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USGS2009

Metal exposure and effects in voles and small birds near a mining haul road in Cape Krusenstern National Monument, Alaska

Voles and small passerine birds were live-captured near the Delong Mountain Regional Transportation System (DMTS) haul road in Cape Krusenstern National Monument in northwest Alaska to assess metals exposure and sub-lethal biological effects. Similar numbers of animals were captured from a reference site in southern Cape Krusenstern National Monument for comparison. Histopathological examination of selected organs, and analysis of cadmium, lead, and zinc concentrations in liver and blood samples were performed. Voles and small birds captured from near the haul road had about 20 times greater blood and liver lead concentrations and about three times greater cadmium concentrations when compared to those from the reference site, but there were no differences in zinc tissue concentrations. One vole had moderate metastatic mineralization of kidney tissue, otherwise we observed no abnormalities in internal organs or DNA damage in the blood of any of the animals. The affected vole also had the greatest liver and blood Cd concentration, indicating that the lesion might have been caused by Cd exposure. Blood and liver lead concentrations in animals captured near the haul road were below concentrations that have been associated with adverse biological effects in other studies; however, subtle effects resulting from lead exposure, such as the suppression of the activity of certain enzymes, cannot be ruled out for some individual animals. Results from our 2006 reconnaissance-level study indicate that overall, voles and small birds obtained from near the DMTS road in Cape Krusenstern National Monument were not adversely affected by metals exposure; however, because of the small sample size and other uncertainties, continued monitoring of lead and cadmium in terrestrial habitats near the DMTS road is advised.

Alaska↗

USGS1955

Origin of the upland silt near Fairbanks, Alaska

Silt mantles the upland slopes and ridge tops throughout the unglaciated interior of Alaska, reaching maximum thickness along the north side of the Tanana River Valley. The silt is probably loess deposited during glacial advances by south winds blowing from the glaciated Alaska Range across outwash plains in the Tanana Valley. In the Fairbanks area, where it is best exposed, the upland silt is 1–80 feet thick on tops of low hills, 50–150 feet above the valley floor. It is 10–100 feet thick on middle slopes of higher hills and thins to a few feet on the higher slopes of ridges 800–2000 feet above the valley. Much silt has been reworked and moved by stream erosion into valley bottoms where it forms a fill 10–300 feet thick. The upland silt is well sorted. The texture and mineral composition are uniform throughout the Yukon-Tanana upland, whether the silt overlies schist, chert, granite or basalt. The silt stands in sheer cliffs and is massive, showing little or no stratification. During the past 50 years, fluviatile, marine, estuarine, lacustrine, residual, and eolian hypotheses have been proposed to explain the origin of the upland silt. The fluviatile marine and estuarine hypotheses never had strong support, but the lacustrine concept was popular in the early part of the century. Most of the upland silt is considered to be wind blown, derived from glacial outwash because: (1) it occurs as a surficial mantle (2) it is lithologically independent of the underlying material, (3) stratification is indistinct or absent, (4) it is associated with sand dunes and ventifacts, (5) it contains fossils of air-breathing land animals, (6) the sorting and texture are similar to upper Mississippi Valley loess and wind-blown dust, (7) the grains are angular and relatively fresh, and (8) loess is being deposited in the region today.

