Showing papers by "Geological Survey of Sweden published in 1993"

Early Caledonian tectonothermal evolution in outboard terranes, central Scandinavian Caledonides: new constraints from U-Pb zircon dates

Abstract: Early Caledonian deformation in an outboard terrane (Storfjallet) in the central Scandinavian Caledonides is constrained to the time-period Arenig to Caradoc on thebasis of U-Pb zircon age determinations. A trondhjemite clast in a conglomerate belonging to a supracrustal sequence that was affected by the early deformational event yields an age of 489 +10 –5 Ma. A granite intruded after this event defines an age of 445 +24 –6 Ma. The clast is similar in age to a trondhjemite intrusion in a contiguous terrane (Gjersvik) dated at 483 +5 –3 Ma and the granite age is in agreement with a Rb–Sr whole-rock isochron of 438 ± 6 Ma determined in a previous study. The three new ages provide further evidence for the separation of igneous events in outboard terranes in the Scandinavian Caledonides into older (Tremadoc to Arenig) and younger (Late Ordovician to Early Silurian) episodes. More critically, they argue against simple correlation of early Caledonian deformational events in outboard terranes with the Late Cambrian to Early Ordovician tectonometamorphic history along the margin of the continent Baltica (the so-called Finnmarkian orogenic phase). This age disparity provides support for significant spatial separation of the Storfjallet terrane from Baltica during the early–mid Ordovician and an influence of the Taconic orogenic event on this outboard terrane is proposed.

A fluid inclusion and light element stable isotope study of the gold-bearing quartz vein system, Falun, Sweden

Abstract: The Falun gold quartz vein mineralization is located ca 230 km NW of Stockholm, Sweden, within the Early Proterozoic volcano-sedimentary sequence of Bergslagen. The mineralization consists of a system with subparallel quartz veins that crosscut the alteration zone to the Falun massive sulphide deposit. Early barren and late gold-bearing quartz veins follow tectonic structures postdating the formation of the massive sulphide ore. Both generations of veins are epigenetic to the massive sulphide ore and were formed by hydrothermal processes. Fluid inclusion study of the gold-bearing quartz veins indicates a low-moderately saline fluid (0.3 to 17.4 equiv wt% NaCl). Heterogeneous trapping is indicated by coexisting inclusions showing a variable CO2 content from 100% CO2 ± CH4 to 100% aqueous fluid. Temperatures of total homogenization also show a wide spread from 116–350°C with a slightly bimodal distribution with peaks at ca 180°C and 280°C. MeasuredδD values — 69 to — 63%0 (SMOW), of inclusion fluid and calculatedδ 18O values of hydrothermal fluids — 7.5 to — 1.4%0 (SMOW), strongly suggest a meteoric origin for the fluids. The quite consistentδD values and the range inδ 18O values indicate that major water-rock interaction led to the evolution inδ18O of the hydrothermal fluids.

Source of metals and age constraints for epigenetic gold deposits in the Skellefte and Pohjanmaa districts, central part of the Fennoscandian Shield

Abstract: The lead isotopic compositions of galena in Early Proterozoic gold deposits have been determined for three districts in northern Sweden and central Finland. The deposits are hosted by a variety of ≈1870–1890 Ma Svecofennian host rocks including the volcanosedimentary succession within the Skellefte District island arc in Sweden as well as I-type tonalites at Jorn (Sweden) and Pohjanmaa (Finland). The deposits are epigenetic in relation to these Svecofennian rocks and are part of a goldbearing metallogenetic belt, which can be followed for 600 km parallel to the southwestern margin of the Archaean Domain. In spite of these epigenetic relationships, the lead isotopic data indicate that the deposits are not dramatically younger than the ≈1870–1890 Ma Svecofennian host rocks (probably not exceeding 10–20 million years). Two principal lead sources were activated when the gold deposits were formed. The most significant source is represented by the I-type tonalites, which constitute a relatively primitive (μ = 9.3) and widely distributed source in the entire metallogenic belt. In addition, the volcanic components in the westernmost part of the Skellefte District constitute an extremely primitive (μ <9.0) source, which only locally was an important contributor to the epigenetic deposits in this metallogenetic belt. The significantly different lead isotopic composition estimated for these sources indicates that the volcanic rocks in the western part of the Skellefte District were not comagmatic with the I-type tonalites recognized at Jorn and central Finland.