Geology Of The Gerlach-Hualapai Flat Geothermal Area .

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GL028887Geology of the Gerlach-Hualapai FlatGeothermal Area,Northwestern NevadaL. T. Grose and R. J. Sperandiowestern Nevada, reaching an elevation of 9,056 feet (2,n7 m)and rising abruptly above the Black Rock Desert and the SmokeCreek Desert playas which are about 3,900 feet (1,170 m) elevation (fig. 1). Hualapai Flat is a relatively small (100 square mileor 200 square km) intermountain basin varying from 4,000 to5,000 feet elevation (1,200 to 1,500 m) (fig. 2).From a geothermal standpoint this area is particularly interesting because: (1) it includes three thermal spring clusters; (2) itis part of a well-defined northeast-trending zone of thermalsprings in northwest Nevada (Hose and Taylor 1974); (3) it lies inthe middle of a region extending from southwest Idaho intonorthern California characterized by modern hydrothermal convection systems with indicated subsurface temperature above150 C (Renner and others 1975); (4) it lies within the BattleMountain Heat Flow province (Sass and others 1971; Diment andothers 1975; Lachenbruch and Sass 1976); (5) it displays abundant evidence of Late Quaternary and Holocene normal faulting,some associated with the hot springs; (6) it is nonvolcanic insofaras Pliocene and Quaternary volcanic activity is concerned, hencevolcanism as a heat source for the modern geothermal systemsthere can be ruled out; and (7) it is typical and representative oflarge regions in the Basin and Range province that have geothermal energy potential without associated volcanism.Geologic field studies have been carried out in concert with thevarious geophysical and geochemical studies described in thisvolume. As work progressed, geological data were released in thefollowing reports: Grose and Keller 1974, 1975; Keller and others1974; Grose 1976; Sperandio and Grose 1976; and Keller andothers 1978.ABSTRACTThe 500-square mile (1,280 square km) area around Gerlachand Hualapai Flat in northwestern Nevada includes PermoTriassic metavolcanic and metasedimentary rocks disposed asroof pendants in Cretaceous granodiorite, Late Oligocene-EarlyMiocene rhyolitic, andesitic, and basaltic sequences, andPliocene(?) and Quaternary alluvial and lacustrine sediments.The volcanic activity is far too old (23 my) to act as a source of heatin the area. Metasedimentary, granitic, volcanic, and alluvialclastic rocks may act as geothermal reservoirs. Thick lacustrineclays and alluvial-fluvial sediments as well as thick tuffaceousunits have strong thermal insulating or cap rock properties.The location of the Gerlach hot springs is controlled by quasiradial spreading effected by contemporaneous activity on intersecting major regional normal faults. The location of Fly Ranchhot springs is also fault controlled and in addition is associatedwith modern tectonic rifting. Geothermal convection systemsoccur in the area, not by virtue of volcanic heating, but by trapping high regional heat flow beneath thermally insulating layersand by creating and maintaining open fractures by recurrenttensional tectonic activity. INTRODUCTIONGEOLOGICFORMATIONSThis report sets out the general geologic framework of about500 square miles (1,280 square km) centered around Gerlach andHu lap i Flat in the southwestern part of the Black Rock Desertr-e lOn m northwestern Nevada. (pI. I). The area lies about 85l"lllies (136 km) north-northeast of Reno Nevada and about 30miles (49 km) due east of the California:Nevada tate line. TheFault-block rrain is typical of many areas in the Basin andRange provmce. The Granite Range is the highest in north-The Gerlach-Hualapai Flat area is underlain by complex anddiversified formations that range from Late Paleozoic to Quater-1

2COLORADO SCHOOL OF MINES QUARTERLYFigure I.-Oblique air view northward of: (1) Granite Range(GR), and its east-west profile reflecting four fault-block levels,and (2) Gerlach area ofjunction of Black Rock Desert (BRD), Sannary (pI. l). Different scales of mapping in the area to date havedelineated different mappable units. More broadly defined unitsare indicated on the reconnaissance maps at scale 1:250,000 byBonham (1969), Tatlock (1969), and Willden (1964), and on theNevada State Geologic Map at scale 1:500,000 by Stewart andCarlson (1974). Greater subdivisions of rock units are presentedon local maps in the general area by Anderson (1977), Crewdson(1974), Olmsted and others (1975), and Sperandio (1978). Thegeologic map accompanying this report (pl. l) was prepared at ascale of 1 :62,500 and is largely based on field mapping by Groseand Sperandio at scale 1:29,000. Ten formations are