Salton Sea Geothermal System Research Articles

NATURE AND SIGNIFICANCE OF IGNEOUS ROCKS CORED IN THE STATE 2–14 RESEARCH BOREHOLE: SALTON SEA SCIENTIFIC DRILLING PROJECT, CALIFORNIA

The State 2–14 research borehole of the Salton Sea Scientific Drilling Project penetrated 3.22 km of Pleistocene to Recent sedimentary rocks in the Salton Sea geothermal system, located in the Salton Trough of southern California and northern Baja California, Mexico. In addition, three intervals of igneous rocks were recovered; a silicic tuff and two sills of altered diabase. The chemical composition of the silicic tuff at 1704 m depth suggests that it is correlative with the Durmid Hill tuff, cropping out 25 km NW of the geothermal system. In turn, both of these tuffs may be deposits of the Bishop Tuff, erupted from the Long Valley caldera of central California at 0.7 Ma. The diabases are similar to basaltic xenoliths found in the nearby Salton Buttes rhyolite domes. These diabase are interpreted as hypabyssal intrusions resulting from magmatism due to rifting of the Salton Trough as part of the East Pacific Rise/Gulf of California transtensional system. The sills apparently intruded an already developed geo‐thermal system and were in turn altered by it.

THE SALTON SEA SCIENTIFIC DRILLING PROJECT

In March 1986 a research borehole, called the “State 2–14,” reached a depth of 3.22 km in the Salton Sea geothermal system of southern California. This was part of the Salton Sea Scientific Drilling Project (SSSDP), the first major (i.e., multimillion dollar) research drilling project in the U.S. Continental Scientific Drilling Program. The principal goals of the project were to investigate the physical and chemical processes of a high‐temperature, high‐salinity, magmatically driven hydrothermal system. The borehole encountered temperatures of up to 355°C and produced metal‐rich, alkali chloride brines containing 25 wt% of total dissolved solids. The rocks penetrated exhibit metamorphism and ore genesis in action. They show a progressive transition from unconsolidated lacustrine and deltaic sediments to hornfelses, with lower amphibolite facies mineralogy, accompanied by pervasive veins containing iron, copper, lead, and zinc ore minerals. The SSSDP included an intensive program of rock and fluid sampling, flow testing, and downhole logging and scientific measurement. The purpose of this paper is to introduce this special section of the Journal of Geophysical Research on the SSSDP, to describe briefly the background of the project and the drilling and testing of the borehole, to summarize the initial scientific results, and to discuss how the lessons learned are applicable to future scientific drilling projects.

Prograde Phase Relations in the State 2–14 Well Metasandstones, Salton Sea Geothermal Field, California

