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C.J. Nutt

Publications and source records attributed to C.J. Nutt.

18 recordsLinked to original sources
USGS2007

Bald Mountain gold mining district, Nevada: A Jurassic reduced intrusion-related gold system

The Bald Mountain mining district has produced about 2 million ounces (Moz) of An. Geologic mapping, field relationships, geochemical data, petrographic observations, fluid inclusion characteristics, and Pb, S, O, and H isotope data indicate that An mineralization was associated with a reduced Jurassic intrusion. Gold deposits are localized within and surrounding a Jurassic (159 Ma) quartz monzonite porphyry pluton and dike complex that intrudes Cambrian to Mississippian carbonate and clastic rocks. The pluton, associated dikes, and An mineralization were controlled by a crustal-scale northwest-trending structure named the Bida trend. Gold deposits are localized by fracture networks in the pluton and the contact metamorphic aureole, dike margins, high-angle faults, and certain strata or shale-limestone contacts in sedimentary rocks. Gold mineralization was accompanied by silicification and phyllic alteration, ??argillic alteration at shallow levels. Although An is typically present throughout, the system exhibits a classic concentric geochemical zonation pattern with Mo, W, Bi, and Cu near the center, Ag, Pb, and Zn at intermediate distances, and As and Sb peripheral to the intrusion. Near the center of the system, micron-sized native An occurs with base metal sulfides and sulfosalts. In peripheral deposits and in later stages of mineralization, Au is typically submicron in size and resides in pyrite or arsenopyrite. Electron microprobe and laser ablation ICP-MS analyses show that arsenopyrite, pyrite, and Bi sulfide minerals contain 10s to 1,000s of ppm Au. Ore-forming fluids were aqueous and carbonic at deep levels and episodically hypersaline at shallow levels due to boiling. The isotopic compositions of H and O in quartz and sericite and S and Pb in sulfides are indicative of magmatic ore fluids with sedimentary sulfur. Together, the evidence suggests that Au was introduced by reduced S-bearing magmatic fluids derived from a reduced intrusion. The reduced character of the intrusion was caused by assimilation of carbonaceous sedimentary rocks. Tertiary faults dismember the area and drop down the upper part of the mineralizing system to the west. The abundant and widespread kaolinite in oxide ores is relatively disordered (1A polytype) and has ??D and ??18O values suggestive of a supergene origin. The deep weathering and oxidation of the ores associated with exhumation made them amenable to open-pit mining and processing using cyanide heap leach methods. ?? 2007 Society of Economic Geologists, Inc.

Economic Geology↗
USGS2003

Alligator ridge district, East-Central Nevada: Carlin-type gold mineralization at shallow depths

Carlin - type deposits in the Alligator Ridge mining district are present sporadically for 40 km along the north-striking Mooney Basin fault system but are restricted to a 250-m interval of Devonian to Mississippian strata. Their age is bracketed between silicified ca. 45 Ma sedimentary rocks and unaltered 36.5 to 34 Ma volcanic rocks. The silicification is linked to the deposits by its continuity with ore-grade silicification in Devonian-Mississippian strata and by its similar δ 18 O values (_e1∼17‰) and trace element signature (As, Sb, Tl, Hg). Eocene reconstruction indicates that the deposits formed at depths of ≤300 to 800 m. In comparison to most Carlin - type gold deposits, they have lower Au/Ag, Au grades, and contained Au, more abundant jasperoid, and textural evidence from deposition of an amorphous silica precursor in jasperoid. These differences most likely result from their shallow depth of formation. The peak fluid temperature (_e1∼230°C) and large δ 18 O H2O value shift from the meteroric water line (_e1∼20‰) suggest that ore fluids were derived from depths of 8 km or more. A magnetotelluric survey indicates that the Mooney Basin fault system penetrates to mid-crustal depths . Deep circulation of meteoric water along the Mooney Basin fault system may have been in response to initial uplift of the East Humboldt-Ruby Mountains metamorphic core complex; convection also may have been promoted by increased heat flow associated with large magnitude extension in the core complex and regional magmatism. Ore fluids ascended along the fault system until they encountered impermeable Devonian and Mississippian shales, at which point they moved laterally through permeable strata in the Devonian Guilmette Formation, Devonian-Mississippian Pilot Shale, Mississippian Joana Limestone, and Mississippian Chainman Shale toward erosional windows where they ascended into Eocene fluvial conglomerates and lake sediments. Most gold precipitated by sulfidation of host-rock Fe and mixing with local ground water in zones of lateral fluid flow in reactive strata, such as the Lower Devonian-Mississippian Pilot Shale.

