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At least 55 records · Page 3Linked to original sources
USGS1985

The plumbotectonics of the West Shasta mining district, eastern Klamath Mountains, California

The tectonic setting comprising the West Shasta mining district has often been compared with that of primitive island arcs. Concentrations of uranium, thorium, and lead and lead isotope compositions were determined for Devonian ores and rocks of the West Shasta district, eastern Klamath Mountains, California, to help evaluate the tectonic classification. The lead isotope pattern is found to be complex. From comparison of the data with those on younger ores and rocks in the region and with those isotopic patterns found in modern tectonic terranes, however, a number of conclusions are possible. A lead isotope point for the Devonian oceanic mantle is now well established from this study on the West Shasta district. This isotopic composition is in agreement with that suggested by Slawson in 1983--i.e., from a sample of massive sulfide ore from the Golinsky deposit--with a 206 Pb/ 204 Pb of 17.830, a 207 Pb/ 204 Pb of 15.450, and a 208 Pb/ 204 Pb of 37.313. If the Devonian mantle is analogous to the modern mantle, the determined value is probably at the more radiogenic end of the Devonian oceanic mantle array.Some samples of volcanic rocks and ores that have elevated values of 207 Pb/ 204 Pb and 208 Pb/ 204 Pb indicate that subducted pelagic sediments or interaction of the magmas with other sediments downsection have been involved to varying degrees in the generation of magmas. The high ratios were not attained by addition of lead to the volcanic rocks or ores during subsequent magmatic events, such as intrusion of the Permian Pit River stock to the east or of the Cretaceous Shasta Bally batholith that crops out to the south and west and may underlie part of the district. Some values of 207 Pb/ 204 Pb are sufficiently high as to suggest that West Shasta may have developed nearer a continent than was previously supposed. Lead isotope data for Quaternary volcanic rocks are somewhat more radiogenic than for Cretaceous ores and plutonic rocks. Permo-Triassic ores are again a bit less radiogenic. The isotopic differences between the Permo-Triassic and Quaternary data could be accounted for by a value for 238 U/ 204 Pb of 10.42 in common source material. Using this value of 238 U/ 204 Pb to calculate even further back to 400 m.y., the resulting ratios are found to fall very close to the "best value" for the lead isotope mixing line between the Devonian mantle and pelagic sediments. We interpret this intersection to be the "mean value" for the assimilated Devonian sediment lead--18.250 for 206 Pb/ 204 Pb and 15.582 for 207 Pb/ 204 Pb. As deduced for the lead isotope composition for the Devonian mantle at this location, the pelagic sediment, lead isotope composition also is toward the more radiogenic end of the values expected for pelagic sediments in the Devonian. Lack of sediments in the observed volcanic section of the West Shasta district suggests a submerged volcanic-arc setting (as interpreted by others for the Troodos and Samail ophiolites) rather than an island-arc situation (such as the Green Tuff area of Japan) or an incipient spreading ridge setting (that might have grown to be a subaerial Iceland). The elevated values of 207 Pb/ 204 Pb for some samples suggest a submerged volcanic are whereas the exceedingly low values of thorium--which is not easily mobilized during alteration events--for both basaltic andesites and plagiorhyolites is suggestive of depleted source material such as is found for most oceanic spreading centers. Elevated values of 207 Pb/ 204 Pb are exceedingly rare in midocean ridge basalts and low contents of thorium are known in island arcs, so a submerged volcanic arc is the preferred interpretation. Some similarity exists between the 206 Pb/ 204 Pb values of Devonian ores having elevated values of 207 Pb/ 204 Pb and those of the Permo-Triassic ores of the East Shasta district, as mentioned by Slawson in 1983. This similarity is now confirmed to be a coincidence. We find it due first of all to some incorporation of sediment lead in the magmas, followed later by some migration of radiogenic lead--probably locally derived--into the ores during subsequent events. Because there was some postdepositional lead migration into the ores, some migration of copper also into the ores cannot be excluded. Lead, especially radiogenic lead, however, is more mobile than copper. As the migrating lead is deduced to have been locally derived from the observed section rather than exotically derived, any copper added was probably locally derived also.

Economic Geology↗
USGS2010

Gravity modeling of the Muertos Trough and tectonic implications (north-eastern Caribbean)

The Muertos Trough in the northeast Caribbean has been interpreted as a subduction zone from seismicity, leading to infer a possible reversal subduction polarity. However, the distribution of the seismicity is very diffuse and makes definition of the plate geometry difficult. In addition, the compressive deformational features observed in the upper crust and sandbox kinematic modeling do not necessarily suggest a subduction process. We tested the hypothesized subduction of the Caribbean plate's interior beneath the eastern Greater Antilles island arc using gravity modeling. Gravity models simulating a subduction process yield a regional mass deficit beneath the island arc independently of the geometry and depth of the subducted slab used in the models. This mass deficit results from sinking of the less dense Caribbean slab beneath the lithospheric mantle replacing denser mantle materials and suggests that there is not a subducted Caribbean plateau beneath the island arc. The geologically more realistic gravity model which would explain the N-S shortening observed in the upper crust requires an overthrusted Caribbean slab extending at least 60 km northward from the deformation front, a progressive increase in the thrusting angle from 8?? to 30?? reaching a maximum depth of 22 km beneath the insular slope. This new tectonic model for the Muertos Margin, defined as a retroarc thrusting, will help to assess the seismic and tsunami hazard in the region. The use of gravity modeling has provided targets for future wide-angle seismic surveys in the Muertos Margin. ?? 2010 Springer Science+Business Media B.V.

