Arsenopyrite compositional variation over variable temperatures of mineralization, Otago Schist, New Zealand

Compositions of arsenopyrite coexisting with pyrite from eight different me sothermal gold-bearing systems in the Otago Schist fall mainly into a narro w band of 30 +/- 1 at. percent As. Intragrain and intergrain compositional variations, combined with analytical uncertainty, preclude resolution of di fferences in arsenopyrite compositions between samples of less than 1 at. p ercent As. Only one of the vein systems has an arsenopyrite compositional r ange that extends below 28 at. percent As. Arsenopyrite from a pyrrhotite-b earing assemblage has compositional range extending above 32 at. percent As . The vein systems have formed over a wide range of temperatures between ab out 200 degrees and 400 degrees C. It is not possible to resolve the influe nce of temperature on arsenopyrite compositions for most samples. Sulfur an d oxygen activity of the hydrothermal fluid may be responsible for the low degree of arsenopyrite compositional variation.