RSG 504: Remote Sensing Applications in Mineral Exploration PDF

RSG 504: REMOTE SENSING APPLICATIONS IN MINERAL EXPLORATION DEPARTMENT OF REMOTE SENSING AND GEOSCIENCE INFORMATION SYSTEM PREPARED BY ADESEKO, A.A  COURSE CONTENTS Classifications Of Mineral Deposi...

RSG 504: REMOTE SENSING APPLICATIONS IN MINERAL EXPLORATION DEPARTMENT OF REMOTE SENSING AND GEOSCIENCE INFORMATION SYSTEM PREPARED BY ADESEKO, A.A  COURSE CONTENTS Classifications Of Mineral Deposits Structural And Lithological Factors Guiding The Occurrence Of Mineral Deposits Spectral Signatures In The Recognition Of Hydrothermally Altered Rocks. Geological Mapping And Interpretation Using Aster And Dem; Microwave And Hyperspectral Remote Sensing For Mineral Prospectivity Modelling; GIS Modelling Systems And Mineral Resources Potential Integration And Analysis Of Exploration Data (Geological Maps, Airphotos, Satellite And Airborne Sensors, Geochemical And Borehole Data  SOME TERMINOLOGIES Mineral: is a “naturally occurring, inorganic, homogeneous element or compound with a definite chemical composition and an ordered atomic arrangement.” Most minerals have distinctive crystalline habit and may occur in well-formed crystals or crystalline aggregates, but a few species are characteristically non-crystalline/amorphous. Minerals can be metallic (gold, silver) or nonmetallic. Rock: is a naturally formed material composed of an aggregate of grains or crystal of one or more minerals. Some rocks may be monominerallic; limestone consists of largely of the mineral calcite and many sandstones are dominantly quartz. Most rock varieties contain a number of mineral species and their classification depends upon the minerals present, their relative abundance, etc. Ore: is a naturally occurring accumulation or deposit of one or more minerals of economic value (more or less mixed with gangue) that can be exploited at a profit. The term is usually restricted to the description of mineral deposits that are of value for their metal content. Nonmetallic deposits although similar to metallic ores generally are valued for the minerals or components present (as with rock salt; Halite) rather than for individual contained elements, though such minerals as “elemental sulfur” are of much value.  TERMINOLOGIES CONTINUES…… Mineral deposit: is any valuable mass of ore. It is a concentration of useful minerals or metals of sufficient size and grade, in the Earth’s crust, that can be exploitable economically, under the prevailing technology and economic conditions. Ore deposits: are accumulations of metals more concentrated than average crustal levels. These accumulations develop as a result of the transport of ore-forming elements, followed by their preferential deposition in a restricted volume. Ore deposit is a mineral deposit that has been tested, is known to be of sufficient size, grade, and accessibility to be mined at a profit. Testing commonly consists of surface mapping and sampling, as well as drilling through the deposit. An ore deposit is an economic term. Note: All ore deposits are mineral deposits but the reverse is not true. Metal: A metal is any of a class of substances (such as gold, iron, and aluminum) that typically are fusible, opaque, and are good conductors of electricity and show a metallic luster. Most metals are malleable, ductile, comparatively heavy, and all are solid (except mercury) at ordinary temperature.  WHY MINERAL EXPLORATION?  The mineral reserves and resources, annual production vs. consumption and index of per capita spending of any commodity are the measures that rank the status of a country as developed, developing or underdeveloped. The per capita consumption of zinc in India during 2008 was very low at 0.43 kg against a world average of 4.3 kg. The higher consumption during the same period was shared between Australia (12.7 kg), South Korea (11.3 kg), Canada (5.6 kg), Japan (5 kg), USA (4.1 kg) and China (2.7 kg).  The policy makers in the government and private sectors allocate funds for long- and short-term exploration plan programs guided by the demand-supply trend of all commodities as a whole. The fund allocation has special significance for strategic and deficient minerals.  Normally, the activity of mineral exploration begins with “reconnaissance” and advances to “detailed reconnaissance” followed by selection of target/targets. If this stage is successful then it leads to “advanced exploration” and finally to “evaluation.” WHY ARE MINERAL DEPOSITS IMPORTANT? Figure 1: Minerals that go into a smartphone  ROCK CYCLE  Rocks are grouped as (1) Igneous rocks, (2) Sedimentary rocks, and (3) Metamorphic rocks.  Igneous rocks are formed by the crystallization of magma (molten rock) and are described as “plutonic” if formed at great depths, “hypabyssal” if formed at intermediate.  