Valles Caldera Research Articles

Evaluation of the thermal regime of the Valles Caldera, New Mexico, U.S.A., by downward continuation of temperature data

Downward continuation of temperature data from 73 wells extending to depths of 250 ft (76 m) provides constraints on the thermal regime of the Valles Caldera. Surface-temperature gradient data and bottom-hole temperatures were used as constraints in the downward continuation. Three factors were found to control the shallow thermal regime: 1. (1) heat associated with the main geothermal source; 2. (2) local topography; and 3. (3) west-southwest groundwater flow. Although the well density is relatively high, comparison with the topography shows that the wells are not randomly distributed and tend to be clustered in valleys. Many details in the thermal regime appear to be related to groundwater drainage in these valleys. Temperature gradients and temperatures generally increase in the same direction as the regional drainage of the caldera suggesting a long-wavelength, shallow component to this regional gradient trend. Inversion of gradient and temperature data show additional deep heat input in the west-southwest sector of the caldera which appears to be spatially associated to the youngest volcanism. A previously reported northeast displacement of the main heat source from the surface anomaly has not been confirmed.

Internal textures of rhyolite flows as revealed by research drilling

Recent drilling of 550- and 140,000-yr-old rhyolite lava flows in the Inyo Dome chain, California, and Valles Caldera, New Mexico, and field inspection of 8 Ma flows in western Arizona reveal a more detailed vertical zonation of lava textures in glassy rhyolite flows than has previously been recognized. The upper vitrophyre of a flow can usually be subdivided into a surficial finely vesicular pumice underlain by obsidian, coarsely vesicular pumice, and a second obsidian layer. Beneath these layers is the crystalline center of the flow, which in turn is underlain by a basal obsidian layer (lower vitrophyre). Differences in lava vesicularity and crystallinity between zones in a flow result from primary effervescence, devitrification, lava rheology, and migration of water vapor.

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.

Travertine deposits of Soda Dam, New Mexico, and their implications for the age and evolution of the Valles caldera hydrothermal system

Uranium-thorium disequilibrium dates and stable isotope analyses of travertine deposits at Soda Dam, New Mexico, have been used to determine the age and to document the evolution of the Valles caldera hydrothermal system. Soda Dam discharges from Paleozoic and Precambrian rocks in San Diego Canyon southwest of the caldera, and the canyon was filled with several hundred metres of Upper Bandelier Tuff during formation of Valles caldera, 1.12 Ma. According to the dates, the Valles hydrothermal system is at least 106 yr old or nearly as old as the caldera. The basic hydrology of the system has thus remained virtually unchanged during this time. Travertine ages indicate pulses in travertine deposition from ∼1.0 to 0.48 Ma, from 0.107 to 0.058 Ma, and from 0.005 Ma to present. The volume of various travertine deposits suggests that the hot-spring system at Soda Dam was once larger than it is today, possibly discharging as much as 5 to 10 times more fluid of Na-HCO3-Cl composition. Stable isotopes of the travertines indicate, however, that the hot-spring system has probably never been more than about 10 °C hotter than the present maximum temperature of 48 °C. Stable isotopes also show that the δ13C of CO2 and δ18O of H2O in hot-spring waters has remained relatively constant for 106 yr. The similar ages between caldera formation and the onset of large-scale hydrothermal circulation and travertine deposition on pre-Bandelier Tuff rocks imply that incision rates by the Jemez River in Bandelier Tuff were relatively rapid. As much as 400 m of tuff were cut by the ancestral Jemez River in 105 yr or less.

