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Introduction
PORPHYRY SYSTEMS (including porphyry deposits sensu stricto,
skarn, carbonate-replacement and sediment-hosted deposits,
high and intermediate sulfidation epithermal deposits) pro-
vide three quarters of the world’s Cu and significant propor-
tions of Mo, Zn, Pb, and precious metals (Au, Ag) to our econ-
omy (Sillitoe, 2010). Therefore, establishing detailed genetic
models for porphyry systems is essential to increase our capa-
bility to discover new metal commodities. Among the different
investigation tools that we can use to construct genetic models
for porphyry systems, geochronology is an essential one be-
cause the timing of ore mineral precipitation, which allows us
to place ore deposit formation within a broader geologic con-
text, can only be quantified by isotopic dating. On a more de-
tailed temporal scale, porphyry systems are complex geologic
environments where mineralizing events relate to dynamically
evolving magmatic and hydrothermal systems (e.g., Sillitoe,
2010). Determining the duration of genetic processes occur-
ring in these deposits is fundamental to understanding how
they form, because the time scales involved are the function of
volumes and rates of magma emplacement in the upper crust,
its resulting thermal structure, and other tectonic and geody-
namic parameters (e.g., crustal stress state, crustal structures,
velocity of subduction). Therefore, establishing the tempo and
the duration of magmatic-hydrothermal events is instrumen-
tal to the development of genetic models for porphyry sys-
tems (e.g., Chiaradia et al., 2009a; Sillitoe, 2010; von Quadt et
al., 2011) and for their exploration. The ore community is be-
coming increasingly aware of the “short” (tens of thousands of
years to a few hundreds of thousands of years) durations of
ore depositional processes as well as of their repetitive, pulsed
nature in porphyry systems (e.g., Ballard et al., 2001; von
How Accurately Can We Date the Duration of Magmatic-Hydrothermal Events
in Porphyry Systems?—An Invited Paper
MASSIMO CHIARADIA,†URS SCHALTEGGER, RICHARD SPIKINGS, JÖRN-FREDERIK WOTZLAW, AND MARIA OVTCHAROVA
Section of Earth and Environmental Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland
Abstract
Determining the absolute duration of magmatic-hydrothermal events leading to the formation of porphyry
systems (i.e., including porphyry copper, skarn, and epithermal deposits) is one of the key questions in ore
geology. This is so because the duration of magmatic-hydrothermal events in porphyry systems is instrumental
to the development of genetic models necessary to explore a category of mineral deposits that provide most of
the copper and significant amounts of base and precious metals to our economy. The problem of determining
the absolute duration of magmatic-hydrothermal events in porphyry systems has been addressed through ther-
mal modeling of cooling intrusions and time needed to precipitate specified metal amounts from active hydro-
thermal systems with known metal concentrations and fluid fluxes. Both these methods have shown that the
likely duration of hydrothermal systems is on the order of a few tens of kilo-annum (ka). Isotopic dating in con-
trast is the only possible way to determine the life span of magmatic-hydrothermal events in fossil porphyry sys-
tems. Analytical and methodological developments during the last decade in the fields of the most robust iso-
topic systems commonly used for absolute dating (U-Pb, 40Ar/39Ar, Re-Os) allow us to date minerals with
internal precisions <0.2% (2σ). For a 10-Ma-old mineral this corresponds to a <20-ka uncertainty, which is mar-
ginally sufficient to discriminate the duration of hydrothermal systems at the tens of kilo-annum scale. How-
ever, many geochronological studies on fossil porphyry systems have shown that these are most often formed
through repeated cycles of several intrusion events, which extend the overall life of the porphyry systems to a
few 0.X and up to ~2 Ma in some cases. Internal precisions of the above mentioned dating methods allow us,
in theory, to comfortably discriminate events at the 0.X scale and the combination of U-Pb, 40Ar/39Ar, and Re-
Os geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic-hydro-
thermal events in porphyry systems.
In this review we discuss some fundamental problems that are systematically overlooked in most geochrono-
logical studies trying to bracket the life span of porphyry systems. We show that if these problems are not
adequately taken into account and tackled the result will be that fundamentally wrong life spans of porphyry sys-
tems will be estimated. We also provide basic guidelines to follow when trying to resolve the duration of mag-
matic-hydrothermal events in porphyry systems with the highest accuracy and precision currently achievable.
† Corresponding author: e-mail, Massimo.Chiaradia@unige.ch
©2013 by Economic Geology, Vol. 108, pp. 565–584 Submitted: May 7, 2012
Accepted: July 18, 2012
BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS
VOL. 108 June–July NO.4
Quadt et al., 2002, 2011; Sillitoe and Mortensen, 2010). Tech-
nical and methodological advances during the previous
decade in the fields of the dating techniques most frequently
used in ore geology (U-Pb, Re-Os, and 40Ar/39Ar) allow us to
date minerals with an internal precision (2σ) better than 0.2%
(<20 ka equiv for a 10-Ma-old mineral, 10 Ma representing an
age which has minimum associated absolute uncertainties for
the above dating techniques: see below) for 40Ar/39Ar and Re-
Os, and ~0.1% (~10 ka equiv for a 10-Ma-old mineral) for U-
Pb (in this review uncertainties/precisions will be always
given at the 2σlevel, unless otherwise stated). These preci-
sions provide, in theory, the temporal resolution that is nec-
essary to understand the evolution of magmatic-hydrothermal
systems associated with porphyry systems.
Mineral assemblages and fluid inclusion data indicate that
porphyry systems form within a broad temperature range
between >700° and <250°C (Sillitoe, 2010 and references
therein), reflecting the emplacement of a source of heat
(magmatic body) and its subsequent cooling. Fluid exsolution
from the magma, the precipitation of ore and gangue miner-
als from such a fluid, and the concomitant fluid-rock and
eventually magmatic fluid-meteoric/basinal fluid interaction
result in defined alteration patterns with precipitation and/or
replacement of new minerals at the expenses of others. Dur-
ing such processes, which are accompanied by an overall ther-
mal decline of the system due to cooling of the magmatic heat
source (which is often punctuated by several thermal rejuve-
nation stages due to repeated intrusive pulses; see below),
minerals are formed that are potentially datable using radio-
metric techniques.
Which isotopic systems and minerals do we choose
for dating?
The low diffusivity of U and Pb in zircon causes these ele-
ments to be retained at temperatures as high as ~900°C,
which are temperatures of magma crystallization (Lee et al.,
1997; Cherniak and Watson, 2001). Zircon thus yields U-Pb
dates that record the upper temperature limit of porphyry
systems, corresponding to a maximum emplacement time of
the magmatic heat source (note that we use the general term
“date” to indicate a numerical value obtained from an analy-
sis that becomes an “age” after evaluation and “acceptance” of
its geologic meaning). Titanite, by virtue of its lower closure
temperature concerning the U-Pb system (~600°−700°C;
e.g., Cherniak, 1993; Scott and St.-Onge, 1995; Verts et al.,
1996), may also yield dates that correspond to either late mag-
matic crystallization (Schaltegger et al., 2009) or early hydro-
thermal alteration (e.g., in skarns: Chiaradia et al., 2009b).
Re-Os ages of molybdenite record the time of crystallization
of this mineral at various early (high T) to late (lower T)
hydrothermal stages (Stein et al., 2001). In contrast to U-Pb
and Re-Os dates, which reflect the time of crystallization of
magmatic zircon/titanite and hydrothermal molybdenite/ti-
tanite, 40Ar/39Ar dates of most minerals found in porphyry sys-
tems correspond to cooling of those minerals below the tem-
perature of Ar retention rather than the time of their
crystallization because Ar can diffuse out of the crystalline lat-
tice of commonly dated minerals at temperatures that are
lower than those of their crystallization (e.g., Harrison et al.,
1985). The closure temperature range of several minerals
(e.g., biotite, sericite, K-feldspar, alunite: Harrison et al.,
1985; Lovera et al., 1997; Love et al., 1998) datable by the
40Ar/39Ar technique overlaps with the lowest temperature
range of the hydrothermal processes that occur in porphyry
systems. Thus, a combination of U-Pb zircon/titanite ages,
molybdenite Re-Os, and 40Ar/39Ar dates of several different
mineral phases is potentially a powerful tool (assuming that
40Ar/39Ar dates are not disturbed by later thermal events: see
below) to bracket the duration of magmatic-hydrothermal
events (Fig. 1) and is widely used by the ore geology commu-
nity (e.g., Maksaev et al., 2004; Deckart et al., 2005; Valencia
et al., 2005; Baumgartner et al., 2009; Schütte et al., 2012).
Alternatively, the maximum duration of magmatic-hydrother-
mal systems can be constrained by U-Pb dating of zircons
566 CHIARADIA ET AL.
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U-Pb zircon
U-Pb monazite
Ar/Ar hbl
U-Pb tit
Rb-Sr WR
Re-Os moly
U-Pb rutile
Rb-Sr musc
Ar/Ar musc
Rb-Sr bio
Ar/Ar bio
FT zircon
Ar/Ar Kf
FT apatite
U-Th/He apatite
FT tit
800
700
600
500
400
300
200
100
0
Closure temperature (°C)
magmatic
stage
hydrothermal
stage
exhumation
stage
Closure temperature (°C)
800
e
g
a
ag
gmatic
Closure temperature (°C)
200
300
400
500
600
700
e
g
ge
sta
ag
o
thermal
ydr
ro
h
hy
sta
gm
ma
ag
Closure temperature (°C)
0
100
200
e
g
sta
ag
exhumation
FIG. 1. Range of closure temperatures for various geochronometers (modified from http://pangea.stanford.edu/~dpol-
lard/NSF/main.html; Re-Os closure temperature in molybdenite is from Suzuki et al., 1996). The combination of U-Pb, Re-
Os, and 40Ar/39Ar dating on various minerals allows us to bracket temporally the evolution of a porphyry system through a
thermal range that goes from the magma emplacement (U-Pb in zircon) to low-temperature hydrothermal alteration.
extracted from syn- and post-ore intrusive rocks that have
clear crosscutting relationships in the field (e.g., von Quadt et
al., 2011). If it happens that the intrusion that we have dated
by the U-Pb method on zircons is not directly associated with
the mineralization (e.g., because the causative intrusion re-
sides at higher depth in the system and has not been sam-
pled), then the Re-Os date constraining the earliest, highest
temperature phase of mineralization will provide a minimum
age for the onset of the magmatic-hydrothermal system.
