Chaffee County Colorado Gold Production
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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 prob¬ably 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).