Wilfred A. Elders

Authigenic layer silicate minerals in borehole Elmore 1, Salton Sea Geothermal Field, California, USA

A combined petrographic/X-ray/electron microprobe and energy dispersive system investigation of sandstone cuttings from borehole Elmore # 1 near the center of the Salton Sea Geothermal Field has revealed numerous regular variations in the composition, texture, mineralogy and proportions of the authigenic layer silicate minerals in the temperature interval 185° C (411.5 m depth) to 361° C (2,169 m). At temperatures near 190° C, dolomite/ankerite+calcite-bearing sandstones contain an illite/mixed layer phase with 10% expandable layers (dolomite/ankerite zone). In shale, the percentage of expandable layers in the mixed layer phase changes from 10–15% at 185° C to 5% at 210° C (494 m). In the interval 250° C (620 m) to 325° C (1,135 m), the calcite+pyrite+epidote-bearing sandstones contain a layer silicate assemblage of chlorite and illite (chlorite-calcite zone). In the shallower portions of this metamorphic zone, the illite contains 0–5% expandable layers, while at depths greater than 725 m (275° C) it is completely free of expandable layers. On increasing temperature, the white mica shows regular decreases in SiIV, Mg and Fe, and increase in AlIV, AlVI, and interlayer occupancy, as it changes gradually from fine-grained illite (=textural sericite) to coarse-grained recrystallized phengitic white mica. In the same interval, chlorite shows decreases in AlVI and octahedral vacancies and an increase in total Mg+Fe. The sandstones range from relatively unmodified detrital-textured rocks with porosities up to 20% and high contents of illite near 250° C to relatively dense hornfelsic-textured rocks with trace amounts of chlorite and phengite and porosities near 5% at 325° C. Numerous complex reactions among detrital (allogenic) biotite, chlorite, and muscovite, and authigenic illite and chlorite, occur in the chlorite-calcite zone.Biotite appears, and calcite disappears, at a temperature near 325° C and a depth of 1,135m. The biotite zone so produced persists to 360° C in sandstone, at which temperature orthoclase disappears and andradite garnet appears at a depth near 2,155 m. Throughout the biotite zone and into the garnet zone, the biotite undergoes compositional changes that are very similar to those observed in illite/phengite in the chlorite-calcite zone, including increases in interlayer occupancy, AlIV, AlVI, and Ti, and decreases in F−, SiIV, and Mg/Fet+Mg, on increasing temperature. Biotite thus changes from a siliceous, K-deficient biotite at the biotite isograd to a typical low-grade metamorphic biotite at temperatures near 360° C. Minor amounts of talc appear with biotite at the biotite isograd in sandstone, while actinolite appears in both sandstone and shale at temperatures near 340° C (1,325 m). Chlorite completely disappears from sandstone at temperatures of approximately 350° C (1,500 m), and diminishes abruptly in amount in the more chloritic shales at the same depth.

Hydrothermal-flow regime and magmatic heat source of the Cerro Prieto geothermal system, Baja California, Mexico

Abstract Our detailed three-dimensional model of the natural flow regime of the Cerro Prieto geothermal field, before steam production began, is based on patterns of hydrothermal mineral zones and light stable isotopic ratios observed in rock samples from more than 50 deep wells, together with temperature gradients, wireline logs and other data. At the level so far penetrated by drilling, this hydrothermal system was heated by a thermal plume of water close to boiling, inclined at 45°, rising from the northeast and discharging to the west. To the east a zone of cold water recharge overlies the inclined thermal plume. Fission track annealing studies show that the reservoir reached 170°C only 10 4 years ago. Oxygen isotope exchange data indicate that a 12 km 3 volume of rock subsequently reacted with three times its volume of water hotter than 200°C. Averaged over the duration of the heating event this would require a flow velocity through a typical cross-section of the reservoir of about 6 m/year. The heat in storage in that part of the reservoir hotter than 200°C and shallower than 3 km depth is equivalent to that which would be released by the cooling of about 1 or 2 km 3 of basalt or gabbro magma. Although this is an extensional tectonic environment of ‘leaky’ transform faulting in which repeated intrusions of basalt magma are likely, for simplicity of computation we have modelled possible heat sources as simple two dimensional basalt intrusions of various sizes, shapes and locations. We have calculated a series of two-dimensional convective heat transfer models, with different heat sources and permeability distributions. The models which produce the best fit for the temperature distributions observed in the field today have in common a heat source which is a funnel-shaped basalt intrusion, 4 km wide at the top, emplaced at a depth of 5–6 km about 40,000–50,000 years ago. Although such forward modelling does not provide a unique solution of the thermal history, it does suggest that the igneous intrusion which supplied the heat to the Cerro Prieto system must be young, large and close. Modelling the hydrothermal system is useful in guiding geothermal resource evaluation; in turn we hope that modelling of the magmatic system will stimulate the search for the parent magma chamber by geophysical means, and eventually by deep drilling.

