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At least 1,747 records · Page 97Linked to original sources
USGS2009

Gold and trace element zonation in pyrite using a laser imaging technique: Implications for the timing of gold in orogenic and carlin-style sediment-hosted deposits

Laser ablation ICP-MS imaging of gold and other trace elements in pyrite from four different sediment- hosted gold-arsenic deposits has revealed two distinct episodes of gold enrichment in each deposit: an early synsedimentary stage where invisible gold is concentrated in arsenian diagenetic pyrite along with other trace elements, in particular, As, Ni, Pb, Zn, Ag, Mo, Te, V, and Se; and a later hydrothermal stage where gold forms as either free gold grains in cracks in overgrowth metamorphic and/or hydrothermal pyrite or as narrow gold- arsenic rims on the outermost parts of the overgrowth hydrothermal pyrite. Compared to the diagenetic pyrites, the hydrothermal pyrites are commonly depleted in Ni, V, Zn, Pb, and Ag with cyclic zones of Co, Ni, and As concentration. The outermost hydrothermal pyrite rims are either As-Au rich, as in moderate- to high- grade deposits such as Carlin and Bendigo, or Co-Ni rich and As-Au poor as in moderate- to low-grade deposits such as Sukhoi Log and Spanish Mountain. The early enrichment of gold in arsenic-bearing syngenetic to diagenetic pyrite, within black shale facies of sedimentary basins, is proposed as a critical requirement for the later development of Carlin-style and orogenic gold deposits in sedimentary environments. The best grade sediment-hosted deposits appear to have the gold climax event, toward the final stages of deformation-related hydrothermal pyrite growth and fluid flow. ?? 2009 Society of Economic Geologists, Inc.

Economic Geology↗
USGS1934

Zoning of ore deposits in and adjoining the Leadville District, Colorado

The ore deposits in the heart of the Leadville district , Colorado , are divisible, as shown by Emmons, Irving, and Loughlin,2 into two main classes on the basis of mineral composition: "contact-metamorphic" silicates accompanied by iron oxides, which adjoin an obscure igneous stock, and mixed sulphides accompanied by manganosiderite and quartz gangue, which cut the "contact-metamorphic" deposits and form a broad zone extending for 2 1/2 miles to the west and for shorter distances in other directions. A detailed consideration of the mineralogy of the mixed sulphide ore deposits clearly shows that they in turn are divisible into groups, referred to as hotter and intermediate mesothermal deposits . These groups, however, because of complications in structural control, are not ideally arranged from the standpoint of zoning .Emmons, Irving, and Loughlin noted that barite was a conspicuous gangue mineral in certain outlying deposits on the west slope of the Mosquito Range. The junior author during 1928-1931 had an opportunity to study the outlying deposits over an area of twenty square miles, which includes mines near the heads of Iowa Gulch and Empire Gulch, and the Weston Pass district (Fig. 1), and found that these baritic deposits form a somewhat distinct outer zone in which the primary ore consists of galena and light-colored zinc blende, with small amounts of silver and practically no gold, in a gangue of barite, jasperoid, and crystalline quartz. There is also recognized a still more remote though poorly defined zone in which the same sulphides are present but jasperoid is less conspicuous and barite is practically absent, whereas dolomite is the characteristic gangue mineral. The deposits in these two outer zones are fissure-fillings and comparatively small replacement bodies. They contrast strongly with the sulphide deposits in the heart of the Leadville district and are assigned to the outer mesothermal and epithermal zones. As they have not been described before, general features of these deposits are given in some detail, followed by their relations to the deposits within the Leadville district . Analogies are also drawn with similar deposits in other districts. In the preparation of this paper the authors have been impressed with the confusion that may result from too simple a conception of zoning . For a proper conception of Leadville , the geologist must have sufficient regard for structural conditions, must make due allowance for successive stages of mineral deposition within a small block of ground or even within a single ore shoot, and must also allow for the reactions between minerals formed at an early stage and solutions introduced during a late stage. The effects of such complications in the Leadville district might give the impression that certain minerals found in one of the inner zones were formed at relatively high temperatures, but close study of paragenesis indicates late deposition at relatively low temperatures. Again, as already stated, there is sufficient inconsistency in the distribution of certain varieties of deposits to give a confused impression of zonal arrangement unless the influence of structure on the circulation of ore -forming solutions is appreciated. The field facts give a far more complex picture than might be anticipated from a simple statement of the theory of zoning .

