Journal of Oceanology and Limnology   2020, Vol. 38 issue(1): 64-74 PDF
http://dx.doi.org/10.1007/s00343-019-8339-y
Institute of Oceanology, Chinese Academy of Sciences
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#### Article Information

LIU He, LIAO Renqiang, ZHANG Lipeng, LI Congying, SUN Weidong
Plate subduction, oxygen fugacity, and mineralization
Journal of Oceanology and Limnology, 38(1): 64-74
http://dx.doi.org/10.1007/s00343-019-8339-y

### Article History

accepted in principle Jan. 18, 2019
accepted for publication Apr. 11, 2019
Plate subduction, oxygen fugacity, and mineralization
LIU He1,2,3, LIAO Renqiang1,2,3,4, ZHANG Lipeng1,2,3, LI Congying1,2,3, SUN Weidong1,2,3,4
1 Key Laboratory of Marine Geology and Environment, Center of Deep-Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;
2 Laboratory for Marine Mineral Resources, Laboratory for Marine Biology and Biotechnology, Pilot National Laboratory for Marine Science and Technology, Qingdao 266071, China;
3 Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China;
4 University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: Plate subduction is the largest natural factory that processes elements,which controls recycling and mineralization of a variety of elements. There are three major ore deposit belts in the world:the circumPacific,the central Asian,and the Tethys belts. All the three belts are closely associated with plate subductions,the mechanism remains obscure. We approached this problem from systematic studies on the behaviours of elements during geologic processes. This contribution summaries the recent progress of our research group. Our results suggest that porphyry Cu deposits form through partial melting of subducted young oceanic crust under oxygen fugacities higher than ·FMQ~+1.5,which is promoted after the elevation of atmospheric oxygen at ca. 550 Ma. Tin deposits are associated with reducing magmatic rocks formed as a consequence of slab rollback. The Neo-Tethys tectonic regime hosts more than 60% of the world's total Sn reserves. This is due to the reducing environment formed during the subduction of organic rich sediments. For the same reason,porphyry Cu deposits formed in the late stages during the closure of the Neo-Tethys Ocean. Tungsten deposits are also controlled by slab rollback,but is not so sensitive to oxygen fugacity. Subduction related W/Sn deposits are mostly accompanied by abundant accessory fluorites due to the breakdown of phengite and apatite. Decomposition of phengite is also significant for hard rock lithium deposits,whereas orogenic belt resulted from plate subduction promote the formation of Li brine deposits. Cretaceous red bed basins near the Nanling region are favorable for Li brines. Both Mo and Re are enriched in the oxidationreduction cycle during surface processes,and may get further enriched once Mo-,Re-enriched sediments are subducted and involved in magmatism. During plate subduction,Mo and Re fractionate from each other. Molybdenum is mainly hosted in porphyry Mo deposits and to a less extent,porphyry Cu-Mo deposits,whereas Re is predominantly hosted in porphyry Cu-Mo deposits and sedimentary sulfide deposits.
Keywords: plate subduction    oxygen fugacity    ore deposits    geochemical behaviors    subduction factory
1 INTRODUCTION

Ore deposits are essentially geologic bodies that are unusually enriched in certain elements. Therefore, the geochemical behaviours of elements are key factors that control mineralization processes. Essentially all the endogenous metallic deposits are associated with magmatism.

