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At least 937 records · Page 52Linked to original sources
USGS1989

Paleohydrologic evolution and geochemical dynamics of cumulative supergene metal enrichment at La Escondida, Atacama Desert, northern Chile

Quantitative limonite mapping within the leached capping of the porphyry copper deposit at La Escondida, Chile, permits reconstruction of the paleohydrologic and chemical evolution of a well-developed supergene ore-forming system. The mineralogy, textures, and relative abundance of supergene limonite minerals (hematite, goethite, and jarosite) are used to reconstruct the former ratio of pyrite to chalcocite and the preoxidation copper grade based on empirical limonite sulfide correlations (after Locke, 1926; Blanchard, 1968; and Loghry, 1972). Estimates of preoxidation copper grades in surface exposures and tops of drill holes at La Escondida are significantly lower than actual copper grades in the underlying enrichment blanket at depth. This apparent inconsistency is explained by a progressive increase in the copper grade of the sulfide enrichment blanket as it descended to its present location in response to a descending water table. This systematic trend of reconstructed grades of supergene-enriched copper sulfide increasing with depth offers the first quantitative proof of cumulative downward enrichment in a supergene profile, as proposed by Locke (1926). The consistent trends of limonite mineralogy and abundance in vertical profiles indicate that water table descent at La Escondida was relatively continuous in space, although not necessarily in time.Evidence for cumulative downward enrichment in vertical profiles through leached capping allows quantitative analysis of chemical mass balance in dynamic supergene systems. Slopes of linear regressions for profiles of reconstructed enriched copper grades vs. depth indicate lateral fluxes into or out of a given vertical profile. This method provides independent verification of conclusions from a previous study of copper mass balance at La Escondida (Brimhall et al., 1985) which showed that lateral fluxes of copper were a significant factor during supergene leaching and enrichment.

Economic Geology↗
USGS1989

The Relief Canyon gold deposit, Nevada: A mineralized solution breccia

The Relief Canyon gold deposit in the Humboldt Range of western Nevada is a low-grade, high-tonnage orebody of Tertiary or younger age. The host rocks include limestones of the Triassic Cane Spring Formation, which are overlain by shales of the Triassic Grass Valley Formation. The rocks were folded and metamorphosed to greenschist grade during Jurassic and Cretaceous regional tectonic activity. Mesozoic thrusting may have occurred along the shale-limestone contact, but evidence has been obscured by later hydrothermal activity. The sedimentary rocks were nominally offset along several Late Tertiary normal faults related to uplift of the range.The upper part of the Cane Spring Formation is composed of a poorly sorted breccia composed of limestone clasts with a clay matrix. Irregular pockets within this zone are filled with clay- to pebble-sized fragments derived from the Grass Valley shale. The enclosing limestone beds were tilted moderately to the southwest during Mesozoic deformation, whereas bedding within these pockets is generally horizontal, indicating post-tilting deposition of the sediments. The sediments show graded bedding and other sedimentary features that indicate deposition from flowing water. Thermally mature carbon derived from the limestone is also concentrated in small pockets in the matrix. The breccia unit is likely the product of low-temperature solution brecciation. Ground water dissolved much of the limestone directly beneath the shales, progressively creating irregular cavities and the breccia. Sediments derived from the overlying Grass Valley shale were fiuvially deposited as a matrix to the developing solution breccia.Episodic pulses of hydrothermal fluids were introduced along faults and possibly mixed with the ground water in the breccia zone. Initially, jasperoids formed along the faults, but later hydrothermal pulses introduced gold, silica, and fluorine into both the early jasperoids and the unconsolidated cave-fill sediments to form the orebody. Continued solution-related brecciation chaotically disrupted the gold deposit.Gold, fluorite, pyrite, silver, calcite, and fine-grained silica are the principal hydrothermal minerals in the deposit. Gold was deposited as micron-sized flakes of native gold and rarely as electrum during a relatively late stage of silicification of the jasperoids, the carbon-rich zones, and the clay-rich matrix of the breccia. Fluorite was deposited with and later than the gold in the jasperoids, and it in part replaced the clay-rich breccia matrix. Antimony, arsenic, mercury, and thallium are directly associated with gold in the orebody.The deposit formed at a relatively shallow depth. On the basis of fluid inclusion data, late-stage hydrothermal fluids related to gold and fluorite deposition were extremely dilute and had temperatures near 200 degrees C. The fluid inclusions in fluorite show no evidence for boiling, but porous crackle breccias in the jasperoids suggest that hydrobrecciation took place.

