Carboniferousseawatersulphate. However,a significant component,oftentermeddeepseated or hydrothermal sulphur, has also been recognized 1 shows thatthesulphurisotopic in mostdeposits.Table composition of the hydrothermal sulphide varies from one deposit to another. Thisstudy aims (a) to determine whether the isotopic composition of diagenetic pyrite within the Lower Palaeozoic strata can account for values attributedto hydrothermalsulphurin(andbelow?)theLowerCar-

Lower Palaeozoic lithologies In general, the Longford-Down inlier of Ireland, and the Southern Uplands of Scotland comprise a variable sequence of Ordovician and Silurian low-grade (prehnite-pumpellyite facies,Oliver 1978) turbiditicgreywackes and shales, with local Ordovician volcanic rocks described by M o m s (1984) and Legget (1987). The tectonic environment of deposition remains controversial (McKerrow 1987). The sequence has also been affected by lateCaledoniangranitic plutonism (Murphy 1987; Stephens 1988). This preliminary study concentrates on samples from the of the Longford-Down lithologiesin the southern portion inlier, and the Moffat Shales of the Southern Uplands. In 715

716

I . K. ANDERSON E T A L . 1. Total ranges in 6 3 of sulphide minerals from the Lower Carboniferous Zn + P6 deposits of centralIreland,andthespecificrangesconsidered tobe of hydrothermal (deep-seated)origin

Table

Hydrothermal Deposit

634sCDT

~ i ~ v e r m i n e s ~ * ~ -8.5 to +7.0 Tynagh4 Keel Ballinalack Tatestown’ Navan

0 to +11.4 Boast +11.4 to-31 -11.0 to +1.0 -15 to 0 -4 to +14.4 0 to +14.1

Total range’ 634sCDT

-45 to +l0

-14 to +1.0 -42 to 0 -24 to +14.4 -38 to +14.0

Source of data Coomer & Robinson (1976) Greig et al. (1971) Boyce et al. (1983) et al. (1981a) Caulfield et al. (1986) Caulfield et al. (1986) Caulfield et al. (1984, 86) Anderson et al. (1986, 87)

This is the total range of 634S sulphides reported by all investigators for these deposits and includes both diagenetic sulphides and ore stage sulphides with a light (bacteriogenic) component of sulphur. * Greig er al. (1971, p. 91) noted only the coincidence between their calculated 634S of the ore fluid (0%) and the value of sulphur of juvenile origin, but nowhere did they state that the origin of the fluid and ore components was magmatic. Coomer & Robinson(1976, p. 161)notedthattwosphaleritesfromtheLowerG orebodyhad 634S of+10%. Coomer & Robinson(1976; p. 160)introducedtheterm ‘deep-seated’ sulphur as appliedto the hydrothermal component in the Irish deposits. 634S of +7.0% and Boast et al. (1981a) noted that the galena and arsenopyrite with +11.4% respectively were sampled from the fault plane 3.62 km eastof the mine. Boast et al. (1981a, p. 43) suggested that the isotopically heavier sulphur component found in late (stage 3) mineralizationat Tynagh resulted from the ‘influx of sulphur from a possibledeep seated (mantle or homogenized crust) source’. They mentioned that the Pb isotope data (Boast er al. 19816, 1983) indicating a magmatic sourceof Pb supported this interpretation. Later Pb isotope data refutes the magmatic origin. Prior to Caulfield et al. (1984), few measurements of sulphides in the Irish deposits with 6”s heavier than +5% had been reported. Caulfield et al. (1986) did not specify the range of 634S values of theheavyhydrothermalsulphurcomponentatTatestown,but sulphur isotope fractionation temperatures ranging from 137 “C to 178 “C were calculated from sulphide pairs in the range 634S+2% to +11%.

