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Magmatic-hydrothermal processes within an evolving Earth: Iron oxidecopper-gold and porphyry Cu Mo Au deposits
Article in Geology · June 2013
DOI: 10.1130/G34275.1
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Magmatic-hydrothermal processes within an evolving Earth:
Iron oxide-copper-gold and porphyry Cu ± Mo ± Au deposits
Jeremy P. Richards1* and A. Hamid Mumin2*
1
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
Department of Geology, Brandon University, 270 18th Street, Brandon, Manitoba R7A 6A9, Canada
2
surrounded and partially overprinted by broad
zones of lower-temperature (<350 °C) more
acidic alteration (sericite-pyrite, clay) generated
by disproportionation of dissolved SO2 to form
H2S and H2SO4.
Porphyry deposits also form temporally and
spatially separated from active arcs in post-subduction settings, from partial melting of previously subduction-modified lithosphere (Solomon, 1990; Hou et al., 2009; Richards, 2009;
Shafiei et al., 2009; Pettke et al., 2010), a process
also recently proposed for MH-IOCG deposits
(Groves et al., 2010). Porphyry systems formed
in such settings range to more alkaline compositions and may be Au rich, leading to alkaline
porphyry Cu-Au and alkalic-type epithermal
Au deposits (Richards, 2009). Such deposits are
typically relatively S poor and Fe oxide rich, and
show the closest overlap with MH-IOCG deposits in terms of tectonic setting, magma composition, and ore and alteration geochemistry.
Porphyry deposits occur most commonly in
Cenozoic and Mesozoic rocks, are less abundant in the Paleozoic, and are very rare in the
ABSTRACT
Iron oxide-copper-gold (IOCG) deposits formed by magmatic-hydrothermal fluids (MHIOCG) share many similarities with, but have important differences from, porphyry Cu
± Mo ± Au (porphyry) deposits: MH-IOCG deposits predominantly occur in Precambrian
rocks, are Fe oxide rich, and have volumetrically extensive high-temperature alteration zones,
whereas porphyry deposits occur almost exclusively in Phanerozoic rocks, are Fe sulfide rich,
and have narrower high-temperature alteration zones. We propose that these deposit types
are linked by common subduction-modified magmatic sources, but that secular changes in
oceanic sulfate content and geothermal gradients at the end of the Precambrian caused a
transition from the predominance of S-poor arc magmas and associated S-poor MH-IOCG
systems, to S-rich arc magmas and associated S-rich porphyry deposits in the Phanerozoic.
Phanerozoic MH-IOCG and rare Precambrian porphyry deposits are explained by local or
periodic fluctuations in oceanic oxidation state and sulfate content, or remobilization of previously subduction-modified lithosphere in post-subduction tectonic settings.
INTRODUCTION
Porphyry Cu ± Mo ± Au (porphyry) and iron
oxide-copper-gold deposits of magmatic-hydrothermal origin (MH-IOCG; sensu Groves et al.,
2010) contain some of the largest concentrations
of Cu, Au, U, Fe, and other metals on Earth,
and are targets of choice for mineral exploration. These deposit types have many similarities
but also some important differences, and their
genetic relationships, if any, have been debated
extensively (Barton and Johnson, 2000; Williams et al., 2005; Pollard, 2006; Groves et al.,
2010; Mumin et al., 2010). Key differences are
that MH-IOCG deposits are S poor and are common in Precambrian rocks, whereas porphyry
deposits are S rich and predominate in the Phanerozoic. We suggest below that a continuum
exists in processes and time between porphyry
and MH-IOCG deposits formed in orogenic and
post-orogenic settings, primarily reflecting the
recycling of volatiles into the lithosphere via
subduction, coupled with a progressive decrease
in lithospheric geotherms and increase in deepseawater sulfate abundance since the Archean.
*E-mails: Jeremy.Richards@ualberta.ca; Mumin
@BrandonU.ca.