Magmatism on Earth is mainly associated with two systems, plate tectonic and mantle plume magmas. With the exception of several types of deposits, e.g., Cu-Ni sulfide, PGE, V-Ti-magnetite, which are mostly related to mantle plumes (Song et al., 2005; Xu et al., 2013; Zhou et al., 2013; Lightfoot and EvansLamswood, 2015; Liao et al., 2016; Yang et al., 2017, Wang et al., 2018), and diamond deposits, which are associated with kimberlites (Shirey et al., 2013; Sun et al., 2018a), most of the endogenous metallic deposits are formed within the plate tectonic regime, mainly related to plate subduction (Carlile and Mitchell, 1994; Kesler, 1997; Sillitoe, 1997; Chiaradia et al., 2004, Sun et al., 2004; Cooke et al., 2005; Mlynarczyk and Williams-Jones, 2005; Lehmann, 2011; Wang et al., 2011; Wilkinson, 2013) and subsequent continental collisions (Hou et al., 2009; Richards, 2011; Chen, 2013; Hou et al., 2015; Hou and Zhang, 2015; Zheng et al., 2019). There are three major ore deposit belts in the world: the circumPacific, the central Asian, and the Tethys belts (Fig. 1). All the three belts are closely associated with plate subduction.

 Fig.1 Global distribution of porphyry Cu deposits and the three major mineralization domains, which are all associated with plate convergences (Modified after Sun et al., 2013) The yellow, blue and red dashed lines show the extents for the circum-Pacific, the central Asian and the Tethys belt, respectively.

Plate subduction is often metaphorized into the "subduction factory" (Hacker et al., 2003; Sun, 2003; van Keken et al., 2009, 2011; Hacker and Abers, 2012; Sun et al., 2014), which is the largest natural factory that processes different elements through dehydration, metasomatism and magmatism on the Earth. Oceanic slab, including both fresh and altered oceanic crust and lithospheric mantle, alongside with sediments are subducted at convergent margins, triggering arc magmatism, changing the composition of the mantle, leading to recycling water, carbon and silicates, and giving rise to mineralization (Yang and Scott, 1996; Sun et al., 2004, 2010, 2011b, 2018c; Zhang et al., 2008; Liang et al., 2009b; Ling et al., 2009, 2013; Li et al., 2017). The behaviours of oreforming elements during plate subduction, however, remain obscure. This contribution briefly reviews our results on the behaviours and corresponding mineralization of several elements, with main interests on endogenous metallic deposits.

2 PORPHYRY CU DEPOSITS

Porphyry Cu deposits are the main sources of Cu, accounting for more than 70% of the world Cu reserves (Singer et al., 2008). It is well known that porphyry Cu deposits are usually associated with high oxygen fugacity magmas (Mungall, 2002; Liang et al., 2009b; Sillitoe, 2010; Sun et al., 2013, 2015a; Zhang et al., 2017a, c ). The speciation of sulfur is controlled by oxygen fugacity, whereas reduced sulfur (sulfide) has stronger influences on Cu than oxidized sulfur (sulfate) (Sun et al., 2017). Why high oxygen fugacity with sulfate as the dominant sulfur species is favorable for porphyry Cu mineralization? The answer is residual sulfide.

Previous results suggest that high oxygen fugacity is not the exclusive requirement for porphyry Cu deposits (Sun et al., 2017). Modelings suggested that partial melting of mantle peridotite even at very high oxygen fugacity forms arc magmas with initial Cu contents that is too low to form porphyry Cu deposits directly (Lee et al., 2012; Wilkinson, 2013). This is clearly illustrated by the absence of porphyry Cu deposits in normal convergent margin magmas, which are mostly highly oxidized (Ballhaus, 1993; Brandon and Draper, 1996; Kelley and Cottrell, 2009; Evans et al., 2012; Sun et al., 2015b). Porphyry Cu deposits are closely associated with adakite (Thieblemont et al., 1997; Oyarzun et al., 2001; Mungall, 2002; Sun et al., 2015a, 2017; Zhang et al., 2017a) formed by partial melting of subducted young oceanic crust (Defant and Drummond, 1990; Sun et al., 2011a, 2012). The subduction of young oceanic ridges is the most favorable geologic process that forms porphyry Cu deposits (Sun et al., 2010). Oceanic crust has Cu concentrations of about 3 times and S concentrations of about 4 times higher than those of the mantle (Sun and McDonough, 1989; McDonough and Sun, 1995). With oxygen fugacity higher than c.a. △FMQ +1.5 to +2.0, sulfur is mainly presented as sulfate, which is about 10 times more soluble than sulfide in magmas (Jugo, 2009; Jugo et al., 2010). Therefore, copper is incorporated into the melts and the residual sulfide is eliminated during partial melting of the subducted oceanic crust under high oxygen fugacity (Sun et al., 2013). This process forms adakite magmas with high initial Cu concentrations (Sun et al., 2011a, 2012, 2015a), favorable for porphyry Cu mineralization (Zhang et al., 2017c). Copper is further enriched during hydrothermal processes (Sun et al., 2004; Wang et al., 2016).

