Minerals prospecting and exploration

CHAPTER 3.2

Minerals Prospecting and exploration José L. Lee-Moreno

inTRoDuCTion

Most outcropping ore deposits have already been discovered, so the modern mineral prospector must use more-subtle markers to recognize the presence of an anomalous mineral occurrence. New mining and metallurgical techniques allow today’s mining industry to evolve beyond limited high-grade production to take advantage of lower-grade, large-volume extraction and processing. The days of the small mine are practically over. An additional objective of mineral exploration today is to find significant extensions of preexisting deposits associated with operating or abandoned mines. Some well-known mining districts, under changing economic conditions and the application of modern exploration techniques, may present new opportunities to extract remaining minerals from them. Increased understanding of ore-deposit modeling using statistical analyses, computer technology, and new laboratory methods provides the explorationist with powerful tools for discovering new deposits in the subsurface including deposits extending from previously known or partially mined ore bodies. Broadly classified, new deposits may be considered either as outcropping (surface) or concealed (subsurface). There is little new ground to be explored in more-developed countries, except in less-accessible, remote areas. The search for outcropping deposits is carried out primarily in developing countries. In some cases, outcropping deposits may have been buried by tropical soils or by some other post-mineralization event, thereby eluding discovery. Discovery of concealed deposits at shallow depths requires the integration of various advanced technological methods for preliminary assessment. Sound exploration methods are the most valued tools for the exploration geologist, whose primary objective is to identify the geological characteristics of a mineralized system that can lead to the discovery of an ore body. These characteristics must include geological, lithogeochemical, isotopic, and petrochemical features; physical properties of both altered and unaltered host rocks; structural and tectonic framework; and any other relevant relationships of the mineralizing events in terms of time and space.

Minerals prospecting and exploration cover a wide range of earth science disciplines including geology, geochemistry, geophysics, and remote sensing (which incorporates satellite imagery and multispectral data interpretation). New laboratory techniques and computer aids are used for modeling and understanding the genesis of ore deposits. These issues will be discussed in detail in this chapter. Prospecting Although prospecting and minerals exploration are widely considered synonymous, there are still some regions in the world where prospecting is considered the initial, moreprimitive phase of exploration, reminiscent of the old goldpanning prospectors, but with the addition of some modern technology. Exploration refers to semidetailed or moreadvanced detailed studies. Prospecting can help locate sites that, after a formal exploration program, may result in an ore deposit discovery. These terms will be used indistinctively here. Ancient prospecting began with panning in creeks and riverbeds and continued with the physical exploration of surrounding hills. These areas were quickly mined out and abandoned but more recently have provided excellent guides for the discovery of source deposits. In modern prospecting, metal detectors and artisanal heavy mineral concentrators are still used. In more-advanced placer mining operations, draglines and high-capacity gravity, magnetic, and centrifugal classification and concentration equipment are used successfully. A new generation of optical and laser mineral separators have also been introduced. Minerals exploration The main objective of minerals exploration is to locate ore deposits, which are anomalous accumulations of one or more minerals that can be mined at a profit. Drilling is the most reliable, more-expensive technique used to confirm or deny the existence of an ore deposit, but new methods and technologies are still being developed.

José L. Lee-Moreno, Adjunct Professor, Department of Mining and Geological Engineering, University of Arizona, Tucson, Arizona, USA