Geothermal exploration of Roccamonfina volcano, Italy

The Quaternary volcano of Roccamonfina, located within the Roman high-potassic province, developed in the time interval 1.5-0.25 Ma, initially as a stratocone which later was partially destroyed by phreatomagmatic explosions. The last evolutionary stage is represented by intra-crater latite domes and trachybasalt flows ( ca. 15 Ma). Geophysical data, primarily from gravity and MT/EM surveys, demonstrate that the volcano formed at a complex structural intersection above a NE-trending Pliocene graben. The same data, confirmed by the result of an 887 m test well, indicate that the volcano has not undergone caldera subsidence of the type exemplified by the Valles Caldera of New Mexico, but more probably had a series of post-stratocone lateral blasts akin to the 1980 Mount St. Helens eruptions. Hot springs near the western foot of the volcano, and the presence of young highly-evolved rocks suggested that the area could have geothermal potential. However, geochemical and isotopic data provide no indication of the present existence of a hydrothermal system, whilst the lack of alteration (both at the surface and in the test well) suggest that no such system existed in the past. Geophysical data show no anomalous features which might be associated with a geothermal reservoir or with hydrothermal alteration. These negative conclusions, strongly supported by the low temperature of the test well (35°C at 886 m), demonstrate fairly conclusively that Roccamonfina is non-prospective for high-enthalpy geothermal resources.

Vent sites and flow directions of the Otowi ash flows (lower Bandelier Tuff), New Mexico

The nonwelded ash-flow tuffs of the 1.4-m.y.-old lower (Otowi) member of the Bandelier Tuff on the flanks of the Jemez Mountains erupted from as many as seven ring-fracture vents bounding the large Toledo Caldera at the center of the volcanic complex. Field evidence for these multiple vents includes (1) broad radial distribution of large volumes of ash-flow tuff; (2) areal variation in the compositions of lithic inclusions, with basalt-andesite clasts more abundant in the western than in the eastern sectors; (3) areal contrasts in welding and zonation, with welding best developed in the western sections; (4) paleotopography; and (5) flow directions determined from field measurements of elongated pumice fragments and charred trees.

New drilling at Vales Caldera

The Continental Scientific Drilling Program at Valles Caldera, N. Mex., enters its second phase in August as drilling begins at Sulphur Springs, an area of acid‐sulfate springs and mudpots on the western flank of the caldera's resurgent dome. The hole will be available for scientific study for 3 to 5 years.

Jemez Mountains volcanic field, New Mexico: Time term interpretation of the CARDEX seismic experiment and comparison with Bouguer gravity

The Jemez Mountains volcanic field (JVF) is centered astride the western flank of the Rio Grande rift in north central New Mexico. Near the center of the volcanic field are the Valles and Toledo calderas, the sites of two cataclysmic eruptions of silicic tephra and subsequent caldera collapses. These calderas are among the largest and most recent (1.1 and 1.4 Ma) of the world's giant volcanic calderas. The Valles caldera, especially, has associated highly developed hydrothermal systems and may still contain residual magma within a cooling, compositionally zoned pluton at shallow crustal depths. The main goal of our study was to estimate the complex shallow crustal structure beneath the Jemez volcanic field using data from a seismic refraction experiment that encompassed the central part of the field. A time term technique was used to process first arrival time data from the network of temporary seismic stations of the Caldera and Rift Deep Seismic Experiment (CARDEX) refraction experiment. The data set, consisting of 290 time‐distance pairs, was processed to yield time terms at 84 locations (78 recording stations and 6 explosion sites). The P wave velocity of the Precambrian basement refractor was estimated to be 5.86 ± 0.01 km/s. Station time terms are highly correlated to Bouguer gravity anomalies and thus tend to corroborate models for which the crystalline basement outcrops along the western margins of the rift and western flank of the JVF and dips generally eastward to depths of at least 600 m below sea level beneath sediment‐filled axial basins of the rift.

Upper crustal structure beneath the Jemez Mountains Volcanic Field, New Mexico, determined by three‐dimensional simultaneous inversion of seismic refraction and earthquake data

