Mineral exploration

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The Prospector by N. C. Wyeth, 1906

Mineral exploration is the process of finding ores (commercially viable concentrations of minerals) to mine. Mineral exploration is a much more intensive, organized and professional form of mineral prospecting and, though it frequently uses the services of prospecting, the process of mineral exploration on the whole is much more involved.

Stages of mineral exploration[edit]

Mineral exploration methods vary at different stages of the process depending on size of the area being explored, as well as the density and type of information sought. Aside from extraplanetary exploration, at the largest scale is a geological mineral Province (such as the Eastern Goldfields Province of Western Australia), which may be sub-divided into Regions. At the smaller scale are mineral Prospects, which may contain several mineral Deposits.

Province scale - area selection[edit]

Area selection is a crucial step in professional mineral exploration. Selection of the best, most prospective, area in a mineral field, geological region or terrain will assist in making it not only possible to find ore deposits, but to find them easily, cheaply and quickly.

Area selection is based on applying the theories behind ore genesis, the knowledge of known ore occurrences and the method of their formation, to known geological regions via the study of geological maps, to determine potential areas where the particular class of ore deposit being sought may exist. Often new styles of deposits may be found which reveal opportunities to find look-alike deposit styles in rocks and terrains previously thought barren, which may result in a process of pegging of leases in similar geological settings based on this new model or methodology. This behaviour is particularly well exemplified by exploration for Olympic Dam style deposits, particularly in South Australia and worldwide based on models of IOCG formation, which results in all coincident gravity and magnetic anomalies in appropriate settings being pegged for exploration.

This process applies the disciplines of basin modeling, structural geology, geochronology, petrology and a host of geophysical and geochemical disciplines to make predictions and draw parallels between the known ore deposits and their physical form and the unknown potential of finding a 'lookalike' within the area selected.

Area selection is also influenced by the commodity being sought; exploring for gold occurs in a different manner and within different rocks and areas to exploration for oil or natural gas or iron ore. Areas which are prospective for gold may not be prospective for other metals and commodities.

Similarly, companies of different sizes (in terms of market capitalisation and financial strength) may look for different sized deposits, or deposits of a minimum size, depending on their will and ability to finance construction. Often the major mining houses will not look for deposits of less than a certain size class because small deposits will not meet their criteria for an internal rate of return. This practise may result in larger mining companies relinquishing control of smaller ore bodies they find, or may preclude them from entering a terrane which is characterised by deposits of a particular type or style. For example, a mining major would not look for a relatively small, high-cost Kambalda style nickel deposit and would direct their efforts toward discovering a Mt Keith style deposit.

Often a company or consortium wishing to enter mineral exploration may conduct market research to determine, if a resource in a particular commodity is found, whether or not the resource will be worth mining based on projected commodity prices and demand growth. This process may also inform upon the Area Selection process as noted above, where areas with small-sized deposit styles will be ruled out based on likely economic returns should a deposit be found. This occurs because often smaller deposits are more expensive to run, and hence, carry greater risks of closure if commodity prices fall significantly.

Area selection may also be influenced by previous finds, a practice affectionately named subsurface control or nearology, and may also be determined in part by financial and taxation incentives and tariff systems of individual nations. The role of infrastructure may also be crucial in area selection, because the ore must be brought to market and infrastructure costs may render isolated ore uneconomic.

The ultimate result of an area selection process is the pegging or notification of exploration licenses, known variously as tenements, claims or licenses.

Political, sovereign and other associated risks[edit]

Any area selected for mineral exploration also carries various forms of sovereign and other associated risks; that is, the risk that even if a commercially viable deposit is present, political, environmental and social factors may make the discovery and development of the mineral resource inviable. This is a complex and varied topic only briefly outlined here, but such risks include, but are not limited to: a change in the security of licence tenure due to changes in legal, political or other factors; changes in local land tenure (such as declaration of various types of conservation zones); outbreaks of social unrest within a country or region (including competition for mineral resources by artisanal miners, who may be operating illegally, or from political resistance from local or non-local organisations, who may or may not be represented or supported locally, but who are opposed to certain, or all, forms of mining); the risk that the process of mineral exploration and assessment may be perceived by various parties as a form of 'mining' (particularly in areas not accustomed to mineral exploration activities) without due respect to environmental or social impacts, (when in fact mineral exploration is simply an early, low impact form of scientific assessment of a nation's mineral resources, which may or may not ultimately lead to mining); a change in government which may be unfavourable to the development of mineral resources or in some cases ideologically opposed in part of whole to private development of mineral resources (i.e. part of whole 'nationalisation' of resources); the prospect that a proposed future mining development may be deemed too environmentally damaging to proceed, meaning that a mining, as opposed to an exploration licence, may not be granted; changes in tax and other financial conditions subsequent to the conditions which were legally in place at the start of exploration; and natural disasters such as volcanoes, earthquakes, floods etc.

