Abstract

The Zinkgruvan stratiform, massive Zn-Pb deposit is located in the southern Bergslagen province, south-central Sweden. This lower Proterozoic, 1.9-Ga ore province hosts massive Zn-Pb-Cu-Ag sulfide ores and banded iron-formations in volcano-sedimentary complexes, consisting of felsic metatuffites mixed with chemical precipitates. These were folded and metamorphosed to upper amphibolite facies during the Svecofennian orogeny.The Zinkgruvan ore deposit is hosted by K-rich metatuffites with intercalated beds of marble, dolomite, and fine-grained quartzite.A stratified disseminated pyrrhotite mineralization occurs 100 m stratigraphically above the Zn-Pb ore zone. The Zinkgruvan ore deposit has a distinctive stratification and a large lateral extent (5 km along the strike, known to a depth of 1,300 m) but is of no great thickness (<20 m). It contains more than 40 million metric tons of crude Zn-Pb-Ag ore.In the central part of the mine the ore zone is underlain by a substratiform Cu stringer mineralization from which a pronounced lateral zonation occurs, Cu-Zn-Pb-(Fe).A huge rock volume of the stratigraphic footwall is altered by hydrothermal solutions to a quartz-microcline rock, enriched in K, Ba, and some Si, but depleted in Na, Fe, Mg, and Mn.The distribution of the ore indicates that the hydrothermal solutions were dense and formed brine pools on the sea floor. The brines were rich in Cl, Si, K, Ca, Mn, Fe, S, Zn, Pb, and precipitated chert, carbonates, sulfides, and some iron oxide. Mg and possibly CO 2 could have been provided by the sea water. It is suggested that the solutions had a pH close to neutral and that the contemporary metatuffites received their high K/Na ratios when they were deposited in and reacted with the brines.The nature of the metasediments suggests that the ore formation took place in a subsiding marine basin at the end of a volcanic period, distal to volcanic centers. The hydrothermal solutions were generated by convective circulation of sea water in the volcanic rock pile. It is supposed that the deep fault fractures initiated the convection cell, which was active for a long period of time and formed, in a 1,000-m-thick rock pile, many vent areas and more than 50 mineralized thin beds.The Zinkgruvan ore deposit with its distal volcanic position and other characteristics seems to take a position in between the classification of massive, volcanogenic sulfide deposits and sediment hosted exhalative Zn-Pb deposits.

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