south for . Mineralization of the CMB came primarily by way of intrusions of Tertiary Period magmas. The primary ores of the CMB were generally deposited as mixed metal sulfide mineral veins containing pyrite, galena, sphalerite, chalcopyrite, and gold, silver, and copper. During the last glacial period native gold was freed from the host rock and became available for placer mining. The district is a highly faulted area, intruded with Tertiary quartz monzonite porphyries, on the east side of the Arkansas River graben, part of the Rio Grande Rift system. The silver occurs associated with manganese and lead in veins, stockworks, and manto-type deposits in the Mississippian Leadville Limestone (here a dolomite), the Devonian Dyer Dolomite, and the Ordovician Manitou Dolomite. Ore minerals are pyrite, sphalerite, and galena, in jasperoid and manganosiderite gangue. In upper levels, the ore minerals are oxidized to cerussite, anglesite, and smithsonite. Leadville was the largest silver-producing district in Colorado. Cumulative production through 1963 was of silver, of gold, of lead, of zinc, and of copper. Geologist R. Mark Maslyn describes the Leadville mining district and the nearby Sherman Mine area saying: Several mining districts surrounding the central Colorado Sawatch Range contain economic deposits hosted by late Mississippian paleokarst features. These are primarily developed in and along the upper surface of the early Mississippian Leadville Formation. Paleokarst features include isolated eaves and sinkholes as well as integrated cavern systems that are mineralized and can be traced from insurgence to outlet. Environmental damage related to mining operations Mining, mineral processing and smelting activities in the area have produced gold, silver, lead and zinc for more than 130 years. Wastes generated during the mining and ore processing activities contained metals such as arsenic and lead at levels posing a threat to human health and the environment. These wastes remained on the land surface and migrated through the environment by washing into streams and leaching contaminants into surface water and groundwater. The site was added to the U.S. Environmental Protection Agency (EPA) National Priorities List in 1983,. Investigation of the site began in the mid-1980s. Since 1995, EPA and the potentially responsible parties have conducted removal and remedial activities to consolidate, contain and control more than of contaminated soils, sediments and mine-processing wastes. Cleanups by the potentially responsible parties have involved drainage controls to prevent acid mine runoff, consolidation and capping of mine piles, cleanup of residential properties and