54”

4‘W

S Salterstown Ballinalock B Keel K LW LeadhillsWanlockhead Navan N S Silvermines TTta t e s t o w n 100 TY TYnagh U km

theformer, lithologiesconsist of well-bedded darkgrits, greywackes,siltstones and fissile shales.Volcanicrocks comprisemugearites,spilitesandkeratophyresand volcaniclastic rocks (Morris 1984). The Moffat Shales are black, graptoliticshaleswith minorchert, ranginginagefrom Ordovician (Llandeillo) to Silurian (Llandovery) (Lapworth 1878; Peach & Horne 1899). The sulphur content of these to shalesisusuallyless than 1wt%, increasinglocally 10.6wt%(Stephens,pers. comm.1988). Inbothareas, of sulphur occurs in the sediments principally in the form diagenetic pyrite. Such pyrite is considered to have formed from the fixing of sulphur, bacteriogenically reduced from seawatersulphate, with reactiveironin thediagenetic environment(Berner, 1984). Lower Palaeozoic seawater 634S valuebetween +25% and +30% sulphate has a (Claypool et al. 1980).Assuminga normal bacteriogenic the fractionation of around -30% to -40% from & contemporaneous seawater sulphate value (Fisher Hudson 1987),LowerPalaeozoicdiageneticpyriteshould fall in the range 0% to -15%.

Fig. 1. Localities of sampling sites and Lower Carboniferousore

Sample descriptions

deposits referred to in text. Hatching represents the outcropof Lower Palaeozoic strata in the Longford-Down inlier (Ireland) and the southern Uplands (Scotland).

Pyritein the Moffat Shale is in theform of veryfine disseminations forming up to 2% (by volume) of the rock; samples were collected from drill core taken within 15 km of

S ISOTOPES, IRELAND & S SCOTLAND

717

produce SO, gas (after Robinson & Kusakabe1975).Thegases an Isospec 64, double collector mass were analysed on raw analyses spectrometer.Standardcorrectionswereappliedto (e.g. Craig1957).Allresults are reported as 6%S per mil (%) relativetotheCaiionDiablotroilite(CDT) standard. Analytical uncertainty, based on 20 repeat analyses (including combustion) of an internal laboratory standard is f0.27%. o

1

zcm

I Summary of sulphur isotope data (Table 2, Fig. 3)

Fig. 2. Sketch of core specimen from Ordovicianstrata beneath the Navan ore deposit. Sedimentary laminations are distorted around pyrite concretion (stipple), implying that pyrite growth occurred prior to complete lithification of the host shale, i.e. diagenetically.

Wanlockhead. The upper part of the MoffatShales is the of chronostratigraphicequivalent of theolderhostrocks Wanlockhead-Leadhills vein system (Pringle 1948; MacKay 1959). AtNavan,samplesweretakenfromthree drillholes which intersectedOrdovicianstratabeneaththedeposit. is presentintwoforms:firstly(hostedin Thepyrite of smallconcregreywackes andsiltstones)asaggregates tionary growths which are possibly framboidal in places; and secondly (hosted in chloritic, sheared shales) as well developed crystals, often forming concretions up to 2 cm in the latter samples, sedimentary diameter (Fig. 2). In laminations are clearly distorted and compacted around the largerconcretions,implyingthatpyritegrowthoccurred prior tothecomplete lithification of thehostrock,i.e. 2). It is possible that the isotopic diagenetically (Fig. composition of this sulphide has been modified by processes related to hydrothermal activity, although we consider this unlikely. Minor Lower Palaeozoic-hosted, microfracture-fill mineralization (ZnS, PbS and CuFeS,) was encountered in only one drillhole below the deposit and is possibly related to the main ore event. Veins fromWanlockheadandSalterstown consist of yellow todarkbrown, crystallinesphaleritecrystalswith associated galena ina quartzJcarbonatematrix.Atboth localities brecciation of the host rock is associated with the mineralization. At Wanlockhead, the veins postdate dykes of late Caledonian age (Temple 1956), and although Ineson & Mitchell (1974) proposeamid-Carboniferousagefrom K-Ar dating of wallrock clays, the mineralization has been of theLowerCarboniferousevent considered tobepart 1976; Samson & Banks which affected Ireland(Russell 1988). Leadisotoperatios of galenasfromSalterstown from those at (O’Keeffe, 1986) are indistinguishable Silvermines, Tatestown and the bulk of Navan (Boast 1983; Caulfield et al. 1986; Mills et al. 1987; LeHuray et al. 1987). Furthermore, Salterstown is situated along structural strike from Navan (Morris 1984) and lies along the extension of the ‘isoplumb’ trend defined by deposits of demonstrably Lower Carboniferous age (Caulfied et al. 1986; LeHuray et al. 1987).