Aims of this review
In this review we present a discussion on how accurately
and precisely we can determine the duration of magmatic-
hydrothermal events in porphyry systems using the U-Pb, Re-
Os, and 40Ar/39Ar methods. Actually the concepts of the pre-
sent discussion, although focused on porphyry systems, can
also be extended to any ore deposit type genetically associ-
ated with intrusive rocks (e.g., greisen, pegmatites, intrusion-
related Au systems). Thermodynamic constraints on the cool-
ing rates of intrusive bodies emplaced at shallow depths, such
as porphyritic stocks (Cathles et al., 1997), imply that the
hydrothermal system that is associated with them will have a
maximum life span of a few tens of thousands of years. These
model constraints are supported by data acquired from active
geothermal systems (Simmons and Brown, 2007), which show
that a few to several tens of thousands of years are required in
principle to form giant deposits (e.g., Ladolam, 1,300 t Au:
Simmons and Brown, 2006), a figure that is also supported by
U-Th disequilibrium dating of young hydrothermal systems
(Lalou et al., 1996; Grimes et al., 1998; You and Bickle, 1998).
The current internal precisions (0.1−0.2%, equivalent to
10−20 ka equiv for a 10-Ma event) of the U-Pb, Re-Os, and
40Ar/39Ar methods are marginally sufficient to resolve events
at the ten thousand year scale in Miocene and younger de-
posits (for older deposits the absolute uncertainties increase
steadily so that age resolution gets worse: see below). On the
contrary, U-Pb zircon, Re-Os molybdenite, and 40Ar/39Ar
dates often indicate durations on the order of ~0.1 to 2.0 Ma
for the formation of Oligocene-Miocene porphyry-type min-
eral deposits (e.g., Ballard et al., 2001; Maksaev et al., 2004;
Padilla-Garza et al., 2004; Deckart et al., 2005; Sillitoe and
Mortensen, 2010; Von Quadt et al., 2011) although mineral
deposits formed in a shorter time range (<0.1 Ma) have also
been reported (e.g., Henry et al., 1997; Pollard et al., 2005).
Clearly, such apparently long-lived magmatic-hydrothermal
events are not consistent with thermal modeling of cooling of
single intrusions or with temporal data acquired from active
geothermal fields and suggest that most fossil economic de-
posits were associated with multi-intrusive histories (e.g., Bal-
lard et al., 2001; Sillitoe and Mortensen, 2010), in agreement
with field observations (Kirkham, 1971; Gustafson, 1978).
The current internal precisions of the U-Pb, Re-Os, and
40Ar/39Ar methods allow us to comfortably discriminate events
even at the lower end (~100 ka) of the duration of multipulse,
fossil magmatic-hydrothermal systems. However, there are
some issues that must be addressed so we can critically eval-
uate our ability to accurately determine the duration of fossil
magmatic-hydrothermal events of porphyry systems using the
above techniques. Obtaining useful ages depends on our abil-
ity to obtain precise ages with each one of the three methods
above, and on their accurate cross calibrations (i.e., the theo-
retical ability of the three methods to yield the same age for
the same geologic event). This depends on the accuracy and
precision of various methodological and analytical proce-
dures, ranging from sample choice and treatment to more
fundamental arguments concerning the accuracy of decay
constants and isotopic tracer calibrations. Accuracy indicates
how close a measurement is to “the real” value and depends
on systematic uncertainties that may have different origins.
Precision indicates the uncertainty related to a certain mea-
surement (e.g., date) and is strictly associated with analytical
uncertainties (e.g., uncertainties in the tracer solution cali-
brations, in the decay constant values, in the counting statis-
tics of the mass spectrometer, etc.). A measurement (a date,
in our case) can be precise but inaccurate, or accurate but im-
precise, or any combination thereof.
We will not address “routine” problems of the three above
mentioned dating techniques in any detail: for example, U-Pb
dating of zircon requires that we avoid analyzing inherited
cores, domains affected by Pb loss, and hydrothermal over-
growth/disturbance (e.g., review papers of Davis et al., 2003;
Parrish and Noble, 2003). 40Ar/39Ar data may be biased by ex-
cess 40Ar and 39Ar recoil problems (e.g., in sericite; Harrison
and McDougall, 1981; Onstott et al., 1997; Baumgartner et
al., 2009), whereas inappropriate sample aliquoting can com-
plicate the interpretation of Re-Os dates obtained from
molybdenite (Selby and Creaser, 2004). We assume that each
mineral dated with either one of the three most used isotopic
systems yields dates with the highest precision obtainable,
and that they are accurate, i.e., they are not affected by any of
the above problems that are commonly known causes of inac-
curacy.
Instead, we will address three more fundamental questions
that, if not tackled appropriately, may have significant impacts
on the accurate determination of the duration of magmatic-
hydrothermal events at porphyry systems: (1) what do U-Pb
zircon ages tell us about the emplacement age of an ore-
causative intrusion? (2) how confidently can we compare
radio-isotopic ages from different isotopic systems (U-Pb,
40Ar/39Ar, Re-Os)? and (3) what is the effect of repeated in-
trusive pulses on the interpretation of combined U-Pb, Re-
Os, and 40Ar/39Ar age data?
Specific Information on the U-Pb, Re-Os, and 40Ar/39Ar
Systems that are Relevant for the Following Discussion
First, we will briefly review some specific information
about the U-Pb, Re-Os, and 40Ar/39Ar dating methods, which
is a useful reminder for the following discussion. Readers who
are interested in general descriptions of these methods are in-
vited to consult the specialized literature (e.g., McDougall
and Harrison, 1999; Dickin, 2004; Faure and Mensing, 2005;
Bowring et al., 2006), bearing in mind that each one of these
techniques is an active area of development and refinement
through community-led initiatives.
U-Pb dating of zircon
U-Pb dating of zircon is currently considered to be the most
accurate and precise dating method available (Steiger and
Jäger, 1977; Begemann et al., 2001; Gradstein et al., 2005;
Schoene et al., 2006) because (1) zircon is a highly refractory
DATING THE DURATION OF MAGMATIC-HYDROTHERMAL EVENTS IN PORPHYRY SYSTEMS 567
0361-0128/98/000/000-00 $6.00 567
mineral that survives most of the geologic processes that
occur after magmatic crystallization (usually between 900°
and 700°C); (2) diffusion of the daughter Pb isotopes is very
slow in zircon up to the higher temperatures (~900°C) of its
crystallization (Cherniak and Watson, 2001)—a combination
of points (1) and (2) implies that U-Pb dates of magmatic zir-
cons correspond to their crystallization ages and are the clos-
est possible proxy for emplacement ages of magmatic intru-
sions; (3) the existence of two different radioactive nuclides of
U (238U and 235U) that decay to different radiogenic Pb iso-
topes (206Pb and 207Pb, respectively) generates two U-Pb ages
for the same mineral, providing a unique internal assessment
for open- versus closed-system behavior (i.e., concordance vs.
discordance); (4) the decay constants of 238U and 235U (Jaffey
et al., 1971) are currently known to a higher precision level
(Begemann et al., 2001) than those of 40K (Beckinsale and
Gale, 1969 compiled in Steiger and Jäger, 1977) and 187Re
(Smoliar et al., 1996), although refinement of these values has
been recently proposed (Schoene et al., 2006; Mattinson,
2010; Hiess et al., 2012), and improvements in the precision
of the 187Re decay constant have also been recently claimed
(Selby et al., 2007; see also below).
U-Pb zircon dating can be performed using both in situ
techniques (SIMS-LA-ICP-MS), which have a high spatial
resolution (≥15−30 μm), and non-in situ analyses of bulk sin-
gle grains or fragments of grains with masses as low as a few
micrograms (ID-TIMS), with a lower spatial resolution but
significantly higher precision (currently down to 0.1% vs.
2−3% for LA-ICP-MS and SIMS: Stern and Amelin, 2003;
Sylvester, 2008).
U-Pb bulk dating techniques by isotope dilution-thermal
ionization mass spectrometry (ID-TIMS)
The last decade has seen dramatic analytical and method-
ological improvements in zircon U-Pb dating by ID-TIMS
that have led to an increase of one order of magnitude in pre-
cision and accuracy determined on natural standard materi-
als, from ca. ±0.4% external reproducibility (Black et al.,
2003) to ±0.1% (e.g., Slama et al., 2008). Such improvements
have occurred on four different levels:
1. The treatment of zircons by chemical abrasion (instead
of the previously widely used mechanical air abrasion: Krogh,
1982) prior to their analysis to mitigate problems associated
with Pb loss (Mundil et al., 2004; Mattinson, 2005). Mattin-
son (2005) has shown that zircons treated by the chemical
abrasion technique yield consistent and reproducible ages,
which are older than those obtained from zircons of the same
rock, which have not been treated, simply leached with HF or
mechanically air-abraded. Clearly, the chemical abrasion ap-
proach circumvents Pb-loss related problems in zircons and
yields reproducible data in the great majority of cases.
2. High levels of accuracy can mainly be attributed to the
use of isotope dilution procedures that use internationally
certified elemental and isotope standards that are used to cal-
ibrate tracer solutions for Pb and U, whose concentration and
U and Pb isotope compositions are precisely and accurately
known and calibrated against the international kilogram. Up
to 60% of the uncertainty in U-Pb zircon ages is due to in-
strumental fractionation of the measured Pb and U isotope
ratios (Schmitz and Schoene, 2007). The recent introduction
and dissemination of a single Pb (205Pb) and double U (233U
and 235U) spike (ET535), and more recently of a double Pb
(202Pb and 205Pb) and double U (233U and 235U) spike (ET2535)
among different laboratories that participate in the EARTH-
TIME project (http://www.earth-time.org, Condon and Mem-
bers of the Earthtime Working Group, 2005) has driven a sig-
nificant increase in the analytical precision of Pb and U
isotope ratio measurements by improving the accuracy relative
to instrumental mass fractionation. This has resulted in an
overall internal uncertainty of <0.1% in the age of a single zir-
con grain, implying that a 10-Ma-old zircon can now be dated
with an uncertainty (2σ) of ±10 ka (e.g., McLean et al., 2011).
3. Technical improvements of mass spectrometers with the
availability of ion counting components (SEM, Daly) that
provide highly linear responses over a wide range of counts
(Richter et al., 2001; Palacz et al., 2011).
4. Improved procedural blanks (<1 pg Pb, e.g., Schoene et
al., 2010a) that have reduced uncertainties in the age calcula-
tion generated by poorly constrained common Pb composi-
tions to the point of relative insignificance compared to other
current major sources of uncertainty.