Marine Geophysical Research↗
USGS2005

Metallogenesis and tectonics of the Russian Far East, Alaska, and the Canadian Cordillera

The Proterozoic and Phanerozoic metallogenic and tectonic evolution of the Russian Far East, Alaska, and the Canadian Cordillera is recorded in the cratons, craton margins, and orogenic collages of the Circum-North Pacific mountain belts that separate the North Pacific from the eastern North Asian and western North American Cratons. The collages consist of tectonostratigraphic terranes and contained metallogenic belts, which are composed of fragments of igneous arcs, accretionary-wedge and subduction-zone complexes, passive continental margins, and cratons. The terranes are overlapped by continental-margin-arc and sedimentary-basin assemblages and contained metallogenic belts. The metallogenic and geologic history of terranes, overlap assemblages, cratons, and craton margins has been complicated by postaccretion dismemberment and translation during strike-slip faulting that occurred subparallel to continental margins. Seven processes overlapping in time were responsible for most of metallogenic and geologic complexities of the region (1) In the Early and Middle Proterozoic, marine sedimentary basins developed on major cratons and were the loci for ironstone (Superior Fe) deposits and sediment-hosted Cu deposits that occur along both the North Asia Craton and North American Craton Margin. (2) In the Late Proterozoic, Late Devonian, and Early Carboniferous, major periods of rifting occurred along the ancestral margins of present-day Northeast Asia and northwestern North America. The rifting resulted in fragmentation of each continent, and formation of cratonal and passive continental-margin terranes that eventually migrated and accreted to other sites along the evolving margins of the original or adjacent continents. The rifting also resulted in formation of various massive-sulfide metallogenic belts. (3) From about the late Paleozoic through the mid-Cretaceous, a succession of island arcs and contained igneous-arc-related metallogenic belts and tectonically paired subduction zones formed near continental margins. (4) From about mainly the mid-Cretaceous through the present, a succession of continental-margin igneous arcs (some extending offshore into island arcs) and contained metallogenic belts, and tectonically paired subduction zones formed along the continental margins. (5) From about the Jurassic to the present, oblique convergence and rotations caused orogen-parallel sinistral, and then dextral displacements within the plate margins of the Northeast Asian and North American Cratons. The oblique convergences and rotations resulted in the fragmentation, displacement, and duplication of formerly more continuous arcs, subduction zones, passive continental margins, and contained metallogenic belts. These fragments were subsequently accreted along the margins of the expanding continental margins. (6) From the Early Jurassic through Tertiary, movement of the upper continental plates toward subduction zones resulted in strong plate coupling and accretion of the former island arcs, subduction zones, and contained metallogenic belts to continental margins. In this region, the multiple arc accretions were accompanied and followed by crustal thickening, anatexis, metamorphism, formation of collision-related metallogenic belts, and uplift; this resulted in the substantial growth of the North Asian and North American continents. (7) In the middle and late Cenozoic, oblique to orthogonal convergence of the Pacific Plate with present-day Alaska and Northeast Asia resulted in formation of the present ring of volcanoes and contained metallogenic belts around the Circum-North Pacific. Oblique convergence between the Pacific Plate and Alaska also resulted in major dextral-slip faulting in interior and southern Alaska and along the western part of the Aleutian- Wrangell arc. Associated with dextral-slip faulting was crustal extrusion of terranes from western Alaska into the Bering Sea.

Professional Paper↗
USGS2010

Thermochronology and tectonics of the Leeward Antilles: Evolution of the southern Caribbean Plate boundary zone

Tectonic reconstructions of the Caribbean Plate are severely hampered by a paucity of geochronologic and exhumation constraints from anastomosed basement blocks along its southern margin. New U/Pb, 40 Ar/ 39 Ar, apatite fission track, and apatite (U-Th)/He data constrain quantitative thermal and exhumation histories, which have been used to propose a model for the tectonic evolution of the emergent parts of the Bonaire Block and the southern Caribbean Plate boundary zone. An east facing arc system intruded through an oceanic plateau during ~90 to ~87 Ma and crops out on Aruba. Subsequent structural displacements resulted in >80°C of cooling on Aruba during 70–60 Ma. In contrast, exhumation of the island arc sequence exposed on Bonaire occurred at 85–80 Ma and 55–45 Ma. Santonian exhumation on Bonaire occurred immediately subsequent to burial metamorphism and may have been driven by the collision of a west facing island arc with the Caribbean Plate. Island arc rocks intruded oceanic plateau rocks on Gran Roque at ~65 Ma and exhumed rapidly at 55–45 Ma. We attribute Maastrichtian-Danian exhumation on Aruba and early Eocene exhumation on Bonaire and Gran Roque to sequential diachronous accretion of their basement units to the South American Plate. Widespread unconformities indicate late Eocene subaerial exposure. Late Oligocene–early Miocene dextral transtension within the Bonaire Block drove subsidence and burial of crystalline basement rocks of the Leeward Antilles to ≤1 km. Late Miocene–recent transpression caused inversion and ≤1 km of exhumation, possibly as a result of the northward escape of the Maracaibo Block.