Mineral exploration is sometimes also conducted in very remote areas, where local enforcement of the law and the legal process may be inconsistent with that at the national or regional level, or inefficiently enforced; this means that in some cases even though mineral exploration is carried out in line with the conditions of a licence as determined at the national, state or local level, it may be unviable to explore or develop an ore deposit due to the inability of local, regional or national governments to properly enforce such laws. This may be particularly the case in countries where a politically separatist element or movement exists, in which local or regional areas may perceive mineral exploration, regulation and development as being administered and/or imposed by a state administration not recognised or supported locally. The benefits that accrue from such mineral exploration and development may also be perceived locally, rightly or wrongly, as being largely for the benefit of a state that is not recognised or supported locally, inflaming local political dissent, even though most modern exploration licences and large-scale mineral projects are routinely required by law to first obtain permission to enter and explore areas from the relevant local landowners and communities, to have proper compensation agreements in place for any impacts that may occur during mineral exploration and development, to have proper rehabilitation programs and plans in place, and with time to negotiate agreed levels of support and funding for such things as local schools and training facilities, medical services and/or hospitals, and various other local community and cultural programs, which funding and support generally increases if and when a commercially viable deposit is found. Moreover, even where a political state may be recognised and fully supported locally, there may be very limited government resources in very remote areas to properly administer mineral exploration and development, particularly in the context of pre-existing civil unrest and local tensions. Moreover, in countries with high levels of endemic corruption, exploration and development may be severely curtailed and increasingly costly due to a variety of reasons, but often including corrupt or inefficient bureaucracies at various local and national levels.

Because of these various and multiple risk factors, many developing countries (which may not in themselves have the financial or technological resources to explore or develop their own nation's mineral resources) are often seen as politically risky places to conduct expensive and risky mineral exploration, as the legal, economic and social conditions, (as well as sometimes also the lack of existing infrastructure), may not be favourable or stable over the longer term to bring a resource discovery into development, leading to a distinct trend towards stronger mineral resource exploration, development, production, and ultimately mineral resource export in more developed countries, and thus also a higher GDP per capita and a greater inequality between more developed, and less developed economies. This is partly a reflection of the naturally irregular distribution of the mineral resources themselves, but is also partly due to the differing level of exploration and development of mineral resources between different countries, as well as in some cases-particularly in less developed countries with high levels of endemic corruption- whereby funds accrued through mineral resource exploration and development may be illegally siphoned off and diverted, often to those in power in poorer countries where levels of corruption are high.

On top of these sovereign-related risk factors, there is also a variety of 'market' risks involved with mineral exploration, such as a drop in demand for a particular commodity which may occur over the years it takes to develop a deposit, which may make a formerly economically 'viable' deposit, uneconomic. (This is often countered by exploration and mining companies seeking to develop a deposit using the process of financial hedging; that is, agreeing to sell a particular commodity at a certain price for a specified time period to insure against a drop in commodity prices, however this can only 'insure' against a drop the buyer is willing to pay, and is not practical in all circumstances). Other 'market' risks include competition from alternative or cheaper commodities, as well as changes in culture or technology which also cause various commodities to drop or increase in demand. Different mineral commodities also have different levels of exploration risk that a commercially viable deposit is actually present or even able to be found, as some deposits or ore (particularly some gold, silver, and various base metal deposits) may show very limited surface expression and may be particularly difficult to find in economic concentrations even if they are present in a region or district, of which there is usually no guarantee, and for some commercially viable deposits many years of mineral exploration may sometimes occur before a commercially viable deposit is even discovered.

