Composition of tetrahedrite-tennantite and "schwazite" in the Schwaz silver mines, North Tyrol, Austria

Universitaet Bern, Mineralogisch-petrographisches Institut, Bern, Switzerland

The hydrothermal fahlore deposits of the Schwaz-Brixlegg district have been mined for silver and copper over many centuries and are famous as the type locality of the mercurian fahlore variety "schwazite". The ore is dominantly monomineralic fahlore and occurs as stratabound, discordant vein, and breccia bodies over a 20 km belt hosted mostly by the Devonian Schwaz Dolomite. The structural style of the mineralization is similar to that of Mississippi Valley type deposits. This study presents the first electron microprobe analyses of the ores and reveals wide variations in fahlore compositions, from 35 to 100 wt.% tetrahedrite end-member in the solid solution series with tennantite. Sb and Zn contents vary between 12.1-28.0 wt.% and 0.1-7.6 wt.%, respectively. Silver contents average 0.5 wt.% and range up to 2.0 wt.%. In the breccia-hosted ores these variations clearly result from a temporal evolution in the ore-forming hydrothermal system: main-stage tetrahedrite is replaced by assemblages of Sb-, Fe-, and Ag-enriched tetrahedrite + enargite, with minor sphalerite+ or -stibnite+ or -cuprian pyrite (< or =25 wt.% Cu). These reactions are deduced to result from either increases in aqueous sulphur activity or falling temperature. Earlier workers recognized strong geographic zonation of fahlore compositions, but our microprobe analyses refute these contentions. The 1167 new microprobe analyses of 51 fahlore samples collected underground or obtained from museum collections yield an average Hg content of 1.8 wt%, and a maximum of 9.4 wt.%. According to modern nomenclature, not even the highest Hg value qualifies as "schwazite". Moreover, it appears that the original and only analysis of "schwazite", reporting 15.6 wt.% Hg (Weidenbusch, 1849), was erroneously performed on a polymineralic aggregate, rather than on a monomineralic fahlore. We conclude that the Schwaz-Brixlegg fahlores are in fact not unusually rich in mercury, and that in all probability there is not, and never has been, any "schwazite" at Schwaz.

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