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At least 811 records · Page 45Linked to original sources
USGS1971

Fluid-inclusion evidence on the environment of formation of mineral deposits of the southern Appalachian valley

Approximately 1,330 fluid inclusions were studied in samples of ore and gangue minerals from both massive ore and late-stage vugs from a series of Appalachian deposits and five active mines in the East Tennessee zinc districts. Most primary inclusions in sphalerite, fluorite, dolomite, and quartz from East Tennessee homogenized at 82 degrees -149 degrees C. Most primary inclusions in fluorite, barite, and sphalerite from the Central Kentucky, Central Tennessee, and Sweetwater barite districts, ranged from 72 degrees -132 degrees C. Pressure corrections to be added are probably less than 10 degrees C.Freezing data were obtained as a crude measure of the salinity of the inclusion fluids. With few exceptions, all primary inclusions contained very strongly saline brines (mostly >20 weight percent salts), with appreciable amounts of at least some salts other than NaCl, and some contained immiscible globules of oil. The exceptions include several quartz, fluorite, calcite, and barite samples that contained only moderately saline brines (12-16 percent), and some inclusions containing essentially fresh water, in vug calcite. Secondary inclusions had lower salinities than adjacent primaries. Eight primary inclusions in the differently colored outer millimeter of one Central Kentucky fluorite had low salinity (4-6 percent).The data indicate that all these deposits formed from hot, saline brines, with sphalerite forming from slightly hotter and more saline fluids than later gangue minerals. This places severe restrictions on possible modes of origin and makes deeply circulating connate brines the most probable ore fluids. The data give no information on the cause or direction of fluid circulation, which would be controlled by topography, salinity, and temperature during ore deposition, and they permit wide latitude in the construction of possible models. Other observations, on "colloform" textures, seem to indicate the admixture of at least small amounts of surface waters with the brines.

Kentucky, Tennessee, Virginia↗
USGS1971

Evidence on the age mineralization in the of east of barite, zinc, and iron lower paleozoic rocks Tennessee

Based on a study of minor occurrences of zinc, barite, and iron in East Tennessee, certain deposits are interpreted to have formed during early Middle Ordovician time. Principal lines of evidence supporting this age are: (1) the occurrence of barite, sphalerite, and pyrite associated with synsedimentary collapse breccias which span the pre-Middle Ordovician unconformity at the Lost Creek barite mine in Union County; (2) barite in basal Middle Ordovician beds (Athens Shale and Lenoir Limestone) in northeastern Tennessee and southwestern Virginia; (3) sphalerite and pyrite in dolomite filling paleocaves in the uppermost part of the Lower Ordovician Mascot Dolomite at the Trotter prospect near Douglas Dam in Sevier County; and (4) oolitic hematite and pyritiferous beds near the base of the Athens Shale and subjacent pyritiferous collapse breccias in the Mascot and Kingsport Formations at several localities along the eastern side of the Appalachian Valley. Thus, it is concluded that ascending solutions formed epigenetic deposits in pre-Middle Ordovician carbonate strata nearly contemporaneously with syngenetic deposits in early Middle Ordovician sediments. These findings are in general accord with those of recent workers in the Mascot-Jefferson City and Copper Ridge districts who have proposed that mineralization in these areas occurred during the time interval of pre-Middle Ordovician erosion. The long-held theory which associates mineralization with late Paleozoic diastrophism is not supported.

Tennessee↗
USGS1971

Problems on the origin of ore deposits in the lower Ordovician formations of east Tennessee

