Presentation on theme: "BACKGROUND: FORMATION AND CLASSIFICATION OF MINERAL DEPOSITS."— Presentation transcript:
BACKGROUND: FORMATION AND CLASSIFICATION OF MINERAL DEPOSITS
Remote Sensing Geophysics Geochemistry Geology Garbage In, Garbage Out Mineral potential maps GIS Analyse / Combine Good Data In, Good Resource Appraisal Out Mineral potential maps GIS Analyse / Combine Remote Sensing Geophysics Geochemistry Geology
database Predictor maps Favorability map MINERAL POTENTIAL MAP MODEL Mineralization processes Conceptual models Knowledge-base Mappable exploration criteria Spatial proxies Processing Overlay Validation Systematic Application of GIS in Mineral Exploration
SOME TERMS Magmatic - Related to magma A complex mixture of molten or (semi-molten) rock, volatiles and solids that is found beneath the surface of the Earth. Temperatures are in the range 700 °C to 1300 °C, but very rare carbonatite melts may be as cool as 600 °C, and komatiite melts may have been as hot as 1600 °C. most are silicate mixtures. forms in high temperature, low pressure environments within several kilometers of the Earth's surface. often collects in magma chambers that may feed a volcano or turn into a pluton.
SOME TERMS Hydrothermal : related to hydrothermal fluids and their circulation - Hydrothermal fluids are hot (50 to >500 C) aqueous solutions containing solutes that are precipitated as the solutions change their physical and chemical properties over space and time. - Source of water in hydrothermal fluids: Sea water Meteroric Connate Metamorphic Juvenile (Magmatic) - Source of heat Intrusion of magma into the crust Radioactive heat generated by cooled masses of magma Heat from the mantle Hydrothermal circulation, particularly in the deep crust, is a primary cause of mineral deposit formation and a cornerstone of most theories on ore genesis.
FUMNDAMENTAL PROCESSES OF FORMATION OF ECONOMIC MINERAL DEPOSITS PRIMARY PROCESSES MAGMATISM SEDIMENTARY (includes biological) HYDROTHERMAL COMBINATIONS OF ABOVE SECONDARY PROCESSES MECHANICAL CONCENTRATION RESIDUAL CONCENTRATION
In order to more readily study mineral deposits and explore for them more effectively, it is helpful to first subdivide them into categories. This subdivision, or classification, can be based on a number of criteria, such as minerals or metals contained, the shape or size of the deposit, host rocks (the rocks which enclose or contain the deposit) or the genesis of the deposit (the geological processes which combined to form the deposit). It is useful to define a small number of terms used in the classification which have a genetic connotation. CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS MAGMATIC Magmatic Deposits are so named because they are genetically linked with the evolution of magmas emplaced into the crust (either continental or oceanic) and are spatially found within rock types derived from the crystallization of such magmas. The most important magmatic deposits are restricted to mafia and ultramafic rocks which represent the crystallization products of basaltic or ultramafic liquids. These deposit types include: Disseminated (e.g., diamond in ultrapotassic rocks called kimerlites) Early crystallizing mineral segregation (e.g., Cr, Pt deposits) Immiscible liquid segregation (Ni deposits) Residual liquid injection (Pegmatite minerals, feldspars, mica, quartz)
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS MAGMATIC – HYDROTHERMAL Deposits formed by precipitation of metals from hydrothermal fluids related to magmatic activity. Porphyry deposits (e.g., porphyry copper deposits) are associated with porphyritic intrusive rocks and the fluids that accompany them during the transition and cooling from magma to rock. Circulating surface water or underground fluids may interact with the plutonic fluids. Volcanogenic massive sulfide (e.g., VMS deposits – Zn and Pb deposits) are a type of metal sulfide ore deposit, mainly Cu-Zn-Pb, which are associated with and created by volcanic-associated hydrothermal events in submarine environments.
