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Fish meal and alternate sources of protein in fish feeds: Update 1993

Close to 12% of the world's 6.5 million metric tons of fish meal is used for aquaculture feeds. If current trends continue, roughly 20% to 25% of total world fish meal production could be used for aquaculture by the year 2000. Fish stocks used in fish meal reduction, however, appear to be in worldwide decline. A growing fish farming industry and a stagnating, if not diminishing, supply of fish meal have sobering economic and technologic implications for fish culture. Unless suitable alternate protein sources are found or other animal feeds begin to rely less on fish meal, fish production costs can be expected to increase dramatically. A precedent was set by the poultry industry, the most economically successful and competitive of the animal agriculture industries. Twenty years ago, the poultry industry consumed up to 80% of fish meal supplies. Through deliberate and well-organized research into alternate protein sources, the industry now uses less than 40% of supplies, and the trend is toward complete independence from fish meal. A comparable research effort is needed for aquaculture. Considering the biotechnologies available, plant proteins, processed to remove enzyme inhibitors and other antinutritional factors and properly supplemented with essential amino acids and minerals where needed, could produce results at least equivalent to those obtained with fish meal.

Fisheries↗

Gold enrichment and the Bi-Au association in pyrrhotite-rich massive sulfide deposits, Escanaba trough, Southern Gorda Ridge

High gold contents (to 10.1 ppm, avg 1.4 ppm, n = 34) occur in pyrrhotite-rich massive sulfide samples from the sediment-covered floor of the Escanaba trough, the slow-spreading, southernmost segment of Gorda Ridge. These concentrations reflect the presence of primary gold, formed during high-temperature hydrothermal activity in mounds and chimneys, and secondary gold deposited during sea-floor weathering of massive sulfide. Primary gold occurs as fine-grained (<2 ??m) gold, electrum, maldonite (Au2Bi), and a rare unnamed mineral (AuBi5S4) and is texturally and temporally associated with sulfarsenides (arsenopyrite, glaucodot, alloclastite, and cobaltite), arsenides (lo??llingite and safflorite), Bi minerals (native bismuth, Bi tellurides, Bi-Te sulfides, and bismuthinite), and galena. Larger (>2 ??m) secondary gold grains have a porous, flaky morphology and occur in samples in which pyrrhotite is oxidized and replaced by Fe oxyhydroxides, Fe sulfate, and sulfur. Mounds and chimneys dominated by pyrrhotite and containing lesser amounts of isocubanite, chalcopyrite, and Fe-rich sphalerite were formed by high-temperature (estimated range 325??-275??C), reduced, low-sulfur vent fluids. The mineral and fluid compositions during this main stage of hydrothermal venting reflect subsurface interaction between circulating hydrothermal fluids and turbiditic sediment containing as much as 1.1 percent organic carbon. As the deposition of pyrrhotite, Cu-Fe sulfides, and sphalerite waned, a volumetrically minor suite of sulfarsenide, arsenide, Bi, and Au minerals was deposited from highly reduced, late main-stage fluids diffusing through mounds and chimneys. The low solubility of Au as a bisulfide complex and the absence of fluid mixing during this stage of hydrothermal activity apparently inhibited the precipitation of gold directly from solution. Instead, gold precipitation is thought to be linked to elevated concentrations of Bi in the late main-stage fluids. The textural relationships of Au and Bi minerals in pyrrhotite-rich samples, low melting point of native bismuth (271.4??C), and recent experimental results on Au and Bi in hydrothermal fluids contribute to the hypothesis that gold was effectively scavenged from the Escanaba trough vent fluids by coexisting droplets of liquid bismuth. Additional phase relationships of alloys in the Au-Bi system indicate that deposition of native bismuth and maldonite occurred at temperatures as low as 241??C. Bismuth droplets trapped in void space between main-stage mineral grains scavenged gold from ambient hydrothermal fluid to a greater extent than bismuth enclosed by late-forming pyrrhotite. The limited solid solution of Au in Bi can explain the apparent exsolution texture in which gold blebs are hosted by native bismuth. The electrum, native bismuth (with gold inclusions), and galena represent the last traces of gold mineralization from late main-stage fluids. During sea-floor weathering and the oxidation of pyrrhotite in the mounds and chimneys, secondary gold formed as aggregates of colloidal particles along pH gradients between acidic pore waters and ambient seawater. Gold was mobilized from earlier formed primary gold minerals and transported as aqueous chloride complexes. The reduction of Au(III) by residual Fe2+ in partly altered pyrrhotite and adsorption of colloids by Fe oxyhydroxides may have influenced the location of secondary gold grains within the alteration front. Solubility differences between gold and silver chloride complexes at low temperature account for the low Ag content of secondary gold grains. The high concentrations of Bi, and thus the association of Au and Bi minerals in pyrrhotite-rich massive sulfide, can be ascribed to the extensive interaction of hydrothermal fluids with sediment in the Escanaba trough. In contrast, the absence of the Au-Bi association in massive sulfides at other ridges, including other sediment-covered sites at Middle Valley and the Guaymas basin

