A quantitative link among mineral deposit modeling, geoscience mapping, and exploration-resource assessment
No abstract available.
Economic Geology↗
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Stable isotope study of water-rock interaction and ore formation, Bayhorse base and precious metal district, Idaho
The Bayhorse base and precious metal district is situated east of the Idaho batholith in south-central Idaho. The ores occur near the Nevada Mountain granitic stock as veins cutting the lower Paleozoic Ramshorn Slate and the Garden Creek Phyllite, and as fillings around breccia fragments within the Bayhorse Dolomite. The veins are dominated by siderite and tetrahedrite, with lesser quartz and galena, whereas the breccia ores dominantly comprise only quartz and galena. Mineralization and intrusive activity were contemporaneous during Cretaceous time. Fluid inclusion and stable isotope data indicate that mineralization formed from hot (ca. 375 degrees -225 degrees C), CO 2 -rich (< or =8.3+ or -1.4 mole %) brines (5-20 wt % NaCl equiv) at confining pressures between 1.1 and 1.7 kbars. Fluid cooling and the resulting CO 2 effervescence were the most important causes of ore deposition.Whole-rock delta 18 O and delta D values from the Garden Creek Phyllite define an isotopically depleted zone (60 km 2 ) around the Nevada Mountain stock and are the result of high-temperature interactions with ancient meteoric waters at water/rock ratios (mass fractions) ranging from 0.002 to 0.09. The delta 18 O (sub H 2 O) and delta D (sub H 2 O) values of the ore fluids ranged from 11.2 to 3.9 per rail and from -55 to -146 per mil, respectively. Comparison of the ore fluid delta 18 O (sub H 2 O) and delta D (sub H 2 O) values with hypothetical waters equilibrated with the Garden Creek Phyllite intricates that the hydrothermal fluids must have also interacted with the basal dolomite of Bayhorse Creek, which underlies the phyllite. Early delta 13 C (sub CO 2 ) values calculated in equilibrium with siderite ranged from -9.1 to -7.4 per mil, whereas later delta 13 C (sub CO 2 ) values obtained from quartz-hosted fluid inclusion extracts range from -4.0 to -1.8 per mil. The delta 13 C (sub CO 2 ) values for the hydrothermal fluids thus also record a transition from early water/rock interactions that were dominated by the Garden Creek Phyllite (organic matter delta 13 C = -23.6 to -20.9ppm) to later interactions that were influenced significantly by the basal dolomite of Bayhorse Creek (delta 13 C = -2.3 to -2.2ppm). The delta 34 S values of main-stage hydrothermal sulfides range from 3.0 to 28.3 per mil and display a distinct mode at approximately 10 per mil. The range of values may be interpreted as either a heterogeneous sedimentary source or mixed sedimentary-magmatic sources. On the basis of mass balance considerations, it is likely that all of the fluid salinity and the dissolved metals were derived from the Garden Creek Phyllite. The paragenetic sequence developed in the ores appears to be related to: (1) processes at the site of deposition, most notably cooling and CO 2 effervescence, and (2) water-rock interactions within the Garden Creek Phyllite in the 60-km 2 isotopically altered area surrounding the Nevada Mountain stock.
