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Ore depositsMichael A. MckibbenDepartment 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 byBurt[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.
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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 resourcescontinues 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 riskbecoming 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 byKesler[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.
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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 byPratt and Sims [1990] andDay 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 Damtype 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 byBliss [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. Boudreauand McCallum [1992] reviewed evidence for PGE enrichments in the reefs of the
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Stillwater layered mafic intrusion in Montana. They proposed a model in which a Cl-
rich fluid phase exsolved from the intercumulus (interstitial to crystals) liquid and
leached PGE from sulfide inclusions in cumulate phases, transporting both PGE and Supward 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 wall-
rock) 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 thedeposit. 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 fromthe 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 byNuelle 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 thePueblo Viejo acid sulfate Au-Ag deposit (Dominican Republic), the world's largest bulk
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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 convectivelycirculating 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 byRona 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 theSouthern 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 toreveal 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 ofViets and Leach, [1990], Shelton et al.,
[1992], andKesler 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 Madridseismic 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
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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 byBerger 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 byJannas et al. [1990]; they found that Au deposition actually occurred from
late, low-salinity geothermal fluids that were different from those of more magma-
hydrothermal 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, butKuehn 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 depositsfound 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 magma-
hydrothermal deposit as proposed earlier. A metamorphosed submarine hydrothermal
Mn deposit in North Carolina whose geochemical signatures survived metamorphism to
amphibolite facies conditions was described byFlohr[1992]. From a textural and
mineralogical standpoint, Craig and Vokes [1993] reviewed the effects ofmetamorphism 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 greenhousewarming.
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Some unusual Ni-Mo-PGE-bearing black shales in China were studied byMurowchick
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/ Svalues 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 anadditional 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.
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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] andBarton [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 techniquesinto ore petrology. The microscopic distribution of metals, ligands and their isotopes
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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 sphalerite-
pyrrhotite-pyrite assemblages which have not suffered retrograde effects. The phase
equilibria experiments ofLusk et al. [1993] have since filled in an important pressure-
temperature region relevant to this geobarometer.
Magmatic and Magma-Hydrothermal Ore-Forming
Processes
Using microbeam analytical techniques,Lowenstern et al. [1991] andLowenstern
[1993] discovered Cu sulfides in CO - and Cl-bearing vapor bubbles in melt inclusionswithin 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 wall-
rock and meteoric waters, and emphasized the episodic, successive input of deep
magmatic volatiles and evolved brine into shallower crustal levels to generate acidalteration and ore deposition. His paper and the review papers by Giggenbach [1993]
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andHedenquist 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 acomplex 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 ofSeyfried 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-Hg-
hydrocarbon 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 ofstate for the P-V-T propoerties of H O-CO mixtures. She found that fluid immiscibility
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(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 pHand redox state must be evaluated in light of the buffering capacity of the entire fluid-
rock 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 ofseafloor 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 byAlt[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
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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, lasermicroprobes) 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 thecaldera 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 sediment-
hosted 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 byRye 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 fractionationfactors 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] andNakai 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 toanalysis. Nonetheless, the diverse age determinations from different deposits suggest
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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-Ndisotopic 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 inthe 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] andMavrogenes 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.
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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 variousgold 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 musthave 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] andMcKibben 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 whoseprimary
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
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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 long-
term effects on U. S. research on ore deposits. Already taking place is a rapid de-
emphasis 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. economicgeology 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 providefertile 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.
References1
7/30/2019 Ore Deposits
16/27
Alt, J. C., A sulfur isotopic profile through the Troodos ophiolite,Cyprus: primary composition and the effects of seawater
hydrothermal alteration. Geochim. Cosmochim. Acta, 58, 1825-1840,1994.
2
Arehart, G. B., Chryssolulis, S. L., and Kesler, S. E., Gold and arsenicin iron sulfides from sediment-hosted disseminated gold deposits---implications for depositional processes. Econ. Geol., 88, 171-185,1993a.
3
Arehart, G. B., Eldridge, C. S., Chryssoulis, S. L., and Kesler, S. E., Ionmicroprobe determination of sulfur isotope variations in iron sulfidesfrom the Post-Betze sediment-hosted disseminated gold deposit,Nevada, USA. Geochimica et Cosmochimica Acta, 57, 1505-1519,1993b.
4
Barton, P. B., Ore textures---problems and opportunities. Mineralog.Mag., 55, 303-315, 1991.
5
Barton, P. B., and Chou, I. M., Refinement of the evaluation of the roleof CO in modifying estimates of the pressure of epithermalmineralization. Econ. Geol., 88, 873-884, 1993.