Uranium-thorium series radionuclides in brines and reservoir rocks from two deep geothermal boreholes in the Salton Sea Geothermal Field, southeastern California

Naturally occurring U and Th series radionuclides have been analyzed in high temperature brines (~300°C, 25 wt% dissolved solids) and associated rocks from two deep geothermal wells located on the northeastern margin of the Salton Sea Geothermal Field (SSGF). These data are part of a study of the SSGF as a natural analog of possible radionuclide behavior near a nuclear waste repository constructed in salt beds, and permit evaluation of some characteristics of water-rock interaction in the SSGF. Rock/Brine concentration ratios (Rc = (dpm/g)rock/(dpm/g)brine) were found to vary from near unity for isotopes of Ra, Pb and Rn to about 5 × 105 for 232Th. The high sorptivity of 232Th is closely followed by that of 238U and 234U (Rc ~ 5 × 104), suggesting that U is retained in the +4 oxidation state by the reducing conditions in the brines. The relatively high solubility of 210Pb and 212Pb is attributed to formation of chloride complexes, while the high Ra solubility is attributed to chloride complexing, a lack of suitable adsorption sites due to the high brine salinity and temperature, and the reducing conditions that prevent MnO2 and RaSO4 from forming. The 228Ra226Ra ratios in the brines are approximately equal to those of their parents (232Th230Th) in associated rocks, indicating that Ra equilibration in the brine-rock system is achieved within the mean life of 228Ra (8.3 years). The 224Ra228Ra ratios in these brines are about 0.7, indicating that either (1) brine composition is not homogeneous and 224Ra decays in fracture zones deficient in Ra and Th as the brine travels to the wellhead or (2) Ra equilibration in the brine-host rock system is not complete within the mean life of 224Ra (5.2 days) because the desorption of 224Ra from the solid phase is impeded. The 228Ac228Ra activity ratio in the SSGF brines studied is <0.1, and from this ratio the residence time of 228Ac in the brine before sorption onto solid surfaces is estimated to be <70 minutes, indicating the potential for rapid removal of reactive isotopes fom brines. The brine is greatly enriched in 226Ra (2–3 dpm/g, about 104–105 times that of its parent 230Th), while reservoir rocks are about 10% deficient in 226Ra relative to 230Th. Material balance calculations for 2226Ra and 18O suggest that brines reside in the reservoir for 102–103 years, that the SSGF formed 10,000–40,000 years B.P., and that porosity cannot be more than 20%.

Origin of a rhyolite that intruded a geothermal well while drilling at the Krafla volcano, Iceland

Magma fl owed into an exploratory geothermal well at 2.1 km depth being drilled in the Krafl a central volcano in Iceland, creating a unique opportunity to study rhyolite magma in situ in a basaltic environment. The quenched magma is a partly vesicular, sparsely phyric, glass containing ~1.8% of dissolved volatiles. Based on calculated H 2 O-CO 2 saturation pressures, it degassed at a pressure intermediate between hydrostatic and lithostatic, and geothermometry indicates that the crystals in the melt formed at ~900 °C. The glass shows no signs of hydrothermal alteration, but its hydrogen and oxygen isotopic ratios are much lower than those of typical mantle-derived magmas, indicating that this rhyolite originated by anhydrous mantle-derived magma assimilating partially melted hydrothermally altered basalts.

Hydrothermal mineral zones in the geothermal reservoir of Cerro Prieto

Abstract Detailed petrologic studies completed to date on ditch cuttings and core from 23 wells in the Cerro Prieto field have led to recognition of regularly distributed prograde metamorphic mineral zones. The progressive changes in mineralogy exhibit a systematic relationship with reservoir temperature. The Cerro Prieto reservoir consists of a series of sandstones, siltstones, and shales composing part of the Colorado River delta. The western part of the field contains relatively coarser sediments apparently also derived from the delta and not from the basin margins as formerly thought. The most abundant detrital minerals in the sediments include quartz, feldspar, kaolinite, montmorillonite, illite, chlorite, mixed-layer clays, calcite, dolomite and iron hydroxides. Some of these minerals were also formed diagenetically. The following progressive stages of post-depositional alteration in response to increasing temperature have been observed: (1) diagenetic zone (low temperature), (2) illite-chlorite zone (above ~ 150°C), (3) calc-aluminum silicate zone (above ~ 230°C) and the biotite zone (above ~ 325°C). These zones are transitional to some degree and can be further subdivided based on the appearance or disappearance of various minerals. One immediate application of these studies is the ability, from a study of cuttings obtained during drilling of a well, to predict the temperatures which will be observed when the well is completed.

