Ore Deposits

Ore deposits
Michael A. Mckibben Department of Earth Sciences, University of California, Riverside

Introduction
Metallic ore deposits constitute the largest geochemical anomalies within the crust. Their study has been critical to understanding the behavior of elements and isotopes in mineral- and rock-forming processes, as well as to deciphering the geochemical differentiation of the Earth through time. The study of ore deposits therefore influences and draws upon virtually every subdiscipline in the Earth Sciences. The four-year research review presented here cannot be comprehensive, given the editor's limit of citing less than 100 research papers by U.S. authors or authors from U.S. institutions. Instead, the intent is to provide the reader with synopses of a representative spectrum of papers containing important advances in U.S. research on ore deposits. In cases where difficult citation choices had to be made the more recent papers on a particular subtopic are usually cited, because they lead the reader back to earlier papers within the 4-year review period. Judicious use of the selected citations in conjunction with standard literature searching tools should allow any reader to quickly find most of the relevant literature on each subtopic. The most recent quadrennial report on ore deposits was made by Burt [1991] for the period from late 1986 to mid-1990, so the present review cites only publications appearing between mid-1990 and mid-1994. Citations are made only to peer-reviewed publications appearing in major journals, periodicals and books. Meetings abstracts, conference and symposia proceedings, field trip guidebooks, open-file reports, and other ``gray'' literature have not been cited. Of the journals whose articles deal mainly with ore deposits and economic geology the most important is Economic Geology, which at the time of this review had published the fourth issue of 1994. Occasional Monographs on special topics are also issued. The quarterly Newsletter of the Society of Economic Geologists, which appeared beginning in April of 1990, is also a valuable source of current research, exploration, mining, and environmental trends in the area of metallic mineral resources. Both the Society of Economic Geologists and the Mineralogical Society of America periodically publish review volumes that emphasize U.S. research on ore deposits. Other major journals that sometimes contain articles about U.S. research on ore deposits include American Journal of Science, American Mineralogist, Canadian Mineralogist, Chemical Geology, Geochimica et Cosmochimica Acta, Geology, Journal of Geochemical Exploration, and Mineralium Deposita. The U. S. Geological Survey (USGS) frequently publishes bulletins, papers, monographs, circulars, and maps on ore deposits, as do many state and county geological surveys.

Status of Ore Deposits Research in the U.S.
Domestic mineral exploration is currently in decline owing to intertwined market, legislative, environmental and political factors. Consequently many U.S. businesses, universities and government agencies involved in mineral resources are reassessing and readjusting their research and development priorities. Many research programs in economic geology and mining are being downsized or phased out, and the number of domestic students pursuing careers in economic geology is diminishing. Based on a recent survey, Prof. Marco Einaudi of Stanford University estmates that only about 80 Ph.D. candidates are now enrolled in the discipline of economic geology at North American universities [ M. T. Einaudi, pers. commun., 1994]. In spite of these trends, our national per capita consumption of mineral resources continues to grow and that of much of the rest of the world is rapidly catching up with ours. Those domestic mineral resources which we can still exploit must be extracted more delicately and their carcasses restored more carefully to an acceptable environmental state. We are increasingly dependent upon, and increasingly competing with, the developing nations for their mineral resources. Within many of these nations, political and environmental constraints on mineral exploration and development are likely to grow with time. In light of these constraints, our need to understand metallogenesis and the occurrence of ore deposits, and our ability to exploit domestic and foreign mineral resources more efficiently and carefully, must remain a high national priority. Otherwise we risk becoming a vulnerable mineral-import dependent nation with no ability to exploit its own resources in times of strife and no ready supply of domestic professionals who can compete on the international scene. Unfortunately, our vulnerability is exacerbated by the fact that the average U. S. citizen has little appreciation of the critical role that mineral and energy resources play in their high standard of living. Efforts to correct this situation must begin early in the educational process, a fact that some government and industry agencies are now vigorously addressing.

General Books and Reviews
An excellent introductory text on mineral resources and economic geology was produced by Kesler [1994]. Compared with many earlier textbooks, he included more emphasis on mineral economics, mining law, exploration, mining methods and the environmental consequences of exploitation. The text is particularly suitable for an introductory survey course on global mineral and energy resources for undergraduates in the sciences and humanities. If every university Earth Science department offered such a survey course, the ultimate result would be voters and decision-makers who have a far greater understanding of global economics and politics and the role that mineral and energy resources play in a nation's wealth.

Studies of Specific Deposits or Districts
This first section highlights U. S. research on mineralized regions or districts, as well as studies of specific mineral deposits.