Seventy‐five core samples of medium‐ to fine‐grained metasandstones from the State 2–14 borehole were studied petrologically and mineralogically to evaluate compositional variations of geothermal minerals and prograde metamorphic reactions. Three distinct metamorphic zones are defined based on the appearance of index minerals. With increasing grade, they are (1) chlorite‐calcite (Chl‐Cc) zone at depths between 610 and 2480 m; (2) biotite (Bt) zone between 2480 and 3000 m; and (3) clinopyroxene (Cpx) zone between 3000 and 3180 m. All of the studied metasandstones at depths greater than 906 m contain epidote (Ep) and quartz (Qtz) with or without albite (Ab). Characteristic assemblages (+ Ep + Qtz ± Ab) are (1) Chl + Cc ± K‐feldspar (Ksp) ± phengitic muscovite for the Chl‐Cc zone; (2) Bt + Chl ± Ksp ± anhydrite (Anh) and Bt + Act ± Chl for the Bt zone; and (3) diopsidesalite + Act ± Bt ± oligoclase ± actinolitic hornblende for the Cpx zone. Epidote exhibits a compositional variation ranging from 0.11 to 0.42 XFe (= Fe/(Fe+Al)). Many amphibole grains of the clinopyroxene zone show patchy or lamellar intergrowths of both actinolite and actinolitic hornblende on a scale less than a few tens of micrometers. These amphiboles show distinct compositional discontinuities such as in Al2O3 contents (2.7 – 4.8 wt %) and Fe/(Fe+Mg) ratios (0.41 – 0.55). The Cpx zone samples thus mark the transition from the greenschist to amphibolite facies. Other important secondary phases include rutile or anatase (depths<1427 m; 2745 – 3020 m), aluminous titanite (>940 m), peristerite (1235 m), and Mn‐salite (2484 m). The observed assemblages and compositions of minerals are chemographically interpreted utilizing an epidote projection in the simple system, Al2O3 ‐ K2O ‐ MgO ‐ CaO ‐ SiO2 ‐ H2O ‐ CO2. Reactions for the appearance of index minerals are elucidated in a qualitative T ‐ XCO2 diagram. The results indicate that (l) the compositions of Chl and Bt are highly variable depending on the bulk composition; however, Chl in the Chl‐Cc zone becomes enriched in Mg with increasing depth; (2) Bt forms by a discontinuous reaction, Ksp + Chl + Cc = Bt + Ep + Qtz + H2O + CO2; hence the first appearance of Bt is greatly affected by the fluid composition; (3) the hydrothermal fluid in equilibrium with the analyzed assemblages is very low in XCO2 (<0.03); and (4) the absence of wairakite and prehnite is attributed to the high fO2 values of the Salton Sea geothermal system.

Metamorphosed Plio-Pleistocene evaporites and the origins of hypersaline brines in the Salton Sea geothermal system, California: Fluid inclusion evidence

The Salton Sea geothermal system (SSGS) occurs in Plio-Pleistocene deltaic-lacustrine-evaporite sediments deposited in the Salton Trough, an active continental rift zone. Temperatures up to 365°C and hypersaline brines with up to 26 wt.% TDS are encountered at 1–3 km depth in the sediments, which are undergoing active greenschist facies hydrothermal metamorphism. Previous models for the origins of the Na-Ca-K-Cl brines have assumed that the high salinities were derived mainly from the downward percolation of cold, dense brines formed by low-temperature dissolution of shallow non-marine evaporites. New drillcores from the central part of the geothermal field contain metamorphosed, bedded evaporites at 1 km depth consisting largely of hornfelsic anhydrite interbedded with anhydrite-cemented solution-collapse shale breccias. Fluid inclusions trapped within the bedded and breccia-cementing anhydrite homogenize at 300°C (identical to the measured downhole temperature) and contain saline Na-Ca-K-Cl brines. Some of the inclusions contain up to 50 vol.% halite, sylvite and carbonate crystals at room temperature, and some halite crystals persist to above 300°C upon laboratory heating. The data are consistent with the trapping of halite-saturated Na-Ca-K-Cl fluids during hydrothermal metamorphism of the evaporites and accompanying solution collapse of interbedded shales. We conclude that many of the salt crystals in inclusions are the residuum of bedded evaporitic salt that was dissolved during metamorphism by heated connate fluids. Therefore, the high salinities of the Salton Sea geothermal brines are derived in part from the in situ hydrothermal metamorphism and dissolution of halides and CaSO 4 from relatively deeply-buried lacustrine evaporites. This fact places important constraints on modeling fluid-flow in the SSGS, as brines need not have migrated over great distances. The brines have been further modified to their present complex Na-Ca-K-Fe-Mn-Cl compositions by on-going sediment metamorphism and water-rock interaction.

