Spectroscopic Mapping of the White Horse Alunite Deposit, Marysvale Volcanic Field, Utah: Evidence of a Magmatic Component

Abstract

Previous studies have demonstrated that the replacement alunite deposits just north of the town of Marysvale, Utah, USA, were formed primarily by low-temperature (100º-170º C), steam-heated processes near the early Miocene paleoground surface, immediately above convecting hydrothermal plumes. Pyrite-bearing propylitically altered rocks occur mainly beneath the steam-heated alunite and represent the sulfidized feeder zone of the H2S-dominated hydrothermal fluids, the oxidation of which at higher levels led to the formation of the alunite. Maps of surface mineralogy at the White Horse deposit generated from Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data were used in conjunction with X-ray diffraction studies of field samples to test the accuracy and precision of AVIRIS-based mineral mapping of altered rocks and demonstrate the utility of spectroscopic mapping for ore deposit characterization. The mineral maps identified multiple core zones of alunite that grade laterally outward to kaolinite. Surrounding the core zones are dominantly propylitically altered rocks containing illite, montmorillonite, and chlorite, with minor pyrite, kaolinite, gypsum, and remnant potassium feldspar from the parent rhyodacitic ash-flow tuff. The AVIRIS mapping also identified fracture zones expressed by ridge-forming selvages of quartz + dickite + kaolinite that form a crude ring around the advanced argillic core zones. Laboratory analyses identified the aluminum phosphate-sulfate (APS) minerals woodhouseite and svanbergite in one sample from these dickite-bearing argillic selvages. Reflectance spectroscopy determined that the outer edges of the selvages contain more dickite than do the medial regions. The quartz + dickite ± kaolinite ± APS-mineral selvages demonstrate that fracture control of replacement processes is more prevalent away from the advanced argillic core zones. Although not exposed at the White Horse deposit, pyrophyllite ± ordered illite was identified using AVIRIS in localized, superimposed conduits within propylitically altered rocks in nearby alteration systems of similar age and genesis that have been eroded to deeper levels. The fracture zones bearing pyrophyllite, illite, dickite, natroalunite, and/or APS minerals indicate a magmatic component in the dominantly steam-heated system.

Lithospheric dismemberment and magmatic processes of the Great Basin–Colorado Plateau transition, Utah, implied from magnetotellurics

“…Surface dip of the Sevier rangefront fault is westward [Stokes, 1986]; we suggest it may be antithetic to the crustal zone we have imaged. The steep conductor does not connect to recently described intense alunite alteration of the Whitehorse mine $3 km to the east [Rockwell et al, 2006], which generates a narrow band of low r yx ( Figure 5) but does not imply structure to depth.…”

“…Surface dip of the Sevier rangefront fault is westward [ Stokes , 1986]; we suggest it may be antithetic to the crustal zone we have imaged. The steep conductor does not connect to recently described intense alunite alteration of the Whitehorse mine ∼3 km to the east [ Rockwell et al , 2006], which generates a narrow band of low ρ yx (Figure 5) but does not imply structure to depth. Another steep conductor appears to connect with the base of the Fish Lake graben, and the easternmost plane surfaces near the base of the eastern topographic scarp of the TZ just east of the Fremont/Thousand Lake fault zone [ Bailey et al , 2007; Schelling et al , 2007].…”

Description and validation of an automated methodology for mapping mineralogy, vegetation, and hydrothermal alteration type from ASTER satellite imagery with examples from the San Juan Mountains, Colorado

“…Alunite-free pyrophyllite occurs along another, sub-parallel fracture system immediately to the south that extends south and west through intense phyllic alteration situated immediately adjacent to and beneath area "A" along Prospect Gulch. Vertical fracture zones and localized, pipelike structures of pyrophyllite ± illite, with natroalunite and (or) dickite ± alumino-phosphate sulfate minerals at higher levels, have been recognized within phyllic alteration situated beneath or adjacent to the Big Rock Candy Mountain (BRCM) and Big Star alunite deposits of the Marysvale volcanic field (Rockwell and others, 2005;2006, Cunningham andothers, 2005). Pyrophyllite replaces sericite at the margins of quartzalunite centers upon outward transition to phyllic alteration in the lower part of the relict hydrothermal systems at Red Mountain no.…”

Silicified layers within the Paleogene volcaniclastic Brian Head Formation, southern Utah: Insights into the origin of silicified beds in nonmarine strata

“…Biek et al, 2012). Heat associated with intrusive bodies in the Marysvale volcanic field established kilometer-scale, meteoric-water, hydrothermal convection cells that were responsible for the formation of alunite deposits (Cunningham et al, 2005;Rockwell et al, 2006). It is likely that Marysvale-related sources of the thermal energy heated meteoric waters, which dissolved microcrystalline quartz from the abundant glassy fragments in the volcaniclastic unit (Wizevich and Braddock, 2012).…”