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Edward A. du Bray

Publications and source records attributed to Edward A. du Bray.

At least 37 records · Page 2Linked to original sources
USGS2015

Magmatism and Epithermal Gold-Silver Deposits of the Southern Ancestral Cascade Arc, Western Nevada and Eastern California

Many epithermal gold-silver deposits are temporally and spatially associated with late Oligocene to Pliocene magmatism of the southern ancestral Cascade arc in western Nevada and eastern California. These deposits, which include both quartz-adularia (low- and intermediate-sulfidation; Comstock Lode, Tonopah, Bodie) and quartz-alunite (high-sulfidation; Goldfield, Paradise Peak) types, were major producers of gold and silver. Ancestral Cascade arc magmatism preceded that of the modern High Cascades arc and reflects subduction of the Farallon plate beneath North America. Ancestral arc magmatism began about 45 Ma, continued until about 3 Ma, and extended from near the Canada-United States border in Washington southward to about 250 km southeast of Reno, Nevada. The ancestral arc was split into northern and southern segments across an inferred tear in the subducting slab between Mount Shasta and Lassen Peak in northern California. The southern segment extends between 42°N in northern California and 37°N in western Nevada and was active from about 30 to 3 Ma. It is bounded on the east by the northeast edge of the Walker Lane. Ancestral arc volcanism represents an abrupt change in composition and style of magmatism relative to that in central Nevada. Large volume, caldera-forming, silicic ignimbrites associated with the 37 to 19 Ma ignimbrite flareup are dominant in central Nevada, whereas volcanic centers of the ancestral arc in western Nevada consist of andesitic stratovolcanoes and dacitic to rhyolitic lava domes that mostly formed between 25 and 4 Ma. Both ancestral arc and ignimbrite flareup magmatism resulted from rollback of the shallowly dipping slab that began about 45 Ma in northeast Nevada and migrated south-southwest with time. Most southern segment ancestral arc rocks have oxidized, high potassium, calc-alkaline compositions with silica contents ranging continuously from about 55 to 77 wt%. Most lavas are porphyritic and contain coarse plagioclase ± hornblende, biotite, and pyroxene phenocrysts. Seven epithermal gold-silver deposits with >1 Moz gold production, several large elemental sulfur deposits, and many large areas (10s to >100 km2) of hydrothermally altered rocks are present in the southern ancestral arc, especially south of latitude 40°N. These deposits are principally hosted by intermediate to silicic lava dome complexes; only a few deposits are associated with mafic- to intermediate-composition stratovolcanoes. Large deposits are most abundant and well developed in volcanic fields whose evolution spanned millions of years. Most deposits are hundreds of thousands to several million years younger than their host rocks, although some quartz-alunite deposits are essentially coeval with their host rocks. Variable composition and thickness of crustal basement is the primary control on mineralization along the length of the southern ancestral arc; most deposits and large alteration zones are localized in basement rock terranes with a strong continental affinity, either along the edge of the North American craton (Goldfield, Tonopah) or in an accreted terrane with continental affinities (Walker Lake terrane; Aurora, Bodie, Comstock Lode, Paradise Peak). Epithermal deposits and quartz-alunite alteration zones are scarce to absent in the northern part of the ancestral arc above an accreted island arc (Black Rock terrane) or unknown basement rocks (Modoc Plateau). Walker Lane structures and areas that underwent large magnitude extension during the Late Cenozoic (areas with Oligocene-early Miocene volcanic rocks dipping >40°) do not provide regional control on mineralization. Instead, these features may have served as local-scale conduits for mineralizing fluids.

