Minerals from the Topeka-Kentucky Giant and Indiana veins, Ouray County, Colorado.
A series of mines along the southwest slope of Brown Mountain in Ouray County, Colorado, has recently yielded specimen-grade crystals of fluorite on milky quartz, and excellent microcrystals of anatase and brookite. These mines exploit two roughly parallel vein systems, the Topeka-Kentucky Giant on the west and the Indiana on the cast. Specimens are found along a series of mines and dumps located on these veins, as well as at the Concave tunnel, a haulage tunnel intersecting the veins at a lower elevation. Because the access road to these mines dead ends at the Lost Day mine on the upper slope of Brown Mountain (as opposed to being a "through" route), there has been relatively little tourist traffic or prior collecting activity.
LOCATION and HISTORY
The mines are located north of Gray Copper Gulch, on the southwest flank of Brown Mountain, at elevations ranging from 10,385 to 11,056 feet. They appear on the U.S. Geological Survey Ironton Quadrangle 7 1/2 minute topographic map as a series of adits and prospects above the Silver Mountain mine, the latter also known as the Concave tunnel.
There is little recorded information on the history and production of this area. The veins were developed by a series of tunnels and shafts in the early 1880's, and some bodies of lead-zinc and "gray copper" (tetrahedrite-tennantite) ore were stoped from the upper workings in the early days of mining (Burbank and Luedke, 1969). The Concave tunnel was later driven as a haulage tunnel to intersect both veins at depth; this work was done prior to 1941. These mines are all patented claims and are therefore private property.
Several buildings and a trestle remain at the haulage level. The tunnel at this level is flooded and extremely dangerous, and under no circumstances should entry be attempted. Little is left at the upper workings other than dumps and some foundations. The Indiana vein was developed by a tunnel about 336 meters in length and 61 meters higher than the Concave tunnel. Tunnels at a higher level have recently been sealed under the Colorado Mined Land Reclamation program. Altogether more than a mile of development was done on these veins (Burbank and Luedke, 1969).
The Indiana and Topeka-Kentucky Giant veins are located within the Silverton Caldera, in volcanic rocks of middle to late Tertiary age that comprise much of the San Juan region (Burbank and Luedke, 1969). Mineralization within the caldera was controlled by intense faulting and fracturing during subsidence. The Indiana and Topeka-Kentucky Giant veins lie within the Bums Member of the Silverton Volcanic Group. The Bums Member is characterized by medium to dark flow breccias, tuffs and flow-banded rocks of mainly rhyodacitic composition (Burbank and Luedke, 1964). The Indiana vein and the Topeka-Kentucky Giant vein are nearly parallel, striking approximately N 22[degrees] E. The Topeka-Kentucky Giant vein, on the west, dips steeply west at approximately 75[degrees], while the Indiana vein on the east dips approximately 70[degrees] east (Burbank and Luedke, 1964 and 1969; Burbank, 1941). At the surface these veins are approximately 31 meters apart.
Anatase is an uncommon species in Colorado. Three earlier reports of crystallized anatase describe dark blue pyramidal crystals to 1 cm in thin veins in dioritic rock from Gunnison County (Larsen and Hunter, 1914); microcrystals as much as 0.1 mm in size at the Ores and Metals mine in Ouray County (Rosemeyer et al., 1988); and microcrystals to 0.7 mm, associated with brookite, from the Silver Link mine in Ouray County (Rosemeyer, 1993). Anatase from the Silver Link mine is similar in habit and association to that reported here.
Anatase at this locality occurs mostly at the Indiana vein, where it appears as minute, 0.05 to 0.2-mm crystals. They are heavily striated and predominantly of bipyramidal habit, showing occasional pinacoid faces. Crystals occur in shades of amber or blue and have a high luster; most are translucent to transparent. Crystals of different color can be in close proximity on the same specimen. The anatase formed concurrently with, to slightly after, the last quartz deposition, as evidenced by mostly surficial crystals and few subsurface crystals. A less common habit has been observed, showing prominent pinacoids, a nearly black color and submetallic luster. Anatase from this locality is usually associated with small quartz crystals (the transparent to translucent, prismatic habit, as opposed to the milky variety that is more tapered and stout), occasionally with brookite, and rarely with barite and pyrite. Identification of anatase was confirmed by X-ray diffraction (P. J. Modreski, written communication, 1993).
