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NARROW
Abstract The Tethyan orogenic belt stretches from the Alps, through the Carpathians and Balkans, Taurides and Caucasus, Zagros, Makran, and Himalayas, to Indochina and into the southwest Pacific Ocean. It represents a complete Wilson Cycle, from opening and closure of the Paleotethys Ocean in the mid-Paleozoic to the Late Triassic, opening of the Neotethys Ocean in the Permian-Early Triassic, and its progressive closure throughout the late Mesozoic and Cenozoic eras. The current state of the orogen includes all stages of convergence from active subduction beneath the Makran and eastern Mediterranean, through advanced continental collision in the Caucasus/Taurides and Zagros, to syn- to postcollisional readjustment in the Carpathians, Balkans, Himalayas, and Indochina (Richards, 2015). The region has been the focus of significant recent attention from geologists interested both in its tectonic evolution and metallogeny, made possible by increased accessibility to many of the geographic sections of the orogen. Key breakthroughs in understanding its tectonic history have come through improved geochronological techniques and expansion of the database of samples and events dated, combined with more accurate paleogeographic and tectonic models. In parallel, an improved understanding of the subtle relationships between tectonomagmatic and metallogenic processes have refined interpretations that were once based on simplistic assumptions (e.g., that porphyry deposits only form above active subduction zones). Indeed, economic geologists have been among the key drivers of these advances by demanding more accurate and predictive tectonomagmatic models for ore formation that can reliably inform mineral exploration. Consequently, the Tethyan orogen is now understood to be the best preserved global example of a collisional orogen, where all stages of convergence can be observed in real or recent geological time, and the detailed relationships to ore formation, commonly reflecting tectonic changes measured on submillion-year timescales, can be accurately documented and modeled. In this volume, we present a selection of papers that showcase this advancement in knowledge, with examples from Eastern Europe to South Asia.Beginning in the Balkans, Knaak et al. (2016) describe the variety of mineral deposits that occur in the emergent worldclass Timok region of eastern Serbia. The origin of the Late Cretaceous Timok Magmatic Complex remains debated, but the authors propose that arc magmatism was focused by dextral transtensional structures, followed by complex structural rearrangement in the Cenozoic. Porphyry Cu-Au deposits, polymetallic replacement deposits, and sedimentary rockhosted Au deposits occur in close spatial, and possibly genetic, relationship to the Late Cretaceous arc rocks. A key contribution of this study is the detailed reconstruction of later Cenozoic fault movements that led to structural dislocation and oroclinal bending, complicating geologic and metallogenic correlations in the region.
Abstract Earth’s near-surface mineralogy has diversified over more than 4.5 b.y. from no more than a dozen preplanetary refractory mineral species (what have been referred to as “ur-minerals” by Hazen et al., 2008) to ~5,000 species (based on the list of minerals approved by the International Mineralogical Association; http://rruff.info/ima). This dramatic diversification is a consequence of three principal physical, chemical, and biological processes: (1) element selection and concentration (primarily through planetary differentiation and fluidrock interactions); (2) an expanded range of mineral-forming environments (including temperature, pressure, redox, and activities of volatile species); and (3) the influence of the biosphere. Earth’s history can be divided into three eras and ten stages of “mineral evolution” (Table 1; Hazen et al., 2008), each of which has seen significant changes in the planet’s near-surface mineralogy, including increases in the number of mineral species; shifts in the distribution of those species; systematic changes in major, minor, and trace element and isotopic compositions of minerals; and the appearance of new mineral grain sizes, textures, and/or morphologies. Initial treatments of mineral evolution, first in Russia (e.g., Zhabin, 1979; Yushkin, 1982) and subsequently in greater detail by our group (Hazen et al., 2008, 2009, 2011, 2013a, b; Hazen and Ferry, 2010; Hazen, 2013), focused on key events in Earth history. The 10 stages we suggested are Earth’s accretion and differentiation (stages 1, 2, and 3), petrologic innovations (e.g., the stage 4 initiation of granite magmatism), modes of tectonism (stage 5 and the commencement of plate tectonics), biological transitions (origins of life, oxygenic photosynthesis, and the terrestrial biosphere in stages 6, 7, and 10, respectively), and associated environmental changes in oceans and atmosphere (stage 8 “intermediate ocean” and stage 9 “snowball/hothouse Earth” episodes). These 10 stages of mineral evolution provide a useful conceptual framework for considering Earth’s changing mineralogy through time, and episodes of metallization are often associated with specific stages of mineral evolution (Table 1). For example, the formation of complex pegmatites with Be, Li, Cs, and Sn mineralization could not have occurred prior to stage 4 granitization. Similarly, the appearance of large-scale volcanogenic sulfide deposits may postdate the initiation of modern-style subduction (stage 5). The origins and evolution of life also played central roles; for example, redox-mediated ore deposits of elements such as U, Mo, and Cu occurred only after the Great Oxidation Event (stage 7), and major Hg deposition is associated with the rise of the terrestrial biosphere (stage 10; Hazen et al., 2012).
