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Geothermal Resources Council, TRANSACTIONS, Vol. 8, August 1984

RELATIONSHIP BETWEEN VOLCANISM AND HYDROTHERMAL ACTIVITY AT ,

Marshall J. Reed

Berkeley Group Inc.

ABSTRACT GEOLOGIC SETTING

Whole-rock analyses of major and minor ele- Cerro Prieto is 7 km northeast of the ments and strontium isotopes show that the Cerro pr,e-Cenozoic crystalline horst of the Sierra Cuca- Prieto volcano, northern Mexico, is derived from pa. In the Cucapz, Bernard (1968) described cord- partial melting of the Cretaceous granitic base- ierite-amphibolite facies metasediments of possi- ment rocks beneath it and not from .differentiation ble Permian age, hornblende-biotite tonalite of of gabbroic intrusions in the Salton Late Jurassic age ( lead-alpha age 140 f 14 Trough--Gulf of rift. The small volume m.y.), and biotite granodiorite of Cretaceous age. of erupted at Cerro Prieto indicates that -argon apparent ages of biotite in two the associated chamber had an insufficient granodiorite samples were 62.6 f 0.4 and 67.1 f volume to retain the heat required to drive the 1.4 m.y. (Krummenacher and others, 1975). The present hydrothermal system. The Quaternary da- biotite granodiorite is predominant along the cite volcanism and current hydrothermal activity northeastern half of the Sierra Cucaps, and deep are both the result of heat transferred to the drilling has shown that it underlies the deltaic crust by gabbroic intrusions, but no mass transfer in the western part of the geothermal from gabbroic to dacitic is detected. field. Just south of the volcano, granodiorite was penetrated in well M-3 at a depth of 2,547 m, in well M-96 at a depth of 2,722 m, and in well INTRODUCTION S-262 at a depth of 1,478 m (Puente and de la Pesa, 1979). The volcano Cerro Prieto (lit. trans. "Dark Hill"), is located on the western margin of the The northeast trending Cerro Prieto and Impe- at 32'25" latitude and 115O15'W rial transform faults form the active branches of longitude. This is one of several small, widely the San Andreas system in this area of the spaced Quaternary volcanic centers in the Gulf of Salton Trough. Both faults show right-lateral California--Salton Trough rift. The Gulf of Cali- displacement in their central sections but have no fornia opened nearly 4.5 m.y. ago in response to apparent displacement at their ends (Majer and shearing motion on the system, others, 1980). A seismic refraction survey across and a chain of spreading centers and transform the Cerro Prieto fault (reported by Majer and oth- faults has formed a zone of crustal rifting and ers, 1980) demonstrated that the granodiorite continuing dilatation over 1,000 km long (Larson, was continuous beneath Cerro Prieto and 1972). The northern end of this zone has been the western part of the geothermal field .but could filled by the rapid of the Colorado not be detected east of the Cerro Prieto fault. River delta. Seismic refraction surveys and grav- Fuis and others (1982) described a similar situa- ity data indicate that late Cenozoic deltaic sedi- tion, 50 km north of Cerro Prieto, where the ments and metasediments, over 10 km thick, overlie sheared eastern edge of the granitic basement and intrusions in the axis of the rocks abut the thick section of rift-filling del- Salton Trough northeast of Cerro Prieto (Fuis and taic sediments. At Cerro Prieto, a secondary set others, 1982). of northeast trending faults appears to have pre- dominantly normal displacement and may be due to The volcano lies on the northwest edge of an tensional stress developed between the major active hydrothermal system with an area of over right-lateral faults. The two eruptive centers of 100 km'. Intense exploration and development for Cerro Prieto developed along one of these north- have provided a great deal of east trending structures. information on the local geology and geophysics (Halfman and others, 1984). Over 100 wells have VOLCANIC ACTIVITY been drilled into the deltaic sediments to depths as great as 3.5 km in order to produce water at In previously published reports, the lithol- temperatures up to 370°C. The Cerro Prieto geo- ogy of Cerro Prieto has been erroneously described thermal field has an electrical generating capac- as , , and rhyodacite. The classi- ity of 180 MW, and additional capacity is under fication as dacite is consistent with the mineral- construction. ogy and chemistry described in this report. Cerro

