Showing papers in "Economic Geology in 1970"

Lateral and vertical alteration-mineralization zoning in porphyry ore deposits

Abstract: Copper and molybdenum deposits, potassic, phyllic, argillic, and propylitic alteration, emphasis on San Manuel-Kalamazoo deposit of Arizona, tabulated data on characteristics of North and South American deposits

Geochemistry of black shale deposits; a summary report

Abstract: Trace-element analysis, correlation statistics, depositional environments, syngenetic, diagenetic, and epigenetic minor element enrichment, samples mainly from United States

Mineralogy, paragenesis and geothermometry of the tin and tungsten deposits of the eastern Andes, Bolivia

Abstract: Numerous geothermometers were applied to ores of the Eastern Andes of Bolivia in order to evaluate temperature as one factor in zoning and paragenesis of the deposits, and also to test the thermometers themselves where results could be checked against available field evidence. Some 225 specimens were analyzed representing 53 deposits and several ore types, chiefly the tin and tungsten ores.The stratigraphic record suggests that ores of the subvolcanic setting of central and southern Bolivia formed at depths on the order of 350 to 2,000 m and at total pressures of approximately 30 to 500 bars. Comparable estimates for the somewhat deeper deposits of plutonic association in the Cordillera Real are depths of 2,000 to 4,000 m and pressures of about 190 to 1,000 bars.A similar paragenesis is indicated for the tin and tungsten deposits and consists of (1) an early vein stage in which most tin and tungsten was deposited, (2) a base-metal sulfide stage, (3) hypogene alteration of pyrrhotite to pyrite, marcasite, and siderite and (4) late crusts and veinlets of siderite, fluorite, and a variety of hydrous phosphate minerals some of which are supergene.The Bolivian ores contain most of the sulfide minerals proposed, at one time or another, as geothermometers, but present attempts to use them have been largely unsuccessful. Pyrrhotite compositions were modified during replacement by pyrite, marcasite and siderite and most do not reflect initial depositional temperatures. Arsenopyrite gave only rough indications of temperature and was essentially useless as a geobarometer. Sphalerite displays complex polytypism in the Bolivian ores, but this could not be correlated with variables of geologic interest. The present iron content of sphalerite reflects initial equilibration with pyrrhotite or pyrite, but its significance in terms of temperature is debatable. Miscellaneous thermometers such as sulfide invariant points, the high-low quartz inversion, melting of native bismuth and sulfide exsolution temperatures are considered and some yield useful temperature limits. Fluid inclusion studies reveal highly systematic trends of salinity and depositional temperature during formation of the tin and tungsten ores. Tin-bearing fluids of the early vein stage were complex, NaCl-rich brines of low CO 2 content. During precipitation of cassiterite and early quartz, salinities reached values as high as 46 weight percent, but the later ore fluids were more dilute and gradually approached fresh water in closing stages of mineralization. Depositional temperatures first increased from about 300 degrees to 530 degrees C during the early vein stage and then declined steadily to less than 70 degrees C in the later stages. Minerals of the main sulfide stage formed as temperatures declined from about 400 degrees C to 260 degrees C. Fluid inclusions in siderite indicate temperatures of 260 degrees C to 200 degrees C for the hydrothermal alteration of pyrrhotite. Late fluorite of the Colquiri lode was precipitated at temperatures between 200 degrees C and 133 degrees C. The late hydrous phosphate minerals formed below 70 degrees C. Many events of the hypogene sequence could be attributed chiefly to cooling of the vein fluids and are so interpreted, but little is known of the wall rock alteration and its influence on paragenesis. Active boiling of the early vein fluids is indicated by the fluid inclusions in quartz and cassiterite. This boiling upgraded salt contents of the early vein fluids while driving CO 2 into the associated vapor phase. This vapor transported some mineral matter, as evidenced by daughter minerals in gas-rich inclusions, but the identity and quantitative significance of this material are unknown. The early boiling favored precipitation of quartz and cassiterite and may explain the restricted vertical distribution of very high-grade tin ore in several deposits of shallow origin in central and southern Bolivia. Evidence of boiling in several major deposits of the Cordillera Real suggests that these ores, though formed in the plutonic setting, were still deposited at depths of less than 3,000 m. This limit is compatible with that based on stratigraphic reconstructions of rock cover at the time of mineralization. All data gathered in this study are consistent with a single, prolonged "event" of mineralization for the individual tin and tungsten deposits throughout Bolivia. The deposits could differ in age, but there is no convincing evidence that they are hybrid products of hydrothermal regenerative processes. A magmatic source is preferred for the mineralizing NaCl brines and their contained metals, but the progressive cooling and dilution of these brines may reflect gradual influx of meteoric water to the magmatic hydrothermal system.