It is also of note, that advanced exploration techniques, properly regulated, are commonly far less environmentally and socially damaging than traditional prospecting (and mining) techniques. This is also a complex and detailed topic, but includes such practises as utilisation of mercury (a highly toxic substance) to explore and extract gold by artisanal gold miners (which is not used in modern mining or exploration), the hazards and high fatality rates involved with digging tunnels and shafts by hand to access ore zones, the poor ventilation of such hand-dug tunnels and shafts, and in some cases the physical stresses to workers carrying heavy lodes of ore (e.g. on their backs up mountains) over longer time periods.

Target generation - Regional Scale[edit]

The target generation phase involves investigations of the geology via mapping, geophysics and conducting geochemical or intensive geophysical testing of the surface and subsurface geology. In some cases, for instance in areas covered by soil, alluvium and platform cover, drilling may be performed directly as a mechanism for generating targets.

Geophysical methods[edit]

Geophysical instruments play a large role in gathering geological data which is used in mineral exploration. Instruments are used in geophysical surveys to check for variations in gravity, magnetism, electromagnetism (resistivity of rocks) and a number of different other variables in a certain area. The most effective and widespread method of gathering geophysical data is via flying airborne geophysics.

Geiger counters and scintillometers are used to determine the amount of radioactivity. This is particularly applicable to searching for uranium ore deposits but can also be of use in detecting radiometric anomalies associated with metasomatism.

Airborne magnetometers are used to search for magnetic anomalies in the Earth's magnetic field. The anomalies are an indication of concentrations of magnetic minerals such as magnetite, pyrrhotite and ilmenite in the Earth's crust. It is often the case that such magnetic anomalies are caused by mineralization events and associated metals.

Ground-based geophysical prospecting in the target selection stage is more limited, due to the time and cost. The most widespread use of ground-based geophysics is electromagnetic geophysics which detects conductive minerals such as sulfide minerals within more resistive host rocks.

Ultraviolet lamps may cause certain minerals to fluoresce, and is a key tool in prospecting for tungsten mineralisation.

Remote sensing[edit]

Aerial photography is an important tool in assessing mineral exploration tenements, as it gives the explorer orientation information - location of tracks, roads, fences, habitation, as well as ability to at least qualitatively map outcrops and regolith systematics and vegetation cover across a region. Aerial photography was first used post World War II and was heavily adopted in the 1960s onwards.

Since the advent of cheap and declassified Landsat images in the late 1970s and early 1980s, mineral exploration has begun to use satellite imagery to map not only the visual light spectrum over mineral exploration tenements, but spectra which are beyond the visible.

Satellite based spectroscopes allow the modern mineral explorationist, in regions devoid of cover and vegetation, to map minerals and alteration directly. Improvements in the resolution of modern commercially based satellites has also improved the utility of satellite imagery; for instance GeoEye satellite images can be generated with a 40 cm pixel size.

Geochemical methods[edit]

Main article: geochemistry

The primary role of geochemistry, here used to describe assaying or geological media, in mineral exploration is to find an area anomalous in the commodity sought, or in elements known to be associated with the type of mineralisation sought.

Regional geochemical exploration has traditionally involved use of stream sediments to target potentially mineralised catchments. Regional surveys may use low sampling densities such as one sample per 100 square kilometres. Follow-up geochemical surveys commonly use soils as the sampling media, possibly via the collection of a grid of samples over the tenement or areas which are amenable to soil geochemistry. Areas which are covered by transported soils, alluvium, colluvium or are disturbed too much by human activity (roads, rail, farmland), may need to be drilled to a shallow depth in order to sample undisturbed or unpolluted bedrock.

Once the geochemical analyses are returned, the data is investigated for anomalies (single or multiple elements) that may be related to the presence of mineralisation. The geochemical anomaly is often field checked against the outcropping geology and, in modern geochemistry, normalised against the regolith type and landform, to reduce the effects of weathering, transported materials and landforms.

Geochemical anomalies may be spurious or related to low-grade or sub-grade mineralisation. In order to determine if this is the case, geochemical anomalies must be drilled in order to test them for the existence of economic concentrations of mineralisation, or even to determine why they exist in the place they exist.

The presence of some chemical elements may indicate the presence of a certain mineral. Chemical analysis of rocks and plants may indicate the presence of an underground deposit. For instance elements like arsenic and antimony are associated with gold deposits and hence, are example pathfinder elements. Tree buds can be sampled for pathfinder elements in order to help locate deposits.

