Boulder Batholith Research Articles

Paleomagnetism of the late Cretaceous Boulder Batholith, Montana

Paleomagnetism of the late Cretaceous Boulder Batholith, Montana

Boulder Batholith, Montana: A Product of Two Contemporaneous but Chemically Distinct Magma Series

Research Article| December 01, 1973 Boulder Batholith, Montana: A Product of Two Contemporaneous but Chemically Distinct Magma Series ROBERT I. TILLING ROBERT I. TILLING 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718 Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT I. TILLING 1U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (12): 3879–3900. https://doi.org/10.1130/0016-7606(1973)84<3879:BBMAPO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROBERT I. TILLING; Boulder Batholith, Montana: A Product of Two Contemporaneous but Chemically Distinct Magma Series. GSA Bulletin 1973;; 84 (12): 3879–3900. doi: https://doi.org/10.1130/0016-7606(1973)84<3879:BBMAPO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Rocks of the Late Cretaceous composite Boulder batholith, though successively emplaced in a relatively small segment of the Earth's crust within a very brief time span (78 to 68 m.y.), can be grouped chemically into two magma series: (1) the main series, defined principally by plutons in the central and northern parts of the batholith; and (2) the sodic series, defined mostly by plutons in the southern part. For any given SiO2 content, the rocks of the main series tend to be higher in K2O and lower in Na2O than rocks of the sodic series. The chemical distinction between the two series proposed is also expressed by variation patterns for U, Th, Rb, and Sr abundances, by lead isotope compositions, but not by strontium isotope compositions.The prebatholith Elkhorn Mountains Volcanics (Late Cretaceous), especially the mafic members, are chemically and isotopically similar to the rocks of the main series, confirming geologic evidence of the genetic association between them. The postbatholith Lowland Creek Volcanics (early Eocene), though chemically more closely related to the sodic series, isotopically are more akin to, but slightly more radiogenic than, the main series. Post–Lowland Creek volcanic rocks (Miocene or Pliocene) are compositionally similar to the sodic series rocks. Spatial distribution of the batholith and the volcanic rocks exhibits a very crude chemical zonation of the region: for a given silica content, relatively more potassic rocks (main series and prebatholith volcanic rocks) tend to occur mainly in the north and east, whereas relatively more sodic rocks (sodic series and postbatholith volcanic rocks) predominate in the south and west.Available field, chemical, and isotopic evidence collectively suggests that the observed compositional variations for the Boulder batholith are most reasonably interpreted in terms of a model involving two magma series derived from two or more magma sources within the lower crust or upper mantle. These source regions are interred to vary chemically and isotopically, either laterally or vertically; in view of the rather small areal extent of the Boulder batholith, however, a vertically zoned source region is more probable. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Variations in Lead-Isotopic Compositions in Mesozoic Granitic Rocks of California: A Preliminary Investigation

