Recycling of metal-fertilized lower continental crust: Origin of non-arc Au-rich porphyry deposits at cratonic edges

Recent studies argue that subduction-modified, Cu-fertilized lithosphere controls the formation of porphyry Cu deposits in orogenic belts. However, it is unclear if and how this fertilization process operates at cratonic edges, where numerous large non-arc Au-rich deposits form. Here we report data from lower crustal amphibolite and garnet amphibolite xenoliths hosted by Cenozoic stocks that are genetically related to the Beiya Au-rich porphyry deposits along the western margin of the Yangtze craton, China. These xenoliths are thought to represent cumulates or residuals of Neoproterozoic arc magmas ponding at the base of arc at the edge of the craton that subsequently underwent high-pressure metamorphism ca. 738 Ma. The amphibolite xenoliths are enriched in Cu (383–445 ppm) and Au (7–12 ppb), and a few garnet amphibolite xenoliths contain higher Au (6–16 ppb) with higher Au/Cu ratios (2 × 10−4 to 8 × 10−4) than normal continental crust. These data suggest that metal fertilization of the base of an old arc at the edge of the craton occurred in the Neoproterozoic via subduction modification, and has since been preserved. The whole-rock geochemical and zircon Hf isotopic data indicate that melting of the Neoproterozoic Cu-Au–fertilized low-crustal cumulates at 40–30 Ma provided the metal endowment for the Au-rich porphyry system at the cratonic edge. We therefore suggest that the reactivated cratonic edges, triggered by upwelling of asthenosphere, have the potential to host significant Au ore-forming systems, especially non-arc Au-rich porphyry deposits. INTRODUCTION Unlike orogenic belts, most cratons have been stable since their formation in the Archean–Proterozoic Eons (Griffin et al., 2013). The initial cratonic crust and subcontinental lithospheric mantle (SCLM) are strongly depleted in magmaphile elements, especially Au and Cu (Rudnick and Gao, 2003), largely due to the liberation of metamorphic fluids from the deep crust (Cameron, 1989) and a high degree of mantle partial melting (Griffin et al., 2013), and have been preserved as a durable, rigid, and buoyant raft. Therefore, the cratons that have not undergone metal fertilization and later activation would be unlikely to host Phanerozoic Cu-Au ore deposits (Groves and Bierlein, 2007). However, recent studies have recognized a suite of Mesozoic–Cenozoic large Au-rich deposits, varying from porphyry Cu-Au (Richards, 2009; Griffin et al., 2013; Hou et al., 2015) to orogenic Au deposits (Goldfarb et al., 2007; X. Sun et al., 2009) that have formed at the margins or in the cratonic interior. They are postulated to be genetically related to non-arc potassic magmas derived from the Proterozoic lithosphere (Lu et al., 2013), metasomatized lower crust (Richards, 2009), SCLM (Griffin et al., 2013), and/or crustal fluids from deep reservoirs (Goldfarb et al., 2007), released during later reactivation. All these non-arc Au-rich deposits are in the category of postcollisional deposits. The occurrence of these Au-rich deposits implies the existence of metal fertilization in cratons during later tectonic episodes (Griffin et al., 2013). However, it is unclear when and how this fertilization process operated, and what factors ultimately control the formation of Au-rich porphyry copper deposits (PCDs) (Richards, 2009). Here we report the occurrence of Cu-Au–rich lower crustal amphibolite and garnet amphibolite xenoliths, hosted by Cenozoic potassic stocks that are genetically related to the Beiya Au-rich PCD in the western Yangtze craton, China. We suggest that these xenoliths represent direct samples of the Neoproterozoic fertilized cratonic lower continental crust, which played an important role in the genesis of Au-rich PCDs at cratonic edges. GEOLOGICAL