Formation Of Ore Deposits Research Articles

Inliers of banded iron‐formation in the Proterozoic Wyloo Group sediments near Paraburdoo, Western Australia

Abstract Two inliers with a total outcrop length of 3000 m and a maximum width of 200 m, consisting of a sedimentary klippe (olistolith) and an olistostrome (both composed of banded iron‐formation and shale belonging to the Hamersley Group) occur within the Mininer Turbidite Member of the Wyloo Group, south of Paraburdoo, W.A., 2500 m from the top of the Hamersley Group proper. The olistostrome is a typical debris slide produced by slumping of unconsolidated material. The klippe was rafted into position as a solid block by a turbidity current. The pattern of mineralisation within the banded iron‐formation part of the klippe, which is identified as being from the Brockman Iron Formation, together with evidence from the basal conglomerate of the Wyloo Group, shows that the formation of the Hamersley iron ore deposits commenced prior to the deposition of the Wyloo Group sediments.

Geothermal zones get close scrutiny

A recent study of the interaction of groundwaters and brines with magma‐heated zones in the crust could result in a number of far‐reaching geophysical implications if the conclusions reported by J.C. Dunn and H.C. Hardee are correct (J. Volcanol. Geotherm. Res., 11, 189–201, 1981). Fluid convection in near‐critical aqueous groundwater solutions is strongly enhanced, resulting in significantly enhanced heat flow. Single‐ and two‐phase flow may lead to processes that cause shallow pooling of magma in the crust. The solutions are chemically active and could initiate formation of extensive hydrothermal ore deposits. Geophysical data and experimental values make it possible to predict depths to superconnecting zones in the crust. Improved heat transfer in these zones could be responsible for the development of geothermal fields in the southwestern United States and other areas in the world that overlie shallow magma zones.

The formation of the volcanic-hosted massive sulfide ore deposit at Rosebery, Tasmania

The Rosebery pyritic zinc-lead-copper-silver-gold orebody occurs in dominantly felsic volcanic rocks of Cambrian age in western Tasmania. Ore formation occurred in a marine environment following eruption of a thick pyroclastic unit consisting mainly of welded, ashflow tuffs, and subsequent subsidence. The main orebody consists of a discontinuous massive sulfide horizon and in the southern part of the mine there is a generalized metal zonation with a lower and central Cu-Fe-rich zone surrounded and overlain by Zn-Pb-Ag-rich ore. Barite-sulfide ore occurs as separate lenses higher in the sequence separated from the sulfide orebody by unmineralized, cleaved siltstone.The orebody is underlain by an extensive zone of alteration marked by depletion of Na and Sr and enrichment in Rb, K, Mg, Mn, and H 2 O. The pyrite and chalcopyrite content of the altered footwall tuff and the Co content of the pyrite is highest beneath the Fe- and Cu-rich zones in the sulfide orebody and a feeder is inferred in this area.The delta 34 S values for sulfides in the sulfide orebody are higher in the richest Zn-Pb-Ag ore (>14.1ppm) than the Fe-Cu ore (>7.8ppm), a pattern paralleled by a decrease in the Fe/Fe + Mg + Mn ratio of chlorite and a decrease in the FeS content of sphalerite. The delta 34 S values for sulfides in the barite ore are even higher (14.5 to 19.8ppm) and the FeS content of sphalerite lower. Barite throughout the barite and sulfide orebodies is fairly constant between 34.6 and 41.2 per mil, but that in the sulfide ore appears to be epigenetic. The delta 34 S values for barite-sulfide pairs indicate a temperature range of 255 degrees to 298 degrees C for the barite orebody, but the temperature of barite ore formation was probably about 250 degrees C.The use of delta 34 S values for sulfides as stratigraphic markers indicates that the sulfide orebody is diachronous. It is possible to account for the metal and delta 34 S distribution by assuming that the ore solutions are buoyant on reaching the sea floor during the Fe- and Cu-rich phases but show reversing buoyancy during the Pb-Zn-rich phase.The coexisting arsenopyrite-pyrite-chlorite-quartz assemblage in the sulfide orebody defines a range of aO (sub 2 (super -) ) T conditions using the six-component solid solution chlorite model of Walshe and Solomon (1981). The maximum indicated temperature of 300 degrees C is consistent with the minimum temperature required by iron and copper solubilities. However, the chlorite compositions may be postdepositional, as seems to be the case for the FeS contents of sphalerite. The delta 34 S (sub Sigma S) for the sulfide orebody is probably derived by almost complete reduction of Cambrian seawater sulfate mixed with sulfur leached from underlying rocks. The delta 34 S values for the barite orebody may represent partial reduction of seawater sulfate.The ore fluid is supposed to have been generated by convective circulation of seawater in a rock pile heated by a Cambrian granitoid pluton.

