Silver-bearing zinc-iron tetrahedrite-tennantite and freibergit fahlores approximating the simplified formula (Ag,Cu)10(Fe,Zn)2(As,Sb)4S13 have been equilibrated with excess electrum (AuxAg1−x) and chalcopyrite + pyrite + iron-bearing sphalerite (CuFeS2 + FeS2 + Fe0.05Zn0.95S) in evacuated silica tubes at 300 °C, in reversed silver-copper exchange experiments with less than 0.1 mg NH4Cl added as a transport medium. A thermodynamic formulation and parameters describing As-Ag incompatibility at 400 °C (Ebel and Sack, 1989), which incorporate large temperature dependencies of standard-state properties and composition-ordering systematics, are shown to apply equally well to these 300 °C results. A generalised graphical model for this mineral assemblage is presented, describing fahlore composition as a function of temperature and the compositions of coexisting electrum and (Fe,Zn)S, which define the Ag(Cu)−1 and Fe(Zn)−1 exchange properties controlling fahlore compositions.

Two types of gold can be distinguished in tetrahedrite from the Veitsch magnesite deposit, Austria. Primary gold present in unfractured massive tetrahedrite, has a grain size up to 18 μm and contains, on average, Cu 8.73, Ag 7.01, Au 78.63 and Hg 2.35 (wt.%). Secondary gold is present in fractures and is directly intergrown with digenite, covellite, Cu-thiosulphate, cuprite and chrysocolla but not with malachite or azurite. The secondary gold is up to 200 µm in size with an average composition of Cu 3.06, Ag 6.82, Au 86.41 and Hg 3.51 (wt.%). It is usually closely intergrown with thiosulphates containing up to 0.21 wt.% Au. This, together with the presence of ‘dirty’ gold with cloudy thiosulphate inclusions, directly indicates the transport and deposition of Au by a thiosulphate ligand. We believe this is the first reported direct evidence of gold transport and deposition by thiosulphate complexes in a natural environment.

The study presents analysis from members of the tennantite–tetrahedrite solid-solution series (‘fahlore’) from 78 locations in the Schwarzwald ore district of SW Germany. Electron microprobe analysis is used to correlate the compositional variations of the fahlores with mineral association, host rock, tectonic history and precipitation mechanisms. Results indicate that most fahlores from gneiss-hosted veins do not have distinctive geochemical characteristics and range from tetrahedrite to tennantite end-member composition with variable trace-element content. However, diagenetically formed fahlore has a near-end-member tennantite composition with very small trace-element content. Red-bed-hosted fahlore formed by fluid mixing is tennantite enriched in Hg that probably has its source in the red-bed sediments. Fahlore formed from granite-related late-magmatic fluids, or from mixing of fluids of which one has equilibrated with granitic basement rocks, is typically As- and Bi-rich (up to 22.2 wt.% Bi). Gneiss-hosted fahlore formed by fluid cooling is Ag-rich near-end-member tetrahedrite. Some fahlores reflect their paragenetic association, e.g. a large Ag content in association with Ag-bearing minerals or a large Co and Ni in association with Co- and Ni-arsenides.

Although they have similar compositions, gneiss-hosted fahlores show systematic variations in Ag contents and Fe/Zn ratios between the Central and the Southern Schwarzwald with Fe-rich fahlore in higher stratigraphic levels (North) and Zn- and Ag-rich fahlore in lower stratigraphic levels (South). We show that fahlore composition varies with precipitation mechanism (cooling vs. mixing vs. diagenesis), depth of formation, paragenetic association and host rock. Comparison with fahlores from other European occurrences indicates that these conclusions are consistent with fahlore systematics found elsewhere, and could be used to infer details of ore-forming processes.

Extreme silver enrichment at the surface of the complex sulphide, tennantite (ideal formula: Cu12As4S13), occurs following exposure to alkaline solutions, and involves the development of an Ag-rich sulphide surface species. The tennantite has a low bulk Ag content of 0.3 at.%, and a percentage surface enrichment of Ag is thirty-six times that of the bulk. The techniques of X-ray photoelectron spectroscopy (XPS) and reflection extended X-ray absorption fine structure spectroscopy show the new phase to be a Ag sulphide species compositionally similar to cupriferous proustite ((Cu,Ag)3AsS3). Solution experiments and XPS depth profiling show that the surface is most depleted in Cu and Zn, and enriched in Ag compared to the bulk tennantite. Selective dissolution and reprecipitation at the tennantite surface cannot explain the enrichment of Ag relative to the bulk. Migration must have occurred and could have been driven by the leaching out of Cu which produces a metal-depleted surface, coupled to the relative incompatibility of Ag in the tennantite lattice. To account for the extreme enrichment at the surface, Ag must have diffused from depths of up to 9 nm, probably via structural weaknesses and vacancies in the tennantite lattice.

Reversed brackets on As and Sb partitioning between tetrahedrite-tennantite fahlores {∼CU10(Fe,Zn)2(Sb,As)4S13} and bournonite-seligmannite solid solutions {CuPb(Sb,As)S3} (400°C evacuated silica tubes, NH4Cl flux) indicate that the maximum nonidealities associated with the As-Sb substitution are about 165 (bournonite) and 250 (fahlore) cal/gfw on a per atom exchange basis. The experimental constraints are consistent with the following calibration of the As-Sb exchange reaction between these phases:

where kcal/gfw, X2 = Zn/(Zn + Fe) (or one-half the number of Zn atoms/formula unit) and X3 = As/(As + Sb) refer to atomic ratios in fahlore, and ΔG°23 = ΔH°23 = 2.59 ± 0.14 kcal/gfw (Raabe and Sack, 1984; Sack and Loucks, 1975; O'Leary and Sack, 1987; Sack, 1992).

Natural samples containing tetrahedrite–tennantite, bournonite–seligmannite and geocronite–jordanite from the Coranda-Hondol ore deposit, Romania, were investigated by Raman spectroscopy to determine its capability to provide estimates of solid solutions in three common and widespread sulfosalt mineral series. Raman measurements were performed on extended solid solution series (Td1 to Td97, Bnn25 to Bnn93 and Gcn24 to Gcn67, apfu). The tetrahedrite–tennantite and bournonite–seligmannite solid solution series show strong correlations between spectroscopic parameters ( position, relative intensity and shape of the Raman bands) and the Sb/(Sb+As) content ratio, while Raman spectra of geocronite–jordanite shows no evolution of Raman bands. In order to simplify the method used to estimate the Sb/(Sb+As) content ratio in tetrahedrite–tennantite and bournonite–seligmannite series, several linear equations of the first-order polynomial fit were obtained. The results are in good agreement with electron microprobe data. Moreover, a computer program was developed as an analytical tool for a fast and accurate determination of Sb/(Sb+As) content ratio by at least one spectroscopic parameter. These results indicate that Raman spectroscopy can provide direct information on the composition and structure of the tetrahedrite–tennantite and bournonite– seligmannite series.