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J. Thomas Nash

Publications and source records attributed to J. Thomas Nash.

At least 37 records · Page 2Linked to original sources
USGS1987

Mineral Resources of the Morey and Fandango Wilderness Study Areas, Nye County, Nevada

The Morey (NV-060-191) and Fandango (NV-060-190) Wilderness Study Areas are located in the northern Hot Creek Range about 25 mi north of Warm Springs, Nev. At the request of the Bureau of Land Management, 46,300 acres of the Morey and Fandango Wilderness Study Areas were studied. In this report, the area studied is referred to as 'the wilderness study area', or simply 'the study area'. Geologic, geochemical, geophysical, and mineral surveys were conducted by the USGS and the USBM in 1984 to appraise the identified mineral resources and to assess the mineral resource potential of the study areas. These studies indicate that there are small identified resources of zinc, lead, and silver at the Lead Pipe property in the Fandango Wilderness Study Area, several areas of high potential for the occurrence of gold resources in the Fandango study area, small areas of low and moderate potential for the occurrence of silver, lead, and zinc resources in the Fandango study area, areas of moderate and high potential for the occurrence of silver, lead, and zinc resources in the Morey study area, and an area of low potential for copper, molybdenum, and tin in the Morey study area. Both study areas have low resource potential for petroleum, natural gas, uranium, and geothermal energy.

Bulletin↗
USGS1985

PROGRESS REPORT ON GEOLOGIC STUDIES OF THE RANGER OREBODIES, NORTHERN TERRITORY, AUSTRALIA.

The Ranger No. 1 and No. 3 orebodies contain about 124,000 tonnes U//3O//8 in highly chloritized metasediments of the lower Proterozoic Cahill Formation within about 500 m of the projected sub-Kombolgie Formation unconformity. In both orebodies, oxidized and reduced uranium minerals occur chiefly in quartzose schists that have highly variable amounts of muscovite, sericite, and chlorite. The effects of several periods of alteration are pervasive in the vicinity of orebodies where biotite and garnet are altered to chlorite, and feldspars to white mica or chlorite. Oxidized uranium minerals, associated with earthy iron oxides, occur from the surface to a depth of about 60 m. Below the oxidized zone, uranium occurs chiefly as uraninite and pitchblende disseminated through thick sections of quartz-chlorite-muscovite schist and has no apparent association with graphite or sulphides. The geologic age(s) of uranium emplacement are obscure because there are few age criteria. Reduced uranium minerals are younger than 1. 8-b. y. -old granite dykes, and some occur locally in 1. 65-b. y. -old Kombolgie Formation.

Conference Paper↗
USGS1982

Preliminary report on the geology and gold mineralization of the Gold Basin-Lost Basin mining districts, Mohave County, Arizona

