Boulder County Tungsten Mining District, Boulder County, Colorado, USAi

Regional Level Types
Boulder County Tungsten Mining District	Mining District
Boulder County	County
Colorado	State
USA	Country

LOCATION AND HISTORY: The Boulder County tungsten district extends in a narrow southwesterly belt 9½ miles long from Arkansas Mountain, about 4 miles west of Boulder, to the Sherwood Flats, a mile northwest of Nederland. Throughout most of this distance the belt is 1 to 2 miles wide, but near its western end it flares out to a width of approximately 3 miles.

The abundant float of a heavy black mineral throughout the tungsten belt was known in the days of the earliest prospecting in Boulder County and was called "heavy iron," "barren silver;' and "black iron" and was assayed again and again for silver and gold. Its identity was not recognized until about 1899, when W. H. Wanamaker, who was familiar with the tungsten ore of the Dragoon Mountains of Arizona, recognized the ferberite float as a tungsten mineral. Wanamaker and his partner, S. T. Conger, obtained a lease on a part of the Boulder County land where the float was abundant, about a mile northwest of Nederland, and 40 tons of high-grade ore were taken from the surface in 1900.

In the same year Conger discovered the Conger vein, which proved to be the most productive vein in the entire district. The discovery of the tungsten ore coincided with the period during which tungsten steel was first introduced in the manufacture of high-speed cutting tools, and prospecting throughout the district was encouraged by the consequent demand for tungsten ore.

In spite of the fact that during the first few years following the discovery the price of tungsten ore was as low as $1 per unit there was much activity throughout the tungsten belt, and most of the productive veins were discovered before 1907. The price paid per unit, which ranged from $2 to $3 in 1901, gradually rose to as much as $14 during the next decade and reached a peak of $100 per unit in 1916 because of the urgent demand during World War I. As early as 1904 the potentialities of the district were recognized by some of the large steel corporations of the East, and a number of well-financed corporations entered the district and acquired property.

The mineral composition of most of the veins in the tungsten belt is very simple. Except in the eastern part, tungsten is the only metal sought. In the eastern part gold-telluride veins are present, and in the extreme northeastern part a substantial amount of high-grade silver-lead ore was taken from the Yellow Pine mine.

Some lead-silver-zinc deposits occur along the northern and southern borders of the tungsten belt but little or no sulfide ore has been mined within it. The total output of the belt is valued at approximately $24,000,000.

Mineralogy: Although many minerals have been found in the tungsten veins, only a few are common. The gangue is microgranular quartz locally known as "horn," and the only ore mineral is ferberite. Sericite and the clay minerals dickite and beidellite are common throughout the district, barite in small quantity is widely distributed, and in certain veins iron sulfides are sufficiently abundant to lower the grade of the ore. Hematite and magnetite occur with some of the ferberite ore at scattered localities in the tungsten belt but are not common. Pyrite, marcasite, galena, and sphalerite are rare but are locally associated with the ferberite as late minerals. Calcite and ankerite occur as early minerals in some of the veins, and adularia is present locally in the veins and wall rocks. The fine-grained quartz gangue in many places contains minute crystals of goyazite ("hamlinite"), dickite, kaolinite, beidellite, hematite, magnetite, and goethite, each min- eral giving a characteristic color to the quartz in which it is disseminated. Opal and chalcedony are late minerals in most of the veins and are usually intimately associated with kaolinite or beidellite. Dickite, the moderately high temperature form of the kaolin group, is very common in vuggy ferberite ore and seems to be nearly contemporaneous with the ferberite. In the eastern part of the district some ferberite is associated with the sylvanite ores, but the sylvanite veins were formed before the tungsten mineralization took place.

Nearly all the tungsten ore mined in Boulder County contains so little manganese that it must be classed as a ferberite, but some ore from Gordon Gulch contained sufficient manganese to be designated wolframite.
Much of the ferberite in the Beaver Creek area is coarsely crystalline and vuggy and shows comb structure. In the Sherwood Creek zone and in the region near the Hurricane Hill fault, although vuggy medium-grained to coarse-grained ferberite is common, much of the ore consists of massive medium-grained ferberite, which occurs as a matrix to country rock fragments. The ferberite becomes finer and finer grained to the east. Scheelite in druses and veinlets in the ferberite ore and in minute veinlets and grains in the late light-colored "horn" is not uncommon in the northern and eastern parts of the district. The ferberite from different areas shows marked differences in color. Much of that in the western part is shiny black, except where it has been somewhat decomposed and in part converted to limonite. Some of the coarsely crystalline ore is highly iridescent. At many localities, especially in the central part of the district, the ferberite has a brownish east and is distinctly softer than the black ferberite. Hess and Schaller suggest that the brown ferberite may have contained specular hematite which has broken down to a hydrous oxide of iron imparting a brownish east to the fresh ferberite.

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