Economic Geology↗
USGS2003

Intrusive rock database for the Digital Geologic Map of Utah

Digital geologic maps offer the promise of rapid and powerful answers to geologic questions using Geographic Information System software (GIS). Using modern GIS and database methods, a specialized derivative map can be easily prepared. An important limitation can be shortcomings in the information provided in the database associated with the digital map, a database which is often based on the legend of the original map. The purpose of this report is to show how the compilation of additional information can, when prepared as a database that can be used with the digital map, be used to create some types of derivative maps that are not possible with the original digital map and database. This Open-file Report consists of computer files with information about intrusive rocks in Utah that can be linked to the Digital Geologic Map of Utah (Hintze et al., 2000), an explanation of how to link the databases and map, and a list of references for the databases. The digital map, which represents the 1:500,000-scale Geologic Map of Utah (Hintze, 1980), can be obtained from the Utah Geological Survey (Map 179DM). Each polygon in the map has a unique identification number. We selected the polygons identified on the geologic map as intrusive rock, and constructed a database (UT_PLUT.xls) that classifies the polygons into plutonic map units (see tables). These plutonic map units are the key information that is used to relate the compiled information to the polygons on the map. The map includes a few polygons that were coded as intrusive on the state map but are largely volcanic rock; in these cases we note the volcanic rock names (rhyolite and latite) as used in the original sources Some polygons identified on the digital state map as intrusive rock were misidentified; these polygons are noted in a separate table of the database, along with some information about their true character. Fields may be empty because of lack of information from references used or difficulty in finding information. The information in the database is from a variety of sources, including geologic maps at scales ranging from 1:500,000 to 1:24,000, and thesis monographs. The references are shown twice: alphabetically and by region. The digital geologic map of Utah (Hintze and others, 2000) classifies intrusive rocks into only 3 categories, distinguished by age. They are: Ti, Tertiary intrusive rock; Ji, Upper to Middle Jurassic granite to quartz monzonite; and pCi, Early Proterozoic to Late Archean intrusive rock. Use of the tables provided in this report will permit selection and classification of those rocks by lithology and age. This database is a pilot study by the Survey and Analysis Project of the U.S. Geological Survey to characterize igneous rocks and link them to a digital map. The database, and others like it, will evolve as the project continues and other states are completed. We release this version now as an example, as a reference, and for those interested in Utah plutonic rocks.

Utah↗
USGS2002

Geologic investigations in the Lake Valley area, Sierra County, New Mexico

At the request of the Bureau of Land Management, the U.S. Geological Survey evaluated the area of the historic Lake Valley mining district and townsite, Sierra County, New Mexico, for its potential for undiscovered mineral resources. The four chapters of this report describe the geology of the area, present the results of geophysical investigations carried out to aid in interpreting subsurface geology, describe the mining history and character of the region's ore deposits, and present geochemical data on potential for contamination from abandoned mine dumps in the mining district.

