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HomeMy WebLinkAboutPreliminary Geothermal Investigations at Manley Hot Springs Alaska - Divison of Geothermal Energy - U.S. Dept of Energy - April 1982 - DOE-ET-27034-TL7- DOE/ET/27 03 4--T1 DE82 020604 Prel i mi nary Geothermal Invest i gat i ons at Manley Hot Springs, Alaska 6: J# 5 '. . denni fer East T rd DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. TABLE OF CONTENTS . u W TABLE OF CONTENTS ....... LIST OF FIGURES ........ w LIST OF TABLES ......... ABSTRACT ........... INTRODUCTION ......... HISTORICAL BACKGROUND ..... PREVIOUS STUDIES ....... Geologic Studies ..... Hot Spring Studies .... .... .... ..e. .... .... .... \ .... .... Page .............. i .............. iii .............. V .............. vi .............. 1 .............. 3 .............. 9 .............. 12 REGIONAL GEOLOGIC SETTING ... Jurassic - Cretaceous Sedimentary Rocks' . 0 0 0'. 0 . 15 Tertiary - Cretaceous Granitic Intrusives .......... 18 REGIONAL SETTING OF THE HOT SPRINGS ................ 19 LO*CAL GEOLOGY. .......................... 23 23 ............. 27 ............. 30 v. . The i8Boulder Ridge Formation" ................ ............ METHODS OF I NVESTIGAT ...... ..... 35 . .'. 0.. . . . . 39 Karshner Creek ........................ 40 Rainwater ........................... 40 # i W v LIST OF FIGURES Page Figure 1 : Figure 2: The Yukon-Tanana upland physiographic province The Manley Hot Springs Hotel with bath house in Archives, Charles Bunnell Collectfon ......... 4 The main springs at Manley Hot Springs, 1910. stone cribbing around one hot spring. Karshner Creek flows through the lower half of photo. University of Alaska Archives, Charles Bunnell Collection. ...................... 5 Poultry and hog barns a anley Hot Springs, 1910. The barns were heated by water carried in buried aqueducts. University of Alaska Archives, Charles Bunnell Collection. 6 cribbing around one hot sprin corner. University of Alaska Archives Collection. ... ............... 7 Open air and control led environment gardenfng at Manley Hot Springs, around 1910, University of Alaska Archives, Charles Bunnel Collection. . . . 8 1915. From L. tconard, 1974. 10 ............. 14 ngs ..... ;.... 24 Hot Springs Dome. 29 ...... 33 Springs. ........ 34 showing the location of Manley Hot Springs ...... viii 4d the background, 191 0. University of A1 aska re 3: Note u Figure 4: w .................. Figure 5: Corn Field and potato fields. ote the stone the lower left Y ’ Recreation of a sketch made by G. A. Waring in w ............. eralired geologic map of, the northwest Yukon- ..... isotherm of grid. Hot Springs ...................... 48 LIST OF TABLES Page b, Y Table 1: Age Determinations for Granittic Plutons in the Northwestern Yukon-Tanana Upland ....... 20 w Table 2: Hot Springs and Seep Temperatures .......... 36 Manley Hot Springs ................. 42 Table 4: Chemical Analysis of Water Samples . . , . . 43 Table 3: Water Wells and Miscellaneous Samples from Y Table 5: Equations for Calculation of Selected Geothermmeters Used in Table 6. C is the Concentration of Silica. All concentrations are in mg/kg ........... 44 Table 6: Calculated Geothermometers in 'C .......... 45 av" 5\ w "? .- I QI w c! w L Y V ABSTRACT Manley Hot Springs is one'of several hot springs which form a belt extending from the Seward Peninsula to east-central Alaska. All of the hot springs are low-temperatur having formed as the result of irculation of meteoric water along deep- seated fractures near or within granitic intrusites. Shal low, thermally disturbed ground at Manley Hot Springs constitutes an area of 1.2 km by 0.6 km along the lower slopes of Bean Ridge on the north side of the Tanana Valley. This area includes 2 springs and seeps and one warm Li v water-domi nated geothermal systems, w # The hottest springs range in temperature from 61" to utilized for space heating and irrigation. This ru' study was designed to Characterize the geothermal system present at Manley Hot Springs and deline likely sites for geothermal drilling. Several surveys were conducte ver a grid system which included shallow ground temperature, helium soi gasI mercury soil and resistivity surveys. ddition, a reconnaissance ound temperature survey and water chemistry sampling program was undertaken. The preliminary results, including some prel 5 mi nary water emistry, show that shallo drothermal activity can v f# 1y hermal system at Manley Hot Sprin Y may act as an impermeable caprock allowing thermal water to reach the surface along fractures.. The study did not conclusively recognize a cr' . 3 INTRODUCTION . . Manley Hot Springs lies within the Yukon-Tanana upland physiographic u W province of the Interior of Alaska, near the junction of the Tanana A-2 and Kantishna River 0-2 quandrangles, latitude 65' 00' N, longitude 150' 38' bl (Fig. 1). By air, Manley is 145 km west of Fairbanks and 71 km east of the village of Tanana. ate Highway 2, known as the Elliott Highway, connects Planley Hot Spr gs with Eureka, livengood and Fairbanks along a 260 km gravel-surfaced road. From Manley Hot Springs, a road continues 21 km northeast to Tofty, an old placer mining dfstrfct. Manley Hot Springs is also connected by a 5 km road to a barge landing on the Tanana River. The village of Manley Hot Springs is situated on the northern margin of the Tanana .long, shallow waterway which drat into the Tanana River. Elevations in the Manley Hot Springs area range from less than 260 feet for the Tanana Valley floor, to 2650 f t far the summit af Hot Springs Dome "?ocated to the northwest. The ome is the highest part of a narrow, 6, w Y lley along Hot Springs Slough, a 73 km IL, a, -* 43 km-long, northeast-trending ridge known as Bean Ridge, which separates the Tanana Valley from Creeks. 43 cupfed by Patterson and Baker * I The Manley Hot Spring frost. Normal thick brush on the upper lopes, and white spruc rush on the lower slop and scattered lowlands con type vegetation. The c 1 of the Yukon River valley; long, cold winters and short, warm summers with a possible range of temperatures from 70' F below zero to 98' F above zero. The annual W 1 precipitation is 25 to 30 cm, most of which falls as rain through the summer months. elementary school. capacity. The town has an airstrip, post office, store, lodge and Power is supplied by diesel generator with a 40 kw The main hot springs are 0.75 km north of the central part of town, and several occurrences of warm seeps are found within a 0.8 km radius of the main springs. base of east-facing slopes of Bean Ridge near the edge of the Tanana Valley. these slopes between Ohio Creek and the highway road to Tofty, Charles In general, the warm springs and seeps occur near the However, they are localized only along a 1.4 km long portion of and Gladys Dart own the hot springs and the surrounding 236 acres. They utilize the thermal water for space heating of their home and the operation of a 30 by 45 rn green house and a small public bath house. The hot springs also serve as the community's principle water source for drinking, washing, and other uses. The greenhouse is located next to the main springs and d primarily for raisfng tomatoes. The tomatoes are sold locally and have also been shipped into Fairbanks where there is always a ready market. Other hothouse vegetables which are sold locally include cucumbers, ggplants and melons. few wells have been drilled adjacent to the and one of these has warm (29' C) water. However no wells Mater is piped thermal water is mixing to Creek dri 1 1 i ng e of the main ter with htg elp delineate targets for drilling of a W geothermal we1 1. LJ I 2 \ HISTORI CAl BACKGROUND- Mertie (1937) reports that the first non-Indian settlement in the W Y Planley Hot Springs area was a trading post established in about 1881, 77 km upstream from Tanana on the Tanana River. Bean's trading post was near the mouth of, or downstream from, the Hot Springs Slough. In 1898 gold was diicovered on Eureka Creek and shortly afterwards Eureka became a recognized mining community. Baker Creek about 8 km from the hot springs was the initial site of another early placer district, the Tofty mining district. For several years the town of Rampart was the sole supply point for the camps in the Manley area.and winter supplies W V w were hauled over the divide at a rate of 4 cents per pound (Mertie, 1934). In 1902 the land around the hot springs was homesteaded by J. M. Karshner and his wife and the s ere known as Karshner Hot Springs. specter named Frank G. Manley formed v n 1906 an enterprising an partnership of sorts with Kar built a large 60-room hotel and several outer enclosed bathing tanks. They developed the hot springs as a resort and also cleared 60 acres of land and established a dairy, poultry and hog farm, and cons ,. w ed several greenhouses ( Figs . 