Download Analysis of tectonic settings of global superlarge porphyry copper

SCIENCE IN CHINA (Series D)
Vol. 46 Supp.
March 2003
Analysis of tectonic settings of global superlarge porphyry
copper deposits
XIA Bin (
), CHEN Genwen () & WANG He (
)
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
Received June 12, 2002
Abstract About three quarters of superlarge porphyry copper deposits throughout the world occur
along the eastern Pacific basin rim, most of which were formed during the Mesozoic-Cenozoic.
Porphyry copper deposits often occur in the upper parts of a subduction zone and in a within-plate
orogenic belt. Some porphyry copper deposits are inconsistent with plate subduction with respect
to their formation time, and most of them in the world are associated with tensional environment.
Metallogenic porphyries originated from the mantle, and the involvement of the lower-crust or
oceanic crust materials have played an important role. Based on the geochemical characteristics
and tectonic settings of the ore-bearing porphyries in the Gandise and Yulong metallogenic zones,
it is proposed that delamination may be the important mechanism of formation of porphyry
copper deposits.
Keywords: superlarge porphyry copper deposit, tectonic setting, metallogenesis, delamination.
Porphyry copper deposits are one of the most important types of copper deposits, and superlarge porphyry copper deposits account for more than 60% of the global superlarge copper ore
reserves. It is of great economic significance in research of superlarge porphyry copper deposits
throughout the world. Porphyry copper deposits are among the metal deposits which have genetic
connections with plate activities. Just as mentioned by Titey et al.[1], in the final several decades of
the 1960s , porphyry copper deposits were best documented and best understood. Meanwhile, it is
during this
period of time that the plate tectonics was developed most rapidly. It is now com-
monly accepted that porphyry copper deposits were mostly formed at the convergent plate
boundaries and are associated with plate subduction. For example, porphyry magma and the main
ore-forming materials
were derived from partial melting of the subduction oceanic crust[2] or
partial melt of the mantle wedge metasomatized by fluids derived from the deeper subduction
oceanic crust[3]. Along with deepening the understanding of metallogenic settings of porphyry
copper deposits, some new problems (e.g. what is the essential factor leading to the occurrence of
most porphyry copper deposits along the eastem Pacific basin rim? What is the genesis of some
porphyry copper deposits farther away from the subduction zone? ). The aim of this paper is to
address the above questions in terms of the distribution rules, regionally geological tectonic settings and petro-geochemical characteristics of superlarge porphyry copper deposits.
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111
Distribution rules of global superlarge porphyry copper deposits
As for their spatial distribution, global porphyry copper deposits are concentrated mainly in
the three large metallogenic zones: the circum-Pacific metallogenic zone, the Tethys-Himalayas
metallogenic zone and the ancient Asia metallogenic zone ( the Central Asia metallogenic zone).
The former can be divided into the east and west zones. The east zone is primarily distributed in
the Cordillera and Andean along the eastern coast of the Pacific. The west zone can be divided
into the inner and outer zones. The inner zone extends from the northern border of Okhotsk of
Russia, via the eastern part of Northeast China and the middle-lower reaches of the Yangtze River
and South China. The outer zone extends from the Japanese archipelago, through Taiwan of China,
Philippines, Kalimantan islands, and Papua New Guinea Solomon archipelago. In addition, a few
porphyry copper deposits were formed at the tectonically active margins of various blocks.
Some of the global porphyry copper deposits were formed during the pre-Cambrian period,
such as the porphyry copper-molybdenum deposits in Malanjkhand, India, Pohjinamaa, Finland,
etc., and some porphyry copper deposits in the Abitibi greenstone belt of Canada; some of the
porphyry copper deposits were formed during the Paleozoic (C-P), including those occurring in
Aerdenituyinobo of Mongolia and Kounradskiy of Kazakhstan; some during the Early-Middle
Mesozoic (T-J); and some during the Tertiary and Quaternary. The metallogenic ages of superlarge porphyry copper deposits were also variable from pre-Cambrian to Quaternary. They were
formed predominantly during the Mesozoic and Cenozoic (table 1), and the copper reserve formed
in this period accounts for 90% of the total history. Coming next is the Paleozoic during which
porphyry copper deposits were largely formed.