Economic Geology↗
USGS1989

Comparison of metasomatic reactions between a common CO2-rich vein fluid and diverse wall rocks: Intensive variables, mass transfers, and Au mineralization at Alleghany, California

The gold deposits at Alleghany, California, are typical of many epigenetic gold-bearing hydrothermal vein systems in metamorphic terranes worldwide. Detailed analyses of alteration halos in serpentinite, mafic amphibolite, and granite wall rocks at Alleghany indicate that widely contrasting deposit types, ranging from fuchsite-carbonate schists to pyrite-albitites, resulted when different wall rocks interacted with the same externally derived CO 2 -rich hydrothermal vein fluid. Patterns of element redistribution within halos and among lithologic units suggest a complex process involving fluid flow along vein fractures and diffusion (+ or - infiltration) normal to the veins. Wall rocks locally controlled both the directions and magnitudes of chemical fluxes across vein walls.Vein fluids at constant temperature and pressure, with relatively constant ratios of mobile species (Na (super +) /K (super +) /H (super +) and CO 2 /H 2 S/H 2 ), caused contrasting ("divergent") reactions in proximal alteration zones because the wall rocks controlled the ratios of relatively immobile species (Al/(Cr + Fe + Mg) and Fe/Mg), and the component activities, of hydrothermal mineral solid solutions. Thus, pyrite and albite were stable with respect to the relatively pure siderite and K mica that occur sparingly in proximal granite zones, but they were not stable with respect to the relatively impure siderite and K mica that characterize proximal serpentinite zones. Partly as a result of divergent reactions, altered rocks exhibit contrasting mobile element concentration gradients (e.g., Ca, Ba, Sr, Rb, Na/K, C/S) and imply local sources and sinks, and possibly short residence times, for some of these components along discordant fluid flow paths.Calculated reaction coefficients for H (super +) , H 2 , and H 2 S indicate that different wall rocks at different stages of alteration may have inhibited or promoted gold precipitation. The distribution of gold and its calculated solubility are consistent with its having been precipitated from Au(HS) 2 (super -) complexes in response to pyritization of granite (fluid desulfidation and reduction), and possibly in part, in response to dolomitization and sericitization of serpentinite (fluid acidification?). Strata-bound disseminated orebodies may be expected in a variety of other host rocks with higher than average Fe/Mg ratios, where pyrite was stable with respect to intermediate magnesite-siderite, as in the granite.It is proposed that the CO 2 -rich vein fluids approached partial equilibrium with alteration assemblages in the average, or dominant, rocks along their flow paths. Simpler, but radically different, proximal assemblages were then produced where the fluids encountered chemically extreme rock types. Fluid-rock interaction equilibria can account for much of the diversity in the mineralogy and chemistry of gold deposits in heterogeneous metamorphic terranes, while maintaining a simple genetic classification based on the distinctive chemistry of the hydrothermal fluids. Comparative studies of multicomponent phase equilibria and mass exchanges can provide both rationalizations and predictions of local wall-rock effects on geochemical anomalies and the locations of ore deposits in lithologically heterogeneous terranes. Simultaneous consideration of contrasting high-variance (phase-deficient) assemblages along discordant veins is suggested as a general method for quantifying fluid-intensive variables in metasomatic systems.

Economic Geology↗
USGS1989

A reinterpretation of the δD H 2 O of inclusion fluids in contemporaneous quartz and sphalerite, Creede mining district, Colorodo: a generic problem for shallow orebodies?