217 Reed

Prieto volcano is formed by a pair of overlapping CHEMICAL ANALYSIS composite domes built up of viscous dacite flows which issued from two vents 500 m apart. The vol- Whole-rock chemical analyses were performed cano reaches a maximum height of 223 m, and the to determine the major- and minor-element and dacite flows and flanking pyroclastic deposits are strontium isotopic compositions. The samples of exposed over an area of 3.5 km2. The eruptive ma- pyroclastic dacite and of granodiorite core were terial exposed at the surface has a'volume of analyzed at the U. S. Geological Survey (USGS) about 0.3 km3, and buried dacite dikes and sills laboratory using the rapid rock analysis method are estimated to contain an additional volume of (Shapiro, 1975). Minor-element concentrations in 0.6 km3. Pyroclastic material exposed on the east the pyroclastic sample were determined by the USGS side of the domes shows an early phase of phreatic using the instrumental neutron activation analysis and phreato-magmatic activity. Several cycles of (INAA) method (Baedecker, 1979). The outcrop sam- pyroclastic deposits contain layers of sedimentary ples of dacite flow and granodiorite were analyzed clasts (deltaic sand and clay), of gray lapilli, at'the Earth Science Laboratory, University of and of poorly sorted black dacite clasts (lapilli Utah Research Institute using an induction coupled to bombs over 1 m in diameter). These pyroclastic plasma (ICP) atomic emission spectroscopy unit deposits underlie the first: extrusions of viscous, (Christiansen and others, 1980), and silica was dark-gray dacite which form the southwest dome. determined separately using the molybdate-blue Continued eruption of overlapping flows built up method. The strontium isotopic ratio of the flow the first dome before the locus of activity shift- dacite was determined at the USGS isotope labora- ed 500 m along a N 40'E fracture. Dacite flows tory in Menlo Park, California. from the northeast dome partly bury the earlier dome. Late-stage brecciated flows and pyroclastic CHEMISTRY deposits from the northeast dome are red to red- dish-gray which indicates that oxidizing gases Table 1 shows that the dacite and granodiorite were present. At the culmination of activity, a are very similar in their major-element composi- crater 290 m across and over 60 rn deep formed in tions. Both rock types are metaluminous, and the the summit of the northeast dome. dacite has an unusually low potassium content. All of the samples are high in titanium, espe- cially so since is the only oxide phase and no occurs in these rocks. Sample 4 Dacite samples were collected from fresh ap- is richer in plagioclase than the average grano- pearing material of the summit flow on the south- diorite, and this is reflected in its chemistry. west dome and from the early pyroclastic deposits The hydrothermally altered granodiorite of sample on the eastern edge of Cerro Prieto. In thin 5 lost sodium and to the circulating hy- section, the dacite exhibits glomeroporphyritic drothermal water but gained silicon. This altered accumulations of andesine, hypersthene, and mag- rock is peraluminous because of the loss of sodium netite in a fine-grained, pilotaxitic and calcium. The minor-element composition of the groundmass of oligoclase, clinopyroxene, and minor rocks is presented in tables 2 and 3. Comparison glass. The andesine phenocrysts show composition- of the major- and minor-element concentrations of al zoning and both penetration and polysynthetic the dacite and granodiorite (tables 1 and 2) indi- twinning. Irregularly shaped vesicles are moder- cates that the dacite was probably derived from ately abundant. Flow banding is visible in zones the granodiorite through partial melting. In the of flattened vesicles and in platy partings in process of partial melting, the sodium concentra- dense rock. The pyroclastic and flow samples are tion increased, but the concentrations of potassi- petrographically very similar, but the more rapid- um, magnesium, lanthanum, and cobalt decreased. ly cooled contains slightly more glass (5 volume percent) and smaller crystals in The strontium concentrations and isotopic the groundmass than the slower cooling flow (2 ratios are essentially identical in the Quaternary volume percent glass). dacite and Cretaceous granodiorite (tables 2 and 4) giving strong evidence of their genetic rela- Granodiorite was collected from the Sierra tionship. The strontium isotopic ratios of Cerro Cucapa', 12 km south-southwest of Cerro Prieto, and Prieto and Sierra Cucapg rocks are heavier than from core taken at a depth of 2,575 m in well M-3. those of the mantle derived, Quaternary volcanic The granodiorite has a medium-grained hypidiomor- rocks of the Salton Trough-- phic granular texture. Outcrop samples are domi- rift from Isla Tortuga (27'26'N, 111'52'W) and the nated by subhedral oligoclase. The other major Salton Buttes (33'12'N, 115'37'W). Among the minerals are , microcline or micro-perthite, rocks listed on table 4, the deltaic sediments, and biotite. Minor amounts of magnetite, , eroded from Cretaceous sedimentary formations of and zircon are present. The core sample has the the Colorado Plateau, have the heaviest isotopic same basic mineralogy and texture, but it shows ratios (Doe and others, 1966). Batiza and others the effects of hydrothermal solution activity (1979) suggest the wide range in strontium isoto- above 200OC. The altered sample exhibits a few pic ratios of the basalt and andesite of Isla medium-sized calcite grains which are poikilitic, Tortuga is the result of contamination. The range containing small grains of magnetite and epidote. in isotopic ratios of the Salton Buttes The calcite and epidote appear to be the altera- may also be due to contamination since sedimentary tion products of oligoclase. Chlorite appears as and plutonic with heavier isotopic ra- an alteration product of the biotite. Alteration tios have been found (Robinson and others, 1976). is strongest in the vicinity of fractures, and Since the Cerro Prieto dacite contains no known many fractures are filled with calcite. xenoliths, isotopic contamination is unlikely.