Economic Geology↗

The Chahnaly low sulfidation epithermal gold deposit, western Makran volcanic arc, southeastern Iran

The Chahnaly low-sulfidation epithermal Au deposit and nearby Au prospects are located northwest of the intermittently active Bazman stratovolcano on the western end of the Makran volcanic arc, which formed as the result of subduction of the remnant Neo-Tethyan oceanic crust beneath the Lut block. The arc hosts the Siah Jangal epithermal and Kharestan porphyry prospects, near Taftan volcano, as well as the Saindak Cu-Au porphyry deposit and world-class Reko Diq Cu-Au porphyry deposit, near Koh-i-Sultan volcano to the east-northeast in Pakistan. The host rocks for the Chahnaly deposit include early Miocene andesite and andesitic volcaniclastic rocks that are intruded by younger dacitic domes. Unaltered late Miocene dacitic ignimbrites overlie these rocks. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology data yield ages between 21.8 and 9.9 Ma for the acidic-intermediate regional volcanism. The most recent volcanic activity of the Bazman stratovolcano involved extrusion of an olivine basalt during Pliocene to Quaternary times. Interpretation of geochemical data indicate that the volcanic rocks are synsubduction and calc-alkaline to subalkaline. The lack of a significant negative Eu anomaly, a listric-shaped rare earth element pattern, and moderate La/Yb ratios of host suites indicate a high water content of the source magma. Gold and electrum are temporally and spatially related to a series of structurally controlled, 030&deg;-trending, subvertical hydrothermal breccias with chalcedony-adularia that cut porphyritic andesite and andesitic volcaniclastic rocks. Gold is associated with pyrite, a siliceous matrix of hydrothermal breccia, and previously formed vein clasts, as well as with iron oxides and hydroxides in oxidized zones. Rare silver minerals include Ag-bearing electrum and naumannite, iodargyrite, an unnamed silver diiodide, and hessite. Hydrothermal alteration is generally well developed surrounding the ore-bearing hydrothermal breccia. The main types of alteration in the area include an inner ~0.5- to 20-m-thick gold-bearing hydrothermal breccia composed of quartz-chalcedonyadularia-illite-pyrite, a ~5- to 50-m-thick zone of quartz, chalcedony, pyrite, illitic phengite, phengite, illitic muscovite, illite, illitic paragonite, paragonite, muscovite, montmorillonite and, rarely, siderite, and a 30- to 70-m outer propylitic zone of Fe-Mg chlorite, calcite, ankerite, dolomite, epidote, palygorskite, and pyrite. The Chahnaly Au deposit formed during the early stages of magmatism. LA-ICP-MS zircon U-Pb geochronology of host andesite and 40Ar/39Ar dating of two samples of gold-associated adularia show that the ore-stage adularia (19.83 &plusmn; 0.10 and 19.2 &plusmn; 0.5 Ma) is younger, by as much as 1.5 million years, than the volcanic host rock (20.32 &plusmn; 0.4 Ma). Therefore, either hydrothermal activity continued well after volcanism or a second magmatic event rejuvenated hydrothermal activity. This second magmatic event may be related to eruption of porphyritic andesite at ~20.32 &plusmn; 0.40 Ma, which is within error of ~19.83 &plusmn; 0.10 Ma adularia. The new LA-ICP-MS zircon U-Pb host rock and vein adularia 40Ar/39Ar ages suggest that early Miocene magmatism and mineralization in the Bazman area is of a similar age to that of the Saindak porphyry and Tanjeel porphyry center of the giant Reko Diq deposit. This confirms the existence of early Miocene arc magmatism and mineralization along the Iranian part of the Makran volcanic arc. Ore, alteration mineralogy, and alteration patterns indicate that the Chahnaly deposit is a typical low-sulfidation epithermal Au deposit, located in a poorly explored part of the Makran volcanic arc in Iran.

Economic Geology↗

Fractionation of minor elements between galena and sphalerite, Darwin lead-silver-zinc mine, Inyo County, California and its significance in geothermometry

Minor element abundances were determined for galena and sphalerite from the Darwin mine, Inyo County, California. The temperature of deposition was calculated from the distribution coefficients determined experimentally by Bethke and Barton (1971) for cadmium and manganese in coexisting galena and sphalerite. The fractionation of cadmium between coexisting sphalerite and galena indicates a temperature of deposition of 416 degrees C+ or -20 degrees , the fractionation of manganese between the same mineral pair a temperature of 377 degrees C+ or -32 degrees . The fractionation of selenium gave unreasonably high temperatures. Three pairs gave temperatures between 568 degrees and 625 degrees C, the fourth pair a temperature of 889 degrees C. The reason for this descrepancy is not known.

