THE INFLUENCE OF STRUCTURAL SETTING ON THE GEOCHEMISTRY OF A FAULT-CONTROLLED HYDROTHERMAL SYSTEM IN THE BASIN AND RANGE PROVINCE OF NEVADA, USA

Leach Hot Springs, Nevada, is a hydrothermal area located in the Basin and Range province of the western US. The area comprises around 20 individual spring vents, roughly aligned along two parallel lineaments associated with a north-trending normal fault. Springs discharging along the easternmost lineament are near boiling in temperature, slightly to moderately acidic, and low in bicarbonate, chloride, and silica. By contrast, springs discharging from the western lineament demonstrate moderate to near boiling temperatures, near neutral pH, and have high concentrations of chloride, silica, and bicarbonate compared to the eastern springs.

It has long been known that these types of differences in chemistry are linked to the presence or absence of boiling conditions in the subsurface; the chemistries observed in the eastern springs are usually associated with boiling conditions in the near subsurface, while those observed in the western springs generally indicate the absence of boiling. Here we discuss the relationship between boiling and non-boiling conditions and the local structural setting: in the study area, boiling conditions appear to be associated with high-permeability regions of the controlling fault, whereas the non-boiling springs are linked to areas of the fault displaying only moderate permeability. Furthermore, the lateral conductivity of the region between the two linear structures can be shown to be relatively low, based on changes in the inferred water table elevation and the distribution of ground-surface temperatures measured at the site. Our observations at Leach Hot Springs provide insight into the near-surface structural controls on fluid flow, and demonstrate a relationship between spring hydrochemistry, the fault hydraulic architecture, and fracture permeability associated with the breakdown regions of the fault.