What have we learnt about orogenic lode gold deposits over the past 20 years?
Robert Moritz
Section des Sciences de la Terre, University of Geneva, Switzerland
Introduction
Gold deposits discussed in this paper have been called
mesothermal gold, metamorphic gold, gold-only, lode
gold,
shear-zone
hosted,
structurally-controlled
deposits, and also include the variety of greenstonehosted and turbidite-hosted deposits. Recently, the
term orogenic gold deposits has been proposed
(Groves et al., 1998), since the term mesothermal
appears to be misused with respect to the original
definition of Lindgren (1933). Why do we have this
proliferation of nomenclature? It certainly reflects some
confusion within the scientific community. Most
certainly the confusion of the terminology does not
stem from a lack of studies. Indeed, since around 1980,
orogenic gold deposits have been studied in great
detail. This research effort is reflected in a large
number of scientific papers and reviews (e.g. Foster,
1991; Macdonald, 1986; Keays et al., 1989; Ramsay et
al., 1998). The reason about the confusion probably
resides in the inherent nature of these gold deposits
hosted by metamorphosed and intensely altered rocks,
that are generally highly deformed and display a
complex geologic evolution.
The orthomagmatic model has been the most popular
for many decades for the formation of the gold deposits
under review (e.g. Emmons, 1937). Syngenetic
deposition of gold in submarine chemical sediments
was invoked in the 1970s and 1980s (e.g. Fripp, 1976;
Hutchinson and Burlington, 1984), possibly followed by
multistage remobilization of gold. Linkage to
metamorphic processes as the driving mechanism for
generating vein-type, shear-zone hosted gold deposits
was envisaged by Fyfe and Henley (1973) with goldbearing quartz veins formed along shear zones that
focused metamorphic fluids expelled from deeper
structural levels in the crust. In his comprehensive
review, Boyle (1979) suggested a model on a more
local scale, with lateral secretion processes resulting in
gold enrichment in dilatant structures during regional
metamorphism.
The dramatic price increase of gold in the late 1970s,
culminating at a high of U.S.$ 620/oz in January 1980,
was certainly a major trigger for the world-wide
launched studies on vein-type and shear zone hosted
gold deposits. The considerable development of
different analytical techniques during the past 20 years
has also allowed us to accumulate a vast data set and
to improve our knowledge of these gold deposits. The
combination of independent lines of investigation,
including fieldwork, laboratory studies, experimental
research and computer modelling, has set important
constraints on the genetic models that are offered
nowadays. However, how far have we progressed in
our knowledge since the early 1980s?
The following review shows that we have gained a fairly
good knowledge about the descriptive aspects of
orogenic gold deposits. It is recognised that they
constitute an independent class of deposits with
recurring characteristics throughout the geologic record
and in different geologic environments. Geochronology
has provided important constraints for fitting confidently
the deposits into a regional tectonic evolution. On the
other hand, despite intense research effort in the past
two decades, sources of fluids and solutes are still
intensely debated. This is reflected by the
disagreement about genetic models.
Regional geologic features
Orogenic gold deposits are present in metamorphic
terranes of various ages, displaying variable degrees of
deformation. The host geological environments include
volcano-plutonic and clastic sedimentary terranes. The
host
rocks
have
been
characteristically
metamorphosed up to greenschist facies conditions,
and locally to amphibolite or granulite facies conditions.
The gold deposits typically occur within or in the vicinity
of regional, crustal-scale deformation zones with a
brittle to ductile type of deformation. Some of them
likely are the present-day expression of ancient major
tectonic boundaries. The geologic structures generally
indicate compressional to transpressional tectonic
settings. The gold deposits can be hosted by any rock
type.
A wide diversity of morphologies
Typically, there is a strong structural control of the gold
deposits and orebodies at all scales. The morphology
can be highly variable, including (1) brittle faults to
ductile shear zones, (2) extensional fractures,
stockworks and breccias, and (3) fold hinges (Hodgson,
1989; Robert, 1996). The orebodies can consist
dominantly of altered host rock with disseminated
mineralization or of fissure-filled mineralization, i.e.
veins, sensu stricto. The deposits can have a
significant vertical extent, exceeding 1 km in many
deposits, reaching even 3.2 km in the Kolar gold field in
India.