Most recent workers in the East Tennessee zinc and barite districts are in general agreement that the host breccias were formed by solution-collapse processes in early Middle Ordovician time, probably in an ancient carbonate aquifer system of regional extent. There is little agreement, however, on the source and nature of the ore fluids from which the epigenetic minerals were formed. In this symposium Hanshaw and his colleagues have presented evidence in support of the concept that much secondary dolomite may have been formed by ground water. On the other hand, Roedder has shown that fluid inclusion studies indicate hot saline brines (75 degrees -150 degrees C) as the source fluids for the ore minerals, and he suggests that they may have been deeply circulating connate brines. Other investigators in the past have proposed a variety of source fluids, including juvenile waters, meteoric waters that have become enriched by the leaching of particular elements from the surrounding rock, and sea water. In the zinc deposits, sphalerite is accompanied locally by minor amounts of galena and pyrite, and occasionally, traces of chalcopyrite. Dolomite is the chief gangue mineral and is associated with lesser amounts of quartz, calcite, barite, fluorite, anhydrite, gypsum, and bitumen. Some deposits containing barite as the dominant ore mineral have various amounts of fluorite and pyrite; sphalerite and galena may be present locally along with the other gangue minerals typical of the zinc deposits. The paragenetic sequences of the minerals have been determined for most of the mines and many of the prospects, and, except for minor refinements, will be little changed by additional studies. The real problem, though, is the correlation of these depositional sequences from mine to mine and from district to district. Perhaps the major difficulty in the correlation is the seemingly uncomplicated mineralogy that disguises a rather complex regional paragenesis. Questions regarding the routes the ore fluids travelled to reach the sites of deposition, the reasons why the ores are concentrated in some areas more than others, and the reasons why the mineralogy varies from area to area are questions that are unanswered or at best only partly answered. Studies to date have produced convincing evidence concerning the nature of the structures and the physical conditions responsible for the localization of ore in the Kingsport Formation (Lower Ordovician). On the other hand, the relation of the Kingsport ores to the smaller deposits in the underlying Lower Ordovician and Cambrian rocks is but little understood, and no final answer to the origin of these deposits can be reached until this relationship has been satisfactorily explained. Fluid inclusion studies, trace element analysis, and isotopic studies of all sorts are needed to further our understanding of these seemingly simple ores. Continued detailed studies of field relations, both regional and local, are of major importance, not only for what they will reveal in themselves, but also to perfect the frames of reference in which to place the growing mass of laboratory data.

Tennessee↗
USGS1971

A method for discriminating between biogenic and chemical origins of the ore-stage pyrite in a roll-type uranium deposit

Some roll-type uranium deposits are marginal to an altered tongue in sandstone beds that originally contained more-or-less uniformly distributed pyrite. Mineralizing solutions percolated through the sandstone, oxidized nearly all the pre-existing pyrite, and then redeposited part of the pyrite downstream in an embryonic ore zone. The pyrite and the entire ore zone continued to migrate downstream in the sandstone, much as a sand dune migrates. The amount of pyrite in mature deposits varies systematically with the position in the ore body. It is postulated that the rate at which the pyrite was redeposited controlled the systematic variation in distribution of pyrite.Biogenic and chemical models which are described in the literature provide alternate explanations for the genesis of roll-type uranium deposits in sandstone. The different theoretical rates for the precipitation of pyrite in the two genetic models provide a distinctive distribution of pyrite that characterizes each process. The theoretical difference between the biogenic and chemical models provides a mathematical technique for identifying the origin of a deposit. Mathematical analysis of the pyrite content of a uranium deposit in the Shirley Basin, Wyoming, illustrates a practical application of the theory. Although a definite conclusion about the origin of roll-type deposits would require considerably more data than are now available, the pyrite content of this deposit does correspond to the theoretical pyrite content of the chemical model, suggesting that a disproportionation reaction was involved in its formation.

Wyoming↗
USGS1971

The effect of salinity on the maximum thermal gradient of a hydrothermal system at hydrostatic pressure

The effect of salinity on the temperature-depth relations of a brine of constant composition, enclosed in a vein system, but freely connected to the surface, and everywhere at the boiling point for the hydrostatic head, was calculated by using a mathematical model. The Na-Ca-K-Cl brines which are found in thermal springs and in fluid inclusions in ore minerals were approximated by the available data for vapor-saturated NaCl-H 2 O solutions. In general, the results are similar to those calculated by D. E. White in 1968 for H 2 O, except that the gradients are steeper because of the increase in density and the decrease in vapor pressure caused by the dissolved salt. As a practical rule, the depth to an isotherm in a 5, 10, 15, 20, and 25 wt percent NaCl brine system is, respectively, 92, 84, 77, 70, and 63 percent (+ or -2 percent) of the depth to the same isotherm in an H 2 O system. From the data presented, the minimum depth to the growth site of crystals containing fluid inclusions which indicate boiling of the brine can be estimated. Among other applications, these results are useful toward the understanding of the behavior of brines in geothermal areas which may or may not contain compositional stratification.

Economic Geology↗
USGS1971

Comment about the origin of micrinite

The granular micrinite in coal may result from the same process that acts during the peat stage to form fusain but dispersed humic matter, rather than organized plant tissue, seems to be involved in micrinite formation. The closely sized micrinite granules that occur in cell lumens of fragments of woody tissue in a few Paleozoic coals are exceptional and may be the result of selective infiltration. The morphologic distinction between micrinite and fusinite should not obscure the fundamental genetic relationship of these macerals.