New Mexico↗
USGS1998

Geology and mineral resources of the Lake Valley area, Sierra County, New Mexico

At the request of the Bureau of Land Management, the U.S. Geological Survey assessed the Lake Valley Area of Critical Environmental Concern (ACEC), which includes the historic Lake Valley townsite and silver-manganese mining district, for undiscovered mineral resources. The Lake Valley ACEC is along the southeastern margin of the Black Range of western Sierra County, New Mexico. The Black Range contains eleven mining districts from which silver, lead, zinc, manganese, copper, gold, and tin have been recovered. As part of the study, an area of over 75 square mi (195 square km) in the area of the Lake Valley ACEC was mapped to understand ore controls and to assess mineral-resource potential. Lake Valley mining district is in Mississippian Lake Valley Formation carbonate rocks, but much of the surrounding terrane consists of volcanic rocks on the southeastern edge of the Emory cauldron which formed about 34.9 Ma. Volcanic rocks are part of the Mogollon-Datil volcanic province which include flows, breccias, ash-flow tuffs, and intrusive rhyolites. The Lake Valley mining district is located about 20 mi (32 km) south of the Late Cretaceous Laramide copper-gold porphyry intrusion at Hillsboro. It is also located at the western boundary of the Rio Grande rift basin.The Lake Valey fault is the major structural feature of the study area. Geological and geophysical data suggest the fault is composed of two segments: a southern, northwest-striking segment that may have a pre-Tertiary history and a northerly-striking segment that is part of the Emory cauldron ring fracture. The mining district is bounded by the southern, northwest-striking segment of the fault which may have up to 800 ft (240m) of normal offset. Deposit types identified in the Black Range, and for which we assess mineral potential, are Laramide porphyry, Laramide skarns, Laramide veins, gold placer, carbonate-hosted, volcanic-epithermal, and rhyolite tin; no Rio Grande Rift baritefluorite-galena deposits are known, but they are included in the assessment. The most likely deposits to be discovered in the Lake Valley district are carbonate-hosted silver-manganese deposits. These deposits could be deposited by hydrothermal fluids or be related to intrusion of Oligocene rhyolite or Laramide porphyry. An aeromagnetic high south of the district probably reflects a small felsic intrusion of unknown age. We assess the Lake Valley ACEC as having low to moderate potential for undiscovered carbonate-hosted deposits associated with volcanic rocks or Laramide porphyry plutons, and low potential for gold placer and rhyolite tin deposits.

New Mexico↗
USGS1987

Age of uranium mineralization at the Jabiluka and Ranger deposits, Northern Territory, Australia: New U-Pb isotope evidence

The Ranger and Jabiluka uranium deposits are the largest in the Alligator Rivers uranium field, which contains at least 20 percent of the world's low-cost uranium reserves. Ore occurs in early Proterozoic metasediments, below an unconformity with sandstones of the 1.65-b.y.-old Kombolgie Formation. This study has used U-Pb isotope data from a large number of whole-rock drill core samples with a variety of mineral assemblages and textures. Ranger samples indicate a well-defined age of 1,737 + or - 20 m.y., consistent with earlier suggestions of the antiquity of this deposit. This age is distinctly pre-Kombolgie, so the Ranger deposit cannot have been formed by processes requiring its presence. The age is consistent, however, with mineralization related to heating associated with either the emplacement of early postmetamorphic granites, or possibly, with intrusion of the nearby Oenpelli Dolerite. In contrast, data for the least-altered Jabiluka ores yield a concordia intercept age of 1,437 + or - 40 m.y.--significantly younger than the Ranger age and also younger than the Kombolgie Formation. This age may correspond to a regional thermal event, as indicated both by mafic dikes of roughly this age and a zircon lower intercept age from a nearby granite-gneiss. Thus our data indicate that there must be at least three distinct periods of major U mineralization in the Alligator Rivers uranium field (including the previously determined approximately 900-m.y. age of the Nabarlek deposit), despite the fact that all of the deposits share strong similarities in their host rock, mineralogy, alteration, and geologic setting.Both Ranger and Jabiluka reflect a common, profound isotopic disturbance at about 400 to 600 m.y. This disturbance, which was especially pronounced at Jabiluka, may correspond to the development of basins and associated basalt flows to the west and southwest, as suggested by Crick et al. (1980).

Economic Geology↗