The phase relations in the ternary system Fe-Pb-S were determined by quenching and differential thermal analysis experiments in rigid silica-glass tubes. The solubilities of FeS in galena and in liquid Pb and the extent of solid solution of PbS in pyrite and pyrrhotite at 700 degrees C are too small to be determined by the methods employed and are probably less than 0.1 wt. percent. Less than 1 wt. percent S is soluble in liquid Pb at 700 degrees C.Ternary compounds do not occur in this system. An invariant point involving galena, pyrite, sulfide liquid, liquid sulfur, and vapor occurs at 716 degrees C. The ternary liquid at this temperature contains about 60 wt. percent Pb, 13 wt. percent Fe, and 27 wt. percent S, and is thus situated slightly on the sulfur side of the PbS-FeS 2 join. At 719 degrees C tie lines between galena and pyrite are replaced by sulfide liquid-pyrrhotite tie lines in an invariant reaction involving vapor. The sulfide liquid field expands rapidly and intersects the PbS-FeS 2 join at 723 degrees C. At 848 degrees C the ternary liquid field intersects the FeS-PbS join. Diagrams are presented to illustrate changes in phase relations from 700 degrees to 1,115 degrees C.The experimental results demonstrate that mixtures of galena (about 70 wt. percent) and pyrite (about 30 wt. percent) can crystallize directly from a liquid at 716 degrees C. Additional components may lower this temperature so that it appears possible that pyrite-galena ores may melt at temperatures below 700 degrees C when exposed to thermal metamorphism.

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