Studies of hydrothermal gold deposition - (pt.) 1, carlin gold deposit, nevada, the role of carbonaceous materials in gold deposition

Abstract: Studies of fresh carbonate host rocks and unoxidized gold ores of the Carlin mine indicate that gold, quartz, barite, pyrite, and other sulfides were introduced into the Roberts Mountains Formation by acid hydrothermal solutions. Laboratory investigations on the carbonaceous materials in the host rocks and ores and studies of reactions between carbonaceous materials and gold-bearing solutions show that the rocks contain: (1) An activated carbon component capable of adsorbing gold chloride or gold cyanide complexes from solution; (2) A mixture of high-molecular-weight hydrocarbons usually associated with the activated carbon components; and (3) An organic acid, similar to "humic acid," containing functional groups capable of interacting with gold complexes to form gold organic compounds. Although the exact structure of the gold organic compound(s) is not known, the most attractive possibility is chelation, where ligands such as N, S, or O in organic acids would easily displace the chloride ion from aurous chloride complexes and form stable gold chelates. Subsequent oxidation of the gold organic compounds destroys the organic component and leads to the formation of metallic gold. The relative amounts and types of carbonaceous materials are of principal importance in determining the chemical state and amount of gold deposited in carbonaceous limestone, although such factors as temperature, pH, and the oxidation state of the system are also involved.Although most of the gold ores at the Carlin mine are in the Silurian Roberts Mountains Formation and are at least several hundred feet stratigraphically below the Roberts Mountains thrust fault, disseminated replacement-type gold deposits could form in the lower-plate Devonian limestones (designated the Popovich Formation at the Carlin mine) and in carbonate and shale units of the upper-plate Ordovician Vinini Formation. In a structural setting that would provide satisfactory channels for movement of gold-bearing solutions, physical characteristics and chemical and mineralogical compositions, including the presence of organic materials, indicate that all three formations are favorable for replacement and deposition of gold.

Cryptic variation and the petrology of the Sudbury nickel irruptive

Abstract: Silica-rich magma, gravitational crystal settling, funnel-shaped body, norites and gabbros, Ontario, Canada

Silver depletion indicated by microanalysis of gold from placer occurrences, western United States

Abstract: Silver-depleted rims on gold grains, removal of silver probably by low-temperature, oxidizing aqueous solutions, possibility of determining lode source by silver content of grain interior, electron microprobe analysis, mineragraphy

Zonal relations of wallrock alteration and sulfide distribution at porphyry copper deposits

Abstract: Most of the porphyry copper deposits are characterized by a similar pattern of lateral zoning involving sulfides and silicate alteration assemblages. A central zone of biotite-orthoclase alteration is surrounded by a partial to complete halo of quartz-sericite alteration. Propylitic alteration commonly forms a further halo if rocks of appropriate composition are present. The hypogene sulfides also are zoned, from a central zone with a low pyrite/chalcopyrite ratio and generally low total sulfides, through a zone of moderate sulfides with dominant chalcopyrite, then to a halo of high pyrite/chalcopyrite and high total sulfides which grades outward to lower total sulfides. The highest copper content normally occurs within the biotite-orthoclase zone or at its outer borders with quartz-sericite alteration. The silicate and sulfide zoning is generally centered on a granitic porphyry stock.Lead and sulfur isotope data, as well as the zoning, suggest that the ore fluid traveled outward from the porphyry, starting at high temperatures and relatively high K/H ratios in the orthoclase field, thereby forming the biotite-orthoclase alteration. In order to form the strong sericite alteration, cooling of the fluid is concluded to be the most probable means of changing mineral equilibria. A model calculation, for a large porphyry copper ore body, of heat losses to wall rock during outward flow of the solution shows that this mechanism cannot be the major means of cooling. The calculation also demonstrates that mineral deposition from solutions as dilute as 1-100 ppm base metals encounters grave difficulties in cooling from temperatures of 500 degrees C or above, because of the huge volumes of solution involved to form a porphyry copper orebody, and the resulting huge amount of heat that must be dissipated. Copper concentrations in the ore fluid on the order of 1,000 ppm or higher are therefore suggested. Cooling by expansion or by mixing with cool ground water seems an attractive means of bringing the high temperature ore fluid into the muscovite stability field. These processes are probably also instrumental in causing metal sulfide precipitation.