Mineralogy of ore & gangue minerals
Veins in the orogenic gold deposits are dominated by
quartz with subsidiary carbonate and sulfide minerals,
and less abundantly, albite, chlorite, white mica
(fuchsite in ultramafic host rocks), tourmaline, and
scheelite. Carbonate minerals consist of calcite,
dolomite and ankerite. Gold occurs in the veins and in
adjacent wallrocks. Gold is usually intimately
associated with sulfide minerals, including pyrite,
pyrrhotite chalcopyrite, galena, sphalerite, and
arsenopyrite. In volcano-plutonic settings, pyrite and
pyrrhotite are the most common sulfide minerals in
greenschist and amphibolite grade host rocks,
respectively, while arsenopyrite is the predominant
sulfide mineral in ores hosted by sedimentary rocks.
Tellurides can be significant ore minerals in some
deposits. Gold to silver ratios typically range between
10:1 and 5:1, less commonly 1:1. Although the vein
systems can be continuous for over 1 to 2 km vertical
extent, there is generally little change in ore grade, and
ore and gangue mineralogy with depth.
development. The mineralized structures display synto post-mineralization displacements, such as striated
slip surfaces with hydrothermal slickenlines, and folding
and boudinage. This indicates that the gold deposits
are syn-tectonic. Veins and structures in the wallrocks
typically record reverse-oblique movements along the
associated shear zones and faults. A combination of
dynamic stress changes and fluid pressure variations is
generally invoked to explain the geometry of the vein
systems (Sibson et al., 1988; Sibson and Scott, 1998).
In
metamorphic
greenschist
grade
settings,
hydrothermal alteration assemblages are typically
superimposed on the regional metamorphic mineralogy
of the host rocks. Therefore, the gold deposits are
interpreted as post-peak metamorphic. However, the
relationship is more ambiguous in higher metamorphic
grade environments where hydrothermal alteration is
reported as syn-metamorphic (Neumayr et al., 1993),
and in other cases as overprinted by deformation and
regional metamorphism (Penczak and Mason, 1997).
Geochronology
Hydrothermal alteration
Hydrothermal wallrock alteration in orogenic gold
deposits is developed in a zoned pattern with a
progression from proximal to distal assemblages. The
alteration intensity decreases with distance with respect
to the orebodies. Scale, intensity and mineralogy of the
alteration is a function of wallrock composition and
crustal level (e.g. McCuaig and Kerrich, 1998).
Alteration is most intensely developed in intermediate
to ultramafic igneous wallrocks. In clastic sedimentary
rocks, alteration is restricted to narrow zones around
the orebodies. The main alteration products of the
wallrocks include (1) carbonate minerals (calcite,
dolomite, ankerite, in some cases siderite and
magnesite), (2) sulfide minerals (generally pyrite,
pyrrhotite or arsenopyrite), (3) alkali-rich silicate
minerals (sericite, fuchsite, albite, and less commonly,
K-feldspar, biotite, paragonite), (4) chlorite and (5)
quartz. Carbonatization, sulfidation and alkalimetasomatism of the wallrocks reflect the addition of
variable amounts of CO2, S, K, Na, H2O, and LILE
during mineralization. Amphibole, diopside, plagioclase
and garnet are recognized at deeper crustal levels
under amphibolite and granulite metamorphic grades,
where carbonate minerals become less abundant.
Geometrical relationships & relative timing
The structures hosting the gold deposits (shear zones,
faults, extensional veins, breccia) are typically
discordant with respect to the stratigraphic layering of
the host rocks, but in some cases they can be parallel
to bedding planes and fold hinges or intrusive contacts.
This reflects the epigenetic nature of the gold deposits
and the influence of strength anisotropy on their
Radiometric dating has certainly provided an absolute
time frame for integrating orogenic gold deposits into
the regional geologic evolution of their host
40Ar/39Ar,
environments.
Indeed,
Sm-Nd, and
conventional or SHRIMP U-Pb dating have allowed us
to refine the absolute time relationships among
tectonic, metamorphic, magmatic, and ore formation
events. The emplacement of orogenic gold deposits
can occur immediately following or just a few million
years after magmatic activity, regional metamorphism
and deformation, to many tens of millions years after
the intial orogenic events of the host rocks (Groves et
al., 1998). Absolute radiometric dating has yielded
diachronous ages for orogenic gold deposits, revealing
that formation of these deposits is episodic and can
occur during a prolonged time-span during the late
orogenic evolution of a given tectonic province, in
general as the host terrains are uplifted and
progressively cooling.
