Howald, TreVor, Department of Earth and Environmental Science, Geology and Geochemistry Programs, 801 Leroy Place, Socorro, New Mexico 87801
The Beowawe sinter terrace is situated along the eastern margin of Whirlwind Valley in the Battle Mountain heat flow high. This geothermal system is characterized by remarkably high shallow heat flow ( > 2000 mW/m2), crustal 3He/4He ratios, and dDwater and δ18Owater compositions near the meteoric water line. Up to 1011 kg of siliceous sinter was deposited along the Malpais Fault zone on the east margin of Whirlwind Valley. About 1 km to the east, the Malpais Fault is occupied by banded chalcedonic quartz veins with anomalous Au. Mass balance calculations based on hot spring silica concentrations and discharge rates led Zoback (1979) to conclude the sinter deposits formed over a period of 200,000 years. Our fluid flow models suggest the duration of the modern Beowawe system is <5,000 years. 14C and 39Ar/40Ar geochronology were used to resolve the age, duration, and rates of fluid flow and silica deposition in the system.
Adularia from a banded chalcedonic quartz vein contains very little radiogenic argon, thus yielding 39Ar/40Ar dates of zero age with large uncertainties. These data are consistent with either a young age for the vein or loss of argon from adularia by subsequent heating.
Conventional radiocarbon ages on organic matter in sinter from six locations along the terrace spanning an elevation of about 500 m vary between 2,298 and14,375 years before present (BP). Two samples separated by 2.4 m near the base of the sinter terrace yield ages of 10,774±50 and 10,560±50 years BP that correspond to an apparent deposition rate of about 1 cm/yr. Two samples separated by 2.5 m near the middle of the sinter terrace yield ages of 14,375±70 years and 8,797±60 BP that correspond to an apparent deposition rate of 0.4 mm/yr. The wide range in deposition rates suggest sinter deposition either varied in space along the fault zone and/or was episodic. The oldest age from near the base of the sinter terrace indicates most of the silica was deposited over the last 14,375 years, which is in between previous estimates by Zoback and us.
The results suggest that both the veins and the sinter deposits are young, fluid discharge sites along the fault zone varied over at least 1 km, and the duration or flux of fluid discharge in our model must be increased by a factor of 3.