California↗

Age of native copper mineralization, Keweenaw Peninsula, Michigan

Amygdaloidal flood basalts and conglomerates are the host for substantial deposits of native copper within the Portage Lake Volcanics in the Keweenaw Peninsula of Michigan. A wide variety of gangue minerals are associated with the regional hydrothermal alteration-mineralization event. Application of the Rb-Sr method to amygdule-filling microcline, calcite, epidote, and chlorite suggests an age of mineralization between 1,060 and 1,047 m.y. (+ or - [asymp] 20 m.y.). These results are supported by a fission track age on epidote of 1,044 + or - 169 m.y. The age of native copper mineralization determined in this study is consistent with geologic evidence which suggests that mineralization postdated the deposition of most or all of the overlying Freda Sandstone. Variable initial 87 Sr/ 86 Sr ratios between the low Rb/Sr phases suggest mixing of isotopically distinct sources of Sr during the generation and equilibration of the hydrothermal solutions.

Michigan↗

Economic significance of revised age relations of rocks in the Cornucopia mining district, Elko County, Nevada

Recent geologic work in the Cornucopia mining district, a small silver-gold mining district in northwestern Elko County, Nev., has resulted in significant revision of the geological interpretation. Rocks formerly thought to be premineralization in age, but unmineralized, are now known to be postmineral extrusives, resting unconformably on the altered andesite in which the ore bodies are found. Extensions of the known productive veins may be expected at shallow depth beneath the younger volcanic rocks, and are separated from the mined part of the veins by postmineral high-angle faults that have brought the younger volcanic rocks into fault contact with the mineralized andesite. Some veins are apparently terminated against premineral faults.

Circular↗

Geologic setting and characteristic of mineral deposits in the central Wasatch Mountains, Utah

Base- and precious-metal deposits in the central Wasatch Mountains southeast of Salt Lake City were mined for more than 100 years beginning in 1868. Deposits present in the Park City, Little Cottonwood, and Big Cottonwood mining districts include Ag-Pb-Zn ± Cu ± Au replacements and veins, a low-grade porphyry Cu-Au deposit, Cu-bearing skarns, a quartz monzonite-type (low F) porphyry Mo deposit, and high sulfidation (quartz-alunite) Au deposits. Most production came from polymetallic replacement and vein deposits in the Park City mining district, which has a recorded production of more than 1.4 million oz Au, 253 million oz Ag, 2.7 billion lbs Pb, 1.5 billion lbs Zn, and 129 million lbs Cu from 1872 to 1978. Production in the Little and Big Cottonwood districts, mostly from Pb-Ag replacement deposits, was much smaller. Most mineral deposits in the central Wasatch Mountains are genetically related to the Wasatch igneous belt, a series of high-K calc-alkaline stocks and cogenetic volcanic rocks that formed about 41(?) to 30 Ma. The mineral deposits mostly formed near the end of magmatic activity between about 36 to 31.4 Ma. A subeconomic porphyry Mo deposit in the Little Cottonwood stock is notably younger having formed about 26 to 23.5 Ma. The intrusive rocks were emplaced mostly along the westward extension of the west-trending Uinta arch during a period of NW-SE-directed extension, and much of the mineralization in the Park City district was controlled by ENE-striking normal faults. About 15 degrees of eastward tilting of the central Wasatch Mountains during Late Cenozoic Basin and Range extension has resulted in progressively deeper levels of exposure from <1 km on the east to about 11 km on the west and in profound variations in the types of mineral deposits exposed in different parts of the range. Most deposits formed at paleodepths ≤5 km, and the most productive deposits in the Park City district formed at depths of 1 to 2 km. The porphyry Mo deposit in the Little Cottonwood stock formed at greater depths of about 6 km.

Utah↗

Thorium investigations, 1950-52, Wet Mountains, Colorado

The Wet Mountain thorium district was discovered in 1950 by members of the U.S. Geological Survey during reconnaissance investigations for uranium made on the behalf of the Atomic Energy Commission The size of this new district is incompletely known, but the deposits found to date are in an area 20 miles long and about 10 miles wide, the southwest boundary of which extends northwest from Querida and Rosita Custer County, into Fremont County. Most of the deposits, however, are in the southeastern half of this area The thorium minerals occur in barite-sulfide veins along northwest-trending shear zones that cut a pre-Cambrian complex or amphibolite, biotite-granite gneiss, metagabbro, migmatite, microcline granites, pegmatite and white granite, and syenite (?) o Along the shear zones are premineralization basic (lamprophyre (?)) dikes. Thorite has been tentatively identified as the principal radio- active mineral It is associated with barite: quartz, galena, fluorite, limonite, and pyrite Some of the shear zones perhaps can be traced for as much as 2 miles, but the largest known thorium-bearing ore body is as much as 300 feet long, 26 feet wide, and 400 feet deep. Samples from the veins contain as much as 1.6 percent equivalent Th0 2 Eleven diamond drill holes, totaling 3,292.4 feet have explored five shear zones on the Haputa Ranch. Three ore bodies of possible economic interest are indicated in two interconnecting shear zones. No other deposits in the wet Mountains have been explored, or even sampled adequately to hazard an estimate as to grade and tonnage of reserves.