Colorado↗
USGS1944

"Ribbon rock", an unusual beryllium-bearing tactite

The beryllium deposits at Iron Mountain, near the northern end of the Sierra Cuchillo in Sierra and Socorro Counties, New Mexico, are unusual products of contact metamorphism. They occur in irregular bodies of tactile formed by replacement of Paleozoic limestone, generally at or near contacts with small intrusive masses of rhyolite, aplite, and fine-grained granite. The metamorphism took place in mid-Tertiary time. Beryllium is present chiefly in the complex silicate minerals helvite and danalite, and is a minor constituent of the garnet grossularite, a boron-bearing idocrase, and chlorite. These minerals are known to occur in noteworthy concentrations in only one type of rock, a peculiar rhythmically layered variety of tactile to which the name "ribbon rock" is given. The structure of such tactite is very conspicuous, and appears in section as thin, finely crenulated bands of magnetite alternating with similar bands of silicate minerals and finely crystalline fluorite. Concentric banding about fluorite-rich pod-like masses is common. Bodies of "ribbon rock" range in size from inch-thick lenses to large masses amounting to thousands of tons; most appear to have been formed along contacts between re-crystallized limestone and massive magnetite-andradite tactite, chiefly by replacing fluids penetrating the limestone from fractures. The layered structure is interpreted as a diffusion effect.The formation of massive and "ribbon rock" tactites can be traced through a range of falling temperature from a stage characterized by deposition from iron-rich vapors to a stage in which hydrothermal solutions were dominant. Both vapors and liquids appear to have been acid. Reducing conditions undoubtedly existed during the latter part of the hydrothermal stage. The occurrence of beryllium in "ribbon rock," but not in typical massive tactite, may signify that its compounds in deposits at or near intrusive contacts are confined to rocks of hydrothermal origin. The occurrence of "ribbon rock" is suggested as a potentially useful clue for recognition of beryllium-bearing contact deposits elsewhere; at least two other occurrences of what apparently is "ribbon rock" have been described in the literature.

New Mexico↗
USGS1990

Genetic implications of regional and temporal trends in ore fluid geochemistry of Mississippi Valley-type deposits in the Ozark region

Fluids extracted from aqueous fluid inclusions in epigenetic gangue and ore minerals record the migration of huge volumes of highly saline fluids throughout the stratigraphic section of the Ozark region. The extracted fluids share many similarities regionally, but there are significant temporal differences which define two geochemically distinct end-member ore-forming fluids that we refer to as the Viburnum Trend main stage or Viburnum Trend type and the Tri-State type.Viburnum Trend-type fluids are enriched in potassium and are associated only with deposits close to the basal Lamotte Sandstone. The main-stage octahedral galena ore of the Viburnum Trend and much of the Old Lead Belt ore is thought to be derived from this type of ore fluid. Galena deposited by Viburnum Trend-type fluids contains less radiogenic lead than galena deposited by Tri-State-type fluids. Sulfides deposited by Viburnum Trend-type fluids also contain isotopically heavier sulfur and significant amounts of copper, cobalt, nickel, and silver.Tri-State-type fluids have a low potassium content when compared with Viburnum Trend-type fluids and are characteristic of deposits where ore-forming fluids migrated through large volumes of carbonate rock. These fluids are thought to have formed the ore deposits of the Tri-State, Northern Arkansas, and Central Missouri districts, the cubic galena-stage ore of the Viburnum Trend, and the many trace occurrences of sphalerite throughout the Ozark region. Galena deposited by Tri-State-type fluids has more radiogenic lead and the sulfides have isotopically lighter sulfur than sulfides deposited by Viburnum Trend-type fluids. A systematic south to north increase of potassium in the Tri-State-type fluids suggests that they migrated from a southerly source such as the Arkoma basin.Possible explanations for the origins of these two end-member fluids include: (1) a single parent brine evolved into two distinct fluids due to reactions with geochemically distinct aquifers during migration, (2) the two distinct fluids reflect normal fluid evolution within a single source basin of a bittern and of later halite dissolution, and (3) the Viburnum Trend and Tri-State-type brines migrated to southeast Missouri from two different source basins. Our data does not preclude any of these possibilities; however, the geochemical similarity of the Viburnum Trend end-member fluid to a bittern may be accounted for by water-rock modifications of the brine during migration. Other evidence strongly supports a southerly source for the ore-forming brines thus limiting possible sources for the Viburnum Trend-type fluid to the Arkoma and/or Black Warrior basins of the Ouachita foreland trough. Viburnum Trend-type fluid flow was probably funneled northward through basal sandstones within the Reelfoot rift and water-rock modifications occurring there may have resulted in its unique geochemistry.

Economic Geology↗
USGS1994

Lead isotope compositions as guides to early gold mineralization: The North Amethyst vein system, Creede district, Colorado

