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The RESTORE Council Monitoring and Assessment Program (CMAP), administered by the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Geological Survey (USGS), spatially and temporally inventoried programs in the Gulf of Mexico focused on water quality and habitat monitoring and mapping.
The RESTORE Council Monitoring and Assessment Program (CMAP), administered by the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Geological Survey (USGS), spatially and temporally inventoried programs in the Gulf of Mexico focused on water quality and habitat monitoring and mapping.
The regional tectonic pattern of the porphyry copper province of northern Sonora, Mexico, is similar to, but more complex than, the Colorado mineral belt. Four northeast-trending shear zones, spaced at 30- to 50-kilometer intervals from Hermosillo, Mexico, north to Nogales, Mexico, are interpreted from analysis of lineament data from Landsat-1 images. From comparison with other areas, these zones are believed to have Precambrian ancestry. North-northwest-trending lineaments, which commonly occur along range fronts, are interpreted to be the principal basin and range faults. North-south- and west-northwest-trending lineaments are believed to be related to lesser faults and fractures. Areas of hydrothermal alteration have been mapped on a regional scale by use of digital image-processing techniques. These altered areas occur on the flanks of the northeast-trending shear zones, generally near intersections with north-northwest-trending lineaments. This tectonic framework with associated alteration is similar to patterns in the Colorado Plateau, the porphyry copper province of Arizona, and central Mexico and is supported by relationships found in limited local mapping. The pattern of northeast-trending shear zones and north-northwest-trending faults in northern Sonora closely resembles the tectonic framework of the Colorado mineral belt. The younger sedimentary and volcanic cover, however, has not been eroded in Sonora as it has been in the Colorado mineral belt. This comparison with the Colorado mineral belt suggests that mineral deposits are most likely to be found in altered areas where a northeast-trending shear zone is intersected by north-northwest-trending faults. Seven areas with these characteristics have been defined in Sonora and are being evaluated by means of regional geophysical and geochemical data.
Results from nine 3 x 3 km study areas in the Rocky Mountains of Colorado, USA demonstrate there is potential for using sporadically acquired Landsat images in combination with daily coarse resolution fractional snow covered area (SCA) images to produce daily high resolution binary SCA images. The results also highlight several challenges to implementing this type of approach. The approach described here consistently yields accurate results in locations with persistent winter and spring snow cover where ten or more partially snow covered images are available to populate the image database, but is less successful in areas with shallower or more ephemeral snow covers or when fewer images are available to populate the image database. This work represents a first step towards developing an algorithm to combine Landsat and MODIS data to produce daily 30 m resolution binary SCA images. Further research should focus on testing the accuracy of this approach across a range of landscape types and snow cover regimes, developing methods to improve prediction accuracy when snow cover is nearly complete or nearly absent, and developing methods to compensate for the effects of canopy cover on SCA retrievals.
Seismic-reflection data were collected in the 1980s as part of the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) to investigate the 1.1 Ga Midcontinent Rift System (MRS). GLIMPCE Line C crosses western Lake Superior from north to south shores (Fig. 1 inset). Many previous workers have interpreted the MRS in Line C as an asymmetric central graben filled with 10–20 km of subaerial basalt flows, overlain by 7-10 km of sedimentary section, and underlain by magmatic underplating. The central graben was interpreted to have formed from extensional normal faults, later reactivated as high-angle reverse faults. The northern part of Line C crosses over a prominent gravity low called the Grand Marais Ridge (GMR; Fig. 1 inset), previously interpreted as an Archean granitic basement high.
