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

Abstract

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 δ 34 S; – 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. δ 34 S of sulphides in the igneous rocks (χ̄ = +0.5 ± 2.4‰ (1 σ ), 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 σ ) n = 9) and the lighter sulphide of the contact zone (χ̄ = –1.1 ± 0.4‰ (l σ ), n = 21 ) . The possibility that the slightly negative δ 34 S values of the contact zone are due to a contribution of 32 S rich sulphur from sulphidic calcareous units is considered unlikely, due to the homogeneity of the contact zone δ 34 S 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 δ 34 S 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.

Carbon in Mineralised Plutons

“…The morphology of the graphite in the intrusion is comparable to the lath-shaped graphite in the South Harris intrusives, though it is more abundant at Bin Quarry and commonly occurs as smaller crystals (0.1-0.3 mm). The Bin Quarry intrusion contains abundant bytownite (plagioclase) phenocrysts (3-5 mm width), as previously identified in other works [29], in which the sample was described as an ultrabasic graphitic pyroxenitic pegmatite. Graphite and sulphides (pyrrhotite) within the Bin Quarry intrusion are present within both the bytownite phenocrysts and the wider groundmass.…”

Terrane and basement discrimination in northern Britain using sulphur isotopes and mineralogy of ore deposits

“…Huntly-Knock gabbroic intrusions of Aberdeenshire have ~34S values ranging from -2%0 to +6%0, the values above +3%o representing contamination with Dalradian sedimentary sulphur (Fletcher et al 1989). Further to the east the Arthrath mafic intrusion has also incorporated crustal sulphur.…”

“…Fletcher et al 1989Fletcher et al , 1997Laouar et al 1990;Lowry et al 1995Lowry et al ,1997, basemetal vein, stratiform and stratabound deposits (Willan & Coleman 1983;Pattrick & Russell 134 D. LOWRY E T A L . There are also distinct variations in the tenor of fi34S across the Iapetus Suture zone, in lithologies of similar age, implying a significant difference in the history of diagenetic sulphide on either side of the palaeo-ocean (Fig.…”

Platinum-group element occurrences in Britain: magmatic, hydrothermal and supergene

“…Studies of PGE in the Littlemill-Auchencrieve ores and more widely in the Huntly and Knock areas were undertaken by Fletcher 12 and Fletcher and Rice. 13 At Littlemill a maximum of 574 ppb Au + Pt + Pd occurs in a remobilized net-textured sulphide ore with 1.5% Cu and 0.7% Ni.…”