218 Reed THERMAL ENERGY CONCLUSIONS

The dominant mechanism for the transfer of The Cerro Prieto volcano is quite small; and, thermal energy into the crustal rocks of the together wit.h a of appropriate size, Salton Trough is the passive emplacement of gab- the energy released during cooling would only broic dikes at temperatures of 1,lOO"C or more. remain for a period of 3,000 years at most. The The gabbroic magma has too high a density to rise age of volcanism may be between 10,000 and 510,000 to the surface through the low-density deltaic years, and the hydrothermal activity is at least sediments, but dikes and sills have been as old as the volcanism. A much greater amount of penetrated by deep wells along the eastern edge of thermal energy has been released through fluid the geothermal field (Goldstein and others, 1982). convection than was available in the dacite magma Hydrothermal has probably increased system. An alternative mechanism exists. Gabbro- the density of the sediments to the point where ic magma, passively emplaced in the lower crust, the mafic magma is buoyant. It is likely that an transfers its heat to the metasediments and sedi- intrusion of gabbroic magma along the Cerro Prieto ments above. Part of the heat was transfered to fault increased the temperature of the basement the granodiorite, partial melting occurred, and granodiorite above 850°C and caused partial melt- dacite magma was mobilized. Only the heat from ing. Cerro Prieto dacite contains moderately the gabbro was transferred to the dacite, and no abundant vesicles, and it is reasonable to assume indications of chemical transfer are detected. that the magma had a high content of volatile con- stituents and that it was probably saturated with REFERENCES water and . The high volatile con- tent would decrease the density of this magma to Baedecker, P. A., 1979, The INAA program of the U. about 2.5 g/cm3, and it is possible that the melt S. Geological Survey (Reston, Virginia), became unstable and mobilized with as little as 10 Carpenter, B. S., Agostino, M. D., and Yule, percent of the magma chamber in the liquid state. H. P., eds., Computers in activation analysis If the estimated volume of extruded and intruded and gama-ray spectroscopy: U. S. Department dacite is correct, the total volume of the magma of Energy Symposium Series 49, p. 373-385. chamber would be less than 10 km3. This volume of rock would have been raised to a temperature of Batiza, R., Futa, K., and Hedge, C. E., 1979, 850"C, but would cool Trace element and strontium isotope character- this body in a reasonably short time. Smith and istics of volcanic rocks from Isla Tortuga: a Shaw (1975) estimate that a slab (length = height young seamount in the Gulf of California: = 10 x width) with a volume of 10 km3 would cool Earth and Planetary Science Letters, v. 43, convectively from 850°C to 300°C in 900 years and p. 269-278. that a cube of the same volume and temperature would cool convectively to 300°C in 3,000 years. Barnard, F. L., 1968, Structural geology of the Sierra de 10s Cucapas, northeastern Baja Cali- AGE RELATIONSHIPS fornia, Mexico, and Imperial County, Californ- ia; Boulder, Colorado University, Ph.D. dis- Attempts at radiometric dating of the Cerro sertation, 160 p. Prieto dacite have been unsuccessful so far, but two indirect methods of dating provide some idea Christiansen, 0. D., Kroneman, R. L., and Capuano, of the age of volcanism. Paleomagnetic studies of R. M., 1980, Multielement analysis of geologic Cerro Prieto (de Boer, 1980) showed that most of materials by inductively coupled plasma-- the eruptive activity occurred during a period of atomic emission spectroscopy: Salt City, normal magnetic polarity, but early pyroclastic Utah, Earth Science Laboratory, University of activity and late emplacement occurred during Utah Research Institute Report ESL-32, 33 p. periods of reverse polarity 100,000 and 10,000 years ago respectively. The rapid sedimentation de Boer, J., 1980, Paleomagnetism of the Quatern- rate and concurent subsidence in the Salton Trough ary Cerro Prieto, Crater Elegante, and Salton also provide useful constraints on the period of Buttes volcanic domes in the northern part of volcanism. A subrounded clast of Cerro Prieto the Gulf of California rhombochasm, Sym- dacite, 5 cm in diameter, was recovered in core posium on the Cerro Prieto geothermal field, from a depth of 1,275 m in well M-26, located 3.2 Baja California, Mexico, 2nd, , Baja km east-southeast of the volcano. This clast is California, Mexico, 1980, Proceedings: Mexi- identical to the early pyroclastic material now Cali, Mexico, Coordinadora Ejecutiva de Cerro exposed on the eastern margin of Cerro Prieto. Prieto, Comision Federal de Electricidad, Precise leveling surveys of the geothermal area p. 91-98. (Lofgren, 1979; de la Pefia, 1981) show that the current rate of tectonic subsidence is 1.3 cm/yr. de la Pe6a L., A., 1981, Results of the first or- At this present rate, the deposition of 1,275 m of der leveling surveys in the Mexicali Valley deltaic sediments represents 98,000 years. A sec- and at the Cerro Prieto field, Symposium ond rate can be calculated from the total accumu- on the Cerro Prieto geothermal field, Baja lation of 10 km of sediments and metasediments California, Mexico, 3rd, San Francisco, Cali- during approximately 4 m.y., and this rate of 2.5 fornia, 1981, Proceedings: Lawrence Berkeley mm/yr gives a time for deposition of about 510,000 Laboratory Report LBL-11967, p. 281-291. years. Many faults occur in the geothermal field, and the possibility of faulted and repeated sedi- mentary sections is very likely.