40Ar/39Ar
For
instance,
and
SHRIMP
U-Pb
geochronology in Australian turbidite-hosted gold
deposits
indicate
episodic
hydrothermal
gold
mineralization during a prolonged time-span in the midPaleozoic at 420-455 Ma synchronous with regional
metamorphism and thrusting, and at 360-380 Ma
overlapping with magmatic activity dated at 360-410
Ma (Arne et al., 1998; Foster et al., 1998). In the
northwestern Alps, 40Ar/39Ar dating on hydrothermal
micas from gold lodes yields progressively younger
ages from 31.6 Ma in the southwest to 10.6 Ma in the
northeast. Thus, the hydrothermal systems depositing
gold were recurrent for about 20 m.y. across the Alpine
belt (Pettke et al., 2000). While Periadriatic plutonism
was coeval with the older deposits, there is no known
magmatic activity contemporaneous with the younger
deposits. Therefore Pettke et al. (2000) concluded that
the gold deposits are related to metamorphic
devolatilization during uplift of the Alpine belt. By
contrast, in other areas, as for example the Massif
Central, France (Bouchot et al., 1997) or the Pataz
province, Peru (Haeberlin et al., 1999), the mineralizing
event apparently lasted only a short lapse of time,
within just a few million years.
Although radiogenic isotopes allow us to obtain
absolute and precise ages, their interpretation can be
subject to debate. For instance, U-Pb, Sm-Nd and
40Ar/39Ar ages of hydrothermal rutile, scheelite and
muscovite, respectively, from gold deposits at Val d’Or,
southern Abitibi greenstone belt, Canada, yield gold
deposition ages about 2600 Ma according to Wong et
al. (1991), Hanes et al. (1992), and Anglin et al. (1996).
Such ages are younger by 70 to 100 m.y. than any
plutonic rocks and metamorphic events of the area. By
contrast, Kerrich and Cassidy (1994) interpret the “late”
2600 Ma ages as resetting of isotopic systems below
their conventional blocking temperatures due to later
fluid migration associated with reactivation of the
structures hosting the gold deposits. Kerrich and
Cassidy (1994) argue that SHRIMP U-Pb ages
between 2680 and 2690 Ma of hydrothermal zircons
from the same gold deposits are more consistent with
the known tectonic, structural and metamorphic
relationships of the gold deposits and the local geologic
environment.
Such a debate shows that care must be taken in the
choice of the minerals to be dated, and that the
radiometric ages must be critically assessed with
respect to the regional geologic evolution of the
environment hosting the gold deposits. Moreover,
attention must be paid to possible thermal and
secondary fluid migration overprints on the gold
deposits.
Recent
developments
in
Re-Os
geochronology may provide an alternative for dating
directly ore minerals in orogenic gold deposits. For
instance, Re-Os dating of vein pyrite at the Sigma Mine
from the southern southern Abitibi greenstone belt
yields a mean age of 2679 Ma (Stein et al., 1999), in
good agreement with the 2680-2690 Ma SHRIMP ages
of the zircons (Kerrich and Cassidy; 1994).
Geodynamic setting
Phanerozoic orogenic gold deposits are spatially
associated with convergent plate boundaries (Nesbitt,
1991; Kerrich and Cassidy, 1994; Goldfarb et al.,
1998). The tectonic setting of the Precambrian deposits
remained more ambiguous until the late 1980s. Based
on abundant geological similarities between the gold
deposits from Phanerozoic orogenic belts and Archean
greenstone belts, a plate convergent setting is also
suggested for Archean deposits (Barley et al., 1989;
Hodgson and Hamilton, 1989; Kerrich and Wyman,
1990). For instance, the Archean Superior Province of
Canada is characterized by the diachronous accretion
from north to south of diverse plutonic, volcanoplutonic, and metasedimentary subprovinces between
2710 and 2670 Ma (Card, 1990). Lode gold deposits
are linked in time and space to the diachronous
accretion events, and typically occur after and during
the later magmatic, metamorphic and deformation
stages of each accretion event (Poulsen et al., 1992).
Goldfarb et al. (2000) recognize a broad correlation of
formation of orogenic gold deposits with thermal events
associated to the growth of new continental crust
during the evolution of our planet. Formation of
orogenic gold deposits on a global-wide scale was
episodic at about 3100 Ma, 2700-2500 Ma, 2100-1700
Ma, and the Phanerozoic. Orogenic gold deposits are
found in both accretionary orogens (e.g. western North
and South America) and collisional orogens (e.g.
Variscan, Appalachian and Alpine orogens), also called
peripheral and internal orogens, respectively (Groves et
al., 1998).
