Mineral exploration dates back to the Bronze Age, but the tools and techniques used by prospectors and geologists to find mineral deposits have changed dramatically over time. 'Boot-and-hammer' prospecting is still essential, but it's no longer the primary method used to search for new mines. Exploration today is a knowledge-based industry that relies on advanced technology, such as satellite imagery and geophysical and geochemical surveys, and on geological studies ranging from plate tectonics to the chemical composition of rocks and minerals. Teamwork and cooperation between geological disciplines is necessary for success.

The element of luck plays a part in any human endeavor and exploration is no exception. While some discoveries owe more to serendipity than science, these are few and far between. Mineral exploration is costly, risky, and more challenging than ever. It has potential for high rewards too, which is why exploration companies rely heavily on the latest technological and scientific advances to identify projects of geological merit. Similarly, investors can increase their chances of success by understanding the basic deposit types, how they're formed and where they're found. A basic understanding of geology helps too.

Exploration is an intellectual journey before it becomes a physical treasure hunt. The journey begins with geological data collected over centuries that show a scientific pattern as to where various types of deposits are found.

Earth is divided into geological regions, which in turn are divided into many geological provinces. Rocks of different types and ages underlie these geological regions, providing for a diversity of mineral resources. The regions are not equally prospective, however. Mineral deposits tend to occur in areas of Precambrian Shield rock - the oldest rocks on earth - or in much younger mountain belts.

In the case of the Precambrian Shield, mineral wealth is related to widespread and intense mountain-building and other tectonic activity that took place between 4.6 billion to half a billion years ago. Erosion has since brought these mineral deposits near surface. After tectonic activity in the Shield ceased, the Cordilleran Region of North and South America (and its counterparts elsewhere in the world) became as active as the Shields once were, resulting in the formation of large mineral deposits.

The Precambrian Shield covers more than half of Canada and contains most of the nation's mines and mineral deposits. Most were found on highly prospective portions of the Shield known as greenstone belts. The Cordilleran Region has also proven to be a great storehouse of mineral wealth. In recent years however, the rate of discovery in North America has slowed because the remaining deposits tend to be deeper and more challenging and costly to find.

Some North American companies are now taking the expertise they've gained at home to the developing world where opportunities for major discoveries are deemed higher. In today's computer age, known mineralization in a given suite of rocks from one geological region -- Canada's Precambrian Shield, for example -- can be fed into a computer and compared with the geology of unexplored regions in other countries to generate targets of geological merit.

Precious metals continue to be the most sought-after commodity for junior companies hoping to become producers or to sell their projects to major companies. Before examining the gold exploration process from discovery through to mine planning, it's helpful to understand the basic deposit types and how they're formed and found.

Gold deposits occur in many forms and in a variety of geological settings. Geologists sometimes make distinctions based on when the minerals were formed relative to the rocks in which they occur. Syngenetic deposits were formed at the same time as their host rocks, while epigenetic deposits form in rocks that already exist. Another classification method is to determine the geological process by which a deposit was formed. Most primary gold deposits were formed by hydrothermal processes, in which fluids carrying gold and other minerals are transported through shear zones and fractures and deposited in rock units, usually with silica or quartz.

Placer gold deposits, or secondary deposits, owe their formation to supergene enrichment, whereby minerals are concentrated by the accumulation of weathered minerals from rock units above the zone of concentration. The gold-bearing conglomerates of South Africa are widely believed to be old gravel placers that have solidified into rock.

Primary gold deposits have many forms depending on their geological environment, but lode deposits found in the Precambrian Shields of Canada, Australia, South America and Africa continue to attract great interest. These deposits are found in greenstone belts -- areas of metamorphosed volcanic and sedimentary rocks -- and tend to cluster around large regional fault zones. The deposits themselves are in zones of intense structural deformation; fracturing, faulting and shearing are common. Gold mineralization is typically hosted in veins and shear zones, but can also form as stockworks or dissemination zones.

