January 2021 LIP of the Month

Carlin-style gold province linked to the Emeishan large igneous province

Jiang Zhu, Zhaochong Zhang, Ziliang Jin

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, 100083, China

Lead author email: zczhang@cugb.edu.cn

Figures, text and tables compiled from:

Zhu, J., Zhang, Z.C., Santosh, M., Jin, Z.L., 2020. Carlin-style gold province linked to the extinct Emeishan plume. Earth and Planetary Science Letters, 530, 115940. https://doi.org/10.1016/j.epsl.2019.115940.

Introduction

The Carlin-style gold deposits were defined as a new type after their discovery near Carlin in Nevada, USA during 1960s (e.g., Cline et al., 2005; Muntean and Cline, 2018; and references therein). Current Au endowment of Carlin-style deposits in Nevada is estimated up to 7,930 tons, and the deposits account for ~6% of the annual global production of Au, thus constituting the second largest auriferous province in the world (Muntean et al., 2011; Large et al., 2016; Muntean and Cline, 2018). The two popular genetic models for the Nevada-type deposits are: Au derived from metal-enriched sedimentary formations through meteoric or metamorphic water (Hofstra and Cline, 2000; Emsbo et al., 2003; Large et al., 2011) and origin through magmatic-hydrothermal activity in metasomatized subcontinental lithospheric mantle (SCLM) associated with oceanic subduction (Muntean et al., 2011; Large et al., 2016). Although some studies proposed a correlation between the Yellowstone mantle plume and Nevada’s Carlin-style Au deposits (Oppliger et al., 1997), this concept has not been widely accepted due to limited evidence. In general, mantle plumes as sources of heat and metals, have the potential to drive and contribute to a variety of giant magmatic and hydrothermal ore systems, such as magmatic Fe-Ti-V oxide, Ni-Cu-PGE (platinum group element) sulfide and Cr deposits hosted in mafic-ultramafic layered intrusions and hydrothermal IOCG (iron oxide-Cu-Au) as well as volcanogenic massive sulfide deposits in large igneous provinces (LIPs) (e.g., Ernst and Jowitt, 2013, 2017; Ernst, 2014).

Following the definition of the Carlin-style deposits of Nevada, similar ore deposits have been discovered in other parts of the world, such as in the Youjiang basin of China, Agdarreh and Zarshouran of Iran, Yukon of Canada and Sarawak of Malaysia (e.g., Muntean and Cline, 2018 and references therein). The Youjiang basin is the second largest province of Carlin-style Au deposits in the world after Nevada (e.g., Muntean and Cline, 2018), and is located within the Emeishan LIP of Yangtze craton in Southwest China. Previous geochronological data on hydrothermal minerals indicate that the Au deposits in this basin were formed at ~235-200 Ma and ~150-130 Ma (e.g., Hu et al., 2017), significantly later than the ~260 Ma Emeishan LIP. The magmatic underplate model suggests that the metal-bearing fluids driven off between ca. 30 and 100 Ma after the Emeishan LIP emplacement contributed to the formation Pb-Zn and native Cu ore deposit (Xu et al., 2014). However, it is difficult to generate extensive magmatic-hydrothermal fluids and related mineralization through the differentiation of basaltic magmas within a relatively short duration of several million years. According to traditional metallogenic concept there are no obvious direct or indirect genetic links between the Emeishan mantle plume and the Au deposits.

Overview of the Emeishan LIP

The ~260 Ma Emeishan LIP is located in the southwestern part of the Yangtze craton (Fig. 1), along the eastern margin of the Tibetan Plateau and northern Vietnam, and covers an area of over ~5×10⁵ km² (e.g., Shellnutt et al., 2014). The thickness of volcanic piles ranges from several hundred meters in the northern and eastern parts of this province to as much as ~5 km in the western-most outcrops (e.g., Zhu et al., 2018). The geophysical data suggest a ~15-20 km thick ultramafic-mafic layer in the western part and ~5-10 km thickness in central-eastern part at the base of the crust or above Moho (~50-40 km in depth) (Fig. 2) (Xu et al., 2015; Chen et al., 2015; Liu et al., 2017b). The major deformation and uplift in this region are associated with the Cenozoic India-Eurasian collision and Himalayan orogeny which resulted in strong erosion in the uplifted central zone, where many large-sized magmatic Fe-Ti-V oxide and middle-small sized Cu-Ni sulfide deposits are hosted in layered mafic-ultramafic intrusions in the Panzhihua-Xichang (Panxi) and Dali areas in the central Emeishan province (Fig. 1) (e.g., Shellnutt et al., 2014). Furthermore, a suite of Mesozoic-Cenozoic gold-rich porphyry deposits were discovered along the Jinshajiang-Ailaoshan suture within western margin and Jinping block of the Emeishan province (Fig. 1), attributed to an oceanic subduction of Mesozoic Paleo-Tethys or an east-west extension followed by the Cenozoic India-Asia collision (e.g., Hou et al., 2017). The Youjiang basin is located along the eastern part of the Emeishan LIP, and is also known as the Guizhou-Yunnan-Guangxi “Golden Triangle”, with more than 200 gold deposits including the world-class Shuiyingdong deposit (~265 tons Au), and a total proven reserve of >800 tons Au (Fig. 1) (Hu et al., 2017). Previous studies proposed a magmatic origin for the formation of the deposits in the basin based on isotopic data from hydrothermal minerals. These include the δ³⁴S range of -2.6‰ to +5.3‰ on ore-stage pyrite at the Shuiyingdong Au deposit (e.g., Tan et al, 2015; Hou et al., 2016). The calcites of the ore-forming stage from several Au deposits including Shuiyingdong have δ¹³C values ranging from -3‰ to -9‰ (e.g., Tan et al., 2015). These isotope compositions are very close to those of mantle sulfur (0 ± 3‰) and carbon (-3‰ to -8‰), indicating a deep magmatic source (Hu et al., 2017). Furthermore, the Au-bearing rims of zoned pyrite grains in the Lannigou deposit are characterized by highly variable δ³⁴S values of 1.1‰ to 18.1‰, inconsistent with the δ³⁴S range of 10‰ to 14‰ from calcareous host rocks, suggesting that the Au-bearing fluids were of magmatic origin with mantle-like δ³⁴S value (~0‰) (Yan et al., 2018).

