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Unusual highto 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 Ag0.97(Fe4.88Ni3.10Cu0.04Co0.01)Σ8.03 (S7.96As0.03)Σ7.99. Zoned irregular aggregates of parkerite, 20–30 μm in size, are typically associated with pentlandite, or overgrow galena and yield empirical formula (Ni3.02Fe0.02Co0.01)Σ3.06(Bi1.73Sb0.05As0.04)Σ1.92 S2.02. 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 (Ni4.57Fe4.14Co0.25)Σ8.96(S8.01As0.02)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.

journal of geosciences

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