Abstract This special volume provides a comprehensive review of the current state of knowledge for rare earth and critical elements in ore deposits. The first six chapters are devoted to rare earth elements (REEs) because of the unprecedented interest in these elements during the past several years. The following eight chapters describe critical elements in a number of important ore deposit types. These chapters include a description of the deposit type, major deposits, critical element mineralogy and geochemistry, processes controlling ore-grade enrichment, and exploration guides. This volume represents an important contribution to our understanding of where, how, and why individual critical elements occur and should be of use to both geoscientists and public policy analysts. The term “critical minerals” was coined in a 2008 National Research Council report (National Research Council, 2008). Although the NRC report used the term “critical minerals,” its focus was primarily on individual chemical elements. The two factors used in the NRC report to rank criticality were (1) the degree to which a commodity is essential, and (2) the risk of supply disruption for the commodity. Technological advancements and changes in lifestyles have changed the criticality of elements; many that had few historic uses are now essential for our current lifestyles, green technologies, and military applications. The concept of element criticality is useful for evaluation of the fragility of commodity markets. This fragility is commonly due to a potential risk of supply disruption, which may be difficult to quantify because it can be affected by political, economic, geologic, geographic, and environmental variables. Identifying potential sources for some of the elements deemed critical can be challenging. Because many of these elements have had minor historic usage, exploration for them has been limited. Thus, as this volume highlights, the understanding of the occurrence and genesis of critical elements in various ore deposit models is much less well defined than for base and precious metals. A better understanding of the geologic and geochemical processes that lead to ore-grade enrichment of critical elements will aid in determining supply risk and was a driving factor for preparation of this volume. Understanding the gaps in our knowledge of the geology and geochemistry of critical elements should help focus future research priorities. Critical elements may be recovered either as primary commodities or as by-products from mining of other commodities. For example, nearly 90% of world production of niobium (Nb) is from the Araxá niobium mine (Brazil), whereas gallium (Ga) is recovered primarily as a by-product commodity of bauxite mining or as a by-product of zinc processing from a number of sources worldwide.

Abstract Magmatic sulfide deposits fall into two major groups when considered on the basis of the value of their contained metals, one group in which Ni, and, to a lesser extent, Cu, are the most valuable products and a second in which the PGE are the most important. The first group includes komatiite- (both Archean and Paleoproterozoic), flood basalt-, ferropicrite-, and anorthosite complex-related deposits, a miscellaneous group related to high Mg basalts, Sudbury, which is the only example related to a meteorite impact melt, and a group of hitherto uneconomic deposits related to Ural-Alaskan–type intrusions. PGE deposits are mostly related to large intrusions comprising both an early MgO- and SiO 2 -rich magma and a later Al 2 O 3 -rich, tholeiitic magma, although several other intrusive types contain PGE in lesser, mostly uneconomic quantities. Most Ni-rich deposits occur in rocks ranging from the Late Archean to the Mesozoic. PGE deposits tend to predominate in Late Archean to Paleoproterozoic intrusions, although the limited number of occurrences casts doubt on the statistical validity of this observation. A number of key events mark the development of a magmatic sulfide deposit, partial melting of the mantle, ascent into the crust, development of sulfide immisciblity as a result of crustal interaction, ascent of magma + sulfides to higher crustal levels, concentration of the sulfides, their enrichment through interaction with fresh magma (not always the case), cooling and crystallization. Factors governing this development include (1) the solubility of sulfur in silicate melts and how this varies as a function of partial mantle melting and subsequent fractional crystallization, (2) the partitioning of chalcophile metals between sulfide and silicate liquids, and how the results of this vary during mantle melting and subsequent crystallization and sulfide immiscibility (degree of melting and crystallization, R factor and subsequent enrichment), (3) how effectively the sulfides become concentrated and the factors controlling this, and (4) processes that occur during the cooling of the sulfide liquid that govern aspects of exploration and mineral beneficiation. These topics are discussed first in general terms and then with specific reference to deposits at Noril’sk, Kambalda, and Voisey's Bay. With regard to Voisey's Bay, quantitative modeling is consistent with the very low PGE concentrations in this deposit being the result of some sulfide having been left behind in the mantle during partial melting. Both the Noril'sk and Voisey's Bay deposits are shown to be economic because of subsequent upgrading of the

