By ROBERT SCHOEN and DONALD E. WHITE, Menlo Park, Calif.

Abstract.-Geothermal waters produced two recognizable pat­ tionships between the compositions of waters and the terns of hydrothermal alteraltion in the rocks in drill hole observed alteration patterns. GS-6. During an early period, the rocks were subjected to The generalized geology of the thermal area is metasomatism that formed K- and celadon­ ite from unstable and ferromagnesian , re­ shown in figure 1. The basement rocks consist of a spectively. A later period of hydrogen metasomatism pro­ pluton of late Mesozoic age intruded into duced mixed-layer· illite-montmorillonite, montmorillonite, and metamorphosed sedimentary and volcanic rocks of kaolinite, probably as a series directly related to the intensity probable early Mesozoic age. Tertiary and Quater­ of alteration. The clays formed during hydrogen metasoma­ nary volcanic and sedimentary rocks were deposited tism are irregularly distributed with depth and probably are related to reactions involving C02 and H2S. This later period intermittently on the evoded surf'ace of this basement, of argillization still may be in progress. and andesite dikes intruded the granodiorite. Out­ crops of the Tertiary volcanic and sedimentary rocks are too small to distinguish in figure 1, but core This paper presents the results of a mineralogic samples of these rocks, from drill hole GS-6, are shown study of GS-6, one of eight holes drilled at Steamboat in figure 2. Springs, Nev., by the U.S. Geological Survey for The oldest Ceno~oic rocks found in the thermal area geologic information. Steamboat Springs, located in are soda flows, breccias, and assocjated the west-central part of Nevada, 10 miles south of sediments of the Alta Foormation of Oligocene ( ?) age. Reno, is an area of active hot springs (fig. 1). Eco­ These were followed by andesite flows, tuff breccias, nomic geologists have been interested in these springs and andesite dikes of the Kate Peak Formation of for about 100 years because minerals of antimony and Miocene ,or Pliocene age. The pre-Lousetown alluvium mercury, together with sman amounts of gold and consists of silt, sand, and gravel. Local patches of silver, precipitate from the hot waters. orthoquartzite, pediment gravels, and chalcedonic hot­ spring sinter are found associated with this alluvium. The Steamboat Springs thermal area straddles the The Steamboat of the Lousetown boundary between the Mount Rose and Virginia City Formation was erupted in early Pleistocene time f.ol­ quadrangles. The regional geology of these quadran­ lowed immediately by domes of pumiceous gles has been described recently by Thompson and of the Steamboat Hills Rhyolite. Pre-Lake Lahontan White (1964). White and others (1964) have pre­ alluvium is a;pproximately middle Pleistocene in age sented the detailed geology of the therm3ll area, in­ and· consists of fragments of older l'ocks, many of cluding an account of earlier studies and the 100'8 of which are cemented by hot-spring sinter. Post-Louse­ an drill holes, including GS-6. b The mineralogy of hydrothermally altered core from town clulIlcedonic sinter consists of sinter older than 4 of the U.S. Geological Survey drill holes has been deposits of Lake Lahontan and now mostly converted described by Sigvaldason and White (1961, 1962), and to chalcedony or cristobalite. Younger hot-spring Schoen and White (1965) similarly ITa ve described an sinter -deposits, consisting largely of uncrystallized additional 2 drill holes. In addition to presenting the , range up to the present in age. Both Post­ detailed mineralogy of core from drill hole GS-6, the Lousetown chalcedonic sinter :and the opaline hot­ present pwper compares the mineralogy of this drill spring deposits are combined under "Sinter" in fig­ hole with the others, and considers briefly the rela- ure 1. Date Pleistocene alluvium that is contempo- U.S. GEOL. SURVEY PROF. PAPER 575-B, PAGES Bll0-B119 BllO SCHOEN AND WHITE BIll

N EXPLANATION LJ Sinter CJ Alluvium • Steamboat Springs 1 .thermal area 119°45' NEVADA Basaltic andesite ~ Mostly metamorphic and granitic rocks