In situ techniques: Laser ablation-inductively coupled
plasma-mass spectrometry (LA-ICP-MS) and secondary
ion-mass spectrometry (SIMS, ion probe dating)
Many geochronologic studies on ore deposits make use of
in situ zircon dating techniques, notably LA-ICP-MS and
SIMS (Ballard et al., 2001; Maksaev et al., 2004: Deckart et
al., 2005; Pollard et al., 2005). Those who are interested in de-
tailed technical aspects of these methods may refer to the ap-
propriate specialized review literature (e.g., Machado and Si-
monetti, 2001; Ireland and Williams, 2003; Sylvester, 2008;
MAC Short Course Volumes 40 and 41 on ICP-MS and
SIMS, respectively). Briefly, these techniques allow dating of
single spots (usually down to 15 μm and 30 μm in diameter
for SIMS and LA-ICP-MS respectively) within zircon crys-
tals, by using either a primary ion beam (usually O−for zircon
dating), or a laser beam. In the LA-ICP-MS technique, a laser
beam (usually 193- or 213-nm wavelength) ablates the min-
eral over a 30- to 60-μm-diam circular area and down to a
depth of a few micrometers from the surface, producing an
aerosol that is carried by an inert gas (usually helium) from
the ablation cell into the plasma source where aerosol parti-
cles are atomized and ionized. In the SIMS technique an ion
beam sputters the mineral surface over a circular area of
≥15-μm diam directly producing secondary ions. In both
cases ions of different elements and their isotopes are ex-
tracted from the source, focused, filtered by an electrostatic
energy analyzer, and finally separated according to their mass-
to-charge ratio. Mass separation may be achieved using a mag-
netic sector (with both SIMS and ICP-MS equipment) or a
quadrupole (in combination with ICP-MS, with a significantly
lower overall precision compared to magnetic sector mass
spectrometry). Ions may be collected by Faraday cups (in
both sequential or simultaneous collection) or by an electron
multiplier (SEM or Daly detector). These in situ techniques
have three considerable advantages over ID-TIMS: (1) the
spatial resolution allowing dating of texturally controlled
areas within single zircon crystals, an essential requirement
568 CHIARADIA ET AL.
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for accurate and meaningful dating of zircons with complex
textures (e.g., inherited cores); (2) easy and fast sample
preparation; and (3) much higher analytical output (with ap-
prox. 20- and 2-min analysis time for SIMS and LA-ICP-MS,
respectively). These advantages are counterbalanced by an
overall lower precision of the date obtained which is, for both
in situ techniques, at least one order of magnitude worse than
for ID-TIMS (i.e., 2−3 vs. 0.1% of ID-TIMS; Stern and
Amelin, 2003; Sylvester, 2008). The poor precision of in situ
techniques results from a combination of factors, including
the smaller amount of material analyzed compared to TIMS
(~103−104μm3for in situ techniques vs. 105μm3for TIMS:
Sylvester, 2008) and, especially, the difficulty to have compo-
sitionally well characterized and homogeneous standards on
which both LA-ICP-MS and SIMS techniques primarily rely
to obtain accurate and precise dates of unknown sample ma-
terial. The need for well-characterized and homogeneous
standards stems from the fact that variably large isotopic and
elemental fractionations are produced in the sources (laser
ablation and plasma for LA-ICP-MS, ion sputtering for
SIMS) for which there is no complete physical understanding
and therefore no mathematical formulation. This problem is
circumvented empirically by analyzing standards and assum-
ing that natural samples behave in the same way as the stan-
dards during the ablation and ionization processes. Such a
procedure requires standards that are chemically and crystal-
lographically as close as possible to the natural samples (ma-
trix-matching) and that they must be homogeneous on a sev-
eral millimeter scale at least, since they are continuously
consumed by analysis. The consequence of the different pre-
cisions inherent to the in situ methods and ID-TIMS is that a
10 Ma age will have an associated uncertainty of 10 ka when
dated by ID-TIMS but of ≥100 Ka when dated by in situ
techniques.
Re-Os dating of molybdenite
Re-Os dating of molybdenite is the only direct method to
routinely date a sulfide mineral with a high internal precision
(<0.2%) throughout the geologic time scale (e.g., Markey et
al., 1998; Stein et al., 2001; Selby et al., 2002). The Re-Os
isochron method is another method to date sulfide minerals
with a good internal precision (<0.6%: e.g., Morelli et al.,
2007). Pb-Pb (e.g., Frei and Petke, 1996; Requia et al., 2003)
and Rb-Sr (Shepherd and Darbyshire, 1981; Nakai et al.,
1993; Petke and Diamond, 1995, 1996; Christensen et al.,
1997; Schneider et al., 2003; Li et al., 2007) isochrons can also
be used to date sulfides, oxides, and fluid inclusions, but with
a lower precision (especially for young minerals), and ambi-
guity regarding whether or not the age relates to the ore min-
eral or to microinclusions within it. 40Ar/39Ar dating has been
sporadically applied to date fluid and mineral inclusions in
sulfides and hydrothermal quartz (Kendrick et al., 2001;
Smith et al., 2001; Phillips and Miller, 2006; Qiu and Jiang,
2007), although there have been too few studies to evaluate it
critically.
Re-Os analyses of molybdenite are considered to be robust
because molybdenite has been shown to survive metamorphic
conditions (Stein et al., 1998) up to the granulite facies (Stein
et al., 2001; Bingen and Stein, 2003). However, in situ LA-
ICP-MS dating of molybdenite has highlighted Re and Os
mobility at the micrometric scale (Kosler et al., 2003; Stein et
al., 2003; Selby and Creaser, 2004), indicating that it is criti-
cal to sample an appropriate quantity of molybdenite using
the Carius tube methodology and ID-NTIMS (isotope dilu-
tion-negative thermal ionization mass spectrometry).
Despite the high internal precision of Re-Os dates (<0.2%),
when comparing Re-Os dates to dates from other isotopic sys-
tems, adequate propagation of the 187Re decay constant un-
certainties must be taken into account. The decay constant
uncertainty of 187Re is reported to be 0.31% (Smoliar et al.,
1996) but may be as high as 1.2% due to nonstoichiometry of
the calibrating solution (Smoliar et al., 1996; Begemann et al.,
2001). Recently, Selby et al. (2007) obtained an 187Re decay
constant value (1.6668 ± 0.0034·10−11 a−1) slightly different
from but overlapping within uncertainty with the value of
Smoliar et al. (1996; 1.666 ± 0.005·10−11 a−1) by cross-cali-
brating molybdenite Re-Os and U-Pb zircon ages. The use of
an Os standard solution with defined (stoichiometric) Os
abundance by Selby et al. (2007) yielded an improved preci-
sion of 0.2% for their 187Re decay constant value. Nonethe-
less, when other laboratories use the 187Re decay constant
value of Selby et al. (2007) they must propagate the ~1% un-
certainty in the calibration of their Os standard used for cali-
bration of their tracer solution.
Single Re-Os molybdenite ages are model ages calculated
under the assumption that no common Os is present in the
analyzed molybdenite (Stein et al., 2001; Markey et al., 2007).
By analyzing several cogenetic molybdenites and plotting
them in an 187Os versus 187Re plot, an isochron age can be ob-
tained that tests the assumption that no initial (common) os-
mium is present in the molybdenites (Stein et al., 2001).
Molybdenite Re-Os dates are crystallization ages that
record the time of molybdenite precipitation from fluids of
various hydrothermal stages. This is because (1) molybdenite
does not overgrow older molybdenite crystals but rather
grows as new crystals deposited in distinct geometric posi-
tions (e.g., crosscutting veins) with respect to predecessors
(Stein et al., 2001), and (2) at an appropriate grain scale, the
Re-Os system in molybdenite can be considered as closed
(Stein et al., 2003; Selby and Creaser, 2004).
40Ar/39Ar dating
The 40Ar/39Ar method is based on the decay of 40K to 40Ar
(40K has a branched decay scheme, with ~10.5% of 40K de-
caying toward 40Ar) and can be applied routinely to many K-
bearing minerals, the most common ones being hornblende,
biotite, muscovite, K-feldspar, alunite, and plagioclase, and
occasionally to mafic groundmass, volcanic glass, and other
exotic minerals (see the summary in McDougall and Harri-
son, 1999). 40K abundance is determined indirectly via fast
neutron irradiation of the sample in a nuclear reactor, which
converts 39K into 39Ar. The 40Ar/39Ar age is then calculated as-
suming a fixed and known 39K/40K ratio, utilizing known decay
constant values and incorporating information concerning the
reactor conditions (e.g., fast neutron flux). The nuclear trans-
formation implies that the user only requires Ar isotope data,
which can be obtained from the same sample aliquot, avoid-
ing sample heterogeneity problems associated with measur-
ing the daughter (40Ar) and parent (40K) isotope abundances
in different sample aliquots.
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Argon gas extraction from the target minerals is usually
achieved by incrementally heating the sample (either by laser
or with a furnace) and measuring the Ar isotope compositions
of the gas released during each step, which are in turn used
to calculate an age for that step. Theoretically, the relation-
ship between the ages of individual steps reflects the distrib-
ution of the daughter isotope (40Ar) throughout the mineral,
relative to variations in diffusion length and diffusivity, when
the mineral is anhydrous (e.g., feldspars). This distribution
can be used to constrain the thermal history of the mineral
(assuming daughter isotopes are lost by Fickian diffusion) and
can indicate whether or not the age is a crystallization age.
When indistinguishable ages are obtained from contiguous
heating steps that span at least 50% of the total 39Ar released,
we can define a plateau, whose corresponding age may be in-
terpreted as the time of rapid cooling through a specific tem-
perature range (e.g., Darlymple and Lanphere, 1974; Lee et
al., 1991). If the mineral crystallized at temperatures that are
below the temperatures of partial Ar isotope loss, then the
plateau age can be a reliable estimate of the time of crystal-
lization. Thermally activated argon diffusion (and loss from
the mineral) is significant in all minerals above temperature
thresholds that vary according to the crystalline structure and
composition of the minerals. Therefore, 40Ar/39Ar dates do
not usually record the crystallization ages of minerals but
rather relate to the cooling history of the mineral below a cer-
tain temperature range.
