Using Infrared Spectrometry to Deduce Fluid History from an Exploration Core, Emigrant Peak Geothermal Prospect, Northern Fish Lake Valley, Nevada, USA

摘要

Infrared (IR) spectrometry and identification of minerals has led to notable progress in quantifying rock alteration in magma-hydrothermal systems. Similar progress may be possible for the amagmatic geothermal systems common in the western U.S., despite their much weaker rock alteration. Infrared spectral scanning of core from a dry hole in such a setting at the Emigrant Peak Geothermal Prospect in northern Fish Lake Valley, Nevada, suggests several discernable fluid migration events at this site. The stratigraphic sequence includes shallow Tertiary intermediate and silicic volcanics, Paleozoic greenschist-facies metamorphosed sheared carbonates and pelites, infrequent lithons of Mesozoic granodiorite, all in detachment fault contact with pre-Cambrian gneissic metamorphic tectonites. IR-detected minerals exhibit a steady downhole progression from smectite to illite to muscovite by TD at 2900 ft., reflecting shallow diagenetic alteration merging smoothly into much older regional metamorphic assemblages. IR scalars, based on ratios of reflectance at diagnostic wavelengths, indicate a steadily increasing alkali phyllosilicate maturity with depth (Illite Spectral Maturity or ISM), largely independent of lithology. At 2400 ft. depth this trend reverses, with increasing immaturity in silicified Paleozoic metasiltstones overlying the Silver Peak-Lone Mountain Detachment Fault (SPLMDF) at 2733 ft. A distinct break to ISM values consistent with the shallow diagenetic zone occurs at and below the detachment. Similar trends are exhibited in the Chlorite Spectral Maturity (CSM) scalar. This suggests a relatively cool hydrothermal event, with fluid temperatures in the range of those observed in nearby amagmatic geothermal systems, primarily restricted to the SPLMDF and below. Reappearance of smectites and iron-oxides in this depth range also indicates relatively epithermal conditions. At an even finer scale, minimum maturity ISM values are spatially correlated with zeolite-smectite mineral occurrences near and below the detachment, potentially indicating now-inactive pathways for the recent amagmatic geothermal circulation.