Water extracted from fluid inclusions in quartz from shallow epithermal ore deposits often has a hydrogen isotope composition (δD) different from that of water extracted from inclusions in associated minerals. This difference is usually attributed to the involvement of primary fluids from multiple sources. Isotopic and homogenization and freezing temperature determinations on fluid inclusions from contemporaneous quartz and sphalerite from the epithermal, silver and base metal orebodies of the OH vein, Creede district, Colorado, suggest an alternative explanation. In near-surface deposits, differences between δD H 2 O of inclusion fluids in ore minerals and quartz may result, instead, from contamination during extraction of the fluids contained in primary inclusions by shallow ground water trapped in pseudosecondary inclusions in quartz. Quartz from the OH vein contains two principal petrographically distinct populations of fluid inclusions: primary and pseudosecondary. The primary inclusions have salinities ranging from 5 to 10 equiv wt percent NaCl, and the salinities of pseudosecondary inclusions cluster between 0 and 1 percent. Primary inclusions in quartz from one locality have a measured δD H 2 O value of -69 per mil, while pseudosecondary inclusions at the same locality have a δD H 2 O value of -102 per mil. Both salinity and isotopic values for primary inclusions in quartz are similar to those for primary inclusions in contemporaneous sphalerite. Homogenization temperatures for primary and pseudosecondary inclusions in quartz range from 191° to 280° C and from 199° to 278° C, respectively. The δD H 2 O value measured on fluid inclusions from bulk crystals ranges between -97 and -85 per mil and represents a mixture of fluids from both primary and pseudosecondary inclusions. We interpret the data to indicate that one or more episodes of abrupt incursion of cooler, overlying ground water into the ore zone caused thermal cracking of the quartz crystals during the time interval of mineralization. Subsequent healing of the fractures trapped heated, low-salinity ground water in pseudosecondary inclusions. The abrupt incursions of overlying ground water are speculated to have resulted from either collapse of a transient vapor-dominated region of the ore zone, or catastrophic venting of the system through hydrothermal eruption(s). The unusually high contrast between the salinities of the ore-depositing fluids and the ground water overlying the ore zone allowed recognition of this phenomenon at Creede. It is likely, however, that Creede is not unique. Similar phenomena may be common in shallow ore zones where rapid fluctuation of an interface between a deep, high-temperature thermal plume and an overlying, cooler ground water may be expected to occur. Careful study of the origins of fluid inclusions, particularly in quartz, is essential to characterize the primary ore fluids and to assess the role of ground water in the hydrology of shallow ore deposits.

Colorado↗
USGS1990

Genesis of the tabular-type vanadium-uranium deposits of the Henry Basin, Utah

Tabular-type vanadium-uranium deposits occur in fluvial sandstones of the Salt Wash Member of the Morrison Formation of Late Jurassic age The mineralized intervals and the weakly mineralized lateral extensions are bounded both above and below by zones rich in dolomite cement. Carbon isotope values of dolomite cements indicate that at least two sources of carbon existed. One source appears to be the same as that which formed the bedded carbonates in the evaporites in the Tidwell Member of the Morrison Formation stratigraphically below the mineralized interval. The second carbon source is typical of terrestrially deposited carbonates generally associated with meteoric water-dominated environments. Oxygen isotope values of these dolomites show the same trend of isotopically light values above the mineralized interval and isotopically heavier values in and below that interval; they indicate that two isotopically distinct fluids were involved in the mineralizing process. Some aspects of the origin of gangue and ore phases are explainable on the basis of processes which occurred solely within the saline fluid, but key aspects of ore genesis involved the interaction of the saline and meteoric waters. It is postulated that the solution interface migrated vertically within the stratigraphic section. -from Authors

Utah↗
USGS1990

Geochemistry of vanadium in an epigenetic, sandstone-hosted vanadium-uranium deposit, Henry Basin, Utah

The epigenetic Tony M vanadium-uranium orebody in south-central Utah is hosted in fluvial sandstones of the Morrison Formation (Upper Jurassic). Although the deposit is mined for uranium, vanadium has a higher average abundance in the ore. Thus, the geochemistry of vanadium in the orebody was studied to characterize ore-forming processes within the inferred ground-water flow regime. Measurements of the relative amounts of V (super +3) and V (super +4) in ore minerals show that V (super +3) is more abundant. Thermodynamic calculations show that vanadium was more likely transported to the site of mineralization as V (super +4) . The ore formed as V (super +4) was reduced by hydrogen sulfide, followed by hydrolysis and precipitation of V (super +3) in oxide minerals (e.g., montroseitc or paramontroseite) or chlorite. Uranium was transported as uranyl ion (U (super +6) ), or some complex thereof, and reduced by hydrogen sulfide, forming coffinite. Detrital organic matter in the rocks served as the carbon source for sulfate-reducing bacteria. It was this bacteriogenic H 2 S that reduced the metals in the mineralization process.Possible sources for the V and U in this deposit have been identified previously. Vanadium most likely was derived from the dissolution of iron-titanium oxides, which liberated Fe as well as V. A zone of titanium-rich remnants is observed updip and up the hydrologic gradient from the deposit (M. Goldhaber and R. L. Reynolds, unpub. data). Uranium probably was derived from the overlying Brushy Basin Member of the Morrison Formation (Northrop, 1982). A preliminary age date for the deposit of 115 Ma (K. Ludwig, 1986, pers. commun.) indicates that the ore formed after deposition of the Brushy Basin Member.Previous studies have shown that the ore formed at the density-stratified interface between a basinal brine and dilute meteoric water. The mineralization processes described above occurred within the mixing zone between these two fluids. Stable isotope analyses of ore-stage dolomite show a progressively heavier carbon and oxygen isotope signature with increasing depth through an ore horizon, consistent with the two-solution interface model.

