Abstract

The Złoty Stok Au-As deposit (SW Poland) hosts two distinct nephrite varieties: ‘type 1’ – a grayish green or pale green, translucent, with waxy to greasy luster nephrite composed of actinolite and abundant löllingite; ‘type 2’ – a deep or intense green, less-translucent, with sub-vitreous to dull luster nephrite composed of tremolite and actinolite, with rare löllingite and arsenopyrite. ‘Nephrite type 1’ shows evidence for more intense metasomatism and recrystallization than ‘type 2’, e.g., more abundant arsenide, sulfosalt, and tungstate minerals, and higher Fe, As, and Co concentrations. The tremolite and actinolite in both types formed at the expense of dolomite—either directly from dolomite or through diopside formation. Calcite is a byproduct of the amphiboles' formation directly from dolomite. Amphiboles have Cr2O3 and NiO concentrations of 0.00–0.04 wt.% and 0.00 wt.%, respectively, typical of dolomite-related nephrites. Diopside composition (up to 51 Wo, 0.00–0.05 wt.% Cr2O3 and 0.00–0.09 wt.% NiO) is similar to other dolomite-related nephrite compositions. Bulk-rock Cr and Ni (<30 ppm and <40 ppm, respectively) and Co, which is less affected by ore mineralization (<10 ppm), are similar to the values in dolomite-related nephrite. Yellow cathodoluminescent diopside is similar to that in other metasomatized dolomitic marbles. Nephrite has δ18O values ranging from +8.3 to +10.4‰ (precision = ±0.1‰), higher than in other typical dolomite-related nephrite. δD values range from –77 to –75‰, similar to the dolomite-related type. Based on these observations, we interpret the Złoty Stok nephrite deposit to have formed as a metasomatic replacement of dolomitic marble. This is the third documented case of a dolomite-related origin (para-nephrite) for nephrite in Europe. Furthermore, the dolomite-related nephrite adds the Lower Silesian Sudetes and its Foreland to rare areas like Wyoming, southern Siberia, China, Korea, and the Alps, where both dolomite-related and serpentinite-related nephrites occur. The Złoty Stok nephrite formed via at least two crystallization events. In the first event, quartz veins and nephrite formed, and the paragenesis included quartz, tremolite, Fe-poor actinolite, Fe-poor diopside and, probably, arsenopyrite. In the second event, Si-depleted actinolite, Fe-enriched diopside, and löllingite formed, and the earlier arsenopyrite was replaced by calcite. The crystallization path reflects progressive metamorphism (i.e., temperature rise) or change in the saturation conditions, e.g., silica and sulfur activity in the fluid. The nephrite formation can be linked with the emplacement and cooling of the neighboring Variscan Kłodzko-Złoty Stok granite at ca. 340–298 Ma.

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