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M.F. Coolbaugh

Publications and source records attributed to M.F. Coolbaugh.

8 recordsLinked to original sources
USGS2007

Assessment of exploration bias in data-driven predictive models and the estimation of undiscovered resources

The spatial distribution of discovered resources may not fully mimic the distribution of all such resources, discovered and undiscovered, because the process of discovery is biased by accessibility factors (e.g., outcrops, roads, and lakes) and by exploration criteria. In data-driven predictive models, the use of training sites (resource occurrences) biased by exploration criteria and accessibility does not necessarily translate to a biased predictive map. However, problems occur when evidence layers correlate with these same exploration factors. These biases then can produce a data-driven model that predicts known occurrences well, but poorly predicts undiscovered resources. Statistical assessment of correlation between evidence layers and map-based exploration factors is difficult because it is difficult to quantify the "degree of exploration." However, if such a degree-of-exploration map can be produced, the benefits can be enormous. Not only does it become possible to assess this correlation, but it becomes possible to predict undiscovered, instead of discovered, resources. Using geothermal systems in Nevada, USA, as an example, a degree-of-exploration model is created, which then is resolved into purely explored and unexplored equivalents, each occurring within coextensive study areas. A weights-of-evidence (WofE) model is built first without regard to the degree of exploration, and then a revised WofE model is calculated for the "explored fraction" only. Differences in the weights between the two models provide a correlation measure between the evidence and the degree of exploration. The data used to build the geothermal evidence layers are perceived to be independent of degree of exploration. Nevertheless, the evidence layers correlate with exploration because exploration has preferred the same favorable areas identified by the evidence patterns. In this circumstance, however, the weights for the "explored" WofE model minimize this bias. Using these revised weights, posterior probability is extrapolated into unexplored areas to estimate undiscovered deposits. ?? International Association for Mathematical Geology 2007.

Natural Resources Research↗
USGS2006

Prediction and discovery of new geothermal resources in the Great Basin: Multiple evidence of a large undiscovered resource base

Geothermal potential maps by themselves cannot directly be used to estimate undiscovered resources. To address the undiscovered resource base in the Great Basin, a new and relatively quantitative methodology is presented. The methodology involves three steps, the first being the construction of a data-driven probabilistic model of the location of known geothermal systems using weights of evidence. The second step is the construction of a degree-of-exploration model. This degree-of-exploration model uses expert judgment in a fuzzy logic context to estimate how well each spot in the state has been explored, using as constraints digital maps of the depth to the water table, presence of the carbonate aquifer, and the location, depth, and type of drill-holes. Finally, the exploration model and the data-driven occurrence model are combined together quantitatively using area-weighted modifications to the weights-of-evidence equations. Using this methodology in the state of Nevada, the number of undiscovered geothermal systems with reservoir temperatures ???100??C is estimated at 157, which is 3.2 times greater than the 69 known systems. Currently, nine of the 69 known systems are producing electricity. If it is conservatively assumed that an additional nine for a total of 18 of the known systems will eventually produce electricity, then the model predicts 59 known and undiscovered geothermal systems are capable of producing electricity under current economic conditions in the state, a figure that is more than six times higher than the current number. Many additional geothermal systems could potentially become economic under improved economic conditions or with improved methods of reservoir stimulation (Enhanced Geothermal Systems).This large predicted geothermal resource base appears corroborated by recent grass-roots geothermal discoveries in the state of Nevada. At least two and possibly three newly recognized geothermal systems with estimated reservoir temperatures ???150??C have been identified on the Pyramid Lake Paiute Reservation in west-central Nevada. Evidence of three blind geothermal systems has recently been uncovered near the borate-bearing playas at Rhodes, Teels, and Columbus Marshes in southwestern Nevada. Recent gold exploration drilling has resulted in at least four new geothermal discoveries, including the McGinness Hills geothermal system with an estimated reservoir temperature of roughly 200??C. All of this evidence suggests that the potential for expansion of geothermal power production in Nevada is significant.

Conference Paper↗
USGS2003

Geothermal GIS coverage of the Great Basin, USA: Defining regional controls and favorable exploration terrains