6
Berger, B. R., and Bonham, H. F., Jr., Epithermal gold-silver depositsin the United States: time-space products of evolving plutonic,
volcanic and tectonic environments.J. Geochem. Expl., 36, 103-142,1990.
7
Berndt, M. E., Buttram, T., Earley, D., III, and Seyfried, W. E., Jr., Thestability of gold polysulfide complexes in aqueous sulfide solutions:100 to 150 C and 100 bars. Geochim. Cosmochim. Acta, 58, 587-594,1994.
8
7/30/2019 Ore Deposits
17/27
Bird, D. K., Brooks, C. K., Gannicott, R. A., and Turner, P. A., A gold-bearing horizon in the Skaergaard intrusion, East Greenland. Econ.Geol., 86, 1083-1092, 1991.
9
Bliss, J. D. (ed.), Developments in mineral deposit modeling. U.S.Geol. Survey Bull., 2004, 168 pp., 1992.
10
Bodnar, R. J., Current research on fluid inclusions---a briefintroduction. Geochim. Cosmochim. Acta, 56, 3-3, 1992.
11
Boudreau, A. E., and McCallum, I. S., Concentration of platinum-groupelements by magmatic fluids in layered intrusions. Econ. Geol., 87,1830-1848, 1992.
12
Bowers, T. S., The deposition of gold and other metals: pressure-induced fluid immiscibility and associated stable isotope signatures.Geochim. Cosmochim. Acta, 55, 2417-2434, 1991.
13
Brannon, J. C., Podosek, F. A., and McLimans, R. K., Alleghenian age ofthe upper Mississippi Valley zinc lead deposit determined by Rb-Srdating of sphalerite. Nature, 356, 509-511, 1992.
14
Burt, D. M., Metallogenesis. Reviews of Geophysics, Supplement,April, 542-553, 1991.
15
Cathles, L. M., Oxygen isotope alteration in the Noranda miningdistrict, Abitibi greenstone belt, Quebec. Econ. Geol., 88, 1483-1511,1993.
16
Chesley, J. T., Halliday, A. N., Kyser, T. K., and Spry, P. G., Directdating of Mississippi Valley-type mineralization---use of Sm-Nd in
fluorite. Econ. Geol., 89, 1192-1199, 1994.
7/30/2019 Ore Deposits
18/27
17
Clendenin, C. W., Niewendorp, C. A., Duane, M. J., and Lovell, G. R.,The paleohydrology of Southeast Missouri Mississippi Valley-typedeposits: interplay of faults, fluids and adjoining lithologies. Econ.Geol., 89, 322-332, 1994.
18
Cunningham, C. G., McNamee, J., Pinto Vasquez, J., and Ericksen, G.E., A model of volcanic dome-hosted precious metal deposits inBolivia. Econ. Geol., 86, 415-421, 1991
19
Craig, J. R., Textures of the ore minerals, in Jambor, J. L., andVaughan, D. J. (eds.), Advanced Microscopic Studies of Ore Minerals.Min. Assoc. Canada, Short Course Vol. 17, 213-261, 1990.
20
Craig, J. R., and Vokes, F. M., The metamorphism of pyrite and pyriticores---an overview. Mineralog. Mag., 57, 3-18, 1993.
21
Day, W. C., and Lane, D. E. (eds.), Strategic and critical minerals inthe midcontinent region, United States. U.S. Geol. Survey Bull., 1989,1992.
22
Dilles, J. H., and Einaudi, M. T., Wall-rock alteration and hydrothermalflow paths about the Ann-Mason porphyry copper deposit, Nevada---a6-km vertical reconstruction, Econ. Geol., 87, 1963-2001, 1992.
23
Doe, B. R., Zinc, copper, and lead in mid-ocean ridge basalts and thesource rock control on Zn/Pb in ocean-ridge hydrothermal deposits.Geochim. Cosmochim. Acta, 58, 2215-2223, 1994.
24
Flohr, M. J. K., Geochemistry and origin of the Bald Knob manganesedeposit, North Carolina. Econ. Geol., 87, 2023-2040, 1992.
25
7/30/2019 Ore Deposits
19/27
Frakes, L., and Bolton, B., Effects of ocean chemistry, sea level, andclimate on the formation of primary sedimentary manganese ore
deposits. Econ. Geol., 87, 1207-1217, 1992.
26
Garven, G., Ge, S., Person, M. A., and Sverjensky, D. A., Genesis ofstratabound ore deposits in the midcontinent basins of North-America. 1. The role of regional groundwater flow.Am. J. Sci., 293,497-568, 1993.
