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

¹ Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB9 lAS, UK
² Isotope Geology Unit, SURRC, East Kilbride G75 0QU, UK

The Caledonian mafic and ultramfic intrusions of the Grampian region of northeast Scotland are synorogenic tholeiitic plutons of middle Ordovician age. They include layered cumulates and xenolithic, contaminated and granular gabbroic varieties. The structurally complex 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 ³⁴S; – 0.1 to —1.7 for disseminated to massive sulphides in the complex and deformed Littlemill-Auchencrieve contact zone; +0.7 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.

³⁴S of sulphides in the igneous rocks ( = +0.5 ± 2.4 (1), n = 36) lie within the range usually indicated for primary magmatic sulphur, i.e. 0 ±3, so that the sulphide system was probably dominated by magmatic sulphur. There is, however, a distinct difference between the isotopically heavier cumulate and granular rocks ( =±2.4 ± 1.2 (1) n = 9) and the lighter sulphide of the contact zone ( = –1.1 ± 0.4 (l), n = 21). The possibility that the slightly negative ³⁴S values of the contact zone are due to a contribution of ³²S rich sulphur from sulphidic calcareous units is considered unlikely, due to the homogeneity of the contact zone ³⁴S 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 ³⁴S values for xenolithic gabbro (+6.5), some of the graphitic and sulphidic pyroxenitic pegmatites (+5.9, +6.0) and possibly a basal olivine cumulate (+4.3). 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.

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