Oscillatory zoning in wolframite from Osanica, near Bor, Serbia

摘要

Wolframite from the Sb-W mineralization at Osanica (Serbia) was investigated using reflected light, EDS electron microanalysis and X-ray powder diffraction. Three paragenetically different types of wolframite with local variations of the chemical composition have been found in Sb-W quartz veins consisting of stibnite and wolframite as the main ore minerals. Wolframite with highly variable Mn/Fe ratios ranging from 8 to 65 mole percent MnWO_4 shows striking oscillatory zoning. Computer-adjusted reflected light images with increased contrast and decreased brightness are a useful, easy method for uncovering the zoning pattern in wolframite. Two modes of chemical changes are established by compositional profiles; high-amplitude oscillations (over 20 mole percent MnWO_4), which caused abrupt changes of the chemical compositions of the zones, and low-amplitude oscillations (up to 10 mole percent MnWO_4), which caused slight changes of the chemical compositions of the zones. Possible interpretations of the formation of these zoning patterns in wolframite are: (i) fluctuations of one of the physical-chemical factors of the hydrothermal solution during crystal growth, (ii) local self-organization crystal growth processes and (iii) decomposition of isomorphic solid solution of the wolframite series.