Jamestown Mining District, Boulder County, Colorado, USAi

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

The Jamestown mining district lies in the central part of Boulder County, 9 miles northwest of Boulder, and is easily accessible over a good automobile road with a gentle grade. Jamestown, the only town in the district, has a population of about 100. The district includes approximately 36 square miles and ranges in altitude from 6,300 to 8,600 feet. It is well watered and has a good supply of native timber.

In the early summer of 1865 Captain Buchanon, Johnny Noop, Hutchinson, and James, after whom the camp was named, left Blackhawk and prospected along the valley of Little James Creek to the west of Jamestown. They returned to Blackhawk in the fall and displayed such rich specimens of gold ore that about 500 people flocked into the new district the following spring. The Buckhorn was one of the first claims located and became one of the chief sources of lead-silver ore in the district. The first mill was erected in 1867 by Willard at the junction of Big and Little James Creeks.

In October 1875 the first telluride ore was discovered on the John Jay property, which was staked by A. J. Vanderen and Edward Fuller. The total value of output during the first year was between $40,000 and $50,000. The Golden Age vein was discovered in the same year, the Smuggler was located the following spring, and the Buena vein was found in 1879.

Jamestown's biggest "boom" came at the beginning of 1883. In January Colonel Straight shipped some rich lead carbonate ore from the Buckhorn and Argo mines, and the rumor soon spread that large blanket deposits of lead-silver ore, similar to those of Leadville, had been discovered at Jamestown. This caused an immediate influx of approximately 3,000 people. The town became a tent city with more than 30 saloons and extended for more than a mile up the gulch. The rumor was soon proved false, and before the summer was over the population had dwindled to about 200.

The presence of commercial deposits of fluorspar was not known until 1903, when a man named Emerson arrived to prospect for it. He found large quantities of fluorspar float on Blue Jay Hill and shipped about 400 tons during the first summer. In June 1916, Ed Lehman built a small mill to treat lead-fluorspar ore from the Alice and Argo mines. A high point in fluorspar output was reached in 1918, when 22,810 tons were shipped, but production fell off sharply in the following years, and from 1930 to 1940 the fluorspar mines were largely idle.

The Jamestown district has been primarily a gold-mining district but has also supplied considerable amounts of fluorspar, lead, and silver and a small amount of copper. The deposits may be divided into four main classes on the basis of age and characteristic minerals. In the order of decreasing age they are lead-silver deposits, fluorspar veins and breccia zones, pyritic gold veins, and telluride veins. These four types are irregularly distributed around the sodic granite porphyry stock in a rough zonal arrangement and appear to be genetically related to that stock. At the outer edge of the telluride zone there are a few lead-silver veins and a few tungsten-bearing veins, both of which appear to be later than the telluride ores. Three factors have caused, or at least influenced, the irregularity of these zones: (1) The stock was intruded at a moderately steep angle from the southwest, and its thrusting force produced faults and open breccia zones in the area adjacent to the steeply pitching roof, thus affording open channels to the early solutions that rose close to the stock; (2) the large competent granodiorite stock to the south was not sufficiently fractured to admit ore-forming solutions except near its borders; and (3) the strong north-westerly faults (breccia reefs) formed early in the Laramide revolution served as channels for some of the ore- bearing solutions and thus exerted an influence on the distribution of the ores.

Lead-silver deposits.- The lead-silver deposits include both veins and irregular and pipe-like bodies in or bordering fluorspar breccia zones. Most of them occur in Silver Plume granite within an area of about a quarter of a square mile on the southwest border of the sodic granite-quartz monzonite porphyry stock. This area lies in the central part of the fluorspar belt. The chief veins of this group are the Buckhorn and the Mount Pleasant, both of which strike northwest and dip steeply. The pipe-like and irregular bodies of lead-silver ore are in the Alice and the Argo mines. In the Alice, a steeply pitching pipe of lead-silver ore, 8 to 20 feet broad and 3 to 8 feet wide, lies along the footwall of an irregular fluorspar vein from 1 to more than 8 feet wide and has been explored to a depth of 400 feet below the surface. In places the ore body is cut by fluorspar veinlets, which clearly indicates that the fluorspar is later. In the Argo mine scattered masses of lead-silver ore a few inches to several feet in diameter are found in a zone 8 to 10 feet wide in the center of the Argo fluorspar breccia zone.

Apparently the lead-silver ore was deposited in a vein or pipe-like body and was then strongly brecciated and surrounded by later fluorspar. In the lead-silver ores the same suite of minerals is found in the veins and in the pipe-like and irregular bodies. Abundant argentiferous galena and gray copper and variable amounts of chalcopyrite, sphalerite, and pyrite are mixed with a gangue of glassy to milky quartz and some fluorspar. Gold seems to be mainly associated with the chalcopyrite, but a little is found in the pyrite. Shipments of this lead-silver ore have ranged in tenor from 0.06 to 1.25 ounces of gold and 2.8 to 47 ounces of silver to the ton, 1 to 40 percent of lead, and 0 to 5 percent of copper.