Formation of Uranium Ore Deposits

Formation of Uranium Ore Deposits

Metal complex formation in aqueous solutions at elevated temperatures and pressures

Despite considerable current interest in metal complex equilibria in aqueous solutions at high temperatures and pressures generated by workers in the fields of geology, corrosion science and hydrometallurgy, there are surprisingly few data available. Geologists are now in a position to tell us much about the conditions of formation of hydrothermal ore deposits (e.g. temperature, pressure, oxidation potential, pH, fluid salinity, etc.), but in order to fully understand the mechanisms of transport and deposition of ore minerals, information about metal complex stability and stoichiometry in high temperature-high pressure aqueous electrolyte solutions is necessary. In addition, the treatment of corrosion problems connected with geothermal power development, nuclear reactor technology and conventional steam generating electricity plant facilities requires an understanding of complex equilibria under extreme conditions. Most of the data available in the literature on metal complex equilibria at elevated temperatures and pressures are represented in Figs 6.1-6.5, and I have intentionally included only experimentally based data. I have arbitrarily chosen to consider only those experimental data where T >/ 100°C and P > 1 bar, although I have naturally included lower temperature data when they were acquired as part of a study over an extended range of temperatures and pressures. Unfortunately, most of the studies have been performed only at the saturated vapour pressure of the system and very few combined high temperature-high pressure experiments have been designed to yield thermodynamic data. Not surprisingly, the majority of the data have been obtained by geochemists during the past decade and for this reason there has been an emphasis in studying the stability of chloro, hydrosulphido (and sulphido) and hydroxo complexes. There are also a few studies of simple metal-sulphato and carbonato complex formation. Because of the ubiquitous distribution of chloride in natural geothermal systems, the study of chloro complexes of heavy metals at elevated temperatures and pressures has been of much interest to geochemists. In addition to the data given in Fig. 6.1, Ivanenko and Pamfilova (1975), have also determined the solubility product and formation constants for AgCI~ -x (x = 1-3) up to 160°C but their values are in only moderate agreement with those of Seward (1976) (Fig. 6.1). Gavrish and Galinker (1964) have determined the concentration stability constants for AsCii-, AgBr£and AgI£ in sodium chloride solutions at 300°C. The formation constants for ZnCI~ were determined in a high temperature anion exchange study by Kraus and Raridon (1960) to be Kl = 370 and 2000 at 115 and 150°C respectively. The species GaClg was also shown to exist in concentrated HC1 solutions up to 150°C. At higher temperatures and pressures, information on metal chloride complex formation becomes even more sparse. Chou and Eugster (1977) have recently treated their data on magnetite solubility in supercritical chloride solutions (T = 500-650°C and P = 2 kbar) in terms of the FeCI~ complex. Hydrosulphido and sulphido complexes at elevated temperatures and pressures have been studied by solubility methods and available data from such experiments are presented in Fig. 6.2 and are reviewed by Barnes (1979). Data on the stability of Pb(HS)3 and Pb(HS)~ up to 100 and 200°C respectively as well as for HPb(HS)~ at 200 and 300°C are given by Giordano and Barnes (1979).