Fault or fissure INDEX MAP $ GS-5 Drill hole or well cr­ Spring

i'S- I( 4800

o 500 1000 1500 FEET L-____~ ____~ ______I


}<'IGURE l.--Generulizpd ~('ologi<; map of Steamuoat Springs thermal area, Washoe l'Ollllty, ::\ey. (;\lodified from detailed map, White aud other,;, I\)CH.) Bl12 MINERALOGY AND PETROLOGY

ORIGINAL ROCK TYPE 2 ------6 - 0------, ------S? \ Z 00: \ OW \ w~ Z \ ...J-U «CI) \ I \ u \ \ Cristobalite \ \ 49 /0 (Including chalcedony) / / / / / / / // w ~ () :J / / :J \ .,--- ~ ~ ...J ~ ...J \ CJ) « \ __ ------"\"1\ \" Cl \ ------\ \ \" z \ \ \ \ \\


FIGURE 2.-Semiquantitative plot of mineralogy from drill hole GS-6. Dashed lines arbitrarily drawn straight between sample points.

raneous with Lake Lahontan, and very small patches the stratigraphy can be found in the paper by White of Recent alluvium in the stream valleys, complete and others (1964). the depositional history of the area. All three aHu­ Drill hole GS-6 is collared at an altitude of 4,838 vial units, Pre-Lake Lahontan alluvium, alluvium feet in an extensive mass of relatively old silicious contemporaneous with Lake Lahontan, and Recent al­ sinter known as Sinter Hill (fig. 1). The depth of luvium, are combined under "Alluvium" in figure 1. the hole is 212 feet, and the depth to the water table During this time of extensive deposition in the is approximately 148 feet. Severrul months after drill­ Cenozoic, the Steamboat Springs thermal area was ing was completed, the water at the bottom of the ~l!lso undergoing episodes of deformation and periods drill hole had a temperature of 102.5°C, a pH of 6.7, of erosion. An account of the Cenozoic geologic his­ a rhloride content of only 12 parts per million, and tory of the area and a more detailed description of a specific conductance of 372 micromhos. This ex- SCHOEN AND WHITE Bl13 tremely dilute water is very different from the fluids dence from thin sections and chemical analyses, pro­ found in drill holes on the three thermally active vide miner'al percentages for each sample of drill core. terraces; typicalJy, the temperature there is about Chemical analyses of altered rocks and fresh rock 170°C, pH is about 6.2, chloride content is about 820 equivalents of the igneous rocks in GS-6 are pre­ ppm, and specific conductance is about 3,000 micro­ sented in truble 1. Additional chemical, normative, mhos. The dilute water of GS-6 is probably precipita­ and lTIiodal anrulyses for other rocks from the Steam­ tion that fell on the area, percolated underground, and boat Springs aTea are presented by White and others was heated moderately in the thermal environment. (1964, tables 1 and 2). Active construction of hot-spring sinter terraces is Good quantitative mineralogic control exists for the taking place today in the Steamboat Springs area at granodiorite that underlies most of the Steamboat lower altitudes where hot chloride water oontaining Springs thermal area, because of its relative homoge­

