Introduction to Ore Deposits

 Introduction

    Steffen Hagemann
     

    Source Material for Reading

      Abbreviations:

      Kesler = Kesler, S.E., 1994, Mineral resources, economics and the environment: MacMillian College Publishing Company, Inc., New York. (553 1997 GEO)

      ODM 1 = Ore deposit models, 1988, Edited by Roberts and Sheahan, Geoscience Canada, Reprint series 6 (553.1 1988 ORE)

      ODM 2 = Ore deposit models, 1988, Edited by Roberts and Sheahan, Geoscience Canada, Reprint series 3. (?)

      MDM = Mineral Deposit Modeling, 1993, Edited by Kirkham, Sinclair, Thorpe and Duke, Geological Association of Canada, Special paper 40. (550.15118 1993 MIN)

      Evans = Evans, A. M., 1993, Ore geology and industrial minerals - an introduction, Blackwell Scientific Publications/Third Edition, 390p. (553.1 1993 ORE)

      Guilbert = Guilbert, J.M., Park, Jr., C.F., 1985, The Geology of ore deposits, Freeman and company, New York, 985p. (553.4 1986 GEO)

      Sawkins = Sawkins, F. J., 1990, Metal deposits in relation to plate tectonics, Springer, New York, 461p. (553.4 1990 MET)

      Barnes = Barnes, H.L., 1998, Geochemistry of hydrothermal ore deposits. (553 1997 GEO)

      Craig+Vaughan = Craig, J.R., and Vaughan, D.J., 1981, Ore microscopy and ore petrography. (549.12 ORE)

      Mason+Berry = Mason, B., and Berry, L.G., 1968, Elements of mineralogy. (549 1968-4)

      #ER=Earth Resources, Groves, D.I., and Vielreicher, R.M., Course Notes, Geology 330

      #OPH= Ore Petrology Handbook, David Groves, Course Notes, Geology 330

      # = not in the Library

    Some definitions

      Kemp, 1909

    Ore is a metalliferous mineral, or an aggregate of metalliferous minerals, more or less mixed with gangue, which from the standpoint of the miner can be won at a profit, or from the standpoint of the metallurgist can be treated at a profit.

    Definitions of ore all emphasize:

      (a) that it is material from which we extract a metal, and
      (b) that this operation must be a profit-making one.

      Economically mineable aggregates or ore minerals are termed orebodies, oreshoots, ore deposits or ore reserves.

      Taylor (1989)
      .......ore is rock that may be, is hoped to be, will be, is or has been mined; and from which something of value may be (or has been) extracted.
        Industrial minerals (after Noetstaller, 1988)
          ...have been defined as any rock, mineral or natural occurring substance of economic value, exclusive of metallic ore, mineral fuels and gemstones
              Gangue
                ...is simply the unwanted material, mineral or rock, with which ore minerals are usually intergrown.

              Growth comparison for mineral products

              Mineral Resource Classification

              • Reserves - material that has been identified and can be extracted at a profit
              • Reserve base - reserves and identified material that might be extractable in the future, depending on geological factors
              • Resources - reserve base plus undiscovered deposits regardless of economic factors

              Geological setting of mineral deposits

              • Earth consists of four global scale divisions:
              • atmosphere
              • hydrosphere
              • biosphere, and lithosphere
              • Mineral deposits are part of the lithosphere

              Some review of definitions:

                • Mineral: control the distribution of elements in earth, naturally occurring solids with a characteristic crystal structure and define chemical compositions.
                  • Solid Earth is divided into the core, mantle, and crust

                    Core: iron and related elements
                    Mantle: ultramafic rocks, contain Cr, Co, Ni
                    Crust: ocean crust: 5-10km thick - mafic rocks (basalts)
                    continental crust: 20 to 70km thick felsic igneous, metamorphic rocks, sedimentary rocks - Mn

                  • Mineral deposits are the direct results of plate tectonics which involves the movement of lithospheric plates about Earths surface
                  • Lithosphere: rigid outer 100km of Earth, including the continental and ocean crust and the uppermost mantle
                  • Lithosphere plates move about on the underlying, more plastic mantle, which is known as asthenosphere
                  • Lithospheric plates have three types of margin:
                    • 1. divergent margins
                    • 2. mid-ocean ridges (ocean) and rifts (continent)
                    • 3. Convergent margins (subductiion zones and island arcs)
                  • Continental collision - obduction zones
                  • Transform margins

                  Kesler, 1994, Fig. 2.2

                  MT = Meteoric water systems
                  MW = Magmatic water systems
                  MM = Metamorphic water systems
                  MG = Magma
                  SW = Seawater systems
                  CW = Basinal water/Hydrocarbon systems

                  Ore deposits and plate teconics - exploration perspective

                  • Implications of plate tectonics to mineral deposits:
                    1. utilization of plate tectonics has never resulted in the discovery of a metal deposits!
                    2. the use of plate tectonic principles has enormous significance to the exploration for metal deposits!
                  • metal deposits are found by the investigation of mineralized or altered outcrops, and follow-up of geochemical and/or geophysical anomalies. Main conceptual tools are that of lithology and structure.
                  • main impact of plate tectonics is to broaden and deepen our understanding of the tectonic environments in which many of these lithologies and structures occur.
                  • clearer idea of ore-generating environment.