Alaska↗

USGS1953

Geology of the Olds Mountain-Clark Peak area, Juneau and vicinity, Alaska

The area under study lies about ten miles east of Juneau, Alaska, and includes the bedded rocks adjacent to the Coast Range batholith, and intrusive rocks related to the batholith. Deep glaciated valleys and glacially scoured rocks are the major topographic features. The relief is about 3,500 feet, and the highest mountain reaches an altitude of 4,453 feet. Remnants of larger glaciers and perennial snow fields cover many of the higher summits, and glacial erratics at elevations above 3,100 feet attest to a former thick ice cover. The bedded rocks are crystalline schists that locally reach katozonal metamorphic rank as indicated by biotite-garnet-kyanite-sillimanite mineral assemblages. The schists are believed to be dominantly a product of regional metamorphism predating the intrusion of rocks related to the Coast Range batholith. It appears, however, that stresses continued to be active during igneous and granitizing activity accompanying the Coast Range orogeny. The major intrusive bodies are a composite quartz-dioritic batholith and quartz-diorite sills that locally reach a thickness of more than two thousand feet. The sills usually are notably gneissic, and appear to have formed by intrusion rather than replacement. The batholith is a composite body formed by migmatization, replacement, and local fusion. A stressed environment during formation is suggested by a primary gneissic structure. A high quartz content and a paucity of potash feldspars is diagnostic of the intrusive bodies. In addition to the bodies of quartz-diorite, small sills and dikes of gabbro, quartz-pegmatite, and unmetamorphosed diabase intrude the schists. Overturned folds in the schists are nearly isoclinal, and some of the folds are large enough to cause repetition of beds in the Clark Peak schists. Two persistent northwest-trending strike faults of small displacement cut the schists. Two east-trending faults of larger displacement are marked by thick gouge zones locally containing graphite. The rocks near the faults are hydrothermally altered and pyritized, and at least two of the diabase dikes are crushed and hydrothermally altered near one of the northwest-trending faults. Faulting and hydrothermal activity probably continued at least into the early Tertiary.

Open-File Report↗

USGS1970

Geologic interpretation of reconnaissance aeromagnetic survey of northeastern Alaska

Aeromagnetic reconnaissance of northeastern Alaska in 1965 covered approximately 100,000 square miles, from lat 64° N. to the Arctic Ocean and from the Alaska-Yukon boundary to long 148°30' W. A magnetic contour map was compiled from data from the 1965 survey and earlier surveys. On the basis of these data, northeastern Alaska may be divided into five areas, each having a distinct magnetic character. A study of specific anomalies within these areas led to the tracing of such features as the Tintina fault zone, Ruby uplift, and Kobuk trench for more than 100 miles. Other features, such as inferred north-trending faults and serpentine belts, had not been identified previously by surface geologic methods. The probable extension of volcanic rocks beneath the Yukon Flats limits the size of a possible Tertiary petroleum basin. Discovery of large magnetic anomalies in the Yukon-Tanana Upland provides new possibilities for exploration for mineral deposits associated with ultramafic rocks.

Alaska↗

USGS1993

Factors controlling the geochemical evolution of fumarolic encrustations, Valley of Ten Thousand Smokes, Alaska

Factor and canonical correlation analysis of geochemical data from eight fossil fumaroles suggest that six major factors controlled the formation and evolution of fumarolic encrustations on the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes (VTTS). The six-factor solution model explains a large proportion (low of 74% for Ni to high of 99% for Si) of the individual element data variance. Although the primary fumarolic deposits have been degraded by secondary alteration reactions and up to 75 years of weathering, the relict encrustations still preserve a signature of vapor-phase element transport. This vapor-phase transport probably occurred as halide or oxyhalide species and was significant for As, Sb and Br. At least three, and possibly four, varied temperature leaching events affected the fumarolic deposits. High-temperature gases/liquids heavily altered the ejecta glass and mineral phases adjacent to the fumarolic conduit. As the fumaroles cooled. Fe-rich acidic condensate leached the ejecta and primary fumarolic deposits and resulted in the subsequent precipitation of Fe-hydroxides and/or Fe-oxides. Low- to ambient-temperature leaching and hydration reactions generated abundant hydrated amorphous phases. Up to 87% of the individual element data variance is apparently controlled by the chemistry of the ejecta on which the relict encrustations are found. This matrix chemistry factor illustrates that the primary fumarolic minerals surrounding the active VTTS vents observed by earlier workers have been effectively removed by the dissolution reactions. Element enrichment factors calculated for the VTTS relict encrustations support the statistical factor interpretations. On the average, the relict encrustations are enriched, relative to visibly unaltered matrix protolith, in As, Br, Cr, Sb, Cu, Ni, Pb, Fe, and LOI (an indirect measure of sample H2O content). ?? 1993.