Economic Geology↗
USGS1972

Borehole activation analysis by delayed and capture gamma rays using a 252Cf neutron source

Theoretical analysis and experimental comparison of the radiative capture and delayed gamma-ray activation techniques indicate the latter to be more efficient for the detection of copper, whereas the radiative capture method is preferable for nickel. A conservative lower detection limit for both copper and nickel is '-0.5%. Borehole spectra by both techniques were made in a copper- and nickel-bearing gabbro, utilizing a Ge(Li) propane-cooled detector. Al, Mn, Na, Mg, Cu, and V were readily activated and detected by the delayed method. H, Fe, Si, and Ni were not usually present in the delayed spectra but they responded well in the capture mode. It is shown that the borehole sonde can be configured to permit simultaneous delayed and capture spectra, permitting detection of all of these elements. Simulated borehole experiments indicate that, in the delayed gamma-ray mode, an infinite sample is achieved when the ore layer has a vertical thickness of ∼20 cm and a horizontal distance of '10 cm into the wallrock. The depth resolution is thus relatively good but horizontal penetration through the wallrock is limited. © 1972 Society of Economic Geologists, Inc.

Economic Geology↗
USGS1972

Massive sulfide copper deposits of the Ergani-Maden area, Southeastern Turkey

The copper deposits of the Ergani-Maden area, in the Taurus Mountains of southeastern Turkey, have been a major source of copper for more than 4,000 years. The area is underlain by gently dipping mudstones interlayered with mafic volcanic rocks and lenticular beds of limestone, mainly of Eocene age. The layered rocks were intruded and slightly metamorphosed by large masses of gabbro and related mafic rocks that are now largely serpentinized. The copper deposits are in an east-trending belt about 20 km long, near the center of which is the town of Maden. A broad geochemical anomaly extends north-northwest and south-southeast from this belt, its axis passing near Maden. Near the mines the main igneous mass has a gently dipping roof and steep sides. Around and south of Maden, where the mass has barely been unroofed, are many roof pendants and remnants of sedimentary and volcanic material. One of these contains the largest mine in the area--the Ana Yatak. The largest deposit, the Aria Yatak ore body, occurs in a canoe-shaped pendant of mudstone that occupies atrough about 1 km long and 1/2 km wide in the top of the intrusive. The present vertical extent is about 170 meters. In the central and western parts of the trough the mudstone is chloritized and partly replaced by ore minerals. The main ore body is about 550 X 300 X 50 m in maximum dimensions. It consists mainly of pyrite and chalcopyrite, and locally it contains abundant magnetite or pyrrhotite. The gangue consists predominantly of chlorite, smaller amounts of quartz, and very minor amounts of calcite and barite. The massive sulfide ore body is underlain by "impregnation ore" in which veinlets and irregular small masses of sulfides are embedded in chloritized mudstone. A zone rich in supergene chalcocite, corellite, and bornite lies between the sulfide mass and the well-developed gossan in many places, and this was extensively mined in ancient times. The Mihrap Dagi mine, about 1 km northwest of the Aria Yatak deposit, is also near a contact between the intrusive and the sedimentary rock. It too contains a massive sulfide ore body accompanied by chloritized rock, but it contains little "impregnation ore." In the Ergani-Maden area, ore deposition followed a long history of sedimentation, lithification, faulting, intrusion of mafic igneous rocks, serpentinization, and further faulting, and was part of a broad-scale introduction of metal into the sedimentary, extrusive, and intrusive rocks. Thus it is epigenetic. The major ore bodies are fracturecontrolled replacement masses. Adularia which occurs in veins a few hundred meters south of the main sulfide ore bodies is inferred to have formed contemporaneously with the sulfide and shows a potassium-argon age of 31.5 +/- 0.8 m.y. If this inference is correcthe deposits formed during middle Oligocene or slightly later.

