Abstract

The genesis of continental collision-related porphyry Cu deposits (PCDs) remains controversial. The most common hypothesis links their genesis with magmas derived from subduction-modified arc lithosphere. However, it is unclear whether a genetic linkage exists between collision- and subduction-related PCDs. Here, we studied Jurassic subduction-related Cu-Au and Miocene collision-related Cu-Mo porphyry deposits in south Tibet. The Jurassic PCDs occur only in the western segment of the Jurassic arc, which has depleted mantle-like isotopic compositions [e.g., (87Sr/86Sr)i = 0.7041–0.7048; εNd(t) as high as 7.5, and εHf(t) as high as 18]. By contrast, no Jurassic PCDs have been found in the eastern arc segment, which is isotopically less juvenile [e.g., (87Sr/86Sr)i = 0.7041–0.7063, εNd(t) < 4.5, and εHf(t) ≤ 12]. These results imply that incorporation of crustal components during underplating of Jurassic magma induced copper accumulation as sulfides at the base of the eastern Jurassic arc, inhibiting PCD formation at this time. Miocene PCDs are spatially confined to the Jurassic arc, and the giant Miocene PCDs cluster in its eastern segment where no Jurassic PCDs occur. This suggests that the arc segment barren for subduction-related PCDs could be fertile for collision-related PCDs. Miocene ore-forming porphyries have young Hf model ages and Sr-Nd-Hf isotopic compositions overlapping with those of the Jurassic rocks in the eastern segment, whereas contemporaneous barren porphyries outside the Jurassic arc have abundant zircon inheritance and crust-like Sr-Nd-Hf isotopic compositions. These data suggest that remelting of the lower crustal sulfide-bearing Cu-rich Jurassic cumulates, triggered by Cenozoic crustal thickening and/or subsequent slab break-off, led to the formation of giant Miocene PCDs. The spatial overlap and complementary metal endowment between subduction- and collision-related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts.

INTRODUCTION

Most porphyry Cu deposits (PCDs) form in magmatic arcs worldwide and are associated with hydrous (>4 wt% H2O) calc-alkaline magmas, derived from an asthenospheric mantle wedge metasomatized by slab fluids (Richards, 2003). Subduction processes are thought to ultimately cause enrichment of metals (Cu, Au) and S (Griffin et al., 2013) and the relatively high oxygen fugacity (forumla) and high H2O contents in arc magmas (Kelley and Cottrell, 2009) that are critical to the formation of PCDs. Recently, such PCDs have been found in collisional zones such as that in Tibet, where they are associated with post-collisional magmas emplaced as isolated complexes within the Jurassic–Cretaceous magmatic arcs (Hou et al., 2004). These magmas are thought to derive by remelting of a thickened juvenile mafic lower crust (Hou et al., 2004) resulting from previous arc magmatism (Richards, 2009; Hou et al., 2009; Li et al., 2011) with subduction-associated PCDs (Tafti et al., 2009). This model implies a spatial relationship between subduction- and collision-related PCDs, but it is unclear whether a genetic linkage exists.

Here, we report new geological, geochemical, and isotopic data for Jurassic subduction- and Miocene collision-related PCDs in southern Tibet. Our new evidence suggests that remelting of sulfide-bearing, lower crustal cumulates of former arc magmas associated with subduction-related PCDs could produce giant collision-related PCDs.

SUBDUCTION- AND COLLISION-RELATED PCDS IN SOUTH TIBET

The India-Asia continental collision zone was built on a complex tectonic collage of terranes accreted onto the southern margin of the Asian continent since the Mesozoic (Fig. 1A; Yin and Harrison, 2000). The Lhasa terrane, bounded by the Meso-Tethyan Bangong-Nujiang suture and the Neo-Tethyan Indus-Yarlung Zangbo suture (Fig. 1A), is composed of Precambrian crystalline basement and Paleozoic–Mesozoic shallow marine clastic strata. Northward subduction of the Neo-Tethyan Ocean beneath the Lhasa terrane formed the Jurassic–Cretaceous volcano-plutonic arcs (Fig. 1B; Chu et al., 2006). The India-Asia collision during the Cenozoic produced a 1500-km-long trans-Himalayan Miocene igneous belt along the southern margin of the Lhasa terrane (Miller et al., 1999), and led to crustal thickening (as much as 80 km) in south Lhasa (Chung et al., 2009).