Economic Geology↗
Organic matter diagenesis as the key to a unifying theory for the genesis of tabular uranium-vanadium deposits in the Morrison Formation, Colorado Plateau
Interstitial, epigenetic amorphous organic matter is intimately associated with uranium in the Grants uranium region on the southern part of the Colorado Plateau in northwestern New Mexico and is considered essential to genetic models for these deposits. In contrast, uranium minerals are intimately associated with authigenic vanadium chlorite and vanadium oxides in amorphous organic matter-poor ores of the Slick Rock and Henry Mountains mining districts on the northern part of the plateau, and therefore, in some genetic models amorphous organic matter is not considered crucial to the formation of these deposits. Differences in organic matter content can be explained by recognizing that amorphous organic matter-poor deposits have been subjected to more advanced stages of diagenesis than amorphous organic matter-rich deposits, during which organic compounds were broken down, solubilized, and removed during various diagenetic stages (e.g., sulfate reduction, methanogenesis, and organic acid production). Most diagenetic alterations (e.g., dissolution of framework grains and cements and precipitation of coarse-grained coffinite, vanadium clays, and vanadium oxides) characteristic of amorphous organic matter-poor deposits occurred when temperatures exceeded 80 degrees C--a temperature typical of the organic acid stage of diagenesis.Two factors combined to assure that a high percentage of the amorphous organic matter was removed from the Morrison Formation during diagenesis: the fact that the organic matter was humic (and therefore highly oxygenated), and that the juxtaposition of authigenic smectite and amorphous organic matter on grain rims facilitated clay mineral-organic catalysis reactions leading to the breakdown of large organic molecules into smaller, soluble molecules.Evidence that amorphous organic matter was involved in the genesis of organic matter-poor, as well as organic matter-rich, deposits includes the presence of leached iron-titanium oxides and strongly etched garnets (both attributed to leaching by organic acids), the gradation from organic matter-rich to organic matter-poor deposits in the Grants uranium region, the inverse correlation between the amount of amorphous organic matter and the intensity of diagenetic alteration, the ubiquitous occurrence of amorphous organic matter inclusions in coffinite, and the similarity in geometry of tabular uranium-vanadium ores and Holocene humate deposits. By recognizing that amorphous organic matter was crucial to the initial concentration of uranium (even in those deposits that presently contain little amorphous organic matter) and by recognizing the effects of the various stages of diagenesis on the ore, one genetic model may be applied to all of the sandstone-hosted, tabular-type uranium-vanadium deposits in the Morrison Formation on the Colorado Plateau.
Economic Geology↗
Pb isotope data indicate a complex, mantle origin for the Noril'sk-Talnakh ores, Siberia
No abstract available.
Siberia, Noril'sk District, Talnakh District, Yeni↗
U-Pb dating of uranium deposits in collapse breccia pipes of the Grand Canyon region
Two major periods of uranium mineralization are indicated by U-Pb isotope dating of uranium ores from collapse breeeia pipes in the Grand Canyon region, northern Arizona. The Hack 2 and 3, Kanab North, and EZ 1 and 2 orebodies apparently formed in the interval of 200 + or - 20 Ma, similar to ages inferred for strata-bound, Late Triassic-hosted uranium deposits in southern Utah and northern Arizona. Samples from the Grand Canyon and Pine Nut pipes, however, indicate a distinctly older age of about 260 Ma. The Pigeon, Orphan, and Arizona-1 deposits were apparently mineralized before 220, 186, and 169 Ma, respectively, but no useful upper age limits can be inferred. There is no evidence in the U-Pb isotope data for uranium mineralization related to Laramide tectonism, mid-Tertiary volcanism, or late Tertiary uplift. The clustering in ages for a variety of uranium deposits at about (or slightly younger than) the age of the lower part of the Chinle Formation (Late Triassic) suggests that uranium in these deposits may have been derived by leaching from volcanic ash in the Chinle and mobilized by ground-water movement resulting from changing hydrologic gradients caused by regional uplift to the southwest. Pb isotope ratios of galenas in mineralized pipes are more radiogenic than those of sulfides from either uranium-poor pipes or occurrences away from pipes. This isotopic contrast suggests that fluids which passed through the pipes had interacted with the Proterozoic basement, possibly through the vertical fractures which influenced the location and evolution of the pipes themselves. Regardless of the source of the common Pb in the uranium-bearing pipes, the generally distinct Pb isotope composition of their galenas (compared to those of barren pipes and nonpipe sulfides in the region) may provide a useful exploration guide.
