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

USGS1977

Mapping of hydrothermal alteration in the Cuprite mining district, Nevada, using aircraft scanner images for the spectral region 0.46 to 2.36µm

Color composites of Landsat Multispectral Scanner ratio images that display variations in the intensity of ferric-iron absorption bands are highly effective for mapping limonitic altered rocks but are ineffective for mapping nonlimonitic altered rocks. Analysis of 0.45- to 2.5-µm field and laboratory spectra shows that iron-deficient opalized rocks in the Cuprite mining district, Nevada, have an intense OH-absorption band near 2.2 µm, owing to their content of clay minerals and alunite, and that this spectral feature is absent or weak in adjacent unaltered tuff and basalt. Altered rocks in the district can be discriminated from unaltered rocks with few ambiguities by use of color-ratio composite images derived from multispectral (0.46 to 2.36 µm) aircraft data. In addition, some effects of mineralogical zoning can be discriminated within the altered area. Only variations in amounts of limonite can be discerned in shorter wavelength aircraft data, Landsat Multispectral Scanner bands, and color aerial photographs.

Nevada↗

USGS2011

Factors affecting groundwater quality in the Valley and Ridge aquifers, eastern United States, 1993-2002

Chemical and microbiological analyses of water from 230 wells and 35 springs in the Valley and Ridge Physiographic Province, sampled between 1993 and 2002, indicated that bedrock type (carbonate or siliciclastic rock) and land use were dominant factors influencing groundwater quality across a region extending from northwestern Georgia to New Jersey. The analyses included naturally occurring compounds (major mineral ions and radon) and anthropogenic contaminants [pesticides and volatile organic compounds (VOCs)], and contaminants, such as nitrate and bacteria, which commonly increase as a result of human activities. Natural factors, such as topographic position and the mineral composition of underlying geology, act to produce basic physical and geochemical conditions in groundwater that are reflected in physical properties, such as pH, temperature, specific conductance, and alkalinity, and in chemical concentrations of dissolved oxygen, radon, and major mineral ions. Anthropogenic contaminants were most commonly found in water from wells and springs in carbonate-rock aquifers. Nitrate concentrations exceeded U.S. Environmental Protection Agency maximum contaminant levels in 12 percent of samples, most of which were from carbonate-rock aquifers. Escherichia coli (E. coli), pesticide, and VOC detection frequencies were significantly higher in samples from sites in carbonate-rock aquifers. Naturally occurring elements, such as radon, iron, and manganese, were found in higher concentrations in siliciclastic-rock aquifers. Radon levels exceeded the proposed maximum contaminant level of 300 picocuries per liter in 74 percent of the samples, which were evenly distributed between carbonate- and siliciclastic-rock aquifers. The land use in areas surrounding wells and springs was another significant explanatory variable for the occurrence of anthropogenic compounds. Nitrate and pesticide concentrations were highest in samples collected from sites in agricultural areas and lowest in samples collected from sites in undeveloped areas. Volatile organic compounds were detected most frequently and in highest concentrations in samples from sites in urban areas, and least frequently in agricultural and undeveloped areas. No volatile organic compound concentrations and concentrations from only one pesticide, dieldrin, exceeded human-health benchmarks.

Alabama;Georgia;Tennessee;North Carolina;Virginia;↗

USGS2015

Summary of the Ahankashan Area of Interest

This report summarizes and interprets results of the work in the Ahankashan Area of Interest in northwestern Afghanistan and four study areas—the Ahankashan Prospect Area, Syahsang-Kushkak, Taghab-Soni, and Zakak-e ‘Olya—delineated for their potential undiscovered mineral occurrences with specific emphasis on porphyry copper and related occurrence types. The Area of Interest is underlain by rocks of three different geologic domains that cross from east to west—the Band-e-Bayan Block/Central Pamirs Domain in the south, the Hindu Kush Domain in the Paropamisus Mountains, and the Afghan Turkestan Domain in the north. The domains are sutured remnants of Tethyan tectonic elements. Interpretation of the geologic maps indicates the presence of thrust faults, strike-slip faults, and granitic intrusions emplaced in ground prepared by faulting. Thrust faulting was followed by strike-slip faulting and then followed by magmatic intrusions. Advanced Spaceborne Thermal Emission and Reflection Radiometer data were used to map minerals that have been altered by hydrothermal fluids typically associated with mineralization to delineate new potential occurrences of copper, gold, and silver. Propylitic-, argillic-, and phyllic-altered intrusive rocks are found in the area, as well as very minor amounts of hydrothermal silica-rich rocks. This area of interest is vastly underexplored and contains only seven known mineral occurrences, of which the Ahankashan copper (gold) skarn occurrence is the best known. Gold has been found in stream sediments near the Ahankashan skarn, in the Taghab-Soni study area, and possibly other parts of the Area of Interest, suggesting potential for at least small-scale placer occurrences.

