Idaho Springs Mining District (Virginia Mining District), Clear Creek County, Colorado, USAi

Regional Level Types
Idaho Springs Mining District (Virginia Mining District)	Mining District
Clear Creek County	County
Colorado	State
USA	Country

The Central City-Idaho Springs district is about 30 miles west of Denver in the southeast part of the Central City quadrangle and the northeast part of the Georgetown quadrangle; it includes parts of Gilpin and Clear Creek Counties. The district has an area of about 25 square miles. Its principal towns are Central City, Blackhawk, and Idaho Springs. The area southwest of Idaho Springs, which is known as the Freeland-Lamartine area, is regarded in this report as part of the Central City- Idaho Springs district, because there is no definite break between the two either in the type of rock or the character and extent of the ore deposits.

The district ranges in altitude from 7,600 to 10,600 feet, and in places it is rugged. The physiography of the district is that of a dissected rolling upland. Much of the area just south of Central City is on the Overland Mountain erosion surface, and supergene enrichment has been especially evident in the upper parts of the veins in this area. A few mountains north of Clear Creek rise to a height of 10,000 feet and are probably monadnocks rising to the level of the Green Ridge surface. In the southwestern part of the district high spurs from the Continental Divide rise to heights of more than 11,000 feet.


HISTORY:

In the winter of 1858 George A. Jackson, of Missouri, discovered the hot soda springs near the present site of Idaho Springs, and on January 7,1859, he washed gold from the "Idaho bar" near the mouth of Chicago Creek. This was the first placer of commercial grade to be found in Colorado, and its discovery led to a rush of prospectors to that region. On May 6, 1859, John H. Gregory made the first lode discovery in the Rockies when he found rich oxidized gold ore along the outcrop of the Gregory vein just east of the site of Central City.

During the summer of 1859 many other lode discoveries were made, and before winter 100 sluices were at work in the vicinity of Gregory's discovery, washing oxidized lode ore and the rich placer ground nearby. The rich oxidized ores were soon removed from the lodes, and by 1864 much of the gold mined was mixed with sulfides and was not amenable to amalgamation. In 1865 J. E. Lyon built the first smelting works in Colorado at Blackhawk and within a short time was successfully treating the sulfide ores so recalcitrant to amalgamation. The district had been nearly at a standstill during the Civil War because of lack of men and the low recovery of gold from the ores, but with the satisfactory smelting of the sulfides in 1866 the district began a long period of continued development and increased production from lode ores. Although the first silver vein found in Colorado was discovered near Montezmna in 1864 by Coley, it was not until 1877 or 1878 that the silver veins of Silver Hill, near Blackhawk, were discovered.


ORE DEPOSITS:

Gold, silver, copper, lead, zinc, and uranium ores occur in the Central City-Idaho Springs district, but shipments have owed their value mainly to gold and silver. The ore deposits are veins and stockworks formed during the Laramide revolution and are genetically related to porphyritic intrusive rocks. The district is on the southeast side of the main porphyry belt at a place where the eastern edge of the mineralized part swings from east-northeast to north. The ores are of two main types-one consisting mainly of pyrite, chalcopyrite, and quartz and the other of galena, sphalerite, chalcopyrite and subordinate pyrite. A zonal arrangement of the ores is distinct.

Gold-silver Ores: Most of the ore deposits of the district are worked primarily for gold and silver, but some copper and lead and a small amount of zinc and uranium are credited to the district's output. The pitchblende ores are only a local variant of the gold- silver veins. In nearly all the primary ores and in all but a few of the supergene ores gold predominates over silver in value. Bastin classified the gold-silver lode ores under four types: Pyritic ores, galena-sphalerite ores, composite ores (pyritic-galena-sphalerite ores transitional in character between the first two types), and telluride ores. The most widely distributed ores of the region are those of the pyritic gold deposits. This type includes most of the ore bodies between Central City on the north and Pewabic Mountain on the south and most of those along Fall River below the mouth of York Gulch. Galena-sphalerite ores occur on Seaton Mountain near Idaho Springs, on Alps Hill, northwest of Russell Gulch, and on Nigger Hill, Maryland Mountain, and Silver Hill north of Central City and Blackhawk. Ores of the composite type are most common in the border zones between areas occupied largely by pyritic ores on the one side and by galena-sphalerite ores on the other. They occur just east and west of Central City and between South Willis Gulch and the summit of Seaton Mountain. The telluride ores are confined to a narrow belt extending southward from Bobtail Hill near Central City to the head of Gilson Gulch and southwestward from this locality to Chicago Creek.