Porphyry-type mineralization in the Lhasa terrane is linked to two distinct magmatic suites. These are the Jurassic arc suites associated with Cu-Au deposits (e.g., Xiongcun, Laze, and Zemoduola; Tafti et al., 2009), and the Miocene post-collisional suites associated with Cu-Mo deposits (e.g., Qulong, Jiama, Tinggong, and Chongjiang; Hou et al., 2009). Numerous 184–158 Ma granitoids and associated Jurassic volcanic rocks form a 600-km-long Jurassic arc (JA) (Geng et al., 2005), onto which a >1000-km-long Cretaceous arc was subsequently superimposed (Fig. 1). This Jurassic arc is divided by a north-south–oriented rift into a western segment (WSJA) and an eastern segment (ESJA), which are dominated by diorite intrusions with andesites, and granitic intrusions with basalt and dacite, respectively. The giant Xiongcun PCD (219.8 Mt Cu resources averaging 0.43% Cu and 0.61 g/t Au; Tafti et al., 2009) with Re-Os molybedenite age of 173 ± 5 Ma (Tang et al., 2010) is associated with a Jurassic quartz diorite porphyry in the WSJA (Fig. 1B). No Jurassic PCDs have been found in the ESJA (Fig. 1B).

The Miocene post-collisional magmas occur as isolated stocks of porphyritic monzogranite and minor granodiorite and granite. The mineralized porphyries developed in the Jurassic arc, whereas contemporaneous barren ones were emplaced outside the arc (Fig. 1B). All porphyries have zircon U-Pb ages of 26–13 Ma with a peak at 16 Ma (Hou et al., 2013a), and the associated PCDs have molybdenite Re-Os ages of 17–14 Ma (Hou et al., 2009), postdating subduction-related Jurassic PCDs by at least 150 m.y.

The Miocene PCDs show a close spatial relationship with the Jurassic arc (Fig. 1B), despite being separated by a time interval of ∼170–130 m.y. For example, the giant Qulong deposit (1420 Mt at 0.5% Cu and 0.03% Mo; Yang et al., 2009) is partly hosted by Jurassic dacitic lavas. The giant Jiama deposit is only 2 km from the nearest outcrop of Jurassic volcanic successions (Fig. 1B). Other PCDs are ∼20–50 km away from Jurassic magmatic rocks (Fig. 1B). Importantly, the coeval Miocene porphyry intrusions emplaced outside of the Jurassic arc are all barren (Fig. 1B).

GEOCHEMICAL AND ISOTOPIC CHARACTERISTICS

Thirty-two key samples from the PCDs and relevant magmatic rocks in south Tibet were analyzed for major and trace element chemistry and bulk-rock Sr-Nd and zircon Hf isotopic compositions (Tables DR1–DR3 in the GSA Data Repository1). These data, combined with previously published results, are presented in Figure 2 and in Figures DR1–DR3 in the Data Repository.

The Jurassic arc rocks are mainly calc-alkaline, and are characterized by enrichment in large-ion lithophile elements (LILE; e.g., Rb, Ba, Sr) and depletion in high field strength elements (HFSE; e.g., Nb, Ta, P, Ti) with flat heavy rare earth element (HREE) patterns (Fig. DR1), thus showing typical subduction-related geochemical features (Hawkesworth et al., 1993). However, the Sr-Nd-Hf isotopic compositions of rocks from the western and eastern segments are distinct. Arc rocks from the WSJA have (87Sr/86Sr)i varying from 0.7041 to 0.7048, εNd(t) from +5.5 to +7.5, and zircon εHf(t) between +11 and +18 (Fig. 2). Those from the ESJA have higher (87Sr/86Sr)i, from 0.7041 to 0.7063, and lower εNd(t) (+1.5 to +4.5; Fig. 2) with variable zircon εHf(t) of +1 to +12 (Wei, 2014).

