SCIENCE CHINA

Earth Sciences

REVIEW April **** Vol.53 No.4: 475 484

doi: 10.1007/s11430-010-0024-0

Ridge subduction and porphyry copper-gold mineralization:

An overview

SUN WeiDong1,2*, LING MingXing1,3, YANG XiaoYong2, FAN WeiMing1,

DING Xing1 & LIANG HuaYing4

1

CAS Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,

Guangzhou 510640, China;

2

Research Center for Mineral Resources, School of Earth and Space Sciences, University of Science and Technology of China,

Hefei 230026, China;

3

Graduate University of Chinese Academy of Sciences, Beijing 100049, China;

4

Key Laboratory for Metallogenic Dynamics, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,

Guangzhou 510640, China

Received July 20, 2009; accepted January 8, 2010; published online March 3, 2010

Many large porphyry Cu-Au deposits are connected to adakitic rocks known to be closely associated with ridge subduction.

For example, there are several subducting ridges along the east Pacific margin, e.g., in Chile, Peru, and South America, most of

which are associated with large porphyry Cu-Au deposits. In contrast, there are much fewer ridge subductions on the west Pa-

cific margin and porphyry Cu-Au deposits are much less there, both in terms of tonnage and the number of deposits. Given that

Cu and Au are moderately incompatible elements, oceanic crust has much higher Cu-Au concentrations than the mantle and

the continental crust, and thus slab melts with their diagnostic adakitic chemistry have systematically higher Cu and Au, which

is favorable for mineralization. Considering the geotherm of subducting slabs in the Phanerozoic, ridge subduction is the most

favorable tectonic setting for this. Therefore, slab melting is the likely link in the spatial association between ridge subduction

and Cu-Au deposits. Geochemical signatures of slab melting and hence maybe ridge subduction in less eroded regions in east-

ern China, the central Asian orogenic belt etc. may indicate important exploration targets for large porphyry Cu-Au deposits.

ridge subduction, porphyry Cu-Au deposits, adakite, slab melting

Citation: Sun W D, Ling M X, Yang X Y, et al. Ridge subduction and porphyry copper-gold mineralization: An overview. Sci China Earth Sci, 2010, 53:

475 484, doi: 10.1007/s11430-010-0024-0

Bronces, La Escondida, Los Pelambres, El Pach n, Rosario,

Porphyry Cu is an important type of deposit, comprising

Radomiro Tomic, El Salvador and Toki, of which, El

>60% of the world s Cu reserves. The circum-Pacific region

Teniente is the world s largest porphyry Cu deposit, with a

is the largest porphyry Cu mineralization province [1, 2].

reserve of 90 million tones of Cu metal [2 6]. There are

For example, Chile on the east Pacific margin hosts more

also many porphyry Cu-Au deposits in the Philippines,

than 10 world class porphyry Cu deposits, with a total re-

southwest Pacific [2, 7]. Most of these deposits are associ-

serve of more than 0.35 billion tones of Cu metal, which

ated with adakites the products of slab melting [4 6, 8],

accounts for >30% of the world s Cu reserves. These de-

and are spatially related to the subduction of ocean ridges or

posits are El Teniente, Chuquicamata, R o Blanco-Los

island chains [2].

In contrast to many other places along the circum-Pacific,

*Corresponding author (email: abpnw3@r.postjobfree.com)

Science China Press and Springer-Verlag Berlin Heidelberg 2010 earth.scichina.com www.springerlink.com

476 SUN WeiDong, et al. Sci China Earth Sci April (2010) Vol.53 No.4

eastern China is poor in Cu, with a total reserve of ca. 10 intersects a trench [18], as was first discovered 4 decades

million tones. Plate reconstruction shows that eastern China ago during studies on orogenic processes in the western

became an active continental margin from the Jurassic [9, America [19]. It is now known that ridge subduction is a

10]. Similar to the South and North Americas, there are also common phenomenon along the circum-Pacific subduction

abundant adakite and/or adakitic rocks in eastern China, zones [20 24], and may have occurred several times since

many of which show Cu-Au mineralization, but large de- the late Mesozoic [25 29]. Because of the unique physical

posits with more than 10 million tones of Cu metal are not and thermal structures of ocean ridges, ridge subduction has

yet known. However, Dexing and the Lower Yangtze River major influence on the subduction angle, the thermal struc-

belt have smaller-scale porphyry and/or skarn Cu (Au) de- ture of the subduction zone, as well as the nature of magma-

posits [11 17]. tism and associated mineralization. Therefore, these proc-

What are the main controls on the distribution of por- esses have been the focus of wide interest [2, 22, 30 34]

phyry Cu-Au deposits in the circum-Pacific region and (Figure 1).