Large porphyry deposits may form through continuous magma re-injections and multiple-pulses of porphyry mineralization. In addition, porphyritic hydrothermal sulfide formed in the early stage of porphyry mineralization is usually associated with iron oxides, which may assist the oxidization of sulfide during re-partial melting, and consequently contribute to porphyry mineralization (Sun et al., 2017).

Interestingly, most of the porphyry Cu deposits are younger than 550 Ma (Xia et al., 2003). This because the formations of porphyry Cu deposits require highoxygen-fugacity magma (△FMQ > +1.5 to +2) (Ballard et al., 2002; Mungall, 2002; Liang et al., 2009a; Sun et al., 2015b; Zhang et al., 2017a). The atmospheric O2 content was enormously increased from less than 1% to nearly 100% of present atmospheric level during the Ediacaran (~635–542 Ma) (Scott et al., 2008; Sahoo et al., 2012; Lyons et al., 2014), resulting in higher Fe3+ in altered oceanic crust. Partial melting of the altered oceanic crust with more Fe3+ forms oxidized adakitic magmas that are beneficial to porphyry Cu mineralization.

Previous researchers proposed that porphyry Cu deposits may form through partial melting of sulfide accumulates at convergent margins (Lee et al., 2012; Wilkinson, 2013; Zheng et al., 2018). Given that the partition coefficient of Cu between sulfide and magmas is very high, partial melting of sulfide accumulates cannot form Cu-enriched magmas due to residual sulfide (Sun et al., 2017).

It has also been proposed that thick overriding continental crust reduces the "leakage" of hydrothermal fluids and is favorable for porphyry mineralization (Chiaradia, 2014). We argue that the thick overriding continental crust is also more difficult for adakatic magma to penetrate (Sun et al., 2017), because porphyry deposits usually form at depths of 2–4 km (Sillitoe, 2010). Therefore, thick continental crust is not necessarily a favorable condition for porphyry Cu deposits. There are many porphyry deposits in places with thin continental crust (Pollard and Taylor, 2002).

It is true that there are giant porphyry Cu deposits in the South American, where the continental crust is thick. We argue that this is nothing but a coincident: slab melting requires the subduction of young oceanic crust, spreading ridges or seamount chains (Sun et al., 2010). In this case, the subducting slab with a shallower subduction angle strongly interacts with the overriding plate, and forms mountain chains as well as the thicker continental crust.

3 TIN DEPOSITS

Magma fractionation, oxygen fugacity, fluids and halogen concentrations are all taken as important conditions that influence Sn mineralization (Lehmann and Harmanto, 1990; Lehmann et al., 1990; Ishihara, 1998, 2000; Hu et al., 2016). However, the controlling factor remain obscure.

It has been observed that Sn concentration in igneous rocks increases with increasing Rb/Sr (Lehmann, 1987), indicating that Sn is enriched during magma fractionations. Based on this, it was further argued that Sn-enriched source materials is not necessary for Sn mineralization (Lehmann and Harmanto, 1990; Lehmann et al., 1990). Tin, however, is not highly incompatible. Why it forms ore deposits while most of the highly incompatible elements do not form ore deposits alongside with Sn during magma evolution? This was explained by the redoxsensitive nature of Sn. More than 70% of the world's Sn deposits are distributed in the Neo-Tethys orogenic belt (Zhang et al., 2017a, b ; Guo et al., 2018a, b ; Sun et al., 2018b). The low oxygen fugacity of magmas within the Neo-Tethys orogenic belt explains the usual abundances of Sn deposits there.

