Pentlandite Exsolution in the Fe-Ni-S System:A Mechanistic Approach to Pentlandite/Pyrrhotite Ore Textures. Available in 2 volumes.

Abstract

In magmatic nickel sulphide deposits, a variety of dominantly pentlandite/pyrrhotite ore textures occur, resulting from the exsolution of pentlandite from monosulphide solid solution (MSS), which subsequently transforms to "pyrrhotite." The formation of these textural types has been studied experimentally and by examination of the natural textures in samples from deposits of different petrotectonic associations (Sudbury, Canada; Merensky Reef, South Africa; Sarqa, Greenland; Kambalda, Australia; and Thompson Mine, Canada). MSS compositions of five Fe/Ni ratios (24, 10, 5, 3, 2) and metal/sulphur ratios from ~1.000 to 0.930 were prepared and isothermally annealed at temperatures between 500 and 200°C, for times ranging from 20 minutes to 2664 hours. Examination of reaction products in polished section established the variations in textural types and their mechanisms of nucleation and growth as functions of composition, temperature, and time. Lower temperatures, higher initial metal/sulphur ratios, and longer times correspond to increased undercooling (ΔT) and pentlandite supersaturation (SSpn), but decreased effective interdiffusion distance for Fe, Ni, and SSpn (YD). Hence, at low ΔT, YD is high, and pentlandite nucleates at MSS grain boundaries as blebs or blades (higher ΔT). Impingement of blebs produces coarser aggregates, including blocky, cell, and rim-type pentlandite. With increasing ΔT, nucleation occurs more rapidly until YD decreases to the point where nucleation within MSS grains is kinetically favoured. Hence, "clusters" of lensoid particles, elongate along basal MSS planes, develop into flames and rosettes. At the highest ΔT, they develop into wiry seriate forms. At higher SSpn, lamellae oriented along pyramidal MSS planes occur. In the natural ores, cooling rate and more diverse available nucleation sites also influence textures, which nevertheless are analogous to synthetic examples. Characterisation of the exsolution controls allowed the variation in the natural textures and their sequences of formation to be interpreted. The influence of these controls on processes operating prior to pentlandite exsolution and during metamorphism is also discussed.