Ahankashan Area of Interest↗

USGS1996

Hydrothermal mineralization along submarine rift zones, Hawaii

This is the first article to describe mineralization of midplate submarine rift zones and hydrothermal manganese oxide mineralization of midplate volcanic edifices. Hydrothermal Mn oxides were recovered from submarine extensions of two Hawaiian rift zones, along Haleakala and Puna Ridges. These Mn oxides form two types of deposits, metallic stratiform layers in volcaniclastic rocks and cement for clastic rocks; both deposit types are composed of todorokite and bimessite. Thin Fe‐Mn crusts that coat some rocks formed by a combination of hydrogenetic and hydrothermal processes and are composed of δ‐MnO 2 . The stratiform layers have high Mn contents (mean 40%) and a large fractionation between Mn and Fe (Fe/Mn = 0.04). Unlike most other hydrothermal Mn oxide deposits, those from Hawaiian rift zones are enriched in the trace metals Zn, Co, Ba, Mo, Sr, V, and especially Ni (mean 0.16%). Metals are derived from three sources: mafic and ultramafic rocks leached by circulating hydrothermal fluids, clastic material (in Mn‐cemented sandstone), and seawater that mixed with the hydrothermal fluids. Mineralization on Haleakala Ridge occurred sometime during the past 200 to 400 ka, when the summit was at a water depth of more than 1,000 m. Hydrothermal circulation was probably driven by heat produced by intrusion of dikces, magma reservoirs, and flow of magma through axial and lateral conduits. The supply of seawater to ridge interiors must be extensive because of their high porosity and permeability. Precipitation of Mn oxide below the seafloor is indicated by its occurrence as cement, growth textures that show mineralizing fluids were introduced from below, and pervasive replacement of original matrix of clastic rocks.

Hawaii↗

USGS2013

Tectonic evolution and Cretaceous gold metallogenesis of southwestern Alaska

Cretaceous gold metallogenesis in southwestern Alaska comprises three distinct episodes related to the accretionary evolution of northwestern North America. The oldest mineralizing event is characterized by 112 Ma Cu-Au-Bi-Te porphyry-type(?) veining in the zoned Bonanza and adjacent plutons that intruded rocks of the Nyac terrane. Tectonic reconstructions and limited geological and geochemical data suggest that Cu-Au mineralization in the Nyac district may be related to terminal subduction during accretion of the Togiak-Koyukuk arc. The subsequent 100 to 89 Ma metallogenic event is a product of subduction-related magmatism immediately following accretion of the Peninsular-Alexander-Wrangellia superterrane and includes formation of the giant Pebble porphyry Cu-Au-Mo deposit. Pebble underwent a complex history of highly oxidized magmatism that is isotopically linked to enriched lithosphere or metasomatized mantle sources. Pebble and other porphyryrelated plutons were emplaced as a consequence of changes in plate motion and onset of dextral transpression along the continental margin to the southeast of their present-day locations. The final 75 to 65 Ma metallogenic event is regionally the most extensive. It followed a ~15-m.y.-long magmatic lull and is related to enigmatic subduction-related magmatism in the western part of the Alaska Range and the Kuskokwim basin. This event resulted in the formation of porphyry, reduced pluton-related, orogenic, and possible epithermal Au deposits. In the better-studied relatively low-relief areas of the Kuskokwim basin, many of the mineralized systems are spatially associated with ilmenite-series monzonite to quartz monzonite composite plutons that have isotopic signatures consistent with derivation from crustally contaminated mantle sources. These pluton-hosted Au deposits comprise low-sulfide stockworks, sheeted veins, and/or breccias in the cupolas of moderately differentiated intrusions that contain high Au; anomalous As, Bi, Sb, and/or Te; and have variable, but not uneconomic, Cu concentrations. In addition, Au, Sb, and/or Hg deposits hosted by competent NE-trending granite porphyry dike-sill complexes or along faults in flysch are widespread in the Kuskokwim basin. These deposits, including the giant Donlin Creek Au deposit, formed at shallow crustal levels and are classified here as epizonal orogenic Au deposits and related Hg and Sb lodes. Mesozonal orogenic Au deposits were formed along the margins of the uplifting Willow Creek pluton that is now exposed along the southern side of the Talkeetna Mountains batholith. Recent discoveries in the rugged Alaska Range comprise less well-studied porphyry Au-Cu (Whistler, Island Mountain), epithermal(?) Au (Terra), and reduced pluton-hosted Au deposits (Estelle). The cause of the broad latest Cretaceous magmatism associated with the final metallogenic event is enigmatic, but indicates widespread high heat flow possibly related to flat-slab subduction, lithospheric mantle delamination, or escape tectonics. Plutonism postdated regional folding and coincided with periods of movement along regional faults and formation of the NE-trending structural fabric. Magmatism resulted from initiation of transpressional faulting in more landward positions during oblique subduction of the Kula plate. Crustal contamination of mantle melts, either near their source or during ascent through the thick flysch of the Kuskokwim basin, produced low oxidation state magmas and controlled much of the metallogeny, particularly for those deposits that display similarities to both reduced porphyry Au-Cu deposits and, to a lesser degree, reduced intrusion-related Au systems. High heat flow also induced crustal melting and metamorphic devolatilization to form orogenic Au deposits. A transition to extension at ~60 Ma recorded elsewhere in Alaska temporally corresponds to termination of gold deposit formation in southwestern Alaska. The multiple periods of gold metallogenesis in southwestern Alaska offer a wide variety of targets for exploration within discrete parts of the region.

