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Geo~h~rmalResources Council, TRANSACTIONS, Vol. 12, October 1988

HYDROT~ERMALALTERATION PATTERNS IN THE BREITENBUSH HOT SPRINGS AREA, CASCADE RANGE, OREGON

Terry E. C. Keith

U.S. Geological Survey, Menlo Park, C A 94025

ABSTRACT DISTRIBUTION OF ALTERATION

Rocks of early Miocene age in the Breitenbush Alteration is extensive in the oldest Hot Springs area have been affected by at least Tertiary (early Miocene) volcaniclastic rocks, two major episodes of hydrothermal alteration, lithic tuffs, and ash-flow tuffs in the map area one of which had temperatures in excess of (Figure 1). Pyroclastic and volcaniclastic rocks 200°C. Afteration minerals in younger Tertiary are altered to quartz, K-feldspar, (+illite, rocks are characteristic of temperatures below chlorite, and calcite) along major faults and 100°C. The most important factor in controlling along much of the axis of the Breitenbush anti- alteration is f.racture permeability. The best cline. Pervasive quartz-K-feldspar-illite guide to interpreting alteration is secondary alteration and recrystallization is apparent €n mineralogy of volcaniclastic and pyroclastic some of the older pyroclastic rocks. . In most of rocks, flow breccia, and vesiculated or fractured the rocks of the Breitenhush Formation of White lavas; alteration effects in interbedded massive (1980), such as are exposed along the Breitenbush lava flows are much more subtle. Hydrothermal River, the outer flanks of the NE-trending minerals from the SUNEDCO 58-28 drill hole Breitenbush anticline, and to the north outward indicate that past temperatures were much hotter from the major NE-trending fault that lies than present and must be relicts of an older immediately west of the antklinal axis, geothermal system; a 116OC aquifer at 780 m alteration is characterized by zeolites (mainly represents the present geothermal system. clinoptilolite and mordenite), smectite, mixed-layer smectitechlorite, and mixed-layer smectite-illite and celadonite. The massive lava flows interlayered with tuffs in the upper part INTRODUCTION of the Breitenbush Formation and in superjacent lava flows are only slightly altered; alteration Reconnaissance mapping of secondary mineral consists of mixed-layer smectite-chlorite or assemblages in the Breitenbush Hot Springs area smectite replacement of mafic phenocrysts (Figure 1) permits the identification of several (especially olivine), interstitial glass, as well types of alteration, including zeolite-clay, as deposition of clays and/or hematite and propylitic, and phyllic/potassic alteration, that goethite along fractures and/or grain are characterized by alteration mineral boundaries. Zeolites (laumontite, analcime, assemblages produced by different processes stilbite, heulandite), clays, epidote, hematite, including geothermal activity, contact effects of quartz, and calcite occur locally in highly plutons, dikes, and lava flows, and deep burial fractured and sheared mafic lavas of the lower (diagenesis) in the thick volcanic pile. The Miocene rocks. extent to which alteration pervades the rocks In contrast to lower Miocene rocks, depends upon primary and secondary permeability of alteration in middle Miocene to Pliocene lavas is the rocks and host rock crystallinity (glass which much less intense. Smectite, zeolites occurs in ash-flow tuffs, air fall tephra, and'is (chabazite, thomsonite, mesolite, scolecite, interstitial in lavas, is more readily altered stilbite, analcime), and calcite colnmonly occur than crystalline rocks), whereas the mineral as replacement minerals and open space filling in assemblages formed during alteration depend vesicular and fractured lavas and flow breccias- largely upon temperatures and fluid chemistry. Pore spaces in the lavas locally contain minor The purpose of studying alteration patterns in * amounts of several of the above metastable the Breitenbush Hot Springs area is to determine zeolites. Massive lavas and local densely welded the alteration sequence in the area, and to tuffs, such as near Mt. Bruno, have no characterize hydrothermal mineral assemblages and recognizable alteration. their relationship to structural and stratigraphic Quaternary rocks are not altered except for controls. Results of this study can be applied to local vapor-phase oxidation and fumarolic the identification and interpretation of past and alteration in vent breccias. Small white EO present geothermal systems and can be extrapolated clear silica (opal and cristobafite) deposits to other parts of the Cascade Range in Oregon and occur in vents where a wet steam phase was southern Washington. present following eruption.