Journal of Volcanology and Geothermal Research↗

USGS2002

Mesozoic thermal history and timing of structural events for the Yukon-Tanana Upland, east-central Alaska: 40Ar/39Ar data from metamorphic and plutonic rocks

We present new 40 Ar/ 39 Ar ages for hornblende, muscovite, and biotite from metamorphic and plutonic rocks from the YukonTanana Upland, Alaska. Integration of our data with published 40 Ar/ 39 Ar, kinematic, and metamorphic pressure ( P ) and temperature ( T ) data confirms and refines the complex interaction of metamorphism and tectonism proposed for the region. The oldest metamorphic episode(s) postdates Middle Permian magmatism and predates the intrusion of Late Triassic (215212 Ma) granitoids into the Fortymile River assemblage (Taylor Mountain assemblage of previous papers). In the eastern Eagle quadrangle, rapid and widespread Early Jurassic cooling is indicated by ~188186 Ma 40 Ar/ 39 Ar plateau ages for hornblende from plutons that intrude the Fortymile River assemblage, and for metamorphic minerals from the Fortymile River assemblage and the structurally underlying Nasina assemblage. We interpret these Early Jurassic ages to represent cooling resulting from northwest-directed contraction that emplaced the Fortymile River assemblage onto the Nasina assemblage to the north as well as the Lake George assemblage to the south. This cooling was the final stage of a continuum of subduction-related contraction that produced crustal thickening, intermediate- to high- P metamorphism within both the Fortymile River assemblage and the structurally underlying Lake George assemblage, and Late Triassic and Early Jurassic plutonism in the Fortymile River and Nasina assemblages. Although a few metamorphic samples from the Lake George assemblage yield Jurassic 40 Ar/ 39 Ar cooling ages, most yield Early Cretaceous 40 Ar/ 39 Ar ages: hornblende ~135115 Ma, and muscovite and biotite ~110108 Ma. We interpret the Early Cretaceous metamorphic cooling, in most areas, to have resulted from regional extension and exhumation of the lower plate, previously tectonically thickened during Early Jurassic and older convergence.

Alaska↗

USGS2014

Structural control of ore deposition at Kasaan Peninsula, southeastern Alaska

Copper‐bearing magnetite deposits and associated copper deposits at Kasaan Peninsula , Prince of Wales Island, southeastern Alaska , have been known for many years and have been mined to some extent for their copper content. At the beginning of World War II, the development of war industries in the Pacific northwest focused attention on the deposits as possible sources of iron ore . From 1942 through 1944 the United States Geological Survey made detailed studies of most of the ore deposits and at a few of the more promising localities diamond‐drilling operations were carried on by the United States Bureau of Mines. The deposits are for the most part small and irregular but of relatively high grade. The chief ore minerals are magnetite, pyrite, and chalcopyrite, and the gangue consists largely of skarn minerals, including garnet, epidote, diopside, and hornblende. The deposits are presumably of pyrometasomatic origin, but whereas deposits of this type are commonly replacement bodies in limestone, at Kasaan Peninsula limestone seemingly was a poor host rock for the ore.

Alaska↗

USGS1962

Epigenetic uranium deposits in the United States, exclusive of Alaska and Hawaii

Productive and unproductive deposits of uranium in the United States (exclusive of Alaska and Hawaii) are shown on the accompanying map. These deposits constitute the actual and some of the potential domestic raw-material sources of uranium. Other potential sources of uranium similar to these known deposits remain to be discovered and are probably distributed, at least in part, in the same general way as the deposits shown on the map.

Mineral Investigations Resource Map↗

USGS1962

Talc and soapstone in the United States, exclusive of Alaska and Hawaii

The locations of talc and soapstone deposits in the United States (exclusive of Alaska and Hawaii) arc shown on the accompanying map. The two types are distinguished by the shape of symbols; the relative importance of the deposits, expressed in broad ranges of tonnage, is indicated by the size of symbols. Two persistent belts in the Southeastern States are shown by patterns, and the locations of the principal deposits within these belts are shown by appropriate symbols

Mineral Investigations Resource Map↗

USGS1998

Natural attenuation of chlorinated-hydrocarbon contamination at Fort Wainwright, Alaska: A hydrogeochemical and microbiological investigation workplan