Active ore formation at a brine interface in metamorphosed deltaic lacustrine sediments; the Salton Sea geothermal system, California

The Salton Sea geothermal system is an area of active hydrothermal metamorphism of Pliocene to Pleistocene deltaic, lacustrine, and evaporitic sediments deposited in a modern continental rift zone. Temperatures up to 365 degrees C, hypersaline metalliferous brines with up to 26 wt percent total dissolved solids and active base metal ore formation accompany low-pressure greenschist and amphibolite facies metamorphism at depths less than 3.2 km. Drill cores recovered by the Salton Sea Scientific Drilling Project shed new light on the nature and origin of ore formation in the eastern portion of the Salton Sea geothermal system.Base metal ore mineralization occurs in vertical fractures that comprise the major form of fluid permeability at 1 to 3 km in the modern geothermal system. Type 1 veins are dominated by carbonate, contain pyrrhotite and other sulfides, are completely sealed, and occur in wall rock showing minimal alteration. Fluid inclusion data indicate that they were deposited under a thermal gradient much higher than that now found in the eastern part of the system. Type 2 veins are dominated by silicates and hematite, are permeable, and occur in wall rocks showing pervasive chloritization and epidotization. Fluid inclusion data indicate that they are forming now from the geothermal brines under the modern thermal gradient.Fluid inclusions record an apparent steep, progressive salinity increase in the vein systems in the upper 2 km of the system. However, fluid production data from Salton Sea geothermal wells indicate that reduced, metalliferous hypersaline brine averaging 23 wt percent total dissolved solids is overlain by more oxidized, metal-poor fluid averaging 5 wt percent total dissolved solids. A sharp interface exists between the fluids at 1 to 2 km, paralleling the 250 degrees C isothermal surface.A model is developed for type 2 vein ore formation involving mixing at the interface between the two fluids. Magmatic intrusion induces fracturing, heating, and the expulsion of reduced connate fluids to cause early type 1 vein mineralization. Following the rising thermal front, deep hypersaline brine rises diapirically and mixes with overlying fluids in vertical fractures transecting the ascending interface, forming type 2 veins. The hypersaline brine rises to a level of density-temperature stabilization, causing pervasive greenschist metamorphism of the host sediments. The diapir then begins to cool and descend. Shallow, oxidized brine is drawn in as the hypersaline brine descends, promoting late-stage hematite in type 2 veins. The eastern Salton Sea geothermal system presently appears to be in this latter mode of cooling and retrograde brine descent.Ore formation in the system is caused by the coincidence of transaxial entry of a major river into an active rift zone, deposition of metal-bearing deltaic sediments to form a closed-basin sedimentary environment, episodic lacustrine evaporite formation, and injection of heat and elements by rift-related magmatic intrusions at depth. The presence of an upwelling brine diapir in the system may be analogous to conditions immediately prior to the exhalation of metalliferous fluids that formed stratiform Fe-Zn-Pb-Cu sulfide and oxide deposits in paleorift zone environments.

Evolution of the thermal cap in two wells from the salton sea geothermal system, California

The Salton Sea geothermal system is located near the center of the sediment-filled Salton Trough where temperatures locally reach 365°C at depths of 2 km. The geothermal reservoir is overlain by a thermal cap of low-permeability rocks that restricts the upward movement of the high-temperature brines. Petrographic and fluid-inclusion studies in two wells drilled in the southern part of the field indicate that the thermal cap consists of an upper layer of lacustrine and evaporite deposits with low initial permeabilities and a lower layer of deltaic sandstones. The sandstones were incorporated into the thermal cap as downward percolating fluids deposited anhydrite and calcite in the pore space of the rocks, reducing their permeabilities. During development of the thermal cap, base-metal sulfides, potassium feldspar and quartz were deposited by brines from higher-temperature portions of the system.

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 Na Ca K Fe Mn Cl 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 Fe Zn Cu Pb 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 δ 18O 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. δ 34S 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.