2-6). were raised on the slopes s, cabbages, surrounding the springs. river to the Iditarod minin re all heated w w er was piped a di hich quickly deve ing camps in the By the early 1910's ho a rapid state of disrepair. Placer mining was on the decline and in the supply point w was falling into 4J 3 springs at Manley Hot Springs, 1910 Karshner Creek fl of photo. Univ slty of Alaska Ar d one hot spring. 11 Collection .. I i I I i t 1 i 1 i I i i I I i I 1 I I C c l c t c c 0- c c I a i rr, April, 1913 the hotel burned to the ground never to be rebuilt. The sketch map of Waring (1917) indicates how the hot springs looked in '9$ Y tion of inves During t fr6m 1300 to 1934. ' V Y f W Figure 7: Recreation of a sketah map made by G. A. Waring in 1915. From L. Leonard, 1974. bodies and Quaternary deposits. w The U.S. Bureau of Mines has investigated mineral occurrences near the summit of Manley Hot Springs Dome (Maloney, 1971). drilled and logged 8 holes which ranged in depth from 243 ft. to 515 ft. In 1953 they v The drill holes are located near the contact between granitic rock and Jurassic-Cretaceous metasediments. Regional geologic studies by Foster and others (1973) include the Manley Hot Springs area as part of the northwestern part of the Yukon- Tanana upland. 1:250,000 geologic map of the Tanana quadrangle. This open-file map incorporated the earlier work done in the Manley Hot Springs area by Hopkins and faber. w In 1975, Chapman and others published a preliminary w Hot Springs Studies Y G. A. Waring, a U.S. Geological Survey geochemist, was the first to conduct a comprehensive geological and geochemical survey of mineral , springs throughout the State of Alaska. His findings are published in a 1917 U.S. Geological Survey water supply paper, and include a short but “f 0 ngs, which he refers ‘cj ring with temperatures W analyzed the water ‘from the western spring (spring Y W 3 i site." On the sketch map (Fig. 7) h the head of Ohio Creek.. This is the same area designated as Site B in hows this spring area as being at U r3 this report. Springs from Site B were not observed by this author to form the head of, or drain into Ohio Creek. Ohio Creek is further to 3 t and does not appea also noted that the springs had no noticeable odor or taste and e water tended to corrode iro o show any surficial thermal activity. essels easily, which he attributed 0 to a high chloride content in the water. The springs were not further studied until the 1970's when the U.S. Geological Survey con in the interior of A1 and others, 1978). Manley Hot Springs yielded 137" C, respective1 IC, w yr, granite-rnetasedimen ished in the near us w hilly and mountain0 River to the north, the Tanana River to the south, and the Alaska-Canada 3 in the eastern an Springs area, constitutes about one-quarter of the entire Yukon-Tanana up1 and LJ border to the east. loess mantles large areas. Rock exposure is poor throughout the region The region is mostly unglaciated, yet Quaternary u J because of loess, a1 1 uvi a1 , col 1 uvi a1 and veget at i ve cover. In general w ay be extensions of the Tintina Fault zone (Foster and others, 1973). According to Chapman and others (1979), the Victoria Creek Fault and parallel faults may be splay faults of the Tintina. blocks may be detached and undergoing variable amounts of right-lateral displacement. Intervening fault A possible splay fault of the Tintina known as the Beaver U ault is located 10 km south st of Manley Hot Springs and hly divergent rock t errain, while to the north lie generally unmetamorphosed Mesozoic sediments. The rocks of the northwestern part of the Yukon- Tanana upland conslst of a seque ce of complexly folded and faulted sedimentary and met asedi ment ary ocks interbedded with a few sequences of volcanic rocks (Hopkins and Taber, 1962). The sedimentary and metasedimentary - rocks range in age from Precamb rocks have bee probable Cretaceous age lutonic rocks ranging South of the fault lies a Paleozoic r3 d or early Paleozoic to middle Tertiary. w a serpentinized ultramafic by a complex sequence of Cretaceous- composition from gabbro to biotite v v b, The 8gBoul der Ridge Formation" and Vut 1 i nana Format i on" form part of ic flysch belt which has a maximum width of 40 km and extends 1 v northeast 240 km from the Tanana River segment between Manley Hot Springs and Tanana, to Victoria Mountain in the northwest corner of the Circle W W quadrangle. Except for the southwestern end of the belt, which has been studied in detail by Hopkins and Taber (1962), the formations are simply grouped as KJs/KJc or KJ by other authors (Chapman and others, 1971; v Chapman and others, 1975). The "Boulder Ridg of Jurassic-Cretaceous age is a sequence of clastic, dominantly marine rocks; chiefly orthoquartzite, slaty siltstone, slate and conglomerate. "eugeosynclinal I@ assemblage of limestones, cherts, volcanics and clastic sedimentary rocks, and is overlain with conformable, possibly interfingering contact try the "Hutlinana Formation". The @180ulder Ridge Formation1@ was fnformally named and described by Hopkins and aber ( 1 962) for its exposures. along the north side of Boulder Ridge and north and south sides of Boulder Creek valley. Beds at the e of the formation include the oldest Mggozoic sedimentary rocks exposed in the area. The top of the "Boulder Ridge Formation" is defined as the stratigraphic level where sedimentary Y It rests unconformably on a Paleozoic r3 Y 0 '9 r than quartzite, kness from several hundred in depth to the south and southwest near the present location of Manley Hot Springs and Dugan Hills. Deeper water sections are believed to have .. * been deposited by turbidity and turbidity-related currents. regional trends in average grain size, the direction of sediment transport was to the south and soutwest. Based on W The "Boulder Ridge Formation1' is probably of early Cretaceous age, W however this age is based on rather scant fossil evidence. The possibility that it is partially or entirely Jurassic cannot be discounted, Rocks of the "Boulder Ridge Formation" have undergone regional and, in part, contact metamorphism. Regional metamorphism has been mild along the margins of the flysch belt, while the central part of the belt, especially the area around the Tofty district, has undergone the most intense metamorphism. Contact metamorphic aureoles of generally high metamorphfc grade and varying widths *are found throughout the area associated with stocks and w Y h the "Hut1 i nana Format ion" i not exposed at or near Manley Hot Springs, it forms the upp part of the Mesozoic flysch belt in the Hutlinana Hot Springs is The IIHutlinana part of the Yukon-Tanana upland. uartzite beds of the "Hutlinana formation1I. Format i on was informal ly named. by Hopkins and Taber ( 1962), and defined hat rests conformably or exposures along the f marine clast upon the t8Boulder Ridge formationta. 1 of Hutlinana C The "Hut1 inana Formation" ly of a monotonous stone, si 1 tston in Hopkins and thick, graded be s ses est 4 mated ters. The "Hutlinana 'I Hut 1 i nan a Format i on Formation" is of probable early Cretaceous age, based on a single Inoceramus w there is still a likelihood that the "Hutlinana Formation' is partially or entirely Jurassic. The "Hutlinana Formation" has undergone mild regional metamorphism nearly everywhere in the study area of Hopkins and Taber. to that shown by rocks of the "Boulder Ridge Formation". Since it does In general the intensity of metamorphism follows a pattern similar 0 not crop out close to intrusive contacts the "Hutlinana Formationt' displays no contact metamorphfc effects. Terti ary-cretaceous Granitic fntrusives V Granltic rocks of the northwest part of the Yukon-Tanana upland have been mainly studied in a reco aissance fashion. They have been mapped by various authors (Hopkins d Taber, 1962; Chapman and others, 1971 ; Ghapman and others, 1975) an briefly described (Chapman and others, 1971), yet little if any work has bee history or other aspects five of the intrusives 60 belt of Cretaceous and Te central Alaska. +& etrology, magmatic -argon ages for Y e part of a large east-west granitic plutons which extend through .. 