The Chumstick Formation (Gresens and others, 1981a,b) comprises Eocene (49 to ~45 Ma) fl uvial sandstone with lesser conglomerate, mudstone, and tuff currently exposed on the east fl ank of the North Cascade Range (fi g. 1). The unit is restricted to the northwest–southeast-trending region Willis (1953) called the Chiwaukum graben. To the northeast, Chumstick is separated from Swakane Biotite Gneiss and Napeequa Schist by the Entiat fault. To the southwest, the Leavenworth fault juxtaposes Chumstick strata with the 90 Ma Mount Stuart batholith, prebatholith rocks of the Chiwaukum Schist, and the Ingalls Tectonic Complex, and, farther south, ~61–51 Ma fl uvial strata of the Swauk Formation. Within the Chiwaukum graben lies a northwest-trending uplift of Swakane Biotite Gneiss termed the Eagle Creek anticline (Willis, 1953) or Eagle Creek fault zone (Evans, 1994).
The influence of permafrost thaw on aquatic ecosystems is complex. Carbon, nutrients, and metals in stream discharge result in changing stream color, chemistry, and temperature. These different processes create a mosaic of spatially and temporally dynamic disturbances across the landscape with complex effects on the ecology.
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Explore the source record for details and available documents.
Zoos and aquariums have been incorporating environmental enrichment into their animal care programs for the past 30 years to increase mental stimulation and promote natural behaviors. However, most attempts to document the effects of enrichment on animal behavior have focused on terrestrial mammals. Staff at the National Aquarium in Baltimore conducted an investigation of the behavioral effects of enrichment on the seven harbor seals and two gray seals housed in the aquarium's outdoor seal exhibit. We expected that enrichment would change the amount of time the animals spent engaged in specific behaviors. The behaviors recorded were: resting in water, resting hauled out, maintenance, breeding display, breeding behavior, aggression, pattern swimming, random swimming, exploration, and out of sight. Activity levels (random swimming and exploration) were expected to increase, while stereotypic behaviors (pattern swimming) were expected to decrease. The frequency and duration of behaviors were documented for 90 hr in both the control phase (without enrichment) and the experimental phase (with enrichment). Statistically significant differences ( P <0.05) in the time spent in pattern swimming, random swimming, exploration, and out of sight were observed between the two phases. With enrichment, pattern swimming and out of sight decreased, while random swimming and exploration behavior increased. These findings demonstrate that enrichment can promote behaviors (random swimming and exploration) that are likely to be normal for phocids in the wild, and that may contribute to the behavioral complexity of these seals in captivity.
In 1955 exploration for base and precious metals was undertaken by Bear Creek Mining Company immediately north of the Main Tintic district , Utah . During the course of this work Bear Creek became interested in the East Tintic district , primarily as a result of the activities of the U.S. Geological Survey in that area . Data published on the East Tintic district by the Survey and others were studied and map data made available from various mining companies were compiled. Preliminary economic studies were made to determine the present day value of the type of ore body discovered previously in the district . Encouraging results from these investigations led to the selection of specific targets for exploration. Recommendations for a project program were made and approved. Negotiations were successfully concluded in mid-1956 for a unit lease agreement on lands in the East Tintic district owned by the Tintic Standard and Chief Consolidated mining companies and their subsidiaries. Of the targets selected for exploration, the Chief Oxide area seemed to be one of the most prominent. Our preliminary work in the Chief Oxide area corroborated the findings of the U.S. Geological Survey described in Part I of this paper. After careful consideration it was decided to gamble the cost of an exploration shaft in this area for the purpose of providing an underground drilling platform. We also hoped by means of underground workings to establish the existence and nature of the postulated fault. A limited amount of surface drilling was done prior to shaft sinking in order to locate a shaft site and also to obtain additional information of subvolcanic structure and alteration. Underground exploration in the Chief Oxide area was started in January, 1957. The Burgin shaft was sunk to a depth of 1,100 feet and by August, 1959, lateral development on the 1050 level totaled 4,721 feet and underground diamond drilling totaled 15,480 feet. Results of the work done to date are as follows: ( a ) Identification of the sedimentary rock section and an interpretation of the structure in the Burgin mine area . (b) Discovery by penetration of the previously postulated East Tintic thrust fault. (c) Discovery of large zones of manganese oxides and carbonates, which were found to be closely related to silver-lead-zinc mineralization. (d) Discovery of ore-grade lead-zinc mineralization within the footwall rocks of the East Tintic thrust. (e) Discovery of high-grade silver-lead ore within the thrust zone. Insofar as ore localization is concerned the most important structural feature in the Burgin mine area is the East Tintic thrust fault- a fact that opens up new ore potential over a large part of the East Tintic district not previously explored. Although the discoveries to date must be attributed essentially to the application of geology to exploration, the tools of geochemical prospecting and geophysics were also used, and the geochemical work, in particular, was found to be a definite aid in the selection of areas for further exploration.
