Introduction to Ore-Forming Processes. Laurence Robb

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Introduction and Aims

      With a global population in 2019 of close to eight billion people, and this figure set to increase to some ten billion by 2050, it is apparent that the world's economies are under growing pressure to meet the demands of an increasingly materialistic lifestyle. The unprecedented growth of human population over the past century has resulted in a dramatic increase in demand for, and production of, natural resources – it is therefore evident that understanding the nature, origin, and distribution of the world's mineral deposits remains a vital and strategic topic. The discipline of “economic geology,” which covers all aspects pertaining to the description and understanding of mineral resources, is, therefore, one which traditionally has been, and should remain, a core component of the university earth science curriculum. It is also the discipline that underpins the training of professional earth scientists working in the minerals and related industries of the world. Unfortunately, a tendency at many universities in the recent past has been to treat economic geology as a vocational topic, and to provide instruction only to those individuals who wished to specialize in the discipline or to follow a career in the minerals industry. There has been a trend, at least in many parts of the world, to sideline economic geology both as a taught discipline and a research topic.

Developments in the early twenty‐first century have indicated how problematic institutional and governmental neglect can be when the security of supply of strategic metals is brought into question. Global demands to reduce greenhouse gas emissions, and to provide a framework for the responsible and sustainable supply of natural resources, have resulted in the realization that all earth scientists need to understand the resource cycle in order to properly advise the public at large, and to manage future programs aimed at the responsible custodianship of the world's finite resources. The conceptual development of earth systems science, a feature of the latter years of the twentieth century, has led to changes in the way in which the earth sciences are taught. A more holistic, process‐orientated approach has led to a much wider appreciation of the Earth as a complex, interrelated system. The understanding of feedback mechanisms has created an awareness that the solid Earth, its oceans and atmosphere, and the organic life forms that occupy niches above, at and below its surface, are intimately connected and can only be understood properly in terms of an interplay of processes. Examples include the links between global tectonics and climate patterns, and also between the evolution of unicellular organisms and the formation of certain types of ore deposits. In this context the teaching of many of the traditional geological disciplines assumes new relevance and the challenge to successfully teaching earth system science is how best to integrate the wide range of topics into a curriculum that provides understanding of the entity. Understanding the processes involved in the formation of the enormously diverse ore deposit types found on Earth is necessary, not only because of its practical relevance to the real world, but also because such processes form an integral and informative part of the Earth's evolution.

      The purpose of this process‐orientated book is to provide a better understanding of the nature and origin of mineral occurrences and how they fit into the Earth system. It is intended for use at a senior undergraduate level, or at a graduate level, and assumes a basic knowledge in a wide range of earth science disciplines, as well as in chemistry and physics. It is also hoped that practicing geologists in the minerals and related industries will find the book useful as a summary and update of ore‐forming processes. To this end the text is punctuated by a number of boxed case studies in which actual ore deposits, selected as classic examples from around the world, are briefly described to give context and relevance to processes being discussed in the main text.

A Classification Scheme for Ore Deposits

There are many different ways of categorizing ore deposits. Most people who have written about and described ore deposits have either unwittingly or deliberately been involved in their classification. This is especially true of textbooks where the task of providing order and structure to a set of descriptions invariably involves some form of classification. The best classification schemes are probably those that remain as independent of genetic linkages as possible, thereby minimizing the scope for mistakes and controversy. Nevertheless, genetic classification schemes are ultimately desirable, as there is considerable advantage to having processes of ore formation reflected in a set of descriptive categories. Guilbert and Park (1986) discuss the problem of ore deposit classification at some length in chapters 1 and 9 of their seminal book on the geology of ore deposits. They show how classification schemes reflect the development of theory and techniques, as well as the level of understanding, in the discipline. Given the dramatic improvements in the level of understanding in economic geology over recent years, the Guilbert and Park (1986) classification scheme, modified after Lindgren's (1933) scheme, is both detailed and complex, and befits the comprehensive coverage of the subject matter provided by their book. In a more recent, but equally comprehensive, coverage of ore deposits, Misra (2000) has opted for a categorization based essentially on genetic type and rock association, similar to a scheme by Meyer (1981). It is the association between ore deposit and host rock that is particularly appealing for its simplicity, and that has been selected as the framework within which the processes described in this book are placed.

Rocks are classified universally in terms of a threefold subdivision, namely igneous, sedimentary, and metamorphic, that reflects the fundamental processes active in the Earth's crust (Figure 1a). The scheme is universal because rocks are recognizably either igneous or sedimentary (generally!), or, in the case of both precursors, have been substantially modified to form a metamorphic rock. Likewise, ores are rocks and can often be relatively easily attributed to an igneous or sedimentary/surficial origin, a feature that represents a good basis for classification. Such a classification also reflects the genetic process involved in ore formation, since igneous and sedimentary deposits are typically syngenetic and formed at the same time as the host rock itself. Although many ores are metamorphosed, and whereas pressure and temperature increases can substantially modify the original nature of ore deposits, it is evident that metamorphism does not itself represent a fundamental process whereby ore deposits are created. Hydrothermal processes, however, are a metallogenic analogue for metamorphism and also involve modification of pre‐existing protoliths, as well as heat (and mass) transfer and pressure fluctuation. A very simple classification of ores is, therefore, achieved on the basis of igneous, sedimentary/surficial, and hydrothermal categories (Figure 1b), and this forms the basis for the structure and layout of this book. This subdivision is very similar to one used by Einaudi (2000), who stated that all mineral deposits can be classified into three types based on process, namely magmatic deposits, hydrothermal deposits, and surficial deposits formed by surface and groundwaters. One drawback of this type of classification, however, is that ore‐forming processes are complex and episodic. Ore formation also involves processes that evolve, sometimes over significant periods of geologic time. For example, igneous processes become magmatic‐hydrothermal as the intrusion cools and crystallizes, and sediments undergo diagenesis and metamorphism as they are progressively buried, with accompanying fluid flow and alteration. In addition, deformation of the Earth's crust introduces new conduits that also facilitate fluid flow and promote the potential for mineralization in virtually any rock type. Ore‐forming processes can, therefore, span more than one of the three categories, and there is considerable overlap between igneous and hydrothermal and between sedimentary and hydrothermal, as illustrated diagrammatically in Figure 1b.

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