Abstract Two recent papers, “Utility of high-altitude infrared spectral data in mineral exploration: Application to northern Patagonia Mountains, Arizona,” by Berger et al. (2003), and “Mapping hydrothermally altered rocks at Cuprite, Nevada, using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), a new satellite-imaging system,” by Rowan et al. (2003), make a distinctive mark on the use of airborne and satellite hyperspectral imaging as an exploration tool. These two papers deal with imaging of the Earth’s surface using the visible (0.4 μ m) to near infrared (2.5 μ m) part of the electromagnetic spectrum to map various mineral species. Depending on their structure and molecular bonding, minerals reflect and absorb the electromagnetic spectrum in unique ways. A large group of minerals have distinct electromagnetic signatures that make it possible to identify them from imaging systems that map the range of the electromagnetic spectrum between 0.5 and 2.5 μ m. These papers represent two distinct approaches. The first paper, by Berger et al., discusses the use of the AVIRIS (Airborne Visible Infrared Imaging Spectrometer) scanner, which provides high-resolution reflectance measurements in the spectral domain (224 channels between 0.4 and 2.45 μ m) and variable spatial resolution (20 m), dependent on aircraft altitude. The second paper, by Rowan et al., discusses the use of the ASTER satellite scanner, which offers a limited range of spectra at three spatial resolutions (15, 30, and 90 m). ASTER measures reflectance radiation in 3 bands within the 0.52- to 0.86- μ m range (visible-near-infrared) at 15-m spatial resolution, and 6 bands between 1.00 and 2.43 μ m (short wave infrared) at 30-m spatial resolution. Emitted radiation is measured in 5 bands between 8.125 and 11.650 μ m (thermal infrared) with a 90-m spatial resolution. The main advantage of the AVIRIS sensor is the level of spectral detail, which provides accurate measurements of reflectance and absorption features of minerals that enables detailed mineral mapping. Its main disadvantages, however, are the extensive processing required to make the reflectance spectra useful, and its limited spatial coverage and acquisition cost based on programmed flights. In contrast, the main advantage of the ASTER sensor is that it measures key portions of the visible, near-infrared, and thermal infrared spectra of minerals for large-scale mapping projects, whereas its main disadvantage is that the data represent only portions of the electromagnetic spectrum and some minerals cannot be distinctively mapped. In addition, the lower spatial resolution in the near-and thermal infrared portions of the spectrum makes it more difficult to map at detailed scales.

Abstract The spark to put together this volume on banded iron formation (BIF)-related high-grade iron ore was born in 2005 during a steamy night in Carajás where the iron research group from the Universidade Federal Minas Gerais, Vale geologists, Carlos Rosière and Steffen Hagemann, were hotly debating the hypogene alteration genesis for the high-grade, jaspilite-hosted Serra Norte iron ore deposits. A couple of caipirinhas later we decided that the time was opportune to put together a volume that captured the new and innovative research that was being conducted on BIF-related high-grade iron ores throughout the world. We had little problem convincing our South African colleagues Jens Gutzmer and Nic Beukes to join the effort and decided that the 2008 biannual Society of Economic Geologists' (SEG) meeting in South Africa would be the perfect place to present this project through a combined field trip and workshop near Sishen. The enthusiastic support that we received from the research community, SEG, and industry to put this volume together was generated by the significant increase in exploration activity, and with it the need for more detailed information on what exactly controls the location of high-grade iron orebodies, and renewed research interest around the world in models for the genesis of BIF-related high-grade iron ore, and particularly the relative importance of hypogene and supergene processes in formation of high-grade ore. This volume concentrates on new research on the characteristics and metallogenesis of BIF-related high-grade iron ores. It contains a state of the art series of papers on established and new iron ore districts and deposits, the different components of the BIF iron mineral system, and how to best explore for this ore type. Although the emphasis of many of the contributions to this volume is on the hypogene aspect of high-grade iron ore formation, it is important to note that most BIF-related iron ore districts have a very pronounced supergene overprint due to deep lateritic weathering. The transformation of many hypogene iron orebodies of reasonable grade and size to the giant deposits exploited today can be related to this geologically recent supergene overprint; most of the past and still much of the present mining of high-grade iron ore relates to soft ore interpreted in most cases to be the direct result of supergene processes. Also mentioned here should be the recent resurgence of a syngenetic model that advocates the formation of chert-free BIF