Colorado↗

Devonian volcanogenic massive sulfide deposits and occurrences, southern Yukon-Tanana Terrace, eastern Alaska Range, Alaska

A belt of volcanogenic massive sulfide deposits extends for over 150 km along the southern margin of the Yukon-Tanana terrane of the eastern Alaska Range. Located north of the Denali fault, the Yukon-Tanana terrane forms a major basement unit in east-central Alaska. The volcanogenic massive sulfide deposits are primarily in the Jarvis Creek Glacier subterrane, one of the highest structural-stratigraphic levels of the Yukon-Tanana terrane. The Jarvis Creek Glacier subterrane consists of a volcanogenic massive sulfide-bearing metavolcanic rock member and a metasedimentary rock member. The volcanogenic massive sulfide deposits and enclosing metavolcanic and metasedimentary schists exhibit two periods of regional metamorphism and penetrative deformation: an older, Early Cretaceous, amphibolite facies event and a younger, mid-Cretaceous lower greenschist facies event.From northwest to southeast, the major volcanogenic massive sulfide districts are the Bonnifield, Trident Glacier, and Delta. The Bonnifield district contains massive sulfide deposits in metavolcanic and metasedimentary rocks that are correlated with the Jarvis Creek Glacier subterrane. The 25-km-long, northwest-trending Trident Glacier district contains several occurrences consisting of massive pyrrhotite and lesser pyrite, minor chalcopyrite with or without sphalerite, and galena and arsenopyrite in south-dipping massive sulfide pods, lenses, and stringers that parallel the foliation of the enclosing schists. Host-rock protoliths were mainly andesite, dacite, rhyodacite tuffs and flows, shale, and limestone, with lesser quartz-rich keratophyre flows and tuffs, volcanic graywacke, and siltstone.The Delta district contains at least 35 deposits in an 800-km 2 area. Thirty-one of the deposits define four mineral trends, 5 to 32 km long, that are subparallel to the west-northwest-striking, southwest-dipping structures and lithologies. The deposits consist of layers and zones containing varying amounts of massive to disseminated pyrite and pyrrhotite, and lesser amounts of chalcopyrite, galena, sphalerite, and arsenopyrite. The larger deposits range between 1.0 and 12.4 million metric tons with combined grades of 5.6 percent total metal (Cu, Pb, Zn), and Ag and Au contents of 56 and 1.9 g/metric ton, respectively. Hostrock protoliths are similar to those found in the Trident Glacier district, but some of the deposits are spatially associated with diorite or gabbro sills that exhibit lower greenschist facies metamorphism and deformation.Sulfur isotope values of 26 pyrrhotite separates from the Trident Glacier district occurrences average 7.7 per mil, the mean value of seven pyrite separates is 8.1 per mil. Forty-one Delta district pyrite separates average 5.9 per mil and ranges of values of the four northwest-trending trends overlap. The relative consistency of sulfur isotope values of iron-bearing sulfides in the Trident Glacier district, and the apparent lithologic relationship of isotope values in the Delta district, may reflect original-rock sulfur isotope values. Alternatively, the 34 S-enriched values may have resulted from the addition of reduced seawater sulfate to the hydrothermal fluid. The sulfide accumulations are interpreted as kuroko-like deposits that formed along a Devonian continental margin igneous arc. Deeper level portions of the arc are exposed in structurally lower levels of the Yukon-Tanana terrane to the north and contain metamorphosed granodiorite and granite plutons of Devonian to Mississippian age. U-Pb isotope data from zircons from metavolcanic rocks hosting the sulfide bodies indicate an age of about 370 Ma. The 207 Pb/ 204 Pb ratios for sulfides from the three districts are similar and indicate lead was in part derived from first-cycle sediments that were eroded largely from Precambrian crystalline rocks. The 207 Pb/ 204 Pb ratios from feldspars in Devonian metavolcanic rocks are similar to those in sulfides and indicate incorporation of radiogenic Precambrian lead into the Devonian magmas. A Precambrian crystalline source for detritus in metasedimentary rocks is indicated by locally abundant quartzite containing approximately 2.3 Ga zircons which were derived from the Hudsonian-age block of the Canadian Shield in southern British Columbia.Cretaceous greenschist facies retrogressive metamorphism and intense penetrative deformation is indicated by reequilibration of U-Pb isotope ratios, metamorphic Rb-Sr mineral isochrons, and K-Ar biotite and muscovite ages of 110 to 115 Ma. The data indicate that the mid-Cretaceous retrogressive metamorphism and associated intense deformation occurred during the accretion of the Yukon-Tanana terrane to Alaska.