A geographic information system (GIS) of geothermal resources, built last year for the state of Nevada, is being expanded to cover the Great Basin, USA. Data from that GIS is being made available to industry, other researchers, and the public via a web site at the Great Basin Center for Geothermal Energy, Reno, Nevada. That web site features a search engine, supports ArcExplorer?? for on-line map construction, and provides downloadable data layers in several formats. Though data collection continues, preliminary analysis has begun. Contour maps of geothermal temperatures, constructed using geothermometer temperatures calculated from a Great Basin geochemical database compiled by the Geo-Heat Center, reveal distinctive trends and patterns. As expected, magmatic-type and extensional-type geothermal systems have profoundly different associations, with magmatic-type systems following major tectonic boundaries, and extensional-type systems associating with regionally high heat flow, thin crust, active faulting, and high extensional strain rates. As described by earlier researchers, including Rowen and Wetlaufer (1981) and Koenig and McNitt (1983), high-temperature (> 100??C) geothermal systems appear to follow regional northeast trends, most conspicuously including the Humboldt structural zone in Nevada, the "Black Rock-Alvord Desert" trend in Oregon and Nevada, and the "Newcastle-Roosevelt" trend in Utah and Nevada. Weights-of-evidence analyses confirm a preference of high-temperature geothermal systems for young northeast-trending faults, but the distribution of geothermal systems correlates even better with high rates of crustal extension, as measured from global positioning system (GPS) stations in Nevada. A predictive map of geothermal potential based only on areas of high extensional strain rates and high heat flux does an excellent job of regionally predicting the location of most known geothermal systems in Nevada, and may prove useful in identifying blind systems.

Conference Paper↗
USGS2003

Atmospheric mercury emissions from mine wastes and surrounding geologically enriched terrains

Waste rock and ore associated with Hg, precious and base metal mining, and their surrounding host rocks are typically enriched in mercury relative to natural background concentrations (<0.1 ??g Hg g-1). Mercury fluxes to the atmosphere from mineralized areas can range from background rates (0-15 ng m-2 h-1) to tens of thousands of ng m-2 h-1. Mercury enriched substrate constitutes a long-term source of mercury to the global atmospheric mercury pool. Mercury emissions from substrate are influenced by light, temperature, precipitation, and substrate mercury concentration, and occur during the day and night. Light-enhanced emissions are driven by two processes: desorption of elemental mercury accumulated at the soil:air interface, and photo reduction of mercury containing phases. To determine the need for and effectiveness of regulatory controls on short-lived anthropogenic point sources the contribution of mercury from geologic non-point sources to the atmospheric mercury pool needs to be quantified. The atmospheric mercury contribution from small areas of mining disturbance with relatively high mercury concentrations are, in general, less than that from surrounding large areas of low levels of mercury enrichment. In the arid to semi-arid west-ern United States volatilization is the primary means by which mercury is released from enriched sites.

Conference Paper↗
USGS2002

Annual emissions of mercury to the atmosphere from natural sources in Nevada and California

The impact of natural source emissions on atmospheric mercury concentrations and the biogeochemical cycle of mercury is not known. To begin to assess this impact, mercury emissions to the atmosphere were scaled up for three areas naturally enriched in mercury: the Steamboat Springs geothermal area, Nevada, the New Idria mercury mining district, California, and the Medicine Lake volcano, California. Data used to scale up area emissions included mercury fluxes, measured in-situ using field flux chambers, from undisturbed and disturbed geologic substrates, and relationships between mercury emissions and geologic rock types, soil mercury concentrations, and surface heat flux. At select locations mercury fluxes were measured for 24 h and the data were used to adjust fluxes measured at different times of the day to give an average daily flux. This adjustment minimized daily temporal variability, which is observed for mercury flux because of light and temperature effects. Area emissions were scaled spatially and temporally with GIS software. Measured fluxes ranged from 0.3 to approximately 50 ng m-2 h-1 at undisturbed sites devoid of mercury mineralization, and to greater than 10,000 ng m-2 h-1 from substrates that were in areas of mercury mining. Area-averaged fluxes calculated for bare soil at Steamboat Springs, New Idria, and Medicine Lake of 181, 9.2, and 2 ng m-2 h-1, respectively, are greater than fluxes previously ascribed to natural non-point sources, indicating that these sources may be more significant contributors of mercury to the atmosphere than previously realized.

Environmental Geology↗
USGS2002

A Geothermal GIS for Nevada: Defining Regional Controls and Favorable Exploration Terrains for Extensional Geothermal Systems

Spatial analysis with a GIS was used to evaluate geothermal systems in Nevada using digital maps of geology, heat flow, young faults, young volcanism, depth to groundwater, groundwater geochemistry, earthquakes, and gravity. High-temperature (>160??C) extensional geothermal systems are preferentially associated with northeast-striking late Pleistocene and younger faults, caused by crustal extension, which in most of Nevada is currently oriented northwesterly (as measured by GPS). The distribution of sparse young (<1.5Ma) basaltic vents also correlate with geothermal systems, possibly because the vents help identify which young structures penetrate deeply into the crust. As expected, elevated concentrations of boron and lithium in groundwater were found to be favorable indicators of geothermal activity. Known high-temperature (>160??C) geothermal systems in Nevada are more likely to occur in areas where the groundwater table is shallow (<30m). Undiscovered geothermal systems may occur where groundwater levels are deeper and hot springs do not issue at the surface. A logistic regression exploration model was developed for geothermal systems, using young faults, young volcanics, positive gravity anomalies, and earthquakes to predict areas where deeper groundwater tables are most likely to conceal geothermal systems.

