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An ISO/16O and D/H Study of Tertiary Hydrothermal Systems in the Southern Half of the Idaho Batholith

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An ISO/16O and D/H Study of Tertiary Hydrothermal Systems in the Southern Half of the Idaho Batholith An ISO/16O and D/H study of Tertiary hydrothermal systems in the southern half of the Idaho batholith R E CRISS* H P TAYLOR J Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 l, JR. } ABSTRACT acteristic of deep-level fluid circulation in geothermal systems such as Yellowstone National Park, Wyoming. In such regions, the During Eocene time, 37 to 49 m.y. ago, a series of large major zones of hydrothermal activity seem to be principally asso- hydrothermal systems was developed around a group of epizonal ciated with either (1) the caldera ring zones or (2) the central plu- granite plutons in the Idaho batholith. These systems involved deep tons (resurgent domes). and extensive circulation of fluids derived from low-Sl80 (~-16) and low-SD (— 120) meteoric waters. Water-rock interactions INTRODUCTION occurred at temperatures of 150 to 400 °C, lowering the ,sO/ leO and D/H ratios in the surrounding Mesozoic rocks (tonalite, gra- A variety of geological evidence (Craig, 1963; Taylor, 1968, nodiorite, and granite), such that the feldspar S,80 and biotite SD 1971, 1977; Taylor and Forester, 1971, 1979) and theoretical models values became as low as -8.2 and -176, respectively. These values (Norton and Knight, 1977; Norton and Taylor, 1979) proves that contrast markedly with the primary isotopic compositions of+9.3 ± large-scale circulation of ground waters commonly occurs around 1.5 and -70 ± 5, respectively. Widespread propylitization of the shallow intrusive bodies in the Earth's crust. Stable isotopic studies Mesozoic plutonic rocks accompanied these isotopic exchange have shown that the circulating fluid is dominantly meteoric water effects. Systematic mapping shows that anomalous SD and 6lsO in subaerial regions (Craig and others, 1956) and ocean water in values occur over more than 15,000 km2, indicating the extensive submarine environments (Craig, 1966; Wenner and Taylor, 1973; lateral dimensions of the ancient circulating systems. The former Gregory and Taylor, 1981 ), although fluids of other derivations are zones of intense hydrothermal activity are marked by low-'80 important in some environments (Clayton and others, 1966; White zones, which were mapped in the vicinity of the margins of several and others, 1973). Similar hot fluids are known to be responsible Eocene plutons (for example, at Rocky Bar) and also within a giant for the formation of many ore deposits (O'Neil and Silberman, (5- to 20-km wide, 60- to 40-km diam) ring zone that surrounds the 1974; Taylor, 1973, 1974a; Sheppard and Taylor, 1974; Ohmoto Sawtooth Mountains. The latter anomaly is coincident with the and Rye, 1970, 1974; White, 1974; Bethke and others, 1976). high-permeability ring fracture zone of an Eocene caldera system. Magaritz and Taylor (1976a, 1976b) discovered that, far from Most of the ore deposits in the southern half of the Idaho batholith being limited only to the shallow plutonic environments mentioned are epithermal and mesothermal Au-Ag veins that are located near above, widespread l80 depletions produced by meteoric-hydro- l8 18 the periphery of the low- 0 zones (that is, near the outermost 6 0 thermal activity were common in a number of deeper-seated plu- = 8 isopleth). This association links these deposits with the Tertiary tonic environments within several of the great Mesozoic granitic hydrothermal activity and has great potential as an exploration tool batholiths of the North American Cordillera. Taylor and Magaritz in the heavily forested region. Evidence is presented that the Eocene (1976, 1978) extended these <5,80 and <5D studies to the Idaho ground-water circulation pattern was affected over large lateral dis- batholith and discovered wide zones of strong D and l80 depletion tances (25 to 50 km) and great depths (5 to 7 km). These conclu- produced by hydrothermal circulation systems associated with a sions, together with the indications that large amounts of water group of crosscutting Eocene plutons. Their principal conclusions, 3 (>7,000 km ) were involved in some systems and that the circula- other than demonstrating the existence and large scale 104 km2) tion patterns probably are related to caldera ring structures, may be of the ancient geothermal systems, were that the aqueous solutions of particular importance in geothermal exploration and exploita- were derived from ordinary meteoric waters and that the Eocene tion of analogous modern systems. For example, the "fossil" magmatic activity provided the requisite heat. They also noted that hydrothermal activity mapped in the Idaho batholith may be char- the areas of ancient hydrothermal activity coincided with large Contribution No. 3575, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology. The Appendix of this paper, which includes all of the isotopic data together with sample descriptions and localities, is available from the authors on request. It may also be obtained by ordering GSA supplementary material 83-4 from Documents Secretary, Geological Society of America, 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301. •Present address: U.S. Geological Survey, Menlo Park, California 94025. Geological Society of America Bulletin, v. 94, p. 640-663, 26 figs., 2 tables, May 1983. 