elsewhere? What is the connection between ridge subduc- Many large porphyry Cu-Au deposits are closely associ-

tion and porphyry Cu deposits? Are there any porphyry Cu ated with ridge subduction. For example, three of the

deposits derived from Pacific subduction in eastern China? world s largest porphyry Cu deposits, El Teniente (94 Mt

To answer these questions, we review previous studies on Cu), R o Blanco-Los Bronces (57 Mt Cu), Los Pelambres-

circum-Pacific large porphyry Cu-Au deposits, discuss the El Pach n (27 Mt Cu), ranking 1, 3, 10 respectively, are

genesis and distribution of these deposits, as well as the located close to each other in central Chile, along the east

connection between ridge subduction and porphyry Cu-Au Pacific margin. All these deposits are spatially associated

deposits from a geochemical perspective. with the subduction of the Juan Fern ndez ridge [2, 36].

Similarly, the Cerro Colorado large porphyry Cu-Au deposit

(21 Mt Cu) corresponds to subduction of the Cocos ridge

1 Ridge subduction and porphyry Cu-Au de-

(Figure 2) [3].

posits along the circum-Pacific region Several large porphyry Cu-Au deposits in the southwest

of the United States, e.g., Bingham (28 Mt Cu, 1603 t Au),

An ocean ridge can be subducted into the mantle when it Butte (35 Mt Cu), are located close to ongoing ridge sub-

Figure 1 Global distribution of large porphyry Cu-Au deposits. Data from refs. [2, 3, 35]. A, B, C are regions dominated by Cu deposits; D, E are regions

dominated by Au deposits. A, central Chile province (El Teniente, R o Blanco-Los Bronces, Los Pelambres); B, northern Chile province (Chuquicamata, La

Escondida, Radomiro Tomic, Rosario, El Salvador, El Abra); C, southwest Arizona-Sonora province (Cananea, Lone Star, Morenci-Metcalf, Pima, Ray); D,

Papua New Guinea-Irian Jaya province (Grasberg, Ok Tedi, Panguna, Frieda River); E, Philippines province (Far South East-Lepanto, Tampakan, Atlas,

Sipalay).

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SUN WeiDong, et al. Sci China Earth Sci April (2010) Vol.53 No.4

Figure 2 Ridge subduction in the eastern Pacific and large porphyry Cu-Au deposits in South America. The tectonic setting is adapted from ref. [20], and

the distribution of ore deposits from refs. [2, 3].

duction in the Baja California [33]. It is however, not clear genetic connection between these particular deposits and the

how closely these deposits are related to the ridge subduc- Iquique ridge subduction, even though most ocean ridges

tion, because they formed at 38 60 Ma. Chuquicamata in are much longer than 3000 km.

central Chile is the second largest porphyry Cu-Au deposit There are several porphyry Cu-Au deposits closely asso-

in the world with a total of 66 Mt Cu metal and 300 t Au. ciated with subduction of the Scarborough ridge in the

About 5 km to the north Chuquicamata, there is another Philippines, southwest Pacific [2]. These are: Lepanto-Far

large porphyry deposit, Radomiro Tomic (20 Mt Cu) [2]. South East (5.48 Mt Cu, 970 t Au) [38], Santo Tomas II (1.2

Both deposits are located above the ongoing subduction of Mt Cu, 233 t Au) [1], Guinaoang (2 Mt Cu, 200 t Au) [3]. In

the Iquique ridge, and formed at 33.6 Ma [2] and 32.7 Ma addition to porphyry Cu-Au deposits, Scarborough ridge

[37], respectively. In general, ridge subduction is slower subduction is also associated with Baguio large low sulfur

than normal plate subduction. Assuming an average rate of epithermal deposit. All these deposits are younger than 3

10 cm/yr, about 3000 km may be subducted within 30 Ma. Ma (Figure 3) [39]. In contrast to ridge subductions along

Therefore, more work is needed to test whether there is any South America, the Scarborough ridge formed by slab tear-

478 SUN WeiDong, et al. Sci China Earth Sci April (2010) Vol.53 No.4

ing [40]. In Japan, Kyushu-Palau ridge subduction [23] spa- 138 5 Ma [13, 14, 17]. These Cu-Au deposits are mainly

tially corresponds to the Hishikari lower sulfur epithermal related to adakite or adakitic rocks, distributed linearly with

deposit and the Nansatsu high sulfur deposits (Figure 4). a roughly east-west trend. More normal calc-alkaline rocks

Both the above mentioned ridge subductions are associated flank both sides of the adakite belt. There are also A-type

with epithermal deposits. There are however, no world class granites whose distribution roughly overlaps with the ada-

porphyry deposits as found in South America. Probably this kites, but they are ca. 10 million years younger, having

is because deposits connected to ridge subduction in the formed mainly at 125 Ma [41 44].