Six alkali feldspar and two whole-rock samples of granitic rocks from the Sierra Nevada batholith and adjacent Klamath Mountains were analyzed for their lead-isotope compositions. The samples represented each of three 87 Sr/ 86 Sr groupings ( 0.706) for granitic rocks north of the Garlock fault in California. The isotopic compositions of lead in the samples from the Sierra Nevada batholith range from 18.73 to 19.37 for 206 Pb/ 204 Pb, 15.61 to 15.71 for 207 Pb/ 204 Pb, and 38.44 to 39.10 for 208 Pb/ 204 Pb. A crude parallel correspondence was found between lead and strontium isotopes, in that the specimens with the most radiogenic strontium also tend to have the most radiogenic lead similar to the previously studied Boulder batholith of Montana. A parallel correspondence is thought to imply characteristics of the source rocks for the plutons rather than consequences of partial melting or natural contamination. Lead-isotopic compositions for the Sierra Nevada batholith and the Boulder batholith differ, average values of 206 Pb/ 204 Pb being at least 18.8 for the Sierra Nevada batholith and about 18 for the Boulder batholith. In the Late Cretaceous part of the Sierra Nevada batholith, the secondary isochron “age” for the lead data in these rocks is about 2,900 m.y., far older than known Precambrian in California. Sources are proposed for these plutons from the lower continental crust and upper continental mantle or dominantly recycled continental materials, probably of intermediate composition and possibly carried down to the zone of melting by subduction. This source material may have been formed in Pre-cambrian times but did not undergo a Precambrian metamorphism greater than upper amphibolite facies which would have reduced the values of 238 U/ 204 Pb in the source rocks and resulted in Mesozoic leads like those found in the Boulder batholith and elsewhere in the Rocky Mountain region. A trondhjemite from the Klamath Mountains has a lead-isotope composition ( 206 Pb/ 204 Pb, 18.57; 207 Pb/ 204 Pb, 15.50; 208 Pb/ 204 Pb, 38.08) similar to that of oceanic volcanic rocks, particularly like those of island volcanics on oceanic ridges. Derivation of this trondhjemite from an oceanic mantle or recycled mantle material is indicated by this observation and supports the conclusion of Kistler and Peterman (1973) based on its alkali abundances and 87 Sr/ 86 Sr value.

Comparison of the Trace Element Content of Plutonic, Volcanic, and Metamorphic Quartz from Southwestern Montana

Systematic differences in the content of Ti, Mg, and Fe, in quartz of plutonic, volcanic, and metamorphic origins are reflected by spectrographic analyses of 36 samples of Holocene fluvial sand from southwestern Montana. The differences should be directly applicable to provenance interpretations of Tertiary basin-fill sand in Montana. More uniform trace-element contents are found in plutonic quartz (Tiσ = 8 ppm, Feσ = 14 ppm, and Mgσ = 3 ppm) than in metamorphic and volcanic quartz (Tiσ = 24 ppm, Feσ = 73 ppm, and Mgσ = 18 pm). Moreover, less Fe and Mg are found in plutonic quartz (Fe = 19 pm and Mg = 5 ppm) than in metamorphic and volcanic quartz (Fe = 107 ppm and Mg = 22 ppm). Mean Ti content is different for quartz derived from different batholiths; Ti = 72 ppm for Boulder batholith quartz versus Ti = 31 ppm for Tobacco Root batholith quartz.