Natural attenuation processes include biological degradation, by which microorganisms break down contaminants into simpler product compounds; adsorption of contaminants to soil particles, which decreases the mass of contaminants dissolved in ground water; and dispersion, which decreases dissolved contaminant concentrations through dilution. The primary objectives of this study are to (1) assess the degree to which such natural processes are attenuating chlorinated-hydrocarbon contamination in ground water, and (2) evaluate the effects of ground-water/surface-water interactions on natural-attenuation processes in the area of the former East and West Quartermasters Fueling Systems for Fort Wainwright, Alaska. The study will include investigations of the hydrologic, geochemical, and microbiological processes occurring at this site that influence the transport and fate of chlorinated hydrocarbons in ground water. To accomplish these objectives, a data-collection program has been initiated that includes measurements of water-table elevations and the stage of the Chena River; measurements of vertical temperature profiles within the subsurface; characterization of moisture distribution and movement in the unsaturated zone; collection of ground-water samples for determination of both organic and inorganic chemical constituents; and collection of ground-water samples for enumeration of microorganisms and determination of their potential to mineralize contaminants. We will use results from the data-collection program described above to refine our conceptual model of hydrology and contaminant attenuation at this site. Measurements of water-table elevations and river stage will help us to understand the magnitude and direction of ground-water flow and how changes in the stage of the Chena River affect ground-water flow. Because ambient ground water and surface water typically have different temperature characteristics, temperature monitoring will likely provide further insight into ground-water/surface-water interactions in the subsurface. Characterization of the unsaturated zone will improve our understanding of interactions among ground water, the unsaturated zone, and the atmosphere. The interactions likely of importance to this study include the migration of water, dissolved contaminants, nutrients, and gases (oxygen, carbon dioxide, and methane) between the saturated and unsaturated zones. We will use the results of ground-water chemical analyses to determine the spatial and temporal distribution of (1) chlorinated-hydrocarbon contaminants and their degradation products, (2) oxidation-reduction indicators, (3) nutrients, and (4) major ground-water ions. These water-quality data will provide insight into ground-water flow directions, interactions between ground water and surface water, attenuation of contaminant concentrations caused by dispersion, and intrinsic microbiological processes. Microbiological analyses will indicate whether microorganisms at the site are capable of degrading the contaminants of interest, and will allow us to estimate their potential to attenuate existing contamination. Physical and chemical data interpreted as part of the analysis of ground water and surface water mixing will improve our understanding of the relationship between water quality and contaminant source mixing.

Alaska↗

USGS2014

The 3D Elevation Program: summary for Oregon

Elevation data are essential to a broad range of business uses, including forest resources management, wildlife and habitat management, national security, recreation, and many others. In the State of Oregon, elevation data are critical for river and stream resource management; forest resources management; water supply and quality; infrastructure and construction management; wildfire management, planning and response; natural resources conservation; and other business uses. Today, high-density light detection and ranging (lidar) data are the primary source for deriving elevation models and other datasets. The Oregon Lidar Consortium (OLC), led by the Oregon Department of Geology and Mineral Industries (DOGAMI), has developed partnerships with Federal, State, Tribal, and local agencies to acquire quality level 1 data in areas of shared interest. The goal of OLC partners is to acquire consistent, high-resolution and high-quality statewide coverage to support existing and emerging applications enabled by lidar data. The National Enhanced Elevation Assessment evaluated multiple elevation data acquisition options to determine the optimal data quality and data replacement cycle relative to cost to meet the identified requirements of the user community. The evaluation demonstrated that lidar acquisition at quality level 2 for the conterminous United States and quality level 5 ifsar data for Alaska with a 6- to 10-year acquisition cycle provided the highest benefit/cost ratios. The 3D Elevation Program (3DEP) initiative selected an 8-year acquisition cycle for the respective quality levels. 3DEP, managed by the U.S. Geological Survey (USGS), the Office of Management and Budget Circular A–16 lead agency for terrestrial elevation data, responds to the growing need for high-quality topographic data and a wide range of other 3D representations of the Nation’s natural and constructed features.