delineatedand are discussed briefly in the following sections. For moreinformation on various units in the area, refer to the above citedreferences.PERMO. TRIASSIC META VOLCANICAND METASEDIMENTARY ROCKSThe oldest formations exposed in the Gerlach-Hualapai Flatarea are Permo-Triassic metavolcanic and metasedimentaryrocks. Three lithologic assemblages are recognized as follows: (1)amphibolite and amphibolite gneiss in the western part of theSelenite Range, (2) meta-basalt and meta-andesite in the northeastern, northern, and northwestern parts ofthe Granite Range,and (3) metasedimentary and metavolcanic rocks interbedded inthe northern part of the Granite Range and in the southwesterntip of the Calico Mountains. These three assemblages crop out asEmidio Desert (SED), Smoke Creek Desert (SCD), and Gerlach(G), Gerlach hot springs (GHS); Calico Mountains (CM). (Photo:LTG 13275)isolated roof pendants several miles apart in Cretaceousgranodiorite, yet their internal bedding strikes rather consistently in an east-northeasterly direction where measurablemostly in the northern part of the area. Dips vary steeply on bothsides of vertical, but appear to be more commonly to the north.The overall sequence crudely extrapolated among the scatteredmasses in the region suggests east-northeast-trending zones consisting of a southern zone of amphibolite, a middle zone of metavolcanic rocks, and a northern zone of interbedded metavolcanicand metasedimentary rocks. Neither top nor bottom nor stratigraphic succession of these rocks are known in the region. Bedding is preserved only in the metasedimentary sequences, thusinternal structures of large areas of metavolcanic and amphibolitic rocks are unknown. The largest mass exposed, that in thenorthern part of the Granite Range, suggests a possibleminimum thickness of25,000 feet (7,500 m). If the widely occurring masses all have near-vertical internal bedding structureand if the rocks are not repeated, the stratigraphic thicknessoriginally would have approached 100,000 feet (30,000 m). Thatunlikely figure plus geometric constraints strongly suggest thatthe sequence has been tectonically thickened by isoclinal foldingand! or thrusting.The roof pendant masses have near-vertical sharp intrusivecontact with adjacent granodioritic rocks, and these contactsappear to indiscriminately cut across various lithologies in thependant masses.Subsurface occurrence of these rocks is inferred by projectionacross covered areas. Both internal structure and alignment of

"GEOLOGY OF THE GERLACH-HuALAPAI FLAT GEOTHERMAL AREA3Figure 2.-View northeastward from crest of Granite Range,toward central and northern parts of Hualapai Flat and CalicoMountains (CM). Light-colored rocks in Calico Mountains arehighly altered Middle Tertiary volcanics. BRD-Black RockDesert; BRR-Black Rock Range; FRHS-Fly Range hot springs .(Photo: LTG 14577)preserved pendant masses strongly suggest that'the central partof Hualapai Flat which includes Fly Ranch hot springs is underlain by metavolcanic and metasedimentary units.Amphibolitic rocks locally in the western part of the SeleniteRange are fine- to coarse-grained, massive, and locally schistoseand gneissose. Hornblende, biotite, and plagioclase are the majorminerals, but in small patches quartz, K-feldspar, and epidoteoccur. While not studied in detail, and. recognizing that correlation is tenuous at best, these rocks are tentatively regarded as amedium-grade, somewhat dynamothermally metamorphosedequivalent of the thermally metamorphosed basalts and andesites that occur several miles north.Large areas of intermediate to mafic flows and flow brecciasoccur in and northeast of the Granite Range. The rocks arecharacteristically dark greeni/ h-gray, massive-directionless,dense, aphanitic to fine-grained, and rarely porphyritic. Greensmudges, veins, and clots, caused largely by epidote, are ubiquitous except for within several hundred meters of contact withgranodiorite, where epidote content decreases and actinolite increases. Original igneous textures are preserved although modified by crystaloblastic amphibole and epidote. Original mineralsare largely destroyed by propylitization and epidotization associated with regional thermal (static) metamorphism accompanying the batholithic intrusions of Cretaceous granodiorite.The modal mineralogy of these rocks (Sperandio 1978) is consistent with that found in the albite-epidote hornfels facies of acontact metamorphic aureole for a mafic rock (Turner 1968, p.]92). Mineralogy of the rocks within 1 km of the granodioriteindicates the higher grade hornblende-hornfels facies. The meta-volcanic rocks in the Gerlach-Hualapai Flat area resemble