The Gold Hill mining district comprises about 12 square miles in the central part of Boulder County, 3 to 8 miles northwest of Boulder and 30 miles northwest of Denver. The district includes several small mining camps or settlements, the largest of which is Gold Hill. Others comprising only a few houses each, are Rowena and Glendale on Lefthand Creek, Sunshine in the eastern part of the district, and Wallstreet, Salina, and Crisman on Fourmile Creek. The district ranges from 5,900 to 8,400 feet in altitude and is well watered by Fourmile Creek, Lefthand Creek, and their tributaries.

Placer gold was discovered near the present site of Gold Hill in January 1859, and during the following summer some of the veins were located. Between 3,000 and 5,000 people flocked to the district during the first few seasons. After 1860 mining in the district rapidly declined but suddenly became active again in 1872, when gold-telluride ore was discovered at the Red Cloud mine and very rich ore was mined on many properties. Mining was further stimulated in 1898 by the completion of a railroad from Boulder to Ward, which lay 3 miles west of Gold Hill.
During the early part of the twentieth century there was a gradual decline in activity in the district, lasting until the fall of 1933, when the sudden rise in the price of gold caused a great increase in activity.

Gold is the chief metal mined in the Gold Hill district, although in most veins minor amounts of silver are associated with the gold. In a few of the mines silver has been the chief product, and some lead and copper have also been produced. Moderate amounts of tungsten ore have been taken from the Logan gold mine in the southwestern part of the district. A pre-Cambrian deposit of nickel ore containing cobalt has been opened near Gold Hill but has had only a small production.

All the ore shipped from the Gold Hill district has come from fissure veins. Most of the veins are of the gold-telluride type, but there are some important pyritic gold veins and a few important silver-lead veins. The silver-lead veins appear to be the oldest; those of the Yellow Pine mine, which have been the most productive in the district, nlay even be related to the breccia reef period. Some of the pyritic gold veins, particularly those of the northern part of the district, are later than the telluride veins, but some may be earlier, as are those in the Jamestown district. The tungsten ore in the Logan mine is thought to be the latest of all the ores.
Gold tellurides are by far the most important ore minerals in the district. Petzite, sylvanite, and calaverite or krennerite are the most abundant, but small amounts of hessite and altaite are common. In most of the telluride ores, two or more of the telluride minerals are so finely inter grown that they cannot be distinguished by the naked eye. The tellurides occur in groups of small blades, in small irregular masses, or in tiny veinlets in horn quartz associated with finely disseminated pyrite. Very small amounts of galena and sphalerite are found with the tellurides in places. In many of the veins free gold is a valuable constituent of the ore. Rusty gold is common in the oxidized parts of the telluride veins, and in several rich primary deposits free gold is found either associated with the tellurides or in separate seams. The Logan mine is well known for its rich free-gold ore, and free gold has been found associated with tellurides in the Slide, Cold Spring, and Red Cloud mines.
In the pyritic gold veins, pyrite and chalcopyrite are the chief ore minerals, but galena, sphalerite, and gray copper are common and locally are abundant enough to form silver-lead ore. The gold appears to be most closely associated with the chalcopyrite, though in places fine-grained pyrite contains a very appreciable quantity of gold. In some of the pyritic gold veins rich pockets of free gold have been found.
In the lead-silver veins, the chief ore minerals are argentiferous galena and gray copper, and in places they are accompanied by some pyrite, chalcopyrite, and sphalerite. Ferberite is the only ore mineral in the tungsten ore and is associated with small amounts of fine-grained pyrite.
The chief gangue mineral throughout the district is quartz. In the lead-silver veins it is sugary to glassy; in the pyritic gold and telluride veins it is sugary to very fine grained (the horn quartz of the miners) but is rarely glassy; and in the tungsten ore it is of the very fine grained horn variety. In places the quartz gangue takes the form of silicified wall rock. Ankerite of variable composition is common in many of the pyritic gold and telluride veins and appears to be the latest mineral present.
The veins in the district are in general not as persistent as those in the central and southwestern parts of the mineral belt. Only a few can be traced as much as a mile along the strike, and most are less than 2,500 feet long. Many have been followed to depths of 300 and 600 feet, and a vein mined through the Slide shaft has been followed to a depth of 1,080 feet. In most mines the depth of mining has been determined not by the pinching out of the veins but commonly by water problems and other operative difficulties that have made exploration unprofitable. The veins range from a few inches to 5 feet in thickness for the most part, but at junctions they locally widen to as much as 15 feet. The ore minerals are not uniformly distributed but are likely to be limited to narrow irregular stringers of quartz scattered throughout the entire width of the vein. In mining the telluride veins it is common practice to sort out these high-grade stringers from the rest of the vein material, which is relatively barren. Almost all the important veins of the district are in the immediate vicinity of the breccia reefs. Some have broken to or nearly to the reefs and others cut through them. Ninety percent of the productive veins lie between the Hoosier and Maxwell breccia reefs. Many productive veins border the northeast side of the Hoosier reef and the southwest side of the Maxwell reef, and there are several along the Fortune reef, the Blue "vein," and the Poorman reef. In the southern part of the district the Logan, Yellow Pine, and Grand Republic veins crop out on the southwest side of the Hoosier reef. The Wood Mountain group is exceptional, lying more than half a mile southwest of the Hoosier, the nearest breccia reef.

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References