SPATIALLY CONTINUOUS ALTERATION MAPPING OVER AN ENTIRE CALDERA: NEW CONSTRAINTS ON THE GENESIS, GEOCHEMISTRY, AND STRUCTURAL CONTROL OF HYDROTHERMAL MINERALIZATION AT LONG VALLEY, CA, USA

Spatially and spectrally continuous airborne remote sensing imagery was used to map and identify hydrothermal alteration minerals over 450km2 of Long Valley Caldera in central eastern California. Normally alteration minerals can be challenging to map in the field over large areas (especially if diffusively distributed). Conventional point surveys in the form of drill holes and surface water and rock sampling provide sparse coverage of surface alteration and rock types. Conversely, each five-meter spatial element of a hyperspectral dataset supplies continuous surface material identification allowing for construction of detailed alteration maps over large areas in short amounts of time. Long Valley alteration assemblages are conclusively identified and mapped throughout the caldera including a preponderance of previously unknown mineralization. The system-scale alteration maps generate new controls on the chemistry, host rock, timing and intensity of alteration stages within the caldera, all of which are captured by caldera-wide maps of sinter, travertine and hydrothermal clays and sulfates. Alteration distribution, density and degree of weathering serve as proxy for alteration timing and intensity, while the comparison of surface alteration with drillcore alteration data at depth provides an innovative way of studying alteration geography and depth to surface zoning. The unprecedented amounts of newly discovered high temperature alteration in the western caldera corroborates hydrothermal circulation models that place a thermal source firmly below the western moat. The western alteration maps ultimately imply a far more intense and complicated alteration regime than previously thought. In addition, hydrothermal discharge maps are generated using linear and point distributions of the alteration assemblages discussed above. These maps mirror known structural networks of Long Valley and add new, previously un-identified hydrothermal conduits including east-west trending faults that allow structurally controlled fluid flow through the central caldera. These refined discharge maps coupled with topography data provide a new framework for hydrothermal circulation modeling and are proving instrumental in current local geothermal exploration efforts.