Variation in sericite compositions from fracture zones within the Coso Hot Springs geothermal system

Two types of white micas are found in drillhole samples within the geothermal system at Coso Hot Springs. Low-permeability zones of the crystalline basement contain coarse-grained relict muscovite, whereas rock alteration near fracture zones at temperatures > 150°C is characterized by abundant finegrained sericite in association with secondary calcite and quartz and unaltered relict microcline. In this hydrothermal sericite there is an increase in interlayer K, octahedral Mg + Fetotal and tetrahedral Al with increasing temperature between ~215° to >250°C. Thermodynamic activity of the Al2Si4O10(OH)2 component of serielles, calculated using site-mixing approximations of Helgeson and Aagaard (1985), decreases with increasing temperature between ~215° and >250°C. As a consequence of the observed variations in the interlayer, octahedral and tetrahedral site occupancies, the activity of KAl2(AlSi3O10(OH)2 is essentially constant in the Coso seriates over this temperature range. The calculated equilibrium distribution of aqueous species in the hydrothermal solutions produced from well 16−8, together with cation-activity phase diagrams that account for variations in sericite composition, requires a pH of ~6.7–6.8 at temperatures between 236° and 250°C. Comparison of predicted and observed phase relations with fluid compositions indicate that seriates are in local equilibrium with the geothermal reservoir fluid, whereas relict metamorphic muscovites are metastable. Although the compositions of hydrothermal sericites are a complex function of temperature, pressure and geothermal fluid composition, compositional relations observed at Coso together with published compositions of hydrothermal dioctahedral layer-silicates from the Salton Sea geothermal system demonstrates that elemental compositions of interlayer K and tetrahedral Al increase systematically with increasing temperature despite the dramatic differences in fluid compositions between these two geothermal systems. This suggests that the observed variations in interlayer and tetrahedral site occupancy is largely dependent on the enthalpy of hydrolysis reactions representing equilibrium between sericite solid solutions and the geothermal reservoir fluids.

Composition and structural state of coexisting feldspars, Salton Sea geothermal field

AbstractActive metamorphism of fine grained sandstone in the c.16000 year old Salton Sea geothermal system has produced a suite of chemically equilibrated coexisting authigenic alkali feldspars and re-equilibrated detrital feldspars in the 250–360°C temperature range. At c.335°C the average compositions, 2 Vs, and (t1o+t1m) and Z ordering parameters of coexisting authigenic feldspars are [Or0.52Ab97.40An2.08, 2Vx = 91.3±4.8, (t1o + t1m) = 0.89±0.05, Z = 0.79±0.09], and [Or94.42 Ab5.10An0.48, 2Vx = 70, (t1o + t1m) = 0.90, Z = 0.81]. At c.360°C authigenic albite becomes more An-rich and less ordered [Or1.21Ab92.83An5.97, 2Vx = 87.5±3.4, (t1o + t1m) = 0.85±0.03, Z = 0.70±0.07] and K-feldspar is no longer stable. Detrital plagioclase (An up to 40%) is preserved metastably to temperatures up to c.190°C in strongly carbonate-cemented sandstone which forms part of a geothermally produced permeability cap. It undergoes rapid alkali exchange at temperatures near 200°C, and by 250°C no plagioclase with An-content over 12% is observed. At &gt; 250°C authigenic and most detrital alkali feldspar compositions are in excellent agreement with the Bachinski and Muller (1971) microcline-low-albite solvus.

Carbonate alteration minerals in the Salton Sea geothermal system California, USA