Tin has two major valance states in natural samples, Sn2+ and Sn4+, which substitute Ca2+ and Ti4+, Fe3+, Ta3+, Nb5+, respectively (Shannon and Prewitt, 1970). It is well known that Sn deposits are closely associated with reducing magmas, e.g., ilmenite granite (Ishihara and Murakami, 2006) (Fig. 2), because under high oxygen fugacity, Sn4+ is the dominant species, so that Sn is taken by early crystallized minerals that contains Ti4+, Fe3+, Ta3+, Nb5+. The problem is that Ca minerals, which may take Sn2+, e.g., plagioclase, may also crystalize early during magma evolution. Moreover, low oxygen fugacity is not a sufficient condition. Most reduced magmas do not form Sn deposits (Zhang et al., 2017a). Why some reduced magmas form ore deposits, while others do not?

 Fig.2 Oxygen fugacity versus Fe contents, showing that Sn deposits are associated with ilmenite series, porphyry Cu deposits with magnetite series W is less sensitive to oxygen fugacity (Modified after Thompson et al., 1999).

Similar to porphyry Cu deposits, most of the Sn deposits are also associated with plate subduction, but favor low oxygen fugacity magmas, away from the convergent margins (Mlynarczyk and WilliamsJones, 2005; Lehmann, 2011; Wang et al., 2011). There is a sequence of iron oxides, porphyry Cu, PbZn and then Sn (W) zonings from the subduction zone to the interior continent (Mlynarczyk and WillaimsJones, 2005; Wang et al., 2011). This is explained by that oxygen fugacity of subduction-related magmas decreases along the same direction, across the magnetite-ilmenite oxygen barometer (Fig. 2).

All these suggests that plate subduction is the most crucial tectonic process that controls Sn mineralization. We propose that the fluorine-rich fluids released from subducting plate are the most favorable agent for Sn mineralization. During plate subduction, fluorine is hosted by lawsonite, phengite and apatite within the slab. Lawsonite decomposes earlier, whereas phengite and apatite decompose at higher temperatures (Schmidt, 1996; Li et al., 2012b; Chen et al., 2016). High-fluorine fluids mobilize Sn and other relevant elements, trigger the formation of A2 type granites, and lower the solidus of melting, resulting in highly evolved granites (Chen et al., 2016). Resulted from the slab rollback, the oxygen fugacity of magmas away from the subduction zone is generally low, where Sn2+ is the dominant species. Meanwhile, plagioclase crystallization may also be delayed by F, resulting in high Sn2+.

Previous authors proposed that S-type granites may also form Sn deposits because of the low oxygen fugacity (Feng et al., 2010). This is yet to be testified. Considering that there are sedimentary Sn deposits, S-type granite may have inherited Sn.

4 TUNGSTEN DEPOSITS

More than half of the world's W reserves are distributed in Southeast China (Hua et al., 2005; Zaw et al., 2007; Wang et al., 2010; Mao et al., 2011a; Jiang et al., 2015; Chen et al., 2016; Li et al., 2016; Yao et al., 2016; Zhang et al., 2017d), with two major W-belts roughly parallel to each other, formed in the Late Jurassic and the Early Cretaceous, respectively (Fig. 3). Abundant Sn-(W) deposits formed in the Late Cretaceous distribute east-west ward (Cheng et al., 2016; Guo et al., 2018a, b ; Zhang et al., 2018). It has long been mysterious why so many W are concentrated in a small granite province of less than one million km2 (Mao et al., 2013; Jiang et al., 2015; Chen et al., 2016; Zhang et al., 2017b). Most of the W deposits are associated with highly evolved F-rich granites (Chen et al., 2016).