299 KEITH

122'1 5' 122000' 12 1*45'

45'00'

EXPLANATION

Quaternary volcanic rocks

Tertiary volcanic rocks

Tcrtiary plutonic rocks

geologic contact

fault, ball and bar on downthrown side

anticlinc

drill hole site 44O45 0- hot spring

0 5 10 Km Q I I 17OREGON

Figure 1. Map showing reconnaissance hydrothermal alteration distribution in the Rreitenbush Hot Springs area, Oregon. Geologic map is from Sherrod and Conrey (1988). Dot pattern shows zeolite-clay alteration, much of which is regional; characterized by a variety of zeolites and abundant smectite, mixed-layer clays, ccladonite, and local calcite. Cross-hatched pattern shows propylitic alteration, mostly associated with intrusive rocks; characterized by chlorite, illite, mixed-layer illite-smectite, mixed-layer smectite-chlorite, hematite, pyrite, quartz, calcite, and local epidote. Line pattern shows phyllic/potassic alteration, localized in major structural zones and in contact and brecciated zones of intrusive rocks; characterized by quartz, chalcedony, illite, sericite, hydrothermal K-feldspar, calcite, local epidote. Checked pattern shows hydrothermally altered plutonic rocks, most of the alteration consists of clay and/or sericite; locally alteration consists of quartz, chalcedony, K-feldspar, 2 epidote, and isolated fractured and brecciated mineralized zones with biotite, tourmaline and sulfides in addition to the other minerals.

300 KEITH

Tertiary Plutons Hydrothermal minerals in the 2458-m deep Alteration associated with Tertiary plutons SUNEDCO 58-28 drill hole (A. F. Waibel, unpub. is common in the western and central part of the data, 1982) indicate much higher past temper- area, where numerous small plutons are exposed atures (maximum temperature measured during throughout the Breitenbush Hot Springs region drilling was 150"C, Blackwell and Steele, 1987) (Cummi'ngs and others, 1987; Hammoncl and others, throughout the drill hole and especially below 1982; Priest and others, 1987; Walker and others, 640 m where epidote occurs in trace amounts. 1985). This alteration locally indicates Laumontite, illite, calcite, and quartz with temperatures above 200°C based on the presence of sporadic epidote occurs from about 795 to 1518 m. epidote in the mineral assemblages including Epidote was not present below 1518 m; however, quartz, hematite, calcite, sericite, and chlorite the other minerals were prevalent along with (Figure 2). Breccias containing tourmaline, chlorite, heulandite, and locally albite, K-feldspar, and biotite are locally associated hematite and pyrite. Below about 1800 m, illite with small plutons in the Little North Santiam and chalcedony were identified, and substantial mining district north of Detroit Lake (Figure 1) recrystallization of ash-flow tuff to quartz, where there is epithermal mineralization (Callaghan and Buddington, 1938; Cummings and others, 1987). Much of the alteration associated with small plutons throughout the region, T"C 0 50 100 150 200 250 however, appears to be relatively lowtemperature (less than 200°C) and perhaps related to a CHABNITE THOMSONITE regional hydrothermal system. MESOLITUSCOLECITE

