By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Boulder County, in north-central Colorado along the east side of the Front Range, ranks ninth among the gold-producing counties of Colorado. Its west boundary is the Continental Divide; its eastern part includes a small area of the Great Plains. All the metal-mining camps are in the mountainous western part.
Gold is the chief metal produced in the county, but considerable silver and tungsten and small amounts of lead and copper have also been produced. Through 1959, Boulder County was credited with an output of about 1,048,200 ounces of gold; all but about 3,000 ounces was from lode mines.
Discoveries of gold deposits in Boulder County, which date from 1858 (Henderson, 1926, p. 38, 105-106), were among the first in the State. Though placers were worked first, most of the gold mined before 1869 was from oxidized free-milling lode ore, and when this was depleted, many mines closed.
Through 1868 the annual production was less than $50,000 in gold. The construction of a new smelter at Blackhawk in 1868 stimulated activity in the mining camps, new veins were discovered, and mines and camps developed rapidly. Gold production increased from about $100,000 (4,838 ounces) in 1870 to $683,941 (33,089 ounces) in 1891 and to a peak production of $982,988 (47,556 ounces) in 1892 (Henderson, 1926, p. 106).
After the panic of 1893, when the price of silver dropped, gold production fluctuated but generally declined; only $16,516 (799 ounces) in gold was produced in 1930. The rise in the price of gold in 1934 stimulated gold mining, and 33,621 ounces was produced in 1940. Activity again declined during World War II and decreased even more after the war.
Boulder County has many mining camps and districts, but to 1959 only five had a total gold output in excess of 10,000 ounces: Jamestown (Central), Gold Hill-Sugarloaf, Ward, Magnolia, and Grand Island-Caribou.
GOLD HILL-SUGARLOAF DISTRICT
The Gold Hill-Sugarloaf district, the largest gold producer in Boulder County, contains several small mining camps within an area of about 12 square miles in the central part of the county, 3 to 8 miles northwest of Boulder. The largest of these is Gold Hill; others are Sugarloaf, Rowena, Salina, and Sunshine. Gold was the chief metal produced in the district, although in most deposits silver is associated with the gold.
Placer gold was discovered in this district in January 1859, very early in the history of mining in the State, and $100,000 (4,838 ounces) worth of gold was worked from these placers during the first summer (Goddard, 1940, p. 106). Gold-bearing veins were discovered nearby during the summer of 1859, and in consequence several thousand people flocked to the district.
The oxidized surface ore yielded free gold and recovery was made by sluice, arrastre, and stamp mill. When these ores were mined out after a few years, activity in the district declined sharply. Mining activity increased markedly in 1872 when the gold-silver telluride, petzite, was discovered at the Red Cloud mine at Gold Hill. In 1873 telluride ore was discovered in the Cold Spring mine. The ore was extremely rich, yielding an average of $1,500 per ton, and in 2 years these two mines produced about $600,000 in gold from about 400 tons of ore (Henderson, 1926, p. 39).
Many more veins were found from 1875 to 1880 and activity was sustained at a high level until 1904, after which mining declined (Goddard, 1940, tables p. 108-109). The district was rejuvenated in the middle and late 1930's but slumped sharply during World War II; after the war it failed to regain its former importance and was almost inactive during 1950-59.
The lode-gold production from the Gold Hill-Sugarloaf district from the time of discovery through 1903 could not be ascertained. According to Henderson (1926, table, p. 106), Boulder County from 1859 through 1903 produced about $13,435,000 worth of gold. It seems reasonable to assume that the Gold Hill-Sugarloaf district produced at least one-fourth of this amount - about $3,360,000 or 162,500 ounces. The minimum total output of the district through 1959 was about 412,000 ounces, mostly from lodes. The placer production probably did not exceed 3,000 ounces (Lovering and Goddard, 1950, p. 240).
The following brief description of the geology and ore deposits of the district is mostly from Goddard (1940, p. 110-139).