The Gold Basin-Lost Basin mining districts are adjacent to each other in northwestern Arizona, south of Lake Mead, and just west of the Grand Wash Cliffs. Most recorded production from lode deposits is credited to mines in the Gold Basin district, which is in the southern White Hills, whereas the bulk of the placer production has been from placers worked along the eastern flank of the Lost Basin range, about 16 km to the northeast across Hualapai Valley. Gold in quartz veins apparently was first discovered in the 1870's. Recorded production from the districts between 1901 and 1942 includes 13,508 oz gold and 6,857 oz silver, and this recorded production has a dollar value of about $359,000 of which 98 percent is credited to gold. Most known occurrences of lode gold in the districts are associated with widespread quartz-cored pegmatite-vein systems, presumably emplaced episodically during Proterozoic X, Proterozoic Y, and Late Cretaceous time into Proterozoic X metamorphic and igneous rocks. The bulk of the veins apparently were emplaced during the Late Cretaceous, and they were localized along both high- and low-angle structures in the Proterozoic X terrane. These veins appear to be associated genetically with presumably Late Cretaceous, two-mica magmatism. A Late Cretaceous two-mica monzogranite crops out in an approximately 4 to 5 km 2 area in the southern part of the Gold Basin district and includes some facies of episyenite. Some gold is found also in small episyenitic alteration pipes, or in veins caught up tectonically along a regionally extensive, low-angle detachment surface which crops out prominently in the southern White Hills, and has been traced for at least 30 km along the western flank of the White Hills. Hydrothermal micas from selected veins in the districts give K-Ar ages of 822, 712, 69, 68, and 65 m.y. (million years), and from the pipes, ages of 130 and 127 m.y. The oldest ages (822 and 712 m.y.) presumably reflect resetting of veins that probably were emplaced penecontemporaneous with emplacement of the 1,400-m.y. granite of Gold Butte, which crops out just to the north of Lake Mead. The latter ages (130 and 127 m.y.) must reflect either the presence of excess radiogenic argon in the hydrothermal environment of the evolving pipes, or contamination of the dated mineral separates by Proterozoic mica and (or) feldspar. Primary white mica from the two-mica monzogranite gives a K-Ar age of 72 m.y.. Most occurrences of gold in the veins and pipes probably reflect either remobilization of gold from gold-bearing, near-surface Proterozoic source areas, or anatectic incorporation of gold into Late Cretaceous, two-mica magmas from very deep gold-bearing Proterozoic sources. Deposition of gold occurred in a mesothermal environment during the galena-, chalcopyrite-, ferroan-carbonate-bearing stages of the veins. Homogenization studies of fluid inclusions prominent in the veins and pipes yield temperatures mostly in the range 150 to 280°C. Early-stage, trapping temperatures at the pipes probably were about 330°C and pressures in the range 500 to 700 bars can be inferred. Fluids were moderately saline, mostly 4 to 16 weight percent NaC1 equivalent, nonboiling, and also contain appreciable amounts of carbon dioxide and, in places, fluorine. Such fluids associated with the deposition of gold in these districts largely bridge the fluid composition interval between many other epithermal precious-metal and porphyry coper deposits. Approximately 350 compositional analyses obtained from native-gold samples from 20 mines in the Gold Basin district and 48 veins in the Lost Basin district show silver contents that range from 6 to approximately 50 weight percent, and copper from 0.01 to 0.5 weight percent. Metal zonation and possible relation to a porphyry copper system at depth can be inferred from some of these chemical data. The differences in the composition of placer gold from 24 occurrences in the Lost Basin district from that of nearby lode sources suggest that other sources contributed gold to the placers or that locally derived grains were enriched by oxidation and weathering of the lodes.

Arizona↗
USGS1981

Geology and concepts of genesis of important types of uranium deposits

Uranium ore deposits occur in nearly every major rock type in the earth’s crust, and nearly all igneous, metamorphic, and sedimentary processes are capable of concentrating or dispersing uranium. However, only three types of deposits account for more than 70 percent of known Western World Reasonably Assured Resources (WWRAR): Precambrian quartz-pebble conglomerate type, Proterozoic unconformity type, and Phanerozoic sandstone type. Igneous-related processes in plutonic, volcanic, and magmatic-hydrothermal environments, considered important 25 years ago, now account for less than 10 percent of world resources known at present. The oldest known ore deposits were formed in conglomerates by placer processes under unique anoxic conditions. For the last 2.2 b.y., since oxygenation of the atmosphere, the genesis of both high- and low-temperature deposits has been dominated by three general geochemical processes: (1) oxidation of uranium to soluble U(VI) species permitting aqueous transport, perhaps most commonly as uranyl-carbonate complexes; (2) reduction, principally by C, S −2 , or Fe +2 species, to U(IV) to allow precipitation of uraninite (pitchblende), and coffinite, although the specific reductant commonly cannot be determined because these three tend to be associated geologically; and (3) igneous and metamorphic differentiation caused by exclusion of uranium from crystal structure of most rock-forming minerals. The geochemistry of uranium ore-forming processes has changed in time because of the evolution of life forms and their impact on the earth’s oxygen and carbon budgets. This evolution is reflected in changing predominance of ore types in geologic time: (1) pre-2.8 b.y. ago—no known uranium ore deposits; (2) ca. 2.8 to 2.2 b.y. ago—the first intràcratonic basins and anoxic atmosphere permitted accumulation of placer deposits of uraninite in quartz-pebble conglomerates; these deposits contain about 19 percent of the western world’s resources; (3) ca. 2.2 to 0.4 b.y. ago—following oxygenation of the atmosphere uranium was oxidized and transported as soluble U(VI) complexes to sites of reduction, commonly in organic carbon-rich marginal marine environments. Diagenesis, metamorphism, and near-surface redox enrichment subsequently formed unconformity-type, ultrametamorphic-type, and vein-type ore deposits which together contain more than 25 percent of the western world’s resources; (4) ca. 0.4 b.y. ago to present—after development of land plants the most important ore-forming process was redox-controlled deposition from ground water in continental sediments. Sandstone-type deposits, characteristic of this stage, contain about 40 percent of the western world’s resources.

