Introduction to Ore-Forming Processes. Laurence Robb

between mineral resources and ore reserves as applied to mineral occurrences. The scheme forms the basis for a more unified description of ore deposits as now required in terms of legislation that has been passed in most major mineral producing jurisdictions.

Some Useful Definitions and Compilations

      General Definitions

      This section is not intended to provide a comprehensive glossary of terms used in this book. There are, however, several terms that are used throughout the text where a definition is either useful or necessary in order to avoid ambiguity. The following definitions are consistent with those provided in the Glossary of Geology (Bates and Jackson 1987) and The Encyclopedia of the Solid Earth Sciences (Kearey 1993).

       Ore: any naturally occurring material from which a mineral or aggregate of value can be extracted at a profit. In this book the concept extends to coal (a combustible rock comprising more than 50% by weight carbonaceous material) and petroleum (naturally occurring hydrocarbon in gaseous, liquid, or solid state).

       Syngenetic: refers to ore deposits that form at the same time as their host rocks. In this book this includes deposits that form during the early stages of sediment diagenesis.

       Epigenetic: refers to ore deposits that form after their host rocks.

       Hypogene: refers to mineralization caused by ascending hydrothermal solutions.

       Supergene: refers to mineralization caused by descending solutions. The term generally refers to the enrichment processes accompanying the weathering and oxidation of sulfide and oxide ores at or near the surface.

       Metallogeny: the study of the genesis of mineral deposits, with emphasis on their relationships in space and time to geological features of the Earth's crust.

       Metallotect: any geological, tectonic, lithological, or geochemical feature that has played a role in the concentration of one or more elements in the Earth's crust.

       Metallogenic Epoch: a unit of geologic time favorable for the deposition of ores or characterized by a particular assemblage of deposit types.

       Metallogenic Province: a region characterized by a particular assemblage of mineral deposit types.

       Epithermal: hydrothermal ore deposits formed at shallow depths (less than 1500 m) and fairly low temperatures (50–200 °C).

       Mesothermal: hydrothermal ore deposits formed at intermediate depths (1500–5000 m) and temperatures (200–400 °C).

       Hypothermal: hydrothermal ore deposits formed at substantial depths (greater than 5000 m) and elevated temperatures (400–600 °C).

      Periodic Table of the Elements

The question of the number of elements present on Earth is a difficult one to answer. There are 94 primordial nuclides present on Earth, these being the elements that formed during nucleosynthesis of the material that makes up the solar system. Most of the element compilations relevant to the earth sciences show 92 elements, the majority of which occur in readily detectable amounts in the Earth's crust. Figure 4 shows a periodic table in which these elements are presented in ascending atomic number and also categorized into groupings that are relevant to metallogenesis. There are in fact as many as 118 elements known to man, but those with atomic numbers greater than 92 (U: uranium) either occur in vanishingly small amounts as unstable isotopes that are the products of various natural radioactive decay reactions, or are synthetically created in nuclear reactors. The heaviest known element, originally called ununoctium (Uuo, atomic number 118), was only fleetingly detected in a nuclear reactor – its existence has now been confirmed and officially named “oganesson” (Symbol Og) after the Russian nuclear physicist, Yuri Oganessian. Some of the heavy, unstable elements are, however, manufactured synthetically and serve a variety of uses. Plutonium (Pu, atomic number 94), for example, is manufactured in fast breeder reactors and is used as a nuclear fuel and in weapons manufacture. Americium (Am, atomic number 95) is also extracted from spent reactor fuel and is widely used as the active agent in smoke detectors.

Figure 4 Periodic table showing the elements with atomic numbers from 1 to 92; classified on the basis of their rock and mineral associations.

      Of the 92 elements shown in Figure 4, almost all have some use in our modern, technologically‐driven societies. Some of the elements (iron and aluminum) are required in copious quantities as raw materials for the manufacture of vehicles and in construction, whereas others (the rare earth elements, for example) are needed in very much smaller amounts for use in the alloys and electronics industries. Only two elements appear at the present time to have little or no commercial use at all (Figure 4). These are francium (Fr, atomic number 87), and protactinium (Pa, atomic number 91). Francium is radioactive and so short‐lived that only some 20–30 g exists in the entire Earth's crust at any one time! Astatine (At, atomic number 85) is another very unstable element that exists in vanishingly small amounts in the crust as a decay chain by‐product or is manufactured synthetically. Astatine has been manufactured in particle accelerators and is occasionally used in various nuclear medical applications.

The useful elements can be subdivided in a number of different ways. Most of the elements can be classified as metals (Figure 4), with a smaller fraction being non‐metals. The elements B, Si, As, Se, Te, and At have intermediate properties and are referred to as metalloids. Another classification of elements, attributed to the pioneering geochemist Goldschmidt, is based on their rock associations and forms the basis for distinguishing between lithophile (associated with silicates and concentrated in the crust), chalcophile (associated with sulfides), siderophile (occur as the native metal and concentrated in the core), and atmophile (occur as gases in the atmosphere) elements. It is also useful to consider elements in terms of their ore mineral associations, with some preferentially occurring as sulfides and others as oxides (see Figure 4). Some elements have properties that enable them to be classified in more than one way – iron is a good example, in that it occurs readily as both an oxide and sulfide.

      Common Ore and Gangue Minerals

      It is estimated that there are about 3800 known minerals that have been identified and classified (Battey and Pring 1997). Only a very small proportion of these make up the bulk of the rocks of the Earth's crust, as the common rock forming minerals. Likewise, a relatively small number of minerals make up most of the economically viable ore deposits of the world. The following compilation is a breakdown of the more common ore minerals in terms of chemical classes based essentially on the anionic part of the mineral formula. Also included are some of the more common “gangue” minerals, which are those that form part of the ore body, but do not contribute to the economically extractable part of the deposit. Most of these are alteration assemblages formed during hydrothermal processes. The compilation, including ideal chemical formulae, is subdivided into six sections, namely native elements, halides, sulfides and sulfosalts, oxides and hydroxides, oxysalts (such as carbonates, phosphates, tungstates, sulfates), and silicates. More detailed descriptions of both ore and gangue minerals can be found in a variety of mineralogical texts, such as Deer et al. (1982), Berry et al. (1983), Battey and Pring (1997), and Wenk and Bulakh (2017). More information on ore mineral textures and occurrences can be found in Craig and Vaughan (1994) and Ixer (1990).