300 ppm or more of Si02 discharges at the surface neity and the a vailability of chemically and petro­ (White and others, 1956). The sinter deposits of graphioally analyzed fresh samples. Therefore, the Sinter Hill prove that high-temperature, silica-bearing granodiorite samples in drill hole GS-6 were evalu­ waters discharged in the past at -altitudes at least 148 ated for semiquantitative mineralogy in the same man­ feet nbove the present water table. The different types ner used for drill holes GS-3 and GS-4 (Schoen and and -ages of the hot-spring sinters have been described ~hite, 1965). The mineralogic oompositions of the in detail by White and others (1964). fresh soda trachyte :and basaltic ·andesite are known I-Iydrothermal alteration of the rocks in the vicinity with less certainty because of their fine-grained of GS-6 was n complex process, but two major stages groundmasses. However, quantitative control is still of alteration can be recognized. An early period of reasonably good. The highly variable original com­ potash metasomatism produced hydrothermal K-feld­ position of the arkosic sediments makes it difficult to spar and celndonite as alteration products. This stage determine semiquantitative mineralogy in. the altered pr6bably coincided with the construction of the sili­ eq ui valents. cious sinter deposit in which GS-6 is collared. During Because of these practical limitations, as well as the later hydrogen metasomatism, olay minerals and, recognized theoretical limitations imposed by assuming locally, opal were produced. This last stage, domi­ a. linear relationship between X-ray peak height and JUtted by argillization, still may be in progress today. volume percent, the diagram of the mineralogy of drill hole GS-6 (fig. 2) is .only a semiquantitative estimate. METHODS The reported percentage for each present may be in er~or by as little as 5 percent for quartz, but X-ray diffraction was the principal tool used to study the error is likely to be greater for most of the others, the mineralogy of whole-~ock and clay fractions of especially the clays. Nevertheless, the determination all sam ples of drill core. Thin-section microscopy of the relative -a-mount of each mineral in a sample and chemical {uld spectrographic aI~alyses of rocks and has been found to be reproducible. This is the major mineral concentrates provided essential additional in­ formation. requirement for a study of alteration where relative changes are important. The mineralogy of the rocks cut by drill hole GS-6 is depicted in figure 2 as a semiquantitative plot of MINERALOGY sample mineralogy versus sample depth. Details of the factors considered in the construction of such a The mineralogy of the drill core is plotted semiquan­ diagram :are given in an earlier paper (Schoen and tit.atively-against depth in figure 2, and chemical analy­ vVhite, 1965). In brief, mod'a.l analyses _and X-ray ses of selected samples are shown in table 1. The field cliffractograms of unaltered rocks were used to evalu­ designated "igneous alkali feldspar" in figure 2 repre­ ate the changes in mineralogy in hyd~othermally af­ sents anorthoclase and in the volcanic rocks, fected rocks. The.decrease in the height of an X-ray in the granodiorite, and mixtures of all three peak for a mineral from an altered rock is assumed in the alluvium. to be linearly related- to the decrease in abundance Post-Lousetown chalced,onic sinter of the mineral in volume percent. Minerals produced The silicious sinter, that extends to a depth of 50 feet by hydrothermal alteration are assumed to be equal in GS-6 (fig. 2), was originally deposited as amorphous in volume percent to the minerrul or minerals they opal (White and others, 1964) but is now composed replaced. Applicatiion of these assumptions to the principally of about equal amounts of chalcedonic X-I'ay diffractograms, together with supporting evi- quartz and opal ({3 or high cristobalite by X-ray). Bl14 MINERALOGY AND PETROLOGY

TABLE l.-Chemical analyses, by rapid methods, of rock from drill hole GS- 6, Sinter Hill, Steamboat Springs, Nev., and of nearby unaltered equivalent rocks [Analysts (U.S. Geol. Survey): Leonard Shapiro (1-12); S. M. Berthold and E. A. Nygaard (3-5,7,9,11,12); H. F. Phillips (1,2,6,8,10); K. E. White (1,2,6,8,10); W. W. Brannock (13)]

Altered rocks of GS-6 Unaltered nearby rocks

Analysis number ______-- 1 2 3 4 5 6 7 8 9 10 11 12 13 ------.. ------.. ------Sample depth (feet) ______2 6 68 100 I 111 I 129 160 I 190 I 205 I 210 Basaltic Description ______Chalced- Chalced- Alluvium Grano- Soda andesite onic onic Soda trachyte of Louse- sinter sinter Steamboat basaltic andesite of trachyte of Alta town Lousetown Formation of Alta Granodiorite Forma- Forma- Forma- tion tion tion

