Abstract Approximately 3.4 billion tons (Gt) of iron ores containing >50 percent Fe were produced from U.S. mines in the Lake Superior region from 1848 until they were exhausted 20 to 30 years ago. The Vermilion Range in Minnesota produced nearly 100 million tons (Mt) of this ore from Archean greenstone belt-hosted iron formation. The remaining production has come from Proterozoic strata including 2.3 Gt from the Mesabi and 100 Mt from the Cuyuna Ranges in Minnesota while Michigan and Wisconsin contributed 230 Mt from the Marquette Range, 290 Mt from the Menominee Range, and 325 Mt from the Gogebic Range. The protore of these direct-shipping ores are carbonate- or oxide-facies banded iron formations that contained 25 to 35 percent Fe prior to undergoing leaching (desilicification), oxidation, and volume loss. The conventional model ascribing these changes to supergene processes has recently been challenged by research showing that hypogene fluids, channeled by faults into structurally favorable horizons and settings, have played a dominant role in producing some of the high-grade (>60% Fe) ores that are presently providing much of the world's iron ore. Descriptions of the North American iron ores, generally starting with the U.S. Geological Survey monographs published at the beginning of the 20 th century provide many tantalizing clues, suggesting that hypogene fluids have indeed played an important role in the evolution of some of these districts. Application of modern geophysical techniques and structural and geochemical analyses may well guide the discovery of new high-grade ores either below or adjacent to the historic mining areas. The time seems to be ripe for exploration to return to the area that can claim to have begun geologists' understanding of this most important ore deposit type.

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.

Abstract Archean orogenic lode gold deposits are the result of large, complex mineralizing systems that have developed within many Archean terrains. Mineralizing systems are defined to include all geologic factors that control the generation and preservation of mineral deposits and emphasize the processes responsible for deposit formation at a variety of scales. Deposits belonging to Archean orogenic lode gold mineralizing systems comprise epigenetic mineralization that formed as a result of focused fluid flow late during active deformation and metamorphism of volcano-plutonic terranes. They can occur in any lithology and formed at a range of paleocrustal levels through site-specific and local physical and chemical processes. All Archean orogenic lode gold deposits formed through broadly similar geologic processes, with the unique character of individual deposits resulting mainly from variations at the depositional site. The key feature of Archean orogenic lode gold systems is a broadly uniform low-moderate salinity, mixed aqueous-carbonic fluid that is capable of carrying Au but has limited capacity to transport base metals. Models for development of orogenic lode gold mineralizing systems are generally poorly constrained, although geologic and geochemical characteristics are consistent with terrane- or larger-scale processes. Archean terranes containing orogenic lode gold systems include accretionary and collisional settings. Mineralization is generally late in the tectonic evolution of the host terranes; is typically syn- to postpeak metamorphism, becoming increasingly postpeak at higher paleocrustal levels; and is indicative of clockwise metamorphic P-T paths and implicating processes involving “deeper later”-type metamorphism. There are few robust absolute ages on mineralization although, in most terranes, available ages indicate mineralization follows major volcanic, sedimentary, and plutonic episodes. In many terranes, mineralization is coincident with mid-crustal felsic magmatism. Young absolute ages recorded in some deposits probably reflect resetting and/or new mineral growth during post-gold mineralization hydrothermal activity and/or slow cooling of host terranes. The source(s) of fluids and metals in orogenic lode gold systems is poorly understood; however, mineral equilibria and isotope tracers implicate sources deeper than presently exposed greenstones. Isotope tracers and mineral equilibria are also consistent with derivation from and/or equilibration of the ore fluid with felsic rocks during transport of hydrothermal fluids to depositional sites. Stable and radiogenic isotope tracers alone do not distinguish between fluid derivation through metamorphic devolatilization and magmatic fluid evolution. Some deposits formed at high paleocrustal levels, however, and record the influx of surface waters. Transport of large volumes of broadly uniform hydrothermal fluids over relatively long distances is implicated and requires channelized fluid flow with minor modification of major molecular components en route from source to depositional sites. Selected major deformation zones that are truly “crustal scale,” as demonstrated by deep seismic profiling, provide ideal fluid pathways for deeply sourced hydrothermal fluids. The largest gold provinces show spatial proximity of world-class lode gold deposits to “crustal-scale” deformation zones (e.g., Boulder-Lefroy, Destor-Porcupine). Linking of active faults is important for fluid focusing and effective transport of hydrothermal fluids. The hydrothermal fluids transport gold along the pathways as one or more neutral and reduced sulfide species. Chemical modeling demonstrates that the inferred hydrothermal fluids can effectively transport gold over long distances and over a significant crustal-depth range. Provided the fluids remain effectively channelized during transport to higher crustal levels, camp- and deposit-scale structural focusing and associated local gold precipitation mechanisms are required to ensure development of economic gold mineralization at the trap site. Archean orogenic lode gold mineralizing systems have distinctive depositional site characteristics at both the camp and deposit scale. Camp-scale features include geochemical signatures related to regional alteration surrounding major deformation zones, large-scale structural inhomogeneities (e.g., bends in major deformation zones, district-scale granitoid-greenstone contacts), and the presence or absence of overlying rock successions that can act as barriers (e.g., seals, aquicludes) to fluid movement. Deposit-scale variables include the local host rocks, structural traps (fault intersections, contacts between contrasting lithologies) typically in zones of low mean stress, and the P-T conditions in the host sequence. Resulting alteration assemblages primarily reflect interaction of host lithologies with the hydrothermal fluid at a particular pressure and temperature. Alteration assemblages generally show enrichment in K, CO 2 and S in deposits irrespective of paleocrustal level. A metal association of Au, Ag ± As, B, Bi, Sb, Te, and W is displayed by most deposits. Fluid-inclusion-derived data at individual deposits show a range in compositions, although salinity is generally low to moderate, with mixed aqueous-carbonic compositions. Variations in CO 2 , CH 4 , N 2 , salinity, and redox-state may reflect district- to local-scale processes and/or specific host rocks rather than differences in fluid source. Deposition at the trap site is likely to reflect catastrophic effects in response to physical changes (e.g., large pressure fluctuations, seismic events), with resultant chemical changes due to local fluid wall-rock interaction, phase separation, and/or fluid mixing. The extreme diversity of Archean orogenic lode gold deposits reflects the complex interplay of physical and chemical processes at a trap (depositional) site localized at various crustal levels ranging from sub-greenschist to upper-amphibolite facies metamorphic environments, with gold precipitation occurring over a correspondingly wide range of pressures and temperatures. The variability in deposit characteristics largely reflects the P-T conditions, variability in host rock, and local changes in ore fluid composition.