Porphyry copper deposits of various geological ages are distributed in different areas. Mesozoic and Cenozoic porphyry copper deposits are distributed mainly in the circum-Pacific zone and
the Tethys-Himalayas zone; Paleozoic porphyry copper deposits mainly in the Mid Asia tectonic
zone, the Appalachia orogenic belt in the west of North America and the Cordilera mountains in
Argentina, and the Tamoaerte-Yaluote rock belt in Australia. Pre-Cambrian porphyry copper ore
zone is located mainly along the ancient continental margins, for example the Tongchangyu porphyry copper deposit in the Zhongtiao Mountain of China, those within the Abitibi greenstone belt
of Canada, and the Archean porphyry copper deposits in the Pulibaer block of Australia.
The Mesozoic and Cenozoic porphyry copper deposits have been better documented at present. However, their metallogenic ages are different from one deposit to another, for instance, porphyry copper deposits of North America were formed mainly during 195175 Ma and 8030
Ma, and also a small number of porphyry copper deposits were formed during 155140 Ma and
11590 Ma[1]; those of Chile were formed dominantly during 6030 Ma and 415 Ma, and a
small number of them were formed during 13295 Ma[7]; those in southwestern Pacific basin rim
were dated at 82 Ma; those in the east of China were formed mainly during 200175 Ma and
10090 Ma, respectively; the mineralization period of the Tethys metallogenic zone was dated at
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2555 Ma and ± 15 Ma. This indicates that the metallogenesis of global porphyry copper deposits is of certain isochroneity, but it is different in intensity from place to place and from one period
to another.
Table 1 Global superlarge porphyry copper depositsa)
Sequence No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
Location
Chile
Chile
Chile
Chile
Chile
Chile
Chile
Chile
Chile
Chile
USA
USA
USA
USA
USA
USA
USA
USA
USA
USA
Canada
Panama
Mexico
Peru
Peru
Colombia
Kazakhstan
Yugoslavia
Mongolia
Iran
India
Indonesia
China
China
Deposit
Reserve (10 × 104ton)
Chuquicamata
6935
El Teniente
6776
La Escondida
2880
Chuqui norte
1655
Collahuasi
1195
Mania Mina
8451170
El Abra
807
Rio Blanco-Disputada
800
El Salvador
860
Andina
500
Bingham
2121
Butte
1800
Morenci
1300
Safford
800
San Manuel-Kalamazoo
708
Santarita
630
Ray
630
Twin Buttes
590
Miami
530
Casa Grande
500
Highland valley
900
Cerro Colorado
1800
Cananea
1340
Cerro Verde
782
Cuajones
600
Pantanos-Pegadorcite
625
Kounradskiy
790
Majdanpek
510
Erdeintyin Obo
1000
Sarchesmah
904
Malanjkhand
655
Grasberg
953
Yulong
650
Dexing Tongchang
492
Tonnage (Cu,%)
0.56
0.68
1.6
0.89
1.2
1.3
0.59
1
1.13
1.25
0.9
0.8
1
0.4
0.75
0.78
0.79
0.7
0.9
1
0.45
1.5
0.7
0.8
1
1
0.6
0.8
0.3-1.5
1.13
0.83
1.41
0.94
0.46
Metallogenic age
E1
E1
E-N
E-N
E-N
E-N
E-N
E
K2-E
N
N
K2
K
−
K2
E
E
E
K2-E
−
T
−
E
E
E
E
C1
E
P
E-N
Pt2
E-N
E-N
J
a) After refs.[48].
2
2.1
Metallogenic settings of superlarge porphyry copper deposits
Factors controlling the domain of formation of superlarge porphyry copper deposits
The majority of porphyry copper deposits occur in the upper parts of the subduction plate,
in cogenesis with the magma arc. Because porphyry copper deposits were formed in the subduction and collision belts, these belts would usually develop into orogenic belts, becoming a main
degradation zone. With the time passing by, the old porphyry copper deposits would be hard to
preserve. For this reason, it is particularly important to analyze the distribution rules of Mesozoic
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113
and Cenozoic porphyry copper deposits. As exemplified by those porphyry copper deposits distributed in the circum-Pacific (table 1), their distribution is extremely heterogeneous in the east
and west, with a proportion of 131. Such heterogeneity is even exhibited in crust thickness that
porphyry copper deposits occurred and ore-forming material composition of porphyry copper deposits. For instance, the porphyry copper deposits occurring at the eastern edges of the Pacific,
where the continent crust is relatively thick, were formed on the basement of the ancient land,
predominated by porphyry copper or porphyry copper-molybdenum deposits. Those occurring
in the island-arc areas of western Pacific, where the crust is thinner, were generated on the transitional crust or oceanic crust, and they are predominated by porphyry copper-gold deposits, often
accompanied with epithermal auriferous deposits. Ore-bearing porphyries in both the metallogenic
zones are different in Sr isotopic composition as well. For example, the porphyry copper deposits
and intrusions, which were formed in North America, have higher initial 87Sr/86Sr ratios, usually
greater than 0.705, and in going toward the continent, the ratios tend to increase progressively[1].