Philippines and Japan are very young, and much less ex- Many workers have proposed that the Mesozoic adakites

posed due to limited erosion. If this is correct, there could in eastern China, including the Lower Yangtze River belt,

be porphyry deposits at depth (Figures 3, 4). Because there were formed by partial melting of thickened or delaminated

are considerably fewer ridge subductions in the west Pacific lower continental crust, based mainly on the enriched iso-

than in the east, this provides a feasible interpretation for topic characteristics [13, 45 51]. If so, the porphyry Cu

much more abundant porphyry Cu-Au deposits along the deposits associated with adakites would have no connection

east Pacific margin. with plate subduction. In contrast, other workers have pro-

Eastern China is located on the west Pacific margin, but posed that the Cu mainly came from the mantle [11, 52].

is now far away from Pacific subduction zones. The Lower Copper is a moderately incompatible element [53, 54],

Yangtze River belt is the most important Cu, poly-metal therefore its abundance in the depleted mantle is lower than

mineralization belt in eastern China, which was formed at that of the primitive mantle (30 ppm) [55]. Interestingly, Cu

Figure 3 Ridge subduction and Cu-Au deposits in the Philippines (modified after ref. [2]).

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SUN WeiDong, et al. Sci China Earth Sci April (2010) Vol.53 No.4

Figure 4 Ridge subduction and Cu-Au deposits in south Japan. The tectonic settings after ref. [23] and the mineral deposit distribution follows ref. [2].

abundance in the continental crust is also low (27 ppm) [56]. ocean ridge between these two plates was subducted in what

This is mainly because Cu and Au are scavenged into fluids is now the Lower Yangtze River region. The ridge subduc-

at the late stage of magma evolution [53, 54]. Because of tion model can well explain the association and distribution

the characteristics of Cu, solely intracontinental processes of adakite, calc-alkaline rocks, Nb enrichment, A-type gran-

or direct addition of mantle components do not necessarily ites and associated mineral deposits [27]. Because oceanic

result in Cu enrichment. crust near a ridge is hotter, it is prone to be partially melted

Alternatively, it has been proposed that the Lower Yang- during ridge subduction, to form adakites. On the other

tze River mineralization belt can be better explained by hand, when older, colder and wetter oceanic crust distal

ridge subduction, whereby associated adakites formed from the ridge is subducted, it undergoes dehydration,

through slab melting have been contaminated by the conti- which triggers flux-melting in the overlying mantle, to pro-

nental crust and enriched mantle [27]. Between ~125 140 duce more typical calc-alkaline arc magmas. As subduction

Ma, the Pacific plate drifted southwestward, whereas the continues, the opening of a slab window along the ridge can

Izanagi plate drifted north-northwestward [57], and the be filled by upwelling asthenosphere, giving rise to high

480 SUN WeiDong, et al. Sci China Earth Sci April (2010) Vol.53 No.4

temperature, anhydrous, A-type granites [33]. A geochemi- 60 125 ppm [76], with an estimated average of 74 ppm [77],

cal character of ridge subduction is the production of much higher than the average abundance of primitive man-

Nb-enriched igneous rocks [33]. The Nb-enriched nature of tle (30 ppm) [55] and the continental crust (27 ppm) [56].

the Lower Yangtze River belt volcanic rocks thus also sup- Therefore, partial melts of oceanic slab should have much

ports the ridge subduction model [27]. higher initial Cu concentrations than melts derived directly

It is worth mentioning that Cretaceous adakites in the from the mantle or from within the continental crust [78,

Dabie Mountains formed at roughly the same time as those 79]. Thus the genetic connection between porphyry Cu de-

in the Lower Yangtze River belt, with high-Mg adakite in posits and adakites links to slab melting, with its elevated

the southeast margin. These adakites, however, are not Cu abundance [79].

mineralized. Geochemical characteristics suggest the Cre- The geochemical behaviors of Cu and Au during mag-

taceous adakite from the Dabie Mountains formed through matism are very similar to each other [53, 54]. Therefore,

melting of the lower continental crust [49], likely triggered fundamentally Au mineralization may be connected to ridge

by ridge subduction [27]. subduction in a way similar to that of Cu. But in contrast,

