A Sulphur Isotope Study of Ni-Cu Mineralization in the Huntly-Knock Caledonian Mafic and Ultramafic Intrusions of Northeast Scotland

Journal of the Geological Society, London, Vol. 146, 1989, pp. 675-684, 9 figs, 3 tables. Printed in Northern Ireland

A sulphur isotope study of Ni-Cu mineralization in the Huntly-Knock Caledonian mafic and ultramafic intrusions of northeast Scotland

T.A. FLETCHER’, A. J. BOYCE’ & A.E. FALLICK’ 1 Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB9 lAS, UK ’Isotope Geology Unit, SURRC, East Kilbride G75 OQU, UK

Abstract: The Caledonianmafic and ultramfic intrusions of theGrampian region of northeast Scotland are synorogenic tholeiitic plutons of middle Ordovician age. They include layered cumulates andxenolithic, contaminated and granular gabbroic varieties. The structurallycomplex Huntly- Knockintrusions contain locally significant quantities of Fe-Ni-Cusulphide, while the associated country rock metasediments are sporadically enriched in Fe-sulphide. Sulphur isotope analyses on sulphide from within and around the intrusions give the following ranges of 6”s; -0.1 to -1.7% for disseminated to massive sulphides in the complex and deformed Littlemill-Auchencrieve contact zone; i0.7 to +4.3% for disseminated interstitial sulphides within cumulate and granular rocks; +1.7 to +6.0% for graphitic and sulphidic pyroxenitic pegmatites; -6.0 to +16.5% for disseminated sulphide from country rock metasediments; -4.0 to +8.2% for sulphides in partially melted sediments. 634Sof sulphides in the igneous rocks (2 = +0.5 f 2.4% (la), n = 36) lie within the range usually indicatedfor primary magmatic sulphur, i.e. Of3%, so thatthe sulphide system was probably dominated bymagmatic sulphur. There is,however, a distinct difference between the isotopically heavier cumulate and granular rocks (j = f2.4 f 1.2% (la) n = 9) and the lighter sulphide of the contact zone (i= -1.1 f 0.4% (la), n = 21). The possibility that the slightly negative 634S values of thecontact zone are due to acontribution of 32S richsulphur from sulphidic calcareous units is considered unlikely, due to the homogeneity of the contact zone 634S values, and so the variation between the two groups is attributed to processes operative within the magma. Locally, an input from country rock sulphur has occurred as suggested by the 634S values for xenolithic gabbro (+6.5%,,), some of the graphitic and sulphidic pyroxenitic pegmatites(i5.9, +6.0%) and possibly a basal olivine cumulate (i4.3L). Although the data from the Littlemill-Auchencrieve contact zone are isotopically distinct from those Ni-Cu deposits dominated by crustal sulphur, petrographic evidence suggests that crustal involvement may have been important in the siting of the ore.

Inhis review of Ni-Cu sulphide deposits, Naldrett (1981) disseminated horizons within cumulates, and sulphide- and concludes thatthe assimilation of crustalsulphur is an graphite-rich pods withinpyroxenitic pegmatites. Fine important process in the formation of most Ni-Cu deposits grained Fe sulphides are also common in the country rock associated with tholeiiticintrusions. 634S datahave been metasediments, particularly in certain calcareous and pelitic particularly useful in determining the relative contributions units. 634S analyses were carried out in order to determine of magmatic and country rock sulphur to the mineralizing whether any of the sulphur in the Fe-Ni-Cu sulphides in system. Althoughsmall, buteconomically significant, the igneousrocks hadbeen derived fromsulphur-bearing depositshave beenproven to be dominated by magmatic country rock metasediments. This was considered likely in sulphur (e.g. the Bruvann ores of the Rana intrusion, see the contact zone where xenolithic rocks and metasediments below), country rock sulphur has been shown tobe a are intimatelyassociated with significantquantities of significant source (up to 30% of sulphur in the intrusions as gabbrolnorite hosted sulphide mineralization. a whole (Ohmoto 1986)) in the giant Ni-Cu ores of Duluth Complex (NorthAmerica) and the Nor’ilskintrusion (USSR). This illustrates the importance of 6%Sin assessing Isotopic background the potential of Ni-Cu prospects. In addition, through an To establish an isotopic framework for the Huntly-Knock understanding of thevariations of 634S withinagiven data, thissection discusses therange of 634S values intrusion,the geochemistcan gain an insight intothe considered typical of magmatic sulphides, and the processes processes operative during sulphide deposition. by which variations from this range can be achieved. Two This study concentrates on a sulphur isotope investiga- case studies are then presented illustrating the isotopic end tion into Fe-Ni-Cu sulphides associated with the Caledo- members of Ni-Cu sulphide ore deposits; the Water Hen niansynorogenic tholeiitic Huntly-Knock intrustions of deposit of the Duluth Complex in which crustal sulphur is northeastScotland (Fig. 1). Disseminated fine grained considered dominant, and the Bruvann deposit of the Rana Fe-Ni-Cu sulphidesare widespread throughoutthese intrusion in which magmatic sulphur dominates. intrusions,but locally greater concentrations occur as disseminated to massive bodies hosted in gabbrohorite in a complexcontact zone,the Littlemill-Auchencrieve zone. Mantle values and variations from the norm This was discoveredin thelate 1960s by Exploration Primary or mantle derived sulphur is normally characterized Ventures Limited(Wilks 1974). Inaddition thereare by 634Svalues close to the meteoritic value 0% (Shima et al. 675

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/4/675/4889671/gsjgs.146.4.0675.pdf by guest on 25 September 2021 676 FLETCHER A. T. ET AL.