GEOLOGY, July 2013; v. 41; no. 7; p. 767–770
|
Volcanic edifice may
or may not be present
Magmatic-hydrothermal
IOCG
Intermediate-low sulfidation
Supracrustal
sequence
High-sulfidation
Intermediate-sulfidation
Sinter, breccia or vein
Au, Ag (As,Hg,Cu,Zn,Pb)
Vein
Cu,Ag,U,Co
Ni,Au,Bi
Propylitic
Py,Hem
Pyrite
Clay-Chl
Sericite
Cu Au Mo
Hematite
Na-Ca
Sericite
K-feldspar
Albite
1 km
Propylitic Hem
Pyrite
Magnetite
1 km
Py Hem
Clay-Chl
Vein Cu,Pb,Zn,Ag,Au
Cu,Au Co Ag U
Porphyry
Breccia pipe
Advanced argillic
or vent
Na-Ca
Ser
K-fsp
FORMATION OF PORPHYRY CU ± MO
± AU DEPOSITS
The formation of porphyry deposits has been
described extensively in the literature (e.g.,
Sillitoe, 1972, 2010; Richards, 2003, 2011;
Seedorff et al., 2005), and only key points are
noted here (Fig. 1; Table 1). Porphyry deposits
are large geochemical anomalies of Fe and S,
with lesser but economically important enrichments in Cu ± Mo ± Au. They consist of veins
and disseminations with variable proportions
of chalcopyrite, bornite, molybdenite, pyrite,
and magnetite, and are formed from magmatichydrothermal fluids exsolved from relatively
oxidized (fayalite-magnetite-quartz buffer
[ΔFMQ] = +1 to +2), S-rich, calc-alkaline to
mildly alkaline, arc-related magmas. Mineralization is mainly associated with central zones of
high-temperature (≥350 °C) potassic alteration
(biotite–K-feldspar–amphibole–magnetite),
Mag+Py
K-fsp
Na
Na-Ca
Basement rocks
Batholith
Vein
Au,Ag
Propylitic
Chl = Chlorite
Hem = Hematite
K-fsp = Potassium feldspar
Mag = Magnetite
Na = Sodic alteration
Na-Ca = Sodic-calcic alteration
Py = Pyrite
Ser = Sericite
Figure 1. Schematic model for magmatic-hydrothermal systems illustrating relationship between S-rich porphyry Cu ± Mo ± Au deposits (right side) and S-poor magmatic-hydrothermal
iron oxide-copper-gold (MH-IOCG) deposits (left side). Approximate hydrothermal mineral
transitions and relative spatial footprint are shown for albite–K-feldspar, K-feldspar–sericite,
and magnetite-hematite transitions. Prograde and retrograde overprinting of alteration assemblages and mineralization is common. Spatial relationships for porphyry deposits after
Seedorff et al. (2005), Sillitoe (2010), and Richards (2011); for MH-IOCG, after Hitzman et al.
(1992), Williams et al. (2005), and Mumin et al. (2010).
doi:10.1130/G34275.1
|
Published online 16 May 2013
©
2013 Geological
America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
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TABLE 1. COMPARISON BETWEEN PORPHYRY AND MAGMATIC-HYDROTHERMAL IOCG DEPOSITS
Characteristic
Major metal association
Minor metal association
Sulfur content
Ore minerals
Ore fluid
Fluid oxidation state; acidity
Source of fluid
Source of metals
Alteration geochemistry
Width of high-temperature (>350–400 °C) alteration
Depth of formation
Geothermal gradient
Regional metamorphism
Magma association; composition; oxidation state
Tectonic setting
Kinematic setting
Age range
Porphyry Cu ± Mo ± Au
[post-subduction porphyry]
Cu, Mo, Au
Ag, Sn, W
High: sulfides and sulfates [lower]
Chalcopyrite, bornite, molybdenite, magnetite;
abundant pyrite with sericite
H2O-NaCl-KCl
Oxidized (ΔFMQ = +3 to +5); neutral to acidic
Magmatic
Subducted slab and asthenosphere [subductionmodified lithosphere]
K-(Na)-Fe-S-SiO2
1–2 km
1–5 km
Normal
Minimal to low grade
Calc-alkaline [mildly alkaline]; intermediate to felsic
[mafic to felsic]; ΔFMQ = +1 to +2
Subduction [post-subduction]
Transpression or transtension
Dominant in Phanerozoic, rare in Precambrian
Magmatic-hydrothermal IOCG
Fe, Cu, Au
U, REE, Co, Ag
Low: minor sulfides
Magnetite, hematite, chalcopyrite, bornite, chalcocite
H2O-CO2-NaCl-KCl
Likely ΔFMQ = +3 to >+5; neutral to mildly acidic,
rarely acidic
Magmatic ± crustal fluids
Subduction-modified lithosphere and fluxing from host
rocks
Na-K-Fe-P-Ca-CO2-SiO2
1 to ≥7 km
Surface to ~10 km
Elevated
Low to high grade
Calc-alkaline to mildly alkaline; mafic to felsic;
uncertain, but likely ΔFMQ = 0 to +2
Distal, back-arc, or post-subduction
Extension to transtension
Dominant in Precambrian, important in Mesozoic
Note: Characteristics of post-subduction porphyry deposits shown in square brackets. IOCG—iron oxide-copper-gold; ΔFMQ—fayalite-magnetite-quartz buffer; REE—
rare earth element.