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Compiling and analyzing preexisting data is indispensable. The exploration geologist must identify specific factors that controlled the mineralization in an area, district, or province and, using multidisciplinary methods, “see through” any post-mineralization cover to determine the possibility of a concealed deposit. Geological surveys, varying in detail and scale, are fundamental to all exploration programs. It is important to accurately identify any geological features of nearby deposits and the general characteristics of their position or emplacement. The features of previously outlined metallogenic provinces are very useful at this stage. Among the most important features considered during this entire exploration process are geochemical, geophysical, and remote sensing signatures; depositional models; mineral alteration; zoning; and other mineralogical guides. The details may be modified and redefined more accurately as the program advances and new indicators are discovered. Geological interpretation must rely on additional techniques and disciplines such as field mapping, structural geology and tectonics, geomorphology, petrology and petrography, sedimentology and stratigraphy, mineralogy, geostatistics, geotechnical engineering, hydrogeology, metallurgy, geothermometry, topography, and geochemistry. The three major methods that integrate all geological observations in formal exploration programs will be reviewed briefly here: exploration geochemistry, geophysical exploration, and ore deposits. Resource and reserve estimations and definitions constitute one of the final stages before entering into production. These methodologies are discussed in detail in other chapters. In regional exploration, geochemistry begins with stream sediment surveys followed by detailed sampling of soil and rock. Hydrogeochemistry, biogeochemistry, and geobotany also should be considered and applied when relevant. Regional aerial geophysics such as magnetic, radioactive, and electromagnetic methods have proven very useful in helping to delineate major structures and in identifying lithology, overburden, and concealed mineralization. In some specific cases, aerial gravimetric technology has been applied, but not with great success. Together with electrical methods, these offer follow-up support to ground geophysical exploration. Some geologists include remote sensing in the area of geophysical exploration, but it is more correctly categorized as a separate method. Aerial photography continues to be extremely useful, and the Internet allows the downloading of mono- and polychromatic images of nearly every part of the earth’s surface, often free of charge. Older methods of low- and high-flight aerial photography have practically disappeared and now are applied only in very specific cases. High-resolution satellite imagery has become the preferred method of aerial photographic imaging. Multispectral data is also available to the public from some government agencies that possess special satellites. Rock alteration and zoning features can be detected easily by manipulating multispectral data. The technology is still under investigation by universities, federal and state geological surveys, and earth science research organizations; a few private firms offer commercial services. In the second phase of an exploration program, analysis of survey data and methodologies are selected and applied. The depth of subsurface exploration depends on the mineral

commodity. Only a very few minerals such as diamonds and high-grade gold- and platinum-group metals have sufficient unit value to support deep exploration. In a subsequent stage, local geological studies collect structural, stratigraphic, lithologic, and petrochemical data. All spatial and temporal relations between different rock types must be defined. Of great significance is the relative time of emplacement of volcanic and/or intrusive rocks in the entire local rock package. Each type of mineral deposit will require specific exploration methods. Porphyry coppers, precious metal epithermal veins, skarns, sediment-hosted deposits, and polymetallics should be studied using different combinations of methodologies. Post-Mineralization Cover

Normal weathering and erosion, as well as tectonic processes, often make direct observation of mineral deposits difficult; however, weathered materials can be excellent indicators of mineralization. Argillization, oxidation, propylitization, and silicification are major alteration varieties that produce significant color and textural changes in outcroppings. Rock alteration therefore is very important at this stage. Detailed geochemistry and geophysics are widely applied in these cases to investigate post-mineralization cover. Remote sensing helps to detect these surface manifestations of altered rock. Computer databases, geographic information systems (GISs), and statistical analytical methods are used to organize and study the geological data. Commercial software packages are available and useful for these purposes. The next step typically is to sample and test the potential target directly by diamond core and reverse circulation drilling. Core samples allow the direct observation of the rock for more-detailed geological, geotechnical, and structural data. Reverse circulation provides a more general idea of lithological variation and permits faster sampling for grade determinations. The primary problem for the exploration geologist is finding anomalous mineral accumulations in accessible locations with grades that will make their extraction, processing, and marketing profitable. Exploration concessions are registered regularly by the thousands, primarily in developing countries. Most are simple “land play” by junior companies or the hope of an old-time prospector to own a mine. Prospect submittals must be selected carefully and technically reviewed before investing any time and capital in further examining such concessions. Modern exploration involves an initial reconnaissance visit, followed by a more-intensive exploration program if warranted. Very few ore bodies are discovered today as a result of only one geological visit. The high cost of exploration can be reduced by applying additional methods of scientific evaluation rationally. World metal prices are the primary factor controlling mineral production; however, complex global economic factors such as politics and armed conflict may cause unexpected and unpredictable price fluctuations. The search for concealed ore deposits is ongoing and conducted in areas hitherto considered prohibitive. These include deeper subsurface ore bodies and those under the oceans. Industrialized countries will continue to be the major consumers of mineral raw materials, while developing countries will continue to be the main providers. Talented exploration geologists will be more in demand, along with the use of

Minerals Prospecting and exploration

technologies, some of which still need defining, refining, and even discovery.

