New Mexico Geological Society Annual Spring Meeting — Abstracts

The Lordsburg Mining District encompasses 125 km² south-southwest of Lordsburg, NM. The district sits on the Santa Rita lineament that includes porphyry copper at Santa Rita (Chino), Bisbee, Copper Flat (Hillsburo) and Tyrone in NM. Along this same lineament are the Douglas Mining District in Arizona and Cananea copper porphyry in Sonora, Mexico. Mineralization in the Lorsburg district occurs as silicified veins and breccias filling northeast and east-west trending faults and fractures in the contact area of the Laramide andesite and a granodiorite stock and is characterized by repeated episodes of hydrothermal brecciation.

The Lordsburg district has been a major producer of copper, gold, and silver and to a lesser extent lead and zinc. Production through 1972 was about 225,000 oz gold, 6.2 million oz silver, and 95 million lb copper. Present mining in the district by Lordsburg mining company is primarily for siliceous fluxing ores grading 0.69% copper, 2.9 ppm gold and 48 ppm silver from newly opened veins in the northern part of the district. Detailed surficial mapping and sampling examined the vein and paragenesis, mineral zoning and alteration style. Metal ratios and metal correlation studies aided in the recognition of metal concentration and the distribution of metals.

Vein mineralization is accompanied by illitic, argillic, and prophylitic alterations, that are zoned from the vein in the order: quartz-illite, illite-chlorite, chlorite-smectite mixed layer and chlorite-albite-calcite. Petrographic observations suggest a retrograde alteration, the propylitic alteration is oldest, followed by argillic and then illitic alterations. The paragenetic sequence, alteration and mineralogical zoning suggest mineralization by a single but complex hydrothermal event. Temperatures of ore-deposition, deduced from the stabilities of various mineral assemblages and fluid inclusion decreased through time from >300°C to <200°C. A greater than 300°C, mildly acidic (pH = 4-5) ore-forming fluids initially flowed outward from the central zone (85 mine) where mostly chalco pyrite, gold and silver were deposited to the peripheral zone depositing sphalerite, galena, silver and some gold at relatively lower temperatures and higher pH (5 - 6). Metal ratios and metal correlation coefficient values suggest a co-precipitation of copper, gold and silver in the central zone and a late-stage overprint deposition of gold in the peripheral lead-zinc zone. The mineralization and alteration styles of the Lordsburg deposit are characteristic of zoned base- and precious-metals lode vein systems associated with intrusives.