Society of Economic Geologists Special Publication↗

USGS2024

Comparison of rapid methods for chemical analysis of milligram samples of ultrafine clays

Two rapid methods for the decomposition and chemical analysis of clays were adapted for use with 20–40-mg size samples, typical amounts of ultrafine products (≤0.5-µm diameter) obtained by modern separation methods for clay minerals. The results of these methods were compared with those of “classical” rock analyses. The two methods consisted of mixed lithium metaborate fusion and heated decomposition with HF in a closed vessel. The latter technique was modified to include subsequent evaporation with concentrated H 2 SO 4 and re-solution in HCl, which reduced the interference of the fluoride ion in the determination of Al, Fe, Ca, Mg, Na, and K. Results from the two methods agree sufficiently well with those of the “classical” techniques to minimize error in the calculation of clay mineral structural formulae. Representative maximum variations, in atoms per unit formula of the smectite type based on 22 negative charges, are 0.09 for Si, 0.03 for Al, 0.015 for Fe, 0.07 for Mg, 0.03 for Na, and 0.01 for K.

Clays and Clay Minerals↗

USGS1979

Evaluation of Landsat multispectral scanner images for mapping altered rocks in the East Tintic Mountains, Utah

The East Tintic Mountains, Utah consist of folded and faulted Paleozoic sedimentary rocks, which are partly covered by Tertiary volcanic rocks. Clastic rocks dominate the lower one-third of the Paleozoic section, whereas carbonate rocks with subordinate amounts of shale and elastic rocks predominate in the remainder. Some of the rocks, especially the Tintic Quartzite and some shales, are commonly limonitic, an important factor in analysis of Landsat MSS images. The volcanic rocks, mainly tuffs, flows, and agglomerates of quartz latitic and latitic composition, are limonitic in a few places where hematite is present in the groundmass. Emplacement of monzonite and biotite monzonite porphyry bodies resulted in several types of altered rocks. Most widespread are argillized and silicified rocks, which are commonly bleached and limonitic. Locally, the intrusive rocks arc also altered. Hydrothermal dolomite is common in the northern part of the area, and in the East Tintic mining district, calcitic, chloritic, and weakly argillized volcanic rocks and pebble dikes are widespread. Volcanic rocks subjected to an early phase of "intravolcanic weathering" in this district are weakly altered but commonly limonitic. Barren as well as mineralized veins are present throughout the study area. In situ spectral reflectance curves representing the most abundant altered and unaltered rocks show that the argillized and silicified rocks generally have intense ferric-iron and hydroxyl absorption bands owing to the presence of iron-oxide and hydroxyl-bearing phases, respectively. These features are generally absent in the unaltered rocks, except the limonitic rocks, which have prominent iron absorption hands. Both spectral features are weakly expressed in the volcanic rocks subjected to accelerated weathering, On the other hand, hydrothermal dolomite and calcitic volcanic rocks generally lack both features, and thus are spectrally similar to the unaltered rocks. Chloritic rocks are of limited distribution and have not been measured spectrally. Most of the silicified and argillized areas are apparent in Skylab S190B, high altitude-, and low altitude color aerial photographs because of the high albedo of these rocks. However, many unaltered rocks have similar albedos and therefore are not distinguishable from the altered rocks. Moreover, very little color information is available in these photographs. These problems are further complicated by brightness variations related to topographic slope. MSS ratio images were generated to subdue the effects of topographic slope and albedo, and combined into several color composite images for displaying the spectral reflectance differences between the most widespread altered and unaltered rocks. The most effective combination proved to be MSS 4/5, MSS 4/6, and MSS 6/7 using blue, yellow and magenta diazo films, respectively, rather than the MSS 4/5, MSS 5/6, and MSS 6/7 combination used so successfully in south-central Nevada. Consideration of schematic frequency distributions of ratio values for these two areas suggests that the lack of enhancement of limonitic rocks in MSS 5/6 images of the present study area is due to the higher frequency of low ratios representing vegetation. Comparison of a limonitic bedrock map produced by scanning the optimum color-ratio composite image with a map of the silicified rocks shows good agreement, except where they are obscured by vegetation. Measurements of vegetation density indicate that shrub cover and. juniper, pinyon, and sage cover greater than 40-50 and 33-43 percent, respectively, obscure limonitic rocks in these images. Argillized rocks, the most widely distributed altered rock type, were consistently detected in exposed areas. On the other hand, hydrothermal dolomite and calcitic and chloritic volcanic rocks are not portrayed in the limonitic bedrock map because of their general lack of limonite. Some altered rocks, especially veins and pebble dikes, are too small to be detected by the MSS except where they are closely spaced and well exposed. Another important limitation is that exposures of unaltered limonitic sedimentary and volcanic rocks are included in the limonitic bedrock map. Analysis of in situ spectral reflectance measurements indicates that this limitation can be largely overcome by obtaining radiance information in the 2.2 and 1.6 μ m regions.