0 Plutons and stocks o anana upland are elliptical with northeast4 + They often f knobs exposed Springs Dome, Rought Mountain, Elep gs Dome. The quartz diori ted and irregularly fractured, medium- to coarse-grained, with lar, porphyritic, and some fine-grained phases. The surrounding country rock is hornfelsed and highly resistant. Associated smal 1 stocks, 18 dikes and sills are composed of gr ite, aplite, pegmatite, rhyolite, monzonite-latite, minette and some mafic differentiates. U u Age determinations for granitic intrusives in this part of the Yukon- Tanana upland are listed in Table 1. Plutons which have been dated are the Manley Hot Springs Dome, Roughtop Mountain-Eureka Dome, Sawtooth Y Mountain, Tolovana Hot Springs Dome, and a small stock on Troublesome Creek. These five localities yield potassium-argon ages which range (LI, from 90 f 10 may. for Roughtop Mountain-Eureka Dome, to 60 m.y. for Manley Hot Springs Dome (Chapman and others, 1971; Hopkins and Taber, 1962). ci REGIONAL SETT NG OF THE HOT SPRINGS Manley Hot Springs forms part of a regional east-west belt of hot springs all of which appear related to anitic plutons. This belt extends through central Alaska and poss 1Y over to the ChUkotka Peninsula of Siberia. As a group, the h hot 'water type of geothermal system and not to the vapor-dominated type. u /'f u ace temperature errnometers range ns show that the thermal waters are ch association with the hot springs in west-c W are quite diverse. of thermal transfer by either a cooling magma chamber, or anomalous This suggests that the hot springs are not a product 9 ., TABLE 1 w Y AGE DETERMINATIONS FOR GRANITIC PLUTONS IN THE NORTHWESTERN YUKON-TANANA UPLAND I% Dating Rock Area Method Mineral Age Compos i t i on Reference + Y Manley Hot K-Ar Biotite 60 my.* Biotite Hopkins and Springs Dome granite Taber, 1962 Manley Hot .Pb-Alpha Zircon 90 f 10 m.y. Biotite Hopkins and Springs dome granite Taber, 1962 Y ' Roughtop Mtn.- K-Ar Biotite 90 m.y.* Quartz Hopkins and Eureka Dome monzonite Taber, 1962 Hopkins and Roughtop Mtn.- Pb-Alpha Zircon 90 f 10 m.y. Quartz Taber, 1962 Eureka Dome monzonite Tolovana Hot K-Ar Bi ot t te 63 f 2.5 m.y. Quartz Chapman and Springs Oome monzonite, others, 1971 Siwtaoth Mtns. K-Ar Biotite 88.3 f 3 m.y. Quartz . Chapman and Y monzonite monzonite, others, 1971 monton i te Chapman and others, 1971 -.* cd W 20 Y radiogenic heat within the plutons. 4 W The bedrock geology associated with the hot spring sites may have a wide range of lithologies but typically consists of massive, competent, well -fractured and generally non-foliated rock. the plutons and the surrounding country rock, if the hot springs are located outside of the pluton. The competent yet fractured nature of the bedrock may allow for good fracture permeability so that meteoric water circulates deeply. been proposed by Miller and others (1975). This is the range of depth required assuming a normal geothermal gradient of water to attain the reservoir temperatures estimated from their chemistry. The northwestern Yukon-Tanana up1 and contains three hot springs : This applies to both Y Depths of circulation on the order of 2 to 5 km have Y to 50' C/km for W Manley Hot Springs, Hutlina ana Hot Springs. A fourth unconfirmed spring h it is quite possible that o Y tle Minook Creek, and t in this region, but ttle Minook Creek r the spring's exact n and Shirley tiss ve not been reported. orted to Waring (191 w ew over the Mfnook Cr to spot any thermal nd Tolovana Hot Springs exhibit s section will briefly y of these neighboring 39 km northeast of Manley J 1 W Hot Springs, along the 'west side of Hutlinana Creek, latitude 65' 13' N, longitude 149' 59' W. The springs issue from the base of a cliff composed of sheared quartzite and hornfelsic greywacke of the "Hutlinana Formation". Waring (1917) noted that the rock showed #'nearly vertical bedding or shearing planes that strike N 25' E and that these would provide likely avenues for the escape of heated water. Hutlinana Hot Spring is about 5 km east of the Elephant Mountain pluton, ch is chiefly porphyritic monzonite and quartz monzonite (Chapman and others, 1971). been mapped in the Hutlinana Hot Springs area. The temperature of the springs is about 43' C, and it has a discharge rate of about 3 l/s. The springs have a slight smell of H2S and the silica and Na-K-Ca geothermometers give reservoir .temperatures of 92' and 98" C, respectively. .eJ v. Y No faults have u w Tolovana Hot Springs is about 107 k ortheast of Manley Hot Srpings, along a creek draining the east 65' 16' N, longitude 148' 50' W pa$t of the Jurassic-Cretaceous sedimentary sequence which Chapman and f Tolovana Hot Springs Dome, latitude e springs are in mudstones which are Y (1971) refer to as KJs. The mudstones probably correspond in part to either the "Boulder Ridge format 5 on" or "Hut 7 i nana Format from the other two springs in th loride, sodium, calcium it is saline, with higher concentrations sium and perhaps lithium, bromide and 22 boron. have high salinities - Tolovana, Pilgrim, Serpentine and Kwiniuk (Miller and others, 1975). All of the saline springs excepting Tolovana, are located on the Seward Peninsula. The high salinity values of these four springs may be 'due to the increased availability of solutes for leaching within the Of the 12 springs sampled in west-central.Alaska, four springs w . use in the past they were subjected to greater It may be that the springs which show lower sa nity values do so nts of leaching or I for longer periods of time than the saline springs (Miller and others, 1975). Si 1 i ca and Na-K-Ca geothermometers for Tolovana Hot Springs yi.eld respective subsurface temperatures of 122' and 162' C. Y LOCAL GEOLOGY The local bedrock geology of the Manley Hot Sprlngs area consists of Y tilt d folded sediments of the Jurassic4 oulder Ridge Formation't, which have been intruded by granitic stock (Fig. 9). These rocks are par co'rluvium, - , I. and alluvium from creeks and the Tanana River floodplain. Surficial deposits and extensive vegetation limit exposures of the bedrock geology to Y cks of the Manley Hot Springs Dome lly mantled by Quaternary loess, 0 .I 6 OUB Kbq-quartzite rocks''8oulder Ridge Formation" ,A. Foliation - Contact K b p- pelp rocks Boulder 9 Ridgb Formation" Recent Qya- younger alluvium Pleistocene Qs- loess and reworked loess U.Cretoce8rs fkg-Hot Sptinas Dome LTBrtiary stock, biotite, gronlte w KBP is composed of pelitic sedimentary rocks which include slate, slatey siltstone and minor interbedded quartzite. Y 4 In the Bean Ridge area the H60ulder Ridge Formationll is approximately 1,070 meters thick. Though exposures are poor, the "FormationB8 probably consqsts of a basal 150 to 300 meter section of thin-bedded quartzite with rare interbeds of chert pebbl conglomerate. The basal section shows graded bottom marking indicative of deposition by turbidity current grades upward into a laminated sequence several 10's of meters thick of fine- 'w . u grained quartzite, quartzose siltstone and slate. The upper part of the "Boulder Ridge Formation" in the Bean Ridge area consists principally of slate and greywacke siltstone with a few thin beds of quartzite. top of the HFormationt* there are lenses, concretions, and thin beds of calcareous siltstone. Near the w Lrf In general, the thin-beded quartzite and slate sequence ranges in color from medium to dark grey, dark g in color with increasing proportions of Si 1 tstone is general ly ium grey or pale olive. Quartzite and . Y conglomerate beds are very high in chert, with a chert:quartt ratio as posed of quartz, chert, feldspa ains of pyrite. Siltstone and slate and sericite. he "Boulder Ridg the silty textured planes have been developed in many slate and siltstone beds along the south 25 ~~ ~ ~~ ~ W side of Bean Ridge, so that pelitic rocks commonly disintegrate into rod and penci 1 -shaped fragments. Beds of quartzose greywacke and conglomerate rich in non-siliceous pebbles also show a poorly developed cleavage resulting development of s 1 i p planes along which micaceous minerals have ligned. Quartzite a d chert -pebble conglomerate beds have been esistant to deformation and rarely show cleavage. Irr' In these beds tz grains may b fractured, strained, and elongated by deformation . w In some of the more highly deformed beds there is evidence of chemical redistribution of the original constituents. This includes pyrite porphyro- blasts in pelitic beds, overgrowths on clastic tourmaline grains, and abundant quartz veining associated wih orthoquartzi te beds. v Contact metamorphism in the Bean Ridge area has resulted in the conver- sion of original slaty rocks to "knotted" slate. Closer to the granitic margins slates have been converted to hornfels, granulite or schist. Sandy-textured beds have been recrystallized, with increasing intensity as i ~ e body is approached. On the southern flank of Manley Hot , contact metamorphosed slatey rocks are characterized by the Y knotted' slates. The "knots" are less than 1 cm in diameter and I on the cleavage planes. In thin section these knots appear us clots or spherules of fine sericite (Mertie, 1932; Hopkins ey Hat Springs Dome, the knots w sericite, hematite, and tourmaline. Quartzite beds close to granitic " marains show recrvstallization of the oriqinal rounded quartz qrains into . 