Scandinavian Caledonide Metallogeny in a plate tectonic perspective

Abstract

Caledonian metallogeny has endowed Scandinavia with abundant metalliferous mineral resources of several genetical and compositional types, many of which have been exploited at various scales from the seventeenth century onwards. Because of this long history of exploration/exploitation, coupled with excellent exposures, the Caledonian orogen in Scandinavia can be taken as a model to illustrate the relationship between metallogenic evolution and plate tectonics. Its orogenic and metallogenic development can be related to a period of plate movements which started about 700–600 Ma ago with rifting and break-up of a Proterozoic megacontinent (Rodinia) followed by the opening-up of a wide ocean (Iapetus), and ending with collision between the Laurentian and Baltic continents (the climactic Scandian phase of orogeny) in Silurian times. Fragments of this sedimentary, magmatic and tectonic history are recorded in various autochthonous, and allochthonous units which were ultimately thrusted eastwards above the Baltic plate margin.

The present consensus is that sequences deposited on the margin of the Laurentian plate are represented in the Uppermost Allochthon (UmA) mostly in northern Norway. The Neoproterozoic and Cambrian history of this margin started with the rifting of Rodinia, followed by the development of an Atlantic-type margin which was characterised by an easterly thickening, continental, shelf-wedge of basal quartz sandstones and an overlying blanket of thick carbonate banks reflecting the drift of Laurentia towards equatorial paleo-latitudes. This setting is considered to have been host to two characteristic types of stratabound-stratiform ores: massive to disseminated Zn-Pb-Cu and Cu-Zn sulphides in sedimentary and mixed sedimentary-volcanic lithologies with original ore reserves up to 5–6 Mt (Bleikvassli, Mofjellet); and numerous, laterally extensive, magnetite-hematite deposits in marble-metapelite lithologies. The economically most important of the latter group are the deposits of the Dunderlandsdal area where resources of around 500 Mt were present.

Early rift basins preserved on the Baltic side of the rifted megacontinent, were filled with coarse clastites prior to establishment of a passive plate margin in Cambrian times. The passive margin was characterised by shallow marine sandstones and later bituminous Alum Shales deposited in a stable, epicontinental sea. Platform sedimentation was accompanied by local magmatic activity of rift-type tholeiitic through to carbonatitic compositions. This plate margin is presently represented in autochthonous and allochthonous sequences in the lower parts of the Caledonian nappe pile. Metalliferous mineral formation related to this time and setting was of limited importance and comprised previously worked Nb, P and Fe ores in carbonatites of the Fen Complex, vast but uneconomic resources of U, Mo, Ni and V in the Alum Shales, and minor stratabound base metal sulphides and orthomagmatic Cr and Ni-Cu-(PGE) occurrences.

Continental margin and oceanic successions, probably developed along the edge of a microcontinent within the Iapetus ocean, are represented in the Gula, Støren and equivalent, sequences in the Upper Allochthon (UA), tectonostratigraphically above and west of the Baltic platform lithologies. These host many Cu-Zn sulphide ores intimately related to tholeiitic basalt units in sedimentary sequences dominated by pelitic and psammitic material, as well as bituminous shales and quartzites of possible ribbon-chert origin. Original ore reserves around 20 Mt were known at the now abandoned Tverrfjell mine and the recently closed Joma deposit. Minor Cu-Ni ores occur in subvolcanic, mafic-ultramafic intrusions.

Major plate convergence is first recorded in the Middle to Late Cambrian (about 505 Ma), when subduction along the outer margin of Baltica affected rocks which are now found in or below the basal parts of the UA. The metallogenic significance of this subduction is uncertain; related magmatic and ore-forming processes have yet to be documented. The earliest subduction-related sequences of major ore-forming importance, however, are slightly younger (Early Ordovician) and are presently found in the UA and partly in the UmA.

One of these is the Stekenjokk-Fundsjø arc sequence (about 490 Ma) which forms a central unit in the mountain chain, comprising bimodal, immature arc-type volcanites and high-level felsic intrusions. The sequence formed in a primary oceanic setting outboard of the Baltica plate while this lay in an approximately east-west position opposite to, and at some distance from, the Siberian plate, and was amalgamated with the Gula Complex in Ordovician times. Abundant VMS deposits are associated with thick, often graphitic, tuffites and are of the Zn-Cu type with generally high Zn/Cu ratios. The biggest known deposit (Stekenjokk-Levi) contained 26 Mt of ore, four others had individual tonnages of about 3 Mt and have been important base metal producers.