Pyritic ores: The pyritic ores are predominantly pyrite and gangue with subordinate amounts of chalcopyrite, tennantite, gold, and in places enargite and other metallic minerals. In these ores gold greatly predominates in value over the other metals. The gold and silver are chiefly associated with chalcopyrite and tennantite. Coarsely crystalline pyrite is proverbially of low grade. The chalcopyrite in most places makes up less than 5 percent of the ore by weight but locally amounts to as much as 50 percent. Silver and gold are rarely visible. Enargite is found in a few veins, in some of which it is as abundant as pyrite. Galena, sphalerite, molybdenite, and native bismuth are present here and there. The predominant gangue mineral is quartz, but locally siderite or fluorite may be abundant, and where the wall rock has been replaced sericite is abundant.

Most of the minerals characteristic of the pyritic ores were deposited during a single period of mineralization, but a prevailing order of crystallization is recognized. Where molybdenite is present, it appears to have been the first mineral to crystallize, but in most of the ore pyrite is the earliest mineral; and apparently it continued to crystallize throughout the period of ore deposition. Chalcopyrite began to form early in the period of mineralization, but its deposition reached a maximum in the later stages I as is shown by its characteristic abundance in the medial portions of certain veins and in or near many vugs. The tennantite shows similar relations, but its period of maximum deposition was later than that of the chalcopyrite. A small amount of galena and sphalerite is apparently contemporaneous with the pyrite and chalcopyrite. Almost nowhere is banding developed. Vugs are only moderately abundant and are usually small in the pyritic ores, which are as a rule irregularly massive in texture.

The average gold content of the smelting ores is usually between 1 and 3 ounces, and the average silver content between 4 and 8 ounces to the ton. The copper content in most ores is below the commercial limit of 1.5 percent, but in some ores it may be 15 to 16 percent. The silica content is extremely variable, but in most lots is between 30 and 70 percent.

Galena-sphalerite ores: In the galena-sphalerite ores the predominant primary sulfides are galena, sphalerite, and pyrite; subordinate chalcopyrite and a little tennantite and bornite also occur. Enargite, native bismuth, and molybdenite are found locally in small amounts. The common gangue minerals are quartz and either calcite or siderite, but rhodochrosite is present in a few veins. Gold and silver, probably alloyed, are finely distributed within the sulfides, and in a few veins visible gold is relatively abundant. In these ores also gold is associated with chalcopyrite and tennantite rather than with the pyrite, but in some ore shoots it is associated with sphalerite. Silver increases with tennantite and galena, and locally chalcopyrite and sphalerite are notably rich in silver. According to Bastin, galena, sphalerite, pyrite, chalcopyrite, bornite, enargite, quartz, siderite, barite, free gold, and rhodochrosite seem to have crystallized contemporaneously in most of the ores. Siderite and calcite are early in a few places, but most commonly are the latest minerals and occur alone or with quartz as linings in vugs. Tennantite is much less abundant in the galena-sphalerite ores than in the pyritic veins and was one of the last minerals to form. These ores are, in general, similar structurally and texturally to the pyritic type, and the wall-rock alteration is similar. The chief differences between alteration along these and the pyritic gold veins are in the more abundant development of calcite and in the presence of sparsely disseminated galena and sphalerite at a few places in the walls.

In the ores of the galena-sphalerite type the variations in metal content are even greater than in the pyritic ores. In general the galena-sphalerite ores are poorer in gold and copper and richer in silver than the pyritic ores. The gold content of the galena-sphalerite ores commonly ranges between 0.15 and 3 ounces to the ton and the average silver content between 5.50 and 40 ounces to the ton. The average percentages of copper, lead, and zinc range from less than 1 to as much as 17 percent of copper, 54 percent of lead, and 32 percent of zinc.

Composite Ores: The composite ores may be the result of dual mineralization, first with the minerals characteristic of the pyritic type and later with minerals characteristic of the galena-sphalerite type. Veinlets of the galena-sphalerite type cut sharply across pyritic ore in several mines in the district, but there are numerous transitions from the composite ores to one or the other of the pure types. Bastin believes the interval between the two stages of mineralization was short but sufficient to permit much local brecciation of the earlier ore before the later ore was deposited. In some ores composed of nearly solid pyrite with some chaleopyrite, the ores have been brecciated and the fragments cemented by quartz, galena, sphalerite, and chalcopyrite. At places in the Centennial-Two Kings vein, pyritic ore is sharply separated from ore of the galena-sphalerite type by a band of comb quartz. The metal content of the ores of the composite type varies greatly with the proportion of the two types present, but in general it is intermediate between the two.