exPloRATion geoCheMiSTRy

Exploration geochemistry (EGCH) is an exploration method based on systematic measurements of the amounts and distribution of chemical elements along with their migration in the earth’s crust. The exploration of minerals is based on the premise that elements or traces of geochemical “pathfinders” migrate out from the original ore body by various means to form anomalous areas or dispersion aureoles that can be detected by highly sensitive methods of analysis (see Table 3.2-1 for some of these elements). Further details are discussed in Chapter 3.4. In general, the larger the anomaly, the larger the source. Also, the intensity of the anomaly is usually proportional to the grade of mineralization; however, the interpretation of a potential ore body depends on the different mobilization capacities of pathfinder elements that occur in various environments. EGCH has important applications in all stages of mineral exploration. In large areas such as metallogenic provinces, regional low-density stream sediment surveys are initially conducted (one sample every 100 km2) followed by moredetailed stream sediment sampling intervals (1–3 km/sample as determined by local topography). Soil and rock geochemical sampling are planned and conducted after an approximate perimeter of the anomaly is delineated. Geochemical data is stored using orthogonal coordinates of sample locations as well as the values of the elements analyzed. Statistical methods are then applied to assist in interpreting the results. Normal (background) and anomalous (above threshold) values within a given geochemical population must be estimated in the initial phase of an investigation. Contrast, which consists of the ratio between a sample and background values, is also commonly used. Profiles of distance versus element value are very useful. Frequency and cumulative frequency distribution plots thus are produced in combination with other basic statistical analyses. Isograd or isopach maps are drafted by contouring selected elements mostly from stream sediment surveys; circles of various sizes represent relative element ranges. Element ratios have proven highly successful in geochemical interpretation. Noneconomic accumulations of metals and human contamination often produce insignificant or spurious geochemical anomalies. The exploration geologist must be alert to these potentialities and filter them out. Although a relatively young technique, EGCH’s economic utility in the discovery of mineralization has a proven track record. EGCH is now universally accepted as the most important and effective exploration method for the discovery of precious metal deposits. Dispersion Models A dispersion model illustrates the abundance and distribution as well as the stability of a chemical in the local environment. Element dispersion is the foundation of EGCH. Dispersion halos are of a primary or secondary nature. Primary dispersion halos are directly related to the place of mineralization, and many sometimes occur as a reflection of the deposit’s alteration halos, but they can be of a much larger extension. Secondary dispersion halos are formed by later migration of traceable guide elements. In large mineral

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Table 3.2-1 elements acting as pathfinders of ore deposits ore Deposits Pathfinders Ag Au Ag As

x

Ba

x

Be

Co

Cu

x

x

x

x

x

li

Mo

ni

Pb

Sn

u

W

x

x x

x

x

Cd

x

Co

x

Cu

x x

x

x

H 2S

x

Mn

x

Mo

x

Ni x

x

x

x

x x x

x

x

x

x

Re Sb

x

x x

Pb

x

x

Se

x

x

Sn

x x

x

SO4 Te

x

x

x

x

x

x

x

x

x

x x

x

x

x

W Z

zn

x n

x

x

x

districts, dispersion halos form “geochemical provinces” with higher than normal background values. The specific applicable methods of EGCH and the associated sampling materials depend on these dispersion halos. These are briefly discussed in the following sections; however, one should always run orientation surveys to determine which group of elements is best to analyze for, along with the types of sampling and sample spacing. Primary Dispersion Halos

There are three main types of primary dispersion halos: 1. Geochemical provinces are specific zones in which the chemical composition is suspiciously different in one or various elements from adjacent zones or from average crustal values. 2. Aqueous fluid dispersions are produced along paths of hydrothermal mineralizing solutions. This results in disseminations on wall rocks and seepage halos along broken ground around the deposit. 3. Gaseous dispersions are produced by high-temperature gaseous fluids that invade adjacent ground. These can be detected later in their gaseous forms or as elemental condensates. Mercury, some radioactive gases, and carbon dioxide or sulfur dioxideare used in the study of gaseous halos. Secondary Dispersion Halos

The physical and chemical properties of the rock-forming minerals and the types of erosion agents control distribution of the disintegrated products of the parent rock. These processes are responsible for forming secondary dispersion halos. The patterns that result are very useful in geochemical exploration and are discussed in the following paragraphs. Weathering is the process whereby rocks are broken down by the action of chemical, physical, and/or mechanical agents.

