Lamartine Mining District, Clear Creek County, Colorado, USAi

Regional Level Types
Lamartine Mining District	Mining District
Clear Creek County	County
Colorado	State
USA	Country

The Chicago Creek area, in Clear Creek County, Colorado, about 3 miles southwest of Idaho Springs forms a part of the Front Range mineral belt, a northeast-trending belt of coextensive veins and porphyry intrusives of Tertiary age. The area occupies about 5 1/2 square miles along the northwest side of Chicago Creek. At least $4.5 million of gold, silver, copper, lead, zinc, and uranium ores has been produced in the area from mesothermal veins of Tertiary age. These veins occupy fractures that cut Precambrian metamorphic and igneous rocks and porphyry intrusive rocks of Tertiary age.

About April 1, 1859, pay gold ore was discovered in a placer near the mouth of Chicago Creek. Soon after this initial discovery a search for gold veins spread up Chicago Creek and into its tributary streams and gulches. Probably the first veins discovered were the Quito and the Little Mattie. Exploration was carried on in these and other mines. Only supergene- enriched ore was shipped at first, and extensive underground development of the veins did not begin until about 1880. By 1884 the Little Mattie, Silver Ring, Charter Oak, Muscovite, Kitty Clyde, Humboldt, Eclipse, and Silver Glance mines were all being worked. Vigorous development work in these and many other mines was carried on until the Silver Panic of 1894, when the silver mines shut down or drastically reduced operations. Mining picked up again after 1900, but it has been intermittent and generally on the decline since about 1910. Several base-metal mines were reopened during World War I and a few gold mines were reopened during the depression years. During the 20-year period between 1920 and 1940, only 2 new mines, the West Gold and the Dixie, were opened and developed to any great extent.

The Chicago Creek area contains veins that bear gold, silver, copper, lead, zinc, and uranium; these deposits were formed as hydrothermal fillings in fault fissures. Replacement of the wall rocks by the ore minerals was unimportant as a method of formation of the ore deposits. Most of the veins have smooth walls; some are lodes, inasmuch as they have a foot-wall vein and a hanging-wall vein separated by a few feet of altered wall rock that at places contains stringers of vein minerals. Slickensides are abundant, and most are nearly horizontal. The vein fractures are fairly regular in strike and dip, and irregularities, where present, commonly provided favorable structures for the deposition of the ore minerals. Repeated opening of the veins is shown by brecciated gangue and sulfides that are cemented by later gangue and sulfides which at places are of different composition from the earlier vein minerals. Vugs, though not uncommon, are generally small, and the veins tend to be filled completely even where they have been fractured and reopened several times. The vein zones, from fresh wall rock on one wall to fresh wall rock on the other, are generally less than 3 feet wide although some, such as the Little Mattie, are as much as 8 feet wide. The metallic ore minerals exclusive of disseminated pyrite commonly are confined to less than 8 inches of the vein zone.

The principal ore minerals are sulfides and sulfosalts of iron, copper, silver, lead, and zinc. Those that are less abundant but common in the area include native gold, native silver, and hydrous uranium phosphates. Ore minerals that have been mined but are rare in the area include tellurides of mercury or of gold and silver and oxides of uranium. The gangue minerals include several varieties of quartz and several kinds of carbonate minerals. The most abundant gangue mineral is vein quartz, but locally a very fine-grained variety, called chalcedonic quartz in this report, forms a major part of the gangue. The chalcedonic quartz commonly is colored tan, brown, or black. Small amounts of opal have been reported from a few mines in the area. Carbonates of calcium, iron, magnesium, barium, or manganese form a small part of the gangue in many mines.

Five principal types and one sub-type of veins have been recognized in the area on the basis of quantitative mineralogy. In general, pyritic types are mined for gold and galena-sphalerite types are mined for lead and silver. Mixtures of 2 or more of the 5 principal types in a reopened vein or in a lode has resulted in some ore bodies that have been mined for gold, silver, and lead. No veins have been mined specifically for copper or zinc, although these metals have been recovered during processing of many of the ores. One vein (shear zone) has been mined for uranium. In general, all types of veins occur in similar vein structures in all types of host rocks.

The Pb/U determinations on pitchblende from veins in the Central City district (about 5 miles northeast of the Chicago Creek area), made by Holmes (1946) and Phair (1952), indicate an age for those veins of about 60 million years, or early Tertiary. As the veins in the Chicago Creek area are in the same geologic setting as the veins in the Central City district, these two groups of veins are probably the same age.