Studies on the pyrrhotite and paragenesis of the Moke creek sulphide lode, Wakatipu district

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Abstract
Part I The geological setting of the lode is briefly described. It consists of thin irregular concordant bands of a variable pyrite-pyrrhotite-chalcopyrite assemblage, with minor sphalerite, in greenschist. Textural relationships are described and the paragenesis and origin of the sulphides briefly discussed. Sulphides in a greenschist band near Queenstown are described for comparison. Part II Pyrrhotite at Moke Creek occurs in both hexagonal and monoclinic modifications, as well as transitional forms. Hexagonal pyrrhotite gave d102 spacings consistent with compositions appropriate to solvus temperatures of 350°C. Heated monoclinic pyrrhotite yielded solvus temperatures of around 500°C. These inconclusive geothermometric results are briefly discussed and their validity questioned. Composition-symmetry relationships suggest that natural hexagonal pyrrhotite tends to be richer in iron than is natural monoclinic pyrrhotite. Thermomagnetic treatment of monoclinic pyrrhotite produced reversible Curie point curves; magnetisation of hexagonal pyrrhotite increases strikingly during such treatment, the increase being associated with inversion to monoclinic symmetry. Significance of Curie points, effect of pyrrhotite alteration products on thermomagnetic curves, and implications for paleomagnetism are briefly examined. High pressure may inhibit inversion of iron-rich pyrrhotite to monoclinic symmetry during cooling. Use of pyrrhotite as a possible environmental pressure indicator is suggested.