Regional Metallogenesis and Mineral Exploration
A review of the economic geology of the U.S. was edited by Gluskoter et al. [1991] as part of the Geology of North America Series of the Geological Society of America. Chapters on the geology of specific mineral commodities, mostly written by USGS experts, covered the major metals and industrial minerals. Three large maps showed the locations of all the deposits and districts discussed in the text. Discovery of South Australia's giant Olympic Dam deposit, comprised of 2 billion metric tons of hydrothermal Cu-U-Au-REE (rare-earth-element) ore within [4] hematitic, granitic breccias, led to a realization that that similar mineralization may be associated with K-rich granites in the Precambrian basement of the U. S. midcontinent. The strategic and critical mineral resources of the midcontinental U.S. were therefore evaluated by a group of USGS, state and industry geologists and the results reported in a series of papers edited by Pratt and Sims [1990] and Day and Lane [1992]. In particular, the middle Proterozoic Pea Ridge deposit of southeast Missouri was recognized to be an Olympic Dam type deposit. The authors summarized the available data and developed exploration strategies for locating other Olympic Dam type deposits in the U. S. midcontinent. Other examples of the development of geologic frameworks and exploration strategies for mineral deposits can be found in a series of papers edited by Scott et al. [1993]. The USGS continued its efforts to develop concise descriptive and grade-tonnage models of mineral deposits for use in exploration, as described in a series of papers edited by Bliss [1992]. New and revised models were developed, mainly for various types of gold deposits. Worksheet templates were provided for ranking the potential of specific occurrences or prospects using the framework of models developed so far. It will be interesting to learn from the minerals industry how useful and successful the models are in conducting exploration and mining.

Magmatic and Magma-Hydrothermal Ore Deposits
The origins of platinum group element (PGE) enriched horizons in mafic layered intrusions are of great interest because such types of mineralization are the main resources of PGE. Bird et al. [1991] described a Au-Pd bearing horizon (Platinova reef) in the Middle zone of the Skaergaard intrusion of east Greenland. Based on textures, the Au appeared to have been trapped at a late magmatic stage as immiscible metal droplets within rims on cumulate silicates. They argued that three distinct fluids must have coexisted at the time of formation of the reef: silicate, sulfide and gold-rich. Boudreau and McCallum [1992] reviewed evidence for PGE enrichments in the reefs of the

Stillwater layered mafic intrusion in Montana. They proposed a model in which a Clrich fluid phase exsolved from the intercumulus (interstitial to crystals) liquid and leached PGE from sulfide inclusions in cumulate phases, transporting both PGE and S upward and re-depositing these elements in mineralization fronts analogous to those in rollfront uranium deposits. These two studies are not contradictory; instead they show that multiple processes within crystallizing mafic magmas can influence the ultimate distribution of PGE observed within layered intrusions. Base metal skarn (coarse calc-silicate) and porphyry deposits typically develop around crystallizing granitic plutons that have been emplaced at moderate to shallow crustal depths above subduction zones beneath their coeval volcanic arcs. A comprehensive review and bibliography on the geology of Au-bearing skarns was provided by Theodore et al. [1991]; they noted that most were calcic exoskarns (developed in wallrock) associated with intense retrograde hydrosilicate alteration. Newberry et al. [1991] expanded and reinterpreted mineralogic, geochemical and isotopic data from the classic Darwin Pb-Zn-Ag skarn deposit in California, showing it to consist of several concentrically zoned sulfide skarn pipes whose ores precipitated in response to large shifts in fluid temperature, pH and oxidation state. Moreover, contrary to earlier studies, they showed that this deposit was genetically unrelated to the nearby, but older, Darwin pluton. Dilles and Einaudi [1992] described the geology and geochemistry of an exposed 5 km vertical section of hydrothermal alteration and mineralization associated with Ann-Mason porphyry copper deposit, one of three such deposits related to the Yerrington batholith in Nevada. From this unique section they were able to reconstruct the flow-paths and thermochemical evolution of hydrothermal fluids which formed the deposit. They identified a dike swarm emanating from a deep granitic cupola as being responsible for the mineralization, and also identified argillic alteration in an adjacent mountain range as representing the paleosurface environment of the deep hydrothermal system. Olympic Dam type deposits may represent the most significant new type of ore deposit whose geology became well-documented during the review period. As a follow-up to their 1990 paper on the Olympic Dam deposit in South Australia, Oreskes and Einaudi [1992] reported fluid inclusion and stable isotopic data from the unusual Fe-rich breccias and Cu-U-Au-Ag ores. They argued that primary magmatic fluids probably deposited early magnetite, but that the mineralized hematitic breccias were formed from the influx of cooler fluids having a more surficial origin. As noted above, Proterozoic Olympic Dam type deposits also occur in granite-rhyolite terranes of the U.S. midcontinent, as described by Nuelle et al. [1992] and Sidder et al. [1993]. These USGS authors concluded that saline magma-hydrothermal fluids derived from Fe-rich trachytes had initially emplaced Fe-silica ore and then subsequently boiled and explosively emplaced rare earth element [4] (REE) bearing breccias into rhyolitic tuffs within a shallow eroded caldera complex. Stable isotopes can be used to ascertain the degree to which magmatic volatiles contributed directly to volcanic-hosted ore deposits. Vennemann et al. [1993] used stable isotopic data to infer a direct role for condensed magmatic fluids in genesis of the Pueblo Viejo acid sulfate Au-Ag deposit (Dominican Republic), the world's largest bulk