Book chapter↗
USGS1979

Geology, petrology, and chemistry of the Leadville Dolomite: host for uranium at the Pitch Mine, Saguache County, Colorado

Newly documented uranium ore in the Pitch Mine occurs chiefly in brecciated Mississippian Leadville Dolomite along the Chester reverse fault zone, and to a lesser extent in sandstone, siltstone, and carbonaceous shale of the Pennsylvanian Belden Formation and in Precambrian granitic rocks and schist. Uranium-mineralized zones are generally thicker, more consistent, and of higher grade in dolomite than in other hosts, and roughly 50 percent of the new reserves are in dolomite. Strong physical control by dolomite is evident, as this is the only lithology that is pervasively brecciated within the fault slices that make up the footwall of the reverse fault zone. Other lithologies tend to either remain unbroken or undergo ductile deformation. Chemical controls are subtle and appear to involve chiefly formation of FeS2 as pyrite and marcasite, which accompany uranium. Leadville Dolomite in the area is about 130 m thick and is predominantly nonfossiliferous dolomicrite. In the Pitch Mine, Leadville Dolomite is bound by faults and maximum known thickness is about 17 m. Mud texture, paucity of fossils and other allochems, thin laminations, and probable algal mat structures suggest sedimentation in a tidal-flat (possibly supratidal) environment. Preservation of mud texture and lack of replacement features indicate that dolomitization was an early, prelithification process, as in modern tidal flats, and produced a chemically and texturally uniform rock over tens of meters with relatively few limestone beds surviving. The sedimentary and diagenetic environment of the tidal-flat dolomite, apparently most favorable for uranium deposits, probably obtained over a large area and should consistute an exploration target over a broad area of central Colorado. Carbonate rocks of the Belden Formation, in contrast to those of the Leadville, contain calcite in great excess of dolomite, more than 5 percent silt-size quartz and clay, and abundant fossils and oolites. Belden limestones (sandy micrite and sandy wackestone) probably were deposited in an intertidal or subtidal environment. Very little uranium ore occurs in these rocks. Chemical aspects, such as the iron, sulfur, and organic carbon contents, are very similar to those of Leadville dolomites, and hence seem favorable, but Belden limestones generally are only mildly fractured. The minor-element content of ore-bearing dolomites is generally normal judging from the relatively scarce data yet published for comparable rocks. Elements enriched in ore include iron, sulfur, molybdenum, and lead. One surface expression of ore in dolomite is ocher-colored, leached, porous gossan that is characterized by residual silica and limonite and by high radioactivity but low chemical uranium content.

Open-File Report↗
USGS1979

Uranium and thorium in granitic rocks of northeastern Washington and northern Idaho, with comments on uranium resource potential