Eastern California and Western Nevada↗
USGS2014

Geochemical and modal data for igneous rocks associated with epithermal mineral deposits

The purposes of this report are to (1) present available geochemical and modal data for igneous rocks associated with epithermal mineral deposits and (2) to make those data widely and readily available for subsequent, more in-depth consideration and interpretation. Epithermal precious and base-metal deposits are commonly associated with subduction-related calc-alkaline to alkaline arc magmatism as well as back-arc continental rift magmatism. These deposits form in association with compositionally diverse extrusive and intrusive igneous rocks. Temperature and depth regimes prevailing during deposit formation are highly variable. The deposits form from hydrothermal fluids that range from acidic to near-neutral pH, and they occur in a variety of structural settings. The disparate temperature, pressure, fluid chemistry, and structural controls have resulted in deposits with wide ranging characteristics. Economic geologists have employed these characteristics to develop classification schemes for epithermal deposits and to constrain the important genetic processes responsible for their formation.

Data Series↗
USGS2014

Geochronology and correlation of Tertiary volcanic and intrusive rocks in part of the southern Toquima Range, Nye County, Nevada

Extensive volcanic and intrusive igneous activity, partly localized along regional structural zones, characterized the southern Toquima Range, Nevada, in the late Eocene, Oligocene, and Miocene. The general chronology of igneous activity has been defined previously. This major episode of Tertiary magmatism began with emplacement of a variety of intrusive rocks, followed by formation of nine major calderas and associated with voluminous extrusive and additional intrusive activity. Emplacement of volcanic eruptive and collapse megabreccias accompanied formation of some calderas. Penecontemporaneous volcanism in central Nevada resulted in deposition of distally derived outflow facies ash-flow tuff units that are interleaved in the Toquima Range with proximally derived ash-flow tuffs. Eruption of the Northumberland Tuff in the north part of the southern Toquima Range and collapse of the Northumberland caldera occurred about 32.3 million years ago. The poorly defined Corcoran Canyon caldera farther to the southeast formed following eruption of the tuff of Corcoran Canyon about 27.2 million years ago. The Big Ten Peak caldera in the south part of the southern Toquima Range Tertiary volcanic complex formed about 27 million years ago during eruption of the tuff of Big Ten Peak and associated air-fall tuffs. The inferred Ryecroft Canyon caldera formed in the south end of the Monitor Valley adjacent to the southern Toquima Range and just north of the Big Ten Peak caldera in response to eruption of the tuff of Ryecroft Canyon about 27 million years ago, and the Moores Creek caldera just south of the Northumberland caldera developed at about the same time. Eruption of the tuff of Mount Jefferson about 26.8 million years ago was accompanied by collapse of the Mount Jefferson caldera in the central part of the southern Toquima Range. An inferred caldera, mostly buried beneath alluvium of Big Smoky Valley southwest of the Mount Jefferson caldera, formed about 26.5 million years ago with eruption of the tuff of Round Mountain. The Manhattan caldera south of the Mount Jefferson caldera and northwest of the Big Ten Peak caldera formed in association with eruption of a series of tuffs, principally the Round Rock Formation, mostly ash-flow tuff, about 24.4 million years ago. Extensive 40 Ar/ 39 Ar dating of about 60 samples that represent many of the Tertiary extrusive and intrusive rocks in the southern Toquima Range provides precise ages that refine the chronology of previously dated units. New geochronologic data indicate that the petrogenetically related Corcoran Canyon, Ryecroft Canyon, and Mount Jefferson calderas formed during a period of about 560,000 years. Electron microprobe analyses of phenocrysts from 20 samples of six dated units underscore inferred petrogenetic relations among some of these units. In particular, compositions of augite, hornblende, and biotite in tuffs erupted from the Corcoran Canyon, Ryecroft Canyon, and Mount Jefferson calderas are similar, which suggests that magmas represented by these tuffs have similar petrogenetic histories. The unique occurrence of hypersthene in Isom-type tuff confirms its derivation from a source beyond the southern Toquima Range.