Precambrian. One explanation for this temporal
distribution is that arc-related porphyry deposits
form in environments of active uplift and erosion, commonly followed by collision, and as
such they are highly susceptible to loss through
erosion (Kesler and Wilkinson, 2006). However,
it is difficult to explain the extreme rarity of
Precambrian deposits by erosion alone, and we
find no credible evidence that porphyry deposits
were in fact formed in abundance prior to the
Phanerozoic. We note that many Precambrian
supracrustal sequences are preserved around the
world, including arc successions, but indisputable porphyry-type deposits in these rocks are
almost nonexistent (Seedorff et al., 2005).
FORMATION OF MH-IOCG DEPOSITS
Major MH-IOCG systems are found in continental orogenic to post-orogenic settings from
the late Archean (e.g., Carajas district, Brazil)
and Proterozoic (e.g., Olympic Dam and Cloncurry districts, Australia; Norrbotten, Sweden;
Great Bear, Canada; Williams et al., 2005;
Groves et al., 2010; Mumin et al., 2010; Skirrow, 2010), to the Mesozoic (e.g., Candelaria–
Punta del Cobre and Manto Verde in Chile;
Raul-Condestable and Mina Justa in Peru;
Marschik and Fontboté, 2001) (Table 1).
MH-IOCG deposits also constitute large
geochemical enrichments in Fe, but mainly as
Fe oxides (± Fe silicates, Fe carbonates) with
relatively minor Fe sulfides; they contain economically important enrichments in Cu ± Au
± U ± REE (rare earth elements) ± Co (Table 1).
They are commonly associated with relatively
oxidized but apparently S-poor, calc-alkaline
to mildly alkaline magmas. They display broad
zones of high-temperature (~600–400 °C)
768
Na (albite-amphibole-pyroxene), Na-Ca-Fe
(magnetite-actinolite-apatite), or K-Fe (K-feldspar–magnetite–biotite–amphibole) alteration (Fig. 1). Unlike porphyry systems, lowertemperature alteration zones (<400 °C; e.g.,
hematite-chlorite-sericite-carbonate) are characterized by persistence of near-neutral to only
mildly acidic pH conditions, with rare highly
acidic alteration, reflecting lower abundances of
H2SO4 in the hydrothermal fluids.
Whereas fluids and metals in porphyry
deposits are derived primarily from underlying
magmatic sources (see the review in Richards,
2011), the origin of IOCG fluids has been widely
debated, ranging from metamorphic and crustal
sources to magmatic hydrothermal fluids. Here,
we restrict our discussion to magma-driven systems involving primary magmatic fluids, albeit
with variable crustal contributions (Barton and
Johnson, 2000; Pollard, 2006; Groves et al.,
2010; Skirrow, 2010).
MH-IOCG systems form primarily during
extensional or transtensional deformation periods, in tectonic settings ranging from back-arc or
distal arc (e.g., Olympic Dam; Skirrow, 2010) to
intra-arc (e.g., Candelaria–Punta del Cobre, Marcona; Marschik and Fontboté, 2001; Chen et al.,
2010), and to post-collision extensional settings
(e.g., Great Bear; Mumin et al., 2013). These
geotectonic environments are shared by arcrelated and post-subduction porphyry deposits.
GENERATION OF FERTILE MAGMAS
IN ARC AND POST-SUBDUCTION
ENVIRONMENTS
A key factor in the generation of magmatic
hydrothermal porphyry and IOCG deposits is the
recycling of H2O into the mantle via subduction
of seafloor-altered oceanic lithosphere. Dehydration of the subducting slab releases aqueous
fluids that metasomatize the overlying asthenospheric mantle wedge and cause partial melting
to form hydrous basaltic magmas. Fractionation
and interaction of these magmas with the upper
plate lithosphere generates evolved, hydrous,
relatively oxidized (ΔFMQ = +1 to +2), calcalkaline magmas. Upon emplacement at shallow crustal depths, volatiles are released due to
cooling, and these fluids may go on to form magmatic-hydrothermal ore deposits (Hedenquist
and Lowenstern, 1994; Richards, 2003).
