Supergene destruction of a hydrothermal replacement alunite deposit at Big Rock Candy Mountain, Utah: mineralogy, spectroscopic remote sensing, stable-isotope, and argon-age evidences

Abstract

Big Rock Candy Mountain is a prominent center of variegated altered volcanic rocks in west-central Utah. It consists of the eroded remnants of a hypogene alunite deposit that, at~21 Ma, replaced intermediate-composition lava flows. The alunite formed in steam-heated conditions above the upwelling limb of a convection cell that was one of at least six spaced at 3-to 4-km intervals around the margin of a monzonite stock. Big Rock Candy Mountain is horizontally zoned outward from an alunite core to respective kaolinite, dickite, and propylite envelopes. The altered rocks are also vertically zoned from a lower pyritepropylite assemblage upward through assemblages successively dominated by hypogene alunite, jarosite, and hematite, to a flooded silica cap. This hydrothermal assemblage is undergoing natural destruction in a steep canyon downcut by the Sevier River in Marysvale Canyon. Integrated geological, mineralogical, spectroscopic remote sensing using AVIRIS data, Ar radiometric, and stable isotopic studies trace the hypogene origin and supergene destruction of the deposit and permit distinction of primary (hydrothermal) and secondary (weathering) processes. This destruction has led to the formation of widespread supergene gypsum in cross-cutting fractures and as surficial crusts, and to natrojarosite, that gives the mountain its buff coloration along ridges facing the canyon. A small spring, Lemonade Spring, with a pH of 2.6 and containing Ca, Mg, Si, Al, Fe, Mn, Cl, and SO 4 , also occurs near the bottom of the canyon. The 40 Ar/ 39 Ar age (21.32F0.07 Ma) of the alunite is similar to that for other replacement alunites at Marysvale. However, the age spectrum contains evidence of a 6.6-Ma thermal event that can be related to the tectonic activity responsible for the uplift that led to the downcutting of Big Rock Candy Mountain by the Sevier River. This~6.6 Ma event also is present in the age spectrum of supergene natrojarosite forming today, and probably dates the beginning of supergene alteration at Big Rock Candy Mountain. O SO 4 values of gypsum show an excellent correlation, with values ranging from 15.2x to À5.2x and 7x to À8.2x, respectively. The stable-isotope data indicate that the aqueous sulfate for gypsum is a mixture derived from the dissolution of hypogene gypsum and alunite, and from the supergene oxidation of pyrite. The aqueous sulfate for the natrojarosite, however, is derived largely from the supergene oxidation of pyrite, with a minor contribution from the dissolution of alunite and gypsum. The exceptional detailed spectral mapping capabilities of AVIRIS led to the recognition of a small amount of jarosite that is probably the top of the steam-heated system that produced the primary hypogene alteration at Big Rock Candy Mountain. Published by Elsevier B.V.

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

“…The White Horse deposit was chosen for this study because of a relatively horizontal level of erosion across the deposit that has preserved a strong concentric "bulls-eye" alteration pattern centered on the replacement alunite. Data from this deposit complement recently published studies of a similar hydrothermal system at nearby Big Rock Candy Mountain (Rockwell, 2004;Cunningham et al, 2005;Rockwell et al, 2005) that has been eroded to deeper levels, providing excellent exposure of the vertical zonation through the system and insight into its natural destruction. Rockwell (2004) examined spectral variations in iron-bearing rocks and soils near the Trinity mine and Big Rock Candy Mountain that are related to variations in grain size and/or mineral abundance.…”

“…The tops of the upwelling limbs of these convective systems are marked by areas of intense advanced argillic alteration in fractured, permeable zones of volcanic rocks near the paleoground surface. Geochronologic studies have indicated that most of the hydrothermal systems and related alteration formed between 23 and 22 Ma (Cunningham et al, 1984) although Big Rock Candy Mountain is slightly younger (Cunningham et al, 2005). Figures 1 and 2 show the distribution of the replacement alunite deposits around the quartz monzonite intrusions.…”

“…Rockwell (2004) examined spectral variations in iron-bearing rocks and soils near the Trinity mine and Big Rock Candy Mountain that are related to variations in grain size and/or mineral abundance. Cunningham et al (2005) integrated data from a variety of analytical sources, including imaging spectroscopy and stable isotopes, to distinguish and characterize primary (hydrothermal) and secondary (weathering) geochemical processes at Big Rock Candy Mountain. presented detailed mineral maps of the Big Rock Candy Mountain and Big Star alunite deposits and more generalized mineral maps of the Tushar Mountains and Antelope Range areas of the Marysvale volcanic field and discussed the surface mineralogy in a geo-environmental context.…”

Identification of quartz and carbonate minerals across northern Nevada using ASTER thermal infrared emissivity data—Implications for geologic mapping and mineral resource investigations in well-studied and frontier areas

“…It is also possible that ASTER visible, NIR, and SWIR data can be used to differentiate chlorite and epidote (as a group) from carbonates by exploiting the deep absorption caused by ferric/ferrous iron centered around 1.0 µm that is present in chlorite and epidote (and many mafi c minerals) but not in pure carbonates (Hewson et al, 2005). In other areas, propylitic calcite and epidote have been identifi ed using ASTER SWIR data (Rockwell et al, 2006), and are readily discernable with spectroscopic data such as AVIRIS (Airborne Visible/Infrared Imaging Spectrometer) (Rockwell et al, 2000Rowan and Mars, 2003;Cunningham et al, 2005), HyMap, or HyperSpecTIR, although intimate mixing of these minerals can cause diffi culties (Dalton et al, 2004). In Paleozoic carbonate rocks, the distribution of hydrothermal dolomite occurrences can be used to map fl ow paths of hot basinal brines that generate SEDEX (sedimentary exhalative) and MVT (Mississippi Valley type) deposits (e.g., Diehl et al, 2005).…”

A study of the deep mineralization evaluation based on field measured spectra in Wushan-copper deposit area

“…Deep drilling cores for the study provides the most direct verifiable information, subject to analysis and testing for a long time, testing costs higher limit, unable to effectively carry out a large number of traditional geological analysis testing. In recent years, with the improvement of hyperspectral sensor accuracy and improved identification of high spectral mineralogical alteration extraction method using visible -near infrared reflectance spectroscopy, fast, economical and more accurate information to carry out geological core drilling research, information provided favorable metallogenic model for deep analysis and prospecting prediction [3][4][5][6][7][8] .…”