The rate at which petroleum deposits were discovered during a 16-yr period (1957–72) was examined in relation to changes in a suite of economic and physical variables. The study area encompasses 11,000 mi 2 and is located on the eastern flank of the Powder River Basin. A two-stage multiple-regression model was used as a basis for this analysis. The variables employed in this model were: (1) the yearly wildcat drilling rate, (2) a measure of the extent of the physical exhaustion of the resource base of the region, (3) a proxy for the discovery expectation of the exploration operators active in the region, (4) an exploration price/cost ratio, and (5) the expected depths of the exploration targets sought. The rate at which wildcat wells were drilled was strongly correlated with the discovery expectation of the exploration operators. Small additional variations in the wildcat drilling rate were explained by the price/cost ratio and target-depth variables. The number of deposits discovered each year was highly dependent on the wildcat drilling rate, but the aggregate quantity of petroleum discovered each year was independent of the wildcat drilling rate. The independence between these last two variables is a consequence of the cyclical behavior of the exploration play mechanism. Although the discovery success ratio declined sharply during the initial phases of the two exploration plays which developed in the study area, a learning effect occurred whereby the discovery success ratio improved steadily with the passage of time during both exploration plays.
A critical part of the exploration for mineral deposits or of quantitative mineral resource assessments is the estimation of how large undiscoveredeposits might be. Typically, this problem is addressed using grade and tonnage models in which a major source of variation in possible sizes is accounted for by the differences in types of deposits (Cox and Singer, 1986; Mosier and Page, 1988; Bliss, 1992). It is clear from studies of petroleum exploration that larger oil fields tend to be found early in the process (Arps and Roberts, 1958). If the same behavior exists in mineral exploration, then tonnage models constructed from local data may be biased estimators of the tonnages of any remaining undiscoveredeposits in the area. Although Singer and Mosier (1981) showed that larger porphyry copper deposits should be found earlier than smaller deposits in a given geologic and exploration environment, there are no definitive studies that we could find which actually test the hypothesis that larger mineral deposits are discovered early in the exploration of a region. In this paper the hypothesis that larger mineral deposits are discovered early in the exploration of a region is tested by examining the relationship between discovery order and size of known mercury deposits in the California Coast Ranges. We then present a new maximum likelihood approach to modeling the size distribution of undiscovered mineral deposits by examining the sizes of the mercury deposits discovered early in the exploration process.
