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- Knock intrusions contain locally significant quantities of Fe-Ni-Cu sulphide, 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. In his 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 been proven 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.