Isotopic characteristics: attempts to
constrain the sources of fluids & solutes
Isotopic studies of a variety of minerals and of fluid
inclusions have been carried out to determine the
source of solutes and fluids, as well as the mechanisms
of ore deposition. Since gold is monoisotopic ( 197Au),
information about the source rocks of gold are inferred
from Sr, Pb and stable isotope systematics. There is
certainly no global consensus about the interpretation
of the enormous data set acquired so far. The
ambiguities attached to the interpretations are reflected
in the wide range of genetic models proposed for the
orogenic gold deposits.
Hydrogen and oxygen isotopes have been popular in
the attempt at constraining the source of the fluids
involved in the formation of orogenic gold deposits.
18O values of the fluids generally range between +6
and +11‰ (SMOW), and D fluid values between –30
and –80‰ (SMOW). Both metamorphic (e.g. McCuaig
and Kerrich, 1998) and magmatic fluids (So and Yun,
1997; de Ronde et al., 2000) reasonably fit the
observed values. 18O and D fluid values from a
number of deposits fall outside the abovementionned
“typical” ranges. In Archean lode-gold deposits from
Western Australia such values are interpreted as
incursion of surface-derived waters (Hagemann et al.,
1994). Low D values of –160 to -80‰ in Phanerozoic
North American Cordilleran and Korean deposits are
interpreted as reflecting the deep circulation of meteoric
waters during formation of the orogenic gold deposits
(Nesbitt et al., 1986; Shelton et al. 1988). By contrast,
other authors attribute such low D values to fluid
reaction with organic matter and reduced gas species
(Peters et al.; 1990; Goldfarb et al., 1997). Goldfarb et
al. (1991) mention that such low D values obtained by
analysis of bulk extraction of fluid inclusions are due to
late generations of secondary inclusions trapping
meteoric water unrelated to gold deposition.
Carbon and sufur isotope data are variable for orogenic
gold deposits of all ages and from different districts
(e.g. Nesbitt, 1991; McCuaig and Kerrich 1998). The
large range in isotopic data may reflect interactions
between different rock types and the ore forming fluids
along their channelways from their source to the gold
deposition site, and variable chemical conditions before
and during ore formation.
Similarly, Pb and Sr isotope systematics of different
minerals from orogenic gold deposits have not allowed
us to define a unique rock reservoir as the source of
gold. In contrast, they indicate heterogeneous sources,
and yield a general picture of a large-scale reservoir
with a variety of rocks providing the components
contained in the orogenic gold deposits (Nesbitt, 1991;
McCuaig and Kerrich, 1998). Moreover, there is no
certainty that gold originates from the same source
rocks as Pb and Sr, since the radiogenic isotopic
compositions may only reflect interaction with different
rocks during fluid migration, thus altering the original
isotopic signature of the fluid (e.g. Moritz et al., 1990;
Pettke et al., 2000). He isotopes can be used to
discriminate among major fluid reservoirs such as
mantle, meteoric and crust (Pettke et al., 2000).
ore types. This feature is cited as evidence for a deep
origin of the ore-forming fluids, like granulitization of the
lower crust (Fyon et al., 1989) or progressive fluid
release under amphibolite facies conditions (Phillips
and Powell, 1993). However, other authors contend
that igneous intrusions can also produce CO 2-rich fluids
(McCoy et al., 1997; Sillitoe and Thompson, 1998).
The fluid groups 2 and 3 described above are typically
interpreted as resulting from unmixing of fluid 1. Phase
separation is often cited as the trigger of gold
deposition. However unmixing is not systematically
associated with gold deposition (e.g. Diamond, 1990),
and other processes such as fluid-rock interaction or
fluid cooling have been invoked. Although less popular,
fluid mixing has been favoured in other cases between
CO2-rich fluids and brines to explain gold deposition
(e.g. Anderson et al., 1992).
Reported salinities of fluids associated to gold
deposition are typically below 10 wt% NaCl equiv.
(Groves et al., 1998; McCuaig and Kerrich, 1998). The
interpretation of more saline fluids with up to 35 wt%
NaCl equiv. (group 4 above), with daughter minerals in
some cases (e.g. halite), is more controversial. In some
cases, such saline inclusions are interpreted as late
brines unrelated to the gold deposits. In other deposits
they have also been considered as resulting from
phase separation of H2O-CO2 fluids. High fluid salinities
are not only diagnostic of magmatic and basinal fluids,
but can also be produced during retrograde
metamorphism (Bennett and Barker, 1992).
