Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sediment-hosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ~100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer.

The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediate-sulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the litho-caps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems.

Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatic-hydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hyper-saline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderate-salinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace.

Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best ore-bodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization.

Porphyry Copper Systems

Orogenic gold and geologic time: a global synthesis

Based on a compilation of published sources, rocks referred to as adakites show the following geochemical and isotopic characteristics: SiO2 ≥56 wt percent, Al2O3 ≥15 wt percent, MgO normally <3 wt percent, Mg number ≈0.5, Sr ≥400 ppm, Y ≤18 ppm, Yb ≤1.9 ppm, Ni ≥20 ppm, Cr ≥30 ppm, Sr/Y ≥20, La/Yb ≥20, and 87Sr/86Sr ≤0.7045. Rocks with such compositions have been interpreted to be the products of hybridization of felsic partial melts from subducting oceanic crust with the peridotitic mantle wedge during ascent and are not primary magmas. High Mg andesites have been interpreted to be related to adakites by partial melting of asthenospheric peridotite contaminated by slab melts. The case for these petrogenetic models for adakites and high Mg andesites is best made in the Archean, when higher mantle geotherms resulted in subducting slabs potentially reaching partial melting temperatures at shallow depths before dehydration rendered the slab infusible. In the Phanerozoic these conditions were likely only met under certain special tectonic conditions, such as subduction of young (≤25-m.y.-old) oceanic crust.

Key adakitic geochemical signatures, such as low Y and Yb concentrations and high Sr/Y and La/Yb ratios, can be generated in normal asthenosphere-derived tholeiitic to calc-alkaline arc magmas by common upper plate crustal interaction and crystal fractionation processes and do not require slab melting. An assessment of several arc volcanic suites from around the world shows that most adakite-like compositions are generated in this way and do not reflect source processes. Similarly, rare adakite-like intrusive rocks associated with some porphyry Cu deposits are the evolved products of extensive crustal-level processing of calc-alkaline basalt-andesite-dacite-rhyolite series magmas. If slab melts contribute to such magmas, their geochemical signatures would have been obliterated or rendered ambiguous by subsequent extensive open-system processes. In Archean terranes, where adakitic and high Al tonalite-trondhjemite-granodiorite (TTG) magma series rocks are more common, porphyry Cu deposits are rare and, where found, are associated with normal calc-alkaline suites rather than adakites. The two different magma series are compositionally distinct in terms of several major and trace element parameters.

Common upper plate magmatic processes such as melting-assimilation-storage-homogenization (MASH) and assimilation-fractional-crystallization (AFC) affecting normal arc magmas can be demonstrated to explain the distinctive compositions of most adakite-like arc rocks, including high Mg andesites and especially those rare examples associated with porphyry Cu deposits. In contrast, slab melting can in most cases neither be proved nor disproved and is therefore unsatisfactory as a unique factor in porphyry Cu genesis.

Special Paper: Adakite-Like Rocks: Their Diverse Origins and Questionable Role in Metallogenesis

article i nfo The Lhasa Terrane in southern Tibet has long been accepted as the last geological block accreted to Eurasia before its collision with the northward drifting Indian continent in the Cenozoic, but its lithospheric architecture, drift and growth histories and the nature of its northern suture with Eurasia via the Qiangtang Terrane remain enigmatic. Using zircon in situ U-Pb and Lu-Hf isotopic and bulk-rock geochemical data of Mesozoic-Early Tertiary magmatic rocks sampled along four north-south traverses across the Lhasa Terrane, we show that the Lhasa Terrane has ancient basement rocks of Proterozoic and Archean ages (up to 2870 Ma) in its centre with younger and juvenile crust (Phanerozoic) accreted towards its both northern and southern edges. This finding proves that the central Lhasa subterrane was once a microcontinent. This continent has survived from its long journey across the Paleo-Tethyan Ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the Qiangtang Terrane to the north and with the Indian continent to the south. Zircon Hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks (e.g., ~50-90%, 0-70%, and 30-100% to the Mesozoic magmatism in the southern, central, and northern Lhasa subterranes, respectively). We suggest that much of the Mesozoic magmatism in the Lhasa Terrane may be associated with the southward Bangong-Nujiang Tethyan seafloor subduction beneath the Lhasa Terrane, which likely began in the Middle Permian (or earlier) and ceased in the late Early Cretaceous, and that the significant changes of zircon eHf(t) at ~113 and ~52 Ma record tectonomagmatic activities as a result of slab break-off and related mantle melting events following the Qiangtang-Lhasa amalgamation and India-Lhasa amalgamation, respectively. These results manifest the efficacy of zircons as a chronometer (U-Pb dating) and a geochemical tracer (Hf isotopes) in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics.

