By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Dona Ana County is in the center of the southernmost tier of counties in New Mexico. The total gold production of the county through 1959 was about 13,500 ounces; almost all production has come from the Organ district, which includes the north end of the Organ Mountains and the extreme south end of the San Andres Mountains.
Ore was discovered in the Organ district in 1849 (Dunham, 1935, p. 185-191), and, from the time of discovery to 1952, the district produced copper, lead, silver, zinc, and gold valued at about $2.5 million (Anderson, 1957, p. 40). Of this total, gold amounts to about $242,227 or 11,435 ounces. Most of the gold output was before 1906; the district was inactive from 1951 through 1959.
The Organ Mountains consist chiefly of quartz monzonite intruded into Paleozoic sedimentary rocks. At the north end of the district the bedrock consists of westward-tilted Paleozoic formations overlying Precambrian granite and metamorphic rocks (Lindgren and others, 1910, p. 205-213; Dunham, 1935, p. 26-89).
The ore deposits in the Organ district occur in or near the quartz monzonite batholith. The principal ore deposits contain mixed sulfides of copper, lead, silver, and zinc with smaller amounts of gold. Locally the values are principally in gold.
The lode ore deposits comprise (1) veins, both in the Precambrian rocks and in the intrusive quartz monzonite, (2) deposits along the Torpedo-Bennett fault, a major fault zone, which extends along the west side of the Organ Mountains, and (3) replacement deposits in the sedimentary rocks (Dunham, 1935, p.111-136).
Most of the gold has been recovered from veins. Veins in the Precambrian rocks are valuable chiefly for gold, and gold has also been recovered from ores along the Torpedo-Bennett fault. A few veins that cut the quartz monzonite are valuable for gold, but most of them are mined for silver.
The ore deposits vary considerably in mineral composition. The veins in the Precambrian rocks consist mainly of pyrite and chalcopyrite in quartz; sphalerite and galena are less common. The fact that the gold content is related to the copper content suggests that the gold is most commonly associated with chalcopyrite; some gold was found as inclusions in pyrite.
Other common ore minerals in other deposits in the district include tetrahedrite, argentite, enargite, and tetradymite (bismuth telluride). The most common gangue minerals in addition to quartz are green fluorite, siderite, and local barite.
In parts of the district there is a crude zonal arrangement of ore with respect to the quartz monzonite (Dunham, 1935, p. 136-140). The ore bodies nearest the quartz monzonite contain copper and are succeeded outward by zinc deposits, which in turn grade into lead deposits. Gold is found chiefly in the copper and zinc zones.
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