Tectonics↗
USGS1989

Back-arc with frontal-arc component origin of Triassic Karmutsen basalt, British Columbia, Canada

The largely basaltic, ???4.5-6.2-km-thick, Middle to Upper Triassic Karmutsen Formation is a prominent part of the Wrangellian sequence. Twelve analyses of major and minor elements of representative samples of pillowed and massive basalt flows and sills from Queen Charlotte and Vancouver Islands are ferrotholeiites that show a range of 10.2-3.8% MgO (as normalized, H2O- and CO2-free) and related increases in TiO2 (1.0-2.5%), Zr (43-147 ppm) and Nb (5-16 ppm). Other elemental abundances are not related simply to MgO: distinct groupings are evident in Al2O3, Na2O and Cr, but considerable scatter is present in FeO* (FeO + 0.9Fe2O3) and CaO. Some of the variation is attributed to alteration during low-rank metamorphism or by seawater - including variation of Ba, Rb, Sr and Cu, but high-field-strength elements (Sc, Ti, Y, Zr and Nb) as well as Cr, Ni, Cu and rare-earth elements (REE's) were relatively immobile. REE's show chondrite-normalized patterns ranging from light-REE depleted to moderately light-REE enriched. On eleven discriminant plots these analyses fall largely into or across fields of within-plate basalt (WIP), normal or enriched mid-ocean-ridge tholeiite (MORB) and island-arc tholeiite (IAT). Karmutsen basalts are chemically identical to the stratigraphically equivalent Nikolai Greenstone of southern Alaska and Yukon Territory. These data and the fact that the Karmutsen rests on Sicker Group island-arc rocks of Paleozoic age suggest to us that: 1. (1) the basal arc, after minor carbonate-shale deposition, underwent near-axial back-arc rifting (as, e.g., the Mariana arc rifted at different times); 2. (2) the Karmutsen basalts were erupted along this rift or basin as "arc-rift" tholeiitite; and 3. (3) after subsequent deposition of carbonates and other rocks, and Jurassic magmatism, a large fragment of this basalt-sediment-covered island arc was accreted to North America as Wrangellia. The major- and minor-elemental abundances of Karmutsen basalt is modeled by first mixing primitive arc magma with enriched basaltic liquid derived either from garnet peridotite or metasomatized mantle, followed by fractionation of olivine, pyroxenes, plagioclase and spinel. ?? 1989.

Chemical Geology↗
USGS2018

Ore-forming adakitic porphyry produced by fractional crystallization of oxidized basaltic magmas in a subcrustal chamber (Jiamate, East Junggar, NW China)

Adakitic intrusions are supposed to have a close genetic and spatial relationship to porphyry Cu deposits. However, the genesis of adakitic intrusions is still under dispute. Here, we describe newly discovered intrusive complex rocks, which are composed of ore-bearing, layered magnetite-bearing gabbroic and adakitic rocks in Jiamate, East Junggar, NW China. These Jiamate Complex intrusions have diagnostic petrologic, geochronologic and geochemical signatures that indicate they were all generated from the same oxidized precursor magma source. Additionally, these layered rocks underwent the same fractional crystallization process as the ore-bearing adakitic rocks in the adjacent Kalaxiangar Porphyry Cu Belt (KPCB) in an oceanic island arc (OIA) setting. The rocks studied for this paper include layered magnetite-bearing gabbroic intrusive rocks that contain: (1) gradual contact changes between lithological units of mafic and intermediate rocks, (2) geochemical signatures that are the same as those found in oceanic island arc (OIA) rocks, (3) typical adakitic geochemistry, and (4) similar characteristics and apparent fractional crystallization relationships of ultra-basic to basic rocks to those in the nearby Beitashan Formation and to ore-bearing adakitic rocks in the KPCB. They also display similar zircon U-Pb and zircon Hf model ages. The Jiamate Complex intrusions contain intergrowths of magnetite and layered gabbro, and the intermediate-acidic intrusions of the Complex display typical adakitic affinities. Moreover, in conjunction with previously published geochronological and geochemistry data of the mafic rocks in the Beitashan Formation and in the KPCB area, additional data generated for the Jiamate Complex intrusions rocks indicate that they were formed from fractional crystallization processes. The Jiamate Complex intrusions most likely were derived from a metasomatized mantle wedge that was underplated at the root of the Saur oceanic island arc (Saur OIA). The ore-bearing adakitic intrusions in the KPCB and the adakitic Jiamate Complex intrusions were both probably generated from the same basaltic parental magmas through fractional crystallization. In addition, characteristics of the layered, magnetite-bearing, oxidized, basaltic Jiamate Complex intrusive rocks indicate that they are likely to be the parental arc magmas for the nearby porphyry Cu deposits. This conclusion is based on new interpretations of the regional and local geology, on interpretation of new geochemical analysis, new stable isotope analysis, new geothermobarometry, and new zircon age dating as well as other techniques and interpretations.

