Jeffrey B. Hulen

The Geysers - Cobb Mountain magma system, California (Part 1): U-Pb zircon ages of volcanic rocks, conditions of zircon crystallization and magma residence times

Abstract Combined U-Pb zircon and 40 Ar/ 39 Ar sanidine data from volcanic rocks within or adjacent to the Geysers geothermal reservoir constrain the timing of episodic eruption events and the pre-eruptive magma history. Zircon U-Pb concordia intercept model ages (corrected for initial 230 Th disequilibrium) decrease as predicted from stratigraphic and regional geological relationships (1σ analytical error): 2.47 ± 0.04 Ma (rhyolite of Pine Mountain), 1.38 ± 0.01 Ma (rhyolite of Alder Creek), 1.33 ± 0.04 Ma (rhyodacite of Cobb Mountain), 1.27 ± 0.03 Ma (dacite of Cobb Valley), and 0.94 ± 0.01 Ma (dacite of Tyler Valley). A significant (∼0.2–0.3 Ma) difference between these ages and sanidine 40 Ar/ 39 Ar ages measured for the same samples demonstrates that zircon crystallized well before eruption. Zircons U-Pb ages from the underlying main-phase Geysers Plutonic Complex (GPC) are indistinguishable from those of the Cobb Mountain volcanics. While this is in line with compositional evidence that the GPC fed the Cobb Mountain eruptions, the volcanic units conspicuously lack older (∼1.8 Ma) zircons from the shallowest part of the GPC. Discontinuous zircon age populations and compositional relationships in the volcanic and plutonic samples are incompatible with zircon residing in a single long-lived upper crustal magma chamber. Instead we favor a model in which zircons were recycled by remelting of just-solidified rocks during episodic injection of more mafic magmas. This is consistent with thermochronologic evidence that the GPC cooled below 350° C at the time the Cobb Mountain volcanics were erupted.

Porosity, permeability and fluid flow in the Yellowstone Geothermal System, Wyoming

Cores from two of 13 U.S. Geological Survey research holes at Yellowstone National Park (Y-5 and Y-8) were evaluated to characterize lithology, texture, alteration, and the degree and nature of fracturing and veining. Porosity and matrix permeability measurements and petrographic examination of the cores were used to evaluate the effects of lithology and hydrothermal alteration on porosity and permeability. The intervals studied in these two core holes span the conductive zone and the upper portion of the convective geothermal reservoir. Variations in porosity and matrix permeability observed in the Y-5 and Y-8 cores are primarily controlled by lithology. Y-8 intersects three distinct lithologies: volcaniclastic sandstone, perlitic rhyolitic lava, and non-welded pumiceous ash-flow tuff. The sandstone typically has high permeability and porosity, and the tuff has very high porosity and moderate permeability, while the perlitic lava has very low porosity and is essentially impermeable. Hydrothermal self-sealing appears to have generated localized permeability barriers within the reservoir. Changes in pressure and temperature in Y-8 correspond to a zone of silicification in the volcaniclastic sandstone just above the contact with the perlitic rhyolite; this silicification has significantly reduced porosity and permeability. In rocks with inherently low matrix permeability (such as densely welded ash-flow tuff), fluid flow is controlled by the fracture network. The Y-5 core hole penetrates a thick intracaldera section of the 0.6-Ma Lava Creek ash-flow tuff. In this core, the degree of welding appears to be responsible for most of the variations in porosity, matrix permeability, and the frequency of fractures and veins. Fractures are most abundant within the more densely welded sections of the tuff. However, the most prominent zones of fracturing and mineralization are associated with hydrothermal breccias within densely welded portions of the tuff. These breccia zones represent transient conduits of high fluid flow that formed by the explosive release of overpressure in the underlying geothermal reservoir and that were subsequently sealed by supersaturated geothermal fluids. In addition to this fracture sealing, hydrothermal alteration at Yellowstone appears generally to reduce matrix permeability and focus flow along fractures, where multiple pulses of fluid flow and self-sealing have occurred.

