New Mexico Mineral Symposium — Abstracts

The Hermosa or Palomas district is in western Sierra County and includes part of the eastern foothills of the Black Range. Harry W. Elliott discovered silver deposits in 1879 at the American Flag and Flagstaff areas, and by 1904 the areas had produced approximately $1.25 million. By 1885, Hermosa was booming and boasted of a 20-tpd concentrator and a 1-tpd adobe smelter. Mining in the district nearly ceased in 1891 with the drop in silver prices. Sporadic activity continued into the 1900s, but never at previous production levels. Last known exploration activity occurred in 1980-81 by Brass Ring Resources, Ltd., followed by Triple S Development Co. in 1981-83. Total production from 1879 to 1957 is approximately 1.25 million oz Ag, 47,600 lbs Pb, 8,000 lbs Zn, 1,850 lbs Cu, and 3 oz of Au worth approximately $2 million from carbonate-hosted Ag-Mn replacement deposits. Much of the early silver production probably was never reported.

Palomas Creek has exposed more than 1,000 ft of Ordovician through Mississippian limestones and dolomites of the Aleman, Cutter, Fusselman, and Oilate Formations that strike N60°E and dip 20-30° NE. Locally Tertiary latite and andesite flows, breccias, and tuffs overlie the sedimentary rocks. Mineralized zones are restricted to three stratigraphic horizons: 1) Upper Ordovician—Silurian Upham, Aleman, Cutter, and Fusselman dolomites, 2) the Lake Valley and Kelly Limestones, and 3) Madera limestone. Two steeply dipping (N38°W and N60-70°E) normal fault systems offset the stratigraphic section and provided conduits for mineralizing fluids.

Mineralized zones occur along or adjacent to the faults as manto-like replacement bodies, fissure veins, and breccia fillings in limestone. Deposits in the Ordovician—Silurian dolomites typically lie beneath the Percha Shale. Individual orebodies range in size from a few short tons to as much as 5,000 short tons. Primary ore minerals include argentite, galena, sphalerite, chalcopyrite, bornite, pyrite, and gold in a gangue of calcite, barite, quartz, clay, and talc. Talc generally surrounds most orebodies. Secondary minerals include azurite, cerussite, chalcocite, chrysocolla, cuprite, descloizite, malachite, mimetite (Pbs(AsO₄)₃Cl), smithsonite, vanadinite, and wulfenite. Wulfenite is locally present as small cubes. Silver is the predominant metal, although lead and zinc occur locally in significant quantities (as much as 40% combined Pb and Zn). Gold rarely exceeds 0.05 oz Au/ton. Silicification is common, especially along faults. Fluid-inclusion and stable-isotopic studies indicate that the mineralization occurred at 220-300°C from low salinity (0-9 eq. wt.% NaCl) meteoric fluids with local magmatic input (Bazrafshan, 1989). Fluid-inclusion studies (Bazrafshan, 1989), alteration assemblages, and textural features (V. T. McLemore, unpublished field notes, December 29, 1995) indicate that boiling probably occurred and may have been a mechanism of deposition.

Shepard (1984) reported quartz pseudomorphs after galena in the American Flag—Flagstaff area. The "pseudomorphs" occur within silicified zones in and near the American Flag, Flagstaff, and Pelican mines. Clusters of pseudocubic to pseudo-octahedral "crystals" as large as 2-4 inches across occur in vugs and solution cavities. The morphological resemblance to galena led Shepard (1984) to believe that they were pseudomorphs after galena. Crystallographic and petrographic analyses of the "pseudomorphs" indicate that they are actually calcite crystals overgrown or replaced by quartz. The thin coating of quartz and subsequent dissolution of the calcite produced the epimorphs ("over forms"). The epimorphs are typically hollow inside and consist of successive layers of chalcedony (initially amorphous silica?) and very fine grained quartz crystals with a final coating of sugary quartz. Freshly broken epimorphs locally contain calcite. "Fossilized" rhombohedral cleavage traces are also preserved by the quartz in some samples. Goniometric measurements of the crystal faces preserved by the epimorphs are consistent for rhombohedral crystals. Interfacial angles measured from complexly twinned galena from the district do not match those of the epimorphs.

One mechanism proposed for the origin of the quartz epimorphs is boiling. Fluid inclusion data are consistent with boiling at the American Flag—Flagstaff mines (Bazrafshan, 1989). In most natural waters, cooling without boiling causes calcite to dissolve. However, when an ascending carbonate-rich fluid boils as a result of cooling adiabatically, then calcite will precipitate as a result of a loss of CO₂. Once CO₂ is lost, boiling stops but the cooler water continues to ascend and dissolves the calcite. On the other hand, quartz is deposited as a result of cooling of hydrothermal waters and therefore, quartz simultaneously replaces the calcite. The deposition of chalcedony (amorphous silica?) prior to drusy quartz is consistent with this interpretation. The rising fluids begin to boil again and the process is repeated until CO2 is completely removed from the system.

References:

1. Bazrafshan, K., 1989, Geology and geochemistry of the Hermosa mining district, Sierra County, New Mexico: Ph.D. dissertation, New Mexico Institute of Mining and Technology, Socorro, 470 pp.
2. Shepard, M. D., 1984, Geology and ore deposits of the Hermosa mining district, Sierra County, New Mexico: Unpublished M. S. thesis, University of Texas (El Paso), 215 pp.