ARSENIAN PYRITE FROM GOLD DEPOSITS - AU AND AS DISTRIBUTION INVESTIGATED BY SIMS AND EMP, AND COLOR STAINING AND SURFACE OXIDATION BY XPS AND LIMS

Ion-probe microanalysis and ion mapping by SIMS have established that submicroscopic ppm-levels of gold in arsenian pyrite from stratabound and stratiform gold deposits are associated exclusively with As-rich g rowth bands; the gold is most probably incorporated in solid solution in arsenian pyrite via As-rich growth surfaces. The nature of the comp osition-specific stain resulting from oxidation of polished surfaces o f pyrite-group minerals with potassium permanganate solution (in 1:1 s ulfuric acid) has been studied by X-ray photoelectron spectroscopy (XP S) and laser ionization mass spectrometry (LIMS). The surface layers o f stained sections of arsenian pyrite and arsenopyrite contain three s pecies of sulfur (disulfide, elemental S, and sulfate), and the propor tion of elemental S in surface layers of pyrite increases with As cont ent of the substrate. The amount of Mn present in surface layers is in significant. Therefore, stain color is attributed to optical interfere nce by a thin surface-layer of elemental S. Surface oxidation of pyrit e by potassium permanganate solution is promoted by As-for-S substitut ion; in this way, color development can be correlated with the distrib ution of As and ''invisible'' Au.