Abstract The northern Pacific Rim—for the purposes of this contribution—comprises the Mesozoic and Cenozoic magmatic-arc and associated terranes of eastern China, Korea, Japan, the Russian Far East, Alaska, Yukon, British Columbia, the western United States, and Mexico. This ~1,800-km-long segment of the Pacific Rim is marked by a broad spectrum of metallogenic environments and mining jurisdictions, which combine to dictate where and how exploration is conducted and the overriding character of any resulting discoveries. This summary report commences with a brief metallogenic overview of the northern Pacific Rim, with particular attention paid to the world-class Mesozoic and Cenozoic ore deposits that define the region’s premier metallogenic provinces. This is followed by a summary of the relative attractiveness of the region’s various mining jurisdictions, as recorded by recent exploration activity. The major discoveries made along the northern Pacific Rim, particularly during the past half century, are then placed in this metallogenic and regulatory context as a basis for determining the successful exploration methodologies employed. This discovery track record is then used to predict what the future of exploration in this vast and varied region may hold. Much of the northern Pacific Rim, from eastern China and the Russian Far East in the northwest through Alaska to western parts of Canada, the United States, and Mexico in the southeast (Fig. 1), is characterized by a complex array of oceanic, accretionary prism, magmatic arc, and back-arc basin terranes and associated microcontinental blocks accreted to the North China, Siberian, Hyperborean, and North American cratons, mainly during Mesozoic times (Coney et al., 1980; Campa and Coney, 1983; Kojima, 1989; Nokleberg et al., 2005; Yakubchuk, 2009). The metallogeny of these tectonic collages is dictated by various combinations of pre-, syn-, and postaccretion ore-forming events, the last of which are generally preeminent, except in British Columbia (Nokleberg et al., 2005; Nelson and Colpron, 2007). Although the Meso-Cenozoic metallogeny of the northwestern and northeastern Pacific quadrants displays some similarities, it is the contrasts that are most marked. The main contrasts stem from the preeminence of tin, tungsten, and antimony in eastern China, Korea, Japan, and the Russian Far East and of copper and silver in Western Canada, the conterminous United States, and Mexico. Nonetheless, both the northwestern and northeastern Pacific quadrants are exceptionally well endowed with gold and molybdenum deposits. The northeasternmost Russian Far East, Alaska, and Yukon Territory display elements of both northwestern and northeastern Pacific metallogeny (Fig. 1). These metallogenic contrasts between the northwestern and northeastern quadrants result in China being the world’s leading producer of tungsten, tin, bismuth, and antimony, mostly from its eastern Mesozoic metallogenic province.
Abstract It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate. A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved. Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained. It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.