Abstract Fluid pathways between metal sources and sites of ore deposition in hydrothermal systems are governed by fluid pressure gradients, buoyancy effects, and the permeability distribution. Structural controls on ore formation in many epigenetic systems derive largely from the role that deformation processes and fluid pressures play in generating and maintaining permeability within active faults, shear zones, associated fracture networks, and various other structures at all crustal levels. In hydrothermal systems with low intergranular porosity, pore connectivity is low, and fluid flow is typically controlled by fracture permeability. Deformation-induced fractures develop on scales from microns to greater than hundreds of meters. Because mineral sealing of fractures can be rapid relative to the lifetimes of hydrothermal systems, sustained fluid flow occurs only in active structures where permeability is repeatedly renewed. In the brittle upper crust, deformation-induced permeability is associated with macroscopic fracture arrays and damage products produced in episodically slipping (seismogenic) and aseismically creeping faults, growing folds, and related structures. In the more ductile mid- to lower crust, permeability enhancement is associated with grain-scale dilatancy (especially in active shear zones), as well as with macroscopic hydraulic fracture arrays. Below the seismic–aseismic transition, steady state creep leads to steady state permeability and continuous fluid flow in actively deforming structures. In contrast, in the seismogenic regime, large cyclic changes in permeability lead to episodic fluid flow in faults and associated fractures. The geometry and distribution of fracture permeability is controlled fundamentally by stress and fluid pressure states, but may also be influenced by preexisting mechanical anisotropies in the rock mass. Fracture growth is favored in high pore fluid factor regimes, which develop especially where fluids discharge from faults or shear zones beneath low-permeability flow barriers. Flow localization within faults and shear zones occurs in areas of highest fracture aperture and fracture density, such as damage zones associated with fault jogs, bends, and splays. Positive feedback between deformation, fluid flow, and fluid pressure promotes fluid-driven growth of hydraulically linked networks of faults, fractures, and shear zones. Evolution of fluid pathways on scales linking fluid reservoirs and ore deposits is influenced by the relative proportions of backbone, dangling, and isolated structures in the network. Modeling of the growth of networks indicates that fracture systems reach the percolation threshold at low bulk strains. Just above the percolation threshold, flow is concentrated along a small proportion of the total fracture population, and favors localized ore deposition. At higher strains, flow is distributed more widely throughout the fracture population and, accordingly, the potential for localized, high-grade ore deposition may be reduced.

Abstract THIS Gold in 2000 volume is organized around a classification of hypogene gold deposits that emphasizes their tectonic setting and relative time of formation compared to their host rocks and other gold deposit types (e.g., Sawkins, 1972, 1990; Groves et al., 1998; Kerrich et al., 2000). The temporal division of orogenic gold deposits into Archean, Proterozoic, and Phanerozoic follows closely the recently published classification of orogenic gold deposits (Groves et al., 1998) which incorporates the previously identified “mesothermal” gold deposits. The newly recognized intrusion-related and sedex gold deposits represent new gold deposit classes even though their exact genetic classification remains open, with more research considered a priority. Proterozoic Au-only and Cu-Au-(Fe) deposits are also a relatively recently recognized class of structurally controlled epigenetic gold deposits. Particularly, the origin and classification of Cu-Au-(Fe) deposits (e.g., Olympic Dam) remains equivocal, as pointed out by Partington and Williams (2000). In fact, Kerrich et al. (2000) discuss the anorogenic iron oxide copper-gold deposits as one of six world-class gold deposit classes. Low- and high-sulfidation and hot spring epithermal gold deposits are dealt with as one genetic gold class. Alkalic epithermal and porphyry gold deposits are dealt with as a separate gold deposit class owing to their specific host-rock association and element enrichment (e.g., Mo, F, Be, Hg, W, and Sn). The gold deposit classes are described from both industry and academic points of view, with emphasis on a balanced account of the descriptive geology, genetic interpretations, exploration significance, as well as open questions and future research avenues. The volume contains 13 papers covering 10 major classes of gold deposits and three summary papers, and was presented as a Society of Economic Geologists-sponsored short course held November 10 and 11, 2000, at Lake Tahoe, Nevada. Orogenic gold ores are associated with regionally metamorphosed terranes of all ages (Kerrich and Cassidy, 1994) and are spatially linked to subduction-related thermal processes (Kerrich and Wyman, 1990)(Fig. 1). These metal concentrations formed during compressional to transpressional deformation processes at convergent plate margins in accretionary (oceanic-continental plate interaction) and collisional (continental-continental collision) orogens (i.e., Bohlke, 1982; Groves et al., 1998). In both cases hydrated marine sedimentary and volcanic rocks have been added to continental margins over a long period of collision (10 to >100 Ma). Accretionary or peripheral orogens contain gold deposits in the Archean of Australia, Canada, Africa, India, and Brazil and the Mesozoic and Cenozoic gold fields of western North America, i.e., the famous Mother Lode belt. Collisional or internal orogens contain gold deposits in the Proterozoic of Australia, North America, West Africa, and Brazil, and the famous Phanerozoic gold fields in the Variscan, Appalachian, and Alpine regions of North America and Europe. In Phanerozoic orogenic gold deposits, subduction- related thermal events, episodically raising geothermal gradients within the hydrated accretionary sequences, initiate and drive long-distance hydrothermal fluid migration.