LITHOS↗
USGS2010

Paleozoic ophiolitic assemblages within the southern New England orogen of eastern Australia: Implications for growth of the Gondwana margin

Several ophiolitic assemblages occur in the southern New England orogen. The development of these rocks and their relations to the rest of the orogen have major implications for the tectonic evolution of eastern Gondwana. A major, narrow but elongate belt of Early Cambrian suprasubduction zone ophiolite crops out along and near the PeelManning Fault System and is juxtaposed against younger arc and subduction complex terranes. No pre-Permian links with the rest of the New England orogen have been established for this terrane. It may represent portions of Lachlan Fold Belt basement which underlies younger, westward overthrust New England terranes, and has been diapirically emplaced at higher crustal levels as serpentinite-matrix melange. Middle to Late Devonian ophiolitic rocks in the Yarras Complex comprise basement to the Birpai subterrane and represent a crustal cross section through a rifted island arc. Correlatives of this terrane also occur within the more extensive Gamilaroi terrane to the west of which deeper crustal levels are not exposed. The various components of serpentinite-matrix melange in the Ngamba terrane at Port Macquarie superficially appear to represent a dismembered ophiolite association. However, the various components of the melange exhibit a wide range of ages, metamorphism, and tectonic affinities, rendering a genetically related origin unlikely. This terrane includes fragments of ocean floor accreted into a Late Devonian subduction complex, which was later affected by Early Carboniferous forearc serpentinite diapirism and high Mg series magmatism. Zircon inheritance in Triassic dikes, which intrude the melange attest to the development or later emplacement of this forearc region over an older Lachlan Fold Belt basement. Ultramafic rocks of the Bundjalung terrane in the east of the New England region probably formed at deep levels in an intraoceanic island arc and are intruded by boninitic dikes. The tectonic development of the NEO was significantly more complicated than has been suggested in earlier published models. Periodic accretion of island arc systems, some of which are now represented by suprasubduction zone ophiolites, to the eastern margin of Gondwana suggests multiple phases of subduction with the possibility of polarity reversals throughout the history of accretion. Lateral accretion was not the only means by which Gondwana continental crust grew, and there was considerable postaccretion continentward overthrusting of younger terranes.

New South Wales↗
USGS2015

Geologic framework of the Alaska Peninsula, southwest Alaska, and the Alaska Peninsula terrane

The Alaska Peninsula is composed of the late Paleozoic to Quaternary sedimentary, igneous, and minor metamorphic rocks that record the history of a number of magmatic arcs. These magmatic arcs include an unnamed Late Triassic(?) and Early Jurassic island arc, the early Cenozoic Meshik arc, and the late Cenozoic Aleutian arc. Also found on the Alaska Peninsula is one of the most complete nonmetamorphosed, fossiliferous, marine Jurassic sedimentary sections known. As much as 8,500 m of section of Mesozoic sedimentary rocks record the growth and erosion of the Early Jurassic island arc. A thinner, but still thick (as much as 5,400 m), sequence of Tertiary sedimentary rocks that are predominantly continental overlies the Mesozoic section. A brief regression in early Tertiary time on the Alaska Peninsula and granodiorite plutonism in the Shumagin, Semidi, and Sanak Islands was followed by deposition of fluvial and minor marine clastic strata. This was followed by deposition of transgressive marine clastic strata and initiation of the Meshik arc, shown by an areally extensive outpouring of volcanic and volcaniclastic rocks and debris between late Eocene and earliest Miocene time. Late Miocene time was marked by another brief transgression and northwest- to southeast-directed compression, followed by renewed volcanism and plutonism which initiated the modern Aleutian magmatic arc. Extensive glacial and glaciomarine deposits of late Pleistocene age create an extensive lowland physiographic province on the northwest side of the Alaska Peninsula and join isolated mountain masses to the Alaska Peninsula on the southwest. Multiple active volcanoes and volcanic peaks dominate the skyline of the Alaska Peninsula and represent the continuation of magmatic activity that has formed the Aleutian arc since late Miocene time. The Alaska Peninsula has had a long and involved history since Paleozoic time. We propose that the Paleozoic and Mesozoic rocks that constitute much of the Alaska Peninsula be called the Alaska Peninsula terrane. Using the concept of subterranes, we divide the terrane into two distinct but tectonically related subterranes: the Chignik and Iliamna subterranes, which share a limited common geologic history. The Iliamna subterrane has served at most times as a source area for the Chignik subterrane; however, some rock units are in common across the subterranes. The Iliamna and Chignik subterranes are in part separated by the Bruin Bay fault system. The Iliamna subterrane is composed of moderately deformed early Mesozoic marine sedimentary and volcanic rocks and schist, gneiss, and marble of Paleozoic(?) and Mesozoic age, and plutonic rocks of the Alaska-Aleutian Range batholith. Characteristic of the Chignik subterrane are little-deformed, shallow-marine to continental clastic sedimentary rocks ranging in age from Permian to latest Cretaceous. However, deep-marine, volcaniclastic, and calcareous rocks form important components of the older rocks in the subterrane. The two subterranes of the Alaska Peninsula terrane are characterized by radically different structural and metamorphic styles. The nonplutonic rocks of the Iliamna subterrane are characterized by metamorphism up to amphibolite-facies grade and intense folding. In the Chignik subterrane, the structural style is dominated by large, open, en echelon anticlinal structures, normal faulting, and thrust and high-angle reverse faults that have minor displacement in a northwest to southeast direction. In the Outer Shumagin and Sanak Islands, rocks assigned to the Chugach terrane are characterized structurally by tight, generally northeast-trending folds. Dips in these rocks tend to be steep, rarely less than 35°, and overturned beds are locally common. The boundaries separating the Alaska Peninsula terrane from other terranes are commonly indistinct or poorly defined. A few boundaries have been defined at major faults, although the extensions of these faults are speculative through some areas. The west side of the Alaska Peninsula terrane is overlapped by Tertiary sedimentary and volcanic rocks and Quaternary deposits.

