Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Jiří Sejkora, Martin Števko, Daniel Ozdín, Stanislav Pršek, Jaroslav Jeleň

Unusual morphological forms of hodrušite from the Rozália vein, Hodruša-Hámre near Banská Štiavnica (Slovak Republic)

Journal of Geosciences, volume 60 (2015), issue 1, 11 - 22

DOI: http://doi.org/10.3190/jgeosci.188

Rare Cu-Bi sulphosalt, hodrušite, occurs in the Rozália vein (levels X-XIV of the Rozália mine, Hodruša-Hámre ore deposit near Banská Štiavnica, central Slovakia) in two unusual morphological forms. The first type are brownish bronze thin acicular striated crystals, up to 3 mm long, in drusy cavities of quartz-hematite gangue, which are usually grouped into chaotic or irregular aggregates. The second hodrušite type comprises flattened columnar aggregates, up to 1.5 cm in length, overgrown by hematite in quartz gangue. These aggregates are distinctly striated with brownish bronze colour and metallic lustre. Abundant W- and Al-rich hematite, chalcopyrite, kaolinite/dickite, siderite, baryte, rare bismuthinite and kupčíkite were found in the association. The earliest columnar aggregates of hodrušite are locally substantially replaced by bismuthinite; these hodrušite-bismuthinite aggregates are further intensively pushed back by hematite displaying W- and Al-rich zones. Acicular crystals of hodrušite in gangue cavities were later than bismuthinite and hematite and their formation was related to remobilisation of Cu and Bi from earlier altered gangue. Powder X-ray diffraction data and chemical composition of both hodrušite types are similar; their unit-cell parameters were refined (monoclinic space group C2/m) as: a 17.552(5), b 3.905(1), c 27.167(9) Å, β 92.44(3)^o, V 1860.5(9) ų (acicular crystals) and a 17.567(2), b 3.9151(7), c 27.155(5) Å, β 92.43(1)^o, V 1865.9(4) ų (columnar aggregates). Cu-for-Bi substitution is characteristic of both hodrušite types; it influences calculated N_chem values (0.98-1.33) to the point that it is impossible to distinguish hodrušite (ideal 1.5) from kupčíkite (ideal 1.0). The problem is resolved by Fe and Ag contents: kupčíkite has more than 2 at. % Fe, cuprobismutite contains more than 2 at. % of Ag; hodrušite usually has Ag and Fe contents below 2 at. %. Hematite is remarkable for its high WO₃ content reaching 4.96 wt. % (0.04 apfu), extremely unusual of a hematite from hydrothermal vein mineralization. Tungsten probably enters the hematite structure via the 2Fe³⁺↔W⁶⁺+   substitution.