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At least 829 records · Page 46Linked to original sources
USGS1957

Reflecting curved-crystal X-ray spectrograph; a device for the analysis of small mineral samples

A curved-crystal reflecting spectrometer of the type described by Birks and Brooks of the Naval Research Laboratories, but adapted for use in mineralogical studies, has been built in the Geological Survey. It has been successfully applied to the analysis of tiny crystals, zones in minerals, and individual grains in mixed-mineral specimens such as thin or polished sections, on grains or areas about 0.5 mm in diameter, X-ray diffraction spindles, and small samples of powder weighing a fraction of a milligram have also been analyzed without any loss or alteration of the sample. Of great value in thin- and polished-section work is the fact that this technique can be used to analyze selected areas without mutilating the specimen by digging out grains.A modification of the curved-crystal spectrometer has made it possible to traverse a standard polished section in synchronization with a recorder, automatically plotting the distribution of various elements along a selected line. This method was applied, for example, to a polished section containing a central core of pyrite intergrown with and surrounded by a marcasite-like mineral. Chemical analysis of a concentrate of these two minerals gave selenium, cobalt, and iron as major constituents. A clear relationship between the cobalt, selenium, and iron was established by the X-ray method, identifying the second mineral as an intermediate member of the FeSe 2 -CoSe 2 series.

Economic Geology↗
USGS1985

Uranium mineralization in the Smith Lake district of the Grants uranium region, New Mexico.

The Mariano Lake and Ruby 1 uranium orebodies, which together comprise much of the uranium ore in the Smith Lake district of the Grants uranium region, New Mexico, occur in sandstones in the lower part of the Brushy Basin Member of the Upper Jurassic Morrison Formation. The orebodies, which are offset by faults of Laramide age, are enriched in an amorphous organic material that was introduced into the host sandstone after deposition. The enrichment by this organic material, in ore, is an important characteristic of the primary uranium deposits in the Grants uranium region. However, the close proximity of the chemically reduced ore zones to oxidized rock is suggestive that the deposits represent accumulations of uranium redistributed by reduction-oxidation processes from preexisting primary deposits.Within the ores, whole-rock abundances of organic carbon correlate positively with uranium contents. This correlation is consistent with petrologic evidence which indicates that uranium is everywhere intimately admixed with the amorphous organic material. Ore zones are also enriched in vanadium (as ore-stage vanadiferous chlorite) and sulfur (as ore-stage iron disulfide minerals with delta 34 S values ranging from -29 to -42ppm).Petrographic observations demonstrate that Smith Lake uranium mineralization occurred early (before major burial compaction) in the paragenetic sequence of host-rock diagenetic alterations but was preceded by precipitation of authigenic iron disulfides (delta 34 S values ranging from -11 to -38ppm), mixed-layered smectite-illite clays, and potassium feldspars.Additional preore alterations included dissolution of detrital sanidine and plagioclase and the leaching of iron from detrital iron-titanium oxide grains. Following mineralization, varying amounts of authigenic calcite and barite formed, both of which were partly replaced by kaolinitc. Oxidation of some previously formed iron disulfide minerals occurred late in the paragenetic sequence, as did localized precipitation of native selenium, pyrite, and very minor amounts of uranium minerals.The positive correlation between contents of uranium and organic carbon and the admixture of uranium with the amorphous organic material indicates that the Mariano Lake and Ruby 1 deposits are primary-type uranium orebodies. The offset of orebodies by Laramide faults and radiometric age determinations of the ores are also consistent with a primary origin for the deposits. Late Tertiary oxygenated ground waters locally modified original chemical and mineralogical characteristics of the ores in part by leaching some uranium. Secondary uranium minerals precipitated from the partly leached primary ores are sparse in the mine area; such recycled uranium appears to represent an insignificant proportion of the total uranium in the Smith Lake district.