27
Giggenbach, W. F., Magma degassing and mineral deposition inhydrothermal systems along convergent plate boundaries. Econ.Geol., 87, 1927-1944, 1992.
28
Gemmell, J. B., Zantop, H., and Meinert, L. D., Genesis of the Aguilarzinc-lead-silver deposit, Argentina: contact metasomatic vs.sedimentary exhalative. Econ. Geol., 87, 2085-2112, 1992.
29
Gluskoter, H. J., Rice, D. D., and Taylor, R. B.(eds.), Economicgeology---U.S. Geol. Soc. Am., P-2, 1991.
30
Graham, U. M., and Ohmoto, H., Experimental study of formationmechanisms of hydrothermal pyrite. Geochim. Cosmochim. Acta, 58,2187-2202, 1994.
31
Graney, J. R., Kesler, S. E., and Jones, H. D., Application of gasanalysis of jasperoid inclusion fluids to exploration for micron golddeposits.J. Geochem. Expl. 42, 91-106, 1991.
32
Hall, D. L., Bodnar, R. J., and Craig, J. R., Evidence for postentrapmentdiffusion of hydrogen into peak metamorphic fluid inclusions from themassive sulfide deposits at Ducktown, Tennessee,Am. Min., 76,1344-1355, 1991.
33
7/30/2019 Ore Deposits
20/27
Hedenquist, J. W., and Lowenstern, J. B., The role of magmas in theformation of hydrothermal ore deposits. Nature, 370, 519-527, 1994.
34
Hemley, J. J., Cygan, G. L., Fein, J. B., Robinson, G. R., and d'Angelo,W. M., Hydrothermal ore-forming processes in the light of studies inrock-buffered systems: I. Iron-copper-zinc-lead sulfide solubilityrelations. Econ. Geol., 87, 1-22, 1992.
35
Hemley, J. J., and Hunt, J. P., Hydrothermal ore-forming processes in
the light of studies in rock-buffered systems: II. Some generalgeologic applications. Econ. Geol., 87, 23-43, 1992.
36
Horrall, K. B., Hagni, R. D., and Kisvarsanyi, G., Mafic and ultramaficplutons associated with the New Madrid Rift complex---a possiblemajor source of the copper-cobalt-nickel mineralization of southeastMissouri. Econ. Geol., 88, 328-343, 1993.
37
Ilton, E. S., and Veblen, D. R., Origin and mode of enrichment inbiotite from rocks associated with porphyry copper deposits: atransmission electron microscopy investigation. Econ. Geol., 88, 885-900, 1993.
38
Jannas, R. R., Beane, R. E., Ahler, B. A., and Brosnahan, D. R., Goldand copper mineralization at the El Indio deposit, Chile.J. Geochem.Expl., 36, 233-266, 1990.
39
Kesler, S. E. (ed.), Fluid inclusion gas analyses in mineral exploration.J. Geochem. Expl., 42, 1-221, 1991
40
Kesler, S. E., Mineral Resources, Economics and the Environment.Macmillan, N. Y., 391 pp., 1994.
41
7/30/2019 Ore Deposits
21/27
Kesler, S. E., Cumming, G. L., Krstic, D., and Lowell, G. R., Leadisotope geochemistry of Mississippi Valley-type deposits of the
southern Appalachians. Econ. Geol., 89, 307-321, 1994.
42
Kuehn, C. A, and Rose, A. W., Geology and geochemistry of wall-rockalteration at the Carlin gold deposit, Nevada. Econ. Geol., 87, 1697-1721, 1992.
43
Lichtner, P. C., and Biino, G. G., A first principles approach to
supergene enrichment of a porphyry copper protore: I. Cu-Fe-Ssubsystem. Geochim. Cosmochim. Acta, 56, 3987-4013, 1992.
44
Lipman, P. W., Ash-flow calderas as structural controls of oredeposits---recent work and future problems. U. S. Geological SurveyBulletin, 2012-L, L1-L12, 1992.
45
Lowenstern, J. B., Evidence for a copper-bearing fluid in magmaerupted at the Valley of Ten-Thousand-Smokes, Alaska. Contr. Min.Petrol., 114, 409-421, 1993.
46
Lowenstern, J. B., Mahood, G. A., Rivers, M. L., and Sutton, S. R.,Evidence for extreme partitioning of copper into a magmatic vaporphase. Science, 252, 1405-1409, 1991.
47
Lusk, J., Scott, S. D., and Ford, C. E., Phase relations in the Fe-Zn-Ssystem to 5 kbars and temperatures between 325 and 150 C. Econ.Geol., 88, 1880-1903, 1993.
