Introduction to Ore-Forming Processes. Laurence Robb
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Название: Introduction to Ore-Forming Processes
Автор: Laurence Robb
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781119232384
isbn:
Source: After Einaudi (2000).
Mineral Resources and Ore Reserves
Throughout this book reference is made to the term “ore deposit” with little or no consideration of whether such occurrences might be economically viable. Although such considerations might seem irrelevant in the present context, it is necessary to emphasize that professional institutions now insist on the correct definition and usage of terminology pertaining to exploration results, mineral resources, and ore reserves. Such terminology should be widely used and applied, as it helps to reduce the incorrect, and sometimes irresponsible, usage of terminology in reports on which, for example, investment decisions might be based. Correct terminology can also assist in the description and identification of genuine ore deposits from zones of marginal economic interest or simply anomalous concentrations of a given commodity.
Although the legislation that governs the public reporting of mineral occurrences varies from one country to another, there is now reasonable agreement globally on a definition of terms. It is widely accepted that different terms should apply to mineral occurrences depending on the level of knowledge and degree of confidence that is associated with their quantification in terms of grade and mass/volume. Figure 3 is a matrix that reflects the terminology associated with an increased level of geological knowledge and confidence, and modifying factors such as those related to mining techniques, metallurgical extraction, marketing, and environmental reclamation. Exploration results can be translated into a mineral resource once it is clear that an occurrence of intrinsic economic interest exists in such form and quantity that there are reasonable prospects for its eventual exploitation. Such a resource can only be referred to as an ore reserve if it is a part of an economically extractable measured or indicated mineral resource. One problem with this terminology is that an economically extractable ore deposit in a developing world artisanal operation may not be viable in a technically developed economy, and vice versa. The term “ore deposit” has no significance in the professional description of a mineral occurrence and is best used simply as a descriptive or generic term.
Figure 3 Simplified scheme illustrating the conceptual difference 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.