Economic Geology↗
USGS1988

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↗
USGS1989

Geology and K-Ar geochronology of the Paradise Peak Mine and the relationship of pre-Basin and Range extension to Early Miocene precious metal mineralization in west-central Nevada

The Paradise Peak mine is a major gold-silver-mercury deposit located in the southwestern part of the Paradise Range near the eastern edge of the Walker Lane in the western Great Basin, Nevada. The Tertiary volcanic rocks of the area can be divided into a sequence of 26- to 24-Ma intermediate lavas, a sequence of 24- to 22-Ma silicic ash-flow tuffs, and a sequence of 20- to 15-Ma intermediate lavas. We classify these rocks as the older, middle, and younger sequences. Sedimentary rocks, and latites and basalts 12 Ma old or younger, locally overlie the younger intermediate lavas south of the mine. Silicified rhyolite tuff in the lower part of the middle tuff sequence is the principal host of the precious metal ore. Most ore occurs under an acid-leached zone of alunitic alteration and is contained in hydrothermal breccias that crosscut early quartz-pyrite and alunite alteration.Field relations and K-Ar ages of hypogene alunite indicate that precious metal mineralization and alunitic and silicic alteration formed at about 19 to 18 Ma, several million years after eruption of the main host rock. Mineralization formed during high-angle faulting related to crustal extension. Low-angle normal faults are present 5 km northeast of the Paradise Peak mine, but they formed more than 2 Ma after formation of the Paradise Peak deposit. Silicic and alunitic alteration and precious metal mineralization are generally absent in areas of low-angle faulting.Regional stratigraphic relations and K-Ar ages indicate that volcanism changed from silicic ash-flow tuffs to intermediate lavas at about 20 to 19 Ma. Regionally extensive angular unconformities indicate that a period of "pre-Basin and Range" crustal extension occurred between about 22 to 19 Ma. This extension was penecontemporaneous with the shift in the style of volcanism and with gold-silver mineralization in the Paradise Peak mine and in the Goldfield and Tonopah districts of western Nevada. The close temporal and spatial relationships of precious metal mineralization with pre-Basin and Range extension suggest that extension was a major factor in the genesis of early Miocene precious metal deposits in the western Great Basin.

Economic Geology↗
USGS1993

Refinement of the evaluation of the role of CO2 in modifying estimates of the pressure of epithermal mineralization

Pressure is the most important of the intensive parameters for relating epithermal mineralization to the geologic setting. This paper describes the limitations on pressure (and therefore depth) of mineralization that may reasonably be derived from simple observations of the behavior of fluid inclusions (i.e., the existence of ice or CO 2 clathrate on the liquidus, the amount of expansion or contraction of the bubble as the host inclusion is crushed in oil on the microscope stage, and the freezing and homogenization temperatures for the inclusion). It is based on the reasonable model that mineralization occurs from a hydrostatically pressured NaCl-CO 2 -H 2 O fluid, consistent with the probability that H 2 O and CO 2 are the only gases contributing significantly to the total pressure. The pressure of CO 2 is, of course, a function of CO 2 content, but, from 100 degrees to 300 degrees C, it is a surprisingly minor function of either temperature or salinity. The presence of the clathrate in freezing studies of fluid inclusions indicates pressures of CO 2 that add at least 1 km to the probable depth of inclusion trapping compared to that estimated from CO 2 -free water. Thus undetected (i.e., no clathrates on cooling) CO 2 in fluid inclusions can nonetheless contribute very significantly to the possible depth of epithermal mineralization. On the other hand, the observation that fluid inclusions crushed in oil have bubbles that do not expand (i.e., <1 atm P (sub CO 2 ) at 25 degrees C), demonstrates CO 2 contents that could add at most a few tens of meters to the depth of mineralization.

