Nickel-(Bi,Ag) sulphide mineralization from NYF Vepice pegmatite, Milevsko pluton, southern Bohemia (Czech Republic) - a reflection of the parental granite chemistry

Abstract

Unusual high-to low-T sulphide mineralization with Ni-(Bi, Ag) phases was examined at the NYF intragranitic pegmatite No. III from Vepice near Kovářov enclosed in the Milevsko Pluton, Moldanubian Zone, Southern Bohemia. Zoned pegmatite dike, up to 30 cm thick, has transitional contact with host durbachite and its internal structure includes transitional unit, border granitic unit, graphic unit, blocky unit (Kfs+Qz), quartz core and small miarolitic pocket. Sulphide mineralization locally associated with calcite and fluorite is represented by common pyrite with minor galena, accessory chalcopyrite, sphalerite, marcasite, native Bi and Ni-(Bi, Ag) minerals -argentopentlandite, parkerite and pentlandite. It is developed in small miarolitic pockets and on pegmatite fractures cutting all pegmatite units. Irregular anhedral grains and aggregates of argentopentlandite, < 200 μm in size, associated with chalcopyrite, have empirical formula Ag 0.97 (Fe 4.88 Ni 3.10 Cu 0.04 Co 0.01 ) Σ8.03 (S 7.96 As 0.03 ) Σ7.99 . Zoned irregular aggregates of parkerite, 20-30 μm in size, are typically associated with pentlandite, or overgrow galena and yield empirical formula (Ni 3.02 Fe 0.02 Co 0.01 ) Σ3.06 (Bi 1.73 Sb 0.05 As 0.04 ) Σ1.92 S 2.02 . Pentlandite is present in two morphological forms: rare lenticular to isometric anhedral grains, ≤ ~200 μm in size, and numerous small tabular crystals, ≤ 20 μm in length, in subparallel arrangement rimming aggregates of other sulphides. Empirical formula is (Ni 4.57 Fe 4.14 Co 0.25 ) Σ8.96 (S 8.01 As 0.02 ) Σ8.03 . Evaluation of relative chronology of the sulphide minerals is complicated mainly due to complex textural relations. Pyrite + marcasite are likely pseudomorphs after pyrrhotite as the earliest phase whereas pentlandite and native Bi are among the latest. Early sulphides (pyrrhotite, chalcopyrite, sphalerite, argentopentlandite) crystallized at T < ~550-400 °C, whereas pyrite, marcasite, galena, parkerite, and native Bi at T < 240 °C. Cooling of the system below T ~200 °C appeared soon after transformation of pyrrhotite to pyrite+marcasite aggregates. Sulphides manifest that concentrations of Ni in residual pegmatite melt and exsolved fluids were high enough to facilitate saturation of Ni-sulphides. Very high Ni/Co ratio in sulphides reflects a much lower concentration of cobalt in durbachite. The examined Ni-(Bi, Ag) sulphides manifest that high concentrations of highly compatible Ni in parental granite may be reflected in accessory minerals from its pegmatite.

Minerály kontaminovaných granitových pegmatitů z lomu Pohled u Havlíčkova Brodu (moldanubikum), část II: prvky a sulfidy

“…V oblasti moldanubika Českého masivu jsou výskyty rudních minerálů ze skupiny prvků a sulfidů v pegmatitech všeobecně vzácné a většinou jde jen o akcesorie. Nejběžnější jsou zejména výskyty pyritu, galenitu, arsenopyritu, pyrhotinu a löllingitu (Špinar 1995;Novák 2005;Cempírek et al 2010;Fuksová et al 2011;Novák et al 2013;Gadas et al 2014;Sejkora et al 2020), výjimečně i sfaleritu, bournonitu, bismutinu, markazitu, covellinu, chalkopyritu, molybdenitu (Staněk 1997, ryzího bismutu a slitin Sb-As (Gadas et al 2014(Gadas et al ), herzenbergitu (Škoda, Čopjaková 2005, telluridů Ag a Bi (Fuksová et al 2011), či sulfidů Ni a Ag (Sejkora et al 2020). Asociace sulfidických (a příbuzných) minerálů v jednotlivých pegmatitech jsou zpravidla velice chudé a zahrnují typicky pouze jednotky minerálních druhů.…”

Pb-Bi mineralizace v amfibolitech z lomu Libodřice u Kolína (kutnohorské krystalinikum, Česká republika)

“…3a-d). Obdobné, avšak xenomorfně omezené objekty popsali i Sejkora et al (2020) v rudní mineralizaci miarolového pegmatitu ve Vepicích v durbachitech masivu Čertova břemene; ke genezi těchto objektů autoři uvádějí, že by mohlo jít o pseudomorfózy po neznámém minerálu. V našem případě se s výše uvedeným názorem zcela ztotožňujeme.…”

Rare minerals of noble metals in the collection of the Mining Museum: new data

“…Argentopentlandite AgFe8S8. It was first identified as a separate mineral species in the ores of the Norilsk region in 1977 [13] and was also described in the deposits of the USSR, Finland [14], Canada [15], the Czech Republic [16] and other countries as silver-enriched pentlandite or a distinct mineral species. Argentopentlandite is represented by lamellar grains and their aggregates 100-150 µm in size in chalcopyrite ore near galena grains, as well as irregularly shaped grains 20-30 µm in size in pentlandite (Fig.…”