Economic Geology↗

U.S. Geological Survey ecosystems science strategy—Advancing discovery and application through collaboration

Executive Summary Ecosystem science is critical to making informed decisions about natural resources that can sustain our Nation’s economic and environmental well-being. Resource managers and policymakers are faced with countless decisions each year at local, regional, and national levels on issues as diverse as renewable and nonrenewable energy development, agriculture, forestry, water supply, and resource allocations at the urban-rural interface. The urgency for sound decisionmaking is increasing dramatically as the world is being transformed at an unprecedented pace and in uncertain directions. Environmental changes are associated with natural hazards, greenhouse gas emissions, and increasing demands for water, land, food, energy, mineral, and living resources. At risk is the Nation’s environmental capital, the goods and services provided by resilient ecosystems that are vital to the health and wellbeing of human societies. Ecosystem science—the study of systems of organisms interacting with their environment and the consequences of natural and human-induced change on these systems—is necessary to inform decisionmakers as they develop policies to adapt to these changes. This Ecosystems Science Strategy is built on a framework that includes basic and applied science. It highlights the critical roles that U.S. Geological Survey (USGS) scientists and partners can play in building scientific understanding and providing timely information to decisionmakers. The strategy underscores the connection between scientific discoveries and the application of new knowledge, and it integrates ecosystem science and decisionmaking, producing new scientific outcomes to assist resource managers and providing public benefits. We envision the USGS as a leader in integrating scientific information into decisionmaking processes that affect the Nation’s natural resources and human well-being. The USGS is uniquely positioned to play a pivotal role in ecosystem science. With its wide range of expertise, the Bureau can bring holistic, cross-scale, interdisciplinary capabilities to the design and conduct of monitoring, research, and modeling and to new technologies for data collection, management, and visualization. Collectively, these capabilities can be used to reveal ecological patterns and processes, explain how and why ecosystems change, and forecast change over different spatial and temporal scales. USGS science can provide managers with options and decision-support tools to use resources sustainably. The USGS has long-standing, collaborative relationships with the Department of the Interior (DOI) and other partners in the natural sciences, in both conducting science and applying the results. The USGS engages these partners in cooperative investigations that otherwise would lack the necessary support or be too expensive for a single bureau to conduct. The heart of this strategy is a framework for USGS ecosystems science that focuses on five long-term goals, which are seen as interconnected components that reinforce our vision of the USGS providing science that is at the forefront of decisionmaking: Improve understanding of ecosystem structure, function, and processes. The focus for this goal is an understanding of how ecosystems work, including the dynamics of species, their populations, interactions, and genetics, and how they change across spatial and temporal scales. Advance understanding of how drivers influence ecosystem change. The challenges here are explaining the drivers of ecosystem change, their spatio-temporal patterns, their uncertainties and interactions, and their influence on ecosystem processes and dynamics. Improve understanding of the services that ecosystems provide to society. Here the emphasis is on the measurement of environmental capital and ecosystem services, and the identification of sources and patterns of change in space and time. Develop tools, technologies, and capacities to inform decisionmaking about ecosystems. This includes developing new technologies and approaches for conducting applications-oriented ecosystem science. A principal challenge will be how to quantify uncertainty and incorporate it in decision analysis. Apply science to enhance strategies for management, conservation, and restoration of ecosystems. These challenges include development of novel approaches to monitoring, assessment, and restoration of ecosystems; new methods to address species of concern and communities at risk; and innovations in decision analysis and support to address imminent ecosystem changes or those that are underway. Closely integrated with the five goals are four strategic approaches that provide the path forward for the USGS Ecosystems Mission Area. These approaches cross-cut all of the goals and are seen as essential to the implementation of this strategy: Assess information needs for ecosystem science through enhanced partnerships. Work with the DOI and other agencies and institutions to identify, design, and implement priority decision-driven ecological research. Promote the use of interdisciplinary ecosystem science. Design and conduct interdisciplinary process-oriented research in ecosystem science. Enhance modeling and forecasting. Build models to forecast ecosystem change, assess future management scenarios, and reduce uncertainties through an adaptive learning process. Support decisionmaking. Use quantitative approaches to assess the vulnerabilities of ecosystems, habitats, and species, and evaluate strategies for adaptation, restoration, and sustainable management. Following the four strategic approaches are a set of proposed actions that represent a sampling of specific USGS activities that align with this strategy and that address the Nation’s most pressing environmental needs. The strategy emphasizes coordination of activities across the USGS mission areas pursuant to these goals. Ecosystem science is inherently interdisciplinary and requires a broad perspective that incorporates the biological and physical sciences, climate science, information technology, and scientific capacity in mission areas across the Bureau. With its emphasis on coordination, this strategy can provide a critical underpinning for integrated science efforts with scientists from multiple mission areas of the USGS working together. Of course, the USGS will continue to conduct discipline-specific and interdisciplinary investigations, and both will continue to be vital parts of the ecosystem science portfolio. Finally, the strategy stresses the importance of coordination with other Federal agencies and organizations in the natural resources community. The USGS collaborates with resource agencies in the DOI and other organizations throughout the world to meet societal needs for species and ecosystem management. Working with these agencies and organizations, the USGS will play a key role in guiding sound decisionmaking during the next decade by advancing the scientific foundation for sustaining the natural resources that diverse, productive, resilient ecosystems provide.