The Lhasa Terrane: Record of a microcontinent and its histories of drift and growth

Metamorphic belts are complex regions where accretion or collision has added
to, or thickened, continental crust. Gold-rich deposits can be formed at all
stages of orogen evolution, so that evolving metamorphic belts contain diverse
gold deposit types that may be juxtaposed or overprint each other. This partly
explains the high level of controversy on the origin of some deposit types,
particularly those formed or overprinted/remobilized during the major
compressional orogeny that shaped the final geometry of the hosting metamorphic
belts. These include gold-dominated orogenic and intrusion-related deposits, but
also particularly controversial gold deposits with atypical metal associations. Orogenic lode gold deposits of Middle Archean to Tertiary age are arguably
the predominant gold deposit type in metamorphic belts, and include several
giant (>250 t Au) and numerous world-class (>100 t Au) examples. Their
defining characteristics and spatial and temporal distributions are now
relatively well documented, such that other gold deposit types can be compared
and contrasted against them. They form as an integral part of the evolution of
subduction-related accretionary or collisional terranes in which the host-rock
sequences were formed in arcs, back arcs, or accretionary prisms. Current
unknowns for orogenic gold deposits include the following: (1) the precise
tectonic setting and age of mineralization in many provinces, particularly in
Paleozoic and older metamorphic belts; (2) the source of ore fluids and metals;
(3) the precise architecture of the hydrothermal systems, particularly the
relationship between first- and lower-order structures; and (4) the specific
depositional mechanisms for gold, particularly for high-grade deposits. Gold-dominant intrusion-related deposits are a less coherent group of
deposits, which are mainly Phanerozoic in age, and include a few world-class,
but no unequivocal giant, examples. They have many similarities to orogenic
deposits in terms of metal associations, wall-rock alteration assemblages, ore
fluids, and, to a lesser extent, structural controls, and hence, some deposits,
particularly those with close spatial relationships to granitoid intrusions,
have been placed in both orogenic and intrusion-related categories by different
authors. Those that are clearly intrusion-related deposits appear to be best
distinguished by their near-craton setting, in locations more distal from
subduction zones than most orogenic gold deposits and in provinces that also
commonly contain Sn and/or W deposits; relatively low gold grades (<1–2
g/t Au); and district-scale zoning to Ag-Pb-Zn deposits in distal zones.
Outstanding problems for intrusion-related deposits include the following: (1)
lack of a clear definition of this apparently diverse group of deposits, (2)
lack of a definitive link for ore fluids and metals between mineralization and
magmatism, (3) the diverse nature of both petrogenetic association and redox
state of the granitoids invoked as the source of mineralization, and (4)
mechanisms for exsolution of the CO 2 -rich ore fluids from the
relatively shallow level granitoids implicated as ore-fluid sources. Gold deposits with atypical metal associations are a particularly diverse and
controversial group, are most abundant in Late Archean terranes, and include
several world-class to giant examples. Most are probably modified Cu-Mo-Au
porphyry, volcanic rock-hosted Zn-Pb-Ag-Au massive sulfide, or Zn-Pb-Ag-Au or
Ba-Au-Mo-Hg submarine epithermal systems, overprinted or remobilized during the
events in which orogenic gold deposits formed, but there is lack of consensus on
genesis. Outstanding problems for these deposits include the following: (1) lack
of a clear grouping of distinctive deposits, (2) lack of published, well
integrated studies of their characteristics, (3) generally a poorly defined
timing of mineralization events, and (4) lack of assessment of metal mass
balances in each stage of the complex mineralization and overprinting events. Both orogenic gold deposits and gold deposits with atypical metal
associations contain a few giant and numerous world-class examples, whereas the
intrusion-related group contains very few world-class examples, and no giants,
unless Muruntau is included in this group. Preliminary analysis suggests that
the parameters of individual world-class to giant gold deposits of any type show
considerable variation, and that it is impossible to define critical factors
that control their size and grade at the deposit scale. However, there appears
more promise at the terrane to province scale where there are greater
indications of common factors such as anomalous subduction-related tectonic
settings, reactivated crustal-scale deformation zones that focus porphyry-lamprophyre
dike swarms in linear volcanosedimentary belts, complex regional-scale geometry
of mixed lithostratigraphic packages, and evidence for multiple mineralization
or remobilization events. There are a number of outstanding problems for all types of gold deposits in
metamorphic belts. These include the following: (1) definitive classifications,
(2) unequivocal recognition of fluid and metal sources, (3) understanding of
fluid migration and focusing at all scales, (4) resolution of the precise role
of granitoid magmatism, (5) precise gold-depositional mechanisms, particularly
those producing high gold grades, and (6) understanding of the release of CO 2 -rich
fluids from subducting slabs and subcreted oceanic crust and granitoid magmas at
different crustal levels. Research needs to be better coordinated and more
integrated, such that detailed fluid-inclusion, trace-element, and isotopic
studies of both gold deposits and potential source rocks, using cutting-edge
technology, are embedded in a firm geological framework at terrane to deposit
scales. Ultimately, four-dimensional models need to be developed, involving
high-quality, three-dimensional geological data combined with integrated
chemical and fluid-flow modeling, to understand the total history of the
hydrothermal systems involved. Such research, particularly that which can
predict superior targets visible in data sets available to exploration companies
before discovery, has obvious spin-offs for global- to deposit-scale targeting
of deposits with superior size and grade in the covered terranes that will be
the exploration focus of the twenty-first century.

