Above 600°C the system Cu-Pb-S is dominated by two-liquid fields in both the sulfurrich and metal-rich portions. A central liquid phase prohibits coexistence of galena and copper sulfides above 523°C. This sulfide liquid, which may be important in the genesis of certain ores, remains stable to 508° ± 2°C, at which temperature a monotectic reaction forms galena, digenite ss, and sulfur liquid. A new ternary phase, A, of the composition Cu14Pb2S9-x;(0<a;<0.15) has been synthesized and is stable between 528° ± 2°C and 486° ± 4°C. Between about 435° and 271° ± S°C the stable assemblages include galena, chalco-cite ss, and native lead; galena, chalcocite ss, and digenite ss; and galena, digenite ss, and covellite. Below 271°C the chalcocite-native lead pair reacts to form galena and native copper.

The phase relations as determined at 200°C must be modified at low temperatures due to the appearance of other copper sulfides such as blue-remaining covellite and djurleite. Experiments containing Fe or Zn in addition to Cu and Pb sulfides indicate that these elements do not stabilize the sulfide liquid phase to temperatures below the minimum in the pure Cu-Pb-S system. Furthermore, the addition of H2O, which has been called upon to depress sulfide melting temperatures, does not have measurable effect on melting relations.

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