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Geology topics

J. Michael O’Neill

Publications and source records attributed to J. Michael O’Neill.

13 recordsLinked to original sources
USGS2014

Geologic map of the Kechumstuk fault zone in the Mount Veta area, Fortymile mining district, east-central Alaska

This map was developed by the U.S. Geological Survey Mineral Resources Program to depict the fundamental geologic features for the western part of the Fortymile mining district of east-central Alaska, and to delineate the location of known bedrock mineral prospects and their relationship to rock types and structural features. This geospatial map database presents a 1:63,360-scale geologic map for the Kechumstuk fault zone and surrounding area, which lies 55 km northwest of Chicken, Alaska. The Kechumstuk fault zone is a northeast-trending zone of faults that transects the crystalline basement rocks of the Yukon-Tanana Upland of the western part of the Fortymile mining district. The crystalline basement rocks include Paleozoic metasedimentary and metaigneous rocks as well as granitoid intrusions of Triassic, Jurassic, and Cretaceous age. The geologic units represented by polygons in this dataset are based on new geologic mapping and geochronological data coupled with an interpretation of regional and new geophysical data collected by the Alaska Department of Natural Resources, Division of Geological and Geophysical Surveys. The geochronological data are reported in the accompanying geologic map text and represent new U-Pb dates on zircons collected from the igneous and metaigneous units within the map area.

Alaska↗
USGS2013

Effect of ultramafic intrusions and associated mineralized rocks on the aqueous geochemistry of the Tangle Lakes Area, Alaska

Stream water was collected at 30 sites within the Tangle Lakes area of the Delta mineral belt in Alaska. Sampling focused on streams near the ultramafic rocks of the Fish Lake intrusive complex south of Eureka Creek and the Tangle Complex area east of Fourteen Mile Lake, as well as on those within the deformed metasedimentary, metavolcanic, and intrusive rocks of the Specimen Creek drainage and drainages east of Eureka Glacier. Major, minor, and trace elements were analyzed in aqueous samples for this reconnaissance aqueous geochemistry effort. The lithologic differences within the study area are reflected in the major-ion chemistry of the water. The dominant major cation in streams draining mafic and ultramafic rocks is Mg 2+ ; abundant Mg and low Ca in these streams reflect the abundance of Mg-rich minerals in these intrusions. Nickel and Cu are detected in 84 percent and 87 percent of the filtered samples, respectively. Nickel and Cu concentrations ranged from Ni <0.4 to 10.1 micrograms per liter (mg/L), with a median of 4.2 mg/L, and Cu <0.5 to 27 mg/L, with a median of 1.2 mg/L. Trace-element concentrations in water are generally low relative to U.S. Environmental Protection Agency freshwater aquatic-life criteria; however, Cu concentrations exceed the hardness-based criteria for both chronic and acute exposure at some sites. The entire rare earth element (REE) suite is found in samples from the Specimen Creek sites MH5, MH4, and MH6 and, with the exception of Tb and Tm, at site MH14. These samples were all collected within drainages containing or downstream from Tertiary gabbro, diabase, and metagabbro (Trgb) exposures. Chondrite and source rock fractionation profiles for the aqueous samples were light rare earth element depleted, with negative Ce and Eu anomalies, indicating fractionation of the REE during weathering. Fractionation patterns indicate that the REE are primarily in the dissolved, as opposed to colloidal, phase.

Alaska↗
USGS2011

Control of Precambrian basement deformation zones on emplacement of the Laramide Boulder batholith and Butte mining district, Montana, United States