Utah↗

USGS1984

Phosphate rock resources of the United States

In 1980, the United States produced about 54 million tons of phosphate rock, or about 40 percent of the world's production, of which a substantial amount was exported, both as phosphate rock and as chemical fertilizer. During the last decade, predictions have been made that easily ruinable, low-cost reserves of phosphate rock would be exhausted, and that by the end of this century, instead of being a major exporter of phosphate rock, the United States might become a net importer. Most analysts today, however, think that exports will indeed decline in the next one or two decades, but that resources of phosphate are sufficient to supply domestic needs for a long time into the future. What will happen in the future depends on the actual availability of low-cost phosphate rock reserves in the United States and in the world. A realistic understanding of future phosphate rock reserves is dependent on an accurate assessment, now, of national phosphate rock resources. Many different estimates of resources exist; none of them alike. The detailed analysis of past resource estimates presented in this report indicates that the estimates differ more in what is being estimated than in how much is thought to exist. The phosphate rock resource classification used herein is based on the two fundamental aspects of a mineral resource(l) the degree of certainty of existence and (2) the feasibility of economic recovery. The comparison of past estimates (including all available company data), combined with the writers' personal knowledge, indicates that 17 billion metric tons of identified, recoverable phosphate rock exist in the United States, of which about 7 billion metric tons are thought to be economic or marginally economic. The remaining 10 billion metric tons, mostly in the Northwestern phosphate district of Idaho, are considered to be subeconomic, ruinable when some increase in the price of phosphate occurs. More than 16 billion metric tons probably exist in the southeastern Coastal Plain phosphate province, principally in Florida and North Carolina and offshore in the shallow Atlantic Ocean from North Carolina to southern Florida. This resource is considered to be hypothetical because it is based on geologic inference combined with sparse drilling data. Total resources of phosphate rock in the United States are sufficient to supply domestic demands for the foreseeable future, provided that drilling is done to confirm hypothetical resources and the chemistry of the deposits is determined. Mining and beneficiation techniques will have to be modified or improved, and new techniques will have to be developed so that these deposits can be profitably exploited.

Circular↗

USGS2010

Mineral resource of the month: salt

The article presents information on various types of salt. Rock salt is either found from underground halite deposits or near the surface. Other types of salt include solar salt, salt brine, and vacuum pan salt. The different uses of salt are also given including its use as a flavor enhancer, as a road deicing agent, and to manufacture sodium hydroxide.