Economic Geology↗
Geochemistry and origin of the Bald Knob manganese deposit, North Carolina
Geochemical and textural evidence for the submarine hydrothermal origin of the Bald Knob Mn deposit, North Carolina, is preserved in a mineralogically diverse suite of rocks that have experienced amphibolite facies metamorphism. Manganese- and Mn-Fe-rich lithologies have high Mn/Fe ratios and low concentrations of Ni, Cu, Zn, U, Th, and rare earth elements that are similar to concentrations reported from other ancient manganiferous deposits and from crusts and sediments which are forming in hydrothermally active regions of the present sea floor. Manganese- and Ca-rich carbonate minerals, a variety of Mn-rich silicates (including tephroite, the Mn humite-group minerals, spessartine, rhodonite, and pyroxmangite), and the oxides galaxite and jacobsite form fine laminations that represent different bulk compositions. No chemical or textural evidence for replacement of Mn-poor by Mn-rich minerals was found and, in some samples, the same lithologic units are repeated on the scale of a thin section, consistent with a primary sedimentary origin for the layering. Chert, which is one of the country rocks at Bald Knob, and Mn- and Ca-rich carbonate rocks represent original sedimentary protoliths. More than one detrital component may be present in the Mn-rich layers. Biogenic detritus was the probable source of at least some of the siliceous material that allowed for the formation of the Mn-rich silicates. An aluminous detrital component, which probably was derived from a continental or andesitic source, is reflected in spessartine-rich layers. Minor Mn and Co sulfide mineralization (alabandite, cattierite, cobaltite) was contemporaneous with deposition of the Mn-rich sediments. The association of the manganiferous lithologies with chert and meta-igneous rocks (amphibolite) is consistent with deposition of the sediments in a rifting environment, such as a back-arc basin, or possibly, a midocean ridge.
Economic Geology↗
Predicting sizes of undiscovered mineral deposits; an example using mercury deposits in California: A reply
No abstract available.
California↗
The Geysers-Clear Lake area, California: Thermal waters, mineralization, volcanism, and geothermal potential
Manifestations of a major thermal anomaly in the Geysers-Clear Lake area of northern California include the late Pliocene to Holocene Clear Lake Volcanics, The Geysers geothermal field, abundant thermal springs, and epithermal mercury and gold mineralization. The epithermal mineralization and thermal springs typically occur along high-angle faults within the broad San Andreas transform fault system that forms the western boundary of the North American plate in this area. The young volcanic rocks overlie Mesozoic marine rocks of the Great Valley sequence which have been thrust above the coeval Franciscan Complex and penecontemporaneously dropped back down along low-angle detachment faults.Many of the waters of the region are nonmeteoric as defined by their isotopic signature. One type of isotopically shifted water emerges from or near Great Valley sequence rocks and is the most chloride rich. It is interpreted to be evolved connate in origin. A second type, evolved meteoric water, has moderate chloride contents, high boron contents, and high B/Cl ratios and is found locally in Franciscan rocks, notably at the Sulphur Bank mercury mine, where it probably results from near-closed-system, repeated boiling of meteoric water in host rocks that also contribute organic components to the water. At the Sulphur Bank mine fracturing of otherwise impermeable Franciscan rocks by faulting has created a localized zone of permeability in which thermal water boils repeatedly with limited venting to the surface. Boron-rich fluids were apparently present at depth in The Geysers geothermal field when intrusion of silicic magma occurred, because the concealed intrusion of felsite is surrounded by a halo of tourmaline-bearing hornfels. The volume of this poorly dated early to middle Quaternary intrusive body probably exceeds the 100 km 3 of erupted Clear Lake Volcanics. Similar intrusions may have occurred in the eastern part of the area at Wilbur Springs and the McLaughlin mine, where gold deposition and evidence of hydrothermal phenomena suggest more magmatic activity than is indicated by the small exposed bodies of early Quaternary basaltic lava. The Clear Lake Volcanics are the present locus of volcanism in the northern Coast Ranges and progressively older volcanic centers occur to the south. Geophysical data suggest that a large silicic magma body may be centered north of The Geysers steam field, providing the heat for the geothermal field.Geothermal power production has peaked at The Geysers and pressure declines indicate significant depletion of the fluid resource. The vapor-dominated field evolved from a preexisting hydrothermal system within fractured, otherwise impermeable Franciscan metamorphic rocks. A deep water table of saline fluid has been postulated to be present under the steam field, but no chloride-rich water has been found at drillable depth. We propose that recently discovered, isotopically shifted steam in the northwest Geysers area indicates the presence not of deep connate water but xather of boiled-down, boron-rich Franciscan evolved meteoric water. This water is likely to be present in limited quantities and will not provide a significant hot water resource for geothermal power production at The Geysers field or from the main Clear Lake volcanic field.