Normal faults are faults that result from horizontal tensional stresses in brittle rocks and where the Hanging Wall block has moved down relative to the Foot Wall block. Faults provide a plane of weakness in the earth's crust for hydrothermal solutions containing gold and other minerals to ascend towards surface.

Epithermal gold deposits provide another attractive exploration target. They're typically found in volcanic rocks in much younger geological environments, such as the Cordilleran and other mineral-rich districts within the 'Pacific Rim of Fire.' The mineralized zones are usually veins in radial or concentric fractures, but mineralization may also extend into the country rocks as disseminations. The narrow veins of epithermal deposits are sometimes mined as high-grade, low-tonnage deposits, though it has become more common in recent years for the veins and country rock to be mined together in large, low-grade operations.

Deposition of epithermal gold: Mineralizing fluids ascend from hot igneous intrusions then mix and interact with meteoric water. Epithermal gold is deposited in the boiling zone (potential bonanza deposit). Alteration of country rocks by invading hot fluids also occurs. Gold can also be deposited in a hot spring environment (mineralizing fluids reach the surface and cool, depositing ore) and in a placer environment (erosion of an ore body, heavy metals redeposited in sedimentary environment).

Replacement deposits (widely considered a subset of epithermal deposits) were formed when ore-bearing fluids migrated out through porous rock, leaving behind enough mineralization in disseminations, fractures and fillings to make ore. The sediment-hosted, disseminated gold deposits of Nevada's Carlin Trend are the most famous example of replacement deposits. Carlin-type deposits pose a challenge in that the 'no-see-um' gold found in primary, unoxidized ore is bound to pyrite and requires autoclaving or special processing before milling to extract the gold. The weathered oxide ores near surface have free gold that is extracted more easily, usually by heap-leaching methods.

Replacement Deposits are formed when hydrothermal fluids, passing through permeable rocks, react with the rocks to dissolve original minerals and replace them with ore and/or other gangue (waste) minerals.

Intrusive rocks also had a hand in forming deposits where gold is found, often with other minerals. In porphyry deposits for example, gold typically occurs with copper. Formed by igneous activity, porphyry deposits tend to be large, but low grade. The mineralization forms in the intrusion itself, and/or in the country rocks around it. Skarn deposits, another deposit type related to intrusions, also host gold. Skarns frequently form on the margins of the intrusions that create porphyry copper deposits.

Gold occurs in a variety of other settings, including the world-famous gold-quartz conglomerates of the Witwatersrand in South Africa. They have produced vast amounts of gold, yet still have immense resources at depth. Their origins are still the subject of debate among geologists.

The first order of business for exploration companies is geological reconnaissance to identify areas prospective for mineral deposits. Governments of most nations lend a helping hand by providing geological reports and maps that identify the most favorable areas. In developing nations where geological information is scant, companies look for evidence of past or recent small-scale mining activity.

Another useful technique at the reconnaissance stage is remote sensing, the use of photographic and radar images taken by satellites or aircraft. This imagery can show large-scale geological features, such as faults or geological contacts where mineralization often occurs. In arid regions or deserts, color changes on satellite imagery may denote changes in rock type, or show areas of rock alteration caused by mineralizing solutions. Gossans -- areas of rusty staining -- may indicate the presence of gold associated with base-metal mineralization.

Once favorable areas are identified, companies carry out mapping and sampling programs more detailed than those carried out by government geologists or prospectors. The goal is to find a geological anomaly, such as an abrupt change in rock type, a change in chemical composition, a zone of faulting or shearing, or even a surface showing.

All deposits are anomalous in that they contain minerals in concentrations many times higher than the average concentration found in the earth's crust, but not all deposits have surface showings that can be easily sampled. Some are covered by overburden or are buried at depth. Finding these deposits requires sophisticated technology, such as geophysics, geochemistry or geological inference.