Techniques Fine-graineddiageneticsulphideswereextracted by crushing and recovery with heavy liquid separation. Other sulphides were examined at high magnification under a binocular microscope, and removed by hand picking or by a fine dental drill. Where necessary X-ray diffraction was used to check the purity of the phases. Sulphides were combusted at 1076“C with an excess of Cu,O to

Pyritein the MoffatShalesexhibitsa range of 634S from -0.6% to -17.1%, as expected ( m e a n = i = -8.4%, n = 9). Incontrast,pyriteinLower Palaeozoic strata +6% to an beneath the Navandeposithasarangefrom lie astonishing +61.7%, however,the bulk of thedata between +6% and +31.3% (3 = +16.2%, n = 7). Minor mineralizationinfracturesin the LowerPalaeozoicrocks of values from +1.8OW to below thedeposithasarange +4.5%. AtWanlockhead,sphaleriteexhibitsa verytight range from -5.1% to -5.9%, and galena from -8.1% to -10.3%. Although the galena and sphalerite are not known to have precipitated cogenetically on the microscopic scale, the consistency of 634S data encouraged us to calculate the range of isotopic equilibrium temperatures using the formulae of Ohmoto & Rye (1979). The speculative temperature range lies between 170 “C and 240 “C, which is generallyhigher thanthe fluid inclusion temperatures of Samson & Banks (1988), butconsistentwith the ‘feeder’ fluidinclusion temperaturesatthe Silverminesdeposits mineralization at (Samson & Russell, 1987). The vein Salterstown has sphalerite 634S of -4.5% and -5.4%0, and galena of - 7 . 3 L to -8.7’40.

Discussion and conclusions (1) Typical diagenetic sulphide produced by bacterial reduction of seawatersulphate in the LowerPalaeozoic sediments would be expected to exhibit 634Svalues between 0% and -15%. However,themodeltested inthisstudy predicted that the heavy 634S range of hydrothermal sulphide in the Navan deposit should correlate with Lower Palaeozoic diagenetic sulphide exhibiting a range of similarlyheavysignatures, andthereforeenriched in 34S relative to the normal bacteriogenic sulphides. The results obtained from the Lower Palaeozoic pyrite below the Navan of deposit arethereforeconsistent with thederivation hydrothermal sulphur within the orebody from this source. However, while the results corroborate the hypothesis, it is not envisaged that the actual samples analysed contributed to the hydrothermal sulphide, as they appear unaltered. One explanation of the anomalously heavy 634Svalues of the diagenetic pyrite involves bacteriogenic sulphate reduction of Lower Palaeozoic seawater sulphate operating under closedsystemconditionswithin thesediment pile +52.7’L and (Hallberg 1985). The veryheavyresults(i.e. +61.7%) could then be produced during the final stages of sulphatereduction,wheretheinitialsulphateinthepore waters had become extremely enriched in 34S. This process requires that the H,S produced throughout the time-span of the closed system reduction was continually fixed as pyrite, ratherthan accumulatingasa pool ofH,S. Inthisway, the initialvalue of the sulphide 6”s valuescanexceed starting sulphate. (2) The largedifferencein therange of 634S values exhibited by diageneticpyritebetweenthedifferentstudy

718

I . K . ANDERSON E T A L .

- hosted pyrite beneath Navan

Lower Palaeozolc

Lower Palaeozolc

- hosted minor mineralization beneath Navan

Salterstown n 2

m

I

,

1

,r

I

I

1

1

1

1

legend

diagenetic pyrite in Moffat Shales

2

m

" 1 -5

-10 -20

m

h

15

10 0

U

I

I

-15

20

5

25

530 5

I

50

b"S

Fig. 3. Histograms of sulphur isotope data from vein and diagenetic sulphides hosted by Lower Palaeozoic strata. Table 2. Sulphur isotope data

Phase Sample

634S,,,

(Z)

(a) Diagenetic pyrite in the Moffat Shales SUGS 10 SUGS 11 SUGS 12 SUGS 13 SUGS 14 SUGS 15 SUGS 16 SUGS 17 SUGS 18