Comparatively speaking, the porphyry copper deposits formed along the island arc of western Pacific have lower initial 87Sr/86Sr ratios, generally lower than 0.705[1]. Porphyry copper deposits
distributed along the continental margins were obviously affected by contamination of crustal materials during their formation.
Such heterogeneity is closely associated with the difference in tectonic settings between
eastern and western Pacific. As viewed from the tectonic settings, the continental margin-type
magma arc was formed in the eastern Pacific basin rim, while the oceanic volcanic arc was formed
in the western Pacific. The two subduction zones are different in a subduction structure. The continental margin type is characterized by a trough-volcano arc-basin range structure; the arc type by
a trough-volcanic arc-back-arc basin-continent structure. These two types of structures have some
similarities, i.e. both were formed in the magma arc and back-arc extensional environments. What
is different is that the magma arc and back-arc extensional environments are different in basement properties, the former suffering from compression and the latter from extension. But the
porphyry copper deposits in the two belts were both formed in the tectonically compressive and
extensional settings. Volcanic islands can be divided into three types[9]: extensive arc, neutral arc,
and compressive arc. Correspondingly, continental marginal arc belongs to the compressive arc
and island arc should be assigned to the extensive arc. In fact, with the exception that porphyry
copper deposits distributed in the southwestern Pacific basin rim were formed in the extensive arc
environment, those in the eastern Pacific basin rim were also produced during the transitional period from compressive arc to extensive arc[10]. The back-arc basin represents the most important
geographic feature in the western Pacific. According to the statistics data, more than 75% of the
marginal sea throughout the world are distributed along the western Pacific[11], and almost all of
the back-arc basins were formed in the west of the Pacific, which adequately indicated that the
stress states are different between the western and eastern Pacific. Differences in tectonic setting
between the western and eastern Pacific also lie in the fact that the East Pacific plate is gradually
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shrinking, the Cocos plate is perishing. But the Northeast Pacific tectonic plate of western North
America has already perished, its mid-ocean ridge is subducting underneath the North America
continent. All pieces of evidence indicate that there are some differences in subduction velocity
for the two Pacific tectonic plates in the geological past, i.e. the East Pacific plate subducted at a
higher speed than the West Pacific plate, which is consistent with the results of modern surveying. In normal cases, volcanic effusion at the mid-ocean ridge is a motive force driving ocean tectonic plate subduction, so it is impossible to subduct underneath the continental tectonic plate.
Therefore, the northern segment of the Pacific mid-ocean ridge perished underneath the western
North America continent, indicating that the North American continent was continuously drifting
westwards. The East Pacific plate was subducting eastwards and the America continent was drifting westwards. It is this relative movement between the two plates that led to a higher subduction velocity of the East Pacific plate than that of the West Pacific plate. Differences in chemical
composition for the different types of volcanic arcs and volcanic-arc rocks generated in the eastern
and western Pacific can be explained by two kinds of ideal subduction models[12]. Developed
along the eastern coast of the Pacific is a Benioff zone, which is characterized as having been
formed by shallow subduction at a high speed and a low angle. The subduction model of the West
Pacific tectonic plate is different from that of the East Pacific plate. The oceanic plate was subducting at a high angle, with no sign of strong earthquake in the subduction belt. The oceanic plate
directly inserted itself into the mantle because of the large-angle subduction, and the magma resultant from melting of the oceanic tectonic plate is hard to ascend. However, because of the static
asthenosphere being obstructed by the subducted ocean slice, the upward convection plum would
be formed, so that the geothermal model similar to the back-arc structure appeared on the side of
the magma arc. Meanwhile, extensive back-arc basins were formed and volcanic activities represented by tholeiite series occurred. Such tectonic settings are also favorable loci for the deposition
of black ores. But porphyry copper deposits were formed in the arc settings in front of the
back-arc basins, for example the Gresberg copper deposit and those epithermal auriferous deposits
distributed in the West Pacific arc area.