Active metasomatism in the Cerro Prieto geothermal system, Baja California, Mexico: A telescoped low-pressure, low-temperature metamorphic facies series

In the Cerro Prieto geothermal field, carbonate-cemented, quartzofeldspathic sediments of the Colorado River delta are being actively metasomatized into calc-silicate metamorphic rocks by reaction with alkali chloride brines between 200/sup 0/ and 370/sup 0/C, low fluid and lithostatic pressures, and low oxygen fugacities. Petrologic investigations of drill cores and cutting from over 50 wells in this field identified a prograde series of calc-silicate mineral zones which include as index minerals: wairakite, epidote, prehnite, and clinopyroxene. Associated divariant mineral assemblages are indicative of a very low pressure/temperature metamorphic facies series which encompasses the clay-carbonate, zeolite, greenschist, and amphibolite facies. This hydrothermal metamorphic facies series, which is becoming increasingly recognized in other active geothermal systems, is characterized by temperature-telescoped dehydration and decarbonation mineral equilibria. Its equivalent should now be sought in fossil hydrothermal systems.

Saline brines and metallogenesis in a modern sediment-filled rift: the Salton Sea geothermal system, California, U.S.A.

The Salton Sea geothermal system (SSGS) is the site of active hydrothermal metamorphism and metallogenesis in the delta of the Colorado River, which partially fills the Salton Trough rift zone at the head of the Gulf of California. Growth of the delta across the rift has isolated the northern part of the Salton Trough since the Pleistocene, forming the evaporative Salton Sea basin whose sediments host the SSGS. More than 70 commercial geothermal wells, including a 3.2 km deep research borehole drilled as part of the Salton Sea Scientific Drilling Project (SSSDP), are yielding a wealth of new data from this system. Within the SSGS, active greenschist facies metamorphism is occuring at temperatures ⩽365°C at only 2–3 km depth, by reaction of NaCaKFeMnCl brines with the deltaic and lacustrine Pilo-Pleistocene sediments. Two kinds of base metal ore mineralization occur at depth: stratabound diagenetic Fe sulfides, and epigenetic vertical veins containing FeZnCuPb sulfides and Fex oxides. The vein mineralization occurs as two types: an older, reduced carbonate-sulfide assemblage, and a modern, oxidized, silicate-hematite-sulfide assemblage. The earlier assemblage formed at temperatures up to 100°C higher than the ambient temperatures measured in the wells today, implying that cooler, oxidized fluids have displaced hot, reduced fluids. A sharp interface between shallow fluids containing <12 wt % TDS and deep hypersaline brines containing 15–27 wt % TDS exists in the SSGS. The deeper hypersaline brines are rich in base metals (Fe ≅ 1500 ppm, Mn ≅ 1000 ppm, Zn ≅ 500 ppm) whereas the overlying lower-salinity fluids contain less than 100 ppm each of Fe, Mn and Zn. The modern silicate-hematite-sulfide vein assemblage is precipitating where the two fluids appear to be mixing. The two fluid types also have distincty different δ18O and δD relations. The shallow lower-salinity fluids have only partially-exchanged oxygen with deltaic and lacustrine host rocls. The deep hypersaline brines have exchanged oxygen extensively at 250°C with the deltaic sediments. δD values of the hypersaline brines are typically lighter than either the lower-salinity fluids or the modern evaporating groundwaters occupying the Salton Trough, implying a different source for the original fluids. The high salinities of the geothermal brines are derived from a combination of evaporation of fossil lakewaters, groundwater dissolution of shallow lacustrine evaporites, and subsurface hydrothermal metamorphism of buried lacustrine evaporites. Episodic filling and desiccation of the closed Salton Sea basin has allowed cold saline brines to form and percolate down into the sedimentary section. In some wells anyhydrite meta-evaporites and interbedded solution collapse breccias occur at 1 km depth. The anyhydrite contains fluid inclusions that are saturated in halite at their homogenization temperatures of 300°C, recording the hydrothermal dissolution of bedded salt. Based on Sr and Pb isotopic data and whole-rock chemical data, the source of the metals in the hypersaline brines is from leaching of the host sediments. However, the origin of reduced S for ore mineralization remains an enigma. δ34S values for vein sulfides cluster uniformly around zero per mil, implying that an isotopically-homogeneous source of reduced sulfide exists in the brines. It is possible that the vein sulfides receive a constribution from a magmatic S source. Movement and mixing of brines of different chemistry and oxidation states play a major role in ore genesis. Our model envisages an early stage in which a deep brine pool accumulated at depth in the sedimentary section by partial section by partial evaporation of basin and dissolution of bedded salts. Intrusion of rift-related basaltic magma into the base of the sedimentary caused heatingand fracturing of the sediments, resulting in precipitation of the carbonate-sulfide veins during pore fluid expulsion. Heating also caused a diapir of the hypersaline brine to rise and displace colder, less saline, shallower pore fluids. This brine intrusion was accompanied by pervasive and extensive mineralization. As this diapir cooled it began to move downward, drawing in shallow, more oxidized fluids and causing the formation of the modern silicate-hematite-sulfide vein ore zone.