Resource evaluation[edit]

Resource evaluation is undertaken to quantify the grade and tonnage of a mineral occurrence. This is achieved primarily by drilling to sample the prospective horizon, lode or strata where the minerals of interest occur.

The ultimate aim is to generate a density of drilling sufficient to satisfy the economic and statutory standards of an ore resource. Depending on the financial situation and size of the deposit and the structure of the company, the level of detail required to generate this resource and stage at which extraction can commence varies; for small partnerships and private non-corporate enterprises a very low level of detail is required whereas for corporations which require debt equity (loans) to build capital intensive extraction infrastructure, the rigor necessary in resource estimation is far greater. For large cash rich companies working on small ore bodies, they may work only to a level necessary to satisfy their internal risk assessments before extraction commences.

Resource estimation may require pattern drilling on a set grid, and in the case of sulfide minerals, will usually require some form of geophysics such as down-hole probing of drillholes, to geophysically delineate ore body continuity within the ground.

The aim of resource evaluation is to expand the known size of the deposit and mineralisation. A scoping study is often carried out on the ore deposit during this stage to determine if there may be enough ore at a sufficient grade to warrant extraction; if there is not further resource evaluation drilling may be necessary. In other cases, several smaller individually uneconomic deposits may be socialised into a 'mining camp' and extracted in tandem. Further exploration and testing of anomalies may be required to find or define these other satellite deposits.

Reserve definition[edit]

Reserve definition is undertaken to convert a mineral resource into an ore reserve, which is an economic asset. The process is similar to resource evaluation, except more intensive and technical, aimed at statistically quantifying the grade continuity and mass of ore.

Reserve definition also takes into account the milling and extractability characteristics of the ore, and generates bulk samples for metallurgical testwork, involving crushability, floatability and other ore recovery parameters.

Reserve definition includes geotechnical assessment and engineering studies of the rocks within and surrounding the deposit to determine the potential instabilities of proposed open pit or underground mining methods. This process may involve drilling diamond core samples to derive structural information on weaknesses within the rock mass such as faults, foliations, joints and shearing.

At the end of this process, a feasibility study is published, and the ore deposit may be either deemed uneconomic or economic.

Extraction[edit]

Main article: Resource extraction
Main article: mining

The ultimate goal of mineral exploration is the extraction, beneficiation and profitable and beneficial sale of mineral commodities.

Extraction methods may vary considerably and it is the discipline of engineers trained in mining engineering to determine the most safe, cost effective and efficient method of mining the ore body.

Mineral exploration and development does not cease upon a decision to mine. Exploration of a brownfields nature is conducted to find near-mine repetitions, extensions and continuity of the existing ore body. In-mine exploration and grade control drilling is a major concern of operating mines and can be an effective tool in adding value to existing mineral operations.

Often the lessons learned from studying an exposed ore body, both empirically and scientifically, are invaluable to the exploration geologist and geophysicist, for they get to see the proof of their concepts and the errors of the assumptions they used in the search for the ore body. It is always the case that the exact nature of the ore body does not exactly match the models used to find it.

Greenfields and brownfields[edit]

Exploration is termed either Greenfields or Brownfields depending on the extent to which previous exploration has been conducted on the tenements in question. Greenfields alludes to unspoilt grass, and brownfields to that which has been trodden on repeatedly. While loosely defined, the general meaning of brownfields exploration is that which is conducted within geological terrain within close proximity to known ore deposits. Greenfields are the remainder.

Greenfields exploration is highly conceptual, relying on the predictive power of ore genesis models to search for mineralisation in unexplored virgin ground. This may be territory which has been drilled for other commodities, but with a new exploration concept is considered prospective for commodities not sought there before.

The success rate of exploration and the return on investment is low because exploration is an inherently risky business. Figures for success rates depend on the commodity in question but a good strike rate can be measured in the oil industry; the supergiant Prudhoe Bay oilfield was found on the 12th well drilled into the area. Within gold deposits a discovery hole may be one in one thousand and within some base metals commodities strike rates range from one in fifty to one in one hundred.

Greenfields exploration has a lower strike rate, because the geology is poorly understood at the conception of an exploration program but the rewards are greater because it is easier to find the biggest deposit in an area earlier, and it is only with more effort that the smaller satellite deposits are found. Brownfields exploration is less risky, as the geology is better understood and exploration methodology is well known, but since most large deposits are already found the rewards are incrementally less.

See also[edit]

References[edit]