thosedescribed by Compton (1958) in the Northern Sierra Nevada(Bidwell Bar area).The sequences of interbedded flows and metasediments comprise up to 5,000 feet (1,500 m) of highly deformed and weaklymetamorphosed flows, limestone, calcareous shale, siltstone,sandstone, and chert. Limestone units in the northern part of theGranite Range are greater than 1,000 feet (300m) in thickness. Inthe northwest part they are 20 to 40 feet (6 to 12 m) thicksandwiched between mafic flows 50 to 200 feet (15 to 60 m) thick.At the southwestern tip of the Calico Mountains a complexlyinterbedded series over 1,000 feet thick (300 m) of chert, limestone, sandstone, fissile shale, mudstone, and mafic flows occurswherein most of the contacts between the units are beddingfaults. Although internally highly deformed, this entirevolcanic-sedimentary sequence appears to be a major basementunit that may extend as a continuous band across the HualapaiFlat area.No identifiable fossils were found in the Permo-Triassic sequences in the Gerlach-Hualapai Flat area. However, Bonham(1969, p. 5) reports that a cast of a Triassic ammonite had beenuncovered in a small mine in the metasedimentary sequenceoccurring in the extreme northwestern end ofthe Granite Range.A volcanic and sedimentary section, remarkably lithologicallysimilar to the section in the Hualapai Flat area, occurs at BlackRuck which is located 15 miles (24 km) on structural trend in aneast-northeasterly direction from similar rocks in the southwestern part of the Calico Mountains. These strata yielded Permian brachiopods (Gianella and Larson 1960; Howe 1975). Other

4COLORADO SCHOOL OF MINES QUARTERLYsections in the region that generally lithologically resemble thePermo-Triassic section in the Hualapai Flat area include theHappy Creek Group and overlying units in the Jackson Mountains (Willden 1964),40 miles (64 km) northeast, and in the PineForest Range (Smith 1973) 60 miles (96 km) north-northeast.CRETACEOUS GRANITIC ROCKSGranitic rocks of batholithic proportions and included roofpendants of metavolcanic and metasedimentary rocks are believed to underlie at least the south half ofthe Gerlach-HualapaiFlat area. The cores of the Granite Range (figs. 1 and 3), theSelenite Range, and the Fox Range-all horst blocks-are composed of granodioritic to dioritic rocks. In the Selenite Range andin the northern part of the Granite Range these rocks are locallyoverlain on a surface of a few hundred feet of relief by LateOligocene to Early Miocene silicic and mafic flows. In manyplaces the plutonic rocks are in fault contact with younger rocks.Granodiorite at the southern end of the Granite Range is especially strongly jointed and faulted. The Gerlach hot springs system (fig. 4) occurs in these highly fractured rocks and alters them(Romberger 1978, this volume).Granodiorite is volumetrically most abundant. Modal analysesindicated petrographic range from granite to diorite (Sperandio1978). Biotite, hornblende, and quartz are ubiquitous minor minerals. It is medium-grained, massive-directionless, uniform, andstrongly jointed in many directions. Near intrusive borders, however, texture varies, a slight gneissic or flow layering occurs,composition changes to quartz diorite and diorite, and aplites,Fig-ure 3.-View south-southeastward along strike of majoreast-dipping normal fault on the eastern side of the Granitepegmatites and xenoliths become abundant. Mineralogical vari.ation between granodiorite and diorite is gradual through a few.hundred feet which suggests origin from cooling a single pluton.Several properties of the batholith exposed in the Granite Rangesuggest that it was emplaced in the lower epizone and uppermesozone at a depth of 3 to 6 miles (5 to 10 km) (Buddington1959). They are: (1) contacts between batholith and wall rock arediscordant; (2) country rock several kilometers from the batholith has been thermally metamorphosed to the albite-epidote-actinolite grade; (3) country rock in close proximity to the batholithhas been contact metamorphosed to the hornblende-plagioclasehornfels facies; (4) there are no known exposures of cogeneticvolcanic rocks; (5) there are no chilled borders or miaroliticcavities in the batholith; (6) planar foliation is generally absentbut is weakly developed in the border zone subparallel to thecontact; and (7) assimilation of wall rocks within the batholith iscommon and obvious.Granitic rocks in the Gerlach-Hualapai Flat area have yieldedK-Ar biotite ages of 89 my (Bonham 1969) and 91 my (Smith andothers 1971). Petrographic, chemical, and geochronologicalcharacteristics of many Cretaceous plutonic rocks in northwestern Nevada are similar, and they resemble granit