Northeastern Washington and northern Idaho is a uranium province in which many Cretaceous and Tertiary granitic plutons contain abnormal amounts of uranium. Mean uranium content of 108 samples of granitic rock is 8.8 parts per million (ppm), more than twice normal for rocks of this composition. The mean thorium content, 20.3 ppm, and mean Th/U, 3.19, are normal. The most uraniferous and fertile rocks are the peraluminous two-mica granitic suite, although not all two-mica plutons are enriched in uranium. The muscovite-bearing suite has mean uranium content of 22.3 ppm, mean thorium content of 22.8 ppm, and mean Th/U of 2.82. Porphyritic quartz monzonite of the Midnite mine, which I interpret to be a two-mica granitic rock, is especially radioactive with mean U of 14.7 ppm, mean Th of 32.1 ppm, and mean Th/U of 2.72. Mean uranium and thorium contents of the two-mica granitic plutons are significantly different from those of the calcalkaline hornblende granitic suite, which are mean U, 5.0 ppm; mean Th, 17.6; and mean Th/U, 3.78. Biotite granitic rocks containing no hornblende or muscovite appear to be an intermediate suite in terms of U and Th, or possibly are variants of both hornblende and muscovite type; mean U is 3.88 ppm, mean Th is 14.4 ppm, and mean Th/U is 3.03 as calculated from the more abundant data of Castor and others (1978). occurrence of uranium and thorium in the muscovite and hornblende suites is systematically different. Many muscovite-bearing rocks are much more enriched in uranium (>15 ppm) than they are in thorium, and have a relatively low Th:U correlation coefficient of +0.409. Many of the uraniferous muscovite-bearing rocks contain less than 20 ppm Th, probably a consequence of forming by anatexis of thorium deficient sedimentary rocks. Uranium and thorium variation is much more regular in the hornblende suite, which has a Th:U correlation coefficient of +0.780. Uranium in the muscovite suite is held primarily in magnetite and biotite, and possibly as minute uraninite grains, whereas in the hornblende suite uranium resides primarily in sphene, zircon, and allanite. Many muscovite-bearing plutons are considered fertile by the following two criteria: high uranium content, and uranium residence in labile phases. The hornblende-bearing granitic plutons are not considered fertile, regardless of uranium content, because uranium resides in refractory phases. Twenty-one sample localities in four plutons are considered highly anomalous according to one or more of the following attributes: (1) Uranium content =115.8 ppm (total population mean plus 1 d). (2) Thorium content =30.3 ppm (mean plus 1 d) and uranium =8.8 ppm (mean). (3) Th/U lower than 1.68 (mean minus 1 d) and uranium content -18.8 ppm. The four identified highly anomalous plutons are, from southwest to northeast, (1) Cretaceous porphyritic quartz monzonite of the Midnite mine; (2) Cretaceous muscovite quartz monzonite east of Deer Lake; (3) Cretaceous quartz monzonite of Hungry Mountain, and (4) Cretaceous quartz monzonite of Granite Pass. The study area is favorable for at least five types of uranium deposits including: intragranitic hydrothermal veins as in the Massif Central, France; intragranitic supergene veins, as at the Daybreak mine; contact zone deposits, as at the Midnite mine; and basal-type sandstone deposits, as at the Sherwood mine. The fifth type is Rossing-type deposits for which the high-grade metamorphic terrane of the Kettle River Range, Ferry County, seems favorable, if sufficient volume of low-tenor rock can be located. The first four deposit types seem most likely in or adjacent to uraniferous plutons, as no other uranium source rocks have been identified in the area. Contact zone deposits and intragranitic hydrothermal veins, both possibly supergene-enriched, and basal-type deposits in Tertiary sandstone and conglomerate, seem most likely to be of economic importance. Exploration and discovery of potential uranium depositions

Open-File Report↗
USGS1975

Fluid inclusion studies of vein, pipe, and replacement deposits, northwestern San Juan Mountains, Colorado