Alaska↗
USGS1990

Volcanism in the Sumisu Rift, I. Major element, volatile, and stable isotope geochemistry

A bimodal volcanic suite with K Ar ages of 0.05–1.40 Ma was collected from the Sumisu Rift using alvin . These rocks are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, and provide a present day example of volcanism associated with arc rifting and back-arc basin initiation. Major element geochemistry of the basalts is most similar to that of basalts found in other, more mature back-arc basins, which indicates that back-arc basins need not begin their magmatic evolution with lavas bearing strong arc signatures. Volatile concentrations distinguish Sumisu Rift basalts from island arc basalts and MORB. H 2 O contents, which are at least four times greater than in MORB, suppress plagioclase crystallization. This suppression results in a more mafic fractionating assemblage, which prevents Al 2 O 3 depletion and delays the initiation of Fe 2 O 3 (tot) and TiO 2 enrichment. However, unlike arc basalts,Fe 3+/ ΣFe ratios are only slightly higher than in MORB and are insufficient to cause magnetite saturation early enough to suppress Fe 2 O 3 (tot) and TiO 2 enrichment. Thus, major element trends are more similar to those of MORB than arcs. H 2 O, CO 2 and S are undersaturated relative to pure phase solubility curves, indicating exsolution of an H 2 O-rich mixed gas phase. HighH 2 O/S, highδD, and low (MORB-like)δ 34 S ratios are considered primary and distinctive of the back-arc basin setting.

Earth and Planetary Science Letters↗
USGS2021

Tectonic and magmatic controls on the metallogenesis of porphyry deposits in Alaska

Porphyry Cu and Mo deposits and occurrences are found throughout Alaska; they formed episodically during repeated subduction and arc-continent collisions spanning the Silurian to Quaternary. Porphyry systems occur in continental-margin and island arcs, which are broadly grouped into pre-accretionary or post-accretionary arcs. Pre-Mesozoic occurrences formed in continental or island arcs prior to accretion onto the margin of North America, whereas Mesozoic and younger systems formed in arcs that developed after terrane fragments were accreted to the margin of North America. As a result, older porphyry systems are typically in the interior and northern metallogenic belts, whereas the younger porphyry systems are predominantly found in the southern third of the state, closer to the modern continental margin. Alaska porphyry formation peaked in the mid-Cretaceous and continued through the Late Cretaceous to Tertiary, in association with continental-margin arcs extending from the eastern interior, into southwest Alaska and along the Alaska Peninsula and Aleutian Islands. Porphyry system formation is not generally recognized in the Early Cretaceous, Triassic, or early Paleozoic – time periods that coincide with continental collisional events or extension. Relatively few pre-accretionary porphyry systems are documented in more deeply exhumed arc segments due to low preservation potential in areas of rapid or repeated bedrock uplift and associated erosion that occurred during later tectonic events. Significant diversity is observed in porphyry occurrences across the state and even within the same region. Occurrences form in association with arc-related intrusions or intrusive complexes, that range in composition from diorite to syenite, but are commonly monzonitic to granitic. Some porphyry occurrences are associated with alkaline intrusive belts that exhibit a stronger crustal contribution to magmatic sources. Intrusions associated with porphyry formation in Alaska are commonly moderately oxidized, however, a distinct group of porphyry systems are associated with more-reduced magmas. Hydrothermal alteration described at many occurrences exhibits zoning from proximal potassic alteration to typically peripheral and(or) later sericitic alteration, that is flanked by large zones of propylitic alteration. Diversity in alteration is observed where sodic and sodic-calcic alteration is present, commonly in more-enigmatic deposits, such as Island Mountain and Chicken Mountain. Advanced argillic alteration is rare, but present in notable examples, such as the Pebble porphyry Cu(-Au-Mo) deposit. Sulfide mineralization is characterized by pyrite, chalcopyrite, molybdenite, and rare bornite hosted in veinlets, veins and disseminations in wallrocks and causative intrusions. Some porphyry systems contain abundant pyrrhotite and(or) arsenopyrite in the mineral assemblages. Systems that exhibit bornite-bearing assemblages containing abundant molybdenite are commonly arsenic- and gold-poor and tend to be associated with more oxidized arc magmas. In contrast, those systems that are pyrrhotite and(or) arsenopyrite dominant tend to be gold, arsenic, and bismuth bearing, and are commonly associated with more-reduced magmas. Exploration for porphyry occurrences in Alaska has experienced a resurgence and currently constitutes about 20% of exploration dollars in the state. Many systems lack complete descriptions, and coupled with the cost of exploration, remain incompletely explored. Additional understanding of known occurrences combined with a framework geologic understanding of porphyry-bearing metallogenic belts will likely result in new discoveries in the future.