The Miocene Cu-Mo–related porphyries are mainly high-K calc-alkaline to shoshonitic (Hou et al., 2004) with enrichment in LILE (e.g., Rb, Ba, Sr) and depletion in HFSE (e.g., Nb, Ta, P, Ti), similar to the Jurassic rocks (Fig. DR1). These porphyries have large isotopic variations with (87Sr/86Sr)t = 15Ma from 0.7048 to 0.7062, εNd(t = 15Ma) from −4.8 to +2.2, and zircon εHf(t) from +2 to +12 (Fig. 2). In contrast, the barren Miocene porphyries have higher (87Sr/86Sr)t = 15Ma (0.7063–0.7101) and lower εNd(t = 15Ma) (−8.1 to −3.1) and zircon εHf(t) (−8 to +5; Fig. 2). In addition, these barren porphyries yield relatively old crustal Hf model ages (average ca. 1100 Ma) and have abundant inherited zircons ranging in age from 30 Ma to 2680 Ma (Table DR3; Fig. DR2).

Compared with the Jurassic rocks, the Miocene Cu-Mo–related porphyries have higher light REE and LILE contents, and lower HREE and HFSE contents with steeper HREE patterns (Fig. DR1). They have Sr-Nd isotopic compositions partly overlapping with those of the Jurassic rocks and plot along a two-component mixing array between Jurassic basalt and old lower crust in Tibet (Fig. 2A). Their zircon Hf isotopic compositions plot within the inferred evolution trend of the Jurassic rocks from the ESJA on plot of εHf(t) versus age (Fig. 2B).

REMELTING OF JURASSIC MAFIC UNDERPLATE

Geochemical and isotope data suggest that the WSJA arc rocks associated with the subduction-related PCDs derive from subduction-modified asthenospheric mantle (Fig. 2; Fig. DR1; Chu et al., 2006). However, the ESJA barren rocks show Sr-Nd isotopic departure from the depleted mantle toward the crust (Fig. 2), indicating the involvement of old crustal components (<15 vol%), consistent with old Hf model ages for these rocks (ca. 700–1500 Ma; Table DR3A).

The Miocene barren porphyries outside the Jurassic arc have crust-like Sr-Nd-Hf isotopic signatures and abundant zircon inheritance, consistent with a source dominated by old continental crust (Fig. 2). By contrast, the Miocene Cu-Mo–related porphyries are free of inherited zircons, and have relatively “depleted” isotopic compositions and younger Hf model ages (average 800 Ma), suggesting a juvenile source (Hou et al., 2004, 2013a).

Importantly, the Miocene Cu-Mo–related porphyries are spatially confined to the Jurassic arc (Fig. 1B), and have overlapping Sr-Nd isotopic compositions with those of the Jurassic rocks from the ESJA (Fig. 2A). The zircon Hf data of the mineralized Miocene porphyries also lie within the evolution trend of the Jurassic rocks from the ESJA (Fig. 2B). These observations are consistent with derivation of these ore-related Miocene porphyries by remelting of the juvenile mafic rocks underplated during the Jurassic magmatism. The trend to lower εNd(t) shown by some ore-related porphyries could reflect mixing with lesser amounts (5%–30%) of old lower crust (Fig. 2A). The remelting scenario is also supported by the subduction-like features of the Miocene ore-related porphyries (such as Nb-Ta negative anomaly), which were inherited from the Jurassic arc magmas (Richards, 2009). In addition, the Miocene Cu-Mo–related porphyries have higher Sr/Y ratios (23–186) and higher (La/Yb)N ratios (11–47) than the Jurassic rocks, showing geochemical affinity with adakite (Hou et al., 2004). This suggests that remelting of the Jurassic cumulates took place in a thickened crust (>50–55 km) within the amphibole and garnet stability field (Rapp and Watson, 1995), consistent with Cenozoic collision-induced crustal thickening in south Tibet (Yin and Harrison, 2000). Pronounced crustal thickening at 45–30 Ma (Chung et al., 2009) and subsequent slab break-off (or tearing) starting at 25 Ma (Hou et al., 2004) triggered this remelting as isotherms rebounded and/or hot asthenospheric melts infiltrated the lithosphere (Richards, 2009).