Cu and Au behave quite differently in fluids, resulting in

diverse types of deposits and spatial distribution for these

2 Adakites and porphyry Cu deposits metals.

Adakite was originally proposed to describe a special type

3 Discussion

of rocks found in Adak Island, Aleutian Islands, which has

components from slab melting [58 60]. The close genetic 3.1 Adakite and ridge subduction

association between porphyry Cu deposits and adakites has

been proposed for more than a decade [8]. Among all the 43 Slab melting is believed to be a major process in the early

deposits or mineralization districts, 38 are connected to history of the Earth, which in those times was responsible

adakites, and often directly occur in adakites. In places for the formation of the continental crust [80 83]. In con-

where adakites coexist with non-adakitic igneous rocks, trast, the thermal structure of modern subducting slab sug-

mineralization is usually associated with the adakites [8]. gests very limited slab melting [60, 84], with it occurring

This model has gained wide support, because most of the mainly during subduction of young, hot oceanic crust [60,

porphyry Cu and epithermal Au deposits are closely related 85].

to adakites both in terms of age and space [7]. Thus in Because of the unique thermal structure of ocean ridges,

Mongolia, the Erdenet porphyry Cu deposit is associated it is then safe to say that ridge subduction is the most fa-

with adakite [61]; whereas many porphyry Cu deposits in vorable geological process for slab melting in the Phanero-

Chile, e.g., Los Pelambres, Chuquicamata, etc., are also zoic. This gives the essential link between the coincidence

connected to adakites [4 6, 62, 63]. In China, the Tongling, of ridge subduction and world class porphyry Cu deposits.

Shaxi, Dexing porphyry or skarn Cu deposits are all associ-

ated with adakite or adakitic rocks [12, 46, 47, 64 66]. It 3.2 Slab melts and porphyry Cu deposits

has even been proposed that adakite should be taken as an

exploration target for Cu deposits [67]. Although the genetic connection between adakite and por-

The genetic connection between adakite and porphyry phyry Cu-Au deposits has been supported by increasing

deposits has been attributed to the high oxygen fugacity of number of studies [4 6, 62, 63], there are still a group of

adakitic magmas [68]; with oxygen fugacity taken as a more scholars who argue strongly against the adakite mineraliza-

important controlling factor than actual adakites for por- tion model [86 88]. The argument is that the geochemical

phyry Cu mineralization [69]. The geochemical behavior of characteristics of adakite do not necessarily need slab melt-

Cu is indeed highly influenced by oxygen fugacity [1, 54, ing, but can be produced through fractional crystallization

70], with mineralized porphyry usually having higher oxy- of normal arc magmas [88 91]. Therefore, magma evolu-

gen fugacity [71]. Most arc rocks, however, have high oxy- tion at crustal levels was taken as a more important factor

gen fugacity, amongst which adakite is not the most oxi- that controls Cu mineralization [88]. This actually argues

dized variety [72 74]. Therefore, the high oxygen fugacity against the genetic relation between slab melting and por-

of arc rocks is not controlled mainly by slab melting, but phyry Cu deposits. According to such model, all types of

more by slab dehydration [75]. Consequently, the close ge- arc rocks are potential candidates to host porphyry Cu de-

netic connection between adakites and porphyry deposits posits. In our opinion, this contradicts with observations so

does not totally depend on high oxygen fugacity. far available. As pointed out in section 2, the Cu concentra-

Copper is a moderately incompatible element during tion in oceanic crust is 2 4 times as high as the Cu abun-

MORB production and flux melting arc magmatism, with a dances in the mantle and the continental crust. Therefore,

partition coefficient similar to the heavy rare earth elements slab melts are more favorable for spawning mineralization

[53, 54]. Copper concentrations in oceanic crust range from [78, 79].

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It is true that not all the adakites are associated with por- disappeared, traces of ridge subduction can only be found in

phyry Cu deposits [49, 92]. For example, there are abundant the magmatic products preserved in the supra-subduction

Mesozoic adakites in eastern China, but large porphyry Cu zone environment. Important for identifying ancient ridge

deposits are only found in Dexing and the Lower Yangtze subduction are the combination of adakite, high-Mg ande-

River belt. However, many of the non-mineralized adakites site and Nb-enriched basalts [93 95]. For locations with

are considered to have formed by partial melting of conti- thick overlying continental crust, there might be Nb-

nental crust [49]. In this context, it is important to note that enriched andesitic rocks. Also, whereas typical arc-type

porphyry Cu deposits are formed at shallow depths (usually calc-alkalic rocks are rare due to limited dehydration near

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