Economic Geology↗
Geochronology of the Sleeper deposit, Humboldt County, Nevada: Epithermal gold-silver mineralization following emplacement of a silicic flow-dome complex
The high-grade gold-silver deposits at the Sleeper mine are low sulfidation, quartz-adularia-type epithermal deposits, formed during the final stages of igneous hydrothermal activity of a small middle Miocene silicic flow-dome complex in north-central Nevada. Potassiumargon and 40 Ar/ 39 Ar ages of alkali feldspar indicate that the rhyolitic flow-dome complex was emplaced before about 16.5 Ma; hydrothermal alteration and mineralization followed, lasting from about 16.2 to 14+ Ma based on ages of adularia associated with electrum. There were multiple pulses of alteration and mineralization but all occurred within a period of less than 2 m.y. Later supergene alteration formed opal and alunite about 5.4 Ma but produced no Au or Ag mineralization other than some remobilization to produce locally rich pockets of secondary Au and Ag enrichment and is unrelated to the older magmatic hydrothermal system. The Sleeper deposit in the northern part of the Great Basin is genetically related to bimodal volcanism that followed a long period of arc-related andesitic volcanism in the same general region.
Economic Geology↗
Devonian volcanogenic massive sulfide deposits and occurrences, southern Yukon-Tanana Terrace, eastern Alaska Range, Alaska
A belt of volcanogenic massive sulfide deposits extends for over 150 km along the southern margin of the Yukon-Tanana terrane of the eastern Alaska Range. Located north of the Denali fault, the Yukon-Tanana terrane forms a major basement unit in east-central Alaska. The volcanogenic massive sulfide deposits are primarily in the Jarvis Creek Glacier subterrane, one of the highest structural-stratigraphic levels of the Yukon-Tanana terrane. The Jarvis Creek Glacier subterrane consists of a volcanogenic massive sulfide-bearing metavolcanic rock member and a metasedimentary rock member. The volcanogenic massive sulfide deposits and enclosing metavolcanic and metasedimentary schists exhibit two periods of regional metamorphism and penetrative deformation: an older, Early Cretaceous, amphibolite facies event and a younger, mid-Cretaceous lower greenschist facies event.From northwest to southeast, the major volcanogenic massive sulfide districts are the Bonnifield, Trident Glacier, and Delta. The Bonnifield district contains massive sulfide deposits in metavolcanic and metasedimentary rocks that are correlated with the Jarvis Creek Glacier subterrane. The 25-km-long, northwest-trending Trident Glacier district contains several occurrences consisting of massive pyrrhotite and lesser pyrite, minor chalcopyrite with or without sphalerite, and galena and arsenopyrite in south-dipping massive sulfide pods, lenses, and stringers that parallel the foliation of the enclosing schists. Host-rock protoliths were mainly andesite, dacite, rhyodacite tuffs and flows, shale, and limestone, with lesser quartz-rich keratophyre flows and tuffs, volcanic graywacke, and siltstone.The Delta district contains at least 35 deposits in an 800-km 2 area. Thirty-one of the deposits define four mineral trends, 5 to 32 km long, that are subparallel to the west-northwest-striking, southwest-dipping structures and lithologies. The deposits consist of layers and zones containing varying amounts of massive to disseminated pyrite and pyrrhotite, and lesser amounts of chalcopyrite, galena, sphalerite, and arsenopyrite. The larger deposits range between 1.0 and 12.4 million metric tons with combined grades of 5.6 percent total metal (Cu, Pb, Zn), and