The original or hypogene ore deposits in the Leadville district have been classified into three main groups: (!) silicate-oxide deposits, (2) mixed sulfide veins, and (3) mixed sulfide replacement bodies (G. F. Loughlin and C. H. Behre, Jr., in Van-derwilt and others, 1947, p. 360-365).
The silicate-oxide deposits are of little economic value; only small amounts were mined in the early days for smelter flux. These deposits are mixtures of magnetite and hematite in a gangue consisting mainly of serpentine, and are replacement deposits in dolomite. The only ore mined has come from pyritic gold veins that cut these deposits and enriched the adjacent wallrocks.
The mixed sulfide veins occur mainly in siliceous sedimentary rocks which predominate in the eastern part of the district, where numerous sill-like bodies of porphyry intrude the grit and shale of Pennsylvanian age near the Breece Hill porphyry stock. The largest veins have been productive to a depth of about 1,300 feet below the surface. Some veins are too small to mine but expand into small replacement bodies where they cut dolomite. The veins that cut siliceous sedimentary rocks consist mainly of pyrite with a little interstitial chalcopyrite in a gangue of quartz. Where they grade into replacement deposits, pyrite and quartz persist for a short distance laterally but grade into a mixture of sphalerite and galena in dense quartz or jasperoid. The veins and the pyritic parts of the replacement deposits have been valuable mainly for gold, some of which is primary but much of which has resulted from enrichment in the secondary sulfide zone. The gold is accompanied by some silver and locally by copper.
Replacement deposits of sulfides in dolomite are common in the western part of the district. These replacement bodies lie along fractures or sheeted zones, known locally as contacts, beneath impervious covers such as sills. The largest replacement bodies are at the top of the Leadville Dolomite (Mississip-pian), and some are more than 2,000 feet long, 800 feet wide, and 200 feet thick.
The mixed sulfide replacement bodies consist of sphalerite and galena with pyrite. The ore contains a few ounces of silver and 0.03 to 0.05 ounce of gold to the ton, but here and there small shoots have been found that are unusually rich in silver and gold and also contain bismuth. Intergrowths of argentite, bismuthinite, and a little galena have been found in this rich ore and also in veins cutting large bodies of the mixed sulfide ore. Tetrahedrite, chalcopyrite, and arsenopyrite also occur locally.
Oxidation and supergene enrichment of the various types of hypogene sulfides produced ores of variable mineralogy. Some of the ores are rich in cerussite and cerargyrite; others, in smithsonite and hemimorphite, manganese-iron-bismuth oxides, chalcocite-covillite-gold-silver, and argentite-silver. Much of the coarse gold in the placers of California Gulch is believed to be of reworked supergene origin.
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