By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Lake County lies near the center of Colorado in a mountainous area that extends from the crest of the Mosquito Range on the east to the crest of the Sawatch Range on the west. The Arkansas River heads in Lake County and flows south across the middle of the county.
Colorado's most important mining districts, Leadville and Climax, are in Lake County; however, only Leadville is important as a source of gold. Relatively small amounts of gold have come from placers in the Arkansas River valley. Through 1959 Lake County produced about 2,983,000 ounces of gold as well as large amounts of silver, molybdenum, lead, zinc, and copper.
ARKANSAS RIVER VALLEY PLACERS
Most of the placer gold production in Lake County has come from California Gulch in the Leadville district, but some has come from many other tributaries of the Arkansas River. The more productive of these were Box and Lake Creeks, which enter the Arkansas River valley from the Sawatch Range in southern Lake County.
Although many of the gold placers in the Arkansas River valley were found in 1859 and in the early 1860's, their discovery and subsequent history and production were obscured by news of the richer discoveries in California Gulch; consequently,, early production is unknown. No placers were worked in Lake County during 1886-1914.
From 1915 to 1924 a bucket dredge, known as the Derry Dredge, operated successfully along the valley of lower Box Creek (Vanderwilt, in Vanderwilt and others, 1947, p. 126). These and later operations along Box Creek yielded about 39,000 ounces of gold. Total production of the district through 1959 was about 41,000 ounces. The placers were virtually idle from 1948 through 1959.
The following brief history of the Leadville mining district and its production was abstracted from reports by Henderson (1926, p. 40-43, 130-176). The early history of Lake County is virtually the history of mining in the Leadville district.
The first ores found and mined in Lake County, as in most of the mining camps in Western United States, were gold placers. At the time of the "Pikes Peak excitement," some of the early prospectors, searching for gold in stream gravels, wandered across the Rampart, Tarryall, and Mosquito Ranges into South Park and the Arkansas River valley. Early in the spring of 1860, placers were discovered in Iowa and California Gulches, tributaries of the Arkansas River, in what was to become the Leadville district.
News of the rich discoveries spread with amazing rapidity, and by July 1860 the placer camp called Oro City boasted a population of 10,000. The placers, though rich, were quickly depleted, and within 3 or 4 years only a few hundred of the more persistent souls remained. It was reported that $2 million in gold was taken out the first summer, and the placers continued to be productive, but at a diminishing rate, until 1886.
In June 1868 gold lodes were discovered which were mined with great success until 1877, after which gold production was overshadowed by silver and lead.
The mining of rich silver-bearing lead carbonate ore began in the summer of 1874 and brought great prosperity to the Leadville district, especially from 1876 until 1902, which was the most productive period in its history. The first railroad to reach Leadville was the Denver and Rio Grande in August 1880. The Colorado Midland Railway, running westward from Colorado Springs and across South Park, reached Leadville in September 1887.
The period of general prosperity, however, was marred by declining silver content of the ores, by the financial depression of 1893, and by labor troubles. As a result, there was increased prospecting for gold which culminated in increased gold production from the Breece Hill area after 1893. Gold continued to be an important commodity through 1917, though its proportion of the total output of the district was overwhelmed by the development of the huge zinc ore bodies after 1903 (Emmons and others, 1927, p. 111-133).
The Leadville mines closed in 1957, and from 1957 to 1959 only small amounts of gold were recovered from fluxing material. The total gold output of the district through 1959 was about 2,970,000 ounces.
According to Behre (1953, p. 18-60), the bedrock in the Leadville district consists of a Precambrian basement complex of gneiss, schist, and granite, overlain by about 500 feet of Cambrian, Ordovician, Devonian, and Mississippian sedimentary rocks and 2,500 feet of Pennsylvanian sedimentary rocks.
The Precambrian and Paleozoic rocks are intruded by numerous sills and dikes and by a few stocks of porphyry and pipelike bodies of volcanic breccia, all of Tertiary age. The igneous rocks are chiefly quartz monzonitic, but some are granitic or rhyolitic. Several irregular, roughly funnel-shaped pipes of Tertiary agglomerate have been partly outlined by mine workings.
The rocks in the Leadville district have been tilted and extensively faulted. According to Tweto (1960), all the porphyries are older than the ores, and nearly all of the faults originated before the ore, although many faults were reactivated after mineralization.
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.