The chemical composition of tetrahedrite-tennantite ores from the prehistoric and historic Schwaz and Brixlegg mining areas (North Tyrol, Austria)
Krismer, Matthias; Vavtar, Franz; Tropper, Peter; Kaindl, Reinhard; Sartory, Bernhard
European Journal of Mineralogy Volume 23 Number 6 (2011), p. 925 - 936
published: Dec 1, 2011
ArtNo. ESP147052306008, Price: 29.00 €
The copper ore occurrences from Schwaz and Brixlegg were mined both in prehistoric and historic times. The ores occur as discordant veins, strata-bound bodies and breccias in the Devonian Schwaz Dolomite of the Northern Greywacke Zone. The ore mineral assemblage is characterized by more or less monomineralic Fe-Zn-Hg-bearing tetrahedrite-tennantite. Especially in the Großkogel mining area in Brixlegg, barite is associated with the tetrahedrite-tennantite ores. To characterize the compositional variations of tetrahedrite-tennantite a comprehensive electron microprobe investigation involving the analysis of 21 elements was conducted. The mining areas of Schwaz and Brixlegg can be distinguished by their chemical and mineralogical compositions. Whereas fahlore from Schwaz is a strongly zoned Fe-Zn tetrahedrite-tennantite solid solution (10.4 % to 59.8 % tennantite component, 21.8-71.8 % Zn end-member) with high Hg (≤5.56 wt%) and low Ag (≤0.80 wt%) concentrations, fahlore from Brixlegg, also strongly zoned, is generally richer in Zn (47.8-97.1 % Zn end-member) and poorer in Hg (≤2.11 wt%) with comparable low Ag concentrations. The mineralogical differences are basically due to the tetrahedrite-tennantite breakdown textures in ores from Brixlegg leading to the assemblage enargite/luzonite-famatinite þ sphalerite þ pyrite þ stibnite þ chalcostibite, or fahlore (second generation) þ enargite/luzonite-famatinite þ sphalerite þ pyrite þ stibnite þ chalcostibite and fahlore (second generation) þ sphalerite þ pyrite þ stibnite þ chalcostibite. The following model reactions explaining these reaction assemblages can be proposed: (1) Cu10Fe2Sb4S13 + Cu10Fe2As4S13 + 13/4S2 → 1/2Cu10Fe2Sb4S13 + 1/4Cu10Fe2As4S13 + 3Cu3AsS4 + Cu3SbS4 + 1/2CuSbS2 + 1/4Sb2S3 + 5/2FeS2 (2) Cu10Fe2Sb4S13 + Cu10Fe2As4S13 + 5S2 → 3Cu3AsS4 + 2Cu3SbS4 + 2CuSbS2 + 4FeS2 (3) Cu10Zn2Sb4S13 + Cu10Zn2As4S13 + 2S2 → 1/2Cu10Zn2Sb4S13 + 1/4Cu10Zn2As4S13 + 3Cu3AsS4 + Cu3SbS4 + 1/2CuSbS2 + 1/4Sb2S3 + 5/2ZnS (4) Cu10Zn2Sb4S13 + Cu10Zn2As4S13 + 3S2 → 4Cu3AsS4 + 2Cu3SbS4 + 2CuSbS2 + 4ZnS Thermodynamic modelling of these breakdown reaction textures shows that either local-scale variations in sulphur fugacity, fS2, and/or decreasing temperature, T, result in the breakdown of the fahlores. The presented chemical and mineralogical data were subsequently compared to ore-fragments, copper slags and raw copper from mining archaeological excavations at the Kiechlberg Early Bronze Age copper smelting site near Innsbruck (Tirol, Austria). Excavated ore fragments contain tetrahedrite-tennantite with similar Fe-Zn-Hg concentrations and breakdown textures as described from the Brixlegg mining district. Several raw copper fragments also yielded similar high Sb-, As- and Ag-concentrations. These observations indicate 1) the use of fahlore-copper smelting and 2) local-scale import of copper-ore from Brixlegg.