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Wittite and cannizzarite, two rare and structurally related nonstoichiometric sulfosalts belonging to the system Pb-Bi-S-Se, were found together around high-temperature fumaroles at La Fossa crater, Vulcano island, Italy. This occurrence makes a comparative study by different methods possible (scanning electron microscope, electron microprobe, and X-ray and electron diffraction). We discuss once more the question whether or not they are distinct mineral species. Cannizzarite was found in samples collected from 1990 to 1995 in assemblages with bismuthinite and galenobismutite (mostly), in some cases with lillianite, heyrovskiite, Se-bearing galena, kirkiite, and the new species mozgovaite. Wittite was revealed only in samples collected in 1995 around the fumarole vent F11 in association with bismuthinite. Both minerals form aggregates of very tiny sheaves of slightly divergent, bladed crystals (cannizzarite up to 0.5 mm in length and 0.07 mm in width; wittite up to 2 mm in length). The electronmicroprobe data obtained (141 analyses) show significant variations in proportions of the main elements. In terms of Bi/(Bi + Pb), the range of composition is 3.14 at.%; the Se content varies from less than 1 to 14.65 wt%. The composition field of the minerals under consideration may be described by the general empirical chemical formula Pb 3+x Bi 4-x (S 9-y Se y) 9-x/2 or Pb 3 (Bi 4-x Pb x) 4 (S 9-y Se y) 9-x/2 , with 0.04 ≤ x ≤ 0.28, and 0.5 ≤ y ≤ 3.5. Electron-diffraction data and X-ray investigations of a sample of wittite containing ~8.5 wt% Se confirm that cannizzarite and wittite have the same structure. Measured unit-cell parameters show that volumes of the H and Q subcells both vary in direct proportion to Se content. No chemical discontinuity between cannizzarite and wittite series has been observed up to 40 at.% Se; consequently, wittite may be validated or discredited as a distinct mineral species only when a definitive crystallographic work will prove that Se exceeds S in the H layers.

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The mineral composition of the Talatui gold deposit has been studied with modern methods. Previously unknown minerals (ilmenite, siegenite, glaucodot, wittichenite, matildite, hessite, pilsenite, zircon, tremolite, cummingtonite, hercynite, and goethite) have been identified in the ore. A high Re content has been detected in molybdenite. The spatiotemporal separation of Au and Ag is caused by different mineral species of these elements and their diachronous precipitation during the ore-forming process. Gold crystallized along with early mineral assemblages, beginning from virtually pure gold (the fineness is 996). Silver precipitated largely at the end of the process as hessite ( Ag 2 Te ) and matildite ( AgBiS 2 ). The temperature of ore deposition varied from 610 to 145 ° C, the pressure was 3370-110 bar, and the salt concentration ranged from 56.3 to 0.4 wt % NaCl equiv. The heterogeneous state (boiling) of fluid at the early stages has been documented. The chemical and isotopic compositions of the fluid testify to its magmatic nature and the participation of meteoric water at late stages in the ore-forming process. Thermodynamic modeling reproduces the main specific features of ore formation, including separation of Au and Ag. A physicochemical model of the gold mineralization in the Darasun ore district has been proposed. On the basis of several attributes, the Talatui deposit has been referred to the porphyry gold-copper economic type.

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