Some aspects of the silver mineralization in the Hällefors region (Bergslagen, Sweden)

Abstract

ABSTRACT. Sulphide and sulphosalt mineralization in the H/illefors silver mining district occurs as stratabound bodies in skarn-like black carbonates and other carbonates in the eastern part of the district, and as epigenetic, tectonically controlled vein mineralization in tuffaceous metavolcanics and, to a lesser extent in slates, in the western part.Previous work has suggested that the Ag-bearing Hfillefors deposits were formed by the action of metamorphic solutions on sediments of volcano-sedimentary origin. At the stages of sulphosalt and sulphide formation, temperature and pressure conditions were probably in the ranges 500-440 K and 4.5-3.5 kbar. Examination of a logas-1/T diagram and of Ag-Sb phase relations combined with study of the ores suggest the paragenesis of the Ag-bearing minerals is as follows: allargentum and dyscrasite as exsolution bodies in silver bearing galena; Ag-containing tetrahedrite --. freibergite ~ miargyrite pyrargyrite ~ stephanite (from hydrothermal solutions).

Conditions of formation of the iron-containing minerals, Hällefors silver mines, Bergslagen, Sweden

“…Descriptions of the mineralization and the paragenetic sequence may be found in Nugteren (1978), Nugteren and Zakrzewski (1984), Sundius et al (1966), andJasifiski (1983).…”

“…In a previous paper (Jasifiski, 1983) calculations were made using pressure values of up to 5 kbar. Recalculations using the more realistic pressures mentioned above resulted in the same mineral sequence, suggesting that pressure does not significantly influence the mineral equilibria.…”

“…Thermodynamic calculations were made using the data of Robie et al (1978), Craig and Barton (1973), Helgeson et al (1978), Vaughan and Craig (1978), Garrels and Christ (1965), Table I. AGr, log as2 and log ao2 were calculated in the same way as in Jasifiski (1983). In order to estimate the lower stability limit of an aqueous fluid on log as~-logao2 diagrams, the following assumptions were made: 1.…”

“…Samples were examined by means of a microscope and an electron-microprobe (Nugteren, 1978;Nugteren and Zakrzewski, 1984;Jasifiski, 1983), and chemically analysed by Boliden Metal AB.…”

“…The lithologies and age determinations were given by Sundius (1923), Welin et al (1981), and Moorman et al (1982). The ore deposits of the H/illefors silver mines are restricted chiefly, but not exclusively, to felsic pyroclastic rocks (Jasifiski, 1983).…”