AbstractActive geothermal systems in fluvial-deltaic sediment of the Salton Trough typically develop a thick, carbonate-cemented sandstone caprock which shows a regular progression of carbonate minerals, mineral reactions, rock-fluid mass transfer, and physical properties on increasing temperature. The Sinclair 3 well contains calcian ankerite at &lt; 175 °C, ankerite from 175 to 195 °C, calcite+minor dolomite from 195 to about 250°C, and calcite at higher temperatures. The carbonate content of sandstone decreases on increasing temperature from &gt; 45% at &lt; 140 °C to about 10% at &gt; 250 °C, The most abrupt decrease occurs in the 140 to 170°C range where significant compaction of sandstone occurs as carbonate is reduced to &lt; 25%, and kaolinite reacts to form chlorite. The overall result is the loss of significant Ca, Fe, Mg, and CO2 from sandstone to the fluid phase on increasing temperature. Reaction ofcalcian ankerite to ankerite near 175 °C, and of ankerite to calcite and minor dolomite in the 195 to 245°C range, takes place on a constant carbonate-volume basis by direct replacement of one carbonate by another. The latter reaction produces significant chlorite, with Al derived from solution of detrital feldspar and from the smectite-to-illite transformation. The equilibrium coexistence of calcite with dolomite and ankerite near 200 °C has allowed construction of an isothermal section in the Ca-Mg-(Fe+Mn) carbonate phase diagram and provided a low-temperature constraint on the calcite limb of the calcite-dolomite solvus.

Fe-Zn-Cu-Pb mineralization in the Salton Sea geothermal system, Imperial Valley, California

The Salton Sea geothermal system is an area of active metamorphism in a Plio-Pleistocene deltaic sedimentary sequence. Within it, hypersaline brines containing up to 250,000 ppm total dissolved solids and temperatures up to 365 degrees C are causing incipient sulfide and oxide mineralization. We have made a detailed analysis of ore mineralization in two boreholes drilled to depths of up to 2,500 m in this system. Major ore minerals observed are, in order of decreasing abundance, pyrite, hematite, sphalerite, chalcopyrite, pyrrhotite, marcasite, and galena. The associated nonore minerals are mainly calcite, quartz, epidote, anhydrite, adularia, and chlorite. Ore mineralization can be divided into three main types. Diagenetic sulfide mineralization, occurring at depths less than 760 m and temperatures less than 250 degrees C, consists of fine-grained stratiform iron sulfides occurring as cement in sandstone and as disseminations and bands in shale. Metamorphic sulfide mineralization, occurring at depths greater than 760 m and temperatures greater than 250 degrees C, includes the progressive development of porphyroblastic pyrite with increasing depth and temperature, its replacement by other Fe-Cu-Zn-Pb sulfides at depths greater than 1,220 m, and its decomposition into skeletal aggregates at depths greater than 1,525 m, concurrent with hornfelsic recrystallization of the host sediments. Vein and related pore-filling ore mineralization is of two contrasting types: hematite-silicate-sulfide + or - sulfate and sulfide-carbonate + or - silicate. The first type occurs in open porous veins in single vertical zones at least 220 m thick in each borehole. The second type occurs in veins that are generally sealed and occur in relatively thin, scattered vertical intervals in both boreholes. Thermodynamic analysis indicates that the modern reservoir brines are near equilibrium with the hematite-silicate-sulfide + or - sulfate vein assemblage at 300 degrees C, with pH = 5.4 and log a (sub O 2 ) = -30. Reactions between hematite, pyrite, and iron-bearing silicates (chlorite, epidote) probably control the redox state of the present brines. Calculated metal-chloride complex solubilities for Fe, Zn, Pb, and Cu agree well with brine analyses. Hematite, pyrite, and chalcopyrite are supersaturated; galena and sphalerite are undersaturated in the modern brines.The phase relations of the presently sealed sulfide-carbonate + or - silicate veins require that earlier fluids, if formed at the same temperature and pH, had to be more reduced and sulfur rich than the present slightly oxidized, sulfur-poor brines. Contrasting fluid redox states may be caused by a late lateral or fault-controlled influx of more oxidized surficial waters. Lack of acid alteration implies boiling has not occurred to produce the oxidized fluids. Early formed diagenetic iron sulfides are commonly replaced by later Cu-Pb-Zn sulfides and iron oxides and enveloped and resorbed by veins, acting as nuclei and sources of S and Fe for later mineralization. Precipitation probably occurs when seismic or hydraulic fracturing allows the metal-rich, sulfur-poor brines to interact with earlier pyrite sulfur in the host rocks. Thus the geothermal system as presently explored is best classified as an incipient stratabound sulfide deposit that is being overprinted by hydrothermal and metamorphic processes.

