New Mexico Geological Society Annual Spring Meeting — Abstracts

The Hermosa mining district is situated on the east slope of the Black Range in northwesten Sierra County, New Mexico. Structurally, regional faults have sliced the district into long and narrow blocks ina north-south direction. Transverse faults with a general east-west strike divide the long and narrow northsouth slices into smaller blocks. Numerous individual smaller faults and joints branch out from the major faults and most mineralization occurs in and along these small faults and joints.

The study area is located between two major structures in the region: Rio Grande rift on the east and Emory caldera on the west. Structural and thermal effects of these two major structures cannot be ruled out as the main cause of the mineralization in the district.

Stratigraphically, rock exposures are dominated by a thick sequence of Paleozoic sediments which range from Ordovician through Permian in age and are flanked on the west by Tertiary volcanic and volcanoclastic rocks of the Mogollon-Datil field and on the east by late Tertiary and Quaternary conglomerates and basalts.

There are three main concentrations of mineralization in the district: Palomas Camp, Antelope Group, and Flag area. The ores occur mainly as small, but high-grade, masses and in tubular to podlike bodies in fractures, faults and bedding planes in Paleozoic sedimentary rocks. Large-tonnage, low-grade replacement and disseminated ores are widespread throughout the district in the jasperoid and Paleozoic carbonates at the erosional exposure districtwide. The pr imary minerals are galena, sphalerite, chalcopyrite, polybasite, pyrargyrite, argenitite, and tetrahedrite. Silver mineralization in the district is associated with galena and sphalerite occurrences. Talc is the most abundant and widespread gangue mineral in the district. A crude zoning of silver, galena, sphalerite, calcite and quartz is present in the district which correlates well with temperature and fluid compos ition changes districtwide.

The results of thermometric and gas analyses of fluid inclusions show that these deposits are shallow-seated, boiling epithermal deposits. A mineralization temperature of 240°-280°C and salinity range of 0 to 9 eq. wt. % NaCI have been obtained from the fluid inclusion analyses. The results of gas analyses show that, besides water, the are forming fluids in the inclusions also contain small amounts of He, H₂, CO₂, H₂S, CH₄ and some other organic compounds.

Based on all geochemical and geological data, it could be concluded that these deposits are the result of igneous heat which was injected into a sedimentary rock-fluid system, and a convective cell was established in the sedimentary fluids. These fluids dissolved rock components at high temperatures and deposited them at lower temperatures.