Conference Paper↗
USGS1994

Geology and geochemistry of Summitville, Colorado: An epithermal acid sulfate deposit in a volcanic dome

Geologic studies during recent open-pit mining at Summitville, Colorado, have provided new information on an epithermal acid sulfate Au-Ag-Cu deposit formed in a volcanic dome. Geologic mapping, geochemical studies of whole-rock samples from blast holes, and geologic and geochemical traverse studies refine the details of the evolution of the Summitville deposit. Six distinct events followed emplacement of the quartz latite volcanic dome and define the development of the Summitville deposit: (1) an early stage of acid sulfate alteration, (2) subsequent Cu sulfide and gold mineralization, (3) widespread hydrothermal brecciation, (4) volumetrically minor, base metal sulfide-bearing barite veining, (5) volumetrically minor, kaolinite matrix brecciation, and finally, (6) supergene oxidation. Events 1 and 2 were the most important for the formation of the Summitville deposit. Acid sulfate altered rock is primarily found in the quartz latite volcanic dome rock and consists of vuggy silica (central), quartz-alunite, quartz-kaolinite, argillic, and propylitic (distal) zones. Ore deposition (event 2) of enargite, luzonite, covellite, pyrite, native sulfur, marcasite, and minor sphalerite, native gold, + or - barite and galena overprinted the acid sulfate altered rocks. Events 3, 4, and 5 are geologically important but rarely formed units large enough to be considered mineable ore by open-pit mining. The final supergene oxidation event affected much of the near-surface portion of the deposit and leached copper and sulfide minerals forming oxide zones. Near-surface oxide zones contain the highest Au grades and decrease progressively with depth.Fractures were important fbr localizing both the acid sulfate altered rock and ore deposition at Summitville. Such fracture control is apparent despite a lack of distinct throughgoing open-space quartz veins; instead, laterally and vertically extensive ore zones are identified by their highly silicified character that can be traced laterally for up to 500 m and vertically up to 200 m. Ore zones strike parallel to regional faults related to rifting and basin and range development, suggesting that hydrothermal fluids followed preexisting fractures. Ore zones follow two northwest trends, a N 30 degrees W + or - 20 degrees trend, and a N 60 degrees W trend. Most fractures and faults in the open pit dip steeply from 65 degrees to vertical. Ore zones intersect near the center of the deposit to form a large zone approximately 150 by 400 m. Ore zones form a rough radial pattern located near the center of the deposit and a discontinuous arcuate feature on the northern side. This radial pattern, arcuate feature, and the intersection of ore zones near the center of the deposit are probably related to the intrusion of a porphyry below the deposit. The overall low Au-grade, high-tonnage character of the Summitville deposit is related to low permeability in the quartz latite volcanic dome rock and absence of well-developed open-space hydrothermal veins.Gold analyses of development drill holes and blast hole samples, in conjunction with geologic mapping in the open pit, indicate that vuggy silica, quartz-alunite, and quartz-kaolinite altered rock constitute most of the Au ore. Vuggy silica averaged 2.4 ppm Au, quartz-alunite averaged 1.2 ppm Au, and quartz-kaolinite averaged 0.86 ppm Au during open-pit mining. Grades for argillically altered rock were usually below the ore cutoff of 0.34 ppm Au. Propylitically altered rock rarely contained detectable Au concentrations (0.03 ppm Au) and constituted only waste rock. Although volumetrically minor, event 4 base metal sulfide-bearing barite veins and event 5 kaolinite matrix breccias contained high Au grades (up to about 800 ppm Au), producing approximately 5 to 15 percent of the total Au mined in the deposit.Geochemical studies of samples collected across traverses of altered and mineralized ore zones indicate that Te, Bi, and Pb are characteristic trace elements, in addition to Au, Ag, Cu, and As that are typical of acid sulfate mineral deposits. Anomalous Te is associated with Cu sulfides and gold in vuggy silica zones. Anomalous Bi and Pb concentrations correlate most consistently with quartz-alunite zones. Thallium is also anomalous at Summitville. Highest concentrations of Tl are found in argillic rock, whereas silicified zones are depleted in it. The anomalous Tl in distal zones is similar to observations in some porphyry Cu systems. Pronounced lateral zonation of Ag/Au ratios is present in the deposit. The Ag/Au ratios vary from about 2:1 near the center of the orebody to greater than 20:1 on the periphery.The Summitville deposit is located in the midlevel portion of a mineralized volcanic dome, with a porphyry system at depth, and a hot springs environment at the surface. Drill holes through the volcanic dome intersected a quartz monzonite porphyry approximately 600 m below the surface that is altered to sericite and pyrite similar to some porphyry deposits. Laterally extensive cristobalite and opal replacing quartz latite are found southwest of the deposit and represent a hot-spring environment. Bedded siliceous sinter, some of which contains plant debris, overlies the cristobalite and opal at some localities. These deposits may represent surface vents for the fluids responsible for acid sulfate alteration or mineralization of the volcanic dome.

Economic Geology↗