Abstract Lead (Pb) isotope compositions of sulfide minerals coupled with rocks associated with an ore deposit provide critical constraints on the source of metals and fluid pathways in a fossil hydrothermal system (Heyl et al., 1966; Stacey et al., 1968; Gulson, 1986; Sanford, 1992). Lead isotope compositions of sulfide minerals also provide chronologic information, either absolute or relative, for ore deposition (for example, Carr et al., 1995) and can also be used as an exploration tool during prospect evaluation (Gulson, 1986; Young, 1995). These varied applications of Pb isotopes to achieve an understanding of the ore genesis process are too diverse to be adequately discussed in a single overview chapter. Instead, this chapter focuses attention on what Pb isotopes tell us about (1) the sources of Pb nd other metals in ore deposits, (2) the interaction between hydrothermal fluids and wall rocks, (3) the influence of basement rocks and tectonic setting on Pb sources in ore deposits in magmatic arcs, and (4) the application f crustal-scale Pb isotope variations to an understanding of regional controls on ore deposition Before Pb isotopes pertinent to understanding ore genesis can be examined, we must review some basic principles of Pb isotope geochemistry (Fig. 1). Elegant discussions of U-Th-Pb geochemistry are presented by Doe (1970), Faure (1977), Zartman and Haines (1988), Garipy and Dupr (1991), and Dickin (1995). The following discussion is simplified from these sources. Three isotopes, 208Pb, 207Pb, and 206Pb, are partly the radiogenic daughter products from the radioactive decay of one isotope of thorium (232Th 208Pb*) and two isotopes of uranium (238U 206Pb* and 235U 207Pb*). (Note that an asterisk (*) after an isotope denotes that it is the product of radioactive decay of a parent isotope over time and is not the total abundance of the isotope in a sample.) The abundance of radiogenic isotopes has grown since the earth formed some 4.56 billion years ago (Fig. 1), building upon an initial concentration. The fourth isotope of Pb, 204Pb, is stable and has no long-lived parent isotope nordoes it decay to another isotope. Time-integrated growth of radiogenic Pb isotopes from an arbitrary starting time, t1, to an ending time, t1, in an environment where there has been no migration of U, Th, and their daughter products, is described by standard decay equations: These equations simply show that the measured present-day Pb isotope composition is equal to the sum of the initial

Abstract The types of mainly metallic mineralization found in metamorphic terranes are reviewed and an attempt is made to define the genetic relations between the mineralization and the metamorphic events.The terms metamorphosed, metamorphic, and metamorphogenic as applied to ores are also considered.The development of thought and the history of investigations on ores in metamorphic terranes aretraced from the early work in the second half of the nineteenth century onward. Early conceptions ofmetamorphism as an ore-forming process (metamorphogenesis) were seemingly not followed up by theiroriginators, contemporaries, or immediate successors and were neglected until comparatively recentyears. The idea of metamorphism as a modifier of preexisting, mainly sulfidic, but also oxidic, mineralizationwon more immediate and general acceptance in the early decades of the present century. InNorth America, emphasis seems to have been mainly on the deformational aspects of the metamorphism,whereas elsewhere, especially in Europe, the textural and mineralogical results of the metamorphic recrystallizationalso received considerable attention and metamorphism as an ore-forming process hadwon a certain degree of acceptance. This difference in emphasis may perhaps be referred to the differentviews held regarding the initial genesis of the ores in the two regions.The late 1940s and the 1950s witnessed a considerable revision of ideas on ore genesis, especially regardingstrata-bound massive sulfide ores. A parallel revival of interest in the role of metamorphism,probably not unrelated to the foregoing, began in the early 1950s, to begin with concerning metamorphosedores. However, new thoughts concerning metamorphogenesis related to granitization or ultrametamorphismas an ore-forming process began to be published.The following decades witnessed an almost explosive increase in the number of publications dealingwith the effects of metamorphism on ore mineralization of practically all types, but with a definite emphasison sulfide ores of the strata-bound type. One of the most significant breakthroughs in this respectconcerned the world-famous Broken Hill deposit, New South Wales, although the metamorphosed natureof ores in the Scandinavian Caledonides, the North American Appalachians, the Lachlan fold beltof eastern Australia, the Sanbagawa terrane of Japan, the Urals, and Proterozoic fold belts in southernAfrica, have all been thoroughly documented.In recent years, however, the interpretation of many massive sulfidic ores in metamorphic terranes asmetamorphosed has been increasingly questioned, and syntectonic, metamorphogenic, origins havebeen advocated. There is obviously a great need to be able to distinguish more

Abstract Anyone studying an ore deposit winds up with a lot of data: field observations in the form of maps, sections and drill logs, chemical analyses, isotope analyses, fluid inclusion data, paragenetic relations, and so on. In addition, there is a vast amount experimental data in the literature on systems relevant to the deposit being studied, in the form of data on the chemical and physical properties of solids and fluids. The investigator then tries to come up with a model of ore formation that is consistent with all these data. Naturally, the model must also be consistent with accepted principles of chemistry and physics, and one of the subjects most useful, in fact essential, in assembling all these data into consistent models is thermodynamics. The purpose of this chapter is to introduce the concepts and terms of chemical thermodynamics that are useful in constructing models of hydrothermal systems. These will be used extensively in the chapters to follow. The concepts covered in this chapter normally occupy a complete book; the coverage is therefore necessarily brief. We can save considerable space, for example, by assuming that we are all familiar with the concepts of energy, work, heat, and temperature. These are in fact quite difficult subjects, but an intuitive understanding is usually sufficient for us. Some Basic Definitions In this chapter we will not describe any natural system, and only one model of a simple natural system (H2-N2). Most of the discussion will be about a model of energy relationships, called thermodynamics. Although natural systems and thermodynamic models of natural systems are described using many of the same terms, there are some subtle differences. To begin with, a natural system is any part of the universe we choose to consider, such as the contents of a beaker, a crystal of quartz, the solar system, or a bacterium. Thermodynamic systems, on the other hand, are not real but conceptual and mathematical, and are of three types. The three types are used to distinguish between the ways that changes in composition and energy content can be effected, and therefore they are defined basically by the nature of their boundaries. Isolated systems can exchange neither matter nor energy with their surroundings. They are therefore described as having walls that are rigid (preventing any change in volume and hence any energy change due to work), and impermeable to matter and energy.