The Geysers-Cobb Mountain Magma System, California (Part 2): timescales of pluton emplacement and implications for its thermal history

Abstract Over 400 ion microprobe U-Pb isotopic ages measured for zircons extracted from 24 geothermal wells that penetrate the Geysers Plutonic complex (GPC) allow us to conclude that the entire known extent of the GPC crystallized during the early Pleistocene. Nine samples of the microgranite porphyry that forms the shallow cupola (100–1,500 m below sea-level, mbsl) of the GPC yield the oldest model U-Pb age (1.75 ± 0.01 Ma after correction for initial U series disequilibrium; errors 1σ). Twelve samples from the main intrusive phase (orthopyroxene-biotite granite) present at depths >1,250 mbsl define a crystallization age of 1.27 ± 0.01 Ma. This coincides with the age determined for a structurally and compositionally distinct body of granodiorite (1.25 ± 0.01 Ma; N = 5 samples) that is intruded over a similar depth range. Two petrographically distinct varieties of orthopyroxene-biotite granite yield ages of 1.46 ± 0.03 (GPC21-6000) and 1.16 ± 0.02 Ma (CA5636 74F 21; three samples). U-Pb zircon ages for dikes intruded in metagraywacke country-rocks overlap with those obtained from the main body of the GPC and include the youngest material identified (dike sample NEGU2 ST1-7700: 1.11 ± 0.03 Ma). Overall, the U-Pb results demonstrate that the main body of the GPC (∼300 km3) was emplaced and crystallized within the upper crust over a short time interval (0.2 Ma) that overlaps with zircon crystallization ages of overlying silicic volcanic units.

Geology and Geothermal Origin of Grant Canyon and Bacon Flat Oil Fields, Railroad Valley, Nevada

Eastern Nevadas Grant Canyon and Bacon Flat oil fields show strong evidence of formation in a still-active, moderate-temperature geothermal system. Modern manifestations of this system include unusually elevated oil-reservoir temperature at shallow depth, 116-122°C at 1.1-1.6 km, and dilute Na-HCO3-Cl thermal waters directly associated with hot oil. Hydrogen and oxygen isotopic compositions indicate that these thermal waters are meteoric in origin, but were probably recharged prior to the Holocene (before 10 ka). The waters apparently ascended to oil-reservoir elevations after deep heating in response to the normal regional thermal gradient; there is no evidence for a modern magmatic heat source. The beginning of oil-reservoir evolution at both fields is recorded by late-stage, fracture-filling quartz in the vuggy, brecciated, Paleozoic dolostone reservoir rocks. Oil and aqueous solutions were trapped as fluid inclusions in the quartz at temperatures comparable to those now prevailing in the reservoirs. Apparent salinities of the aqueous inclusions closely match the actual concentrations of current oil-field waters, and the quartz has the oxygen isotopic composition predicted for its crystallization from these waters at contemporary temperatures. Present-day and fluid-inclusion temperatures define essentially coincident isothermal profiles through and beneath the oil-reservoir interval, a phenomenon consistent with near-constant convective heat transfer since inception of the eothermal system. Textural and mineralogic clues indicate that hot waters circulating in this system also increased porosity by dissolving carbonate minerals, and helped seal reservoir margins by precipitating silica and kaolin. More importantly, the rising thermal waters may have aided oil transport and accelerated source-rock maturation through an increase in the shallow (<3 km) local thermal budget. Along with the aforementioned fluid-inclusion and isotopic evidence, radiometrically dated life spans for numerous extinct geothermal systems (epithermal ore deposits) in the Basin and Range make it likely that the Grant Canyon-Bacon Flat system and associated oil reservoirs are no older than 2.5 Ma.

Preliminary numerical analysis of the magma-hydrothermal history of The Geysers geothermal system, California, USA

Abstract Numerical approximations to the thermal history of a representative 2-D semi-infinite section through a cooling pluton equivalent in size, configuration, and composition to the Quaternary-age Geysers plutonic complex (generally known as the felsite) provide new insights into the evolution of the associated, pre-vapor-dominated, magmatic-hydrothermal system. A reference state model demonstrates that 300 o C peak paleotemperatures preserved in metagraywacke 1.4 km above the felsite (in core from scientific drillhole SB-15-D) could not have been attained by pure conductive heat transfer. The paleotemperatures are readily matched, however, by a convective-cooling model in which fluids above the igneous heat source can migrate within a permeable (0.05 millidarcy, or 5 × 10 −17 m 2 ) reservoir the size and geometry of the modern vapor-dominated regime intermittently open to the surface within a narrow vertical conduit proxying the Big Sulphur Creek strike-slip fault zone. Under these conditions, the thermal maxima are achieved within only 55–70 ky post-intrusion. Model temperatures thereafter cool to 235°C (the modern reservoir value for SB-15-D) within 650 ky and to 130°C within 1 My. The latter value is close to the 1.2–1.1 Ma calculated crystallization age for explored portions of the felsite. These relationships suggest that a 1.2–1.1 Ma felsite-heated hydrothermal system, at the levels penetrated by SB-15-D, should have cooled to the currently prevailing temperature by about 0.5 Ma. This clearly did not happen, and in the simplest analysis (in accordance with previous investigations), the large temporal discrepancy points to a more complex intrusive history for the felsite than age-dating has thus far revealed.