Earth↗

USGS1954

The copper and uranium deposits of the Coyote district, Mora County, New Mexico

The copper and uranium-vanadium deposits of the Coyote district, Mora County, N. Mec, are confined to the lower 2,000 feet of the Sangre de Gristo formation of Pennsylvanian and Permian age. A narrow belt of deposits in steeply dipping or overturned rocks extends for 7 miles along Coyote Creek south of Guadalupita. Earlier studies showed that the copper deposits contained uranium, but both the reserves and the uranium content of the copper-bearing shale are too low to permit the recovery of uranium. However, small, commercial grade uranium deposits have been discovered in sandstone. Small lenses of copper-bearing carbonaceous shale, siltstone, limestone or sandstone, interbedded with predominantly red rocks, are present at intervals at 12 or more stratigraphic levels. The better deposits, in carbonaceous shale, average about 2 percent copper. The copper content of the other rocks is usually lower, but small concentrations may contain 6 percent copper. The principal copper minerals are chalcocite and malachite. Chalcocite replaces wood and forms nodules that contain small, variable amounts of pyrite, bornite, covellite, and rarely, uraninite. The uranium deposits occur as small closely spaced pockets that are commonly localized by sedimentary structures within one or more fluviatile arkosic sandstone beds near the middle of the formation, particularly" where carbonized wood, clay galls, and rock fragments are abundant. The uraniferous sandstone is commonly stained pink by hematite that probably was introduced with the uranium. The color increases in intensity with the radioactivity. The outcrops of the uranium deposits are typically inconspicuous, but close inspection shows they contain malachite, chalcopyrite, black vanadium minerals of micaceous habit, metatyuyamunite and microscopic grains of an unidentified black uraniferous substance. The proportion of copper, uranium, and vanadium is variable and any metal may be dominant. The metals probably were derived from pre-Cambrian granitic rocks. Copper and minor amounts of uranium were deposited in local stagnant basins by reaction with hydrogen sulfide. and decaying organic material. The uraniferous shales and the copper deposits are believed to be syngenetic, or nearly so, but the uranium deposits in sandstone are epigenetic and probably were deposited from ground waters with a possible hydrothermal admixture. The uranium and vanadium may have been reconcentrated from earlier, low-grade, syngenetic deposits.

New Mexico↗

USGS2025

Widespread anhydrite saturation in Laramide-age arc magmas of southwestern USA

Anhydrite is considered a rare mineral phase in magmas, with only ∼33 documented occurrences worldwide. However, anhydrite readily decomposes in the near-surface environment, making it difficult to recognize its former presence in rocks collected at or near Earth’s surface. In such samples, only small anhydrite inclusions fully shielded within other minerals can have survived. During a recent field trip to the southwestern USA, we sampled 17 Laramide-age (ca. 40−80 Ma) magma systems, most of which are associated with porphyry copper deposits. A systematic search for anhydrite inclusions preserved within apatite, amphibole, plagioclase, and quartz phenocrysts in ∼100 rock samples by optical microscopy and Raman spectroscopy revealed that each of these 17 magma systems was at least temporarily anhydrite-saturated. Also, most previously identified magmatic anhydrite-bearing intrusions are associated with porphyry copper deposits, and both intrusive and volcanic rocks containing magmatic anhydrite show high Sr/Y ratios. These observations suggest that anhydrite saturation and porphyry copper formation are linked via magma fractionation at high pressure. Compared to average arc magmas, anhydrite-bearing magmas are unusually oxidized and sulfur-rich and seem to also be unusually water-rich. Hence, our preferred interpretation is that magma generation and/or fractionation at high pressure promotes the formation of superhydrous and oxidized magmas, which in turn promotes high sulfur contents and ultimately the precipitation of anhydrite. The high mineralization potential of these magmas does not need to result from their high sulfur content but could be caused by other properties of high-pressure magmas.

Arizona, New Mexico↗

USGS2010

Secular variation in economic geology

The temporal pattern of ore deposits on a constantly evolving Earth reflects the complex interplay between the evolving global tectonic regime, episodic mantle plume events, overall changes in global heat flow, atmospheric and oceanic redox states, and even singular impact and glaciation events. Within this framework, a particular ore deposit type will tend to have a time-bound nature. In other words, there are times in Earth history when particular deposit types are absent, times when these deposits are present but scarce, times when they are abundant, and still other times for which we lack sufficient data. Understanding of such secular variation provides a critical first-order tool for exploration targeting, because rocks that have formed or were deformed during a certain time slice may be very permissive for a given deposit type, whereas identification of rocks of less favorable ages would help eliminate large areas during exploration programs. Secular analysis, therefore, is potentially a powerful tool for mineral resource assessment in poorly known terranes, providing a quick filter for favorability of a given deposit type using age of host rocks. Factors bearing on the known age distribution of a particular type of deposit include the following: (1) uneven preservation, (2) data gaps, (3) contingencies of plate motions, and (4) long-term secular changes in the Earth System. The present special issue of Economic Geology is focused on the latter factor, although all of these are interrelated. The selective preservation of certain mineral deposit types and the greater susceptibility for shallowly formed ores in tectonically active environments to be lost to erosion define a pattern that is superimposed on the secular formational trends (e.g., Groves et al., 2005a , b ; Kerrich et al., 2005 ). With improved geochronological methods and the availability of information on important mineral deposits from most parts of the world, data gaps for defining broad temporal distributions of ore types are becoming smaller. It has been increasingly recognized that ore deposit formation is also correlated with plate tectonic setting. Nevertheless, a complex Earth history of supercontinent assembly and breakup has led to the fragmentation of many Paleozoic and Precambrian mineral provinces. The use of plate reconstructions in economic geology, although extremely controversial and conjectural before the late Paleozoic, is critical for defining these provinces prior to the added complications resulting from post-ore plate motions. It is now well established that the temporal patterns of many types of mineral deposits (Fig. 1 1 ) reflect the formation or break-up of supercontinents and the preservation potential of deposits formed during these periods ( Barley and Groves, 1992 ; Titley, 1993 ; Kerrich et al., 2005 ; Goldfarb et al., 2009 ). Approximate time periods for such formation and break-up, respectively, include 2800–2500 and 2450–2100 Ma for Kenorland, 2100–1800 and 1600–1300 Ma for Nuna/Columbia, 1300–1100 and 850–600 Ma for Rodinia, and 600–300 Ma and 200–60 Ma for Gondwanaland-Pangea. A new supercontinent, Amasia, has begun to form during the past 250 m.y., thus overlapping Pangea break-up. Many of the formation-preservation patterns are themselves controlled by progressive cooling of Earth, the change from a mantle-plume buoyancy style to subduction-dominated tectonics, a decreasing buoyancy of the subcontinental lithospheric mantle, and depth of ore formation. In general, orogenic Au, volcanogenic massive sulfide (VMS), epithermal Au-Ag, and porphyry Cu±Au and Mo porphyry deposits form in active margins during periods of supercontinent assembly. Numerous other ore deposit types show an association with supercontinent formation, but develop inland of the active margin. These include many of the MVT Pb-Zn deposits and unconformity-type U deposits. The Tertiary Carlin-type deposits within the deformed shelf sequences along the North American craton margin also appear to have formed during the ongoing growth of Amasia. Those ores associated with periods of supercontinent breakup or attempted breakup are more difficult to define. They probably include diamond, Bushveld-type Ni-Cu-PGE, IOCG, and clastic-dominated Pb-Zn (or SEDEX) deposits in intracontinental areas of failed rifting, and other clastic-dominated Pb-Zn deposits in areas of actual breakup. In all cases, however, these temporal/spatial distributions are ultimately controlled by the secular character of Earth history.

