Linking Mesozoic lode gold deposits to metal-fertilized lower continental crust in the North China Craton: Evidence from Pb isotope systematics

Journal Pre-proof Linking Mesozoic lode gold deposits to metal-fertilized lower continental crust in the North China Craton: Evidence from Pb isotope systematics

Le Xiong, Xinfu Zhao, Junhao Wei, Xiaoye Jin, Lebing Fu, Zuwei Lin PII:

S0009-2541(19)30569-8

DOI:

https://doi.org/10.1016/j.chemgeo.2019.119440

Reference:

CHEMGE 119440

To appear in:

Chemical Geology

Received date:

29 July 2019

Revised date:

23 November 2019

Accepted date:

4 December 2019

Please cite this article as: L. Xiong, X. Zhao, J. Wei, et al., Linking Mesozoic lode gold deposits to metal-fertilized lower continental crust in the North China Craton: Evidence from Pb isotope systematics, Chemical Geology (2019), https://doi.org/10.1016/ j.chemgeo.2019.119440

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© 2019 Published by Elsevier.

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Linking Mesozoic lode gold deposits to metal-fertilized lower continental crust in the North China Craton: Evidence from Pb isotope systematics

Le Xiong a, Xinfu Zhao a,*, Junhao Wei a, Xiaoye Jin a, Lebing Fu a, Zuwei Lin a

School of Earth Resources, China University of Geosciences, Wuhan 430074, China

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*Corresponding author: e-mail, [email protected]

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a

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Abstract

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The source(s) of lode gold deposits formed in Precambrian cratons related to

sequences

or

deep

crustal/sub-crustal

origin,

remains

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volcano-sedimentary

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accretion/collision and cratonic reactivation, formerly attributed to either supercrustal

controversial largely because gold deposits are spatially related to metamorphosed

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rocks, but geochemical data somewhat indicate a poorly understood deep sources. Reconciling such conflicts is important to better understand the main factor controlling the formation of ore deposits and their genetic link with specific tectonic settings. Giant Late Mesozoic lode gold provinces in North China Craton (NCC) were formed ca. 1.7 Ga later than cratonization metamorphism, and contemporaneous with intensively felsic to mafic magmatism related to cratonic reactivation. In this study, we conduct a comprehensive Pb isotope study on major gold deposits from the eastern Yanshan belt, northern margin of the NCC. In order to constrain the source(s) of gold, we attempt to map regional Pb isotope variations of lower continental crust (LCC), Page 1 / 72

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and develop a two stage quantitative model (punctuated by three prominent geological events at 2.80 Ga, 1.85 Ga and 0.16 Ga) to reproduce time-integrated Pb isotopic signatures of deep-seated lithospheric reservoirs to make a comparison with Pb isotopic signatures of the gold mineralization. Gold-bearing pyrites from different types of host rocks have relatively uniform Pb isotopic ratios, which are significantly

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different to high-grade metamorphosed host rocks, but similar to those of spatially associated Late Mesozoic granitic rocks. Our data show that Pb isotopic signatures of

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gold deposits vary consistently with presumed regional Pb isotopes of the LCC during

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the Late Mesozoic. Pb isotopic heterogeneity of the LCC was likely caused by

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underplating of mafic magmas derived from mantle sources. During underplating

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highly chalcophile elements (e.g. Au, Ag and Cu) were concentrated at the base of the

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LCC due to sulfide saturation from mafic magmas. Integrating petrological, geochemical, geochronological, and considering chalcophile element solubility, we

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propose a new genetic model to describe the formation of Late Mesozoic gold deposits in the NCC: (1) early formation of sulfide-bearing cumulates with high Au/Cu ratios during magma differentiation at the base of the LCC; (2) subsequent fluid-fluxed remelting of these cumulates at the onset of lithospheric extension and release and ascent of ore-forming fluids to the site of precipitation. Considering the metallogenic characteristics of other Au ore-forming systems, we suggest that metal fertilization in deep-seated reservoirs and subsequent tectonic decompression are important factors controlling the development of Au-rich ore deposits worldwide. This study demonstrates how Pb isotope can be employed to trace source(s) of gold Page 2 / 72

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deposits. Keywords: LA-ICP-MS micro-analysis; lode gold deposit; North China Craton; Pb isotope.

1. Introduction

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Lode gold deposits are one of the most economically important gold deposit types and account for nearly one third of historical and current resources (Frimmel, 2008).

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They have been widely recognized in both Phanerozoic orogenic belts and

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Precambrian cratonic blocks worldwide and usually formed in middle to shallow crust,

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at or above the brittle-ductile transition (Phillips and Powell, 2010; Goldfarb and

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Groves, 2015). Previous studies have shown that these deposits are temporally and

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spatially related to greenschist- to amphibolite-facies metamorphism and/or magmatism in accretionary and collisional orogens (Kerrich and Wyman, 1990;

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Groves et al., 1998). Thus, two plausible sources of gold, i.e., metamorphosed rocks and intrusive magmas, have been proposed and debated for decades mainly based on mineral assemblages, spatial and temporal relationship with granitoids, regional structural controls, tectonic settings, and geochemical characteristics (briefly reviewed by Tomkins, 2013). In recent years, deep-seated anomalously enriched source regions, i.e. a lower crustal or upper mantle source, have been considered to be critical for the formation of Au ore-forming systems (Richards, 2009). Formation of these deep-seated transient Au storage zones can be caused by magmatic differentiation processes operating in lower crust (Lee et al., 2012; Chiaradia, 2014; Hou et al., 2017; Page 3 / 72

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Jenner, 2017) or infiltration of plume-related melts in lithospheric mantle (McInnes et al., 1999; Webber et al., 2012; Tassara et al., 2017). Later melts/fluids can scavenge the Au and migrate towards the overlying uppermost crust through trans-lithospheric faults to form giant gold province (Tassara et al., 2017). The North China Craton (NCC) contains a large number of lode gold deposits that cluster in several districts around the eastern part of the NCC. Numerous isotopic 40

Ar/39Ar and U-Pb isotopes) have revealed that these

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dating studies (e.g. Re-Os,

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deposits were mainly formed in the Late Jurassic to Early Cretaceous, broadly

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contemporaneous with the peak of changes in lithospheric thickness, thermal state and

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chemical compositions (Li et al., 2012; Zhu et al., 2015), and nearly 1.7 billion years

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later than cratonization metamorphism (e.g. Zhai and Santosh, 2011). These gold

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deposits are commonly associated with intermediate to mafic dykes that are roughly contemporaneous with ore mineralization. A significant portion of orebodies are

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hosted in slightly older granitoid plutons that commonly contain abundant coeval mafic enclaves (Yang et al., 2003; Liu et al., 2013; Jiang et al., 2016; Gao and Zhao, 2017). The ore-fluids have low to moderate salinity and are CO2-rich, similar to those involved in orogenic gold deposits that have been considered to have a metamorphic origin. Carbonate alteration is not abundant or widespread (Goldfarb and Santosh, 2014). Noble gas and carbon isotope data have shown that a significant amount of mantle-derived components was involved in the ore formation (Mao et al., 2008; Zhang et al., 2008; Li et al., 2012; Tan et al., 2018). Based on these observations, different models have been proposed for the source(s) of ore-forming metals, Page 4 / 72

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including exsolution from coeval magmas, or extraction from Precambrian basement rocks by circulated hydrothermal fluids, or derivation from deep-seated reservoirs by ascending mantle or slab fluids (Yang and Zhou, 2001; Mao et al., 2008; Li et al., 2012; Goldfarb and Santosh, 2014). Lead isotope systematics are powerful tools can be used to fingerprint metal

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sources involved in the formation of ore deposits, because of comparable geochemical behavior of Pb, Zn and Cu in hydrothermal fluids and its close association with Au

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and Ag (Tosdal et al., 1999). The application of Pb isotopes also has the advantage

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that fluid-mineral interactions under natural physico-chemical conditions do not cause

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measurable Pb isotope fractionation (Potra and Macfarlane, 2014). Pb isotopic

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compositions can be easily measured on both ore minerals and silicate rocks, which

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can be useful for determining how strong the interaction of hydrothermal fluids with crustal basement is, and thus allows us to evaluate the contributions from crustal

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basement to ore deposits (Chiaradia et al., 2006). U, Th and Pb have different geochemical behaviors during melting, metamorphism and fluid-assisted processes, which may cause various U/Pb and Th/U ratios resulting in distinct time-integrated Pb isotopic compositions in different deep-seated reservoirs (Kramers and Tolstikhin, 1997; Pettke et al., 2010; Xu et al., 2017). Isotopic signatures of these deep-seated sources could be revealed somehow by investigating geochemical characteristics of certain rocks (e.g. crustal basement outcrops and igneous rocks) that are considered to have deep lithospheric origin, and emplaced into uppermost crust (Macfarlane, 1999), thus investigation into Pb isotopic compositions of these rocks may allow us to Page 5 / 72

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determine ore metal provenance, especially for ore deposits supposed to have a deep-seated origin. Application of Pb isotopic terrane maps is also an important method to understand large-scale magmatic and tectonic events that shaped regional metallogenic character (Tosdal et al., 1999; Potra and Macfarlane, 2014). Native gold, electrum, galena and chalcopyrite are common mineral assemblages

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enclosed in euhedral and subhedral pyrites formed in the main mineralization stage of gold deposits in the NCC (Jian et al., 2015; Kong et al., 2015). LA-ICP-MS trace

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element analyses indicated that lattice-bound gold concentrations of pyrites are very

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low (~0.06–2 ppm, Zhao et al., 2011; Yang et al., 2016; Lin et al., 2019). The close

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association of high-grade native gold ores with base metal sulfides cannot be formed

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by dissolution-reprecipitation processes triggered by addition of exotic base

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metal-rich fluids, which means that the source(s) of gold could be inferred from those of associated base metal sulfides. Pb isotope systematics are therefore potentially

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feasible approaches for gold source tracing. In this study, we present a comprehensive Pb isotope study on three representative gold deposits (the Niuxinshan, Jinchanggouliang-Erdaogou and Paishanlou) from the northern margin of the NCC, the third largest gold district in the NCC. Using published Pb isotope data of coeval magmatic rocks, we attempt to unravel regional variations of Pb isotopic compositions of the lower continental crust (LCC) beneath this district, and build a new quantitative Pb isotope growth model using calculations to make a comparison with Pb isotopic signatures of the gold mineralization. We propose that ore-forming metals of the Mesozoic lode gold deposits in the NCC originated from metal-fertilized Page 6 / 72

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lower crust. The results also emphasize that prior metal fertilization in deep source and subsequently tectonic decompression are critical factors controlling Au-rich ore deposits formation worldwide.