The North Amethyst vein system, which is hosted by approximately 27 Ma Carpenter Ridge Tuff and approximately 26 Ma Nelson Mountain Tuff, has two mineral associations separated by brecciation and sedimentation in the veins. The early association consists of quartz, rhodonite, hematite, magnetite, electrum (Au (sub 0.3-0.5) Ag (sub 0.7-0.5)) , and Mn carbonate, Au-Ag sulfide, Ag sulfosalt, and base metal sulfide minerals. The later mineral association cuts the Mn- and Au-bearing assemblages and consists of quartz, calcite, sericite, chlorite, hematite, adularia, fluorite, base metal sulfides, and Ag-bearing tetrahedrite.Our detailed studies show that the Pb isotope compositions of paragenetically early galenas associated with Au-rich mineralization in the North Amethyst vein system are relatively unradiogenic ( 206 Pb/ 204 Pb: 18.826-18.881, 207 Pb/ 204 Pb: 15.588-15.602, and 208 Pb/ 204 Pb: 37.790-37.926) compared to Pb isotope compositions of galenas formed later at about 25 Ma during Ag and base metal mineralization ( 206 Pb/ 204 Pb: 19.041-19.115, 207 Pb/ 204 Pb: 15.627- 15.672, and 208 Pb/ 204 Pb: 37.829-38.057). New Pb isotope data for the central and southern parts of the Creede district, which are located 5 to 7 km south of the North Amethyst area, agree with the results of a regional study by Doe et al. (1979) that included five samples from the main part of the Creede district. Galenas and adularia from the central and southern Creede district and galenas from the Bondholder district, 7 km north of the North Amethyst area, are similar to the Pb isotope compositions of galenas formed later in the North Amethyst area during Ag and base metal mineralization. Galenas from the Alpha-Corsair vein, which was mined for Ag prior to 1910, are isotopically similar to galenas associated with North Amethyst Au-stage mineralization. This isotopic similarity suggests that unexplored segments of the Alpha-Corsair structure may have the same mineralogy as the North Amethyst Au stage; thus, the Alpha-Corsair structure has the potential for high gold contents.Pb isotope compositions from the late stage of the North Amethyst vein system and from the Bondholder and central and southern Creede mining districts are more radiogenic than the host volcanic rocks of the central cluster of the San Juan volcanic field. Our Pb isotope results indicate that early Au mineralization of the North Amethyst area may represent the product of an older and relatively local hydrothermal system distinct from that of the younger base metal and Ag mineralization found throughout the region. Fluids that deposited Au minerals may have derived their Pb isotope composition by a greater degree of interaction with shallow, relatively less radiogenic volcanic wall rocks. The younger, base metal and Ag-rich mineralization that overprints the Au mineralization in the North Amethyst area clearly has a more radiogenic isotopic signature, which implies that the later mineralization derived a greater component of its Pb from Proterozoic source rocks, or sediments derived from them.Paragenetically early sulfide-rich vein assemblages have the least radiogenic galenas and generally also have the highest Au contents. Thus, identification of paragenetically early vein assemblages with relatively unradiogenic Pb isotope compositions similar to those of the North Amethyst area provides an additional exploration tool for Au in the central San Juan Mountains area.

Colorado↗
USGS2003

Geology and geochemistry of the Reocín zinc-lead deposit, Basque-Cantabrian Basin, Northern Spain

The Reocín Zn-Pb deposit , 30 km southwest of Santander, Spain , occurs within Lower Cretaceous dolomitized Urgonian limestones on the southern flank of the Santillana syncline. The Reocín deposit is one of the largest known strata-bound, carbonate-hosted, zinc - lead deposits in Europe. The total metal endowment of the deposit , including past production and remaining reserves, is 62 Mt of ore grading 8.7 percent Zn and 1.0 percent Pb. The epigenetic mineralization consists of sphalerite and galena, with lesser marcasite and trace pyrite with dolomite as gangue. Microprobe analyses of different generations of dolomite revealed nonstoichiometric compositions with various amounts of iron (up to 14 mol % of FeCO 3 ). Replacement of host dolomite, open-space filling of fractures, and cementation of breccias derived from dissolution collapse are the principal types of ore occurrence. Detailed cross-section mapping indicates a stratigraphic and structural control on the deposit . A stratiform morphology is present in the western part of the orebody (Capa Sur), whereas mineralization in the eastern part is highly discordant but strata bound (Barrendera). Stratigraphic studies demonstrate that synsedimentary tectonic activity, related to the rifting of the North Atlantic (Bay of Biscay), was responsible for variation in sedimentation, presence of unconformities (including paleokarsts), local platform emergence and dolomitization along the N60 fault trend. In the Reocín area, two stages of dolomitization are recognized. The first stage is a pervasive dolomitization of the limestone country rocks that was controlled by faulting and locally affected the upper part of the Aptian and the complete Albian sequence. The second dolomitization event occurred after erosion and was controlled by karstic cavities. This later dolomitization was accompanied by ore deposition and, locally, filling of dolomite sands and clastic sediments in karstic cavities. The circulation of hydrothermal fluids responsible for sulfide deposition and the infilling of karst cavities were broadly contemporaneous, indicating a post-Albian age. Vitrinite reflectance data are consistent with previously measured fluid inclusion temperatures and indicate temperatures of ore deposition that were less than 100°C. Carbon and oxygen isotopic data from samples of regional limestone, host-rock dolostone and ore-stage dolomite suggest an early hydrothermal alteration of limestone to dolostone. This initial dolomitization was followed by a second period of dolomite formation produced by the mixing of basinal metal-rich fluids with local modified seawater. Both dolomitization events occurred under similar conditions from fluids exhibiting characteristics of basinal brines. The δ 34 S values of sulfides are between -1.8 and +8.5 per mil, which is consistent with thermochemical sulfate reduction involving organic matter as the main source of reduced sulfur. Galena lead isotope compositions are among the most radiogenic values reported for Zn-Pb occurrences in Europe, and they are distinct from values reported for galena from other Basque - Cantabrian deposits. This suggests that a significant part of the lead was scavenged from the local underlying Asturian sediments. The stratigraphic and structural setting, timing of epigenetic mineralization, mineralogy, and isotopic geochemistry of sulfide and gangue minerals of the Reocín deposit are consistent with the features of most of Mississippi Valley-type ore deposits.

northern Spain↗
USGS1994

Emperor goose (Chen canagica)