Economic Geology↗

USGS2004

Stable isotopic constraints on fluid-rock interaction and Cu-PGE-S redistribution in the Sonju Lake intrusion, Minnesota

The Sonju Lake intrusion, part of the 1.1 Ga Midcontinent rift-related Beaver Bay Complex, is a 1,200-m-thick, strongly differentiated, layered sequence of mafic cumulates located in northeastern Minnesota. Basal melatroctolite and dunite layers are overlain by troctolite, gabbro, Fe-Ti oxide-rich gabbro, apatite diorite, and monzodiorite. Stratigraphic intervals rich in Pt + Pd, Cu, and S occur over ???500 m in the Fe-Ti oxide-rich gabbro and apatite diorite units. Peak concentrations show offsets that are similar to those found in other tholeiitic layered intrusions. Concentrations of Pd in excess of 100 ppb are confined to the lowermost 25 m of the interval. Copper shows a sharp increase to 630 ppm above the Pd-rich interval. Sulfur contents are low (<375 ppm) in the Cu-rich interval, but they increase to values as high as 3,150 ppm above in the apatite diorite. Disseminated sulfides in the intrusion have ??34S values that range from -2.2 to 3 per mil Vienna-Canyon Diablo Troilite (V-CDT) and suggest that contamination by country rock sulfur was not an important process in the formation of the metal-rich interval. ??18O values of plagioclase from the intrusion range from 5.6 to 12.0 per mil (V-SMOW) and indicate that a relatively low-18O fluid (??18O ???3-5 ???) interacted with the rocks of the intrusion at temperatures less than ???275??C. Clinopyroxene and Fe-Ti oxides (ilmenite with minor amounts of titanomagnetite) show much more restricted ranges in ??18O values (4.6-5.7 and 5.5-6.7 per mil, respectively) and attest to the kinetic control of the oxygen isotope exchange process. The externally derived fluid that interacted with rocks now enriched in platinum group elements (PGE) + Cu- and Fe-sulfide minerals locally liberated sulfur and replaced chalcopyrite and pyrite with goethite. In the Cu-rich zone, goethite that replaces chalcopyrite may contain up to 8.5 weight percent Cu. It is evident that hydrothermal alteration resulted in a decoupling of copper and sulfur, with sulfur being transferred out of the Cu-rich interval. Interaction between rocks in the PGE-Cu-S interval of the Sonju Lake intrusion and an externally derived fluid at low temperatures modified what appears to have been a primary stratigraphic metal-sulfur zonation. The effects of hydrothermal alteration on PGE and base-metal sulfide mobility and redistribution must be understood before models of primary zonation processes can be meaningfully applied. ?? 2004 by Economic Geology.

