New Mexico Geological Society Annual Spring Meeting — Abstracts

The Red River district encompasses a belt 24 km long and 8 km wide situated along the southeast edge of the Questa caldera in the Sangre de Cristo Mountains of northern New Mexico. Caldera formation and associated plutonic activity was the final stage in the evolution of the Latir volcanic field. Southernmost of the Rocky Mountain volcanic fields, the Latir field was active from 29 to 19 Ma and is localized near the intersection between the Jemez lineament and the eastern margin of the Rio Grande rift.

Along the southern margin of the caldera are post-caldera plutons which are characteristically rich in molybdenum. The Questa molybdenum mine and precious metal mineralization are both products of this period of plutonism. Precious metal veins occupy radial and ring fractures associated with the caldera and normal faults associated with early rift related extension.

Alteration in the district occurred in three stages: 1) regional propylitic alteration and smaller areas of intense acid alteration, both associated with molybdenum mineralization; 2) local argillic alteration and silicification along structures, associated with the precious metal mineralizing event; and 3) supergene alteration generated by weathering of the two previous stages.

Mineralization occurs as quartz-cemented breccia zones and banded, massive or vuggy quartz veins, both of which occupy fault zones. Gangue mineralogy includes quartz, calcite and rare fluorite. Metallic mineralogy consists of pyrite, native gold, sphalerite, galena, chalcopyrite, pyrargyrite and argentite. Homogenization temperatures from two-phase liquid dominant inclusions average 237°C and salinity averages 1.1% equivalent wt. NaCI. Salinity vs. homogenization temperature data is consistent with mixing between fluids of contrasting salinity.

Epithermal precious metal mineralization is an indirect product of Mo-rich plutonism of the Questa caldera. The elevated geothermal gradient was sufficient to generate a convective cell of meteoric water, which leached the intermediate volcanic pile and deposited silica and gold in tensional openings produced by caldera activity.