Schists and gneiss of the Idaho Springs Formation are intruded by a batholith of Boulder Creek Granite and dikes of Silver Plume Granite, all Pre-cambrian in age. Sedimentary rocks of Pennsyl-vanian age unconformably overlie the Precambrian rocks about 2 miles east of the district. The Precambrian rocks have been cut by a series of porphyry dikes of Laramide age that range in composition from diabase to alaskite. The mineral deposits are chiefly in the northern part of the Boulder Creek batholith; most of the veins are in the granite and a few in the western part of the district extend into the schist.
The distribution of ore deposits was strongly influenced by conspicuous silicified, hematite-stained breccia zones, called breccia reefs. The most prominent of these are nearly vertical and trend N. 25°-50° W.; others can be grouped into sets that trend N. 70°-80° W., N. 60°-75° W., and N. 5°-30° E. The gold deposits are in telluride and pyritic veins that occupy fissures, most of which strike northeast. Ore is localized where these veins cross the breccia reefs.
Most of the productive veins are more than half a mile long and from 1 to 5 feet wide, but some are from 10 to 30 feet wide. The order of deposition of the veins is not certain. Silver-lead veins appear to be the oldest in the district; these are followed by the gold telluride veins, and then by the pyritic gold veins. A few of the silver-lead veins, however, seem to be related to the pyritic gold veins.
Gold tellurides, the most abundant of which are petzite and sylvanite, are the most important ore minerals in the Gold Hill-Sugarloaf district, but free gold is also abundant. Other tellurides occurring in small amounts are hessite, altaite, and coloradoite. Fine-grained pyrite and very small amounts of galena and sphalerite are associated with the ore minerals. Horn quartz and sugary quartz are the chief gangue minerals. Roscoelite is closely associated with the tellurides and free gold. Anker-ite and other carbonates also are associated with the telluride ores but are younger than the telluride minerals.
In the pyritic gold veins, pyrite and chalcopyrite are the most abundant ore minerals, but free gold is abundant in some veins. The chief gangue mineral is sugary to glassy quartz; ankerite is found in some veins.
GRAND ISLAND-CARIBOU DISTRICT
The Grand Island-Caribou district is in southwest Boulder County, about 17 miles west of Boulder and 4 miles northwest of Nederland.
Silver is the chief metal produced in the district, but moderate amounts of lead and some lode gold have also been produced. Prospectors discovered veins near Caribou in about 1860 (Henderson, 1926, p. 38) ; however, they did not recognize the silver ore until 1869, when one of them, after seeing some silver ore from Nevada, returned to the district and made the first location. Other claims were staked the same year, and by the end of 1871 most of the rich lodes in the district had been found and production increased rapidly.
Ore was produced from the district until 1893, when a drop in the price of silver forced most mines to close; however, some of the richer gold mines resumed operation in 1898. Since 1900 activity in the area has been limited to sporadic attempts to reactivate certain mines or to mill dump material from some of the larger mines (Moore and others, 1957, p. 521-522). The district was almost dormant from 1952 through 1959.
Nearly all the output of the district has come from lead-silver veins containing a little gold, though some gold-silver ore has been mined in outlying areas. The ore in the upper levels of many of the mines was very rich in silver, probably because of secondary enrichment (Moore and others, 1957, p. 521).
There is no record of the early gold production from the Grand Island-Caribou district. It has been estimated (Moore and others, 1957, p. 522) that the total value of lead and silver produced before 1924 was about $6 million, but no figures are given from which to estimate the gold production, though it was probably small. The gold production from 1932 through 1959 was 10,006 ounces.
The eastern and western parts of the district are underlain by schist and gneiss of the Idaho Springs Formation and by small bodies of Boulder Creek Granite. These rocks, which are of Precambrian age, were intruded by a composite stock of calcic monzonite and quartz monzonite which occupies the central part of the district. A striking feature is the occurrence of numerous masses of pyroxenite, titaniferous magnetite, and hornblendite in the stock (Smith, 1938, p. 171, 174).
