By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
San Miguel County, in northeastern New Mexico, adjoins Santa Fe County on the east. Deposits of gold, silver, copper, lead, and zinc are found in the mountainous northwest corner of the county in the Willow Creek district. The total gold production of the county through 1959 was 178,961 ounces.
WILLOW CREEK DISTRICT
Almost the entire production of the Willow Creek (Pecos) district came from the Pecos mine, formerly known as the Hamilton or Cowles mine. Discovered in 1881, the deposit was developed slowly, and in the early 1900's, it was developed primarily as a copper mine. Intensive exploration that began in 1916 later revealed large reserves of lead-zinc ore at depth. From 1926 through 1939, under the ownership of American Metal Co., the mine produced $36 million in gold, silver, copper, lead and zinc. Aside from minor activity in 1943-44, the district was dormant from 1940 through 1959. Total gold production for the district through 1959 was roughly 178,300 ounces.
Precambrian diabase, granite, and schist are exposed in the deeper canyons of the district where they are unconformably overlain by the Magdalena Formation of Pennsylvanian age. The unconformity and overlying sedimentary rocks dip 7Â°-12Â° SW. (Krieger, 1932, p. 351-352).
The ore deposits are in a mineralized shear zone in the Precambrian schist. Both the shear zone and ore are believed to be of Precambrian age, for neither extends into the overlying Pennsylvanian rocks. Pyrite and sphalerite are the most abundant ore minerals; chalcopyrite and galena are less abundant, and pyrrhotite, bornite, argentite, and proustite are sparse (Krieger, 1932, p. 462, 463). Harley (1940, p. 84) reported minor roscoelite, and Lasky (in Lasky and Wootton, 1933, p. 93) reported sparse tetrahedrite. The gangue minerals are quartz, chlorite, actinolite, sericite, and tourmaline. The gold is associated with quartz and chalcopyrite (Harley, 1940, p. 84).
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