Fig. 1. Caledonian mafic and ultramafic intrusions in northeast Scotland. Loca- tion of sulphur isotope metasediment samples (this study). Intrusions; A, Arnage & Haddo House; B, Belhelvie; Bo, Boganclough; H, Huntly; I, Insch; K, Knock; M, Maud; M-C, Morven Cabrach; P, Portsoy; T, Tarland.

1963). Slight fluctuations (f2‘%’, Ohmoto & Rye 1979) are basalts can lead to negative 634Sshifts in the residual melts. possible dueto isotopicfractionation occurring during Many mafic rockshave 6”s valuesoutside the range magmaticcrystallization. Sulphides whichhave formed in 0 f 3”A; for example, 0 to +17OA for the Muskox intrusion situ by magmatic segregation from an overlying silicate melt (Sasaki 1969) and +l1 to +16%for the Water Hen commonly show 32S enrichments in the early formed massive intrusion,Duluth Complex (Mainwaring & Naldrett1977) ore with respect to later sulphides. This effect has been well (Fig. 2). Ohmoto (1986) concludes thatthese deviations documented in layeredintrustion e.g. Insizwa, Transkei, from 0 f 3% aredue to assimilation of crustalsulphur (Shima et al. 1963) andthe Deer Lake Complex, USA, during emplacement of the mafic magmas. However, (Ripiey1983) andmay be attributed to progressive fo, Chaussidon et al. (1987) suggested thatsulphur isotope increaseduring magmaticfractionation (Seccombe et al. variations in primary magmatic sulphides (-4.9% to +9.5% 1981). However this is notnecessarily diagnostic of fromsulphide globules or inclusionsin mantle derived magmaticfractionation. As Ohmoto & Rye (1979) and rocks)may be caused by the contamination of the upper Seccombe et al. (1981) point out, the reverse situation can mantle by altered oceanic crust or by isotopic fractionation occur at lower fo, conditions,where sulphide minerals at magmatic temperatures either with gaseous phases (HZS, crystallizing from a silicate melt will tend to be enriched in SO,) or during exsolution of sulphides from silicate melts. ”S with respect toHS- (the dominant sulphur phase in These new data mustcontest the validity of Ohmoto’s mafic melts (Burnham 1979)).Fractional crystallization of magmatic range. these sulphides would decrease 6”s of the residual melts. However, at these high temperatures, isotopic fractionation between crystals and sulphide liquid should not exceed 2% Examples of sulphur isotopic studies from other Ni-Cu (Ohmoto & Rye 1979). Ohmoto (1986) concludedfrom deposits these observations that the range 0 f 3% is representative of In theWater Hen intrusion of theDuluth Complex primary mantlesulphur. Loss of gaseousphases from (Mainwaring & Naldrett 1977) Cu-Ni sulphide mineraliza- magmascan also causeisotope fractionation; for example tion occurs in basal dunites and contaminated dunites and Faure (1986) described how outgassing of SO, at low fo, in peridotites. 6”s forthese sulphides range from +l1 to

++ VirginiaFormation metasediments Intrusion, Water Hen sulphides Duluth Complex

k schist4 (-9.0) black -(-5.9) noritedisseminationwth graphite -18 - Intrusion Rana graphite 1(-12.8) with sulphide massive -(t2.1) massive sulphide ,(tl.1)cumulate dissemination

IIII/I,/,, -14 -12 -10 -8 -6 -4 -2 -0 +Z r4 t6 +8 +l0 +l2 +l4 +l8116

6 34 S %D Fig. 2. Sulphur isotope ranges from the Rana intrusion, Norway, means in parentheses (Boyd & Mathieson 1979), and the Water Hen Intrusion, Duluth Complex U.S.A. (Mainwaring & Naldrett 1977).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/4/675/4889671/gsjgs.146.4.0675.pdf by guest on 25 September 2021 S ISOTOPES,HUNTLEY-KNOCK,SCOTLAND NE 677

+16% (Fig. 2). Sulphidesfrom the Virginia Formation Geological setting footwall metasediments give 6”s values around+18%. Mainwaring & Naldrett (1977) conclude that a large portion The regional geology of northeast Scotland is discussed in of thesulphur in the intrusion wasderived from these Trewin et al. (1987) and references therein. The geology of footwall metasediments. the Huntly-Knockintrusions is describedin Watt (1914), The Bruvann deposit of the Caledonian Rana intrusion, Read (1923) andMunro (1970, 1984). The effects of Norway,contains Cu-Ni sulphide mineralization as postmagmaticdisruption and deformation are discussedin disseminations and massive ore within peridotite, pyroxenite Munro & Gallagher (1984), andon aregional scale by andnorite cumulates (Boyd & Mathieson 1979). Con- Ashcroft et al. (1984). A moredetailed account of the tamination has occurred locally as testified by the presence geology and mineralization is being prepared for publication of megascopicgraphite. 6”s forcumulate disseminations by one of the authors (TAF) and a colleague. and massive ore lie within the range -1 to +4% (Fig. 2). In the Huntly area (Fig. 1) poorly exposed amphibolites, Country rockblack schists, -9.0 to -14%and igneous pyroxenites andserpentinites, layered periodititic to sulphideswith visible associated graphite -3.0 to -18%, gabbroiccumulates, granular gabbros and norites, and are considerably lighter. Boyd & Mathieson (1979) conclude xenolithic rocks (Fig. 3a) underlie an area of 70 km2. They that external sulphur was not significant in the formation of wereemplaced into Middle andUpper Dalradian the Cu-Ni mineralization, and only very locally (in samples metasedimentsaround 489Ma (Pankhurst 1970), more or with visible graphite) has it had any obvious influence. less synchronously with the Caledonian metamorphic peak.