Sur la migration des éléments par thermodiffusion. Etat et perspectives d'un modèle géochimique

In the lithosphère, particularly in great geothermal activity zones, there exist possibilities of quasi permanent thermal diffusion natural convection couplage which is able to develop a thermogravitational effect. The study of concrete cases requires a good knowledge of physical parameters introduced in thermogravitational-effect theoretical relations, as well as hydrothermal convective circuits, the nature of solutions, and local thermal gradient values. A thermogravitational schematic model has been developed and applied to Salton-Sea geothermal system. This model is supported by some simple geometric dispositions fitted to convective cell development and underlining the effect of geometric and thermal parameters on estimate concentration extent. This model provides the rough estimate concentrations of brines, like those observed in-situ, in agreement with most geochemical and geological local contingencies. The refining of this model will be linked to a better knowledge of the organization and stability of convective circuits in underground fluids.

Calc-silicate mineralization in active geothermal systems

Detailed study of the calc-silicate mineralogy in the Cerro Prieto geothermal system was used to demonstrate examples for thermodynamic evaluation of phase relations among minerals of variable composition and to calculate the chemical characteristics of hydrothermal solutions compatible with the observed calc-silicate assemblages. In general there is a close correlation between calculated and observed fluid compositions. Calculated fugacities of O 2 at about 320 degrees C in the Cerro Prieto geothermal system are about five orders of magnitude less than those at the nearby Salton Sea geothermal system. This observation is consistent with the occurrence of Fe (super +3) -rich epidotes and andradite garnets in the latter system and the presence of prehnite at Cerro Prieto.--Modified journal abstract.

Theoretical prediction of phase relations among aqueous solutions and minerals: Salton Sea geothermal system

Thermodynamic calculations of compositional relations among aqueous solutions and minerals in the system Na2O-K2O-CaO-MgO-Fe2O3 Al2O3-SiO2-H2O-CO2 HCl at pressures and temperatures corresponding to liquid-vapor equilibrium of H2O permit quantitative description and interpretation of phase relations among rock forming minerals and aqueous solutions in magma-hydrothermal systems. The extensive data base on the Salton Sea geothermal system provides an exemplary case for predicting the chemical characteristics of geothermal fluids associated with metasomatic mineral zones observed in deep drillhole samples. Near the Elmore No. 1 well aqueous species activity ratios of aNa+aH+ and aK+aH+ vary several tenths of a log unit with increasing depth and temperature from ∼ 0.6 km and ∼250°C to ∼ 2.2 km and ∼350°C, whereas aCa2+a2H+ decreases ∼ 2 orders of magnitude for a comparable range in depth and temperature. The fugacity of CO2 gas is ∼1.5–6 bars at ≲ 310°C. Calculated values of aSio2(aq), aNa+aK+, aCa2+aMg2+ and fCO2(g), in the fluid phase coexisting with observed mineralogic phase relations are in remarkably close agreement with measured solute concentrations in geothermal fluids produced from deep drillholes near the Salton Sea.Hydrolysis reactions representing observed phase relations and written with alkali and alkaline earth cations as products have negative standard molal enthalpies (ΔH0P,T,r) and volumes (ΔV0P,T,r) of reactions; consequently, aNa+aH+, aK+aH+, aCa2+a2H+, and aMg2+a2H+ decrease with increasing temperature at constant pressure, but increase with increasing pressure at constant temperature. An approximate linear relationship exists among these activity ratios and the reciprocal of absolute temperatures because ΔH0P,T,r varies only slightly with increasing temperature at ≲ 250 to 300°C. However, at ≳300°C, ΔH0P,T,r and ΔV0P,T,R decrease dramatically as a consequence of extrema in the thermodynamic and electrostatic properties of the solvent near the critical region of H2O. Hence, empirical solute geothermometers based on linear regressions of geothermal fluid compositions at ≲250–300°C cannot be confidently extrapolated to higher temperature systems.