2. Geological background

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The NCC is well known as one of the oldest (3.85–3.2 Ga, Liu et al., 1992; Zhai and Santosh, 2011) Archean cratons in the world. It is composed of an

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Archean-Paleoproterozoic metamorphosed basement overlain by Mesoproterozoic to

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Cenozoic cover. The basement can be divided into the Eastern and Western Blocks,

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separated by a tectonic belt called Trans-North China Orogen (Fig. 1, inset). Collision

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between the two blocks at ~1.85 Ga has led to final amalgamation and stabilization of

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the craton (Zhao et al., 2001). The craton was magmatically and tectonically quiescent until two major Phanerozoic orogenic belts, the Qingling-Dabie-Sulu Orogenic Belt to

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the south and the Hingan-Mongolian Orogenic Belt (southern part of the Central Asia Orogenic Belt) to the north, were accreted during the Paleozoic-Early Mesozoic. Following the collision events, the eastern part of the NCC experienced intense tectono-thermal activities (e.g. development of intra-continental pull-apart basins, exhumation of metamorphic core complexes, extensive felsic to mafic magmatism and mineralization) during the Mesozoic-Cenozoic (Zhu et al., 2012), which is interpreted as the results of thinning and destruction of the lithosphere beneath the craton, because an ancient, thick (ca. 200 km), refractory, cold lithospheric root was replaced by a young, thin (80–120 km), fertile, and hot lithospheric root during this Page 7 / 72

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period (Menzies et al., 1993; Griffin et al., 1998). The EW-trending Yanshan belt is located in the northern part of the NCC ( Fig. 1). The basement rocks of the eastern Yanshan belt consist predominantly of Neoarchean high- and low-grade TTG gneisses and ~ 2.5 Ga syntectonic granitoids, with minor amounts of ultramafic to mafic volcanic and metasedimentary rocks, and have been

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considered to be part of the Eastern Block of the NCC (Zhao et al., 2001). The western Yanshan belt has different lithological assemblages, and has been classified as

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a part of the Trans-North China Orogen. It is composed of felsic gneisses,

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amphibolites, marbles and mica schists that have undergone low amphibolite-facies,

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and locally upper amphibolite- to granulite-facies metamorphism. A suite of 1.75–

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1.68 Ga anorthosite-mangerite-alkali granitoid-rapakivi granites, formed in a

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post-collisional environment following the assembly of the Eastern and Western Blocks, has been recognized along major regional faults (Zhang et al., 2007; Wang et

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al., 2013). From the Mesoproterozoic to Late Paleozoic, shallow-marine carbonate platform sediments were continuously deposited. The Yanshan belt was tectonically reactivated during the Mesozoic, and was affected by post-collisional extension during the Early Mesozoic (Zhang et al., 2009), intra-continental contractional deformation during the Mid-Late Jurassic, and large-scale extension during the Early Cretaceous (Zhao et al., 2004). Large volumes of Mesozoic volcanic rocks and clastic strata

unconformably

overlie

Precambrian

basement

rocks

and

Paleozoic

shallow-marine carbonate platform strata. The Middle Jurassic-Early Cretaceous volcanic rocks are basaltic to andesitic-rhyolitic in composition, and consist mainly of Page 8 / 72

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the Haifanggou Formation (~173 Ma, Ma et al., 2015), Lanqi/Tiaojishan Formation (166–148 Ma, Yang and Li, 2008; Ma et al., 2015) and Yixian Formation (126–120 Ma, Yang and Li, 2008). Plutons were also emplaced from the Middle Jurassic to Early Cretaceous.

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3. Gold mineralization There are more than 60 Mesozoic lode gold deposits along or nearby regional faults

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of the Yanshan belt (Fig. 1). These deposits have a total resource in excess of 520 t Au

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with an average grade of 4–20 g/t. Most of them are hosted in metamorphosed

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Neoarchean basement rocks, Mesozoic granitoids, and minor in Mesozoic volcanic

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rocks. These Neoarchean country rocks consist mainly of amphibolites, amphibolite

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gneisses and TTG gneisses, and are known as Zunhua Group and Jianping Complex in the eastern Yanshan belt. The granitoids are commonly intermediate to felsic in with

Jurassic

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composition

to

Cretaceous

ages.

Field

relationships

and

geochronological data have shown that gold deposits formed slightly later than granitic intrusions (Nie, 1997; Yao et al., 1999; Luo et al., 2001a). Orebodies of the Niuxinshan gold deposit, hosted in granites with an zircon SHRIMP U-Pb age of 172 Ma (Luo et al., 2001a), are cut by lamprophyre dykes with a K-Ar age of 166 Ma (Sun et al., 1989). The white and red granites, with zircon SHRIMP U-Pb ages of 175–174 Ma (Luo et al., 2001a), in the Yu’erya gold deposit are cut by sulfide-quartz veins with a gold associated hydrothermal sericite

40

Ar/39Ar plateau age of 169 Ma

(Chen et al., 2019). The biotite monzogranite, located in the northern part of the Page 9 / 72

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Paishanlou gold deposit with hydrothermal biotite and K-feldspar plateau ages of 124–117 Ma (Luo and Zhao, 1997; Wang et al., 2008), is dated with zircon U-Pb ages of 128–124 Ma (Luo et al., 2001b; Sun et al., 2012). Auriferous sulfide-quartz veins of the Jinchanggouliang-Erdaogou ore field, with a hydrothermal sericite

40

Ar/39Ar

plateau age of 140 Ma (Pang and Qiu, 1997), are hosted in felsic lavas with a zircon

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U-Pb age of 145 Ma (Hou et al., 2012). The gold deposits consist of auriferous quartz veins and subordinate disseminated

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or stockwork ores. All major orebodies are hosted in brittle or brittle-ductile faults,

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which are subsidiary faults of regional NE- or EW-trending faults (Hart et al., 2002).

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Hydrothermal alteration haloes are well developed along the auriferous quartz veins,

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with a common width of several centimeters to 1–2 meters. Alteration in the wall

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rocks varies within different country rocks. Quartz, sericite, chlorite, epidote, ankerite and calcite are common alteration minerals in metamorphosed rocks, whereas quartz,

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sericite, K-feldspar and carbonate are common in granites (Yao et al., 1999; Hart et al., 2002; Zhang X. H. et al., 2005). Most gold deposits contain four stages of hydrothermal alteration, including an early stage of milky quartz + coarse-grained pyrite, two main gold mineralization stages of quartz + fine-grained pyrite and quartz + sphalerite + chalcopyrite + galena + pyrite ± tellurobismuthite, and a late stage of barren carbonate ± quartz minerals (Trumbull et al., 1992; Yao et al., 1999). Gold occurs mainly as native gold and electrum within fractures of pyrite and quartz, and is partly enclosed within pyrite, sphalerite and chalcopyrite as mineral inclusions or distributed along grain boundaries of base metal sulfides (Fig. 2, Kong et al., 2015). Page 10 / 72

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Isotopic dating has revealed that gold deposits in the eastern Yanshan belt were formed at 169–118 Ma (Sun et al., 1989; Trumbull et al., 1992; Luo et al., 2001b; Miao et al., 2003; Zhang X. H. et al., 2005; Chen et al., 2019).

4. Geochemical characteristics of Mesozoic igneous rocks and constraints on lead

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isotope signatures of deep lithosphere of the Yanshan belt Bulk rock Pb isotope, Nd isotope and major and trace elements data of the Middle

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Jurassic-Early Cretaceous igneous rocks from the Yanshan belt were considered

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(Supplementary Table S1). All whole rock isotope data were calculated at their

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emplacement or eruption age. For each sample, if available, Pb isotope, Nd isotope,

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major and trace elements are obtained in the same rock sample, except for three

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Jurassic granitoids (Yu’erya by Qiu et al., 1994; Bajiazi by Zhao et al., 2003; Niuxinshan by Wang, 2012), whose average SiO2 contents are obtained from

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published data of the same intrusions elsewhere. Compositional systematics of the compiled data are plotted in Figs. 3 and 4. Only Pb isotope and Na2O/K2O ratios are plotted if Nd isotope data are not available (e.g. samples from Dashitougou, Sun et al., 2012). One of the main observations in Fig. 3 is that the igneous rocks from the eastern Yanshan belt have clear systematic decreasing 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios from 17.245, 15.373 and 37.114 to 16.035, 15.067 and 35.513 with increasing SiO2 from ~51 wt.% to ~68 wt.%. Pb isotope ratios show a clear negative linear correlation with SiO2. Similarly, Nd isotope and Na2O/K2O ratios also display clear Page 11 / 72

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decreasing changes with increasing SiO2. These trends can be explained by variable degrees of mixing between mafic and felsic magmas or assimilating of mafic melts by different volumes of lower crustal materials. This interpretation is based on the geochemical and isotopic studies for the Lanqi and Yixian formations, which were considered to be formed by underplating of mantle-derived basaltic melts at the