Abstract More than 150 Moz of gold has been added in production and resources from Proterozoic deposits in the last ten years, and many Proterozoic basins are now considered high priority exploration targets. The bulk of Proterozoic gold is produced from lode gold and Cu-Au (U-REE-Ba-F) deposits which are found in northern Australia, South Dakota, West Africa, Canada, South Africa, Scandinavia, and Central America. Proterozoic lode gold deposits are restricted to late collisional stages in the development of Proterozoic orogenic belts. They appear to have a systematic sequence of events in common and occur in linear belts associated with regional ductile structures at, or near, the greenschist facies brittle-ductile transition. Gold occurs in a large variety of rock types and has a close spatial association with regional-scale domes, anticlines, strike-slip shear zones, duplex thrusts, and in some deposits, geochemically distinct granites. Deposit styles can be subdivided into several types, directly related to the host structure and to contrasts in host-rock competency and mineralogy. These deposits have fluids and geochemical associations that overlap those of Archean lode gold deposits. Proterozoic Cu-Au-(Fe) deposits formed in a broader range of crustal and tectonic environments and display a great variety of structural and host-rock controls and styles. It is evident in all districts where the timing relationships are known that these deposits have spatial and temporal relationships to granites. These deposits display a range of fault and shear zone controls and are commonly associated with regions of geometric complexity, structural intersections, or regionally anomalous structural orientations. There is considerable evidence of variable fluid chemistry in Cu-Au-(Fe) deposits. Districts are commonly characterized by regional metasomatism and alteration at both regional and deposit scale which is commonly intense. Fe oxide-Cu-Au environments tend to produce similar alteration assemblages in all aluminous rock types. The influence of magmas as sources of fluid and ore components appears to have been greater in at least some Cu-Au-(Fe) systems and the associated granitoids are typically oxidized and include both mafic and felsic varieties. Sodic alteration styles are commonly prevalent regionally; the larger ore systems in particular are hosted specifically within substantial bodies of rock that are depleted in Na and enriched in K-Fe-(H).

Abstract Phanerozoic lode gold deposits are invariably associated with convergent plate margins and occur within close proximity to major translithospheric structures or compressional to transpressional-transtensional shear zones. The deposits are almost entirely structurally controlled and the nature of the immediate host rock does not generally play an integral part in ore formation. Nonetheless and unlike the majority of their Archean and Proterozoic analogues, Phanerozoic lode gold deposits are primarily hosted in several kilometer-thick sequences of marine sedimentary rocks which accumulated on pre-collision continental margins and/or in prograding arc-trench complexes. The sedimentary successions are commonly under lain by, and interspersed with, bimodal volcanogenic rocks which formed as a result of magmatic processes related to spreading, arc formation, plate collision, and subduction. The largest Phanerozoic lode gold systems are found in sub- to medium-grade greenschist metamorphosed terranes which have been caught up in the accretion of one or more allochthonous microplates and associated oceanic crust to an active continental margin. Mineralization in these collisional settings closely follows peak meta-morphism of the immediate host rocks and is temporally associated with exhumation of the orogen and addition of heat into the thickened crust via lithospheric delamination processes. Generation of CO 2 -rich aqueous ore-forming fluids involves metamorphic devolatilization of subcreted hydrated crust and the devel op ment of laterally and vertically extensive hydrothermal plumbing systems. Rich Phanerozoic lode gold deposits display a very close spatial and temporal relationship with syn- to post-tectonic felsic intrusive rocks but generally predate the emplacement of the granitoids. The deposits typically consist of quartz gold lodes in fault and shear systems at or above the brittle-ductile transition and form at P-T conditions of 1 to 3kbars and 250° to 400°C: they are characterized by relatively straightforward parageneses and a lack of pronounced vertical mineral or ore zonation. Episodic brittle reactivation in response to short-lived tectonic pulses is common and can result in remobilization of pre-existing mineralization and the formation of secondary lode systems. Alteration halos around Phanerozoic lode gold systems vary from a few centimeters to several tens of meters and reflect variations in the host-rock lithology and reactivity, permeability and porosity, orientation of bedding in metasedimentary rocks relative to auriferous veins, and fluid composition. On the deposit scale, lithogeochemical information obtained from wall-rock alteration assemblages represents by far the most valuable exploration tool. Broad bleached zones characterized by carbonate, sulfide, and sericite altera tion surrounding mineralized zones provide an exploration target of increased magnitude. Geochemical traverses generally indicate depletion of Na 2 O and increased values of CO 2 , H 2 O, K 2 O, S, As, Au, and possibly Sb, within five to several tens of meters from the auriferous lodes.

Abstract The source of the ore fluid and hence of the gold in orogenic lode gold deposits remains unresolved. The consensus that the ore fluid of orogenic deposits of all ages and settings is a low-salinity mixed aqueous-carbonic fluid, and hence different from that for most other gold deposit types, stands up on examination of available fluid inclusion data. A minority of deposits have only low salinity aqueous fluid inclusions. The composition and isotope chemistry of the fluid is uniform in many respects. For many components it appears that the fluid is in approximate equilibrium with the host-rock sequence, although not with the immediate wall rock. There is evidence that fluid compositions varied in space and time in the deposits with respect to a number of critical components. Assessment of mass balance of fluid-rock interaction in these hydrothermal systems shows that fluid compositions should not be expected to reflect the source after the fluid travel distances implied but rather to reflect fluid-rock interactions along the fluid pathway or mixed signatures of the source and the wall rocks. With this principle, it is seen that either a granitoid magmatic or a metamorphic devolatilization model for the fluid source is allowable given our present knowledge.