Geochemistry is an efficient tool used routinely in the exploration process. It's aimed at finding anomalous concentrations of elements in a geological environment, particularly the halos of trace elements around mineralized bodies. There may be a concentration of gold itself that can be defined by its content, thereby giving an indication of the possible shape and size of the deposit. While geochemistry is mainly directed at picking up the 'signature' of mineral deposits, it also reveals attributes of the mineralization (fineness, grain size, etc.) and the nature of associated 'pathfinder elements' and host rocks.

The primary aim of geophysics is to measure the physical properties of subsurface rocks and detect anomalies in the data that may indicate the presence of mineralization. These features can be surveyed and measured by instruments that employ seismic, gravity, magnetic, electrical, electromagnetic, radiometric and radar methodologies. Like geochemical surveys, airborne and ground-based geophysical programs are carried out on a grid pattern.

The gravity method involves measuring the acceleration due to the earth's gravitational field. A gravity meter or gravimeter measures the variations in the earth's gravitational field. The variations in gravity are due to lateral changes in the density of the subsurface rocks in the vicinity of the measuring point. Because the density variations are very small and uniform, gravimeters are very sensitive and they measure one part in 100 million of the earth's gravity field (980 gals or 980,000 milligals) in units of milligals or microgals. Gravity Surveys are used for modeling the earth's crustal structure, for locating bedrock fractures, buried topographic valleys, subsurface voids such as abandoned mines, caverns, and sinkholes, and for determining overburden thickness and depth to bedrock. Section AA is the line along which gravity survey readings were taken. Section BB runs through the centre line of an old shaft in plan view, followed by the gravity meter readings plotted in cross-section. Lastly, the overlying sediments and underlying bedrock (solid) are also shown in cross section. Certain gravitational patterns may be indicative of surface or buried mineral ore bodies.

No matter what methods are used, the goal of early-stage exploration is to find a geological anomaly that warrants further testing. The target is then sampled on surface or drilled at depth, and the samples sent for a chemical analysis called an assay to determine their mineral content. If results are positive, a program of diamond or reverse-circulation drilling takes place to test the mineralized body at depth and along strike on a grid-like pattern. This allows geologists and engineers to piece together a three-dimensional picture of the deposit.

ADVANCED EXPLORATION: IS IT ORE?

A mineral deposit does not a mine make. Some deposits are too deep or too low grade to be mined, while others have refractory or complex mineralization not amenable to low-cost extraction processes. The goal of advanced exploration therefore, is to determine if a mineral deposit is 'ore' -- material that can be mined at a profit -- and to prove that there are sufficient reserves to justify the capital costs of a mining operation. Advanced exploration programs are tailored to the deposit type being tested, because what's encouraging at one project may not be at another. For example, a 50-metre intersection grading 5 grams gold per tonne would be of economic interest in Nevada, but not necessarily so in rugged areas lacking roads and mining infrastructure. Bulk-tonnage deposits typically have low grades over long intervals, while underground projects have higher grades over narrower intervals. Some vein-type deposits have bonanza grades, but caution should be exercised, as these veins often pinch and swell, or disappear altogether. To prove up reserves, vein-type deposits must be drilled on a tighter grid than bulk-tonnage deposits with evenly disseminated mineralization.

A mineral project enters the feasibility stage once a significant resource has been outlined. Extensive drilling programs are carried out to turn resources into proven and measured reserves. Mining companies typically hire independent engineering firms to oversee feasibility studies, which include tests to determine the best mining method, and metallurgical tests to determine the best processing method. Risks associated with geology (size, shape and grade of the deposit), metallurgy (how much metal can be recovered) mining methods (dilution) and economics (metal prices, transportation costs, etc.) are assessed, along with political risk and environmental challenges.

During the exploration stage, companies take steps to ensure that their footprint on the land will be small. Once a deposit has been outlined, environmental specialists investigate the project's potential impacts on wildlife, water, soils and vegetation, and air quality and climate. The resulting 'environmental impact statements' are released for comment and submitted to government agencies responsible for mine permitting. Reputable companies also keep local communities informed of their activities, whether at home or abroad. The consultation process sets the stage for good relations once mine planning begins.