Base and precious metal ores valued in excess of $300 million have been mined from vein, pipe, and replacement deposits located on the northwest side of, and structurally related to, the middle Tertiary Silverton caldera. A belt of normal faults radial to the caldera contains veins which have been mined for as much as 15,000 feet (4,600 m) along strike and 3,700 feet (1,100 m) downdip. Structural relations, metal distributions, and fluid inclusion data suggest that many of the veins were open at the same general time and received the same mineralization. Fluid inclusions in quartz and sphalerite of the main ore stage of the Argentine and adjoining veins have filling temperatures ranging from 315 degrees to 249 degrees C and salinities from 7.9 to 0.1 wt percent NaCl equivalent. However, the majority of inclusions from base metal and gold stages are in a relatively narrow range near 280 degrees to 290 degrees C and salinities are unusually low (less than 2 wt %). Filling temperatures in postsulfide quartz and fluorite are as low as 153 degrees C and salinities are generally less than 0.5 wt percent. A pressure correction of about +25 degrees C should be added to the filling temperatures. Ore fluids were physically homogeneous liquids over the 3,700-foot (1,100 m) vertical and 11,200-foot (3,400 m) lateral range studied and no systematic variations in filling temperatures or salinity in this space are noted, but significant fluctuations are evident locally. Recently developed base metal replacement deposits occur in the Eocene Telluride Conglomerate adjacent to many metal-bearing veins. Calc-silicate alteration of the conglomerate host was probably by moderately saline fluids that ranged in temperature from 320 degrees to about 400 degrees C. Quartz-bearing clasts in altered zones contain secondary halitebearing inclusions with filling temperatures of about 250 degrees C and salinities about 34 wt percent; such inclusions are not known from the base metal stage and are interpreted to have been preore. Hydrothermal quartz, sphalerite, and calcite from replacement pods contain inclusions with filling temperatures ranging from 204 degrees to 309 degrees C and salinities from 11.2 to 0.2 wt percent. That the replacement zones and throughgoing veins are genetically related is suggested by the similarity of filling temperatures and compositions of base metal-stage fluid inclusions and by structural evidence. Small high-grade pipe deposits of base and precious metals occur along the ring-fault zone of the Silverton caldera. Filling temperatures for inclusions in quartz, sphalerite, and barite of the main ore stage of these deposits range from 216 degrees to 308 degrees C and salinities range from 1.6 to 0.2 wt percent NaCl. Quartz in altered wall rocks and silicified capping contains moderately saline secondary inclusions with relatively uniform liquid: vapor ratios that suggest filling temperatures of about 225 degrees to 300 degrees C. From the available samples there is no fluid inclusion evidence for boiling. Throttling, which had been hypothesized to explain the telescoped nature of the pipe ores, is neither substantiated nor discredited by the inclusion data.

Colorado↗
USGS1975

Geochemical studies in the Park City district; II, Sulfide mineralogy and minor-element chemistry, Mayflower mine

Fissure-filling and replacement Pb-Zn-Cu-Au-Ag ores of the Mayflower mine occur in calcareous sedimentary and intrusive wall rocks over a vertical interval of 3,000 feet. The ores are exceptional in the district for their chalcopyrite and gold content and for the unusual associated gangue minerals anhydrite, chlorite, and hematite. High oxygen fugacities are indicated for many stages of mineralization. Sphalerite compositions are highly variable in the range 0.09 to 5.9 weight percent Fe, an indication of large fluctuations in sulfur fugacity. Silver is carried in tetrahedrite-tennantite (0.2 to 16 wt %), enargite (0.5 to 0.8 wt %), in coupled substitution with Bi + Sb in galena (0.02 to 2.9 wt %), and in the minerals argentite, matildite, chalcocite, and bornite. Content of As, Se, Te, and other minor elements is small. Fractionation of Cd between sphalerite and coexisting galena is relatively consistent whereas the fractionation of Mn between sphalerite and galena is variable; calculated temperatures of formation are high and unreliable.

Economic Geology↗
USGS1975

Geology of the Midnite uranium mine, Stevens County, Washington; a preliminary report

The Midnite mine is one of only two mines in the United States currently producing uranium from discordant deposits in crystalline host rocks. Ore bodies are in metamorphosed steeply dipping Precambrian pelitic and calcareous rocks of a roof pendant adjacent to a Cretaceous(?) porphyritic quartz monzonite pluton. Production during 14 years, of operation has been about 8 million pounds of U3O8 from oxidized and reduced ores averaging 0.23 percent U3O8. Uranium deposits are generally tabular in form and dimensions range up to 380 m long, 210 m wide, and 50 m thick. Deposits are bounded on at least one side by unmineralized intrusive ribs of granitic rock, and thickest mineralized zones invariably occur at depressions in the intrusive contact. Upper limits of some deposits are nearly horizontal, and upper elevations of adjacent mineralized zones separated by ribs of granite are similar. Near surface ore is predominantly autunite, but ore at depth consists of pitchblende and coffinite with abundant pyrite and marcasite. Uranium minerals occur as .disseminations along foliation, replacements, and stockwork fracture-fillings. No stratigraphic controls on ore deposition are recognized. Rather, mineralized zones cut across lithologic boundaries if permeability is adequate. Most ore is in muscovite schist and mica phyllite, but important deposits occur in calc-silicate hornfels. Amphibolite sills and mid-Tertiary dacite dikes locally, carry ore where intensely fractured. High content of iron and sulfur, contained chiefly in FeS2, appear to be an important feature of favorable host rocks. Geometry of deposits, structural, and geochemical features suggest that uranium minerals were deposited over a span of time from late Cretaceous to late Tertiary. Ore occurs in but is not offset by a shear zone that displaces mid-Tertiary rocks.. Economic zones of uranium are interpreted to have been secondarily enriched in late Tertiary time by downward and lateral migration of uranium into permeable zones where deposition was influenced by ground water controls and minerals that could reduce or neutralize uranium-bearing solutions.