u)W PHILLIPSITE Hot Springs k STILBITE Lava flows at Breitenbush Hot Springs have J 0 HEULANDITEGROUP WN been affected little by alteration except LAUMONTITE adjacent to fractures where interstitial glass MORDENITE and, locally, where mafic phenocrysts are altered ANALCIME to smectite. Fractures clearly control fluid access to the rock. The fractures are coated SMECTITE first with a layer of smectite, then silica SMECTITOILLITE (chalcedony), and finally calcite; hot spring F CELAWNlTE deposits surrounding numerous small orifices 5 ILLITE consist of amorphous silica and/or calcite. 0 CHLORITE SMECTITOCHLORITE Structural Controls Major faults and anticlinal axes in the SERlClTE southern part of the map area have fractured, (WHITE MICA) brecciated zones that are cemented by multiple OPAL I3-CRISTOBALITE stages of quartz deposition along with chalcedony, calcite, illite, smectite, pyrite, a-CRISTOBAUTE and hematite. The general area on the east side c"v of the main northeast-trending structures has QUARTZ been faulted up and exposes the oldest rocks of CALCm the map area which were buried beneath a thick EPIDOTE volcanic pile. Subordinate faults and shear ADULARIA zones in mafic volcanic rocks throughout the map 4 SUNEDCO 58-28 area are commonly highly altered to zeolites d 6 CTGH-1 (laumontite, analcime, stilbite, heulandite), 0 I EWEB-5 smectite, mixed-layer smectite-chlorite, and calcite. Structures are more abundant in the pre-Quaternary rocks (Sherrod and Conrey, 1988). Figure 2. Selected hydrothermal minerals found The major altered zones may represent fossil in the Breitenbush Hot Springs area plotted geothermal systems that were fracture-controlled. against temperatures of occurrence in well-studied active geothermal systems. At the SUMMARY OF DRILL HOLE ALTERATION bottom, the inferred maximum temperature range of alteration in three drill holes in the map area Cuttings from the EWEB drill holes (Keith and are plotted relative to the secondary minerals Boden, 1980a, 1980b, 1981a, 1981b) and core from identified in them; present maximum measured CTGH-1 (Bargar, 1988) have been studied for temperature is indicated by a solid line; alteration. Virtually no hydrothermal alteration inferred previous maximum temperature is shown by was found in EWEB 3, 4, and 6, but minor zeolitlc a dashed extension of the line. Main sources of alteration (heulandite) was encountered below mineralogical information are: zeolites and clays 46 m in EWEB 5 (Figure 2). Lowtemperature (KristmannsGBttir and TBmasson, 1978); clay smectite-zeolite alteration was prevalent in the minerals (SrSdon and Eberl, 1984; Horton, 1985); 1463 m-deep CTGH-1 hole, although there was a celadonite (Keith and others, 1978; Odom, 1984); change in zeolite assemblages at about 890 m epidote (Bird and others, 1984); adularia (Browne (Rargar, 1988). and Ellis, 1970).