A significant problem with the 40Ar/39Ar method is to accu-
rately estimate the quantity of 39Ar that is generated in the
nuclear reactor. Direct determination is usually not per-
formed due to the difficulties associated with accurately mea-
suring neutron flux and neutron capture by 39K. Conse-
quently, an indirect method is used where (1) primary
standards of known ages, or (2) secondary standards with ages
intercalibrated against the primary standards are irradiated
together with the unknown samples, generating an irradiation
specific “J” factor to account for the nuclear parameters and
decay branching ratios of 40K.
An additional problem arises in 40Ar/39Ar dating of young
and/or very low K/Ca minerals. We can define two sources of
40Ar in minerals, which are (1) 40Ar that forms by radioactive
decay of 40K, and (2) 40Ar that was trapped by the mineral as
it crystallized from the magma or precipitated from a fluid.
The 40Ar/39Ar method assumes that the nonradiogenic,
trapped component, which must be accounted for, has an at-
mospheric composition, and violation of this assumption usu-
ally leads to 40Ar/39Ar ages that are too old. Isotope correlation
plots provide a means to test this assumption, within limits
(e.g., Kuiper, 2002). However, debate exists about the com-
position of argon isotopes in air, and inaccuracies can arise if
the wrong value is used, especially in young and/or high Ca,
low K material (Renne et al., 2009). Argon composition in air
is usually provided as a 40Ar/36Ar ratio, and a value of 295.5 ±
0.5 was adopted by the International Union of Geological Sci-
ences in 1976 (Steiger and Jäger, 1977; although it was deter-
mined by Nier, 1950). More recently, Lee et al. (2006) re-
ported a more precise value of 298.56 ± 0.31. Renne et al.
(2009) showed that an overwhelming majority of 40Ar/39Ar
ages are insensitive to the isotopic composition of atmospheric
argon and significant inaccuracies will only affect Quaternary
minerals, unless the target minerals have unusually low K/Ca
ratios. Nevertheless, we recommend that the ore community
use the atmospheric argon composition reported by Lee et al.
(2006), which is 298.56 ± 0.31 (40Ar/36Ar) and 0.1883 ± 0.0003
(38Ar/36Ar), due to the superior precision of these measure-
ments and the more advanced methods used to determine
them (e.g., Renne et al., 2009).
Resolving Magmatic-Hydrothermal Events in
Porphyry Systems: The Effect of Age
With increasing age, the geochronological methods will
yield a lower age resolution when determining the life span
of a magmatic-hydrothermal system. This occurs because the
relative uncertainty associated with a date translates into in-
creasingly larger absolute uncertainties with increasing ab-
solute ages (i.e. a ±0.2% uncertainty corresponds to ±0.003,
±0.03, ±0.3, and ±3.0 Ma absolute uncertainties for 1.5, 15,
150, and 1500 Ma dates, respectively). Therefore, younger
porphyry systems will be dated with a higher resolution be-
cause the absolute uncertainties associated with their dates
(assuming that they are accurate) will be smaller. However,
this theoretical advantage has a trade-off because younger
isotopic systems show a lower degree of ingrowth of radi-
ogenic daughter isotopes (e.g., 206Pb, 207Pb, 187Os, and 40Ar).
Therefore, dating very young systems requires precise mea-
surement of low amounts of radiogenic isotopes, which relies
on (1) the sensitivity of the mass spectrometer; (2) low pro-
cedural blanks (i.e., the amount of the isotope which is not
inherent to the sample itself but introduced artificially dur-
ing sample manipulation, preparation, and analysis, includ-
ing desorption from the mass spectrometer); (3) the maxi-
mum quantity of material that can be analyzed (this is
particularly significant when dating <100-ka mafic ground-
mass with the 40Ar/39Ar method); and (4) high parent nuclide
concentrations and parent/daughter nuclide ratios. Beyond a
certain age threshold (which cannot be precisely quantified
because it varies depending on the amount of radioactive
parent incorporated into the mineral and quantity of mater-
ial available), it is not possible to obtain ages with a useful
precision.
Summarizing, the absolute precision of increasingly younger
dates below a certain threshold tends to degrade (instead of
steadily improving) because the relative precision of the ra-
dioactive methods significantly deteriorates as the quantity of
radiogenic isotopes produced decreases. Therefore, in gen-
eral terms, there is a minimum in the uncertainty associated
with single dates obtained with the U-Pb, Re-Os, and
40Ar/39Ar dating methods. This characteristic is illustrated in
Figure 2 for the U-Pb system, based on real zircon U-Pb data
from the University of Geneva (Fig. 2a). The same concept
applies to the 40Ar/39Ar and Re-Os systems. The diagram
shows that the minimum uncertainty occurs for ages of 2 to 4
Ma for an average zircon grain weighing ~12 μg and contain-
ing 250 ppm U (Fig. 2b). Absolute age uncertainties strongly
increase for younger ages and, with lower gradients, toward
older ages (Fig. 2b). Decreasing the sample size and/or the U
content increases the age uncertainty and shifts it to older
ages (Fig. 2b). Increasing the sample size and/or the U con-
tent decreases the age uncertainty and shifts it to younger
ages (Fig. 2b).
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This simulation shows that, as a rule-of-thumb, the time
range of the minimum absolute uncertainty for U-Pb dating
of “average” zircons (i.e., 5−20 μg, 125−500 ppm U) lies be-
tween 1 and 10 Ma. According to Marsh et al. (1997) the op-
timum peak for the 40Ar/39Ar method lies between 500 ka and
5 Ma. Exceptions occur when zircons (for U-Pb dating) or
other minerals (for 40Ar/39Ar and Re-Os dating) are particu-
larly enriched or depleted in U (K, Re) and/or have unusually
large or small grain sizes, which shift the optimum “valley” re-
gion to lower or higher ages, respectively. In fact, there are
some examples of precise dating of Pleistocene ages both by
zircon U-Pb (Crowley et al., 2007; Bachmann et al., 2010) and
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volume = 2500000 μm3, U = 250 ppm
0
0.005
0.01
0.015
0.02
0.025
0.03
0 2 4 6 8 101214161820222426283032343638404244464850
Age (Ma)
Absolute error (Ma)
Measured 206Pb/204Pb
double volume or double U half volume or half U
2σage (%) measured
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 200 400 600 800 1000 1200 1400 1600 1800 2000
A
B
FIG. 2. Simulation showing how absolute errors change with zircon ages based on real data obtained in our laboratory
(Department of Mineralogy, University of Geneva). Age data collected at our laboratory on >150 single zircons spanning an
age interval between 1.4 and 232 Ma show that that there is a good correlation between decreasing 206Pb/204Pb values mea-
sured in a zircon grain and the corresponding relative precision of its 238U/206Pb age (A); in other words the higher the
206Pb/204Pb values measured in a zircon, the smaller is the relative error of the associated 238U/206Pb age (the relative errors
here considered do not include tracer calibration and decay constant uncertainties, which are however systematic errors that
can be added without affecting the systematics of the correlation). Converting relative age errors into absolute age errors by
means of equations of power law curves best fitted to the trend defined in the 206Pb/238U vs. relative error space (A) allows
us to plot absolute age errors vs. sample age (B), assuming that the 206Pb/204Pb values are linearly correlated with age (this
implies that we assume that all 206Pb/204Pb values were measured on zircon grains with the same size and same U content;
this is not true but gives us an idea of the behavior of an average zircon with an average grain size and U content). The plot
shows that there is a “valley” in the absolute age error, which, for an average zircon grain weighing ~12 μg and containing
250 ppm U, lies between 4 and 2 Ma. Absolute age errors strongly increase for younger ages (becoming much larger than
the age itself and therefore indicating that dating of such young zircons is simply meaningless) and increase in a more gen-
tle way for older ages. Decreasing the sample size and/or the U content results in a shift of the minimum absolute age errors
to older ages and to higher absolute values. Increasing the sample size and/or the U content results in a shift of the minimum
absolute age errors to younger ages and to smaller absolute values.
by 40Ar/39Ar (Singer and Pringle, 1996). On the other hand,
porphyry systems that are ≤~1 Ma old are relatively rare
(e.g., Ok Tedi, Ladolam, FSE Lepanto: Page, 1975; Sillitoe,
1994; Arribas et al., 1995), which is probably also an expres-
sion of exposure (Wilkinson and Kesler, 2009), and thus the
problem of decreased absolute precision arising from dating
such young deposits is not a routine issue.
An additional problem associated with dating zircons with
the U-Pb technique is the correction for initial Th disequilib-
rium, which is again particularly relevant for young zircons
(Schärer, 1984). Because Th is less easily accommodated in
the crystalline lattice of zircon than U, 230Th, an intermediate
product of the decay series of 238U, which ultimately decays
into stable 206Pb, will be depleted in zircon during magmatic
crystallization with respect to its parent 238U, resulting in a
final deficit of 206Pb and yielding a younger than real
238U/206Pb age. The contrary occurs in monazite, where 230Th
becomes enriched, leading to an excess in 206Pb. The correc-
tion can be significant when dating young zircons, resulting in
an age variation of >10% in ~0.8-Ma old zircons of the Bishop
Tuff (Crowley et al., 2007). The accuracy of such a correction
depends on the accuracy of the estimation of the Th/U value
of the magma from which the zircons have crystallized, which
is usually inferred and not measured, and relies on the deter-
mination of the Th/U value of the zircon, directly calculated
from measured radiogenic 208Pb concentrations. Generally,
large uncertainties on the estimated magmatic Th/U value
have an insignificant impact on the correction of progressively
older rocks (being much smaller than the ~0.1% external un-
certainty inherent to the U-Pb method), but they have signif-
icant impacts on young zircons, where they introduce an un-
certainty of the same order of magnitude as that which is
inherent to the method (Fig. 3a). Figure 3b shows that the
age uncertainty resulting from Th disequilibrium corrections
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0.1
1
10
0 2 4 6 8 10 12 14
Δt/t (%)
t (Ma)
Th/Umagma=4
Th/Umagma=2
uncorrected for initial 230Th disequilibrium
corrected for initial 230Th disequilibrium using Th/Umagma = 2
corrected for initial 230Th disequilibrium using Th/Umagma = 4
207Pb/235U
~5 Ma
5.04
5.12
5.00
5.08
~10 Ma 10.12
10.04
10.00
10.08
~100 Ma100.2
100
206Pb/238U
A
B
FIG. 3. A. Concordia plots for zircons ~5, 10, and 100 Ma old, both uncorrected for initial Th disequilibrium and cor-
rected for Th/Umagma values of 2 and 4, showing that the magnitude of correction is strongly dependent on age (more signif-
icant for younger ages). Note that error ellipses (2σof 0.1% for 206Pb/238U and 1−2% for 207Pb/235U) are shifted only along
the vertical axis by the Th disequilibrium correction. This is because the Th correction concerns 230Th, a product of the decay
chain of 238U. B. Variation of the relative age correction (Δt/t*100) for Th disequilibrium with absolute age of the zircon. The
plot shows that the relative age correction becomes increasingly higher with younging ages, being >8% for zircons <1 Ma
old. The correction becomes <1% for zircons older than 10 Ma and progressively decreases to insignificant values for older
zircons. The plot also shows how the correction is affect by different Th/U values. For instance, for a 1-Ma-old zircon Th/U
values of 2 and 4 result in ages differing by 1.4%. For progressively older zircons the curves of different Th/U values tend to
converge and therefore the relative differences become insignificant.
for Th/U values that range between 4 and 2 are 1.4% for a 1-
Ma sample (i.e., significantly more than the 0.1% external un-
certainty of the U-Pb method), 0.14% for a 10-Ma sample
(i.e., on the same order of magnitude as the external uncer-
tainty of the method), and (not shown) 0.014% for a 100-Ma
sample (i.e., insignificant). Establishing methods to accu-
rately and precisely determine magmatic Th/U ratios (e.g., by
measuring Pb-rich minor or trace mineral phases, or melt in-
clusions in zircon) should become a field of active research
because it is a prerequisite to accurately date young zircons
using the U-Pb method.