Early naturalists exploring western Alaska were intrigued to find a stocky, blue-gray species of coastal goose unique to that area and nearby Russia. As E. W. Nelson (1887) wrote, "Among the various species of birds more or less peculiar to Alaska this goose is perhaps the most noteworthy." The Emperor Goose nests in the extensive coastal salt marsh habitats of arctic and sub-arctic Alaska and Russia and winters primarily on coastal beaches along ice-free areas of the Aleutian Islands and the Alaska Peninsula. Locally known as the "Beach Goose" from its habit of roosting and feeding near the waters' edge, the diet of this species consists largely of clams, mussels, and algae when wintering and staging in marine and estuarine habitats. When nesting in terrestrial habitats, it eats mostly roots, bulbs, and shoots of vegetation. Although the breeding biology and habitat requirements of this goose have been examined in detail, mostly on the Yukon-Kuskokwim Delta (Y-K Delta) of Alaska, there have been no intensive studies of its wintering or staging biology. Recent studies of migration have confirmed pathways and timing between the Y-K Delta and staging areas on the Alaska Peninsula and wintering areas largely on the Aleutian Islands (Hupp et al. 2007). Observations of substantial molt migrations to the Chukotka Peninsula in eastern Russia (Hupp et al. 2007) suggest a possible change in distribution, or at least a clarification, and raise the possibility of harvest mortality on both continents. Alaska's Emperor Goose population declined precipitously from 139,000 in 1964 to 42,000 in 1986, although its numbers have recovered slightly since then (Fischer et al. 2008). The factors responsible for this quick decline and slow recovery remain poorly known. Hunting, especially subsistence hunting by Native Americans, is probably a factor (Hupp et al. 2008b); coastal oil pollution could also be reducing the survival of overwintering individuals (Byrd et al. 1995); and interspecific competition among brood rearing geese may limit recovery through recruitment (Schmutz and Laing 2002, Lake et al. 2008).

Birds of North America↗
USGS2010

Changes in agriculture and abundance of snow geese affect carrying capacity of sandhill cranes in Nebraska

The central Platte River valley (CPRV) in Nebraska, USA, is a key spring-staging area for approximately 80 of the midcontinent population of sandhill cranes (Grus canadensis; hereafter cranes). Evidence that staging cranes acquired less lipid reserves during the 1990s compared to the late 1970s and increases in use of the CPRV by snow geese (Chen caerulescens) prompted us to investigate availability of waste corn and quantify spatial and temporal patterns of crane and waterfowl use of the region. We developed a predictive model to assess impacts of changes in availability of corn and snow goose abundance under past, present, and potential future conditions. Over a hypothetical 60-day staging period, predicted energy demand of cranes and waterfowl increased 87 between the late 1970s and 19982007, primarily because peak abundances of snow geese increased by 650,000 and cranes by 110,000. Compared to spring 1979, corn available when cranes arrived was 20 less in 1998 and 68 less in 1999; consequently, the area of cornfields required to meet crane needs increased from 14,464 ha in 1979 to 32,751 ha in 1998 and 90,559 ha in 1999. Using a pooled estimate of 88 kg/ha from springs 19981999 and 20052007, the area of cornfields needed to supply food requirements of cranes and waterfowl increased to 65,587 ha and was greatest in the eastern region of the CPRV, where an estimated 54 of cranes, 47 of Canada geese (Branta canadensis), 45 of greater white-fronted geese (Anser albifrons), and 46 of snow geese occurred during ground surveys. We estimated that a future reduction of 25 in available corn or cornfields would increase daily foraging flight distances of cranes by 2738. Crane use and ability of cranes to store lipid reserves in the CPRV could be reduced substantially if flight distance required to locate adequate corn exceeded a physiological maximum distance cranes could fly in search of food. Options to increase carrying capacity for cranes include increasing accessibility of cornfields by restoring degraded river channels to disperse roosting cranes and increasing wetland availability in the Rainwater Basin to attract snow geese using the CPRV. ?? The Wildlife Society.

Journal of Wildlife Management↗
USGS1988

Length of stay and fat content of migrant semipalmated sandpipers in eastern Maine

Semipalmated Sandpipers (Calidris pusilla) stop at coastal staging areas in the Canadian maritime provinces and northeastern United States to replenish fat reserves before initiating a nonstop transoceanic flight of at least 3,200 km to wintering areas in South America. The relationship between estimated fat content at capture and length of stay (days between marking and last observation) of Semipalmated Sandpipers at one of these staging areas in eastern Maine was studied during 1980-1982. Total body mass and wing chord length were used to estimate fat content. When data were analyzed by week of initial capture, mean length of stay of both adults and juveniles decreased with increasing fat content. This supports the assumption that resumption of migration is affected by fat content at staging areas for long-distance nonstop flights. However, fat content at capture was a poor predictor of length of stay, which suggests that other factors are more important in determining length of stay.

Condor↗
USGS1995

Disarticulation of turtle shells in north-central Florida: How long does a shell remain in the woods?

From 1985 through 1991, I examined and categorized the sequential shell disarticulation patterns of 80 turtle specimens, representing six species and three families, in N-central Florida. Shells were allowed to disarticulate on land under natural environmental conditions. Turtle shells were observed from 5-54 mo. Based on examinations of photos, nine stages of disarticulation were identified. Although it is relatively easy to classify turtle shells into disarticulation stages, the amount of time between death and shell discovery cannot be determined precisely due to individual and species-specific variation in disarticulation rates. Shells do not necessarily pass sequentially through well-defined disarticulation stages. In general, shells of the most commonly observed species, Gopherus polyphemus, Pseudemys nelsoni and P. floridana, routinely remained intact from 12-30 mo, depending on species. Shells usually disarticulated completely by 40 mo or longer.