Oxygen isotope studies of the geothermal system at Wairakei, New Zealand

Volcanic rocks in the Wairakei geothermal field have undergone extensive oxygen isotope exchange with the thermal waters, resulting in an O 18-depletion averaging about 4%. A lower limit on the ratio of the mass of water to rock in the exchange system is 4·3, at least ten times greater than the corresponding figure for the Salton Sea geothermal system. Carbonates, present as alteration products in most samples, are found to be in equilibrium with waters at present-day temperatures in some wells, and to record higher ‘fossil’ temperatures in others. Quartz phenocrysts and xenocrysts remain unexchanged, and only new hydrothermal quartz is in isotopic equilibrium with geothermal solutions.

Source fluids for the Salton Sea geothermal system; discussion

The proposal by Helgeson \[ibid., v. 266, no. 3, p. 129-166, 1968\] that the source fluid for the Salton Sea geothermal brine was interstitial connate Colorado River water in the sediments is evaluated quantitatively in terms of the dynamics of the geothermal system and the isotopic composition of the various waters involved. Neither batch nor continuous evaporation processes operating on original interstitial water of present day Colorado River composition can produce the geothermal brine characteristics presently observed. The isotopic evidence indicates that the source fluid was meteoric water derived from local precipitation and circulating downward through the sediments.

Active Metamorphism of Upper Cenozoic Sediments in the Salton Sea Geothermal Field and the Salton Trough, Southeastern California

The Salton Sea geothermal system is entirely within Pliocene and Quaternary sediments of the Colorado River delta at the north end of the Gulf of California. At the time of deposition, these sediments consisted of sands, silts, and clays of uniform original mineralogic composition, but under the elevated temperatures and pressures of the geothermal system they are being transformed to low-grade metamorphic rocks of the greenschist facies. We have studied these transformations by X-ray, petrographic, and chemical analyses of cuttings and core from deep wells that penetrate the sedimentary section. Temperatures within the explored geothermal system range up to 360° C at 7100 feet. The wells produce a brine containing over 250,000 ppm dissolved solids, primarily Cl, Na, Ca, K, and Fe, plus a host of minor constituents. The original sediments consisted of detrital quartz, calcite, K-feldspar, plagioclase, montmorillonite, illite, dolomite, and kaolinite. Discrete montmorillonite is converted to illite at temperatures below 100° C, and illite-montmorillonite is completely converted to K-mica at temperatures below approximately 210° C. Ankerite forms by the conversion of calcite, dolomite, or both, at temperatures as low as 120° C, possibly as low as 80° C. Dolomite, ankerite, kaolinite, and Fe ++ (probably from the brine) react to produce chlorite, calcite, and CO 2 at temperatures as low as 180° C and possibly as low as 125° C. At temperatures greater than approximately 290° to 310° C, iron-rich epidote and K-feldspar become abundant, calcite disappears, and K-mica is sporadic. Detrital Na-Ca plagioclase persists throughout the explored system, and at depth exists out of equilibrium with metamorphic albite. The most common metamorphic assemblage at temperatures of 300° C and above is quartz + epidote + chlorite + K-feldspar + albite ± K-mica. Pyrite, sphene, and hematite are also sporadically present. Similar metamorphism occurs in the sedimentary section penetrated by the Wilson No. 1well, drilled to a depth of 13,433 feet 22 miles south-southeast of the geothermal field. The lower-temperature reactions observed in the Salton Sea geothermal field also occur in Wilson No. 1, but at much greater depths owing primarily to the lower temperature gradient. Temperatures in this well reach only 260° C, insufficient for the formation of epidote and the destruction of calcite and K-mica. The mineralogical transformations taking place in the Salton Sea geothermal field are metamorphic responses to the elevated temperatures and pressures. Some transformations such as the reaction of dolomite, ankerite, and kaolinite to produce chlorite, calcite, and CO 2 are regional in extent and pose no metasomatic requirements other than that the system be open to H2 O and CO 2 . Other relationships, such as the destruction of calcite and K-mica and the complementary formation of epidote, may involve interchange of elements with the brine. The Salton Sea geothermal system displays a continuous transition from sediments through indurated sedimentary rocks to low-grade metamorphic rocks of the greenschist facies. This transition encompasses transformations commonly considered as diagenetic, and takes place without the formation of zeolites.