Abstract Carlin-type gold deposits are restricted to a small part of the North American Cordillera, in northern Nevada and northwest Utah, and formed over a short interval of time (42–30 Ma) in the mid-Tertiary when the Yellowstone mantle plume is inferred to have been located below the subduction zone. They formed after a change in plate motions (43 Ma) at, or soon after, the onset of extension in an east-west-trending, subduction-related magmatic belt. The deposits do not show consistent spatial relationships to mid-Tertiary magmatic centers, rather, most are located along long-lived, deep crustal structures inherited from Late Proterozoic rifting and formation of a passive margin. These structures influenced subsequent patterns of sedimentation and deformation and localized multiple episodes of igneous and hydrothermal activity, many of which contain anomalous concentrations of gold. The mid-Tertiary surface topography was relatively flat and many systems were located below large shallow lakes. Most deposits are hosted in a Paleozoic miogeoclinal carbonate sequence that is either structurally overlain by a eugeoclinal siliciclastic sequence, the Roberts Mountains allochthon emplaced in Early Mississippian time, or stratigraphically overlain by a miogeoclinal siliciclastic sequence deposited in the resulting foredeep. These siliciclastic sequences are less permeable than underlying carbonate rocks and apparently caused fluids ascending along major structures to flow laterally into permeable and reactive rocks below them. In these areas, gold ore is localized at intersections of a complex array of structures with permeable and reactive strata. The common alteration, mineralogy, and geochemical signature of these deposits is a direct expression of the P, T, and composition of ore fluids. The deposits generally formed at depths of >2 km at temperatures of 250° to 150°C, from moderately acidic (pH ≈5), reduced fluids containing <6 wt percent NaCl equiv, <4 mole percent CO 2 , <0.4 mole percent CH 4 , and >0.01 mole percent H 2 S. The H 2 S concentration was critical because it suppressed the solubility of Fe, base metals, and Ag as chloride complexes and enhanced the solubility of Au and associated trace elements (e.g., As, Sb, Tl, and Hg) as sulfide complexes. Gold was transported as AuHS° and/or Au(HS) 2 −1 complexes. The main ore stage formed during cooling and neutralization of ore fluids by reactions with the host rocks. It is characterized by carbonate dissolution, argillization of silicates, sulfidation of ferroan minerals, and silicification of limestone. Gold occurs as submicron inclusions or solid solution in arsenian pyrite and precipitated as H 2 S was consumed by sulfidation of Fe released from ferroan minerals. The other common trace elements (e.g., Sb, Tl, Hg) also reside in arsenian pyrite. The ideal host rock consists of permeable ferroan carbonate that is completely dissolved and its contained iron completely sulfidized such that all that remains is gold-bearing arsenian pyrite. Accordingly, large tonnage, low-grade gold deposits (e.g., Gold Quarry) are in siliceous rocks with low carbonate and reactive iron contents, and small tonnage, high-grade gold deposits (e.g., Meikle) are in carbonate rocks with high concentrations of reactive iron. Late ore-stage quartz, calcite, orpiment, realgar, stibnite, and barite occur in open fractures and pores and their abundance varies tremendously from deposit to deposit. These minerals precipitated as the systems cooled and ore fluids mixed with local ground water. Boiling was generally not important. Isotopic data from different districts yield conflicting indications as to the source of ore fluids. Abundant stable isotope data (δD, δ 18 O, δ 13 C, δ 34 S) and limited radiogenic isotope data (Pb, Sr, Os) from the major trends and districts are consistent with models involving the circulation of meteoric water through sedimentary rocks. In contrast, δD, δ 18 O, and δ 13 C data from the Getchell trend suggest that gold was introduced by a deep-sourced fluid that was of metamorphic or magmatic origin. The apparent lack of mid-Tertiary intrusions in this district argues for a metamorphic fluid, although the characteristics of certain portions and stages of the deposits suggest there was a magmatic fluid component characterized by higher Cl, Fl, K, Fe, and Cs contents. N 2 /Ar/He ratios of fluid inclusions suggest there were inputs of mantle He. Carlin-type deposits do not fit neatly into any one of the models proposed for them. Although variably evolved meteoric water is present in all of them, they are deeper than low-sulfidation epithermal veins and there is little or no evidence of boiling. They are shallower than orogenic veins and metamorphic fluids have only been detected in one district. Magmatic models call upon concealed intrusions that are so far removed from the deposits that no coeval contact metamorphic rocks, breccia pipes, or zoned geochemical halos are recognized at current levels of exposure or drilling. If the numerous similarities among Carlin-type deposits reflect the presence of a common ore fluid, then only one of the fluids detected by isotopic methods can be the ore fluid and the others must be due to contamination. In this case, we find the metamorphic fluid model most attractive, because both Carlin-type and orogenic gold deposits form in broad thermal anomalies, are distributed along major crustal structures, form during a change in stress regime, have similar ages over wide areas, have monotonous geochemical signatures, and contain similar endowments of gold. If we rely on the best data available from each district, a variety of models is needed and the only common factor is the geologic setting. These considerations suggest that Carlin-type deposits are unique, or too complex, to neatly fit into any one of these models.