American Midland Naturalist↗
USGS1990

Time allocation by northern fulmars during the breeding season

Averaged over the whole breeding cycle (pre-laying through mid-chick stage), breeding fulmars spent about 39% of their time at the breeding site and 61% of their time at sea. Annual means of site occupancy before egg-laying were positively correlated with breeding success, suggesting that time allocation was a sensitive indicator of food availability in different years. Nonbreeding site-holders spent about half as much time at the colony as breeders; their attendance was highest early in the season when pair bonds and site ownership were established. The attendance of failed birds after egg or chick loss was positively correlated with colony-wide breeding success in six years. Males spent more time at the breeding site than females at every stage of the season. Most days were devoted to foraging in the pre-laying period (69% in males, 82% in females). An extended absence from the colony just before laying (the pre-laying exodus) averaged 12.2 d in males and 17.6 d in females. Changes in body weight indicated that neither sex incurred an energy deficit on a seasonal basis. A greater investment by the male in site attendance during incubation offset the female's investment in egg production and assured that both sexes entered the demanding chick-feeding stage in good condition.

Alaska↗
USGS1997

Wetland use, settling patterns, and recruitment in mallards

The correlation between number of May ponds in the Prairie Pothole Region (PPR) of North America and size of the continental mallard (Anas platyrhynchos) breeding population the following spring weakened from the 1950s to the 1980s, suggesting possible changes in suitability of prairie ponds for meeting reproductive needs. We studied wetland use and preferences of radioequipped female mallards by reproductive stage (1988-90) in eastern North Dakota and westcentral Minnesota and evaluated effect of land use on pair distribution in eastern North Dakota (1987-91). May pond density varied among years and study areas, with changes in number of temporary and seasonal ponds accounting for 93% of variation in total ponds. During all reproductive stages, semipermanent basins were used most by females, but temporary and seasonal ponds were preferred during prenesting and egg production. Accounting for number of relocations, number of ponds used varied by year, by reproductive stage and with pond density during egg production. Numbers of breeding mallard pairs in stratum 46 in eastern North Dakota increased as May ponds increased from 1963 to 1985, but 33,659 fewer breeding pairs on average were present in 1971-85 than in 1963-70. Number of breeding pairs declined relative to May ponds from the 1960s to the 1980s, probably because fewer pairs settle in temporary and seasonal ponds as the percent of landscape in cropland increases. Waterfowl managers in the PPR should target efforts to increase duck production on landscapes where non-cropped temporarily and seasonally flooded wetland habitats are plentiful, thereby increasing cost effectiveness of management actions taken to increase nest success rate.

Journal of Wildlife Management↗
USGS1990

Conditioning of sandhill cranes during fall migration

Body mass of adult female and male sandhill cranes (Grus canadensis) increased an average of 17 and 20%, respectively, from early September to late October on staging areas in central North Dakota and varied by year. Increases in body mass averaged 550 and 681 g among female and male G. c. canadensis, respectively, and 616 and 836 g among female and male G. c. rowani. Adult and juvenile G. c. rowani were lean at arrival, averaging 177 and 83 g of fat, respectively, and fat reserves increased to 677 and 482 g by mid-October. Fat-free dry mass increased by 12% among juveniles, reflecting substantial growth, but remained constant among adults. The importance of fall staging areas as conditioning sites for sandhill cranes, annual variation in body mass, and vulnerability of cranes to habitat loss underscore the need to monitor status of fall staging habitat in the northern plains region and to take steps to maintain suitable habitat where necessary.

Journal of Wildlife Management↗
USGS2001

Breeding bird response to juniper woodland expansion

In recent times, pinyon ( Pinus spp. )-juniper ( Juniperus spp. ) woodlands have expanded into large portions of the Southwest historically occupied by grassland vegetation. From 1997-1998, we studied responses of breeding birds to one-seed juniper ( J. monosperma ) woodland expansion at 2 grassland study areas in northern Arizona. We sampled breeding birds in 3 successional stages along a grassland-woodland gradient: un-invaded grassland, grassland undergoing early stages of juniper establishment, and developing woodland. Species composition varied greatly among successional stages and was most different between endpoints of the gradient. Ground-nesting grassland species predominated in uninvaded grassland but declined dramatically as tree density increased. Tree- and cavity-nesting species increased with tree density and were most abundant in developing woodland. Restoration of juniper-invaded grasslands will benefit grassland-obligate birds and other wildlife.

Journal of Range Management↗
USGS2001

The geology of a part of Acadia and the nature of the Acadian orogeny across Central and Eastern Maine