Barite occurs as thin, tabular, gray crystals, commonly with subparallel blades, as large as 2.5 cm across. They are typically zoned in alternating shades of gray, and have a bone-white rim. Massive white barite is frequently found, and is associated rarely with anatase.
Brookite is found as tabular, amber-colored crystals that range in size up to 0.26 mm. Some are distinctly color zoned. Identification was made by scanning electron microscope with energy-dispersive spectrometry (SEM-EDS) capability, which showed that only Ti and O were present (P. J. Modreski, written communication, 1993), and by optical methods. Deposition was contemporaneous with, to slightly after, the last stage of quartz crystallization. Brookite is uncommon in Colorado. It has only been reported from two places in the Iron Hill (Powderhorn) district in Gunnison County, where it occurs as crude, bluish-black crystallized aggregates associated with anatase near the Lot mine (Ray Ranstrom, personal communication, 1993) and as thin films (with anatase) in parts of the gabbro dike in the Iron Hill composite stock (Olson and Wallace, 1956), and has also been noted in a similar occurrence from the Silver Link mine in Ouray County (Rosemeyer, 1993). The locality near the Lot mine is likely the one referred to by Larsen and Hunter (1914), discussed under anatase above.
Pale to medium green, botryoidal to hemispherical fluorite is, besides quartz, one of the more conspicuous minerals here. Two generations can be distinguished. The first occurs either as radiating aggregates composed of individual blades of fluorite, which form intergrown hemispheres typically no more than 7 mm across (or isolated hemispheres as much as 2.2 cm across), or as rough tetrahexahedrons as large as 1.5 cm. Early-generation fluorite is found on matrix fragments or on milky quartz crystals, where it commonly occurs coating one side of the specimen, a result of a directional fluid flow in the cavity that deposited fluorite on the side of the specimen facing the direction of flow. The quality of this fluorite is variable, ranging from dense, lustrous material to somewhat more porous with a chalky appearance when dry.
The later generation is characterized by colorless, transparent microcrystals as much as 2 mm in size. They occur on earlier-formed fluorite or quartz and show a distinct dodecahedral or dodecahedron/ cube combination with sharp, lustrous crystal faces.
Goyazite was observed on a single specimen as microscopic, pale, yellowish tan crystals as much as 1 mm in size, interstitial to small, prismatic quartz crystals. Identification was by optical and SEM-EDS methods. Goyazite has rarely been reported from Colorado; a similar occurrence was documented from the Sweet Home mine in Park County, where it occurs as orange crystals as much as 6 mm across, associated with fluorite, rhodochrosite, and apatite (Kosnar, 1979).
Gypsum occurs as late-formed, microscopic, prismatic crystals to about 1 mm in length that form radiating aggregates coating fluorite or quartz; the aggregates are usually tightly intergrown, but occasionally free-standing prismatic crystals are found. Their identity was confirmed by optical methods.
Halotrichite was observed on a single specimen as radiating acicular tufts within a cavity in massive barite, associated with anatase. individual needles are as large as 0.46 mm in length, and are transparent and colorless with a milky cast. It was identified by SEM-EDS and X-ray diffraction. This species has seldom been documented in Colorado, but is doubtlessly more prevalent than the few reports in the literature indicate.
Pyrite is occasionally found as sharply formed, lustrous pyritohedrons as much as 5 mm across, in thumbnail to (rarely) miniature-size clusters. It is a ubiquitous constituent in wallrock and vein material, as disseminated anhedral grains and as small subhedral grains interstitial to quartz.
Quartz is the most conspicuous mineral at this locality, although large, well-formed crystals are uncommon. Three habits are present:
(1) Medium to deep purple massive amethyst is sometimes seen underlying later-formed milky quartz. The amethyst is translucent and a few specimens have small transparent areas.
(2) The most noteworthy mineral at this locality is porcelain-white milky quartz that is associated with green fluorite. The quartz crystals are tapered, highly lustrous, and commonly have minute drusy quartz crystals near the base or along three adjacent prism faces. Except for the fluorite association, some of these crystals would be nearly identical to those reported from a prospect in the amphitheater east of Ouray (Stoufer and Rosemeyer, 1989; Kile et al., 1991).