What are the roles of deep Precambrian basement deformation zones in the localization of subsequent shallow-crustal deformation zones and magmas? The Paleoproterozoic Great Falls tectonic zone and its included Boulder batholith (Montana, United States) provide an opportunity to examine the importance of inherited deformation fabrics in batholith emplacement and the localization of magmatic-hydrothermal mineral deposits. Northeast-trending deformation fabrics predominate in the Great Falls tectonic zone, which formed during the suturing of Paleoproterozoic and Archean cratonic masses approximately 1,800 mega-annum (Ma). Subsequent Mesoproterozoic to Neoproterozoic deformation fabrics trend northwest. Following Paleozoic through Early Cretaceous sedimentation, a Late Cretaceous fold-and-thrust belt with associated strike-slip faulting developed across the region, wherein some Proterozoic faults localized thrust faulting, while others were reactivated as strike-slip faults. The 81- to 76-Ma Boulder batholith was emplaced along the reactivated central Paleoproterozoic suture in the Great Falls tectonic zone. Early-stage Boulder batholith plutons were emplaced concurrent with east-directed thrust faulting and localized primarily by northwest-trending strike-slip and related faults. The late-stage Butte Quartz Monzonite pluton was localized in a northeast-trending pull-apart structure that formed behind the active thrust front and is axially symmetric across the underlying northeast-striking Paleoproterozoic fault zone, interpreted as a crustal suture. The modeling of potential-field geophysical data indicates that pull-apart?stage magmas fed into the structure through two funnel-shaped zones beneath the batholith. Renewed magmatic activity in the southern feeder from 66 to 64 Ma led to the formation of two small porphyry-style copper-molybdenum deposits and ensuing world-class polymetallic copper- and silver-bearing veins in the Butte mining district. Vein orientations parallel joints in the Butte Quartz Monzonite that, in turn, mimic Precambrian deformation fabrics found outside the district. The faults controlling the Butte veins are interpreted to have formed through activation under shear of preexisting northeast-striking joints as master faults from which splay faults formed along generally east-west and northwest joint plane orientations.

Scientific Investigations Report↗
USGS2008

Geologic Field Notes, Geochemical Analyses, and Field Photographs of Outcrops and Rock Samples from the Big Delta B-1 Quadrangle, East-Central Alaska

The U.S. Geological Survey, in cooperation with the Alaska Department of Natural Resources Division of Mining, Land, and Water, has released a geologic map of the Big Delta B-1 quadrangle of east-central Alaska (Day and others, 2007). This companion report presents the major element oxide and trace element geochemical analyses, including those for gold, silver, and base metals, for representative rock units and for grab samples from quartz veins and mineralized zones within the quadrangle. Also included are field station locations, field notes, structural data, and field photographs based primarily on observations by W.C. Day with additions by J.M. O'Neill and B.M. Gamble, all of the U.S. Geological Survey. The data are provided in both Microsoft Excel spread sheet format and as a Microsoft Access database.

Open-File Report↗
USGS2007

Geologic Map of the Big Delta B-1 Quadrangle, East-Central Alaska

Geologic mapping and U-Pb age dating of rocks from the Big Delta B-1 quadrangle, east-central Alaska, have yielded new insights into the geology and gold mineral resource for the headwater region of the Goodpaster River, northeast of Delta, Alaska. The area lies within the Yukon-Tanana Upland and is underlain by Paleozoic and Cretaceous crystalline bedrock and contains several gold mines and prospects. The Paleozoic units include biotite gneiss, quartzite interlayered with metapelite, and amphibolite gneiss. The Paleozoic units were intruded during the Devonian by tonalitic to granitic plutons, which, as a result of regional Mesozoic metamorphism and tectonism, are now augen gneiss and biotite orthogneiss. The Mesozoic regional metamorphism and ductile deformation of the entire Yukon-Tanana Upland culminated by the Late Cretaceous (about 116 Ma) as a result of northwest-directed regional transpression along the southern margin of the North American craton. This dynamothermal episode was followed by invasion of syn- to post-tectonic granodioritic to granitic batholiths during the Late Cretaceous (about 113-107 Ma), followed by a pulse of 100-95 Ma quartz feldspar porphyry intrusions. Gold mineralization is spatially associated with various post-tectonic Late Cretaceous granitic dikes and batholiths throughout the quadrangle. A northeast-trending structural corridor, described herein as the Black Mountain tectonic zone, both controlled the emplacement of some of the Cretaceous intrusive rocks, gold deposits, and prospects, as well as formed a deep-seated crustal conduit along which a subsequent rhyolite flow-dome complex erupted during the Paleocene. Tertiary uplift and erosion resulted in the development of extensive erosional pediments. Quaternary alpine glaciation carved beautiful, broad valleys in the eastern part of the quadrangle, leaving behind terminal moraines in the headwater region of the Goodpaster river drainage. Continued Holocene to Recent deformation along the Black Mountain tectonic zone has offset Tertiary terraces, as well as Quaternary fluvial and alluvial deposits, indicating that the area has a long, complex, and ongoing tectonic history.