Environments of generation of some base-metal ore deposits

The origin of the ore fluids, their dissolved constituents, the force that drives them, and the causes for deposition of their constituents are reviewed for five districts: Providencia, Mexico; Mississippi Valley (Pb-Zn); Salton Sea geothermal system; Red Sea geothermal system; Nonesuch Shale, Michigan. The compositions (chemical and isotopic) and temperatures, determined directly for brines and from fluid inclusions for ancient deposits, reveal significant differences between the Providencia deposit (close magmatic affiliations) and the others, in which connate water and constituents dissolved from country rock appear to be important. Deposits formed by complex processes, with multiple sources for materials, may well be more common than simple 9end-member9 deposits.

Geologic and thermodynamic characteristics of the Salton Sea geothermal system

The area is in the San Andreas Rift Valley of southern California. The geothermal reservoir is in an arkosic sandstone more than 2,000 feet thick and below an insulating shale 2,000 to 3,000 feet deep. Interstitial concentrated NaCl-CaCl ₂ -KCl brine occurs; no steam is present. At 3,000 feet, the temperature exceeds 300 degrees C and is 360 degrees C at 7,000 feet. Mineral reactions and alterations are described. Below the shale, the temperature-depth profiles approach adiabatic gradients. Thermal convection of the pore fluids is the primary mechanism of heat transfer. The temperature gradient is equated to salinity with changes in depth. Various other physical features are evaluated. The brine formed by the evaporation of Colorado River water trapped in the reservoir sands.

Thermodynamic and Geochemical Characteristics of Salton Sea Geothermal System: ABSTRACT

The Salton Sea geothermal area is in a rift valley of the San Andreas fault system in southern California. The geothermal reservoir consists of more than 2,000 ft of arkosic sandstone containing interstitial concentrated NaCl-CaCl2-KCl brines. No steam is present underground. The sandstone is overlain by shale from the surface to a depth of 2,000-3,000 ft. The temperature in the subsurface exceeds 300°C at 3,000 ft and 360°C at 7,000 ft. Thermal convection of pore fluids in the sandstone appears to be the primary mechanism of heat transfer in the reservoir. The salinity and temperature of the pore fluids decrease outward from the center of the geothermal reservoir. The enthalpy of the brines in the reservoir ranges from 220 to 275 cal/g, which is 45-9 cal/g less than the enthalpy of pure water at equivalent temperatures and pressures. Although the origin of the brines is obscure, it appears likely that they formed by evaporation of Colorado River water originally trapped in the pore spaces of the reservoir sands. Thermodynamic calculations indicate that reaction of the interstitial brines with the sediments in the geothermal reservoir (which is below 3,000 ft) has resulted in substantial mass transfer (more than 40 g/1,000 g of water) during metamorphism. Geochemical considerations suggest that the reservoir fluids in the geothermal system are in chemical equilibrium with the mineral assemblage in the enclosing rocks. Chemical reactions between the original pore fluid in the rock and the sedimentary mineral assemblage have led to a relatively low pH and enrichment of K, Ca, Fe cations, and other constituents in the aqueous phase. The total dissolved solids in the brine have been concentrated by removal of water from the system. End_of_Article - Last_Page 563------------