Ergani-Maden↗
USGS1973

Geochemical studies in the Park City district; I, ore fluids in the Mayflower mine

Ore bodies in the Mayflower mine, Park City district, Utah, are localized along a normal fault zone which cuts Mississippian sedimentary and Tertiary intrusive rocks. Fissure filling and replacement Pb-Zn-Cu-Ag-Au mineralization occurs in both sedimentary and intrusive host rocks over a known vertical distance of 3,000 feet. The paragenesis of three major recognized veins in the mine is nearly the same, consisting of early quartz, anhydrite, hematite, and pyrite + or - chalcopyrite, followed by sphalerite and galena, grading into pyrite + chalcopyrite + hematite, in turn followed by quartz + carbonate and minor anhydrite, followed by sphalerite and chalcopyrite. Deep, early veins outside of the Mayflower ore zone are characterized by quartz, K-feldspar, biotite, pyrite, and anhydrite, and some also contain magnetite, amphibole or chalcopyrite; these veins contain halite-bearing and gas-rich inclusions which are not known from the ore zone. The deep, early fluids had 34 to 44% salinity, homogenization temperatures of 315 degrees to 430 degrees C, and at times were boiling. Fluid inclusions in samples from the three ore-bearing veins, which contain only simple two-phase inclusions with consistent phase proportions, indicate a marked change in the fluids prior to ore deposition. Homogenization temperatures range from 220 degrees to 300 degrees C, and probably require a pressure correction of approximately + 10 degrees C. Freezing tests indicate salinities in the range 0.3 to 11 wt %NaCl equivalent; no CO 2 -bearing phases were detected at reduced temperatures. Near-surface veins, presumably contemporaneous with the Mayflower ore zone, show evidence for boiling and suggest that there was approximately 90 bars pressure at the present Mayflower vein outcrop. The distribution of fluid densities, temperatures, boiling, and key minerals in time and space indicates a dramatic change from very hot dense post-magmatic fluids to cooler, relatively low salinity fluids at the onset of economic basemetal deposition, probably concurrent with normal faulting. The changes in the physical properties of the hydrothermal fluids are believed to reflect the structural and magmatic evolution of the area.

Utah↗
USGS1973

Gold abundance in igneous rocks; bearing on gold mineralization

Review of quantitative data, restricted range in gold content (rarely more than 10 ppb, generally below 5 ppb), mafic rocks have more, so do early crystallizing minerals, no use in exploration, factors other than concentration determine mineralization; examples

Economic Geology↗
USGS1973

Gold-bearing arsenian pyrite determined by microprobe analysis, Cortez and Carlin Gold Mines, Nevada

Studies of polished sections and chemical analyses made by electron microprobe show that gold and arsenic in the unoxidized ores from the Cortez and Carlin mines are most abundant in pyrite. Gold, as particles too small to be seen under the microscope, along with arsenic is concentrated in tiny pyrite grains (<0.005 mm) and in thin rims of larger pyrite grains. It is concentrated also in arsenopyrite which is sparsely distributed in the Cortez ore. Traces of gold are contained in sphalerite and chalcopyrite sparsely disseminated in Carlin ore. Mercury and antimony occur also in the pyrite, and antimony is in illite as well. Little if any gold or arsenic is contained in quartz, carbonate, clay, and carbonaceous material. In oxidized ore, the sulfur and carbonaceous material have been removed and gold and arsenic occur in iron oxide pseudomorphs after pyrite. Gold can be seen in the oxidized ore, indicating that it has been mobilized and concentrated. No association was found between gold and the carbonaceous material. During mineralization, calcite was removed from the fractured silty carbonate of the Roberts Mountains Formation, creating micropore space in which pyrite, quartz, and illite were deposited. The average tenor of the pyrite, if all the gold were in the pyrite, would be 0.10 percent at Cortez and 0.14 percent at Carlin. These calculated values are similar to the analytical values. The calculated tenor of the carbonaceous material, however-if such material were considered to be the mineral host of the gold-would be 0.28 percent at Cortez and 1 percent at Carlin. These figures are unreasonable when compared with the analytical data, which show that the carbonaceous material contains no detectable amounts of gold. © 1973 Society of Economic Geologists, Inc.

Nevada↗
USGS1973

Silica-carbonate alteration of serpentine: Wall rock alteration in mercury deposits of the California Coast Ranges

Chemical, isotopic, and thermodynamic properties have been measured of CO 2 -rich ground waters in the central California Coast Ranges. The acidic CO 2 -rich waters react with serpentine to form silica-carbonate rock, the host rock of many mercury deposits in the Coast Range of California. In part the waters are of a metamorphic origin and in part the waters are locally derived meteoric waters. The CO 2 is entirely derived from metamorphic reactions at depth. Depending on the relative importance of several reactions, the relative abundances of silica and carbonate minerals vary in the silica-carbonate rock. If the CO 2 -rich fluids react directly with peridotite or dunite, massive magnesite deposits may form. © 1973 Society of Economic Geologists, Inc.

California↗