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The following main products of weathering are important in EGCH: • Primary residual products are more stable, only partially disintegrated, and help in defining the parent rock. • Secondary residual products occur close to the source; hydrolysates such as clays and iron oxides may carry tracks of nearby mineralization. • Soluble products are unstable products that travel considerable distances from their source(s) as dissolved solids (e.g., calcium and magnesium carbonates, sodium and potassium salts, manganese and iron oxides, and other basic metal salts). Surface waters carry visible to microscopic particles of organic and inorganic compounds with traces of chemical elements incorporated along the way. Element mobility is the main factor controlling this type of dispersion. It is the basis for hydrogeochemical prospecting. Groundwater involves the same principles as in surface waters. Shallow and deeper water wells and spring waters are used for sampling. Stream sediments are clastic materials derived from ore deposits and are often transported large distances by fluvial water, then deposited by mechanical or chemical effects, particularly near confluences. Stream sediment geochemistry has been a very effective method in exploration. Anomalous values are followed upstream to discover the source. Soils retain chemical and mineralogical characteristics of their parent material and often capture elements that travel through them. Most soils have a great capacity for absorption and so they become useful in EGCH. One should recognize whether soils are transported or residual in nature to correctly interpret anomalous patterns and possible sources. In soil sampling, always sample from the same soil horizon; however, a new methodology analyzes soil samples at the same depth regardless of horizon. This topic is examined further in the following section. Vegetation may indicate soil chemistry because some plants grow only where certain nutrients occur. Others are natural absorbers and accumulators of specific elements. These travel through soils and fractured rocks and are taken up by vegetation. In some cases, plants grow long, deep roots that may reach subsurface dispersion halos and take up some of their chemical components. In other cases, deleterious elements and compounds may result in detectable atrophies or hypertrophies in plants that can lead to discovering mineralization. Organic accumulations resulting from some organic materials that possess a high ionic exchange capacity can cause them to act as traps for certain cations. This is usually observed in swampy areas near water feeder channels. Animal actions are less important in EGCH, and only in a few isolated cases have they been helpful. Burrowing animals such as foxes and ants, for instance, or mound builders such as termites may bring subsurface metal-bearing minerals or precious-gem indicators to the surface where they can be sampled and observed easily. Glacier sediments may be explored similarly to stream sediments. Terminal and lateral moraines may contain mineral fragments from upstream. Also useful are fine clays, which should be collected and analyzed. If an anomaly is detected, any additional geochemical sampling would be

difficult because of the glaciated terrain; therefore, indirect geophysical methods can be used to investigate further. Meltwater actions may carry traces of certain anomalous elements trapped in the pore spaces in rocks. Freezing produces upward flows of some groundwater that could be helpful in locating concealed deposits. This method is only of limited advantage. geochemical Analytical Methods Although geochemical methods are routinely updated and redefined, the exploration geologist must keep three main factors in mind: 1. Sensitivity 2. Precision 3. Cost All samples from a particular area must be analyzed by the same technique and preferably by the same laboratory for consistency. In some cases, incorrect results have caused additional unnecessary expenses, or worse, unjustified project abandonment. Duplicate sets of samples should be prepared and delivered to the lab, with one blank and one known standard provided for each set of twenty samples. Double-checking the analyses at different laboratories is highly recommended. EGCH is based on the detection of very small amounts of certain elements, making use of many modern methods of analysis. The most common are the following: • Regular fire assay remains a preferred method for gold analysis. • Spectrometric methods are mostly applied for multielemental analyses: – Atomic absorption, developed in the 1950s, is still widely used in EGCH when a single or small number of elements need to be determined. In atomic absorption the electrons of an element are promoted to higher orbitals for a short time by absorbing a quantity of energy specific to that particular element. This gives the technique its elemental selectivity. The amount of energy is measured and is directly related to the concentration of the element in the prepared sample. – Inductively coupled plasma–mass spectrometry (ICP-MS) is the preferred method in modern geochemistry that allows for quick and precise multi-elemental analysis. ICP-MS determines the elemental composition of samples by counting the number of ions at a certain mass of the element. – Inductively coupled plasma–optical emission spectrometry (ICP-OES), a complement to the mass spectrometer, is also used in detecting lighter elements of the periodic table. • Mobile Metal Ion is the low-level chemical analysis of soils by use of special extractant solutions and later concentration determination by ICP-MS. Soil samples are taken from equal depths regardless of soil horizon. Initially applied by the Geochemistry Research Centre of Australia, this method is now widely used. • X-ray fluorescence handheld analyzers allow for fairly accurate detection of low elemental concentrations of most types of solid geochemical samples in the field. • The colorimetric method was one of the pioneering methods of analysis several years ago. It was replaced