mineable deposit of this type. The late Cretaceous deposit was formed at shallow crustal depth within a maar-diatreme setting. Although metals and fluids were derived largely from magmatic vapors, it may be that shallow mixing with and cooling by convectively circulating meteoric or seawaters caused precipitation of the ores and acid alteration assemblages. The Pueblo Viejo hydrothermal system may have been similar to modern magma-hydrothermal systems such as White Island, New Zealand.

Hydrothermal Mineral Deposits
Recent U.S.-Canada research along the Gorda Ridge in the NE Pacific led to a special Economic Geology issue on seafloor hydrothermal mineralization, edited by Rona and Scott [1993]. Zierenberg et al. [1993] described Besshi-type massive sulfide deposits forming on a sediment-covered spreading center, the axial Escanaba Trough on the Southern Gorda Ridge. The deposits form along the margins of uplifted sediment fault blocks generated by intrusion of MORB (mid-ocean ridge basalt) laccoliths. Because of hydrothermal fluid interactions with sediments, the deposits are enriched in group IV, V and VI elements, thermogenic hydrocarbons, and radiogenic Pb compared with those deposits forming in sediment-free spreading centers. Doe [1994] analyzed and discussed source rock control on the Zn, Cu and Pb contents of ocean-ridge hydrothermal fluids; in particular the relatively low Pb contents of mid-ocean ridge basalts lead to a predominance of Zn- and Cu-rich sulfide deposits in sediment-starved ridge systems. Studies of Mississippi Valley-type carbonate-hosted lead-zinc deposits continued to reveal the geologic, hydrologic and geochemical complexities of these fascinating epigenetic ore deposits, which form the major U. S. resources of lead and zinc. There are still many unresolved apects of the origin of these deposits. Several papers presented various geochemical arguments for the influence of multiple fluid sources and/or aquifers in the genesis of some deposits or districts (for example, compare and contrast the databases and conclusions of Viets and Leach, [1990], Shelton et al., [1992], and Kesler et al., [1994]). Structural and tectonic controls of ore genesis in the Southeast Missouri lead belt were examined by several authors. Horrall et al. [1993] suggested that much of the Cu, Co, Ni and siderophile element enrichments in the southeast Missouri Pb-Zn district were derived by basinal brine leaching of alkali mafic and ultramafic plutons occurring along the margins of the Reelfoot rift (New Madrid seismic zone). Clendenin et al. [1994] argued that local and micro-structural controls on fluid flow were important in localizing ore, and that stratigraphic units do not behave as homogeneous aquifers as is commonly assumed in many numerical fluid-flow models. Nonetheless, Garven et al. [1993] developed numerical simulations for Late Paleozoic regional gravity-driven groundwater flow triggered by uplift after the Alleghanian orogeny in the midcontinental region of North America. They used temporal and geographic variations in uplift to explain variations in the timing and directions of regional fluid flow and discharge, and the resulting genesis of carbonate-hosted Pb-Zn mineral deposits. Much like diamonds, gold continues to garner a level of interest that is out of proportion to its true relative importance in technology and industry. Nonetheless, important

contributions to the U. S. literature on hydrothermal gold deposits were made during the review period. A volcano-tectonic framework for epithermal Au-Ag deposits in the western United States was presented by Berger and Bonham [1990]. Lipman [1992] reviewed how structures in calderas influence and localize ore deposition. The geology and origin of the high-grade acid sulfate Cu-Au vein deposit at El Indio, Chile was described by Jannas et al. [1990]; they found that Au deposition actually occurred from late, low-salinity geothermal fluids that were different from those of more magmahydrothermal affinity that deposited the enargite and alunite. Cunningham et al. [1991] described a conceptual genetic model for the diversity of volcanic-dome hosted precious metals deposits in Bolivia, which should prove more generally applicable. In a paper with important exploration significance, Nelson [1990] evaluated the geochemistry of jasperoids from Carlin-type sediment-hosted Au deposits in the western United States. He found that elements characteristic of metalliferous marine black shales can be used to distinguish ore-bearing from barren systems. Acid alteration and oxidation are frequently cited as evidence of boiling of epithermal fluids in such deposits, but Kuehn and Rose [1992] showed that Au deposition at Carlin, Nevada, was structurally and stratigraphically controlled and occurred well before widespread oxidation, the latter of which is supergene (shallow weathering) in origin and not caused by shallow boiling during ore formation.