Abstract This paper summarizes the characteristics of epithermal gold deposits and discusses potential ore deposition mechanisms. Epithermal deposits mostly form at shallow crustal levels (<1 km) in subaerial volcanic settings. There are two classes of epithermal deposits which can be discriminated in terms of their geologic environments, alteration mineralogy, and fluid chemistry: (1) low-sulfidation epithermal deposits which are spatially associated with magmas, where ore deposition generally occurs several kilometers above the site of intrusion, and quartz-adularia-sericite-carbonate-alteration assemblages are characteristic; and (2) high-sulfidation epithermal deposits which have a closer spatial association with degassing calc-alkaline magmas and are characterized by residual quartz and hypogene advanced argillic alteration assemblages (quartz-alunite-kaolinite-pyrophyllite). The waters that precipitate low-sulfidation epithermal mineralization have isotopic compositions that are consistent with a predominantly meteoric source of water, and a magmatic volatile source for sulfur and carbon. Alteration and vein mineral assemblages are consistent with near-neutral pH conditions. H 2 S (aq) is the predominant sulfur species in the mineralizing solutions. Temperatures of ore deposition are less than 300°C and salinities are low (<3.5 wt % NaCl equiv). High dissolved gas concentrations (CO 2 , H 2 S) can increase the depth of initial boiling. High-sulfidation epithermal mineralization precipitates from waters that are predominantly magmatic in origin, based on oxygen, hydrogen, and sulfur isotope compositions. Oxidizing, acidic waters form via the disproportionation of magmatic SO 2 (g) , which generates abundant sulfuric acid and minor H 2 S. Temperatures vary widely (>400°–100°C), but salinities are generally low (<5 wt % NaCl equiv). There is rare fluid inclusion evidence for magmatic brines in some deposits. There is substantial mineralogical, fluid inclusion, and stable isotope evidence for boiling in low-sulfidation epithermal systems, and it can be a highly effective process for electrum deposition. Fluid mixing is less likely to be important for ore formation. Mixing within the ore zone is generally restricted to late-stage collapse of the hydrothermal system, which allows descent of steam-heated waters into the mineralized environment and produces barren carbonate or sulfate gangue. Some high-sulfidation deposits contain unequivocal isotopic and fluid inclusion evidence for fluid mixing. However, it remains unclear whether mixing is responsible for ore deposition, gangue deposition, or enrichment of metals in ground waters. For any given high-sulfidation deposit, if acid chloride brines transport gold as a chloride complex, then dilution, cooling, and/or pH increase can cause gold to deposit, possibly during mixing with ground waters. By contrast, if gold is transported as a hydrosulfide complex in dilute acidic waters, then gold may deposit in response to boiling or to mixing-induced oxidation, but not to temperature, salinity, and pH changes.

Abstract The successful exploration geologist uses knowledge of geologic relationships and ore-deposit styles, tempered by experience, to interpret all information available from a given prospect in order to develop an understanding of its mineral potential. In the case of exploration for epithermal gold deposits, this understanding can be augmented by familiarity with active hydrothermal systems, their present-day analogues. Just as geological skills and exploration experience are the defining elements of a philosophy of exploration, the needs of a company determine, as much as the funding and skills available, which level of exploration it pursues and where: grassroots, early-stage or advanced targets. Epithermal gold deposits have size, geometry, and grade variations that can be broadly organized around some genetic classes and, therefore, influence the exploration approach or philosophy. Nearly 80 years ago, Waldemar Lindgren defined the epithermal environment as being shallow in depth, typically hosting deposits of Au, Ag, and base metals plus Hg, Sb, S, kaolinite, alunite, and silica. Even before this, Ransome recognized two distinct styles of such precious-metal deposits, leading to the conclusion that the two end-member deposits form in environments analogous to geothermal springs and volcanic fumaroles, which are dominated by reduced, neutral-pH versus oxidized, acidic fluids, respectively. The terms we use are low- and high-sulfidation to refer to deposits formed in these respective environments. The terms are based on the sulfidation state of the sulfide assemblage. End-member low-sulfidation deposits contain pyrite-pyrrhotite-arsenopyrite and high Fe sphalerite, in contrast to pyrite-enargite-luzonite-covellite typifying high-sulfidation deposits. A subset of the low-sulfidation style has an intermediate sulfidation-state assemblage of pyrite-tetrahedrite/tennantite-chalcopyrite and low Fe sphalerite. Intermediate sulfidation-state deposits are Ag and base metal-rich compared to the Au-rich end-member low-sulfidation deposits, most likely reflecting salinity variations. There are characteristic mineral textures and assemblages associated with epithermal deposits and, coupled with fluid inclusion data, they indicate that most low-sulfidation and high-sulfidation deposits form in a temperature range of about 160° to 270°C. This temperature interval corresponds to a depth below the paleowater table of about 50 to 700 m, respectively, given the common evidence for boiling within epithermal ore zones. Boiling is the process that most favors precipitation of bisulfide-complexed metals such as gold. This process and the concomitant rapid cooling also result in many related features, such as gangue-mineral deposition of quartz with a colloform texture, adularia and bladed calcite in low-sulfidation deposits, and the formation of steam-heated waters that create advanced argillic alteration blankets in both low-sulfidation and high-sulfidation deposits. Epithermal deposits are extremely variable in form, and much of this variability is caused by strong permeability differences in the near-surface environment, resulting from lithologic, structural, and hydrothermal controls. Low-sulfidation deposits typically vary from vein through stockwork to disseminated forms. Gold ore in low-sulfidation deposits is commonly associated with quartz and adularia, plus calcite or sericite, as the major gangue minerals. The alteration halos to the zone of ore, particularly in vein deposits, include a variety of temperature-sensitive clay minerals that can help to indicate locations of paleofluid flow. The areal extent of such clay alteration may be two orders of magnitude larger than the actual ore deposit. In contrast, a silicic core of leached, residual silica is the principal host of high-sulfidation ore. Outward from this commonly vuggy quartz core is a typically upward-flaring advanced argillic zone consisting of hypo gene quartz-alunite and kaolin minerals, in places with pyrophyllite, diaspore, or zunyite. The deposit form varies from disseminations or replacements to veins, stockworks, and hydrothermal breccia. During initial assessment of a prospect, the first goal is to determine if it is epithermal, and if so, its style, low-sulfidation or high-sulfidation. Other essential determinations are: (1) the origin of advanced argillic alteration, (i.e., hypogene, steam-heated, or supergene), (2) the origin of silicic alteration (e.g., residual silica or silicification), and (3) the likely controls on grade (i.e., the potential form of the orebody), because this is one of the most basic characteristics of any deposit. These determinations will define in part the questions to be asked, such as the relationship between alteration zoning and the potential ore zone, and will guide further exploration and eventual drilling, if warranted. Observations in the field must focus on the geologic setting and structural controls, alteration mineralogy and textures, geochemical anomalies, etc. Erosion and weathering must also be considered, the latter masking ore in places but potentially improving the ore quality through oxidation. As information is compiled, reconstruction of the topography and, hence, hydraulic gradient during hydrothermal activity, combined with identification of the zones of paleofluid flow, will help to identify ore targets. Geophysical data, when interpreted carefully in the appropriate geological and geochemical context, may provide valuable information to aid drilling by identifying, for example, resistive and/or chargeable areas. The potential for a variety of related deposits in epithermal districts has exploration implications. For example, there is clear evidence for a spatial, and in some cases genetic relationship between high-sulfidation epithermal deposits and underlying or adjacent porphyry deposits. Similarly, there is increasing recognition of the potential for economic intermediate sulfidation-state base metal ± Au-Ag veins adjacent to high-sulfidation deposits. By contrast, end-member low-sulfidation deposits appear to form in a geologic environment incompatible with porphyry or high-sulfidation deposits of any economic significance. The explanation for these empirical metallogenic relationships may be found in the characteristics of the magma (e.g., oxidation potential) and of the magmatic fluid genetically associated with the epithermal deposit. For effective exploration it is essential to maximize the time in the field of well-trained and experienced geologists using tried and tested methods. Understanding the characteristics of the deposit style being sought facilitates the construction of multiple working hypotheses for a given prospect, which leads to efficiently testing each model generated for the prospect, using the tools appropriate for the situation. Geologists who understand ore-forming processes and are creative thinkers, and who spend much of their time working in the field within a supportive corporate structure, will be best prepared to find the epithermal deposits that remain hidden.

