By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Clear Creek County is in north-central Colorado in the Front Range, west of Denver, and immediately east of the Continental Divide.
Clear Creek County ranks seventh among the gold-producing counties of Colorado with a total of about 2,400,000 ounces through 1959, yet gold is second to silver in the value of minerals mined in the county.
Substantial amounts of lead, zinc, and copper are also produced. Most of the precious metals have been recovered from siliceous ores in lodes, but in some deposits they were byproducts of base-metal ores. From 1859 to 1864 gold was almost wholly derived from placer deposits or from the hydraulicking of decomposed outcrops of veins.
Prospecting parties working westward from the first gold discoveries in Colorado near Denver discovered gold in Clear Creek County in early April 1859 along Chicago Creek, just above its junction with Clear Creek near Idaho Springs. Prospectors stampeded to the region, and shortly afterwards the first gold-bearing lodes were discovered and located. Later that year lodes were discovered in the Empire district and as far west as Georgetown.
In the middle 1860's discoveries of silver veins in the Argentine and Georgetown-Silver Plume districts drew many mining people to western Clear Creek County (Spurr and others, 1908, p. 173).
When the free-milling oxidized ores were mined out, the mining industry waned until 1868 when the Blackhawk smelter opened and successfully treated the sulfide ores. The ever-widening network of railroads also encouraged mineral exploitation at this time. After 1900 mining activity was accelerated during World War I and in the 1935-41 period.
The country rock in Clear Creek County is a complex of Precambrian metamorphic and igneous rocks cut by Tertiary stocks and numerous dikes of porphyries. The most common porphyries range in composition from quartz monzonite to bostonite and alaskite. The ore deposits are also Tertiary in age and are genetically related to the porphyries.
The following six districts in the county have each produced more than a total of 10,000 ounces of gold: Alice, Empire, Idaho Springs, Freeland-Lamartine, Georgetown-Silver Plume, and Argentine. Of these the Idaho Springs district is the largest producer.
The Alice district is about 7 miles west-northwest of Central City in north-central Clear Creek County and extends into southwest Gilpin County.Most of the production of the district has come from the Alice mine, though little is known about its early history. The deposit probably was first mined in 1883. It was first worked as a placer by hydraulicking, and it yielded $60,000 in gold (2,903 ounces). Later the oxidized ore was treated in a stamp mill, and part of the free gold was recovered by amalgamation. The mill operated at a profit for 3 years until the oxidized ore was exhausted.
Attempts to treat the unoxidized ore by concentration met with indifferent success; only about $10,000 (484 ounces) in gold concentrates was shipped (Bastin and Hill, 1917, p. 120, 325-326). The only other production recorded from the district is that of the North Star-Mann mine, which produced ore worth about $116,000 (about 5,610 ounces) through 1916 (Bastin and Hill, 1917, p. 329).
Apparently the district was abandoned for many years after the oxidized ore had been removed. The increased price of gold caused a brief revival from 1935 to 1941, during which time the upper enriched part of the sulfide zone was mined (Lovering and Goddard, 1950, p. 164). The district was largely idle from 1943 through 1959. Total gold production was at least 23,000 ounces through 1959.
The country rock of the Alice district consists mainly of schist of the Idaho Springs Formation which is interfingered with granite gneiss and Boulder Creek Granite, all of Precambrian age. A stock of quartz monzonite porphyry of Tertiary age intrudes the Precambrian rocks. The Alice mine ore body is a gold-bearing pyritic stockwork in the quartz monzonite porphyry; other mines in the district are in quartz-pyrite veins in the Precambrian rocks. The deposits were oxidized near the surface and contained 1 to 2% ounces of gold per-ton.
A zone of supergene sulfide enrichment, containing 0.20 ounce of gold per ton, was found beneath the oxidized zone. The primary sulfide zone, beneath the supergene zone, contains only 0.03 ounce of gold per ton and is of too low grade to mine. Quartz and pyrite, some chalcopyrite, a little arsenopyrite, and local bismuth sulfide are the constituents of the primary ore. Sooty chalcocite and bornite are locally abundant in the supergene zone (Lovering and Goddard, 1950, p. 164-165; Bastin and Hill, 1917, p. 323-330).
The Argentine district, located at the heads of Leavenworth and Stevens Creeks in southwestern Clear Creek County, is 6 to 8 miles southwest of Georgetown and Silver Plume, just east of the Continental Divide. The mineralized area is on Kelso and McClellan Mountains. Lovering and Goddard (1950, p. 135) also included in the district an area in Summit County at the head of Peru Creek on the west side of the Continental Divide, but this area has produced insufficient gold to be considered in this report.