Metamorphosed Ore Deposits
Because of the obliterative effects of metamorphism, the primary origins of ore deposits found in metamorphic rocks are often controversial. Slack et al. [1993] studied mineralization in the large Broken Hill District of Australia, and demonstrated that the metamorphosed base metal ores originally formed during interaction of hydrothermal fluids with non-marine evaporitic sediments in a Proterozoic continental rift setting. Gemmell et al. [1992] described a stratigraphically conformable Zn-Pb-Ag deposit in Argentina whose geologic and isotopic features support an origin as a shallow marine sedimentary exhalative sulfide deposit, rather than a contact metamorphic magmahydrothermal deposit as proposed earlier. A metamorphosed submarine hydrothermal Mn deposit in North Carolina whose geochemical signatures survived metamorphism to amphibolite facies conditions was described by Flohr [1992]. From a textural and mineralogical standpoint, Craig and Vokes [1993] reviewed the effects of metamorphism on pyritic ores.

Sedimentary Mineral Deposits
A series of papers on manganese metallogenesis appeared in a journal issue edited by Frakes and Bolton [1992]. They also reviewed the mode of origin of Phanerozoic sedimentary manganese deposits and correlated their occurrences with variations in ocean chemistry, sea level and paleoclimate. They concluded that extensive Mn carbonate and oxide precipitation occurred during periods of regression; these periods promote oxidation of seafloor organic matter, release of CO and global greenhouse warming.

Some unusual Ni-Mo-PGE-bearing black shales in China were studied by Murowchick et al. [1994], who concluded that they formed via venting of metalliferous hydrothermal fluids into an anoxic, phosphogenic basin. Large variations in ion microprobe S/ S values for pyrite implied that bacteriogenic seawater sulfate reduction associated with organic matter decomposition was an important mechanism for ore deposition. Most of our domestic uranium resources occur in Tertiary non-marine sandstone deposits that are thought to have formed by groundwater transport and deposition. Sanford [1994] developed a four-layer finite difference model for the formation of tabular sandstone uranium deposits. His results indicated that regional fluid flow was gravity-driven, with discharge concentrated at lake shorelines or playa margins. Inferred zones of mixed local and regional groundwater discharge were associated with the ore zones; these data support a fluid interface mixing mechanism for ore deposition. Precambrian conglomerates rich in detrital pyrite, uraninite and quartz continue to challenge economic geologists as well as paleoclimatologists, because such a combination of minerals cannot survive fluvial transport in our present oxygen-rich atmosphere. Vennemann et al. [1992] found variable O values in adjacent quartz pebbles and their contained fluid inclusions in Archean conglomerates from the Witwatersrand (South Africa) and Huronian (Canada) districts. They concluded that the pebbles preserved their predepositional oxygen isotopic compositions and fluid inclusion chemistry. Both areas exhibited quartz pebble O modes consistent with derivation from erosion of Archean granites and pegmatites. However, the Witwatersrand pebbles exhibited a broader, heavier range in O values, suggesting an additional source of quartz from erosion of Archean greenstone belt lode gold deposits. This provenance difference may explain the presence of both Au and U in the Witwatersrand ores, but only Au in the Huronian ores. The O values and fluid inclusion characteristics of the quartz pebbles were inconsistent with previous proposals for their derivation from Archean exhalative deposits.

Ore-Forming Processes and Tools Used in Their Study
This section highlights research that was centered more on general assessments of processes of ore formation than on evaluating the genesis of specific deposits or districts.
• • • • • • • • •

Ore Petrology and Phase Equilibria Magmatic and Magma-Hydrothermal Ore-Forming Processes Metals in Hydrothermal Fluids Ore Mineral Precipitation Hydrothermal Alteration Low-Temperature Processes and Weathering Light Stable Isotopes Radiogenic and Heavy Isotopes Fluid Inclusions

U.S. National Report to IUGG, 1991-1994 Rev. Geophys. Vol. 33 Suppl., © 1995 American Geophysical Union