Abstract Gold deposits associated with alkaline rocks include high-grade, gold-rich epithermal deposits, porphyry-type Cu(Au) and Mo(Au) deposits, and several other deposit types more speculatively linked to alkaline magmatism. These deposits can be large and high grade; several contain >1,000 tonnes Au. Alkaline rocks associated with gold deposits range from mafic-ultramafic lamprophyres to fractionated alkaline rhyolites, they have widely variable K/Na ratios, and they are found in a variety of tectonic settings, most notably in arc environments and in areas of extensional tectonics. Alkaline Cu (Au) deposits tend to form in volcanic arcs with thin or mafic crust, whereas Mo(Au) deposits are typically found in areas of thickened continental crust. Alkaline rocks are found to be as old as the Archean, but alkaline-related gold deposits are usually associated with shallow-level Phanerozoic alkaline magmatism, particularly within Cenozoic orogenic zones. In many cases deposits are found in clusters or in regions with recurring episodes of alkaline magmatism. Key characteristics of alkaline rocks associated with gold deposits are their hydrous and oxidized nature, as well as their ability to produce hydrothermal systems with ideal chemistries for transporting gold. Magmatic endowments may be variable but are not likely to exceed tens of parts per billion. In addition to their association with a distinctive group of igneous rocks, gold deposits related to alkaline magmatism are characterized by telluride-rich mineralization, extensive carbonation, and voluminous K metasomatism. Hydrothermal quartz is much less prominent in many alkaline systems than in most sub-alkaline systems and is absent in some high-temperature alkaline deposits. Likewise, hydrolytic (acid) alteration tends to be poorly developed in many alkaline systems. Where sericitic alteration is observed, it is commonly accompanied by carbonate minerals and a significant gain of K 2 O. These features and other geochemical data (such as isotopes and fluid inclusions) reflect formation from hydrothermal fluids of predominantly magmatic origin. Alkaline porphyry-style systems in which SiO 2 < 60 wt percent tend to develop deposits rich in Cu and platinum-group elements (PGE), whereas more felsic systems were enriched in Mo, and ultimately, F, Be, Hg, W, and Sn in the most evolved systems. Alkaline gold deposits also exhibit distinctive metal ratios and zonations. Both Cu(Au) and Mo(Au) porphyry-type deposits may grade upward or outward into telluride-rich epithermal deposits. Epithermal parts typically are base metal poor and have Au > Ag, whereas porphyry-style parts contain significant gold but have Ag > Au in a base metal-rich core. Both epithermal and porphyry-type deposits typically have low total sulfides. Exploration for these deposits is encouraged by their large sizes and high grades and because they are environmentally favorable to mine (low total sulfides and high acid-buffering potential). The most productive deposits show evidence for voluminous metasomatism and multiple magmatic and hydrothermal events, in addition to structurally focused zones of high-grade mineralization. Geophysical and geochemical signatures of these deposits are variable, but their characteristic styles of mineralization and alteration can be recognized in almost all examples, providing an effective exploration tool.

Abstract Gold-rich porphyry deposits worldwide conform well to a generalized descriptive model. This model incorporates six main facies of hydrothermal alteration and mineralization, which are zoned upward and outward with respect to composite porphyry stocks of cylindrical form atop much larger parent plutons. This intrusive environment and its overlying advanced argillic lithocap span roughly 4 km vertically, an interval over which profound changes in the style and mineralogy of gold and associated copper mineralization are observed. The model predicts a number of geologic attributes to be expected in association with superior gold-rich porphyry deposits. Most features of the descriptive model are adequately explained by a genetic model that has developed progressively over the last century. This model is dominated by the consequences of the release and focused ascent of metalliferous fluid resulting from crystallization of the parent pluton. Within the porphyry system, gold- and copper-bearing brine and acidic volatiles interact in a complex manner with the stock, its wall rocks, and ambient meteoric and connate fluids. Although several processes involved in the evolution of gold-rich porphyry deposits remain to be fully clarified, the fundamental issues have been resolved to the satisfaction of most investigators. Exploration for gold-rich porphyry deposits worldwide involves geologic, geochemical, and geophysical work but generally employs the descriptive model in an unsophisticated manner and the genetic model hardly at all. Discovery of gold-rich porphyry deposits during the last 30 yr has resulted mainly from basic geologic observations and conventional geochemical surveys, and has often resulted from programs designed to explore for other mineral deposit types. The tried-and-tested approach is thought likely to provide most new discoveries for the forseeable future, although more rigorous and innovative application of the descriptive and genetic models can only improve the chances of success.