Nevada↗
USGS2013

Petrologic, tectonic, and metallogenic evolution of the southern segment of the ancestral Cascades magmatic arc, California and Nevada

Ongoing arc magmatism along western North America was preceded by ancestral arc magmatism that began ca. 45 Ma and evolved into modern arc volcanism. The southern ancestral arc segment, active from ca. 30 to 3 Ma, adjoins the northern segment in northern California across a proposed subducted slab tear. The east edge of the Walker Lane approximates the east edge of the southern arc whose products, mostly erupted from stratovolcanoes and lava dome complexes arrayed along the crest of the ancestral arc, extend down the west flank of the Sierra Nevada. Southern arc segment rocks include potassic, calc-alkaline intermediate- to silicic-composition lava flows, lava dome complexes, and associated volcaniclastic deposits. Northern and southern segment rocks are similar to other convergent-margin magmatic arc rocks but are compositionally distinct from each other. Southern segment rocks have lower TiO 2 , FeO*, CaO, and Na 2 O contents and higher K 2 O contents, and exhibit less compositional-temporal variation. Compositional distinctions between the northern and southern segment rocks reflect the composition and thickness of the crust beneath which the associated magma systems were sourced. Northern segment rock compositions are consistent with generation beneath thin, primitive crust, whereas southern segment rocks represent magmas generated and fractionated beneath thicker, more evolved crust. Although rocks in the two arc segments have similar metal abundances, they are metallogenically distinct. Small porphyry copper deposits are characteristic of the northern segment whereas significant epithermal precious metal deposits are most commonly associated with the southern segment. These metallogenic differences are also fundamentally linked to the tectonic settings and crustal regimes within which these two arc segments evolved.

California, Nevada↗
USGS2012

Miocene magmatism in the Bodie Hills volcanic field, California and Nevada: A long-lived eruptive center in the southern segment of the ancestral Cascades arc

The Middle to Late Miocene Bodie Hills volcanic field is a >700 km 2 , long-lived (∼9 Ma) but episodic eruptive center in the southern segment of the ancestral Cascades arc north of Mono Lake (California, U.S.). It consists of ∼20 major eruptive units, including 4 trachyandesite stratovolcanoes emplaced along the margins of the field, and numerous, more centrally located silicic trachyandesite to rhyolite flow dome complexes. Bodie Hills volcanism was episodic with two peak periods of eruptive activity: an early period ca. 14.7–12.9 Ma that mostly formed trachyandesite stratovolcanoes and a later period between ca. 9.2 and 8.0 Ma dominated by large trachyandesite-dacite dome fields. A final period of small silicic dome emplacement occurred ca. 6 Ma. Aeromagnetic and gravity data suggest that many of the Miocene volcanoes have shallow plutonic roots that extend to depths ≥1–2 km below the surface, and much of the Bodie Hills may be underlain by low-density plutons presumably related to Miocene volcanism. Compositions of Bodie Hills volcanic rocks vary from ∼50 to 78 wt% SiO 2 , although rocks with <55 wt% SiO 2 are rare. They form a high-K calc-alkaline series with pronounced negative Ti-P-Nb-Ta anomalies and high Ba/Nb, Ba/Ta, and La/Nb typical of subduction-related continental margin arcs. Most Bodie Hills rocks are porphyritic, commonly containing 15–35 vol% phenocrysts of plagioclase, pyroxene, and hornblende ± biotite. The oldest eruptive units have the most mafic compositions, but volcanic rocks oscillated between mafic and intermediate to felsic compositions through time. Following a 2 Ma hiatus in volcanism, postsubduction rocks of the ca. 3.6–0.1 Ma, bimodal, high-K Aurora volcanic field erupted unconformably onto rocks of the Miocene Bodie Hills volcanic field. At the latitude of the Bodie Hills, subduction of the Farallon plate is inferred to have ended ca. 10 Ma, evolving to a transform plate margin. However, volcanism in the region continued until 8 Ma without an apparent change in rock composition or style of eruption. Equidimensional, polygenetic volcanoes and the absence of dike swarms suggest a low differential horizontal stress regime throughout the lifespan of the Bodie Hills volcanic field. However, kinematic data for veins and faults in mining districts suggest a change in the stress field from transtensional to extensional approximately coincident with the inferred cessation of subduction. Numerous hydrothermal systems were operative in the Bodie Hills during the Miocene. Several large systems caused alteration of volcaniclastic rocks in areas as large as 30 km 2 , but these altered rocks are mostly devoid of economic mineral concentrations. More structurally focused hydrothermal systems formed large epithermal Au-Ag vein deposits in the Bodie and Aurora mining districts. Economically important hydrothermal systems are temporally related to intermediate to silicic composition domes. Rock types, major and trace element compositions, petrographic characteristics, and volcanic features of the Bodie Hills volcanic field are similar to those of other large Miocene volcanic fields in the southern segment of the ancestral Cascade arc. Relative to other parts of the ancestral arc, especially north of Lake Tahoe in northeastern California, the scarcity of mafic rocks, relatively K-rich calc-alkaline compositions, and abundance of composite dome fields in the Bodie Hills may reflect thicker crust beneath the southern ancestral arc segment. Thicker crust may have inhibited direct ascent and eruption of mafic, mantle-derived magma, instead stalling its ascent in the lower or middle crust, thereby promoting differentiation to silicic compositions and development of porphyritic textures characteristic of the southern ancestral arc segment.