26 Y coarser, granoblastic quartz aggregates and conversion of original micaceous minerals to biotite. Within several hundred feet of the margins of Hot Springs Dome, rocks of the "Boulder Ridge Formation" are intimately injected with hydrothermal veinlets composed of quartz, biotite, tourmaline and Y W minor sulfides. res are essentially nonexistent at Manley Hot Springs. Quaternary loess and vegetation covers most of the surrounding slopes and hi 1 1 s. Boulders present in Karshner Creek are primari ly hornfel sed quartti te and biotite granite. Several hundred meters upstream, near the foot of the Y . ide of Karshner Creek valley, several small weathered outcrops of W hornfelsed "Boulder Ridge Forma on" can. be observed. Approximately 500-800 meters upstream from Manley Hot prings the pluton-country rock contact is crossed. In general, rocks e "Boulder Ridge Formation" upstream of Manley Hot Springs are predomi nant 1 siltstone. Rocks collected within 40 meters of the Hot Springs Dome stock were medium to dark grey, tbin-bedded quartzites with well-defined bedding planes'about 1 cm in thickness. The quartz grains appeared to be recrystallized and there were 1-2% medium sited grains composed of hematite or limonite. g and may be sites'of earlier uartzite with lesser slate and W e. u w W d, light colored biotite granite. Y Some small areas of tourmaline granite in or near the border zone were also 1 u- noted. Leucocratic dikes are common and may be hydrothermally altered. The dikes are composed of rhyolite, aplite, alaskite and rare pegmatite. The Manley Hot Springs Dome has K-Ar age of 60 m.y. measured on biotite and a Pb-alpha age of 90 f 10 m.y. measured on zircon (Hopkins and Taber, r3 1962). Small metal-bearing lode deposits, located near the summit of Hot Y Springs Dome, have been known to exist since the 1890's. Mineralization is within metasediments along the northern margin of the stock and is associated with shear zones up to 15 meters wide. Drilling was conducted near the summit by the U.S. Bureau of Mines in 1953. Approximately 975 meters of drill hole was completed in 8 hobs with depths from 74 to 157 meters. W lhe gic sections for these 8 holes are published in a Bureau of Mines open- report (Maloney, 1971). It noted by Maloney that the shear zones w near the northern contact of the stock are parallel to the pluton's margin. also noted that none of the drill holes intersected primary sulfides I with the exception of one smal 1 otected pocket. Oxidation was practical1 +! 0 ce, which was the greatest depth reached. e is good permeability and fracturing in the of at least 136 meters. 62), the Manley Hot Springs Dome is ts are associated wit eved to be shallow, of 30 to 40 degrees. The pluton 28 may have been emplaced at fairly shallow levels in the crust, based upon the abundance of fine-grained associated dikes. The Roughtop Mountain CI d complex to the north may be in part a ring dike complex which also suggests hypabyssal intrusion (D. Hopkins, pers. corn., 1981). much of the Manley Hot , Springs area is mantled by luvium, colluvium and loess. The loess forms deposlts of 5 to 30 meters or more along low hills and slopes. massive, homogeneous eolian silt, buff to tannish grey when dry and brown when wet. The period of loess deposition was approximately 25,000 to 10,000 years ago (D. Hopkins, per$. corn., 1981). is found in the lower part of Karshner valley at the hot springs. Creek has cut through at least 10 meters of loess, yet it is not certain whether the creek is close to bedrock, or is resting on several meters of The loess is W Loess of an undetermined thickness Karshner 0 loess. The loess-metasediment contact is not exposed in Karshner Valley. r _. w The floor of Karshner Val ley is covered by vegetation and a1 1 uvium. The alluvium is composed dominantly of sand and gravel, but ranges in size complexly folded and faulted. of Bean Ridge strike east-kest or northeast, subparallel to the trend of the ridge. Faults on the vertical displacement (Ho Few faults have been Springs dome. It may sim of outcrop. However, the that a deep-seated fault system may be supplying the plumbing for the thermal water at Manley. An alternate possibility is that well-developed fracture permeability in the bedrock may be responsible for the springs. Faults along the crest and the northern side e are characteri zed by predomi nantly Taber , 1 962). V the' southern f 1 ank of Manley Hot hat faults were undetected due to the lack of the hot springs in this area suggests w Using aerial photographs, Hopkins and Taber (1 962) mapped several 1 inear w within granitic rock upstream of the hot springs in the Karshner Creek drainage. These linears strike approximately N 50" E. An arcuate fault shown upstream from the hot springs in Figure 9 was also mapped by Hopkins and Taber on the basis of aerial photographs. Fairlt, located 10 km to the southeast, may be a splay of the Tintina Fault. Therefore the possibility of fault activity in the Manley Hot Springs area w The Beaver Creek t ',, y1 cannot be discounted.' the Beaver Creek Fault appears to die out to the southwest underneath south of the Tanana Valley. Rocks are not faulted out Hot Springs taking aeri a1 photographs an ples. Evaluation of this report has be ngoing through the fall and , Y spring semesters of 1981-1982. The final report presented as a University of Alaska Master's Thesis is forecast for completion in December, 1982. ' . The thermistor, resistivity instruments and helium equipment were U 4 loaned from the Geophysical Institute, University of Alaska, Fairbanks. The Alaska Division of Geological and Geophysical Surveys (DGGS) provided water U sampling equipment. Helium and water analyses were done respectively through Western Systems Lab, Inc. and the Alaska DGGS. Mercury soil simple analysis was done by the author during the fall of 1981, using a Jerome Instruments mercury detector provided by the Geophysical Institute. Y GRID SYSTEM Y A grid system was surveyed early in the field season with northsouth and east west coordinates and a spacing interval of 15 m (figs. 11-12). The north-south and east-west base lines intersect 15 m northwest of the greenhouse. The grid was surveyed with a tape and Brunton compass. v The network was to cover t thermally disturbed ground at the main ngs (Site A), which was app ent in aerial photos taken earlier that ng. It was also intended that the grid extend out from the main springs 0 far enough to reach 'b During the surveying, it was decided that the grid southwest (Site B) and th-south baseline of 330 m, and contains ple points. As grid. The grid also provided springs, seeps, creeks and other geographic features. Y '6, c NLEY HOT SPRINGS NG LOCATION OF GRID RROUNDING SPRINGS AND WELLS w . Approximate rcole 1:6000 .I. .. .. Site A contains 11 measured hot springs and seeps which range in w Id temperature from 60.7. C to 16.3' C. They lie at a mean elevation of 94.5 m, along the walls and floor of Karshner Creek valley. Springs A,l-3 constitute the main springs, possessing higher temperatures and rates of flow than any other springs at Manley. These three springs are utilized presently for spaceheating, irrigation and other purposes. The water chemistry of springs A,1-3 has been analyzed several authors (Wari ng, 191 7; Mi 1 ler and others, 1975; Mariner and others, 1978). The three main springs and several smaller ones (A,1-7) issue from a knoll or terrace along the north wall of Karshner Valley. Their points of issuance have a spread in elevation of about 7 meters, from the lowest to the highest spring. The J Y 3 rs to be composed of unconsolidated sandy and silty gravel alluvium, and may represent an older alluvial terrace of Karshner Creek. In March, 1981, Moorman and Liss of the DGGS measured a total flow rate of approximately 1418 l/min for springs A,1-7. w .' *. i ., Other seeps of Site A, springs A,9-11, are located on the south side rshner Creek downslope from the maln spring area. They issue from ase of a low knoll. and from the valley floor. Y Springs A,8-11 are terized by low rates of flow, probably less then 5-10 l/min. A11 W prings and seeps from site A drain to Karshner Creek, and increase ek considerably. There