Proterozoic metallogeny of the Ladoga region in Karelia
|Location||International Geological Congress,oslo 2008|
|Author||Sundblad, Krister۱; Ivashchenko, Vasil۲; Lehtila, Tommi۱|
|Holding Date||03 September 2008|
The Karelian schist belt in the Ladoga region consists of pre-Svecofennian Palaeoproterozoic supracrustal (and intrusive) units. These units, as well as root-less slabs of Archaean crust, were emplaced as allochthonous nappes on the Archaean continent during the c. 1.85 Ga Svecokarelian orogeny, thus creating exotic terranes comparable to the much younger Caledonian and Alpine orogenies, something that was understood already by Wegmann in 1928. Two generations of stitching plutons (1.8 Ga Late Svecofennian and 1.54 Ga anorogenic rapakivi granites) intruded into the Karelian nappes. Several ore types are associated with these units.
1. Deposits in Lower-Upper Kaleva
a. Anomalous contents of Au, PGE, V and other metals occur at several sites in the Soanlahti black schists.
b. Ag-Pb occurrences, associated with silicified marbles at Sarka and Vuohenlampi, represent a new mineralization type of uncertain extension, age and genesis.
c. The Hammaslahti Cu-Zn-bearing VMS deposit occurs in gabbro-intruded meta-turbidites of the Höytiäinen basin (Pälkjärvi unit). Comparison of this deposit type with VMS deposits in similar settings in the Scandinavian Caledonides (e.g. at Røros and Ankarvattnet) suggest that they are ancient analogues to the Escanaba type of sulphide deposits in the northeastern parts of the Pacific.
d. The 28 Mt Outokumpu Cu-Zn deposit was mined during the 20th century and is hosted by serpentinites, amphibolites and black schists, a rock sequence that can be correlated with the 1.95 Ga Jormua ophiolite. This geological setting may suggest a Cyprus type, but an origin related to black schists have also been suggested.
e. The black schists are also hosts for the 336 Mt low-grade Ni-Zn-Cu-Co ore at Talvivaara.
2. Structurally controlled deposits
a. Orogenic Au-quartz-carbonate veins have been recognized in Archaean rocks at Jalonvaara and in several Palaeoproterozoic rock types (gabbro-dolerite at Roikonkoski and tonalite at Alattu). The ore-forming structures were formed during the Svecokarelian orogeny and are connected with the Au-bearing structures in the Archaean Hattu schist belt in the Ilomantsi region.
b. U occurs as unconformity deposits at the base of the Jotnian sandstones at Karku and as Proterozoic shear/fracture-controlled deposits along the Archaean-Proterozoic boundary (Paukkajanvaara).
3. Stitching plutons
a. W-bearing exoskarn deposits (e.g. Kesälahti and Latvasyrjä) occur frequently in marble horizons east of the 1.80 Ga Pyhäjärvi granite and were formed from fluids emanating from km-sized granitic plutons of the same age.
b. Numerous exoskarn deposits, related to the 1.54 Ga Salmi batholith, were mined for Fe, Cu, Zn, Pb and Ag during the 19th and 20th centuries at Pitkäranta. Although these deposits to a large extent already are exhausted, the high contents of iron, base metals and rare elements (e.g. Ag and In) encourage exploration of polymetallic ores also elsewhere in the rapakivi regions, both in Russia and Finland.