California, Nevada↗
USGS2012

Synthesis of petrographic, geochemical, and isotopic data for the Boulder batholith, southwest Montana

The Late Cretaceous Boulder batholith in southwest Montana consists of the Butte Granite and a group of associated smaller intrusions emplaced into Mesoproterozoic to Mesozoic sedimentary rocks and into the Late Cretaceous Elkhorn Mountains Volcanics. The Boulder batholith is dominated by the voluminous Butte Granite, which is surrounded by as many as a dozen individually named, peripheral intrusions. These granodiorite, monzogranite, and minor syenogranite intrusions contain varying abundances of plagioclase, alkali feldspar, quartz, biotite, hornblende, rare clinopyroxene, and opaque oxide minerals. Mafic, intermediate, and felsic subsets of the Boulder batholith intrusions are defined principally on the basis of color index. Most Boulder batholith plutons have inequigranular to seriate textures although several are porphyritic and some are granophyric (and locally miarolitic). Most of these plutons are medium grained but several of the more felsic and granophyric intrusions are fine grained. Petrographic characteristics, especially relative abundances of constituent minerals, are distinctive and foster reasonably unambiguous identification of individual intrusions. Seventeen samples from plutons of the Boulder batholith were dated by SHRIMP ( S ensitive H igh R esolution I on M icroprobe) zircon U-Pb geochronology. Three samples of the Butte Granite show that this large pluton may be composite, having formed during two episodes of magmatism at about 76.7 &plusmn; 0.5 Ma (2 samples) and 74.7 &plusmn; 0.6 million years ago (Ma) (1 sample). However, petrographic and chemical data are inconsistent with the Butte Granite consisting of separate, compositionally distinct intrusions. Accordingly, solidification of magma represented by the Butte Granite appears to have spanned about 2 million year (m.y.). The remaining Boulder batholith plutons were emplaced during a 6-10 m.y. span (81.7 &plusmn; 1.4 Ma to 73.7 &plusmn; 0.6 Ma). The compositional characteristics of these plutons are similar to those of moderately differentiated subduction-related magmas. The plutons form relatively coherent, distinct but broadly overlapping major oxide composition clusters or linear arrays on geochemical variation diagrams. Rock compositions are subalkaline, magnesian, calc-alkalic to calcic, and metaluminous to weakly peraluminous. The Butte Granite intrusion is homogeneous with respect to major oxide abundances. Each of the plutons is also characterized by distinct trace element abundances although absolute trace element abundance variations are relatively minor. Limited Sr and Nd isotope data for whole-rock samples of the Boulder batholith are more radiogenic than those for plutonic rocks of western Idaho, eastern Oregon, the Salmon River suture, and most of the Big Belt Mountains. Initial strontium (Sr i ) values are low and epsilon neodymium (&epsilon; Nd ) values are comparable relative to those of other southwest Montana basement and Mesozoic intrusive rocks. Importantly, although the Boulder batholith hosts significant mineral deposits, including the world-class Butte Cu-Ag deposit, ore metal abundances in the Butte Granite, as well as in its peripheral plutons, are not elevated but are comparable to global average abundances in igneous rocks.