Economic Geology↗
USGS1994

Fluid chemistry evolution and mineral deposition in the main-stage Creede epithermal system

This paper presents results of chemical speciation and reaction path calculations that model fluid chemistry evolution and ore deposition in the main-stage Creede, Colorado, epithermal system. An extensive geologic, mineralogic, and geochemical framework for mineralization has been developed by many researchers for the central and southern district vein systems (OH and P veins; central and southern Amethyst and Bulldog Mountain vein systems) and is used to constrain and guide the modeling presented in this paper. Previous studies have shown that the central base metal sulfide-rich and southern barite- and silver-rich Creede ores were deposited by hydrothermal brines with temperatures as high as 285 degrees C and salinities as high as 13 wt percent NaCl equiv. Fluid inclusion studies indicate that mixing with dilute steam-heated ground waters was the dominant ore deposition mechanism, although boiling did occur during some stages. Speciation calculations confirm that the hydrothermal fluids, due to their high salinities, were relatively acidic (pH near 5.5) and transported significant quantities of base metals (i.e., up to 10-2 m total concentrations of Zn) as chloride complexes. Reaction path calculations show that strong north-south mineralogical variations in main-stage mineralization are best accounted for by variable boiling of the hydrothermal brines, followed by lateral mixing of the brines with overlying dilute, steam-heated ground waters. The extent of boiling prior to mixing was a function of the temperature and gas contents of the hydrothermal fluids as they first entered the district ore zones from depth, and of the thickness of the steam-heated ground-water column overlying the hydrothermal fluids. The calculations indicate that limited amounts of boiling (during stages when the overlying ground-water column was relatively thick) produced a chlorite-pyrite-hematite-sphalerite-galena-chalcopyrite + or - adularia assemblage in the central district vein systems (OH and P veins; central Amethyst and Bulldog Mountain vein systems). More extensive boiling (during a stage in which the ground-water table apparently dropped considerably) deposited quartz, fluorite, adularia, and hematite with only minor sulfides in the central district veins. Boiling ceased when the saturation pressure of the hydrothermal fluids dropped below hydrostatic pressures generated by the overlying ground-water column. Following boiling, lateral mixing with overlying steam-heated ground waters initially produced sphalerite- and galena-rich assemblages in the central district vein systems. With continued mixing to the south, the hydrothermal fluids deposited abundant barite, subordinate sphalerite, galena, and quartz, and lesser native silver, acanthite, and sulfosalts in the district's southern vein systems (southern Amethyst, southern Bulldog Mountain vein systems).Modeling results for Creede and other epithermal fluid compositions show that epithermal ore grades, mineral assemblages, and mineral zoning patterns are strongly influenced by shallow hydrologic processes such as boiling and fluid mixing. As a result, epithermal mineral assemblages and zoning patterns can be used to reconstruct the paleohydrology of the hydrothermal systems from which they were deposited, and thus provide useful tools for epithermal ore exploration.

Economic Geology↗
USGS2011

Measurement of net nitrogen and phosphorus mineralization in wetland soils using a modification of the resin-core technique

A modification of the resin-core method was developed and tested for measuring in situ soil N and P net mineralization rates in wetland soils where temporal variation in bidirectional vertical water movement and saturation can complicate measurement. The modified design includes three mixed-bed ion-exchange resin bags located above and three resin bags located below soil incubating inside a core tube. The two inner resin bags adjacent to the soil capture NH 4 + , NO 3 - , and soluble reactive phosphorus (SRP) transported out of the soil during incubation; the two outer resin bags remove inorganic nutrients transported into the modified resin core; and the two middle resin bags serve as quality-control checks on the function of the inner and outer resin bags. Modified resin cores were incubated monthly for a year along the hydrogeomorphic gradient through a floodplain wetland. Only small amounts of NH 4 + , NO 3 - , and SRP were found in the two middle resin bags, indicating that the modified resin-core design was effective. Soil moisture and pH inside the modified resin cores typically tracked changes in the surrounding soil abiotic environment. In contrast, use of the closed polyethylene bag method provided substantially different net P and N mineralization rates than modified resin cores and did not track changes in soil moisture or pH. Net ammonification, nitrifi cation, N mineralization, and P mineralization rates measured using modified resin cores varied through space and time associated with hydrologic, geomorphic, and climatic gradients in the floodplain wetland. The modified resin-core technique successfully characterized spatiotemporal variation of net mineralization fluxes in situ and is a viable technique for assessing soil nutrient availability and developing ecosystem budgets.

