Using infrared spectrometry to deduce fluid history from a geothermal energy exploration core at Emigrant Peak Geothermal Prospect, Northern Fish Lake Valley, Nevada, U.S.A.




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The purpose of this study was to test the possibility of using infrared (IR) spectrometry in mineral identification from a core (EM 17-31), from the Emigrant Peak geothermal prospect in northern Fish Lake Valley, Nevada. The use of IR spectrometry in the 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 western U.S., despite their much weaker rock alteration. The stratigraphic sequence includes shallow Tertiary intermediate and silicic volcanic rocks, Paleozoic greenschist-facies metamorphosed sheared carbonates and pelites, infrequent lithons of Mesozoic granodiorite, all in detachment fault contact with Precambrian gneissic metamorphic tectonites. IR-detected minerals exhibited a steady downhole progression from smectite to illite to muscovite (total depth of 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 decreasing maturity in silicified Paleozoic metasiltstones overlying the Silver Peak-Lone Mountain Detachment Fault at 2733 ft. A distinct change to ISM values consistent with the shallow diagenetic zone occurs at and below the detachment. Chlorite Spectral Maturity (CSM) scalar exhibits similar trends. 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 detachment fault and below. The 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 smectite mineral occurrences near and below the detachment, suggesting the movement of relatively cool fluids younger than 5 Ma indicating pathways for recent amagmatic geothermal circulation.



Fish Lake (Nev.), Infrared spectroscopy, Geothermal resources, Drill core analysis, Hydrothermal alteration


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