Montana↗
USGS2011

Petrologic, tectonic, and metallogenic evolution of the Ancestral Cascades magmatic arc, Washington, Oregon, and northern California

Present-day High Cascades arc magmatism was preceded by ∼40 m.y. of nearly cospatial magmatism represented by the ancestral Cascades arc in Washington, Oregon, and northernmost California (United States). Time-space-composition relations for the ancestral Cascades arc have been synthesized from a recent compilation of more than 4000 geochemical analyses and associated age data. Neither the composition nor distribution of ancestral Cascades magmatism was uniform along the length of the ancestral arc through time. Initial (>40 to 36 Ma) ancestral Cascades magmatism (mostly basalt and basaltic andesite) was focused at the north end of the arc between the present-day locations of Mount Rainier and the Columbia River. From 35 to 18 Ma, initial basaltic andesite and andesite magmatism evolved to include dacite and rhyolite; magmatic activity became more voluminous and extended along most of the arc. Between 17 and 8 Ma, magmatism was focused along the part of the arc coincident with the northern two-thirds of Oregon and returned to more mafic compositions. Subsequent ancestral Cascades magmatism was dominated by basaltic andesite to basalt prior to the post–4 Ma onset of High Cascades magmatism. Transitional tholeiitic to calc-alkaline compositions dominated early (before 40 to ca. 25 Ma) ancestral Cascades eruptive products, whereas the majority of the younger arc rocks have a calc-alkaline affinity. Tholeiitic compositions characteristic of the oldest ancestral arc magmas suggest development associated with thin, immature crust and slab window processes, whereas the younger, calc-alkaline magmas suggest interaction with thicker, more evolved crust and more conventional subduction-related magmatic processes. Presumed changes in subducted slab dip through time also correlate with fundamental magma composition variation. The predominance of mafic compositions during latest ancestral arc magmatism and throughout the history of modern High Cascades magmatism probably reflects extensional tectonics that dominated during these periods of arc magmatism. Mineral deposits associated with ancestral Cascades arc rocks are uncommon; most are small and low grade relative to those found in other continental magmatic arcs. The small size, low grade, and dearth of deposits, especially in the southern two-thirds of the ancestral arc, probably reflect many factors, the most important of which may be the prevalence of extensional tectonics within this arc domain during this magmatic episode. Progressive clockwise rotation of the forearc block west of the evolving Oregon part of the ancestral Cascades magmatism produced an extensional regime that did not foster significant mineral deposit formation. In contrast, the Washington arc domain developed in a transpressional to mildly compressive regime that was more conducive to magmatic processes and hydrothermal fluid channeling critical to deposit formation. Small, low-grade porphyry copper deposits in the northern third of the ancestral Cascades arc segment also may be a consequence of more mature continental crust, including a Mesozoic component, beneath Washington north of Mount St. Helens.

California, Oregon, Washington↗
USGS2010

Episodic intrusion, internal differentiation, and hydrothermal alteration of the Miocene Tatoosh intrusive suite south of Mount Rainier, Washington

