Geothermal Fluids and Detachment Faults

Movement of crustal fluids along large faults is a crucial part of the formation of geothermal systems, ore and petroleum deposits, and is related permeability changes in and adjacent to the faults. When these fluids are relatively cool, as is the case for deep fluid recharge in transtensional settings, the geological evidence for fluid circulation can be subtle. Analysis of the distribution of neoformed clay minerals (illite-smectite series) was performed on core samples from exploratory geothermal well EM-17-31, that intersects a low-angle detachment fault at depth, within Fish Lake Valley, Nevada. IR spectrometry, SEM-EDS, thin section photomicrography, and X-ray CAT suggest that the geothermal fluid flow was largely localized to the fault plane, and the zone immediately proximal to the detachment fault. Furthermore, SEM photomicrography and element mapping suggests the development of the geothermal system proceeded in stages. First, early stage of profound silicification, followed by silica dissolution with a corresponding increase in porosity. Subsequently, precipitation of low-temperature hydrothermal clay minerals in the aforementioned secondary porosity and concomitant decrease in total porosity.