Economic Geology↗
USGS1990

Sulfur isotope studies of the Stillwater Complex and associated rocks, Montana

The Stillwater Complex in south-central Montana is a late Archean layered, ultramafic to mafic intrusion emplaced into middle to late Archean metagraywacke, metashale, and ironformation. Sulfide minerals are concentrated near the base of the intrusion in some chromitite layers, podiform pegmatoids, discontinuous discordant pods or pipes, and numerous thin, stratiform layers. This investigation focuses on the isotopic composition of sulfur in metamorphosed sedimentary rocks, Stillwater-associated sills and dikes, sulfide accumulations near the base of the complex, and platinum-group element-enriched sulfide mineralization that constitutes the J-M reef and the Picket Pin deposit.The delta 34 S values for 233 samples analyzed in this study have a mean of 0.4 per mil, a standard deviation of 1.7, and a maximum and minimum of-3.8 and 7.8 per mil, respectively. Despite the very narrow range of values, analysis of variance calculations confirms that the data comprise three statistically distinct groups. These groups are (1) metagraywackes and metashales (mean = 2.8ppm); (2) iron-formation, Stillwater-associated sills and dikes, and basal sulfide accumulations in the complex (mean = -0.7ppm); and (3) disseminated sulfide minerals that occur above the basal accumulations in the Stillwater Complex (mean -- 1.2ppm). The isotopic composition of sulfur in the basal sulfide accumulations indicates that assimilation of substantial amounts of sulfur from iron-formation may have occurred. The low tenor of the basal sulfide ores is also consistent with assimilation of sulfur into a limited volume of magma near the base of the intrusion.

Economic Geology↗
USGS1990

Organic matter and thermochemical sulfate reduction in the Viburnum Trend, southeast Missouri

The role of organic matter in Mississippi Valley-type Pb-Zn deposits has been studied by systematically sampling and characterizing various types of organic matter in the Upper Cambrian Bonneterre formation in lead-zinc mines from the Viburnum Trend and from rocks as far as 20 km away from the Trend. Organic matter that is several kilometers from ore consists of insoluble disseminated kerogen in carbonates. Kerogen from brown dolostones yields mainly n-alkane pyrolysis products up to approximately n-C 20 H 42 , whereas gray and white carbonates that are poorer in organic carbon give much smaller quantities of pyrolysis products. Within meters to centimeters of ore in the Milliken mine, at the south end of the Viburnum Trend, organic matter occurs as solid, partly soluble tacky bitumen and insoluble hard blebs of millimeter to centimeter size. These hydrocarbon materials also yield n-alkane pyrolysis products up to C 20 . In contrast, the solid insoluble organic matter in intimate contact (intergrown) with ore (galena and chalcopyrite) is friable and brittle. Products from pyrolysis-gas chromatography-mass spectrometry of this material are quite different from the kerogen in rocks or the hydrocarbon blebs; they are substituted aromatic molecules such as toluene and methyl naphthalene, rather than n-alkanes.The sulfur content of solid organic matter from the Milliken mine varies and is lowest for tacky material away from ore, intermediate for blebs near ore, and highest for friable material in intimate contact with ore.Pyrolysis-gas chromatography of this sample suite documents the progression of kerogen (far from ore) through solid petroleumlike material (near ore) to degraded organic matter (in contact with ore). The change in the organic matter from n-alkane to a hydrogen poorer aromatic character with a higher sulfur content when associated with sulfide ores, suggests that chemical reactions such as nonbiological (thermochemical) sulfate reduction, which consume hydrogen and produce reactive sulfur, were responsible for organic degradation.

Economic Geology↗
USGS1990

Genetic implications of regional and temporal trends in ore fluid geochemistry of Mississippi Valley-type deposits in the Ozark region