48
Marcantonio, F., Reisberg, L., Zindler, A., Wyman, D., and Hulbert, L.,An isotopic study of the Ni-Cu-PGE-rich Wellgreen intrusion of theWrangellia terrane: evidence for hydrothermal mobilization ofrhenium and osmium. Geochim. Cosmochim. Acta, 58, 1007-1017,1994.
49
7/30/2019 Ore Deposits
22/27
Mavrogenes, J. A., and Bodnar, R. J., Hydrogen movement into andout of fluid inclusions in quartz: experimental evidence and geologic
implications. Geochim. Cosmochim. Acta, 58, 141-148, 1994.
50
McCandless, T. E., and Ruiz, J., Rhenium-osmium evidence forregional mineralization in southwestern North-America. Science, 261,1282-1286, 1993.
51
McKibben, M. A., and Eldridge, C. S., Radical sulfur isotope zonation in
pyrite accompanying boiling and epithermal gold deposition: aSHRIMP study of the Valles Caldera, New Mexico. Econ. Geol., 85,1917-1925, 1990.
52
McKibben, M. A., Williams, A. E., and Hall, G. E. M., Solubility andtransport of platinum-group elements and Au in saline hydrothermalfluids: constraints from geothermal brine data. Econ. Geol., 85, 1926-1934, 1990.
53
McKibben, M. A., Hall, D. L., Goldstein, R. H., Introduction to currentresearch on fluid inclusions (PACROFI IV), Geochim. Cosmochim. Acta,58, 1051, 1994.
54
Meeker, K. A., Chuan, R. L., Kyle, P. R., and Palais, J. M., Emission ofelemental gold particles from Mount Erebus, Ross Island, Antarctica.Geophys. Res. Lett., 18, 1405-1408, 1991.
55
Murowchick, J. B., Marcasite inversion and the petrographicdetermination of pyrite ancestry. Econ. Geol., 87, 1141-1152, 1992.
56
Murowchick, J. B., Coveney, R. M., Jr., Grauch, R. I., Eldridge, C. S.,and Shelton, K. L., Cyclic variations of sulfur isotopes in Cambrianstratabound Ni-Mo-(PGE-Au) ores of southern China. Geochim.Cosmochim. Acta, 58, 1813-1823, 1994.
57
7/30/2019 Ore Deposits
23/27
Nakai, S., Halliday, A. N., Kesler, S. E., Jones, H. D., Kyle, J. R., andLane, T. E., Rb-Sr dating of sphalerites from Mississippi Valley-type
(MVT) ore deposits. Geochim. Cosmochim. Acta, 57, 417-427, 1993.
58
Nelson, C. E., Comparative geochemistry of jasperoids from Carlin-type gold deposits of the western United States.J. Geochem. Expl.,36, 171-196, 1990.
59
Newberry, R. J., Einaidi, M. T., and Eastman, H. S., Zoning and genesis
of the Darwin Pb-Zn-Ag skarn deposit, California: a reinterpretationbased on new data. Econ. Geol., 86, 969-982, 1991.
60
Nuelle, L. M., Day, W. C., Sidder, G. B., and Seeger, C. M., Geologyand mineral paragenesis of the Pea Ridge iron ore mine, WashingtonCounty, Missouri---origin of the rare-earth-element- and gold-bearingbreccia pipes. U. S. Geological Survey Bulletin 1989-A, A1-A11, 1992.
61
Ohmoto, H., Kaiser, C. J., and Geer, K. A., Systematics of sulphurisotopes in recent marine sediments and ancient sediment-hostedbasemetal deposits. In Stable Isotopes and Fluid Processes inMineralization, H. K. Herbert and S. E. Ho, Eds., Univ. West. Austr.,Publ. 23, 70-120, 1990.
62
Ohmoto, H., Hayashi, K.-I., and Kajisa, Y., Experimental study of thesolubilities of pyrite in NaCl-bearing aqueous solutions at 250-350 C.
Geochim. Cosmochim. Acta, 58, 2169-2185, 1994.63
Oreskes, N., and Einaudi, M. T., Origin of hydrothermal fluids atOlympic Dam---preliminary results from fluid inclusions and stableisotopes. Econ. Geol., 87, 64-90 1992.
64
Peters, E. K., O enriched waters of the Coast Range Mountains,northern California: connate and ore-forming fluids. Geochim.Cosmochim. Acta, 57, 1093-1104, 1993.
7/30/2019 Ore Deposits
24/27
65
Pratt, W. P., and Sims, P. K., The midcontinent of the United States---permissive terrane for an Olympic Dam-type deposit? U. S. Geol.Survey Bull., 1932, 81 pp., 1990.