Dalradian metasediment Amphi boli te , m minorserpentinite and pyroxenite m Mainlypicrite and troctolite cumulates Mainly olivine 0gabbro cumulates Transitionalcumulate types ,o, Orthopyroxene cumulates m Mainlygranular gabbro/nori tes

Contactrocks

Granite

__--- Conjectural - Inferred -- Fault

OK Knock OR Rothiemay Station

OH Huntly AL Littlemill-Auchencrieve Ore Zone (Location of contact zone sulphurisotope samples)

Location of igneoussulphur isotope samples

0 1 2

Fig. 3. (a) Geological map of the Huntly-Knock area with (b) structural inset map, and location of igneous sulphur isotope samples. Map data compiled from field observations (T. A. Fletcher), reassessment of EVL company data (Wilks 1974), and petrological and mapping data of I. M. Munro (pers. comm.).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/4/675/4889671/gsjgs.146.4.0675.pdf by guest on 25 September 2021 678 T. A. FLETCHER ET AL.

Theypost-date two regional deformation events in the together with the poor exposure present major barriers to a country rocks, but are themselves locally deformed by N-S comprehensivesynthesis of the geology and isotopic strikingregional ductile shear zones (Ashcroft et al. 1984; characteristics. Munro & Gallagher 1984) (Fig. 3b). In the igneous rocks, deformation is manifested as internal and marginal zones of Sulphide mineralization shearing and mylonitization that vary from centimetres to kilometres in width and which developed under amphibolite Littlemill-Auchencrieve contact zone. Mineralization faciesconditions (Kneller & Leslie 1984). This,together occursintwo zoneson the farms of Littlemill and with later faulting, has produced a complex distribution of Auchencrieve(Fig. 3a).The Littlemill zone,a roughly rock types. comformableunit of irregularlenses of disseminated to massivesulphide, lies within complexa NNW-dipping Country rock metasediments sequence of granulargabbros and norites, transitional cumulates (often graphite- and/or xenolith-bearing), norite Thecountry rock metasedimentsstrike NNE andbelong cumulatesand metasediments (Fig. 4). Disseminated predominantlyto the Argyll group(Middle Dalradian), sulphides occur in most lithologies, while the submassive to although the eastern margin of the Huntly body comes into massive sulphides appear to be associated with a particular contact with Southern Highland Group (Upper Dalradian) noritic unit. pelites and greywackes. The Argyll group is further The Auchencrieve zone is less well defined, and contains sub-divided intothe Portsoy Group (whichconsists of a similar sequence of rock types with in addition a number pelites,limestones, quartzites and blackschists thatoccur of picriticunits. Disseminated sulphides arecommon along thewestern margin of the intrusionsand probably throughout picritic andgabbroic rocks(Fig. 5), while within the Littlemill-Auchencrievecontact zone)and, to submassivesulphide is restricted toa number of short the east, the Cowhythe Gneiss (a high grade semi-pelitic to sectionsincluding a 0.5 m sectionin a picrite anda 5 m pelitic biotite gneiss, with quartzite and calcareous bands). section at a metasediment-norite contact. Host rocks in both zones show signs of intense ductile Mafic and ultramafic rocks deformation,secondary alteration and faulting. Net to massivesulphide may containeuhedral and deformed Amphibolites,pyroxenites and serpentinite sheets, with silicate crystals and variably deformed rounded and angular variably hydratedand deformed textures lieswithin and fragments of both gabbroic and metasedimentary material definea regional N-S shearzone system(Ashcroft et al. (Figs 6,7) and sulphidesthemselves may showsigns of 1984) (Fig. 3a), and probably represent deformed parts of ductile deformation. the intrusions (Ashcroft et al. 1984). A number of cumulate Gabbro and picrite hosted sulphides consist dominantly bodiesoccur, with picrite, troctolite, olivine gabbro and of pyrrhotitewith lesser amounts of pentlanditeand norite being the most importantrock types. Igneous layering chalcopyrite(Fig. 5). Cubanite occurswithin disseminated and lamination are common, with generally a N-NNE trend sulphides in picrites. Primary magnetite and ilmenite occur andsteep to overturned attitude (Fig. 3b).They possibly within some massivesulphides. Graphite is common represent discrete chambers or sheets, or more likely parts throughoutthe contact zone and is usuallyintimately of a larger intrusion (Munro 1984). Transitional cumulates associatedwith, andreplaces, sulphides anddeformed showtextural affinities and gradational and interlayered zones. contacts with both cumulate and granular rock types. Vague Due to poor geological control, it is unclear at this stage modaland sometimes textural and grain-sizelayering are whetherthe mineralization occursin a roof zoneor a observed.Fine-grained granular gabbros andnorites tectonically disrupted basal unit. outcrop extensively. Thesegranular rocks locallyshow transitional contacts with the cumulates, but they usually cut Disseminated sulphides within cumulates and granular cumulates and each other. Some granular rocks may have rocks. Disseminatedsulphides are common in cumulates originated from a separate magma pulse that caused some and to a lesser extent in granular rock types. Concentrations disruption of thecumulates (Munro 1984). Alternatively, of up to 15% may occur locally in the cumulates, especially they may represent residual melts remobilized by deforma- in olivinerich (peridotitic) units olivinein gabbro tionprior to crystallization. Contact faciesrocks outcrop cumulates. Sulphide mineralogyis pyrrhotite with lesser mainly along the eastern side of the intrusion and include pentlandite,chalcopyrite andminor cubanite. In granular contaminatedand xenolithic gabbros and partially melted rocks coarse blebs and fine disseminations of pyrrhotite are sediments(Gribble & O’Hara 1967). Thelatter are often occasionally seen. highly xenolithic andgraphitic and may backvein and intrude cumulate and granular rocks. A number of graphite- Sulphide- and graphite-bearing pyroxenepegmatites. and sulphide-bearing noritic to orthopyroxenitic pegmatites Sulphide and graphite form irregular pods of disseminated up to 3.5 m in width cross-cut layering in cumulates. They to submassivemineralization. The sulphidemineralogy is have alteredmargins and sometimes contain a deformed dominated by pyrrhotite with lesser amounts of pentlandite matrix. and chalcopyrite (Fig. 8). Graphite occurs as fine to coarse The overalldistribution, mineralogy and crystallization patches and laths and is intimately associated with sulphide, sequence of rock types suggests younging from west to east. which itoften replaces.Locally it may accountfor up to The mineral chemistry data of Munro (1984) support this. 40-50% of the rock. The present form and attitude of the igneous bodies in the Huntly-Knock area is aresult of acomplex history of Metasediments and partially melted sediments. Pyrite is multiple magma emplacement, shearing and faulting, which the main sulphide present and is especially common in pelitic