Distribution of hydrothermal mineral zones in the Cerro Prieto geothermal field of Baja California, Mexico

Abstract Our ongoing studies of water-rock interaction at Cerro Prieto have now been extended to include samples from 40 wells. We have confirmed the regular sequence of progressive hydrothermal alteration zones previously described, and have mapped these alteration zones across the geothermal field. Our earlier work showed the relationships between hydrothermal mineralogy, temperature and permeability, in that alteration occurs at lower temperature in sandstone than in the less permeable shales. The effects of chemical parameters such as silica activity and differences in CO 2 fugacity have also become apparent when mineral assemblages are compared in sandstones from different wells at the same temperature. A rather complete picture of the shape of the reservoir and the nature of its boundaries is developing, and we have begun to identify patterns in the observed hydrothermal mineral zonations which are characteristic of different temperature gradients. We infer such different gradients to be indicative of different parts of the hydrothermal flow regime. In certain wells mineral zones are closely spaced, indicating steep thermal gradients, while in others they are much farther apart. We believe that patterns characteristic of recharge, discharge and upwelling zones as well as areas of primarily horizontal flow can be recognized. The geothermal circulation system at Cerro Prieto appears to be rather young and shows no indication of retrograde reactions due to cooling. The pattern of fluid flow does not seem to be significantly affected by faults, stratigraphic horizons or by the presence of a cap-rock. The mineral zones define a thermal dome which is apparently fed from the east and spreads westward.

Hydrothermal Minerals Record CO2 Partial Pressures in the Reykjanes Geothermal System, Iceland