Abstract Environmental issues have become important, if not critical, factors in the success of proposed mining projects worldwide. In an ongoing and intense public debate about mining and its perceived environmental impacts, the mining industry points out that there are many examples of environmentally responsible mining currently being carried out (e.g., Todd and Struhsacker, 1997). The industry also emphasizes that the majority of mining-environmental problems facing society today are legacies from the past when environmental consequences of mining were poorly understood, not regulated, or viewed as secondary in importance to societal needs for the resources being extracted. On the other hand, environmental organizations (e.g., Mineral Policy Center, 1999) point to recent environmental problems, such as those stemming from open-pit gold mining at Summitville, Colorado, in the late 1980s (see Summitville summaries in Posey et al., 1995; Danielson and Alms, 1995; Williams, 1995; Plumlee, 1999), or those associated with a 1998 tailings dam collapse in Spain (van Geen and Chase, 1998), as an indication that environmental problems (whether accidental or resulting from inappropriate practices) can still occur in modern mining. Recent legislation imposing a moratorium on new mining in Wisconsin, and banning new mining in Montana using cyanide heap-leach extraction methods further underscore the seriousness of the debate and its implications for mineral resource extraction. In this debate, one certainty exists: there will always be a need for mineral resources in developed and developing societies. Although recycling and substitution will help meet some of the world’s resource needs, mining will always be relied upon to meet the remaining needs. The challenge will be to continue to improve the ways in which mining is done so as to minimize its environmental effects. The earth, engineering, and life sciences (which we group here under the term “earth-system sciences,” or ESS for short) provide an ample toolkit that can be drawn upon in the quest for environmentally friendly mineral resource development. The papers in this two-part volume provide many details on tools in the scientific toolkit, and how these tools can be used to better understand, anticipate, prevent, mitigate, and remediate the environmental effects of mining and mineral processing. As with any toolkit, it is the professional’s responsibility to choose the tool(s) best suited to a specific job. By describing the tools now available, we do not mean to imply that all of these tools need even be considered at any given site, nor that

Abstract There would be few geological studies in which, at some stage, there did not arise a question of timing. The answer is often to be found through direct observation; the principles of superposition and crosscutting relationships apply in determining the order of events on all scales from the microscopic to the macroscopic, from crystallization history to continental assembly. By augmenting those principles with the means to establish sequence and correlation provided by palaeontology, the geologist has the capability, through observation and logical reasoning alone, to determine the relative ages of a great range of geological processes. However, while these techniques make it possible to place geological events in time order, they do not provide an absolute measure of time itself. The measurement of absolute time in geology— geochronology—requires a quantifiable physical process that takes place continuously at a known rate from the time of the event to be dated to the present day. Some cyclic processes, such as the passage of the seasons, leave their imprint in parts of the geological record and can provide detailed, accurate measurements of elapsed time intervals, but they do not permit the measurement of absolute time (age) unless the record is unbroken to the present day or the age of one of the cycles is known by some independent means. The number of annual growth bands in a fossil coral, for example, tells how long that coral once lived, but not when. To measure absolute geologic time, one needs a process that is continuous and unidirectional. The most widely utilized of such processes is natural radioactivity. The concept behind radioisotope geochronology is quite simple. Some of the elements in rocks and minerals have isotopes (atoms of the same atomic number but different mass numbers) that are naturally radioactive-the nuclei of those isotopes are unstable, and liable to break down spontaneously (decay) to an isotope of a different element. If the newly-formed isotope is also unstable, the process continues until a stable nucleus forms. Radioactive decay occurs at rates characteristic of each element and isotope. As far as is known, those rates are independent of any chemical or physical parameters (e.g., pressure, temperature, chemical state etc. ) . The probability that a given nucleus of a given isotope will decay in any given time period is a constant, so the number of decays occurring per unit time is proportional to the number of atoms of that