Fluorine compounds are essential in numerous chemical and manufacturing processes. Fluorspar is the commercial name for fluorite (isometric CaF 2 ), which is the only fluorine mineral that is mined on a large scale. Fluorspar is used directly as a fluxing material and as an additive in different manufacturing processes. It is the source of fluorine in the production of hydrogen fluoride or hydrofluoric acid, which is used as the feedstock for numerous organic and inorganic chemical compounds. The United States was the world’s leading producer of fluorspar until the mid-1950s. In the mid-1970s, the U.S. fluorspar mining industry began to decline because of foreign competition. By 1982, there was essentially only a single U.S. producer left, and that company ceased mining in 1996. Consumption of fluorspar in the United States peaked in the early 1970s, which was also the peak period of U.S. steel production. Since then, U.S. fluorspar consumption has decreased substantially; the United States has nonetheless increased its imports of downstream fluorine compounds, such as, in order of tonnage imported, hydrofluoric acid, aluminum fluoride, and cryolite. This combination of no U.S. production (until recently) and high levels of consumption has made the United States the world’s leading fluorspar-importing country, in all its various forms. The number of fluorspar-exporting countries has decreased substantially in recent decades, and, as a result, the United States has become dependent on just a few countries to supply its needs. In 2013, the United States imported the majority of its fluorspar from three countries, which were, in descending order of the amount imported, Mexico, China, and South Africa. Geologically, in igneous systems, fluorine is one of a number of elements that are “incompatible.” These incompatible elements become concentrated in the residual magma while the common silicates crystallize upon magma ascent and cooling, leading to relatively high fluorine concentrations in the more evolved or differentiated igneous rocks and in hydrothermal deposits associated with those evolved igneous rocks. In sedimentary rocks, fluorine’s highest concentrations are found in phosphorites because fluorine substitutes for hydroxyl ions in apatite, which leads to fluorine concentrations of, typically, from 2 to 4 weight percent in phosphorites. Because of the presence of fluorine, phosphate fertilizer manufacturers can produce a fluorosilicic acid byproduct. Most deposits mined for fluorine are hydrothermal, however, and consist of fluorine minerals that precipitated from hot water. Magmatic brines and brines from deep within sedimentary basins that have high concentrations of dissolved fluoride are the mineralizing fluids for various types of hydrothermal fluorspar deposits. Relatively dilute hydrothermal fluids that formed in some volcanic rocks can also transport sufficient fluoride to form a high-grade fluorspar deposit. Fluorite has low solubility in a common range of hydrothermal temperatures, particularly from about 160 degrees Celsius (°C) down to 60 °C. The increasing fluorite solubility below 60 °C partly explains why some water with exceptionally high levels of dissolved fluorine are found even at ambient temperatures in evaporitic lake basins in some East African Rift valleys in Kenya and Tanzania. The geologic conditions that led to the high concentrations there are known to exist in a number of other places in the world as well, including, perhaps, places in the Basin and Range province of the United States. Eight minerals or mineral groups have sufficient fluorine in their structures to be considered as possible ores of the element; they are bastnaesite (also spelled bastnäsite; and other fluorocarbonates), cryolite, sellaite, villiaumite, fluorite, fluorapatite (in phosphorites), various phyllosilicates, and topaz. Fluorite is currently the only mineral that is mined for fluorine, and nomineral except fluorite is likely to become a source of commercially produced fluorine as a primary product as long as supplies from relatively thick and high-grade fluorite deposits continue to be available. At least seven classes (which include one subclass) of hydrothermal fluorite deposits are recognized; they are classified according to their tectonic and (or) magmatic settings, as follows: (1) carbonatite-related fluorspar deposits; (2) alkaline-intrusion-related fluorspar deposits; (3) alkaline-volcanic-related epithermal fluorspar deposits; (4) Mississippi Valley-type fluorspar deposits (and a subclass of salt-related carbonate-hosted fluorspar deposits); (5) fluorspar deposits related to strongly differentiated granites; (6) subalkaline-volcanic-related epithermal fluospar deposits; and (7) fluorspar deposits that appear to be conformable within tuffaceous limy lacustrine sediments. An eighth class (not hydrothermal) is that of fluorspar deposits concentrated in soils and weathered zones; that is, residual fluorspar deposits. Generally, fluorspar deposits related to strongly differentiated granites have larger tonnages and lower grades than carbonatite-related fluorspar deposits, which, in turn, have larger tonnages and lower grades than fluorspar vein deposits from various other classes. The United States has a few identified resources of fluorspar, most notably the Klondike II property in the Illinois- Kentucky fluorspar district located about 8 kilometers southwest of Salem, Kentucky, which has a large vein that contains at least 1.6 million metric tons at a grade of 60 percent CaF 2 (Feytis, 2009). Additional fluorspar resources of lower grade but larger tonnage have been identified at Hicks Dome in the Illinois-Kentucky fluorspar district and at Lost River near the western tip of the Seward Peninsula in Alaska, along with a couple of dozen smaller, higher grade resources. Internationally, new mines that either opened before the beginning of 2013 or were scheduled to open soon after that time include the Nui Phao tungsten-fluorspar-bismuth-copper-gold deposit in northern Vietnam; the St. Lawrence project in Newfoundland, Canada, which is located in a well-known fluorspar district; the Bamianshan deposit, which is related to a strongly differentiated granite in northwestern Zhejiang Province, China, near some of that Province’s large, subalkaline-volcanic-related epithermal veins; and the Nokeng project in South Africa, which is also related to a strongly differentiated granite. Other deposits in northwestern Australia, Nevada (United States), Norway, South Africa, and Sweden have been identified and could be put into production within just a few years. Among undiscovered resources, an interesting possibility might be to produce a fluorine product from evaporitic, high-fluorine, high-pH sodium-carbonate brines like Lake Magadi (Kenya) and Lake Natron (Tanzania) in Africa’s Eastern Rift Valley. In addition, apparently conformable fluorspar deposits in tuffaceous limy lacustrine sediments, such as those in Italy, are likely to occur in similar young alkalic volcanic settings elsewhere in the world. Modern geophysical and geochemical exploration techniques have typically not been brought to bear in exploration for new fluorspar deposits, although such techniques are likely to be used in future exploration. The tendency for fluorine to dissolve in significant concentrations in water at low temperature allows both surface water and groundwater to be used as sampling media in geochemical exploration. Evolved granite-related fluorspar deposits may be particularly susceptible to geophysical exploration methods because crystalline rocks that form a basement to sedimentary sections can be approximately defined with gravity and magnetic methods, and magnetite-bearing skarns can be directly detected with magnetic surveys. Environmental considerations of fluorine mining focus especially on drinking water, where high fluorine concentrations can lead to tooth decay; dental and skeletal fluorosis; and bone and cartilage conditions, including genu valgum, which is the crippling bone deformity more commonly known as knock knee. Trace amounts of other elements in fluorspar ores are a concern at some deposits; for example, high beryllium concentrations in alkaline-volcanic-related epithermal deposits or high cadmium concentrations associated with Mississippi Valley-type and salt-related carbonate-hosted fluorspar deposits. Future research might include testing whether fluorine can be extracted economically from high-pH, sodium-carbonate brines and exploring for new occurrences of apparently conformable fluorspar deposits in tuffaceous limy lacustrine sediments outside of the Latium Region of Italy. Other promising new areas of research could be studies of fluorspar deposit fluid inclusion compositions by quadrupole mass spectrometry, by noble gas mass spectrometry on irradiated fluid inclusions, or by chlorine isotopes, while also measuring the chemistry of the same fluid inclusions either by bulk crush-and-leach methods or by laser ablation-inductively coupled plasma mass spectrometry. Advanced studies of fluid inclusion chemistry could be applied beneficially to some of the enigmatic large epithermal fluorspar veins at various places in the world, where they might determine those deposits’ possible relationships to igneous intrusions, or to dissolved salt, or to heated meteoric water in volcanic sections, or perhaps to all three. This knowledge could help focus new exploration.