is a proliferation of arshner Creek after reous sinter up the springs of depression- on the side of a hi1 1 , V approximately 200 meters west of Site A. The site is at a mean elevation t 113 m. Nine seeps were measured at Site 8, with a range of 37 temperatures from 32.0. C to 17.7. C. The seeps form a northeast-trending belt along the lower half of an open and marshy meadow. seeps at Site B are characterized by very low rates of flow, less than 5 l/min. Site B may be an example of a eutrophic spring, consisting of a single point of issuance for the thermal water {Mary Moorman, pers. comm.., 1981). The spring however, becom through an upper organic-rich soil horizon. This leads to the surface expression of several cooler seeps, rather than a single hot spring. There is a conspicuous absence of trees in the area surrounding Site 8 which might be explained by poor drainage. The seeps at Site 6 flow downslope and coalesce into a single drainage which enters the Hot Springs Slough just west of the bridge. There are bluegreen algae and small amounts of calcareous sinter present near the mouths of seeps A,5-8. The individual v d. Y cooled and diffused as it filters Y Y ud Site C consists of several small groups of seeps and springs which are southeast of the main springs and distributed along the edge of the main (Thana) e. valley. Site C is the lowest of the four sites, with a mean ., Y elevation of of 85 m. Nine springs and seeps were measured, with seeps of site C flow southwest along the edge of the main valley and eventually drain into the lower portion of Karshner Creek. Minor amounts W U 38 , w of blue-green algae and calcareous sinter are associated with them. U 4 Site D is about 0.8 north of the main springs. The site is at a mean elevation of approximately 152 m, making it the highest of the four sites. Two seeps were measured, with temperatures of 25.4. C and 21.7. C. U The seeps are located near the heads of two adjacent valleys and form the first signs of running water along the creek floors. The valleys are narrow and steep-walled. The surrounding hills consist of loess which is covered us by trees and lush vegetation. Although there is a thick vegetative cover the loess appears to be undergoing extensive erosion expressed by steep, irregular topography suggestive of dl ands topography. Seeps D,1-2 have low rates of flow, less than eventually mixing with co valley west of the road t calcareous sinter . Wells * 'There are six water wells present within the eastern part of the study area. One or two more wells may be present whfch the author was unable to v The thermal water flows downstream, r surface runoff water, which drains into the ofty. Seeps at Site 0 show minor amounts of L 0 - r 0 are plotted on Fig. 11 and a summary of well 199 The table and text notation for wells is as letter symbols are followed ells were located was lowered down insi ~ in the container was 3 All of the wells are under private ownership, except for the highway W maintenance yard well. Of the six wells in the study area all but one 39 . are cold water wells. CW-5 is capped so little information was obtained on it, yet it is probably a cold well. currently in use. by the Highway Department. V 1 Of the cold water wells two are Well CW-3 is for residential use and well CW-4 is used Y The warm water well HW-1 is about 0.6 km northeast of the main hot springs, located on the side of a hill, The temperature measured was 29.1' C, however this is the temperature of standing water about 0.5 m below the water surface. The well was drilled several years ago and is not in use. A water sample was taken of HW-1 for analysis of major chemical constituents and oxygen isotopes. Karshner Creek 'cs W Karshner Creek is one of several creeks which drains the southern side of Bean Ridge and is the only c drains an area of approximately months. Ouring the summer the 2-5' C, measured upstream from the$'lot springs. probably the result of snow and ice melt. Water was collected and analyzed nt in the study area. The creek nd is fast-moving in the summer erature of Karshner Creek is approximately w Much of the creek water is u 3 three other sites (A,l; A,2; B,l; C,5; D,l). In addition, waters were analyzed from the warm well (HW-1) and from Karshner Creek (CS-1) upstream v of the hot spring area. All of the samples were analyzed by Mary Moorman the Alaska OGGS. Oxygen isotope samples were also collected at each of the above localiti as well as from rainwater. However, these data are not yet available at the time of this report's publication. v A list of the major chemic onstituents is given in Table 4. In addition, a cold water well sample (CW-4) was analyzed by Northern Testing Laboratories, Fairbanks, Alaska, for the community of Manley Hot Springs. Some of the data from this analysis are also llsted, although it is by no means complete. A ternperature-calibrated pH could not be obtained from HW-1 as the well is not presently being pumped. In addition, the silica value from this well is anoma water from Karshner Creek. S could be that the silica has ree completely explain the very low Si02 value W v sly low, even when compared with the is standing in the well, it , however this still does not W It would be worthwhile to is well and resample after a w is obtained. v Several geothermometers were calculated for the water, using the sults are listed 5n Table 6. The equations listed in Table 5. able reservoir temperature estimates e main spring samples (A,1 and and others, 1978, .' 1 chemistry by constraints. However, , water chemistry i nte oxygen isotopes in a later thesis report. oming along with a study of the Water I W TABLE 3 We1 1s and Miscell aneous Samples from Manley Hot Sprlngs Type of Analysis Water Temp . Sample ('C) Comments HW-1 29.1 Warm well Water, Isotope W Frank Shelton, owner Water level 53 ft below surface Approximate?y 100 ft deep cs-1 . 1.5 Karshner Creek Water, Isotope f hot springs RW-1 U Rainwater Isotope cw-1 12 .o Greg Neubauer, owner cw-2 9.0 Approx . temp. U Water level 36.5 ft below surface Robert Miller, Owner Water level 37 ft below surface Robert Mi 1 1 er , owner Well presently used Y I 15.0 Approx . temp. cw -3 Y TABLE 4 w d Chemical Analysis of Water Samp A, 1 A, 2 891 c, 5 091 W Temp ('C) 59.5 58.7 32.0 33.1 25.4 es Hw-1 cs-1 cw-4 Y GROUND TEMPERATURE SURVEYS Two ground temperature surveys were conducted at Manley Hot Springs u" . (Figures 13 and 14). An earlier temperature survey was done over the establ ished grid system and concentrated on thermal anomal ies around Site A and to a lesser extent Sites 8 and C (Fig. 13). thermally disturbed ground extended outside of the grid, a 1 ater temperature survey was conducted which covered a larger area including Site 0 and established ttbackground't or non-thermally disturbed ground (Fig. 14). Because of time constraints the temperature surveys were shallow, utilizing a probe which measured the ground temperature at a depth of 0.50 meters below the surface. As shown in Figs. 13 and 14, it appears that shallow level geothermal activity can be adequately delineated at this measurement depth. The temperature probe was designed by Tom Osterkamp of the Geophysical Institute, University of Alaska, Fairbanks, and constructed at the Institute's machine shop. Several modifications were later made to the original probe by Cy Hetherington and Joe Redington, Jr. of Manley dot Springs. The probe consists of a hollow length of steel which is pounded into the ground. thermistor is lowered down insi e the probe and the probe is then filled with water to allow for better heat he probe and the thermistor. It takes 2 to 3 minutes for the temperature to Y Since the area of Y Y Y A * Y The temperature ading is taken fro read to an accuracy m was not significantly It was affected by the time of day at which the temperature was taken. noted however that as the summer progressed, the ground at a depth of E-W thermal ground Approximate scote 1:6000 kiEi3; r::rs, 0.5 m gradually warmed due to the accumulation and storage of solar heat. Y Two control points measured at the end of 12 days showed an average increase in temperature of approximately 8.0%. Another point measured after 38 days showed an Sncrease in temperature of 17.0%. To minimize the effects of long-term solar acc out over the shortest possible in 6 days, while the reconnaissance survey was carried out in 8 days. v Y Temperature measurements over the grid area were taken at regularly spaced intervals of 15 m. The temperatures were then plotted and hand- contoured, using a contou (Figure 13). Areas of the grid which have a samplin dashed isotherms to indic Y is approximate d as areas enclosed Figure 13. The largest and 0 hottest anomaly, referre loses the main springs and the area of spring Site two small lobes with orthwest of the greenhouse . 