The geology, mineralisation (Ni-Cu-PGE) and isotope systematics of Caledonian mafic intrusions near Huntly N.E. Scotland
The Caledonian mafic and ultramafic intrusions of the Grampian region of N.E. Scotland are a suite of synorogenic tholeiitic plutons of mid-Ordovician Age. They include layered cumulates, granular gabbronorites, quartz biotite norites and xenolithic contact facies lithologies. They postdate two regional deformation events in the enclosing Late Precambrian Dalradian metasediments, but are themselves locally deformed by a major regional ductile shear zone system. A detailed study of the Huntly-Knock area was undertaken combining geological mapping, petrological, geochemical and stable isotope techniques. In the study area, layered peridotitic to gabbroic cumulates, transitional cumulate types, granular gabbronorites quartz biotite norites and complex xenolithic contact facies rock types are present as a series of disrupted bodies formed by multiple intrusive events and subsequent deformation of a laccolithic and sheeted intrusive complex. Progressive cryptic fractionation trends are observed from basal peridotites to quartz biotite norites in the 'roof' of the intrusion. The chemistry and mineralogy of the rocks places them in the Lower and Middle Zone of the regional Younger Basic 'stratigraphy', although isolated pockets of Upper Zone may occur. Fine grained disseminated Fe-Ni-Cu sulphides are widespread throughout the mafic and ultramafic rock types. Richer sulphide concentrations locally occur as: gabbronorite hosted disseminated to massive bodies in the structurally complex, Littlemill-Auchencrieve contact zone; disseminated horizons within cumulates; disseminated to submassive graphite-rich pods in pyroxenitic pegmatites. The sulphide assemblage is dominated by pyrrhotite with minor pentlandite and chalcopyrite. Sulphide textures are attributed to magmatic processes with local modification by ductile deformation and hydrothermal reworking. Field, textural and Cu/Cu+Ni relations of certain submassive-massive sulphides is consistent with their derivation from an ultramafic parent. Maximum Ni and Cu levels are 3.02% and 6.46% respectively. The highest combined Pt+Pd+Au values occur in remobilised net sulphide (574ppb) and graphitic pyroxenite (700ppb). These metal values are generally low and comparable to other orogenic Caledonian intrusions. Sulphide immiscibility occurred many times during cooling of the tholeiitic parent magma(s), however early sulphide melts are generally of most economic importance. While there is abundant evidence for magma/country rock interaction, only locally is there evidence for involvement of metasediment sulphur, the system being dominated by a magmatic signature. In the Littlemill-Auchencrieve contact zone, crustal involvement may have been the principal factor controlling sulphide immiscibility. Subsequent hydrothermal reworking within ductile shear zones under amphibolite facies metamorphic conditions modified metal values. Depletion, especially of Au, Pt and Pd was mainly observed but local significant zones of enrichment may also be present.