The Miocene Tatoosh intrusive suite south of Mount Rainier is composed of three broadly granodioritic plutons that are manifestations of ancestral Cascades arc magmatism. Tatoosh intrusive suite plutons have individually diagnostic characteristics, including texture, mineralogy, and geochemistry, and apparently lack internal contacts. New ion-microprobe U-Pb zircon ages indicate crystallization of the Stevens pluton ca. 19.2 Ma, Reflection-Pyramid pluton ca. 18.5 Ma, and Nisqually pluton ca. 17.5 Ma. The Stevens pluton includes rare, statistically distinct ca. 20.1 Ma zircon antecrysts. Wide-ranging zircon rare earth element (REE), Hf, U, and Th concentrations suggest late crystallization from variably evolved residual liquids. Zircon Eu/Eu*–Hf covariation is distinct for each of the Reflection-Pyramid, Nisqually, and Stevens plutons. Although most Tatoosh intrusive suite rocks have been affected by weak hydrothermal alteration, and sparse mineralized veins cut some of these rocks, significant base or precious metal mineralization is absent. At the time of shallow emplacement, each of these magma bodies was largely homogeneous in bulk composition and petrographic features, but, prior to final solidification, each of the Tatoosh intrusive suite plutons developed internal compositional variation. Geochemical and petrographic trends within each pluton are most consistent with differential loss of residual melt, possibly represented by late aplite dikes or erupted as rhyolite, from crystal-rich magma. Crystal-rich magma that formed each pluton evidently accumulated in reservoirs below the present level of exposure and then intruded to a shallow depth. Assembled by episodic intrusion, the Tatoosh intrusive suite may be representative of midsized composite plutonic complexes beneath arc volcanoes.

Washington↗
USGS2009

Geochemical database for igneous rocks of the ancestral Cascades Arc— Southern segment, California and Nevada

Volcanic rocks that form the southern segment of the Cascades magmatic arc are an important manifestation of Cenozoic subduction and associated magmatism in western North America. Until recently, these rocks had been little studied and no systematic compilation of existing composition data had been assembled. This report is a compilation of all available chemical data for igneous rocks that constitute the southern segment of the ancestral Cascades magmatic arc and complement a previously completed companion compilation that pertains to rocks that constitute the northern segment of the arc. Data for more than 2,000 samples from a diversity of sources were identified and incorporated in the database. The association between these igneous rocks and spatially and temporally associated mineral deposits is well established and suggests a probable genetic relationship. The ultimate goal of the related research is an evaluation of the time-space-compositional evolution of magmatism associated with the southern Cascades arc segment and identification of genetic associations between magmatism and mineral deposits in this region.

California, Nevada↗
USGS2009

Geochemical database for the Boulder batholith and its satellitic plutons, Southwest Montana

Geochemical data presented in this report pertain to Cretaceous igneous intrusions of the Boulder batholith and its satellitic plutons in southwest Montana. The geographic area addressed in this compilation is approximately bounded by lats 45.6 deg and 46.7 deg N. and longs 112.75 deg and 111.5 deg W. These data were compiled in order to establish the geologic framework for world-class mineral deposits of the Butte district. Although these deposits and their host rocks have been the subject of many investigations, the petrologic characteristics of associated intrusive rocks have not been systematically compiled, synthesized, or interpreted. Abundant late Mesozoic intrusions in the study area are probably byproducts of subduction-related processes, including back-arc magmatism that prevailed along the west edge of the North American plate during this interval. The ultimate goal of this effort will be an evaluation of the time-space-compositional evolution of Mesozoic magmatism associated with the Boulder batholith and identification of genetic associations between magmatic and mineralizing processes in this region.

Data Series↗
USGS2009

Complete Analytical Data for Samples of Jurassic Igneous Rocks in the Bald Mountain Mining District, Nevada

This report presents all petrographic, major oxide, and trace element data for a set of 109 samples collected during an investigation of Jurassic igneous rocks in the Bald Mountain mining district, Nevada. Igneous rocks in the district include the Bald Mountain stock, quartz-feldspar porphyry dikes, basaltic andesite dikes, aplite sills, and rare lamprophyre dikes. These rocks, although variably altered near intrusion-related mineral deposits, are fresh in many parts of the district. Igneous rocks in the district are hosted by Paleozoic sedimentary rocks.

Open-File Report↗
USGS2007

Geochemical Database for Intrusive Rocks of North-Central and Northeast Nevada

North-central and northeast Nevada contains numerous large plutons and smaller stocks but also contains many small, shallowly emplaced intrusive bodies, including dikes, sills, and intrusive lava dome complexes. Decades of geologic investigations in the study area demonstrate that many ore deposits, representing diverse ore deposit types, are spatially, and probably temporally and genetically, associated with these igneous intrusions. However, despite the number and importance of igneous intrusions in the study area, no synthesis of geochemical data available for these rocks has been completed. This report presents a synthesis of geochemical data for these rocks. The product represents the first phases of an effort to evaluate the time-space-compositional evolution of Mesozoic and Cenozoic magmatism in the study area and identify genetic associations between magmatism and mineralizing processes in this region.