Ore Petrology and Phase Equilibria
Compared with other types of petrologists, ore petrologists spend a large fraction of their microscope time looking at the ``opaques.'' However, with the decline in domestic economic geology programs, fewer courses on reflected-light microscopy will probably be available to geology students. This is unfortunate, because oxide and sulfide minerals often record critical information on the conditions of rock genesis. Craig [1990] and Barton [1991] reviewed examples of textures in ores and their interpretation. Barton illustrated how careful interpretation of disequilibrium textures can reveal many aspects of mineralizing processes, including the duration of geological processes. Craig and Vaughan [1994] produced a second edition of their widely-used textbook on ore microscopy and petrography; perhaps a future edition will contain more examples illustrating the growing importance of elemental and isotopic microanalytical methods in ore petrology. Murowchick [1992] summarized textural criteria that can be used in assessing the ancestry of pyrite and marcasite, as well as the pH and temperature of their formation. Economic geologists continue to integrate the use of new microanalytical techniques into ore petrology. The microscopic distribution of metals, ligands and their isotopes

within and among crystals in ores is relevant not only to understanding ore genesis, but also to ore beneficiation and processing. For example, because the specific crystal chemical hosts for Au in sediment-hosted disseminated gold deposits are uncertain, Arehart et al. [1993a] used backscattered electron and secondary ion imaging techniques to document a direct correlation of Au with metastable arsenian pyrite on a microscopic scale. They proposed that Au is present as Au in solid solution, having been deposited from aqueous bisulfide complexes by coupled Au oxidation and As reduction. Such data have important ramifications for processing and extracting the gold from these types of ores. Sphalerite is the only common ore mineral whose composition can record the pressure of mineralization; it is thus an important geobarometer. Toulmin et al. [1990] reviewed the basis and status of the sphalerite geobarometer, noting that experimental and theoretical discrepancies at low temperatures and high pressures need to be resolved, but also that the geobarometer could be successfully applied to equilibrium sphaleritepyrrhotite-pyrite assemblages which have not suffered retrograde effects. The phase equilibria experiments of Lusk et al. [1993] have since filled in an important pressuretemperature region relevant to this geobarometer.

Magmatic and Magma-Hydrothermal Ore-Forming Processes
Using microbeam analytical techniques, Lowenstern et al. [1991] and Lowenstern [1993] discovered Cu sulfides in CO - and Cl-bearing vapor bubbles in melt inclusions within phenocrysts in pantellerites and rhyolites, thus demonstrating that melt Cu could be strongly partitioned into an early magmatic vapor phase in [4] phenocryst-poor magmas. The possibility of strong partitioning of Cu into an early vapor phase, prior to extensive crystallization of phases that would otherwise remove Cu from the melt, means that crystallization-induced volatile saturation (second boiling) is not necessary for the creation of metal-rich fluids in shallow H O- or CO -rich silicic magma chambers. They also argued that volcanic contributions of Cu to the atmosphere may be more significant than previously thought. Meeker et al. [1991] identified crystalline elemental gold and gold chloride particles being emitted from Mount Erebus in Antarctica. This plus consistent Au/Cl ratios of aerosols from the volcano suggested that the gold is transported as a chloride gas species. Transport of trace metals in volcanic gases from Mount St. Helens was modeled by Symonds and Reed [1993], who likewise concluded that most were volatilized from shallow magma as simple chlorides and deposited as sublimates upon cooling as oxides, sulfides, halides, tungstates and native elements. Rye [1993] summarized the evolution of magmatic-hydrothermal ore-forming fluids based on many years of stable isotopic research on such ore deposits. He reviewed evidence for high-level interactions of deep magmatic components with shallow wallrock and meteoric waters, and emphasized the episodic, successive input of deep magmatic volatiles and evolved brine into shallower crustal levels to generate acid alteration and ore deposition. His paper and the review papers by Giggenbach [1993]

and Hedenquist and Lowenstern [1994] are perceptive, complementary evaluations of the processes that form magma-hydrothermal ore deposits.

Metals in Hydrothermal Fluids
Both experiment and empirical observation contributed to advances in knowledge about the geochemistry of metalliferous hydrothermal fluids. Hemley et al. [1992] studied the solubility of Fe, Pb, Zn and Cu sulfides in chloride solutions that were rock-buffered in pH, fS and fO from 300-700 C and 0.5-2.0 bars (5 x 10 to 2 x 10 Pa). Hemley and Hunt [1992] applied the results to conclude that for quasi-adiabatic transport conditions, the pressure effect on rock-buffered solubilities compensates for the temperature effect, allowing metal transport over long distances from deep-seated crystallizing plutons. The outward Cu-Zn-Pb zoning typically seen around mineralized plutons forms in a complex manner dictated by the intersection of transport pathways with metal sulfide saturation surfaces, caused by thermal and chemical changes and their temporal variations. Hemley and Hunt [1992] also present an insightful discussion of paragenesis and zoning in space and time that should be read by everyone interested in ore genesis. Other experimental studies of hydrothermal base [4] metal solubility and speciation include those of Seyfried and Ding [1993] on the relative solubilities of Fe and Cu in Na-K-Cl fluids and Ohmoto et al. [1994] on the solubility of pyrite in Na-Cl solutions, and references cited therein. Platinum-group elements and gold were the focus of experimental studies by Wood et al. [1994], Berndt et al. [1994] and references cited therein. As a complement to experimental studies, McKibben et al. [1990] described similar dissolved concentrations but contrasting precipitation mechanisms of gold and PGE in boiling hot brines within geothermal wells, thus providing empirical evidence for significant differences in the transport mechanisms of these two metals in natural hydrothermal brines. Peters [1993] described the connate origins and Au-Ag-Hghydrocarbon contents of hot spring waters in the California Coast Ranges, and related them to the genesis of hot spring precious metals deposits such as McLaughlin.

