Middle Tertiary ore deposits of significant size and number replace the Eocene Telluride Conglomerate adjacent to northwest-trending base-metal veins on the north-west side of the Silverton caldera in the San Juan Mountains, Colorado.Sphalerite, galena, chalcopyrite, and pyrite are the major sulfide minerals, and the ores contain some cadmium, bismuth, silver, and gold. Major gangue minerals are quartz, green epidote, chlorite, rhodonite, pink manganese-bearing epidote, and carbonates; there are lesser amounts of sericite and clay minerals. Most of the ore is characterized by complete replacement of the original conglomerate, and consists of 70 to 80 percent massive sulfides and about 20 percent fine-grained greenish gangue minerals. A lesser amount of conglomerate-type ore contains 15 to 20 percent coarse sulfides disseminated in the matrix and in pods of sulfides and gangue; the original conglomerate host is still clearly evident in this type of ore. Some siltstone-type ore is characterized by sulfides disseminated in local sandy and silty units; this type of ore lacks any sulfide-gangue pods.Alteration related to the mineralization changed normally reddish units in the conglomerate to gray and green as chlorite and sericite formed in the matrix. Near the ores the grays and greens give way to pinks and pale green where manganese-bearing epidote and carbonates, rhodonite, and green epidote formed. Sulfide minerals occur as small grains disseminated through the matrix of the altered conglomerates and in pods composed of chlorite, rhodonite, and carbonates. Toward the ore zone, the altered pods are more numerous and the disseminated sulfide grains become larger.All the known replacement ore deposits are adjacent to base-metal veins which acted as the conduits for the mineralizing fluids. Where dikes and veins coincide, replacement ore, if present, is located where a vein is alongside rather than within a dike. Permeable, calcareous, conglomeratic rock units appear to have been the most susceptible rocks for replacement. Permeability was enhanced locally by minor fractures and bedding planes. On a regional scale the replacement ores are part of a zonal pattern of altered and mineralized rocks that formed progressively outward from a probable source for the mineralizing and altering fluids in the vicinity of the Red Mountains altered area.Replacement ore is most common in the Telluride Conglomerate, but some occurs in the underlying Cutler and Dolores Formations as well.