The Western San Juan Mountains: Their Geology, Ecology, and Human History

7. Ore Deposits and Minerals

CHAPTER 7 ORE DEPOSITS AND MINERALS SCOTT FETCHENH/ER Gold! Silver' The fabulous riches of the San Juans were discovered in the last quarter of the nineteenth century, and hordes of prospectors poured into the area. Every miner had romantic visions of wealth and dreamed of enjoying the success of Leadville's Horace Tabor or Silverton's Stoiber brothers (of Silver Lake Mine fame). El Dorado layover the next ridge, just waiting to be found. Mining towns and civilization sprang up with each new discovery. Some disappeared almost as quickly, products ofthe boom-and-bust economics ofthe mining and metal industries. Gold and silver were the driving forces that caused men to throw caution to the wind and to migrate westward from their homes and families in the East. Some of the greatest mines in Colorado, such as the Sunnyside , Camp Bird, and Idarado, have produced millions of tons of ores containing gold, silver, lead, zinc, and copper over the last 100 to 120 years. But the settling of southwestern Colorado and the great mining discoveries of the past would not have occurred had it not been for geologic events that happened millions ofyears ago (see Chapter 6). GEOLOGY STRUCTURAL CONTROLS The Silverton and Lake City Calderas, along with a downdropped block ofground between them called the Eureka Graben, were the 80 Ore Deposits ond Minerals 81 most important structural features for localization oflater ore bodies in this area (see Chapter 6, Fig. 6.7). Doming and collapse ofthe Silverton Caldera was accompanied by radial and concentric faulting. These faults, when open, acted as a plumbing system for later upward migration of hot, acidic, mineral-laden waters. These hydrothermal solutions, charged with metals such as gold, silver, lead, zinc, and copper, leached from the surrounding rocks and percolated upward over long periods of time. As these ore fluids encountered lower temperatures and pressures near the surface, they precipitated minerals along the sides of the faults as veins and altered and leached the surrounding host rock. These veins were often shattered by recurrent fault movement and then cemented by subsequent episodes of ore deposition. The most productive ore deposits in the San Juans were open, space-filled vein deposits. Vein orcs were most often confined within fault walls or within a zone of multiple fractures within a faulted area. Veins are tabular, greater in length (strike) and depth below ground than in thickness. They often pinch and swell both vertically and horizontally and change direction depending on the brittleness and composition ofthe surrounding host rocks. LITHOLOGIC CONTROLS The San Juan Tuff, Eureka Rhyolite, Burns Latite, Telluride Conglomerate , and varied limestone and sandstone layers in the older sedimentary series were the most favorable host rocks for vein ore bodies. Smaller amounts of production came from replacement ore bodies in sedimentary rocks. Hydrothermal solutions, migrating upward along veins, would, upon encountering a favorable host rock such as limestone, eat away and replace the original rock with ore minerals. Many of the deposits in Rico, Ouray, and the lower levels ofthe Camp Bird and Idarado Mines were ofthis type and are called replacement bodies. Intrusions of molten laccoliths, stocks, dikes, and sills also exerted local control on ore deposition, especially in the La Plata Mountains, Rico, Ouray, the Red Mountain breccia pipes, western Ophir, and the Yankee Boy Basin southwest of Ouray. 82 The Western San Juan Mountains MINERALOGY The San Juan ore deposits contain a wide variety of minerals and native elements. Gold and silver occur in their native forms and in complex combinations of tellurides. Silver also occurs in tetrahedrite /tennantite, pyrargyrite, proustite, argentite, and argentiferous (silver-bearing) galena. Base metals are found in galena (lead), sphalerite (zinc), chalcopyrite (copper), pyrite (iron), and hubnerite (tungsten), as are subsidiary amounts oftess economic minerals. White, milky "bull" quartz is the most common gangue, or waste mineral, accompanied by rhodonite, rhodocrosite, calcite, barite, and fluorite. Ransome identified more than seventy-seven mineral species in his study of the San Juan mining districts in 1901. SILVERTON AREA Silverton is synonymous with the famous Sunnyside Mine, one of Colorado's largest gold mines, which contained more than 60 miles of underground workings. The original Sunnyside vein was discovered by R. J. McNutt and George Howard in August 1873 on the northern shores of Lake Emma. The Sunnyside Mine lies within the downdropped block, or graben, between the Silverton and Lake City Calderas. The numerous veins...