Classifications Of An Ore Deposits

Many years ago, when I was studying geology, I tried to find some simple and synthetic classification of an ore deposits. I found a lot, but none of them were simple… An investor who wants to explore the nature of ore faces a problem that is difficult to understand even for many specialists in this field as geoscientists or engineers. To assess the economic potential of an ore deposit, you must know all its parameters such as: type of ore deposit, morphology, grade and concentration of the valuable mineral, depth at which the deposit is located, geological structure, as well as many other factors such as infrastructure, access to water, infrastructure and electricity, etc. The economic model of the mining project is based on those factors. The difficulty is that each deposit has its own unique set of features and it is not possible to construct very accurate and universal models that will be repeated schematically with new  discoveries.

There are many different classifications of ore deposits. Every classification has a different criterion according to which it is divided into types or subtypes. The most important parameters, according to which we describe and divide the deposits, include:

• (1) The type of rock in which the deposit is formed (a host-rock)
• (2) Morphology
• (3) Depth of formation
• (4) Geotectonic environment
• (5) Age of formation
• (6) Ore-forming process
• (7) Mineral composition

The first and most important assumption of mining business is to find the desired elements in the rock, in such an accumulation and concentration that they can be exploited profitably now or in the foreseeable future. Therefore it is necessary to take into account many different parameters listed above. They are the result of many geological processes that often overlap in millions or even billions of years, ultimately creating a complex structure.It’s a big problem for mining engineers. There are no simple models that can be used in the construction of each mine – every project requires an individual approach.

Criteria and definitions according to which scientists and engineers divide ore deposits are sometimes ambiguous. Below You can find pros and cons of each classification.

Rock type – a host-rock. It’s easy, but mostly not so useful…

There are three basic types of rock: igneous, metamorphic and sedimentary. Some rock are somewhere between igneous and metamorphic, igneous and sedimentary or sedimentary and metamorphic – but it is minority.

Classification of deposits according to this criterion is rarely used, because morphology of an ore deposit can go beyond one specific type of rock. Although descriptive classifications directly refer to the type of rock, however, as we will see later, more accurate division on subtypes requires some other parameters.

Many deposits are closely related to the formation of the rock itself and its original morphology. These are called syngenetic deposits. Examples are Nickel, Copper or PGE (platinum group elements) deposits in ultra-basic intrusions or Au or Ti deposits in some sedimentary rocks called placers. These deposits are formed simultaneously with the rock in which they occur. In this case knowledge of the geological structure is enough to be able to start a mining project.

Few types of ore deposits are formed during final stage of cooling magma as a result of two co-existing processes: (a) rock-forming crystallization and (b) activity of hydrothermal fluids transporting mineral from place to place. Example of such deposits are coarse crystalline rocks called pegmatites that may contain deposits of Lithium, Tin, Vanadium or REE (rare earth elements). These deposits are sometimes described as magmatic and sometimes as hydrothermal.

The second type is epigenetic deposits and arises in the rock when it is fully formed. They are formed as a result of the activity of hydrothermal fluids within existing rocks. Also virtually all oil and gas deposits (except most of shale hydrocarbons) belong into this category. Some deposits occur within one type of rock like huge deposits of copper in porphyries. A lot of epigenetic ore deposits may cut through few types of rocks and form its own morphology independently of the surrounding rock formations. Examples are veins or stockworks that may contain gold deposits.

Second drawback of this criterion is that economically important elements are disproportionately distributed among different types of rocks and many of them do not have valuable minerals.

The morphology of the deposit. Usefull, but hard to estimate…

This category is a continuation of thoughts from the previous paragraph.

There are two main types of ore deposits in terms of relation of the orebody morphology to the host rock: concordant orebodies and discordant orebodies.

The first imitate roughly the shape of the rock formation in which the deposit was formed. To this type belong:

• (a) stratiform deposits (parallel to bedding sedimentary deposits)
• (b) igneous rocks deposits (stratiform, lenticular or sheetlike in shape Cu-bearing volcanic deposits or layered deposits of chromite or nickel resulted due to gravitational segregation of partly crystallized minerals in magma chamber)
• (c) residual deposits develop during concentrations of some minerals due to removal the other ones (iron-brearing bauxites or aluminum-bearing laterites)

Discordant, on the other hand, are orebodies whose morphology is irregular and often independent of the structure of the host-rock or boundaries between individual types of rocks. To this type belong:

• (a) tabular orebodies (veins and lodes with mineralization developed along fissures, fractures, cracks and other open spaces) (b) tubular orebodies bodies (pipes or chimneys sometimes mineralized with bismuth, molybdenum, tungsten or tin)
• (c) disseminated deposits (point-scattered diamonds deposits in kimberlite pipes or network of closely-spaced anastomosing mineralized zones in stockworks)
• (d) replacement deposits are formed by replacement of pre-existing rocks (skarns formed by replacement of carbonate rich sediments at contact with hot igneous intrusions) Likewise syngenetic deposits, concordant ore deposits are relatively easy to describe due to the convergence of their shapes with the shape of a rock formation. However discordant ore deposits are sometimes very irregular and therefore it require many specialized researches. Moreover you have to keep in mind that the morphology is not associated with any particular element. Au can occur both in the form of regular layers in sedimentary rocks (Au placers), as well as in the form of quartz veins formed at a depth of several hundred meters. Finally this criterion is very useful, but you have to recognize it first.

Depth of deposit formation. Very important for science, not really  for mining business…

This category applies only for deposits arising from the action of hydrothermal fluids. They operate at various depths and (associated with it) different temperatures, which determine first of all type of minerals formed.