Abstract Volcanic-associated massive sulfide deposits (VMS) are predominantly stratiform accumulations of sulfide minerals that precipitate from hydrothermal fluids at or below the sea floor, in a wide range of ancient and modern geological settings (Figs. 1, 2). They occur within volcanosedimentary stratigraphic successions, and are commonly coeval and coincident with volcanic rocks. As a class, they represent a significant source of the world's Cu, Zn, Pb, Au, and Ag ores, with Co, Sn, Ba, S, Se, Mn, Cd, In, Bi, Te, Ga, and Ge as co- or by-products. The understanding of ancient, land-based VMS deposits has been heavily influenced by the discovery and study of active, metal-precipitating hydrothermal vents on the sea floor. During the last three decades, excellent descriptions of sea-floor sulfides and related vent fluids and hydrothermal plumes have provided modern analogs for the landbased VMS deposits (Rona, 1988; Rona and Scott, 1993; Hannington et al., 1995). Conversely, the geology and mineralogy of land-based deposits have provided insight into the plumbing systems and sulfide mineral paragenesis of sulfide deposits relevant to sea-floor hydrothermal systems. This volume capitalizes on the complementary nature of ancient, land-based VMS deposits and active, metal-precipitating hydrothermal systems on the sea floor, much as the Reviews in Economic Geology Volume 2 (Berger and Bethke, eds., 1985) did with epithermal deposits and active, subaerial geothermal systems, and draws equally from land-based and sea-floor VMS research. This volume attempts to provide a balanced view of VMS systems, with descriptions of the processes involved in VMS formation and of important examples representing a variety of VMS deposits and districts, in modern and ancient settings. It is not meant to be a comprehensive review; rather, it presents a spectrum of current ideas based on research since the benchmark paper of Franklin et al. (1981). The contributions are divided into two parts. In Part I, reviews of the most significant geological, physical, and chemical processes involved in the formation of landbased and sea-floor VMS deposits are presented. These include: the volcanology of subaqueous settings and the relationship between volcanology and VMS systems by Gibson et al. (1999); structural aspects of magmatism and hydrothermal circulation in ocean floor and ophiolitic settings by Harper (1999); the relationship between magma chemistry and hydrothermal venting, with emphasis on the thickened oceanic crust in the Galapagos area by Perfit et al.(1999), and more generally in bimodal volcanic settings by Barrett and MacLean (1999); hydrothermal alteration of the oceanic crust by Alt (1999); fluid-rock interactions in VMS systems as recorded by stable isotope systematics by Huston (1999); the metal transport capabilities of hydrothermal fluids by Seyfried et al. (1999); precious metal enrichment associations and processes in VMS systems by Hannington et al. (1999); and heat and fluid flow in VMS systems by Barrie et al. (1999a).

Abstract The assooatwn of organic matter with ore minerals, gangue, and host rock in many low-temperature ( 120C) o moderate-temperature (120-350C) ore deposits is a well-known phenomenon (Saxby, 1976; Leventhal, 1986; Parnell et al., 1993; Giordano, 1996; Gize, 1999) and was recognized early in the twentieth century (Siebenthal, 1915; Harder, 1919; Schneiderholm, 1923; Bastin, 1926; Fowler, 1933). The study of organic constituents in ores, particularly if coupled with studies of other ore components and conditions, can provide much information on both active and passive roles of organic matter before,during, and after ore genesis, and in some cases can leadto the development of valuable exploration techniques.By the 1950s, it was recognized that biological sequesteringof metals, sulfide production by sulfate-reducing bacteria,biological precipitation of metals, sorption of metals byorganic colloidal particles, modification of geochemicalenvironments by organic processes, and the mobilizationof metals by metal-organic complexes were all potentiallyimportant roles played by organic matter in the concentrationof metals to form metalliferous shales and certaintypes of ore deposits (Berger, 1950; Krauskopf, 1955). Bythe 1960s, it was recognized that dead organic matter(organic matter not in living organisms) may be a powerfulreducing agent for sulfate and thus may provide asource of sulfide for ore-forming systems (Barton, 1967;Skinner, 1967). Roedder (1967) reported the presence ofhydrocarbons and sulfate in fluid inclusions from oredeposits. This observation was cited by Barton (1967) asstrong evidence that organic matter was present at thetime of ore formation and that thermodynamic equilibrium(which predicts hydrogen sulfide and carbon dioxide)was not attained in the ore fluid because of sluggishkinetics at the low temperature of ore formation. Hoering(1967) summarized his pioneering work on organic matterassociated with gold and uranium in the Carbon LeaderFormation of the Witwatersrand district, South Mrica.Because it was relatively immature Precambrian organicmatter (rather than graphite), it was suitable for analysis ofsimple and complex chemical compounds and led the wayfor future studies of organic matter in ore deposits andPrecambrian rocks (Leventhal et al., 1975).It was not until the 1970s and early 1980s that majorefforts on a worldwide scale were initiated to study theroles of organic matter in ore genesis (Breger, 1974; Leventhalet al. 1975; Connan and Orgeval, 1976; Saxby,1976; Giordano, 1978; Connan, 1979; Estep eta!., 1980).As a consequence of this major