Emplacement Age of Intrusions Inferred from
Zircon U-Pb Dating by ID-TIMS
Recently improved analytical procedures of the ID-TIMS
method have introduced new challenges to the accurate in-
terpretation of the temporal evolution of porphyry systems.
Analyses of several Cretaceous to Tertiary zircon grains from
the same intermediate to felsic magmatic rock carried out
with these improved methods almost invariably reveal a series
of discrete (not overlapping within uncertainty), concordant
(or close to concordant) ages over a 104- to 106-year time
range (e.g., Matzel et al., 2006; Miller et al., 2007; Simon et
al., 2008, Schaltegger et al., 2009). We are confident that such
a spread is not the result of Pb loss because (1) the applica-
tion of chemical abrasion has been shown to consistently and
efficiently remove damaged domains within zircon that are
prone to Pb loss (Mattinson, 2005); (2) the zircons that yield
these different ages have geochemical and isotopic (Hf) vari-
ations that are consistent with their formation from evolving
magmas (e.g., Schoene et al., 2010a); (3) zircons that crystal-
lized from mafic magmas (i.e., magmas that have not under-
gone recycling and mixing processes, and in which zircons are
likely to crystallize “simultaneously” as a result of Zr satura-
tion in residual melt pockets) display reproducible ages
within the high precision of the single grain analyses
(Schoene et al., 2010b); and (4) U-Th ages (SIMS and TIMS)
of zircons from Quaternary volcanic rocks show also a spread
of 0.X Ma within the same sample (e.g., Brown and Fletcher,
1999; Charlier and Zellmer, 2000; Bacon and Lowenstern,
2005; Bachmann et al., 2007).
The age spread observed in Cretaceous to Tertiary zircon
grains from single samples of intermediate to felsic magmatic
rocks indicates zircon crystallization in earlier melt batches of
a magmatic system (antecrystic zircon) or protracted growth
in the present melt volume (autocrystic zircon; terminology
after Miller et al. 2007). Because zircon antecrysts are vari-
ably incorporated from partly crystallized portions of incre-
mentally constructed plutonic systems at depth, most felsic
intrusions do not host zircons with a single age population.
Consequently, calculation of a weighted mean chemical abra-
sion (CA)-ID-TIMS U-Pb age from these high precision
dates would violate statistical rules and be meaningless with
respect to the timing of emplacement of the intrusion. How-
ever, dating the same Cretaceous to Tertiary zircons with a
lower precision procedure than that achievable with state-of-
the-art CA-ID-TIMS, e.g., by LA-(MC)-ICP-MS or SIMS,
will produce larger uncertainties on individual zircon grain
ages (>1%). These lower precision data could statistically de-
fine a single age population (indicated by an MSWD of ~1),
such that a weighted mean age might be calculated. However,
an apparent weighted mean age calculated from these less
precise data does not represent a geologically significant em-
placement age of a given intrusion but rather corresponds to
the average age of a protracted period of zircon crystallization
associated with crustal magmatism. The low-precision weighted
mean age is thus, to some extent, a geologically meaningless
age and the associated age uncertainty will underestimate the
real time period of protracted zircon growth and incremental
pluton construction.
An example of such a problem is illustrated in Figure 4
where single zircon U-Pb dates from the Miocene porphyry
of Chaucha, Ecuador, which is associated with a porphyry Cu-
Mo deposit (Schütte et al., 2010), were obtained with the
ET535 spike (green bars) and are compared to the same dates
with artificially inflated uncertainties (2.5%: white bars) cor-
responding to those obtainable by the LA-(MC)-ICP-MS and
SIMS techniques. In the latter case, a statistically significant
mean 238U/206Pb age can be obtained (15.09 ± 0.13, MSWD =
0.95) by pooling together the eight points with lower preci-
sion, which may be incorrectly interpreted as the “emplace-
ment age” of the intrusion, although, in reality, it underesti-
mates the full time range of zircon crystallization and crustal
magmatism. On the contrary, no statistically significant mean
age (15.11 ± 0.13 Ma, MSWD = 19) can be obtained from the
data obtained using CA-ID-TIMS and the ET535 spike be-
cause the eight data points scatter well beyond their signifi-
cantly smaller analytical uncertainties. Importantly, the
youngest date of the eight points, which most closely ap-
proaches the youngest zircon crystallization event and thus
provides an upper age limit to the porphyry intrusion (14.84
± 0.07 Ma), is significantly younger than the statistically
meaningful mean age (potentially interpretable as the intru-
sion age) obtained with lower precision techniques (15.09 ±
0.13). A similar mismatch between high-precision single zir-
con grain ID-TIMS ages and mean 238U/206Pb LA-(MC)-ICP-
MS and SIMS ages measured on large zircon populations has
been reported also by von Quadt et al. (2011), for the case of
Bajo de Alumbrera. They show that the difference between
their ID-TIMS (7.216 ± 0.018) and LA-ICP-MS dates (7.92
± 0.14, MSWD 1.67: Harris et al., 2004) is even larger (>0.54
Ma) in this case. These differences relate to the lifetime of a
magmatic cycle in a specific area and to different degrees of
recycling of zircons from previous magmatic pulses belonging
to that cycle. However, differences on the order of 0.X Ma be-
tween the two different approaches discussed above are likely
to be common. Given the time scale of magmatic-hydrother-
mal events in porphyry systems (on the order of 0.1−0.X Ma:
see above), such an inaccuracy in dating the intrusion age is
significant (up to several 100%) and may lead to estimations
of the life span of a magmatic-hydrothermal system that are
too long.
Another example is given in Figure 5, although on a much
smaller time scale: it occurs within the ID-TIMS method by
using either the “old” 205-235 UNIGE spike (i.e., a single Pb
and single U spike) or the ET2535 spike for analyzing zircons
extracted from an ash bed at the Triassic-Jurassic boundary in
the Utcubamba valley, northern Peru. In the case of the
analyses carried out with the 205-235 UNIGE spike uncer-
tainties associated with individual zircon grains allow pooling
DATING THE DURATION OF MAGMATIC-HYDROTHERMAL EVENTS IN PORPHYRY SYSTEMS 573
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all individual dates into a weighted mean 206Pb/238U age
(201.58 ± 0.17 Ma) with a statistically significant MSWD of
0.87 (Schaltegger et al., 2008). In contrast, zircons from the
same ash bed analyzed later with the ET2535 spike have so
small associated uncertainties that prevent the possibility of
pooling them together into a single statistically meaningful age
(Schoene et al. 2010b). The zircons from the coeval North
Mount Basalt crystallized instantaneously from a supersat-
urated residual melt and allow calculation of a statistically
significant average (Schoene et al., 2010b). This example shows
how continuous methodological and analytical progress is
allowing us to narrow down uncertainties of single zircon ages
to the point that we are gaining insight into the timing of mag-
matic processes that could not be traced just a few years ago.
To summarize this section, dramatic improvements in the
precision of U-Pb zircon dating by ID-TIMS during the last
10 years show that zircon grains from the same sample of
intermediate to felsic magmatic rocks (such as porphyry in-
trusions which are generally associated with a mineralized
system) almost invariably yield a scatter on the order of a few
0.X Ma or higher, implying that the magmas from which these
rocks crystallized are the result of complex mechanisms of
574 CHIARADIA ET AL.
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14.0
14.2
14.4
14.6
14.8
15.0
15.2
15.4
15.6
15.8
16.0
mean = 15.09±0.13 [0.87%] 95% conf.
MSWD = 0.95, probability = 0.47
mean = 15.11±0.13 [0.83%] 95% conf.
MSWD = 19, probability = 0.000
15.6
15.4
15.2
15
14.8
14.6
0.00224
0.00228
0.00232
0.00236
0.00240
0.00244
0.011 0.013 0.015 0.017 0.019
206Pb/238U
data-point error ellipses are 2σ
207Pb/235U
206Pb/238U age (Ma)
A
B
FIG. 4. A. Concordia plot of single zircon crystal ages from sample E07003, taken from the Chaucha porphyry (Ecuador;
Schütte et al., 2010). Zircons from this sample were analyzed using the ET535 spike and show distinct ages covering a 500-
ka range (14.84 ± 0.07−15.32 ± 0.06 Ma). B. 206Pb/238U ages measured by TIMS using the ET535 spike do not yield a single
zircon population (mean age of 15.11 ± 0.13 Ma, with an MSWD = 19). However, if we apply to the same ages uncertainties
(2.5%) that correspond to those typical of an in situ technique (LA-ICPMS, SIMS) we obtain a statistically significant (15.09
± 0.13, MSWD = 0.98) mean 206Pb/238U age. This age differs significantly from the ID-TIMS age of the youngest single zir-
con (14.84 ± 0.07 Ma), which is an upper limit of the intrusion age.
recharge and recycling of previous magma batches and of
their crystal (including zircon) cargo. If our purpose is to pre-
cisely and accurately constrain the duration of a magmatic-
hydrothermal system, pooling together the U-Pb ages of
these zircons with a technique with relatively low temporal
resolution (LA-ICP-MS, SIMS) may yield a statistically valid
but geologically meaningless and inaccurate age for the intru-
sion. This occurs because porphyry systems are invariably as-
sociated with intermediate to felsic intrusions, which have
been shown by various petrographic and geochronologic
studies to have evolved via reworking of previous magmatic
pulses on the 0.X Ma scale (e.g., Bachmann et al., 2007; Miller
et al., 2007; Michel et al., 2008; Schaltegger et al., 2009). Con-
sequently, LA-ICP-MS and SIMS ages may differ by up to
several 0.X Ma from the youngest age of a zircon population
measured with state-of-the-art ID-TIMS techniques, which
will more accurately approach an upper limit for the intrusion
age. If additional minerals with a lower U-Pb closure temper-
ature (e.g., titanite), crystallized from the same evolving magma,
are dated with a similar precision (Schaltegger et al., 2009;
Schoene et al., 2012), the age of intrusion can be estimated
more accurately (because the titanite closure temperature for
Pb is around the solidus in magmatic systems), even though
the relatively large amount of common Pb in titanite results
in a higher uncertainty on the obtained age. Since the dura-
tion of multistage, magmatic-hydrothermal events that ulti-
mately lead to economic mineralization is on the order of 0.1
to several 0.X Ma (see above), an inaccuracy of 0.X Ma in dat-
ing the intrusive event that initiates the magmatic-hydrother-
mal event will lead to fundamentally wrong conclusions about
the life span of porphyry systems (and more generally to all
ore systems associated with magmatic intrusions).
