3. Exploration of mineral deposits
The mining life cycle starts with the search for, and discovery of, mineral deposits.
Prospecting is the most basic level of the search for minerals, often carried out by individuals, using unsophisticated techniques and equipment. Rural communities are often aware of the presence of useful or valuable mineral deposits and exploit them at village level, for example, coal and gemstones in Afghanistan, gold in Zimbabwe, often with scant regard for legislation or licensing concerns. Sometimes the presence of minerals has been detected by ancient communities and traces of early mining may be visible, dating back centuries or even millennia.
Prospecting at this level often leads to or is accompanied by exploitation at artisanal level. The persons involved may have little or no formal knowledge of geology or of safe and efficient mining methods, but they are often resourceful and ingenious, finding and profitably mining deposits that might be ignored by the formal sector. Artisanal workings can be a useful guide for formal exploration teams to the location of deposits and the structural geology of an area, for example, cobalt mining in the Democratic Republic of Congo.
Scientific exploration for minerals starts with regional geological mapping which may be incorporated into geographic information system (GIS) databases. As part of their efforts to promote mining investment, governments carry out geological mapping of their territories and publish maps at varying levels of detail, making them commercially available to interested parties. Whilst primarily of interest for exploration purposes, geological maps are used by many other disciplines such as civil engineering.
Mining investment and exploration companies study the geology of entire regions to identify prospectivity, i.e. the likelihood of finding economic minerals. Computer simulations are used to compare unexplored regions with geologically similar areas and to make mathematical predictions of the probabilities of desired economic outcomes of exploration. They use these models to plan exploration investment programmes.
The decision to undertake exploration in any area is based primarily on geological prospectivity. This is not, however, the only factor. After identifying geologically favourable conditions, investors will consider other country factors and will assess the investor-friendliness of the venue under consideration. With advances in technology, particularly remote sensing by satellite, there are many geologically attractive targets available globally. Countries are competing for investors more than investors are competing for concessions.
Having identified an area for exploration, the investor’s next step is to obtain the necessary government and related approvals in the form of licenses, contracts or other mechanisms. The ease or otherwise of accomplishing this may be a decisive factor. Time frames can be important to investors. The process may include a requirement to complete an Environmental and Social Impact Assessment (ESIA).
For budgeting purposes, exploration is different from many other business activities. Expenditure is progressive, with each phase dependent on the outcomes of the previous one. The budget is based on assumptions that during reconnaissance and preliminary exploration, areas will be found of sufficient interest to justify more detailed techniques, at higher cost. If such targets are not identified, further work may not proceed.
Exploration is a highly speculative activity. Governments should exercise caution in requiring minimum expenditures, which may be a disincentive to potential investors. Exploration license holders can be encouraged not to delay their programmes by a requirement to relinquish portions of a license area after prescribed time intervals have elapsed. Apart from the potential benefits of mine development, governments benefit from exploration from the detailed information which investors are normally obliged to disclose following exploration.
Exploration companies may supplement existing geological mapping with satellite imagery or aerial mapping. Satellites can now provide high-resolution images suitable for multiple purposes, at a fraction of the cost of other techniques.
From satellite mapping, exploration may move to reconnaissance with field work for sampling and more detailed geological mapping. Aerial magnetometer surveys may be used to study structures and to search for magnetic and gravimetric anomalies which may indicate the presence of potential orebodies. More intensive techniques include geophysical prospecting, which embraces a variety of techniques, including seismic surveys.
Geochemical prospecting involves field teams taking soil and rock samples on a pre-planned grid. The samples are analysed and anomalies are plotted, i.e. areas with exceptionally high concentrations of certain elements. An anomaly is not in itself necessarily an economic concentration but an indicator of the possible presence of one. The presence of one element or mineral may be an indicator of the possible presence of another.
The final stage is drilling, conducted on targets considered to have a reasonable probability of yielding economic deposits. Various types of drilling machines and techniques are in common use, ranging from percussion drilling to core drilling or diamond drilling. Percussion drilling yields pulverised rock for sampling and assay.
