Time and space are perhaps the two most critical reference points from which ore deposits are described and understood. Timing is not simply an instantaneous point in geologic time, but must take into account the multiplicity and duration of events that form ore deposits. In reality, timing marks the turning on and turning off of paralleled, staggered, or sequenced processes. In this paper, we present Re-Os ages for molybdenite that strongly suggest a superposition in space of two temporally and genetically distinct ore-forming events at Pitkäranta, Russia. The first event (W-Mo) is related to Early Proterozoic orogenic magmatism at about 1800 Ma, and the second event (Sn-Zn-Pb-Cu) is related to Late Proterozoic anorogenic magmatism at about 1540 Ma. Both events formed skarn-related ore deposits that overlap in geologic space. As a result, all metals were previously assumed to be consanguineous and derived from the adjacent 1543 Ma Salmi batholith.
The Sn-Zn-Pb-Cu (W-Mo) skarn deposits at Pitkäranta in westernmost Russia have been periodically mined for more than a century. Located at the eastern end of the Svecofennian Domain along the northeast margin of Lake Ladoga, these deposits are among the most well studied contact-metamorphic ores in Fennoscandia. Notably, Törnebohm (1891), Trüstedt (1907), Saksela (1951), and Eskola (1951) provided some of the early contributions on the district. The Pitkäranta ores are hosted in a 1.9 Ga supracrustal sequence which overlies and outcrops as rings or mantles around 2.7 Ga basement gneiss domes. This supracrustal sequence was subjected to Early Proterozoic metamorphism associated with the Svecofennian orogeny (ª1.85-1.83 Ga). Emplacement of Late Svecofennian granites followed, though only their associated pegmatites are exposed in the Pitkäranta region. Along the northwest margin of Lake Ladoga in the Sortavala region, some 50 km west of Pitkäranta, W-Mo skarns, genetically related to the emplacement of Late Svecofennian granites, are common (Khazov and Ivashchenko, 1981; Ivashchenko, 1987). A U-Pb zircon age for the Late Svecofennian Puurivesi granite, located 50 km west of Sortavala and 100 km west of Pitkäranta, on the Russian-Finnish border, yielded 1797 ± 19 Ma (Huhma, 1986). The most striking feature in the Pitkäranta area is the large 1543 ± 8 Ma (U-Pb, zircon) Salmi batholith, an anorogenic rapakivi granite characteristic of this region of Russia and southern Finland (Neymark et al., 1994). The Pitkäranta ores are located almost entirely within the ~1.9 Ga supracrustal rocks, but they are spatially associated with the western margin of the Salmi batholith. A Sm-Nd isochron for the skarn assemblage of garnet, pyroxene, and whole rock yielded an age of 1546 ± 20 Ma and a Sm-Nd isochron for fluorite, topaz, oligoclase, muscovite, protolithionite, and whole rock yielded an age of 1491 ± 42 Ma (Amelin et al., 1991). The genesis of the Pitkäranta ores, therefore, has been attributed to the intrusion of the Salmi batholith.
We present Re-Os ages for molybdenite that indicate the presence of Svecofennian-related W-Mo ores as far east as the Pitkäranta region of Russia. Economically important, Late Svecofennian W-Mo deposits are well known and studied in
the Bergslagen area of Sweden at the western margin of
the Svecofennian Domain, but these deposits have not been recognized nor temporally distinguished in the Pitkäranta area. We also obtained ~1800 Ma Re-Os ages for the Wigström
W-Mo skarn deposit in the Bergslagen district of Sweden.
The Re-Os data for Pitkäranta, in triplicate based on three size fractions from the Kaivos Schwartz mine, are presented in the table below. The younger age is based on a revised 187Re decay constant (Smoliar et al., 1996).
Amelin, Y., Belyaev, A., Larin, A., Neymark, L., & Stepanov, K., Geol. Surv. Finland Guide, 33, 57 pp (1991).
Eskola, P., Ann. Acad. Sci. Fennicæ, Series A, III 27, 90 pp (1951).
Huhma, H., Geol. Surv. Finland Bull., 337, 52p p (1986).
Khazov, R.A., & Ivashchenko, V.I., Akad. Nauk. USSR Pr. Geol. Ser., 1, 96-106 (1981).
Ivashchenko, V.I., Nauka, Leningrad, 240 pp (1987).
Neymark, L.A., Amelin, Yu.V. & Larin, A.M., Mineral. and Petrol., 50, 173-193 (1994).
Saksela, M., Geol. Surv. Finland Bull., 154, 181-231 (1951).
Smoliar, M.I., Walker, R.J., & Morgan, J.W., Science (in press) (1996).
Törnebohm, A.E., Geol. Fören. Stockholm Förh., 13, 313-334 (1891).
Trüstedt, O., Geol. Surv. Finland Bull., 19, 333 pp (1907).
Sample, Run # Grain Size Re, ppm 187Os, ppb common Os, ppb Age, Ma Age, Ma
SW93-PK4 (M-55) mm pieces 169.9 (3) 3240 (8) 1.6 1822 ± 5 1795 ± 5
SW93-PK4 (M-58) -40+60 mesh 176.2 (2) 3348 (8) 4.4 1816 ± 5 1789 ± 5
SW93-PK4 (M-59) -60 mesh 183.5 (2) 3489 (8) 1.4 1817 ± 5 1790 ± 5