Conducting mineral exploration requires expertise that can rarely be found condensed into a single volume. Mineral exploration techniques have considerably evolved over the past few decades. It may be difficult, quite a challenge in fact, to try to summarize them all in a single document. But we have decided to take up the challenge in order to produce a simple, convenient, and relatively succinct document that should enable a layperson to understand how metal and mineral deposits are formed, and especially how they are found.

 

To prepare this document, we did find some inspiration in the "Guide de la prospection", a handbook published in 1993 (now out of print) by the Groupe de communication PAT, the Corporation de formation et de développement en exploration minière and the Cégep de l’Abitibi-Témiscamingue. In fact, many AEMQ members were actively involved in the preparation of this handbook. Certain tables and figures in this document are reproduced in our guide. But as opposed to the "Guide de la prospection", our Mineral Exploration Guide is a vulgarization tool designed for those who are not in the field per se, meaning for example, investors, members of the financial sector, members of the media, and the general public.

 

The purpose of this guide is not to enable the reader to perform mineral exploration work on his own, but rather to help the reader understand what mineral exploration is all about and how it's done. After reading this guide, the reader should be able to better understand the content of our website, and may even be better able to understand press releases issued by mineral exploration companies. He/she will we able to distinguish geophysics from geochemistry, granite from peridotite, a prospectus offering from a financing by way of offering memorandum. He/she may even be in a position to discuss the value of a mineral exploration project or its results with an industry representative.

     
THE CYCLE OF FORMATION OF ROCKS 

   Geology, from ancient Greek (gê-, "earth") and λογος (logos, "speak", "reason"), is the science that deals with the composition, structure, history and evolution of the Earth's internal and external layers and the processes that shape it. Geology is an important discipline among Earth sciences.

 

The materials that make up the Earth's crust are composed of minerals and rocks. Some may contain or be, in and among themselves, substances that have a certain economic value, meaning they may be mined at a profit. To discover these substances, we must be able to identify the geological environments and structures that are favourable for the concentration of such mineralization. Mineral explorationists work to discover these orebodies.

     
New rocks are continually formed on, in, and underneath the Earth's crust. At the Earth's surface, rocks are subjected to erosion and weathering conditions, that break them down. Particles are carried toward a sedimentation setting where, after compaction, they will become sedimentary rocks. Thick accumulations will eventually bring the sedimentary rocks to greater depths, where they will become metamorphic rocks. At the base of the Earth's crust, the rocks may melt, become magma once again and subsequently migrate toward the surface to form plutonic and volcanic igneous rocks. This repetitive formation process constitutes the cycle of formation of rocks. This cycle is directly related to the process of formation and deformation of the Earth's crust driven by plate tectonics.
     
PLATE TECTONICS 

  In order to fully understand the rock cycle, it is important to first grasp the concept of plate tectonic movements and the ways in which such movements can deform geological formations. Plate tectonics (also known as continental drift) is a theoretical model that explains the inner workings of our planet. It describes how convection within the Earth’s mantle affects the surface of our planet. The lithosphere, Earth’s outermost layer, is divided into rigid plates that float and move over the more ductile asthenosphere below.

Tectonic plates are carried along by the movements of the underlying asthenospheric mantle, and the plates interact with each other in three main ways:
     

Divergence: This type of movement causes plates to move away from each other, allowing the mantle to rise up between them. Divergent boundaries create oceanic ridges where new oceanic lithosphere is formed. Volcanic activity along these boundaries is generally basaltic. This type of movement is responsible for creating new lithospheric crust (the rigid layer above the upper mantle).

 

Convergence: This type of movement causes plates to move toward each other, thus compensating for widening oceans elsewhere around the world. There are three types of convergent boundaries : subduction zones, where one plate (usually the denser one) plunges under the other, less dense plate. Volcanic activity above subduction zones is generally andesitic with calc-alkaline geochemistry; collisional zones, where two plates collide; and finally, obduction zones, where oceanic lithosphere is pushed up onto a continent.

 

 
   
     
Transform: At transform plate boundaries, two plates slide horizontally alongside each other. The western coast of California is an example of this type of boundary. 

In all these types of tectonic movements, the forces are great enough to fold rock formations and create great faults. An example of a highly deformed region in Quebec would be the Appalachians. Part of a geologist’s work in mineral exploration is to measure the orientation and inclination of geological layers, and to study folds, faults, shear zones, joints (fractures) and veins.
     
FAULTS AND FOLDS

  Faults are breaks in the Earth’s crust along which the blocks on either side move with respect to each other. The length of a fault can range considerably, from metres to kilometres. The San Andreas Fault in California, which runs along part of the west coast of North America, is famous for its earthquakes caused by fault-related movements.
     