Fluids extracted from aqueous fluid inclusions in epigenetic gangue and ore minerals record the migration of huge volumes of highly saline fluids throughout the stratigraphic section of the Ozark region. The extracted fluids share many similarities regionally, but there are significant temporal differences which define two geochemically distinct end-member ore-forming fluids that we refer to as the Viburnum Trend main stage or Viburnum Trend type and the Tri-State type.Viburnum Trend-type fluids are enriched in potassium and are associated only with deposits close to the basal Lamotte Sandstone. The main-stage octahedral galena ore of the Viburnum Trend and much of the Old Lead Belt ore is thought to be derived from this type of ore fluid. Galena deposited by Viburnum Trend-type fluids contains less radiogenic lead than galena deposited by Tri-State-type fluids. Sulfides deposited by Viburnum Trend-type fluids also contain isotopically heavier sulfur and significant amounts of copper, cobalt, nickel, and silver.Tri-State-type fluids have a low potassium content when compared with Viburnum Trend-type fluids and are characteristic of deposits where ore-forming fluids migrated through large volumes of carbonate rock. These fluids are thought to have formed the ore deposits of the Tri-State, Northern Arkansas, and Central Missouri districts, the cubic galena-stage ore of the Viburnum Trend, and the many trace occurrences of sphalerite throughout the Ozark region. Galena deposited by Tri-State-type fluids has more radiogenic lead and the sulfides have isotopically lighter sulfur than sulfides deposited by Viburnum Trend-type fluids. A systematic south to north increase of potassium in the Tri-State-type fluids suggests that they migrated from a southerly source such as the Arkoma basin.Possible explanations for the origins of these two end-member fluids include: (1) a single parent brine evolved into two distinct fluids due to reactions with geochemically distinct aquifers during migration, (2) the two distinct fluids reflect normal fluid evolution within a single source basin of a bittern and of later halite dissolution, and (3) the Viburnum Trend and Tri-State-type brines migrated to southeast Missouri from two different source basins. Our data does not preclude any of these possibilities; however, the geochemical similarity of the Viburnum Trend end-member fluid to a bittern may be accounted for by water-rock modifications of the brine during migration. Other evidence strongly supports a southerly source for the ore-forming brines thus limiting possible sources for the Viburnum Trend-type fluid to the Arkoma and/or Black Warrior basins of the Ouachita foreland trough. Viburnum Trend-type fluid flow was probably funneled northward through basal sandstones within the Reelfoot rift and water-rock modifications occurring there may have resulted in its unique geochemistry.

Economic Geology↗
USGS1990

Fine-grained rutile in the Gulf of Maine: Diagenetic origin, source rocks, and sedimentary environment of deposition

The Gulf of Maine, an embayment of the New England margin, is floored by shallow, glacially scoured basins that are partly filled with late Pleistocene and Holocene silt and clay containing 0.7 to 1.0 wt percent TiO 2 , chiefly in the form of silt-size rutile. Eleven basins in the gulf are estimated to contain 479 X 10 6 metric tons of TiO 2 (to a depth of 10 m) in the U.S. exclusive economic zone and 168 X 10 6 in Canada, based on analyses of surface sediment and of cores 10 to 20 m long. The U.S. annually consumes approximately 1 X 10 6 metric tons of TiO 2 , of which 73 percent is imported. The inferred amount of fine-grained TiO 2 in the basins is large, and we interpret it to be predominantly rutile; but no attempt has been made yet to mine and beneficiate the fine-grained ore.Sedimentary rocks of Nova Scotia, New Brunswick, and northern Maine contain fine-grained rutile crystals and have been the major source of the fine-grained sediment in the Gulf of Maine basins during and after the Wisconsinan glaciation. We conclude that much of the rutile in the Gulf of Maine mud formed diagenetically in poorly cemented Carboniferous and Triassic coarse-grained sedimentary rocks of Nova Scotia and New Brunswick after the dissolution of titanium-rich detrital minerals (ilmenite, ilmenomagnetite). Another major source of rutile is the generally finer grained Paleozoic sedimentary rocks of northern Maine (and possibly northwestern New Brunswick) which contain recycled diagenetic rutile that most likely originated in sandstones. Additional Wisconsinan sources of fine-grained, diagenetic TiO 2 probably include sedimentary rocks of inferred Triassic age that underlie the Gulf of Maine.The diagenesis of rutlie in coarse sedimentary rocks (especially arkose and graywacke) followed by erosion, segregation, and deposition (and including recycling of fine-grained rutile from shales) can serve as a model for predicting and prospecting for unconsolidated deposits of fine-grained TiO 2 .Gulf of Maine mud is comparable in TiO 2 content to typical shale. A determination of whether the fine-grained TiO 2 in shales primarily occurs as detrital titanium minerals from metamorphic and igneous sources or as recycled, diagenetic titanium oxides from sedimentary sources may assist in interpreting shale provenance and environment of deposition.