Depths, and “extrusive” if found at or on the surface of the earth. Igneous rocks are broken down into tiny particles/sediments by surface process of weathering/erosion and are carried away by wind/water/gravity to be deposited once again at a lower elevation to form sedimentary deposits. The action of softening, disintegration, partial solution of the mineral matter, etc. takes place depending upon the mineral composition of igneous rocks.  When the sediments are buried by more sediment and subjected to pressure and temperature or both, finally lithify as sedimentary rocks. continued burial of sedimentary rocks (and igneous rocks) added with temperature and pressure or both recrystallize and become metamorphic rock.  These rocks that are exposed at the surface will also weather to form sedimentary rocks. when the metamorphic rocks become very hot, they melt and form magma again, completing the rock cycle. Figure 2: A sketch map of the rock cycle. Source: USGS (2005)  GEOLOGICAL REQUIREMENTS FOR ANY MINERAL DEPOSIT TO FORM  Four basic requirements for mineral deposit formation  A source for the ore components (metals and ligands).  A mechanism that either transports these components to the ore deposit site and allows the appropriate concentration or removes non ore components to allow residual concentration.  A depositional mechanism (trap) to fix the components in the ore body as ore minerals and associated gangue.  A process or geological setting that allows the ore deposit to be preserved. Figure 3 : The four basic geological requirements for any mineral deposit to form. Source: McQueen (2009).  CLASSIFICATION OF MINERAL DEPOSITS  BASED ON FORM: It includes veins, replacements and disseminated.  Veins: Are either filling of fissures, fractures, or faults, which are transverse to structural features such as bedding in the containing rocks.  Replacements: May be highly irregular in form giving “Chimneys” or “Pipes” and filling in around or replacing irregular fragments of country rock.  Disseminations: Consist of minute or large units of economic mineral with or without an associated gangue, scattered irregularly through the country rock. Figure 4: Primary dispersion and hydrothermal vein system. From McQueen (2009)  CLASSIFICATION OF MINERAL DEPOSITS  BASED ON ORIGIN AND THEORY  MAGMATIC DEPOSITS  PEGMATITES  KIMBERLITES  CUMULATES (MAGMATIC SULFIDE DEPOSITS)  HYDROTHERMAL DEPOSITS  PORPHYRY  SKARN  VOLCANOGENIC MASSIVE SULFIDE (VMS)  SEDIMENTARY EXHALATIVE (SEDEX)  EPIGENETIC  SEDIMENTARY DEPOSITS  PLACERS  IRON ORES  EVAPORITES Figure 5: A simplified genetic classification of all ore deposit types. From McQueen (2009).  LATERITES  PHOSPHORITES   MAGMATIC DEPOSITS  These originate by the aggregation of the desired mineral particles or crystals at one or several stages during the cooling and solidifying of molten rock.  The heavier metal-rich liquids sink and concentrate at the base of the intrusive body, while lighter silicate liquid tends to rise.  They are commonly disseminations, locally richer, however, than the main body of such rock; they may also form continuous masses of varying form and size.  Many Chromite, Nickel, Platinum, Ilmenite, and Magnetite bodies have had this origin.  Sometimes geological processes concentrate ore minerals in Vein deposits consisting of veins that are centimeters to meters thick. if ore is distributed in many small veins, geologists call the deposit a Lode deposit.  Vein deposits account or most of the world’s Gold Figure 6: Geodynamic setting of mineral deposits. and Silver mines, and also some Copper and Lead- Zinc mines.  PEGMATITES  During crystallization, some minerals crystallize before others.  Consequently, a late-stage magma will not be the same composition as an original magma.  Pegmatite is the name given to coarse-grained igneous rocks that form during the final stage of magma crystallization.  Common pegmatites have an overall granitic composition and comprise mostly quartz, feldspar, and mica.  But pegmatites share another important characteristic, which is that, they also commonly contain minerals made of relatively rare elements that did not go into the early formed minerals.  Pegmatites are often mined for minerals rich in Boron, Cesium, Lithium, Molybdenum, Niobium, Tantalum, Tin, Tungsten, or other elements. For example, Pegmatites are sometimes sources of Spodumene (an important lithium mineral) and beryl (an important beryllium ore mineral).  Additionally, Pegmatites may be a source of gem minerals. gems from Pegmatites include Emerald and Aquamarine (varieties of the mineral Beryl), Amazonite (variety of Feldspar), Apatite, Chrysoberyl, Garnet, Spodumene, Lepidolite, Topaz, Tourmaline, Zircon, and others. Figure 7: A pegmatite containing a large black tourmaline crystal withe K-feldspar  Most Pegmatite mines make their money from mineral specimens and gems rather than from the ore minerals they and quartz contain.  KIMBERLITES Figure 8: Kimberlite from  Kimberlites, named after the town of Baffin Island that contains coarse crystals of Kimberly, South Africa, where they were chromediopside, small first described, are volcanic rocks that crystals of red garnet, and originate in Earth’s mantle. They are mined include fragments of exclusively for diamonds. limestone.  