The upper crustal structure beneath the Jemez Mountains volcanic field (JVF), New Mexico, was determined using a simultaneous inversion of earthquake and seismic refraction travel time data. One‐, two‐, and three‐dimensional inversions were sequentially applied to derive a detailed velocity model of the upper 10 km of the crust beneath the JVF. Travel time observations from six shot points recorded at as many as 72 temporary seismograph stations and from 27 earthquakes recorded at from 6 to 12 network stations were simultaneously inverted to determine both compressional wave velocity structure and hypocentral parameters. The velocity model was described by a grid of velocity values (unknowns in the inversion procedure) with velocities between grid points given by linear interpolation. Ray tracing through two‐dimensional slices of the three‐dimensional model was performed to calculate theoretical travel times for each source‐receiver pair. A damped least squares iterative inversion was used to derive the velocity model and hypocenters from the differences between observed and theoretical travel times. The relocation of the earthquakes resulted in changes in epicentral location of up to 16 km, although most events were moved less than 7 km. Hypocentral depths of the relocated events ranged from sea level to 17 km, averaging about 8 km. The final three‐dimensional model provided a good fit to the travel time observations. The velocity model displays a prominent low‐velocity, approximately cylindrically shaped body beneath the Valles caldera within the JVF. The velocity anomaly is defined by a lateral decrease in velocity from about 6.0 km/s to as low as 5.6 km/s. The low‐velocity body has a diameter of about 15 km. We interpret the velocity anomaly beneath the Valles caldera to be the result of the combined effects of a silicic intrusion in the upper crust and velocity reduction caused by elevated temperature.

A geological and geophysical appraisal of the Baca geothermal field, Valles Caldera, New Mexico

Abstract The Baca location #1 geothermal field is located in north-central New Mexico within the western half of the Plio-Pleistocene Valles Caldera. Steam and hot water are produced primarily from the northeast-trending Redondo Creek graben, where downhole temperatures exceed 260°C at depths of less than 2 km. Stratigraphically the reservoir region can be described as a five-layer sequence that includes Tertiary and Quaternary volcanic rocks, and Mesozoic and Tertiary sediments overlying Precambrian granitic basement. Production is mainly controlled by fractures and faults that are ultimately related to activity in the Rio Grande Rift system. Geophysically, the caldera is characterized by a gravity minimum and a resistivity low in its western half. A 40-mgal gravity minimum over the caldera is due mostly to the relatively low-density volcanics and sediments that fill the caldera and probably bears no relation to deep-seated magmatic sources. Two-dimensional gravity modeling indicates that the depth to Precambrian basement in Redondo Canyon is probably at least 3 km and may exceed 5 km in eastern parts of the caldera. Telluric and magnetotelluric surveys have shown that the reservoir region is associated with low resistivity and that a deep low-resistivity zone correlates well with the depth of the primary reservoir inferred from well data. Telluric and magnetotelluric data have also identified possible fault zones in the eastern and western sections of the production region that may form boundaries to the Redondo Creek reservoir. These data also suggest that the reservoir region is located at the intersection of lineaments that trend north-south and northeast-southwest. Magnetotelluric results indicate deep low resistivity at the western edge of the caldera which may be associated with deep hot fluids. On the basis of geophysical and well data, we make three estimates of reservoir dimensions. The estimates of the areal extent of the reservoir range from 10 to 30 km 2 , depending on whether the Sulphur Creek or the Valle Seco area is considered to contain deep geothermal resources.

Initial results from VC‐1, First Continental Scientific Drilling Program Core Hole in Valles Caldera, New Mexico

Valles Caldera 1 (VC‐1) is the first Continental Scientific Drilling Program (CSDP) core hole drilled in the Valles caldera and the first continuously cored well in the caldera region. The objectives of VC‐1 were to penetrate a hydrothermal outflow plume near its source, to obtain structural and stratigraphie information near the intersection of the ring fracture zone and the precaldera Jemez fault zone, arid to core the youngest volcanic unit inside the caldera (Banco Bonito obsidian). Coring of the 856‐m well took only 35 days to finish, during which all objectives were attained and core recovery exceeded 95%. VC‐1 penetrates 298 m of moat volcanics and caldera fill ignimbrites, 35 m of precaldera volcaniclastic breccia, and 523 m of Paleozoic carbonates, sandstones, and shales. A previously unknown obsidian flow was encountered at 160 m depth underlying the Battleship Rock Tuff in the caldera moat zone. Hydrothermal alteration is concentrated in sheared, brecciated, and fractured zones from the volcaniclastic breccia to total depth with both the intensity and rank of alterations increasing with depth. Alteration assemblages consist primarily of clays, calcite, pyrite, quartz, and chlorite, but chalcopyrite and sphalerite have been identified as high as 450 m and molybdenite has been identified in a fractured zone at 847 m. Carbon 13 and oxygen 18 analyses of core show that the most intense zones of hydrothermal alteration occur in the Madera Limestone above 550 m and in the Madera and Sandia formations below 700 m. This corresponds with zones of most intense calcite and quartz veining. Thermal aquifers were penetrated at the 480‐, 540‐, and 845‐m intervals. Although these intervals are associated with alteration, brecciation, and veining, they are also intervals where clastic layers occur in the Paleozoic sedimentary rocks.