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/4/675/4889671/gsjgs.146.4.0675.pdf by guest on 25 September 2021 S ISOTOPES,HUNTLEY-KNOCK, NE SCOTLAND 679

NNW D.D.H. RD15.14 D.D.H. HK 7'5 l

Fig. 4. Cross-section through Littlemill m ORE ZONE AND NORITICUNIT m NORITIC CUMULATES - - FAULT ore zone. Rocks are variably deformed 0MOTTLED UNIT m METASEDIMENT 50 m from weak to mylonitic. D.D.H. TRANSITIONALGRANULAR AND XENOLITHIC ROCKS RD15/14 and HK5/7, diamond drill GABBROS/NORITES +/- OLIVINE m DRIFT holes.

and calcareous units of the Portsoy Group, where it occurs pyrrhotite.Distribution is sporadicand locallyrich asfine partings, disseminations and occasional coarser concentrations occur. lenses. Sulphide disseminations may reach up to 5% in some instances,especially in a black schist unit, which may be stratigraphically equivalent to the Perthshire mid-Dalradian Sulphide genesis BenEagach schist (Harris & Pitcher 1975), (aunit well known for itsassociation with basemetal mineralization; Origin of igneous sulphies Willan & Coleman 1983). Within partially melted sediments The Fe-Ni-Cu sulphidesfound inthe Huntly-Knock and their xenoliths, sulphides occur as fine to coarse blebs of igneousbodies, consist mainly of pyrrhotite, withlesser pentlanditechalcopyrite andtrace cubanite. The sulphide assemblage andtextures indicate a magmaticorigin

. 0.2 mm Fig. 5. Disseminated sulphide bleb in olivine gabbro fromthe Fig. 6. Submassive ore from the Littlemill zone, containing silicate Auchencrieve zone. PO, pyrrhotite; Pn, pentlandite; and crystals and partly digested gabbro fragments(gf). PO, pyrrhotite; remobilized Cp, chalcopyrite. Pn, pentlandite; S, serpentine fracture veinlet.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/4/675/4889671/gsjgs.146.4.0675.pdf by guest on 25 September 2021 T. A. FLETCHER ET AL.

the magma may have modified metasediment sulphide (e.g. Hall (1986) described how the transformation of pyrite to pyrrhotite may be induced during progressive metamorphism), or it may have been directly responsible for generating pyrrhotite.

