Ore bearing fluids

Ore bearing fluids

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  1 ORE-BEARING FLUIDS BY PROF.A.BALASUBRAMANIAN An"ore mineral" is a mineral used for the extraction of one or more metals. An "ore”, is that part of a geological body from which the concentrated metal or metals may be extracted commercially for profit. Minerals are formed by changes in chemical energy in systems which contain one fluid or vapor phase. In nature, minerals are formed by crystallisation or precipitation from concentrated solutions. These solutions are called as ore-bearing fluids. Ore-bearing fluids are characterised by high concentration of certain metallic or other elements. Fluids are the most effective agents for the transport of material in the mantle and the Earth's crust. Supersaturation and consequently crystallisation or precipitation are controlled by the thermodynamic environment of the system. Composition, volume, viscosity, depth, temperature and pressure are the most important factors of the ore-bearing fluids. At greater depth, these fluids may be magmatic fluids or aqueous hydrothermal fluids. Hydrothermal fluids play a major role in many metamorphic reactions and magmatic processes. Aqueous hydrothermal fluids are responsible for the formation of many ore deposits. These metal- bearing fluids migrate at different levels in the earth’s crust. Metals are largely derived from the mantle or crust by partial melting and fluid-related leaching. Ligands can be provided from the same sources, or from the atmosphere, hydrosphere and biosphere. Transport is mainly by mechanical or mass transfer mechanisms done by the fluids. Hydrothermal fluids are a major transport medium for many ore systems. These fluids essentially contain the major viscous mass, water, with lesser and variable amounts of CO2, H2S, SO2, CH4, N2, NaCl and other salts, as well as dissolved metal complexes. The Source and character of ore-bearing fluids are very essential aspects to be studied. The include: a) Sources of the ore constituents b) How they are placed in fluid or in solution form c) Migration of those ore-bearing fluids d) Manner of deposition of the ore minerals from them. Ore-forming fluids are derived from a variety of sources, including i) water-rich silicate melts, ii) circulated sea, connate and meteoric waters, iii) formational, diagenetic and metamorphic fluids. At upper crustal levels, the fluids are typically hotter than the rocks they traverse and in which they deposit their ores. pH is the hydrogen ion concentration of the fluids. Eh is the electrochemical potential controlling the oxidation-reduction processes of the phases. Ore-bearing fluids have variable pH and Eh. They may be charged with a range of metal complexing agents including Cl- and HS-. Cooling, crystallisation, precipitation, and solidification are the major mechanisms involved in the formation of ore bodies. These are common to igneous, sedimentary and metamorphic processes. On
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  2 and near thesurface, biological processes can also concentrate and transport ore components or remove non-ore components. Deposition of ore minerals results from changes to physiochemical parameters, including temperature, pressure, pH, Eh- redox state and total concentration of ligands. These changes are associated with such processes as addition of components by contamination, phase separation, cooling across a temperature gradient, decrease in pressure increase or decrease in volume, mixing of fluid and the reaction happening with the host rocks. The vast number of ore deposits, their types and their particular elemental compositions result from a complex interplay of favourable combinations of source, transport and depositional variables. Formation of minerals In most cases, the formation of minerals involves a chemical reaction subject to pressure and temperature. This may be brought about in several ways: 1. By reactions between liquids or liquid solutions. 2. By reactions between gases or gaseous solutions. 3. By reactions between liquids or liquid solutions and gases. 4. By reactions between solids and liquids or Liquid solutions or gases. Few minerals are formed below the freezing point of water. Their upper limit of development is marked by the temperature at which they become unstable or melt. Pressure: The decrease of pressure, which results when solutions ascend in the earth's crust, will be favorable to precipitation. The effects of uniform, or hydrostatic, pressure are much less marked than are the effects of stress, or unequal pressure. Under conditions of stress, a given pressure will lower the melting temperature far more rapidly than when the pressure is equal from all sides. Influence of Temperature: In a solution of various salts in water or in a silicate melt changes in temperature are far more effective in producing precipitation than changes in pressure. In the great majority of cases, increase of temperature promotes the solubility of salts, and decreasing temperature due to cooling of ascending thermal waters or of magmas will promote precipitation. In any hot, complex solution, decreasing temperature will cause precipitation of some mineral; with continued cooling a series of other minerals may also be precipitated. Precipitation by Evaporation of the Solvent: The salts contained in a solution are naturally precipitated when evaporation at the surface is increased. Precipitation by Reaction between Solutions: Mingling of different solutions is one of the most common occurrences in nature. Precipitation by Reactions between Aqueous Solutions and Solids: In nature, solutions act constantly upon solid minerals. Precipitation by Reactions between Gases or between Gases and Solutions.—Gases may produce precipitation in solutions. Nature of ore component- Crystalline Minerals: The minerals may be precipitated as crystalloids or as colloids. In mineral deposits formed in depth and at temperatures higher than those prevailing at the surface crystalloids are almost exclusively