Minerals Prospecting and exploration

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Table 3.2-2 Main applications of geophysical methods of exploration Method

Application Media

Main exploration Applications

Gravimetric

Ground, marine

Heavy minerals deposits, iron ores, pyrite, chalcopyrite, chromite, salt domes, intrusive vs. volcanic or sedimentary rocks, structural mapping

Magnetic

Ground, marine, airborne, drill-hole logging

Iron ores, magnetite, pyrrhotite, black sands, kimberlites, chromite ores, mafic intrusives, basement irregularities, geological and structural mapping

Electrical: Resistivity

Ground, marine, drill-hole logging

Sulfide deposits, conductive vs. resistive rocks, massive sulfides, base metals, graphite, quartz and calcite veins, salt domes, coal beds, underground conductive fluids, tectonics

Electrical: Induced polarization

Ground, drill-hole logging

Nonconductive polarizable mineralization; disseminated and massive sulfides; porphyry copper and gold, and silver deposits; tin; zinc; stockworks

Electrical: Self-potential

Ground, drill-hole logging

Sulfide ores; tin, cobalt, nickel, gold and silver deposits; massive sulfides

Electromagnetic: Coil sensors

Ground, airborne, drill-hole logging

Sulfides and oxides, magnetite, graphite, base metals, kimberlites, shear zones, geological and hydrological mapping

Electromagnetic: Superconductive quantum interference device (SQUID) sensors

Ground, airborne, drill-hole logging

Deep conductive deposits, massive sulfides

Radioactive

Ground, airborne, drill-hole logging

Radioactive minerals: uranium, thorium, potassium; coal; phosphates; monazite; structural mapping; differentiation of intrusives

Seismic

Ground, marine

Coal, uranium, heavy minerals, buried placer deposits, sand and gravel deposits, fractured rocks, lithological changes

Remote sensing

Airborne, satellite

Geological and structural mapping, tectonics, alteration

by more-accurate methods; however, it is still applied in remote areas where on-site results are required quickly. Data interpretation Statistical analysis is the best method to assist the exploration geologist in interpreting large amounts of geochemical data. Numerous commercial software packages exist, ranging from simple statistical calculations (e.g., the mean and standard deviation, and normal and log-normal frequency distributions) to more-advanced geostatistical analysis including Kriging, cluster analysis, discriminant analysis, factor analysis, correlation, and multiple regression analysis. All data are entered in digital databases and later integrated into GISs and spatially related through the Global Positioning System. This allows for compiling various layers of information to facilitate interpretation. Many software programs, either canned or proprietary, are used for plotting different kind of maps, profiles, and block models.

geoPhySiCAl MeThoDS of exPloRATion

Geophysical exploration is based on measuring the contrast between natural and induced physical properties of materials. Geophysical surveys begin with airborne reconnaissance methods to outline broad geologic features. Radiometric, magnetic, electromagnetic, and, more recently, gravimetric methods have been applied successfully in airborne surveys. The exploration geologist continues to use more-detailed geophysical methods that provide more-detailed information, including downhole measurements, searching directly for indications of concealed mineralization. Geophysical exploration methods have gained considerable popularity in the last few years. More precise and easyto-use instruments have been developed. Modern surveying techniques and software aid in interpreting results faster and with more reliability than previously. Table 3.2-2 shows the main geophysical methods used in the exploration of various minerals. Survey design and the

nature of the data are dependent on the characteristics of the detecting instruments. The geophysical detection of mineral deposits requires that they possess physical properties different than those of their host rocks. geophysical Methods Highly sensitive gravimeters can detect anomalies caused by differences between the local measured gravitational acceleration and the regional expected value for a given point on the earth’s surface. Multiple corrections (for geographical and topographical effects) to the field data, however, must be calculated before a final interpretation can be made. Gravity data is analyzed using digital techniques to obtain regional anomalies, first and second derivatives, residual anomalies, and horizontal gradients. Magnetic Surveys

Magnetic surveys are based on the earth’s magnetic field. Similar to gravimetric surveys, magnetic surveys show anomalies caused by differences between values obtained locally from survey readings and that calculated for the normal intensity of the earth’s magnetic field at any given point. Aeromagnetic surveys have proven very useful in regional reconnaissance exploration. The application of remote magnetometry has also been used on research ships in oceanographic studies, and, even more remotely, from artificial satellites. Modern magnetometers are highly sensitive, can produce results in a short amount of time, and are relatively inexpensive to operate. In addition to the detection of ferromagnetic mineral deposits (mainly magnetite and pyrrhotite), remote magnetometry helps in regional mapping of lithological, structural, and tectonic features. Downhole magnetic logging has been widely used when magnetic minerals are present in detectable amounts in the survey area. Magnetometric methods can also help define concealed formation contacts, unconformities, major faults, and thicknesses of sedimentary cover.

