Origin of the Kleva Ni-Cu sulphide mineralisation in Småland, southeast Sweden

Sammanfattning: The Kleva Ni-Cu sulphide deposit is situated within a gabbro-diorite intrusive complex in southeast Sweden. The basement north of the intrusive complex is dominated by 1.81–1.77 Ga granites of the Palaeoproterozoic Transscandinavian Igneous Belt (TIB). Slightly older (1.83–1.82 Ga) rocks of the Oskarshamn Jönköping Belt, which hosts numerous syngenetic and epigenetic base metal mineralisations, occur just south of the Kleva intrusive complex. The aim of this PhD-thesis is to deduce the origin of the Kleva deposit, the mineralisation itself as well as its host rocks through geochemical, geochronological and petrological studies.
U-Pb age determination of zircon dates igneous crystallisation to 1.79 Ga, which is the age of the Kleva intrusive complex and confirms its temporal association with the voluminous TIB magmatism. Major- and trace element systematics are in accordance with a basaltic magma that formed through partial melting of a metasomatically refertilised mantle wedge underneath an Andean-type continental magmatic arc. Lu-Hf signatures of zircon, together with other rocks of Palaeoproterozoic Fennoscandia indicate alternating stages of extension and compression across the subduction zone, facilitating ascent of the mafic magma. Evidence for contamination of the magma through crustal assimilation during its ascent are inconclusive. Low IPGE/Ni together with high S/Se, indicate sulphide melt saturation prior to final emplacement, possibly induced by crustal contamination. Nb/La vs La/Sm indicate contamination with mid-crustal rocks, and radiogenic Os of magmatic pyrite suggests <10% contamination with Archean crust. OJB aged rocks are thus unlikely contaminants, despite the numerous rock inclusions of similar geochemical composition within the intrusive complex. δ34S of Kleva mineralised rocks and the country rocks corresponds with the mantle range, and local or mantle origin of S can neither be proven nor rejected.
Sulphide melt segregated from an evolved magma and partially accumulated into massive lenses, which is in accordance with a magmatic conduit setting. The mineralisation contains massive, net-textured and disseminated sulphides of typical magmatic association and is interpreted to be contemporaneous with silicate melt crystallisation, consistent with a Re-Os 1.71 ±0.2 Ga isochron for massive pyrite with magmatic texture. Re-Os isochrons of secondary pyrite indicate metamorphic disturbance of the mineralisation at least twice; at c. 1.61 Ga and 1.39 Ga, which can be linked to orogenic events further to the south and west. The mineralisation was heterogeneously affected by tectonic disturbance, resulting in remobilisation of chalcopyrite into veins, plastic deformation of sulphides and host rock, micro-faulting and brittle deformation of oxides and sulphides and recrystallisation of pyrite in fractures. To summarise, the deposit is an example of a subduction related magmatic Ni-Cu mineralisation affected by multi-stage deformation and alteration.