Intra- and Intercalibration of the U-Pb, 40Ar/39Ar,
and Re-Os Systems
The formation/equilibration of ore and alteration minerals
at different temperatures in a porphyry system has led to the
widespread use of combined geochronological tools to date
different magmatic-hydrothermal stages of porphyry systems.
The most commonly used tools are U-Pb in zircon (and titan-
ite) to date the intrusive and high-temperature hydrothermal
events, Re-Os in molybdenite to date the sulfide mineraliza-
tion stages, and 40Ar/39Ar to date different hydrothermal al-
teration stages according to the different closure tempera-
tures of the analyzed minerals. The combined use of these
dating techniques allows us to bracket the duration of mag-
matic-hydrothermal events associated with porphyry systems
(Fig. 1).
However, several problems are usually overlooked when
comparing ages acquired by these different methods, and ages
obtained using the same method but in different laboratories.
Intramethod reproducibility: the U-Pb lesson
The U-Pb, Re-Os, and 40Ar/39Ar methods have reached a
high degree of internal precision, which is usually better than
0.2%. However, complications arise when comparing dates ob-
tained using the same isotopic system in different laboratories.
The U-Pb, Re-Os, and 40Ar/39Ar methods are indirect dat-
ing methods because they rely on the precision and accuracy
of measurements of other intensive variables. The U-Pb and
Re-Os methods make use of isotopic tracer solutions to mea-
sure the concentrations and the atomic ratios of the isotopes
needed to calculate the ages. The tracer solutions must be ac-
curately calibrated, i.e., their isotopic composition and ele-
ment concentrations must be precisely and accurately known.
Ages obtained with the same tracer solution within the same
laboratory may be inaccurate if its calibration is inaccurate,
but this does not affect the relative relationships among the
obtained dates because they are all affected by the same sys-
tematic uncertainty. In contrast, a comparison of ages ob-
tained by the same method in different laboratories, which
use tracer solutions that have been calibrated differently, is
problematic because the different degrees of accuracy of cal-
ibration of the two tracer solutions will ultimately yield dif-
ferent ages for the same sample. Such interlaboratory bias is
addressed, for example in the U-Pb zircon dating community,
by intercalibration exercises using natural zircon materials
such as the Temora zircon (Black et al., 2003) or the Plesovice
standard zircon (Slama et al., 2008), or by repeated analyses
of synthetic U-Pb solutions (Condon et al., 2008).
The problem of interlaboratory bias is being tackled in the
U-Pb community by the use of a common tracer solution
that has been disseminated among different laboratories
(EARTHTIME ET535 and ET2535 tracer solutions: http://
www.earth-time.org, Condon and Members of the Earthtime
Working Group, 2005). This approach eliminates systematic
uncertainties that relate to the calibration of different tracer
solutions in different laboratories, and the reproducibility of
standard 206Pb/238U ages among these laboratories is within
0.1% (accuracy; Fig. 6). Such an improvement in precision
DATING THE DURATION OF MAGMATIC-HYDROTHERMAL EVENTS IN PORPHYRY SYSTEMS 575
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FIG. 5. 206Pb/238U ages and relative 2σerror bars of zircons from an ash
bed at the Triassic-Jurassic boundary in the Utctubamba Valley, northern
Peru (data from Schaltegger et al., 2008, and Schoene et al., 2010b). The zir-
cons on the left-hand side of the plot were analyzed using the 205-235
UNIGE tracer, whereas those on the right-hand side were analyzed using the
ET2535 spike: note the difference of the associated errors bars in the two
cases. Also shown are zircons from the North Mountain Basalt of the CAMP
province analyzed with the ET2535 tracer. Note that whereas zircons from
the felsic magma chamber, whose eruption has produced the Utcubamba ash
layer, spread an age range of about 600 ka, those of the North Mountain
Basalt have indistinguishable ages. This is the result of magmatic recycling at
the X00-ka scale to produce felsic magmas in the crust as opposed to the lack
of magmatic refinement in basaltic magmas.
199
200
201
202
203
204
Schoene et al. (2010)
Mean=201.375±0.02 Ma
MSWD=0.7
ET2535 tracer
Schaltegger et al. (2008)
Mean=201.58±0.17
MSWD=0.87
205-235 UNIGE tracer
Schoene et al. (2010)
MSWD=23
ET2535 tracer
Sample 86
Ash bed at the Triassic-Jurassic
boundary Utcubamba valley,
Northern Peru
Sample NMB-03-1
North Mt. Basalt, CAMP
Nova Scotia, Canada
206Pb/238U age (Ma)
204
203
Ash bed at the
Northern P
boundary Utcubamba valle
rias
T
Tr
Ash bed at the
Sample 86
eru
P
Pe
,
y
y,
mba valle
ey
assic
ur
ra
ssic-J
Ju
Sample 86
ET2535 tracer
MSWD=23
Schoene et al. (2010)Schoene et al. (2010)
202
201
U age (Ma)
238
Pb/
North Mt. Basalt, CAMP
Schoene et al. (2010)
va Scotia, CanadaNo
North Mt. Basalt, CAMP
Sample NMB-03-1
Schoene et al. (2010)
va Scotia, Canada
North Mt. Basalt, CAMP
Sample NMB-03-1
201
200
199
Pb/
206
205-235 UNIGE tracer
MSWD=0.87
Mean=201.58±0.17
gger et al. (2008)Schalte
205-235 UNIGE tracer
Mean=201.58±0.17
gger et al. (2008)
ET2535 tracer
MSWD=0.7
Mean=201.375±0.02 Ma
Schoene et al. (2010)
Mean=201.375±0.02 Ma
Schoene et al. (2010)
199
and accuracy is both the result of using a double isotope spike
for U and Pb (improving the precision of the isotope ratio
measurements), and using the same tracer solution calibrated
against gravimetrically calibrated reference solutions (im-
proving the reproducibility of the age of the same standard in
different laboratories). Evidently, using U-Pb ages obtained
from several laboratories with a reproducibility worse than
0.1% to determine the geochronologic history of magmatic-
hydrothermal events in a porphyry system can inhibit the ac-
curate and precise estimation of the life span of the system.
Similar problems are to be expected when comparing Re-Os
ages from different laboratories, which make use of different
tracer solutions. Indeed, a ~1% uncertainty in the calibration
of the Os standard used for calibration of tracer solutions is
typical of most Re-Os laboratories (see also above).
Intramethod reproducibility: the 40Ar/39Ar case
40Ar/39Ar is an indirect dating method that relies on the
known ages of standards (which are also referred to as fluence
monitors), interspaced with unknown samples in the irradia-
tion package, to calculate the neutron fluence parameter J,
which is a function of nuclear irradiation parameters such as
the neutron flux (e.g., Renne et al., 1998; McDougall and
Harrison, 1999). The calculation of accurate 40Ar/39Ar ages
relies on a well-constrained interpolation of the J value be-
tween the fluence monitors, which can be achieved by using
a large number of standards intermixed with the samples
during irradiation (Renne et al., 1998). A meaningful com-
parison of 40Ar/39Ar ages obtained in different laboratories
that use different standards requires that either (1) primary
standards (e.g., GA-1550 biotite, McDougall and Roksandic,
1974) are accurately intercalibrated using U-Pb or, tradition-
ally, K/Ar dating; or (2) secondary standards (e.g., Fish
Canyon Tuff sanidine [FCTs], Renne et al., 1998) are accu-
rately intercalibrated with primary standards. Renne et al.
(1998) intercalibrated some of the most common standards
used as neutron fluence monitors, and the intercalibration
factors they obtained indicate a small contribution (<1%) to
the overall uncertainties in the ages that arise from such a
process. A significantly higher percentage of the absolute in-
ternal uncertainty can be attributed to analytical uncertainty
that arise while determining the 40Ar*/39ArKvalue (i.e., the
ratio between the radiogenic daughter isotope and the proxy
for the parent isotope, 40K) of the unknown sample relative to
that of the standard (Renne et al., 1998).
Intermethod comparisons: U-Pb (Re-Os) versus 40Ar/39Ar
When comparing U-Pb (Re-Os) and 40Ar/39Ar geochrono-
logical dates which have high internal precisions (<0.2%), un-
certainties in the decay constants, tracer calibrations, and
ages of the standards used as neutron fluence monitors (for
40Ar/39Ar) must be taken into account. This very important
step is often neglected, also in geochronological studies of ore
deposits.
At present the 238U and 235U decay constants are known
with better precision (0.1% and 0.14% respectively, 2σ; Jaffey
et al., 1971; Steiger and Jäger, 1977; Begemann et al., 2001),
than for 40K (≥2%; Min et al., 2000; Begemann et al., 2001)
and 187Re (≥0.31%, Smoliar et al., 1996; Begemann, 2001;
~0.2%, Selby et al., 2007). This difference is one of the main
reasons why U-Pb ages are taken as a benchmark against
which 40Ar/39Ar and Re-Os ages are calibrated (Steiger and
Jäger, 1977; Begemann et al., 2001).