219 Reed Doe, B. R., Hedge, C. E., and White, D. E., 1966, Silver, L. T., 1979, Regional investigation of Preliminary investigation of the source of strontium isotopic distribution in Southern lead and strontium in deep geothermal brines California and Baja California: Geological underlying the geothermal area: Society of America Guidebook, Map. Economic Geology, v. 61, p. 462-483. Smith, R. L., and Shaw, H. R., 1975, Tgneous- Fuis, G. S., Mooney, W. D., Healey, J. H., Mc related geothermal systems, White, D. Mechan, G. A., and Lutter, W. J., 1982, E., and Williams, D. L., cds., Assessment of Crustal structure of the geothermal resources of the United States-- region, in The Imperial Valley, California, 1975: U. S. Geological Survey Circular 726, earthquakyof October 15, 1979: U. S. Geolog- p. 58-83. ical Survey Professional Paper 1254, p. 25-49.

Goldstein, N. E., Wilt, M. J., and Corrigan, D. J., 1982, Analysis of the Nuevo Leon and its possible relation to the Cerro Prieto magmatic-hydrothermal system, in Sym- posium on the Cerro Prieto geothermal field, Baja California, Mexico, 4th, Guadalajara, Table 1. Major-element composition of volcanic Jalisco, Mexico, 1982, Proceedings: Mexicali, and plutonic rocks at Cerro Prieto, Mexico Mexico, Coordinadora Ejecutiva de Cerro [concentration in weight percent] Prieto, Comision Federal de Electricidad, V. 1, p. 35-41. Sample Identification Halfman, S. E., Lippmann, M. J., Zelwer, R., and Oxide Dacite Granodiorite Howard, J. H., 1984, Geologic interpretation of geothermal fluid movement in Cerro Prieto 1 2 3 4 5 field, Baja California, Mexico: American Association of Petroleum Geologists Bulletin, Si02 66.1 65.6 67.4 63.4 68.1 v. 68, no. 1, p. 18-30. A1203 14.2 14.8 14.3 16.5 15.5 1.4 ------0.94 Krummenacher, D., Gastil, R. G., Bushee, J., and Fe203 Doupont, J., 1975, K-Ar apparent ages, Penin- Few 2.9 4.33 2.57 4.34 1.6 sular Ranges batholith, Southern California MgO 0.84 0.81 1.06 1.75 1.8 and Baja California: Geological Society of CaO 3.6 4.54 3.12 5.20 2.9 America Bulletin, v. 86, no. 6, p. 760-768. Na20 5.17 5.86 4.07 4.24 3.9 1.51 K2° 1.80 3.46 1.86 2.8 Larson, R. L., 1972, Bathymetry, magnetic anomal- H20- 0.16 ------0.17 ies, and plate tectonic history of the mouth 2.0 ------1.0 of the Gulf of California: Geological Society H20e Ti02 0.51 0.49 0.43 0.91 0.54 of America Bulletin, v. 83, no. 11, p . 