Ag and Au contents of 56 and 1.9 g/metric ton, respectively. Hostrock protoliths are similar to those found in the Trident Glacier district, but some of the deposits are spatially associated with diorite or gabbro sills that exhibit lower greenschist facies metamorphism and deformation.Sulfur isotope values of 26 pyrrhotite separates from the Trident Glacier district occurrences average 7.7 per mil, the mean value of seven pyrite separates is 8.1 per mil. Forty-one Delta district pyrite separates average 5.9 per mil and ranges of values of the four northwest-trending trends overlap. The relative consistency of sulfur isotope values of iron-bearing sulfides in the Trident Glacier district, and the apparent lithologic relationship of isotope values in the Delta district, may reflect original-rock sulfur isotope values. Alternatively, the 34 S-enriched values may have resulted from the addition of reduced seawater sulfate to the hydrothermal fluid. The sulfide accumulations are interpreted as kuroko-like deposits that formed along a Devonian continental margin igneous arc. Deeper level portions of the arc are exposed in structurally lower levels of the Yukon-Tanana terrane to the north and contain metamorphosed granodiorite and granite plutons of Devonian to Mississippian age. U-Pb isotope data from zircons from metavolcanic rocks hosting the sulfide bodies indicate an age of about 370 Ma. The 207 Pb/ 204 Pb ratios for sulfides from the three districts are similar and indicate lead was in part derived from first-cycle sediments that were eroded largely from Precambrian crystalline rocks. The 207 Pb/ 204 Pb ratios from feldspars in Devonian metavolcanic rocks are similar to those in sulfides and indicate incorporation of radiogenic Precambrian lead into the Devonian magmas. A Precambrian crystalline source for detritus in metasedimentary rocks is indicated by locally abundant quartzite containing approximately 2.3 Ga zircons which were derived from the Hudsonian-age block of the Canadian Shield in southern British Columbia.Cretaceous greenschist facies retrogressive metamorphism and intense penetrative deformation is indicated by reequilibration of U-Pb isotope ratios, metamorphic Rb-Sr mineral isochrons, and K-Ar biotite and muscovite ages of 110 to 115 Ma. The data indicate that the mid-Cretaceous retrogressive metamorphism and associated intense deformation occurred during the accretion of the Yukon-Tanana terrane to Alaska.
Economic Geology↗
Origin and significance of tourmaline-rich rocks in the Broken Hill district, Australia
Tourmaline-rich rocks are widespread minor lithologies within the Early Proterozoic Willyama Supergroup in the Broken Hill district, Australia. Tourmaline concentrations occur in strata-bound and local stratiform tourmalinites, clastic metasedimentary rocks, quartz-gahnite lode rocks, stratiform Pb-Zn-Ag sulfide ores, garnet quartzites, strata-bound scheelite deposits, quartz-tourmaline nodules, discordant quartz veins, and granitic pegmatites. Most of the tourmaline-rich rocks are within the Broken Hill Group that hosts the main Pb-Zn-Ag ores.At the Globe mine along the northeast end of the main lodes, tourmalinites are closely associated with Pb-Zn-Ag mineralization and in places are interbedded with Mn-rich garnet quartzites. Galena and other ore minerals occur locally in the cores of recrystallized tourmaline grains in these tourmalinites, indicating that tourmaline and sulfides were present together prior to deformation and high-grade metamorphism. Electron microprobe analyses of tourmalines intergrown with Fe sulfides at the Globe mine show Mg-rich compositions