Tertiary Stratigraphy, Structure, and Geologic History, Jefferson Basin, Montana

The Jefferson basin trends northward, elongated parallel to the southeastern border of the Boulder batholith on the west, and the linear fault-controlled flanks of the Tobacco Root and Bull Mountain areas on the east. The otherwise linear eastern margin of the basin is interrupted by an embayment that extends eastward between the Tobacco Root and Bull Mountain areas to coincide with the projected westward trace of the ancient Willow Creek fault zone in the Three Forks basin. Continental Tertiary basin-fill deposits of fluvial, lacustrine, and eolian origin (Bozeman Group, Robinson, 1963) overlie pre-basin rocks with erosional and angular unconformity. The Bozeman Group, represented by two unconformable and lithologically distinct depositional sequences is subdivided into two formations: the Renova Formation (new name), 0 to 3,500 + ft thick, partly of early and middle Oligocene age; and the Sixmile Creek Formation (Robinson, 1967), 0 to 2,400 + ft thick, of late Miocene to middle Pliocene age. The Renova Formation, characterized by fine-grained strata deposited in low-energy flood plain and pond environments, is subdivided into: the Bone Basin Member (new name), predominantly interbedded micrite and montmorillonite mudstone; the Climbing Arrow Member, predominantly montmorillonite mudstone; and the Dunbar Creek Member, predominantly vitric siltstone. The Sixmile Creek Formation, characterized by coarse-grained strata which accumulated in relatively high-energy ephemeral and perennial channel environments, is typified by locally derived fanglomerate near the basin margin; and by pebbly arkose, vitric arenite, and subordinate rounded conglomerate in more central parts of the basin. Renova strata along the western margin of the basin dip eastward from unconformable contacts with pre-basin rocks and disappear beneath gently dipping Pliocene Sixmile Creek strata in the central part of the basin. North of the Tobacco Root Mountains, Renova strata repeated by displacement along the Tobacco Root fault, are moderately folded and faulted and overlain by tilted Miocene and Pliocene Sixmile Creek strata, that dip eastward into the Mayflower Gulch fault. Both Renova and Six-mile Creek strata are unconformably overlain by thin Quaternary deposits of diverse origin. The Jefferson basin had its beginning when east-west and north-south zones of weakness produced by Laramide faulting were eroded subsequently by an eastward-flowing drainage system to produce an irregular erosion surface cut sufficiently deep to expose the Boulder batholith. By the Oligocene, drainage was impaired and lower and middle Oligocene Renova Formation strata were deposited and subsequently folded probably by adjustment along buried faults. Post-Renova erosion removed a large volume of Renova strata before the oldest Sixmile Creek strata accumulated in the late Miocene. Sixmile Creek strata continued to accumulate until the late Pliocene when a new cycle of basin excavation was initiated. During basin excavation, but before the late Pleistocene, displacement along the Tobacco Root fault raised the Bull Mountain and Tobacco Root Mountain blocks relative to the basin area to the west. The Tobacco Root block was independently rotated eastward, probably by concurrent faulting along the Mayflower Gulch and Jefferson faults, tilting Renova and Sixmile Creek strata at the north end of the Tobacco Root Mountains eastward. Following deformation, continued excavation of the basin left Renova and Sixmile Creek strata deeply eroded and the irregular pre-basin erosion surface partly exhumed.

On the Nature of the Boulder Batholith of Montana

In a recent review of the nature of batholiths, Hamilton and Myers (1967) interpreted the Boulder batholith of western Montana to be “in effect a gigantic mantled lava flow .... only a few kilometers thick,” that flowed, under a crust of its own ejecta, across a broad structural basin. Such an interpretation is inconsistent with abundant geologic and geophysical data. The main mass of the batholith, the Butte Quartz Monzonite, does not have the characteristics of a lava flow or a laterally emplaced sheet. Its volcanic cover was not a floating cap but a laterally stable roof that was part of a volcanic plateau which occupied at least twice the area of the batholith. It does not thin toward its edges but is generally steep sided. Its flow structures are predominantly steep rather than near horizontal. It is separated from two smaller flanking plutons by thin vertical septa kilometers long. Its emplacement required more than 4 m.y., a rate orders of magnitude too slow for a single sheet only a few kilometers thick, however extensive. The batholith is more than a few kilometers thick. Recent gravity studies (Burfeind, 1967; Bonini, 1969) suggest a maximum thickness of 9 to 15 km to their authors, but the calculations are based on (1) assumed lateral and vertical homogeneity of the batholith, whereas in reality the Butte Quartz Monzonite core is discontinuously rimmed by more mafic, denser plutons; and (2) inappropriate densities, leading to excessive apparent density contrasts. The gravity data suggest to us that the batholith is more than 15 km thick. Heat flow, cooling rate, and seismic data also are compatible with a thickness of at least 15 km, but are difficult to reconcile with a thickness of only a few kilometers. Convincing examples of extrusive or quasi-extrusive thin batholiths must be sought elsewhere.

