By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Gold deposits are scattered throughout Imperial County in the numerous desert mountain ranges, but the bulk of the production has come from the eastern third of the county, particularly the Cargo Muchacho, Picacho, and Tumco districts. Other smaller producing districts are the Chocolate Mountains and the Paymaster districts.
Data on production are incomplete, but mining was done in the Picacho district as early as 1857. From 1907 to 1941 about 81,000 ounces of gold was mined in the county (Sampson and Tucker, 1942, p. 110-111) ; however, Henshaw (1942, p. 152) credited the Cargo Muchacho district alone with about $4 million in gold (193,500 ounces). Minimum total production for the county is about 235,000 ounces: only a small amount was produced from placers, the rest came from lodes.
Sampson and Tucker (1942, p. 112-113) gave brief accounts of the geology of the auriferous areas. In the Cargo Muchacho Mountains, the gold deposits consist of small gold-bearing quartz veins that cut metasediments and dioritic and monzonitic intrusives.
The Picacho Mountains are composed of lavas, tuffs, and conglomerates underlain by gneisses and schists that contain the auriferous veins. A few small gold-producing localities are in the Chocolate Mountains, but their production has been minor. Through 1959, only the Cargo Muchacho district had produced more than 10,000 ounces of gold.
CARGO MUCHACHO DISTRICT
The Cargo Muchacho district is in the southeast corner of Imperial County, immediately northeast of Ogilby.
Mining began here on a fairly large scale in 1879, but previously there had been desultory activity by explorers and Mexicans. From 1892 to 1938 the district produced $2,437,760 in gold (Henshaw, 1942, p. 153). Total production of the district to 1938 is conservatively estimated at about $4 million, or about 193,500 ounces (Henshaw, 1942, p. 152). From 1938 through 1959, the output was 31,200 ounces of gold, mostly from lodes, but some was from dry placers. The district was idle in 1959.
Rocks exposed in the district are predominantly quartzite, schists, and arkosite of possible Precambrian age (Henshaw, 1942, p. 149) that have been separated into the Vitrefrax and Tumco Formations. These were intruded by quartz diorite, quartz monzonite, and granite of probable Mesozoic age and andesite dikes of probable Tertiary age (Henshaw, 1942, p. 153-190).
The ore deposits are tabular bodies of quartz, gold, and copper sulfides that are alined along major north-trending faults in the area. Where fractures were open, as in the metamorphic rocks and quartz diorite, economic concentrations of metals were deposited, but in the granite or quartz monzonite the fractures were rather tight and very little material was deposited.
Gold occurs as fine grains disseminated through the country rock, as wire gold and grains in quartz veins, as microscopic grains in pyrite, and as placer nuggets (Henshaw, 1942, p. 184).
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