The most majority of porphyry copper deposits are controlled regionally by large-sized linear
structures or by the confluence of the linear structures, as viewed in the field. For example, the
porphyry copper deposits of northern Chile are closely associated with the SN-trending fault systems of Domeyko (fig. 1). Especially at the confluence of faults, porphyry copper deposits are
highly concentrated, and that is the favorable locus for the formation superlarge porphyry copper
deposits. Fig. 1 shows the distribution of porphyry copper deposits in the northern part of Chile[13].
Porphyry copper deposits in western North America are developed in two directions[14]. The ore
belts generally extend along the active margins of the pre-Cambrian craton in southwestern North
America, and display regular changes in metallogenic time; they all display a distribution pattern
along NE-tending linear structures in regard to a specific ore-concentrated area. Four NE-tending
linear structures in western North America are the large-scale, pre-Cambrian crustal structures, and
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TECTONIC SETTINGS OF GLOBAL SUPERLARGE PORPHYRY COPPER DEPOSITS
115
they can extend over different distances from
the margins of pre-Cambrian shield southwestward.
The Slave tectonic zone is a trench and
also a large NE-SW-tending shear zone developed along the margins of a shield, which
extends till the foot of the Cordillera range
above the Paleozoic cover strata. Near the
margins of the shield, there appeared syngenetic sea reefs, which controlled the Peiyinbote Pb-Zn deposit, and the Rainbow oil/
gas field in northwestern Albera, indicating
that the fault, which had been formed at 1.9
Ga ago, once activated during the Devonian
period and it could be produced by declining
collision of two plates. Other several
near-parallel linear structures are all continental rifts and collision zones formed durFig. 1. Sketch map showing the distribution of porphyry copper
ing the Middle Proterozoic. The locations deposits in northern Chile from ref. [13].
where these zones are intersected with modern subduction zones are most favorable to ore deposition. The Dexing porphyry copper deposit is
controlled by the deep fault zone in northeastern Jiangxi between the Jiangnan anteclise and the
Qiantang depression, and it is also situated in the northwestern segment of the fault. Regionally,
the Yulong copper deposit is controlled by the Jinshajiang ultra-crust fault. It is suggested that the
porphyry copper deposits are most likely to have been formed in tectonically weak zones.
2.2 Temporal-spatial variations of ore-bearing porphyries in inconsistency with migration direction of magma arc
According to the theory of plate tectonics, magma arc should migrate toward the continental
margin with the proceeding of plate subduction. Chile’s copper ore belt tends to become younger
and younger from west to east (off the ocean), and the deposits formed during the same metallogenetic period are in linear parallel with the subduction zone (fig. 1). The formation of the deposits seems to be consistent with the migration direction of magma arc. Porphyry copper deposits
in North America show a relatively complicated distribution pattern, and they are bound by the
NE-tending Wollaston tectonic zone. The deposits tend to become older and older from west to
east at the northern boundary and then tend to become younger from west to east in the south. The
porphyry copper deposits in North America are not strictly in parallel with the subduction zone as
in Chile with respect to their distribution, and they are primarily controlled by a few NE-tending
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Vol. 46
tectonic zones[14]. Magma activities in South China during the Mesozoic and Cenozoic tended to
become younger toward the ocean. Obviously, the metallogenetic time and spatial development of
porphyry copper deposits either in North America or in South China are inconsistent with the migration direction of magma arc produced by tectonic plate subduction. For this reason, it cannot be
explained merely by partial melting of the crust or mantle caused by tectonic plate subduction.
2.3
Obvious time gap between the formation of porphyry copper deposits and plate subduction
According to the fact that porphyry copper deposits are usually formed in the upper part of
subduction plate, Sillitoe[2] proposed that the formation of porphyry copper deposits is attributed
to the strong enrichment of ore-forming elements in the oceanic crust in response to partial melting of the subducted oceanic crust caused by friction heat. Studies of McMillan and Panteleyev[15]
indicated that there is a time gap of several tens of millions of years between the mineralization of
some porphyry copper deposits and tectonic plate subduction. The Dexing porphyry copper deposit of China can be taken as a most typical example. It was controlled by the fault zone in the
northeast of Jiangxi Province, but it is now commonly accepted that this fault belt is the suture
zone formed during the Late Proterozoic[16
Early Yanshanian
ü18]
. The Dexing porphyry copper deposit formed at
[19]
and by that time the subduction zone had already drifted to the Changle-Shao’an area. Its distance from the northeast of Jiangxi Province also reached 400 km. Such
regionally controlling characteristics are also obviously seen in the Tethys metallogenetic zone.
