Composition of tetrahedrite-tennantite and 'schwazite' in the Schwaz silver mines, north Tyrol, Austria

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 dominant ly monomineralic fahlore and occurs as stratabound, discordant vein, and br eccia bodies over a 20 km belt hosted mostly by the Devonian Schwaz Dolomit e. The structural style of the mineralization is similar to that of Mississ ippi 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.% tetrah edrite end-member in the solid solution series with tennantite. Sb and Zn c ontents 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 o res these variations clearly result from a temporal evolution in the ore-fo rming hydrothermal system: main-stage tetrahedrite is replaced by assemblag es of Sb-, Fe-, and Ag-enriched tetrahedrite + enargite, with minor sphaler ite + stibnite +/- cuprian pyrite (less than or equal to 25 wt.% Cu). These reactions are deduced to result from either increases in aqueous sulphur a ctivity or falling temperature. Earlier workers recognized strong geographi c zonation of fahlore compositions, but our microprobe analyses refute thes e contentions. The 1167 new microprobe analyses of 51 fahlore samples collected undergroun d or obtained from museum collections yield an average Hg content of 1.8 wt .%, and a maximum of 9.4 wt.%. According to modem nomenclature, not even th e highest Hg value qualifies as 'schwazite'. Moreover, it appears that the original and only analysis of 'schwazite', reporting 15.6 wt.% Hg (Weidenbu sch, 1849), was erroneously performed on a polymineralic aggregate, rather than on a monomineralic fahlore. We conclude that the Schwaz-Brixlegg fahlo res are in fact not unusually rich in mercury, and that in all probability there is not, and never has been, any 'schwazite' at Schwaz.