/pdf/gold-deposits-in-metamorphic-belts-overview-of-current-pxhffwpeq8.pdf

Gold Deposits in Metamorphic Belts: Overview of Current Understanding,Outstanding Problems, Future Research, and Exploration Significance

This volume presents new data on a group of gold deposits that are hosted primarily within or in the immediate wall rocks to intrusions, and which have recently been suggested to comprise a distinct class of magmatic-hydrothermal system. These deposits have been called 'porphyry gold deposits' (Hollister 1992; Bakke 1995), 'intrusion-related stockwork-disseminated deposits' (Sillitoe 1991), 'plutonic-related gold deposits' (Newberry et al., 1988; McCoy et al., 1997) and 'intrusion-related gold deposits' (Thompson et al. 1999). Lang et al. (2000) preferred the term 'intrusion-related gold systems' because it reflects a tendency common to all magmatic-hydrothermal environments to form ores that manifest multiple styles, metal assemblages and spatial associations with their related intrusive centres. Although in its infancy, investigation and exploration of intrusion-related gold systems has accelerated markedly in the last five years, due in part to their global distribution and to the large number of deposits that contain a gold resource of >30 metric tons (Fig. 1). Major deposit examples include Fort Knox (~210 t Au), Donlin Creek (~315 t Au), Pogo (~160 t Au), and Dublin Gulch, True North and Brewery Creek (~40 t Au each) in Yukon and Alaska, as well as Mokrsko, Czech Republic (~120 t Au), Vasilkovskoe, Kazakstan (~300 t Au), Salave, Spain (~30 t Au), Korri Kollo, Bolivia (~160 t Au) and Kidston, Australia (~140 t Au).
A paucity of detailed descriptions of individual intrusion-related gold systems, the plutonic provinces that host these systems, and the genetic processes critical to their formation currently limits our ability to either develop precise criteria for their definition or to formulate well-constrained geological and exploration models. The principal discussions (Sillitoe 1991; Hollister 1992; Newberry et al. 1988 and 1995; Lang et al. 1997; McCoy et al. 1997; Thompson et al. 1999; Lang et al. 2000; Goldfarb et al. 2000; Newberry 2000) suggest that there are several features common to most intrusion-related gold deposits and provinces, including: 1) metaluminous, subalkalic intrusions of intermediate to felsic composition that span the boundary between ilmenite- and magnetite-series, 2) carbonic hydrothermal fluids, 3) a metal assemblage which variably combines Au with elevated Bi, W, As, Mo, Te and/or Sb and low concentrations of base metals, 4) a low sulphide content (<5 volume %) with a reduced ore mineral assemblage that typically comprises arsenopyrite, pyrrhotite and pyrite, and which lacks magnetite or hematite, 5) areally restricted, commonly weak hydrothermal alteration, except in systems formed at the shallowest depths spanned by these deposits, 6) a tectonic setting of continental magmatism well-inboard of inferred or recognized convergent plate boundaries, and which commonly contains coeval intrusions of alkalic, metaluminous calc-alkalic and peraluminous compositions, and 7) a location in magmatic provinces best or formerly known for W and/or Sn deposits. Deposits that can be confidently included in the group formed during much of the Phanerozoic, but inclusion of some Proterozoic and even Archean deposits has also been proposed (e.g., Robert, this volume).
The defining criteria as presently recognized show that intrusion-related gold systems and their associated plutonic provinces are globally widespread (Fig. 1), but adequate descriptions of the contained deposits are only now beginning to emerge. The Tintina Gold Belt of Alaska and the Yukon Territory in the northern part of the North American Cordillera (Fig. 1) is thus far the most extensively studied intrusion-related gold systems province. This belt is ~1000 km in length and contains gold deposits of Early Cretaceous to Eocene age (McCoy et al. 1997; Newberry et al. 1995; Lang et al. 2000; Goldfarb et al. 2000) that, as a group, span much of the globally recognized variation among these systems. As such, the Tintina Gold Belt is currently the primary standard against which deposits in other provinces can be compared, and is therefore emphasized in this introduction.
This paper briefly considers the status of our knowledge of intrusion-related gold systems, and is intended only as an introduction to the major characteristics of these deposits rather than a comprehensive description. The discussion emphasizes the nature of the associated igneous rocks and their tectono-magmatic setting, the styles of deposits and their spatial distribution, the characteristics and evolution of hydrothermal fluids, and structural controls. It concurrently highlights some of the important gaps in our understanding of these systems that will be fruitful areas of investigation in ongoing and future research programs. The paper concludes with comments on deposit classification and the relationship of intrusion-related gold systems to other types of magmatic-hydrothermal systems.