Kimberlite eruptions are gas-powered explosive events.  The magmas originate at depths of 150 to 450 km, deeper than other igneous rocks.  Most kimberlites are in small vertical columns called Kimberlite pipes although some rare sills are known. Figure 9: Large diamond crystal in kimberlite. The largest crystal is about 7 mm  These pipes are the most important source across. of diamonds today.  If kimberlite weathers and erodes, the diamonds may become concentrated in sedimentary deposits.  CUMULATES (MAGMATIC SURPHIDE DEPOSIT)  Sulfides have greater densities than silicate minerals and the mafic or ultramafic melts. The denser sulfide minerals will over time, begin to sink.  Eventually, after more cooling and crystallization, significant deposits of sulfide minerals may accumulate on the bottom of a magma chamber.  The deposits, which may form centimeters-, or meters-thick layer called a Cumulate, are often entirely, or nearly entirely, composed of sulfide minerals.  This process produces Magmatic Sulfide deposits, which are the most important sources of Platinum, Palladium, Chromium, and several other metals. Cumulate Sulfide minerals include Pentlandite (Fe, Ni)9S8, Chalcopyrite (CuFeS2), Pyrrhotite (Fe1-xS), and Pyrite (FeS2). Figure 10: Forming a Cumulate deposit  HYDROTHERMAL DEPOSITS As a melt cools and crystallizes, hot, water-rich fluids may be released. These hydrothermal fluids are rich in Sulfur, Sodium, Potassium, Copper, Tin, Tungsten, and other elements with relatively high solubility. Hydrothermal fluids dissolve other elements as they flow through rocks and eventually cool to deposit minerals in Hydrothermal deposits. These deposits fall into four or five categories: Porphyry deposits, Skarn deposits, Volcanogenic massive sulfide deposits (VMS), Sedimentary Exhalative deposits (SEDEX), and Epigenetic deposits. They have distinctly different origins and vary in size from huge networks of veins covering many square kilometers to small veinlets only centimeters wide. Hydrothermal deposits generally form at mid-ocean ridges, in subduction zone, or next to plutons. There is a source of heat that drives fluid circulation. The exceptions are epigenetic deposits that may form in Figure 11: Gold-bearing quartz vein at the Nalunaq Gold Mine in Southern Greenland. Note: continental interiors. Meter stick for scale.  Examples of hydrothermal mineralization Figure 12: Hydrothermal veins of Molybdenite (MoS2) from the Keystone Figure 13 :Hydrothermal Gold from the Mother Lode of the Sierra Nevada Mine, Colorado Mountains  PORPHYRY DEPOSITS  Porphyry deposits are a special kind of hydrothermal deposit.  They form when hydrothermal fluids, derived from magmas at depth, carry metals toward the surface and deposit minerals to create disseminated ore deposits.  These deposits are important sources of Copper, Molybdenum, and Gold. they may also yield Tungsten or Tin.  Porphyry deposits, ore minerals are in small veins within a hydrothermally altered host rock, generally a porphyritic felsic to intermediate composition intrusive rock. Figure 14: The mine workings at Morenci, Figure 15: Porphyry copper deposits Source: word Arizona press.com  SKARN DEPOSITS  Skarns are contact metamorphic zones that develop around an intrusion.  They may be thin or thick, and their formation often involves metasomatism.  Skarns can form in any kind of rock, but most are associated with Limestone or Dolostone.  Common Skarn minerals include Calcite and Dolomite, and many Ca-, Mg-, and Ca-Mg- silicates. Some Skarns, however, are valuable mineral deposits containing Copper, Tungsten, Iron, Tin, Molybdenum, Zinc, Lead, and Gold.  Skarns account for nearly three quarters of the world’s Tungsten production.  Less commonly, Skarns produce Manganese, Nickel, Uranium, Silver, Boron, Fluorine, and Rare-earth elements. Figure 16: Contact metamorphism around a pluton  Porphyry deposits and Skarn deposits are both the results of hydrothermal activity, and a continuum exists between the two types.  EXAMPLES OF SKARN DEPOSITS Figure 17: A rock altered by hot, chemically-active fluid Figure 18: Rich Gold-bearing Skarn deposit Source: 911 mining  VOLCANOGENIC MASSIVE SULFIDE (VMS)  Volcanic massive sulfides: are deposits formed as massive lens-like accumulations on or near the sea floor in association with the volcanic activity (Felsic volcanic hosted—Cu-Pb-Zn-Ag-Au; Mafic volcanic hosted—Cu (Zn, Au); Mafic volcanic/sedimentary—Cu-Zn (Au)).  When hydrothermal fluids create ore deposits at, or near, Earth’s surface, we call the deposits Exhalatives.  What makes VMS deposits especially intriguing is that we can see them being created today.  Hot hydrothermal waters, mixing with ocean waters, create fine particles of sulfide minerals and produce massive ore deposits. The Iron sulfides that are the most common minerals created, are black, so they are referred to as Black Smokers.  