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.
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