Sulphur isotope analyses Forty eight sulphur isotcjpe analyses were camed out on various igneousand metasediment sulphide samples, taken mainly from boreholelocalities. Due to the paucityof locally outcropping metasediment, equivalent units were sampled along strike. Samples from hornfelsed calcareous xenoliths in partially melted sediments ?,l,: wereall collected from the contactzone. Sample locations, cm descriptions and results are shown in Figs 1 and 3a. Tables 1-3 and Fig. 9. Fig. 7. Gabbro/norite (gn) containing calcareous xenoliths(cx) and Sulphides were extracted using a fine-tipped dental drill, or by disseminated to net sulphide (d). crushingand heavy liquid separation for some metasediment samples. TheSO, gas was produced by reacting an intimate mixture of thesulphide with excess Cu,O at 1070°C(after Robinson & with evidence of modification by subsequent shearing and Kusakabe 1975). The preapared gas was then analysed on a 12 cm hydrothermal activity. These later remobilisation processes radiusmodel double collector mass spectrometer (Isospec 44 have led to the formation of replacement and deformation modified for SO,). The ion beams monitored are m/z 66 and 64, textures seen in certain contact zone, massive, net and vein withstandard correction factors applied to raw 6’% ratios (e.g. sulphides andin some submassivesulphides in pyroxene Craig 1957). The entire instrument is operatedat 110°C. Within run pegmatites. However some contact zone sulphide may have precisionis typically fO.O8% (24 or better, witha long term been remobilized at the magmatic stage via injection of a reproducibility (including sample preparation) of f0.27% based on 20 replicate analyses of the internal laboratory standard (including sulphideliquid prior solidification,as indicated by the combustion). All 6%S results are reported as %O variations relative presence sharpore confacts and sulphide ‘breccias’in of to Canon Diablo troilite (CDT). submassive to massive sulphides. Sulphide immiscibility canbe induced bydecreasing pressure,temperature, increasing fs, and fo,, increasing Discussion of Sj4S distributions in the Huntly-Knock SiO,, Na,O, K,O, MgOand AI,O3 anddecreasing Fe0 area activity (Maclean 1969; Shamazaki & Clark 1973; Haughton The 6-S of sulphides in the cumulate and granular rocks et al. 1974; Buchanan et al. 1979). Many of these changes and in the contact zone (including pyrite veins) give a mean could be brought about by country rock assimilation and this value of f0.5 f 2.4% (la: n = 36): a tightly defined range. may havebeen an important process in the formation of Asthe expected 6”s rangefor sulphide derived from sulphides in the contact zone. partially meltedmantle is Of3% (Ohmoto, 1986), the isotopic evidence is consistent with derivation of the great Origin of sulphide in partially melted sediments majority of sulphide within themafic and ultramafic rocks of In partially melted sediments, the dominant sulphide phase the Huntly-Knock area from a magmatic source. is pyrrhotite,and in the equivalent metasediment lithol- Althoughthe sulphides in the cumulates and granular ogies, the sulphide phase is pyrite. Thermal interaction with rocks are isotopicallyindistinguishable, there isa distinct differencebetween these (range + 0.7% to +4.3%, f = +2.4 f 1.2%: la, n = 9) and the contact zone sulphides (range-0.1% to -1.7%, f=-1.2*0.4%: la, n=21), and this variation is discussed below. An approximate west to east (i.e. younging) traverse was taken through the cumulate sequence (samples 20, 21, 15, 16 and19); however no systematic 6”s variationwas detected by this study. The heavy 6?S (withrespect to the typicalmagmatic range) of sulphides within the xenolithic gabbro (+6.5%), pyroxenepegmatite (+5.9%, +6.0%) and basalpicrite (+4.3%, +4.3%) imply that contamination froman external sulphursource was locally important. The presence of xenolithsin thegabbro, locallyextensive amounts of graphite in the pegmatite (cf. Rana) and the location of the picrite cumulates at the base of the intrusion are consistent with this theory. In the case of the pegmatite, the positive 634Svalues, and the association with graphite, suggest that incorporation of some sulphur from graphitic blackschist units took place. Why crustal interaction should lead to local uptake of

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/4/675/4889671/gsjgs.146.4.0675.pdf by guest on 25 September 2021 S ISOTOPES,HUNTLEY-KNOCK, NE SCOTLAND 68 1

Table 1. Sulphur irrotope results from metasediment/partially melted sediments.

634s%0 Sample No. Grid Ref Location/depthLithology Ref Grid No.Sample Py PO

Metasediments 1 NJ58676631 Portsoy limestone -6.0 2 NJ58446642 Portsoy black shist +16.5 3 NJ56456115 BD7/40 m calcareous unit +10.5 11 nla BH7/13 m black schist +16.1 12 NJ56743664 12 Adamston cleaved mudstone -3.9 8 NJ51844516 Cumrie pelitic hornfels +8.7

Partially melted sediments 4 NJ51884750 HK7/24 m +1.4 5 -4.0 xenoliths calcareous Hornfelsed HK7/22 m 6 NJ52394802sedimentmelted partially RD21/73 in m +8.2 7 NJ52284804 RD14/86 m +1.8 9 NJ49334210 HK12/54 m +4.5 NJ49224371 HK18/67 m}I Partiallymelted sediment 10 -2.6

Py, pyrite; PO, pyrrhotite. Grid Ref, (Grid reference) and outcrop localities from 1: loo00 scale Ordnance Survey maps. See Fig. 1 for locations. n/a not available.

sulphur in the above cases and not in the contact zone is (similarbutless thanthemetasediments) and the uncertain, but is suggests that on emplacement, the contact observations of Hall (1986) (see above), preclude a totally zonemagma was already close to sulphursaturation or magmatic origin and suggest the values represent those of indeed that immiscible sulphides had already formed before modified metasediment sulphide. However, incorporation of external sulphur could be assimilated. variable amounts of magmatic sulphur cannot be discounted Pyrite in black schist gave constant 634Sof +16.1% and and this makes it difficult to ascertain to what degree they +16.5%. The values are typical for this mid-Dalradian retain their original metasediment 634Svalues. lithology (Hall et al. 1988). In contrast 634Sanalyses on two With the possible exception of the two samples of pyrite calcareous units give values of -6.0% and +10.5%. Such a (+0.2%and +0.3"%) whichoccur in laterserpentine- spreadhas been noted in mid-Dalradian schists and bearingfracture veinlets where they cut massive ore, carbonates by Willan & Coleman (1983). A cleaved deformation and hydrothermal alteration do not appear to mudstone of UpperDalradian age (Southern Highland have affected the sulphide 6'"s to any significant extent. Group) gives a 634Sof -3.4%. Sulphidesinpartially meltedsediment (-2.6% to 634Svariation between the contact zone and +4.5%, f = +0.95 f 3.6%: la, n = 2) and from hornfelsed calcareousxenoliths within the matrix (-4.0% to +8.2%, cumulatelgranular rocks X = + 1.85 f 4.3%: la, n = 4) exhibit a considerable spread The assimilation of crustalsulphur by mafic magmas is in 634S. Althoughthe means lie within therange of considered an important condition in the formation of many magmatic sulphur, the considerably greater variation in 6?3 Ni-Cu deposits(Naldrett 1981). In the contact zone, the