Exploration in the central Tethys region of Turkey, Armenia, Azerbaijan, Georgia, Iran, and weste rn Pakistan has led to the identification of the giant Reko Diq (24 Mt Cu and 1300 t Au), Sar Cheshmeh (8.9 Mt Cu and 0.46 Mt Mo), Sungun (5.1 Mt Cu and 0.20 Mt Mo), and Kadjaran (4.6 Mt Cu, 0.94 Mt Mo, and 1100 t Au), and 10 other large (1–2 Mt Cu) porphyry deposits including Saindak, Cevizlidere, Teghout, Meiduk, and Halilağa. Continued exploration efforts have also resulted in the development of porphyry-related gold deposits such as Kişladağ (9.6 Moz Au), Çöpler (3.7 Moz Au), Aği Daği (1.7 Moz Au), and Sary Gunay (3.0 Moz Au), and in the generation of several other promising exploration projects. The distribution in space and time of porphyry deposits in the central Tethys region was shaped by complex pre- to post-mineral tectonic, igneous, collisional, uplift and burial events. These events are represented by a partially-overlapping and variably exhumed and covered collage of twenty-six Early Jurassic to Holocene magmatic belts permissive for the occurrence of porphyry deposits (porphyry tracts and sub-tracts). Twelve tracts or sub-tracts are characterized by compressional continental arcs that formed on drifting terranes or continental margins, 10 developed in compressional to extensional intra-oceanic arc and backarc-rift settings, and 4 formed in extensional post-collisional environments over amalgamated terranes. Eight of these belts were variably affected by coeval and younger metamorphic, fold-and-thrust, and extensional faulting events. Fifty-four porphyry Au-(Cu), Cu-Au, Cu-Mo, Mo-Cu deposits, 15 porphyry-related Au, Au-(Mo) and W-(Mo-Au) deposits, 239 porphyry prospects, and 68 other porphyry-related mineral sites were identified in the study region. Of the 376 porphyry and porphyry-related sites, about 11% formed in island arc, 42% in continental arc, 20% in backarc, and 27% in post-collisional settings. Of the 69 porphyry and porphyry-related deposits, 7% developed in intra-oceanic arc, 41% in continental arc, 27% in backarc, and 25% in post-collisional settings. The largest occur in either compressional continental arc (18 deposits including the Reko Diq and Sar Cheshmeh giants) or post-collisional (13 deposits including the Kadjaran and Sungun giants) environments. Ninety percent of the largest porphyry or porphyry-related deposits occur in only 9 of the 26 permissive porphyry tracts or sub-tracts. Moreover, 88, 90, and 77% of the identified Cu, Mo, and Au resources are contained in porphyry deposits that occur in only 4 of these 9 tracts. Of these 4 tracts, 3 outline arc settings, and one delimits a post-collisional environment. The compositional diversity of porphyry intrusions in these tectono-magmatic environments generally varies from island arc settings with the most restricted range (partly alkaline but mainly calc-alkaline dioritic to granodioritic-tonalitic), to continental arc (calc-alkaline dioritic-quartz dioritic, granodioritic, quartz monzonitic-granitic, and less commonly mildly alkaline), to backarc (mildly alkaline and calc-alkaline dioritic to granitic), to post-collisional settings with the most expansive range (alkaline and calc-alkaline mafic to felsic, and weakly peraluminous). Metal associations also vary broadly as a function of porphyry intrusion composition from weakly peraluminous to metaluminous felsic (Mo[±W ± Cu]; <2% of porphyry-related systems [i.e., Tyrnyauz]), to metaluminous felsic and intermediate (Cu-Mo[±Au]; 85% [i.e., Cevizlidere, Haft Cheshmeh, Kahang, Sar Cheshmeh, Sungun, Teghout, Reko Diq, Saindak]), to mildly alkaline felsic and intermediate (Cu-Au[±Mo] [i.e., Agarak, Kadjaran, Kale Kafi]) and mafic (Au-Cu; 12% [i.e., Çöpler]), and to alkaline felsic (Au-Mo; 1% of porphyry-related systems [i.e., Kişladağ). Tectonic changes were critical in triggering the formation of large porphyry deposits in the region. Large porphyry deposits were preferentially emplaced in continental arc settings shortly before major collisional events (Dar Alu, Kahang, Meiduk, Now Chun, and the giant Sar Cheshmeh and Reko Diq deposits), or in post-subduction environments shortly after collision (Bakirçay, Güzelyayla, Haft Cheshmeh, Masjed Daghi, and the giant Kadjaran and Sungun deposits) or during periods of prominent extension (Aği Daği, Halilağa, Kişladağ, Sari Gunay, and Zarshuran porphyry-related deposits). Collision-induced uplift, erosion, and removal of coeval volcanic rocks favorably exposed the hypabyssal level of subduction-related porphyry deposits. Extensional structures that developed parallel and orthogonal to the compressional principal stress component along transtensional or transpressional strike-slip faults or in pull-apart basins commonly