Deep hypogene oxidation of porphyry copper potassium-silicate protore at Butte, Montana; a theoretical evaluation of the copper remobilization hypothesis

The oxidation effects of potassium-silicate protores from heated meteoric water have been thermodynamically computed and confirm previously described copper vein-forming mechanisms involving hypogene leaching and enrichment of chalcopyrite-bearing protore (Brimhall, 1979). Intense copper leaching of protore chalcopyrite is shown to be related to a state of peroxidation in which the only stable minerals are quartz, pyrite, and muscovite following the complete destruction of the wall-rock oxygen fugacity buffer assemblage including hematite dissolution. Because modification of the protore continues to higher fluid/rock ratios, copper is extracted from the fluid phase and again is fixed in the rock as sulfides, first as bornite and ultimately as chalcocite-covellite associated with the advanced argillic alteration assemblage. This end-stage assemblage should occur only where extremely high fluid/rock ratios are reached in zones of unusually high permeability near the source of the heat responsible for the fluid motion.The results of the calculations are interpreted to mean that certain intervals of fluid/rock ratio are characterized by net base metal leaching and other intervals by metal fixation. Attainment of only intermediate fluid/rock ratios may result in the major amount of the copper in a protore being remobilized into solution and leaving the system through ground-water circulation. In contrast, in highly evolved systems where the fluid/rock ratio has increased to extremely high values, the presence of an advanced argillic alteration assemblage with chalcocite and covellite strongly suggests the presence within a district of a partially oxidized, potassium-silicate, disseminated sulfide protore, including a strongly leached and sericitized quartz-pyrite protore relict.Near-surface vein mineralization may conceivably be related to primary or unoxidized protore at depth in two distinct ways, depending upon the origin of the intrusives and the fracture systems responsible for circulation of oxidizing meteoric water. First, late intrusives which are cogenetic with the magmatic heat source of the protore may remobilize ore-forming elements upon deep hypogene oxidation of pyritic disseminated sulfides. Second, much later, totally unrelated intrusive activity and deep crustal faulting, perhaps related to entirely different petrotectonic regimes, may provide access for meteoric water to penetrate much older protores, resulting in the formation of near-surface ore deposits by element recycling.It is therefore suggested that near-surface hydrothermal base and precious metal deposits be explored to considerable depths for possible low-grade protore potential.

Intrachamber differentiates of granitic intrusions and their role in the formation of ore deposits (as in Kirgizia)

Intrachamber differentiates of granitic intrusions and their role in the formation of ore deposits (as in Kirgizia)

Conditions of formation of ore deposits of volcanogenic belts

Conditions of formation of ore deposits of volcanogenic belts

Thermoluminescence of Uranium Host Rocks in Ambrosia Lake Area: ABSTRACT

Exposure to radiation during the formation or redistribution of uranium ore deposits is expected to affect the thermoluminescence (TL) properties of the host rock. For example, TL measurements on samples from a traverse of a Wyoming roll-type deposit display the following intensity-versus-distance pattern. Starting from barren oxidized ground the intensity increases gradually with decreasing distance to ore, is extremely high in ore, and drops abruptly with distance in reduced ground to a level substantially below that in oxidized ground. This pattern is in accord with current hypotheses concerning the genesis of this type of deposit. Measurements of both natural and artificial TL made on quartz separates from drill cores and samples collected underground in the Ambrosia Lake area, because of the intensity and structure of their respective glow curves, permit discrimination among oxidized ground, reduced ground, and mineralized areas. In addition, samples from oxidized ground show TL intensities and glow-curve characteristics that have been correlated with structural features that controlled the redistribution of ore. Although the data show some statistical variation, the general intensity-versus-distance pattern has been observed in many cases. All currently available results suggest that TL can be developed into a viable tool for uranium exploration. End_of_Article - Last_Page 689------------