Faults are classified according to the inclination of the fault plane and the relative movement between the blocks on either side. There are three main types of faults: normal, reverse and thrust. When a reverse fault is almost horizontal, it becomes a thrust fault, and in certain cases, these types of faults can move blocks of rock many kilometres.

 

The forces responsible for creating faults also cause the rocks on either side to break into pieces and form fault breccias. If the intensity of the deformation is great enough to cause partial recrystallization of the rocks, a mylonite will form. A shear zone is a band of highly fractured rocks commonly associated with a fault. Faults and shear zones provide permeable sites through which mineralizing hydrothermal fluids can circulate. In the Abitibi region, fluid movement along faults and shear zones gave rise to scores of gold-bearing quartz and carbonate veins. Several major faults in the Abitibi region—the Cadillac–Larder Lake, Destor–Porcupine, Casa Berardi and Lac Doré faults—are particularly famous for their mineral deposits.

Folds are undulations in rock formations. They range in size from millimetres to kilometres. There are two main families of folds: anticlines and synclines. Anticlines are folds in which the oldest rocks occur in the centre, whereas in synclines, the youngest rocks occupy this position.     
     

MINERALS

The Earth’s crust is made up of a great variety of rocks, each of which contains a mixture of minerals giving the rock a specific chemical composition and crystalline structure. A mineral is generally an inorganic substance defined by its chemical composition and the arrangement of its atoms into an orderly pattern and specific symmetry, all of which is reflected by the space group and crystal system to which that mineral belongs. A mineral is typically solid at normal temperature and pressure conditions. Minerals are combined in various proportions to form the rocks making up the Earth’s crust and, in a broader sense, the entire lithosphere.

Mineral classification and properties

There are more than 4,000 minerals divided into eight classes: native elements, sulphides and sulphosalts, oxides and hydroxides, halides, carbonates and nitrates, sulphates, phosphates, and silicates.

 

In Quebec, the most intense exploration is for native elements, like gold (Au), silver (Ag), graphite (C) and diamond (C), as well as sulphide minerals, like chalcopyrite (copper), sphalerite (zinc), pentlandite (nickel) and galena (lead), and the oxide minerals hematite (iron), ilmenite (titanium) and chromite (chromium).

 

Carbonate minerals are a major constituent of some sedimentary rocks used in the manufacture of lime and cement, namely limestones and dolomites. As for the silicate minerals, they account for 95% of the crust. Feldspar and quartz are used in the manufacture of ceramics and glass. Chrysotile asbestos (a magnesian silicate) was once an important industrial mineral, but demand has dropped dramatically over the past few decades.

Minerals can be identified based on their diagnostic physical properties. Identification usually relies on a combination of several of the mineral’s characteristics. The main physical properties that can be easily observed are lustre, the colour of the mineral or its streak, shape, hardness, cleavage and density. Other properties require special equipment to be identified: magnetism, electrical conductivity and radioactivity are some examples.

ROCKS
A rock is a natural aggregate of minerals, fossils and/or components derived from other rocks. Rocks are grouped into three main families: igneous, sedimentary and metamorphic.

     
Igneous rocks
Igneous (magmatic) rocks are derived from molten rock called magma. Magma that solidifies within the Earth’s crust before reaching the surface forms plutonic (intrusive) rocks. This type of rock may eventually be exposed at the surface if deformation or crustal uplift comes into play, followed by erosion. The minerals in plutonic rocks are visible to the naked eye. On the other hand, magma that moved up along fractures in the crust to emerge on the surface as lava flows will solidify to form volcanic (extrusive) rocks. 
   
     

The rapid cooling of magma at the surface forms glassy rocks or rocks with microscopic to very fine grains. Volcanic explosions, which throw lava and rock fragments into the air, create fragmental pyroclastic rocks. Pyroclastic rocks are typically layered and exhibit a wide range of fragment types.

Bodies of intrusive rocks are defined by their size or location within the surrounding rock formations: for example, intrusive rocks can form dykes, sills, batholiths (plutons) or stocks. Dykes and sills are thin tabular bodies ranging from about a metre to several hundred metres thick and up to many kilometres long. Dykes are discordant bodies (that is, they are oblique to the geological layering in surrounding rocks), whereas sills are concordant (parallel to the layering). Batholiths, which by definition cover a surface area of at least 100 km2, and stocks are also discordant bodies because they cut through the layering in the surrounding rock formations.

Igneous rock classification
Igneous rock classification is based on a rock’s mineralogical and chemical composition, according to the amount of silica it contains.