controlled porphyry-related deposits in post-collisional settings. The latter deposits typically exhibit shallow epithermal levels of emplacement because of preservation by burial. Seventeen porphyry deposits and one porphyry-related deposit in the study region are reported to contain significant supergene resources. Relatively mature levels of secondary copper enrichment in dominantly granodioritic to granitic porphyry deposits occur in areas where large pyrite-rich quartz-sericite alteration zones have been preserved and exposed to surface oxidation (Güzelyayla and Ulutaş in northeastern Turkey; Agarak, Ankavan, Dastakert, Kadjaran, and Teghout in Armenia; Ali Javad in northern Iran; Kale Kafi in central Iran; Darreh Zar, Meiduk, Now Chun, and Sar Cheshmeh in southeastern Iran; and Tanjeel in southwestern Pakistan). Chalcocite blankets also developed over porphyry deposits in regions where significant post-mineral faulting has occurred (Muratdere and Sarıçayıryayla in western Turkey). Normal faulting also enhanced secondary enrichment of gold in the Halilağa porphyry and Sary Gunay porphyry-related deposits located respectively in western Turkey and northern Iran. Evaluation of provincial as well as local controls strongly suggests that continued exploration in the region will lead to the identification of additional porphyry and porphyry-related deposits. These deposits will likely be found under younger cover formations in porphyry belts that are already known, and in association with superjacent high- and intermediate-sulfidation epithermal deposits, or increasingly peripheral skarn, carbonate-replacement, and sediment-hosted deposits. Application of suitable exploration techniques to detect concealed and/or deformed deposits in porphyry belts that remain under-explored may also prove productive.
Advancements in aeromagnetic acquisition technology over the past few decades have led to greater resolution of shallow geologic sources with low magnetization, such as intrasedimentary faults and paleochannels. Detection and mapping of intrasedimentary faults in particular can be important for understanding the overall structural setting of an area, even if exploration targets are much deeper. Aeromagnetic methods are especially useful for mapping structures in mountain-piedmont areas at the margins of structural basins, where mineral exploration and seismic-hazard studies may be focused, and where logistical or data-quality issues encumber seismic methods. Understanding if the sources of aeromagnetic anomalies in this context originate from sedimentary units or bedrock is important for evaluating basin structure and/or depth to shallow exploration targets. Advancements in aeromagnetic acquisition technology over the past few decades have led to greater resolution of shallow geologic sources with low magnetization, such as intrasedimentary faults and paleochannels. Detection and mapping of intrasedimentary faults in particular can be important for understanding the overall structural setting of an area, even if exploration targets are much deeper. Aeromagnetic methods are especially useful for mapping structures in mountain-piedmont areas at the margins of structural basins, where mineral exploration and seismic-hazard studies may be focused, and where logistical or data-quality issues encumber seismic methods. Understanding if the sources of aeromagnetic anomalies in this context originate from sedimentary units or bedrock is important for evaluating basin structure and/or depth to shallow exploration targets. Although explorationists have surmised that subtle, narrow, linear aeromagnetic anomalies or gradients are caused by intrasedimentary faults, the nature of the magnetic sources has been debated. A common and intuitive explanation for the linear anomalies considers that the magnetic properties of the fault zone have been altered by secondary chemical processes, either through the growth or destruction of magnetic minerals. However, comprehensive, multidisciplinary studies of partially exposed intrasedimentary faults in basins within the central Rio Grande Rift, New Mexico, have shown that the anomalies can be completely explained by the tectonic juxtaposition of strata of differing magnetic properties at the fault (summarized in Grauch and Hudson, 2007 , 2011 ). Whereas a reduction in magnetic susceptibility was detected at some fault zones in the laboratory, the slight reduction and small volume of material affected were insufficient to produce aeromagnetic anomalies ( Hudson et al., 2008 ). A key finding of the studies summarized by Grauch and Hudson (2007 , 2011 ) is that multiple magnetic contrasts (sources) can be vertically stacked at one fault. This situation requires rethinking of