In the Naica mining district of N-central Mexico, pyrometasomatic and mesothermal replacement bodies of base-metal sulfides with minor amounts of Ag and Au occur in a thick sequence of locally marmorized Cretaceous limestone. The only igneous rocks known in the Sierra de Naica, consisting of thin, discontinuous dikes and sills of very fine-grained quartz and feldspar, have themselves been altered and partly replaced during ore emplacement. Ore emplacement in the district was controlled by dikes and sills, and steeply dipping NW and NE fractures. Movement took place on faults belonging to both of these fracture sets before, during, and after the period of mineralization. Two structural types of ore body are exposed - gently dipping silicate-sulfide sheets or "mantos," and steeply plunging chimneys of sulfide ore with or without minor amounts of silicates, localized along steep NE or NW faults. The main manto consists of a central zone with wollastonite, grossularite, and vesuvianite (idocrase), minor scheelite. This zone grades laterally into ore containing manganhedenbergite, quartz, and calcite, that was deposited in cooler ground adjacent to the central conduit. The fluids that deposited these marginal zones were depleted in Al, W, and Ag, and relatively enriched in Fe by the deposition of silicates and sulfides in the central zone of the ore body. The silicates and sulfides were introduced into the manto by a single fluid that did not change in composition during the course of formation of this ore body. Silicates were deposited first, followed by sulfides that crystallized at temperatures on the order of 500 degrees -550 degrees C. The mantos were cut by silicate-bearing chimneys of sulfide ore that were in turn succeeded by mesothermal massive sulfide ore during the last stages of mineralization in the district. Most of the mesothermal ore appears to have been deposited at the same temperature and pressure as the earlier pyrometasomatic ore, suggesting that in this instance, the difference between the 2 types of ore is due to progressive changes with time in the composition of the ore-forming fluid, rather than to changes in the physical conditions of deposition. The silica, Al, Mg, W, and Ag content of the fluid decreased relative to the content of Pb, Zn, Fe, and S. In addition, the amount of Fe carried by the fluid increased markedly with respect to Pb and Zn during the course of mineralization in the district. While the oxidation state of the fluid as a whole remained nearly constant with time, it did change, due solely to the drop in temperature, during the course of crystallization within each successive ore body. The inferred changes in the mineralizing fluid with time are virtually identical to the changes in the mantos produced by the progressive reaction of the fluid with limestone.