Abstract There seems to be general consensus throughout much of the global mining industry that the supply of base and precious metals and some other commodities (e.g., ferrous metals, uranium) is reasonably well assured into the oreseeable future because increases in total resources continue to keep pace with or outstrip global consumption. The basic assumption is that market forces and technological advances will combine to promote and perpetuate this trend (e.g., Tilton, 2003; Crowson, 2008). Others disagree, however, andpredict that shortages are inevitable if metal consumption continues to escalate (Beaty, 2010). It is already becoming clear that many known resources seem unlikely to be mined, irrespective of commodity prices, because of their low grade and/or quality. Hence, many mineral resources that were uneconomic in the early 2000s are likely to remain so, both today and into the foreseeable future because of increases in both the direct (e.g., energy, labor) and indirect (e.g., environmental, social) production costs. This situation is being further exacerbated by the perceived decrease, over at least the past decade, in the discovery rate of base and precious metal resources measured in terms of both the number of major discoveries made and the exploration dollars spent per discovery (e.g., Dummett, 2000; Horn, 2002; Schodde, 2004). There is also a suggestion that the discoveries made are, on average, becoming both smaller and lower grade. Therefore, it seems reasonable to ask whether current exploration practices and success rates are going to be adequate to provide for the massive increases in metal consumption that world population growth, rising living standards, and rapid industrialization and urbanization in China, India, and other emerging markets appear to portend. For example, Rio Tinto's projections suggest that "by 2030 the additional supplyrequired will be equivalent to replicating the iron ore output of the Pilbara region of Australia every five years, adding another aluminium production complex the size of Canada's Saguenay every nine months, and developing another copper mine the size of Escondida in Chile each year. Future energrequirements are such that an entire Hunter Valley coal supply chain needs to be created each year plus a uranium mine the size of Ranger every four years" (Albanese, 2010, p. 7). Clearly, the exploration business has to become increasingly effective if it is to rise to the challenge of finding mineral resources of the right caliber to assure that this burgeoning demand can be adequately satisfied.
Abstract V OLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.
Abstract At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km 2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years. Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting. As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.
Nickel Deposits of the Yilgarn Craton: Geology, Geochemistry, and Geophysics Applied to Exploration
Abstract The Yilgarn craton is one of the world's major nickel provinces, containing 31.5 million tonnes (Mt) of Nimetal with an in situ value of about $350 billion on a pre-mining basis, amounting to 13.6 percent of the world'scurrently known Ni resources. This entire resource inventory has been discovered since 1966. This chapterpresents an analysis of the 40-year discovery history, which isideal as a province-scale case study in mineralexploration. The province experienced a major peak in exploration activity between 1966 and 1971, the "nickel boom," which accounted for more than half of all NiS deposits, and all of the giant (>1 Mt Ni) NiS deposits so far discovered. Almost 70 percent of those discoveries were related to direct surface prospecting methods, commonly based on recognition of magnetic ultramafic rocks as favorable hosts. Since the end of the nickel boom, the dominant discovery method has been follow-up exploration around significant known mineralization. From about the mid-1990s, electromagnetic (EM) surveying, which had been considered ineffective during the nickel boom phase, became a demonstrably successful technique for the detection of sulfide deposits. An improved understanding of geologic processes and controls has played an important role in sustaining exploration success since the end of the early, surface-prospecting phase of the nickel boom. Most nickel laterite deposits were first found during the nickel boom but not considered at that time to be economically significant. A large surge in exploration activity, much of it focused on resource delineation rather than true green-fields exploration, occurred between 1996 and 2001, triggered by the advent of the Pressure Acid Leach (PAL) technology in the mid-1990s. The discovery record of the Yilgarn province exhibits many patterns typical of an exploration province: early discovery of both the largest deposits and most of the metal and generally increasing discovery costs as the province matures. The average discovery costs for nickel in the Yilgarn have been 5.2 c/lb for sulfide nickel and 0.6 c/lb for laterite nickel. The Yilgarn province offers two examples of exploration expenditure booms arising from the coincidence of an upturn in commodity price with the opening up of new exploration parameter spaces. These are the initial discovery of komatiite-hosted nickel sulfide at Kambalda in 1966, and the mid-1990s recognition of the potential of PAL technology to treat laterite ore deposits.