Data Series↗
USGS2004

Geochemistry and Geochronology of Middle Tertiary Volcanic Rocks of the Central Chiricahua Mountains, Southeast Arizona

Middle Tertiary volcanic rocks of the central Chiricahua Mountains in southeast Arizona are the westernmost constituents of the Eocene-Oligocene Boot Heel volcanic field of southwestern New Mexico and southeastern Arizona. About two dozen volumetric ally and stratigraphically significant volcanic units are present in this area. These include large-volume, regionally distributed ash-flow tuffs and smaller volume, locally distributed lava flows. The most voluminous of these units is the Rhyolite Canyon Tuff, which erupted 26.9 million years ago from the Turkey Creek caldera in the central Chiricahua Mountains. The Rhyolite Canyon Tuff consists of 500-1,000 cubic kilometers of rhyolite that was erupted from a normally zoned reservoir. The tuff represents sequential eruptions, which became systematically less geochemically evolved with time, from progressively deeper levels of the source reservoir. Like the Rhyolite Canyon Tuff, other ashflow tuffs preserved in the central Chiricahua Mountains have equivalents in nearby, though isolated mountain ranges. However, correlation of these other tuffs, from range to range, has been hindered by stratigraphic discontinuity, structural complexity, and various lithologic similarities and ambiguities. New geochemical and geochronologic data presented here enable correlation of these units between their occurrences in the central Chiricahua Mountains and the remainder of the Boot Heel volcanic field. Volcanic rocks in the central Chiricahua Mountains are composed dominantly of weakly peraluminous, high-silica rhyolite welded tuff and rhyolite lavas of the high-potassium and shoshonitic series. Trace-element, and to a lesser extent, major-oxide abundances are distinct for most of the units studied. Geochemical and geochronologic data depict a time and spatial transgression from subduction to within-plate and extensional tectonic settings. Compositions of the lavas tend to be relatively homogeneous within particular units. In contrast, compositions of the ash-flow tuffs, including the Rhyolite Canyon Tuff, vary significantly owing to eruption from compositionally zoned reservoirs. Reservoir zonation is consistent with fractional crystallization of observed phenocryst phases and resulting residual liquid compositional evolution. Rhyolite lavas preserved in the moat of the Turkey Creek caldera depict compositional zonation that is the reverse of that expected of magma extraction from progressively deeper parts of a normally zoned reservoir. Presuming that the source reservoir was sequentially tapped from its top downward, development of reverse zonation in the rhyolite lava sequence may indicate that later erupted, more evolved magma contains systematically less wallrock contamination derived from the geochemically primitive margins of its incompletely mixed reservoir. New 40Ar/39Ar geochronology data indicate that the principal middle Tertiary volcanic rocks in the central Chiricahua Mountains were erupted between about 34.2 and 26.2 Ma, and that the 5.2 m.y. period between 33.3 and 28.1 Ma was amagmatic. The initial phase of eruptive activity in the central Chiricahua Mountains, between 34.2 and 33.3 Ma, was associated with a regional tectonic regime dominated by subduction along the west edge of North America. We infer that the magmatic hiatus, nearly simultaneous with a hiatus of similar duration in parts of the Boot Heel volcanic field east of the central Chiricahua Mountains, is related to a period of more rapid convergence and therefore shallower subduction that may have displaced subduction-related magmatic activity to a position east of the present-day Boot Heel volcanic field. The hiatus also coincides with a major plate tectonic reorganization along the west edge of North America that resulted in cessation of subduction and initiation of transform faulting along the San Andreas fault. The final period of magmatism in the central Chiricahua Mountains, between 28.1 and 23.2 Ma, ap

