Tin, tungsten, and molybdenum
These three metals generally form in a certain type of granite, via a similar mechanism to intrusive-related gold and copper. They are considered together because the process of forming these deposits is essentially the same. Skarn type mineralisation related to these granites is a very important type of tin, tungsten and molybdenum deposit. Skarn deposits form by reaction of mineralised fluids from the granite reacting with wall rocks such as limestone. Skarn mineralisation is also important in lead, zinc, copper, gold and occasionally uranium mineralisation.
Greisen granite is another related tin-molybdenum and topaz mineralisation style.
Rare earth elements, niobium, tantalum, lithium
The overwhelming majority of rare earth elements, tantalum and lithium are found within pegmatite. Ore genesis theories for these ores are wide and varied, but most involve metamorphism and igneous activity. Lithium is present as spodumene or lepidolite within pegmatite.
Carbonatite intrusions are an important source of these elements. Ore minerals are essentially part of the unusual mineralogy of carbonatite.
Phosphate
Phosphate is used in fertilisers. Immense quantities of phosphate rock occur in older sedimentary basin, generally formed in the Proterozoic. Phosphate deposits are thought to be sourced from the skeletons of dead sea creatures which accumulated on the seafloor. Similar to iron ore deposits and oil, particular conditions in the ocean and environment are thought to have contributed to these deposits within the geological past.
Phosphate deposits are also formed from alkaline igneous rocks such as nepheline syenites, carbonatites and associated rock types. The phosphate is, in this case, contained within magmatic apatite, monazite or other rare-earth phosphates.
Mineral exploration
Mineral exploration is the process undertaken by companies, partnerships or corporations in the endeavour of finding ore (commercially viable concentrations of minerals) to mine. Mineral exploration is a much more intensive, organised and professional form of mineral prospecting and, though it may at times use the methods of hobby prospecting, the process of mineral exploration is much more involved.
Stages of mineral exploration:
· Area selection
· Target definition
· Resource evaluation
· Reserve definition
· Extraction
Area Selection
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.
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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.
This process applies the disciplines of basin modelling, 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.
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.
Area selection may also be influenced by previous finds, a process informed heavily by 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 as tenements.
Target generation
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.
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Geophysical methods
Instruments play a large role in gathering geological data. Instruments are used in geophysical surveys to check for variations in gravity, magnetism, electromagnetism and a number of different other variables in a certain area. Geiger counters and scintillometers are used to determine the amount of radioactivity. Ultraviolet lamps may cause certain minerals to give off a different color. Airborne magnetometers are used to search for magnetic anomalies in the Earth's crust. The anomalies are an indication of concentrations of iron minerals and associated metals.
Remote sensing
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 two and was heavily adopted in the 1960's onwards.
Since the advent of cheap and declassified Landsat images in the late 1970's early 1980's, 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. Improvement in the resolution of modern commercially based satellites has also improved the utility of satellite imagery; for instance IKONIS satellite images can be generated with a 30cm pixel size.
Geochemical methods
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.
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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
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 a 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.
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Resource estimation may require pattern drilling on a set grid, and in the case of sulphide 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
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, floatbility 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.
Greenfields vs Brownfields
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 terranes 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.
Mineral resource classification is the systematic organization of information on ores and other mineral deposits which contain economic value. The process guides governmental and industrial planning on how to manage the resources.
Not all mineralisation meets these criteria. The specific categories of mineralization in an economic sense are:
- mineral occurrences or prospects which are of geological interest but may not be of economic interest
- mineral resources, include those which are potentially economically and technically feasible, and those which are not
- ore reserves, must be economically and technically feasible to extract
The common terminology for mining, "ore deposit", by definition must have an 'ore reserve', and may or may not have additional 'resources'.
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