Reservoir processes and fluid origins in the Baca Geothermal System, Valles Caldera, New Mexico

At the Baca geothermal field in the Valles caldera, New Mexico, 19 deep wells were drilled in an attempt to develop a 50‐MWe (megawatts electric) power plant. The chemical and isotopic compositions of steam and water samples have been used to indicate uniquely the origin of reservoir fluids and natural reservoir processes. Two distinct reservoir fluids exist at Baca. These fluids originate from the same deep, high‐temperature (335°C), saline (2500 mg/kg Cl) parent water but have had different histories during upflow. One fluid (from wells 4 and 13) is isotopically light, high in radiogenic noble gases, CO2 and HCO3, and low in Ca. It has a temperature of 290°–295°C and a reservoir chloride near 1900 mg/kg. This fluid resulted from rapid upward flow through 1.1‐ to 1.4‐m.y.‐old Bandelier Tuff reservoir rocks after long residence in pre‐Bandelier (>7 m.y.) sediments and Precambrian basement rocks and 25% dilution with high‐altitude cold groundwater from Redondo Peak. The other water (from wells 15, 19, and 24) moved slowly through the Bandelier Tuff and cooled conductively (with minor steam loss for well 19) from 335°C to 280°–260°C. Apparently, short residence in old basement rocks has left this water with low radiogenic gases. Conductive cooling without mixing has kept the original chloride and relatively heavy isotope composition of the deep water. The recharge to the deep parent water is not well understood but may be from lower elevation precipitation outside the Valles caldera area. Gases are in equilibrium in all‐liquid reservoir fluids at near reservoir temperatures, and the concentrations of atmospheric gases are similar to those of air‐saturated water, indicating little boiling and steam loss. All water, solutes, and gases in the reservoir fluids originate from air‐saturated meteoric recharge water, watermineral reactions, and rock leaching, with the possible exception of excess 3He that must have an ultimate mantle source. This gas could originate directly from magma or from leaching of intrusive volcanic rocks beneath the Bandelier.

Explosive rhyolitic volcanism in the Jemez Mountains: Vent locations, caldera development and relation to regional structure