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  3 present. Crystalline mineralsdevelop best by slow precipitation in solutions contained in open spaces. Colloids: A number of minerals are formed both as crystalloids and as colloids. Colloid state is not a separate kind of matter but simply crystalloids in a state of dispersion ranging from comparatively coarse suspension down to almost molecular subdivisions. A colloid mixture is thus a two-phase, heterogeneous system. Observations which helped to decide all these aspects are: a) Field observations , b) Thermal springs c) Volcanic gases, d) Emanations e) Accompanying gangue minerals f) Lab studies, g) Isotopic and geochemical studies. 1. Magma and Magmatic fluids: a) Molten melts- composition, deepness, high temperature, heterogeneous nature b) Generated, transported and solidified c) Cooling of magma- resulting rocks and minerals ( Fractional crystallisation, magmatic differentiation, petrological differences, partly crystallized magma, filter pressing, magmatic injection, intrusion, extrusion, ore magmas, solution, liquid immiscibility, etc) 2. Hydrothermal fluids: Magmatic waters(juvenile)- mineralizers, hydrothermal fluids, contain volatiles, mobile elements, dissolved minerals. The fluid properties- viscosity, density, solubility of ore compounds. 3. Meteoric waters ( atmospheric) Water of whatever origin that has equilibrated with the atmosphere is called meteoric water. In equilibrating with the air, water dissolves air and thus N, O, CO2 and traces of rare gases. These waters have specific H and O isotopes and are easily tagged. SMOW: Standard Mean Oceanic Water. (Essential to supergene processes.) Waters of surficial origin- rain, snow, mist, dew, frost. Dissolved constituents- tags. 4. Seawater- ore forming fluids- evaporates, phosphorites, submarine exhalites, manganese nodules, oceanic crust deposits. 5. Connate waters- Fossil water. This water has been out of contact with either the atmosphere or hydrosphere for a geologically significant period of time. Mostly contain Cl. If metamorphosed, could be significant carriers of metals. 6. Metamorphic fluids- metamorphic waters- recrystallisation, volatile and mobile constituents. Role of High temperature. 7. Thermal springs- solfataras. Hot springs, geysers. Fumeroles, groundwater. 8. Mine waters. Movement of ore-bearing fluids:  How various fluids move through rocks?. All fluids migrated at all levels in the earth’s crust.  Movement under the ground is considered as a very significant factor in ore genesis.  Concentration of oil, gas, emplacement of dikes,etc  Viscosity, density, permeability, structures, depth  Reason for upward movement. 1. Migration of magma a. Moved upward to areas of lowest pressure and temperature b. Contained dissolved gases and water c. injected into overlying rocks, breaking rocks- Sills and dikes- d. Mechanism of intrusion.
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  4 Magma- Melting, buoyant Largescale structures, Assimilation, Engulfing Homogenisation of blocks and of country rocks Gas fluxion, gas cooling, Collapse of wall rocks into gas filled pockets. Tectonic squeezing(Tooth paste tube effect). 2. Origin of porosity and permeability Primary and secondary porosity Primary and secondary permeability Intrinsic=primary Induced= secondary Capillary openings a) Super Capillary > 1 mm b) Capillary 0.01 – 1.0 mm c) Sub-capillary < 0.01mm. 3. Migration of hydrothermal fluids at greater depth 4. Migration of hydrothermal fluids at shallow depth 5. Ground penetration 6. Structural controls 7. Primary and intrinsic permeability 8. Secondary or superimposed permeability 9. Hydrothermal flow mechanism 10. Sources of ore deposit components 3. Deposition of ores: Magmatic segregation deposits, Deposition of carbonatites, Deposition from hydrothermal fluids Depositional forms and textures- replacement textures, exsolution, open-space-filling- open-space filling textures, colloidal- colloform textures; Chemical control- cobalt, limestone-lead and zinc, sulphur, etc 4. Wall-rock alteration and gangue minerals - relation between alteration, gangue and mineralisation - temperature, pressure and composition gradients -reactions between wall-rocks and fluids -alteration assemblages -Presentation of alteration details -quantification of alteration -Distribution of altered assemblages Alteration associated with: a) Porphyry- base metal deposits/ Magmatic deposits b) Skarn deposits/ Vein deposits/ Epithermal deposits c) Pegmatites/ Volcanogenic massive sulphide deposits/ Other deposits of lead-zinc, uranium, etc. Gangue minerals 5. Paragenesis Sequences Zoning in ore deposits- mineral zoning, assemblage zoning,etc Paragenetic sequence- tin zone, copper zone, lead-zinc zone, iron-antimony sulfosalt zone, Zoning- regional zoning, district zoning, ore-body zoning.
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  5 6. Geothermometry Geobarometry Isotope studies-stable isotope studies, radioisotope studies.