Si0 ______2 97. 9 96.4 91. 3 58. 3 60. 9 61. 3 67. 7 68. 9 72.2 74.0 65. 6 65. 0 54.38 Ah03_ ------.06 .30 .84 19.6 19.6 18. 5 13.8 15. 1 11. 4 12. 8 16.5 18.6 17.83 Fe203 ___ ------.12 . .08 .04 1.6 1.0 1.2 .5 .05 .3 .02 2.2 1.5 2. 57 FeO ______.0 .0 .17 .87 .23 .70 .30 .81 .60 .74 2.1 2.0 4.97 .00 .01 .10 .14 .26 .10 .15 .90 .40 .53 1.7 .74 4. 12 ~ag~~======.00 .14 .40 .50 1. 1 1.8 1.2 1.0 1.2 1. 1 3. 2 3.1 6. 58 N a 0 ___ - ___ --- ____ 1.6 2.5. 1.6 1.8 1.8 2.1 K O2 ______.22 .20 .11 .78 3. 6 5. 0 4. 03 2 .10 .10 .10 10. 9 2. 8 8. 8 8. 7 4.4 4.7 4.8 2. 9 3. 2 2. 50 H20 ______~_ 1.1 2. 8 5. 7 3.5 7. 8 1.9 .0 4.2 2.9 1.9 .55 .25 .28 Ti02____ -- ______-- .00 .00 .58 1.4 1.5 1.5 .42 .34 .35 .28 .55 .46 1. 89 0 .00 .00 .01 .46 .45 .56 .10 .06 .11 .06 .15 .21 .57 P2~nO5------______.00 .00 .02 .05 .01 .01 .05 .. 02 .04 .02 .06 .14 .11 FeS2 ______- _ - ____ .02 .02 ------1.9 1.3 .62 4.5 1.9 3. 7 2.1 ------.00 CO ___ -- ___ --- __ -- <.05 <.05 <.05 <.05 <.05 <.05 <.05 .09 .01 S032 ______<.05 <.05 <.05 ------.12 <.2 .35 ------.62 ------.00 ------TotaL ______100 100 100 100 99 100 100 100 100 100 99 100 100 ------Specific gravity (powder) ______2. 59 2. 26 2.16 2. 60 2. 40 2. 62 2.63 2.64 2. 68 2. 67 2. 69 2. 58 2. 80 Specific gravity (lump) ______2. 36 1. 80 1. 95 2.41 2. 04 2. 50 2. 58 2.20 2.41 2. 54 2. 62 2.52 2. 59