301 KXITH

K-feldspar, and illite (phyllic/potassic overlap than zeolites but they often occur where alteration) are seen in a few available thin zeolites are not found because of other stability secttons. A thermal (116°C) aquifer fn the upper factors (such as zeolites are favored by slightly part of the Breftenbush Formation at about alkaline conditions and they do not form in high 752-782 m was identified during drilling pC02 environments). (Blackwell and Steele, 1987), but hydrothermal Zeolites and clay minerals can form quickly alteration at those depths does not reflect this in glassy rocks where porosity is such that water temperature; rather, the minerals are is held in pore spaces, but the rock may have low representative of an older, higher-temperature permeability overall. Two of the minerals appear geothermal system. Only cuttings are available to be more chemically dependent: mordenite occurs from the rotary-drilled SUN~~CO58-28 hole, which in greater abundance and at higher temperatures is unfortunate because drill cores might have in slliceous rocks, and celadonite, though it has provided more textural information to interpret the illite structure, forms at lower temperatures superimposed alteration stages. where abundant K and Fe are available. Low-temperature zeolite-smectite alteration DISCUSSION encountered in most of the geothermal drill holes in the Breitenbush and Austin Hot Springs area Alteration in the Breitenbush Hot Springs could be developing at present conditions. area clearly is controlled mainly by rock Secondary minerals identified in the drill holes permeability; fracturing is the most important (except SUNEDCO 58-28) indicate that past factor in obtaining sufficient permeability for temperatures were no hotter than present measured the circulation of hydrothe~alfluids. temperatures. Lavas in WEB-1 drill hole (about Temperatures and fluid compositions are important 30 km to the south of the Breitenbush Hot Springs controls when fluids are able to penetrate the area) and in CTGH-1 (Figure 1) contain small rock. Volcaniclastic rock, ash-flow tuff, flow amounts of several metastable zeolites that are breccia, and fractured and vesicular lava flows not greatly different in total chemical show more alteration effects than interbedded composition (Kefth and Boden, 1981c; Rargar, massive lava flows. The glass components of 1988). In addition to temperature, these volcanic rocks alter more readily than zeolites are subtle differences in fluid crystalline components, and poorly welded glassy composition, pH, temperatures, and perhaps tuffs alter more readily than Zctasely welded nucleation effects in vesicles and pore spaces in tuffs. When glass quickly alters to clays and lavas (Keith and Staples, 1985). zeolites or is recrystallized to higher ~gh-temper~tu~ehydrothe~al alteration in temperature minerals, the permeability is the Breitenbush Hot Springs area may be severely reduced. Volcaniclastic rocks, in associated with a fossil (late Tertiary ?) contrast to pyroclastic rocks, commonly contain geothermal system that was localized along the very little glass and do not alter as readily northeast-trending anticlinal axis and faults to under lowtemperature hydrothermal conditions. In the west of the SUNEDCO 58-28 drill site and most massive lava flows the only conspicuous Breitenbush Hot Springs (Figure 1). These alteration effect is interstitial glass and mafic structures expose the oldest (lower Miocene) phenocrysts (especially olivine) locally altered rocks in the area, and the age of folding is to clays. Thus, the alteration state of massive constrained to 18-12 Ma (Sherrod and Conrey, lava flows of various ages is often difficult to 1988). In the SUNEDCO 58-28 drill hole, differentiate in the field. extensive recrystallization and the presence of Secondary mineral assemblages can be key epidote, chlorite, K-feldspar, and illite below factors in interpreting the alteration history of 640 m are evidence of past temperatures that were the area. Many zeolites are metastable phases significantly higher than the maximum temperature that are of lowtemperature origin (Figure 2); of 150°C measured during drilling. These rocks however, a few persist to about 200°C. Clay are ash-flow tuff and are stratigraphically in minerals also are important as a guide to the same part of the Breitenbush Formation as the temperatures. The clay mineral structure must be high-temperature altered ash-flow tuffs exposed determined by some method such as X-ray along the northeast-trending fault and anticlinal diffraction or differential thermal analysis to axis. The high-temperature fossil geothermal be diagnostic; color and field identifications system could be 18-12 Ma or younger but probably are not satisfactory. Secondary minerals found is not Quaternary because most of the deformation in the map area are plotted with depositional that would be called upon to control the temperature ranges derived from active geothermal geothermal system is pre-Quaternary (Sherrod and areas where they are used as guide minerals Conrey, 1988). The faulted and folded area west (Figure 2). Formation temperatures for zeolites of Rreitenbush Hot Springs contafns in mafic volcanic rocks in the Rreitenbush Hot phyllic/potassic alteration in the same rocks Springs area can be compared with temperatures of (mostly ash-flow tuffs) that are characterized by formation in Icelandic geothermal systems as zeolite-clay alteration outward from the documented by Kristmannsddttir and Tdmasson structures (1978) (Figure 2). A minor modification is the The area of high-temperature alteration in the extension of the range of laumontite to lower lower Miocene rocks corresponds to the area of temperatures (McCulloh and others, 1981; Keith present high heat flow delineated by Blackwell and Staples, 1985). Clay minerals have more and Baker (1988). The present Breitenbush Hot

