By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Located in the southwestern part of the State, Sierra County contains many mining districts, which in earlier years made the county one of the foremost mining centers in New Mexico. Gold has been produced from numerous districts, but only the Hillsboro district has yielded more than 10,000 ounces. Total gold production of the county through 1959 was about 183,900 ounces.
The Hillsboro (Las Animas) district is about 25 miles southwest of Truth Or Consequences. Both lodes and placers were discovered in 1877 and were worked intensively from 1884 through 1905 (Harley, 1934, p. 139-140). Mining was resumed from 1931 through 1942, but was spasmodic from the end of World War II through 1959. Harley (1934, p. 140-141) estimated gold production from 1877 through 1931 at $2,200,000 (106,400 ounces) from placers and $4,700,000 worth of combined metals from lode mines.
Total gold production through 1959 was probably about 149,000 ounces, roughly two-thirds of which was from placers.
The gold placer deposits of the district cover an area of about 18 square miles of dissected alluvial fans east of the Animas Hills, about 6 miles northeast of the town of Hillsboro (Harley, 1934, p. 125, 137-138, 166-169). The most productive deposits were the Luxemburg placers at the apex of the fan now drained by Grayback, Hunkidori, and Greenhorn Gulches.
The most productive lode mines are in the southwest part of a mineralized area in the Animas Hills, north of the Rio Percha. Bedrock is predominantly extrusive andesite and latite, intruded by two small monzonite masses and related dikes. Drill holes about 1,150 feet deep have penetrated lower Paleozoic limestone beneath the volcanic rocks (Harley, 1934, p. 131).
Most of the lode gold has come from quartz veins, 2 to 8 feet wide, that are along dikes of latite in the andesite (Harley, 1934, p. 133-139). The ore minerals are chalcopyrite, bornite, and pyrite, accompanied by gold and silver. Molybdenite is an accessory ore mineral and tetradymite is found locally.