By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Chaffee County is near the central part of Colorado. It borders the Continental Divide on the west and extends eastward across the valley of the upper Arkansas River. Through 1959 the county produced about 370,500 ounces of gold, mostly from lode deposits. Placers and base-metal ores yielded small quantities of gold.
Among the earliest gold discoveries in Colorado were placer deposits in early 1859 near the headwaters of the Arkansas River in Chaffee County and along Cache and Clear Creeks near Granite (Henderson, 1926, p. 9). By late 1860 most of the stream valleys in the county probably had been prospected, and gold placers were reported in places along the Arkansas River from Buena Vista southeast for 25 miles to the Fremont County line and in the northern part of the county near Granite and northward into Lake County.
Other placers were found along Lost Canyon Gulch, Chalk Creek, Cottonwood Creek, Pine Creek, Bertscheys Gulch, Gold Run Gulch, Gilson Gulch, Oregon Gulch, and Ritchey's Patch. The relative importance of these stream placers is not given, but through 1869 Chaffee County is credited with a production of placer gold amounting to $400,000 (Henderson, 1926, p. 107). The small estimated annual production - $80,000 or less through 1904 - indicates that there were no rich placers, and the large number of placer deposits listed above indicates that few if any of them had a large production.
The deposits along the Arkansas River and Cache Creek near Granite probably were the most productive, but whether they produced more than 10,000 ounces cannot be ascertained. Some of this production probably came from placers just north of Granite in Lake County.
The date of the first discovery of lode deposits in Chaffee County has not been recorded. There was some lode mining in 1867-68 and the lode mines at Granite produced $60,000 in gold in 1870 (Henderson, 1926, p. 43, 107). The large lode deposits were not discovered until the early 1870's and later, and very little work was done on these deposits until 1883, when railroad facilities became available.
Mining activity was accelerated during the 10 years following 1883 and continued at a high level until the close of World War I, after which production decreased rapidly. Gold mining remained at a low ebb through 1959.
About 15 districts in Chaffee County have produced gold, but of these probably only the Chalk Creek and Monarch districts have produced more than 10,000 ounces. Henderson (1926, p. 107) credited Chaffee County with a gold production through 1923 valued at $7,401,354 (358,072 ounces), of which $1,548,179 (74,900 ounces) represented placer gold and $5,853,175 (283,172 ounces), lode gold. Most of this production was achieved before 1904 when reliable and fairly complete records were not kept, and the source of much of this gold can only be conjectured.
Total gold production of Chaffee County through 1959 was about 370,500 ounces. Most of the lode gold came from the Chalk Creek district, but an appreciable amount, probably 50,000 to 75,000 ounces, came from many small mines and districts scattered throughout the county.
CHALK CREEK DISTRICT
The Chalk Creek district is in western Chaffee County near the headwaters of Chalk Creek, 16 miles west of Nathrop along the Arkansas River. The ores of the district contain gold, silver, lead, zinc, and a little copper. The Chalk Creek district probably is the only one in the county that has produced more than 100,000 ounces of gold.
The date of discovery of ore in the district probably was in the late 1860's. The Mary Murphy mine, the largest and most important in the district, is believed to have been in continuous operation from 1870 to 1925; thereafter it was operated intermittently (Dings and Robinson, 1957, p. 98). Although there are at least 20 other mines in the district, records of their history are fragmentary; most of them were inactive from 1901 through 1959.
The Mary Murphy mine, from which has come about 75 percent of the output of the district, produced a total of about 220,400 ounces of gold in addition to large amounts of silver, lead, zinc and a little copper (C. S. Robinson, oral commun., 1961). Total gold production of the district through 1959 was roughly 275,000 ounces.
Two masses of Tertiary quartz monzonite are the most abundant bedrock of the Chalk Creek district: the Mount Pomeroy, which is the older, and the Mount Princeton. In the southern part of the district, bodies of Tertiary andesite, quartz latite porphyry, quartz monzonite porphyry, and granite aplite cut these rocks; along the western edge of the district a large dike of quartz monzonite porphyry cuts the Mount Princeton Quartz Monzonite (Dings and Robinson, 1957, pi. 1).