Ore Mineral Precipitation
The precipitation mechanisms and stabilities of iron sulfide minerals are not completely understood. The conditions and rates of precipitation of marcasite and pyrite from hydrothermal solutions were studied experimentally by Schoonen and Barnes [1991] and Graham and Ohmoto [1994], who found that these minerals form via precursors of crystalline FeS or liquid S. Relying on a computational approach, Bowers [1991] developed a model for the deposition of Au and other metals during pressure-induced fluid immiscibility. Her model used the EQ3/6 speciation and mass transfer software package with extensions accounting for O, H and S isotopic fractionation, plus the Redlich-Kwong equation of state for the P-V-T propoerties of H O-CO mixtures. She found that fluid immiscibility

(volatile loss) could induce metal deposition under a variety of conditions. However, she also showed that the influence of volatile loss on metal deposition must be evaluated in the context of realistic constraints: the effects of volatile loss on fluid pH and redox state must be evaluated in light of the buffering capacity of the entire fluidrock system.

Hydrothermal Alteration
Feldspar hydrolysis is a common pervasive type of hydrothermal alteration. The thermodynamics of hydrothermal alkali feldspar-mica-aluminosilicate equilibria were evaluated by Sverjensky et al. [1991], who derived an internally consistent set of thermodynamic data for these minerals and relevant aqueous species that will be useful in modeling fluid-rock interactions. Revised values for the dissociation constant of HCl, an important source of acidity, were also derived. The spatial scale of hydrothermal circulation and alteration in crustal rocks is important because of its implications for the volume of rock that can be leached of metals to form concentrated ores. Cathles [1993] used O isotopic data on altered rocks to conclude that a major, long-lived hydrothermal convection cell, centered around a pluton in the Noranda district, Quebec, had penetrated to depths of greater than 8 km. The most intense and coherent zones of O depletion coincided with the highest tonnage massive sulfide deposits. Hydrothermal alteration of oceanic crust is important to understanding the formation of seafloor massive sulfide deposits, the geochemical cycle of sulfur in the oceans, and ultimately the origin of magmatic sulfur erupted from volcanic arcs over subduction zones. Sulfur mass balance and isotopic systematics accompanying hydrothermal alteration of oceanic crust by seawater were developed by Alt [1994], based on studies of ophiolite complexes. Sulfur is redistributed from the lower dike and gabbro sections to the upper dike section, and additional sulfur is added to the upper dike section by reduction of convecting seawater sulfate. However, this latter gain is balanced by oxidative loss of sulfur from the volcanic section. These processes result in exchange of crustal sulfur for seawater sulfur, resulting in enrichment of S in altered crust.

Low-Temperature Processes and Weathering
Weathering of exposed or near-surface ores can result in the redistribution and zoned concentration of valuable metals. Lichtner and Biino [1992] developed a numerical model for metasomatic supergene enrichment of porphyry copper protore. They were able to reproduce elemental zoning seen in the field, particularly the high Cu grades seen in the upper zone of enrichment blankets formed by weathering. Using transmission electron microscopy, Ilton and Veblen [1993] showed that inclusions of native copper found in mica in rocks associated with porphyry copper deposits were formed during weathering. Previously, such copper had been thought to

be the product of primary magma-hydrothermal mineralizing processes related to the emplacement of copper-rich magma. The timing of ore deposit weathering can also be used to evaluate paleoclimates, because some weathering products are rich in potassium and retain its radiogenic decay products. Vasconcelos et al. [1994] used laser-heating Ar/ Ar dates on the mineral jarosite, formed during progressive weathering of sulfide ores, to identify a global late Miocene oxidation and weathering event responsible for weathering and supergene enrichment of several ore deposits.