Professional Paper↗
USGS2002

SHRIMP U-Pb and 40Ar/39Ar age constraints for relating plutonism and mineralization in the Boulder batholith region, Montana

The composite Boulder batholith, Montana, hosts a variety of mineral deposit types, including important silver-rich polymetallic quartz vein districts in the northern part of the batholith and the giant Butte porphyry copper-molybdenum pre-Main Stage system and crosscutting copper-rich Main Stage vein system in the southern part of the batholith. Previous dating studies have identified ambiguous relationships among igneous and mineralizing events. Mineralizing hydrothermal fluids for these types of deposits and magma for quartz porphyry dikes at Butte have all been considered to be late-stage differentiates of the Boulder batholith. However, previous dating studies indicated that the Boulder batholith plutons cooled from about 78 to 72 Ma, whereas copper-rich Main Stage veins at Butte were dated at about 61 Ma. Recent efforts to date the porphyry copper-molybdenum pre-Main Stage deposits at Butte resulted in conflicting estimates of both 64 and 76 Ma for the mineralizing events. Silver-rich polymetallic quartz vein deposits elsewhere in the batholith have not been dated previously. To resolve this controversy, we used the U.S. Geological Survey, Stanford, SHRIMP RG ion microprobe to date single-age domains within zircons from plutonic rock samples and 40 Ar/ 39 Ar geochronology to date white mica, biotite, and K-feldspar from mineral deposits. U-Pb zircon ages are Rader Creek Granodiorite, 80.4 ± 1.2 Ma; Unionville Granodiorite, 78.2 ± 0.8 Ma; Pulpit Rock granite, 76.5 ± 0.8 Ma; Butte Granite, 74.5 ± 0.9 Ma; altered Steward-type quartz porphyry dike (I-15 roadcut), 66.5 ± 1.0 Ma; altered Steward-type quartz porphyry dike (Continental pit), 65.7 ± 0.9 Ma; and quartz monzodiorite of Boulder Baldy (Big Belt Mountains), 66.2 ± 0.9 Ma. Zircons from Rader Creek Granodiorite and quartz porphyry dike samples contain Archean inheritance. The 40 Ar/ 39 Ar ages are muscovite, silver-rich polymetallic quartz vein (Basin district), 74.4 ± 0.3 Ma; muscovite, silver-rich polymetallic quartz vein (Boulder district), 74.4 ± 1.2 Ma; muscovite, early dark micaceous vein (Continental pit), 63.6 ± 0.2 Ma; biotite, early dark micaceous vein (Continental pit), 63.6 ± 0.2 Ma; potassium feldspar, early dark micaceous vein (Continental pit), 63 to 59 Ma; and biotite, biotite breccia dike (Continental pit), 63.6 ± 0.2 Ma. Outlying silver-rich polymetallic quartz veins of the Basin and Boulder mining districts probably are directly related to the 74.5 Ma Butte Granite, whereas Steward-type east-west quartz porphyry dikes and Butte pre-Main Stage deposits are parts of a 66 to 64 Ma magmatic-mineralization system unrelated to emplacement of the Boulder batholith. The age of the crosscutting Main Stage veins may be about 61 Ma as originally reported but can only be bracketed as younger than the 64 Ma pre-Main Stage mineralization and older than the about 50 Ma Eocene Lowland Creek intrusions. The 66 Ma age for the quartz monzodiorite of Boulder Baldy and consideration of previous dating studies in the region indicate that small ca. 66 Ma plutonic systems may be common in the Boulder batholith region and especially to the east. The approximately 64 Ma porphyry copper systems at Butte and gold mineralization at Miller Mountain are indicative of regionally important mineralizing systems of this age in the Boulder batholith region. Resolution of the age and probable magmatic source of the Butte pre-Main Stage porphyry copper-molybdenum system and of the silver-rich polymetallic quartz vein systems in the northern part of the Boulder batholith documents that these deposits formed from two discrete periods of hydrothermal mineralization related to two discrete magmatic events.

Montana↗