PY PY PY PY PY PY PY PY PY

-17.1 -3.1 -0.6 -10.0 -4.7 -14.4 -7.3 -8.5 -9.7

(b) Vein mineralization at Salterstown ANAG-l LP-A LP-B LP-c -8.7 LP-D

€P

SP SP gn gn

-7.8 -4.5 -5.4 -7.3

(c) Vein mineralization at Wanlockhead

-8.3

gn gn

17 18 19 SP

-8.4

gn 31

SUS 15 -8.8 SUS 16 -8.7 SUS SUS SUS sus 20 sus 21 sus 22 SUS 23 -8.8 SUS 24 SUS

gn gn -8.1

SP SP SP gn SP

-5.6 -5.3 -5.5 -5.1 -5.3

sp, sphalerite; cp, chalcopyrite; gn, galena.

634S,,, (Z)

Phase Sample

(c) Vein mineralization at Wanlockhead -5.2 -5.6 -5.9 -5.4 -5.9 -8.4 -10.3 -9.5

SUS 32

sus 33 sus 34 sus 35

SUS 36 sus 37 SUS 38 sus 39

(d) Diagenetic sulphides beneath the Navan ore deposit U80-G45 +61.1 PY U80-G45R +61.7 (repeat) PY U80-54 m +31.3 PY U80-96.1 m +52.7 PY U80-96.1 mR +52.7 (repeat) PY U80-55 m +7.2 PY U80-88.6 +7.1 PY N168-1 +24.1 PY N168-2 +6.0 PY N837-2 +15.5 PY N837-3 +18.8 PY N837-1.3 +19.2 PY (e) Minor mineralization beneath the Navan ore deposit LP-l LP-2 LP-3 LP-3

+1.8

SP SP CP gn

+3.6 +3.5 +4.5

60

,

65

S I SI R OE TL OA PN ES D.

areas is significant in that it indicates geographical a of hydrothermal sulphur heterogeneity the insource ore deposit. The differencein available for apotential hydrothermal sulphide signatures observed in the Irish deposits may be explained in this way. (3) We consider the vein sulphides at Wanlockhead to represent leached and homogenized Lower Palaeozoic diagenetic sulphide. The mean composition of pyrite in the sulphur-richMoffatShales is almost identical tothe 6'"s exhibited by the veins, lending credence to this hypothesis. The similarlightisotopic range shown by the Salterstown mineralization carries the implication that diagenetic pyrite in its surroundingareashouldbe isotopicallylighter than thatseen atNavan,eventhoughthese veins occur only 30 km from the latter: this provides a test of the hypothesis. Interestingly, minor Lower the Palaeozoic-hosted microfracture-fillmineralization beneath Navan is isotopically heavy comparedtoeither of theabove veins,again agreeing with the observed presence of heavy sulphur in the nearby Lower Palaeozoic sediments. (4) It is clearfromthetwoareasstudied in this reconnaissancesurvey thatmajor differencesexistin the 634S of diagenetic pyrite within the Lower Palaeozoic sequence. The observed variation probably reflect differences in the depositional environment (i.e. facies control) affecting the availability of sulphate (see 1 above), reactiveiron andsuitableorganicmatter, which in turn influences the extent of bacteriogenic sulphur isotope fractionation (Fisher & Hudson 1987). The results from our preliminarysurvey of diagenetic sulphide in Lower Palaeozoic strata and its possible relationship tothehydrothermalsulphur in the overlying base-metal deposits have not refuted our hypothesis. However, we are well aware of the limited nature of our geographical and stratigraphic survey in terms of its coverage. Fortunately, given the intensive mineral exploration which has taken placein Ireland over the past thirty years, considerable drillcore coverage is available to supplement outcrop in order to pursue this problem. We thank Clemente Recio Hernandez for technical assistance, and Tara Mines Ltd and B. P. Minerals for access to drillcore material. We also thank D. Manning for his patienceandencouragement. TheIsotopeGeologyUnit,SURRC isfunded by theNatural Environment Research Council and the Scottish Universities.

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Received 15 August 1988; revised typescript accepted 17 October 1988.