A meaningful comparison of 40Ar/39Ar dates with U-Pb
and Re-Os dates requires that we have an accurate knowl-
edge (Renne et al., 1998) of (1) the ages of the primary and
secondary standards used for 40Ar/39Ar dating accuracy, and
(2) decay constants for the decay of 40K to 40Ar (λε) and 40Ca
(λβ). Inaccuracies in the ages of the standards and decay
constants propagate into the calculated age. These require-
ments have proven to be problematic, as is shown by dis-
agreements concerning the age of the FCTs standard, which
is one of the most frequently used standards in 40Ar/39Ar
geochronology.
576 CHIARADIA ET AL.
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336.4
336.6
336.8
337.0
337.2
337.4
337.6
337.8
338.0
206Pb/238U date (Ma)
UNIGE MIT NIGL BSU
average = 337.15 ± 0.31 (2σ) (0.092%)
±0.1%
FIG. 6. Reproducibility of the ID-TIMS U-Pb age of the Plesovice zircon standard in four different laboratories (UNIGE
= University of Geneva; MIT = Massachusetts Institute of Technology; NIGL = NERC Isotope Geosciences Laboratory;
BSU = Boise State University) using the same calibrated ET2535 tracer is at the 0.1% level (Slama et al., 2008).
External uncertainties attached to 40Ar/39Ar dates may in-
crease to >2% when decay constant and standard age uncer-
tainties are considered (Min et al., 2000; Kuiper et al., 2008).
Such a high external uncertainty is significantly higher than the
internal precision of 40Ar/39Ar dates and inhibits useful com-
parisons of 40Ar/39Ar dates with those acquired by the U-Pb and
Re-Os methods to resolve the time scale of processes occur-
ring in porphyry systems. An uncertainty of ≥2% for a 10-Ma
age corresponds to >0.2 Ma. This can be a significant propor-
tion (up to more than the totality) of the expected magmatic-
hydrothermal life span of many porphyry systems, precluding
the use of the 40Ar/39Ar method for such a purpose. Unfortu-
nately, this is frequently ignored in geochronological studies of
ore deposits (however, see Parry et al., 2001, for a discussion
of 40Ar/39Ar dates calculated with different Fish Canyon Tuff
ages reported in the literature at that time: see also below).
There have been several attempts to quantify and increase
the accuracy of the 40Ar/39Ar method with respect to U-Pb
and astronomically tuned ages during the last decade. A com-
parison of 40Ar/39Ar dates with U-Pb and astronomically tuned
ages from the same lava flow or volcanic tuff layer reveals a
systematic offset of 40Ar/39Ar dates of ~1% toward younger
values (in reality 40Ar/39Ar dates would overlap within uncer-
tainty with U-Pb and astronomically tuned ages if an uncer-
tainty of >2% was considered for the 40Ar/39Ar dates). This
offset has been attributed to inaccurate ages of the standards
(in particular, the age of the commonly used secondary FCTs
standard), and inaccurate values of the 40K decay constant
(Min et al., 2000; Kuiper et al., 2008; Renne et al., 2010). To
solve this problem, new absolute ages of the standards (in
particular of FCTs) have been proposed that force 40Ar/39Ar
dates to agree within uncertainty with the U-Pb and astro-
nomically tuned ages (Min et al., 2000; Kuiper et al., 2008).
Kuiper et al. (2008) proposed an extremely precise age of
28.201 ± 0.023 Ma (1σ) for the FCTs standard, which is older
and more precise than the previously commonly used ages of
28.02 ± 0.16 Ma (internal uncertainty, ±1σ; Renne et al.,
1998), and 28.03 ± 0.08 Ma (±1σinternal uncertainty), which
was recently proposed by Jourdan and Renne (2007), using
the decay constant for 40K of Steiger and Jäger (1977).
Min et al. (2000), Jourdan and Renne (2007), and Renne et
al. (2010) have shown that knowledge of the accurate age of a
standard only partly contributes to an accurate calibration of
the 40Ar/39Ar system, and that accuracy of the decay constants
of 40K should also be taken into account. Renne et al. (2010)
used a statistical optimization approach to propose an older
age for the secondary FCTs standard (28.305 ± 0.036 Ma) and
a revised value for the decay constants of 40K (λβ4.9737 ±
0.0093*10−10 a−1, λε0.5755± 0.0016*10−10 a−1), based on in-
tercalibrations with U-Pb ages. These proposed values of the
age of FCTs and the decay constants of 40K were claimed to
improve external errors of 40Ar/39Ar ages by one order of mag-
nitude, rendering them comparable to external errors ob-
tained by the U-Pb method (Renne et al., 2010). Subse-
quently Renne et al. (2011) have slightly revised the 40K decay
constant values following a comment by Schwarz et al. (2011)
who highlighted the sensitive dependence of liquid scintilla-
tion counting data (used by Renne et al., 2010, in the compu-
tation of the 40K decay constant values) on the branching ratio
(λβ/λε) for determination of the 40K half-life. This shows how
the uncertainties relating to the values of the 40K decay con-
stants are far from being solved.
Nonetheless, the impact of using different 40K decay con-
stant values for late Paleogene-Neogene events (<35 Ma),
which are the most relevant for aiming to solve the duration
of magmatic-hydrothermal events in porphyry systems (see
above), is minimal (<<0.0X%) and becomes relevant (e.g.,
>0.0X%) above 80 Ma, steadily increasing for older ages (Fig.
7a; see also Min et al., 2000).
In contrast, the impact of using different ages of standards
in the 40Ar/39Ar method is very significant, also for young ages.
Figure 7b shows how dating of a ~15-Ma-old sample can yield
dates that vary by as much as 0.25 Ma, depending on the cho-
sen age for the FCTs as a secondary standard. Again this may
represent ≥100% of the duration of the magmatic-hydro-
thermal events in porphyry systems. Note incidentally that
changes due to using different proposed values for the total
40K decay constant have an insignificant impact on ages of
~15 Ma (Fig. 7b), as discussed above.
Considering the uncertainties and continuous evolution in
the determination of accurate standard ages and 40K decay
constant values it is not advisable to recommend the use of a
specific set of values for the standard age (in particular the
widely used FCTs) and the 40K decay constants to the ore
community. For the time being, whereas the combination of
parameters of Renne et al. (2010, 2011), which are inter-
linked and should not be used in isolation, different from the
widespread use of various FCTs age values relating mostly to
the 40K decay constant value of Min et al. (2000), appears to
be optimal throughout the full geologic time scale, U-Pb
(Rivera et al., 2011) and astronomically tuned dates (Kuiper
et al., 2008) suggest that the Kuiper et al. (2008) FCTs age
might be the most accurate for Neogene ages (when com-
pared to U-Pb ages of the FCT). Therefore, the FCTs age of
Kuiper et al. (2008) should perhaps be preferred when trying
to resolve the duration of magmatic-hydrothermal events of
porphyry systems, which is best done for Neogene events (see
above), given the insignificant impact of decay constant val-
ues for such young ages. Regardless, the ore geology commu-
nity should be aware of these problems and propagate all un-
certainties related with a 40Ar/39Ar date to allow an accurate
comparison with dates from the U-Pb and Re-Os systems and
avoid an overestimation of the duration of magmatic-hydro-
thermal events in porphyry systems.
Intermethod comparisons: U-Pb versus Re-Os
Internal precisions of ages obtained using the Re-Os
method are now routinely below 0.2% (e.g., Stein et al., 2001).
However, the external uncertainty of the Re-Os system needs
to take into account the uncertainty in the 187Re decay con-
stant value, which, for the value of Smoliar et al. (1996), can
be as high as 1.2% (Begemann et al., 2001), due to the non-
stoichiometry of the ammonium hexachloro-osmate used by
Smoliar et al. (1996) to prepare the Os standard solution for
calibrating the Os spike. Selby et al. (2007) used an Os stan-
dard solution with a defined (stoichiometric) Os abundance
and, through cross calibrations between molybdenite Re-Os
and zircon U-Pb ages, obtained a slightly different decay con-
stant value for 187Re (but within uncertainty of that of Smoliar
et al., 1996) with a reduced uncertainty of 0.2% (2σ). Taking
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into account the 187Re decay constant uncertainties and the
uncertainty in the calibration of the Os standard used for the
calibration of tracer solutions (usually ~1% in most laborato-
ries), the overall external uncertainty of the Re-Os method is
≥~1%.
Intercalibration between the Re-Os and U-Pb systems on
the same rock samples seems to be good (Selby and Creaser,
2005), so any intercomparison between dates acquired by
these two systems should be reliable. Nonetheless, systematic
studies with the combined use of U-Pb and Re-Os methods
should be carried out in order to test the interaccuracy of the
two methods at the finer time scale (i.e., 0.X Ma) discussed in
this review, in a similar way as is being done for the U-Pb and
40Ar/39Ar systems.
578 CHIARADIA ET AL.
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15.00
15.05
15.10
15.15
15.20
15.25
15.30
15.35
15.40
FCT = 27.84 Ma
(Deino and Potts, 1999)
FCT = 28.02 Ma
(Renne et al., 1998)
FCT = 28.201 Ma
(Kuiper et al., 2008)
FCT = 28.305 Ma
(Renne et al., 2010)
λ of Steiger and Jäger (1977)
Age (Ma)
10.30
10.34
10.38
10.42
10.46
10.50
±0.15% (2σ)
Absolute age (Ma)
104.4
104.5
104.6
104.7
104.8
104.9
105.0
1008
1010
1012
1014
1016
1018
1020
1022
±0.15% (2σ)
±0.15% (2σ)
~ 10 Ma mineral
~ 100 Ma mineral
~ 1000 Ma mineral
λ of Steiger and Jäger (1977) (5.543*10-10 a-1)
λ of Min et al. (2000) (5.37*10-10 a-1)
λ of Renne et al. (2010) (5.549*10-10 a-1)
A
B
FIG. 7. A. Differences in the 40Ar/39Ar ages calculated for minerals with ages of ~10, 100, and 1,000 Ma, using the same FCTs
age (28.305 Ma: Renne et al., 2010) and different 40K decay constant values (Steiger and Jäger, 1977; Min et al., 2000; Renne
et al., 2010). The age difference arising from the use of different 40K decay constants is insignificant (i.e., 2.5% of the internal
method uncertainty, here assumed to be at a fixed value of 0.15%: bar on top left of each plot) for an age of 10 Ma, becomes
significant (i.e., ~25% of the internal uncertainty) for an age of 100 Ma and is largely dominant (250% of the internal uncer-
tainty) for an age of 1000 Ma. B. Differences in the 40Ar/39Ar ages calculated for a mineral with an age of ~15 Ma, resulting from
the use of different proposed ages (Renne et al., 1998, 2010; Deino and Potts, 1999; Kuiper et al., 2008) of the secondary FCTs
standard (bars are ±0.15% uncertainties). In contrast to the insignificant effect of using different 40K decay constant values, the
use of different FCTs ages results in significantly different calculated ages for the mineral at this young age.