Biogeochemistry↗
USGS2005

Post-precipitation bias in band-tailed pigeon surveys conducted at mineral sites

Many animal surveys to estimate populations or index trends include protocol prohibiting counts during rain but fail to address effects of rainfall preceding the count. Prior research on Pacific Coast band-tailed pigeons (Patagioenas fasciata monilis) documented declines in use of mineral sites during rainfall. We hypothesized that prior precipitation was associated with a short-term increase in use of mineral sites following rain. We conducted weekly counts of band-tailed pigeons at 19 Pacific Northwest mineral sites in 2001 and 20 sites in 2002. Results from regression analysis indicated higher counts ???2 days after rain (11.31??5.00% [x????SE]) compared to ???3 days. Individual index counts conducted ???2 days after rain were biased high, resulting in reduced ability to accurately estimate population trends. Models of band-tailed pigeon visitation rates throughout the summer showed increased mineral-site counts during both June and August migration periods, relative to the July breeding period. Our research supported previous studies recommending that mineral-site counts used to index the band-tailed pigeon population be conducted during July. We further recommend conducting counts >3 days after rain to avoid weather-related bias in index estimation. The design of other population sampling strategies that rely on annual counts should consider the influence of aberrant weather not only coincident with but also preceding surveys if weather patterns are thought to influence behavior or detection probability of target species.

Wildlife Society Bulletin↗
USGS1998

Revising U.S. Geological Survey mineral-resource assessment methods

As a result of public controversy over recommendations related to the wilderness preservation system, the U.S. Geological Survey (USGS) conducted a series of reviews of its mineral-resource assessment methods. The first review panel recommended several short- and long-term modifications to improve future mineral-resource assessments (Harris and Rieber, 1993). Implementation of some of these recommendations requires the development of new tools to augment USGS assessment procedures. The second panel identified the need to improve the classification of known mineral deposits according to USGS deposit models and to improve the delineation of areas that are favorable for the occurrence of particular mineral-deposit types (Barton and others, 1995). The panels' major recommendations, however, were to develop two new tools as a first priority -- empirical rate-of-occurrence models for important mineral-deposit types (which aid in the subjective assessment of the number of undiscovered deposits in a given area) and economic cost filters.

Information Handout↗
USGS1987

Mineral resources of the East Fork High Rock Canyon Wilderness Study Area, Washoe and Humboldt counties, Nevada

The part of the East Fork High Rock Canyon Wilderness Study Area (CA-020-914/NV-020-006A) included in this study encompasses 33,460 acres in the northwestern part of Nevada. Throughout this report, "wilderness study area" and "study area" refertothe 33,460 acres for which mineral surveys were requested. The U.S. Geological Survey and the U.S. Bureau of Mines conducted geological, geophysical, and geochemical surveys to assess the mineral resources (known) and the mineral resource potential (undiscovered) of the study area. Fieldwork for this report was carried out in 1985 and 1986. No mines, significant prospects, or mining claims are located inside the study area, and no identified resources were found. The wilderness study area has moderate mineral resource potential for gold, silver, and mercury and for zeolite minerals. A low potential also exists for geothermal energy resources, and potential for oil and gas is unknown.

Nevada↗
USGS1988

Mineral resources of the Turtle Mountains Wilderness Study Area, San Bernardino County, California

At the request of the U.S. Bureau of Land Management, approximately 105,200 acres of the Turtle Mountains Wilderness Study Area (CDCA-307) were evaluated for mineral resources (known) and resource potential (undiscovered). In this report, the area studied is referred to as "the wilderness study area" or simply "the study area"; any reference to the Turtle Mountain Wilderness Study Area refers only to that part of the wilderness study area for which a mineral survey was requested by the U.S. Bureau of Land Management. The wilderness study area is in southeastern San Bernardino County, Calif. Gold, silver, copper, and lead have been mined within and adjacent to the study area. Copper-zinc-silver-gold mineral occurrences are found in the southern part and gold-silver mineral occurrences are found in the northern part of the study area; identified low- to moderate-grade gold-silver resources occur adjacent to the study area along the western boundary. Six areas in the south-central and northwestern parts of the study area have high resource potential, two broad areas have moderate resource potential, and part of the southwest corner has low resource potential for lode gold, silver, and associated copper, lead, zinc, molybdenum, and tungsten. Alluvium locally within one of these areas has moderate resource potential for placer gold and silver, and the entire area has low resource potential for placer gold and silver. There is low resource potential for perlite, ornamental stone (onyx marble and opal), manganese, uranium and thorium, pegmatite minerals, and oil and gas within the study area. Sand and gravel are abundant but are readily available outside the wilderness study area.