                  • metallogenesis now understood within the context of continental evolution, this in turn permits the conception of exploration programs aimed at specific types of deposits.
                    • utilization of plate tectonics in exploration provides a meaningful framework within which the geologic and geochemical processes that lead to economic metal concentrations can be more fully understood.
                      • more specific: increased understanding of paleogeography, metallogenic zoning in arc systems, arc segmenation, intraplate magmatism, and nature and products of rifting.

                      Geological processes that form mineral deposits

                      • Most of the geologic processes that form mineral deposits involve chemical changes in rocks and minerals where elements or compounds that were dispersed through large volumes of rock are collected and concentrated to form ore minerals or compounds.
                        • The most effective agents for chemical changes of this type are water and magmas both of which dissolve elements and crystallize new minerals.
                          • Because hot water is a better solvent than cold water, waters that form many mineral deposits are hot and are known by the special name of hydrothermal solutions.

                          • The formation of ore minerals and compounds by magmas and water can take place at Earth’s surface or at depth in the crust or mantle.
                            • In many cases, these chemical processes make a mineral deposit that is rich enough to be an ore deposit. In other cases, further concentration is needed by an additional chemical process or a physical process.

                            Geological Processes and Mineral Deposits

                            Surface and near-surface ore forming processes :

                            Weathering
                            Physical sedimentation
                            Chemical sedimentation
                            Organic sedimentation

                            Subsurface ore forming processes:

                            Involving water (hydrothermal)
                            Involving magmas

                            Ore-bearing fluids can be subdivided into six categories:

                              1. silicate-dominated magmas or derived oxide, carbonate, or sulfide-rich magmatic liquids
                              2. water-dominated hydrothermal fluids that separate from magmas
                              2a. meteoric waters
                              2b. seawater
                              2c. connate waters trapped in pore spaces in sediments
                              3. fluids associated with metamorphic processes

                              If the fluid is heated and liquid the aqueous fluids can be considered a hydrothermal solution
                              If the fluid is a gas, it is called pneumatolitic above the critical point, we are dealing with supercritical fluids

                            Magma and magmatic fluids

                            Magma is “naturally occurring mobile rock material, generated within the Earth and capable of intrusion and extrusion....” (AGI Glossary of Geology, 1980).

                              As a magma cools it crystallizes and separates into fractions by complicated processes of fractional crystallization or igneous differentiation. metallic elements can be concentrated by rock-forming mechanisms in various portions of the resulting igneous assemblages.
                              Locally, specific minerals such as chromite may be so abundant that the resulting igneous rock itself constitutes ore.
                              If a partly crystallized magma is subjected to differential external stresses, the fluid fraction can be squeezed away from the crystalline mush. This process is called filter pressing.
                              If the liquid is squeezed out into the surrounding rocks, the process is known as magmatic injection (magmatic injection deposit).

                            Schematic sequence of magmatic events leading to ore magmas and their emplacement.

                            Igneous rocks, differentiation, and hydrothermal fluid involvement associated with fractional crystallization

                            Hydrothermal fluids

                            • Magmatic fluids
                            • Meteoric fluids
                            • Seawater
                            • Connate waters
                            • Metamorphic fluids
                            • Thermal springs

                            Processes that deposit ore minerals from hydrothermal solutions

                            Magmatic fluids

                            • Mineralizers:
                            • mobile elements LIL Large ion lithophile such as Li, Be, B, Rb, Cs
                            • also significant quantities of alkalis, alkali earths and volatiles such as:
                            • Na, K, Ca, Cl, and CO2 metals such as Fe, Cu, Zn
                            • Water is the principle mobile constituent in all magmas, increases in amount with increasing differentiation and plays an important part in the transportation of many ore components.
                            • Estimates of water in magmas range form 1 to 15%.
                            • Micas, clay minerals, zeolites, and amphiboles contain small amounts of chemically bonded water
                            • water that has passed through and equilibrated with the atmosphere is called meteoric waters.
                            • meteoric waters and convection cells - circulation
                            • stable isotope signature - meteoric water line

                            Seawater

                            • seawater as an ore-forming fluid are best described in the contexts of evaporates, phosphorites, submarine exhalalites
                            • deposits related to seawaters are:
                            • Alpine Peridotite chromite,
                            • VMS
                            • BIF
                            • Mt Isa and Broken Hill???
                            • Sedimentary iron deposits
                            • Phosphate deposits
                            • Evaporites
                            • Manganese modules

                            Connate waters

                            • water trapped in sediments at the time they were deposited is known as connate water
                            • connate waters are fossil waters
                            • observed in oil field exploration
                            • rich in sodium and chloride, also considerable amounts of calcium, magnesium, and bi-carbonate, and many contain strontium, barium and nitrogen compounds
                            • can also contain light hydrocarbons
                            • stable isotope ratios near SMOW
                            • Mississippi Valley type deposits

                            Metamorphic fluids

                            • recrystallization of hydrous and anhydrous phases
                            • volatile and mobile constituents are activated during metamorphism and forced from the rock to migrate toward cooler

                            Seawater

                            Ore fluids and geothermobarometry

                            • Necessary to understand the nature of ore-component transporting media: liquid or gases
                            • Deeper ore-bearing fluids are studied by observations of:
                            • thermal springs,
                            • volcanic gases,
                            • studies of ores (ore petrology, thermodynamics) and gangue,
                            • from laboratory studies of appropriate synthetic systems (synthetic fluid inclusions, sulfide systems),
                            • isotopic analyses, and
                            • geochemical studies.