Richards (2009, 2011) proposed that magmatic-hydrothermal ore deposits can also form
temporally and/or spatially distal to active subduction zones by remelting of lower crustal
amphibolitic cumulates and/or metasomatized
mantle lithosphere residual from precursor arc
magmatism. A significant amount of the chalcophile (e.g., Cu) and highly siderophile element
(e.g., Au and platinum group elements [PGE])
content of the arc magma flux may be left as
residual sulfides in these deep lithospheric
cumulate or metasomatized zones. Second-stage
partial melting of these amphibolitic rocks may
occur at any later time due to tectonic processes
that increase temperature or reduce pressure,
such as crustal thickening or lithospheric mantle
delamination during arc or continent collision,
post-collisional extension, or back-arc rifting.
Lacking a fresh supply of subduction-sourced
sulfur, derivative partial melts will be relatively
S poor, and will dissolve a significant proportion of any residual Cu ± Au–rich sulfides; such
magmas may go on to form relatively S-poor
magmatic-hydrothermal Cu ± Au deposits
(Richards, 2009). Porphyry and MH-IOCG
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~ Paleo heat production (% present value)
B
basins where conditions were oxidizing; Fig. 2),
but favoring the development of S-poor MHIOCG systems. Conversely, S-rich Phanerozoic
arc magmas favor the formation of porphyrytype deposits, although periodic swings back to
anoxic deep-ocean conditions, particularly evident in the Mesozoic (Holland, 2006), could lead
to the intermittent formation of S-poor magmas
favorable for MH-IOCG formation.
The Role of Decreasing Geothermal
Gradients
Not all differences between MH-IOCG and
porphyry deposits can be explained on the basis
of sulfur content, including the full nature and
extent of high-temperature alteration in MHIOCG deposits, and their different minor-element contents (e.g., U and REE). Given that
the source magmas and therefore initial fluid
temperatures for both deposit types are similar
(~700 °C), one explanation for these further differences may be that ambient rock temperatures
in the upper crust were higher under steeper Precambrian geothermal gradients, due to higher
radiogenic heat output and higher primordial
heat loss (Araki et al., 2005). Some estimates
suggest that geothermal gradients were approximately twice current values at 2 Ga (Richter,
1988; Plant and Saunders, 1996), while Hand
et al. (1999) argued for highly variable paleogeotherms depending on the concentration of
radioactive elements (U, K, Th) in Proterozoic
rocks. They calculated that high-heat-producing granites such as those associated with the
Olympic Dam and Cloncurry mining districts
could have had geothermal gradients double the
2
2
2
Deep ocean O2 (mM)
Fe-Oxide Versus Fe-Sulfide Deposits: The
Role of Seawater Sulfate
A key difference between Fe oxide–rich MHIOCG deposits and Fe sulfide–rich porphyry
deposits reflects a difference in magmatic sulfur
content in otherwise similar hydrous, relatively
oxidized, calc-alkaline to mildly alkaline source
magmas. Phanerozoic arc magmas and related
porphyry and high-sulfidation epithermal deposits are S rich, with the bulk of the sulfur being
derived from seawater via the subduction cycle
(De Hoog et al., 2001; Wallace and Edmonds,
2011). In contrast, Prouteau and Scaillet (2013)
have shown that arc magmas generated in the
Precambrian were relatively S poor, and this
correlates with the scarcity of S-rich Precambrian porphyry deposits.
A secular increase in the sulfur content of
arc magmas may relate to a parallel increase in
the abundance of dissolved sulfate in seawater
over time. Seawater sulfate concentrations are
estimated to have been ~0.2 mM from the late
Archean to earliest Proterozoic, with a gradual
increase to the early Phanerozoic, followed
by a rapid increase to modern day values of
~28 mM (Fig. 2; Kah et al., 2004; Gill et al.,
2007). The increase in seawater sulfate content
is directly linked to the availability of biogenic
oxygen and the onset of oxidative weathering,
which released continent-derived sulfate to the
oceans beginning at ca. 2.3 Ga (Kasting, 2001).
A dramatic spike in both seawater and seafloor
sulfate content occurred in the Phanerozoic due
to sulfide oxidation by ocean ventilation and
marine bioturbation (Canfield and Farquhar,
2009; Shields-Zhou and Och, 2011). Subduction of this oxidized material would introduce
abundant sulfate to the mantle wedge for the
first time, and generate S-rich arc magmas.
Thus, we propose that subduction-related magmas would have been S poor in the Precambrian,
inhibiting the formation of S-rich porphyry-type
deposits (except in locally anomalous environments, such as subduction of shallow marine
A
Seawater sulfate concentration (mM)
DISCUSSION
Despite the similarities noted above, porphyry and MH-IOCG deposits vary somewhat
in both economic and trace-element signatures,
are distinct in their alteration footprints, and vary
in abundance over geological time. We argue
below that these differences may be explained
by secular changes in the abundance of seawater
sulfate entering magmatic-hydrothermal systems, and decreasing lithospheric geothermal
gradients since the Precambrian.