relative to tourmalines in sulfide-free assemblages from the same area, suggesting early (premetamorphic) introduction of boron and Mg enrichment of tourmaline by sulfide-silicate reactions during metamorphism.Combined field and geochemical data indicate that the district tourmalinites represent normal clastic sediments that were metasomatically altered by boron-rich hydrothermal fluids at or below the sediment-water interface. Whole-rock chemical analyses of 33 tourmaline-rich rocks show linear trends of data for major and trace elements that closely resemble the trends observed for unmineralized elastic metasedimentary rocks of the district. Average Fe/Al, Mg/Al, Na/Al, and Ti/Al molar ratios of the tourmaline-rich rocks and clastic metasediments are very similar; the average K/Al molar ratio of the tourmaline-rich rocks is significantly lower than that of the clastic metasediments, reflecting the loss of K during tourmalinite formation. Chondrite-normalized patterns of rare earth elements (REE) in the quartz-rich tourmalinites are generally similar to those of the clastic metasediments, except for minor depletions of light REE; local positive and negative Ce anomalies suggest tourmalinite formation in the presence of seawater or a seawater-derived pore fluid. The geochemical data imply relative immobility of Al, Ti, Cr, and heavy REE during hydrothermal alteration and later metamorphism. Boron isotope analyses of 52 tourmaline separates show a total range of delta 11 B values from -26.8 to -17.0 per mil. Fine-grained, euhedral, nonpoikilitic tourmalines from tourmalinites in the andalusite-muscovite zone in the northern part of the district (e.g., Black Prince mine) have delta 11 B values from -21 to -17 per mil, whereas coarse granoblastic and poikilo-blastic tourmalines from the sillimanite and two-pyroxene granulite zones in the southern part of the district (e.g., Globe mine) have delta 11 B values of-24 to -19 per mil. Tourmalines in strongly retrogressed tourmalinites have delta 11 B values from about -27 to -20 per mil. The observed variations in delta 11 B are consistent with prograde and retrograde metamorphic fractionation of boron isotopes, in which the fluid phase is preferentially enriched in the heavier isotope ( 11 B). Premetamorphic hydrothermal fluids that deposited the Black Prince tourmalinites had delta 11 B values of-8 to -5 per mil at 200 degrees to 300 degrees C, suggesting a boron source from nonmarine evaporite borates.Tourmalinites in the Broken Hill district apparently formed by the same submarine hydrothermal processes as the main Pb-Zn-Ag lodes and the siliceous ferromanganese protoliths of the garnet quartzites. In our model, the hydrothermal system(s) acquired abundant boron by leaching evaporitic borates within the Thackaringa Group, the stratigraphic sequence that underlies the Broken Hill Group and most of the tourmaline concentrations. We suggest that evaporites of the Thackaringa Group provided a source of readily extractable boron for formation of the tourmalinites and also the source of the fluoride, sulfur, and perhaps the carbonate in the main lodes; such evaporites may have been critical for increased metal chloride complexing and transport necessary for deposition of the high-grade Pb-Zn-Ag ores. The Broken Hill deposit may have formed contemporaneously with the Mount Isa and McArthur River Pb-Zn-Ag deposits in similar evaporite-bearing sequences during widespread Early Proterozoic continental rifting.
Economic Geology↗
Genesis of massive sulfide deposits on a sediment-covered spreading center, Escanaba Trough, southern Gorda Ridge
No abstract available.