5% located just north and s near the base of the knoll, located upslope on the top of the A forms a rough rectangle which is and minor axes The southeast end of the an w its' upper, northwest end lies along the knoll and part of the northeast of the valley. Another anomaly defined by the 20'C isotherm is present near spring site B and is referred to as A omaly 8. several of the seeps. Anomal cated east of seep B,l. Th is oriented parallel to the s The anomaly is centered around , 8 has a maximum ground temperature of 26.5'C Y hill. Three anomalies exist on the eastern side of the grid at lower elevations in the general area of spring site C. anomalies referred to as anomalies C1 and C2, are each less than 25 m2 in area. They are centered on seeps.& along the margin of the main valley. north side of Karshner valley s is approximately 105 ins .3 areas where the in excess 'of 30'C. Anomaly X differs from the other shall anomalies within the grid in that it is not closely associated with surface thermal water. Afte {on of road which intersects The two northernmost Y to the main valley. The anomaly melt (Charles Dart, Pers. and largest e thermal waters of spring sites A, B and C. Anomaly X lies along and runs parallel to Karshner Creek valley. The anomaly may be associated with W very shallow level hydrothermal activity. Reconnaissance Ground Temperature Survey W d The ground temperature survey done on the grid system made it possible ately del i neate local, higher temperature anomal i es. However, the U not extend into what was considered to be true "background" temperature (thermally undisturbed ground) for the Manley Hot Springs ies A, 6, C1, C2 and X appear to be superimposed on a low- c, level thermal anomaly which covers a major portion of the grid and extends well beyond its boundaries. Geothermal areas outside of the grid such as warm seeps of site D and the warm well HW-1 are also part of the geothermal w system present at Manley Hot Springs. boundaries of the low-level thermal anomaly and to tie in geothermal areas outside of the grid, a reconnaissance temperature survey was conducted. The locations of temperature measurements were plotted on ELM 1:6,000 aeri a1 photos. In order to determine the actual W W It was concluded from the reconnaissance temperature survey that the temperature for local "background" at a depth of 0.50 m is generally less w is a wide valley where temperatures drop off gradually and vegetation changes to black spruce and other cool soil plants, also scattered permafrost just west of the Tofty Road. West of 'the regional temperature anomaly, along the base of the slopes of Bean Ridge, temperatures drop off gradually. In a westerly direction temperatures are consistently less than or equal to about 10.0'C for 0.7 km. The extent of the northern boundary is not well defined, and is shown as a questioned line in places in Figure 14. The northern boundary is shown as upslope of an old fire break and is located within dense forest. Accurate mapping on aerial photos was difficult, and the northern boundary may be mislocated by as In this area there is * I U Y Y. anomaly shown in Figure 14 may be helpful in indicating a higher probability of geothermal potential, This map is only based on shallow temperature measurements however, and should not be construed as definitively indicative of areas which do or do not have r3 eothemal potenti a1 . 0 MERCURY SOIL SURVEY ome highly mobile soil sample was collected at each of Each of the samples the grid points - a total of 287 samples (Figure 15). was collected below the organic horizon at a standard depth of 10 to 15 cm. 52 .- re, U Samples were dried ?fn the shade, sifted through a 80 mesh screen, and then clr stored in'glass vials. Analysis was done from October to December, 1981 on a Jerome Instruments Model 301 gold film mercury detector, with a detection level of about 1 part per billion (ppb). A low-temperature method of analysis was done on the Model 301 mercury detector, in which v e heated to temperatures of 290'C rather than the high-temperature volving heating to 800'C. The low temperature analysis drives off less mercury so that mercury values are about 40.60% lower than those w using the higher temperature analysis. This method of reduced soil heating ive, not absolute values for mercury and is often perferred because of improved reproducibility and ease of analysis. were run in duplicate in order to insure reproducibility of results. Values for duplicate samp s were generally within f 048% of each other. Results of the mercury an ysis were plotted and handdcontoured, using a contour interval of 10 ppb mercury (Figure 15). Samples of 0.25 g v The majority of samples had values of about 5 to 15 ppb. Background mercury values for the grid are about 4 to 9 ppb. However it should be kept re relative and not absolute values. ury contents ranging from 107 to 6,300 ppb, it is believed that these are not the eothermal activit ct of soil contamination. site of collecti alue was coll nery close t two large areas e losed by the 10 ppb contour. One of the areas is along the northern side hill north of the valley. The other anomaly is U-shaped and covers part of the southern side of Karshner valley and a portion of the hillside south of the valley. d Cy Values on the order of 20-60 ppb are probably associated with geothermal activity. The exception to this might be the two anomalous areas on the south-central edge of the grid. They show values of 52 and 72 ppb and are located on a wooded hillside which is not close to any hydrothermal area. The other 20-60 ppb anomalies are near or within hot spring sites. Several of the apparently geothermal -re1 ated mercury anomalies are 1-5 m upslope of spring or-seep sites. A 14 ppb anomaly is located just upslope and west of springs of sfte 8. A 32 ppb anomaly is upslope of springs A,lO-11. A 24 ppb mercury value is upslope of springs A,l and A,3-4, and a 27 ppb value is above springs C,l-3. This may be due to higher temperatures allowing mercury to be driven off. There are o large mercury anomal les in soi 1 associated with the main hot springs. This may be due to htgher temperatures allowing mercury to be driven off. There is a 77 ppb anomaly just east of the greenhouse whlch is located above the knoll on the north wall of w w Y v ' 0, , east -west trending anomaly art of Karshner valley and ower or higher than average. ~~~ ~~ ~~~ W Y I EM31 SHALLOW-LEVEL RESISTIVITY SURVEY In order to determine lateral and vertical variations of ground resistivity at Manley Hot Springs, a shallow resistivity survey was run over the grid system. The Geonics EM31 instrument consists of coplanar transmitter and receiver coils located at opposite ends of a 3 me The transmitter coil induces circular eddy current loops in the earth, such that the magnitude of any one of the current loops is directly pro- portional to the terrain conductivity in the vicinity of that loop. A magnetic field is generated by each of the current loops which is propor- tional to the value of the current flowing within that loop, The receiver coil intercepts a part of th voltage which is also linear to the terrain conductivity. Assuming the earth is uniform, the instrum However, if the earth is layered values, the instrument reads an i accuracy of f 5% at 20 millimhos per meter. U u Y agnetic field and results in an output 0 eads the actual terrain conductivity. layers of different conductivity ediate value. The EM31 has a recorded 0 In general, the EM31 aiquires . most of its conductivity res nse from shallow ground levels wit e from deeper levels. for example, the ground below 6 m s about 28% of the total conductivity response, while the ground 4th the ground so 1 meter from s per meter which to resistivity units of ohm-meters, The transmitter was kept oriented towards the east. Y It was unnecessary to apply drift corrections to the w 56 II) data, as base station readings throughout the period of the survey showed re1 atively 1 ittle change. Meter readings tended to fluctuate wildly where stations were located next to metallic pipes. At these locations the data I w ither disregarded or the instrument was moved far enough so that readings were not affected. U The results of the EM31 survey are shown in Figure 16. The data are 6 given in units of ohm-meters and are hand-contoured using a contour interval of 20 ohm-meters. 14 ohmmeters. 