Abstract Skarns containing Au are present worldwide and in a variety of geologic settings. Most are associated with plutons emplaced into shallow levels of the earth's crust. Common features of the 60 Au skarns included in this review are biotite hornfels, garnet-pyroxene alteration, clastic- and/or volcaniclastic-rich protoliths, and an Au-As-Bi-Te geochemical signature. Of the four major subdivisions of Au skarns, both reduced and oxidized Au skarns are related to shallow Phanerozoic plutons with depth estimates of ≤5 km, broadly similar to the general environment of porphyry-type deposits. Plutons associated with reduced Au skarns tend to be ilmenite-bearing mafic diorites and granodiorites, whereas plutons associated with oxidized Au skarns tend to be more silicic, alkalic, and magnetite bearing. The biotite ± K feldspar (potassic) alteration that surrounds most Au skarns is one of the characteristics of this deposit type. In most cases, the biotite ± K feldspar alteration forms in relatively fine grained, clastic host rocks, resulting in a hornfels texture surrounding the proximal relatively coarse grained garnet-pyroxene skarn. This proximal skarn zone typically is zoned from garnet dominant close to the pluton or fluid pathway to pyroxene dominant away from the pluton or fluid pathway. The relative proportion of garnet and pyroxene is a complex function of protolith composition, activity of components in the hydrothermal fluid, and overall oxidation state as influenced by magmatic sources, wall-rock composition, and mineral reactions. The Au-As-Bi-Te geochemical signature of Au skarns also can be related to geochemical variations involving temperature, f (O2) , and f (S2) . The fluids associated with most Au skarns are high-temperature brines of demonstrably magmatic origin. Fluid inclusion homogenization temperatures from skarn minerals are typically in the 400° to 600°C range and salinities are in the 30 to 70 wt percent NaCl equiv range, with locally important quantities of other salts including KCl, CaCl 2 , and MgCl 2 .

Abstract The role of intrusions in the formation of many types of gold deposits has been widely debated. Magmatic- hydrothermal processes are proposed by some authors for gold-rich porphyry systems, while many other authors claim that intrusions only serve as convective heat engines or structural hosts for mesothermal gold deposits. Based largely on well-documented examples from Alaska and the Yukon Territory, we recognize a class of deposits that display a spatial and temporal relation to reduced (low f O2 ) granitoids. Most intrusion-hosted and intrusion-proximal deposits and prospects of this class display a consistent and striking Au-Bi-Te-As (W,Mo,Sb) metal association; evidence for a series of alteration and mineralization events spanning a significant range of temperatures (>500°–<300°C); a consistent pattern of early feldspathic (albite and/or K feldspar) alteration and younger sericite-carbonate alteration; evidence for change in sulfidation state from well below pyrite-pyrrhotite (early) to pyrite-arsenopyrite (late); and the presence of high-CO 2 and/or high-salinity fluid inclusions. Fluid inclusion and other geobarometric data indicate formation over a wide range of pressures. Deposits that formed at pressures >1 kbar generally lack evidence for rapid magmatic water exsolution (e.g., porphyritic textures, random stockworks, magmatic breccias). Deposits horizontally and vertically distal from intrusions within a given belt or district typically lack the strong Au-Bi association of proximal deposits and contain higher As, Sb, and base metals and only yield evidence for relatively low-temperature and low-salinity fluids. These latter types possess some characteristics of orogenic (mesothermal), epithermal, or Carlin-type deposits; however, their spatial and temporal association with the higher-temperature deposits suggests a common origin. Although the clearest examples of these reduced intrusion-related deposits (e.g., Fort Knox, Alaska; Dublin Gulch, Yukon Territory) are only of moderate size (<150 tonnes Au), worldwide deposits of apparently similar character and origin host major amounts of gold. Significant global examples include Mokrsko in the Czech Republic, Kidston in Australia, and possibly the world-class Murantau deposit in Kazakstan. Belts hosting deposits of this type occur in a variety of continental arc, back-arc, and collisional settings, most of which contain intrusion-related Sn-W deposits. A key characteristic of this class of deposit is the occurrence of a wide variety of mineralization styles, depending on formation depth, distance from the parent intrusion, and structural control. It is likely that new variants of this type of deposit are yet to be found in many areas of the world, an assertion supported by the relatively recent discovery of the unusually high-grade Pogo deposit in interior Alaska.

Abstract Although generally considered a poor cousin of Au-rich deposits such as orogenic or epithermal deposits, a significant number of volcanic-hosted massive sulfide (VHMS) deposits are significant repositories of Au. Several of these deposits had original Au resources exceeding 8 Moz and in some recently discovered deposits Au, not base metals, is the primary economic metal. Although most Au in volcanic-hosted massive sulfide districts is hosted by massive sulfide lenses, recent discoveries, both on land and on the ocean floor, indicate that significant Au occurs outside of these lenses. In most deposits, Au has a metallogenic association with either Cu or Zn. When associated with Cu, Au is concentrated toward the base of the massive sulfide lens. Gold-rich deposits of this metallogenic assemblage commonly are associated with (metamorphosed) advanced argillic assemblages and are inferred to have formed from acidic, high-temperature (>300°C), oxidized fluids. These deposits have been equated to high-sulfidation epithermal deposits and may be detected using recently developed spectral techniques such as PIMA (Portable Infrared Mineral Analyzer) and airborne hyperspectral scanners. When associated with Zn, Au is concentrated near the top of massive sulfide lenses, in some cases in baritic zones. Gold-rich deposits of this metallogenic assemblage tend to be formed from low-temperature (200° ± 50°C) and/or near-neutral fluids as indicated by fluid inclusion studies or by alteration assemblages (e.g., K feldspar or carbonate). A small number of deposits cannot be classified into the Au-Zn or Au-Cu association. In these deposits, Au is concentrated in pyritic zones that contain relatively low amounts of base metals. Moreover, a consistent relationship with Zn or Cu is not present. Although this group is small, it includes deposits such as Horne. Mineralogically Au can occur in electrum or native gold, Au tellurides, or auriferous pyrite or arsenopyrite. In deposits of the Au-Cu association, Au tends to occur as native gold or tellurides, whereas electrum and auriferous pyrite and/or arsenopyrite is more common in the Au-Zn association. Metamorphic recrystallization tends to liberate Au held in auriferous pyrite or arsenopyrite, potentially enhancing metallurgical recoveries.

