The Horseshoe and Sacramento districts lie near the Alma and Leadville districts, Colorado. Paleozoic sedimentary rocks intruded by Tertiary (?) porphyry sills are folded into a northwestward trending asymmetrical anticline, whose steep west limb is broken by the London reverse fault, with an apparent vertical displacement of around 3,000 feet.All the known ores, which form small silver-lead replacement deposits along small fissures in pre-Pennsylvanian dolomites, occur on the east or hangingwall side of the London fault. These favorable stratigraphic horizons, which lie just below those that contain the valuable gold-quartz veins on the footwall side of the London fault in the nearby London mine, lie far below the surface west of the fault in the Horseshoe and Sacramento districts.Mineralization includes widespread recrystallization of the dolomites, less widespread replacement of dolomite by jasperoid, and deposition of ore accompanied by additional local wall rock alteration. Field and, particularly, microscopic observations dealing with all these processes are discussed.The ores mainly contain pyrite, sphalerite, galena, tennantite, and argentite in a gangue of quartz, barite, carbonate, and recrystallized and silicified country rock; manganiferous ankerite, luzonite, chalcopyrite, and an unidentified sulphide locally occur in small amounts. These minerals exhibit a zonal distribution as to mineral species, textures, and compositional variations. Following the precedent established for the Leadville region, non-baritic ores in the central zone of ore deposition may be classed as "intermediate mesothermal," baritic ores more distant as "cooler mesothermal," and the most distant as "telethermal." The productive area is confined to the "intermediate" and the inner portion of the "cooler mesothermal" zones.The jasperoids are grouped into three textural types: (1a) anhedral-nodular, (1b) anhedral-granular, and (2) euhedral. Particularly significant is their zonal arrangement, which reveals the finest-grained textures nearest the center of ore deposition.Economic interest pertains not only to discovery of new orebodies within the area of known mineralization but also to the possibilities at depth in favorable horizons west of the London fault. The question of possible southward continuation of the London gold-quartz veins from the Alma district is highly important but no significant data pertaining to it have been obtained.

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