The zone of Acadian collision between the Medial New England and Composite Avalon terranes is well preserved in Maine. A transect from northwest (Rome) to southeast (Camden) crosses the eastern part of Medial New England comprising the Central Maine basin, Liberty-Orrington thrust sheet, and Fredericton trough, and the western part of Composite Avalon, including the Graham Lake, Clarry Hill, and Clam Cove thrust sheets. U-Pb geochronology of events before, during, and after the Acadian orogeny helps elucidate the nature and distribution of tectonostratigraphic belts in this zone and the timing of some Acadian events in the Northern Appalachians. The Central Maine basin consists of sedimentary and volcanic rocks of Middle Ordovician (∼470 to ∼460 Ma) age overlain with probable conformity by latest Ordovician(?) through earliest Devonian marine rift and flysch sedimentary rocks; these are intruded by weakly to undeformed plutonic rocks of Early and Middle Devonian age (∼399–378 Ma). The Fredericton trough consists of Early Silurian gray pelite and sandstone to earliest Late Silurian calcareous turbidite, deformed and variably metamorphosed prior to the emplacement of Late Silurian (∼422 Ma) and Early to Late Devonian (∼418 to ∼368 Ma) plutons. The Liberty-Orrington thrust sheet consists of Cambrian(?)-Ordovician (>∼474 to ∼469 Ma and younger) clastic sedimentary and volcanic rocks intruded by highly deformed Late Silurian (∼424 to ∼422 Ma) and Devonian (∼418 to ∼389 Ma) plutons, possibly metamorphosed in Late Silurian time (prior to ∼417 Ma), and metamorphosed to amphibolite facies in Early to Middle Devonian time (∼400 to ∼381 Ma). The Graham Lake thrust sheet contains possible Precambrian rocks, Cambrian sedimentary rocks with a volcanic unit dated at ∼503 Ma, and Ordovician rocks with possible Caradocian Old World fossils, metamorphosed and deformed in Silurian time and intruded by mildly to undeformed Late Silurian (∼421 Ma) and Late Devonian (∼371 to ∼368 Ma) plutons. The Clarry Hill thrust sheet consists of poorly studied, highly metamorphosed Cambrian (?) rocks. The Clam Cove thrust sheet contains highly deformed Precambrian limestone, shale, sandstone, and conglomerate, metamorphosed to epidote amphibolite facies and intruded by a mildly deformed pluton dated at ∼421 Ma. Metamorphism, deformation, and voluminous intrusive igneous activity of Silurian age are common to both the most southeastern parts of Medial New England and the thrust sheets of Composite Avalon. In contrast to Medial New England, the thrust sheets of Composite Avalon show only modest effects of Devonian deformation and metamorphism. Regional stratigraphic relations, paleontologic findings, and U-Pb geochronology suggest that the Graham Lake, Clarry Hill, and Clam Cove thrust sheets are far-traveled allochthons that were widely separated from Medial New England in the Silurian. One hundred nine (109) new U-Pb analyses of zircon, monazite, and sphene from 25 samples of metamorphosed, stratified, and intrusive igneous rocks are used to decipher the history of events along the transect. In our view, many of the structures within Medial New England were formed during the closing of the back-arc ocean during the waning stages of the Taconian orogeny. These features include the precursors to the upright folds found in Silurian sedimentary rocks, the west-facing Liberty-Orrington thrust sheet, and the Silurian plutons of Medial New England. Acadian tectonic features, including flysch sedimentation, igneous activity, deformation with nappe emplacement, and metamorphism record the progressive loading of Medial New England by a stack of thrust nappes emplaced in latest Silurian to Middle Devonian time. The Acadian orogeny is a prolonged event, lasting from earliest Late Silurian to the Late Devonian, whose evolution involved: (1) convergence between Medial New England and Composite Avalon along an east-dipping subduction zone from earliest Late Silurian to Early Devonian time; (2) collision and concurrent delamination of lithospheric mantle beneath Medial New England in Early Devonian time resulting in deformation, high-grade metamorphism, and intrusive igneous activity in the most eastern part of Medial New England and the western parts of Composite Avalon; (3) Early to Middle Devonian northwest-migrating penetrative deformations of the Acadian Main stage, including northwest-directed thrusting and recumbent folding followed by tightening of folds possibly produced in the waning stages of the Taconian orogeny and forming folded wave trains of isoclinal folds (∼419 to ∼404 Ma), (4) asymmetrical folds produced by east-west shortening (∼399 to ∼380 Ma); and (5) final westward emplacement of Composite Avalon thrust sheets onto Medial New England in Late Devonian time (∼380 to ∼371 Ma). The present boundary between rocks of Medial New England and Composite Avalon at the surface is the Sennebec Pond fault, a high-angle fault that cuts the Graham Lake and Clarry Hill thrusts, and is intruded by the Mt. Waldo pluton (∼371), one of several Late Devonian plutons (∼371 to ∼367 Ma) that mark the end of the Acadian orogeny in coastal Maine. The thrust sheets of Composite Avalon carried rocks of peri-Gondwanan affinity and with possible Old World fauna many kilometers to the west over Medial New England, thus potentially hiding the original suture at depth under the Gulf of Maine.

Maine↗
USGS1972

Determination of channel capacity of the Mokelumne River downstream from Camanche Dam, San Joaquin and Sacramento Counties, California

