Origin and Evolution of Vein Minerals in the High-Temperature Geothermal Reservoir of the Northwest Geysers, California

摘要

Vein mineral assemblages in contact metamorphosed biotite hornfelsic metagreywacke in the high-temperature vapor-dominated geothermal reservoir of the Northwest Geysers have been analyzed using a combination of transmitted and reflected light petrography, fluid inclusion microthermometry, scanning electron microscopy, energy dispersive spectroscopy and cathodoluminescence in order to characterize their origin and evolution. In this study, we report data from two core samples, the first from 11,051-11,067 ft in the L'Esperance-2 well and the second from 9940-9945 ft in well Prati-5 St-1. These new data are supplemented with oxygen isotope data from vein quartz and the adjacent altered wall rock from L'Esperance-2 and whole-rock samples in both wells. The vein assemblages and paragenesis in both wells are similar, even though they are located ~3.5 km apart. The veins contain early biotite, (± clinopyroxene, amphibole, tourmaline and apatite), this assemblage is overprinted by quartz (± actinolite and plagioclase) and late stage Fe>Zn-Cu sulfide minerals (± chlorite and quartz). All fluid inclusion measurements were made on vein quartz. Fluid inclusion microthermometry and energy dispersive spectrometry data suggests that the inclusion fluids contain Cl, Na, Ca, Fe, K, F and potentially Mn, C, B, S and P. The abundance of vapor-rich inclusions in both samples, the relationship of increasing salinity with decreasing homogenization temperature in L-Esperance-2 and the presence of hypersaline fluid inclusions with up to at least 14 daughter salts in Prati-5 St-1 are interpreted to be the result of boiling connate derived water during the transition from liquid-dominated to vapor-dominated reservoir conditions. Modern measured temperatures logged in these wells fall within the range of total fluid inclusion homogenization temperatures (T_(h(tot))). In Prati-5 St-1 the maximum measured temperature was 314°C and T_(h(tot)) ranged from 298 to 386°C. In L'Esperance-2 the maximum measured temperature was 296°C and T_(h(tot)) ranged from 255 to 353°C. These temperature relationships may indicate that the quartz veins are relatively young, and/or that temperatures in the Northwest Geysers have been relatively stable since quartz deposition. Oxygen isotope data from vein quartz in L'Esperance-2 (+9.8 to +10.4‰) were used in conjunction with the homogenization temperatures to calculate a range of values for the formation fluids (+2.5 to +5.2‰). These values are similar to the oxygen isotopic composition of saline hot springs fluids issuing from the Franciscan Assemblage, such as Wilbur Springs (+3.2 and +5.6‰), which have been interpreted as connate/metamorphic fluids. In addition, the early steam condensate produced from many wells in the Northwest Geysers in the vicinity of Prati-5 St-1 ranged from 0 to +3.2%o. Thus, we suggest that vein quartz precipitated from saline f connate/metamorphic fluids that were progressively concentrated by boiling.