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Bismuth tellurides and sulphosalts from the Larga hydrothermal system, Metaliferi Mts., Romania: Paragenesis and genetic significance

Geological Survey of Norway, N-7491 Trondheim, Norway

View larger version (74K): [in a new window]   FIG. 1. (a) Geological sketch map of the Metaliferi Mts., showing the principal ore zones, including the Zlatna- Stnija volcanic zone that hosts the Faa Bii-Hane-Breaza-Larga-Trîmpoiele orefield. The dashed line shows the perimeter of the ‘golden quadrangle’. (b) Geological-metallogenetic map of part of the Zlatna- Stnija volcanic zone, indicating the location of ores and underground galleries referred to in the text (projected to surface). Redrawn and modified after Borco et al.(1989).

View larger version (66K): [in a new window]   FIG. 2. Geological sketch map of the Horia Gallery and other underground workings accessed from the +516 m level (plan view). The detail is essentially based on mapping carried out by one of the present authors (CLC; Intreprinderea de Prospeciuni Geologice i Geofizice, Bucharest, unpubl. rep., 1991). The numbers in boxes are the locations of specific samples mentioned in the text and figures.

View larger version (115K): [in a new window]   FIG. 3. Photomicrographs in reflected light showing paragenetic relationships of ore minerals in Larga ores. (a) Inclusions of gold (Au) within an intergrowth of arsenopyrite (Apy) and relict löllingite (Lo). Sample: 52 (0 Est gallery). (b) Association of tellurides in pyrite (Py) and arsenopyrite (Apy) within typical vein mineralization. Ttd: tetradymite, Hs: hessite, Ts: tsumoite. Sample: 3M (Vein 37 at 625 m, 0 Vest gallery). (c) Inclusions of tetradymite (Ttd) within a matrix of arsenopyrite (Apy) and pyrite (Py). Note the co-existence of hessite (Hs) and galena (Gn). Sample: 3M (Vein 37 at 625 m, 0 Vest gallery). (d) Typical occurrence of bismuth sulphosalts, in this case dominantly composed of Sb-bearing lillianite (Li) as inclusions in chalcopyrite (Cp). Sample: 145 (vein in Padina gallery). (e) Association of gold (Au) and rucklidgeite (Rkl) in quartz within the Trîmpoiele porphyry deposit. Sample: T960. (f) Inclusions of chalcopyrite and pyrrhotite crystallized from intermediate solid solution (Cp/Po) within Hane veins dominated by sphalerite (Sp). Sample: 1963.

View larger version (78K): [in a new window]   FIG. 4. BSE images showing the habit and association of Bi-tellurides in Larga ores. (a) Association of tetradymite (Ttd) and tsumoite (Ts) as inclusions within arsenopyrite (Apy) in vein ores. Note curvilinear equilibrium boundary between the two Bi-minerals. Sample: 3M (Vein 37 at 625 m, 0 Vest gallery). (b) Association of sylvanite (Sy), native tellurium (Te) and tellurobismuthite (Tbs) within pyrite (Py). Sample: 3M (Vein 37 at 625 m, 0 Vest gallery). (c) Assemblage containing coexisting altaite (Alt) and galena (Gn), with minor hessite (Hs). Sample: 881 (skarn, Padina gallery). (d) Equilibrium association of tetradymite (Ttd) and tellurobismuthite (Tbs) in calcite (Cal) from skarn ores. Sample: 12P (skarn, Padina gallery). (e) Larger grain of tetradymite (Ttd) within calcite from skarn ores, containing exsolved galena (Gn). Sample: 12P (skarn, Padina gallery). (f) Narrow rim of tetradymite (Ttd) surrounding pyrite (Py) in calcite (Cal) from skarn ores. Gn: galena. Sample: 12P (skarn, Padina gallery).

View larger version (70K): [in a new window]   FIG. 5. BSE images showing the habit and association of Bi-tellurides in ‘geode’ type ores; all from sample: 71 (0 Est gallery). (a) Included blebs of tellurides (bright white) within pyrite (Py). Sp: sphalerite; Cal: calcite. (b) Composite inclusions of hessite (Hs) and altaite (Alt) within pyrite (Py). (c) Similar bleb-like telluride inclusions within pyrite (Py), consisting of hessite (Hs), altaite (Alt) and volynskite (Vl). (d) Composite, bleb-like inclusions in pyrite (Py), consisting of hessite (Hs), altaite (Alt) and a decomposition intergrowth with a bulk composition approximating to Ag3Au(S,Te)2. Note curvilinear equilibrium boundaries.

View larger version (37K): [in a new window]   FIG. 6. (a) Composition of Bi-tellurides/tellurosulphides from Larga (tetradymite, tsumoite, tellurobismuthite, rucklidgeite), plotted in terms of Bi(+Pb)– Te– (S+Se). (b) Composition of tellurides and tellurosulphides in the Bi2Te3-Bi2Se3-Bi2S3 system demonstrating the extent of solid solution along the Bi2Te2S-Bi2Te2Se join between tetradymite, Bi2Te2S and kawazulite, ideally Bi2Te2Se. (c) Compositional variation expressed in terms of Bi-Pb-(S+Se+Te), demonstrating compositions of Pb-free rucklidgeite (vein ores) and rucklidgeite (porphyry mineralization).

View larger version (85K): [in a new window]   FIG. 7. BSE images showing the habit and association of Bi-sulphosalts in Larga ores. (a) Gustavite (Gu) as a lamellar inclusion in pyrite (Py) and containing exsolution of krupkaite (Kr). Sample: 145d (Padina vein). (b) Coarse crystals of Sb-gustavite (Sb-Gu), within a cavity (dark areas) on a polished specimen. Gn: galena, Py: pyrite. Sample: 145d (Padina vein). (c) Treasureite (Tr) as small elongate blades within hessite (Hs), intergrown with tetradymite (Ttd), enclosed within pyrite (Py). Sample: 1983/1 (Vein 37, 625 m on 0 Vest gallery). (d) Similar assemblage as in c. Note the apparent equilibrium boundaries between treasureite and hessite. Sample: 1983/1 (Vein 37, 625 m on 0 Vest gallery).

View larger version (26K): [in a new window]   FIG. 8. Plot of compositions in (Cu+Ag)-Pb-(Bi+Sb) ternary space.

View larger version (30K): [in a new window]   FIG. 9. Compositions of pavonite homologue (5P, pavonite?) plotted in Bi-Ag-(Pb+Cu) ternary space. Two backscattered electron images show the morphology of the mineral. Pav: pavonite (?), Li: lillanite, Py: pyrite.

View larger version (26K): [in a new window]   FIG. 10. Telluride-sulphide stability diagram at 300°C in f Te2-f S2space, after Afifi et al. (1988a,b), for mineral phases relevant to the Larga system. The grey arrow indicates the trend of mineralizing conditions from the base of the system upwards. Domains marked ‘1’ and ‘2’ correspond to the intermediate (i.e. Horia +516 m) and uppermost levels respectively. In the latter, Au-telluride-rich levels, calaverite and native tellurium are stable. Based on prevailing major and accessory assemblages. Abbreviations: Py: pyrite, Po: pyrrhotite, Cp: chalcopyrite, Bn: bornite, Apy: arsenopyrite, Lö: löllingite, Fo: frohbergite.

View larger version (38K): [in a new window]   FIG. 11. Application of the geothermometric method based on the composition of coexisting stannite-sphalerite pairs in altered skarn ores from Larga. (a) and (b) BSE images showing two examples of stannite-sphalerite pairs used for geothermometry. Abbreviations: Sp: sphalerite, Stn: stannite, Qz: quartz, Cal: calcite, Se-Gn: seleniferous galena. Sample 71 (0 Est gallery). (c) Compositional data plotted on isotherms according to the calibration of Nakamura and Shima (1982) cited by Shimizu and Shikazono (1985). Smaller grains (e.g. a), give temperatures close to 350°C.