The f3 cristobalite commonly is filled with inclusions, Steamboat basaltkandesite of Lousetown Formation has an index of refraction ranging between 1.460 and The Steamboat basaltic andesite flow was pene­ 1.480, and is characterized by a strong 4.10A X-ray trated from a depth of 91 feet to 133 feet (fig. 2). peak. Some a· (low) cristobalite may be present at a Where unaltered this rock consists of of depth of 49 feet where the mineral grains contain few plagioclase and in a cryptocrystalline ground­ inclusions, have indires of refraction above 1.480, and mass composed largely of monoclinic alkali feldspar. produce a strong 4.06A X-ray peak. Cristoba1ite is The plagioclase phenocrysts are rimmed by over­ absent from the upper few feet of the core and from growths of anorthoclase that, in turn, may grade into other zones (irregularly distributed throughout the sanidine in the groundmass; a change that pr.obably sinter) that have been completely converted to chalced­ occurred during the rapid cooling associated with ony. eruption. Quartz is very rare in the fresh rock, and Pre-Lake Lahontan alluvium the large iamount in the sample at 95 feet is caused Alluvial sediments occur from a depth of 50 feet by the downward settling of rulluvium into the ­ to 91 feet. At 68 feet, where the alluvium has been ceous top of the flow. extensively opalized, its present chemical and minera­ K-metasoniatism of the basaltic andesite produced logic composition is very similar to that of the over­ nearly pure hydrothermal K-feldspar from all the lying sinter (table 1, fig. 2). St~biconite, a hydrous primary plagioclase phenocrysts and exchanged K for antimony oxide, is present at a depth of 68 feet in N'a in most. of the original anorthoclase and saliidine. sufficient amount to color powdered samples of allu­ Small amounts of celadonite and pyrite formed, dur­ vium a pinkish yellow and produce four characteristic ing K-metasomatism, from primary olivine, aluminum X-ray diffraction lines. Rounded fragments of basal­ released in the breakdown of plagioclase, and sulfide tic andesite 'are common in the. lower part of the ions in the thermal water. Few minerals susceptible alluvium, and they are generally rimmed by the green to further alteration rem'ained in the altered basaltic -rich mica, celadonite. andesite, thus partially explaining the paucity of clay SCHOEN AND ~TE Bl15 minerals produced during a later period of H-metaso­ Volcanic plagioclase and some plutonic plagioclase matism. were converted to hydrothermal K-feldspar during The sample from a depth of 111 feet requires special K-metasomatism of the -alluvium. Celadonite and py­ discussion because it does not fit the patterns described rite formed as products' ·of the thermal water and the in the preceding paragrruph. The original plagioolase breakdown of all primary ferromagnesian minerals phenocrysts are completely replaced by disordered except , which in general simply was bleached. kaolinite; hydrothermal I(-feldspar alid celadonite are Later H-metasomatism of the alluvium produced clay absent. Because elsewhere plagioclase is easily kaoli­ minerals from the most susceptible minerals, especially nized in an acid environment but I(-feldspar is com­ remnant plutonic plagioclase. monly metastable and is the last silicate mineral to Alta Formation be altered, it is ooncluded that the basaltic andesite An altered soda-trachyte lava of the Alta Forma­ fI'om 111 feet in depth near the center of the flow tion occurs between a depth of 160 feet and 170 feet. was insulated from the early episode of K-metasom·a­ Fresh trachyte from nearby areas consists of plagio­ tism. Subsequently, during an episode of H-metaso­ clase phenocrysts and numerous tiny oriented sodic matism, while the I(-feldspathized basaltic andesite plagioclase crystals in a very fine grained groundmass was metastable, all plagioclase at 111 feet was kaoli­ of anort.hoclase and quartz. and biotite nized. The massive impermeahle nature of the flow, are common but minor in amount. Quartz makes up and fracturing caused by structural deformation that about 15 percent of the rock. . occurred between the two periods of alteration, prob­ K-metasomatism of the trachyte converted all pla.­ ably explain the difference in access of the altering gioclase and some of the anorthoclase to. hydrotherma.l fluids. K-feldspar. Subsequent H-metasomatism (argilli.7.n.­ The identification of the small amount of kaolinite tion) was relatively ineffective in 'altering the K-fAlil-. in the sample nt 100 feet is questionruble. The mineral spar and residual anorthoclase to clay minerals. has a. 7A periodicity, a doo1 spacing of 7.15A, and is A thin bed of arkosic sediment extends from a depth dioctahedrnl. Its inq,ex of refraction, however, is of 170 feet to 177 feet. This rock was originally com­ slightly nbave 1.57 nnd, therefore, the mineral may posed entirelY' of granodioritic debris. K-metasoma­ be dioctahedrnl septechlorite rather than. kaolinite. tism converted some plagioclase to hydrothermal Pre-Lousetown alluvium I(-feldspar, but most of the plagioclase persisted until From a depth of 133 feet to 160 feet, arkosic allu­ later when it was changed to clay minerals by H-meta­ somatism .. It is possible, however, that some of the vium contains ~rubundan:t recognizable fragments of clay minerals, especially kaolinite, might have formed grnnodiorite and soda trachyte. Other fragments in­ by weathering of plagioclase prior to hydrothermal cl ude andesite and metamorphic rocks. alteration. The sample from a depth of 148 feet presents evi­ dence that suggests n period of hydrothermal altera­ Granodiorite tion predating the deposition of these sediments. A Granodiorite extends from 177 feet to the bottom fragment of grnnodiorite contains chlorite that is of the hole at 212 feet. Fresh granodiorite contains pseudomorphic ·after primary biotite. 'rhe fragment plagioclase, quartz, orthoclase, biotite, hornblende, is enclosed by a fine-grained containing many and chlorite in order of decreasing abundance. bleached biotite flnkes but none altering to chlorite. I(-metasomatism changed 'some of the plagioclase to The pseudomorphic nlterntion of biotite to chlorite hydrothe_rmal I(-feldspar in the samples from depths is ComlTIon in drill holes GS-3 nnd GS-4 as nn enrly of 205 feet and 210 feet. But at 180 and 190 feet no product of H-metnsomntism (Schoen and White, hydrothermal I(-feldspar was produced and, instead, 1965). In GS-6, however, chlorite is very rare, ap­ most of the plagioclase was nltered to clay minerals, parently becnuse I(-metnsomntism either bleached the probably during later argillization. As mentioned in biotite or converted it to celndonite. It is, therefore, the preceding pnrngraph, the kaolinite may have possible that Ulis grnnodiorite fragment was hydro­ formed by weathering prior to hydrothermal altera­ thermnlly tl.ltered prior to its erosion and deposition. tion. This suppor,ts White's conclusion that some hydro­ Clay mineralogy thermnl activity predated the extrusion of the Steam­ The clay minernls in these rocks deserve special boat basaltic andesite (White and others, 1964, p. mention because they. are apparently related to differ­ B33). ences in the alteration process. Bl16 M~ERALOGY AND PETROLOGY

