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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. Geothermal Resources Council, TRANSACTIONS, Vol. 8, August 1984 RELATIONSHIP BETWEEN VOLCANISM AND HYDROTHERMAL ACTIVITY AT CERRO PRIETO, MEXICO Marshall J. Reed Berkeley Group Inc. ABSTRACT GEOLOGIC SETTING Whole-rock analyses of major and minor ele- Cerro Prieto volcano 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 Quaternary gabbroic intrusions in the Salton Late Jurassic age (zircon lead-alpha age 140 f 14 Trough--Gulf of California rift. The small volume m.y.), and biotite granodiorite of Cretaceous age. of dacite erupted at Cerro Prieto indicates that Potassium-argon apparent ages of biotite in two the associated magma 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 sediments in the western part of the geothermal from gabbroic to dacitic magmas 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- Salton Trough 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 fault 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 San Andreas fault 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, basement 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 sedimentation 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 diabase and gabbro 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- geothermal energy 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 basalt, andesite, 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- PETROLOGY cially so since magnetite is the only oxide phase and no ilmenite 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 calcium 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 phenocrysts 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 pyroclastic rock 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).