302 KEITH

Springs are within the area of high heat flow, in Blackwell, D.D. and Baker, S., 1988, Thermal fact,, they are located in the zone of highest analysis of the Breitenbush geothermal system: heat flow gradient for the area (Blackwell and -in Sherrod, D.R., ed,., Geology and geothermal Baker, 1988). Geochemical calculations indicate resources of the Breitenbush-Austin Hot Springs a reservoir temperature of as much as 195°C area, Clackamas and Marion Counties, Oregon: (Brook and others, 1979) which, on the basis of Oregon Department of Geology and Mineral mineralogy, is still cooler than the fossil Industries Open-File Report 0-88-5, in press. geothermal system. The Breitenbush Hot Springs appear to be an Brook, C.A., Mariner, R.H., Mabey, D.R., Swanson, surface expression of the present geothermal J.R., Guffanti, M., and Muffler, L.J.P., 1979, system. A second expression of the present Hydrothermal convection systems with reservoir system is the 116OC aquifer between 752 and 782 m temperatures -9O"C, kMuffler, L.J.P., ed., depth in the SUNEDCO 58-28 drill hole (Blackwell Assessment of geothermal resources of the and Baker, 1988) in a distinctive quartz-rich United States--1978: U.S. Geological Survey ash-flow tuff. Priest and others (1987) Circular 790, p. 18-85. correlate th€s distinctive ash-flow tuff with outcrops in the faulted and folded area to the Browne, P.R.L. and Ellis, A.J., 1970, The west of the present hot springs. In the drill Ohaki-Broadlands hydrothermal area, New hole and in outcrop the distinctive ash-flow tuff Zealand: Mineralogy and related geochemistry: has been altered to a quartz, K-feldspar, illite, American Journal Science, V. 269, p. 97-131. -+epidote assemblage by an older, much hotter, system. Callaghan, E. and Buddington, A. F., 1938, The rocks affected by high-temperature Metalliferous mineral deposits of the Cascade alteration consist for the most part of anhydrous Range in Oregon: U.S. Geological Survey minerals that have been at least partly Bulletin 893, 141 p. recrystallized. These rocks are more easily fractured and sustain open fractures more readily Cummings, M.L., Mestrovich, M.K., Pollock, J.M., than rocks affected by low-temperature alteration and Thompson, G.D., 1987, Geothermal systems in where the minerals are mostly hydrous and the the Cascade Range in Oregon: insights from a rock is less brittle. As with the fossil fossil system, North Santiam mining area, geothermal system, the most likely major control western Cascades: Geothermal Resources Council of the present geothermal system is a complex Transactions, v. 11, p. 235-241. system of fractures, perhaps in the same general horizon. Primary permeability in the rocks has Hammond, P.E., Geyer, K.M., and Anderson, J.L., been significantly decreased by hydrothermal 1982, Preliminary geologic map and alteration. cross-sections of the upper Clackamas and North On the basis of very low-temperature Santiam Rivers area, northern Oregon Cascade alteration in late Tertiary and Quaternary Range: Portland, Oregon; Portland State volcanic rocks in drill holes EWEB-6 and CGTH-1, University Department of Earth Sciences, scale the young stratovolcanoes along the crest of the 1: 62,500. High Cascades Range appear to have little effect on regional hydrothermal alteration. The Horton, DOG., 1985, Mixed-layer illite/smectite as abundance of cold surface water percolating a paleotemperature indicator in the Amethyst downward must have a major cooling effect on vein system, Creede district, Colorado, USA: subsurface thermal waters (Muffler and others, Contributions to Mineralogy and Petrology, V. 1982). Also the sporadic small magma bodies that 91, p- 171-179. fed the volcanoes preclude the development of major shallow hydrothermal convection systems. Keith, T.E.C. and Boden, J.R., 1980a, Volcanic Perhaps good hydrothermal convection systems stratigraphy and alteration mineralogy of drill would be developed deeper beneath the volcanic cuttings from EWEB 3 drill hole, Clackamas arc County, Oregon: U.S. Geological Survey Open-File Report 80-877, 19 p. REFERENCES CITED , 1980b, Volcanic stratigraphy and Bargar, K.E., 1988, Secondary mineralogy of core alteration mineralogy of drill cuttings from from geothermal drill hole CTGH-1, High Cascade EWEB 4 drill hole, Clackamas County, Oregon: Range, Oregon: Geothermal Resources Council U.S. Geological Survey Open-File Report 80-891, Transactions, this volume. 8 P-

