TECTONIC STYLES OF EARLY PROTEROZOIC ORE DEPOSITION IN THE FENNOSCANDIAN SHIELD

The Early Proterozoic metallogenic events of the Fennoscandian Shield are connected with two major stages of crustal evolution: the Karelian continental development (2.5-2.1 Ga) and the Svecofennian orogeny (2.0-1.75 Ga). The Karelian continental development was dominated by deposition of terrestrial sediments and tholeiitic volcanism on Archaean crust. A tensional regime, probably associated with a mantle plume and partial melting of the upper mantle, culminated in the generation of an ENE-WSW-trending rift zone which was intruded by tholeiitic magma 2.44 Ga ago. Some 15 layered igneous bodies, hosting Cr, PGE, Ni-Cu and Fe-V-Ti deposits are known in the Kemi-Suhanko and Koillismaa belts. These mafic-ultramafic bodies originally formed either several lopoliths or one single layered igneous complex with a minimum length of 260 km. NW-SE- and NNW-SSE-trending dolerite dykes intruded the rifted and attenuated Archaean crust 2.25-2.0 Ga ago preceding ocean opening. The coeval NE-SW-trending dolerite dykes in the northern part of the shield are interpreted as manifestation of an aulacogen, the failed arm of a triple-armed rift system. Oceanic crust is preserved in 1.96-Ga-old ophiolite complexes. The Cu, Co, Zn, Ni and Cr contents of the Outokumpu-type deposits derive from hydrothermal leaching of oceanic crust and were discharged, together with chemogenic cherts and carbonates, from ascending hydrothermal fluids on the seafloor above fissure vents. In the northern part of the shield, rifting and probably ocean opening resulted in the formation of the layered tholeiitic-sedimentary complex of the Petsenga Ni-Cu district, intruded by mineralized mafic-ultramafic magma. The age of this event is poorly constrained. The basin was subsequently closed and continental collision of the Kola Suture was accompanied by southward obduction of the Lapland Granulite Belt on the Archaean crust. During the Svecofennian orogeny the divergent continental margin was converted into an active margin. Early subduction of oceanic crust produced oceanic island arcs intruded by 1.93-1.9-Ga-old tonalites and trondhjemites. Subsequent subduction of oceanic crust generated 1.9-1.87-Ga-old continental island arcs associated with massive sulphide Cu-Zn-Pb-Ag-Au deposits. The island arcs were accreted to the Archaean crust in a concave belt containing the ore districts of Kiruna, Skellefte, Vihanti-Pyhäsalmi, Aijala-Orijärvi and Bergslagen. Bimodal volcanic suites, with felsic calc-alkaline and mafic tholeiitic volcanites, are attributed to rifting of the overriding plate above subduction zones. Ni-Cu deposits in synorogenic, 1.9-1.87-Ga-old, mafic-ultramafic intrusions occur in linear, fault-controlled belts at the margin of the Archaean crust. The generation of Ni-bearing tholeiitic to komatiitic magma is explained by partial melting of the mantle wedge above the early subduction zone, magma differentiation and magma upwelling along transcurrent faults on the continental margin. Sulphur was probably assimilated from the supracrustal wall rocks. 1.89-1.87-Ga-old Early Svecofennian granitoids are associated with porphyry-type Cu ± Mo ± Au deposits. Widespread Late Svecofennian S-type (1.84-1.75 Ga) granitic magmatism generated minor Mo, U, W, Sn and REE deposits. Emphasis is laid on the importance of geotectonic models as a basis for further exploration in the Fennoscandian Shield. © 1990.