California↗
USGS1988

Mineral resources of the Whipple Mountains and Whipple Mountains Addition Wilderness Study Areas, San Bernardino County, California

At the request of the U.S. Bureau of Land Management, approximately 85,100 acres of the Whipple Mountains Wilderness Study Area (CDCA-312) and 1,380 acres of the Whipple Mountains Addition Wilderness Study Area (AZ-050-010) were evaluated for identified mineral resources (known) and mineral resource potential (undiscovered). In this report, the Whipple Mountains and Whipple Mountains Addition Wilderness Study Areas are referred to as simply "the study area." Most of the mines and prospects with identified resources in the Whipple Mountains Wilderness Study Area are within areas designated as having mineral resource potential. The area in and around the Turk Silver mine and the Lucky Green group and the area near the northwest boundary of the study area have high mineral resource potential for copper, lead, zinc, gold, and silver. An area along the west boundary of the study area has moderate resource potential for copper lead, zinc, gold, and silver. An area in the east adjacent to the Whipple Mountains Addition Wilderness Study Area has moderate resource potential for copper, gold, and silver resources. One area on the north boundary and one on the southeast boundary of the study area have low mineral resource potential for copper, lead, zinc, gold, and silver. Two areas, one on the north boundary and one inside the east boundary of the study area, have moderate resource potential for manganese. A small area inside the south boundary of the study area has high resource potential for decorative building stone, and the entire study area has low resource potential for sand and gravel and other rock products suitable for construction. Two areas in the eastern part of the study area have low resource potential for uranium. There is no resource potential for oil and gas or geothermal resources in the Whipple Mountains Wilderness Study Area. Sites within the Whipple Mountains Wilderness Study Area with identified resources of copper, gold, silver, manganese and (or) decorative building stone are located at the Stewart mine, New American Eagle mine, Turk Silver mine, Twin Lode mine, decorative stone property, Lucky Green group, Blue Cloud mine, Nickel Plate mine, Crescent mine, Quadrangle Copper group, and the Copper Basin mine. The Whipple Mountains Addition Wilderness Study Area has moderate resource potential for copper, gold, and silver resources and low resource potential for sand and gravel and other rock products. There is no resource potential for oil and gas or for geothermal energy in the Whipple Mountains Addition Wilderness Study Area. Although there are no identified resources in the Whipple Mountains Addition Wilderness Study Area, sites within and immediately adjacent warrant further study because of gold assays from widespread, numerous samples.

California↗
USGS1995

Mineral and energy resources of the Roswell Resource Area, East-Central New Mexico

The sedimentary formations of the Roswell Resource Area have significant mineral and energy resources. Some of the pre-Pennsylvanian sequences in the Northwestern Shelf of the Permian Basin are oil and gas reservoirs, and Pennsylvanian rocks in Tucumcari Basin are reservoirs of oil and gas as well as source rocks for oil and gas in Triassic rocks. Pre-Permian rocks also contain minor deposits of uranium and vanadium, limestone, and gases. Hydrocarbon reservoirs in Permian rocks include associated gases such as carbon dioxide, helium, and nitrogen. Permian rocks are mineralized adjacent to the Lincoln County porphyry belt, and include deposits of copper, uranium, manganese, iron, polymetallic veins, and Mississippi-Valley-type lead-zinc. Industrial minerals in Permian rocks include fluorite, barite, potash, halite, polyhalite, gypsum, anhydrite, sulfur, limestone, dolomite, brine deposits (iodine and bromine), aggregate (sand), and dimension stone. Doubly terminated quartz crystals, called 'Pecos diamonds' and collected as mineral specimens, occur in Permian rocks along the Pecos River. Mesozoic sedimentary rocks are hosts for copper, uranium, and small quantities of gold-silver-tellurium veins, as well as significant deposits of oil and gas, carbon dioxide, asphalt, coal, and dimension stone. Mesozoic rocks contain limited amounts of limestone, gypsum, petrified wood, and clay. Tertiary rocks host ore deposits commonly associated with intrusive rocks, including platinum-group elements, iron skarns, manganese, uranium and vanadium, molybdenum, polymetallic vein deposits, gold-silver-tellurium veins, and thorium-rare-earth veins. Museum-quality quartz crystals are associated with Tertiary intrusive rocks. Industrial minerals in Tertiary rocks include fluorite, vein- and bedded-barite, caliche, limestone, and aggregate. Tertiary and Quaternary sediments host important placer deposits of gold and titanium, and occurrences of silver and uranium. Important industrial commodities include caliche, limestone and dolomite, and aggregate. Quaternary basalt contains sub-ore-grade uranium, scoria, and clay deposits.