Copper content of biotites from the Boulder Batholith, Montana

Copper content (by instrumental neutron activation analysis) of sixty-one biotites from rocks of the Late Cretaceous, composite Boulder batholith ranges from 6 to 4,395 ppm and averages 700 ppm. High copper values for the samples studied can generally be accounted for by the presence of minute inclusions of sulfide minerals in the biotite, rather than to extensive isomorphic substitution of copper in the crystal structure.Although the distribution pattern is variable, the highest copper contents in biotite are observed for samples from the Butte Quartz Monzonite, the largest single pluton of the batholith and the host rock for the rich deposits at Butte and many other major ore deposits of the region. Plutons older or younger than the Butte Quartz Monzonite have less copper (fewer or no sulfide inclusions) in their biotites. Within the Butte unit itself, higher values of copper in biotite tend to be found for rocks from a broadly defined, NNE-SSW-trending belt, which parallels the long dimension of the mass and contains the Butte district in the south and the Wickes and Clancy districts in the north.

Gravity studies on Montana, Wyoming, and Washington

Since the previous report [Bonini, 1965] the Princeton gravity program has been directed mostly toward detailed studies to aid in the solution of specific geologic problems. Some stations have been observed, however, purely to fill in blank areas as a contribution to the preparation of state maps. Worden meter W‐57 was used before 1966 and LaCoste‐Romberg instruments were used in 1966 and later.Figure 1 identifies by numbers the areas covered by specific projects, summarized below, for which some preliminary unpublished interpretations are presented. Area 1 covers the Teton Range and Jackson Hole, Wyoming, where Princeton gravity data have been incorporated with additional gravity, seismic‐refraction, and aeromagnetic data into a comprehensive report [Behrendt et al., 1968]. Similarly, the gravity map and interpretation of area 2, the Boulder batholith of Montana, have been published [Biehler and Bonini, 1969].

Lead and strontium isotope studies of the Boulder Batholith, southwestern Montana

Late Cretaceous batholith, quartz monzonite ore, Precambrian isotope age, partial-melting source material model; Isotope ratios of Pb in feldspar range widely from pluton to pluton: 16.9-18.1 for 206/204, 15.4-15.7 for 207/204, and 37.7-38.5 for 208/204.Whole-rock initial Sr-87/Sr-86 is 0.706-0.709. Variation in Pb isotope ratios within Butte Quartz Monzonite, plus differences in ratios between megacryst and groundmass feldspars in Donald pluton suggest incomplete isotopic mixing; similarity of Butte ore Pb to feldspar Pb of Butte Quartz Monzonite, therefore, has genetic significance. Isotope data suggest that batholith rocks result from partial melting of lower crust or upper mantle; assimilation of upper crustal material is necessary if source material resembled oceanic tholeiites.

K-Ar ages and time span of emplacement of the Boulder Batholith, Montana

<h3>ABSTRACT</h3> Learned associations between stimuli in different sensory modalities can shape the way we perceive these stimuli (Mcgurk and Macdonald, 1976). During audio-visual associative learning, auditory cortex is thought to underlie multi-modal plasticity in visual cortex (McIntosh et al., 1998; Mishra et al., 2007; Zangenehpour and Zatorre, 2010). However, it is not well understood how processing in visual cortex is altered by an auditory stimulus that is predictive of a visual stimulus and what the mechanisms are that mediate such experience-dependent, audio-visual associations in sensory cortex. Here we describe a neural mechanism by which an auditory input can shape visual representations of behaviorally relevant stimuli through direct interactions between auditory and visual cortices. We show that the association of an auditory stimulus with a visual stimulus in a behaviorally relevant context leads to an experience-dependent suppression of visual responses in primary visual cortex (V1). Auditory cortex axons carry a mixture of auditory and retinotopically-matched visual input to V1, and optogenetic stimulation of these axons selectively suppresses V1 neurons responsive to the associated visual stimulus after, but not before, learning. Our results suggest that cross-modal associations can be stored in long-range cortical connections and that with learning these cross-modal connections function to suppress the responses to predictable input.