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crust-mantle boundary to assimilate LCC materials, and/or partial melting of the LCC (Yang and Li, 2008; Ma, 2013). The basaltic underplating event was also recorded by

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zircon U-Pb and Hf isotopic studies for lower crustal xenoliths hosted by Phanerozoic

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volcanic rocks in the Yanshan belt (Fuxin and Hannuoba, Zheng et al., 2012; Zhang et

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al., 2013). The decreasing trend of Pb and Nd isotope ratios with increasing SiO2

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appears to cease at SiO2 ≥ ~68 wt.%. Noticeably, several Early Cretaceous igneous

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rocks (135–128 Ma; highlighted in brown circles, samples from Guancaishan by Zhang et al., 2003; Duimiangou by Fu et al., 2012; and Dashitougou by Sun et al.,

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2012) have clearly radiogenic Pb isotope and Nd isotope ratios at certain range of SiO2 (~61–71 wt.%), and plot above the mixing/assimilation trends. A possible explanation is that these rocks were formed by melting of mixed sources composed of ancient and juvenile components formed by underplating of mafic magmas (Fu et al., 2012). However, The igneous rocks (~143–110 Ma) from the western Yanshan belt do not show systematic changes of Pb isotope and Na2O/K2O ratios with increasing SiO2 from ~55 wt.% to ~78 wt.%; only Nd isotope ratios display decreasing trend with increasing SiO2. These isotopic systematics are different from those of igneous rocks Page 12 / 72

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from the eastern Yanshan belt, which may be related to the different geological evolutionary histories of the above two tectonic units before the assembly of the NCC. Mafic magmas are normally considered as primary melting products of a mantle source, and felsic magmas could be primary melts from continental crust, or formed by crustal assimilation and/or fractional crystallization of mafic melts. The Middle

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Jurassic-Early Cretaceous igneous rocks from the eastern Yanshan belt show a negative linear correlation between Pb isotope ratios and SiO2 contents, which was

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explained by mafic melts assimilating LCC components, or mixing with melts derived

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from the LCC (Yang and Li, 2008; Ma, 2013). These rocks can therefore be used to

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roughly estimate the Pb isotopic compositions of presumed lower crustal and mantle

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source at specific SiO2 values according to the above linear trends. One potential

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problem to be considered is that possible magma differentiation could promote silicate melts to have higher SiO2 contents. Considering that fractional crystallization

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of silicate melts almost cannot change their Pb isotopic compositions, the estimated isotopic compositions of the above two end-members only can deviate significantly from the “desired values” when extrapolating these linear trends to very high (or low) SiO2 values, so suitable extrapolation of the linear trends is acceptable. Prior to the calculation of Pb isotopic compositions of the above two end-members, an important premise is to identify the nature of the presumed mantle end-member. We note that the most basic samples (e.g. Wulahada, ~52 wt.% SiO2, ~17.0

206

Pb/204Pb, and –6 εNd(t),

Zhang et al., 2003) show unradiogenic Pb isotope and Nd isotopic compositions when compared to asthenospheric mantle-derived melts (e.g. ~100 Ma Jianguo with 18.2– Page 13 / 72

Journal Pre-proof 18.4 206Pb/204Pb, 4.1–4.8 εNd(t), and ~109–93 Ma Zhanglaogongtun basalts with 18.2– 18.5 206Pb/204Pb, 4.3–5.7 εNd(t), Zhang et al., 2003; Yang and Li, 2008) from the same region. The unradiogenic Pb and Nd isotopes are similar to those of the Smoky Butte lamproites interpreted to have an EM1-type subcontinental lithospheric mantle (SCLM) source (Fraser et al., 1985), and those of the EM1-type mantle xenoliths (–

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6.9 to –10.6 εNd(t), Ma and Xu, 2006) identified in Cenozoic alkaline basalt from the Taihang region. This is also consistent with the conclusions made by Yang and Li

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(2008) and Ma (2013) that ancient SCLM play a significant role in the genesis of

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volcanic rocks of the Lanqi and Yixian formations. Hence, the mafic end-member can

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be regarded as derivations of a presumed SCLM. Thus, we chose SiO2 values of 48

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wt.% being in the field of basalts as the mafic end-member (end-member B in Fig. 3).

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Previous experimental melts produced by 13–44% melting of mafic LCC, analogous to the average LCC estimated by Rudnick and Gao (2003) and Condie and

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Selverstone (1999), would have SiO2 values in a range of ~63–74 wt.% (Qian and Hermann, 2013), we therefore choose SiO2 values of 68 wt.% as the felsic end-member (end-member A). Thus, based on the regression lines through the presenting mixing/assimilation trends in Fig. 3, we can obtain the Pb isotopic compositions of the above two end-members, which can be roughly considered as an approximation to the Pb isotopic compositions of presumed ancient LCC and SCLM beneath the eastern Yanshan belt during the Middle Jurassic-Early Cretaceous.

5. Lead isotope variations of lower crust beneath the Yanshan belt Page 14 / 72

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Granitic rocks resembling compositionally average upper continental crust have been widely considered to be produced by partial melting of crustal rocks of various compositions (Brown, 2001; Chen and Grapes, 2007; Sawyer et al., 2011). Previous studies have suggested that the “adakitic” signature of Jurassic igneous rocks in the Yanshan belt is inherited from ancient LCC (Ma et al., 2015). The widespread Middle

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Jurassic-Early Cretaceous igneous rocks in the Yanshan belt therefore can be used to explore the spatial variation of Pb isotopic compositions of the LCC. Melting of

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typical crustal rocks usually produces ~30–40% granitic melts that move from their

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source region (Chen and Grapes, 2007). As mentioned above, 13–44% melting of

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LCC would produce melts with SiO2 values of ~63–74 wt.%. Hence, igneous rock

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samples with SiO2 values of 63–74 wt.% are selected with attempt to map regional

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variation of Pb isotopic compositions of the presumed LCC. The geographic distribution of the compiled lead isotope data is shown in Fig. 5.

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The lead isotopic compositions of the LCC beneath the western Yanshan belt are significantly more radiogenic than those of the LCC beneath the eastern part; such characteristics are consistent with the subdivision scheme for basement rocks of the NCC proposed by Zhao and Cawood (2012). The recommended boundary between the Trans-North China Orogen and Eastern Block is generally distributed along certain contour lines with

206

Pb/204Pb values of ~16.6–16.8 (Fig. 5b). The remarkable

unradiogenic Pb isotope beneath the eastern Yanshan belt could be inherited from the intrinsic characteristics of the ancient craton. The study of ancient amphibolite- to granulite-facies xenoliths has revealed similar unradiogenic Pb isotopic compositions Page 15 / 72

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of the LCC beneath the Wyoming Craton (Bolhar et al., 2007). Noticeably, it displays clearly radiogenic Pb isotopic compositions along the northern part of the eastern Yanshan belt (Jianping to Fuxin), as also shown by those samples with relatively radiogenic Pb isotopes in Figs. 3 and 4. Because the presumed SCLM and asthenospheric mantle-derived melts (Zhang et al., 2003; Yang and Li, 2008) in this

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region have more radiogenic Pb isotope ratios than ancient LCC, the relatively radiogenic isotopic compositions may be related to modification of the LCC by input

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of mantle-derived juvenile crustal materials, due to upwelling of asthenospheric

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mantle (Fu et al., 2012) or modification of lithospheric mantle wedge by previous

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subduction of oceanic slab (Zhang et al., 2003). Given that this region is located close

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to the northern boundary (Chifeng-Kaiyuan fault) of the NCC, it was argued that

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modification of the LCC may have been induced by southward subduction of the Paleo-Mongolian Ocean and Late Permian-Early Mesozoic collision between the

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NCC and the Mongolian composite terranes (Xiao W. J. et al., 2009).

6. Samples and methods 6.1 Sample selection

Lead isotopic compositions were studied on vein pyrite hosted in Late Mesozoic granites and Neoarchean metamorphosed rocks, and on whole rock amphibolites from the eastern Yanshan belt. Large auriferous quartz-sulfide veins hosted in amphibolites were selected in the Jinchanggouliang-Erdaogou gold ore field from the northern part of the eastern Yanshan belt for pyrite Pb isotope analysis. Auriferous sulfide-quartz Page 16 / 72

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veins hosted in granites and amphibolites were collected in the Niuxinshan deposit from the southern part of the eastern Yanshan belt for pyrite Pb isotope analysis. All samples were prepared as thick sections (50–70 μm) and were then examined using reflected-light microscopy and back-scattered electron imaging before performing micro-analyses. Euhedral pyrite grains enclosing native gold, galena and chalcopyrite

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as mineral inclusions (Fig. 2) were chosen for in-situ laser ablation analysis. As a predominant country rock type, amphibolites samples were collected from the

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Niuxinshan deposit for whole rock Pb isotope and trace element analysis.