A study of precious metals in the Sudbury nickel irruptive ores

Abstract: Ru-Os-Ir enrichment in hanging wall, Pd-Au enrichment in footwall, Pd and Au fractionated between chalcopyrite and pyrrhotite, no strong fractionation of Ru, Os, and Ir, geochemistry and thermodynamics of ore genesis, neutron activation analysis, Strathcona mine, Ontario

Strength and deformation of pyrite and pyrrhotite

Abstract: Brittle fracture of pyrite, twinning and translation-gliding plastic deformation of pyrrhotite

Native sulfur deposition in the Castile Formation, Culberson County, Texas

Abstract: Permian, sulfur-calcite and associated anhydrite, petroleum migration, bacterial reduction of anhydrite

The Bluebell Mine, British Columbia; I, Mineralogy, paragenesis, fluid inclusions, and the isotopes of hydrogen, oxygen, and carbon

Abstract: Pressure, temperature, and depth of sulfide mineralization, geochemistry and origin of hydrothermal solutions, earlier fluids possibly juvenile, later fluids meteoric

Solubility of gold in hydrothermal alkaline sulfide solutions

Abstract: Formation of AuS (super -) as transport mechanism in near-neutral low salinity hydrothermal solutions, effective means for genesis of epithermal gold deposits

Stability of the tetragonal polymorph of Cu 2 S

Abstract: Inversion to chalcocite (orthorhombic polymorph) at atmospheric pressure, chalcocite inversion to hexagonal polymorph at 103.5 degrees C and cubic polymorph at 420 degrees C (at atmospheric pressure)

Similarities, differences, and some genetic problems of the wyoming and colorado plateau types of uranium deposits in sandstone

Abstract: Uranium deposits of the Wyoming roll type and the Colorado Plateau peneconcordant type are the principal domestic resources of uranium. Both types occur in lenticular sandstone beds of continental origin, have a similar suite of elements and minerals, are associated with mildly altered rock, and are thought to have formed by reduction precipitation from ground waters before significant regional deformation. They differ in several respects. The Wyoming deposits are in unlithified sandstone that is highly arkosic, whereas the Colorado Plateau deposits are in lithified sandstone that is only slightly to moderately arkosic. Uranium is the only ore metal in the Wyoming deposits, whereas vanadium or copper is more abundant than uranium in some of the Plateau deposits. The Wyoming deposits are elongate crescent-shaped bodies that extend vertically through, or partly through, a sandstone unit and which are scattered, like widely spaced beads on a string, along miles-long interfaces between oxidized (altered) and unoxidized sandstone, whereas the Plateau deposits are thin tabular layers that are nearly concordant to bedding and which occur as discrete bodies, like raisins in raisin bread, enveloped in rock altered by reduction. The Wyoming ore rolls and interfaces were dynamic, having been pushed downdip by downward-moving oxygen-bearing water that passed through the interfaces and deposited the ore minerals on the reducing side, whereas the Plateau deposits seemingly formed as static bodies, localized by intensive reducing "patches" in a mildly reducing environment. These differences focus attention on genetic problems relating to the Eh of the ore-bearing and altering solutions, the shape and localization of deposits, and the source of the uranium.

Multiple regression as a method of estimating exploration potential in an area near terrace, b.c

Abstract: The aim of this study was to investigate the usefulness of multiple regression in defining target areas for detailed mineral exploration, given only general geological information and production and/or reserve data for all known deposits of a specific type in a given mining camp. An area near Terrace, B.C., for which reserve and production information for 50 mineral deposits was available, was divided into 128 4-mile by 4-mile cells by random positioning of an appropriate grid. Geological variables were measured and estimates of value obtained for each cell. Cell "value" was stated in terms of tonnage (production and/or reserves), average value per ton, and total value, the latter two in 1969 dollars.Two control areas of 35 and 28 cells, respectively, were chosen somewhat arbitrarily. Measured variables from the control areas were transformed by means of relatively simple transformations, to produce near-normal frequency distributions. Linear equations were then obtained for the control areas by stepwise multiple regression using various estimates of value as the dependent variable, and geological measurements as independent variables.Equations obtained for cell data from one of the control areas appear reasonable both geologically and statistically. Consequently, the equations were applied to the remaining cells to estimate "exploration potentials" or values. High calculated values represent high priority cells and vice versa.