Model for evolution of rocky mountains east of Idaho Batholith

Studies of thick, synorogenic, continental conglomerates in south-western Montana indicate that by Mid-Cretaceous time erosion of the roof of the Cretaceous Idaho Batholith had caused extensive exposure of Precambrian quartzites in a high mountainous terrain occupying part of the former geosyncline in central Idaho. Evidently, emplacement of the batholith had been accompanied by rapid regional uplift, creating a large tumor or undation. The gravity potential provided in this manner represents at least one known source of energy for the contemporary and subsequent orogenic processes in the northern Rocky Mountains. The Late Cretaceous to Paleocene folds and thrusts east of the batholith may be seen as the result of compressive shortening compensating for extension in the domed roof due to down-flank gravitative spreading. Compressive stress was propagated toward the edge of the cratonic shelf as earlier thrusts were folded and locked and newer thrusts formed farther east, finally involving the conglomerates. Local uplifts gave rise to epidermal gliding sheets. On a smaller scale, the Boulder Batholith in Montana played a somewhat similar role in causing westward directed folds and thrusts. The undation model for the northern Rockies accounts well for the radially directed tectonics around the northeast corner of the Idaho Batholith, for the dominant westward transport on the Pacific side of the Cordilleran belt, and for the recent seismic data indicating that the Sial in this belt not only lacks a root, but is actually thinner than normal by 5–20 km. The alternative model fitting the field data would require longdistance centrifugal overthrusting of the solidified batholithic core itself, or centripetal underthrusting of the craton and ocean basin in eastward, south-ward, and westward directions beneath this core. This alternative is difficult to accept from a mechanical point of view and also would lead to a thickening of the Sial, in contradiction to the seismic data.

Zonal Distribution of Variations in Structural State of Alkali Feldspar within the Rader Creek Pluton, Boulder Batholith, Montana

Zonal Distribution of Variations in Structural State of Alkali Feldspar within the Rader Creek Pluton, Boulder Batholith, Montana

Radiometric Dating of the Bearpaw Sea: ABSTRACT

Sampling of the bentonites included in the Upper Cretaceous Bearpaw formation of southern Alberta and adjacent areas has provided material for a geochronological investigation of this marine sequence of strata. K-Ar dating of biotite and sanidine included in the bentonites has indicated that the Bearpaw sea invaded most of the southern Alberta Plains 72-73 million years ago. The transgression of the sea was probably rapid and the base of the formation may be isochronous over most of the area, with the possible exception of areas in southern Saskatchewan and northern Montana where the sea might have transgressed somewhat earlier. The regressive upper boundary of the Bearpaw formation is set at 68 million years in the westernmost plains and at 66 million years farther east n the Cypress Hills region. The geochronological picture is compatible with the paleogeography of the Bearpaw. The bentonites intercalated with the normal sediments represent ashfalls produced by relatively remote volcanic eruptions. Study of the phenocrysts in the sand-size fraction, provided that contamination by detrital material has been negligible, has indicated that most of the bentonites are remarkably uniform in petrologic type and are dominantly andesitic. A source area is suggested in the eastern Cordilleran belt of northern Montana, where strong volcanism throughout most of the Late Cretaceous accompanied the gradual emplacement of the Boulder batholith. The andesitic nature of the Bearpaw bentonites is compatible with granodioritic magmatism in the postulated source area. End_of_Article - Last_Page 525------------

Crystallization Sequence of Minerals Leading to Formation of Ore Deposits in Quartz Monzonitic Rocks in the Northwestern Part of the Boulder Batholith, Montana

The crystallization succession of minerals in quartz monzonitic rocks in the northwestern part of the Boulder batholith from the magmatic to deuteric to hydrothermal stages is described. Ore minerals began crystallizing in stable-feldspar or deuteric-stage assemblages irrespective of structures and in hydrothermal veins with associated sericitization and argillization of the wall rocks. Tourmaline, which began crystallization in the deuteric stage and continued crystallizing into the hydrothermal stage, provides an important link in demonstrating the continuity of crystallization of the ore minerals from the deuteric into the hydrothermal stage. Pyrite, tourmaline, molybdenite, and rare chalcopyrite occur in stable-feldspar or deuteric-stage associations in pegmatitic zones in aplite, in locally derived segregation pods, and in primary joints which postdate aplite. The same minerals with quartz occur in veins where the feldspars are altered to sericite and clay minerals. Sphalerite, galena, silver, and gold crystallized at relatively cooler temperatures and somewhat farther from the point of derivation; this accounts for the localization of base metals in open spaces in veins near the top of the batholith and in replacement bodies in limestone adjacent to the batholith. The writer proposes that the sulfur, iron, copper, zinc, and lead were derived from magma in the normal course of magmatic crystallization.