Bird, D.K., Schiffman, P., Elders, W.A., Williams, , 1981a, Volcanic stratigraphy and A.E., and McDowell, S.D., 1984, Calc-silicate alteration mineralogy of drill cuttings from mineralization in active geothermal systems: EWEB 5 drill hole, Clackamas County, Oregon: Economic Geology, V. 79, p. 671-695. U.S. Geological Survey Open-File Report 81-91, 18 pa Blackwell, D.D. and Steele, J.L., 1987, Geothermal data from deep holes in the Oregon Cascade Range: Geothermal Resources Council Transactions, v. 11, p. 317-322.

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, 1981b, Volcanic stratigraphy and Odom, I.E., 1984, Glauconite and celadonite, alteration mineralogy of drill cuttings from Bailey, S.W., ed., Micas: Mineralogical Society EWEB 6 drill hole, Elackamas County, Oregon: of America Reviews in Mineralogy, v. 16, p. U.S. Geological Survey Open-Fllc Report 81-168, 545-57 2 15 p. Priest, G.R., Woller, N.M., and Ferns, M.L., 1987, , 1981c, Volcanic stratigraphy and Geologic map of the Breitenhush River area, alteration mineralogy of drill cuttings from Linn and Marion counties, Oregon: Oregon EWEB 1 drill hole, Linn County, Oregon: U.S. Department of Geology and Mineral Industries Geological Survey Open-File Report 81-250, 9 p. Geologlcal Map Series GMS-46, scale 1:62,500

Keith, T.E.C. and Staples, L.W., 1985, Zeolites Sherrod, D.R. and Conrey, R.M., 1988, in Eocene basaltic pillow lavas of the Siletz Stratigraphy and structure of the River Volcanics, central Coast Range, Oregon: Breitenbush-Austin area, Cascade Range, Clays and Clay Minerals, V. 33, p. 135-144. north-central Oregon: 2 Sherrod, D.R., ed., Geology and geothermal resources of the Keith, T.E.C., White, D.E., and Beeson, M.H., Breitenbush-Austin Hot Springs area, Clackamas 1978, Hydrothermal alteration and self-sealing and Marion Counties, Oregon: Oregon Department in Y-7 and Y-8 drill holes in northern part of of Geology and Mineral Industries Open-File Upper Geyser Basin, Yellowstone National Park, Report 0-88-5, in press. Wyoming: U.S. Geological Survey Professional Paper 1054-A, 26 p. frSdon, J. and Eberl, D.D., 1984, Illite, in Bailey, S .W. , (ed. ) , Micas : Mineralogic= Kristmannsdtjttir, H. and Tbmasson, J., 1978, Society of America Reviews in Mineralogy, V. Zeolite zones in geothermal areas in Iceland: 16, p. 495-5440 -in Sand, L. B. , and Mumpton, F. A., eds., Natural Zeolites: Occurrence, properties, use: Walker, G.W., MacLeod, N.S. , and Blakely, R.J., Pergamon Press, New York, p. 277-284. 1985, Mineral resource potential of the Bull of the Woods Wilderness, Clackamas and Marion McCulloh, T.H., Frizzell, V.A., Jr., Stewart, Counties, Oregon: U.S. Geological Survey R.J., and Barnes, I., 1981, Precipitation of Open-File Report 85-247, 27 p. laumontite with quartz, thenardite, and gypsum at Sespe Hot Springs, western Transverse White, C.M., 1980, Geology of the Breitenbush Hot Ranges, California: Clays and Clay Minerals, v. Springs quadrangle, Oregon: Oregon Department 29, p. 353-364. of Geology and Mineral Industries Special Paper 9, 26 p* Muffler, L.J.P., Bacon, C.R., and Duffield, W.A., 1982, Geothermal systems of the Cascade Range: Proceedings of Pacific Geothermal Conference 1982, Auckland, New Zealand, November 8-12, 1982, po 337-343.

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