Economic Geology↗
Titanium, vanadium, and niobium mineralization and alkali metasomatism from the Magnet Cove complex, Arkansas
The Christy Ti-V-Nb deposit is hosted by the Arkansas Novaculite, adjacent to the carbonatite-alkaline Magnet Cove Complex. Mineralized rock contains up to 8 wt percent Ti, 2.68 wt percent V, and 0.124 wt percent Nb. Minor amounts of Li and Mo are present, and there is evidence for the preferential concentration of the middle to heavy rare earth elements (REE) in altered clay-rich dikes. Mineralized rock consists of (1) thermally recrystallized novaculite that is partly replaced by actinolite, sodic amphibole, taeniolite (KLiMg 2 Si 4 O 10 F 2 ), pyrite,and siderite, (2) brookite-bearing amphibole- and taeniolite-rich rock that contains siderite and pyrite, (3) quartz-brookite aggregates, and (4) goethite-rich rocks that form the oxidized zone of the deposit. Unaltered rock is scarce at the Christy deposit. Comparison of mineralized lithologies from the Christy deposit with those from two other areas of mineralization associated with the Magnet Cove Complex, the Magnet Cove Titanium Corporation deposit ("Rutile deposit") and the Hardy-Walsh prospect, provides mineralogical and chemical evidence for the source of the mineralizing fluids at the Christy deposit.Massive clay-rich (kaolinite + or - smectite) dikes are in contact with the mineralized zones of the Christy deposit. The presence of relict K feldspar and pyrite in the least altered dikes and certain characteristic trace elements (e.g., Ba, Rb) suggest that the Christy dikes are related to less pervasively altered rutile-mineralized, pyrite-bearing carbonate-feldspar dikes fromthe Rutile deposit. The feldspar-carbonate dikes from the Rutile deposit may be the products of intense alkali metasomatism of alkaline igneous silicate rocks that were emplaced prior to the intrusion of the Magnet Cove carbonatite.The Christy deposit formed through a series of complex processes. The initial phase of mineralization is directly related to the infiltration of novaculite by alkali-rich fluids that were probably derived from carbonatite magma. Introduction of the metasomatic fluids preceded emplacement of massive feldspar- and pyrite-bearing dikes that were subsequently altered to the clay-rich masses Titanium, V, Nb, and Li were introduced by the alkali-rich fluids at temperatures that were, as indicated by fluid inclusion data (Willis et al., 1991), as high as 600 degrees C. During the initial stage of mineralization, V was concentrated in aegirine and sodic amphibole, Li was concentrated in taeniolite, minor amounts of Ti were concentrated in aegirine, and pyrite formed. The replacement of novaculite by the aforementioned minerals yielded excess silica, which precipitated as quartz in veins and as clear overgrowths on recrystallized, inclusion-rich (partly replaced) quartz. Niobium- and V-bearing brookite precipitated with the quartz. Minerals formed during the first stage reacted with a second fluid at temperatures of 100 degrees to 300 degrees C, as indicated by fluid inclusion data (Willis et al., 1991), and V was then concentrated in smectite and goethite. The isotopic composition of siderite [delta 18 O SMOW = 23.8-25.0ppm, delta 13 C PDB = -7.1 to -8.9ppm suggests that the second fluid was a mixture of low-temperature metasomatic fluid and ground water. Vanadium was further concentrated in clay minerals (Breit et al., 1992), in goethite, and in vug minerals as low-temperature alteration proceeded. Brookite is not totally resistant to weathering and it is at least partly replaced by other oxides in the oxidized zone.
Economic Geology↗
A quantitative model of ground-water flow during formation of tabular sandstone uranium deposits
Tabular sandstone uranium deposits constitute the largest uranium resource type in the United States. A major point of contention has been the nature and direction of the ground-water flow. This paper presents a quantitative simulation of regional ground-water flow during uranium deposition in the Westwater Canyon Member and Jackpile Sandstone Member of the Upper Jurassic Morrison Formation in the San Juan basin. Input for the four-layer finite-difference model was from thicknesses and sandstone/mudstone ratios in each layer from 1,600 drill holes and 100 measured sections and from palcotopographic reconstructions. Topographic slope, shoreline position, and density contrasts in the lake and pore fluids controlled the directions of flow and recharge-discharge areas. The most important results for uranium ore deposit formation are that regional ground water discharged throughout the basin, regional discharge was concentrated along the shore line or playa margin, flow was dominantly gravity driven, and compaction dewatering was negligible. A strong association is found between the tabular sandstone uranium deposits and major inferred zones of mixed local and regional ground-water discharge. The results of ground-water modeling favor the brine interface model over the lacustrine-humate model for uranium deposition.