Lead isotopes in ores and rocks of Butte, Montana

The isotopic compositions of leads in ores and rocks of Butte, Montana, suggest that the ore metals were neither late stage concentrates from the crystallizing magma that produced the Boulder batholith, nor concentrates of trace metals from nearby sedimentary rocks. The ore metals probably were concentrated and isolated in several non-shallow locations about 400 m.y. ago and were then transported into the batholith by the late stage fluids of the magma, after the batholith was emplaced 60 m.y. ago. Mixing of host rock Pb and ore mineral leads occurred in and near the ore zone.

Potassium-Argon Dates of Biotites from Cordilleran Granites

Biotites from 20 plutons of the Cordillera of North America, principally from British Columbia, have been dated using the potassium-argon method. Samples were selected from well-known intrusive masses in an attempt to compare physically measured dates with relative age established by geological mapping; most dates confirm the geological age interpretations. The Cordillera underwent granitic intrusion or orogeny at five times separated by long periods of quiescence. The earliest orogeny, of Devonian (Acadian) age, occurred 350-360 million years ago and resulted in intrusion of plutons as widely separated as the Ice River syenite of the Rocky Mountains and the Fitton granite of the northern Yukon. Early Mesozoic orogeny resulted in the emplacement of the upper Triassic or lower Jurassic Guichon (186 m.y.) and Topley (163 m.y.) batholiths of central British Columbia. The major Cordilleran intrusion occurred 95–100 million years ago, in about the middle of the Cretaceous, with emplacement of the Coast Range batholith and early phases of the Nelson batholith of British Columbia, and the Cassiar and Itsi batholiths of Yukon. In late Cretaceous time, 80 million years ago, the Bayonne pluton, British Columbia, and Boulder batholith and Marysville stock of Montana were emplaced. During Rocky Mountain orogeny (Eocene), 50–60 million years ago, late phases of the Nelson batholith and a number of plutons such as Coryell were emplaced. Eighteen million years ago, in the Miocene, small granitic bodies intruded the Cascade Range; these are believed to be the youngest exposed batholithic rocks in North America.

Lead isotopes in ores and rocks of Butte, Montana; discussion of paper by V. R. Murthy and C. C. Patterson, 1961

Discussion of a paper by Murthy and Patterson. The isotopic composition of Pb of the Boulder batholith is based on the composition of Pb from quartz of one sample (S-1) and may not be representative of the whole quartz monzonite mass. Rejection of feldspar Pb data by Murthy and Patterson because of supposed Pb contamination may be unwarranted. Slight cloudiness of the K feldspar is a primary or at most a deuteric feature. If the batholithic model is not restricted to 2 dimensions but includes depth as well, the hypothesis of Murthy and Patterson need not necessarily depart from the classic hydrothermal epigenetic theory.