Economic Geology↗

USGS2011

Investigation of the potential for concealed base-metal mineralization at the Drenchwater Creek Zn-Pb-Ag occurrence, northern Alaska, using geology, reconnaissance geochemistry, and airborne electromagnetic geophysics

In 2005, the U.S. Geological Survey, Bureau of Land Management, and State of Alaska cooperated on an investigation of the mineral potential of a southern part of the National Petroleum Reserve in Alaska, Howard Pass quadrangle, to provide background information for future land-use decisions. The investigation incorporated an airborne electromagnetic (EM) survey covering 1,500 mi 2 (~3,900 km 2 ), including flight lines directly over the Drenchwater Creek sediment-hosted Zn-Pb-Ag occurrence, the largest known base-metal occurrence in the survey area. Samples from the mineralized outcrop and rubblecrop contain metal concentrations that can exceed 11 percent Zn+Pb, with appreciable amounts of Ag. Soil samples with anomalous Pb concentrations are distributed near the sulfide-bearing outcrops and along a >2.5 km zone comprising mudstone, shale, and volcanic rocks of the Kuna Formation. No drilling has taken place at the Drenchwater occurrence, so alternative data sources (for example, geophysics) are especially important in assessing possible indicators of mineralization. Data from the 2005 electromagnetic survey define the geophysical character of the rocks at Drenchwater and, in combination with geological and surface-geochemical data, can aid in assessing the possible shallow (up to about 50 m), subsurface lateral extent of base-metal sulfide accumulations at Drenchwater. A distinct >3-km-long electromagnetic conductive zone (observed in apparent resistivity maps) coincides with, and extends further westward than, mineralized shale outcrops and soils anomalously high in Pb concentrations within the Kuna Formation; this conductive zone may indicate sulfide-rich rock. Models of electrical resistivity with depth, generated from inversion of electromagnetic data, which provide alongflight-line conductivity-depth profiles to between 25 and 50 m below ground surface, show that the shallow subsurface conductive zone occurs in areas of known mineralized outcrops and thins to the east. Broader, more conductive rock along the western ~1 km of the geophysical anomaly does not reach ground surface. These data suggest that the Drenchwater deposit is more extensive than previously thought. The application of inversion modeling also was applied to another smaller geochemical anomaly in the Twistem Creek area. The results are inconclusive, but they suggest that there may be a local conductive zone, possibly due to sulfides.

Professional Paper↗

USGS1974

K-Ar Age Relations of Granodiorite Emplacement and Tungsten and Gold Mineralization near the Getchell Mine, Humboldt County, Nevada

A granodiorite stock intrudes complexly folded and thrust-faulted Paleozoic sedimentary rocks in the Osgood Mountains of eastern Humboldt County, Nevada. Within the metamorphic aureole surrounding the pluton, the sedimentary rocks are converted to cordierite hornfels and marble; tungsten-bearing tactites developed along the contacts of the granodiorite. Cutting the granodiorite and sedimentary rocks is the Getchell fault, along which the disseminated gold ore bodies of the Getchell mine are localized. K-Ar ages of the granodiorite, andesite porphyry, tungsten-bearing tactites, and altered granodiorite from the Getchell mine indicate that emplacement, alteration, and mineralization are all part of a magmatic-thermal episode which took place approximately 90 m.y. ago.

Nevada↗

USGS2015

Qualitative assessment of selected areas of the world for undiscovered sediment-hosted stratabound copper deposits: Chapter Y in Global mineral resource assessment

A qualitative mineral resource assessment of sediment-hosted stratabound copper mineralized areas for undiscovered copper deposits was performed for 10 selected areas of the world. The areas, in alphabetical order, are (1) Belt-Purcell Basin, United States and Canada; (2) Benguela and Cuanza Basins, Angola; (3) Chuxiong Basin, China; (4) Dongchuan Group rocks, China; (5) Egypt–Israel–Jordan Rift, Egypt, Israel, and Jordan; (6) Maritimes Basin, Canada; (7) Neuquén Basin, Argentina; (8) Northwest Botswana Rift, Botswana and Namibia; (9) Redstone Copperbelt, Canada; and (10) Salta Rift System, Argentina. This assessment (1) outlines the main characteristics of the areas, (2) classifies known deposits by deposit model subtypes, and (3) ranks the areas according to their potential to contain undiscovered copper deposits. An analytic hierarchy process (AHP) was used to rank assessment areas according to their potential for undiscovered copper deposits. Once the main characteristics of each area were compiled (age of host rock, geologic setting, stratigraphy, host lithology, deposit subtype(s), known deposits and occurrences, and mineral system components), three criteria (mineralization, extent of study area, and lithostratigraphic framework, each with multiple subcriteria) were scored for all assessment areas. Relative weights and scores were assigned to all criteria by three geologists. In addition, the assessment areas were ranked for comparison exclusively on the basis of professional opinion. The AHP and professional opinion lists are similar but not the same. Both the professional opinion and the cumulative AHP lists rate the Northwest Botswana Rift in Botswana and Namibia as the area most likely to contain the most undiscovered copper deposits. The Salta Rift System in Argentina is rated lowest among the 10 qualitatively assessed areas.