Figure 2. Global changes
Present-day sulfate concentration
28
affecting magmatic-hydrothermal ore deposits
from Archean to present.
24
300
A: Increase in seawater
sulfate
concentration
~ De
20
crea
and decrease in heat prose in
pale
duction relative to preso he
at pr
ent-day values. Sulfur
16
Porphyry 200
oduc
tion
concentrations from Kah
Cu most
abundant
et al. (2004) and Gill et al.
Porphyry Cu rare
12
(2007); paleo-heat production from Plant and
8
IOCG Deposits
Saunders (1996), Hand
100
et al. (1999), and Araki
Initial
et al. (2005). IOCG—iron
4
biospheric
ate
oxide-copper-gold.
B:
er sulf
oxygen
eawat
S
Atmospheric O2 levels as
0
0
a percentage of present
0
1.5 Age Ga 1.0
2.0
0.5
2.5
atmospheric level (%PA;
Oxic upper ocean
blue dashed line) super0.5
imposed on correspond100
0.4
Anoxic
deep
ocean
with
widespread
ing ocean redox condisulfidic bottom waters
0.3
tions (after Canfield,
10 Anoxic
Oxic deep
O Max
O -%PA
?
2005; Shields-Zhou and
0.2
ocean
deep
Och, 2011). Following the
1 ocean
O Min 0.1
Neoproterozoic Oxygen0.0
Sulfate accumulation
Ocean basalt and sediments
ation Event, deep ocean
0.1
O2 levels (black short
dashed lines) varied rapidly over short time intervals (Holland, 2006), resulting in variable
sulfate additions to oceanic crust.
O2 (% present atmosphere)
deposits have now been recognized from all of
these post-subduction tectonic settings (Pollard,
2006; Hou et al., 2009; Richards, 2009; Groves
et al., 2010; Skirrow, 2010), and it is here that
we observe the closest overlap between these
two broad deposit types.
Proterozoic average at the time of mineralization (1.6 Ga).
Most MH-IOCG deposits are associated with
extensional environments, and many are close
to their source batholiths, which places them
in host rocks with significantly higher-thannormal thermal gradients (e.g., Hitzman, 2000).
In contrast, most porphyry deposits are formed
at shallow levels (~1–5 km depth) above deeper
batholiths (~5–10 km), and thus in cooler country rocks. High ambient temperatures in the host
rocks to MH-IOCG deposits would facilitate the
formation of extensive zones of high-temperature alteration, from which a significant flux of
crustally derived lithophile elements such as Fe,
K, Ca, U, and REE could be derived.
Temporal Overlap between MH-IOCG and
Porphyry Deposits
This model does not preclude the formation
of rare porphyry deposits in the Precambrian
where magmatic sulfur contents were locally
high, for example due to subduction of oxidized
shallow-marine sediments. It also does not preclude the formation of MH-IOCG deposits in
the Phanerozoic from S-poor magmas generated
from partial melting of relatively S-poor asthenospheric or subduction-modified lithospheric
sources that may be linked to periods of deep
ocean anoxia, or in extensional zones with high
thermal gradients.
CONCLUSIONS
Porphyry Cu ± Mo ± Au and magmatichydrothermal IOCG deposits share many attributes, such as their association with calc-alkaline
769
to mildly alkaline magmas, broad geotectonic
relationships, and metal contents; and yet they
remain distinctive. The closest overlap between
these two deposit types is found in post-subduction alkalic porphyry Cu-Au deposits, which are
magnetite rich and relatively S poor. We propose that higher geothermal gradients and lower
seawater sulfate contents in the Precambrian
favored the formation of S-poor arc magmas and
MH-IOCG deposits during that period, whereas
the reverse was true in the Phanerozoic, favoring S-rich porphyry formation. Exceptions exist,
however, and we explain the overlap by local
variations in geothermal gradients and source
sulfur contents.
ACKNOWLEDGMENTS
This paper is dedicated to the memory of Rob Kerrich, an innovative thinker and inspiring mentor. We
acknowledge the support of Discovery Grants from the
Natural Sciences and Engineering Research Council
of Canada, our industry collaborators, and discussions
with numerous colleagues that have challenged us to
try to make sense of the porphyry-IOCG conundrum.
We particularly thank Mark Barton, David Cooke, and
Adam Simon for perceptive and constructive reviews.
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Manuscript received 29 November 2012
Revised manuscript received 11 February 2013
Manuscript accepted 14 February 2013
Printed in USA
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