Economic Geology↗
USGS1990

Ground-water control of evaporite deposition

Topographically closed basins may be hydrologically open as a result of seepage losses to underlying or surrounding ground-water systems. In such cases, these losses can have a substantial control over the suite and the thicknesses of evaporite minerals formed in the basin. The ratio of ground-water outflow to inflow (flux ratio) in hydrologically open basins is as important in determining the mineralogy and thicknesses of evaporite deposits as the solute composition of the inflow water. Attainment of steady state flux ratios permits large thicknesses of two or three minerals to form rather than thin veneers of many minerals. The presence or absence of glauberite, mirabilite, halite, bloedite, polyhalite, and hexahydrite, caused by subtle changes in the ground-water seepage is illustrated using an example from the Southern High Plains of Texas and New Mexico. However, the model is general and is applicable with any solute composition including that of seawater and the use of surface rather than ground water. An analytical, lumped parameter, solute mass balance model is developed to define the concept of a ground-water flux ratio as it applies to topographically closed basins in which evaporation exceeds precipitation. Diffusion, advection, and density-driven flow are proposed as mechanisms by which solutes can escape to the ground water from these closed basins. The geochemical reaction computer program PHRQPITZ is used to document the effects of various flux ratios on the mineralogy and thickness of deposits. Solute analyses used in conjunction with the model can be used to screen prospective basins as well as to provide insights for exploratory drilling program.

Texas, New Mexico↗
USGS1990

The Ag-Mn-Pb-Zn vein, replacement, and skarn deposits of Uchucchacua, Peru; studies of structure, mineralogy, metal zoning, Sr isotopes, and fluid inclusions

Uchucchacua is an Ag-Mn-Pb-Zn vein, replacement, and skarn mineral district in the central Andes of Peru. Host rocks are massive Jumasha Formation shelf limestones of Turonian age that have been folded into an asymmetric northeast-verging anticline of Andean trend. Strata near the fold crest are cut by minor dacitic intrusions and have been displaced by a conjugate set of steep wrench faults that strike northwest-southeast and northeast-southwest. Most ore occurrences are restricted to host rocks that lie below marly limestone at the top of the middle Jumasha Formation. Vein ores located along the fracture system have formed by fissure infill and by replacement of limestone wall rocks. Larger sheetlike replacement orebodies are parallel and adjacent to a large fault. Replacement is in zones of brecciation adjacent to fault bends which were dilatent during sinistral slip of the master fracture. Such fracture belts may have been subject to paleokarst solution processes before mineralization.Four paragenetic stages have been identified. Fe, Mn, and Si were introduced at the exoskarn stage (I) as the anhydrous silicates ferroan tephroite, johannsenite, rhodonite, and bustamite. During the early main stage (II) ferroan tephroite was replaced by friedelite and magnetite under oxidizing conditions at a relatively low pH. Pb, Zn, Fe, Cu, and B were introduced; principal sulfides are pyrrhotite, Fe-rich sphalerite, Mn-rich wurtzite, alabandite, galena, chalcopyrite, and tetrahedrite. Pyrrhotite was replaced by other sulfides during later stage II. Main gangue minerals were calcite, kutnohorite, rhodochrosite, and quartz. Ag, As, and Sb were introduced during the late stage (III) in the form of sulfosalts, principally pyrargyrite. Redistribution of metals introduced at stage II resulted in the growth of Fe-poor sphalerite and alabandite accompanied by calcite gangue. Decreasing Fe contents of alabandite and sphalerite during late stage II and stage III, together with the appearance of pyrite, indicate an increase in sulfur fugacity and/or decrease in temperature over this period. The supergene stage (IV) affects the upper 30 to 150 m of most veins and involves the growth of Mn hydroxides, goethite, orpiment, marcasite, cerussite, and siderite.Distribution patterns of metal ratios and high metal values define ore bands, with a succession of antiforms and synforms. Ore-band locations are determined by vein width, itself a function of wall-rock reactivity, and fracture permeability. Metal ratio distributions in the Luz vein show symmetric dispersion of metals at right angles to the ore-band axis. Ag shows the least dispersion with progressively greater dispersion of Pb and Zn.Geologic considerations indicate mineralization at a minimum depth of 1,600 m. Most primary fluid inclusions in calcite of probable stage II origin were trapped over a temperature range of about 200 degrees to 337 degrees C (hydrostatic pressure) or about 225 degrees to 362 degrees C (lithostatic). Fluid inclusions from late stage II quartz were trapped at about 165 degrees to 205 degrees C (hydrostatic) or 190 degrees to 230 degrees C (lithostatic). Salinity ranged from 0.5 to 29.7 equiv wt percent NaCl-CaCl 2 with little relation to temperature. Calcite 87 Sr/ 86 Sr values of 0.70743 to 0.71122 extend well outside the range of values both determined for magmatic rocks of the region and estimated for the host limestones but are probably compatible with older sedimentary rocks of the mine district. Salinity levels and NaCl/CaCl 2 ratios are similar to basinal brines associated with Mississippi Valley-type deposits. It is concluded that basinal brine, expelled from the deforming Mesozoic sequence, was a significant component of the ore fluid. This fluid was probably heated by dacitic magmas or cooling dacitic intrusions; a polygenetic high-salinity magmatic-basinal fluid then rose along existing strike-slip fractures toward the mineralization site where it mixed with heated low-salinity ground water.