 Fig.3 Distribution of major Jurassic metallogenic belts in South China and comparison with the metallogenic belts in South America (modified from Zhao et al., 2017) a. the grey, yellow, orange and green polygons show the extents of Jurassic Fe oxide, porphyry Cu, Pb-Zn-Ag and Sn metallogenic belt, respectively. The black arrow line shows the proposed moving direction of Paleo-Pacific plate subduction during the period 180-125 Ma; b. distribution of Fe oxide, porphyry Cu, Pb-Zn-Ag and Sn metallogenic belts in South America.

Tungsten has very low abundances in the silicate Earth. Under the oxygen fugacity of the mantle and the crust, tungsten is a highly incompatible lithophile trace element, which may be dramatically enriched through magmatism processes (McDonough and Sun, 1995). Therefore, highly evolved high-F granites are the most favorable source of W deposits (Xiong et al., 2002a, b ). Previous studies suggested that those granites formed during the initial rollback of a flatly subducting oceanic slab and consequent decomposition of phengite (Schmidt, 1996) ± apatite (Schmidt and Poli, 2014) as a result of abruptly elevated temperatures caused by asthenosphere upwelling, releasing F and Li (Li et al., 2012b; Chen et al., 2016). Tungsten usually presents as water soluble complex, [WO4]2-, which may be further enriched during hydrothermal processes. Subduction also provides water that enable hydrothermal mineralization.

5 LITHIUM DEPOSITS

More than 75% of the world Li resources comes from brine type Li deposits formed through weathering and evaporation, while other Li resources are hosted by pegmatite-type Li deposits (Xu et al., 2018). Although Li is a moderately incompatible element, it may be highly enriched during the late magmatic evolution, likely due to hydrothermal fluids that enriches Li. Therefore, pegmatite is important for hard rock lithium deposits.

Similar to F, lithium may also be carried several hundred kilometers away from the subduction zone by phengite. The high-F, high-Li granites in Southeast China have been attributed to decomposition of phengite during slab rollback (Li et al., 2012b; Chen et al., 2016). Meanwhile, plate subduction often forms accretionary orogenic belts, resulting in large basins coupled with arid climate, which collects weathering products from the high-Li granites and forms brine Li deposits. This explains the brine type of Li deposits in the west margin of the American continents (Munk et al., 2016). Similarly, southeast China experienced uplifting (Li and Zou, 2017) likely also due to plate subduction. Cretaceous basins near the Nanling Li-Fgranite belt are favorable targets for brine Li deposits.

6 MOLYBDENUM DEPOSITS

Most of the Mo resources are hosted in porphyry deposits in Qinling-Dabie orogenic belt (Mao et al., 2011b; Li et al., 2012a; Chen, 2013) and eastern Pacific continental margins (Sun et al., 2016). There are three main types of porphyry Mo deposits, high-F porphyry Mo, low-F porphyry Mo and porphyry CuMo deposits (Fig. 4).

 Fig.4 Distribution of low-F Mo, Climax porphyry Mo and porphyry Cu-Mo deposits in the American continent (Modified after Sun et al., 2016) Data were download from the USGS database (https://mrdata.usgs.gov/). a. distribution of Mo deposits in the American continent. The white rectangles show the extents of figure a and b; b. the magnified map showing names and locations of low-fluorine Mo deposits; c. the magnified map showing the name and locations of Climax-type porphyry Mo deposits.