   
     

     
On the other hand, the naming of pyroclastic rocks is based instead on the size of the fragments making up the rock. The main classes are breccia (fragments with diameters greater than 64 mm), lapillistone (from 2 to 64 mm), and tuff (less than 2 mm).

When conducting mineral exploration work, geologists look carefully for signs of alteration and mineralization. Hydrothermal fluids responsible for depositing economic minerals (mineralization) may also cause notable changes in the minerals of the host rocks, a process known as alteration. For example, massive sulphide deposits in volcanic rocks are often surrounded by a halo of rocks containing greater than normal concentrations of the minerals chlorite and sericite. Finding these types of alteration zones suggests to a geologist that mineralized zones may be close at hand.
     
SEDIMENTARY ROCKS

  When exposed to atmospheric conditions (that is, conditions at the surface of the Earth), all rocks eventually disintegrate into fragments, tiny particles, and ions dissolved in water or air. The process begins as erosion, followed by weathering, and the resulting physical particles range widely in size and type. These rock particles travel along waterways, eventually settling and accumulating in thick layers in sedimentary basins. After the layers are compacted and cemented—the two processes that constitute diagenesis—they form sedimentary rocks. Most sedimentary rocks form in a marine environment, but some—like evaporites (gypsum, halite and sylvite)—form by precipitation processes in continental or shallow marine environments
     
Sedimentary rocks are composed of minerals and a wide variety of rock fragments, rounded to various degrees and held together by natural cement. Fossils are often present, and these represent the remains or imprints of ancient animals or plants. The main constituents of sedimentary rocks are quartz, feldspars, micas, clay minerals, and carbonate minerals. Most sedimentary rocks have a stratified or layered appearance. This stratification is the result of changes in mineral composition, colour, texture or porosity.

Sedimentary rocks classification

Sedimentary rocks are divided into two groups: detrital rocks, composed of rock or mineral fragments, and chemical rocks, formed by the precipitation of dissolved ions in water, by either chemical means or biological activity. The classification of detrital rocks is based on particle size, and the nature and diversity of the constituents. Conglomerate, sandstone and shale are the main types of detrital rocks.

Detrital sedimentary rocks may display a change in grain size known as graded bedding. In rocks with graded bedding, layers display a gradual change from coarser grains at the bottom of a bed to finer grains at the top. There are many other types of sedimentary structures, including cross-bedding, desiccation cracks (shrinkage features in drying clay), and ripple marks (ancient ripples), similar to features observed on modern-day beaches.

METAMORPHIC ROCKS

In contrast to detrital rocks, chemically formed sedimentary rocks are not derived from pre-existing rocks, but were instead formed by the precipitation of ions dissolved in water or by the accumulation of organic matter (petroleum, coal, etc.) in oceans or shallow seas. Chemical sedimentary rocks can be identified by their physical properties or by the minerals making up the rock. These rocks have variable porosity and permeability, and their textures range from microscopic to coarsely crystalline. The main types of chemical sedimentary rocks are carbonate rocks, siliceous rocks, carbonaceous rocks, various types of rock salts, and ironstones.

Deep under the Earth’s crust, the temperatures and pressures are high enough to transform existing rocks into metamorphic rocks. These transformations occur in the solid state, producing significant changes in the original rock’s mineralogy and texture (the size and arrangement of its grains). Igneous and sedimentary rocks can undergo metamorphism, and previously metamorphosed rocks can be re-metamorphosed.

Metamorphism caused by continental-scale or regional-scale deformation of the Earth’s crust is known as regional metamorphism, whereas metamorphism caused by faults or intrusions is called cataclastic or contact metamorphism. Contact metamorphism occurs along the edges of a magmatic chamber where the surrounding rocks are subjected to the high-temperature effects of the molten rock, creating a metamorphic halo. Along faults and shear zones, metamorphism is characterized by relatively minor changes in mineralogy accompanied by variable degrees of fragmentation. This type of metamorphism is local in scale, limited to the immediate zone of deformation.

Minerals that only form under specific temperature and pressure conditions are known as metamorphic indicators. Even though they may only be present in small amounts, metamorphic indicator minerals can be used to determine the intensity of the metamorphism that affected the rock.

Types of metamorphic rocks
Many metamorphic rocks display an alignment of flaky or needle-like minerals. This arrangement produces foliated or banded textures. Other metamorphic rocks display a massive texture, without any preferential mineral direction. Some metamorphic rocks are schistose, causing them to split into thin sheets or slabs. The main metamorphic rocks are slate, schist, gneiss, amphibolite, marble, quartzite and serpentinite.