Recent research shows that there would exist two parallel porphyry copper ore zones in Tibet. One
is the Yulong metallogenetic zone, and the other is the Gandise porphyry copper ore zone[20]. The
Yulong copper deposit is sandwiched in the Qiangtang-Changdu micro-continental block between
the Lancangjiang great fault and the Jinshajiang great fault, and its ore-forming age is 3755
Ma[19]. The Qiangtang-Changdu micro-continental block was amalgamated with the Yangtze Old
Land in the Jinshajiang suture zone during the Middle Triassic, with a time gap of about 100 Ma
relative to the metallogenetic age of the Yulong copper deposit. The Gandise porphyry copper ore
zone was formed in the intermountain basin of Gandise magma arc. Qu et al.[20] used the Re-Os
method to determine the age of the Nanmu Cu-Mo deposit in the middle run of the ore zone
(14.46 ± 0.20 Ma). But the whole Tethys Ocean had already closed at 50Ma ago, and at least there
is a time gap of more than 400 Ma between the formation of the collision zone (continental crust
shortening caused by plate collision) and the metallogenesis of the Gandise copper ore zone. The
time relations are hard to define between the formation of a great number of porphyry copper deposit and tectonic plate subduction, but, according to the statistics of Muller and Groves[21], quite a
number of porphyry copper deposits and porphyry copper-gold deposits are closely associated
with potash igneous rocks generated along the eastern and western coasts of the Pacific, including
the Bingham copper deposit of the United States, the Chuquicamata copper deposit, El Salvador
copper deposit and Escondida copper deposit of Chile, the Grasberg Cu-Au deposit of Indonesia
and the Ok Tedi Cu-Au deposit of Papua New Guinea. And it is believed that these deposits were
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TECTONIC SETTINGS OF GLOBAL SUPERLARGE PORPHYRY COPPER DEPOSITS
117
formed largely in the continental arc and post-collision tectonic environments, so that it is hard to
explain by using the tectonic plate subduction model of porphyry copper deposits established by
Sillitoe[2].
2.4
Tensional settings where porphyry copper deposits were formed
While great attention has been paid to the relationship between superlarge porphyry copper
(molybdenum) deposits and subduction-induced magma arc, there is usually a kind of prejudice,
believing that porphyry copper (molybdenum) deposits are more closely associated with the
magma arc of the East Pacific, but the marginal arc of the East Pacific is a compressive arc.
Therefore, porphyry copper deposits are thought to have been formed in the compressive environment, but it is known that porphyry copper (molybdenum) deposits are hard to form in the tensional arc environment as compared with massive sulfide deposits (e.g. black ores)[2]. Lowell[10]
noticed long ago the fact that the porphyry copper deposits of southwestern North America were
controlled by extensive structures. The case is true as is recognized in the Chile copper ore zone.
The Chile region is made up of five tectonic zones from west to east: the Paleozoic magma arc,
the Mesozoic magma arc, the central valley, the Mesozoic-Cenozoic magma arc and the modern
volcanic arc, respectively. On the eastern side of the Cenozoic magma arc was developed an intermountain tableland basin where red-layer rock series was formed during the Tertiary, and regional fault activities were intensive. Meanwhile, a westward-inclining normal fault system was
also developed. With violent volcanic activities, this zone continued to rise during the Paleocene.