The early wave of frenzied gold prospectors overlooked the potential wealth of silver in Colorado; consequently, silver lodes lay unnoticed until 1864 when the Belmont silver lode in the Argentine district was discovered. A rush to the area ensued, resulting in additional discoveries in the Georgetown-Silver Plume district and in the Montezuma district in Summit County to the west.
The Belmont and Baker mines were developed, slowly at first, but after 1869 they showed enough promise to attract numerous prospectors to the district. The district flourished during the 1870's, but activity declined and became intermittent thereafter (Lovering, 1935, p. 66-67, 69, 73).
Production records for the Argentine district are fragmentary. According to incomplete mine production figures listed by Lovering (1935, p. 68-116), the district had a minimum gold output through 1928 of 21,990 ounces. From 1932 through 1957 the district had a recorded production of 3,373 ounces of gold, or a total minimum of 25,400 ounces, all of which was a byproduct of silver ores.
The Argentine district is underlain by schist and gneiss of the Idaho Springs Formation into which were intruded masses of Silver Plume Granite, of Precambrian age. Dikes and plugs of Tertiary quartz monzonite porphyry and rhyolite and dacite porphyry cut the Precambrian rocks. The ore bodies are in veins that trend north-northeast and contain galena, pyrite, sphalerite, chalcopyrite, silver sulfantimonides, and gold (Lovering and Goddard, 1950, p. 135-136). Quartz, carbonates, and locally occurring fluorite are the most common gangue minerals.
The Empire district is in north-central Clear Creek County about 9 miles west of Idaho Springs and 4 to 5 miles south-southwest of the Alice district. It includes an area of about 8 square miles, but the main productive veins are about IV2 miles north of Empire.
Oxidized and disintegrated gold-bearing quartz veins were discovered in the district in 1860 by prospectors from the Central City district (Spurr and others, 1908, p. 172-173). The disintegrated material could be easily sluiced and treated the same way as placer gravel and these operations proved very profitable. The oxidized material extended to a depth of about 40 feet and there gave way to sulfide ore which carried some free gold. Amalgamation of the sulfide ore presented difficulties, and after 1875 activity declined, although sporadic activity continued to 1924 (Henderson, 1926, p. 31).
The district gained prominence again in 1934 when a group of veins north-northwest of Empire was developed in the Minnesota mine. Production increased from 272 ounces in 1932 to 16,693 ounces in 1940. The mines closed during World War II, and there was only minor sporadic production from 1945 through 1957. Total minimum gold production of the district was 165,000 ounces, roughly half of which came from the Minnesota mine from 1934 through 1943.
The following summary of the geology and ore deposits of the district was taken from Spurr, Garrey, and Ball (1908, p. 383-386) and from Lovering and Goddard (1950, p. 156-161).
The country rock of the Empire district consists chiefly of Precambrian rocks intruded by stocks and dikes of early Tertiary age. The Precambrian rocks include schist of the Idaho Springs Formation, hornblende gneiss, granite gneiss, and both the Boulder Creek and Silver Plume Granites. The Boulder Creek Granite occupies the greater part of the area. The Tertiary stocks are quartz monzonite, and the dikes range in composition from bostonite to alaskite.
The ore deposits of the district are in pyritic quartz veins that are chiefly in the Boulder Creek Granite near the contact of the granite with a quartz monzonite stock. The chief metal produced is gold but in some veins copper also is of value. Few veins have been traced for more than 1,000 feet along the surface or to depths greater than 500 feet.
The chief minerals are pyrite, chalcopyrite, and quartz, although small amounts of sphalerite and galena are found in some veins. The gold seems to be associated with the chalcopyrite. The ore ranges from about 0.2 to 0.4 ounce of gold per ton, and silver ranges from a few ounces to 20 ounces per ton.
The Freeland-Lamartine (Trail) district includes about 4 square miles of the Colorado mineral belt and is about 3 miles west of Idaho Springs in central Clear Creek County.
Soon after the discovery of gold placers near the mouth of Chicago Creek in 1859 (Spurr and others, 1908, p. 311), the search for gold spread to the area along Trail Creek, which lies north of Chicago Creek. The first veins were discovered in 1861, but these were not developed until 1868, after the successful smelter operation at Blackhawk.
In 1870 railroad facilities became available to the region, and mining activity was stimulated still further. From about 1910 through 1933, mining in the district was intermittent and generally on the decline (Harrison and Wells, 1956, p. 36), but it was revived when the price of gold was increased in 1934. The mines in the district were relatively idle from 1944 through 1959.
Almost all production has come from lode deposits in the Lamartine and Freeland mines. Harrison and Wells (1956, p. 74) estimated that from 1868 to 1905 ore valued at about $5 million was produced, and the output from 1905 to 1953 was valued at $13 million in gold, silver, copper, lead, and zinc. Total gold production through 1959 was about 220,000 ounces, about 100,000 of which was mined from the Lamartine and Freeland mines between 1905 and 1953 (Harrison and Wells, 1956, p. 74).
The country rock in the district consists of the Idaho Springs Formation, which is composed of schist and gneiss of sedimentary origin and is intruded by quartz diorite, granite, and pegmatites, all of Precambrian age. During Tertiary time the Precambrian rocks were intruded by dikes and plugs of porphyries that range in composition from quartz monzonite and bostonite to alaskite. The structure is complex and involves two periods of Precambrian folding and Tertiary arching, fracturing, and faulting (Harrison and Wells, 1956, p. 37-67).
The ore deposits of the district are Tertiary mesothermal fissure veins deposited in fractures near porphyritic intrusive rocks. Two principal varieties of veins are recognized as pyrite-gold and galena-sphalerite.
Locally, as observed in the Lamartine tunnel, a transition zone between the two types contains composite ore. The primary minerals of the pyrite-gold veins are pyrite (partly auriferous), chalcopyrite, tetrahedrite-tennantite, and minor galena and sphalerite in a gangue of quartz and carbonate. The galena-sphalerite veins contain galena (partly argentiferous), sphalerite, and pyrite, with subordinate amounts of chalcopyrite and tetrahedrite-tennantite, and quartz-carbonate gangue. Native gold is present in both types (Harrison and Wells, 1956, p. 74, 75).
GEORGETOWN-SILVER PLUME DISTRICT
The Georgetown-Silver Plume (Griffith) district consists of about 25 square miles centered around the towns of Georgetown and Silver Plume, in western Clear Creek County.
Precious metal lodes were first discovered near Georgetown in 1859, soon after the placer discoveries along Chicago Creek near Idaho Springs. In 1864 when rich silver ores were found in the Argentine district, southwest of Silver Plume, many prospectors were attracted to the region and uncovered significant silver deposits in the Georgetown area. With the successful introduction of smelting at Blackhawk, the sulfide veins were mined after the surface ores were exhausted.
As the mines reached greater depths, silver and lead were the main commodities, whereas the surface ores were richer in gold. By 1880, the district was the principal silver producer in Clear Creek County. This trend was maintained, reaching a peak in 1894, after which a gradual decline began (Spurr and others, 1908, p. 173-175). Zinc became a major component of the metal output after 1903, especially during World Wars I and II. Between the two wars the base-metal and silver mines were mostly idle, and most of the sporadic activity was from gold mines.
The raise in the price of gold in 1934 failed to effect any marked increase in gold production. From the end of World War II through 1959 the lead-silver mines yielded a few ounces of gold annually.
The total gold output of the district through 1959 was about 145,000 ounces, most of which was mined before 1900.
Bedrock in the Georgetown-Silver Plume district is generally similar to that in the other mining districts in Clear Creek County. Contorted schist of the Idaho Springs Formation is cut by several types of Precambrian igneous rocks, including dikes and sheets of hornblende gneiss, bodies of gneissic quartz monzonite resembling Boulder Creek Granite, and large masses of Silver Plume Granite which makes up about half the bedrock in the district. Stocks and dikes of Tertiary dacite, quartz monzonite porphyry, alaskite porphyry, and granite porphyry were intruded into the Precambrian rocks (Lovering and Goddard, 1950, p. 138-140).
The ore deposits of the district are of two mineralogical types: silver-lead-zinc veins and pyritic gold veins. The rich silver-bearing veins are found chiefly in the very productive area just north of Silver Plume, but some are also found south and northeast of Georgetown. The gold-bearing veins lie in a narrow belt between the two silver belts. North of Silver Plume, the veins carry silver, lead, and zinc, and almost no gold, whereas the gold-bearing veins contain some silver and locally their silver content is higher than gold.
The most abundant minerals of the silver-lead-zinc ores are galena, sphalerite, and pyrite. These ores commonly contain less than 0.10 ounce of gold per ton. The chief gangue minerals are quartz and brown carbonates. The principal minerals in the pyritic gold deposits are pyrite, chalcopyrite, gold, small amounts of silver minerals, and quartz gangue. Small amounts of galena and sphalerite are in most of these veins, but where galena is more abundant the veins contain larger amounts of gold and silver. Locally hessite and argentite have been found, and in one mine platinum and iridium were noted (Lovering and Goddard, 1950, p. 141-142).
