A Portion of the System Fe-S-O between 900 and 1080 {degrees}C and its Application to Sulfide Ore Magmas

The portion of the Fe–S–O system including pyrrbotite,wüsite, magnetite, and iron has been studied between 900and 1080 °C by modifed silica-tube techniques. At 900 °C,tie lines extend from pyrrhotite containing between 63.53 and62.8±0.2 wt. per cent Fe to wüstite solid solutionand form pyrrhotite containing between 62.8 and 60.0 wt. percent Fe to magnetite. A ternary eutectic, troilite-wüstite-iron-liquidoccurs at 915±2 °C. A ternary invariant point, wherepyrrhotite (composition 62.8±0.2 wt. per cent Fe)+wüstite magnetite+liquid occurs at 934 °C. Pyrrohotite compositionstrongly influences the temperature of thee magnetite-pyrrhotitesolidus. Magnetite-pyrrhotite assemblages begin to melt at 934°C when the pyrrhotite contains 62.8 wt. lper cent Fe, at1010 °C when it contains 62.5 wt. per cent Fe, at 1030 °Cwhen it contains 62.0 wt. per cent Fe, and at 1050 °C whenit contains 60.5 wt. per cent Fe. Craig & Naldrett (1967) have shown that up to 20 wt. percent nickel substituting for iron in pyrrhotite solid solutionon a weight per cent basis has little effect on magnetite-pyrrhotitesolidus temperature and that up to 2 wt. per cent copper substitutingin a sunukar way lowers the solidus less than 20 °C. Byredetermining the solidus in the presence of H₂O at 2 kb totalpressure Naldrett & Ricahrdson (1967) have show that, withinexperimental accuracy (± 10 °C), water has no effecton melting temperature. since natural iron-sulfide magmas rarelycrystallize pyrrhotites containing more than 62.5 wt. per centtotal metal, the temperature range of from 1010 to 1050 °Cdetermined in this study is probably within 20 °C of theminimum temperature of introduction of a large number of magnaticsulfide ores. Comparison of the melting temperatures of ores with those ofthe rocks with which they are associated suggest that crystallizationunder different water pressures is responsible for the presenceof sulfides disseminated as rounded ‘buck-shot’type spherules in some rocks ans as an interstittial fillingin others. The composition of an iron sufide-oxide ore magma settling fromits associated silicate magma is controlled by the sulfur andoxygen fugacities of the silicate magma at the moment when equilibrationbetween the two ceases. In the case of large bodies of massivesulfide ore, equilibration probably ceased when the ore settledout of its host; the sulfide to magnetite ratio of such orewill depen on how far below its liquidus temperature the sulfide-oxideliquid was at the moment of separation. In the case of sulfide-richdroplets remaining disseminated throughtot the plutonic hostrock, equilibration probably continued to subsolidus temperatures;under these conditions it is possible that the droplets maylose all of their oxygen to the host rock. Finally in the caseof sulfife-rich droplets trapped within rapidly cooled volcanicrocks complete re-equilibration was probably prevented by therate of cooling and consequently these droplets retain muchof their original oxygen as magnetite.