Light Stable Isotopes
Microanalytical techniques developed within the past decade (ion microprobes, laser microprobes) are allowing not only elemental analysis on a fine scale, as noted above, but also stable isotopic analysis on a fine (micron) scale. In particular, microbeam studies of the zonation of S isotopes within and among individual crystals in sulfide ores are providing important clues to deposit origins. Using secondary ion microprobe mass spectrometry, McKibben and Eldridge [1990] found that hydrothermally altered rhyolites within the Valles [4] Caldera contained strongly S/ S-zoned authigenic pyrite crystals (enriched cores, depleted rims) at depths coinciding with elevated Au contents. They concluded that boiling and oxidative H S destruction had caused Au deposition coincident with progressive Rayleigh S isotopic depletion in the growing crystals. The micron-scale isotopic zoning may have recorded a large-scale geologic event, breaching of the caldera wall and draining of the former caldera lake, which triggered the boiling and Au deposition. Arehart et al. [1993b] also found large variations and late-stage depletions in S/ S values for arsenian pyrite in fine-grained ores from the Post/Betze sedimenthosted disseminated gold deposit in Nevada. Several important advances were made based on conventional (bulk sample) analyses of stable isotopes in ore deposits. An elegant paper by Rye et al. [1992] worked out the stable isotopic systematics of acid sulfate alteration. The characteristic mineral alunite contains four stable isotope sites in its crystal structure, making it a very useful mineral for reconstructing mineralizing conditions and processes. A companion paper by Stoffregen et al. [1994] reported experiments that determined O and H fractionation factors between alunite and water. Ohmoto et al. [1990] reviewed the sulfur isotopic systematics of modern marine sediments and sediment-hosted base metal deposits.

Radiogenic and Heavy Isotopes
As with stable isotopes, researchers continue to ``push the sample size envelope'' for radiogenic isotopes. Brannon et al. [1992] and Nakai et al. [1993] used Rb/Sr isotopic data on sphalerites to date the age of mineralization of several Mississippi Valley-type Pb-Zn ore deposits; the technique is complicated by differential brine-mineral partitioning of Rb and Sr and the consequent need to remove inclusion fluids prior to analysis. Nonetheless, the diverse age determinations from different deposits suggest

that the timing of Paleozoic orogenic activity and resulting regional brine migrations in North America, thought to be responsible for forming these types of ore deposits, may be more complicated than was previously assumed. Chelsey et al. [1994] used Sm-Nd isotopic data on fluorites to date the age of mineralization in the Illinois-Kentucky fluorite district; they obtained a Permian age identical to that obtained by Brannon et al. [1992] for Upper Mississippi Valley sphalerites, supporting a model for large-scale fluid movement from the Illinois basin related to the Alleghenian-Ouachita orogenies in North America. Re-Os isotope systematics are proving useful in ore deposit studies because of the Os isotopic contrast between mantle and crust, the occurrence of Re in molybdenite, and the occurrence of Os in PGE deposits. Marcantonio et al. [1994] used Re-Os, Nd-Sm, Rb-Sr and O isotopic systematics to demonstrate that primary magmatic PGE mineralization in the Wellgreen intrusion, Yukon Territory, had been overprinted by post-crystallization hydrothermal processes which remobilized radiogenic crustal Re and Os from sedimentary wall-rock sources. They questioned earlier studies which had concluded that radiogenic or variable Os isotopic compositions in magmatic PGE deposits must reflect mantle heterogeneities or crustal assimilation, rather than hydrothermal remobilization from radiogenic crustal sources after magma emplacement. McCandless and Ruiz [1993] applied Re-Os isotopic systematics to determine the ages of molybdenite-bearing porphyry base metal deposits associated with the Laramide orogeny in Arizona. They used a variety of analytical techniques to identify molybdenites which had not been affected by post-crystallization remobilization. They found that in each deposit ore deposition consistently occurred in the late stages of magmatic activity. Two narrow episodes of mineralization were delineated: 74-70 million years ago (largely within older Precambrian basement) and 60-55 million years ago (largely within younger Precambrian basement). The synchroneity of this widespread mineralization implied that some type of fundamental crust-mantle interaction resulted in regional genesis of the metal-enriched magmas responsible for the deposits. Walker et al. [1994] applied Re-Os, Nd-Sm and Pb isotopic systematics to magmatic Cu-Ni-PGE sulfide ores and associated igneous rocks from three Permian Noril'sk-type deposits in Siberia. They found that the isotopic data required a hot-spot type asthenospheric mantle source for the primary igneous melts and PGE, with little or no crustal contribution for these elements.

Fluid Inclusions
One of the fundamental assumptions in using fluid inclusion data to study mineral genesis is that the inclusions have behaved as closed systems since their formation. Hall et al. [1991] and Mavrogenes and Bodnar [1994] showed that H diffusion into and out of fluid inclusions during metamorphism or laboratory heating could significantly modify the chemical compositions of the inclusions. Diffusion will be most rapid at high temperatures and when the hydrogen fugacity difference between the inclusion and its surroundings is large. Failure to recognize or expect diffusion problems could result in flawed reconstruction of the formation conditions of some minerals and ores.

