New Mexico Geological Society Annual Spring Meeting — Abstracts

There are 10 mining districts in the Black Range area, central New Mexico. Carbonate-hosted Ag and Pb-Zn deposits are found in the Georgetown, Carpenter, Kingston, Tierra Blanca, Hermosa, and Lake Valley districts. Epithermal vein and carbonate-hosted Pb-Zn deposits are found in the Macho and Cuchillo districts. Porphyry-copper, vein, and carbonate-hosted deposits are found at Hillsboro. Epithermal
vein deposits are found at Chloride. The predominant structural feature of the Black Range is the Emory cauldera (about 35 Ma) and other workers have attributed much of the mineralization to this feature.

Carbonate-hosted Ag and Pb-Zn replacement deposits in the Black Range include replacements in carbonate rocks with little or no calc-silicate minerals, small skarns, and veins in carbonate rocks. These replacement deposits are typically Ag or Pb-Zn dominant, with by-product Cu, Mn, and Au. Calc-silicate minerals typical of skarn deposits are rare to absent in most ofthese deposits. The Ag deposits typically consist of an oxidized assemblage ofnative silver, cerussite, vanadinite, wulfenite, and smithsonite. Hypogene mineralization is obscured by oxidation, but when preserved consists of argentite, argentiferous galena, polybasite, pyargyrite, stephanite, sphalerite, and
chalcopyrite. Galena and sphalerite are the predominant ore minerals in the Pb-Zn deposits, with lesser amounts ofchalcopyrite. Paleozoic limestones and dolomites, typically belonging to the Fusselman Dolomite (Silurian) and Lake Valley Limestone (Mississippian), host the mineral deposits.

These deposits are found near igneous intrusions, but occur more distal from the intrusions than typical skarn deposits. The precise ages of these deposits are unknown; geologic relationships and published age dates of nearby igneous intrusions suggest they are 20 to 75 Ma. The oldest deposits are found at Hillsboro where the Copper Flat quartz monzonite porphyry has been dated as 75.1 ± 2.5 Ma (K-Ar date, Hedlund, 1985). A dike from Georgetown has a ³⁹Ar/⁴⁰Ar isocbron date of 71 ± 2 Ma; but the relationship of the mineralization to the dikes is unknown. These deposits also display textures and other features similar to those found in volcanic-epithermal vein deposits. Field relationships suggest that several ages of igneous activity and mineralization occurs in some districts.

The evidence suggests that these deposits were formed by large, convective hydrothermal systems related to local igneous activity at different times. The oldest of these systems is associated with Laramide igneous activity (ie. Hillsboro, possibly Georgetown). Formation of the Emory cauldera created additional fracturing and faulting in the Black Range and allowed multiple intrusion of magmas that provided heat and fluids to form additional deposits at Carpenter, Kingston, Tierra Blanca, Hermosa, Cuchillo, Macho, and Chloride. The main difference between the Laramide deposits and the Mid-Tertiary deposits is one oftectonic setting. The Laramide deposits were emplaced under compressional tectonics, whereas the Mid-Tertiary deposits were formed under extensional tectonics, ie. along fractures formed by the Emory cauldera,