Telluride ores: The telluride ores show more diversity in mineral character than the ores already described and probably formed at a later stage in the Laramide revolution. So far as known, no tellurides of gold and silver occur in either the pyritic or galena-sphalerite ores. The most important shipments of telluride ore have been from the War Dance and East Notaway mines near Central City, the Gem vein near the Gem Shaft, the Jewelry Shop vein, and the, Treasure Vault mine near Idaho Springs. These telluride shoots all lie in a narrow north-northeasterly zone extending from near the mouth of Chicago Creek to Bobtail Hill and are at the intersection of the Dory Hill fault with northeasterly veins in the northern half of the zone. The telluride ores consist of gold and silver tellurides in a gangue of blue-gray fine- grained horn quartz and small amounts of fluorite, ferruginous calcite, and fine pyrite. The pyrite is less abundant in the veins than in the wall rocks. In general no very consistent sequence was observed, although fluorite and calcite line some vugs and were there the last minerals to form. Although the gold-telluride ore mined on the Gem vein came from the 12th level, most of the telluride ore thus far mined has been obtained within 125 feet of the surface and has contained free gold in even greater abundance than tellurides. This gold is partly and perhaps wholly the result of the oxidation of gold-telluride minerals. The tellurides occur chiefly as flakes or plates of pale brass-yellow color. Altaite, coloradoite, krennerite, petzite, and sylvanite have been identified by M. N. Short in ore from the Jewelry Shop mine, which is typical of the rich ores of this group. The ratio of gold to silver in the telluride ore shipped from the War Dance mine commonly ranged between 1:1 and 3:1, and the average for 36.8 tons was 16.31 ounces of gold and 13.06 of silver to the ton. In the War Dance and Jewelry Shop mines the telluride ore formed in a network of small veinlets and as an irregular replacement of the wall rock near fractures. In the East Notaway mine the telluride veins are characteristically 1 to 3 inches wide and consist of dark fine-grained gray "horn" quartz, fine-grained pyrite, some antimonial tennantite, and varying amounts of the tellurides. All the vein minerals are essentially contemporaneous, but the sulfides are more abundant near the borders of the veinlets, and sylvanite is more abundant near the center.

Ueanium ores: Pitchblende associated with sulfides occurs in several veins on the south slope of Quartz Hill. The pitchblende is largely restricted to a north-south zone about a quarter of a mile wide and a mile long and occurs in the east-northeast veins that cross it. Two north-northwesterly bostonite dikes cut diagonally across the productive belt, and, according to Alsdorf the parts of the dikes are strongly radioactive. It is possible that the uranium-bearing solutions are related to a deep bostonite magma apexing in a narrow northerly zone at depth and that these fluids first followed fissures or dikes transverse to the east-northeasterly veins in which the ore has been found. For many years a small and sporadic output has come from this group and has been used mainly for specimens and for experiments. Pitchblende has been reported from the Alps, Beleher, Calhoun, German, Kirk, Leavenworth, Mitchell, Pewabik, Wood, and Wyandotte mines. The pitchblende occurs as a minor component of the pyritic ores, and most of these mines have been worked for gold and silver rather than pitchblende. Alsdorf believed that the pitchblende veins were earlier than the pyritic veins, but according to Bastin it is evident in a number of specimens that pitchblende, chalcopyrite, and minor amounts of pyrite and gray quartz crystallized contemporaneously. Other specimens contain sulfides that are later than the pitchblende. A rich specimen from the Calhoun mine contains abundant pitchblende sharply cut by veinlets of sphalerite, pyrite, and galena. Bastin believes that the pitchblende ores were deposited during the early pyritic mineralization and that ores of the galena-sphalerite types were later deposited in the fractures. The pitchblende ores are believed to represent a local and unusual variation of the main sulfide mineralization of the district.

Placers: Although placer mining within the district has persisted to the present day, it had ceased to be of major importance before the end of the eighteen sixties.

Productive operations were started in 1933 at the junction of Clear Creek and North Clear Creek, 5 1/2 miles east of Idaho Springs. A dry-land dredge operated here in gravel that yielded 50 cents a yard in 1935. The most extensive placer workings in the district were in the valleys of Chicago Creek and Clear Creek near Idaho Springs, and they developed not only the present stream gravels but also glacial outwash gravels and gravels of possible pre-glacial age. These deposits, which are described by Spurr and Garrey have not been productive for some time. Intermittent placering continues in the district southeast of Central City, however, chiefly in the valley of North Clear Creek and in Russell Gulch, in spite of the fact that nearly all this ground has been worked and reworked several times. As would be expected, present yields are low.