In spite of these diffusion problems, it is still possible to retain primary gas ratios in fluid inclusions from unmetamorphosed low-temperature ores. Graney et al. [1991] studied the gas compositions of fluid inclusions in epithermal jasperoids from various gold deposits. They noted a correlation of high H S/CO and other gas parameters with mineralized jasperoids, suggesting the utility of the technique for exploration. In the case of aqueous fluids trapped as inclusions during boiling, knowledge of the temperature of boiling and fluid composition can lead directly to an estimate of the depth of formation if the P-V-T properties of the fluid are known. Many economic geologists have use the pure H O system as a proxy to interpret data from low-salinity fluid inclusions in epithermal gold deposits. Barton and Chou [1993] reviewed P-V-T data for the H O-CO system and demonstrated that large errors in hydrostatic paleodepth reconstructions of epithermal systems may occur if the presence of significant amounts of CO in fluid inclusions is not recognized. For example, if one observes the formation of CO clathrates upon freezing of an inclusion, then the inclusion must have formed under relatively high CO pressure. This pressure would add at least 1 km to the paleodepth that would otherwise be estimated if one used P-V-T data for boiling of pure water. Therefore, if CO is not detected because clathrate does not form or is not recognized upon freezing, then large errors may result when reconstructing the original depth of formation of the host minerals. Kesler [1991], Bodnar [1992] and McKibben et al. [1994] edited special journal issues containing several other papers on U.S. research on fluid inclusions applied to ore deposits.

Conclusions
It is ironic and unfortunate that the study of ore deposits---the largest geochemical anomalies in the earth's crust---is in a tumultuous state at a time when our technological abilities to collect, manipulate, depict and analyze geologic data are accelerating rapidly. We have never been in a better technical position to understand the Earth's geologic processes and history. Tremendous advances are being made in the applications of ion- and laser-microprobes, supercomputers, and global satellite positioning systems. The current interest in global climate change is stimulating a renewed interest in those mineral deposits whose occurrence may reflect past climatic and environmental changes. Also, we are increasingly seeking more efficient ways of extracting, consuming and recycling mineral resources, to minimize the impacts on our environment. Given these conditions, there should be growing research opportunities for geologists who have an understanding of mineral deposit genesis. Yet there is a clear downturn in current domestic opportunities for those researchers whose primary avocation is studying how and where mineral deposits form and applying that knowledge to finding new resources. This downturn has been prompted by the evolving and sometimes conflicting economic, political, environmental and legislative

constraints on domestic mineral exploration and exploitation, in the context of an increasingly global economy. To prosper as a nation we must continue to produce trained experts who understand how and where mineral deposits form, but this capability is threatened. The current declines in domestic mineral exploration and the consequent decreased domestic university enrollments in the discipline of economic geology will have several longterm effects on U. S. research on ore deposits. Already taking place is a rapid deemphasis of pure economic geology research programs at many universities, companies and government agencies. Many retiring senior economic geologists are either not being replaced or are being replaced by geologists with different specialties (often some aspect of environmental geology). Many excellent recent Ph.D.s are surviving on a year-to-year basis in post-doctoral positions, instead of landing permanent faculty and research positions. Some economic geology research programs and their graduates will survive by shifting their emphases to the environmental contamination and remediation aspects of mineral resource exploitation. Others may conduct some ore deposits research under the banner of paleoenvironmental research. In the near-term, many domestic graduates interested in mineral exploration may find better employment opportunities overseas; those seeking domestic careers will need to emphasize the environmental and legislative aspects of resource exploitation and clean-up. An increasing number of graduate students in economic geology will likely come from developing foreign countries, where the need for trained individuals is great. Graduate students in U. S. economic geology programs may be more likely to work on mineral deposits in foreign countries, because of more active exploration and better access to new deposits. Studies of domestic deposits can be hampered by lack of both access to deposits and industry logistical support, prompted in part by liability and fiscal considerations. Although the current domestic situation regarding ore deposits research is not rosy, the long-term outlook is by no means bleak. Economic geologists must of necessity be familiar with almost all aspects of the earth sciences, as well as with basic economics and politics, and therein lies their credentials for making continued contributions to science and human prosperity. Because ore deposits are the ultimate examples of geochemical diferentiation and enrichment in the Earth's crust, they will always provide fertile research ground for those who must understand the geochemistry of the elements and petrogenesis. By studying ore deposits, we learn fundamental aspects of geochemistry and geology that can be applied broadly to problems beyond the origin and occurrence of mineral resources. Acknowledgments. The author thanks the anonymous reviewers, as well as the editors, for making the final version of this paper much more readable.

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