Circular↗

Mineral discrimination using a portable ratio-determining radiometer

The instrument has ten bands in the visible and near-infrared portion of the spectrum (0.5-2.4 mu m). Measurements and statistical analyses were performed on 66 samples, which were characterized by microscopic and X-ray diffraction analyses. On the ability to discriminate between 16 mineralogical groups, 91 percent classification accuracy was achieved.

Economic Geology↗

Summary of exploration geochemical and mineralogical studies at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska: Implications for exploration under cover

Exploration geochemical and mineralogical studies by the U.S. Geological Survey at the Pebble porphyry Cu-Au-Mo deposit were designed to (1) determine whether the concealed deposit can be detected with surface samples, (2) better understand the processes of metal migration from the deposit to the surface, and (3) test existing methods for assessing concealed mineral resources and/or develop new ones. Surface water (ponds, streams, and springs), pond and stream sediment, soils subjected to various leaching techniques, and glacial till samples were collected. The tilted nature of the undisturbed orebody, varying depth of cover, and later glacial processes, strongly influence the geochemical responses and processes active on the various sample media. The multimedia approach aids in identifying possible processes that caused the significant geochemical variations within and among the various media. These processes include the following: In the Pebble West zone, thin cover and local exposure of the orebody have facilitated the oxidation of pyrite and other sulfides, and associated ferrous-ferric iron reactions, resulting in the local natural acidification of ponds observed in the West zone, and in associated metal anomalies in waters, sediments, and soils. In contrast, the East zone is concealed by both glacial deposits and underlying thick cover rocks, which precludes the oxidation of sulfides in the underlying orebody. Low-level geochemical anomalies in circumneutral spring and pond waters from the East zone are discernible only by using high resolution-inductively coupled plasma-mass spectrometry with lower limits of determination two and perhaps three orders of magnitude lower than traditional methods. A variety of partial leaches of soils over the East zone reveal geochemical anomalies in a similar suite of elements that may be related to upwelling waters from depth along graben-bounding faults. The indicator minerals gold, jarosite, and andradite in till reveal a displaced mineralogical anomaly to the west and south of the Pebble orebody, as ore-related minerals were scraped from the orebody and deposited in till downice of the deposit. Geochemical anomalies in pond water and sediment over the displaced till are attributed to the ore-related minerals in till. This orientation study demonstrates the strong control of local geologic and geochemical settings on the effectiveness of different traditional and newer reconnaissance geochemical exploration techniques and thus has important implications for exploration.

Alaska↗

Silver-bearing black calcite in western mining districts

The name black calcite has been applied from time to time to a dark gray to black variety of calcite or aragonite in metalliferous ore deposits in the Western States. Most of the material shows curved, roughly rhombic cleavage. The color is due to dispersed minute grains of one or more oxides of manganese, so dispersed that they rarely show any relation to cleavage surfaces or outward crystal forms.Recent study shows that the black calcite from numerous metalliferous deposits in the Western States contains small amounts of several metals, especially zinc, lead, and silver; in places, the crude black calcite contains as much as one percent silver and the black residue remaining after solution of the calcite as much as 1,500 ounces of silver to the ton. Black calcite is recognized in some deposits of Late Cretaceous to early Tertiary age where the host is Paleozoic carbonate rocks; thus far, this variety contains the larger amounts of silver. It is also present in some vein deposits where the host is generally volcanic rocks of middle Tertiary age; these are known in many districts in the southwest and even though the lead and zinc content is noteworthy, the silver content rarely exceeds one ounce to the ton (8).By studying polished sections of black calcite from the Aurora mine, White Pine County, Nevada, three silver-bearing manganate minerals have been recognized; (1) an argentian chalcophanite containing 7.5 percent silver (Ag 2 O), to which, in a following paper, the name aurorite is given; (2) argentian todorokite containing 3.9 percent silver (Ag 2 O), and (3) hydrous silver-bearing lead manganese oxide, containing 1.18 percent silver (Ag 2 O).

