By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
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La Plata County is in southwestern Colorado just north of the New Mexico State line. The mineral deposits lie in the mountainous west-central part of the county, on the southwest end of the mineral belt of Colorado. Gold has been the most valuable mineral mined in the county, but silver and small amounts of lead and copper have also been recovered.
The total metal production through 1959 was valued at about $6,230,000, of which about $4,825,000 (215,375 ounces) represented its gold production. The sole important gold-producing district in the area has been the La Plata district in the La Plata Mountains (Henderson, 1926, p. 52).
LA PLATA DISTRICT
Spanish explorers who visited the La Plata Mountains in the 18th century may have found gold, but mining in the region did not begin until 1873 when placer gold was found along the Animas River near the present site of Durango (Eckel, 1949, p. 51). In that same year placer gold was discovered along La Plata River. Production during the early years in the La Plata district is not known, but it was probably small.
Lode gold, which has been the chief commodity of the district, was also discovered in 1873, and by the end of 1881 many locations had been made. The output from lodes was small through 1901, but with the discovery of new deposits in 1902, output increased sharply, and through 1914 annual gold production exceeded $100,000 (5,000 ounces). Thereafter production fluctuated considerably and rarely exceeded $100,000 annually(Eckel, 1949, p. 54).
Since 1938, mining activity has declined and, except for a few years, output usually has been less than 500 ounces annually. The total gold production of the district through 1959 was about 215,000 ounces, chiefly from lodes.
The La Plata district lies within the La Plata Mountains, a rugged mountain group about 15 miles in diameter, between the San Juan Mountains to the east and the Colorado Plateau on the west. The La Plata Mountains were carved from a domal uplift of sedimentary rocks caused by intrusion of numerous stocks, dikes, and sills of igneous rocks. Superimposed on the general dome is a curving open anticline whose horseshoe-shaped axis opens southward. Several faults of large displacement cut the outer parts of the dome, and within the dome there are many small faults and fractures.
The sedimentary rocks exposed within the district are more than 4,500 feet thick and range in age from Pennsylvanian through Late Cretaceous. From oldest to youngest they are the Hermosa Formation of Pennsylvanian age, the Rico and Cutler Formations of Permian age, the Dolores Formation of Late Triassic age, the Entrada Sandstone, Wanakah Formation, Junction Creek Sandstone, and Morrison Formation of Jurassic age, and the Dakota (?) Sandstone and Mancos Shale of Cretaceous age (Eckel, 1949, p. 7-31).
The igneous rocks, which are of Late Cretaceous or Tertiary age, are of two general types - porphyritic and nonporphyritic. The more abundant porphyritic rocks are intermediate between diorite and monzonite in composition; the nonporphyritic rocks consist of syenite, monzonite, and diorite. The country rocks were silicified during the doming and intrusion of the nonporphyritic stocks, which occurred later than the intrusions of porphyritic bodies (Eckel, 1949, p. 50-51).
Most of the output of the district has come from veins and replacement deposits of gold and silver-bearing telluride ores. Of lesser importance are disseminated deposits of platinum-bearing chalcopyrite, gold-bearing contact-metamorphic bodies, replacement and breccia bodies of pyritic gold ore, veins of mixed base-metal sulfides with silver or native gold, chalcocite veins, and veins of ruby-silver ore. The gold-bearing placers have not been very productive (Eckel, 1949, p. 60).