New Mexico↗
USGS2002

Argon thermochronology of mineral deposits; a review of analytical methods, formulations, and selected applications

40Ar/39Ar geochronology is an experimentally robust and versatile method for constraining time and temperature in geologic processes. The argon method is the most broadly applied in mineral-deposit studies. Standard analytical methods and formulations exist, making the fundamentals of the method well defined. A variety of graphical representations exist for evaluating argon data. A broad range of minerals found in mineral deposits, alteration zones, and host rocks commonly is analyzed to provide age, temporal duration, and thermal conditions for mineralization events and processes. All are discussed in this report. The usefulness of and evolution of the applicability of the method are demonstrated in studies of the Panasqueira, Portugal, tin-tungsten deposit; the Cornubian batholith and associated mineral deposits, southwest England; the Red Mountain intrusive system and associated Urad-Henderson molybdenum deposits; and the Eastern Goldfields Province, Western Australia.

Bulletin↗
USGS1969

Gold in minerals and the composition of native gold

Gold occurs in nature mainly as the metal and as various alloys. It forms complete series of solid solutions with silver, copper, nickel, palladium, and platinum. In association with the platinum metals, gold occurs as free gold as well as in solid solution. The native elements contain the most gold, followed by the sulfide minerals. Several gold tellurides are known, but no gold selenides have been reported, and only one sulfide, the telluride-sulfide mineral nagyagite, is known. The nonmetallic minerals carry the least gold, and the light-colored minerals generally contain less gold than the dark minerals. Some conclusions in the literature are conflicting in regard to the relation of fineness of native gold to its position laterally and vertically within a lode, the nature of the country rocks, and the location and size of nuggets in a streambed, as well as to the variation of fineness within an individual nugget.

Circular↗
USGS1984

Interlaboratory comparison of mineral constituents in a sample from the Herrin (No. 6) coal bed from Illinois

