Volcanic-epithermal deposits in the Mogollon-Datil volcanic field, west-central New Mexico
— Virginia T. McLemore

Most volcanic-epithermal precious-metal deposits in New Mexico are restricted to Oligocene to Miocene volcanic rocks and to areas immediately adjacent to volcanic centers in the Mogollon-Datil volcanic field in west-central New Mexico. These deposits occur in areas of complex regional and local structures that appear to control emplacement of magmas and subsequent flow of hydrothermal fluids. Although the mechanism of ground preparation varies, most districts occur along the ring-fracture zones of calderas, in the vicinity of rhyolitic domes, and/or along faults associated with regional crustal lineaments. Two types of volcanicepithermal deposits occur in the Mogollon-Datil volcanic field. Most deposits are of the low-sulfidation (quartz-adularia) class and formed by multiple cycles of low salinity (<5 eq. wt.% NaC1), slightly acidic to neutral pH fluids, at temperatures between 150° and 300°C, at relatively shallow depths (<1500 m) and low pressure (<150 bars). The mineral deposits at Alum Mountain are high-sulfidation (acid-sulfate) deposits. In addition, acid-sulfate alteration similar to that found in high-sulfidation deposits is present in several districts and may indicate that additional high-sulfidation precious-metal deposits may occur in these areas as well. Limited geologic and geochemical data are consistent with formation of acid-sulfate alteration by acidic fluids at temperatures below 340 °C at relatively shallow depths (<1.5 km) in a magmatic-hydrothermal environment. The acid-sulfate alteration characteristic of the high-sulfidation deposits appears to be restricted to 27-33 Ma in the Mogollon-Datil volcanic field, whereas the low-sulfidation, volcanic-epithermal vein deposits are younger than 27 Ma. The economic resource potential for low-sulfidation, volcanic-epithermal vein deposits in many districts is good because many favorable structures have not been adequately explored. Also, there is good potential for the discovery of high-sulfidation precious-metal deposits in areas of acid-sulfate alteration.