Abstract A variety of metals and deposit types define the metallogeny of the Andes from Colombia through Ecuador, Peru, and Bolivia to Argentina and Chile, although porphyry copper and epithermal gold deposits undoubtedly predominate and will continue to do so. Discoveries over the last 30 yrs or so, predominantly in the central Andes and especially Chile, have been made using routine, field-based geologic and complementary geochemical methods, a situation that is considered unlikely to change radically in the foreseeable future. The only clearcut evolutionary change is the increased number of deposits being discovered beneath pre- and postmineral cover. The predictive capacity of conceptual geology has had minimal impact on the Andean discovery record but is thought to offer much promise for the future. This introductory article selects mineralization styles and relationships as well as some broader metallogenic parameters as simple examples of geologic concepts that may assist exploration. Emphasis is placed on porphyry copper ± molybdenum ± gold and high-, intermediate-, and lowsulfidation epithermal gold ± silver deposits, although reference is also made to several carbonate rock-hosted precious and base metal deposit types and styles as well as subvolcanic tin, volcanogenic massive sulfide, and slate-belt and intrusion-related gold deposits. Particular emphasis is placed on the potential for exceptionally high grade porphyry copper, porphyry gold, epithermal gold, and subvolcanic tin deposits. Deposits resulting from the oxidation, enrichment, and chemical transport of copper and zinc and mechanical transport of gold and silver during supergene weathering are also briefly highlighted. Si bien la metalogenia de los Andes de Colombia, Ecuador, Perú, Bolivia y Chile se encuentra definida por una gama de metales y estilos de mineralización, son los depósitos tipo pórfido de cobre y epitermal de oro los que dominan en el presente y continuarán prevaleciendo en el futuro. Los descubrimientos de los últimos 30 años, predominantemente en los Andes centrales y especialmente en Chile, han sido realizados mediante métodos geológicos rutinarios de campo, generalmente complementados satisfactoriamente por métodos geoquímicos. Se estima que esta situación difícilmente experimentará variaciones radicales en un futuro cercano. El único cambio destacable en esta historia evolutiva está dado por el aumento apreciable de descubrimientos de depósitos cubiertos, bajo cobertura pre o postmineral. A nivel andino, la capacidad predictiva de la geología conceptual ha tenido un impacto mínimo en el número total de descubrimientos, aunque se piensa que su uso debiera garantizar buenas perspectivas futuras. El presente artículo
Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth
Abstract To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic. The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.
Abstract Global political and economic developments shape both the demand for minerals and primary metals and their supply. Overall, demand has moved broadly in step with economic activity over the past 30 years. Notwithstanding the collapse of the Soviet Union and Eastern Bloc countries, demand grew more rapidly in the second half of the period than the first. The performance of individual products within this general trend largely reflects the specific nature of their main end uses. The geographic center of demand has shifted away from the mature industrial economies of North America, Western Europe, and Japan toward the newly industrializing countries of the Pacific Rim, China, and India. Mine production rose with demand, but not always in precise step. New capacity was required not just to meet demand, even where that was static, but also to offset the continuing effects of ore depletion. There were also changes in the location of production in response to geopolitical forces, the depletion of ore reserves, and the changing economics of extraction and processing. The number of mines contracted, especially during the 1990s, and the scale of mining operations was increased in order to achieve the requisite cost savings. Prices fluctuated in response to changing balances between supply and demand, trending downward from the early 1970s until the early 2000s. Most products witnessed at least one sharp price spike during the period, usually with continuing repercussions. Prices picked up from 2003, but generally not back to their earlier peak in real terms. Profitability varied according to the products concerned. In many years the average rates of return on capital employed have been insufficient to cover the risks involved.