Sedimentary and Tectonic Controls of Uranium Mineralization in Morrison Formation (Upper Jurassic) of South-Central Utah: ABSTRACT

Sedimentologic studies in the Henry basin of south-central Utah indicate that uranium deposits in sandstones of the Morrison Formation are related to the depositional environment in which the host sandstone bed and nearby mudstones were deposited and to the tectonic setting at the time of deposition. Thus, an understanding of environments of deposition and contemporaneous tectonism may be helpful exploration guides for uranium deposits in sandstones. Discontinuous tabular uranium orebodies are present in fluvial and marginal lacustrine sandstone beds that lie just above, below, or a short lateral distance from offshore lacustrine gray mudstone beds. The mudstones are present only in the distal part of the lowermost alluvial-plain sequence of the Morrison where fluvial energy regimes and rates of sedimentation were low, allowing lakes to form while the Henry basin was slowly subsiding. The lakes were restricted to the east side of the basin where the least amount of clastic sedimentation by streams occurred. Palynomorphs, carbonized plant debris, and scarce pyrite in the mudstones indicate that the lakes were sufficiently persistent and deep during most of their lifetimes for reducing conditions to persist in the offshore muds. The also indicate that the mudstones were originally gray and are not bleached red mudstones. The gray mudstones are not present in other alluvial-plain sequences of the Morrison because little or no basin subsidence occurred after deposition of the lowermost sequence. Plant debris and palynomorphs in the gray lacustrine mudstones suggest deposition in humus-producing lakes in which humic and fulvic acids (degradation products of plant tissues) were generated in the lake sediments. The close spatial association of the ore-bearing sandstones and the gray mudstones suggests that pore fluids containing these organic acids were expelled by compaction or seepage from the mudstones into nearby sandstones where they were fixed as tabular humate deposits. Subsequently, uranium in groundwater passing through the sandstone was concentrated by the humate to form the ore deposits. Gray mudstones which cannot be used as exploration guides are bleached red mudstones and primary gray mudstones containing the alga Botryococcus. Sandstone beds adjacent to these gray mudstones are barren of uranium because (1) the gray color in bleached red mudstones is an alteration feature and does not indicate primary reducing conditions, and (2) Botryococcus thrives in humus-free lakes whereas humus-producing lakes are considered necessary to form ore deposits. Thus, the requisites for mineralization appear to have been active crustal downwarping to cause ponding of fluvial sediments, and formation of humus-producing lakes on the alluvial plain, in the most sediment-starved part of the area undergoing basinal downwarping. End_of_Article - Last_Page 837------------

A theoretical study of hydrothermal convection and the origin of the ophiolitic sulphide ore deposits of Cyprus

Hydrothermal circulation of seawater has been suggested as a mass transport mechanism for the formation of sulphide ore deposits in the ophiolitic rocks of Cyprus. Since ophiolitic sequences are generally regarded as fragments of oceanic crust and upper mantle, hydrothermal circulation of a form inferred from geological observations on Cyprus may be analogous to that thought to occur in oceanic crust at spreading ridges. The hypothesis that ore deposits were formed in ascending plumes of hot, buoyant fluid is examined by considering thermal convection in a permeable medium. To match the inferred pattern of circulation, finite amplitude convection in a cylindrical geometry is studied using finite difference approximations. These results combined with available geological and geochemical data are applied to understand better the physical controls on mineralisation. A simple model for the formation of the hydrothermal ore deposits of Cyprus is discussed. The model is semi-quantitatively reasonable in terms of vertical fluid flow rate, thermal structure, permeability and basal heat flow, and predicts volumes of maximum mineralisation similar to those observed. Three factors are identified which were important in confining mineralisation to a small volume immediately beneath the sea water/rock boundary: (1) hot fluid was confined to a narrow core zone of a rising plume, (2) the upward fluid flux was greatest in this same core zone, and (3) significant temperature decrease occurred within a thin surface boundary layer.