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6.2 Analytical methods

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In-situ Pb isotope analyses on pyrite were performed on a Neptune Plus

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MC-ICP-MS equipped with a Geolas 2005 excimer ArF laser ablation system at the

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State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences (CUG), Wuhan. Analytical conditions and procedures for

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high precision Pb isotope analyses with this instrument are described in detail elsewhere (Hu et al., 2015; Zhang et al., 2016). All data were acquired in single spot mode, with variable spot size (ranging from 44 to 90 μm dependent on Pb signal intensity), constant repetition rate of 8 Hz, and a laser fluence of ~6 J/cm2. A natural sphalerite standard Sph-HYLM (Pb = ~394 μg/g, Zhang et al., 2016) was used to monitor the precision and accuracy of the measurements after ten sample analyses. The obtained accuracy is estimated to be equal to or better than ± 0.2 ‰ for 208

Pb/204Pb,

207

Pb/204Pb and

206

Pb/204Pb compared to the solution value by

MC-ICP-MS, with a typical precision of 0.4 ‰ (2σ). Page 17 / 72

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Whole-rock U, Th and Pb concentrations were measured by inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7700e) at Wuhan Sample Solution Analytical Technology Co., Ltd. The samples were crushed and powdered in an agate ring mill to greater than 200 meshes (~74 μm). 50 mg whole-rock powders were dissolved in sealed Teflon bombs with a mixture of concentrated HF and HNO3. The

of

sealed bombs were kept in an oven at 190 °C for 24 h and dried at 140 °C. The samples were then dissolved with HNO3 again at 190 °C for 3 days. Dissolved

ro

samples were diluted to 100 g using 2% HNO3. Indium was used as an internal

-p

standard to monitor signal drift during measurement. Four standards (AGV-2,

re

BHVO-2, BCR-2 and RGM-2) were analyzed to calibrating element concentrations.

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The obtained accuracy of measurements is better than ± 10%, ± 5% and ± 3% for Pb,

na

Th and U concentrations compared to reference values of the above four standards. Approximately 100 mg whole-rock powders were digested in sealed Teflon bombs

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with a mixture of concentrated HNO3, HF and HClO4. The sealed bombs were kept in an oven at 190 °C for 48 h. The decomposed samples were then dried at 140 °C followed by adding concentrated HNO3 and HCl. Pb was separated using standard ion exchange procedures (Bio-Rad AG1-X8, 200-400 mesh resin) in HBr-HCl media. After chemical separation, NBS 981 (NIST SRM 981) Pb standards and sample solutions were spiked with NIST-SRM 977 Tl. The samples and standards were adjusted to a consistent Pb/Tl ratio of 3:1 to reach appropriate ion currents in the range of 8–12 V total Pb. A modified Tl normalization technique (Woodhead, 2002) was used to correct for the mass bias. In this method, the natural Tl-isotopic Page 18 / 72

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composition was assumed, and a series of reference samples were then run to define the mathematical relationship between Tl and Pb mass bias. This relationship could then be applied to the unknowns, providing a robust correction for any mass bias related to either instrument drift or matrix effects. The ion beam intensities for

202

Hg

were always below 0.17 mV for all runs, corresponding to a correction of less than 0.1 mV in mass for

204

Hg. The samples were analyzed with the reference material NBS

of

981 (208Pb/204Pb = 36.7262 ± 31, 207Pb/204Pb = 15.5000 ± 13, 206Pb/204Pb = 16.9416 ±

ro

13, Baker et al. 2004) run every two samples. Pb isotopic compositions were

-p

determined using static mode on a multi-collector ICP-MS (MC-ICP-MS, Neptune

re

Plus) at the GPMR. Repeated measurements of the NBS 981 Pb isotope standard were

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7. Results

na

(2σ) for 208Pb/204Pb.

lP

16.915 ± 4 (2σ, n = 8) for 206Pb/204Pb, 15.485 ± 2 (2σ) for 207Pb/204Pb and 36.694 ± 3

7.1 Lead isotopes

Fifty-eight spots on pyrites associated with visible gold particles in five samples from the Niuxinshan and Jinchanggouliang gold deposits were investigated for their Pb isotopic compositions (Supplementary Tables S2 and S3). Sulfides from the Niuxinshan gold deposit have Pb isotope ratios varying over a narrow range (Fig.6), with

206

Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 15.91–16.27, 15.13–15.21, and

35.78–36.10, consistent with previously reported data of bulk sulfide mineral separates (206Pb/204Pb = 15.91–16.25,

207

Pb/204Pb = 15.15–15.22, and

Page 19 / 72

208

Pb/204Pb =

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35.81–36.12, Song et al., 2012; Xiong, 2017). Isotopic compositions of sulfides hosted in granites and Neoarchean amphibolites show slight differences, but generally cluster around the lower crustal end-member (end-member A, Fig. 6). Bulk rock Pb isotope data of amphibolites, corrected to an age of 160 Ma based on measured U/Pb (0.005–0.021) and Th/Pb values (0.013–0.219), are enriched in

207

Pb relative to

206

Pb

of

and significantly less radiogenic when compared with those of sulfides, whereas those of Mesoproterozoic cover (dolomite and chert, Wang et al., 1997) are more radiogenic

ro

and plot to the right of the geochron. However, gold-bearing sulfides have Pb isotopic

-p

signature close to the spatially associated Middle Jurassic magmatic rocks.

re

Pb isotope data of sulfides hosted in Neoarchean metamorphosed rocks from the 206

Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb

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Jinchanggouliang-Erdaogou ore field, having

na

values of 16.86–17.17, 15.33–15.39, and 36.96–37.25, align along a linear trend defined by end-members A and B (Fig. 7). They are significantly more radiogenic

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than those of metamorphosed rocks. The data for bulk sulfide mineral separates hosted in Mesozoic granite and volcanic rocks, like those in the Erdaogou and Changgaogou deposits (206Pb/204Pb = 17.07–17.30, 208

207

Pb/204Pb = 15.40–15.44, and

Pb/204Pb = 37.24–37.47, Hou et al., 2012), also conform to the same trend, and

indicate relatively radiogenic signatures close to the spatially associated Mesozoic igneous rocks. These entire sulfide data plot to the left of the geochron, and the end-member B roughly lies in the central part.

8. Discussion Page 20 / 72

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8.1 U-Th-Pb evolution model of deep lithosphere beneath the eastern Yanshan belt In order to explain the unradiogenic Pb isotopic compositions of the LCC beneath the eastern Yanshan belt, we conducted numerical calculations to build a quantitative U-Th-Pb evolution model for the deep lithosphere in this study area. The model includes two long-lasting evolution stages punctuated by three geological events,

of

which are prominent for the NCC (2.80 Ga, 1.85 Ga and 0.16 Ga), and can produce appropriate Pb isotope ratios for the presumed LCC (end-member A) and SCLM

ro

(end-member B) defined in Fig. 3. The initial Pb isotope ratios at 2.80 Ga for the

-p

model is calculated from a hypothetical Neoarchean mantle reservoir (with variable μ

re

values) that evolved through a one-stage evolutionary history (4.55–2.80 Ga,

lP

including ~30 Ma Earth’ core segregation, Kleine et al., 2002; Yin et al., 2002),

na

starting with a Canyon Diablo troilite Pb isotopic compositions (Tatsumoto et al., 1973). Detailed parameters used in the calculations and modeling results are given in

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Table 1. The modeling was operated using a Microsoft Visual Studio 2005 C++ compiler, and designed codes can be seen in online Supplementary materials (Section S1). Because there are more than three variables in the modeling calculations, only two target points (end-members A and B) at 0.16 Ga cannot tightly constrain on the modeling results. In order to rule out other possibilities, the 1.75–1.68 Ga anorthosite-mangerite-alkali granitoid-rapakivi granites suites (highlighted in black snowflakes in Fig. 1), which have been proposed to be derived from EM1-type lithospheric mantle followed by assimilation of minor LCC components (Zhang et al., 2007), were chosen as a constraint condition. Their Pb isotope ratios show a well Page 21 / 72

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linear trend in uranogenic diagram (1.72 Ga isochron, Fig. 8a). Thorogenic Pb systematics are only shown as complementary information, and are consistent with the model. The preferred modeling results are indicated in Fig. 8, in which two boundary conditions for the model are also present, including the 1.72 Ga isochron and the two target points at 0.16 Ga. End-members A and B roughly plot on the 0.16

of

Ga isochron defined by the Mesozoic igneous rocks. All of the Mesozoic igneous

corresponding to age of the Earth of 4.55 Ga.

-p

8.1.1 Neoarchean Event

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rocks have unradiogenic Pb isotopic compositions that plot to the left of the geochron

re

The oldest basement rocks within the Eastern Block can be dated back to 3.85–3.2

lP

Ga (Liu et al., 1992; Zhai and Santosh, 2011), but only few outcrops are preserved

na

nowadays. The metamorphosed mafic rocks and orthogneisses with > 3.0 Ga Nd isotopic model ages from the NCC account for only ~15% of the total volumes,

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whereas rocks with model age ranging from 3.0 to 2.5 Ga account for up to ~78% (Jahn and Zhang, 1984; Wu et al., 2005; Zhai and Santosh, 2011). The zircon Hf isotopic model ages are also concentrated in the ranges of 3.0–2.6 Ga with a peak at ~2.82 Ga (Yang et al., 2008). Hf and Nd model ages of the rocks are normally considered as the timing of extraction of the protolith magmas from the mantle. The starting point of stage 1 is therefore assumed at 2.80 Ga representing an event of significant crustal-mantle differentiation, and in good agreement with the period of continental crust growth worldwide (Condie, 1998; Hawkesworth et al., 2010). Initial Pb isotopic compositions in steps 4 and 6 (and steps 1 and 3) of the model Page 22 / 72

Journal Pre-proof calculations at 2.80 Ga cannot be produced at appropriate μ values after Earth’s core segregation (~30 Ma after 4.55 Ga, Kleine et al., 2002; Yin et al., 2002). In contrast, a μ value of 8.1–8.3 (modeling steps 2 and 5) for the Neoarchean mantle reservoir is required to produce the initial Pb isotopic compositions at 2.80 Ga. The relatively higher μ values than that of the Primitive Mantle (7.0, calculated using the

of

recommended U/Pb ratios by Sun and McDonough, 1989) may be caused by recycling of some older crustal materials. This is also similar to that of the

ro

hypothetical enriched mantle reservoir underlying the Neoarchean Wyoming Province

-p

(with μ values of 8.5–8.65, Frost et al., 2006). The Neoarchean crust produced by

re

Event 1 subsequently underwent widespread metamorphism and deformation at 2.6–

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2.5 Ga (Zhai and Santosh, 2011), accompanied by some syntectonic crustal growth,

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resulting in a moderate μ value for the Neoarchean-Paleoproterozoic crust in stage 1 (from 2.80 to1.85 Ga, Fig. 8).