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Electrical Methods

Electrical methods operate by measuring natural or induced electrical fields. Although there are many varieties of electrical methods, resistivity, induced polarization, and self-potential are most commonly applied in mineral exploration. Electrical methods may be used to define structural and tectonic features, but they are more useful in the search for metallic conductors associated with ore bodies. Downhole logging is used widely and successfully to define ore bodies. In the proper geological setting, surface data should be employed in conjunction with downhole survey data. Resistivity is controlled by porosity and fluid content of the rock and the presence of conductive minerals; therefore, different rock types will have different natural resistivities. The depth of penetration and investigation is dependent on the energy injected into the ground by the power source, and by the spacing and array of electrodes. Resistivity has been used broadly in detecting all kinds of mineral conductors and in groundwater investigations. Induced polarization is often applied in parallel with regular resistivity surveys, making use of the same electrode array or with minor modification. It is based on the detection of electrochemical activity and/or the polarization at a mineral interface brought about by the application of an electric or magnetic field. Induced polarization is very effective in detecting disseminated mineralization such as the sulfide zone in porphyry coppers or disseminated ore minerals in precious metal deposits. Self-potential is used when the effect of a “natural battery” is produced, such as occurs by the flow of groundwater and, in the case of minerals exploration, by the interface of a sulfide ore body and its oxide zone. These occurrences are related to outcropping ore bodies or those that have been oxidized corresponding to phreatic zone fluctuations. Electromagnetic Methods

Several coil arrays are used in electromagnetic methods to measure a combination of induced long-wave electrical fields and the earth’s natural electromagnetic fields. An electromagnetic field is generated by a transmitter creating a secondary magnetic field, which is registered together with the primary magnetic field. Subsequent reductions and corrections are calculated so that a final interpretation is done in conjunction with geological and geochemical data. A superconducting quantum interference device (SQUID) uses an extremely sensitive magnetometer that registers very low magnetic fields. Sometimes it is used in mineral exploration together with coil sensors to detect deeper conductors and to isolate the signal from that coming from overlying conductive cover. Many successful applications have occurred in exploring for deep massive sulfide deposits. Radioactive Methods

Radioactive methods measure natural or artificial radioactivity. Uranium and thorium are the main sources of radioactive emissions. Regular radioactivity detectors can only reach shallow depths of no more than a few meters; however, emanometry, which detects radon emissions, can “see” deeper into the subsurface and is sometimes used together with regular radioactivity detectors.

Airborne gamma-ray surveys have wide applications exploring over large areas and are useful in geological mapping, taking advantage of minor radioactive emissions from naturally occurring rock-forming minerals. Decaying radioactive elements produce gamma rays, which are easily detected by modern gamma-ray spectrometers. Potassium, uranium, and thorium are common radioactive trace elements that occur in many rock-forming minerals. As they decay, they provide natural sources of gamma rays that are detected in these surveys. Seismic Surveys

With limited applications in mineral exploration, seismic methods measure changes in the velocity of shock waves produced by explosive charges or mechanical vibrators (even those from sledgehammer impacts). Reflection and refraction waves are recorded and used to define formational and structural contacts in the subsurface. Seismic surveys are relatively expensive but can be applied directly on the earth’s surface or from sea vessels. They are more widely used in petroleum exploration. Remote Sensing

Infrared sensors register changes of temperature, which have been used successfully in environmental studies and in delineating groundwater zones and exothermic zones of mineral alteration. High-resolution satellite imagery and imaging spectroscopy are gradually displacing traditional aerial photographic methods. Imaging spectrometry uses multispectral detecting devices. The National Aeronautics and Space Administration’s Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) is among the most advanced in use with 224 channels. Other sensors are listed as follows: • HyperCam infrared hyperspectral imaging camera (ground based or taken by aircraft) • Moderate Resolution Imaging Spectroradiometer (MODIS; aboard the Terra EOS AM-1 satellite) • Medium Resolution Imaging Spectrometer (MERIS; aboard the European Space Agency’s Envisat satellite) • Hyperspectral Digital Imagery Collection Experiment (HyDICE) spectrometer • Hyperion sensor (aboard Earth Observing-1 satellite) The large multichannel feature gives the sensors the capability to observe atmospheric and planetary phenomena in environmental studies and mineral exploration. Absorption of spectral energy caused by chemical bonds allows the detection of different types of vegetation and many synthetic materials, minerals, and mineral assemblages. Interpretation is not simple and requires advanced training and experience along with the use of sophisticated software. Costs are relatively high and depend on the size and characteristics of the area to be covered. The popularity of imaging spectrometry is certain to increase in the future.