Intrusion-related gold systems: the present level of understanding

An under-recognized and economically important class of intrusion-related gold deposits, which occur within magmatic provinces best known for tungsten and/or tin mineralization, is described with reference to seven major deposits (Fort Knox, Mokrsko, Salave, Vasilkovskoe, Timbarra, Kidston and Kori Kollo). These gold deposits contain a metal suite that includes some combination of bismuth, tungsten, arsenic, tin, molybdenum, tellurium and antimony, and contrasts with that found in the more widely-developed gold-rich porphyry copper and related deposits. The gold deposits associated with tungsten and/or tin provinces are located in cratonic margins, in a landward or back-arc position relative to continental margin arcs (where recognized), or within continental collisional settings. The deposits are related genetically to felsic domes, stocks or plutons of intermediate oxidation state, both magnetite- and ilmenite-series magmas are represented. The intrusion-hosted gold deposits are most commonly of sheeted vein/veinlet type, although greisen-like, disseminated and breccia deposits are also described. Gold may also be concentrated more distally (1–3 km) with respect to the intrusions, where deposits may be of skarn, disseminated replacement or vein types. K-feldspar, albite and/or sericitic alteration assemblages, commonly including carbonate, accompany the gold mineralization. In sheeted vein deposits, alteration is normally restricted to narrow envelopes around veins, whereas more pervasive alteration occurs in greisen-like, disseminated and shallow ( 100 tonnes (3 million oz) of gold, thereby highlighting the gold potential of intrusion-related deposits beyond the more traditionally explored gold and copper provinces in arc terranes.