Abstract Newly recognized gold-rich sedimentary-exhalative (sedex) mineralization in Nevada, with an average gold grade of 14 g/tonne (t), and the occurrence of significant amounts of gold in classic sedex deposits like Rammelsberg, Germany (30 Mt at 1 g/t), Anvil, Canada (120 Mt at 0.7 g/t), and Triumph, Idaho (? at 2.2 g/t) demonstrate that basin brines can form gold ore. The sedex Au mineralization in Nevada represents a previously unrecognized end member in a spectrum of sedex deposits that also includes large Zn-Pb, intermediate Zn-Pb-Ba ± Au, and barite deposits. Study of ore deposits, modern brines, and chemical modeling indicates that variation in metal ratios and their abundance in sedex deposits are dominantly controlled by the concentration and redox state of sulfur in brines. For example, Au and Ba solubilities are highest in H 2 S-rich, SO 4 -poor fluids, whereas base metal solubilities are highest when H 2 S is not present. Chemical modeling indicates a typical reduced brine (15 wt % NaCl equiv, pH = 5.5, H 2 S = 0.01 m) at 200°C is capable of transporting as much as 1 ppm Au in solution. The H 2 S content in brines is controlled by the rate of its production through thermochemical reduction of sulfate by organic matter and the rate of its removal from the fluid through the sulfidation of reactive Fe in the sediments. Thus, sedimentary basins with high organic carbon and sulfate in rocks low in reactive Fe, such as carbonates and shales, are most likely to produce H 2 S-rich brines that may form gold-rich sedex deposits. Because of the tremendous scale of sedex hydrothermal systems, evidence that basin fluids can transport gold identifies a new mechanism for concentrating gold in sedimentary basins and opens extensive areas to further gold exploration.

Abstract The Witwatersrand gold fields of South Africa account for more than a third of the world's total gold production since mining started there in 1886: 48,000 t Au. These gold fields dominated production throughout the twentieth century, but production peaked at 1,000 t in 1970 and has steadily declined since that time. The regional geologic framework is well understood as a consequence of intensive study and virtually unrivalled three-dimensional information derived from drilling and underground openings, supplemented by seismic data. Advancements of the last decade have come from the integration of stratigraphy, sedimentology, structural evolution, and a much greater appreciation of thermal and fluid processes. Gold has been produced from seven major gold fields located around the northern and western margin of the 350-km-long Witwatersrand basin. Each gold field consists of one major and commonly several minor reef horizons that have been mined semicontinuously for up to 400 km2. Mineralized zones range from 1 cm to several meters in thickness, and the host rock varies from quartz pebble conglomerate and carbon seam to polymictic conglomerate and pyritic quartzite. All reefs are either on or within a few meters above unconformity surfaces, and the major reefs are part of a very distinctive reef package of footwall, unconformity, conglomerate, quartz-rich sandstone and/or shale. The actual distribution of gold has been poorly represented in the literature owing to grade coding, inappropriate averaging, and omission of important features that are critical to understanding the controls on the gold. High grades and remarkable lateral continuity have nevertheless facilitated economic exploitation. The mineralogy of the Witwatersrand reefs is dominated by pyrite with lesser pyrrhotite and arsenopyrite, widespread nickel and cobalt sulfarsenides, and low base metal sulfides. A distinctive mineral assemblage of pyrophyllite-chloritoid-muscovite-chlorite-quartz-rutile-pyrite is found in and around the reef package in all gold fields. This assemblage has been used to constrain peak metamorphic temperatures to just above 300°C to the south and east of the basin, and closer to 400°C in the northwest corner west of the basin near Johannesburg. Two characteristics of the metamorphic event are the nearly strata-parallel distribution of metamorphic grade and an extremely high geothermal gradient. The presence of pyrophyllite and chloritoid in conglomerate, quartzite, shale, basalt flows, and some dikes has been used to indicate an alteration halo embracing the gold fields and approximately 300 km by 50 km (into the basin) by 3 km (of sequence). Quartz veins of up to a few centimeters thickness are common within this alteration halo especially in the reef horizons, but veins of meters thickness are rare. Ubiquitous pyrite throughout the alteration zone indicates elevated levels of sulfur in solution. The elements most closely associated with gold are Fe and C; uranium is also associated with many reefs and extensively mined as a coproduct. A consensus appears to be developing in support of a paragenetic sequence of uranium, then hydrocarbons, then gold. The origin of the Witwatersrand has been of great interest to economic geologists and a source of great contention. Historically, placer and hydrothermal theories have competed, and much more regional and mine-scale data have been used in the debate during the last decade. In the 1980s and especially the 1990s, models shifted from the unmodified to the modified placer model, and from both placer models to a hydrothermal model. Challenges still facing the placer models include regional alteration, structural control on gold, the chemical process of gold remobilization, and the poor match with modern gold placer processes. Moreover, the outstanding issues remain a source area for the gold and a convincing demonstration that processes during sedimentation concentrated gold at all. The hydrothermal replacement model invokes transport of gold into the Witwatersrand Supergroup during metamorphism and associated widespread alteration by a reduced, low-salinity fluid, analogous to other gold-only deposits. Fluids were channeled by structures, unconformity surfaces, and bedding, and gold precipitation was dominated by reaction with carbon- or iron-bearing rocks. These Fe- and C-rich rocks are concentrated immediately above unconformity surfaces. This model proposes that crustal rocks (probably mafic) beneath the Witwatersrand Supergroup are the source of gold. The hydrothermal model links the tectonic evolution of the basin to mineralizing processes, and can thus be used to target other basins with potential for similar mineralization. From the hydrothermal model arise a number of factors that contribute to the enormous size of the Witwatersrand gold fields: The age of the Witwatersrand basin, which predates a major period of gold introduction, The retroarc foreland basin, Major thrust structures around the basin margin, Unconformities in the upper Witwatersrand, Iron-rich nodules on unconformity surfaces, Carbon distribution, High geothermal gradient during gold deposition, Metamorphism and regional alteration, and Limited erosion. Special source regions for detritus and special sorting processes during sedimentation do not appear to have been critical. The Witwatersrand gold formed by the optimization of a set of processes found in other gold provinces, not by unique processes unrepresented elsewhere.