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8.1.2 Late Paleoproterozoic Event

Collision between the Eastern and Western blocks along the Trans-North China Orogen led to the final amalgamation and stabilization of the NCC at ~1.85 Ga (Zhao et al., 2001; Faure et al., 2007). The existence of an active eastward-dipping subduction zone into the base of the Eastern Block before the collision was proposed by Zhao et al. (2001) and Zhang et al. (2015). Re-Os data for peridotite xenoliths also indicate that the original Archean SCLM has been partly replaced during the Late Paleoproterozoic (Gao et al., 2002; Wu et al., 2006; Liu et al., 2015). Hence, significant influence on the chemical compositions of deep lithosphere would be Page 23 / 72

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expected. Unlike tectonic processes characterized by direct recycling of crustal materials into lithospheric mantle during the Archean, however, previous studies have led to the consensus that the Trans-North China Orogen is most likely a modern-style continent-to-continent collisional orogen (Zhao et al., 2001; Zhang et al., 2015; Xu et al., 2019). Dehydration and melting of the subducting oceanic crust are both

of

important processes of element fractionation in subduction zones. Incorporation of aqueous fluids released by dehydration of subducting crust at low temperatures and

ro

depths ≤ 160 km into the above mantle wedge will dramatically lower its U/Pb ratios,

-p

and produce significantly elevated Rb/Sr ratios and moderate enrichment of Sr

re

relative to La and Ce (Kogiso et al., 1997; Kessel et al., 2005). However, Mesozoic

lP

lithospheric mantle-derived magmas show both moderately depleted U and Th

na

relative to Pb, and moderate depletion of Sr relative to La and Ce (Yang and Li, 2008; Xiong et al., 2018). In addition, the unradiogenic Sr isotopic compositions of these

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magmas do not support significant incorporation of slab aqueous fluids either. Instead, addition of slab melts at relatively higher temperatures into the mantle wedge could produce the above observed geochemical characteristics (Fig. 1 in Kessel et al., 2005) and enriched Sr-Nd isotopic compositions, and most importantly, led to moderately depressed U/Pb ratios (or depressed μ values) in the Proterozoic lithospheric mantle. Additionally, collision-related metamorphism of the LCC under high pressure and temperature conditions (like granulite facies) will cause pervasive depletion of U ± Th due to loss of grain boundary fluids and breakdown of accessory phases (DePaolo et al., 1982; Rudnick et al., 1985; Bolhar et al., 2007), which could produce extremely Page 24 / 72

Journal Pre-proof low μ values. In fact, the modeling μ values of 3.6–4.2 (calculation step 5) for the lower crust in stage 2 (from 1.85 to 0.16 Ga, Fig. 8) are similar to the μ values of 4.6 calculated using the average U/Pb ratios of metamorphosed basement rocks from the eastern Yanshan belt (Supplementary Fig. S1a). The μ values are also identical to values (3.7–4.2) for a second stage evolution model for the LCC in the Wyoming

of

Craton (Bolhar et al., 2007). In order to assess if the modeled LCC in stage 2 could actually have remained thermally stable, one-dimensional conductive heat model

ro

(using heat-producing elements based on the equations (1) and (2) in Kramers et al.

-p

(2001)) is employed to calculate steady-state geotherm for the LCC after regional

re

metamorphism at 1.85 Ga (detailed parameters used in the calculations can be seen in

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Supplementary materials Section S2). The modeling results show that a two-layer

na

crust of 40–50 km thickness can have remained thermally stable and mechanically strong in state 2 (Fig. S4). Consequently, we propose that the Pb isotopic

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compositions of the presumed LCC and SCLM during the Late Mesozoic (~0.16 Ga) can be generated by the above two long-lasting evolution stages. 8.1.3 Late Mesozoic Event

The Yanshan belt was tectonically reactivated during the Mesozoic as a result of remove and modification of lithospheric root of the eastern NCC (Zhang et al., 2003; Yang and Li, 2008; Ma et al., 2015). Large volumes of Middle Jurassic-Early Cretaceous magmatic rocks record significant underplating of mafic melts derived from the SCLM at the crust-mantle boundary, and reworking and partial melting of the ancient LCC in the eastern Yanshan belt (Yang and Li, 2008; Ma, 2013), which Page 25 / 72

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may have produced the mixing/assimilation trends of lead isotopic compositions in Figs. 3 and 8. In addition, the lower crust beneath the northern part of the eastern Yanshan belt was modified by southward subduction of the Paleo-Mongolian Ocean and Late Permian-Early Mesozoic collision between the NCC and the Mongolian composite terranes (Zhang et al., 2003; Xiao W. J. et al., 2009; Fu et al., 2012), which

of

led to relatively radiogenic isotopic compositions of the LCC beneath this region (Fig. 5). Our modeling results and achieved regional variation of lead isotopes of the LCC

ro

clearly demonstrate how long-lasting geological evolution histories imprint on the Pb

-p

isotopic compositions of the deep lithosphere, and also indicate recently local

re

modifications of the ancient LCC. These results provide significant constraints on the

na

lP

origin of contemporary gold deposits within a more regional context.

8.2 Constraints on the metal sources of Mesozoic lode gold deposits

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8.2.1 Lead isotope evidences

It has been suggested that gold, sulfur, and other ore components are mobilized by metamorphic fluids under prograde greenschist- to amphibolite-facies metamorphism and lode gold ores are deposited in a post peak metamorphic P-T environment near the brittle-ductile transition (Phillips and Powell, 2010; Goldfarb and Groves, 2015). However, isotope data has revealed that gold deposits in the eastern Yanshan belt were formed in the Middle Jurassic-Early Cretaceous (Trumbull et al., 1992; Luo et al., 2001b; Miao et al., 2003; Zhang X. H. et al., 2005), approximately two billion years later than regional high-grade metamorphism of the host rocks (e.g. Zhai and Santosh, Page 26 / 72

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2011). Because the Neoarchean-Paleoproterozoic basement rocks have already experienced amphibolite- to granulate-facies metamorphism in the study area during cratonization of the NCC, syntectonic devolatilization of Precambrian basement rocks appears unlikely to provide required metal sources for the Late Mesozoic gold deposits. In contrast, the formation age range of gold deposits overlaps with that of

of

the widespread Middle Jurassic-Early Cretaceous igneous event in the eastern Yanshan belt.

ro

Because high-grade gold ores commonly contain various quantities of base metal

-p

sulfides (e.g. sphalerite, chalcopyrite and galena), and native gold is partly included in

re

sphalerite and chalcopyrite as mineral inclusions or distributed interstitial to base

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metal sulfides (Fig. 2). The close association of high-grade native gold ores with base

na

metal sulfides is unlikely the result of re-precipitation of gold from pyrite remobilization by addition of exotic base metal-rich fluids, considering low

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lattice-bound gold contents of pyrites (Zhao et al., 2011; Yang et al., 2016; Lin et al., 2019). The source(s) of gold could therefore be inferred from those of associated base metal sulfides, and detailed analysis of sulfide lead isotopes can provide some constraints on the sources of lead and gold. In order to evaluate the effect of water-rock interaction on the sulfide Pb isotopic compositions at the site of deposition, the Pb contents of Precambrian basement rocks from the eastern Yanshan belt are estimated. The rocks contain low Pb contents within a narrow range of 1.0–25.0 ppm with average contents of 11 ppm (Supplementary Fig. S1c); such low values will have very limited influence on the Pb isotopic Page 27 / 72

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compositions of large quartz vein-hosted sulfides. Indeed, sulfides hosted in granite and amphibolites from the Niuxinshan deposit have a narrow range of Pb isotope ratios, similar to values for the Middle Jurassic magmatic rocks (Fig. 6). Pb isotopic compositions of sulfides, hosted in different country rock types, from the Jinchanggouliang-Erdaogou ore field show little variability. Sulfides hosted in

of

Mesozoic igneous rocks (like those in the Erdaogou and Changgaogou deposits) have relatively uniform Pb isotope ratios close to those of the Early Cretaceous granite,

ro

whereas sulfides hosted in metamorphosed rocks present a mixing trend toward the

-p

metamorphosed rocks, indicating a small amount of metamorphosed wall rock

re

component was involved (Fig. 7). Additionally, Pb isotope ratios of pyrites from the

lP

Paishanlou deposit plot around the Early Cretaceous granite, but bulk-ores that occur

na

as disseminations and stockwork sulfide veinlets have Pb isotope ratios plotting close to those of Archean metamorphosed rocks and Proterozoic dolomitic marbles

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(Supplementary Fig. S2, Zhang X. H. et al., 2005). Considering that the orebodies and Mesozoic igneous rocks were both emplaced into the widespread amphibolite- to granulite-facies basement rocks, if the crustal basement rocks contribute dominantly components to ore metals, sulfides would have Pb isotope ratios close to those of high-grade metamorphosed basement rocks, rather than the Mesozoic igneous rocks. Hence, the above observations suggest that sulfides deposited from original ore fluids may

have

Pb

isotopic

signature

significantly

different

from

high-grade

metamorphosed rocks, but similar to those of spatially associated Mesozoic igneous rocks, which possibly indicate an identical source region. Page 28 / 72

