Nickel metallogeny related to tectonics in the central baltic shield

Abstract

Nickel-copper deposits in the Central Baltic Shield were formed in four tectonic events culminating nt times 2,700-2,800 Ma, 2,440 Ma, 1,970-2,100 Ma and 1,882-1,892 Ma, during which deep-reaching crustal processes brought upper mantle material into high crustal levels: 1. Archean nickel-copper deposits are associated with tholeiitic and komatiitic rocks in a typical greenstone belt environment. The nickel bearing magma ascended in the initial stage of the greenstone-belt evolution, 2,700-2,800 Ma ago, along N-S trending faults connected with rifting of the early Archean ensialic crust. 2. Consolidated upper Archean crust fractured at the beginning of the Proterozoic era and tholeiitic magma intruded an E-W trending tensional fracture zone. Mafic layered intrusions were formed with low-grade nickel-ccpper-platinum mineralizations in their basal parts. It is assumed that an originally more coherent belt of layered intrusions was faulted and fragmented during subsequent crustal movements. 3. Fragments of oceanic crust, generated in the period from 1,970 Ma to 2,100 Ma, were added to the Proterozoic continental crust by plate tectonic processes during a compressional event 1,880-1,900 Ma ago. Low-grade nickel deposits in skarn around serpentinite bodies are interpreted as part of a dismembered ophiolitc complex. 4. Nickel-copper deposits, associated with synorogenic mafic to ultrarnafic intrusions dated 1.882-1.892 Ma. occur in long fault controlled, straight or curvilinear belts. The nickel belts are located between linear zones of gravity highs and gravity lows and run parallel to high-grade metamorphic belts. It is proposed that trondhjemitic and tholeiitic magmas were generated by subduction of the oceanic crust below the Archean continent. The intrusion of the nickel bearing tholeiitic magma was controlled by transcurrent faults trending parallel to the ancient continental margin.