Abstract There are six distinct classes of gold deposits, each represented by metallogenic provinces having hundreds to more than 1,000 tonnes (t) gold production. These deposit classes are as follows: (1) orogenic gold; (2) Carlin and Carlin-like gold deposits; (3) epithermal gold-silver deposits; (4) copper-gold porphyry deposits; (5) iron oxide copper-gold deposits; and (6) gold-rich volcanic-hosted massive sulfide to sedimentary-exhalative (sedex) deposits. This classification is based on ore and alteration mineral assemblages, ore and alteration metal budgets, ore fluid pressure(s) and compositions, crustal depth or depth ranges of formation, relationship to structures and/or magmatic intrusions at a variety of scales, and relationship to the P-T-t evolution of the host terrane. The classes reflect distinct geodynamic settings. Orogenic gold deposits are generated at midcrustal (4–16 km) levels proximal to terrane boundaries, in transpressional subduction-accretion complexes of cordilleran-style orogenic belts; other orogenic gold provinces form inboard by delamination of mantle lithosphere or by plume impingement. Carlin and Carlin-like gold deposits develop at shallow crustal levels (<4 km) in extensional convergent margin continental arcs or back arcs; some provinces may involve asthenosphere plume impingement on the base of the lithosphere. Epithermal gold and copper-gold porphyry deposits are sited at shallow crustal levels in continental margin or intraoceanic arcs. Iron oxide copper-gold deposits form at middle to shallow crustal levels; they are associated with extensional intracratonic anorogenic magmatism. Proterozoic examples are sited at the transition from thick refractory Archean mantle lithosphere to thinner Proterozoic mantle lithosphere. Gold-rich volcanic-hosted massive sulfide deposits are hydrothermal accumulations on or near the sea floor in continental or intraoceanic back arcs. The compressional tectonics of orogenic gold deposits are generated by terrane accretion; high heat flow stems from crustal thickening, delamination of overthickened mantle lithosphere inducing advection of hot asthenosphere, or asthenosphere plume impingement. Ore fluids advect at lithostatic pressures. The extensional settings of Carlin, epithermal, and copper-gold porphyry deposits result from slab rollback driven by negative buoyancy of the subducting plate, and associated induced convection in asthenosphere below the overriding lithospheric plate. Extension thins the lithosphere, advecting asthenosphere heat; promotes advection of mantle lithosphere and crustal magmas to shallow crustal levels; and enhances hydraulic conductivity. Siting of some copper-gold porphyry deposits is controlled by arc-parallel or orthogonal structures that in turn reflect deflections or windows in the slab. Ore fluids in Carlin and epithermal deposits were at near-hydrostatic pressures, with unconstrained magmatic fluid input, whereas ore fluids generating porphyry copper-gold deposits were initially magmatic and lithostatic, evolving to hydrostatic pressures. Fertilization of previously depleted subarc mantle lithosphere by fluids or melts from the subducting plate, or incompatible element-enriched asthenosphere plumes, is likely a factor in generation of these gold deposits. Iron oxide copper-gold deposits involve prior fertilization of Archean mantle lithosphere by incompatible element enriched asthenospheric plume liquids, and subsequent intracontinental anorogenic magmatism driven by decompressional extension from far-field plate forces. Halogen-rich mantle lithosphere and crustal magmas form, and likely are the causative intrusions for the deposits, with a deep crustal proximal to shallow crustal distal association. Gold-rich volcanic-hosted massive sulfide deposits develop in extensional geodynamic settings, where thinned lithosphere extension drives high heat flow and enhanced hydraulic conductivity, as for epithermal deposits. Ore fluids induced hydrostatic convection of modified seawater, with unconstrained magmatic input. Some gold-rich volcanic-hosted massive sulfide deposits with an epithermal metal budget may be submarine counterparts of terrestrial epithermal gold deposits. Real-time analogues for all of these gold deposit classes are known in the geodynamic settings described, excepting iron oxide copper-gold deposits.

Abstract The academic economic geology research community contains an abundance of bright and motivated individuals. Yet many economic geology researchers are finding it increasingly difficult to maintain funding due to substantial cutbacks in government and industry support. These falling levels of support for economic geology research have been attributed to government policies overly focused on economic rationalism, and to the steady, albeit cyclical, decline of commodity prices over time. However, few consider an equally important reason: that the relevance of academic economic geology research to industry has, on average, steadily declined over the past 10 to 15 years. The gap between economic geology research outputs and exploration industry goals is evidenced by the proliferation of consulting agencies that have filled this niche. This paper explores the premise that there is now a fundamental misalignment between the goals of academic economic geology research and the goals of the mineral exploration industry, and how the two may be brought back into alignment. In essence, the industry is seeking better ways to predict or detect the spatial location of mineralization at all scales, in the most cost-effective manner. In order to attract industry funding, academic research should be focused on adding value to this prediction (or detection) process. In contrast, recent research is pervaded by studies that appear to be model-pushing rather than model-building, are driven by single subdisciplines at the expense of multidisciplinary syntheses, lack effec tive communication of results, and often have a misperception of the relevance of their results to the exploration industry. It is proposed that this misalignment of academic and industry goals has arisen due to (1) a lack of understanding of the exploration industry by economic geology researchers, and (2) the structure of the academic environment, including a lack of accountability for research funding, or accountability to inappropriate criteria, and fragmentation of research programs. In order to maintain funding levels into the future, successful economic geology researchers will become more value focused, and will concentrate on identifying and addressing industry needs. Better communication and team building with industry partners will be required. Secondments of industry people to help direct research groups should become commonplace and these assignments should be viewed as prestigious positions. Future economic geology research should involve the establishment of multidisciplinary groups concentrating on holistic models of ore-forming processes and should have a strong applications-development focus.

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.