The Reykjanes Peninsula in southwest Iceland is the landward extension of the Mid-Atlantic Ridge spreading center. At present seawater penetrates the coastal Reykjanes geothermal system at depth, where the highest recorded temperature is ∼320°C. It mixes with magmatic volatiles and reacts with the basaltic host rock to form secondary hydrothermal minerals in progressively higher-grade mineral alteration zones with increasing depth. Within the epidote-chlorite and portions of the epidote-actinolite zones of alteration, epidote-prehnite-calcite-quartz-fluid constitutes a quadra-variant assemblage that, under conditions of specified temperature, pressure, and activity of H2O allows prediction of geothermal fluid PCO2 as a function of the composition of the solid solution minerals epidote or prehnite. This assemblage is typically found at temperatures >250°C and ≲310°C, and potentially provides a mineralogical recorder that constrains fluid CO2 concentrations based on compositional zoning in hydrothermal epidote. Analysis of epidote crystals separated from drillhole-cuttings from three geothermal wells (RN-9, RN-10, RN-17) display complex chemical zoning, generally with Fe(III)-rich cores and Al-rich rims. The Fe(III)-mol fraction of epidote at depths between 0.5 to 1 km ranges from 0.21 to 0.38, between 1 to 2 km depth the range is 0.17 to 0.48 and between 2 to 3 km it is 0.17 to 0.30. The Fe(III)-mol fraction of prehnite ranges from 0.11 to 0.59 in the upper portions of drillhole RN-17, where the highest Fe(III) content in epidote, 0.36, serves as the upper Fe(III) limit for epidotes coexisting with prehnite in this study. Because most observed prehnite crystals in the drillhole-cuttings are too small for electron microprobe analyses (<20μm), we employed a sigmoidal correlation of available compositional data from active geothermal systems to calculate the Fe(III)-Al composition of prehnite using measured compositions of epidote in the Reykjanes system. In drill cuttings that contain epidote, prehnite, quartz and calcite, using measured epidote compositions between the reference temperatures of 275°C and 310°C, calculated values of PCO2 for the geothermal fluids range from ∼0.6 to ∼6.2 bars. When only epidote, prehnite and quartz are observed in the drill cuttings, the calculated range of PCO2 is from ∼1.3 to ∼6.8 bars, which provides the maximum value of PCO2 at which calcite will not be present. The present day PCO2 values of geothermal fluids from the Reykjanes system were derived from analytical data on liquid and vapor samples collected at the surface from wet-steam well discharges using both the WATCH and SOLVEQ speciation programs. The geothermal fluids at reference temperature between 275°C and 310°C have PCO2 concentrations ranging from 1.3 bars to 4.0 bars. The calculated PCO2 values based on epidote compositions are in close agreement with present-day fluid PCO2 in the Reykjanes geothermal system. 72 percent of the calculated PCO2 values based on epidote compositions where the assemblage of epidote, prehnite, quartz and calcite are observed in drill cuttings are within the range of measured present-day fluids, while 58 percent of the calculated PCO2 values fall within the range when calcite is not present in the drill cuttings. Therefore, our method for calculating fluid PCO2 is proven quite reliable when all four index minerals are present. Additionally, if only epidote, prehnite and quartz are observed, our model calculations still serve as a moderately accurate predictive proxy for maximum fluid PCO2 composition in the Reykjanes geothermal system. Ultimately, these correlations between the measured and calculated PCO2 fluid compositions will in the future provide a method, based on compositional variation and paragenesis of hydrothermal minerals in mafic lithologies, by which to characterize spatial and temporal concentrations of CO2 in both active and fossil hydrothermal systems and in low-grade metamorphic environments. The generally observed zoning pattern in epidotes in the Reykjanes geothermal system from Fe(III)-rich cores to Al-rich rims suggests that if the zoning formed while equilibrium was maintained among the epidote-prehnite-calcite-quartz assemblage under near isothermal conditions, there must have been an increase in PCO2 with time. This geochemical signature may then be employed to make large scale inferences concerning the evolution of the Reykjanes geothermal system. Analysis of geothermal fluids collected at the wellhead suggest that the four index minerals that comprise the assemblage are in equilibrium with the fluids, thus enabling the application of the PCO2 predictive method discussed in this study to modern epidote samples. In addition to aiding in understanding the history of reactions that involved natural sequestration of CO2 derived from magmatic degassing, this study may also provide useful insights into reactions that could result from the injection of industrial CO2-rich fluids into hydrothermal environments in basaltic rocks.

Vitrinite reflectance geothermometry and apparent heating duration in the Cerro Prieto geothermal field

Abstract Vitrinite reflectance measured in immersion oil ( R o ) on kerogen extracted from hydrothermally altered mudstones in borehole M-84 at the Cerro Prieto geothermal field exhibit an increase in mean reflectance ( R o ) from 0.12 per cent at 0.24 km depth to 4.1 per cent at 1.7 km depth. Downhole temperatures measured over this interval increase from about 60° to 340°C. These R o data plotted against temperature fall along an exponential curve with a coefficient of determination of about 0.8. Other boreholes sampled in the field show similar relationships. A regression curve calculated for temperature and R o in borehole M-105 correctly predicts temperatures in other boreholes within the central portion of the geothermal system. The correlation between the reflectance values and logged temperature, together with consistent temperature estimates from fluid inclusion and oxygen isotope geothermometry, indicates that changes in R o are an accurate and sensitive recorder of the maximum temperature attained. Therefore, vitrinite reflectance can be used in this geothermal system to predict the undisturbed temperature in a geothermal borehole during drilling before it regains thermal equilibrium. Although existing theoretical functions which relate R o to temperature and duration of heating are inaccurate, empirical temperature- R o curves are still useful for geothermometry. A comparison of temperature- R o regression curves derived from nine boreholes within the Cerro Prieto system suggests that heating across the central portion of the field occurred penecontemporaneously, but varies near margins. Boreholes M-93 and M-94 appear to have cooled from their maximum temperatures, whereas M-3 and Prian-1 have only recently been heated. Comparison of the temperature- R o data from the Salton Sea, California, geothermal system indicates that the duration of heating has been longer there than at the Cerro Prieto field.