66
Rona, P. A. and Scott, S. D., A special issue on sea-floor hydrothermalmineralization---new perspectives---preface. Econ. Geol., 88, 1935-1976, 1993.
67
Rye, R. O., The evolution of magmatic fluids in the epithermalenvironment---the stable isotope perspective, Econ. Geol., 88, 733-753, 1993.
68
Rye, R. O., Bethke, P. M., and Wasserman, M. D., The stable isotopegeochemistry of acid sulfate alteration. Econ. Geol., 87, 225-262,1992.
69
Sanford, R. F., A quantitative model of gound-water flow duringformation of tabular sandstone uranium deposits. Econ. Geol., 89,341-360, 1994.
70
Schoonen, M. A. A., and Barnes, H. L., Mechanisms of pyrite andmarcasite formation from solution: III. Hydrothermal processes.Geochim. Cosmochim. Acta, 55, 3491-3504, 1991.
71
Scott, R. W., Detra, P. S., and Berger, B. R. (eds.), Advances related toUnited States and international mineral resources: developingframeworks and exploration technologies, U. S. Geol. Survey Bull.,2039, 277 pp., 1993.
72
Seyfried, W. E., and Ding, K., The effect of redox on the relativesolubilities of copper and iron in Cl-bearing aqueous fluids at elevated
temperatures and pressures: an experimental study with application
7/30/2019 Ore Deposits
25/27
to subseafloor hydrothermal systems. Geochim. Cosmochim. Acta,57, 1905-1918, 1993.
73
Shelton, K. L., Bauer, R. M., and Gregg, J. M., Fluid-inclusion studies ofregionally extensive epigenetic dolomites, Bonneterre dolomite(Cambrian), southeast Missouri---evidence of multiple fluids duringdolomitization and lead-zinc mineralization. Geol. Soc. Am. Bulletin,104, 675-683, 1992.
74
Sidder, G. B., Day, W. C., Nuelle, L. M., Seeger, C. M., and Kisvarsanyi,E. B., Mineralogic and fluid-inclusion studies of the Pea Ridge iron-rare-earth-element deposit, Southeast Missouri. U. S. Geol. Survey.Bull., B2039, 205-216, 1993.
75
Slack, J. F., Palmer, M. R., Stevens, B. P. J., and Barnes, R. G., Originand significance of tourmaline-rich rocks in the Broken-Hill District,Australia. Econ. Geol., 88, 505-541, 1993.
76
Stoffregen, R. E., Rye, R. O., and Wasserman, M. D., Experimentalstudies of alunite: I. O- O and D-H fractionation factors betweenalunite and water at 250-450 C. Geochim. Cosmochim. Acta, 58, 903-916, 1994.
77
Sverjensky, D. A., Hemley, J. J., and D'Angelo, W. M., Thermodynamicassessment of hydrothermal alkali feldspar-mica-aluminosilicate
equilibria. Geochim. Cosmochim. Acta, 55, 989-1004, 1991.78
Symonds, R. B., and Reed, M. H., Calculation of multicomponentchemical equilibria in gas-solid-liquid systems: calculation methods,thermochemical data, and applications to studies of high-temperaturevolcanic gases with examples from Mount St. Helens.Am. J. Sci., 293,758-864, 1993.
79
Theodore, T. G., Orris, G. J., Hammarstrom, J. M., and Bliss, J. D., Gold-bearing skarns. U.S. Geological Survey Bulletin, 1930, 61 pp., 1991.
7/30/2019 Ore Deposits
26/27
80
Toulmin, P., Barton, P. B., and Wiggins, L. B., Commentary on thesphalerite geobarometer,Am. Min., 76, 1038-1051, 1991.
81
Vasconcelos, P. M., Brimhall, G. H., Becker, T. A., and Renne, P. R.,Ar/ Ar analysis of supergene jarosite and alunite: implications to thepaleoweathering history of the western USA and West Africa.Geochim. Cosmochim. Acta, 58, 401-420, 1994.
82
Vennemann, T. W., Kesler, S. E., and O'Neil, J. R., Stable isotopecompositions of quartz pebbles and their fluid inclusions as tracers ofsediment provenance: implications for gold- and uranium-bearingquartz pebble conglomerates. Geology, 20, 837-840, 1992.
83
Vennemann, T. W., Muntean, J. L., Kesler, S. E., O'Neil, J. R., Valley, J.W., and Russell, N., Stable isotope evidence for magmatic fluids in thePueblo-