CONTROLS ON METAL ENDOWMENT

The lack of Miocene PCDs outside of the Jurassic arc suggests that there are no sulfide-bearing Jurassic cumulates to remelt there. In contrast, localization of Miocene PCDs within the Jurassic arc implies that metal-rich Jurassic cumulates could release Cu into the Miocene porphyry systems (Lee et al.., 2012). This is supported by the east-west spatial variation in the average εHf(t) values of magmatic zircons from the Jurassic and Miocene magmatic rocks (Fig. DR3). The average zircon εHf(t) values of the Jurassic rocks decrease significantly from the WSJA (average +16) to the ESJA (average +6), suggesting distinct intra-crustal magmatic processes along the arc. High forumla of the arc magma in the WSJA, evidenced by appearance of magmatic anhydrite in the Xiongcun host porphyry (Tang et al., 2010), suppressed the formation of significant amounts of Cu-rich sulfides (e.g., Richards, 2009), which led to Cu enrichment in the evolving magma, thus forming the Jurassic PCDs (Fig. 3). By contrast, incorporation of older crust by the ESJA basaltic rock, indicated by the lower and variable zircon εHf(t) (+1 to +12), may be responsible for lowering magmatic forumla (e.g., Ripley and Li, 2013), thus inducing sulfide saturation (Tomkins et al., 2012) and sequestering Cu-rich sulfides (Nadeau et al., 2010) in the Jurassic lower-crustal arc cumulates (Fig. 3). The complementary metal endowment of the Jurassic and Miocene magmatic suites (Fig. 1) suggests that enrichment of Cu in the juvenile mafic lower crust, though inhibiting the Jurassic PCD formation, could have provided abundant metals for the Miocene giant PCDs by breakdown of the host sulfides during post-collisional melting.

IMPLICATIONS FOR EXPLORATION

Our study highlights a spatial overlap and genetic linkage between giant collision-related PCDs and non-economic volcano-plutonic arcs. This is applicable to other orogenic belts, where former arc magmatism left sulfide-bearing metal-rich cumulates at the base of the crust (e.g., Lee et al., 2012), which provided abundant metals and S for post-subduction or collision-related PCDs during later remelting (e.g., Richards, 2009). Typical examples include: the Miocene post-collisional Kerman porphyry Cu belt, occurring within a non-economic Eocene magmatic arc in Iran (Shafiei et al., 2009); the Cretaceous giant PCDs, developed along the Mesozoic Qinling orogen in central China (Dong et al., 2011); and the Jurassic giant Dexing PCD, formed on a Mesozoic intra-continental orogen in east China (Hou et al., 2013b). Therefore, the spatial overlap and complementary metal endowment between subduction- and collision-related porphyry suites could be used for predicting PCD occurrence and assessing the mineralization potential of other tectonically composite orogens.

Our data also demonstrate that in a given magmatic suite the PCDs are associated with the isotopically “primitive” magmas with high bulk-rock εNd and zircon εHf values (Figs. 2 and 3). Hf isotopes might, therefore, be an important tool for assessing the metallogenic fertility of porphyry magmas (Fig. DR3). We suggest that systematic zircon εHf measurement could be effective in identifying the most prospective areas during regional targeting of PCDs.

This work was funded by National Basic Research Program of China (2011CB403104), IGCP/SIDA-600, National Science Foundation of China (41221061, 41320104004, and 41273051), and the Ministry of Land and Resources of China (201011011). We thank Chris Hawkesworth, Jeremy Richards, Rui Wang, and two anonymous reviewers for their constructive comments. This is contribution 383 from the CCFS and the Innovation Center of Continental Tectonics, Northwest University (China).

1GSA Data Repository item 2015088, Tables DR1–DR3 and Figures DR1–DR3, is available online at www.geosociety.org/pubs/ft2015.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.