Deposits formed by (bio-)sedimentary processes, that is, deposition of sediments in basins. The term sedimentary mineral deposit is restricted to chemical sedimentation, where minerals containing valuable substances are precipitated directly out of water. Examples: Evaporite Deposits - Evaporation of lake water or sea water results in the loss of water and thus concentrates dissolved substances in the remaining water. When the water becomes saturated in such dissolved substance they precipitate from the water. Deposits of halite (table salt), gypsum (used in plaster and wall board), borax (used in soap), and sylvite (potassium chloride, from which potassium is extracted to use in fertilizers) result from this process. Iron Formations - These deposits are of iron rich chert and a number of other iron bearing minerals that were deposited in basins within continental crust during the Early Proterozoic (2.4 billion years or older), related to great oxygenation event. SEDIMENTARY DEPOSITS CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
SEDIMENTARY HYDROTHERMAL These deposits form by precipitation of metals from fluids generated in sedimentary environments. Example: SEDEX Deposits (e.g., Pb-Zn deposits of Rajasthan)
CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS HYDROTHERMAL These deposits form by precipitation of metals from hydrothermal fluids generated in a variety of environments Example: Orogenic Gold Deposits (e.g., Kolar, Kalgoorlie)
SECONDARY DEPOSITS: Formed by concentration of pre-existing deposits MECHANICAL CONCENTRATION RESIDUAL CONCENTRATION CLASSIFICATION OF ECONOMIC MINERAL DEPOSITS
FORMATION OF MINERAL DEPOSITS COMPONENTS Ligand source Metal source Model I Model III Trap Region Energy (Driving Force) Transporting fluid Residual Fluid Discharge No Deposits Mineral System ( 500 km) Deposit Halo Deposit ( 10 km) ( 5 km) 1. Energy2. Ligand 3. Source4. Transport5. Trap 6. OutflowINGREDIENTS
GOLD DEPOSIT FORMATION
Orogenic gold deposits Close to trans-lithospheric structures (vertically extensive plumbing systems for hydrothermal fluids) Related to accretionary terranes (collisional plate boundaries) Temperature of formation – C Major deposits form close to: – Fault deflections – Dilational jogs – Fault intersections – Regions of low mean stress and high fluid flow (permeable regions) – Greenschist facies metamorphism (low-grade metamorphism, low temperature-pressure conditions)
Orogenic gold deposits characteristics High Au (> 1 PPM) and Ag; Au/Ag 5 Associated with – hydrated minerals (micas, chlorite, clay) – Carbonate minerals (calcite, dolomite) – Sulfides (pyrite etc) Enrichment of semi-metals (As, Sb, Bi, Sn) Depletion of base and transition metals (Zn, Cu, Pb)
Leaching of Gold in Source Areas By hydrothermal fluids that contain suitable ligands for complexing gold as Au(HS) 2 –, HAu(HS) 2 0 and Au(HS) 0 Hydrothermal fluids are: – aqueous (H 2 O)-CO 2 -CH 4 – dilute – carbonic – having low salinity (<3 Wt% NaCl) – Source rocks – typically crustal rocks (granites)
Transportation of Gold Gold is transported in the form of sulfide complex Au(HS) 2 –, HAu(HS) 2 0 or Au(HS) 0 Low Cl and high S in hydrothermal fluids account for high Au and low Zn/Pb in hydrothermal solutions Transportation pathways – permeable structures such as faults, shear zones, fold axes focus vast volumes of gold-sulfide bearing fluids into trap areas.
Gold trapping – (precipitation) Key precipitation process: -break soluble gold sulfide complexes (Au(HS) -1 ) How? - Take sulfur out of the system How? - by changing physical conditions - by modifying chemical compositions
Gold trapping – (precipitation) Physical mechanism: - Fluid boiling through pressure release - Catastrophic release of volatiles, particularly, SO 2 - Removal of sulfur breaks gold sulfide complexes leading to the precipitation of gold - Pressure release could be by seismic pumping or by brittle failure of competent rock
Gold trapping – (precipitation) Chemical mechanism: - Gold-sulfide complexes react with iron, forming pyrite and precipitating gold - Rocks such as dolerite, banded iron formations are highly enriched in iron and therefore form good host rocks for trapping gold
LEAD-ZINC SULFIDE DEPOSITS 100m 60 km km
LEAD-ZINC SULFIDE DEPOSITS – SEDEX or Sedimentary Exhalative Deposits PbCl x (2-x) + H 2 S PbS +2H + + xCl -
Nickel deposit formation Nickel-rich source magma (ultramafic) Transportation of the source magma through active pathways Deposition of nickel- sulfide through sulphur saturation Shallow sills and dyke complexes Mid-crustal magma chamber Magma plumbing system Deep level magma chamber CSIRO, Australia Slide Km Sub-volcanic staging chambers Magmatic nickel sulfide deposits form due to saturation of nickel-rich, mantle-derived ultramafic magmas with respect to sulfur, which results in formation and segregation of immiscible nickel sulfide liquid.
Uranium deposit formation Uranium deposit Uranium Ore Transported as U +6 (uranyl) Deposited as U +4 (uraninite)
Coal, Oil And Natural Gas Formation The carbon molecules (sugar) that a tree had used to build itself are attacked by oxygen from the air and broken down. This environment that the tree is decaying in is called an aerobic environment. All this means is that oxygen is available. If oxygen is not available (anaerobic environment), the chains of carbon molecules that make up the tree are not be broken down. If the tree is buried for a long time (millions of years) under high pressures and temperatures, water, sap and other liquids are removed, leaving behind just the carbon molecule chains. Depending on the depth and duration of burial, peat, lignite, bitumen and anthracite coal is formed.
Difference between coal and oil Crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Like coal, forms by anerobic decay and break down of organic material. However, while coal is solid, crude oil is liquid. Coal contains massive molecules of carbon rings derived from plant fibres that can be very long, sometimes metres long or more. The carbon chains in oil are tiny by comparison. They are the structural remains of microscopic organisms and so they are ALL very small
Oil And Natural Gas Formation Kerogen
Oil and Natural Gas System An oil and natural gas system requires timely convergence of geologic processes essential to the formation of crude oil and gas accumulations. These Include: Mature source rock Hydrocarbon expulsion Hydrocarbon migration Hydrocarbon accumulation Hydrocarbon retention (modified from Demaison and Huizinga, 1994)
Cross Section Of A Petroleum System Overburden Rock Seal Rock Reservoir Rock Source Rock Underburden Rock Basement Rock Top Oil Window Top Gas Window Geographic Extent of Petroleum System Petroleum Reservoir (O) Fold-and-Thrust Belt (arrows indicate relative fault motion) Essential Elements of Petroleum System (Foreland Basin Example) (modified from Magoon and Dow, 1994) OO Sedimentary Basin Fill O Stratigraphic Extent of Petroleum System Pod of Active Source Rock Extent of Prospect/Field Extent of Play