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In general, Pb isotopic compositions of sulfides from gold deposits within the eastern Yanshan belt plot along the 0.16 Ga isochron and to the left of the geochron (Fig. 9). More interestingly, sulfides from gold deposits in different locations have Pb isotopic compositions quite consistent with the spatial variation of Pb isotopic compositions of the LCC (Fig. 5). Gold deposits in northern part of the eastern

of

Yanshan belt show relatively radiogenic Pb isotopic compositions that mostly plot to the right of 0.16 Ga isochron and are more radiogenic than the lithospheric mantle

ro

end-member (Fig. 9); such characteristics are in good agreement with the occurrence

-p

of modified LCC beneath these locations. In addition, gold deposits in the southern

re

part of the eastern Yanshan belt are characterized by unradiogenic Pb isotopic

lP

compositions overlapping with the lower crustal end-member. However, the estimated

na

ancient lower crustal end-member has Pb isotope ratios more radiogenic than Precambrian basement rocks (Fig. 6), which may imply overestimate on the

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compositions of the ancient LCC; such characteristics also indicate significant addition of mafic materials at the base of the LCC. Thus, Pb isotope characteristics here suggest that ore-forming metals of these gold deposits were mainly derived from modified lower crust. 8.2.2 Additional geochemical evidences There is increasing evidence that metal fertilization in the lower crust may play a significant role in the formation of some Au-rich deposits worldwide (Richards, 2009; Lee et al., 2012; Hou et al., 2017). Recent studies on subduction-related volcanic rocks have shown that mantle-derived melts usually reach sulfide saturation during Page 29 / 72

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differentiation at the base of the continental crust prior to ascent (Chiaradia, 2014; Locmelis et al., 2016; Jenner, 2017), which leads to the sequestration of highly chalcophile elements (such as Au, Ag and Cu) into lower crustal cumulates. Formation of sulfide-bearing cumulates possibly result from earlier magnetite fractionation (oxygen fugacity decrease) under continental crust with thickness > 30

of

km (Chiaradia, 2014) or high pressure differentiation of their parental magmas (Jenner, 2017), because the stability field of sulfide shifts towards more oxidizing conditions increasing pressure (Matjuschkin

et

al.,

2016),

ro

with

which

means

that

-p

crystalline-sulfide fractionation is more likely to occur under high pressure conditions.

re

The partition coefficient of Au between sulfide phases and silicate melts is

lP

significantly higher than that of Cu (Peach et al., 1990), thus Au could be

na

preferentially partitioned into sulfide-bearing cumulates to generate high Au/Cu ratios. Detailed studies on lower crustal amphibolite and garnet amphibolite cumulates have

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also demonstrated that reactivation of metal pre-fertilized lower crust at the cratonic edge has the potential to produce Au ore-forming systems in a non-arc environment (Hou et al., 2017).

It is notable that the NCC has undergone episodic reactivation and modification of the LCC (Zheng et al., 2012; Zhang et al., 2013), related to subduction of the Paleo-Asian and Paleo-Pacific Ocean and collision with the Yangtze Craton in the Phanerozoic. Thus, the LCC beneath the cratonic edge may have undergone significant metal fertilization and liberated metals during later reactivation. Previous studies have discovered that hornblende phenocrysts in Early cretaceous dioritic Page 30 / 72

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dykes, coeval with the formation of gold deposits in the Jiaodong Peninsula, host abundant sulfide inclusions in the Fe-rich (low-Mg) cores and at the contact between low-Mg cores and high-Mg mantle zones (Tan et al., 2012). Precipitation of sulfides from dioritic dykes has been thought to be triggered by mixing with mafic magma in their source region (Tan et al., 2008). Most importantly, these sulfide inclusions show

of

elevated Au/Cu ratios (4.8 × 10-4), which may indicate sulfide-bearing cumulates fractionated by earlier mafic magmas were retained in the lower crust and

ro

subsequently remelted to produce these dioritic dykes. Coexistence of sulfide and

-p

anhydrite inclusions suggests that redox condition was probably near the

re

sulfide-sulfate transition (Tan et al., 2012), at which silicate melts have the highest

lP

potential to extract gold from their source (Botcharnikov et al., 2011; Jégo and

na

Pichavant, 2012). In addition, noble gas isotopic data have also revealed that significant amounts of mantle-derived components were involved in the formation of

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Mesozoic lode gold deposits across the eastern part of the NCC (Zhang et al., 2008; Li et al., 2012; Tan et al., 2018). The size of resources of gold deposits in the Jiaodong Peninsula have a clearly positive correlation with their 3He/4He ratios (Tan et al., 2018) and negative correlation with their δ13CPDB values (Mao et al., 2008). This relationship may indicate that the larger gold deposits tend to involve a greater contribution of mantle-derived components (Tan et al., 2018), which is consistent with addition of different volumes of mantle-derived magmas into the LCC prior to liberating ore-forming metals. Gold deposits clustered in the northern part of the Mesozoic Linglong complex also have slightly more radiogenic Pb isotope ratios than sparsely Page 31 / 72

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distributed gold deposits in the southern part (Hou et al., 2006; Zhang et al., 2014; Wu, 2016), consistent with the correlations between 3He/4He, δ13CPDB values and gold reserves. The Mesozoic volcanic rocks (~173 Ma Haifanggou, 166–148 Ma Lanqi and 126– 120 Ma Yixian Formation) in the eastern Yanshan belt partly have systematically

of

higher total Fe2O3 at a given MgO contents (~0.5–4.0 wt.%, Fig. 4c), when compared with those of melting experimental results on average LCC (Qian and Hermann, 2013)

ro

and average arc magma evolution trend with crustal thicknesses > 30 km (Chiaradia,

-p

2014). Crustal melts commonly have low Fe2O3 and MgO contents (Zimmer et al.,

re

2010), in agreement with experimental melting results (Qian and Hermann, 2013).

lP

Thus, the features of significantly elevated Fe2O3 contents can be ascribed to

na

occurrence of cumulates and their melting in deep crustal levels (Chin et al., 2018). Mafic to intermediate lower crustal xenoliths clearly record two accumulation events

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and addition of mantle-derived materials into the LCC (237–218 Ma and ~169 Ma, Shao et al., 2000, 2006; Zheng et al., 2012; Zhang et al., 2013), nearly simultaneous with or prior to eruption of the Middle Jurassic-Early Cretaceous volcanic rocks and formation of gold deposits in this region. It is also evident from the elevated Pb isotope ratios of our estimates of the ancient lower crustal end-member (Fig. 6), with values more radiogenic than Precambrian basement rocks, possibly due to addition of mafic magmas and subsequent fractionation at the base of continental crust during the Early Mesozoic (Ma et al., 2012; Zheng et al., 2012; Zhang et al., 2013; Xiong et al., 2018). These gold deposits in the eastern Yanshan belt combined have δ34S values in Page 32 / 72

Journal Pre-proof the range of –2.8 to 7.5‰ with average values of 2.7 ± 2.4‰ (Supplementary Fig. S3), which are close to those of sulfides in meteorites (Ohmoto, 1972). As mentioned above, lead isotope characteristics of sulfides from gold deposits also suggest derivation for ore-forming metals from lower crustal sources. Based on the above observations, we therefore propose that these gold deposits were mainly derived from

of

lower crust that had been previously affected by intrusion and modification with

ro

mafic magmas followed by fractionating sulfide-bearing cumulates at the crustal base.

-p

8.3 A possible genetic model

re

Base on above observations, here we propose a genetic model for the generation of

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the Mesozoic lode gold deposits in the NCC. Firstly, highly chalcophile elements

na

dissolved in mantle-derived mafic magmas could be partitioned into sulfides during high pressure differentiation (at the base of thick continental crust) and thus led to the

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sequestration of these elements (with elevated Au/Cu ratios like Liuhe-Beiya xenoliths, Hou et al., 2017) in lower crustal cumulates (Fig. 10a). Because the ancient LCC has Pb isotopic compositions less radiogenic than the SCLM (Fig. 3), this process could produce more radiogenic Pb isotopic compositions in the LCC than those of Precambrian basement rocks. Then, lower crust would become hotter with time as basaltic magmas continue to be injected into the ‘deep crustal hot zone’ during later tectono-thermal event (Annen et al., 2006). At the onset of regional extension, exsolved volatiles (like H2O, CO2, Cl and S) from basaltic melts can flux overlying lower crustal rocks and cause melting to Page 33 / 72

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the extent of ~20–45% at temperatures up to 1000–1100 ℃ (Fig. 10b), which would produce dacitic to andesitic (trachyandesitic) melts with total FeO contents of ~4–8 wt.% (e.g. dacite and andesite in Haifanggou and Lanqi Formation, Yang and Li, 2008; Ma, 2013). Meanwhile, sulfides in cumulates would breakdown and mobilized by later magmas. Previous studies have documented that the solubility of sulfur in

of

silicate melts saturated with sulfides have strong dependence on temperature, oxygen fugacity, melt compositions (especially FeO contents) and pressure (Mavrogenes and

ro

O’Neill, 1999; Jugo et al., 2005; Liu et al., 2007). Au solubility in silicate melts show

-p

significant dependence on sulfur fugacity and would reach highest level at redox

re

conditions corresponding to the sulfide-sulfate transition (~FMQ + 1.2 for andesitic

lP

melt at 200 MPa and 1050 ℃, where the FMQ is the fayalite-magnetite-quartz buffer,

na

Botcharnikov et al., 2011; Jégo and Pichavant, 2012). At pressure of 1.0 GPa (corresponding to a depth of ~35 km), redox conditions for the sulfide-sulfate

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transition in silicate melts would be at oxygen fugacity of ~NNO + 1.2 (where the NNO is the nickel-nickel oxide buffer, Matjuschkin et al., 2016). Production of silicate melts at an oxygen fugacity of ~FMQ + 1 (like dioritic dykes in the Jiaodong Peninsula, Tan et al., 2012), would have facilitated the transfer of S, Au and other chalcophile elements into the resultant silicate melts from their source region. Most importantly, lithospheric extension could result in significant pressure relief and therefore resorption of these accumulated sulfides into silicate melts. The produced melts would have Pb isotopic signatures similar to the juvenile LCC, and would become undersaturated in sulfides during near-adiabatic ascent. Page 34 / 72

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Once the silicate melts ascend to shallow crustal levels near the brittle-ductile transition, decompression would induce volatile saturation in silicate melts, and they would release a single-phase CO2-, Cl-, H2S- and Au-bearing aqueous fluid at oxygen fugacity of ~FMQ + 1. This ore fluid probably had a salinity ranging from ~2 to 12 wt.% (Muntean et al., 2011; Runyon et al., 2017), and relatively high Au contents due

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to its high partition coefficients between exsolved fluid and silicate melt (Simon et al., 2005). Exsolved ore fluids would have evolved through decompression and cooling

ro

without significant phase separation during ascent (see Section S3 in Supplementary

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Materials). Thus, pyrite would precipitate during fluid ascent along regional faults. Pb

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isotopic compositions of the ore fluid would not be significantly affected by

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fluid-rock interaction due to very low Pb contents of ambient basement rocks.