The ore deposits in the district are veins chiefly in the Mount Princeton Quartz Monzonite, although veins also occur in the other intrusive rocks. Most of the ore mined was from pyritic quartz veins which range from mere stringers to lodes 50 feet thick and more than a mile long. The Mary Murphy vein has been worked through a vertical range of about 2,200 feet.
Galena and sphalerite are the principal ore minerals and occur in streaks 1 to 12 inches wide within the pyritic quartz lodes. Some chalcopyrite is generally present. Free gold is reported to occur in much of the ore and probably occurs chiefly in the oxidized ore. The chief gangue mineral is white quartz, locally accompanied by calcite, rhodonite, rhodochrosite, barite, and fluor-ite. A few veins have been developed as molybdenum prospects, and these consist of white quartz with small amounts of molybdenite, molybdite, pyrite, magnetite, beryl, and muscovite (Dings and Robinson, 1957, p. 95-101).
The Monarch district is centered around the town of Garfield about 17 miles west of Salida. The district has produced chiefly silver, lead, and zinc, some copper, and a small amount of byproduct gold.
The first ore was discovered in the Monarch district in 1878. Other discoveries followed shortly, and by 1882 most of the large ore deposits in the region had been found. Transportation difficulties inhibited early development, but by 1883 a railroad was built to Monarch which permitted cheap and rapid transportation to the smelter at Pueblo.
Oxidized ore rich in silver and lead was mined during the next 10 years, and production was large. In 1893, when the price of silver dropped, most mines were closed, and the district was nearly deserted (Crawford, 1913, p. 195-196). A demand for zinc revived the district, and in 1906 shipments of zinc carbonate ore began and continued for many years (Henderson, 1926, p. 43), especially during World War I. Between 1924 and 1940 activity was only intermittent. During World War II a few mines reopened, but these were closed again shortly after the war.
The Madonna mine, which is credited with almost 50 percent of the total output of the district, produced 4,652 ounces of gold between 1883 and 1911 (Crawford, 1913, p. 239). Total gold production of the district through 1959 was probably from 15,000 to 20,000 ounces.
The bedrock of the district is of three ages - metamorphic and igneous rocks of Precambrian age, sedimentary rocks of Paleozoic age, and intrusive rocks of Tertiary age. The Precambrian rocks are schists and gneisses and intrusive masses of Pikes Peak and Silver Plume (?) Granites. The Paleozoic rocks, which contain productive ore horizons, are about 6,000 feet thick and range in age from Cambrian to Permian (?).
The sedimentary rocks have been eroded from a large part of the area, but patches have been preserved in synclines and in down-faulted blocks. The intrusive rocks of Tertiary age, the largest of which is the Mount Princeton batholith, are chiefly of quartz diorite and quartz monzonite composition (Dings and Robinson, 1957, p. 5-27).
The ore deposits in the Monarch district are replacement bodies, chiefly in the Paleozoic limestone and dolomite beds, and fissure veins. The Manitou Dolomite of Ordovician age and beds in the upper part of the Leadville Limestone of Mississippian age are especially favorable host rocks. The largest and richest replacement deposits are along faults in these beds near or adjacent to the Mount Princeton batholith.
The chief sulfide minerals are galena, pyrite, sphalerite, some chalcopyrite, and local pyr-rhotite. Much of the pyrite is gold bearing, and practically all the galena carries some silver, in places large amounts. The gangue consists of quartz and recrystallized limestone or dolomite. The bulk of all ore mined was at least partially oxidized, and the ore in the replacement deposits was more thoroughly and deeply oxidized than the ore in veins. Oxidation in the replacement deposits extends to a depth of 1,000 feet or more, whereas the oxidized ore in the veins is only a few feet deep.
Most of the veins in the Monarch district occur in the Mount Princeton Quartz Monzonite and in the sedimentary rocks of Pennsylvanian age; a few are found in some of the older sedimentary rocks and in the Precambrian rocks. The mineral assemblage in the veins is very similar to that of the replacement bodies. The unoxidized parts of the veins consist chiefly of galena, sphalerite, and pyrite, and some chalcopyrite in a gangue of white quartz. Silver is present in most of the ores, but gold is erratically distributed (Dings and Robinson, 1957, p. 81-85).