oRe DePoSiTS

The exploration geologist must have an idea of the type of ore deposit being sought prior to starting a mineral exploration program. In regional studies, the general geologic setting provides the first clues. The proximity to any mineral district or predefined metallogenic province also helps to anticipate the type of mineralization.

Minerals Prospecting and exploration

ore Deposit Models and Classification Most modern authors refer to the initial, and still popular, classification of ore deposits proposed by Lindgren in 1933, or that proposed by Schneiderhohn (and later modified by Niggli in 1929). Many other classifications have been proposed but these still prevail. Modern studies have established geological and geophysical ore-deposit models whose parameters provide important guides for mineral exploration. Each deposit, however, should be studied or explored on its own merit. The most widely accepted classification schemes are based on genetic characteristics. Five groups are defined: magmatic, hydrothermal, sedimentary, metamorphic, and surface accumulations. Morphological classifications, such as vein type, disseminated, massive, or strata-bound are normally subdivisions of these. Each group is subdivided into classes, families, or types, and very often there is some overlapping between two or more groups. Some ore-body classifications are based on mineralogical associations. Although there may be some economic or genetic implications, the result has limited relevance in exploration. Greater significance is placed on classifications based directly on geological setting and oreformation processes. In all cases, it is very useful to model the deposit under exploration after a similar well-known or previously studied deposit. The geologist should be prepared to confirm or change the model as more data become available. When evaluating an exploration prospect (or any mining property), the explorationist must adhere to terms and definitions for resources and reserves, which are now required and regulated internationally. The most widely accepted standards are those in National Instrument NI-43-101 (2005), proposed by the Canadian Institute of Mining, which is similar to the Joint Ore Reserves Committee (JORC) Code from Australia (AusIMM 2004). Both are accepted in international mining stock exchanges. The two documents also define a “qualified person” who is required to sign-off on any technical reports. zoning Mineralogical and geochemical zoning constitute valuable guides in mineral exploration. They reveal subtle clues of certain indicator chemical elements or accumulations of ore minerals. Zoning occurs in regional, district, or ore-body scales. Regional zoning is related to metallogenic provinces; district zoning is observed in areas with several mines; ore-body zoning is particular to an identified deposit type and is more relevant in mineral exploration. Ore-body zonal arrangements are manifest in three dimensions. Their proper identification is especially helpful in lateral or vertical mine planning, in the selection of new exploration targets, and in decisions to terminate drilling and mine-development programs, or both. Generalized zoning patterns have been recognized for most types of ore deposits; however, each case is different. Most ore-body zonation is characterized by several pulses of mineralization, which often cause patterns of overlapping zones. These must be identified, registered, and properly mapped as soon as the information becomes available. It is not uncommon to follow two or more zoning patterns in mine production. Figure 3.2-1 shows an idealized zoning pattern for a hydrothermal gold–quartz vein deposit.