640 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/640/3444703/i0016-7606-94-5-640.pdf by guest on 29 September 2021 TERTIARY HYDROTHERMAL SYSTEMS, IDAHO BATHOLITH 641 regions of rocks with anomalously young K-Ar ages, as mapped in reconnaissance by R. L. Armstrong (1974). Following up the reconnaissance studies of Taylor and Maga- ritz (1976, 1978), the goal of the present investigation was to care- fully map the l80/l60 and D/H distributions in the southern one-half (Atlanta Lobe) of the Idaho batholith. A companion study of the K-Ar age relationships (Criss and others, 1980, 1982) was also undertaken to determine whether any detailed correlations exist among these different isotopic variables. For a number of Figure 1. Generalized geologic map of Idaho, modified after King and Beikman (1974), Bond (1978), and Rember and Bennett (1979). EXPLANATION | Cu | upper Cenozoic undifferentiated Tertiary intrusive rocks 1111111 lower Tertiary volcanic and sedimentary rocks Mesozoic intrusive rocks Mesozoic and Paleozoic undifferentiated Precambrian undifferentiated Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/94/5/640/3444703/i0016-7606-94-5-640.pdf by guest on 29 September 2021 642 CR1SS AND TAYLOR geological and logistic reasons, we believed that the Idaho batholith and involve reaction of the silicate with excess fluorine gas in a was one of the best areas in which to make such a detailed study of vacuum line, purification of the released oxygen, combustion of the these types of processes. oxygen to carbon dioxide, and analysis on a double-collecting mass This paper discusses the geographic distribution of the stable spectrometer. Hydrogen extractions from hydrous silicates were isotopic variations, the implications of these data with respect to the performed both at Caltech and in the stable isotope laboratory of geology of the ancient geothermal systems in Idaho, and their bear- the U.S. Geological Survey at Menlo Park, in a manner similar to ing on the genesis and localization of associated economic minerali- that outlined by Friedman (1953) and Godfrey (1962); this tech- zation. Criss and Taylor's (1978) suggestion that a major Eocene nique involves thermal decomposition of the mineral in a vacuum caldera developed in the Sawtooth Mountains region will be exam- line, reduction of the released water to H2 with hot uranium metal, ined in detail, and it will be shown that the present-day Yellowstone and mass spectrometer analysis. All analyses are reported as per mil geothermal area is a close analogue to the Sawtooth hydrothermal deviations from the SMOW standard (Craig, 1961a). More than system. The stable isotopic data from the eroded terrane of the 400 5lsO and 75 <5D measurements are discussed in this text, includ- Idaho batholith provide information on the otherwise inaccessible ing the earlier data of Taylor and Magaritz (1976, 1978), which are deepest levels of modern geothermal systems associated with large incorporated in all figures. Complete data tables, including detailed silicic volcanic centers. Such information currently cannot be descriptions and exact localities for 238 samples, are given in the obtained from modern systems because of the difficulties associated Appendix, which is available on request from the authors and with deep drilling in such hot terranes. which is also stored in the data bank of the Geological Society of America (see footnote on p. 640). GEOLOGIC SETTING ISOTOPIC RELATIONSHIPS IN The Idaho batholith is a large (~40,000-km2) composite mass, THE IDAHO BATHOLITH made up of numerous granitic plutons, located in the northern Rocky Mountains of central Idaho and adjacent portions of Mon- Sl80 and 5D measurements allow "normal" (primary mag- tana (Fig. 1). Most of the batholith consists of rather uniform gran- matic) rocks to be distinguished from their hydrothermally altered ite and granodiorite (Larsen and Schmidt, 1958; Ross, 1963), counterparts; this forms the basis for mapping the ancient geother- although rocks of the western margin are predominantly tonalite mal systems. Moderate-temperature interactions of crustal rocks and quartz-diorite (Moore, 1959; Schmidt, 1964).
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