This study evaluates the adequacy of a 39-mile reach of the Mokelumne River in San Joaquin and Sacramento Counties, California, to carry planned flood releases between Camanche Reservoir and the Bensons Ferry Bridge near Thornton. The flood releases from Camanche Reservoir are to be restricted, insofar as possible, so that the flows in the Mokelumne River will not exceed 5,000 cfs (cubic feet per second) as measured at the gaging station below Camanche Dam. Areas of inundation and computed floodwater profiles are based on channel conditions in late 1970 and on observed water-surface profiles during flood releases of about 5,000 cfs in January 1969 and January 1970. The inundated area shown on the maps (appendix A) and the water-surface elevations indicated on the cross sections (appendix G) are for the flood releases of those dates. The following conclusions are contingent on there being no levee failures during periods of high flow and no significant channel changes since the flood release of January 1970. 1. High tides in San Francisco Bay and, to a greater degree, flood stages on the Cosumnes River, cause backwater in the study reach. Severe backwater conditions occurring simultaneously with a flow of 5,000 cfs in the Mokelumne River can increase the flood stage 4 to 6 feet at Bensons Ferry Bridge (cross section 1). Backwater effects decrease in an upstream direction and are less than 0.5 foot at cross section 35, a river distance of 8.6 miles upstream from cross section 1, and 1.5 miles downstream from the Peltier Road bridge. 2. In the reach between cross sections 1 and 35, a 5,000 cfs release from Camanche Reservoir with maximum backwater effect (measured at cross section 1 at the mouth of the Cosumnes River) is confined within the natural or leveed banks except on the right bank flood plain between cross sections 12 and 19. 3. Upstream from cross section 35, there is overbank flooding at a flow of 5,000 cfs between cross sections 48 and 51, and 62 and 67.5. An increase in flow from 5,000 to 6,000 cfs will cause flooding between cross sections 43 and 47, 52 and 56, and 73 and 85. 4. A discharge of 5,000 cfs will pass through all bridge openings in the study reach except that of the Western Pacific Railroad Co. bridge at cross section 4. If large amounts of debris lodge on the railroad bridge when backwater from the Cosumnes River occurs, the debris could cause higher stages and flooding along the right bank between cross sections 5 and 12.

