Introduction to Ore-Forming Processes. Laurence Robb
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.
Figure 1 (a) Classification of the principal rock types and (b) a simple classification of ore deposits also based on host rock type – Parts 1, 2, and 3 represent the breakdown of sections in this book. Photographs illustrate examples representing the main ore forming processes. (c) Igneous: magmatic layering and chromitite seams, Critical Zone, Bushveld Complex, South Africa. (d) Sedimentary: Au‐ and U‐bearing conglomerate from the Witwatersrand Basin, South Africa. (e) Hydrothermal: quartz‐carbonate vein network in metasedimentary host rocks of the Lily gold mine, Barberton greenstone belt, South Africa.
The main part of this book is subdivided into three sections termed Igneous (Part I), Hydrothermal (Part II), and Sedimentary/Surficial (Part III) (Figure 1a–e). Part I comprises Chapters 1 and 2, which deal with igneous and magmatic‐hydrothermal ore‐forming processes respectively. Part II contains Chapter 3 and covers the large and diverse range of hydrothermal processes not covered in Part I. Part III comprises Chapter 4 on surficial and supergene processes, as well as Chapter 5, which covers sedimentary ore deposits, including a section on the fossil fuels. The final chapter of the book, Chapter 6, is effectively an addendum to this threefold subdivision and is an attempt to describe the distribution of ore deposits, both spatially in the context of global tectonics and temporally in terms of crustal evolution, through Earth history. This chapter is relevant because the plate tectonic paradigm, which has so pervasively influenced geological thought since the early 1970s, provides another conceptual basis within which to classify ore deposits. In fact, modern economic geology, and the scientific exploration of mineral deposits, is now firmly cast into the frame of global tectonics and crustal evolution. Although there is still a great deal to be learnt, the links between plate tectonics and ore genesis are now sufficiently well established that studies of ore deposits are starting to contribute to a better understanding of the Earth system.
What Makes a Viable Mineral Deposit?
Ore