Fluid inclusion characteristics
Fluid inclusion microthermometry has been abundantly
used to define fluid compositions, ore forming
processes and pressure-temperature conditions.
Problems associated to fluid inclusion studies in
syntectonic, orogenic gold deposits are their possible
reequilibration due to overprinting deformation and
uplift/exhumation following formation of the deposits.
Uncertainties
in
reconstructing
the
pressuretemperature conditions (isochore calculations) are also
attached to small, often undetectable dissolved gas
species. Despite these problems, fluid inclusion studies
have revealed the presence of four main types of fluids
in the orogenic gold deposits: (1) two- to three-phase
H2O-CO2 fluids with variable quantities of CH4, N2 and
dissolved salts; (2) CO2-rich inclusions with variable
contents of CH4, N2 and H2S and low to undetectable
H2O contents; (3) low to medium saline two-phase H2O
fluids with or without CO2; and (4) very saline H2O
inclusions with a solid salt phase in some cases.
The presence of CO2 in the fluids is entirely consistent
with the carbonatization of the host rocks of orogenic
gold deposits. This CO2-rich nature distinguishes the
fluids associated to the orogenic gold deposits with
respect to fluids encountered in other major classes of
In Alpine and Variscan deposits, halogen systematics
based on bulk fluid extraction has been interpreted by
Yardley et al. (1993) and Boiron et al. (1996) as
reflecting deep circulation of meteoric water during
gold ore formation. However, in the case of the Alpine
deposits the meteoric fluid interpretation based on
halogen systematics (Yardley et al., 1993) contrasts
with He-Ar isotope systematics ruling out meteoric
water during ore formation (Pettke et al., 2000).
Insufficient database and knowledge of halogen
systematics may explain such diverging interpretations
(McCuaig and Kerrich, 1998). In addition, analyses of
bulk extracted fluid inclusions should be interpreted
with caution because of possible mixing of various fluid
inclusion populations. Some of these fluids maybe
totally unrelated to gold deposition. In the future, single
fluid inclusion analyses (e.g. LA-ICP-MS) may give us
more confidence about the nature of the fluids
genetically related to orogenic gold deposits.
The combination of the fluid inclusion and other data
suggest a pressure-temperature range of ore formation
of 1-3 kb and 250-350°C. Some shallow deposits may
have formed as low as 0.5 kb and 150°C, and deeper
deposits under higher pressures up to 5 kb and higher
temperatures of 700°C (Hagemann and Brown, 1996).
Modelling and experimental studies
In recent years, various types of modelling have been
applied to orogenic gold deposits. The models are
developed and applied as a working tool, they provide
information about the complexity of the system, and
guidance on the type of future useful measurements or
observations. For instance, the improvement of
internally consistent thermodynamic databases for
mineral and volatile phases has allowed us to refine the
modelling of physico-chemical conditions of the oreforming fluids. Based on phase equilibria, and fluid
inclusion, isotope and thermodynamic data, Mikucki
and Ridley (1993), and Mikucki (1998) have calculated
that the orogenic gold deposits were in general formed
from moderately reduced fluids with a nearly neutral to
weakly alkaline pH at all crustal levels. The fluids
maintained approximate thermal equilibrium with the
rocks through which they circulated, but their chemical
composition was progressively modified through fluidwallrock interaction and/or mineral precipitation during
their ascent. Such thermodynamic calculations lend
support to models advocating a crustal continuum of
gold deposition, with a single fluid moving from deep to
shallow crustal levels (Groves, 1993).
Modelling of metamorphic processes, fluid production
and fluid circulation may explain conflicting temporal
relationships between gold-bearing quartz veins and
peak-metamorphism (Stüwe et al., 1993; Stüwe, 1998).
In Barrovian-type metamorphic terrains, lower crustal
levels experience prograde metamorphism later than
shallow crustal levels, with a time difference of up to 50
m.y. Such calculations show that veining in upper
crustal levels can be coeval with metamorphism at
deeper crustal levels, therefore explaining the postpeak metamorphic setting of orogenic gold deposits,
and the relatively long-time gap in certain cases
between peak-metamorphism of the immediate host
rocks and the gold-bearing quartz veins. Similar
calculations for terrains where magmatic underplating
or magmatic intrusions are the cause of metamorphism
indicate that quartz veining can be both pre- and postmetamorphic.