common assumptions when modeling faults as simple steps or when interpreting depth-estimation results. The multiple contrasts in these case studies arise from the tectonic juxtaposition of stratified sediments with differing magnetic properties. Multiple, vertically stacked magnetic sources also can occur where volcanic layers are interbedded with the sedimentary section or where faults offset both shallow bedrock and its overlying sedimentary cover. The latter situation is common at basin margins. Herein, we summarize and expand on an investigation of the sources of aeromagnetic anomalies related to faults along the eastern margin of the San Luis Basin, northern Rio Grande Rift, Colorado ( Grauch et al., 2010 ). Similar to the faults examined in the central Rio Grande Rift, magnetic sources can be completely explained by tectonic juxtaposition and produce multiple, vertically stacked magnetic contrasts at individual faults. However, the geologic sources are different. They arise from both the sedimentary cover and the underlying bedrock rather than from stratified sediments. In addition, geologic evidence for secondary growth or destruction of magnetic minerals at the fault zone is lacking.
The weights-of-evidence analysis, a quantitative mineral resource mapping tool, is used to delineate favorable areas for epithermal gold deposits and to predict future exploration activity of the mineral industry for similar deposits in a four-county area (222 x 277 km), including the Okanogan and Colville National Forests of northeastern Washington. Modeling is applied in six steps: (1) building a spatial digital database, (2) extracting predictive evidence for a particular deposit, based on an exploration model, (3) calculating relative weights for each predictive map, (4) combining the geologic evidence maps to predict the location of undiscovered mineral resources and (5) measuring the intensity of recent exploration activity by use of mining claims on federal lands, and (6) combining mineral resource and exploration activity into an assessment model of future mining activity. The analysis is accomplished on a personal computer using ArcView GIS platform with Spatial Analyst and Weights-of-Evidence software. In accord with the descriptive model for epithermal gold deposits, digital geologic evidential themes assembled include lithologic map units, thrust faults, normal faults, and igneous dikes. Similarly, geochemical evidential themes include placer gold deposits and gold and silver analyses from stream sediment (silt) samples from National Forest lands. Fifty mines, prospects, or occurrences of epithermal gold deposits, the training set, define the appropriate a really-associated terrane. The areal (or spatial) correlation of each evidential theme with the training set yield predictor theme maps for lithology, placer sites and normal faults. The weights-of-evidence analysis disqualified the thrust fault, dike, and gold and silver silt analyses evidential themes because they lacked spatial correlation with the training set. The decision to accept or reject evidential themes as predictors is assisted by considering probabilistic data consisting of weights and contrast values calculated for themes according to areal correlation with the training sites. Predictor themes having acceptable weights and contrast values are combined into a preliminary model to predict the locations of undiscovered epithermal gold deposits. This model facilitates ranking of tracts as non-permissive, permissive or favorable categories based on exclusionary, passive, and active criteria through evaluation of probabilistic data provided by interaction of predictor themes. The method is very similar to the visual inspection method of drawing conclusions from anomalies on a manually overlain system of maps. This method serves as a model for future mineral assessment procedures because of its objective nature. To develop a model to predict future exploration activity, the locations of lode mining claims were summarized for 1980, 1985, 1990, and 1996. Land parcels containing historic claims were identified either as those with mining claims present in 1980 or valid claims present in 1985. Current claim parcels were identified as those containing valid lode claims in either 1990 or 1996. A consistent parcel contains both historic and current claims. The epithermal gold and mining claim activity models were combined into an assessment (or mineral resource-activity) model to assist in land use decisions by providing a prediction of mineral exploration activity on federal land in the next decade. Ranks in the assessment model are: (1) no activity, (2) low activity, (3) low to moderate activity, (4) moderate activity and (5) high activity.