The Jemez volcanic field straddles the western margin of the Rio Grande rift where the rift is intersected by the Jemez lineament in north central New Mexico. The field has a record of volcanism extending back to before 13 Ma. Initial basaltic activity was related to active rifting, with minor rhyolitic eruptions occurring along N‐S rift‐bounding faults. Between 10 and 7 Ma, voluminous andesitic volcanism took place in the central Jemez Mountains area, overwhelming contemporaneous basaltic and rhyolitic magmatism. An apparent tectonic lull took place from 7 to 4 Ma, accompanied by lower eruption rates. During this interval, dacitic magmas were erupted to form the Tschicoma volcanic center, but mafic and rhyolitic volcanism virtually ceased. Since 4 Ma, accompanying resumption of rifting, a growing silicic magma system has been present under the central part of the Jemez Mountains, ultimately evolving to the magma body that produced the voluminous rhyolitic Bandelier tuffs. Explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 Ma. Early eruptions, 3.6–2.8 Ma, produced high‐silica rhyolite ignimbrites restricted to the southwest part of the Jemez Mountains; formation of these units may have been accompanied by caldera collapse. These events were followed by the two caldera‐forming Bandelier Tuff ignimbrite eruptions, 1.45 and 1.12 Ma. Post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both Bandelier events. Vent and caldera locations for the rhyolitic eruptions during 3–4 Ma have been inferred from grain size characteristics, dispersal patterns, and facies variations in the Plinian deposits and ignimbrites. Lithic breccia zones of the pre‐Bandelier ignimbrites indicate possible caldera sources in the southwest part of the present Valles caldera. During the eruption of both Bandelier tuffs, initial plinian falls and early pyroclastic flows emanated from vents centrally located in the Jemez Mountains. In the lower Bandelier Tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows, but there is no strong evidence to suggest such a transition occurred during the upper Bandelier eruption. Calderas associated with the lower and upper Bandelier tuffs (Toledo and Valles, respectively) are almost identical in location, as are the Plinian vent sites for these two large eruptions. The Toledo embayment, northeast of the Valles caldera, contains lava domes from up to 3.6 Ma and may be a caldera or crater associated with early explosive dacitic volcanism in the Tschicoma volcanic center. Post‐1.4 Ma lava domes also fill this depression. The main volcanic features of the Jemez Mountains field, including the Valles caldera complex, eruption vents, and the apical graben of the post‐Valles‐Redondo resurgent block, appear to be aligned along the NE‐SW trending Jemez fault zone. This zone, the local expression of a Precambrian basement feature (the Jemez lineament), has exerted strong control on the location and style of eruptions from the Bandelier rhyolitic magma system.

Introduction to Special Section on the Valles Caldera and Jemez Mountains Volcanic Field

The Jemez Mountains volcanic field, New Mexico, is one of the best known volcanic centers in the world because of pioneering efforts by C. S. Ross, R. L. Smith, R. A. Bailey, and their coworkers in the U.S. Geological Survey [Ross et al., 1961; Bailey et al., 1969; Smith et al., 1970]. The field contains roughly 2000 km3 of basaltic through rhyolitic rocks, dominated volumetrically by andesites. The most significant activity was formation of the Toledo and Valles calderas during eruption of over 600 km3 of rhyolitic ash flow tuffs, the Bandelier tuff, during two phases 1.5 and 1.1 m.y. ago [Smith and Bailey, 1966; Doell et al., 1968; Izett et al., 1981]. Many concepts of pyroclastic flows and ignimbrites were based upon work in this volcanic field by Ross and Smith [1961].

Intracaldera volcanic activity, Toledo Caldera and Embayment, Jemez Mountains, New Mexico

The Toledo caldera was formed at 1.47±0.06 Ma during the catastrophic eruption of the lower member, Bandelier Tuff. The caldera was obscured at 1.12±0.03 Ma during eruption of the equally voluminous upper member of the Bandelier Tuff that led to formation of the Valles caldera. Earlier workers interpreted a 9‐km‐diameter embayment, located NE of the Valles caldera (Toledo embayment), to be a remnant of the Toledo caldera. Drill hole data and new K‐Ar dates of Toledo intracaldera domes redefine the position of Toledo caldera, nearly coincident with and of the same dimensions as the younger Valles caldera. The Toledo embayment may be of tectonic origin or a small Tschicoma volcanic center caldera. This interpretation is consistent with distribution of the lower member of the Bandelier Tuff and with several other field and drilling‐related observations. Explosive activity associated with Cerro Toledo Rhyolite domes is recorded in tuff deposits located between the lower and upper members of the Bandelier Tuff on the northeast flank of the Jemez Mountains. Recorded in the tuff deposits are seven cycles of explosive activity. Most cycles consist of phreatomagmatic tuffs that grade upward into Plinian pumice beds. A separate deposit, of the same age and consisting of pyroclastic surges and flows, is associated with Rabbit Mountain, located on the southeast rim of the Valles‐Toledo caldera complex. These are the surface expression of what may be a thicker, more voluminous intracaldera tuff sequence. The combined deposits of the lower and upper members of the Bandelier Tuff, Toledo and Valles intracaldera sediments, tuffs, and dome lavas form what we interpret to be a wedge‐shaped caldera fill. This sequence is confirmed by deep drill holes and gravity surveys. This fill accumulated in depressions formed during precaldera rifting and episodes of caldera collapse. We interpret the Toledo‐Valles caldera complex to be a pair of nearly coincident trapdoor calderas, with the hinge on the west side and thick caldera fill in the east.