100 ohm-meters, while anomalous values have been designated as about 60 ohm- meters or less. The 60 ohm-meter contour encloses two large areas. One area is centered over the knoll, including the greenhouse and some of the main springs. 7his anomaly responds quite osely with the floor of Karshner valley from 20 meters northwest of the greenhouse to about 25 meters southeast of the greenhouse Another anomalous area enclosed partially by the 60 ohm-meter ontour is located on the north edge of the . main-valley with a into Karshner valley. There is Values range from a high of 500 ohm-meters to a low of In general, background is considered to be greater than 80- Y w s of site 6. spond closely with surface or near- ng waters accounts for the low which is slow-moving, murky water, es and dissolved constituents, ear or over water of Karshner Creek before thermal mixing has taken place is not anomalously Based on exposures along the walls of Karshner valley and steep slopes bordering the main valley, it is believed that the loess forms continuous deposits at least 10-12 meters thick on the hillsides surrounding the hot springs. Therefore, on the hillsides the depth to bedrock is at least 10- 12 meters. Assuming the EM31 is seeing a homogeneous earth on the hillsides, the resistivity of the loess iqbelieved to be about 200 ohm-meters. Upstream from t by thermal water, the resistivity is similar to that of the loess-covered hills. v hot spring area, where readings are not strongly affected u EM16R (RAOIONM) RESISTIVITY SURVEY An attempt was made to'measure variations in resistivity at deeper v levels than those obtained by the EM31 survey. The Geonics EM16R was the instrument used r this purpose. To determine the electrical resistivity of the ground, the horizontal elec distant VtF (very 1 resistivity there is a phase angle of 45 between the electric aid magnetic field components, a true terrain resistivity. The effective depth of io and the phase angle between w Ids of the wave propagated by tters. If the earth is of uniform 16 the electrical resistivity itself tbe transmitting station. The penetration. If ation depth at a frequency of 1 layers of differ t resistivities the EM16R is deeper than the n intermediate resistivity e 45'. In a two-layer case, if value and the ph the resistivity of either layer or the th 'cui W then from the apparent resistivity and the phase angle the other two parameters, e.g. the thickness of layer 1 and the resistivity values of the other layers can be found by comparison with a series of two-layer . model curves or by calculations. Calculations for three or more layers are quite complex. v The EM16R makes electrical contact with the ground by means of 2 probes spaced 10 meters apart. The probes are aligned with the direction of the VtF transmitter. The transmitter used for this study is based in Seattle, Y with a transmitted frequency of 18.6 kHz. Phase angle and resistivity readings were obtained by locating an audio nullpoint. Stations which Y were covered by the EM16R survey are shown in Figure 17. A total of 61 stations were measured covering segments of 5 east-west grid lines. Profiles for these lines are shown n Figure 18A-F. line, the station locations and the variations in phase angle and resistivity. Phase angle is read from the right side of the graph, and resistivity values from the left. The phase angle readings are indicated by a dashed line. The horizontal dotted line indicates the 45' phase angle of the homogeneous, one-layered case. Each profile gives the grid v Y Cooking at the profiles, apparent resistivity 50 ohm-meters. The .45'. At some stations phase omogeneaus ground. ine is located on n the knoll, which area of high thermal 3 rJ Figure 18A-F. EM16R re&ivity profiles aloq segtents of east-st lines. &--a& resistivity (pa) values are read fra the left of grqh, phase angle ($1 values are read fran right. =Pa "9, 3 -- A. Y 200 Pa 100 U disturbance associated with high temperatures and flow activity of the main springs, indicating that hot water fills the loess and aluvium to 20-25 m depth. .id Most of the stations show phase angles greater than 45' and low apparent resistivities. must be assumed that p2 is less than PI. The layer 1 resistivity was calculated to be greater than or equal to about 100 ohm-meters, Using this value, the resistivity of layer 2 is small, averaging about 10-40 ohm- meters, and the thickness of ayer 1 ranges from 1-10 meters. the EM-16R results agree with he EM31 data which has a shallower depth of penetration. For several othe a pi of 10-30 ohm-meters. Cor ohm-meters at depth of 0.5-15 met s. of near surface geology, The EM16R resistivities certainly are affected by thermal disturbance in the area. the instrument is seeing 3 or more lay s. In order to obtain a solution using a two-layer case, it 13 Y In general b tations a solution was obtained by assuming ponding ~2 values ranged from 25-90 These values may not agree with projections W It nay also be that at many of the stations Due to these complications, M16R data are insufficient o completely determine the depth of alluvium, the depth to bedrock'in Karshne ley, and the depth to be hillsides. They do however sho as of probable hot water more rn in depth. / anomalously hi g as a by-product of the radioactive decay of uranium and thorium. These elements are present -in minor amount in most rocks and can be enriched W P in granitic intrusive rocks. The solubility of helium in water increases with temperatures above 30°C, so that thermal water coming from depth may act as an efficient helium scavenger (Mazor, 1972). As the water reaches a near-surface reservoir, it undergoes cooling and a drop in pressure. Both of these conditions effectively release the helium which may then rise towards the surfac -i Y 0 There are several ds for helium sampling. Samples of gas can either be extracted from the soil by a driven tube, or by canning soil samples. The gas within the thermal water can also be collected and 0 analysed for helium. At spri A,2 one helium water sample was taken. The helium survey at Manley Hot Springs primarily consisted of canning soil samples. The soil was collected from 0.6 m depth with a soil auger and then canned'as quickly as possible to minimize the loss of helium. Cans were shipped to Western Systems, Inc. in Colorado for mass spectrometric He analysis, with results (ppm) helium with a precision of 10 par 0 0 13 I Figure 17 shows the g d stations where hellurn soil samples were collected and the corre ium in ppm. A total of 33 n along eastowest grid lines. aturf! values t Springs range fr onsidered anomalous e 90s. The station is rings A,lO-11.- The neighboring The soil was collected The main spring area sample was also anomalous, with a value of 8.0 ppm. from gravelly and sandy alluvium of Karshner Creek. 4J 64 Fc I. .. .. I shows maximum values of 6.0 and 6.2 ppm, and a water sample collected at spring A,2 had a value of 31.9 ppm helium. Water samples generally run .crJ! igher than si1 values, but 31.9 ppm is anomalously high (Turner and Wescott, 1982). The other high helium value was taken in soil near spring C,2 and contains 9.7 ppm. W The highest soil reading of 9. pm helium was taken fro a shallow temperature of 21.5.C, while the two high values near the main springs had ground temperatures of 30.0. and 42.5.C. Since helium is an indicator of thermal water which has ascended from depth, these localities may represent areas where fracturing or faulting has allowed the deeply circulating water to ascend to the near surface. As such, these areas might be likely targets for geothermal drilling. Y Y 0 SUMMA The Manley Hot Springs are s located in the northwestern part of the Yukon-Tanana upland. The hot s ngs are situated within hornfelsed sediments adjacent to a granitic pluton, a setting similar to that of many (0 * Sprl ngs the bedrock Manley Hot Springs area. The nearest major fault is It may be a splay the Beaver Creek Fault located 10 km to the southeast. fault of the Tintina Fault, which has documented movement in Recent times. Based on aerial photos, Hopkins and Taber (1962) have mapped several linear trends upslope of Manley Hot Springs. These linears strike about NSOE, transverse to slope dip, and are in granitic rocks. d W 32 hot springs and seeps were measured during the period of this study, with temperatures ranging from 60.7OC to 16.3"C. springs are located on Karshner Creek and are utilized by the Darts for space heating, irrigation and operation of a bath house. All of the hot springs and seeps are located near the base of south-facing slopes of Bean Ridge, between Ohio Creek and the road to Tofty. They are all on land owned by the Darts. The 32 springs