Kaolinite, montmorillonite, and mixed-layer illite­ Othe,r drill holes montmorillonite are the common clay minerals in Oom'parison of the miner.alogy of drill hole GS-6 GS-6. The kaolinite is well crystallized except in with the six other drill holes already described in core samples from above a depth of 155 feet, where detail (Sigvaldason and White, 1961, 1962; Schoen it .is disordered. The montmorillonite gives a 12.4A and White, 1965), indicates generally similar altera­ basal X-ray spacing which probaJbly indicates natural saturation

I I I I I I A I I D I G I I I I I I I I I I I 7.20A I I II I I I I I I I I I I I I I 12.1A I I I I I I (8-1-2) I (32-0.6-2) I (8-1-2) I aiotit~ I I I I I I I I 7.18A I II I I I I I I I I I I a.OA: I I I I I I I I I I I I I I KOH I I I I I I I I I I I I I I I 190 I I 155 I I H I I I I I I I I Air dried I Air dried I I I I I I I I I I I I I I (8-1-2) I I I I I I I I I I I I B I I I E 31.5A I I I I I I 127.6A I I I I I I I I I I I I I I I I I I I I I I I I " I 17 1 I I 1 . "1 I I I I I I I I I I I (16-0.6-2) I I I (32-0.6-2) I Glycolated I I I I I I I I 7.20A I I I I I I I II I I I I I I I I 11.5A I I I I I I I II I 14.0 I I I I ISiotite I I I I a.9SA I II I (8-1-2) I I I I I I I I I I I 155 Glycolated Glycolated I I I I I I I I C I F J I I I I I 11O.02A I (8-1-2) (16-1-2) 10.2A (8-1-2) I

NaCI Glycolated I I I I I 190 I 155 I Baked at 5500 C : Baked at 5500 C

'-_____..I-._..I-._-'--_-'--_-'- __---' ______I ____ ~ __->--1_--1.. ___ -1- ______--'-_--'-_--'-_-'- ___--1 12 10 8 6 4 12 10 8 6 4 10 8 6 4 DEGREES 29

I"IGUIU~ a.-x-ruy (Uffrnctograms of representative clay-mineral fractions, drill hole GS-6. A.. -C, sample collected at 190 feet, D-J, snmple collected at 155 feet. The nature of the treatment each sample w:ts given prior to X-ray analysis is, listed below each diffrnctogrnm. The numbers in parentheses show the scale factor, multiplier!,~U1d time constant, respectively, used during the recording of the X-ray diffractogram. B118 M~ERALOGY AND PETROLOGY