Another series of arc sequences are found at higher levels in the nappe pile, in southwestern Norway, the western Trondheim Region, the western Grong District, Leka and Lyngen. Mafic or bimodal volcanic and plutonic lithologies comprise many types characteristic of immature arcs and often include ophiolitic successions. They all formed in oceanic arc-marginal basin systems on the Laurentian side of the Iapetus ocean between 500 and 480 Ma. VMS deposits of the Cu-Zn type are abundant and several of these were important base metal producers in the past. The biggest deposit is Løkken which contained 30 Mt of ore; six fall in the 1–10 Mt class and include the Skorovas and other deposits in the western Grong District and in southwest Norway. Orthomagmatic ores occur in the form of PGE mineralisation in ultramafic cumulates in the basal parts of ophiolite successions and Cu-Ni-PGE mineralisation related to high-level intrusions of boninitic affinity; none of these have hitherto been of economic significance.

The immature arc-marginal basin systems near Laurentia continued to evolve and in Early to Middle Ordovician times magmatism gradually changed to predominantly calc-alkaline igneous activity characteristic of an active continental margin. Major VMS ores seem to be scarce in these sequences which are most extensively developed in central and southwest Norway. Known metalliferous deposits are predominantly iron-formations ranging from magnetite- to pyrite-dominated types with varying base-metal contents; Zn-Pb-Cu mineralisation is subordinate. Major granodioritic plutons which intruded the mature-arc sequences at an advanced stage of development (c.460 Ma) occasionally contain vein- and stockwork-type Cu-Mo mineralisation, so far with no economic significance.

By the end of the Ordovician, convergence of the Laurentian and Baltic continents had resulted in considerable narrowing of the Iapetus ocean and obduction of the arc/arc-basin systems on to the edge of the Laurentian plate. Material derived from the uplifted sequences supplied detritus to the remaining, narrow, basin. At about the Ordovician/Silurian boundary these thick clastic, often calcareous, sequences and their crustal substrata were intruded by rift-type, mainly tholeiitic to alkaline magmas and minor subduction-related melts; local volcanism also accompanied this activity. This important magmatic event is interpreted to reflect a paleotectonic setting characterised by transcurrent movements and development of local transtensional regimes and fault-controlled sedimentary basins during the initial, oblique, interference of the Baltic and Laurentian plate margins. Its metallogenic manifestation is seen partly in occasional Ni-Cu deposits in mafic intrusions, including the Bruvann, Råna, deposit presently being exploited. More abundant, however, are stratabound sulphides which are associated with the volcanic rocks, or hosted in the sedimentary successions often closely related to mafic intrusions. The most important past-producers were found in the Røros and Sulitjelma districts, and included several individual deposits ranging from 1 up to nearly 9 Mt in size and dominated by Cu and Zn in highly variable proportions and grades.

Partly overlapping in time with the rift-related magmatism, an array of large, mafic to granitic batholiths was emplaced in rock sequences which are presently found in the Uppermost and Upper Allochthons. This activity continued through the Late Ordovician and Early Silurian, presumably along the Laurentian plate margin in response to westward subduction of the edge of Baltica. Its metallogenic significance is limited to small and uneconomic skarn-, pegmatite- and quartz-vein mineralisation of Zn-Pb-Cu, Mo, W and Be.

The ultimate collision between Laurentia and Baltica occurred during the Silurian, when the accretionary prism with granitic batholiths was obducted onto the Baltic margin. The obduction and subsequent uplift of the subducted edge of Baltica led to the formation of intramontane ORS basins in the Early and Middle Devonian. The Caledonian orogeny terminated with the deformation of the ORS basins in the Late Devonian.

The collisional stage is metallogenically represented by three types of deposits, i.e. lead sandstone deposits (Pb, Zn, Ba, F and Cu), carbonate-hosted base-metal deposits, and vein deposits with variable proportions of Au, Ag, Pb, Cu, Zn, Fe, As, Sb, W, Mo and/or U. The first type, which comprises deposits with up to 5.0 Mt lead metal (Laisvall) is by far the most important economically. The vein deposits have in the past been the site for small-scale mining of precious metals and molybdenite. Although the exact timing of the different ore-forming events is poorly constrained, all types seem related to a continuous process of tectonically induced fluid flow during episodes of crustal shortening and uplift. Brief comparisons with other parts of the now-rifted Caledonian-Appalachian orogen reveal both similarities and differences in the relative importance of the ore types generated during the time span covered in the present account.