BACKGROUND The Yangtze craton, southwest China, underwent Neoproterozoic lithospheric accretion and Cenozoic tectonic reactivation with Au-Cu mineralization at its margin (Fig. 1). It is therefore an ideal place to study Au-rich systems at cratonic edges. The remnants of 1000–740 Ma voluminous arc plutons and volcanic rocks in the western part of the craton (Fig. 1B) suggest that the oceanic subduction beneath the craton occurred in the Neoproterozoic (W. Sun et al., 2009). An ~1000-km-long potassic magmatic belt of Eocene–Oligocene intrusive and associated volcanic rocks (40–30 Ma) along the cratonic edge (Fig. 1B) record significant reworking by Indo-Asia collision that started ca. 65 Ma (Lu et al., 2013). The Beiya Au-rich PCD (304 t Au, 2.4 g/t Au; 0.6 Mt Cu, 0.48% Cu) is the largest among several PCDs associated with the Eocene–Oligocene collision-related intrusive stocks (ca. 37 Ma; He et al., 2015). The Beiya porphyries are thought to have formed by remelting of thickened mafic lower crust, whereas the Liuhe syenite stocks formed by remelting of the metasomatized SCLM during collision (Lu et al., 2013). Geophysical data reveal that asthenosphere upwelling appears along the cratonic edge with 42–45-km-thick crust, and upwelling is thought to trigger melting of the cratonic lithosphere during collision (Lu et al., 2013). LOWER CRUSTAL XENOLITHS AND THEIR ORIGIN Abundant xenoliths have been found with Eocene stocks and associated volcanic rocks at six locations exposed along the cratonic edge (Fig. 1B). Amphibolites and garnet amphibolites are the primary types, the former hosted by Liuhe syenites and Beiya monzogranite porphyries (Fig. 2A), and the latter widely occurring in the Liuhe stock (Fig. 2B). Their mineralogical and whole-rock compositions, and zircon, oxide, and sulfide geochemical *E-mails: [email protected]; [email protected] GEOLOGY, June 2017; v. 45; no. 6; p. 1–4 | Data Repository item 2017181 | doi:10.1130/G38619.1 | Published online XX Month 2017 © 2017 eological Society of A erica. For permission to copy, contact [email protected]. 2 www.gsapubs.org | Volume 45 | Number 6 | GEOLOGY data are listed in Tables DR1–DR11 in the GSA Data Repository1 and presented in Figure 2 and Figures DR1–DR6 (the Data Repository). Sulfide phases (<0.3 vol%) in the garnet amphibolites are dominated by pyrrhotite with chalcopyrite rims and pyrites. The pyrrhotite occurs as globules enveloped by garnet grains that lack any fissures and hydrothermal alteration (Fig. 2C). Their globular shape, sharp boundaries, and the coexistence of pyrrhotite with chalcopyrite suggest original entrapment as a magmatic sulfide melt (Nadeau et al., 2010). Magnetite occurs inside and outside of amphibole crystals as globules, suggesting dissolution of the sulfide melt by a volatile phase, which was likely oxidized (Fig. DR1g; Nadeau et al., 2010). Pyrite occurs as an interstitial phase within biotite-orthoclase assemblages in the amphibolites and garnet amphibolites, and is irregular in shape, suggesting a secondary or metamorphic origin. Microprobe analyses of some pyrites show Au enrichment of 170–580 ppm (just above the detection limit of 140 ppm; Table DR2). The lack of quenched margins (Figs. 2A and 2B) and the occurrence of metamorphic mineral assemblages (Fig. 2D; Figs. DR1a–DR1d) in all of these xenoliths indicate that they are unlikely to be autoliths formed during magma crystallization or enclaves formed by injection of mafic melts into the felsic magma chamber. The garnet amphibolite xenoliths show typical retrograde textures, including (1) symplectite composed of fine-grained diopside, pargasite, and magnetite (Fig. DR1e) formed by the decompressional breakdown of garnet (Zhao et al., 2003) and (2) finegrained assemblages of anhedral albite, pargasite, and magnetite around coarse diopside grains (Fig. DR1f), likely