By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Santa Fe County is in north-central New Mexico, along the western edge of the Great Plains physiographic province.
Gold has been mined from placers and lodes along the western boundary of the county in the Cerrillos Hills, Ortiz Mountains, and San Pedro Mountains. These ranges are circular in outline and were formed by doming of Carboniferous and Cretaceous sedimentary rocks by laccolithic sheets of Tertiary monzonite porphyry (Lindgren, in Lindgren and others, 1910, p.163-165).
Gold placers were mined as early as the 1830's; therefore, the early production can only be estimated. The principal gold districts are the Old Placer and the New Placer; each produced an estimated $2 million in gold (Lindgren, in Lindgren and others, 1910, p. 75, 168). Total county gold production through 1959 probably was between 150,000 and 200,000 ounces.
NEW PLACER DISTRICT
The New Placer (San Pedro) district is on the west side of the San Pedro Mountains between the towns of San Pedro and Golden. Most of the gold production came from placers that were mined before 1880, but after 1904 there was also an appreciable output of lode gold. The placer deposits were discovered in 1839 (Lindgren, in Lindgren and others, 1910, p. 174), 11 years after those of the Old Placer district were discovered. The placers are in Lazarus Gulch and in branches of Tuerto Creek near Golden on the west side of the mountains.
It has been estimated that the New Placer district produced about $2 million (96,759 ounces) from beginning of production to 1904 (Lindgren and others, 1910, p. 75). This figure represents chiefly placer gold but it may also include some lode gold; figures on lode production before 1904 are not available. From 1904 through 1957 the district produced 2,931 ounces placer gold and about 16,000 ounces of lode gold. Total gold production was about 115,700 ounces. There was no recorded production from 1957 through 1959.
The San Pedro Mountains consist of Carboniferous formations at least 700 feet thick intruded by a laccolith, sheets, and dikes of monzonite and related porphyries probably of Late Cretaceous or early Tertiary age (Lindgren and others, 1910, p. 170-175). Numerous small gold-bearing veins, containing quartz, calcite, pyrite, and free gold, occur both in the porphyry and in the sedimentary rocks. In the lower part of the laccolithic roof are found small copper-bearing contact metamorphic deposits, and in limestone farther away from the igneous contact are found replacement deposits rich in galena.
The placer gold is found in alluvial fans along the mountain front and in the gravels of streams that have reworked the alluvial fans.
OLD PLACER DISTRICT
The Old Placer (Ortiz, Dolores, Cerrillos) district is on the east side of the Ortiz Mountains. Placer deposits were found in the Old Placer district in 1828 - probably the first gold discoveries in New Mexico. Gold-quartz veins were discovered in the district 5 years later. The richest placers were found at the mouth of Cunningham Gulch near the old town of Dolores where the gravels form a mesa, a remnant of the upper part of an alluvial fan.
Lower grade placers were mined in Dolores Gulch, west of Cunningham Gulch, and on the south side of the Ortiz Mountains. Unrecorded but probably small amounts of gold were mined from lodes which were known as early as 1833 (Lindgren and others, 1910, p. 168-169). Total gold output of the district is about 99,300 ounces, most of which was mined from placers before 1900.
The Ortiz Mountains and Cerrillos Hills were formed from laccolithic masses of monzonitic rock intruded into sedimentary rocks, chiefly shale, of Cretaceous age (Lindgren, in Lindgren and others, 1910, p. 164-170). The monzonite is laced with numerous small veins, some of which cut across the contact into the surrounding shales. Rich shoots containing free gold were found in the oxidized zone, and below it the gold is probably contained in the sulfides, which include pyrite, chalcopyrite, sphalerite, silver-bearing galena, some arsenopyrite, and locally molybdenite. The veins also contain spec-ularite and magnetite.
Gangue minerals are quartz and calcite. The contact-metamorphic ores are found in garnetized limestone which contains grains of gold-bearing chalcopyrite.