There is a consensus that the main complex
responsible for gold transport in orogenic gold deposits
is Au(HS)2- (Seward, 1984; Mikucki, 1998). Au(HS)0
may become predominant under low pH conditions
(Benning and Seward, 1996). Gold-chloride complexes
are generally considered to be negligible under the
physico-chemical conditions prevailing in orogenic gold
deposits. However, based on thermodynamic
calculations, Mikucki (1998) suggests that AuCl2- will
eventually supersede Au(HS)2- as the dominant goldcomplex at temperatures above 550°C, even in lowsalinity fluids. Experimental data are certainly
necessary to understand the stability of gold-complexes
under amphibolite and granulite facies conditions.
Conclusion: no consensus about genetic
models
Because of the difficulties in defining a unique source
of gold and the ore-forming fluids, a number of genetic
models have been proposed. The main models are: (1)
granulitization of the lower crust due to CO2-enriched
fluids from the mantle accompanied by felsic
magmatism (Fyon et al., 1989; Hodgson and Hamilton,
1989); (2) magmatic fluids exsolved from tonalite trondjemite - granodioritic intrusions (Burrows and
Spooner, 1987); (3) fluids produced by metamorphic
processes (e.g. Kontak et al., 1990; Phillips and Powell,
1993; Kerrich and Cassidy, 1994); and (4) deep
circulation of meteoric water (Nesbitt et al., 1986;
Boiron et al., 1996). Lamprophyres have also been
suggested as a gold and fluid source, but their spatial
association with orogenic gold deposits is downplayed
to a merely structural relationship (Rock et al., 1989).
There has been a tendency in the late 1980s and early
1990s to classify gold deposits located within
deformation zones under a single category named
mesothermal, with the understanding that they were
formed by the same process regardless of their age
and geographic location (Kerrich and Wyman, 1990;
Nesbitt, 1991). It is clear that with such a unifying
classification, the origin of some deposits has remained
contentious, in particular in higher grade metamorphic
settings. Some authors have ascribed a deep origin to
such deposits, suggesting a syn-metamorphic origin
(e.g. Neumayr et al., 1993), therefore supporting a
crustal continuum model for the orogenic gold deposits
(Groves, 1993; Groves et al., 1998). In contrast, other
authors favour a shallow origin for such deposits,
subsequently overprinted by deformation and regional
metamorphism at deeper structural levels (e.g.
Morasse et al., 1995; Penzcak and Mason, 1997).
Orogenic gold deposits hosted by upper amphibolite to
granulite metamorphic grade rocks have only been
recognised in Archean terrains, and seem to be absent
in Phanerozoic orogenic belts (Groves et al., 1998).
Presently, the genetic models ascribed to a number of
gold deposits hosted by deformed host rocks are reexamined, in particular those with styles of alteration,
ore assemblages and metal associations departing
from the features typical of orogenic gold deposits cited
above. Such deposits are reinterpreted using intrusionrelated, porphyry-like Au-Cu, Au-rich volcanogenic
massive-sulfide or low sulfidation epithermal models
(Robert and Poulsen, 1997; Robert, 2000). This
approach maybe an alternative to the crustal continuum
model proposed by Groves (1993). In particular, the
intrusion-related model has recently gained new
attention. Sillitoe and Thompson (1998) and Thompson
et al. (1999) identify various types of metal associations
with gold in intrusion-related systems (e.g.: Au-W-Sn,
Au-Fe-Cu, Au-As-Bi-Sb, etc.). Gold deposits can be
hosted by the genetically associated intrusion or be
located in more distal settings (1-3 km). According to
Sillitoe and Thompson (1998) and Groves et al. (1998),
both intrusion-related and orogenic gold deposits are
spatially associated in the same geodynamic setting.
Therefore, a convergence of regional geological
features is likely. Assigning systematically a
metamorphic or meteoric model to gold deposits
located in deformation zones is certainly an
oversimplification. On the other hand, caution should
be applied in deducing a genetic relationship for gold
deposits spatially associated with intrusions. An
example for the latter is provided by gold veins hosted
by a batholith in the Pataz province of Peru. This
spatial setting may suggest a genetic relationship
between the gold veins and the host batholith (Sillitoe
and Thompson, 1998). However, such a genetic
relationship is questionable based on recent detailed
40Ar/39Ar dating of sericite from alteration zones of the
veins revealing a mineralization age of 312-314 Ma
which is much younger than the 329 Ma U-Pb age of
zircon from the host batholith (Haeberlin et al., 1999). It
underscores that absolute dating is required to test for
genetic relationships between spatially associated
intrusions and vein type gold deposits.
granodiorite stock, Mink Lake, northwestern Ontario. Econ. Geol., 82,
1931-1957.
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