Distribution of silver and copper in placer gold derived from the northeastern part of the colorado mineral belt

Abstract: Placer gold grains from the modern streams originating in the Colorado Mineral Belt were examined for silver and copper content on a quantitative basis utilizing the electron microprobe The variation among grains from a particular locality is large, but the mean silver content of the interior of the placer gold grains from each locality and (or) the variation in copper content may be of value in distinguishing lode sources and gold mining districts Microprobe analysis of the interior of gold grains is independent of chemical actions that affect the border of placer gold grains during their transport history, and it is shown that distinct compositional groups of different lode sources may be identified even in a single sample--information that may aid in recognizing the existence of concealed lodes that once contributed to a placer environment

Ore solution chemistry - (pt.) 3, solubility of cus in sulfide solutions

Abstract: The solubility of covellite (CuS) was measured in aqueous sulfide solutions under varying conditions: up to 4.95 molal HS (super -) ; pH 3.5 to 13.5; 20 degrees to 200 degrees C; and 2 to 83 atm total pressure. The solubility of covellite is a function of the concentration of HS (super -) , H 2 S pressure, and temperature. Three important copper-bisulfide complexes are formed, the stability of which depends on pH and temperature. The following reactions represent the formation of the complexes predominating in the pH ranges given, at the temperatures, pH9s, and ionic strength (mu ) indicated:At 25 degrees C: pH 2 S(g) + HS (super -) = Cu(HS) 3 (super -) K = 10 (super -4.40+ or -0.20) (mu = 0)K9 = 10 (super -4.25+ or -0.20) (mu = 1.0)pH > 7.3 CuS(s) + 3HS (super -) = CuS(HS) 3 (super 3-) K = 10 (super -3.67+ or -0.20) (mu = 0)K9 = 10 (super -3.67+ or -0.20) (mu = 1.0)At 200 degrees C: pH 2 S(g) + 2HS (super -) = Cu(HS) 4 (super 2-) K = 10 (super -2.30+ or -0.3) (mu = 0)K9 = 10 (super -3.60+ or -0.3) (mu = 1.0)pH > 6.6 CuS(s) + 3HS (super -) = CuS(HS) 3 (super 3-) K = 10 (super -2.13+ or -0.20) (mu = 0)K9 = 10 (super -2.12+ or -0.20) (mu = 0)The boundary between the complexes Cu(HS) 3 (super -) and CuS(HS) 3 (super 3-) is dependent on the total sulfur concentration (Sigma S = HS (super -) + H 2 S) of the solution and ionic strength. The pH limits given here are for mu = 1.00. The pH boundary at 200 degrees C is relative to a neutral pH of 5.6. There is no evidence for the existence of CuS-H 2 S complexes in the very low pH region ( approximately 3), or CuS-S (super 2-) complexes in the high pH region (10-13.5). Solubilities calculated using the above data compare well with those observed in these experiments.If it is assumed that a solubility of at least 10 ppm Cu is necessary for significant ore transport, bisulfide complexes are potential ore carriers at 200 degrees C and a total sulfur concentration of 0.25 molal, or greater, at neutral to weakly alkaline pH9s. However, complexes between Cu and either NH 3 or Cl (super -) in bisulfide solutions are not geologically important, at least at low temperatures.Covellite transported as bisulfide complexes would be deposited by any mechanism that lowers the stability of these complexes, such as a decrease in temperature or pH, oxidation, or dilution. The conditions of covellite deposition obtained in the laboratory experiments agree well with those deduced from natural occurrences, although additional solubility data are needed for the more common copper sulfides digenite and chalcocite.