Intrusion-related gold deposits associated with tungsten-tin provinces

The chemistry of brine, vapor, and low-salinity fluid inclusions measured by proton-induced X-ray emission from the Bismark skarn deposit, Mexico, is consistent with an evolving magmatic-hydrothermal system with no evidence for external fluid inputs. The results support a model that invokes early phase separation of magmatic fluids into brine and vapor (two-phase field) at high temperature and lithostatic pressure, followed by the entrapment of a low-salinity magmatic fluid (one-phase field) at lower temperature and hydrostatic pressure. The early brine and vapor inclusions contain high Pb and Zn concentrations and low Cu; however, the vapor contains significantly more Cu than the brine and was likely transported as a sulfur complex. The fluid phase changes observed and behavior of Cu are comparable to those of porphyry Cu systems. The later, low-salinity fluid at Bismark represents a distinct pulse of magmatic fluid with a high K/Ca ratio in which base metals, including Cu, were transported by chloride complexes. Paragenetic relationships and variations in the relative concentrations of Cu, Pb, and Zn suggest that this fluid was primarily responsible for ore deposition. The relative Cu, Pb, and Zn concentrations in fluid inclusions at Bismark are consistent with those measured from base-metal ores in high-temperature carbonate- replacement deposits throughout Mexico, and suggest that the bulk-metal budget of these deposits is primarily controlled by magmatic-hydrothermal fluids deficient in Cu.

Composition and evolution of ore fluids in a magmatic-hydrothermal skarn deposit

AN EXPLORATION MODEL FOR Intrusion-Related Gold Systems

Major element, trace element, and Nd-Sm and Rb-Sr isotope data are described from igneous rocks in 11 multiphase magmatic complexes of Laramide age in Arizona. Eight complexes contain barren stocks as well as productive plutons associated with the formation of porphyry base metal deposits, whereas in three complexes porphyry orebodies have not been recognized. Most of the complexes evolved through similar stages. Early, dominantly intermediate volcanic rocks were intruded by multiphase complexes temporally characterized by increases in SiO 2 , changes in the ferromagnesian mineral assemblage, increasingly porphyritic textures, and a progression from barren to subproductive to productive intrusions. Within complexes the sequence is independent of both the absolute age of initiation of local magmatism and its duration. Analytical data and field relationships are combined to show elements of the petrogenetic history of these complexes, potential relationships between productive and barren magmas, and factors which may have influenced formation of the characteristic magmatic hydrothermal sequence. The isotopic composition of Laramide igneous rocks forms an array from epsilon NdT and ( 87 Sr/ 86 Sr) initial values of 0 and 0.704. to -14 and >0.710, respectively. Early volcanic rocks have a more primitive signature consistent with retention of a significant mantle component. Subsequent intrusions were derived predominantly from isotopically heterogeneous. Precambrian lower crust but probably also contain an undefined amount of mantle component. Individual complexes do not exhibit any consistent isotopic trends over time and unique types of source rocks are not indicated for productive magmas. In contrast, rare earth elements (REE) in several complexes display temporally systematic behavior. Progressively younger intrusions in a given complex follow a path of decreasing concentration of REE, steepening profiles, greater upward concavity in heavy REE profiles, and changes in the Eu anomaly from negative to either markedly less negative or positive. The REE data from individual complexes suggest an increasing petrogenetic involvement of amphibole, either in the restite of the source region or as a fractionating phase, and similar paths of petrogenetic evolution in spite of chronologic and trace element evidence which argues against completely comagmatic development. Productive and barren intrusions cannot be clearly distinguished by the isotopic data, but the systematic REE trends parallel increasing productivity of magmas in individual complexes. Models which reconcile isotopic variation, systematic REE behavior, protracted local but largely noncomagmatic magmatism and the increasing development of porphyry mineralization include: (1) progressive metasomatism of the crustal component, (2) increasing assimilation of amphibolitic crust by mantle basalts, and (3) migration of crustal anatexis from deep anhydrous rocks to shallower amphibolites. The relative importance of these processes cannot yet be clearly assessed from the data, but a plausible metallogenic impact of each would be upon the concentration, and possibly the composition, of magmatic volatiles in the primary melts and/or during subsequent fractionation. In this light, existing models which propose that the principal control upon intrusion productivity is a balance between crystal fractionation and the timing of saturation and release of magmatic hydrothermal fluids may be strongly influenced by processes operative either in their source regions or during their interaction with the crust. Initiation of such processes may be a response to abrupt changes in tectonic regime in the volcanic-are environment.

James R. Lang

Papers

Intrusion-related gold systems: the present level of understanding

Intrusion-related gold deposits associated with tungsten-tin provinces

Composition and evolution of ore fluids in a magmatic-hydrothermal skarn deposit

AN EXPLORATION MODEL FOR Intrusion-Related Gold Systems

Isotopic and geochemical characteristics of Laramide magmatic systems in Arizona and implications for the genesis of porphyry copper deposits