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Zoning is often related to paragenetic sequences and therefore is greatly influenced by changes in temperature and pressure, and by the composition and stability of the mineralizing fluids. The concepts are well known and cited abundantly in the technical literature. Among the subtle zoning guides mentioned previously are geochemical indicators. Geochemical zoning reflects the elements present in mineralizing solutions, and these depend on their relative mobility in and through the rock environment. Wall-Rock Alteration Wall-rock alteration is related to the action of hydrothermal fluids in epigenetic and high-temperature remobilization processes, which are observed most often in felsic rocks. It may also be caused by some metamorphic or diagenetic processes. Weathering of outcropping deposits may cause “strong rocks alteration” as well, due to the oxidation of iron-rich minerals and some argillization. All alteration processes are the result of chemical exchange and mineralogical modification. Alteration halos are typically present in zonal arrays emanating from mineralized circulation channels outward to fresh country rock. They often extend outward several times the perimeter of the ore deposit, making them easier to detect. However, they may also be of very limited dimensions, depending on the reactivity and permeability of the rock, and the amount of circulating fluid. The most common alteration assemblages are as follows: • Potassic involves the introduction of K-spar and other potassium minerals. • Sericitic, also referred to as phyllic, consists of the presence of micaceous potassic silicates derived from primary felsic minerals. • Argillic is represented by kaolinite and montmorillonite after plagioclases and amphiboles. Advanced argillic is a variation that results from strong acid-leaching. • Silicic involves the abnormal injection of siliceous solutions (and the almost universal presence of free quartz). • Propylitic is produced by low temperature–pressure formation of epidote, chlorite, and calcite. • Fe oxidation, although not properly a result of hydrothermal alteration, gives rise to the common decay or alteration of Fe-containing minerals. These are highly visible and widely used in minerals exploration. This phenomenon has developed into the study of leached and oxidation caps (or gossans), which, in their diverse assemblages, provide excellent clues to the presence of ore deposits and their sources. All these alteration processes produce color changes in the original rocks. White to bright reddish and yellow are common, passing through shades of green and pink. They all constitute primary guides for the explorationist. fluid inclusion Studies In idealized deposition models for many ore deposits, temperature and chemical zoning becomes useful in understanding mineralization and its origin. Identifying these features can be accomplished through the study of fluid inclusions, particularly during the drilling of a prospect, or when the upper portions of an ore body have been eroded away, or when structural or tectonic movements have modified the original ore-body’s position.

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Surface

Au

As

Bi

Ag

Pb

Sb

Cu

Be

Mo

Co

Zn

Sb, Hg As Ag, Au

Vein or Emission Center

Barren Ag, Mn Pb Zn

Irregular Se and Te Close to Origin

Cu Bi

TI Present on All Occasions

Mo, W Au, Ag

Distances Are Variable

Co Be

Telescoping or Overlapping May Be Present

Barren

figure 3.2-1 idealized vertical and lateral zoning in gold–quartz veins

Fluid inclusion studies are based on the collection of transparent mineral crystals that have trapped gases and/or liquids in vacuoles at the time of deposition. The vacuoles are observed under the microscope and then subjected to temperature manipulation by special heating–cooling stages to determine the temperature of homogenization. Their chemical compositions can be defined by one of many analytical techniques, such as laser spectrometry or electron microprobe. isotopic Studies Some isotopes have been used to study the sources and compositions of mineralizing fluids in many ore deposits and to determine their geochronology. Isotopes of hydrogen, carbon, oxygen, sulfur, strontium, and lead are most commonly studied. Distinctions between barren and productive intrusive and volcanic packages have been made successfully by this method. Isotopic studies are performed mostly as academic research because of a lack of knowledge and experience of the technique in the mining industry; however, its importance in mineral exploration is becoming more relevant as more experimental data (and its interpretation) are published. geometallurgy Geometallurgy is a relatively recent technique in mineral exploration. It is based on precise quantitative mineralogical and chemical measurements using electron microprobes or similar instruments, followed by statistical analyses to determine variabilities in physical, mineralogical, and geochemical characteristics of a mineral prospect. The information

obtained is used in all stages of exploration, as well as in the development of an ore deposit, reserve and resource evaluation, and metallurgical processing. Geometallurgy may be applied to stream sediment samples, mineralized outcrops, drill cores, or even in productive mine stopes. Ore-deposit models can be proposed, and strategies for further exploration may then be designed. It is also very useful in ore-body modeling, predicting possible extensions and determining the quality of mineral accumulations.

ACknoWleDgMenTS

The author is particularly grateful to Stan Krukowski of the Oklahoma Geological Survey for his help in reviewing this chapter. Monica and Steve Rich, and Gil Colgate also assisted.

RefeRenCeS

AusIMM (Australasian Institute of Mining and Metallurgy). 2004. The 2004 Australasian Code for Reporting Exploration Results, Mineral Resources and Ore Reserves (The JORC Code). Gosford, NSW: Joint Ore Reserves Committee. Lindgren, W. 1933. Mineral Deposits. New York and London: McGraw-Hill. National Instrument NI-43-101. 2005. Standards of Disclosure for Mineral Projects. Montreal: Canadian Institute of Mining, Metallurgy and Petroleum. Niggli, P. 1929. Ore Deposits of Magmatic Origin: Their Genesis and Natural Classification. Translated by H.C. Boydell. London: Thomas Murby and Company.