Compositional variations in the bismuthinite-stibnite solid solution, metal ratios, and bulk semimetal ratios provide a record of the chemical evolution of the ore fluids responsible for mineralization of the Julcani mining district in Peru. Bismuthinite-stibnites become more antimony rich along the flow paths (defined by changes in the ratio of lead to copper in the bulk ore) of the veins at the Mimosa and Herminia mines. Rapid changes in the composition of the solid solution coincide with the silver bonanza zone at Herminia and with the appearance of complex sulfosalts of silver and bismuth.Bismuthinite-stibnites with 5 to 45 mole percent Bi were encountered; such compositions lie within the natural gap proposed by previous workers. Although not common, these samples are consistent with experimental studies which document a complete solid solution.Comparison of bulk semimetal concentrations in the ores of the two mines reveals differences along the respective flow paths. At Mimosa the ratio of antimony to bismuth increases, consistent with a normal zoning of the semimetals. In contrast, at Herminia antimony nearly disappears from the ore assemblage in the zone of silver bonanzas and rapid changes in bismuthinite-stibnite compositions. Farther along this flow path, antimony concentrations increase in the ores, a return to the normal pattern of semimetal zonation. Volatilization of antimony (semimetal complex boiling) from the ore fluid may be responsible for the anomalous behavior of antimony at Herminia. Antimony trihalides are volatile molecular species and potentially can boil from an ore fluid and later recondense, permitting a resumption in the precipitation of antimony-bearing phases.