GEOCHEMIST-RY ized by relatively rapid changes in response to chang­ ing environments. Environmental sensitivity of the The general patterns of mineral distribution shown random mixed-layer illite-montmoriUonite of a sample in figure 2 can be correlated with the kind and se­ from 155 feet in GS-6 is shown by X-ray diffracto­ quence of alteration reactions that occurred in the grams after soaking in IN KOH at room temperature rocks. for 24 hours (fig. 3G); the same sample after glyco­ K .. metasomatism lation (fig. 3H) showing the loss of expansibility com­ The distribution of hydrothermal K-feldspar is pared to figure 3E; the same sample after soaking in primarily related to the original abundance of min­ IN N aCI at room temperature for 43 hours (fig. 3/) ; erals most sensitive to K-metasomatism. Volcanic and the same sample glycolated (fig. 3J) now appar­ plagioclase is the feldspar most susceptible to replace­ ently restored to the condition shown in figure 3E. ment by K-feldspar during K-metasomatism, fol­ If the clay minerals in GS-6 had formed prior to IC­ lowed hy plutonic calcic plagioclase, sodic plagioclase, metasomatism, they should have been converted to volcanic aJkali feldspars (anorthoclase and sanidine), IC~mica or possibly even to IC-feldspar. The fact that and finally orthoclase, which is inert during K­ clays are now present in GS-6 indicates that they metasomatism. formed after K -metasomatism. Olivine, hornblende, and were destroyed ICaolinite, however, may be an exception. Its con­ during the early stages of the K-metasomatism, and the stant iron- and mag:r..esium-free composition and gen­ iron and liberated were used in forming erally coarse grain size may Inake it less sensitive to celadonite. Therefore, the original distribution of alteration than other clay minerals. Thus, the abun­ celadonite and hydrothermal K-feldspar was somewhat dant kaolinite that formed at the expense of plagio­ similar. Later alteration has tended to argillize most clase in the upper part of the granodiorite may be a of the celadonite, which may account for its apparent result of ancient suhaeri'al weathering. However, the absence below a depth of 155 feet (fig. 2). Small presence of residual biotite in this zone seems inimical alnounts of celadonite, however, are recognizable in to this hypothesis. The origin of the unique mineral thin sections of most samples to the bottom of the assemblage from a depth of 180 feet is still in doubt. hole. Celadonite is rare or absent in all other drill core from Steamboat Springs, and the abundance of Genesis and distribution of clays celadonite in OS-6 is probably related to the high initial The three clay minerals in GS-6 (mixed-layer illite­ abundance of iron and magnesium in the rocks. montmorillonite, montmorillonite, and kaolinite) prob­ Biotite was converted to a bleached mineral pseu­ ably formed in response to decreasing K+l/H+l ratios domorphic after the original biotite, during K-meta­ and decreasing temperatures in the aqueous environ­ somatism, and exhibits a broadened and reduced lOA ment. Both variables favor the formation of mont­ X-ray periodicity that does not expand with ethylene morillonite from mixed-layer illite-montmorillonite and glycol. This bleached biotite is more resistant to kaolinite from montmorillonite. This is in accord with H-metasomatism than some plutonic plagioclase, as Hemley's experimental work (Hemley and Jones, 1964) evidenced by its presence in the highly argillized upper as well as the field observations of Altschuler and parts of the granodiorite. others (1963). Several factors controlled the abundance and dis­ H .. metasomatism tribution of clay minerals in 0S-6. JVfost important The experimental work of Hemley (1959) indicates was the saturation of the rock with w"ater of 10"­ that the clay minerals K-mica (illite) and kaolinite cation/H+l ratio. A low ratio of other cations to H+l in precipitate from solution at moderate to "low K+ljH+l the water is fundamental to the formation of clays ratios, respectively, and that K-feldspar precipitates (Hemley, 1959). Except for the sample from 111 feet, at high K+IjH+l ratios. We conclude from this that clay minerals are abundant only below a depth of about the coexisting clay minerals and hydrothermal K-fe1d­ 150 feet, which is the level of the present-day water spar in 0S-6 formed at different times and from table (fig. 2). As an example, in the sample from 155 different solutions. Because clay minerals form at feet, which is belm,- the water table, all plagioclase that the expense of feldspars in fluids with relatively low was not previously altered to hydrothermal K-feldspar ratios of other cations to hydrogen ions (Hemley and has been argillized. The sample fronl 133 feet and Jones, 1964), the term H-metasomatism has been used above the ,,-ater table still contains un replaced plagio­ in this report to describe this argillization. clase and little clay. Although K-feldspar can persist metastably in non­ A second factor is the relative abundance of easily equilibrium environments, clay minerals are character- alterable minerals at the time H-metasomatism began. SCHOEN AND WHITE Bl19