Some effects of deformation on sulfide-rich layers in lead-zinc ore bodies, Mount Isa, Queensland

Abstract: Folded galena-rich and sphalerite-rich layers, solid state processes of mineral migration with minor fluid transport in galena layer, recrystallization and local mineral transport in sphalerite layer, low-grade regional metamorphism, mineralogy, textures, chemical analyses, Australia

Content of zinc and copper in some fluid inclusions from the Cave-in-Rock District, southern Illinois

Abstract: Hydrothermal fluids trapped in fluorite, quartz, and barite at time of sulfide ore deposition, interfering boundary relations indicative of simultaneous growth of minerals, paragenesis, atomic absorption analysis for Zn and Cu, x-ray fluorescence for Cl, variations in elements with time and space

Uranium mineralization and alteration, Orphan Mine, Grand Canyon, Arizona

Abstract: Occurrence in pipe structure (probable solution collapse feature), ore solutions possibly from distant hydrothermal source with subsequent mixing with ground water, lithology, mineralogy, fluid inclusion geothermometry, hydrothermal alteration, isotope age of 141 m.y. (upper Jurassic) for mineralization

A suggested volcanigenic origin for certain gold deposits in the slate belt of the North Carolina piedmont

Abstract: Geological studies in the Piedmont of central North Carolina have suggested several lines of evidence that certain gold deposits may be related to volcanism in genesis. The area considered here, in south-central North Carolina, is underlain by rocks of the Carolina slate belt and is bounded on the west by the Charlotte belt plutonic complex, and on the east by the Deep River Triassic basin. Rocks of the Carolina slate belt include a basal sequence of felsic pyroclastics overlain by argillite, tuffaceous argillites, and one or more later volcanic sequences. The region has been gently folded, weakly metamorphosed, and intruded by minor mafic sills and dikes.Lode gold deposits in this part of the Piedmont commonly are zones of silicified and weakly pyritic strata bearing low-grade gold ore. The deposits are largely conformable and are almost entirely within the basal volcanic unit or a short distance stratigraphically above it. In as much as the area has no prominent granite intrusives, is only weakly metamorphosed and has only minor evidences of subsequent hydrothermal mineralization; the strata-bound gold lodes seemingly must be referred to their enclosing volcanic strata for their origin. A volcanigenic origin for the gold deposits in the waning exhalations of the basal felsic volcanic pile seems to best account for the form, composition, and areal distribution of these deposits. Such an origin has certain similarities to many epithermal precious-metal deposits located in more recent volcanic fields in the western United States.

Natural work-hardening in galena, and its experimental reduction

Abstract: Hardness as a deformation effect, varying degrees of deformation and recrystallization in co-existing ore components

Statistical decision in regional exploration; application of regression and Bayesian classification analysis in the southwest Wisconsin zinc area

Abstract: A statistical decision model based on regression and Bayesian classification techniques is designed to help predict the most favorable drilling targets in regional exploration programs. The procedure is applied to a 225 square mile portion of the southwest Wisconsin zinc area hitherto little explored. The analysis led to the selection of 22 targets with high success probabilities (0.80 or better) and high regression estimates.The validity of the method depends on the quality of field data and on a careful selection of relevant factors rather than the nature of the field techniques used for the coverage. The aim of the method is to assist the exploration management in making objective decisions fully consistent with the initial goals of the programs.

Phase relations below 575 degrees C in the system Ag-As-S

Abstract: Quenching experiments were used to define the thermal stabilities of phase assemblages in the system Ag-As-S. Extensive liquid immiscibility occurs in this system at high temperatures and immiscible liquids are present even below 400 degrees C. Five ternary solid phases were encountered: proustite and xanthoconite (both Ag 3 AsS 3 ), smithite and trechmannite (both AgAsS 2 ), and Ag 7 AsS 6 . The phase relations of proustite were studied in detail. The maximum temperatures at which proustite can coexist with either Ag 7 AsS 6 , Ag 2 S, As, or smithite are, respectively, 480+ or -2 degrees , 469+ or -3 degrees , 452+ or -2 degrees , and 409+ or -4 degrees C. These minimum-melting temperatures define upper-temperature limits (un-corrected for confining pressures) for deposition of corresponding assemblages in ores. The xanthoconite --> proustite transition occurs between 180 degrees and 202 degrees C (confirming previous data) and the assemblage proustite + silver is unstable relative to Ag 2 S + arsenic when heated above about 190 degrees C. Both reactions provide useful indicators for geothermometry. Equilibrium assemblages of smithite + realgar and smithite + orpiment could not be synthesized, although both assemblages occur in nature; phase relations show that the AgAsS 2 modification associated with either realgar or orpiment would be trechmannite if equilibrium were maintained during low-pressure deposition and subsequent cooling.

Genesis of manganese deposits in northern Chile

Abstract: Deposition by circumvolcanic hot springs during Pleistocene, deposits at modern erosion levels due to secondary mobilization