INFRARED SPECTROMETRY AT DETACHMENT FAULT GOUGE FOR FLUID PATHWAY ANALYSIS, SILVER PEAK RANGE AND SYLVANIA MOUNTAINS, NEVADA, USA

Major faults in the Basin and Range Province have long been understood to be pathways for both upwelling (hot) and downwelling (cool) water, but exactly how this occurs is debatable. This project utilized the TerraSpec Halo portable infrared mineral identifier, which uses infrared and near-infrared wavelengths to measure the hydrous silicates present in the samples, providing mineral content and spectral signatures. Profiles were scanned from hanging wall to footwall, including the gouge. Mineral indicators and spectral peaks of these profiles can be used to determine relative thermal maturity of the gouge, which gives an idea of fluid temperatures and alteration history.

The principal spectral indicator used is illite spectral maturity (ISM), which is the ratio of infrared absorption by the AlOH bond to absorption by bound OH^-. Cooler fluids result in lower ISM values, while hotter fluids lead to higher values. Previous studies in the classic turtlebacks of Death Valley (Haines and van der Pluijm 2012, Brikowski 2017) have shown noteworthy changes in mineralogy and infrared signature across and along detachment faults, and similar results are found in this study, on largely unexplored faults. In some cases there is little lateral variation in ISM, while in others there are large changes in ISM values along with mineral content changes in less than 200 meters. At one particular site this is clearly evident, with ISM values ranging from near zero to around 10 between profiles in the region (values above 1 show a thermally mature history). This suggests upwelling could be limited to zones of only a few hundred meters, having worthwhile implications for geothermal, economic geology, and even petroleum geology.