Uchucchacua↗
USGS1991

Oxygen, hydrogen, and sulfur isotope studies in the Juneau gold belt, southeastern Alaska: Constraints on the origin of hydrothermal fluids

The delta 18 O values of gold-bearing quartz from the Juneau gold belt range from 15.2 to 20.8 per mil, indicating that ore fluid values ranged from 7.2 to 12.8 per mil at an estimated temperature of 300 degrees C. Hydrothermal micas from many of the deposits are characterized by delta D values of -75 to -53 per mil, and ore fluids were calculated to have values of -35 to -15 per mil. In contrast, extracted fluid inclusion waters have a broad range of delta D values, from -48 in relatively undeformed quartz to about -110 per mil for some of the more deformed quartz veins. This range of more than 60 per mil reflects various mixtures from inclusions containing isotopically heavy, primary ore fluids and those containing low-temperature, isotopically light meteoric waters that were trapped in late fractures during uplift of the veins to shallow crustal levels. These results indicate a deep crustal source for the ore fluids, most likely of metamorphic origin. Values of delta D for muscovite, biotite, and hornblende from country rocks, as well as of delta 18 O for quartz-feldspar and quartz-plagioclase, provide little evidence of deep circulation of meteoric water along the length of the gold belt.The provinciality of delta 34 data suggests a regional metamorphic fluid of approximately -6 per mil that acquired much of its sulfur from lithologies near sites of ore deposition. Sulfur ratios of sulfide minerals from auriferous quartz range from -17.8 to -5.6 per mil in black phyllite hosts, from -6.0 to -1.3 per mil in less reduced metasedimentary rocks, and from -3.8 to +1.2 per mil in relatively oxidized igneous rocks. The regional fluid is interpreted to have driven desulfidation reactions in relatively 34 S-depleted sulfur reservoirs in the phyllites and 34 S-enriched reservoirs in the more oxidized lithologies.

Economic Geology↗
USGS1991

A method for the concentration of fine-grained rutile (TiO2) from sediment and sedimentary rocks by chemical leaching

Quaternary marine sediment in the Gulf of Maine basins contains 0.7 to 1.0 wt percent TiO 2 (determined by X-ray fluorescence spectrometry). Most of this TiO 2 exists in the form of silt-size rutile crystals that are visible by using the petrographic microscope with transmitted light (Valentine and Commeau, 1990). The identification of rutile was confirmed by using a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS) system. To quantify the amount of TiO 2 in the sediment contributed by rutile and its polymorphs, anatase and brookite, it was necessary to eliminate as many of the other minerals as possible, especially titanium-bearing minerals such as ilmenite, ilmenomagnetite, biotite, hornblende, oyroxene, and sphene. We accomplished this by developing a method of chemical dissolution that removed the bulk of the raw material and left the TiO 2 minerals intact. Many methods using acids and bases have been developed over the years to dissolve rocks, minerals, and sediments for chemical analysis or to concentrate specific minerals (Dolcater et al., 1970; Church, 1971; Campbell, 1973, and the references cited therein). The method developed by Raman and Jackson (1965) to concentrate rutile and anatase in soils and sediments requires digesting the sample in concentrated hydrofluoric acid (HF) for 24 hours. However, Campbell (1973) found that digestion in HF for more than 2 hours results in a loss of anatase. The method of Dolcater et al. (1970) for concentrating titanium as a free oxide requires the use of hydrofluotitanic acid (H 2 TiF 6 ), which is difficult to find on the commercial market. The acid can be prepared by the reaction of concentrated HF (48%) with an excess of TiO 2 , but the procedure requires 36 hours to complete and should be attempted with caution because it is highly exothermic. Jackson (1979) provides a detailed method for digesting soils, but many of the recommended pretreatment steps employ sodium compounds such as sodium bicarbonate (NaHCO 3 ), sodium citrate (Na 3 C 6 H 5 O 7 -2H 2 O), and sodium dithionate (Na 2 S 2 O 4 ), which are used for the removal of calcium carbonate, iron oxides, and phosphates. In combining the methods of Dolcater and Jackson, sodium compounds must be thoroughly washed from the sample because they form sodium fluotitanate (Na 2 TiF 6 ) in the presence of hydrofluotitanic acid (Fig. 1K). French and Adams (1973) described an inexpensive method for decomposing silicates by HF digestion in polypropylene containers. Their technique was effective for a wide variety of rock types. However, it did not address the problem caused by the precipitation of insoluble fluorides, nor did it outline a procedure to concentrate any residue that remained. As no one method gave the results we required, we modified procedures described in the literature and developed a process that removes 96 to 98 wt percent of the raw sample material. The residue is composed of rutile and minor amounts of micro- and cryptocrystalline TiO 2 (Fig. 1A-J), barite (Fig. 1L), elemental carbon (coal), and insoluble fluorides (Fig. 1J). The fluorides precipitate during the decomposition of siliceous material in hydrofluoric acid (e.g., MgF 2 and MgAlF 5 -2.7H 2 O; Lanmyhr and Kringstad, 1966). Most of the sample analyzed by the method described were marine muds collected from the Gulf of Maine (Valentine and Commeau, 1990). The silt and clay fraction (up to 99 wt% of the sediment) is composed of clay minerals (chiefly illite-mica and chlorite), silt-size quartz and feldspar, and small crystals (2-12 um) of rutile and hematite. The bulk sediment samples contained an average of 2 to 3 wt percent CaCO 3 . Tiher samples analyzed include red and gray Carboniferous and Triassic sandstones and siltstones exposed around the Bay of Fundy region and Paleozoic sandstones, siltstones, and shales from northern Maine and New Brunswick. These rocks are probable sources for the fine-grained rutile found in the Gulf of Maine.

