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 Au- and Ag-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 3 mineral assemblages: sphalerite-pyrite-chalcopyrite-tennantite-galena, sphalerite-pyrite-tennantite-galena, and sphalerite-pyrite-enargite-tennantite-galena. Quartz and, locally, other gangues are present. The sphalerite samples contain from 12 to 0.05 weight % Fe and detectable amounts of a restricted suite of minor elements, principally Mn, Cd, Cu, and Pb. Mn correlates directly with Fe content, but the other minor elements have random correlations. The Fe content of Central City sphalerite is interpreted to be mainly a function of activity of S 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 Fe in the sphalerite. The Fe content of the sphalerite and fluid inclusion studies indicate that mineralization occurred over a temperature range from at least 620 degrees C. to about 150 degrees 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 S decreased with temperature, but to an extent such that more S-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 accommodate it. Mn varies widely because 1) it is geochemically much more abundant than is Zn and 2) it can also enter other minerals on a large scale. Conversely, because the amount of Cd is small relative to that of Zn and because it enters only sphalerite in quantitatively significant amounts in hydrothermal environments, the Cd content of sphalerite is constant. The Cu content of the sphalerites is low and in good agreement with recent experimental data of Priestley Toulmin 3d.