The ores mined from the Kidd, Windy Craggy, Rio Tinto, in Canada, and Spain respectively and other massive sulfide deposits owe their origins to black smokers. Figure 19: A black smoker on the ocean floor  The smokers cover huge regions of the ocean floor and did so in the past. After forming, they later became uplifted and incorporated into the continents where we find them today.  EXAMPLES OF VMS MINNING SITES Figure 21: Acid mine drainage from the Rio Tinto Mine Figure 20: The Kidd Mine near Timmins, Ontario Figure 22: Rio Tinto Mine. The view is 10 km across. Source : Google Earth  SEDIMENTARY EXHALATIVE (SEDEX)  Sedimentary Exhalative (SEDEX) deposits are close cousins to VMS deposits.  The difference is that the host rocks in Sedex deposits are sedimentary rocks.  These deposits are rare compared with the other deposit types already discussed.  They have produced significant amounts of Zinc, Lead, Silver and sometimes Copper. But, most of them are not economical to mine.  The figure on the right shows Copper ore (mostly chalcopyrite and Bornite) from the Rammelsberg SEDEX deposit in Germany.  At Rammelsberg, the hydrothermal ores are in Shale, (A sedimentary rock). Figure 23: Chalcopyrite and Bornite from the Rammelsberg deposit, Germany  EPIGENETIC DEPOSITS  Epigenetic deposits are believed to have come much later than the host rocks in which they occur.  When a hydrothermal deposit is not directly associated with a pluton, we call it an epigenetic deposit.  They have been introduced into the pre-existing country rock/host rock after its formation via migration of metal-bearing fluids. A mineral vein is a good example.  Fracturing and breaking of the rock along weak plane (or fault) at a depth ranging from surface to several km below surface is a pre-requisite.  Hydrothermal solutions pass through open spaces/fractures and deposit the minerals. Figure 24: Galena, Calcite, and Fluorite from the Rogerley Mine, County Durham, England  These deposits have various forms like fissure veins and sheet-like form.  SEDIMENTARY DEPOSITS  These originate from surface or near-surface processes such as evaporation (Evaporites—most deposits of Gypsum, Halite, etc.); from biochemical extraction and precipitation usually in enclosed water bodies.  More than 90% of world’s Iron deposits are tied up in Banded Iron formation (BIF);  It can also be from physical concentration of solid particles from weathering of primary minerals and transported by stream (Placers, yielding Gold nuggets, Diamond, etc.);  Leaching of rock leaves residual minerals behind (Laterites, Al, Ni, Fe).  Coal is a sedimentary mineral deposit.  The deposits could be in the form of placers, Iron Figure 25: Formation of placer Gold deposit ores, Evaporites, Laterites and Phosphates.  PLACER DEPOSITS  Gravity may be an important force that concentrates economic minerals.  Heavy minerals, weathered from igneous, sedimentary, or metamorphic rocks, can be picked up and rivers may transport them long distances before they become concentrated in placers.  Placer deposits, also just called placers, form when one or more minerals concentrate in this way to become an ore deposit.  The word placer is Spanish for Alluvial sand. Placer minerals must be both dense and durable to be deposited and remain in place without decomposing.  Native metals such as Copper or Gold, Sulfide minerals such as Pyrite or Pyrrhotite, and Oxide minerals such as Magnetite or Ilmenite are all dense and likely to be found in placers. Metal oxides, especially Magnetite (Iron oxide), are common and especially dense and durable, and often dominate such deposits.  Gold is dense and extremely resistant to any kind of Figure 26: The loci of placers concentration (A) Below waterfalls, (B) Inside meander loops, (C) weathering and so can accumulate in stream and Downstream from a tributary, (D) Behind undulations on ocean floor, (E) Behind rock bars, and (F) river sediments. In rock holes. PLACER DEPOSIT CONTD………. Figure 27: Placer gold in the Draper Museum of Natural History, Cody, Wyoming  IRON ORES  Sedimentary ore deposits also form by chemical precipitation; Banded Iron Formations (BIF), found in Figure 28: The Precambrian shields are examples. Soudan Iron Formation in  BIFs are massive in scale, in places covering hundreds northern Minnesota. of square kilometers, and perhaps being tens to Note: Hammer hundreds of meters thick. for scale.  If they contain especially significant amounts of Magnetite and Hematite, they are profitably mined.  Typical Banded iron formation contains repeating layers of black to Silver iron oxide (Magnetite), and red Chert (microcrystalline quartz). The overall red color is because the Chert contains inclusions of Figure 29 : Iron Hematite. mine at Tom Price, Western  BIFs include Oxides, Silicates, and Carbonates of Australia Iron. They are most commonly rich in Magnetite (Fe3O4) and Hematite (Fe2O3) but Siderite (FeCO3), and the Iron hydroxides, Goethite and Limonite are sometimes ore minerals.  