Molybdenum is a very rare element with an abundance of ~50 ppb in the silicate Earth (McDonough and Sun, 1995). It is a moderately incompatible chalcophile element with incompatibilities similar to that of Ce during mantle magmatic processes (McDonough and Sun, 1995; Sun et al., 2003c), and is enriched in the continental crust (~0.8×10-6) (Rudnick and Gao, 2003). In addition to magmatism and hydrothermal processes, Mo is also efficiently concentrated through the oxidation- reduction cycle on the Earth's surface, enriched in organic-rich sediments, e.g., Ocean Anoxic Event (OAE) sediments (Jenkyns, 2010; Sun et al., 2016). In particular, Mo enrichment became more pronounced after the elevation of atmospheric oxygen during the Ediacaran ~550 Ma ago (Scott et al., 2008; Sahoo et al., 2012; Lyons et al., 2014).

There are 9 major OAEs since the Jurassic (Jenkyns, 2010). Most of the OAE Mo-enriched sediments have been subducted beneath the western American Continents, resulting in Mo-enrichment in the mantle wedge (Sun et al., 2015a, 2016). This provides a feasible explanation to the abundant Mo resource in the east Pacific continental margins, e.g., Climax high-F and low-F porphyry Mo deposits (Ludington and Plumlee, 2009) and porphyry Cu deposits (Ludington and Plumlee, 2009), accounting for about half of the world's Mo reserves.

The Ediacaran-Cambrian black shale in the South China block has high Mo concentrations. The QinlingDabie porphyry Mo metallogenic belt is attributed to the subduction of Mo-rich Precambrian black shales in the South China block (Sun et al., 2016).

7 RHENIUM RESOURCES

Independent Re deposit is rare, although rhenite has been found in northwest Pacific islands (Korzhinsky et al., 1994). Most of the Re resources are hosted in molybdenite of porphyry coppermolybdenum deposits (Cooke et al., 2005) and sulfides in the reducing sedimentary rocks (Jiang et al., 2008). At present, nearly half of the rhenium reserves in the world were contained by the porphyry Cu-Mo deposits in Chile (Schulz et al., 2017). As discussed in Section 6, this may be plausibly explained by the involvement of OAE sediments.

Rhenium has very low abundance in the silicate Earth (Esser and Turekian, 1993; McDonough and Sun, 1995; Peucker-Ehrenbrink and Jahn, 2001; Sun et al., 2003a, b ). It is sensitive to sulfur and oxygen fugacities. Similar to Mo, Re may be highly enriched though oxidation-reduction cycle, and get enriched in reducing sediments, e.g., black shales or other organic-rich sediments (Sun et al., 2016). OAE sediments formed in the Jurassic and Cretaceous (Jenkyns, 2010) provide most of the Re to the circumPacific subduction zones, such that most of the world's Re reserves are hosted along the east Pacific continental margin (Fig. 5).

 Fig.5 Cartoon illustrating recycling of the Re or Moenriched sediments during plate subduction (modified from Sun et al., 2016) As mentioned above, the northward subduction of black shales in the South China block was responsible to the world largest porphyry Mo belt, the Qinling-Dabie Mo belt (Sun et al., 2016). However, most of these porphyry Mo deposits contain very low Re, with the exception of the Triassic Huanglongpu carbonatite Mo deposit (Stein et al., 1997), suggesting Re and Mo fractionates from each other during plate subduction (Sun et al., 2016), which is yet to be studied.
8 CONCLUDING REMARKS

Plate subduction is the "factory" for mineral resources. Water, carbon, oxygen fugacity, mobility of elements, and behaviours of geological solvents during plate subduction are the keys for mineralization associated with convergence. In general, mobility of elements is controlled by stabilities of host minerals and activities of geological solvents. Water controls the formation of all hydrothermal deposits, ranging from porphyry, skarn, to epithermal deposits, etc. Oxygen fugacity has major influences on the formation of porphyry Cu deposits, Sn deposits and a variety of other relevant deposits. In addition, plate subduction also forms orogenic belts, and consequently changes the erosions and climate patterns, which in turn influences hypergene processes and related metallogenesis, forming brine deposits and redox-controlled deposits.

9 DATA AVAILABILITY STATEMENT

All data generated and/or analyzed during this study are available from the corresponding author upon reasonable request.

References