Nevada↗

Similarities, differences, and some genetic problems of the Wyoming and Colorado plateau types of uranium deposits in sandstone

Uranium deposits of the Wyoming roll type and the Colorado Plateau peneconcordant type are the principal domestic resources of uranium. Both types occur in lenticular sandstone beds of continental origin, have a similar suite of elements and minerals, are associated with mildly altered rock, and are thought to have formed by reduction precipitation from ground waters before significant regional deformation. They differ in several respects. The Wyoming deposits are in unlithified sandstone that is highly arkosic, whereas the Colorado Plateau deposits are in lithified sandstone that is only slightly to moderately arkosic. Uranium is the only ore metal in the Wyoming deposits, whereas vanadium or copper is more abundant than uranium in some of the Plateau deposits. The Wyoming deposits are elongate crescent-shaped bodies that extend vertically through, or partly through, a sandstone unit and which are scattered, like widely spaced beads on a string, along miles-long interfaces between oxidized (altered) and unoxidized sandstone, whereas the Plateau deposits are thin tabular layers that are nearly concordant to bedding and which occur as discrete bodies, like raisins in raisin bread, enveloped in rock altered by reduction. The Wyoming ore rolls and interfaces were dynamic, having been pushed downdip by downward-moving oxygen-bearing water that passed through the interfaces and deposited the ore minerals on the reducing side, whereas the Plateau deposits seemingly formed as static bodies, localized by intensive reducing "patches" in a mildly reducing environment. These differences focus attention on genetic problems relating to the Eh of the ore-bearing and altering solutions, the shape and localization of deposits, and the source of the uranium.

Arizona, Colorado, New Mexico, South Dakota, Utah,↗

The lead-zinc veins of the chilete mining district in northern Peru

Lead-zinc-silver veins in the Chilete mining district in the Department of Cajamarca in northern Peru have been worked sporadically since the 17th century, but the greatest activity dates only from 1951. The ore deposits are in a thick section of andesitic volcanic rocks that overlie with marked unconformity a sequence of Cretaceous limestone, shale, and quartz-ite. The volcanic rocks form a gently north-dipping block of apparently simple structure. The Chilete deposits are quartz-sulfide veins consisting essentially of sphalerite, pyrite, and galena in a gangue of quartz and silici-fied and pyritized andesite. Wall rocks are calcitized, chloritized, pyrit-ized, and sericitized. The veins were formed by fissure filling, breccia filling, and replacement. Veins strike either northwest or east-northeast and dip steeply. They range from a few centimeters to 4 m in width and the longest has a strike length of about 1,700 m. The veins are eut by a number of post-mineral strike-slip faults of small displacement. Thicker sections of the principal northwest-striking vein, Murcielago, tend to occur where the strike swings slightly to the north. Vein intersections are not favorable loci for ore deposition, and several intersections seem to be dis-tinctly unfavorable. Ail the veins have roughly the same combined content of lead and zinc at ail levels, but the proportion of zinc to lead increases notably at the lowest levels of exploration, 200-250 m below the outerops. © 1955 Society of Economic Gelogists, Inc.

Economic Geology↗

Report on S.E.G. Symposium on the chemistry of the ore-forming fluids August-September,1964

Sessions included presentations and discussions of the following topics: mineral stability and phase assemblages at ore - forming temperatures and pressures; high-temperature solution chemistry of sulfides, carbonates, and silicates, with consideration of complexing and stability, and the generation of ore fluids ; hydrothermal fluids as deduced from wallrock alteration, from fluid inclusions and hot springs, from isotopic studies, and from mine studies, zoning, and so forth.

Economic Geology↗

Laramide alteration of proterozoic diabase: A likely contributor of copper to porphyry systems in the dripping spring mountains area, Southeastern Arizona

Proterozoic diabase of the Dripping Spring range occurs as sills in the Proterozoic Apache Group and the Troy Quartzite and as intrustive sheets in basement rocks. The aggregate thickness of the diabase sills and intrusive sheets averages about 450 m in the part of the range showing little mid-Tertiary extension. Laramide alteration is of two types, dominated by chlorite and actinolite, respectively, and formed mostly from clinopyroxene. Actinolite-dominated assemblages are higher in Na and Ca. Hydrothermal biotite is common in the central areas of both alteration types. Laramide alteration forms two distribution patterns: a subsequent pattern centered on Laramide intrusions and small porphyry deposits, characterized by actinolitic alteration, and a more extensive branching linear pattern that follows Laramide structures, centered on the larger Ray porphyry deposit, extending toward other Laramide districts and showing both alteration types. Alteration has apparently mobilized copper and other metals from diabase. The freshest diabase samples average about 120 ppm copper with little variation. In chloritic alteration, about 100 ppm of this copper is expelled in the most completely altered rocks. In actinolitic alteration, diabase may either gain or lose copper during alteration. Chloritic alteration constitutes roughly 70 percent of the diabase alteration in the study area, where alteration averages 41 percent complete. This implies liberation of about 9X10 6 tons (t) copper from diabase alteration, significantly less than the 16X10 6 t copper in Laramide mineral deposits of the superdistrict Ray, Superior. Chilito. Christmas. However, diabase alteration may have been a significant component of the supply of copper to the Laramide mineral districts of the area. Synmineral magmatic sources of copper are not documented in this area. The distribution of Proterozoic diabase coincides with the central part of the southeastern Arizona copper province, which may thus owe much copper availability to an unusual abundance of diabase. However, many unanswered questions remain about metal supply from altering diabase.