Approximately 20 kg of the Herrin (No. 6) coal was collected from a strip mine in St. Clair County, Ill. A 10-kg portion was ground to -60 mesh, homogenized, and riffled into 128 splits of 70-80 g each. Homogeneity of these splits was confirmed by moisture, ash, and sulfur analyses of six randomly selected splits. Results of these analyses were within the ASTM (American Society for Testing and Materials) guidelines for interlaboratory precision. Splits of the Herrin (No. 6) coal were then transmitted to more than 30 laboratories for analysis. Low-temperature plasma oxidation was used to isolate inorganic matter for quantitative chemical and mineralogical analysis. Despite a wide variation in ashing conditions, only minor variations in ash yields were obtained; these variations were attributed to differences in operating temperature and moisture content. Mineralogical analyses of low-temperature ash (LTA) concentrates prepared by five different laboratories indicated variations within the limits of analytical error. The mean values, in weight percent, for the major minerals are as follows: calcite, 9; quartz, 20; pyrite, 23; kaolinite, 14; and illite+mixed-layer clays, 31. Normative mineralogical calculations and Fourier transform infrared analysis (FTIR) yielded results similar to those obtained from X-ray diffraction (XRD). Choosing appropriate mineral standards was found to be critical for the proper use of analytical techniques such as XRD and FTIR. Good interlaboratory agreement was obtained for most major, minor, and trace elements despite differences in analytical procedures and in the type of sample analyzed (coal, high-temperature ash, or LTA). Discrepancies between analyses for zinc, strontium, manganese, and iron may be attributed to sampling inhomogeneity problems. Mossbauer spectroscopy showed that approximately 44 percent of the pyritic sulfur was lost through weathering in the first year after preparation of the interlaboratory sample. Szomolnokite and possibly coquimbite and jarosite were also identified. Scanning electron microscopy studies indicated ubiquitous pyrite framboids and, less commonly, euhedral crystals, skeletal grains, irregularly shaped particles, and vein fillings. Minor accessory minerals such as rare-earth phosphates and possibly silicates, zircon, barium sulfate, titanium oxide, and sphalerite were also found. The textural evidence indicates that the minerals in the banded material are detrital whereas the minerals occurring as vein and pore fillings are authigenic. Magnetic measurements indicate that coal crushed in a steel pulverizer is contaminated by small quantities of abrasion fragments from the crusher, which seriously affect the measured magnetic properties of the coal.

Circular↗
USGS2017

Isotopic data for Late Cretaceous intrusions and associated altered and mineralized rocks in the Big Belt Mountains, Montana

The quartz monzodiorite of Mount Edith and the concentrically zoned intrusive suite of Boulder Baldy constitute the principal Late Cretaceous igneous intrusions hosted by Mesoproterozoic sedimentary rocks of the Newland Formation in the Big Belt Mountains, Montana. These calc-alkaline plutonic masses are manifestations of subduction-related magmatism that prevailed along the western edge of North America during the Cretaceous. Radiogenic isotope data for neodymium, strontium, and lead indicate that the petrogenesis of the associated magmas involved a combination of (1) sources that were compositionally heterogeneous at the scale of the geographically restricted intrusive rocks in the Big Belt Mountains and (2) variable contamination by crustal assimilants also having diverse isotopic compositions. Altered and mineralized rocks temporally, spatially, and genetically related to these intrusions manifest at least two isotopically distinct mineralizing events, both of which involve major inputs from spatially associated Late Cretaceous igneous rocks. Alteration and mineralization of rock associated with the intrusive suite of Boulder Baldy requires a component characterized by significantly more radiogenic strontium than that characteristic of the associated igneous rocks. However, the source of such a component was not identified in the Big Belt Mountains. Similarly, altered and mineralized rocks associated with the quartz monzodiorite of Mount Edith include a component characterized by significantly more radiogenic strontium and lead, particularly as defined by 207 Pb/ 204 Pb values. The source of this component appears to be fluids that equilibrated with proximal Newland Formation rocks. Oxygen isotope data for rocks of the intrusive suite of Boulder Baldy are similar to those of subduction-related magmatism that include mantle-derived components; oxygen isotope data for altered and mineralized equivalents are slightly lighter.

Montana↗
USGS2011

Geophysical, geochemical, and mineralogical data from the Pebble Cu-Au-Mo porphyry deposit area, southwest Alaska: Contributions to assessment techniques for concealed mineral resources

In 2007, the U.S. Geological Survey began a multidisciplinary study in southwest Alaska to investigate the setting and detectability of mineral deposits in concealed volcanic and glacial terranes. The study area hosts the world-class Pebble porphyry Cu-Au-Mo deposit, and through collaboration with the Pebble Limited Partnership, a range of geophysical and geochemical investigations was carried out in proximity to the deposit. The deposit is almost entirely concealed by tundra, glacial deposits, and post-mineralization volcanic rocks. The discovery of mineral resources beneath cover is becoming more important because most of the mineral resources at the surface have already been discovered. Research is needed to identify ways in which to assess for concealed mineral resources. This report presents the uninterpreted geophysical measurements and geochemical and mineralogical analytical data from samples collected during the summer field seasons from 2007 to 2010, and makes the data available in a single Geographic Information System (GIS) database.

Alaska↗