Economic Geology↗
USGS1992

Hydrothermal ore-forming processes in the light of studies in rock- buffered systems: I. Iron-copper-zinc-lead sulfide solubility relations

Experimental studies, using cold-seal and extraction vessel techniques, were conducted on Fe, Pb, Zn, and Cu sulfide solubilities in chloride solutions at temperatures from 300 degrees to 700 degrees C and pressures from 0.5 to 2 kbars. The solutions were buffered in pH by a quartz monzonite and the pure potassium feldspar-muscovite-quartz assemblage and in f (sub S 2 ) - f (sub O 2 ) largely by the assemblage pyrite-pyrrhotite-magnetite. Solubilities increase with increasing temperature and total chloride, and decrease with increasing pressure. The rise in solubility is particularly steep between 300 degrees and 500 degrees C and between 1,000 and 500 bars. With increasing temperature at any given pressure, or with decreasing pressure at any given temperature, metal solubility eventually passes through a maximum due to increasing competition for chloride by the alkali, hydrogen, and base metal ions and because intersection with a two-fluid region eventually occurs. In that portion of the two-fluid region encountered in the study, metal solubilities in the brine were very high, but solubilities in the gas phase also were significant. In a system controlled by the potassium feldspar-muscovite-quartz buffer, 1-m total Cl (super -) , and the assemblage pyrite-pyrrhotite-magnetite-sphalerite-galena-chalcopyrite, solubilities in ppm at 1 kbar and 300 degrees , 400 degrees , and 500 degrees C were 237, 1,216, and 5,636, for Fe; 51, 613, and 3,105 for Pb; 36, 423, and 2,649 for Zn; and 11, 40, and 113 for Cu, respectively. At 400 degrees C, 0.5 and 2 kbars, the values were 2,627 and 500 for Fe; 1,262 and 194 for Pb; 983 and 120 for Zn; and 60 and 29 for Cu, respectively. All of the above were in the single-fluid region. Single-metal solubilities also were investigated to assess the influence of iron on the solubility of the other metals and to corroborate preliminary dissociation constants for the metal chloride complexes involved. The effect of increasing chloride concentration on solubility reflects primarily a shift to lower pH via the silicate buffer reactions. The effect of decreasing pressure reflects primarily the relative change in the dissociation constants of the chloride complexes involved. Increasing sulfur fugacity lowers solubility, but in systems controlled at relatively low values by an f (sub S 2 ) buffer or wall-rock sulfidation reactions, solutions of high metal content relative to reduced sulfur will tend to develop at high chloride concentrations. Similarity in behavior with respect to the temperature and pressure of Fe, Zn, and Pb sulfide solubilities points to similarity in chloride speciation, and the neutral species appear to be dominant in the high-temperature region. At 500 degrees C and 1 kbar, the log K D values for FeCl degrees 2 , PbCl degrees 2 , ZnCl degrees 2 , and CuCl degrees are, respectively, -8.76, -9.14, -10.86, and -6.22.

Economic Geology↗