Economic Geology↗
Isotopic composition of Pb in ore deposits of the Betic Cordillera, Spain; origin and relationship to other European deposits
The Betic Cordillera in southern Spain is a complex Alpine fold belt that resulted from the Cretaceous through Cenozoic collision of Africa with Europe. The region is illustrative of one of the characteristics of the Alpine-Mediterranean orogen: the occurrence over a limited area of mineral deposits with a wide variety of host rocks, mineralization ages, and styles. The metamorphic basement in the Betic zone is characterized by a nappe structure of superimposed tectonostratigraphic units and consists of lower Paleozoic to Lower Triassic clastic metasedimentary rocks. This is overlain by Middle to Upper Triassic platform carbonate rocks with abundant strata-bound F-Pb-Zn-(Ba) deposits (e.g., Sierra de Gador, Sierra Alhamilla). Cretaceous to Paleogene subduction-related compression in southeastern Spain was followed by Miocene postcollisional extension and resulted in the formation of the Almeria-Cartagena volcanic belt and widespread hydrothermal activity and associated polymetallic mineralization. Typical Miocene hydrothermal deposits include volcanic-hosted Au (e.g., Rodalquilar) and Ag-rich base metal (e.g., Cabo de Gata, Mazarron) deposits as well as complex polymetallic veins, mantos, and irregular replacement bodies which are hosted by Paleozoic and Mesozoic metamorphic rocks and Neogene sedimentary and volcanic rocks (e.g., Cartagena, Sierra Almagrera, Sierra del Aguilon, Loma de Bas).Lead isotope compositions were measured on sulfide samples from nine ore districts and from representative fresh samples of volcanic and basement rock types of the region. The results have been used to evaluate ore-forming processes in southeastern Spain with emphasis on the sources of metals. During a Late Triassic mineralizing event, Pb was leached from Paleozoic clastic metasedimentary rocks and incorporated in galena in strata-bound F-Pb-Zn-(Ba) deposits ( 206 Pb/ 204 Pb = 18.332 + or - 12, 207 Pb/ 204 Pb = 15.672 + or - 12, 208 Pb/ 204 Pb = 38.523 + or - 46). The second episode of mineralization was essentially contemporaneous (late Miocene) throughout the region and did not involve remobilization of less radiogenic Triassic ore Pb. Lead isotope data indicate a dominantly Paleozoic metasedimentary source for polymetallic vein- and manto-type deposits that formed by hydrothermal circulation through the Betic basement, driven by Miocene intrusions ( 206 Pb/ 204 Pb = 18.747 + or - 20, 207 Pb/ 204 Pb = 15.685 + or - 9, 208 /Pb/ 204 Pb = 39.026 + or - 37). Lead in Au-(Cu-Te-Sn) ores is isotopically indistinguishable from that of the calc-alkalic volcanic host ( 206 Pb/ 204 Pb = 18.860 + or - 9, 207 Pb/ 204 Pb = 15.686 + or - 8, 208 Pb/ 204 Pb = 38.940 + or - 27). In contrast, the Pb in volcanic-hosted Pb-Zn-Cu-(Ag-Au) veins was derived from Paleozoic metamorphic and Miocene volcanic rocks ( 206 Pb/ 204 Pb = 18.786 + or - 5, 207 Pb/ 204 Pb = 15.686 + or - 2, 208 Pb/ 204 Pb = 38.967 + or - 9).A comparison of the Pb isotope data from southeastern Spain with published data from selected Pb-Zn deposits in southern Europe (including Les Malines, L'Argentiere, and the Alpine, Iglesiente-Sulcis, and Montagne Noire districts) indicates the importance of a metasedimentary basement as a common source of ore Pb.
Economic Geology↗
Structure and bulk composition of the Tin Mountain pegmatite, Black Hills, South Dakota
Construction of structure contour and isopach maps has yielded a rather simple and orderly interpretation of the structure and has led to calculations of the tonnage of each of the internal units and of the bulk composition of the pegmatite. The outcropping part of the pegmatite trends S 75° E for 90 m and then connects with an arm of the pegmatite that plunges gently N-NW for approximately another 90 m. An albite-quartz-muscovite wall zone completely surrounds four inner units. Though these four inner units are termed zones, in accordance with standard practice, differentiation is more in a vertical direction than in a concentric arrangement. The upper two of these units are perthite rich.
North Dakota↗