Diamond drilling can be carried out to depths of several thousand metres. “Directional drilling” is used to ensure that holes penetrate as close as possible to the intended target. Deflection techniques may be used in deep holes to obtain multiple intersections of a target area. The holes are surveyed so that the actual position of the hole at each elevation can be accurately plotted and data incorporated into a computer block model.
“Reverse circulation” or “RC” drilling is an enhanced diamond drilling technique which increases efficiency and core recovery, which is the percentage of intact core recovered per metre of hole depth. Core diameters commonly vary from about 12mm to 120mm. Diamond drilling is an extremely costly process but major investment decisions are made on the basis of analysis of samples totalling, perhaps, a few hundred kilograms of core. It is essential to maintain strict records of all data obtained and for the core itself to be kept safe, stored securely in the order in which it was extracted from the hole.
As exploration progresses from the early stages of mapping towards the final technique of drilling, always based on positive results from the preceding stage, the costs tend to escalate exponentially. The area being examined also tends to reduce dramatically. Initial reconnaissance and mapping might cover an area of hundreds of square kilometres, whilst drilling, if it happens at all, might be confined to a few hectares.
Of every target explored in any depth, globally, perhaps one in a hundred might yield a deposit that can sustain a viable mining operation.
Drill core is “logged” by geologists, a process in which the core is examined and a detailed record made of rock type, minerals identified, the dip and direction of structural features and other characteristics. Using a diamond saw, the core is split along its axis, with one half being kept and carefully stored for future reference. The other half of selected portions is sent to a laboratory for assay for valuable minerals.
Where logging and assay indicates that a hole has entered or left a mineralised zone, these points are called intersections. Geologists join the intersections from a drilling programme and, using geological interpretation of structures, create a model of a potential orebody. This has been done manually in the past on sections and plans but is now almost exclusively done on computer models in two common formats: block models and wireframes. A block model uses each unit of data to create blocks, typically cubes with sides of 15 to 25m, assigning uniform characteristics to a block, including ore grades. The blocks are aggregated to produce an overall model of a deposit.
A number of commercial software packages are available for these purposes, including SURPAC, DATAMINE, Vulcan, Lynx and others. The same packages have modules which are used for the design of open pit and underground mines based on the initial geological model. These models are highly sophisticated, capable of being turned and viewed from any direction and of generating plans and sections on any selected plane. They can generate complete mine plans and schedules and financial models.
In addition to geological and assay data, drill cores yield a host of other data which are useful to mine planners, including specific gravity, hardness, metallurgical properties and geotechnical data. The latter are used to assess rock strength for stability of excavations.
See the Topic 'Technical Aspects of Mining' for more detail.
Although a typical mine life cycle includes exploration and exploitation as two distinct phases, exploration activities actually continue throughout the life of the mine up to very near the end of the life. Open pit and underground mining activities progressively expose the orebody, creating opportunity for detailed mapping and sampling. Usually supplemented by continued drilling, these data are used to continually update the geological model. Larger mines have a geology department for these purposes.
In addition to generating more detailed knowledge of an orebody or orebodies being exploited, mine operators often continue exploration for new resources once exploitation is in progress. These may be separate occurrences or extensions of an orebody planned for extraction. For example, once an underground mine has been developed, it may be possible to explore for extensions at depth by drilling from the lowest levels, at considerably less cost than deep drilling from surface. Extensions of an orebody may lead to an increased mine life in years or to a decision to increase annual production rate.
It is common practice to conduct sufficient exploration within a license area to prove an economic resource, design, construct and operate a mine while leaving a portion of a license area relatively unexplored. Known or potential mineral occurrences present in the area may initially be afforded low priority. It is legitimate for investors to focus initially on targets most likely to yield a viable mine, and to take these through the stages leading to exploitation, before conducting wider exploration, which may be funded by profits from mining.