3345-3359. '2'5 0.14 0.14 0.10 0.21 0.15 MnO 0.08 0.09 0.05 0.06 0.05 ------Lofgren, B. E., 1979, Measured crustal deformation c02 0.50 0.29 in the Imperial Valley, California, in Sym------posium on the Cerro Prieto geothermal field, Total 99.2 98.2 96.6 98.5 99.7 Baja California, Mexico, lst, , Cali- Samples: fornia, 1978, Proceedings: Lawrence Berkeley 1. Pyroclastic dacite from eastern margin of Cerro Prieto. Laboratory Report LBL-7098, p. 141-145. 2. Dacite flow from southwest summit of Cerro Prieto. 3. Granodiorite from 12 km south-southwest of Cerro Prieto. Majer, E. L., McEvilly, T. V., Albores, A., and Diaz C., S., 1980, Seismological studies at 4, Granodiorite from 12 km south-southwest of Cerro Prieto. Cerro Prieto: Geotherrnics, v. 9, no. 1/2, 5. Granodiorite core from 2575-m depth in well M-3. p. 79-88. Note: * Total iron is given as FeO for samples 2, 3, and 4. Puente C., I., and de la PeEa L., A., 1979, Geol- ogy of the Cerro Prieto geothermal field: Geothermics, v. 8, no. 3/4, p. 155-176.

Robinson, P. T., Elders, W. A., and Muffler, L. J. P., 1976, Quaternary volcanism in the Salton Sea geothermal field, Imperial Valley, California: Geological Society of America Bulletin, v. 87, no. 3, p. 347-360.

Shapiro, L., 1975, Rapid analysis of silicate, carbonate, and phosphate rocks--revised edi- tion: U. S. Geological Survey Bulletin 1401, 76 p.

220 Reed

Table 2. Minor-element composition of volcanic Table 4. Strontium isotopic ratios of rocks in and plutonic rocks at Cerro Prieto, Mexico the Salton Trough--Gulf of California rift [concentration in ppm weight]

Sample Identification Sample 87~r/86~r Ref. Element Dacite Granodiorite 1 2 3 4 Cerro Prieto Dacite 0.7056 1 Li 12 23 14 24 Cucapas Granodiorite 0.706 2 Sr ' -- 319 363 594 Ba 1070 860 870 850 Deltaic sediments 0.7129-0.7150 3 -- 1.8 2.3 1.9 Be 0.7043-0.7050 3 Zr 252 246 -- -- Salton Buttes La 27 23 34 33 Isla Tortuga 0.7024-0.7035 4 Ce 56 32 42 49 co 4 6 13 13 Zn 73 68 46 96 References: 1. 2 1 2. : This study, sample of tables and Samples 2. 1979 1. Pyroclastic, eastern margin of Cerro Prieto. Silver, 3. 1966 2. Flow, southwest summit of Cerro Prieto. Doe and others, 4. 1979 3. 12 km south-southwest of Cerro Prieto. Batiza and others, 4. 12 km south-southwest of Cerro Prieto.

Table 3. Minor-element composition of pyroclastic dacite at Cerro Prieto, Mexico [concentration in ppm weight]

Element Concentration Element Concentration

Rb 20 Tb 0.71 cs 0.6 Yb 2.9 Ta 0.51 sc 10.8 Hf 5.6 Lu . 0.44 'U 3.2 Sm 5.7 Th 3.5 Gd 5.9 Nd 28 Ho 0.7 Eu 1.33 Tm 0.42

221