This is well illustrated by eomparing the sample from altered rock, resulting in thermal waters near the sur­ 160 feet (trachyte) with the sample from· 171 feet face having compositions high in H+t; (2) lowering of (arkose). The volcanic plagioclase in the sample from the water table below the iand surface, which greatly 160 feet was previously replaced completely by hydro­ reduced the throughflow of mineralized thermal water thermal K-feldspar, which remained metastable during and allowed evolved H 2S to form sulfuric acid above the subsequent argillization, whereas much of the plutonic water table that subsequently drained downward to be plagioclase in the sample from 171 feet resisted K­ incorporated in part by the water circulating at depth; feldspathization and was then still available for later and (3) sealing of the deep channels by deposition of argilliza tion. minerals resulting in further decreases in the supply of The anomaly of abundant kaolinite in the sample thermal water. Both decreasing K+l/H+l ratios and from 111 feet is, in part, related to the proximity of a decreasing temperatures favored the formation of clay fault zone (White and others, 1964, p. B17) that minerals from the readily alterable minerals remaining served as a channel for H-metasomatizing fluids. in the rocks. This argillization still may be going on in the vicinity of drill hole GS-6. SYNTHESIS The alteration of the rocks cut by drill hole GS-6 REFERENCES was a complex process that left fragmentary evidence. Altschuler, Z. S., Dwornik, E. J., and Kramer, Henry, 1963, The conclusions based on this evidence are necessarily Transformation of montmorillonite to kaolinite during tentative, but we believe these conclusions have the weathering: Science, v. 141, p. 148-152. fewest inconsistencies. Brindley, G. W., 1956, Allevardite, a swelling double-layer mica Hydrothermal alteration probably began in the mineral: Am. Mineralogist, v. 41, p. 91-103. Hemley, J. J., 1959, Some mineralogical equilibria in the system vicinity of GS-6 after deposition of the Pre-Lake K 20-AbO;r-SiOr-H20: Am. Jour. Sci., v. 257, p. 241-270. Lahontan alluvium. Hot silica-laden water rose and Hemley, J. J., and Jones, W. R., 1964, Chemical aspects of discharged onto the land surface, forming opaline sinter hydrothermal alteration with emphasis on hydrogen metaso­ deposits. At the same time that sinter was forming at matism: Econ. Geology, v. 59, p. 538-569. the surface, K-metasomatism was probably taking MacEwan, D. M. C., 1956, A study of an interstratified illite­ place at depth. montmorillonite clay from W orcestershire, : Clays and Clay Minerals, Proc. 4th Natl. Conf., NAS-NRC pub. The high ratios of K+l/H+l necessary to produce 456, p. 166-172. K-metasomatism by the water were probably aehieved Schoen, Robert, and White, D. E., 1965, Hydrothermal altera­ by two mechanisms that were active at this time A~1 tion in GS-3 and GS-4 drill holes, Main Terrace, Steam boat the hot water rose from depth, the decrease in confining Springs, Nevada: Econ. Geology, v. 60, p. 1411-1421. pressure allowed boiling to occur, and volatiles such as Sigvaldason, G. E., and White, D. E., 1961, Hydrothermal C02 and H S were rapidly lost to a vapor phase. The alteration of rocks in two drill holes at Steamboat Springs, 2 Washoe County, Nevada: Art. 331 in U.S. Geol. Survey loss of CO2 caused a decrease in the H+l activity and a Prof. Paper 424-D, p. D116-D122. c'onsequent increase in the K+l/H+l ratio. Such a -- 1962, Hydrothermal alteration in drill holes GS-5 and mechanism is probably responsible for restricting K­ GS-7, Steamboat Springs, Nevada: Art. 153 in U.S. Geol. metasomatism to the upper part of the spring system. Survey Prof. Paper 450-D, p. D113-D117. A eontributory factor in eausing high K+l/H+t ratios Thompson, G. A. and White, D. E., 1964, Regional geology of the during the very early stages of activity may have been Steamboat Springs Area, Washoe County, Nevada: U.S. Geol. Survey Prof. Paper 458-A, p. A1-A52. the freshness of the rock adjacent to deep channels in Weaver, C. E., 1956, The distribution and identification of mixed­ the spring system. The reaction of thermal water with layer clay in sedimentary rocks: Am. Mineralogist, v. 41, fresh roc.k should have consumed more of the available p. 202-221. H+l while also assuring enough K+l to. yield fluids with White, D. E., Brannock, W. W., and Murata, K. J., 1956, Silica higher K+l/H+l ratios than are no,,- found in the in hot-spring waters: Geochim. et Cosmochim. Acta, v. 10, thermal waters. p.27-59. White, D. E., Thompson, G. A., and Sandberg, C. H., 1964, ~1any factors contributed toward ending K-metaso­ Rocks, structure, and geologic history of the Steamboat matism in the vicinity of GS-6. Among the more Springs thermal area, Washoe County, Nevada: U.S. Geol. important are: (1) insulation of the deep channels by Survey Prof. Paper 458-B, p. B1-B63.