Alaska↗
USGS2017

Undiscovered porphyry copper resources in the Urals—A probabilistic mineral resource assessment

A probabilistic mineral resource assessment of metal resources in undiscovered porphyry copper deposits of the Ural Mountains in Russia and Kazakhstan was done using a quantitative form of mineral resource assessment. Permissive tracts were delineated on the basis of mapped and inferred subsurface distributions of igneous rocks assigned to tectonic zones that include magmatic arcs where the occurrence of porphyry copper deposits within 1 km of the Earth's surface are possible. These permissive tracts outline four north-south trending volcano-plutonic belts in major structural zones of the Urals. From west to east, these include permissive lithologies for porphyry copper deposits associated with Paleozoic subduction-related island-arc complexes preserved in the Tagil and Magnitogorsk arcs, Paleozoic island-arc fragments and associated tonalite-granodiorite intrusions in the East Uralian zone, and Carboniferous continental-margin arcs developed on the Kazakh craton in the Transuralian zone. The tracts range from about 50,000 to 130,000 km 2 in area. The Urals host 8 known porphyry copper deposits with total identified resources of about 6.4 million metric tons of copper, at least 20 additional porphyry copper prospect areas, and numerous copper-bearing skarns and copper occurrences. Probabilistic estimates predict a mean of 22 undiscovered porphyry copper deposits within the four permissive tracts delineated in the Urals. Combining estimates with established grade and tonnage models predicts a mean of 82 million metric tons of undiscovered copper. Application of an economic filter suggests that about half of that amount could be economically recoverable based on assumed depth distributions, availability of infrastructure, recovery rates, current metals prices, and investment environment.

Urals↗
USGS2005

Geology of the Payette National Forest and vicinity, west-central Idaho

Before the Late Cretaceous, the eastern and western parts of the geologically complex Payette National Forest, as divided by the Salmon River suture, had fundamentally different geologic histories. The eastern part is underlain by Mesoproterozoic to Cambrian(?) rocks of the Laurentian (Precambrian North American) continent. Thick Mesoproterozoic units, which are at least in part equivalent in age to the Belt Supergroup of northern Idaho and western Montana, under-went Mesoproterozoic metamorphic and deformational events, including intrusion of Mesoproterozoic plutons. During the Neoproterozoic to early Paleozoic, the western edge of Laurentia was rifted. This event included magmatism and resulted in deposition of rift-related Neoproterozoic to Lower Cambrian(?) volcanic and sedimentary rocks above Mesoproterozoic rocks. The western part of the forest is underlain by upper Paleozoic to lower Mesozoic island-arc volcanic and sedimentary rocks. These rocks comprise four recognized island-arc terranes that were amalgamated and intruded by intermediate-composition plutons, probably in the Late Jurassic and Early Cretaceous, and then sutured to Laurentia along the Salmon River suture in the Late Cretaceous. The Salmon River suture formed as a right-lateral, transpressive fault. The metamorphic grade and structural complexity of the rocks increase toward the suture from both sides, and geochemical signatures in crosscutting plutonic rocks abruptly differ across the crustal boundary. Having been reactivated by younger structures, the Salmon River suture forms a north-trending topographic depression along Long Valley, through McCall, to the Goose Creek and French Creek drainages. During the last stages of metamorphism and deformation related to the suture event, voluminous plutons of the Idaho batholith were intruded east of the suture. An older plutonic series is intermediate in composition and preserved as elongated and deformed bodies near the suture and as parts of roof pendants to younger intrusions to the east. A younger magma series consists of undeformed, marginally peraluminous plutons that formed east of the suture after accretion. After suture-related compression, crustal extension resulted in voluminous volcanic and plutonic rocks of the Eocene Challis magmatic complex on the east side of the forest. Extension, from the Late Cretaceous to post-Miocene, uplifted the area of the Idaho batholith relative to the western part of the forest and formed dominant highlands along the Snake River. Extensional basins also formed such that, in the Miocene, the Columbia River Basalt Group and related basaltic lavas flowed over most of the lower elevations on the western side of the forest and redirected erosional debris into north-trending, fault-controlled drainages and young sedimentary basins.

Idaho↗
USGS1991

Deep crustal structure of the Precambrian basement beneath northern Lake Michigan, midcontinent North America

A deep seismic-reflection profile in northern Lake Michigan, midcontinent North America, provides a cross section of the crust across the 1850 Ma Penokean orogen, in which an Early Proterozoic island-arc complex was deformed between two converging Archean continental masses. The island-arc crust is about 40 km thick and has a few kilometres of intensely reflective rocks near its base, above which it is variably reflective to transparent. The Archean terranes have thicker crust, as much as 50 km, the lower 20-25 km of which is strongly reflective. Abrupt offsets of Moho near terrane boundaries may have been preserved since accretion during the Penokean orogeny.

Geology↗
USGSDate not supplied

Early Paleozoic composite melange terrane, central Appalachian Piedmont, Virginia and Maryland; Its origin and tectonic history

