Eagle County Colorado Gold Production
The replacement deposits in the limestones consist of chimneys of pyritic silver-copper ore and mantos of base-metal sulfide ores. The chimneys are downward-tapering pipes that extend from the ends of manto ore bodies near the top of the Leadville Limestone downward into the Parting Quartzite Member of the Chaffee Formation. The chimneys are roughly circular or elliptical and are as much as 300 feet in diameter at the top and taper downward. There is no physical break between the two types of ore bodies, but there is a pronounced mineralogic difference. The chimney ore, the chief source of the gold, consists of a core of pyrite containing minor quantities of other minerals which make them valuable for silver, copper, and gold. The chief copper mineral is chalcopyrite. Silver and gold are associated with chalcopyrite, galena, and with a group of late copper and silver minerals including tetra-hedrite, freibergite, polybasite, stromeyerite, bournonite, and schapbachite. Late accessory minerals include manganosiderite, dolomite, barite, apatite, and quartz. The galena contains small inclusions of hessite and a little petzite, and the petzite contains minute blebs and veinlets of free gold.
The manto ore bodies are valued mainly for zinc. The minerals of these deposits are sphalerite, pyrite, manganosiderite, minor galena, and accessory chalcopyrite, barite, dolomite, and quartz. Oxidized parts of these deposits contain considerable lead, silver, and gold. The mantos are 50 to 300 feet wide, 5 to 150 feet thick, and as much as 4,000 feet long. All are in the Leadville Limestone (Ogden Tweto and T. S. Lovering, in Vanderwilt and others, 1947, p. 381-385).