Table 2. Sulphur botope results from igneous rocks

634S%o

Sample No. Grid Ref Location/depthLithology Ref Grid No. Sample PO

13 NJ53184822 HK8/70 m norite +1.4 14 NJ52604720 14 RD3/26 m picrite +1.0 15 NJ48114442 15 Sinsharnie gabbroolivine +3.0 16 NJ49224371 16 HK18/131 m gabbroolivine +0.7 17 NJ49874312 17 Bin quarry pyroxenepegmatite +5.9 18 NJ49924307 18 Bin quarry pyroxenepegmatite +6.0 19 NJ49964284 19 BH5/148 m gabbro/norite +2.3 20 NJ48594172 20 +4.3 Broadlands picrite 21 NJ48634085 21 +4.3 CD1/175 m picrite 22 NJ51694078 22 Bridge of Gibston noritegranular +2.5 23 NJ52934195 23 Robieston noritegranular +1.9 24 NJ53804107 24 Hazelcraig pyroxenepegmatite +1.7 25 NJ55184189 25 Hill of Mosstown xenolithic+6.5gabbro

PO, pyrrhotite. Grid Ref, (Grid reference) and outcrop localities from 1: loo00 scale survey maps. See Fig. 3a for locations.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/4/675/4889671/gsjgs.146.4.0675.pdf by guest on 25 September 2021 682 T. A. FLETCHER ET AL.

Table 3. Sulphur isotope results for contact zone gabbrolnorite hosted sulphides

634s%0

Sample No. Grid Ref Location/depthRef GridSample No. Sulphide type PO Pn Cp Py

L1 NJ52284804 L1 RD14/112.9 m vein -1.3 L2 NJ51854758 L2 RD15/121.5 m massive -1.2-1.6 L3 RD15/121.7 m massive -1.7 -1.3 L4 RD14/105.3 m vein -1.3 -1.4 L5 RD14/109.2 m disseminated - 1.6 L6 RD14/124.0m vein in mylonite -1.5 L7 RD15/121.2 m net/disseminated -0.9 L8 RD15/125.9 m disseminated -1.6 L9 RD15/125.1 m disseminated -1.5 L10 NJ.51884750 L10 HK7/78.0 m submassive -1.2 L11 NJ51884750 HK5/46.0 m submassive -1.4 L12 HK5/51.0 m massive -0.9 L13 RD15/117.3 m disseminated -1.4 L14 RD15/125.1 m remobilized blebs -0.8 L15 RD15/121.2m net -0.9 A1 NJ52284805 HK3/77.5 m submassive -0.6 A2 NJ.52284804 RD13/63.3 m submassive cut by -0.1 pyrite vein +0.2 vein pyrite A3 -0.3 RD13/64.9 m submassive A 4 RD13/63.0 A4 m pyrite vein cutting massive ore +0.3

L, Littlemill zone, A, Auchencreivezone, PO, pyrrhotite; Pn, pentlandite; Cp, chalcopyrite;Py, pyrite. Grid Ref. (Grid reference) and outcrop localities from 1: loo00 scale Ordnance Survey maps. See Fig. 1 for locations.

homogeneity of 6"s valueswithin themagmatic range 634S betweeencontact zone sulphides(range -0.l"A to indicatesnegligible a input of sedimentarysulphur. -1.7%, X = -1.2 f0.4%: la, n =21) andthose of the However, the assocation of metasedimentary material may cumulate andgranular rocks (range +0.7% to 4.3%, have beenimportant during formation of suplhides(i.e. X = +2.4 f 1.2%: la, n = 9) is thereforeattributed to inducingsulphide immiscibility). The distinctdifference in magmaticisotopic fractionation processes. Uncertaintyin

634Sof sulphide Mineralogy in contact zone Disseminated to massive ore PO v CP Pyrite veins PY m veins Pyrite IIIIIII

n 634S of sulphide l in igneous rocks 4i I- Cumulate/granular Pyroxene pegmatite j PO Xenolithicgabbro IIIII

Calcareous xenoliths in oartiallv melted sedirnents 2 .. Fig. 9. Sulphur isotope data from ... Partially melted sediment :P,' ... Huntly-Knock intrusions and surround- IIIIIIIIIIII Hornfelsedpelite ing country rocks. Sulphide mineralogy: -6 -4 -2 0 +6 +l6 t2 t4 t8 +l0 +l2 +l4 PO, pyrrhotite; Pn, pentlandite; Cp, 6 34 S %a chalcopyrite; Py, pyrite.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/4/675/4889671/gsjgs.146.4.0675.pdf by guest on 25 September 2021 S ISOTOPES,HUNTLEY-KNOCK.SCOTLAND NE 683