Two distinct types of mélange deposits, distinguished by their matrix, occur within a collage of thrust slices in the Piedmont of the central Appalachians. They crop out in a northeast-trending belt that extends from at least central Virginia northeastward through most of Maryland. One type is a block-in-phyllite mélange that constitutes the Mine Run Complex (new name) of Virginia. It consists of a variety of metaplutonic, metavolcanic, mafic, and ultramafic blocks enclosed within a matrix of phyllite or schist and metasandstones of feldspathic or quartz metagraywacke. The Mine Run Complex is interpreted to consist of four imbricated thrust slices, each with its own distinctive exotic block content. The blocks in one of these mélange subunits (III) are almost exclusively mafic and ultramafic rocks, such as serpentinite, greenschist, metabasalt, and talc schist. The second mélange type within this Piedmont terrane, a metadiamictite, contains a less extensive variety of exotic blocks, the most common being mafic and ultramafic blocks. Such exotic blocks are enclosed in a micaceous quartzofeldspathic matrix, which has contemporaneously deposited schist and quartz-lump fragments as its characterizing features. The Sykesville Formation of Maryland and Virginia is typical of this type of mélange. Several varieties of metadiamictite that have some lithologic differences from the type locality of the Sykesville have been recognized in Virginia and are described as the Lunga Reservoir and the Purcell Branch Formations (new names). Mélanges of the block-in-phyllite and the metadiamictite types are interpreted as having been formed in a Cambrian-Ordovician back-arc or marginal basin that lay on the continentward side of an island-arc system (central Virginia volcanic-plutonic belt and the James Run Formation of Maryland) that had formed in Cambrian time. This Cambrian-Ordovician back-arc basin is assumed to have been underlain, at least initially, by a transform-segmented spreading ridge. The metadiamictite deposits now occur, for the most part, along the flanks and at the southeastern end of the Baltimore-Washington anticlinorium or antiform. This spatial relation is interpreted to be a consequence of thrusting of the diamictite across, as well as onto, the cover rocks and basement of the anticlinorial core terrane (“Baltimoria”) during the Taconic orogeny. The chaotic-textured diamictite formed as a sedimentary slump or slide apron or aprons somewhere to the east of “Baltimoria.” The source area for this diamictite probably was another crystalline landmass that lay east of “Baltimoria.” The matrix rocks of the block-in-phyllite mélange (Mine Run Complex) may have accumulated contemporaneously, in part with diamictite, probably by along-trough basin sedimentation. The block-in-phyllite mélanges of the Mine Run Complex occur south of the Baltimore-Washington anticlinorium metadiamictite terrane and are not associated directly with metadiamictite mélanges. The metavolcanic and metaplutonic blocks within the eastern parts of the Mine Run Complex probably were shed from the island-arc terrane as it was thrust westward during the progressive tectonic telescoping of the back-arc basin in Cambrian and Ordovician time. The mafic and ultramafic blocks in both types of mélanges are believed to have been derived, in part, from the “oceanic” back-arc basin floor from various sources and different processes. Ultramafic protrusions, for example, may have been emplaced periodically along transform faults. Some of the mafic-ultramafic rock may have formed talus rubble along steep submarine scarps. Others may represent blocks broken and segmented by faults from what once were mafic sills emplaced within the sediments accumulating in the basin.

Maryland, Virginiia↗
USGS2019

Detrital zircon geochronology along a structural transect across the Kahiltna assemblage in the western Alaska Range: Implications for emplacement of the Alexander-Wrangellia-Peninsular terrane against North America

The Kahiltna assemblage in the western Alaska Range consists of deformed Upper Jurassic and Cretaceous clastic strata that lie between the Alexander-Wrangellia-Peninsular (AWP) terrane to the south, and the Farewell and other peri-cratonic terranes to the north. Differences in detrital zircon populations and sandstone petrography allow geographic separation of the strata into two different successions, each consisting of multiple units, or petrofacies, with distinct provenance and lithologic characteristics. The northwestern succession was largely derived from older, inboard peri-cratonic terranes and correlates along strike to the southwest with the Kuskokwim Group. The southeastern succession is characterized by volcanic and plutonic rock detritus derived from Late Jurassic igneous rocks of the AWP terrane and mid to Late Cretaceous arc related igneous rocks and is part of a longer belt to the southwest and northeast, here named the Koksetna-Clearwater belt. The two successions remained separate depositional systems until the Late Cretaceous, when the northwestern succession overlapped the southeastern succession at about 81 Ma and they were deformed together by about 80 Ma by southeast-verging fold-and-thrust style deformation interpreted to represent final accretion of the AWP terrane along the southern Alaska margin. We interpret the tectonic evolution of the Kahiltna successions as a progression from forearc sedimentation and accretion in a south-facing continental magmatic arc to arrival and partial underthrusting of the backarc flank of an active, south-facing island arc system (AWP terrane). A modern analogue is the ongoing collision and partial underthrusting of the Izu-Bonin-Marianas island arc beneath the Japan Trench-Nankai Trough on the east side of central Japan.

Alaska↗
USGS1992

Geology of the d'Entrecasteaux-New Hebrides arc collision zone: Results from a deep submersible survey

