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

[摘要] 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. Zonedpegmatite 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. Sulphidemineralization locally associated with calcite and fluorite is represented by common pyrite with minor galena, accessorychalcopyrite, 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 anhedralgrains and aggregates of argentopentlandite, <200 μm in size, associated with chalcopyrite, have empirical formulaAg0.97(Fe4.88Ni3.10Cu0.04Co0.01)Σ8.03 (S7.96As0.03)Σ7.99. Zoned irregular aggregates of parkerite, 20–30 μm in size, are typicallyassociated with pentlandite, or overgrow galena and yield empirical formula (Ni3.02Fe0.02Co0.01)Σ3.06(Bi1.73Sb0.05As0.04)Σ1.92S2.02. 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 mineralsis complicated mainly due to complex textural relations. Pyrite + marcasite are likely pseudomorphs after pyrrhotiteas 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 nativeBi 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 highenough to facilitate saturation of Ni-sulphides. Very high Ni/Co ratio in sulphides reflects a much lower concentrationof cobalt in durbachite. The examined Ni-(Bi, Ag) sulphides manifest that high concentrations of highly compatible Niin parental granite may be reflected in accessory minerals from its pegmatite.

[学科分类] 自动化工程

[关键词] argentopentlandite;parkerite;pentlandite;EPMA data;NYF pegmatite;Czech Republic