Belt-Purcell Basin, Benguela Basin, Cuanza Basin, ↗

USGS1980

The Boulder Creek Batholith, Front Range, Colorado

The Boulder Creek batholith is the best known of several large Precambrian batholiths of similar rock composition that crop out across central Colorado. The rocks in the batholith belong to the calc-alkaline series and range in composition from granodiorite through quartz diorite (tonalite) to gneissic aplite. Two rock types dominate': the Boulder Creek Granodiorite, the major rock unit, and a more leucocratic and slightly younger unit herein named Twin Spruce Quartz Monzonite. Besides mafic inclusions, which occur mainly in hornblende-bearing phases of the Boulder Creek Granodiorite, there are cogenetic older and younger lenses, dikes, and small plutons of hornblende diorite, hornblendite, gabbro, and pyroxenite. Pyroxenite is not found in the batholith. The Boulder Creek Granodiorite in the batholith represents essentially two contemporaneous magmas, a northern body occurring in the Gold Hill and Boulder quadrangles and a larger southern body exposed in the Blackhawk and the greater parts of the Tungsten and Eldorado Springs quadrangles. The two bodies are chemically and mineralogically distinct. The northern body is richer in CaO and poorer in K2O, is more mafic, and has a larger percentage of plagioclase than the southern body. A crude sequence of rock types occurs from west to east in the batholith accompanied by a change in plagioclase composition from calcic plagioclase on the west to sodic on the east. Ore minerals tend to decrease, and the ratio potassium feldspar:plagioclase increases inward from the western contact of the batholith, indicating that the Boulder Creek batholith is similar to granodiorite batholiths the world over. Emplacement of the Boulder Creek batholith was contemporaneous with plastic deformation and high-grade regional metamorphism that folded the country rock and the batholith contact along west-northwest and north-northwest axes. Also, smaller satellitic granodiorite bodies tend to conform to the trends of foliation and fold axes in the country rock, suggesting that emplacement was controlled by preexisting structures in the country rock. On a gross scale, chemical equilibrium in the Boulder Creek Granodiorite is expressed by a near 1:1 ratio, or straight-line relationship in the distribution of iron, magnesium, and manganese in biotite and hornblende. General mineralogical trends in the Boulder Creek Granodiorite indicate that modal biotite, hornblende, and plagioclase tend to increase and quartz and microcline tend to decrease as CaO increases. These trends were not found in the Twin Spruce Quartz Monzonite. Differentiation is believed to have played a major role and assimilation a minor role in the development of the Boulder Creek batholith. The Boulder Creek Granodiorite is of probable mantle or lower crust origin, and, based on the scant data available, the Twin Spruce Quartz Monzonite may be of crustal origin, but the magma was extensively altered by contaminants of ambiguous origin. Mafic inclusions, possibly derived from a dioritic magma which was an early differentiate associated temporally with the Boulder Creek Granodiorite and (or) the Twin Spruce Quartz Monzonite, were in jected into the Boulder Creek Granodiorite during the mush stage and before the batholith was completely crystallized. Biotite, hornblende, and potassium feldspar were studied extensively. Their chemistry and petrology indicate a homogeneity throughout the batholith not believed possible by a casual observance of the batholithic rocks in the field. The accessory minerals, where investigated, also tend to indicate this same pervasive homogeneity.

Professional Paper↗

USGS1971

A paleohydrologic model for mineralization of the White Pine copper deposit, northern Michigan

Pertinent physical properties of the upper Keweenawan rocks can be measured or inferred within a sufficiently narrow range to make the quantitative evaluation of various paleohydrologic models for the origin of the White Pine copper deposit feasible. The approach is illustrated here by calculations for models that involve lateral migration of fluids through the subjacent Copper Harbor Conglomerate to the site of the deposit and stripping of copper from these solutions where they percolated upward through the Nonesuch Shale. The calculations reveal limitations to theories of origin that would not be evident from purely qualitative consideration; some of these limitations could be useful to exploration. For example, if the water was yielded by compaction of the Copper Harbor Conglomerate and contained 50 ppm Cu, there must have been significant convergence of solution paths toward White Pine. Surface water entering the Copper Harbor Conglomerate on the north limb of the Lake Superior syncline is an adequate source if it could be shown that the point of entry was once significantly higher in altitude than the water table at White Pine; this model implies a major copper deposit at great depth north of the axis of the syncline. Ground water entering the Nonesuch Shale up dip from White Pine is not a possible source of mineralizing solutions.

Michigan↗