New Mexico Geological Society Annual Spring Meeting — Abstracts

The Cochiti or Bland Mining District is located about fifty miles north of Albuquerque on the southeast flank of the Jemez Mountains. The majority of mining activity was conducted from 1897 to 1903 and again from 1914 to 1916. A total of at least 185,000 tons of ore grading .20 oz/t gold and 4.0 oz/t silver has been produced.

Mid-to late Miocene andesitic volcanic and felsic hypabyssal intrusive rocks underlie the area. This sequence of rocks may reflect the early evolution of the adjacent Valles-Toledo caldera complex. Numerous north-south trending, steeply dipping faults and fractures cut all rock types and maybe related to extensional tectonism of the Rio Grande rift.

Veins and stockwork zones are generally associated with these north-south trending structures and are composed primarily of sugary to chalcedonic quartz with only minor calcite. Multiple periods of vein filling and brecciation can be demonstrated. At least thirteen separate vein systems have been mapped, some extending for over two miles along strike. Veins locally exceed sixty feet and typically are at least ten feet in width. Ore shoots are localized at flexures and dilatant zones along veins and ore grades generally increase with vein width. Vein sulfides are quite sparse and are primarily pyrite, gold, electrum, argentite, ruby silver and minor base metal sulfides. Alteration zoning towards veins grades from regional propylitization through advanced argillic to silicification.

Fluid inclusion data suggest that precious metal deposition occurred at or above the zone of boiling, typically at temperatures between 240 and 315°C from dilute hydrothermal fluids with greater than 1% H₂S. Mineralization occurred between about 6.5 and 1.5 m.y.