Sphalerite, pyrite, and pyrrhotite were recrystallized together in a saturated ammonium iodide solution at 150 ø to 325øC and 0.4 to 5.1 kbars. The data were obtained from ten sets of experiments comprising gold capsules stacked consecutively within the bores of cold seal pressure vessels which were heated for periods of between 300 and 750 days. Five capsules contained magnetite as an additional reactant.A P-T-composition map has been constructed for sphalerite buffered by pyrite and hexagonal pyrrhotite. The upper P-T range for the present study includes the lower temperature boundary for the sphalerite geobarometer which has been located near 285øC at 0.5 kbars and 300øC at 5 kbars. The provisional lower P-T range for the present data extends from approximately 180øC at 0.5 kbars to 190øC at 5 kbars. Within the investigated P-T interval, isobaric composition curves lbr sphalerite show a progressive decrease in iron content with increasing pressure and decreasing temperature.Hexagonal pyrrhotite was the only iron monosulfide identified in the experimental reaction products and was found to increase in iron content with increasing pressure. Its occurrence in these long-term experiments supports earlier predictions that monoclinic pyrrhotite may be a metastable phase down to low temperatures. Hexagonal pyrrhotite may stably invert to monoclinic pyrrhotite at temperatures below 150øC under isotropic stress conditions; however, at higher temperatures monoclinic pyrrhotite appears to be favored by anisotropic stress conditions which produce lattice strain. Under these conditions it may form and persist metastably as the preferred phase below the sphalerite geobarometer boundary. The presence of magnetite does not affect the compositions of sphalerite or pyrrhotite under the conditions of study.The present calibration may be applied to hydrothermal vein deposits provided codeposition of phases can be proven; however, it is expected to have limited application for massive sulfide ores that have prograded or retrograded to temperatures around or below 300øC. This is because the calibration requires buffering by pyrite and hexagonal pyrrhotite, whereas most lower temperature assemblages appear to have been buffered by pyrite and monoelinic pyrrhotite over a range of temperatures, pressures, and anisotropic stress conditions. This latter buffer has restricted sphalerite compositions between around 11 and 12 mole percent FeS.

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Archean lenticular nickel sulfide deposits of volcanic association bear a striking resemblance lo volcanic-exhalative Cu–Zn massive sulfide deposits. They have similar morphologies, show well developed mineralogical layering, and exhibit close stratigraphic relationships with intimately associated volcanics and intra-volcanic sediments of greenstone belts. The nickel sulfide deposits are associated with early ultramafic/mafic volcanics. and massive Cu–Zn sulfide deposits with intermediate to silicic volcanics that formed later in volcanic-sedimentary cycles.It is suggested that a magmatic volcanic-extrusive origin does not explain features of massive nickel sulfide deposits as well as an alternative volanic-exhalative origin, which provides a direct link between sulfide ores and intimately associated exhalative sediments. It also satisfactorily accounts for a number of important features, including mineralogical layering, a relative enrichment in pyrite, and the local abundance of millerite.Outpouring of extremely hot ultramafic lavas across top surfaces of deposits would partially melt the Ni–Cu sulfides, thereby generating the igneous textures observed. Later regional metamorphism has modified these earlier features.