EVAPORITES  When a body of water is trapped, evaporation can lead to precipitation of Halite and other salts. Thick Evaporite deposits of Halite, Sylvite, Gypsum, and Sulfur have formed in this way.  Evaporites are mined for many things, most notably Halite, Sylvite, and Gypsum.  Gypsum (CaSo4 2H2o), Anhydrite (CaSo4), Halite (NaCl), and Sylvite (KCl) consist of common elements.  Gypsum and Anhydite have high solubility; Halite and Sylvite have even higher solubility. So, their chemical components are common as dissolved species.  As water evaporates, perhaps in a closed inland basin or an isolated sea, these four minerals may Figure 30: Salt deposits on the shore of Utah’s Great Salt Lake precipitate to form thick beds of Evaporite minerals.  Evaporites are found in many parts of the world.  LATERITES  The weathering of pre-existing rock may expose and concentrate valuable minerals.  Over time, water leaches rocks and soils, dissolving and carrying away soluble material. The remains, called Residuum, may be rich in Aluminum, Nickel, Iron, or other insoluble elements.  In tropical climates extreme leaching has produced soils called Laterites, which are rich in Aluminum or, some times, Nickel.  If Aluminum-rich Laterite lithifies to become rock, we term it Bauxite.  Laterites and Bauxites are mined from open pits to produce Nickel and Aluminum and, Figure 31: Bauxite (red) above sandstone (white) at Pera Head, Weipa, sometimes as a secondary commodity, Iron. Australia  PHOSPHORITES  Phosphorus, like potassium, is an essential nutrient for all living things and people have used it as a fertilizer for centuries.  Most phosphorus comes from Phosphorite  Phosphate-rich chemical sedimentary rock forms in several different marine environments.  The most common depositional environments are in shallow, near-shore marine settings, including beaches, intertidal zones, and estuaries. Figure 32: Phosphate mining in Togo, West Africa  CLASSIFICATION BASED ON GEOGRAPHIC LOCALIZATION  “Province” or “Metallogenic province” is a large specific area having essentially notable concentration of certain characteristic metal or several metal assemblages or a distinctive style of mineralization to  Province be delineated and developed as economic deposits. The metallogenic province can be formed on various processes such as plate tectonic activity, subduction, igneous intrusive, metal-rich  Region epigenetic hydrothermal solution and expulsion of pore water enriched in metals from  District sedimentary basin. The examples of metallogenic provinces are Zn-Pb-Ag-bearing.  Belt  “Region” is similar to province but relatively smaller in size, controlled by stratigraphy and/or  Deposit structure, for occurrence of specific mineral(s) at commercial quantity. The examples are Kalgoorlie Goldfield, Esperance region of Western Australia, Zn-Pb region of Mississippi Valley, United States,  Block Copper region of Chile and Peru, Diamond bearing region of Northern Minas Geraes, Brazil, Diamond bearing region of Kimberley, South Africa, Pacific and Central coal-bearing region of US and Rubies in high-grade metamorphic rocks of Kashmir region of India.  “District” is comprised of one geographical area popularly known for occurrence of particular mineral e.g. Aeolian soils of Blayney District, NSW, Australia, Baguio mineral district in Philippines for copper deposits, New Mexico for Uranium deposits, Singhbhum district for copper and Salem district for magnesite, India. CLASSIFICATION BASED ON GEOGRAPHIC LOCALIZATION CONTD….  “Belt” is a narrow linear stretch of land having series of deposits of associated minerals, such as, Colorado gold-molybdenum belt, US, Grant uranium mineral Belt, New Mexico Khetri  Belt Copper belt, Rajpura-Dariba-Bethumni Zinc-Lead-Silver belt, Rajasthan, Sukinda Chromite belt, Orissa, India, Ilesha Schist belt associated with Gold and other Schist belts in the NW-  Deposit SW of Nigeria.  Block  “Deposit” is comprised of a single or a group of mineral occurrences of sufficient size and grade separated by natural narrow barren parting e.g. Broken Hill group of zinc-lead deposits, Australia, Zawar group of zinc-lead deposits, India, Red Dog zinc-lead deposit, Alaska, OK Tedi copper deposit, Papua New Guinea, Olympic Dam copper-gold-uranium- silver deposit, South Australia, Neves Corvo poly-metallic deposit, Portugal Stillwater group of platinum deposit, US and Gold Deposit in Iperindo, SW Nigeria.  “Block” is a well-defined area having mineral concentration wholly or partly of economic value, such as Broken Hill main, Australia, Bailadila deposit-14, Central Mochia, India. The blocks in underground mining are subdivided to “Level” (say: upper level, lower level, 500- 700 and 300-500 mRL). The levels are further split into “Stope” (say: West 301 stope, North 101 stope, Valley stope). These terms are locally convenient to use for attention and allocation of work activities in mineral exploration and sequencing mine production block. EXAMPLE OF A BELT: SCHIST BELT OF NIGERIA  The schist belts comprise low grade, metasediment-dominated belts trending N–S which are best developed in the western half of Nigeria.  