Early Miocene lamproite from the Colorado Plateau tectonic province, Southeastern Utah, USA

Abstract Newly discovered olivine phlogopite lamproite dikes intrude Jurassic siliciclastic strata in the Green River Desert subregion of the western Colorado Plateau tectonic province in southeastern Utah. The dikes yield an age of 22 Ma both from 40 Ar/ 39 Ar step-heating of phlogopite and from isochron modeling of laser-fused sanidine. This age is similar to those of mica-rich minettes and melanephelinites of the Wasatch Plateau about 125 km northwest and within the age range of the Navajo potassic volcanic field about 150 km to the southeast. The dikes intruded a pre-existing, northwest-oriented fracture system containing previously introduced bitumen, indicating that some regional lineaments of this trend are Early Miocene or older. The dikes are highly LREE-enriched, and display lamproite-specific REE ratios and phlogopite and sanidine compositions. Incompatible element and radiogenic isotope (Nd–Sr–Pb) ratios suggest that lithospheric source material modified by ancient subduction processes, together with younger asthenospheric source components, produced the melt. Timing of the intrusion coincides with the transition from Early–Middle Cenozoic, calc-alkaline plutonism to the dominantly mafic, Basin and Range type volcanism of the Late Cenozoic. While the lamproite occurrence indicates thermal input from the mantle, model non-uniqueness for both magma source depths and geophysical structure prevents quantitative comparison of Early Miocene with present-day lithospheric thickness.

Hydrologic characterization of reservoir metagraywacke from shallow and deep levels of The Geysers vapor-dominated geothermal system, California, USA

Abstract Hydrologic and geochemical properties were measured on metagraywacke plug samples of matrix rock from shallow and deep cores from The Geysers geothermal field. Gas permeability ranged from 2×10 −22 to 4×10 −20 m 2 and Klinkenberg slip factor ranged from 1 to 20 MPa. Porosities ranged from 1.9 to 2.2%. Capillary-pressure curves were measured and van Genuchten fitting parameters were used to estimate relative-permeability curves. Shallow samples were weakly altered, but deeper ones were intensely altered and mineralized to chlorite, secondary potassium feldspar and various calc-silicate minerals. Deeper samples were more permeable, and this difference is related to chlorite–potassium feldspar alteration and related porosity enhancement.

Molybdenum mineralization in an active geothermal system, Valles caldera, New Mexico

Shallow, sub-ore-grade molybdenite mineralization has been discovered in the active, high-temperature geothermal system penetrated by Continental Scientific Drilling Program corehole VC-2A at Sulfur Springs, in the western ring-fracture zone of the Valles caldera, New Mexico. This mineralization is hosted by fractured, quartz-sericitized, intracaldera ash-flow tuffs younger than 1.12 Ma. The molybdenite is an unusual, poorly crystalline variety that occurs in vuggy veinlets and breccia cements also containing quartz, sericite (illite), pyrite, and fluorite, as well as local sphalerite, rhodochrosite, and chalcopyrite. Fluid-inclusion data suggest that this assemblage was deposited from very dilute solutions at temperatures near 200/sup 0/C. Geochemical modeling indicates that under restricted pH and fO/sub 2/ conditions at 200/sup 0/C, the molybdenite and associated phases would be in equilibrium with hydrothermal fluids now circulating in the deep subsurface. The shallow molybdenite zone intersected in VC-2A may be the near-surface expression of deep, Climax-type stockwork molybdenum mineralization.

Water adsorption at high temperature on core samples from The Geysers geothermal field, California, USA