These types include four basic deposit categories:

• (a) Telethermal (temperature +/- 100˚C), forming near the earth’s surface in rocks near lava flows. They may contain zinc or gadolinium.
• (b) Epitermal (50-200˚C), forming to a depth of 1.5 kilometers in fracture systems, as veins or stockworks. They may contain silver, gold, uranium or copper.
• (c) Mesothermal (200-300˚C), formed at a depth of 1.5 -4.5 kilometers near the hot magmatic intrusions associated within regional tectonic fracture zones as tabular bodies, chimneys, stockwork or fracture fillings. They may contain silver, gold, uranium, cobalt or molybdenum.
• (d) Hypothermal (300-600˚C) deposits are formed at a depth of 3-15 km surrounded by deep and hot intrusions and fracture zones as a tabular or irregular form and may contain gold, tin, tungsten and many others.

Ore deposits formed at depth of 10 km, may have be uplifted to the surface. As I wrote earlier – it is very interesting for geoscientist, but not for an investor. You don’t have to drill and dig to the hell, since it came to you itself.

Geotectonic environment. Useful and important, but too general…

Geotectonic environments are large areas of the Earth’s crust in regional or even global range. Examples are orogenic areas, rifts, precambrian cartons, and sedimentary basins. This criterion is of great scientific value and is also extremely important for recognizing potentially valuable areas on a large scale. They are synthetic models that are created as a result of the compilation of hundreds of thousands field data. A very large amount of metal or hydrocarbon deposits is associated with such characteristic areas or specific structures in the earth’s crust. Examples:

• – most of gigantic copper deposits related to the porphyry – rocks occurring abundantly in active orogenic zones, such as the Andes,
• – iron deposits in precambrian cratons, such as in Western Australia,
• – hydrocarbons in sedimentary basins with particular parameters.

There are several basic types of geotectonic environments that are defined differently. These environments you can divide into the following general categories:

• (a) active margin environments (orogenic areas and oceanic trenches),
• (b) passive margin areas (west coast of Africa, west coast of South America),
• (c) areas of the cratonic shield (e.g. Western Australia, Brazil, South Africa, northeastern Canada),
• (d) oceanic rifts (Atlantic or Pacific ranges) and continental rifts (eastern Africa),

(e) sedimentary basins (Canadian Foreland Basin, North Sea, West Siberia, Caspian Sea). In project of the mine geotectonic models is too general, but history of mining or oil&gas exploration shows how important is this classification. One of best example comes from diamond mining industry. In the first half of the 20^th Century, the main producer, or rather a monopolist, was South African DeBeers, who controlled 90% of the diamond market. In the second half of the twentieth century, geological researches led many geologists, engineers and mining managers to the conclusion that diamonds occur in the so-called kimberlite pipes –
vertical channels connecting deep parts of the earth’s crust with the surface and filled with rocks called kimberlites. At the end of the 20^th Century, geological knowledge of kimberlite pipes was so large that next significant amounts of diamond deposits were discovered year by year. DeBeers was not able to take over them and its monopolistic position on the market dropped to just 30%.

Age of the deposit. Important for geoscientist, but not for investors…

Again. Likewise in previous category, age of the rock is very important for geoscientist, but not for investors… However in searching of new rich in valuable minerals areas it is worth knowing, that some minerals were more often formed during some periods of geological time. Many of the deposits were related to global events that took place during the evolution of the Earth. Examples:

• dominance of a particular type of magma in some geological eras (iron deposits in Precambrian cratons)
• hydrocarbon or coal deposits resulted from global biogeochemical changes related to the evolution of living organisms (most of oil deposits are within rocks of Cretaceous, Jurassic and Tertiary age; gas within Silurian and Permian,  coal were formed mostly during Carboniferous)

Some deposits were formed at repeated time intervals every several hundred million years. Others, however, were concentrated around single episodes.

Origin of an ore deposit. Best classification of all, but too complex…

The most comprehensive and valuable classification is based on all individual features of an ore deposit and the most complete picture of the distribution of the mineralization within the rock body. It combines practically all classifications mention above (i.e.: mineral, morphology, geological age, space distribution, hydrothermal fluids, type of rock and others). The problem is that formation of an ore can be the result of many overlapping processes spanning in broad range of time and space. The final effect can be very complex. Drilling, geophysical, geochemical and geological researches and statistical modeling give us almost always incomplete information just about the nature of an orebody – not about the processes that formed the ore deposit.

For this reason most authors create their own genetic classification with unique name based on different criteria, one-time categories, or ore deposits occurring in just one place in the world.

Mineral composition. Very important for Investors, but it is actually no classification…

Many investors are interested in investing in a specific element such as gold, copper or silver. If someone would like to know more deeply how mining companies make profit from gold exploration, he faces a problem that is the essence of the entire article – the lack of simple models of deposits. The same element may occur in ore deposits of a different genesis, shape, geological settings, or in different minerals.

So, we return to the starting point.

Descriptive criterion. Finally something simple, but why so many types?

It was made by Cox and Singer from the United States Geological Survey in 1986. It is very useful and contains all the characteristics feature of an ore deposits. It based on different parameters and contains almost 90 types. Ore-forming process is of secondary importance. General it is based on rock types, because it is easy to define. More detailed division uses other criteria like geotectonic environment or type of geological process.

Finally thoughts           

Theoretically the whole path is simple:

• (a) looking for gold deposits,
• (b) exploring area by scientific methods,
• (c) analyzing data,
• (d) making model of the orebody,
• (e) mine design and construction,
• (f) exploitation and sale of gold. Practically it looks like the same, but with invalid models of ore deposits and much more uncertainty in future profits.

Several thoughts within this article is just tip of the iceberg. In next ones we’ll find out how it is big under the water.