The schist belts have been mapped and studied in detail in the following localities: Maru, Anka, Zuru, Kazaure, Kusheriki, Zungeru, Kushaka, Isheyin Oyan, Iwo, and Ilesha where they are known to be generally associated with gold mineralization. Schist belt localities within the context of the Geology of Nigeria (After Woakes et al.,1987) STRUCTURAL CONTROL OF MINERAL DEPOSITS  Joints And Fractures  Structure, tectonics and surface weathering play a vital role over geological time  Fold as a passage for hydrothermal flow of mineralized fluids, accumulate and concentrate at suitable location, remobilize and re-orientate as post-genetic  Fault activity. The features related to mineralization control are deformation,  Shear Zone weathering, joints, fractures, folds, faults, breccias and plate tectonics.  Breccia  Subduction  STRUCTURAL CONTROL OF MINERAL DEPOSITS  Joints and Fractures Many deposits show varied degree of deformation, contemporaneous to formation or after effect. “Joints” and fractures are caused by regional stress, break in the rock along which little or no movement have occurred. Mineralization often concentrates along these regular and irregular planes. Lennard shelf Zinc- Lead deposit, Western Australia, is an example of cavity filled along major fault zone. Figure 32: Hot fluids that pass through fractures in deep rock can crystallize and fill the fracture to form  Fold mineral veins (fracture filled gold veins; Photo: Gandhi). Directed compression of the crust, resulting in a semi- plastic deformation, creates “folding” of strata (“fold”). The fold closure, limb in-flex zone and axial planes are suitable for mineral localization. Mineral deposits are often folded during or after formation e.g. Rajpura-Dariba Zinc-Lead-Copper deposit, Agnigundala Lead-Copper deposit, Sukinda Chromite belt, India. Figure 33: Stratiform Pyrite-Zinc-lead mineralization folded with mineral concentration at crests presenting saddle reef structure, Rajpura- Dariba deposit,  CONTD…… SHEAR ZONE Shear is the outcome of rock deformation generating particular texture like intense foliation, deformation and micro folding due to compressive stress. A “shear zone” is a wide zone of distributed shearing in crushed rock mass with width varying between few centimeters and several kilometers. The interconnected openings of shear zone serve as an excellent channel ways for mineral-bearing solutions and subsequent formation of Figure 34: Layered Chromite (black) and Magnesite (white) veins developed in shear zones, deposits. Many shear zones in orogenic belts host ore Sinduvally, Karnataka, India. deposits. Breccia is commonly used for clastic sedimentary rocks composed of large sharp-angled fragments embedded in fine-grained matrix of smaller particles or mineral cement. Breccia generated by folding, faulting, magmatic intrusions and similar forces are called “tectonic breccia”. Tectonic breccia zones are represented by crush, rubble, crackle and shatter rock mass. Figure 35: Irregular fragmented chromite (black with white rims) in matrix of Pt-Pd-bearing gabbro from the tectonic breccia zone, Boula- Nausahi underground mine, Orissa, India.  CONTD…..  Subduction is the process of two converging tectonic plate movement. The plates of continental margin arcs, oceanic lithosphere and volcanic island arcs collide and one slides under the other. In the process, the heavier oceanic crust stoops under the lighter continental crust or the volcanic island arc forming a “subduction zone”.  The formation of subduction zone is closely associated with multidimensional tectonic Figure 36: Likely occurrence of mineral deposits in different plate tectonic settings activities like shallow and deep focuses earthquakes, melting of mantle, volcanism, rising magma resulting volcanic arc, plutonic rocks of ophiolite suites, platinum-chromium- bearing, peridotite-dunite-gabbro-norite, movement of metal-bearing hydrothermal solution and metamorphic dewatering of crust.  WHY REMOTE SENSING IN MINERAL EXPLORATION?  Remote sensing has been in use extensively in order to develop regional scale geological maps for use in small- scale survey, scheduling ground activity for large-scale surveying and to study the field geological features together with their geographical locations and association of various geological units on surface.  Multiple data sources are required to be integrated in order to have a comprehensive understanding of litho- stratigraphy.  Three-dimensional view of the local relief will be provided by stereo imagery to facilitate delineation and identification of units.  Aerial photographs and satellite imageries are carried along for use in the form of base maps in the field for field analysis and ground truth.  Aerial photographs generally provide high-resolution information (like weathering, drainage patterns, etc.) for site- specific analysis. Large coverage area and moderate resolution are required for regional overview.  