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USGS1933

Metalliferous deposits of the greater Helena mining region, Montana

The ore deposits described in this bulletin are distributed through a region of about 3,000 square miles surrounding the city of Helena, Mont. In general the surface of this region is mountainous, but it includes several large intermontane valleys. Large areas in the northern and eastern parts of the region sire underlain by sedimentary rocks of the Algonkian Belt series, and on the northeast and southwest the Belt rocks are overlain without any noticeable angular unconformity by Paleozoic and Mesozoic beds. Oligocene, Miocene, and possibly Pliocene sediments, composed chiefly of volcanic ash and land waste of local origin, occupy large areas in the intermontane valleys and lie unconformably upon Cretaceous and older rocks. A thin veneer of Pleistocene and Recent alluvium generally overspreads the Tertiary. In the extreme northern part of the region are large deposits of glacial drift that represent two stages of the Pleistocene. The principal igneous body of the region is the northern part of the early Tertiary or late Cretaceous Boulder batholitb of quartz monzonite. The main exposure of this body occupies an area of nearly 1,200 square miles and extends southward beyond the limits of the particular region considered. Smaller areas of similar rocks are clustered around this exposure. Most of the exposures probably represent bodies that are connected in depth to form a single mass. The late Cretaceous and older sedimentary rocks are involved in a series of northwestward-trending folds. Along the east side of the region overthrust faults related to the great Lewis overthrust of Glacier National Park cause Belt rocks to overlie rocks of Paleozoic and Mesozoic ages. Large normal faults occur near Marysville and faults of moderate displacement near Helena. The Tertiary beds are slightly deformed by folds and faults that are unrelated to the structure of the older rock. The geologic history of the region includes two contrasting periods, the earlier of which was characterized by the accumulation of marine sediments and the later by mountain building and erosion. The later period began with folding and elevation in late Cretaceous or early Eocene time, followed by overthrust faulting and the intrusion of the Boulder batholith. Next, there ensued a period of crustal stability, during which erosion reduced the region to a surface of low relief and cut away at least 10,000 feet of strata in the area north of Helena. In Oligocene, Miocene, and Pliocene (?) time sediments composed of land waste and volcanic ash were deposited, and this event was followed by warping and faulting that elevated the present mountains. During Pliocene and Pleistocene time the mountains were maturely dissected, and in middle and late Pleistocene time local glaciers formed in the higher mountains and large valley glaciers invaded the extreme northern part of the region. The ore deposits include lodes and placers that have yielded gold, silver, lead, copper, and zinc to a value of at least $176,860,000. The placers were formed mostly during interglacial stages of the Pleistocene. They have been almost entirely exhausted. Most of the lodes are classified as regards age in two groups, an older and a younger. The older lodes are related in origin to the Boulder batholith or some of the neighboring intrusive granitic bodies and were probably formed during early Eocene time. The younger lodes were formed after dacite of probable Miocene age was erupted. They are possibly related in origin to some unexposed intrusive granitic rock. For convenience in description the region is divided into three parts districts north of Helena, districts in the Belt Mountains, and districts south of Helena. The districts north of Helena include a 50-mile stretch of the Continental Divide that forms a broad ridge surmounted with considerable areas of flat or gently sloping surface at a general altitude of 7,000 feet. Narrow valleys 1,000 to 2,000 feet deep are cut into this surface and lead out to the neighboring wide intermontane valleys. The area is underlain mostly by shale, sandstone, and limestone of the upper part of the Belt series. Beds of Paleozoic and Mesozoic age occur south of the Belt area and extend from Helena west and northwest. The igneous rocks of the area include diorite and gabbro sills and dikes of probable Cretaceous age, extrusive andesite that is probably Oligocene or Miocene, and stocks of quartz monzonite, granodiorite, and quartz diorite, probably of Oligocene or Miocene age. The ore deposits of the northern districts are chiefly lodes that are valuable for gold and silver but contain some lead and copper. In the Ophir district bodies of gold and silver ore occur mainly in limestone near a body of quartz monzonite. In the Scratchgravel Hills and Grass Valley districts veins of gold quartz and veins containing lead-silver ore occur in quartz monzonite and in the adjoining metamorphic rocks. In the Austin district lodes containing gold; silver, lead, and copper are found in limestone near intrusive quartz monzonite. An unusual mineral in one of these lodes is corkite, a hydrous sulphate of lead containing arsenic. A small stock of quartz diorite in the Marysville district has invaded and domed Belt rocks. Marginal and radial fractures formed during the cooling and contraction of the igneous body became the receptacles of gold and silver veins, one of which, the Drumlummon, has produced $16,000,000. The veins filled open fractures and are characterized by a gangue of platy calcite and quartz. Lodes in Towsley Gulch in the western part of the district contain lead in addition to gold. In the Gould district a small stock of the granodiorite has invaded the Belt rocks and caused the deposition of veins similar to those near Marysville. In the Heddleston district lodes valuable for gold, silver, lead, and copper occur in Belt sedimentary rocks and diorite, some of them associated with porphyry dikes. In the Wolf Creek district veins in Belt rocks have produced copper ore composed mainly of chalcopyrite or chalcopyrite and tennantite accompanied by pyrite and a gangue of quartz and barite. Placer deposits along the western slope of the Belt Mountains have produced $17,500,000 in gold. Sapphires were formerly obtained from some of these deposits. The central part of the Belt Mountains is a plateaulike area considered to be the remnant of a surface produced by erosion during Tertiary time. This surface was elevated and has been deeply trenched by narrow, transverse valleys that are bordered with remnants of low terraces in which most of the placer deposits occur. Most of the western slope of the mountains is underlain by sedimentary rocks of the Belt series. At the foot of the mountains these give place to Paleozoic rocks, and these in turn are overlain unconformably in Townsend Valley by Tertiary and later deposits. The main structural feature is a great arch called the York anticline, which occupies most of the west side of the mountains. At the west foot of the mountains this fold is bordered by a series of small synclines that are tightly squeezed, faulted, and overturned as a result of pressure exerted from the west or southwest along a fracture described as the El Dorado overthrust On another fracture called1 the Scout Camp overthrust the Belt rocks composing the western slope of the mountains are thrust eastward over Paleozoic beds. Both faults are regarded as branches of the Lewis overthrust of Glacier Park. Igneous rocks that probably range in age from early Eocene to Pleistocene are widely but sparingly distributed.' They include sills and dikes of quartz dibrite, porphyry dikes, small stocks of quartz monzonite and quartz diorite, and surface flows of andesite and basalt. The deposits in the Belt Mountains that are of most interest at the present time are lodes that are chiefly valuable for gold. Most of them are found in the vicinity of York and Confederate Gulch. Nearly all are small quartz veins formed along fractures in diorite dikes and stocks or on bedding planes in the adjoining Belt sedimentary rocks. An exception is the Golden Messenger, a replacement deposit of large size but low grade, formed along fractures in a quartz diorite dike. Other veins in the same dike belong to the rather uncommon class called ladder veins. Many of the small veins contain shoots and bunches of rich ore in their upper parts. Downward- enrichment in gold is indicated to have occurred in some of the veins near York that lie below an old erosion surface. Elsewhere the origin of the placer deposits from erosion o'f the lodes during interglacial stages of the Pleistocene is indicated. Lodes containing chalcopyrite occupy tension fractures in the Belt shales that were produced by lateral movements of the mass composing the mountain front. In the districts south of Helena mining began with the discovery, on July 14, 1864, of rich placer deposits at the present site of the city of Helena, on Last Chance Creek. Since then the placer and lode deposits of these districts have produced metals worth $130,000,000 or more, of which about one-third was gold, the remainder chiefly lead and zinc. Sedimentary rocks ranging in age from Algonkian to Cretaceous underlie parts of the region, and other parts are underlain by a bedded series of andesite and latite tuffs, breccias, and flows. These rocks have been intruded and severely metamorphosed by the quartz monzonite of the Boulder batholith, the exposures of which occupy a large area. Rocks later than the intrusion of the batholith are chiefly a series of late Tertiary dacites and rhyolites. The placers of the southern districts have been almost entirely worked out. The lodes have yielded metals worth $111,600,000, but many of them are still productive. They include veins and contact-metamorphic deposits. Some of the contact deposits contain copper ore, and others contain iron ore valuable for fluxing. The veins are of two ages. The older veins have yielded most of the metallic production of the region. Their ores in general are heavy sulphide aggregates composed mainly of galena, sphalerite, and pyrite. Arsenopyrite is generally present; tetrahedrite and chalcopyrite are less common. Many of the veins are distinguished from the usual type of ore body by the occurrence of abundant tourmaline. The, metals produced are chiefly silver, lead, gold, and zinc, with some copper. The younger veins are essentially precious-metal deposits. They are mainly fissure veins but include some disseminated deposits of low grade. They are widely distributed and include several productive bodies. A distinguishing feature is the occurrence in the gangue of cryptocrystalline quartz and lamellar calcite. A dominant eastward trend of the vein fractures of the older group indicates them to be tension cracks in the crust block lying west of the Lewis overthrust that were produced by stretching in a direction at right angles to the thrust.

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