Individual milky quartz crystals have been found as much as 10 cm in length; smaller crystals (up to 3 cm) grouped in rounded clusters have also been recovered in cabinet-size specimens. The crystals are almost always partly to mostly covered with intergrown (and occasionally isolated) fluorite hemispheres that sometimes impart an attractive "halo" appearance to the specimen.
(3) Quartz also occurs as translucent prismatic crystals (with transparent tips) as large as 10 cm in length. Prominent scepters are occasionally found, the largest seen being 4.2 cm long. Delicate, acicular crystals are occasionally seen in cavities within massive quartz, associated with small botryoidal to hemispherical fluorite aggregates or crude fluorite tetrahexahedrons.
Galena, chalcopyrite, and other unidentified sulfides or sulfosalts are occasionally noted as subhedral grains interstitial to quartz, and as small crystals, generally less than 2-3 mm, associated with small, translucent, prismatic quartz crystals. A hydrous iron sulfate (determined by SEM-EDS) was noted in minute amounts, on a single specimen, as orange-red prismatic microcrystals. Its X-ray diffraction pattern did not match any known species.
This locality has yielded an interesting suite of minerals and associations not previously reported from the San Juans. Although some areas have been carefully collected, the locality should continue to provide specimen-grade material, barring future restrictions to access. Cabinet-size specimens from this locality are quite rare, but thumbnail-size specimens are not uncommon.
Based on underground occurrences noted by local collectors (prior to tunnel closure) and a careful survey of dump material, it appears that anatase and brookite occur in both the Indiana and Topeka-Kentucky Giant veins, while fluorite and milky quartz occur only in the Topeka-Kentucky Giant vein. Transparent quartz crystals occur in both veins.
[Figures 1 to 11 ILLUSTRATION OMITTED]
The authors thank Eugene E. Foord (U.S. Geological Survey) for identification of halotrichite and goyazite and analysis of the unidentified hydrous iron sulfate, Peter J. Modreski (U.S. Geological Survey) for SEM and XRD analyses of anatase and brookite, and E. E. Foord, D. S. Collins (U.S. Geological Survey), and G. W. Robinson (Canadian Museum of Nature) for reviewing the manuscript .
BURBANK, W. S. (1941) Structural control of ore deposition in the Red Mountain, Sneffels, and Telluride districts of the San Juan Mountains, Colorado. Colorado Scientific Society, Proceedings, 14, 141-261.
BURBANK, W. S., and LUEDKE, R. G. (1964) Geology of the Ironton quadrangle, Colorado. U.S. Geological Survey Map GQ-291.
BURBANK, W. S., and LUEDKE, R. G. (1969) Geology and ore deposits of the Eureka and adjoining districts, San Juan Mountains, Colorado. U.S. Geological Survey Professional Paper 535, U.S. Government Printing Office, Washington, DC, 73 p.
KILE, D. E., MODRESKI, P. J., and KILE, D. L. (1991) Colorado quartz: occurrence and discovery. Rocks and Minerals, 66, 374-406.
KOSNAR, R. A. (1979) The Sweet Home mine. Mineralogical Record, 10, 333-338.
LARSEN, E. S., and HUNTER, J. F. (1914) Melilite and other minerals from Gunnison County, Colorado. Washington Academy Of Science Journal, 4, 473-479.
OLSON, J. C., and WALLACE, S. R. (1956) Thorium and rare-earth minerals in the Powderhorn district, Gunnison County, Colorado. U.S. Geological Survey Bulletin 1027-0, 693-723.
ROSEMEYER, T., RAINES, E., and STOUFER, R. (1988) Anatase on quartz from Ouray, Colorado. Mineralogical Record, 19, 103-104.
ROSEMEYER, T. (1993) Brookite, anatase and rutile from the Silver Link mine, Ouray County, Colorado. Rocks and Minerals, 68, 327-330.
STOUFER, R., and ROSEMEYER, T. (1989) The Ohio mine, Ouray County, Colorado: A famous quartz locality rediscovered. Rocks and Minerals, 64, 210-214.