Detailed studies of sphalerite, as a part of a larger study of the Central City district, Colorado, have been undertaken to learn something of the physico-chemical environment of ore deposition. More than 90 samples have been analyzed by chemical and spectrochemical methods and these data are interpreted in the light of experimental information.Sphalerite is a widespread and moderately abundant constituent of the gold- and silver-rich veins of the district. It was deposited during one stage of mineralization, in all environments of the concentrically zoned district except in the core. On a district-wide basis it occurs in three mineral assemblages: sphalerite-pyrite- chalcopyrite-tennantite-galena, sphalerite-pyrite-tennantite-galena, and sphalerite-pyrite-enargite-tennan-tite-galena. Quartz and, locally, other gangues are present.The sphalerite samples contain from 12 to 0.05 weight percent iron and detectable amounts of a restricted suite of minor elements, principally manganese, cadmium, copper, and lead. Manganese correlates directly with iron content, but the other minor elements have random correlations.The iron content of Central City sphalerite is interpreted to be mainly a function of activity of sulfur and temperature. Total pressure and minor elements that may enter the structure of either sphalerite or coexisting pyrite are thought to have negligible effects on the amount of iron in the sphalerite.The iron content of the sphalerite and fluid inclusion studies indicate that mineralization occurred over a temperature range from at least 620° C to about 150° C. In general, the temperatures tended to decrease from the vicinity of the central zone outward toward the peripheral zone. The thermal pattern, however, was complex, and marked by local irregularities.The activity of sulfur decreased with temperature, but to an extent such that more sulfur-rich mineral assemblages could form toward the margins of the district.The minor-element content of the sphalerite is governed by the activities of the various components and by the ability of the host mineral to accomodate it. Manganese varies widely because (1) it is geochemically much more abundant than is zinc and (2) it can also enter other minerals on a large scale. Conversely, because the amount of cadmium is small relative to that of zinc and because it enters only sphalerite in quantitatively significant amounts in hydrothermal environments, the cadmium content of sphalerite is constant. The copper content of the sphalerites is low and in good agreement with recent experimental data of Priestley Toulmin 3d.