Oregon↗

USGS2007

The biogeochemistry and occurrence of unusual plant species inhabiting acidic, metal-rich water, Red Mountain, Bonnifield district, Alaska Range: Chapter J in Recent U.S. Geological Survey studies in the Tintina Gold Province, Alaska, United States, and Yukon, Canada--results of a 5-year project

This report presents results on the occurrence and biogeochemistry of unusual plant species, and of their supporting sediment, in an undisturbed volcanogenic massive sulfide deposit in the Tintina Gold Province (see fig. 1 of Editors’ Preface and Overview). The extraordinary plant assemblage found growing in the acidic metal-rich waters that drain the area is composed predominantly of bryophytes (liverworts and mosses). Ferricrete-cemented silty alluvial sediments within seeps and streams are covered with the liverwort Gymnocolea inflata, whereas the mosses Polytrichum commune and P. juniperinum inhabit the area adjacent to the water and within the splash zone. Both the liverwort-encrusted sediment and Polytrichum thalli have high concentrations of major- and trace-metal cations (for example, Al, As, Cu, Fe, Hg, La, Mn, Pb, and Zn). Soils in the area do not reflect the geochemical signature of the mineral deposit, and we suspect that they are most influenced by the chemistry of airborne dust (aeolian material) derived from outside the area.

Alaska;Yukon↗

USGS1993

Reconnaissance guidelines for gold exploration in Central Alaska

Distribution of more than 300 gold-bearing samples from the Livengood (Tolovana) and parts of the Fairbanks and Rampart mining districts in central Alaska, USA, indicate that the concentration of gold in placers is spatially related both to structural features and to Late Cretaceous and (or) Tertiary felsic plutons. The regional consistency of these spatial relationships is demonstrated by proximity analysis using a Geographic Information System (GIS), and suggests a genetic association between faults, felsic plutons, and gold occurrences. The local presence of gold within several of the plutons indicates that these are the source of some of the gold. In addition, some gold occurs proximal to faults where plutons are not present, suggesting that some of the gold was also derived from the country rock. We envision a model whereby weakly mineralized solutions, thermally driven by latent plutonic heat, were enriched by circulation through clastic units that may have had a naturally elevated gold background. The resultant enriched solutions were channeled and reconcentrated along or adjacent to large-scale fault systems. Future exploration to define individual target areas should be directed toward areas where Late Cretaceous and (or) Tertiary felsic plutons occur near major faults. ?? 1993.

Journal of Geochemical Exploration↗

USGS1978

Electron-microprobe study of chromitites associated with alpine ultramafic complexes and some genetic implications

Electron-microprobe and petrographic studies of alpine chromite deposits from around the world demonstrate that they are bimodal with respect to the chromic oxide content of their chromite. The two modes occur at 54 ? 4 and 37 ? 3 weight per cent chromic oxide corresponding to chromite designated as high-chromium and high-aluminum chromite respectively. The high-chromium chromite occurs exclusively with highly magnesian olivine (Fo92-97) and some interstitial diopside. The high-aluminum chromite is associated with more ferrous olivine (Fo88-92), diopside, enstatite, and feldspar. The plot of the mole ratios Cr/(Cr+Al+Fe3+) vs. Mg/(Mg+Fe2+) usually presented for alpine chromite is shown to have a high-chromium, high-iron to low-chromium, low-iron trend contrary to that shown by stratiform chromite. This trend is characteristic of alpine type chromite and is termed the alpine trend. However, a trend similar to that for startiform chromite is discernable on the graph for the high-chromium chromite data. This latter trend is well-developed at Red Mountain, Seldovia, Alaska. Analysis of the iron-magnesium distribution coefficient, Kd=(Fe/Mg)ol/(Fe/Mg)ch, between olivine and chromite shows that Kd for the high-chromium chromite from all ultramafic complexes has essentially the same constant value of .05 while the distribution coefficient for the high-aluminum chromite varies with composition of the chromite. These distribution coefficients are also characteristic of alpine-type chromites. The constant value for Kd for the high-chromium chromite and associated high-magnesium olivine in all alpine complexes suggests that they all crystallized under similar physico-chemical conditions. The two types of massive chromite and their associations of silicate minerals suggest the possibility of two populations with different origins. Recrystallization textures associated with the high-aluminum chromite together with field relationships between the gabbro and the chromite pods, suggest that the high-aluminum chromite was formed by metamorphic recrystallization of the ultramafic rocks and adjacent gabbro.