formed by the breakdown of the Ca-Tschermaks components in pyroxene (Core et al., 2006). These 1 GSA Data Repository item 2017181, petrography of xenoliths, Figures DR1– DR6, and Tables DR1–DR11, is available online at http://www.geosociety .org /datarepository /2017/ or on request from [email protected]. observations indicate that the xenoliths underwent high-pressure metamorphism and later retrograde metamorphism during exhumation (Figs. DR1g and DR1h). The clinopyroxene geothermobarometer (Ravna, 2000) and garnet-clinopyroxene Fe-Mg geothermometer (Mercier, 1980) yield temperature-pressure estimates from 642 °C to 675 °C and 1354 MPa to 1560 MPa for the garnet amphibolites (Table DR3); this suggests highpressure eclogitic facies metamorphism at the crustal base (~41–52 km, based on amphibolite density of 3.0 g/cm3). By contrast, the amphibolite xenoliths show massive and gneissic structures and formation at 832 MPa (Al-in-amphibole barometer; Table DR4), corresponding to a metamorphic depth of ~27 km. We therefore argue that the garnet amphibolite and amphibolite xenoliths were derived from the bottom (~45 km) and upper part (~27 km) of the lower continental crust, respectively. Zircon age populations provide further evidence for the origin of these xenoliths. Two groups of zircons with distinct ages have been recognized in these xenoliths (Fig. DR2; Table DR5). Older inherited zircons show a prominent age cluster ca. 773 Ma in the Beiya amphibolites (Fig. DR2a) and ca. 794 Ma in the Liuhe garnet amphibolites (Fig. DR2b) that approximate the peak of Neoproterozoic arc magmatism in the western Yangtze craton (ca. 813 Ma; W. Sun et al., 2009). These U-Pb ages also coincide with the crustal growth period (1000–740 Ma) of the craton (W. Sun et al., 2009). Younger zircons from the Liuhe xenoliths occur as overgrowth rims around older zircons, or as interstitial grains, both yielding similar U-Pb age clusters at 35.6 Ma (Fig. DR2c). This age group is slightly older than the host syenite (34.6 ± 0.4 Ma; Fig. DR2c), suggesting the growth of younger zircons during igneous remobilization of the xenoliths. Most Neoproterozoic and Cenozoic zircon samples are typical of igneous zircons (Fig. DR3). By contrast, one Neoproterozoic zircon grain (ca. 738 Ma) lacks an Eu anomaly, and has a flat heavy rare earth element (HREE) pattern, suggesting its growth in the presence of garnet (Fig. DR3). This metamorphic zircon suggests that the eclogitic facies metamorphism occurred ca. 738 Ma. Whole-rock analyses indicate that these xenoliths show a close geochemical affinity with the Neoproterozoic arc plutons (Fig. DR4; Table DR6). They have relatively high Y (>15 ppm) and low Sr/Y (< 30) (Fig. DR4b) and show enrichment in large ion lithophile elements and depletion in high field strength elements (Fig. DR4c), which are characteristics typical of arc magmas derived from the metasomatic mantle wedge. The Figure 1. A: Significant orogenic Au deposits (black dots) in the North China craton (NCC, 130–120 Ma; Goldfarb et al., 2007) and Yangtze craton (YC, 38–32 Ma; X. Sun et al., 2009). CAO—Central Asia orogen; TM—Tarim block; CCO—Central China orogen; AHO—Alpine-Himalaya orogen; SGO—Songpan Ganzi orogen; CC—Cathaysia craton. B: Locations of the lower crustal xenoliths (six stars) are shown; xenoliths at Beiya and Liuhe were sampled for study. The tectonic framework and distribution of Cenozoic porphyry Cu-Au and orogenic Au deposits in the western Yangtze craton are shown. Figure 2. A, B: The xenoliths and their mineral assemblages. C: Electron probe–scanned element distribution of the globular sulfide phases in garnet. Cp—chalcopyrite; Gt—garnet; Po—pyrrhotite. D: Garnet amphibolite (sample LH14–53) consisting of two stages of amphibole (Am), garnet, and minor plagioclase (Pl), pargasite, and biotite. Ap— apatite; Rt—rutile; Di—diopside.