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Mineralization caused by this type of ore fluid would have led to forming of numerous large quartz-pyrite veins and extensive pyrite-sericite-quartz alteration.

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Native gold could be found to occasionally occur as mineral inclusions in coarse-grained pyrite. As ore fluids continued to be released from silicate melts, gold would have precipitated as native gold and electrum in fractures of coarse-grained pyrite and quartz, and partly enclosed in sphalerite and chalcopyrite or distributed along grain boundary of base metal sulfides. Gold-associated pyrite formed by this process would have inherited Pb isotopic signatures consistent with the corresponding LCC.

8.4 Implications for Au ore-forming systems in extensional environments Page 35 / 72

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Deep-seated metal sources have also been documented by lead isotopic studies of ore-related sulfides from Precambrian blocks elsewhere (McNaughton et al., 1993). A significant number of Precambrian lode gold deposits from South Africa, India, Western Australia (Kolb et al., 2015), and even Phanerozoic deposits from the French Massif Central and the north-western margin of the Yangtze Craton (Bouchot et al.,

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2005; Zhao et al., 2019) have been discovered to occur in mid- to upper-amphibolite facies domains. These characteristics call for deep-seated metal sources below the

ro

supracrustal sequences. Gold was broadly deposited during late- to post-metamorphic

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times during a change in far-field stress from compression to transpression or

re

transtension (Groves et al., 2019). The required gold source and specific tectonic

lP

setting can be adequately explained by the proposed genetic model, in which metal

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fertilization in lower crust and upper mantle plays an important role. Such fertilization in deep-seated reservoirs is also continuously evidenced by some other types of Au

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ore-forming systems worldwide formed during a similar change in far-field stress, including the Eocene-Oligocene Au-rich porphyry deposits along the western margin of the Yangtze Craton (Hou et al., 2017), the Late Jurassic epithermal Au-Ag deposits of the Deseado Massif in Patagonia (Sillitoe, 2008; Tassara et al., 2017), the Early Cretaceous epithermal Au-Ag deposits in Coastal Region of the South China Block (Wang et al., 2016), and the Eocene Carlin-type gold deposits in Nevada (Muntean et al., 2011). Thus, understanding of long-lasting geological histories of the continental lithosphere is vital to unravel metallogenic characters of a region, and can help in successful gold exploration. Page 36 / 72

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9. Conclusions Lead isotope data for pyrites from major lode gold deposits, combined with published Pb isotope data of coeval magmatic rocks, in the Yanshan belt, northern margin of the NCC, provide new constraints on ore metal provenance within a

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regional context. Geochemical data of igneous rocks indicate that ancient LCC and SCLM beneath the eastern Yanshan belt have variable Pb isotopic signatures, which

ro

can be modeled by two long-lasting U-Th-Pb evolutionary stages separated by three

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prominent geological events. As revealed by regional Pb isotope mapping results, the

re

ancient LCC was locally modified by subduction and collision prior to the Late

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Mesozoic. Lead isotope data for gold-associated pyrites hosted in different country

na

rocks suggest that water-rock interaction did not impact on Pb isotope ratios. Sulfides deposited from original ore fluids have Pb isotopic compositions similar to those of

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spatially associated Late Mesozoic granitic rocks. Their Pb isotopic signatures vary consistently with correspondingly regional isotope variation of the LCC that experienced heterogeneous modification by mantle-derived magmas, which may indicate that metal fertilization in the LCC plays an important role in the generation of the Late Mesozoic gold deposits, because sulfide saturation is believed to be a common mechanism that sequesters chalcophile elements from mafic magmas at the base of thick crust. Subsequently lithospheric extension resulted in significant pressure relief and resorption of these metals during later remelting, which permitted magmas with high metal contents and Au/Cu ratios to ascend to shallow crustal levels Page 37 / 72

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and release ore-forming metals. In order to verify this model, further work is needed using lower crustal cumulates to understand in more detail the mechanisms that cause the transfer of ore-forming metals from deep crustal levels.

Acknowledgement

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We thank journal reviewers Robert Bolhar and Richard M. Tosdal, and editor Balz Kamber for their useful comments and suggestions, which helped improve the

ro

presentation of the manuscript. This study was financially supported by the National

re

lP

(Grant No. 41902087 and 41822203).

-p

Key R&D Program of China (Grant No. 2016YFC0600104); and NSFC project

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Declaration of Interest

We wish to confirm that there are no known conflicts of interest associated with

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this publication and there has been no significant financial support for this work that could have influenced its outcome.

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Craton. Lithos 149, 61–78. doi: 10.1016/j.lithos.2012.04.025.

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Zhu R. X., Yang J. H. and Wu F. Y. (2012) Timing of destruction of the North China

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Craton. Lithos 149, 51–60. doi:10.1016/j.lithos.2012.05.013. Zhu R. X., Fan H. R., Li J. W., Meng Q. R., Li S. R. and Zheng Q. D. (2015) gold

deposits.

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Decratonic

Sci.

China

Earth

Sci.

58,

1523–1537.

doi: 10.1007/s11430-015-5139-x. Zimmer M. M., Plank T., Hauri E. H., Yogodzinski G. M., Stelling P., Larsen J., Singer B., Jicha B., Mandeville C. and Nye C. J. (2010) The role of water in generating the calc-alkaline trend: New volatile data for Aleutian magmas and a new tholeiitic index. J. Petrol. 51, 2411–2444. doi: 10.1093/petrology/egq062.

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Figure Captions Figure 1. Simplified geological map of the Yanshan belt, modified after Ren et al. (2015). NE-trending dashed dark thick line shows general boundary between the Trans-North China Orogen and Eastern Block (Zhao and Cawood, 2012). Snowflake symbols denote location of the 1.75–1.68 Ga anorthosite-mangerite-alkali

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granitoid-rapakivi granites suite (Zhang et al. 2007; Wang et al., 2013). Two different sizes of yellow circles represent gold deposit with gold resources exceed or less than

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20 t. The white/blank colour on the map represents Mesoproterozoic to late Paleozoic

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shallow-marine sediments and Mesozoic continental clastic strata. The inset shows

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tectonic setting and location of the Yanshan belt. YZ – Yangtze Block; CAOB –

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Central Asian Orogenic Belt; Jiao-Liao-Ji – Jiao-Liao-Ji continental rift.

Figure 2. Field photographs and reflected-light photomicrographs of ore-related

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mineral assemblages. Auriferous quartz-sulfide vein associated with dioritic dyke (a) and disseminated and stockwork ores hosted in granitic rocks (b). Gold and galena (and chalcopyrite) enclosed in pyrites (c and d) and gold coexistent with chalcopyrite, galena and sphalerite (e and f). Abbreviations: Ccp – chalcopyrite; Gn – galena; Py – pyrite; Qz – quartz; Sph – sphalerite.

Figure 3. Diagrams of

206

Pb/204Pb (a), 207Pb/204Pb (b) and

208

Pb/204Pb (c) versus SiO2

of Middle Jurassic-Early Cretaceous igneous rocks from the Yanshan Belt. Systematic variation of Pb isotope ratios of igneous rocks (yellow and red circles in shaded fields) Page 65 / 72

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from the eastern Yanshan belt with SiO2 between 51 and 68 wt.% is consistent with mixing/assimilation trends. End-members A and B are calculated based on the regression lines (solid dark blue lines in shaded fields) through the Middle Jurassic-Early Cretaceous igneous rocks at SiO2 values of 68 and 48 wt.%, respectively. Elevated Pb isotope ratios of part Early Cretaceous igneous rocks from

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the northern part of the eastern Yanshan belt (brown circles, Zhang et al. 2003; Fu et al. 2012; Sun et al. 2012) may indicate derivation from a modified LCC. Dotted grey

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lines correspond to SiO2 contents of 63 and 74 wt.%. Data sources: Jurassic igneous

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rocks from the eastern Yanshan belt: Yu and Jia (1989), Zhang (1989), Li et al. (2001),

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Yang and Li (2008), Li et al. (2010); Early Cretaceous igneous rocks from the eastern

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Yanshan belt: Li et al. (2002), Zhang et al. (2003), Cai et al. (2003, 2005), Zhang H. et

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al. (2005), Yuan et al. (2006), Yang and Li (2008), Wang (2009), Sun et al. (2012), Fu et al. (2012); Early Cretaceous igneous rocks from the western Yanshan belt: Xiao Y.

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Y. (2009), Li and Jiang (2013), Liang et al. (2013), Xia et al. (2016), Wu et al. (2017).