On the western side the tensional normal fault system was further enlarged; until the Oligocene, a
great deal of porphyry mostly penetrated into these normal faults, indicating that the MesozoicCenozoic magma arc was still under extensive state during the Paleocene and Oligocene. Richard[13] also found in his study of porphyry copper deposits in the Escondida area that it is the relaxation of regional stress during the Late Eocene in this area that led to the emplacement of diorite porphyry. Take the Tethys metallogenetic zone as another example. It is found that the crust
of the Gandise massif has gradually thickened starting from 21 Ma and the tableland began to rise
rapidly. During 20 14 Ma, nearly SN-tending rift basins and potassic basalt series and
ore-bearing alkaline porphyries were formed because of the occurrence of extensive EW-tending
tension in Gandise and southern Tibet. At the same time, Miocene delaminational rock series were
produced in southern Tibet. The case is true in northern Tibet. In the Yulong copper mine, tensional stress is clearly shown on the ground, the down-warped basin was then formed in the eastern region, where Tertiary red sandstone was received. The intercontinental basins were formed
around the Yulong and Xuzhong areas. To the west of the ore zone is developed the ChangduMangkang inner continental depression, and the regional alkaline intrusion activities during this
period also suggested that its tectonic setting is a tensional environment[22]. Therefore, we think
that porphyry copper deposits were formed in the compressive environment during the tensional
period, or in a transitorily stretching period following violent compression. Compared to plate
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compression, the intrusion of porphyries is relatively late, as is also reflected in many porphyry
mining districts.
2.5 Geochemical characteristics of metallogenetic porphyries
As compared with ordinary granites, ore-bearing porphyries are characterized as being deep
in source but shallow in emplacement, as well as relatively low initial 87Sr/86Sr ratios. The initial
87
Sr/86Sr ratios in the island-arc ore-bearing porphyries are within the range of 0.7020.705[1], so
it is believed that the rocks are of mantle source. In western North America, the initial
87
Sr/86Sr
ratios in ore-bearing porphyries are about 0.70550.7109[1], suggesting the involvement of crust
materials. The initial 87Sr/86Sr ratio of the Dexing porphyry, China, is 0.7044[19], that of porphyry
copper deposits hosted in the granodiorite is 0.7054[19] in Mt. Duobaoshan, and that of
trondhjemite is 0.70369[23]. In the Yulong copper deposit, the initial 87Sr/86Sr ratios of potassic
alkaline porphyry are within the range of 0.70510.7068, as analyzed by Zhang et al.[24,25], which
are much higher than the value of the primitive mantle (0.7045). The 143Nd/144Nd ratios vary between 0.512427 and 0.512552. It is believed that the ore-bearing porphyry was derived from an
enriched mantle. The sulfur isotopes for global porphyry copper deposits vary over a narrow range,
with δ 34 S values being distributed near zero, for instance, the δ 34 S values of d porphyry deposits
are within the range of −3.5‰ + 3‰. This indicates that the ore-forming materials were derived
largely from the deep interior of the Earth. The diagrams made in terms of the trace element (fig. 2)
and rare earth element (REE) data (fig. 3) provided by previous authors[13, 20] indicated that the
potassic ore-bearing porphyries in Gandise and Escondida share much in common with respect to
their REE and trace element distribution patterns. The rocks that possess such kind of REE and
trace element distribution patterns may have resulted from oceanic basalts, or partial melting of the
mantle in which crustal materials had been involved[28].
Fig. 2. The trace element distribution patterns of the Gandise porphyry copper ore zone, and Escondida ore-bearing porphyries
of Chile. Samples IM154, ESC1, ESC3, and ZAL1 were collected from the Escondida ore-bearing porphyries of Chile. The
original data were cited from ref. [13]. The other samples were collected from the Gandise ore-bearing porphyry. The original
data were cited from ref. [20]. The chondrite-normalized trace element distribution patterns were based on ref. [26].
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TECTONIC SETTINGS OF GLOBAL SUPERLARGE PORPHYRY COPPER DEPOSITS
119
Fig. 3. The REE distribution patterns of Gandise porphyry copper ore zone and the Escondida ore-bearing porphyries of Chile.
Samples IM154, ESC1, ESC3, and ZAL1 were collected from the Escondida ore-bearing porphyries of Chile. The original data
were cited from ref. [13]. The other samples were collected from the Gandise ore-bearing porphyries. The original data were
cited from ref. [20]. The chondrite-normalized REE distribution patterns were based on ref. [27].