Since the various elements of concern are not dynamic, frequency of imaging has been never an issue in geological exploration.  By integrating different source of image data (optical, radar) at an appropriate scale, remote sensing is optimally used. Worldwide data are readily available in imageries captured through satellites and are of reasonable price (price per km).  IMAGE INTERPRETATION: PURPOSE  Image interpretation is carried out for a number of reasons as required by different field of applications. Some of the requirements are: 1. Lithological-Structural interpretation: can sometimes include;  Geomorphological interpretation (origin)  Denudational/Structural  Fluvial  Coastal/Marine  Karst  Glacial 2. Delineation of lithologic units 3. Mineralogical mapping 4. Extraction of structural trends  They are interrelated and knowledge about all is often relevant doing interpretation for any. It involves terrain analysis and classification of terrain units.  IMAGE INTERPRETATION FOR LITHOLOGICAL- STRUCTURAL PURPOSE  Terrain analysis and classification involves analysing and evaluating terrain units, putting them into classes for;  its potential use  geologic mapping  Terrain analysis is concerned with arrangement and grouping of different areas of the earth’s surface into variety of categories on the basis of similarity of the type of surface and near surface attributes.  Terrain analysis/classification has geomorphology as its basic input.  Geomorphology is the study of landforms and in particular their nature, origin, process of development and material of composition.  Image Interpretation for Lithological-Structural Purpose  Landform can be defined as features formed by natural processes which have a definable composition and a range of physical and visual characteristics which occur wherever landform is found.  Land forming forces can either be;  Exogenic: acting within the atmosphere, hydrosphere and biosphere earth surface e.g denudation, transportation, deposition.  Endogenic: forces acting from within the earth surface e.g. plate tectonics; faulting, folding, volcanism.  Polygenetic: combination of several processes acting together e.g. structural-denudational  Lithological-Structural Interpretation: On-Paper  For analogue and digital aerial photo, printed Stereo pair can be used  Before undertaking interpretation task, the analyst should consider a) Good lightning b) Access to magnification gadgets  The image should to be viewed, systematically labelled and indexed to facilitate cross-referencing to other data source  Delineation should be made on overlays  Depending on the type of overlay, pencil or permanent marker can be used  Very fine pencils or indelible pens should be used so that not too thick lines are drawn. Graphite pencils are used.  Refrain from pressing hard on transparencies over photographs LITHOLOGICAL-STRUCTURAL INTERPRETATION : ANALOGUE PHOTO  The following colours are used to annotate the corresponding features Color Code Explanation Blue Water bodies (river, stream, lake, marsh etc Yellow Alluvial/Quaternary deposits Orange Alluvial/Quaternary deposits Red Geological features e.g. strike, dip, fold, fold axis, joints, fold traces, faults Pink Doubtful structural features as above Purple Stratigraphic contacts and unconformities Green Stratigraphic breaks e.g keybeds. Although a single colour can be sometimes used for the whole interpretation, however a key is used for each lithologic unit. Drawing terrain boundaries: Example  LITHOLOGICAL-STRUCTURAL INTERPRETATION : ON-PAPER Drawing terrain boundaries: Example  LITHOLOGICAL-STRUCTURAL INTERPRETATION : DIGITAL AERIAL PHOTO/SATELLITE IMAGERY  In analogue aerial photo, tone is used in combination with other interpretation elements. However in digital aerial photograph and satellite images, colour is used in combination with other interpretation elements. The use of colour comes from the combination of image bands acquired by different spectral channel of a sensor over a spectral band for which it is sensitive. Note:  It is possible to display a single band of satellite image in grey scale in which colour is absent and tone is present instead.  On the computer, 3 bands must be displayed together to enable perception of colour.  Pseudo colour which uses a single band for display is used occasionally to enable perception of colour. LITHOLOGICAL-STRUCTURAL INTERPRETATION : DIGITAL AERIAL PHOTO/SATELLITE IMAGERY  LITHOLOGICAL-STRUCTURAL INTERPRETATION : DIGITAL AERIAL PHOTO/SATELLITE IMAGERY  The display of the computer is designed to have 3 channels: RGB  The image bands inserted into the different channels of the computer determines the type of composite image the is seen on the screen  e.g Landsat TM (Right) LITHOLOGICAL-STRUCTURAL INTERPRETATION : DIGITAL AERIAL PHOTO/SATELLITE IMAGERY  IMAGE INTERPRETATION FOR GEOLOGIC INFORMATION The kinds and amount of information that can be obtained depends primarily on a) Type of terrain b) Stage of geomorphic cycle c) Climatic environment (degree of exposure) The following types of geologic information can be derived from remote sensing data a) Combined lithological and structural information (Aerial photo, MSS) b) Lithological information (MSS, Aerial photo) c) Structural information (Aerial photo, MSS) d) Mineralogical information (MSS, Hyperspectral data) Interpretation of analogue or digital aerial photographs and satellite images (usually in digital format) for lithological and structural information can be done either; 1. on paper: using either printed photo/image 2. on-screen  IMAGE INTERPRETATION FOR LITHOLOGICAL-STRUCTURAL PURPOSE Lithological-structural interpretation is possible mostly in regions with: Arid climate where there is sufficient exposure of rock outcrops Sedimentary (to some extent in metamorphic) terrains where the layers have significant attitude (in particular dip) Structural or denudational environment: development of folds, faults, joints e.t.c Matured geomorphic cycle: different drainage patterns would have developed within different lithologies in such areas. Also weathering and erosion would have developed features which can be used to identify rocks with different resistance in the region.  STRUCTURAL INTERPRETATION (VISUAL)  Structural interpretation/mapping can be done either on visually or automated.  Visual interpretation can be done either on paper or on-screen. However, focus will be on-screen interpretation.  The following images can be used in combination i. Hillshade (generated from DEM) ii. Colour composite (generated from MSS image)  Where it is possible, the following images can also be combined for interpretation iii. Stereo image (from Aerial photograph, or generated from MSS imagery) iv. Anaglyph (generated by fusion of DEM and one band of MSS imagery/aerial photograph) LITHOLOGICAL-STRUCTURAL INTERPRETATION : COLOUR COMPOSITE FROM DIGITAL AERIAL PHOTO/SATELLITE IMAGERY INTERPRETATION PROCEDURE: DEM INTERPRETATION PROCEDURE: ANAGLYPH IMAGE  THE MAIN INDICATIONS OF FAULTS ON RS IMAGE  Scarps: Morphologically, the most common landforms are fault scarps.  Triangular facets: when erosion advances and cuts up the fault scarp into residual landforms, parts of the original scarp will remain and indicate the original approximate position of the fault plane.  Block forming: relatively elevated and depressed surfaces can form a pattern called a block mosaic. Such an area is called block faulted.  Truncation: abrupt termination of landforms, or drainage pattern or sudden changes in photographic tone, texture along a straight line or linear feature.  Trenches or linear depressions: straight, incised narrow valleys or grabens in hard rock like igneous, dissolved cracks in limestone.  Controlled drainage; every straight, angular stream course if not strato-subsequent, should be considered fault controlled: streams are known to meander, any straight segment invariably raises suspicion of fault controlled.  Igneous features: linear arrangement of extrusive sand dikes where magma intrudes into major fractures or extrudes along them.  Lines of faults in limestone usually support development of sinkholes: where limestone is thrown against impermeable rocks, local drainage may disappear into lines of sinkholes marking the fault, alignment of sinkholes and vegetation springs  THE MAIN INDICATIONS OF FAULTS ON RS IMAGE Lineaments: These are large scale linear features, which are the topographic expression of underlying structural features such as fault-controlled valley, joint-controlled valleys or streams, fronts of mountain ranges, straight and narrow mountain or hill ranges, ridges, lines of isolated hills, linear igneous intrusions, and lines of volcanoes  Offset streams, rock units, and other linear features.  Abrupt changes of dip on monoclines, or a sudden change of dip, strike or both, along a line. SPECTRAL REFLECTANCE CURVE  We can establish for each material of interest a spectral reflectance curve by recording the amount of reflection for several wavelength band between a particular wavelength range (e.g 0.4 –2.4µm).  Sensors composed several sensing channels are able to measure reflectance in numerous wavelength bands generating a spectral reflectance curve  Such curves show the fraction of the incident radiation that is reflected as a function of wavelength.  Measurement can be carried out in the laboratory or in the field using spectrometer and can also be done using airborne spectrometers called hyperspectral sensors. Fig. 45: Typical spectral reflectance curves for selected common natural objects - water, vegetation, soil and limonite SPECTRAL REFLECTANCE CURVE The proportion of the radiation reflected in the different parts of the spectrum depends on leaf pigmentation, leaf thickness and composition (cell structure) Spectra of Soils a.Organic dominated b. Minimally altered The main factors influencing the reflectance are soil c. Iron altered d. Organic affected colour, moisture content, the presence of carbonates, e. Iron dominated and iron oxide content.