are divided into 4 sites. The sites vary in elevation from about 85 m to 150 m above sea level. Most have associated small deposits of travertine and moderate to extensive growth of blue-green algae. to issue from loess. The hottest of these w 0 In general, spring sites are high1 The hottest group of springs issue from a knoll. The is composed of gravel alluvium and may represent a stream terrace of e east several wells have been drilled, including rature of 29.1. C (84.4' F). No 1 ow-temperatur el ineate li kel re survey were located ain springs and had a hallow (0.5 m) ground vity, and helium soil gas. Water from several springs and seeps, as well as from the warm well and Karshner u Creek were collected and analyzed. The results of the water chemistry and oxygen isotopes will be dealt with more extensively in a forthcoming thesis report. The results of the shallow ground temperature survey on the grid define several anomalies enclosed by the 20'C isotherm. These temperature anomalies are coincident with seeps and springs except for one anomaly, yd U referred to as Anomaly X. Although not associated with surface water, near-surface thermal Anomaly X is probably due to water. The reconnaissance w temperature survey defined an area of thermally disturbed shallow ground about 1.2 km long and 0.8 kin wide, which occurred along the base of south slopes of Bean Ridge. Results of the mercury survey showed several anomalies defined as areas enclosed b the 20 ppb contour, which are upslope of Y several of the springs and seeps, as well as temperature Anomaly X. Resistivity surveys were of two types. The EM31 survey measured clr, shal low ground resistivity with an effective depth of penetration of about 6 meters. Results of the EM31 delineated areas of low resistivity which appear to be closely associated with the partially saline thermal water. The EM31 shallow resistivity survey also yielded a resistivity value for Assuming this value, it follows that 0 present under allu e substantiated b The EM16R resistivity survey ve results which general , ved in the geolog 1 ow resist i vi ty vey del i neated ent in canned soil gas samples. um-enriched gas is migrating W 68 to the surf ace above subsurf ace hot water reservoir( 5). The Manley Hot Springs Dome stock is characteristically massive and well-jointed. Springs Dome discovered almost complete oxfdation of rock to a depth of 136 meters which was the deepest hole drilled. This suggests that the fracture permeability of the granite allows for migration of ground water Drilling done by the Bureau of Mines near the summit of Hot W to substantial depth from the dome summit, and quite possibly, the slopes of the dome. Dome stock as dipping moderately to gently underneath the "Boulder Ridge Formation" in the Manley Hot Springs area. metasediment contact with the surface is approximately 0.6-0.8 kin upslope of the hot springs. thin-bedded quartzite and "knotted' slates overlie granitic rocks in the Manley Hot Springs area. within these rocks, or water thin-bedded quartzite. Hopkins and Taber (1962) show the margin of the Hot Springs 0 The intersectlon of the granite- W Hornfelsed sediments which include recrystallized, Contact metamorphism may Rave increased fracturing be' migrating along bedding planes in w. 0 All of the springs and seeps appear to be issuing from surficial deposits of etther es loess. The loess is omposed of massive ures are prese ver it is not believed widens near the mai and several hot springs flow near the base of one of the knolls. springs and seeps appear to issue from Other s or at the base of loess cliffs. 69 w In general, the springs and the shallow, thermally disturbed ground are distributed over a 1.2 km long, northeast trending belt. Variations in elevation of the springs suggest that they may be structurally and not topographically control led. d, W CONCLUSION above evidence, a model is proposed for the low temperature geothermal system present at Manley Hot Springs. southeast slopes of Bean Ridge enters joints and fractures in granitic rocks of the Manley Hot Springs Dome stock. The water migrates deeply enough in the granite to be heated by a normal geothermal gradient of 30-50'C/km. Given a reservoir temperature of 137'C, derived from the cation Ground water along the 0 (19 . geothermometers, this would imply migration to depths of about 2.5- As water is heated, it circulates towards the surface, eventually Y along bedding planes or fractur hornfelsed "Boulder Ridge Formation" metasedimentary rocks. The overlying loess apparently acts as a caprock, allowing the hot ater to mwate along the 1oess-metasediment Interface. 49 I Areas of fracturing in the loess allow for final escape of e surface, expressed as hot springs and seeps of sites apparently involves ay be the case sures are poor, so that if faulting does exist it is well-hidden. Future analysis of the water chemistry will aid in interpretation of ulc’ sub-surface water-rock reactions, as well as the extent of mixing of thermal water wlth ground water. A seismic survey would aid in delineating the depth to basement in Karshner valley, as well as possible faulting. More extensive helium surveys‘could be useful in defining areas of hot water source migratfon and detection of the fault or fracture system which may control the Manley Hot Springs geothermal area. v Y Based on findings from the helium, temperature, mercury, and resistivity surveys, three localities at Manley Hot Springs were chosen as likely sites for a geothermal well (Figure 19). e first and most promising site is the area just north to northwest of the eenhouse, referred to as site 1. the area is an obvious choice, since the hottest springs are located here. Helium soil gas values are anomalously high, as are shallow ground temperature 0 nd shallow resistivity values. based on anomalous helium values. valley near the intersection of drainages of sites A,10 and A,11. the third most likely drilling Site 2 he second most likely site ated on the floor of Karshner 9 Site 3, is located near temperature Anomaly orth side of Karshner Creek Valley. ous temperature and resistivity values and I Y .. I w a small population center in the interior of Alaska. and Manley in the early part of the century attests to the fact that Manley Hot Springs, as well as other hot springs of the Interior, can be utilized on a much larger scale than they are presently. Agricultural production, spaceheating and even the generation of small amounts of electricity by geothermal means coul d be hi ghl y benef i ci a1 to surrounding communi t i es . The work of Karshner ' w W ACKNOWLEDGEMENTS The U.S.. Dept. of Energy provided funding for this study under Cooperative Agreement No. DE-FC07-79-ET27034 with E. M. Wescott and D. L. o-principal investigators. While at Manley Hot Springs housing and partial board was kindly provided by Charles 'and Gladys Dart. 3 istance in g und temperature and mercury soit collection as Y well as grid surveying was ably .provided by geoscience student then enrolled at the Uni The EM-31 resistivity survey was assisted by Becky Pettinger, an undergraduate geophysics student then enrolled at New Mexico Tech. University, New Mexico. Mary Moorman with the Alaska 06 ity Of Alaska, 0 8 ied instruction and partial assistance n water sampling of the hot springs. lso like to express her vice, insight and supp Cy Hetherington, Steven 0' Bri en Jo residents of I would especially like to thank the following people: Charles and Gladys Dart, Dr. David Hopkins of USGS, Mary Moorman and Roman Motyka of 73 W B I BL IOGRAP HY Chapman, R. M., W. E. Yeend, W. P. Brosgg, and H. N. Reiser, 1975, Preliminary geologic map of the Tanana and northeastern part of the Kantishna River quadrangles, Alaska: U.S. Geol . Survey open-file report 75-337, scale 1:250,000. r, Chapman, R. M., F. R: Weber, M. Churkin, Jr., and'C. Carter, 1979, The Livengood Dome Chert, A new Ordovician formation in central Alaska, and its relevance to displacement on the Tintina Fault: U.S. Geol. Survey Prof. Paper 1126-F, p. 1-13. Chapman, R. M., F. R. Weber, and B, Taber, 1971, Preliminary geologic map of u the Livengood quadrangle, Alaska: U.S. Geol Survey open-file report 71-66, scale 1:250,000. Foster, H. L., F. R. Weber, R. 8. Forbes, and E. E. Brabb, 1973, Regional Y geology of Yukon*Tanana Upland Mem., no. 19, p. 3 Hopkins, D. M. and 8. faber, 1962, Stratigraphy of the pre-Quaternary bedded V rocks of the Manley Hot Springs area, Alaska: (prelim. and uripub.) U.S. Geol. Survey Bu vol. 6, no. 4, p. 37. lnvesti gat i ons of open-fi le report 8-71, 28 p. R. 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