Iron-deficient Low-temperature Pyrrhotite

Abstract

IN a recent communication1, Sawkins, Dunham and Hirst discussed the occurrence of pyrrhotite in low-temperature veins in the northern Pennine orefield. They found that, on experimental inversion and application of the hexagonal pyrrhotite–pyrite solvus curve2, monoclinic pyrrhotite in these deposits yields temperatures of deposition markedly in excess of those indicated by fluid inclusion studies. Gronvold and Haraldsen3, however, have demonstrated that at temperatures below c. 300° C the stable phases in the iron-rich part of the FeS–S system include troilite, intermediate hexagonal pyrrhotite (Fe0.936–0.900S), and monoclinic pyrrhotite (Fe∼0.877S). These three phases have very restricted ranges of composition, although the compositions of the iron-deficient synthetic phases differ somewhat from those observed in nature4. In the light of these relations it seems unreasonable to expect that primary monoclinic pyrrhotite of low-temperature origin co-existing with pyrite will show an iron-deficiency related to temperature of formation by the hexagonal pyrrhotite-pyrite solvus (which probably terminates at some temperature between 250° and 300° C). In the absence of very iron-deficient hexagonal pyrrhotite, the occurrence of troilite and intermediate pyrrhotite, with or without associated monoclinic pyrrhotite, in pyrite-bearing ores may be accepted as evidence that either the initial crystallization or the re-equilibration of the assemblage occurred at temperatures below c. 250° C. I have recently described5,6 the pyrrhotite assemblages in the Ylojarvi copper–tungsten deposit, Finland, and have suggested that cooling of hexagonal pyrrhotite from 500° to 565° C to below 300° C resulted in the inversion of much of the pyrrhotite to the monoclinic + hexagonal and monoclinic forms7, the formation of hexagonal pyrrhotites having compositions in the range Fe0.887–0.907S, and the development of troilite and intermediate hexagonal pyrrhotite (Fe0.912S). The last-mentioned two phases form characteristic lamellar intergrowths showing transitions to granular aggregates and monomineralic patches and vein-lets. The formation of mackinawite8 (tetragonal FeS), largely by replacement of magnetite and iron- and copper-sulphides, apparently postdated the crystallization of troilite. In this deposit, the monoclinic and monoclinic + hexagonal pyrrhotites have compositions in the narrow range, Fe0.869–0.873S. If inversion from the hexagonal form was iso-chemical, these compositions would indicate temperatures of formation of 510°–515° C. Because natural monoclinic pyrrhotite shows little departure from the composition, Fe0.875S, however, the close coincidence of the crystallization temperatures deduced from the compositions of the co-existing hexagonal and mono-clinic pyrrhotites may be fortuitous9. In other sulphide deposits, low-temperature re-equilibration of hexagonal pyrrhotite has resulted in the formation of monoclinic and monoclinic + hexagonal-pyrrhotite, and hexagonal pyrrhotite having a composition in the approximate range Fe0.896–0.899S. Intermediate pyrrhotite (Fe0.912S) and, especially, troilite may be subordinate or absent in such assemblages (unpublished work).