Effects of Repeated Intrusive Pulses in Porphyry Systems
Many recent geochronological studies have shown that
large porphyry systems are associated with several intrusive
pulses that occur over a range of several tens of kilo-annum to
a few million years (see above). This is consistent with con-
clusions reached in older studies that used field observations
to show that porphyry systems were associated with multi-
phase intrusions (Kirkham, 1971; Gustafson, 1978). These
pulses can be identified by crosscutting relationships in the
field (e.g., Muntean and Einaudi, 2001; Seedorff and Ein-
audi, 2004), and their relative and absolute timing can be de-
termined by U-Pb dating of zircons. Notwithstanding the is-
sues discussed above, related ore events can be recorded by
Re-Os dates of molybdenites, which are sufficiently refrac-
tory to not be reset by subsequent hydrothermal events (e.g.,
Selby and Creaser, 2001; Stein et al., 2001; Bingen and Stein,
2003; Selby et al., 2002, 2003). In contrast, minerals with
lower closure temperatures for argon, which are dated by the
40Ar/39Ar technique, can be subject to isotopic disturbance
during sustained thermal anomalies caused by repeated mag-
matic injections and may yield mixed ages or record the final,
waning stages of the magmatic-hydrothermal system.
An example of this is shown in Figure 8, where U-Pb zircon
ages, Re-Os molybdenite ages, and 40Ar/39Ar ages of various
minerals with different closure temperatures are reported for
the El Teniente porphyry system (Maksaev et al., 2004; Can-
nell et al., 2005). It is evident that each intrusive pulse
(recorded by the U-Pb zircon ages) is followed by molybden-
ite mineralization and high closure temperature mineral equi-
libration in the 40Ar/39Ar system (i.e., hornblende). However,
only the last intrusive pulse is followed by abundant 40Ar/39Ar
ages of low closure temperature sericite, indicating a sustained
and prolonged thermal anomaly of the magmatic-hydrother-
mal system until its thermal decline and cooling below some
250°C due to the cessation of magmatic input into the shal-
low crust (see also Seedorff and Einaudi, 2004). This example
shows that 40Ar/39Ar ages must be interpreted in combination
with field observations of crosscutting relationships between
repeated cycles of magmatic and hydrothermal events, and
accurate U-Pb dating of the various intrusive events, other-
wise erroneous interpretations may lead to an “anomalously”
long, single magmatic-hydrothermal system. The same incor-
rect conclusion may arise if one of the intrusive pulses is not
identified, sampled, or dated. Unfortunately, crosscutting re-
lationships are not always visible in the field due to poor out-
crop conditions or the lack of drill cores, and the sustained
thermal input into magmatic-hydrothermal systems may be
related to deep intrusions that are not exposed.
Concluding Remarks and Guidelines
Accurate dating of magmatic-hydrothermal events is of
fundamental importance for reconstructing the genetic evo-
lution of porphyry systems and evaluating their duration.
Continuous development and refinement of the currently
most used and reliable geochronological tools (U-Pb, Re-Os,
and 40Ar/39Ar) provide an unprecedented precision and accu-
racy in dating the duration of magmatic-hydrothermal sys-
tems. We have examined some aspects that should be care-
fully taken into account when attempting to bracket the
duration of magmatic-hydrothermal systems, so as to take full
advantage of the potentially high precision and accuracy of
the above geochronological tools. We advise that the follow-
ing points are taken into account when trying to resolve the
time scale of magmatic-hydrothermal processes in porphyry
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4
4.5
5
5.5
6
6.5
7
A porphy
C qz-diorite
N qz-diorite
A porphy
moly bx
C qz-diorite
Sewell stock
Sewell stock
moly vein Sewell
moly vein
Braden pipe
bio rock
Braden pipe
N qz-diorite
Teniente porph
Sewell stock
Teniente porph
Braden pipe
Teniente porph
qz-ser alt
bio rock
Braden pipe
Braden pipe
bio rock
C qz-diorite
qz-ser alt
Braden pipe
Gabbro
moly TP
moly
Sewell stock
Age (Ma)
Event
U-Pb zircon
Re-Os molybdenite
Ar-Ar bio/hbl
Ar-Ar sericite
FIG. 8. Compilation of U-Pb, Re-Os, and 40Ar/39Ar data for the El Teniente porphyry Cu deposit (data from Maksaev et
al., 2004, and Cannell et al., 2005). For discussion see text.
systems, using a combination of U-Pb, Re-Os, and 40Ar/39Ar
methods.
1. The absolute precision of the geochronometers most
used in ore geology (U-Pb, 40Ar/39Ar, and Re-Os) is at optimal
levels, under normal conditions, for the Neogene time range
(theoretically between 1 and 10 Ma for average zircons dated
by the U-Pb method). Therefore, porphyry systems within
this time frame are those that provide the best resolution of
magmatic-hydrothermal events and the highest precision of
the estimated duration of the ore processes. Bear in mind
however, that for these young ages, Th disequilibrium correc-
tions applied to U-Pb zircon dates are critical and that uncer-
tainties on the Th/U ratios of the magma from which zircon
crystallized may result in significant age variations.
2. High precision and accuracy in chemical abrasion ID-
TIMS U-Pb zircon dating provides the resolution to record
processes that occur within magma chambers, by identifying
zircons with distinct ages (in a range of few 0.X to 1−2 Ma)
and precisions (0.1%, i.e., few tens of ka for Paleogene to
Neogene zircons) that do not define single age populations.
The maximum emplacement age of the youngest magma is
approximated by the 206Pb/238U date of the youngest zircon.
U-Pb ages that are distinguishable by ID-TIMS within an in-
terval of several hundreds of thousands of years would be sta-
tistically pooled together by less precise methods (e.g., LA-
ICP-MS or SIMS). This process statistically associates older
zircons with zircons that crystallized within the youngest
batch of magma. The resulting age may be geologically mean-
ingless and is usually several 0.X Ma older than the youngest
single zircon age obtainable by ID-TIMS, leading to an ex-
tended and incorrect result for the life span of the magmatic-
hydrothermal and porphyry system (up to tens to hundreds of
percent of the real life span). Clearly, the duration of mag-
matic-hydrothermal events of porphyry systems can only be
accurately approached using ID-TIMS ages obtained from
single zircon grains (see also von Quadt et al., 2011).
3. The combined use of the three most used dating meth-
ods for bracketing the duration of magmatic-hydrothermal
events is a powerful tool. We have shown, however, that there
is a need for an effort in the dating community to use uniform
intensive variables (tracer solutions, ages of 40Ar/39Ar stan-
dards, values of decay constants, principally for 40K), which
determine the accuracy of the ages. Recent studies of argon
isotope systematics consistently show that the age of one of
the most critical secondary standards in 40Ar/39Ar geochronol-
ogy (FCTs) is older than what was thought in the previous
decade and that the 40K decay constant must be smaller, in
order for 40Ar/39Ar ages to be compared with U-Pb and astro-
nomically tuned ages (Kuiper et al., 2008; Renne et al., 2010).
The ore dating community should take advantage of these
critical improvements by adopting the recently proposed val-
ues of these intensive variables for the 40Ar/39Ar system. We
have shown that for Neogene ages (which are relevant for
many porphyry systems and are the most suitable ones to
resolve the duration of magmatic-hydrothermal events) the
choice of different 40K decay constants is not critical. In con-
trast the choice of the FCTs age is critical and will signifi-
cantly affect the calculated 40Ar/39Ar age. At the time of writ-
ing, it would seem that whereas the combination of FCTs age
and 40K decay constant proposed by Renne et al. (2010, 2011)
is the most appropriate for the entire Earth’s time scale, the
FCTs age of Kuiper et al. (2008) is perhaps the most accurate
for Neogene ages. Failure to pay heed to these issues can ren-
der comparisons of 40Ar/39Ar ages with U-Pb and Re-Os ages
meaningless at the 0.X Ma time scale necessary to resolve the
duration of magmatic-hydrothermal events in porphyry sys-
tems. Future studies should continue to determine reliable
intra- and intersystem cross calibrations between the different
isotopic methods that are used to date the different compo-
nents of porphyry magmatic-hydrothermal systems (including
cross calibrations between the U-Pb and Re-Os systems).
4. Crosscutting relationships that crop out in the field and
in drill cores are the most robust form of evidence for relative
temporal relationships that can be used to test the quality of
isotope-based geochronological data. Accurate dating of all of
the different intrusive events observed within the field and
their associated ore and alteration minerals constitutes the
basis of a successful estimation of the duration of single mag-
matic-hydrothermal events, and of the integrated lifetime of
a porphyry system that formed via multi-intrusive events. Re-
Os dates constraining the earliest, highest temperature and
latest, lowest temperature ore mineral phases of the mineral-
ization provide the most accurate minimum time duration of
the magmatic-hydrothermal system. U-Pb dating of zircons
from intrusive rocks that, based on crosscutting relationships,
are shown to be respectively pre-/syn- and post-ore and there-
fore bracket an ore event in porphyry systems is the most ac-
curate way to obtain the maximum time duration of the mag-
matic-hydrothermal event (e.g., von Quadt et al., 2011). The
accuracy of combined U-Pb, Re-Os and 40Ar/39Ar ages deter-
mined on minerals that formed at different stages of the mag-
matic-hydrothermal event can eventually be verified against
the time interval bracketed by the U-Pb ages of such pre-
/syn- and post-ore intrusions.
Acknowledgments
We thank the Chief Editor of Economic Geology, Larry
Meinert, for the invitation to write this review, following a talk
given by MC on occasion of the 9th SGA Biennial Meeting
held in Antofagasta, Chile, between 26 and 29 September
2011. The ideas expressed in this review have been forged
through several years of discussion with various researchers
working at our Department. Among these we would like to
thank Blair Schoene, Philip Schütte, Kalin Kouzmanov, Bryan
Sell, Alexey Ulianov, and Cyril Chelle-Michou. Reviews of
Stephen Noble (British Geological Survey, Keyworth, U.K.)
and David Selby (University of Durham, U.K.) contributed to
improve the final version of this work. MC and US acknowl-
edge the Swiss National Science Foundation for granting the
research that has led to the formulation of this review paper.
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