the geological relationships between the cumulate/granular involvement hada negligibleeffect on 6”s values,it rocks and the contact zone precludes a rigorous discussion probablyplayed animportant role during formation of of the 6”s variation at this stage, but three possibilities may sulphide mineralization by inducing sulphide immiscibility. be considered: Shearing and hydrothermal processes do not appear to (i) If the contact zone magma represents a residual melt have contributed to the observed variations. that had already suffered 6”s depletion by earlier pyrrhotite precipitation (Ohmoto & Rye 1979) in a parental chamber Exploration Ventures Limited are thanked for providing access to wherecumulates were forming, then the earlier cumulate drill core, company data and financial assistance. Sulphur isotope sulphides would be heavier im6”S with respect to the later analyses were carried out at SURRC, East Kilbride. A. G. Gum, contact zonesulphides. However, at magmatic tempera- A. J. C. Hogg and C. M. Rice are thanked for constructive criticism tures,isotopic fractionation betweenpyrrhotite and HS- on earlier draughts. Technical assistancewas provided by B. Fulton, (Burnham 1979) is very small (less than 17/00at 700 “C).Even W. Ritchieand S. Reid (Aberdeen), and E. Tweedie(SURRC). after90% of thesulphur in the originalmagma had T. A. Fletcher was supported by NERC/CASE award GT4.84.GS.4. precipitated, the 6”s of the residual melt would be no more than 2% lighter thanthe original(assuming a Rayleigh distillationprocess, Ohmoto & Rye 1979). Thusthe References differencein 634S betweenthe two groupsand the ASHCROFT, W.A., KNELLER,B. C., LESLIE,A. G. & MUNRO,M. 1984. Major insignificant depletion of Ni-Cu sulphide in the contact zone shear zonesand autochthonous Dalradian in the NorthEast Scottish relative tothe cumulates (i.e. volumetrically thecontact Caledonides. Nature, 310, 760-762. BOYD,D. R. & MATHIESON, C. D.1979. The Nickelmineralization of the zone has greater than 10% of the sulphur in the intrusion), Ranamafic intrusion, Nordland, Norway. Canadian Mineralogist, 17, do not favour this model. 287-298. (ii) If the contact zone represents a basal unit, emplaced BUCHANAN,D. L. & NOLAN,J. 1979.Solubility of sulphur and sulphide prior to the magma that went on to form the cumulate and immiscibility in synthetic tholeiitic melts and their relevance to Bushveld Complex rocks. Canadian Mineralogist, 17, 483-494. granularrocks (cf. the Insizwa intrusion, Lightfoot et al. BURNHAM,C. W. 1979. Magmas and hydrothermal fluids. In: BARNES,H. L., 1984), thenthe isotopic fraction processesdescribed by (ed.) Geochemistry of Hydrothermal OreDeposits, second Edition, Shima et al. (1963) and Ripley(1983) may explainthe Wiley Interscience, New York, 71-136. variationin In this model,the early segregation of CHAUSSIDON,M,, ALBAREDE,F. & SHEPPARD,S. M. F. 1987. Ion microprobe evidencefor 634S variationsinprimary magmatic sulphides from sulphidein thecontact zone is followed by thelater diamonds, ultramaficand mafic rocks. In: Mineral Deposits Studies precipitation, under increased fo2 conditions (see above) of Group; Stable isotopes in ore genetic studies,Abstracts, Newcastle, slightly 634Senriched sulphide in the cumulate and granular December 1987. rocks. CRAIG,H. 1957. Isotopic standards for carbon andoxygen and correction factorsformass-spectrometric analysis of CO,. Geochimica et (iii) Another possibilityconsiders differences in the Cosmochimica Acta, 0, 133-149. environment of precipitationbetween the two sites, in FAURE,G. 1986. Sulphur. In: FAURE,G. (ed.) Principles of Isotope Geology, particular variations in fo,, which exerts a major control on Second Edition. John Wiley and Sons, New York. the SO, :H,S ratio of escaping volatiles in the melt (Ohmoto GRIBBLE,C. D. & OHARA,M. J. 1967. Interaction of basicmagma with pelitic materials. Nature, 214, 1198-1201. & Rye 1979). Significantly large isotope fractionations exist HALL,A. J. 1986. Pyrite-pyrrhotite redox reactions in nature. Mineralogical between SO, and reduced sulphur species, even at magmatic Magazine, 50, 223-229. temperatures.Thus, if SO, is asignificant voltaile phase, -, BOYCE,A. J. & FALLICK,A. E. 1987. Iron sulphides in metasediments: considerable 634S variations will occur between the voltaile Isotopic support for a retrogressive pyrrhotite to pyrite reaction. fluid andthe residualmelt. In particular under low Chemical Geology (Isotope Geoscience Section), 65, 305-310. fo, HARRIS,A. L. & PITCHER,W. S. 1975. The Dalradian Supergroup. In conditions, SO, release from the magma leaves the residual HARRIS,A. L., SHACKELTON,R. M,, WATSON,J., DOWNIE, C.,HARLAND, meltdepleted in 634S (Faure 1986). Theoccurrence of W. B. & MOORBATH,S. (eds) Correlation of Pre-Cambrian rocks in the graphitic metasediments in thecontact zone magma may British Isles. Special Report of the Geological Society, London, 6, 52-76. HAUGHTON,D.R., ROEDER, P. L. & SKINNER,B. J. 1974.Solubility of havecreated fo2 conditions low enoughto produce the sulphur in mafic magmas. Economic Geology, 69, 451-462. observedvariation inStudies onthe mineral stability KNELLER,B. L. & LESLIE,A. G. 1984. Amphibolite facies metamorphism in relationships(magnetite-ilmenite) may help elucidate shearzones in the Buchan area of North East Scotland. Journal of differencesin fo, regimes,and may help to clarify the Metamorphic Geology, 2, 83-94. distinctions between the two groups. LIGHTFOOT,P. C., NALDRETT,A. J. & HAWKESWORTH,C. J. 1984. The geology and geochemistry of the Waterfall Gorge section of the Insizwa Complexwith particular reference to the origin of the nickel sulphide deposits. Economic Geology, 79, 1857-1879. Conclusions MAINWARING,P. R. & NALDRETT,A. J. 1977. Country rock assimilation and The majority of sulphidesin the igneous rocks have 6”s the genesis of Cu-Ni sulphides in the Water Hen intrusion, Duluth valuesconsistent with derivation from a magmatic source. Complex, Minnesota. Economic Geology, 72, 1269-1284. MACLEAN, W. H. 1969. Liquidus phase relationships in the FeS-FeO-Fe,O,- The 634S values fromthe contact zone and from the SiO,system, and their application in geology. Economic Geology, 64, cumulates and granular rocksplot as two discrete groups, 865-884. and thisvariation is attributedto isotopic fractionation MUNRO,M. 1970. A reassessment of the ‘YoungerBasic’ igneous rocks processes occurring at magmatic temperatures either during between Huntly and Portsoy based on new borehole evidence. Scotfish Journal of Geology, 6, 41-52. segregation of sulphidesfrom parent melts or via SO, - 1984. Cumulate relationshipsin the ‘YoungerBasic’ masses of the release under low fo2 conditions. Huntly-Portsoy area, Grampian Region. Scottish Journal of Geology, While there is abundant evidence for the interaction of U), 343-359. magma with the country rock, only locally is there evidence -, & GALLAGHER,J.W.1984. Disruption of the ‘Younger Basic’ masses in the Huntly-Portsoy area, Grampian Region. Sconish Journal of for involvement of metasediment . sulphur(the xenolithic Geology, 20, 361-382. gabbro,graphitic pyroxene pegmatites and the basal NALDRE~,A. J. 1981.Nickel sulphide deposits: Classificationcomposition picrites). However in thecontact zone although crustal and genesis. Economic Geology, 75th Anniversary Volume, 76,628-685.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/4/675/4889671/gsjgs.146.4.0675.pdf by guest on 25 September 2021 684 T. A. FLETCHER ET AL.