Constraints on the age of heating at the Fenton Hill Site, Valles Caldera, New Mexico

Subsurface samples and temperature data from the Fenton Hill drilling site, adjacent to the Valles caldera, New Mexico, allow an investigation of the thermal history of this site through analyses of the behavior of isotopic dating systems in changing thermal regimes and through thermal modeling. Estimates of the age of the heating event at Fenton Hill responsible for the current elevated temperatures vary from about 1 to 4 Ma. However, new 40Ar/39Ar analyses and thermal modeling indicate that the age of heating may be much younger than previously believed. The 40Ar/39Ar analyses of microcline separates from five samples, ranging in drill hole depth from 1.13 to 4.56 km and in situ temperatures from 110°C to 313°C, indicate thermal events at the site at around 1030 and 870 Ma and a very recent thermal event related to the thermal development of the caldera. The maximum estimates of peak heating duration during this recent thermal event are between 3 and 60 ka. Two distinct thermal perturbations are recognized in the subsurface temperature data, a shallow disturbance, probably related to heating from a lateral flow of hot water just above the Precambrian surface at a depth of about 730 m and a deep thermal disturbance, the source of which is unknown. Model studies indicate the age of the shallow thermal disturbance to be about 10 ka. With reasonable constraints on the source of the deep thermal disturbance, it must lie within a few kilometers of the site, and its maximum age is estimated to be less than about 40 ka. Thus heating at the Fenton Hill site appears to be much younger than the main caldera event at about 1 Ma and is probably related to a magmatic and/or hydrothermal event very close to Fenton Hill during the last few tens of thousands of years.

Tectonics of the Jemez Lineament in the Jemez Mountains and Rio Grande Rift

The Jemez lineament is a NE trending crustal flaw that controlled volcanism and tectonism in the Jemez Mountains and the Rio Grande rift zone. The fault system associated with the lineament in the rift zone includes, from west to east, the Jemez fault zone southwest of the Valles‐Toledo caldera complex, a series of NE trending faults on the resurgent dome in the Valles caldera, a structural discontinuity with a high fracture intensity in the NE Jemez Mountains, and the Embudo fault zone in the Española Basin. The active western boundary faulting of the Española Basin may have been restricted to the south side of the lineament since the mid‐Miocene. The faulting apparently began on the Sierrita fault on the east side of the Nacimiento Mountains in the late Oligocene and stepped eastward in the early Miocene to the Canada de Cochiti fault zone. At the end of the Miocene (about 5 Ma) the active boundary faulting again stepped eastward to the Pajarito fault zone on the east side of the Jemez Mountains. The north end of the Pajarito fault terminates against the Jemez lineament at a point where it changes from a structural discontinuity (zone of high fracture intensity) on the west to the Embudo fault zone on the east. Major transcurrent movement occurred on the Embudo fault zone during the Pliocene and has continued at a much slower rate since then. The relative sense of displacement changes from right slip on the western part of the fault zone to left slip on the east. The kinematics of this faulting probably reflect the combined effects of faster spreading in the Española Basin than the area north of the lineament (Abiquiu embayment and San Luis Basin), the right step in the rift that juxtaposes the San Luis Basin against the Picuris Mountains, and counterclockwise rotation of various crustal blocks within the rift zone. No strike‐slip displacements have occurred on the lineament in the central and eastern Jemez Mountains since at least the mid‐Miocene, although movements on the still active Jemez fault zone, in the western Jemez Mountains, may have a significant strike‐slip component. Basaltic volcanism was occurring in the Jemez Mountains at four discrete vent areas on the lineament between about 15 Ma and 10 Ma and possibly as late as 7 Ma, indicating that it was being extended during that time.