Primary mineralization of uranium-bearing "siliceous reef" veins in the Boulder Batholith, Montana--Part 2, The veins

Primary mineralization of uranium-bearing "siliceous reef" veins in the Boulder Batholith, Montana--Part 2, The veins

Primary mineralization of uranium-bearing "siliceous reef" veins in the Boulder Batholith, Montana--Part 1, The host rocks

The siliceous reef uraniferous veins of the Boulder batholith contain, in decreasing order of abundance, finely crystallized pyrite, sphalerite, galena, uraninite, tetrahedrite, argentite, chalcopyrite, and covellite. Microcrystalline quartz comprises over 90% of the vein material; opal and barite are minor. The color of the veins varies from light gray to black due to variation in size and concentration of the microscopic grains of opaque minerals. Microcrystalline quartz was deposited in several stages, separated by intervals of brecciation. Quartz of the later stages is generally dark gray to black. It occurs as fracture fillings in massive early microcrystalline quartz of light color, and as a cementing matrix about breccia fragments of the lighter quartz. The U content of nearly 100 vein samples was determined by alpha counting and fluorimetry. Radioactive equilibrium of the U series is indicated by agreement between measured Ra content (analyzed by New Brunswick Laboratory, AEC) and that required for equilibrium, with U determined in 2 samples, and by general agreement between fluorimetric and alpha-activity determinations of U. Semiquantitative spectrographic analysis of the same samples provided data on the content and distribution of 28 minor elements in the veins. In most cases, the darker the vein quartz, the higher is the content of those elements generally occurring in opaque vein minerals: Ag, As, Ba, Co, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Sn, U and Zn. Elements that showed no significant correlation with vein color are Al, B, Be, Ca, Cr, Cs, Ga, Mg, Na, P, Sr, Ti, V, and Zr. Five elements - Fe, As, Co, Mo, and Pb - showed a marked positive correlation with U.

Deuteric alteration of some aplite-pegmatities of the Boulder Batholith, Montana, and its possible significance to ore deposition

The Boulder batholith contains many texturally complex leucogranitic aplite-pegmatite intrusives. Both the aplite-pegmatites and the enclosing quartz monzonites were hydrothermally altered along steep zones of closely spaced fractures that are in part the hosts for vein deposits. Mineralogically and chemically similar alterations of different origin are common in some aplite-pegmatite intrusives near Boulder, Mont. These internal alterations are small, irregular, and vaguely outlined and are commonly related spatially to pegmatitic textures. The altered rocks were formed during crystallization of the aplite-pegmatites and are thus deuteric in nature. These deuteric alterations and other mineralogic changes related to textural variations imply the physical-chemical conditions of crystallization in the batholith. Because hydrothermal alterations in the quartz monzonite are closely similar to the deuteric ones, the deuteric alterations have some significance to the origin of ore deposits.The geometrically irregular textural variants and deuteric alterations in the aplite-pegmatites may be the result of increased vapor pressure and of concentration of mineralizers in local, hermetically sealed volumes of rock. This increase led to coarsened grain, recrystallization, to the occurrence of tourmaline and sulfides, and/or to major stability adjustments of mineral composition and crystal structure. Mineral assemblage is systemmatically related to textural type as shown by the progressive decrease of plagioclase and coarsening of grain until only separate bodies of quartz and of K-feldspar rock remain. Chemically deuteric alteration resulted in similar differentiation.The differentiation pattern of the aplite-pegmatites closely resembles Neuman9s hypothetical process of hydrothermal differentiation, leading to aplites and pegmatites and also to an aqueous product, possibly rich in ore metals. The similarity of deuteric alteration of aplite-pegmatites to hydrothermal alteration of quartz monzonite and aplite-pegmatites clearly indicates that residual magmatic fluids alone may cause all the observed features of hydrothermal alteration in the quartz monzonites. Theoretically, the migration of residual fluids into volumes of lessened pressure resulting from thermal contraction may cause the observed hydrothermal alterations of the batholith and may also transport ore-metals. These fluids also may gain additional metals released by alteration of the original rock minerals. If hydrothermal alteration releases ore metals from rock minerals, several hundred thousand tons of metal could have been made available to ore solutions from an estimated 5 cubic miles of altered batholithic rock.These three processes of ore deposition--hydrothermal differentiation, migration of fluids, and alteration release of metals--may be amenable to geochemical-geological testing. Systematic analyses of rocks of varying chemistry and structural positions are proposed as a basis for such tests.