OHMOTO,H. & RYE, R. D. 1979. Isotopes of sulphur and carbon. In: Lunnon and Juan Main Shoots, Kambalda: Textural and sulphur isotopic BARNES,H. L. (ed.) Geochemistry of Hydrothermal Ore Deposits, evidence. Economic Geology, 76, 1675-1685. Second Edition, Wiley Interscience, New York, 509-567. SHAMAZAKI,H. & CLARK,A. 1973.L. Liquidus relations in the - 1986. Stable isotope geochemistry of ore deposits. In: VALLEY,J. W., FeS-FeO-SiO,-NqOsystem and geological implications. Economic TAYLOR,H. P. & O’NEILL,J. R. (eds.) Stable Isotopes in High Geology, 68, 79-96. Temperature Geological Processes. Mineralogical Society of America SHIMA,M,, GROW,W. H. & THODE, H. 1963. Sulphur isotope abundances in Review in Mineralogy, 16, 491-556. basic sills, differentiated granites and meteorites. Journal of Geophysical PANKHURST, R.1970. J. The geochronology of the basic igneous complexes. Research, 68(9), 2835-2847. Scottish Journal of Geology, 6, 83-107. TREWIN,N. H., KNELLER, B. C. & GILLAN,C. 1987. Excursion guide to the READ,H. H. 1923. The geologyof the country around Banff,Huntly and geology of the Aberdeea area. Geological Society of Aberdeen, Scottish Turrif. Memoir of the Geological Survey, U.K. Academic Press. RIPLEY,E. M. 1983. Sulphide mineralogy and sulphur isotope geochemistry WATT, W. R. 1914. The geology of the country around Huntly, of layeredsills in the Deer Lake Complex, Minnesota. Mineralium Aberdeenshire. Quarterly Journal of the Geological Society of London, Deposita, 18, 3-15. 70, 266-293. ROBINSON B.W. & KUSAKBE,M. 1975. Quantitative preparation of SO, for WILLAN,R. C. R. & COLEMAN,M. L.1983. Sulphur isotopes of the 34S/3iS analysesfrom sulphides by combustionwith cuprous oxide. Aberfeldy baryte, zinc, lead, deposit and minor sulphide mineralization Analytical Chemistry, 47, 1179-1181. in the Dalradian metamorphic terrain, Scotland. Economic Geology, 78, SASAKI, A.1%9. Sulphur isotope study of the Muskox intrusion, District of 1619-1650. Mackenzie. Geological Survey of Canada, Paper 68. WILKS,G. 1974. Exploration Ventures Limited:Ni exploration programme SECCOMBE,P. K., GROVES,D. l., MARSHAM,R. J. & BARRE-IT,F. M. 1981. over the Buchan area of north east Scotland. Open File Report, British Sulphide paragenesis and sulphur mobility in Fe-Ni-Cu sulphide ores at Geological Survey, Murchison House, Edinburgh.

Received 29 July 1988; revised typescript accepted 27 September 1988

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/4/675/4889671/gsjgs.146.4.0675.pdf by guest on 25 September 2021