In the southern part of the district, south and south- west of Nugget Hill, there are a few lead-silver veins of another type, which have been somewhat productive. These lead-silver veins strike east-northeast, dip steeply southeast, and contain argentiferous galena, sphalerite, and pyrite in a gangue of horn quartz, ankerite, and some barite. This ore commonly contains 0 to 1.7 ounces of gold and 4 to 118 ounces of silver to the ton. Sorted lead ore from the Tippecanoe vein carried 187 ounces of silver to the ton and 25 percent of lead. These veins are probably later than the telluride veins, for in some of the telluride veins of the Gold Hill district just to the south the same type of lead-silver ore is later than the horn quartz that contains the tellurides.

Fluorspar deposits.-The fluorspar deposits consist of breccia zones and veins that form a northwestward-trending belt in granite and granodiorite about 2 miles long and half a mile wide on the southwest side of the sodic granite porphyry stock. Most of these strike northwest, but a few trend north or northeast; all dip steeply, most of them to the south. The belt seems to have been an early fault zone, probably due in large measure to the forces produced by the intrusion of the sodic granite porphyry stock.

The veins and breccia zones are filled with purple to deep violet fluorspar, both granular and coarsely crystalline, and contain some quartz, clay minerals, disseminated pyrite, and small amounts of galena, gray copper, sphalerite, and chalcopyrite. Minute grains of pitchblende are found in some of the deposits, notably the Blue Jay vein. In many of the deposits coarsely crystalline fluorspar has been brecciated and cemented by fine-grained sugary fluorspar mixed with variable amounts of quartz, clay minerals, sericite, brown carbonate, and small amounts of pyrite, galena, and sphalerite. In the breccia zones the fine-grained fluorspar and associated minerals surround fragments of granite or granodiorite a fraction of an inch to several feet in diameter and in places fragments of lead-silver ore that represent earlier veins or chimneys. There has been some replacement of the granite breccia fragments by fluorspar, and in many places the ground-up granite matrix, which has been altered to a mixture of clay minerals, sericite, and quartz, appears to have been replaced also.


The breccia zones range from 10 to 70 feet in width and from 50 to 350 feet in length. Many of them are parts of larger barren breccia zones 50 to 200 feet wide and 500 to 2,000 feet long. Most of the spar is low-grade, containing 5 to 60 percent of CaF2 , but many of the breccia zones enclose relatively high-grade veins or pockets that contain 50 to 85 percent of CaF2.

The fluorspar veins range from a few inches to 16 feet in width and from 150 to 1,000 feet in length. They are mostly of higher grade than the breccia zonesand in many of them material containing GO to 85 percent of CaF2 has been mined, but in some places the veins are of very low grade.

Pyritic gold veins.-The pyritic gold veins fill the later fault fissures, which trend northeast and dip southeast. Most of them appear to be younger than the early lead-silver deposits, and they in turn are older than the telluride veins. The vein fining consists chiefly of glassy, milky, or sugary quartz and coarse-grained pyrite and chalcopyrite. Roscoelite is intergrown with the quartz in some of the veins in the southern part of the district. Galena and sphalerite are present in some veins and are locally abundant. The gold occurs free or is intimately associated with chalcopyrite and less intimately with pyrite. The most productive veins are in a zone half a mile wide, 1 1/2 to 2 miles east of the sodic granite porphyry stock, but there are also numerous low-grade veins along the north and south borders of the stock and a few small veins within the stock itself.

Another group of low-grade veins lies in the vicinity of Nugget Hill, 2 to 2½ miles south of the stock. The veins commonly range in width from a few inches to 3 feet, but some of the veins of the southern group are mineralized zones 10 to 30 feet wide. The length ranges from 300 feet to more than a mile.

Telluride veins.-The telluride veins also have a northeasterly trend, but some of the most important of them strike, north-northeast and dip west instead of southeast. They are younger than the pyritic gold veins, for in the Golden Age mine the Sentinel telluride vein cuts the Golden Age pyritic gold vein.

The telluride veins range in width from a fraction of an inch to as much as 10 feet, and in the Buena mine some ore bodies at vein junctions are as much as 10 or even 30 feet wide. The veins range in length from a few hundred feet to more than a mile, but only small parts of the longer ones are productive. The veins contain gray jaspery (horn) quartz, finely disseminated pyrite, and a variety of telluride minerals. In some of the veins appreciable amounts of free gold are associated with the tellurides. Commonly the veins are made up of numerous interlacing seams of horn quartz, a fraction of an inch to 18 inches wide, in which the tellurides are unevenly distributed; the intervening wall rock is nearly barren. The most abundant tellurides in the district are krennerite (or calaverite) and petzite, but there are also significant amounts of sylvanite and altaite and small amounts of hessite, coloradoite, native tellurium, and rickardite. In most of the veins two or more telluride minerals are microscopically intergrown, and rarely is anyone telluride exclusively present. One outstanding exception is the ore from the Buena Mine, where in much of the ore krennerite appears to be the only telluride, except for very small amounts of petzite. Minute grains of free gold are scattered through some of the tellurides, and apparently gold was the latest ore mineral deposited. Very small amounts of galena, sphalerite, and chalcopyrite are commonly associated with the telluride ore. Associated with the ore in the Rip Van Dam, King Wilhelm, and Gladiator veins are small amounts of roscoelite, and in the John Jay vein there are small amounts of brown carbonate.