Scientific Investigations Map↗
USGS2007

The Black Mountain tectonic zone--a reactivated northeast-trending crustal shear zone in the Yukon-Tanana Upland of east-central Alaska: Chapter D in Recent U.S. Geological Survey studies in the Tintina Gold Province, Alaska, United States, and Yukon, Canada--results of a 5-year project

The Black Mountain tectonic zone in the YukonTanana terrane of east-central Alaska is a belt of diverse northeast-trending geologic features that can been traced across Black Mountain in the southeast corner of the Big Delta 1&deg;×3&deg; degree quadrangle. Geologic mapping in the larger scale B1 quadrangle of the Big Delta quadrangle, in which Black Mountain is the principal physiographic feature, has revealed a continuous zone of normal and left-lateral strikeslip high-angle faults and shear zones, some of which have late Tertiary to Quaternary displacement histories. The tectonic zone includes complexly intruded wall rocks and intermingled apophyses of the contiguous mid-Cretaceous Goodpaster and Mount Harper granodioritic plutons, mafic to intermediate composite dike swarms, precious metal mineralization, early Tertiary volcanic activity and Quaternary fault scarps. These structures define a zone as much as 6 to 13 kilometers (km) wide and more than 40 km long that can be traced diagonally across the B1 quadrangle into the adjacent Eagle 1&deg;×3&deg; quadrangle to the east. Recurrent activity along the tectonic zone, from at least mid-Cretaceous to Quaternary, suggests the presence of a buried, fundamental tectonic feature beneath the zone that has influenced the tectonic development of this part of the Yukon-Tanana terrane. The tectonic zone, centered on Black Mountain, lies directly above a profound northeast-trending aeromagnetic anomaly between the Denali and Tintina fault systems. The anomaly separates moderate to strongly magnetic terrane on the northwest from a huge, weakly magnetic terrane on the southeast. The tectonic zone is parallel to the similarly oriented left-lateral, strike-slip Shaw Creek fault zone 85 km to the west.

Alaska;Yukon↗
USGS2004

Geologic map of the Hebgen Lake quadrangle, Beaverhead, Madison, and Gallatin counties, Montana, Park and Teton counties, Wyoming, and Clark and Fremont counties, Idaho

The geology of the Hebgen Lake Quadrangle was mapped as part of an Ongoing research effort by the USGS to understand the geologic framework of the area in and around Yellowstone National Park. Prior to 1965 the regional geology was known only from reconnaissance surveys. Two important components of this effort are an evaluation of (1) the seismic risk hazard and (2) the mineral resource potential.

Idaho;Montana;Wyoming↗
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↗
USGS1985

Character and regional significance of Great Falls tectonic zone, east-central Idaho and west-central Montana

The Great Falls tectonic zone, here named, is a belt of diverse northeast-trending geologic features that can be traced from the Idaho batholith in the Cordilleran miogeocline, across thrust-belt structures and basement rocks of west-central and southwestern Montana, through cratonic rocks of central Montana, and into southwesternmost Saskatchewan, Canada. Geologic mapping in east-central Idaho and west-central Montana has outlined a continuous zone of high-angle faults and shear zones. These structures (1) extend more than 150 km (93 mi) northeastward from near Salmon, Idaho, toward Anaconda, Montana, (2) had recurrent movement from middle Proterozoic to Holocene time, (3) controlled the intrusion and orientation of Late Cretaceous to early Tertiary dike swarms, and (4) ontrolled the uplift and orientation of the Anaconda-Pintlar Range. Recurrent fault movement in this zone and strong structural control over igneous intrusion suggest a fundamental tectonic feature that has influenced the tectonic development of the Idaho-Montana area from at least middle Proterozoic time to the present.

Idaho, Montana↗