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. Grønvold 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 (Fe_{0.936–0.900}S), 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 described^5,6 the pyrrhotite assemblages in the Ylöjärvi 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 Fe_{0.887–0.907}S, and the development of troilite and intermediate hexagonal pyrrhotite (Fe_0.912S). 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, Fe_{0.869–0.873}S. 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, Fe_0.875S, 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 Fe_{0.896–0.899}S. Intermediate pyrrhotite (Fe_0.912S) and, especially, troilite may be subordinate or absent in such assemblages (unpublished work).