An ¹⁸O/¹⁶O, D/H and K-Ar Study of the Southern Half of the Idaho Batholith

Citation

Criss, Robert Everett (1981) An ¹⁸O/¹⁶O, D/H and K-Ar Study of the Southern Half of the Idaho Batholith. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/dc2n-5n03. https://resolver.caltech.edu/CaltechTHESIS:05302023-213210418

Abstract

δ¹⁸O and δD measurements provide powerful tools for the study of igneous rock petrogenesis. Among the noteworthy contributions of such studies have been the demonstration that massive quantities of hydrothermal fluids have interacted with considerable portions of the earth's upper crust, that the majority of such fluids are derived from ordinary meteoric surface waters, and that similar fluids appear to be involved in the formation of many ore deposits. This thesis presents the results of a combined δ¹⁸O, δD, K-Ar and petrographic study, whose principal goal was to elucidate the characteristics and thermal history of a series of hydrothermal systems developed within the Idaho batholith during the Eocene, about 40-45 m.y. ago.

The hydrothermal activity of concern is related to a period of intense magmatism and tectonism in Idaho termed the Eocene event. This period was characterized by intrusion of epizonal granite batholiths, formation of the cogenetic Challis volcanic field, block faulting, ring faulting, and ore deposition. The effects are very widespread but are most conspicuous in the east-central portion of the southern half (Atlanta lobe) of the Idaho batholith, the region which was studied in most detail.

The effects of the widespread hydrothermal activity are easily monitored against the relatively uniform primary character of the Mesozoic granitoids, which originally had whole-rock δ¹⁸O values of 9.5±1.5 and biotite δD values of -70±5 permil. The Eocene meteoric waters in this region had δ¹⁸O and δD values of about -16 and -120 permil respect­ively, and interaction and exchange of the rocks with deep circulating fluids derived from these waters produced major heavy isotope depletions in the rocks, such that the whole-rock δ¹⁸O and biotite δD values became as low as -4.5 and -176 permil. However, the rates of isotopic exchange of various minerals with the fluid are not identical, and the new results prove that feldspar exchanged ¹⁸O with the fluid at least four times as fast as did coexisting quartz, and that biotite exchanged D with the fluid at a much faster rate than did coexisting muscovite. These differential effects are important because they allow the primary isotopic compositions to be deduced from altered rocks, and have the still unproven potential of indicating something of the temperature-time history of the hydrothermal interactions.

Widespread propylitization of the rocks occurred concurrently with the δ¹⁸O and δD exchange effects. The most commonly observed petro­ graphic changes are the development of chlorite and sericite, whose quantities generally increase in crude proportion to the amount of ¹⁸O exchange. Such hydration effects are generally not conspicuous in previously studied hydrothermal areas associated with gabbroic plutons, which indicates that the hydrothermal interactions in Idaho occurred at relatively low temperatures (generally <350°C, see below).

Systematic mapping of the δ¹⁸O and δD depletions of the rocks has provided some startling new insight on the ancient hydrothermal systems in Idaho. The largest well-studied low-¹⁸O anomaly, termed the Sawtooth Ring Zone (SRZ), is a 5 to 15 km wide annulus of low ¹⁸O rock (average ~2 permil) which has an outer diameter of 40 X 60 km. D/H effects in the rocks are often discernible more than 25 km outside of the periphery of this feature, the larger size being attributable to the extreme sensitivity of the δD value to alteration under conditions of low water/rock ratios. The low-¹⁸O ring is centered on the Eocene Sawtooth batholith and its outlying plutons, and a large aeromagnetic anomaly suggests that Eocene rocks in fact underlie the entire SRZ region. These data provide good evidence that the SRZ is coincident with a high-permeability ring fracture zone associated with a giant Eocene caldera.

Several smaller low-¹⁸O zones have also been mapped in the Atlanta lobe, and their clearcut spatial association with the Eocene intrusives provides an important mapping and interpretative tool in the region. Furthermore, most of the productive mines in the Atlanta lobe are located near the periphery of these low-¹⁸O zones (along the "outer" +8 permil δ¹⁸O contour); this association links these deposits with the Tertiary hydrothermal activity and has great potential as an exploration tool.

K-Ar age data provide important complementary information about the geology of the region and on the thermal characteristics of the ancient geothermal systems. The apparent ages of Mesozoic biotites systematically decrease from about 95 m.y. along the east and west margins of the Atlanta lobe to <45 m.y. near the Eocene plutons, the latter ages being approximately concordant with those of the Eocene plutons themselves. However, there is a significant "age gap" between 45 and 53 m.y. No strong correlations of the apparent ages with the δ¹⁸O, δD or K₂O contents of the "biotite" separates were noted, and the majority of the apparent ages in any given locality are in fact correlated simply with the elevation. These observations suggest that most of the ages were produced by uniform, constant uplift (~0.15 mm/yr) of the batholithic terrane through the Ar "blocking temperature" (~300°C) during the early Tertiary. This uplift continued through the Eocene but was modified by the formation of a regional dome probably related to the intrusion of a gigantic volume (~25,000 km³) of Eocene magma. However, rocks collected near the contacts with the Eocene plutons, or at the lowest elevations in the overall region, definitely suffered catastrophic Ar loss during the Eocene; the apparent ages of these rocks do not correlate with the elevation and indicate that temperatures were high (>300°C). A model of the ancient temperature distribution is derived from these data and indicates that 1) most of the hydrothermal activity occurred at temperatures of 150-350° C and persisted to depths of at least 7 km below the earth's surface, 2) many of the outcropping Eocene intrusives were intruded at depths of 5 to 7 km, and 3) a significant proportion (>1/3) of the heat provided to the circulating fluids was provided by the ordinary geothermal gradient in the older rocks, although the driving force for the circulation was clearly provided by the thermal energy of the Eocene plutons.

All of the above properties bear on the nature of modern geothermal systems at deep levels which are presently inaccessible to view, and a close analogy of the SRZ may be made with the region at Yellowstone, Wyoming. The evidence presented for the extensive lateral migration of fluids, the depth of circulation and the thermal properties of these fluids are of particular importance, and helps explain the incredibly high fluid and thermal discharge rates currently observed within the Yellowstone caldera. The lack of definite evidence for low-¹⁸O magmas in Idaho contrasts with the recent discovery of voluminous extrusions of such magmas at Yellowstone, and suggests that such magmas are produced at the top of silicic magma chambers and are inevitably erupted, commonly as H₂O-rich ash flow sheets. last, the intense deep-level circulation patterns of the annular SRZ zone contrast with the surface intercaldera discharge systems currently observed at Yellowstone, and may indicate that significant geothermal reserves may be tapped at deep levels by drilling into the ring fracture zones of modern caldera-related systems.

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