Abstract For the first time, water sorption on representative geothermal reservoir rocks from The Geysers steam field has been determined in the laboratory at actual reservoir temperature. The Oak Ridge National Laboratory (ORNL) isopiestic apparatus has been used to measure quantities of water retained at various temperatures and relative pressures by plug samples of three representative reservoir metagraywacke cores. The measurements were made at 150, 200 and 250°C as a function of relative pressure in the range 0.00 ⩽ p / p 0 ⩽ 0.98, where p 0 is the saturated water vapor pressure. Both adsorption (increasing pressure) and desorption (decreasing pressure) runs were made in order to investigate the phenomenon of hysteresis. Low-temperature gas adsorption analyses were completed on the same rock samples. Nitrogen or krypton adsorption and desorption isotherms at 77 K were used to obtain BET (Brunauer, Emmet, Teller) specific surface areas and pore volumes and their distributions with respect to pore sizes. Mercury-intrusion porosimetry was also used to obtain similar information extending to very large pores (macropores). A qualitative correlation was found between the surface properties obtained from nitrogen adsorption and the mineralogical and petrological characteristics of the solids. In general, however, there is no direct proportionality between BET specific surface areas obtained from nitrogen adsorption and the capacity of the rocks for water adsorption at high temperatures. An analysis of the temperature dependence of adsorption/desorption indicates that multilayer adsorption rather than capillary condensation is the dominant water storage mechanism in The Geysers reservoir rocks at high temperatures.

Evolution of the Western Valles Caldera Complex, New Mexico: Evidence from intracaldera sandstones, breccias, and surge deposits

Scientific core drilling in the Pleistocene Valles caldera complex (encompassing the Valles (1.13 Ma) and coaxial Toledo (1.50 Ma) calderas) of north central New Mexico has provided new insight into the origins of sandstones, breccias, and pyroclastic surge deposits interbedded with the thick intracaldera ignimbrite sequence. These rocks were previously interpreted from geothermal drill cuttings as dominantly fluvial in origin. As such, representing significant erosional intervals, they formed much of the basis for subdividing the intracaldera ignimbrite sequence (up to 2000 m in apparent thickness where drilled) into four major units: the lower tuffs (> 1.50 Ma); the Otowi (1.50 Ma) and Tshirege (1.13 Ma) members of the Bandelier Tuff; and a new unit, the upper tuffs, believed to be post-Bandelier in age (<1.13 Ma). All but the upper tuffs correspond to mapped outflow facies ignimbrite sheets. However, Continental Scientific Drilling Program core holes VC-2A (completed in 1986) and VC-2B (completed in 1988), in the Sulphur Springs area of the Valles caldera, have provided continuous cores, revealing for the first time that some intracaldera rocks previously thought to be exclusively clastic actually have multiple origins. Some of these rocks are probably pyroclastic surge deposits; others could be lithic-rich breccias of various origins incorporated nearly instantaneously in ignimbrites during ash flow eruption and concomitant caldera collapse. These new findings demonstrate the value of continuous core for subsurface characterization and correlation of complex intracaldera lithologies; they also necessitate revision of Nielson and Hulens (1984) cuttings-based intracaldera stratigraphic framework. For example, the hematitic S2 “sandstone” was initially interpreted as marking an erosional interval between the Tshirege Member of the Bandelier Tuff and the overlying, petrographically similar upper tuffs. Core from VC-2A and VC-2B, however, shows that the S2 cuttings could also represent disaggregated, Permian red bed-rich, lithic lag breccias or caldera collapse mesobreccias. If this is the case, then most or all of the upper tuffs are actually uppermost Tshirege Member ignimbrites. In similar fashion and upon review of previously applied correlation criteria the “lower tuffs” of the western Valles caldera complex could represent both genuine pre-Bandelier ignimbrites and those of the lowermost Otowi Member. The core, however, shows that in the Sulphur Springs subsurface the lower tuffs are separated from overlying ignimbrite sheets by prominent erosional and eruptive breaks; they appear to be slightly more mafic than the overlying tuffs and host distinctive pumice lapilli. At this site the lower tuffs almost certainly predate the Otowi Member and are probably correlative with the outflow facies San Diego Canyon ignimbrites (1.78 Ma). Cores from VC-2A and VC-2B support earlier interpretation of the S3 “sandstone” as a major marker horizon separating the intracaldera Otowi and Tshirege members of the Bandelier Tuff but clearly shows that this important unit is not, as previously thought, entirely a simple intracaldera epiclastic apron. In VC-2A the S3 has the superficial appearance of a sandstone but contains abundant blocky shards as well as accretionary and armored lapilli; it is also soft-sediment-deformed and invades overlying nonwelded tuff as small clastic dikes. We believe that here the S3 was emplaced by a wet pyroclastic surge. In nearby corehole VC-2B the S3 consists of a basal, massive, sediment-gravity-flow (?) sandstone overlain by sandstone and dacite breccias with accretionary and armored lapilli-bearing tuffaceous matrices. These deposits are probably caldera collapse mesobreccias that were formed simultaneously with early Tshirege Member ash flow eruptions through or into a Toledo caldera lake.