Open-File Report↗

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°×3° 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°×3° 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↗

USGS2013

The giant Pebble Cu-Au-Mo deposit and surrounding region, southwest Alaska: Introduction

The Pebble deposit is located about 320 km southwest of and 27 km northwest of the village of Iliamna in Alaska (Fig. 1A). It is one of the largest porphyry deposits in terms of contained Cu (Fig. 2A) and it has the largest Au endowment of any porphyry deposit in the world (Fig. 2B). The deposit comprises the Pebble West and Pebble East zones that represent two coeval hydrothermal centers within a single system (Lang et al., 2013). Together the measured and indicated resources total 5,942 million metric tons (Mt) at 0.42% Cu, 0.35 g/t Au, and 250 ppm Mo with an inferred resource of 4,835 Mt at 0.24% Cu, 0.26 g/t Au, and 215 ppm Mo. In addition, the deposit contains significant concentrations of Ag, Pd, and Re (Northern Dynasty Minerals, 2011).

Alaska↗

USGS2015

Soil surface organic layers in Arctic Alaska: spatial distribution, rates of formation, and microclimatic effects

Organic layers of living and dead vegetation cover the ground surface in many permafrost landscapes and play important roles in ecosystem processes. These soil surface organic layers (SSOLs) store large amounts of carbon and buffer the underlying permafrost and its contained carbon from changes in aboveground climate. Understanding the dynamics of SSOLs is a prerequisite for predicting how permafrost and carbon stocks will respond to warming climate. Here we ask three questions about SSOLs in a representative area of the Arctic Foothills region of northern Alaska: (1) What environmental factors control the thickness of SSOLs and the carbon they store? (2) How long do SSOLs take to develop on newly stabilized point bars? (3) How do SSOLs affect temperature in the underlying ground? Results show that SSOL thickness and distribution correlate with elevation, drainage area, vegetation productivity, and incoming solar radiation. A multiple regression model based on these correlations can simulate spatial distribution of SSOLs and estimate the organic carbon stored there. SSOLs develop within a few decades after a new, sandy, geomorphic surface stabilizes but require 500–700 years to reach steady state thickness. Mature SSOLs lower the growing season temperature and mean annual temperature of the underlying mineral soil by 8 and 3°C, respectively. We suggest that the proximate effects of warming climate on permafrost landscapes now covered by SSOLs will occur indirectly via climate's effects on the frequency, extent, and severity of disturbances like fires and landslides that disrupt the SSOLs and interfere with their protection of the underlying permafrost.

Alaska↗

USGS1987

Petrogenesis of gabbronorite at Yakobi and northwest Chichagof Islands, Alaska

On Yakobi Island and at Mirror Harbor on the northwest coast of Chichagof Island, gabbronorite occurs as irregular bodies, as much as 5.5 km in maximum dimension, mostly within a 40 to 43 m.y. composite pluton consisting largely of tonalite. The gab-bronorites are the host rocks for a magmatic nickel-copper sulfide deposit consisting predominantly of pyrrhotite, pentlandite, and chalcopyrite. The gabbronorites characteristically have more orthopyroxene than augite and have a significant amount of hornblende. Rock types mapped as gabbronorite range from hornblende pyroxenite to hornblende-pyroxene gabbronorite to quartz-bearing norite and gabbronorite. The tonalite pluton is composed of hornblende diorite, biotite-hornblende diorite, hornblende quartz diorite, biotite-hornblende tonalite, and biotite granodiorite. Contacts between types of gabbronorite are generally gradational on a scale of centimetres to metres; contacts between gabbronorite and the tonalite pluton are gradational on a scale of metres to tens of metres. Rock textures, pyroxene-hornblende relations, and rock and mineral chemistry of the gabbronorites show systematic changes as the gabbronorites grade into the tonalites. The field, petrographic, and chemical data, including trace-element abundances, of the gabbronorites and tonalite pluton rocks can best be explained by either (1) crystallization of gabbronorite from a tholeiitic magma with subsequent assimilation by tonalite that was simultaneously undergoing fractional crystallization or (2) fractional crystallization of a quartz diorite parent magma yielding the range of gabbronorites and tonalite pluton rocks.

Geological Society of America Bulletin↗