IDAHO SPRINGS DISTRICT
An unbroken succession of gold deposits extends from Idaho Springs in Clear Creek County to Central City and Blackhawk in Gilpin County. The deposits form a geologic entity, separated into the Idaho Springs and Central City districts only by the county line. The area has been the largest source of gold in both counties and includes the Chicago, Ute, and Cascade Creek camps.
Gold placers, which were found in early 1859 along Chicago Creek near Idaho Springs, attracted many prospectors who combed the nearby gulches and surrounding mountains and who soon uncovered additional placers in Nevada and Illinois gulches and Missouri Flats, as well as rich gold quartz veins, notable among which were the Gregory, Russell, Bates, Bobtail, and Mammoth lodes (Henderson, 1926, p. 27, 28).
After the oxidized ores were depleted, the mines were shut down. In 1868 the Blackhawk smelter began treating the sulfide ores; the district was rejuvenated and experienced a long period of intense activity, exemplified by the 4y2-mile-long Argo tunnel, which was started in 1904. By 1918 a marked decline was evident, and this trend continued until 1932. The period 1932-42 was one of high production, and it was followed by a period of steady decline after World War II.
The total minimum gold production of that part of the district in Clear Creek County was about 1,805,000 ounces (R. H. Moench, written commun., 1963). The production of the Central City part of the district, in Gilpin County, is given in the Gilpin County section of this report (p. 100).
The area is underlain by interlayered metamorphic gneiss, migmatite, and roughly concordant sheets of granodiorite similar to Boulder Creek Granite and granite similar to Silver Plume Granite (Moench and others, 1962, p. 37-38), all of Precambrian age. These Precambrian rocks are intruded by a variety of Tertiary plutons and dikes of porphyritic igneous rocks of the granodiorite, quartz monzonite, bostonite, and hornblende granodiorite groups (Wells, 1960, p. 232).
Two episodes of Precambrian deformation are recognized: a plastic deformation that recrystallized the rock minerals and produced large open folds whose axes trend north-northeast, and a younger cataclastic deformation that was characterized by asymmetrical folds whose axes trend N. 55° E., weak recrystallization of minerals, and intense granulation. The major structures near Central City are the Central City anticline and smaller subparallel folds formed in the older period (Moench and others, 1962, p. 39-54).
The younger folding is recognized only near Idaho Springs as part of a belt of shearing called the Idaho Springs-Ralston Buttes cataclastic zone (Tweto and Sims, 1960, p. B8). Large northwest-trending faults of Precambrian ancestry, displaying Laramide movement, and three sets of small faults of Laramide age also cut the Precambrian rocks.
Most of the ore deposits are mesothermal sulfide veins in fault fissures. Veins are grouped according to their mineral assemblages into (1) pyrite-quartz veins, (2) pyritic copper veins containing quartz, pyrite, chalcopyrite, tennantite, and minor galena and sphalerite, (3) pyritic lead-zinc veins containÂ¬ing quartz, pyrite, galena, sphalerite and subordinate chalcopyrite and tennantite, and (4) lead-zinc veins containing quartz, carbonates, galena, sphalerite, and small amounts of chalcopyrite, tennantite, and pyrite (Sims, 1956, p. 743-744; R. H. Moench, written commun., 1963).
All four types contain gold, but the most important mines are in the pyritic lead-zinc veins. Gold occurs partly as discrete fine particles and is partly tied up in the sulfide minerals. A few gold telluride veins are found in the southeast part of the Central City district in Gilpin County.
The veins are arranged in a concentric pattern of zones with pyrite and pyritic copper veins occupying an elliptical central area which is 2 to 3 miles wide and extends from Blackhawk south to the Idaho Springs district. This is surrounded by successive zones containing pyritic lead-zinc veins and galena-sphalerite veins (Sims, 1956, p. 744-745; R. H. Moench, written commun., 1963).
About a mile southwest of Central City is the well-known "stockwork" named "The Patch - a pipe or chimney of brecciated country rock cemented by ore minerals. This mineralized body extends from the surface to a depth of 1,600 feet where it is intersected by the Argo tunnel. At this depth the pipe has not decreased in size, but the grade of ore is diminished. The ore is of two types: one is characterized by pyrite, chalcopyrite, quartz, and a little tetrahedrite; and the other, by galena, sphalerite, chalcopyrite, and subordinate pyrite (Bastin and Hill, 1917, p. 96-97).