Abstract Major oil companies have been utilizing techniques of quantitative basin analysis in exploration for a decade or more. Ore-forming processes in stratiform, sediment-hosted ore deposits commonly involve sedimentary processes, diagenesis, basinal brines, and paleohydrology. Like the maturation and migration of hydrocarbons, their formation is an integral part of basin history. Consequently, applying comprehensive basin analysis to mineral exploration is a logical and helpful approach to understanding sediment- hosted ore deposits and predicting their occurrence, location, and origin. When the Society of Economic Geologists' Short Course Committee contacted the writer in 1985 to develop a short course on sedimentary processes of ore formation, ft seemed to me that such a course would provide an excellent opportunity to introduce the concept of comprehensive basin analysis as an exploration tool for sediment-hosted mineral deposits. As Sawkins pointed out (1990, p. 333), “Meaningful exploration in extensional tectonic paleo-environments will increasingly require the integration of surface, subsurface, and geophysical data, and enlightened programs of basin analysis similar to those practiced by the petroleum industry will be increasingly needed.” Sediment-hosted ore deposits include sedimentary gold and other heavy mineral accumulations; evaporites; syngenetic to late diagenetic base metal and barite deposits in clastic and carbonate rocks, including epiclastic volcanic rocks; banded iron formations; Clinton-minette-type iron and manganese ores; unconformity-related and sandstonetype uranium deposits; and Mississippi Valley-type leadzinc deposits. Some sediment -hosted ore deposits were formed at various stages of basin history and are multistage. This short course focuses on (1) the types of basins in which major sediment - hosted ore deposits occur, and (2) the controls of basin types on ore-hosting sedimentary environments and ore-forming processes. The precise role of sedimentary processes in the formation of ore deposits has been debated by geologists around the world; this debate has affected the manner and success of exploration program s . Skinner (1979, 1987) traced the origins of the polarization of thought on the genesis of ore deposits to Agricola, who expounded on lateral secretion and precipitation of metals from circulating ground waters, and to Descartes, who perceived the earth as an outgassing star and believed that metals were not derived from host rocks. The neptunist theories of Werner (1750-1817) may have evolved from Agricola and the plutonist theories of Hutton (1726-1797) from Descartes. L. C. Graton, whom the Graton-Sales volume Ore Deposits in the United States 1933-1967

Abstract Magmatic sulfide ores are thought to form as the result of droplets of an immiscible sulfide-oxide liquid forming within silicate magma and then becoming concentrated in a particular location. Certain elements, notably the Group VIII transition metals Fe, Co, Ni, Pd, Pt, Rh, Ru, Ir and Os together with Cu and Au, partition strongly into the sulfide- oxide liquid, and thus become concentrated with it. A number of factors may influence the concentration of this liquid, but the dominant one is gravitational settling, since the liquid has a density of >4 in comparison with a value of <3 for its host silicate magma. To help in the understanding of deposits of this type, in this book we first discuss the phase relations of simple sul- fide-oxide liquids and activity-composition relations within them. We then discuss the solubility of sulfide in mafic and ultramafic melts, followed by the partitioning of elements between silicate magma and sulfide-oxide liquid. The oxidation state and volatile content of a silicate magma can have a major influence on the segregation of a sulfide-oxide liquid and the distribution of metals so that this forms the focus of a second chapter. Magmatic sulfide deposits can be viewed in terms of their associated mafic or ultramafic bodies and the tectonic settings into which these were emplaced. The scheme shown as Table 1. 1 is adapted from that of Naldrett, (1989). In it, bodies are divided into whether they were emplaced in a rifted continental environment (category II), a cratonic environment (category III) or an active orogenic belt (category IV) . Archean greenstone belts still represent an enigma in terms of present-day tectonics. For example, were komatiites erupted through continental crust (Arndt, 1986a; Compston et al., 1986) or do they represent the floor of a primitive ocean (de Witt et al., 1987)? Thus a separate category (category I) has been created for the syn-volcanic activity in this environment. Experience in Archean greenstone belts has shown that mafic and ultramafic bodies fall into two main classes, komatiites and tholeiites, and that the tholeiites constitute two distinct sub-classes, one with picritic average compositions and chilled margins and the other very rich in anorthositic gabbro. The komatiites are host to Ni sulfide ores in Western Australia, Zimbabwe and Canada; these ores and their origin are discussed by C.M. Lesher in this volume. Examples of mineralization associated with the picritic sub-class of tholeiites include

