Spar caves as fossil hydrothermal systems: Timing and origin of ore deposits in the Delaware Basin and Guadalupe Mountains, New Mexico and Texas, USA

Abstract

Studies of sulfuric acid hypogene speleogenesis have contributed significantly to understanding the history of the Guadalupe Mountains of southeast New Mexico and west Texas for at least the past 12 Ma. A recently published hypothesis of supercritical CO2 spar cave genesis provides information that constrains the timing of the start of uplift to between 27 and 16 Ma, and helps to explain landscape evolution of this region for the last 185 Ma. This new speleogenetic model is summarized here and shows that U-Pb dating of crystals from different spar caves reveal different ages, and that a majority of the spar crystals were deposited during ignimbrite flare-up episodes at the end of the Basin and Range extension and onset of Rio Grande Rift extension between 36–28 Ma. During cave spar formation, geothermal gradients ranged from 50 to 70°C/km. Stable isotope data for δ13C, δ18O, and δ88Sr support that parent waters of cave spar were of low hydrothermal origin and mixed with gases emanating from shallow magma conduits; nearby outcrops of Tertiary igneous dikes of the same age as the spar support this hypothesis. Supercritical CO2 hydrothermal systems driven by magma intrusion on the western fringe of the Delaware Basin were responsible for the formation of small caves containing large, euhedral calcite crystals. Hydrothermal deposits from these types of systems are sometimes used to locate economic ore deposits, however, since this area has been uplifted, any indication of fossil hydrothermal systems, like travertine deposits, have eroded. Spar caves are remnants of hydrothermal processes and are related to and coeval with ore deposition and hydrocarbon generation in the Guadalupe Mountains and Delaware basin. These spar caves can be used as both proxies for landform evolution and to locate economic mineral deposits.