2.6
Delamination is an important mechanism of formation for porphyry copper deposits
It is hard to explain the mechanism of formation of some porphyry copper deposits far away
from subduction zones merely on the basis of oceanic plate subduction and partial melting of
oceanic crust. Meanwhile, it is also hard to explain the inconsistency in time between plate activities and the formation of porphyry copper deposits. And the 87Sr/86Sr ratios and δ 34S values also
indicate that the metallogenic porphyries were derived from deep mantle. Porphyry copper deposits along the western margins of North America and those in the Yulong and Gandise metallogenic
zones of China all have relatively high 87Sr/86Sr ratios, indicating that probably part of the lower
crust or oceanic basalt[29, 30] was involved in partial melting. Take porphyry metallogenesis on the
Qinghai-Tibetan Plateau for example. There obviously have been developed two high-K igneous
rock zones there. The first zone is distributed along the Kunlun Mountains, Hoh Xil, Jinshajiang,
and Ailaos, Mt. constituting an alkali-rich intrusive rock zone[24,25,31
ü33]
. The other zone is distrib-
[20,34]
. These porphyries are closely associated with
uted along the Karakorum, and Gandise ranges
the formation of the famous Yulong copper ore zone, the Qulong and Jiama copper deposits,
which are still under exploitation. The K-high porphyries in this region are special in geological
setting in addition to their special geochemical characteristics and formation time mentioned
above. The Qinghai-Tibetan Plateau possesses special lithospheric structures with thick crust[35]
and thin lithosphere generally. There are two explanations[32]. One is that thermal melting of asthenosphere resulted in thinning of the lithosphere, but it would take a long period of time. The
other is that rapid thinning of the lithosphere is related to the delamination of its basement. This
process of delamination provoked upwelling and underplating of the asthenospheric materials to
form the “crust-mantle mixed layer” in the region. Deng argued that the “crust-mantle mixed
layer” is the source of potassic rocks in this region[32]. Yuan et al.[36] thought that in south-central
Gandise there had occurred two phases of tectonic activity in their fission track studies of the
Gandise tectonically active zone. During 37.218.5 Ma and 18.58 Ma , there had occurred
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Vol. 46
discrepancy uplift early and rapid uplift late. This period is consistent with the metallogenic age of
porphyry copper deposits in the Gandise arc, as is ascertained by Qu[20] (about 14 Ma). From 16 to
13Ma, it is the period during which the Gandise block experienced extension and Miocene delaminational series were formed in the south of Tibet[37]. It is usually considered that the rise of
crust and the uplift of asthenosphere resulted from delamination, thereafter leading to depressurized melting and magmatic activities and making the crust further uplift to form the plateau. Then
there took place the collapse of orogenic belt and the extensional thinning of lithosphere. All these
processes are consistent with geological activities on the Qinghai-Tibetan Plateau, especially in
the Gandise block during the Cenozoic. Many researchers have recognized the delamination on
the Qinghai-Tibetan Plateau during the Mesozoic[38]. Bird[39] thought that under the background of
regional compression, recent uplift and E-W extension of the Qinghai-Tibetan Plateau can be explained by delamination and he also held that volcanic activities and granitic magmatism are just
the result of delamination. Research on the attenuation and velocity of earthquake waves indicates
that the asthenosphere mantle has uplifted to the shallow levels in the north of the Qinghai-Tibetan
Plateau[40]. This lends support to the hypothesis of delamination on the Qinghai-Tibetan Plateau.
Ore-bearing porphyries formed in the intermountain basins at Gandise are characterized by high
contents of potassium, belonging to the K-high series and K-basalt series. Trace elements show a
transitional feature between I-type granites and A-type granites. This shows that the porphyry
magmatism is related to delamination and upwelling of the associated asthenospheric materials[41]. And ore-bearing kaliporphyries are the outcome of delamination. Therefore, it is no doubt
that delamination is an important mechanism of formation of porphyry copper deposits. The Andean ranges, the Cordillera ranges, the Qinghai-Tibetan Plateau and the Alpines are the main locations where lower-crust delamination took place and are also the dominant distribution areas of
porphyry copper deposits.
In regard to the east and west of the Pacific, great subduction speed, small subduction angle
and great crust thickness in the east of the Pacific are more favorable to the proceeding of
large-scale delamination, as well as to the involvement of the lower crust or oceanic crust with
abundant ore-forming materials in partial melting. This may be one of the factors leading to the
production of porphyry copper deposits on a large scale along the eastern coast of the Pacific.
Acknowledgements
The authors wish to thank academician Tu Guangzhi with the Institute of Geochemistry, the
Chinese Academy of Sciences (CAS) and chief scientist Zhao Zhenhua with Guangzhou Institute of Geochemistry, the CAS for
their great concern and help with this research. This work was supported by the National Climbing Program (95-Y-25), Knowledge Innovation Program of the CAS (Grant Nos. KZCX2-SW-117 and KZXC2-101) and the National Natural Science Foundation of China (Grant No. 40072022)
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