Open-File Report↗
USGS1975

Geology, geochemistry, and fluid-inclusion petrography of the Sapo Alegre porphyry copper prospect and its metavolcanic wallrocks, west-central Puerto Rico

The Sapo Alegre prospect, a small porphyry copper-molybdenum occurrence in west-central Puerto Rico, is characterized by distinct zones of alteration and mineralization of quartz diorite porphyry. A biotite-chlorite zone in the porphyry near its contact with surrounding metavolcanic rocks contains copper, molybdenum, gold, silver, selenium, and tellurium. A quartz-sericite-pyrite zone within the porphyry contains abundant sulfur and traces of selenium, but metals of the biotite-chlorite zone are nearly absent and zinc, manganese, nickel, sodium, calcium, and magnesium contents are very low. Quartz grains in the biotite-chlorite zone contain abundant fluid inclusions in which halite crystals are common, whereas in the quartz-sericite-pyrite zone inclusions are less abundant and rarely contain halite inclusions. Metavolcanic rocks northwest of the mineralized porphyry are altered to amphibolitic hornfels near the contact. Biotitic alteration is strong outward from the hornfels and grades into chlorite alteration. The hornfels has very low sulfide content, and the biotite zone has moderate amounts of copper and sulfur, decreasing outward. The chlorite zone is characterized by an abrupt increase in zinc and manganese content. Fluid-inclusion data suggest temperatures between 300° and 400°C for hydrothermal fluids in the biotite-chlorite zone of the porphyry and 250° to 300°C for fluids in the quartz-sericite-pyrite zone. Pressures equivalent to about 1.5 km of burial are indicated.

Río Víví district↗
USGS1974

Fluid inclusion and light stable isotope study of the climax molybdenum deposits, Colorado

The Climax mine contains three molybdenite ore bodies and widespread late quartz-pyrite-sericite barren mineralization, each related to separate intrusive phases of the Climax stock. Alteration zones spatially related to each molybdenite ore body include a silica zone below, a potassium-silicate zone that approximately coincides with ore, and overlying quartz-sericite-pyrite-topaz, argillic, and propylite zones . Delta 18 O values of quartz range from +8.4 to 10.9 per mil; delta 18 O values of muscovite and sericite range from --0.8 to +7.5 per mil. Eight of ten sericite values are between +5.8 and +7.5. All of the quartz and most of the sericite is in isotopic equilibrium with a calculated water at 350 degrees C with delta 18 O values of +3.0 to 5.5 per mil. Delta 18 O values of K-feldspars range from --4.5 to +7.5 per mil. This wide range indicates partial reequilibration of the feldspar with later, isotopically light, waters. Two muscovites have delta D values of --146 and --91 per mil; sericite ranges from --168 to --116 per mil. The calculated delta D values of water at 275 degrees C in equilibrium with sericite are --144 to --92. This range is the same as that of fluid inclusions in ore samples.Kaolinite in the argillic zone has delta 18 O values of --0.9 to --2.2 and delta D values of --162 per mil. Calculated water at 250 degrees C in equilibrium with kaolinite delta 18 O values of --4.7 to -6 and delta D of about -130 per mil. The isotopic and fluid inclusion data best fit a model whereby the Climax ore bodies were formed from a hydrothermal fluid that originated by mixing light and heavy waters. The light water is isotopically similar to present-day meteoric water; the heavy water has the isotopic composition postulated for magmatic water, although it could be meteoric water that was heavier than present-day meteoric water.