During the SUBPSO1 cruise, seven submersible dives were conducted between water depths of 5350 and 900 m over the collision zone between the New Hebrides island arc and the d'Entrecasteaux Zone (DEZ). The DEZ, a topographic high on the Australian plate, encompasses the North d'Entrecasteaux Ridge (NDR) and the Bougainville guyot, both of which collide with the island-are slope. In this report we use diving observations and samples, as well as dredging results, to analyse the geology of the Bougainville guyot and the outer arc slope in the DEZ-arc collision zone, and to decipher the mechanisms of scamount subduction. These data indicate that the Bougainville guyot is a middle Eocene island arc volcano capped with reef limestones that appear to have been deposited during the Late Oligocene to Early Miocene and in Miocene-Pliocene times. This guyot possibly emerged during the Middle and Late Miocene, and started to sink in the New Hebrides trench after the Pliocene. The rocks of the New Hebrides arc slope, in the collision zone, consist primarily of Pliocene-Recent volcaniclastic rocks derived from the arc, and underlying fractured island-arc volcanic basement, possibly of Late Miocene age. However, highly sheared, Upper Oligocene to Lower Miocene nannofossil ooze and chalk are exposed at the toe of the arc slope against the northern flank of the NDR. Based on a comparison with cores collected at DSDP Site 286, the ooze and chalk can be interpreted as sediments accreted from the downgoing plate. East of the Bougainville guyot an antiform that developed in the arc slope as a consequence of the collision reveals a 500-m-thick wedge of strongly tectonized rocks, possibly accreted from the guyot or an already subducted seamount. The wedge that is overlain by less deformed volcaniclastic island-arc rocks and sediments includes imbricated layers of Late Oligocene to Early Miocene reef and micritic limestones. This wedge, which develops against the leading flank of the guyot, tends to smooth its high-drag shape. A comparison between the 500-m-thick wedge of limestones that outcrops southeast of the guyot and the absence of such a wedge over the flat top of the guyot, although the top is overthrust by island-arc rocks and sediments, can be interpreted to suggest that the wedge moves in the subduction zone with the guyot and facilitates its subduction by streamlining.

Tectonophysics↗
USGS1995

Geology of the Blue Mountains region of Oregon, Idaho, and Washington: Petrology and tectonic evolution of pre-Tertiary rocks of the Blue Mountains region

This Professional Paper contains 14 chapters on the Blue Mountains region of Oregon, Idaho, and Washington. The authors discuss petrology and tectonic evolution of an island arc that formed in the ancestral Pacific Ocean during the Permian to Cretaceous interval. The island arc was accreted to cratonal North America in the Early Cretaceous and thereby became one of the several exotic terranes in western North America.

Idaho, Oregon, Washington↗
USGS1985

Geologic setting, petrology, and geochemistry of stratiform sphalerite-galena-barite deposits, Red Dog Creek and Drenchwater Creek areas, northwestern Brooks Range, Alaska

Similar stratiform sphalerite-galena-barite deposits occur in the Red Dog Creek area, De Long Mountains quadrangle, and in the Drenchwater Creek area, Howard Pass quadrangle, northwestern Brooks Range, Alaska. The deposits, located approximately 180 km apart, are hosted by Mississippian and Pennsylvanian strata of the Kagvik structural sequence of late Paleozoic and early Mesozoic age. This sequence is situated in the lowermost structural plate of a series of predominantly east-striking, shallow-dipping thrust plates that characterize the regional structure. The deposits occur in shale, tuff, tuffaceous sandstone, altered chert, and quartz exhalite. These units are in places interlayered with or adjacent to bedded or nodular barite or submarine keratophyre flows. Chert and shale commonly contain abundant highly altered, fine-grained feldspar, pumice lapilli, and mafic volcanic rock fragments.The main types of occurrences at Red Dog Creek are: (1) disseminated and stratiform sulfides thinly bedded in organic-rich Mississippian and Pennsylvanian shale and chert, (2) massive sulfide veins and breccia fillings in silicified Mississippian and Pennsylvanian shale, and (3) stratiform sulfide-bearing lenses subdivided into (3a) quartz-exhalite with up to several percent sulfides and barite, (3b) massive sulfide-quartz lenses with up to several percent barite, and (3c) barite-quartz lenses with up to several percent sulfides.At Drenchwater Creek sulfides and barite occur as: (1) disseminations in shale, chert, tuff, and tuffaceous sandstone; (2) disseminations to aggregates in quartz exhalite; and (3) occurrences in veins crosscutting cleavages in brecciated chert and shale.The stratiform sulfides occur in zones up to several thousand meters long and several tens of meters thick. Preliminary grades at Red Dog Creek are 17.1 percent zinc, 5.0 percent lead, and 2.4 oz silver per ton contained in at least 85 million tons.The range of sulfide sulfur isotope values at Red Dog Creek is -16.6 per mil to +3.6 per mil. Sphalerite averages 0.8 per mil. Pyrite is isotopically lighter than coexisting sphalerite in quartz exhalite and massive sulfide lenses. This isotopic relationship is compatible with paragenetic relationships which show pyrite crystallized both earlier and later than other sulfides. Galena is isotopically lighter than coexisting sphalerite. Three groups of geologically and isotopically distinct barites exist including isotopic values appropriate for derivation from late Paleozoic seawater sulfate.Early in the depositional sequence isotopically light pyrite formed from organically reduced seawater sulfate. Sphalerite, galena, late pyrite, and some barite precipitated from two sulfur-bearing solutions that mixed at the mineralization site. Isotopically light barite associated with sulfides precipitated from oxidized H 2 S without seawater sulfate contribution. Lead isotope data from Red Dog and Drenchwater Creeks are compatible with generation of lead in a Phanerozoic mature island-arc setting, a typical orogene, or possibly a back-arc type intracratonic basin.The stratiform sphalerite-galena-barite deposits probably formed in an incipient island-arc environment near a continental margin, containing magmas of keratophyric and andesitic composition, where metal-laden hydrothermal fluids discharged into a deep (?) water, low-energy environment. Later intense deformation, associated with plate docking and thrusting, disrupted and partly remobilized and stratiform deposits.

Alaska↗