Figure 4. Diagrams of εNd(t) (a) and Na2O/K2O (b) versus SiO2 of the Middle Jurassic-Early Cretaceous igneous rocks from the eastern Yanshan belt at SiO2 between 51 and 68 wt.% also suggest mixing/assimilation trends (igneous rocks with yellow and red circles in shaded field) and possible derivation of modified LCC (igneous rocks with brown circles). (c) Diagram showing total Fe2O3 versus MgO. At MgO between 0.5 and 4.0 wt.%, the Middle Jurassic-Early Cretaceous igneous rocks show higher Fe2O3 contents when compared to those of obtained from melting Page 66 / 72

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experimental results using average LCC and average arc magma evolution trend. Data sources in (a) and (b) as for Figure 2. Data sources in (c): Haifanggou and Lanqi Formation: Li et al. (2001), Wu et al. (2004), Yuan et al. (2006), Yang and Li (2008), Ma (2013); Yixian Formation: Li et al. (2002), Ji (2004), Wang et al. (2005), Yang and Li (2008); magma trend in island arc with crustal thickness > 30 km is from Chiaradia

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(2014); melting experimental results (melt compositions: granodiorite to tonalite; melting residues: amphibolite, garnet granulite, two-pyroxene granulite, garnet

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pyroxenite, pyroxenite; melt fractions: 13–69; H2O contents: 1.5–6.0 wt.%;

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temperatures: 800–1050 ℃; pressures: 10–15 kbar) using average LCC is from Qian

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and Hermann (2013).

Regional variation of

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Figure 5. Geographic distribution of rock samples analyzed for lead isotopes (a) and 206

Pb/204Pb (b),

207

Pb/204Pb (c) and

208

Pb/204Pb (d) ratios of the

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LCC. Igneous rocks with SiO2 contents of 63 to 74 wt.% (between the two dotted lines in Figure 2) are selected for mapping. The mapping is conducted using Surfer v.8.0 based on krigging interpolation algorithm. The geographic distribution of igneous rocks is obtained from the general sampling locations in referred literatures, which are plotted at corresponding rock units in Figure 1. Variation of the mapping results caused by positional deviation of sample locations at each rock unit is imperceptible.

Figure 6. Pb isotopic compositions of sulfides and host rocks from the Niuxinshan Page 67 / 72

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gold deposit in the southern part of the eastern Yanshan belt. Gold-bearing sulfides have Pb isotopic compositions close to spatially associated Middle Jurassic magmatic rocks and cluster around end-member A. The Neoarchean amphibolites have less radiogenic Pb isotopic compositions than gold-bearing sulfides, whereas the Mesoproterozoic dolomite and chert have more radiogenic Pb isotopic compositions.

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Note that relatively radiogenic Pb isotopic compositions of the end-member A than those of Neoarchean amphibolites may suggest overestimate on the compositions of

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lower crust. Isotope data of bulk sulfide mineral separates for the Yu’erya deposit

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located in the same district are also present for comparison. Data sources: sulfide data

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for the Niuxinshan and Yu’erya deposits: Qiu et al. (1994), Song et al. (2012) and this

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study; K-feldspars of middle Jurassic magmatic rocks: Yu and Jia (1989); Neoarchean

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amphibolites: Lin and Guo (1985) and this study; Mesoproterozoic dolomite and chert: Wang et al. (1997). More details regarding the geology of gold deposit is provided in

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Yao et al. (1999) and Kong et al. (2015).

Figure 7. Pb isotopic compositions of sulfides and host rocks from the Jinchanggouliang-Erdaogou gold ore field in the northern part of the eastern Yanshan belt. Sulfides in gold ores hosted in Mesozoic granite and volcanic rocks, like those in the Erdaogou and Changgaogou deposits, have radiogenic Pb isotopic compositions than end-member B and consistent with the Early Cretaceous magmatic rocks. Pb isotopic compositions of sulfides in gold ores hosted in Neoarchean metamorphosed rocks, like those in the Jinchanggouliang deposit, show a mixing trend between the Page 68 / 72

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Early Cretaceous igneous rocks and metamorphosed rocks. Data sources: sulfide data for the Jinchanggouliang, Erdaogou and Changgaogou deposits: Wang et al. (1994), Hou et al. (2012) and this study; Early Cretaceous magmatic rocks: Fu et al. (2012); Neoarchean metamorphosed rocks: Wang et al. (1994). More details regarding the geology of the Jinchanggouliang-Erdaogou ore field can be found in Hou et al.

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(2012).

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Figure 8. Two-stage quantitative uranogenic (a) and thorogenic (b) Pb isotope

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evolution models that can produce appropriate Pb isotope ratios for the end-members 206

Pb/204Pb,

207

Pb/204Pb and

Pb/204Pb ratios of 13.19, 14.47 and 32.76 for plotting. The 1.72 Ga isochron is

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208

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A and B. The starting point of stage 1 is selected with

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regressive as a constraint condition for numerical calculations using the Pb isotope data of the 1.75–1.68 Ga anorthosite-mangerite-alkali granitoid-rapakivi granites suite

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(Shachang rapakivi granites and alkali granitoids, Xie, 2005; Ren et al., 2006). μ and κ values denote

238

U/204Pb and

232

Th/238U, respectively. Numbers labeled adjacent to

the growth curves with increments of 0.4 are ages in billion years (Ga) before present. The grey stars indicate the beginning and end of stages 1 (from 2.8 to 1.85 Ga) and 2 (from 1.85 to 0.16 Ga). Geochron with an age of the Earth of 4.55 Ga is shown for comparison. The equations of the above two regression lines: 206

Pb/204Pb +10.25, R = 0.85 (1.72 Ga isochron);

+11.32, R = 0.64 (0.16 Ga isochron).

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207

207

Pb/204Pb = 0.320 *

Pb/204Pb = 0.238 *

206

Pb/204Pb

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Figure 9. Comparison of Pb isotopic compositions of sulfides from gold deposits in the eastern Yanshan belt with Pb isotope evolutionary curves for lower crust and lithospheric mantle. Data sources as for Figure 6–7 and S3.

Figure 10. Schematic illustration of the model for generation of lode gold deposits.

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This model includes (a) early formation of sulfide-bearing cumulates at the base of continental crust during basaltic underplating, (b) lithospheric extension-induced

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remelting of these cumulates to variable degrees and followed by releasing

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ore-forming fluids from the resultant silicate melts near the brittle-ductile transition

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and then ascending to the site of deposition to form gold ores. Note that the

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lower crust.

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geothermal gradient increases with continuous heating by injection of basalt into the

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Tables Table 1. Parameters used in numerical calculations of Pb isotope evolution and modeling results

–

2

8.1-8.3

3

–

4

–

5

8.1-8.3

6

–

2.90– 2.70 2.90– 2.70 2.90– 2.70 2.90– 2.70 2.90– 2.70 2.90– 2.70

11.00– 13.35 12.93– 13.44 13.02– 14.88 11.00– 13.34 12.93– 13.42 13.02– 14.79

Pb/204Pb (at T1)

13.65– 14.57 14.33– 14.60 14.36– 15.00 13.66– 14.56 14.33– 14.58 14.37– 14.98

Pb/204Pb (at T1)

31.00– 32.93 32.50– 33.00 32.61– 34.20 31.00– 32.93 32.50– 32.99 32.61– 34.17

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NO. denotes the number of modeling steps.

208

37.102.

206

T2 (Ga)

μ2

κ2

9.2– 19.9 8.7– 11.0 1.4– 10.5 7.3– 19.9 7.1–9.0

4.0–4.7

1.0–8.5

4.5– 14.4

1.90– 1.80 1.90– 1.80 1.90– 1.80 1.90– 1.80 1.90– 1.80 1.90– 1.80

5.5– 6.4 5.6– 6.4 5.5– 6.4 3.5– 4.3 3.6– 4.2 3.5– 4.3

3.8– 3.9 3.8– 3.9 3.8– 3.9 4.4– 4.5 4.4– 4.5 4.4– 4.5

4.5–4.8 4.5– 10.6 3.9–4.6 4.4–4.7

Pb/204Pb = 16.060,

Pb/204Pb = 17.060,

207

207

Pb/204Pb = 15.160,

Pb/204Pb = 15.360,

208

Pb/204Pb =

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Pb/204Pb = 36.182; end-member B:

κ1

206

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Pb isotope ratios of target points, end-member A: 208

μ1

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1

207

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T1 (Ga)

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Pb/204Pb (at T1)

μ0

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206

NO.

Modeling steps 1-3 and 4-6 aimed to reproduce the Pb isotope signatures of end-members B and A, respectively.

Earth accretion started 4.55 Ga ago with a Canyon Diablo troilite Pb isotope composition: 206

Pb/204Pb = 9.307, 207Pb/204Pb = 10.294, 208Pb/204Pb = 29.476 (Tatsumoto et al., 1973).

μ and κ values prior to Earth’s core segregation at 4.52 Ga used in the modeling are 0.85 and 3.92, respectively (after Pettke et al. (2010) and Wipperfurth et al. (2018)). μ0 and κ0 denote the μ and κ (values used in the modeling are 3.92) values after core segregation. μ1 and κ1 denote the μ and κ values of stage 1. Page 71 / 72

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μ2 and κ2 denote the μ and κ values of stage 2. T1 and T2 signify the beginning of stages 1 and 2 in billion years (Ga) before present. 206

Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb denote the range of initial Pb isotope ratios at T1.

Decay constants of

238

U,

235

U and

232

Th are 1.55125 × 10-10 a-1, 9.8485 × 10-10 a-1 and 4.9475 ×

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lP

re

-p

ro

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10-11 a-1, respectively (Steiger and Jäger, 1977).

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10