Colorado↗
USGS1974

Alteration and fluid inclusion studies of the porphyry copper ore body at Bingham, Utah

Distribution patterns for biotitic alteration, sericitic alteration, and distinctive fluid-inclusion types in igneous host rocks of the porphyry copper ore body at Bingham, Utah, have been determined by petrographic examination of about 300 samples. These patterns are related to differences in original rock composition, variations in physical-chemical conditions during periods of intrusion and mineralization, and spatial position within the ore body. The distribution of biotitic (potassium-silicate) alteration assemblages and high-salinity fluid inclusions generally follows the crudely triangular form of the disseminated copper ore zone. Variations in abundance of hydrothermal biotite are attributed to differences in original mafic mineral content of the igneous host rocks. Biotitic alteration and initial copper mineralization were accomplished by high-salinity fluids concentrated during final crystallization of the monzonitic parent magma; genetic continuity between magmatic and hydrothermal stages is indicated . Pervasive sericitic alteration of plagioclase is confined to rocks in the northern one-third of the Bingham stock; a subzone of argillic alteration in the north-central part of the ore body occurs within the broader area of sericitic alteration. Fluids responsible for sericitic and argillic alteration were channeled by a broad zone of northeast-trending fractures. Hydrothermal minerals and high-salinity fluid inclusions occur within a large volume of shattered rock. Boiling of fluids during crystallization of the aplitic porphyry may account for the shattering. Sericitic (and argillic) alteration were apparently super-imposed on the earlier biotitic assemblage as the hydrothermal system cooled. Cooling and hydrolytic alteration were promoted by progressive introduction of meteoric waters. The many generations of inclusions trapped from boiling fluids in the temperature range 400 degrees to 600 degrees C suggest that the system was recharged repeatedly during the period of mineralization. Estimated fluid pressures of about 800 bars in the early stages of mineralization correspond to a lithostatic load of about 3 km; pressures were even lower (less than 200 bars) in the later stages and were probably controlled by hydrostatic conditions. Distribution patterns for biotitic alteration, sericitic alteration, and distinctive fluid-inclusion types in igneous host rocks of the porphyry copper ore body at Bingham, Utah, have been determined by petrographic examination of about 300 samples. These patterns are related to differences in original rock composition, variations in physical-chemical conditions during periods of intrusion and mineralization, and spatial position within the ore body.The distribution of biotitic (potassium-silicate) alteration assemblages and high-salinity fluid inclusions generally follows the crudely triangular form of the disseminated copper ore zone. Variations in abundance of hydrothermal biotite are attributed to differences in original mafic mineral content of the igneous host rocks. Biotitic alteration and initial copper mineralization were accomplished by high-salinity fluids concentrated during final crystallization of the monzonitic parent magma; genetic continuity between magmatic and hydrothermal stages is indicated.Pervasive sericitic alteration of plagioclase is confined to rocks in the northern one-third of the Bingham stock; a subzone of argillic alteration in the north-central part of the ore body occurs within the broader area of sericitic alteration. Fluids responsible for sericitic and argillic alteration were channeled by a broad zone of northeast-trending fractures.Hydrothermal minerals and high-salinity fluid inclusions occur within a large volume of shattered rock. Boiling of fluids during crystallization of the aplitic porphyry may account for the shattering. Sericitic (and argillic) alteration were apparently super-imposed on the earlier biotitic assemblage as the hydrothermal system cooled. Cooling and hydrolytic alteration were promoted by progressive introduction of meteoric waters. The many generations of inclusions trapped from boiling fluids in the temperature range 400 degrees to 600 degrees C suggest that the system was recharged repeatedly during the period of mineralization. Estimated fluid pressures of about 800 bars in the early stages of mineralization correspond to a lithostatic load of about 3 km; pressures were even lower (less than 200 bars) in the later stages and were probably controlled by hydrostatic conditions.

Utah↗