Uranium-lead isotope systematics in uraniferous alkali-rich granites from the Granite Mountains, Wyoming; implications for uranium source rocks

Evaluation of U-Th-Pb isotopic analyses of whole-rock granite samples from the Granite Mountains, Wyoming, suggests that uranium has been mobilized at least twice since the intrusion of the granite. Whole-rock Th-Pb, whole-rock Rb-Sr, and zircon U-Pb ages are concordant at 2,600 m.y., but the whole-rock Pb-Pb age is distinctly older. This apparent age is attributed to a 10 to 45 percent uranium loss between 1,400 and 1,700 m.y. ago when the granite was subjected to an ill-defined metamorphic event that was probably of a thermal nature.A second and more extensive uranium loss occurred after the granites were exposed to near-surface conditions by the Laramide event. This loss has left whole-rock U-Pb ages variable and generally impossibly old. Granites that exhibit the largest recent uranium loss are characterized by high silica, thorium, and alkali contents and moderate biotite contents. These features are suggested as more diagnostic of potential granitic uranium source rocks than present-day uranium contents because the general 70 percent or greater recent loss of uranium has left many surface samples with a uranium content that has been considered typical of many granitic units.A few samples were found to have concentrations of some of the mobilized uranium (as much as 1,460 ppm U). Although these samples are not petrographically or chemically distinguishable from those that lost uranium, most are highly fractured. It therefore appears that fracturing may be an important control in forming ore deposits in granites.

大館市，北鹿地域孔井ＨＯ‐１粗粒玄武岩々床の分化

The bore hole HO-1 dolerite sill, which is about 120m thick and well-differentiated sill, intruded into the Okuzu formation (middle Miocene), about 5km NNW of Odate city, northeastern Japan in the middle Miocene age, after the formation of the Kuroko ore deposits in this district. After the intrusion, a magma having about 10 per cent by volume of olivine and auigte phenocrysts formed several segregation veins composed of soda monoznite and/or soda syenite subparallel to the contact by fractionation in situ. Thirteen representative rocks have been analyzed chemically (Table 1). Petrographically, the HO-1 dolerite belongs to the calc-alkalic basalt having high-alkali tholeiite (Kuno, 1968b) or olivine tholeiite (Yoder and Tilley, 1962) in composition. The compositions of olivine dolerite-soda syenite suite vary widely with the fractionation, showing the clear increase of SiO2 and Na2O and the decrease of CaO and MgO. The crystallization trend of the dolerite magma does not follow that of calc-alkalic series, but follows alkalic series. Such unusual trend would be attributable to the presence of small amounts of water in the magma.

An analysis of the cooling of intrusives by ground-water convection which includes boiling

A finite difference model of the cooling of an igneous intrusive of limited volume is developed and used to investigate the relation between igneous intrusion, the formation of liquid and vapor dominated geothermal systems, and the formation of porphyry-type ore deposits. The model takes into account the properties of pure water and accommodates the phenomena of boiling and condensation. Permeability, level of intrusion, and pluton volume are systematically varied. Pressure, temperature, and fluid velocity are computed as functions of time.It is found that a self-supported, vapor dominated steam zone is commonly (but briefly) formed above the intrusive. Condensed water bounds the steam zone above, and if the hydrothermal solutions are saline, a zone of boiling bounds the steam zone below. For pure water condensation is far more important than boiling--the solutions circulate around the critical point of water to become gaseous without boiling. Despite large temperature variations, convection causes fluid pressures throughout the whole uniform permeability system to be close to normal cold-water hydrostatic values. Thus, even in an active converting system with moderate permeability variations, fluid pressure will tend toward normal hydrostatic values. Fluid circulation appears easily sufficient to produce a typical porphyry copper ore shell, but base metal precipitation probably must be controlled by mechanisms other than simple temperature drop.