Lead-silver veins in the monzonite stock in the vicinity of Caribou Hill have been the most productive in the district. The gold veins are in the Precambrian rocks and are older than the lead-silver veins. Quartz, pyrite, chalcopyrite, covellite (?), and minor galena and sphalerite are the predominant minerals of the gold veins. The lead-silver veins chiefly contain quartz, pyrite, sphalerite, galena, chalcopyrite, argentite, pyrargyrite, carbonates, and secondary azurite, malachite, native silver, and limonite (Moore and others, 1957, p. 526-528).
The Jamestown (Central) district is in the Front Range in central Boulder County, about 9 miles northwest of Boulder. Gold ore was discovered in 1865 (Lovering and Goddard, 1950, p. 255), and sometime between 1876 and 1881 the town of Jamestown was founded (Henderson, 1926, p. 40).
Little else is known of the early development of the district, even though it is one of the major gold-producing areas in Boulder County. The Jamestown, Gold Hill, and Ward districts were the chief gold producers in the county from 1883 to 1912. Many mines were closed thereafter but were reopened during 1934-42. From World War II through 1959 gold mining again declined. The total gold production from the district through 1959 was about 207,000 ounces. This figure includes an estimate by E. N. Goddard (in Vanderwilt and others, 1947, p. 324) of $4,700,000 worth of gold ore produced through 1943.
The geology of the district is summarized chiefly from E. N. Goddard (in Vanderwilt and others, 1947, p. 323-327) and Lovering and Goddard (1950, p. 255-279).
The Jamestown district, at the northeast end of the Colorado mineral belt, is underlain chiefly by schist of the Idaho Springs Formation and by some hornblende gneiss, intruded by the Boulder Creek and Silver Plume Granites. These Precambrian rocks have been intruded by stocks and a variety of dikes of porphyritic igneous rocks of early Tertiary age. The rocks are cut by silicified brecciated zones known as breccia reefs, which are older than the Tertiary porphyries, and by normal faults, the vein fissures, which are younger than the porphyries.
An early set of vein fissures, trending northwestward, contains lead-silver and fluorspar deposits, and a later set, trending northeastward, encloses pyritic gold and gold telluride deposits. The ore deposits appear to be genetically related to a small quartz monzonite porphyry stock. Though their distribution around the stock is irregular, there is, nevertheless, a rough mineralogical zoning of the deposits. The lead-silver and fluorspar deposits are close to the border of the stock and the pyritic gold and gold telluride deposits are successively farther away.
The chief minerals in the pyritic gold veins are pyrite and chalcopyrite in a quartz gangue. The gold is free or is intricately associated with chalcopyrite and in small amounts with pyrite. Galena and sphalerite are present in some veins. Silver is usually present in about equal amounts with the gold.
The telluride veins consist of jaspery quartz, finely disseminated pyrite, free gold, and a variety of telluride minerals - predominantly krennerite and petzite, smaller amounts of sylvanite and alta-ite, and locally small amounts of hessite, coloradoite, and native tellurium.
The pyritic gold veins range in width from a few inches to 3 feet, but some shoots occur in mineralized zones 10 to 30 feet wide. The telluride veins are as much as 10 feet wide, and some shoots at vein junctions are as much as 30 feet wide. The deepest workings are only about 500 feet below the surface, and on many veins the deepest workings are only 100 to 200 feet deep. Many veins are as strong at the bottom level as at the surface and are considered favorable for exploration at greater depth (E. N. Goddard, in Vanderwilt and others, 1947, p. 327).
The Magnolia district, about 4 miles southwest of Boulder along the east side of the Front Range, is small, and most of the productive veins crop out in an area of less than 1 square mile.
Gold telluride ore was discovered in the district in 1875, 3 years after the discovery of gold telluride in the Gold Hill camp (Wilkerson, 1939, p. 82), and most of the known veins were being worked by 1877. Small amounts of tungsten ore also have been mined. The productive life of the district was largely spent by 1905, and even the increased price of gold in 1934 failed to excite more than a spark of revival.