Gold enrichment and remobilization in juvenile lower crust

Our results combined with the multidisciplinary geophysical investigations indicate that cumulate rocks of > 5 km thickness formed by high degree of fractional crystallization (~80-95 wt.%, Fig. 3) of plume-derived magmas were emplaced at ~50-40 km in depths beneath the Youjiang basin. The bulk-rock geochemical signature of basalts from the eastern Emeishan province displays Fe-rich and Si-poor evolution (Fig. 4), indicating a Fenner trend in the deep magma chamber. Petrographic observations show that the phenocrysts in the basalts from the eastern Emeishan province are dominantly plagioclase and that all the Fe-Ti oxides are hosted in the matrix (Hou et al., 2011, Liao et al., 2012). These geochemical and petrographic features suggest that the early stage of magma evolution occurred at a low oxygen fugacity and H₂O-poor environment beneath the eastern Emeishan LIP, which delayed the onset of magnetite crystallization and promoted Fe-enrichment in magmas at the late stage. These features demonstrate that early stage of magma evolution occurred at low oxygen fugacity and sulfide saturation, which is favorable for the enrichment of chalcophile elements within the cumulate rocks beneath the eastern Emeishan province. At the Jinbaoshan Pt-Pd sulfide deposits in the central part of the Emeishan province, several primary grains of native gold and platinum-group minerals were reported in the sulfide-bearing mineralized wehrlites (Tao et al., 2007), providing direct evidence for Au enrichment in the deep cumulate rocks. More importantly, pure native Au blebs (30 μm) were found enclosed within fresh olivine phenocrysts from picritic lavas, which also carry native copper grains and moissanite (SiC) (Zhang et al., 2006). All this evidence supports the crystallization of native Au from the Emeishan plume-derived magmas at low oxygen fugacity during the early stage within the cumulate rocks. Therefore, we speculate that the massive cumulate rocks at the base of the crust beneath the eastern Emeishan province, as revealed by geophysical data, might be enriched in Au, forming a potential Au reservoir.

The high B/Sr ratios of fluid inclusions from the world-class Shuiyingdong ore field in the Youjiang basin (Su et al., 2009) are consistent with those of typical porphyry deposits (Large et al., 2016). Geophysical investigations reveal that this ore field lies above a felsic pluton ~5 km below the surface (Xie et al., 2018). Fluid inclusions from the Shuiyindong and Yata Au deposits contain significant amount of CO₂ and are estimated to have formed at depths of ca. 4-6 km (Su et al., 2009). This is consistent with the emplacement depth of the felsic pluton as revealed from geophysical investigations. Moreover, sulfur and carbon isotopic data from ore-stage pyrite and calcite also suggest a deep magmatic or mantle-like origin for Au deposits in the Youjiang basin (Hu et al., 2017; Yan et al., 2018). Taken together, we conclude that the ore-forming fluids are characterized by mantle-like origin and significantly oxidized in the Youjiang basin, and were evolved from magmatic (mantle-like) magmas, similar to that of porphyry deposits. It has been proposed that flat subduction of the Paleo-Pacific slab occurred beneath the South China Block over a region of 1000 km during the Middle Triassic to the Middle Jurassic (~235-155 Ma) (Li and Li, 2007). Another flat subduction of the Cretaceous Neo-Tethyan slab reached up to the Ailaoshan suture (Liu et al., 2017a), both suggesting that the Youjiang basin was located in a back-arc setting. Recent studies on the Cretaceous igneous rocks of ca. 140-115 Ma within the Ailaoshan suture, close to the Youjiang basin, indicate that they formed in a back-arc extension setting associated with the Neo-Tethyan oceanic subduction (Liu et al., 2017a), which is coupled with the later ore-forming stage (~150-130 Ma) in the Youjiang basin.

In conjunction with the geological framework in the Youjiang basin, we propose the following model for the formation and remobilization of Au-rich crustal magmatic underplate. (i) The primary Emeishan plume-derived magmas (~260 Ma), underwent high degrees of fractional crystallization, and generated massive Au-rich cumulate rocks at the base of the crust beneath the basin. (ii) The favorable ore-controlling systems, reverse fault on the flank of anticline (dome) or anticline (dome), were formed during the Indochina-China continental collision event (~247-237 Ma). (iii) The back-arc extensions and oxidized fluids associated with multiple oceanic subduction events (~ 235-155 Ma and ~140-115 Ma) triggered the melting of enriched juvenile lower crust, and contributed to the formation of Au-rich felsic magmas by oxidized fluids into the upper crust. (iv) The Au-rich magmatic fluids were released from felsic plutons, transported into shallow crust, mixed with meteoric water and ultimately precipitated within the favorable ore-controlling systems generated by continental collision event (Fig. 5).

Summary

Overall, our study raises the possibility of a potential Au reservoir in the crustal magmatic underplate that provided the ore-forming materials for the formation of large-scale Carlin-style Au province and other types of Au deposits in the Youjiang basin in the Emeishan and other LIPs. Our work modeled on the basis of the Emeishan LIP has important implications in formulating new concepts on metallogeny associated with mantle plume, and also offers new insights into the formation of large auriferous province within stable cratonic interior.

References