Hydrothermal alteration in the Baca Geothermal System, Redondo Dome, Valles Caldera, New Mexico

Thermal fluids circulating in the active hydrothermal system of the resurgent Redondo dome of the Valles caldera have interacted with their diverse host rocks to produce well‐zoned alteration assemblages, which not only help locate permeable fluid channels but also provide insight into the system's thermal history. The alteration shows that fluid flow has been confined principally to steeply dipping normal faults and subsidiary fractures as well as thin stratigraphic aquifers. Permeability along many of these channels has been reduced or locally eliminated by hydrothermal self‐sealing. Alteration from the surface through the base of the Miocene Paliza Canyon Formation is of three distinctive types: argillic, propylitic, and phyllic. Argillic alteration forms a blanket above the deep water table in formerly permeable nonwelded tuffs. Beneath the argillic zone, pervasive propylitic alteration is weakly developed in felsic host rocks but locally intense in deep intermediate composition volcanics. Strong phyllic alteration is commonly but not invariably associated with major active thermal fluid channels. Phyllic zones yielding no fluid were clearly once permeable but now are hydrothermally sealed. High‐temperature alteration phases at Baca are presently found at much lower temperatures. We suggest either that isotherms have collapsed due to gradual cooling of the system, that they have retreated without overall heat loss due to uplift of the Redondo dome, that the system has shifted laterally, or that it has contracted due to a drop in the water table. The deepest Well (B‐12, 3423 m) in the dome may have penetrated through the base of the active hydrothermal system. Below a depth of 2440 m in this well, hydrothermal veining largely disappears, and the rocks resemble those developed by isochemical thermal metamorphism. The transition is reflected by temperature logs, which show a conductive thermal gradient below 2440 m. This depth may mark the dome's neutral plane, which separates an upper permeable zone of extensional fracturing from a lower, less permeable compressional regime. The Baca hydrothermal system is similar to those which have formed ore deposits in other calderas: particularly, Creede (Colorado) type epithermal silver base metal veins and stockworks. Recent scientific drilling has also intersected a deep zone of strong phyllic alteration and molybdenum mineralization in the Valles caldera's ring fracture system, a setting which localized a large stockwork molybdenite orebody in the nearby Questa caldera.

An interpretation of the alteration assemblages at Sulphur Springs, Valles Caldera, New Mexico

Sulphur Springs, located near the western edge of the Valles caldera, north central New Mexico, is the only vapor‐dominated surface manifestation of any significance found in the Valles. The percolating acid sulfate waters and fumarolic gases have leached and oxidized the country rock near the hot springs to a bleached wasteland. The near‐surface mineral assemblages at 90°C are restricted to one and two phases occurring in contact with a variety of solutions and gases. The conditions of local equilibrium at 90°G are approximately halotrichite + alunogen, log fO2 −45, log fS2−12, pH 0.3; szomolnokite + pyrite, log fO2 −46, log fS2−16, pH 0.8; alunite, log fO2−50, log fS2−10, pH 1–2; kaolinite + sulphur, log fO2 −53, log fS2 −10, pH 1–2, log aK+ = 0.0012 m, log aSiO2 = 0.0033 m; and pyrite, log fO2 −60, log fS2 −19, pH 4.3. This study has demonstrated that local equilibrium is closely approached even under the low‐temperature and ‐pressure conditions found near the surface.

Chemical and isotopic characteristics of fluids within the Baca Geothermal Reservoir, Valles Caldera, New Mexico

Geochemical and isotopic fluid data from 10 wells completed in the Baca geothermal reservoir indicate the presence of both a normal enthalpy, lower‐chloride fluid and an excess enthalpy, higher‐chloride fluid. This chemical difference, along with enrichment of deuterium and oxygen 18 in the excess enthalpy fluid, can be explained by adiabatic cooling or boiling at 170°C during upward convection along the central fault system in the reservoir followed by conductive reheating during downward movement. Chemical modeling using the EQ3NR computer code indicates chemical stability with the mineral assemblage quartz, albite, K‐mica, epidote, chlorite, calcite, anhydrite, and pyrite, which is in agreement with observed minerals in well cuttings. This correlation, along with chemical geothermometers, implies a constant temperature range of 260°–320°C during evolution of the reservoir.