The production of the district before 1906 was valued optimistically at $2,815,000 and was mostly in gold (Lovering and Goddard, 1950, p. 227). Total gold production through 1959 was about 130,000 ounces.
Almost all the district is underlain by gneissic Boulder Creek Granite of Precambrian age, which is cut by numerous aplite and pegmatite dikes (Wilkerson, 1939, p. 84). Most of the ore deposits are in gold telluride fissure veins that trend west or northwest, and ore seems to be localized at intersections of fissures.
The Magnolia district, whose ore minerals consist chiefly of gold tellurides, was the first in Colorado to produce considerable quantities of telluride ore. Much of the ore was very rich. The district is of interest also because of the variety of the telluride minerals and the unusual association of gold tellurides with tungsten, vanadium, and molybdenum minerals (Lovering and Goddard, 1950, p. 228).
Sylvanite is the main ore mineral but it is almost everywhere accompanied by one or more of the following tellurides: calaverite, hessite, petzite, coloradoite, and altaite. Other minerals found in the district are native gold, lionite, magnolite, nagyagite, henryite, tellurite, ferro-tellurite, melon-ite, native tellurium, ferberite, molybdenite, and roscoelite, and some galena, sphalerite, pyrite, mar-casite, calcite, and fluorite (Lovering and Goddard, 1950, p. 228). Gangue minerals are present in only minor amounts and consist mainly of light- to dark-colored varieties of extremely fine grained quartz or "horn."
The Ward district is in western Boulder County, west of the Gold Hill-Sugarloaf district and about 9 to 13 miles northwest of Boulder. It comprises 12 square miles in the headwaters of Lefthand and Fourmile Creeks and includes the mining camps of Sunset and Copper Rock.
Gold was first discovered in 1861, and by 1870 most of the major lodes had been located. The Niwot and Columbia mines were the largest gold producers; however, Worcester (1920, p. 56) noted that there were more than 50 mines in the district that had produced more than $5,000 worth of ore. Gold mining in the district declined after 1893; it revived briefly during 1936-42, but it waned from 1943 through 1959. The exhaustion of the rich oxidized ores left only low-grade sulfide ores that have thus far resisted all attempts at successful treatment.
The early gold output of the Ward district can only be estimated. Using Henderson's (1926, p. 106) figure of $15,954,999 for Boulder County from 1859 to 1923 and Worcester's (1920, p. 70) assumption that Ward produced 20 to 24 percent of the total dollar value of Boulder County mineral production through 1915, we can credit the district with a minimum of $3,191,000 or about 154,400 ounces of gold through 1923. Total gold produced through 1959 was roughly 172,000 ounces, mostly from quartz veins. The small placer deposits were exhausted long ago.
The country rock in the Ward district is largely Precambrian in age. Gneiss and schist of the Idaho Springs Formation predominate in the southern part of the district, whereas Silver Plume Granite is the major bedrock in the northern part. Several stocks of diorite and monzonite porphyry and smaller masses of sodic andesite and diorite porphyry and a wide variety of dikes of Tertiary age intrude the Precambrian rocks throughout the district (Lovering and Goddard, 1950, p. 203).
Nearly all the ore in the Ward district occurs in veins or in shoots or chimneys that appear to be local enlargements of veins. Most of the productive veins are in the granite or granite gneiss; many veins feather out in the schist. The veins either follow or are closely associated with felsite, dacite, quartz monzonite, latite, or quartz latite dikes.
Gold, silver, and lead have been mined in appreciable quantities; and copper, zinc, and tungsten have been produced in small amounts. Most of the gold has been derived from quartz veins rich in chalcopyrite; lesser amounts have come from quartz-pyrite veins with minor molybdenite and wolframite. Native gold and gold alloyed with silver occur in small amounts in ores that contain sphalerite and argentiferous galena as the chief constituents. Gold telluride ores are found in mines in the eastern part of the district (Lovering and Goddard, 1950, p. 203-207).
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