Economic Geology↗

Geochronology and mapping constraints on the time-space evolution of the igneous and hydrothermal systems in the Taurus Cu-Mo district, eastern Alaska

The Taurus porphyry Cu-Mo district contains four mineralized porphyry centers in the eastern interior of Alaska. All four centers were emplaced during a magmatic episode that spanned from ca. 72 to 67 Ma, with seven distinct igneous suites. Each igneous suite resulted in hydrothermal alteration and mineralization, with younger pulses overprinting older pulses. Each magmatic-hydrothermal system is not present at all four mineralized centers. Apart from the Dennison occurrence, each mineralized center records pulses of repeated intermediate-silicic magmatism and associated alteration and mineralization. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon crystallization ages indicate that an early quartz porphyry dike swarm ranges in age from ca. 71 to 70 Ma and is associated with potassic, sericitic, and propylitic alteration. Quartz latite intrusions were emplaced at ca. 69 Ma and exhibit early sodiccalcic alteration overprinted by potassic, sericitic, and propylitic alteration. The Taurus monzonite suite is cut by quartz latite but yielded an ca. 70 Ma emplacement age and exhibits the largest footprint of potassic and sericitic alteration. Feldspar porphyry dikes were emplaced ca. 69 Ma and have significant tourmaline-bearing potassic and sericitic alteration. This suite was followed by development of an igneous breccia with a monzonitic igneous matrix. Sodic-calcic alteration was associated with the igneous brecciation. A small stock of monzonite was emplaced at ca. 68 Ma causing locally pervasive sericite-tourmaline-pyrite alteration. The youngest suite of magmatism dated in the district is a series of granodiorite porphyry dikes with weak sodic-calcic and propylitic alteration that truncates earlier alteration assemblages. Mineralization in the district consists of chalcopyrite and molybdenite associated with sugary quartz veins with potassium feldspar and biotite alteration envelopes (A veins). Less common banded quartz-molybdenite veins (B veins) occur with potassium feldspar envelopes. Gold occurs throughout the district and is strongly correlated with copper grade. Sericitic alteration contains lower copper contents and is predominantly associated with quartz-pyrite veins with sericite envelopes (D veins). Pyrrhotite and local arsenopyrite are present in sericitic assemblages. Pyrrhotite also occurs as inclusions in pyrite within D veins. Magmas across the district exhibit oxidized characteristics, evidenced by the presence of abundant magnetite, rare titanite, and elevated Eu/Eu* and Ce/Ce* in zircon. Zircon Th/U and Yb/Gd compositions suggest a fractionation path controlled by apatite, titanite, and hornblende. Zircon rare earth element ratios and trace element data indicate two distinct batches of magma evolved from mafic parental compositions to monzonite and granodioritic compositions via fractional crystallization. In the early pulse of magma (ca. 72–69 Ma), fractional crystallization was key to ore formation. Earlier, better mineralized suites evolve to less negative Eu anomalies (Eu/Eu* > 0.7), indicating more oxidized and higher-water-pressure conditions evidenced by the suppression of plagioclase crystallization, compared to later, more poorly mineralized suites. The temporal and spatial evolution of the district was determined from mapping and U-Pb and Re-Os geochronology. Mapping of igneous and hydrothermal assemblages indicates that the locus of the intrusive suites and hydrothermal systems shifted spatially over time, based on the presence of high-temperature (K-silicate–dominant) alteration, which is coincident with the highest Cu and Au grades. The earliest hydrothermal system was centered at Bluff and East Taurus and transitioned to West Taurus during emplacement of the second magmatic suite. Emplacement of the third magmatic suite was centered back at East Taurus, and the fourth and fifth suites were centered at West Taurus. The latest suites were widespread without a core of high-temperature alteration marking a central locus. East Taurus contains the overlap of six of the seven magmatic and hydrothermal suites and has the highest intersected grades and tonnages in the district. The Bluff and Dennison occurrences exhibit fewer igneous suites and hydrothermal assemblages with weak mineralization. Sodic-calcic alteration, common on the deep and distal flanks of porphyry systems, is only present at West Taurus and is indicative of a localized source of high-salinity nonmagmatic fluids.

Alaska, Yukon Territory↗