Abstract Measurements of the optical properties of opaque minerals using a reflected-light polarizing microscope are difficult for the unskilled investigator because they require demanding and exacting techniques. Unlike transparent minerals, which can be easily identified by tests using a transmitted-light petrographic microscope, opaque minerals have few optical properties that can lead to unambiguous identification. The principal properties used to identify opaque minerals are color and color intensity, but these involve subjective decisions on the part of the investigator. Color is determined in part by the colors of the surrounding minerals; color intensity is influenced by reflectance. This, in turn, is influenced by sample preparation. Many teaching institutions, private consultants, and professional companies do not possess microscopes which have the additional equipment to test microhardness, or photometers to test the reflectance of minerals; however, microhardness and reflectance data have been included for those people who have access to such equipment. The following two tables, one for colored minerals, one for noncolored minerals, include 95 common opaque minerals, 13 transparent minerals, and 8 minerals that range from transparent-translucent to opaque. The tables are by no means complete because they are designed to serve as a rapid and efficient way of identifying the common opaque minerals. We have found by experience that unless told the identity of a mineral, the unskilled user often finds alphabetical tables of opaque mineral properties imposing and time consuming. In an attempt to overcome this problem, we use an identification scheme which enables the user to identify a mineral by a process of elimination. The scheme is straightforward and utilizes a tree diagram (Fig. 1) in conjunction with a list of the mineral properties that can be easily observed under the microscope. The mineral is identified by moving from the top of the tree diagram to the bottom through a series of decisions based on optical properties. These properties are, in order: color intensity, pleochroism, anisotropism and its intensity, internal reflection, and relative hardness. A final choice of the mineral’s identity can be made when the end of the branch is reached. In only a small number of cases are all five decisions required to make this choice. Since this procedure may not always give a unique answer, additional features are included that will help elucidate the mineral’s identity. These are: the presence or absence of cleavage and twinning, the composition of commonly associated minerals, and a summary of key identifying criteria.

Abstract As mineral exploration becomes increasingly difficult, costly and competitive, success is essential; there is no room for waste or inefficiency. Exploration must be truly cost effective. The present book is concerned ultimately with the interpretation of geochemical surveys. However the data to be interpreted are the product of the field survey and thus only as good as the work that went into these earlier phases. The truism "garbage in—garbage out" is as relevant here as anywhere. These are independent yet interdependent functions. Failure to execute one step correctly will negate all efforts in the succeeding steps. By and large the function that is most costly, and certainly most difficult to repeat, is the field survey. Any deficiences at this stage will have fatal effects on the remainder of the project. Analysis of the samples is, indeed, costly and an area of necessary concern. However, if samples have been collected properly it is not unreasonable to suppose that they can be reanalyzed should this be deemed necessary or useful. Ultimately interpretation, provided that sampling and analysis are reliable, is an exercise that can be repeated many times using a variety of techniques or models depending on supplementary information available and the skills and prejudices of various geologists or geochemists. The design and execution of a geochemical survey is thus crucial to its success. Surveys can be, and are, optimized to find specific targets in particular environments. Such fine tuning requires an understanding of applied geochemistry, knowledge of the environment in which the survey will be carried out and an appreciation of the target being sought. Before considering these points in more detail, it is worth defining the nature of a geochemical survey in more general terms and establishing clearly the role of the survey in an exploration program. The basic premise of exploration geochemistry is that the systematic sampling and analysis of naturally occurring materials will reveal features indicative of the presence of potentially economic mineralization. This is a deceptively simple statement for it begs the questions—what materials should be sampled?; how and for what entities should these samples be analyzed?; and what features will be revealed? We will consider these points in more detail in later sections. The key wording here is “systematic sampling and analysis.” Regular and consistent application of a technique across a property should produce a common database, a synoptic picture of the distribution of elements or compounds, that will meet

Abstract In the context of exploration for epithermal deposits, why study geothermal systems at all? After all, not one exploited system to date has been shown by drilling to harbor any economically significant metal resource--but then until recently not one had been drilled for other than geothermal energy exploration.* The latter involves drilling to depths of 500-3000 meters in search of high temperatures and zones of high permeability which may sustain fluid flow to production wells for steam separation and electricity generation. In many cases such exploration wells have discovered disseminated base-metal sulfides with some silver and argillic-propylitic alteration equivalent to that commonly associated with ore-bearing epithermal systems (Browne, 1978; Henley and Ellis, 1983; Hayba et al., 1985, this volume). In general, however, geothermal drilling ignores the upper few hundred meters of the active systems and drill sites are situated well away from natural features such as hot springs or geysers, the very features whose characteristics (silica sinter, hydrothermal breccias) are recognizable in a number of epithermal precious-metal deposits (see, for example, White, 1955; Henley and Ellis, 1983; White, 1981; Berger and Eimon, 1983; Hedenquist and Henley, 1985; and earlier workers such as Lindgren, 1933). Knowledge of the upper few hundred meters of active geothermal systems is scant and largely based on interpretation of hot-spring chemistry. Tantalizingly, in a number of hot springs, transitory red-orange precipitates occur which are found to be ore grade in gold and silver and which carry a suite of elements (As, Sb, Hg, Tl) now recognized as characteristic of epithermal gold deposits (Weissberg, 1969).