Abstract The dominant forces that affect the mining industry in today's economy are globalization, entrepreneurship, and unprecedented minerals availability. The current malaise and diminished economic importance of the base and precious metal mining industry in the world economy are the result of ongoing, longterm trends. Shifting market forces within a changed geopolitical landscape have resulted in excess supplies and reduced demands for high-unit-value mineral products and for geologists and engineers in the exploration and mining industries. Although these changes are still evolving, professionals and the mining companies that employ them must realize that these changes are irreversible. Mining has five dominant characteristics: (1) it is essential to society, (2) it is effective in keeping society supplied with abundant, low-cost mineral products, (3) historically, it has been wealth destructive for investors, (4) it has been subsidized, and (5) it is shaped by social and political forces. What is different now is that instead of being subsidized, the industry is being handicapped by governments and abandoned by investors. The industry has reacted to a marketing problem with an inappropriate strategy of cost reduction and increased production. The increased supply has resulted in decreased prices. Mining companies are earning low or negative rates of return. Remedies for the larger mining companies and the high-unit-value metals businesses include: Exemplary behavior to regain the trust of the public, governments and investors; Continued consolidation to become sector leaders; Profit through value-added vertical integration and direct marketing to consumers Development of innovative consumer-based financing mechanisms, especially for exploration ventures Individuals and small groups that have been displaced from the mainstream must become entrepreneurs. They must engineer their own survival by shifting careers or by finding, developing, and capitalizing on exploration and production opportunities that are unrecognized by, or are too small for, the major companies. The mining industry is here to stay. It is too necessary to society to be abandoned; however, the future size and shape of the industry will probably be unrecognizable to most of us. It will be leaner and more opportunistic. It will be characterized by entrepreneurial corporations of all sizes that dominate niches and sectors and that anticipate and profit from the changing needs of society
Abstract Fluid inclusion and metal-ratio data have been compiled for 52 low-sulfidation precious metal and base metal-rich low-sulfidation epithermal deposits in Mexico. Precious metal deposits typically have inclusion salinities that range from 0 to 7.5 wt percent NaCl equiv, whereas base metal-rich deposits have salinities that are as high as 23 wt percent NaCl equiv. Salinities are typically high in fluids included in sphalerite, suggesting a genetic relationship between base metal mineralization and saline fluids. Silver/gold and Ag + Au/Pb + Zn + Cu ratios correlate with fluid inclusion salinity, a relationship that underscores the importance of chloride complexing in base metal transport and polymetallic mineralization. Fluid inclusion gas chemistry of 21 low-sulfidation epithermal deposits plotted on N 2 -Ar-He and N 2 -Ar-CH 4 diagrams indicate that meteoric, mantle or evolved crustal, and magmatic fluids were present in the ore-forming hydrothermal systems, although in different proportions in individual deposits. The N 2 /Ar ratios of sulfide mineral fluid inclusions are all higher than that of air-saturated water, indicating a mag-matic source, whereas a significant proportion of inclusions in barren gangue minerals have N 2 /Ar ratios near that of air-saturated water. Plots of N 2 /Ar vs. H 2 S/Ar show a correlation between N 2 and H 2 S concentrations. The data suggest that low-sulfidation epithermal deposits in Mexico comprise both meteoric waters and magmatic waters, with a significant contribution of H 2 S of magmatic origin. New oxygen and hydrogen isotope data are presented for seven deposits. Fluids responsible for precious metal and base metal deposition contain consistently heavy oxygen isotope signatures and shifts as high as +10 to +20 per mil from the meteoric water line, regardless of host rock type. Boiling and/or water-rock interaction processes alone cannot explain adequately the consistently heavy oxygen isotope signatures of Mexican low-sulfidation deposits. Rather, these results are best accounted for by a significant contribution of magmatic waters to the deep fluid, subsequently modified by water-rock interaction, boiling, and mixing with meteoric water. A classification of low-sulfidation deposits of Mexico is presented based on depth of formation and whether or not boiling is thought to have occurred in the system. Three end-member types are recognized: shallow with boiling, deep with boiling, and deep without boiling. In shallow-formed deposits boiling fluids rise to depths of <500 m below the paleowater table, and ore occupies a vertical range of a few hundred meters. In deep-formed deposits, boiling occurs at temperatures that may exceed 300°C, and ore is generally deposited between 400 and 1,000 m from the paleowater table as fluids rise within the area of liquid-vapor immiscibility.
Abstract Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages: 1. Normal subduction extended into Eocene time. 2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera. 3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle. 4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab. 5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner