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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

^* E-mail: Nigel.Cook{at}ngu.no

Intermediate levels of the Larga-Faa Bii field, Metaliferi Mts., Romania, are host to epithermal vein mineralization, small skarn-like bodies and ‘geode’-like replacement ores, that together form a complex hydrothermal system rooted in the subjacent andesite stock. Silver-Pb-Bi sulphosalts, some with Sb and/or Se, are intimately associated with hessite and a range of Bi-tellurides with Bi/(Te+S+Se) 1 (tetradymite, tellurobismuthite, tsumoite, Pb-free rucklidgeite) within all these ore types. Lillianite homologues, both ordered ⁴L and ⁷L derivatives (lillianite, gustavite), and disordered varieties are the most abundant sulphosalts, with subordinate bismuthinite derivatives (aikinite, krupkaite, bismuthinite). An exceptionally Ag-rich lillianite homologue, intimately intergrown with tetradymite and hessite, is identified as treasureite on the basis of chemical composition. Porphyry-style mineralization from 600 m beneath the studied level contains rucklidgeite as the single stable Bi-telluride. The trace mineralogy of the system is suggestive of a hydrothermal system strongly zoned with respect to f_Te2 and, to a lesser extent, also f_S2. Mineralogical and textural evidence suggests that initial crystallization took place at temperatures in excess of 400°C. The association of gold with Bi-minerals is linked to the role of Bi_melt as a scavenger for gold, and deposition within ‘droplets’ simultaneous with the formation of arsenopyrite from löllingite + pyrrhotite. Both Bi-tellurosulphides and sulphosalts are commonly non-stoichiometric, a fact that can be correlated with order-disorder in both series. When widely distributed, as in the Larga system, trace amounts of Bi-minerals (sulphosalts, tellurides/ tellurosulphides) have considerable, untapped potential as tracers of the physical-chemical character of an evolving hydrothermal system.

KEYWORDS: tellurides, Bi-sulphosalts, compositional data, Larga, Metaliferi Mts., Romania

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