硫黄同位体温度計による対州，豊羽，神岡Ｐｂ‐Ｚｎ鉱床の生成温度の推定

The 34S/32S ratios of coexisting sphalerite and galena from Taishu, Toyoha and Kamioka Pb-Zn ore deposits have been measured to estimate the temperature of formation of ore deposit. All of the sphalerite samples were enriched in 34S compared with galena. Sulfur isotopic fractionation between sphalerite and galena ranges from 1.5 2.0 permil in the Taishu and Kamioka deposits, while in the Toyoha deposits ranges mostly from 2.5 to 3.0 permil. The fractionation of coexisting samples from early stage vein in Toyoha deposits is smaller than those from later stage vein. Using these isotopic fractionations and experimental temperature scale (Kiyosu, 1973), the temperature of formation is estimated to be 350° to 400°C in Kamioka deposits, 300° to 350°C in Taishu deposits and 200° to 250° in Toyoha deposits. These isotopic temperatures are slightly higher than filling temperatures determined from fluid inclusions of quartz and sphalerite.

Formation of Uranium Ore Deposits. Proceedings of a Symposium, Athens, 6–10 May, 1974

Formation of Uranium Ore Deposits. Proceedings of a Symposium, Athens, 6–10 May, 1974

Well water trace element reconnaissance, eastern maritime Canada

A geochemical survey involving the collection and analysis of about 2000 well water samples from an area of roughly 25,000 km 2 was carried out during the 1975 field season over parts of the Carboniferous basin of eastern Canada. This report describes the results of 1721 regional well sites. Three samples of water were collected from each site and analyzed in four different laboratories for twenty constituents. The distribution and relationships of U, Rn, He F, CH 4, Zn, Cu, Pb, Mn, Fe, pH, suspended matter, depth of well, conductivity and alkalinity are described. The elements U, Rn, He, F, along with conductivity and alkalinity, show systematic regional patterns indicating broad regional belts of element enrichment which are being leached by circulating groundwaters. The close spatial association of these elements over many tens of miles suggests chemical reaction cells or fronts similar to those observed in the uranium mining districts of Colorado and Wyoming in the United States and elsewhere. Their relatively weak character suggests an intermediate cycle in the cyclic enrichment hypothesis believed to be responsible for the formation of epigenetic uranium ore deposits. The heavy elements Zn, Cu, Pb, Mn, and Fe show positive correlation with each other but their spatial distribution is more spotty than that of the uranium elements. Broadly speaking the anomalies of these elements cluster mainly on the southern border of the survey area. Most of the anomalies can be explained in terms of known mineral occurrences The most prominent CH 4 anomaly is located south of Moncton and is believed to be due to the old St. Josephs gas and oil field. Several weaker CH 4 anomalies are probably generated by swamps and peat bogs.

Compendium on ancient fluid “Bubbles” published

A report on fluid inclusions, which are microscopic fluid ‘bubbles’ trapped inside mineral crystals for as much as a billion years, has been published. Geologists have found that these bubbles are useful clues in the search for ore deposits.According to the author, Edwin Roedder, the U.S. Geological Survey report is the most comprehensive summary of the subject in the English language. Roedder explained that fluid inclusions might be considered as miniature Rosetta Stones or Dead Sea Scrolls of geology. ‘They are clues to geologic events of the fat past,’ he said, ‘when ore deposits were forming in baths of salty water. Evidences of the formation of ore deposits lie in the common occurrence of microscopic droplets of fluids that became trapped in growing mineral crystals deposited by the hot water solutions moving through cracks in rocks millions or even more than a billion years ago.’

The time factor in formation of stratiform ore deposits

The time factor in formation of stratiform ore deposits

Abundance and distribution of manganese in the western part of the Philipsburg Batholith, Montana

Increase in manganese content from interior toward manganese replacement deposits at western edge of batholith, evidence for formation of ore deposits by local solutions from batholith at time of intrusion