1.3. Scientific background and aims of this study
A number of studies have speculated that upwelling mantle plumes are
generally rich in Au (e.g., Sisson et al., 2003; Webber et al. 2013),
which can lead to globally different styles of Au mineralization, such
as Carlin-type deposits in the US (e.g., Oppliger et al. 1997) and China
(Zhu et al. 2020) and porphyry deposits in South America (e.g., Tassara
et al. 2017). However, it is not clear how the upwelling mantle plume
leads to Au mineralization. Recently, receiver function and tomography
(Lei, 2012, He, 2020) have defined a vestige of an upwelling Mesozoic
mantle plume beneath the NCC that corresponds well to the region of gold
mineralization, which implies that the upwelling mantle plume may be
related to the gold mineralization in this area. In this study, the
dynamic process of the upwelling mantle plume is analysed in the NCC,
and the relationship between the upwelling mantle plume and gold
mineralization is discussed. Finally, it is proposed that the upwelling
mantle plume might lead to gold mineralization in the NCC.
Although many researchers have considered that magmatism may be related
to the formation of the polymetallic province in the SCB (e.g., Deng et
al., 2014), it is not clear what generated the large-scale magmatism and
how it induced the metallogeny in the SCB. Recently, receiver function
analysis and tomography have revealed a vestige of the large-scale
mantle upwelling beneath the SCB in the Mesozoic (He and Santosh, 2016,
2021) that corresponds well to the Mesozoic magmatism and metallogeny
region in the SCB, which implies that mantle upwelling may have resulted
in the large-scale magmatism and metallogeny in the SCB. However, the
relationship between the mantle upwelling and metallogeny need to be
further clarified. In this study, a thorough examination of the tectonic
evolution and a detail analysis of the mantle upwelling are carried out
in this area. Finally, it is suggested that the mantle upwelling might
have led to large-scale magmatism and resulted in the metallogeny in the
Mesozoic in the SCB.
Data and method of receiver function and tomography
He
et al. (2014) performed H-k stacking of receiver functions using 441
teleseismic events with magnitude mb ≥5.8 recorded by 314 seismic
stations. For each event-station pair, data were selected within the
ranges of 30°-95° and initially
windowed 15 s before and 120 s after the P -wave
pick. Data were filtered using a zero-phase Butterworth bandpass filter
with corner frequencies of 0.03-3 Hz . A modified frequency domain
deconvolution was carried out, and
the
Gaussian factor and water level were set to 3 and 0.01. One of the
results indicates that the bulk Vp/Vs ratio identified in this
study show higher values for the regions in the northern part of the
eastern NCC and the Trans-North China Orogen (>1.76) than
for other regions, which might represent mafic and ultramafic lower
crust. This implies that an upwelling mantle plume led to lower crustal
underplating and magma intrusion in this region.
He (2020) used 647 teleseismic events recorded by the China Earthquake
Network with 671 seismic stations from July 2007 to March 2014 and data
from the Incorporated Research Institutions for Seismology (IRIS) to
carry out tomography and common conversion point (CCP) stacking of
receiver functions. Seismic events with magnitudes >6.0 and
epicentral distances ranging from 30°-85° were selected. Waveforms were
cut from digital seismograms 15 s before and 50 s after the first P-wave
arrival and filtered between 0.3 and 3 Hz. Finally, 102421
first
P-wave arrivals were extracted using the time cross-correlation method.
A damping value of 20.0 was selected for use in the tomographic
inversion following a trade-off or L-shaped curve. One of results
revealed a mushroom-like low-velocity anomaly beneath the Eastern Block
of the NCC, which might be a vestige of the upwelling mantle plume from
the Mesozoic. The CCP technique was employed to stack 85,015
high-quality receiver functions and to image the mantle transition zone
beneath the NCC. The results show that the location of the mantle plume
corresponds well to the region where both the 410 and 660 km
discontinuities become deeper.
He et al. (2013) applied H-k stacking of receiver functions in the South
China Block using 424 teleseismic events recorded by 254 seismic
stations; the data processing and method were similar to those of He et
al. (2014). One of the results shows a region with high Vp/Vs ratio in
the Cathaysia Block, which might be associated with magmatic intrusions
generated by mantle upwelling.
He and Santosh (2016) collected 57813 P-wave arrivals from 449
teleseismic events recorded by 382 seismic stations with event-station
epicentral distances ranging from 30° to 85° and performed tomography in
the South China Block; the data processing and method were similar to
those of He (2020). One of the results indicates that there is a
large-scale low-velocity anomaly beneath the Cathaysia Block, which may
be connected with mantle upwelling.
He and Santosh (2021) carried out CCP stacking of receiver functions in
the South China Block using 1053 teleseismic events recorded by 330
permanent seismic stations of the China Seismic Network; the data
processing and method were similar to those of He (2020). One of the
results demonstrated that the thinning region of the mantle transition
zone corresponds well to the upwelling mantle defined by He and Santosh
(2016).
Mantle plume upwelling and gold mineralization in the NCC
Wilson (1963) proposed that the lithosphere moving over a stationary
mantle plume or hot spot in the mantle formed the Hawaiian Islands.
Generally, mantle plume upwelling is considered to originate from the
core-mantle boundary (Campbell, 2005), and it can be represented as
either a low-viscosity and low-density fluid injected into a
high-viscosity and high-density fluid or the lighter material of the
lower boundary layer upwelling into the overlying mantle (Campbell,
2005). Therefore, upwelling mantle plumes may play a key role in
material transfer and convection (Morgan, 1971), and their vestiges can
be retained for several million to billion years in the mantle and be
detected by seismic techniques (e.g., Phillips et al., 2018). For
example, the Eifel volcanic field in the North Atlantic was induced by
an upwelling mantle plume between 700,000 and 10,800 years ago as
revealed by a Rayleigh wave tomography (Pilidou et al., 2005) and the
Ross Island (Antarctica) lava material during the Archean to early
Proterozoic associated with an upwelling mantle plume has been revealed
by recent global tomographic modelling (Phillips et al., 2018).
In early Earth’s geological history, due to gravitational processes,
lighter elements rose upwards, whereas heavier elements sank to the
core-mantle boundary or into the core of Earth; examples of heavier
elements include nickel, iron, silver and gold, which are mostly
concentrated at the core-mantle boundary or in the core of the Earth
(Trubitsyn, 2019). Mantle plumes act as channels that connect
deep-seated (or core-mantle boundary) ore-forming material with the
shallow lithosphere (Santosh et al., 2009). When a mantle plume upwells,
gold or other metals retained at the core-mantle boundary and in the
core begin to rise (Tassara et al., 2017) in a gaseous state (Alexander,
2014; Munteen et al., 2011) together with the hot material flow of
mantle plumes into the shallow mantle or the base of the
lithosphere. Finally, gold or other
metals are transferred through fluid and magmatic conduits and preserved
in
favourable structural regions, thus forming mineral deposits (Pirajno
and Santosh, 2015).
Hayden and Watson (2007) and Saunders et al. (2018) also proposed that
rising plumes may potentially add Au into the sub-continental
lithospheric mantle (SCLM) or introduce metal to the SCLM during their
final ascent. However, this process does not exclude the upwelling
transport of gold or other metals in another state. Gold is more likely
to be mobilized by silicate melts than hydrous fluids (Saunders et al.,
2018), and the recycling of crustal materials may contribute to the
placement of gold deposits (McInnes et al., 1999).
The relationship between mantle convection and surface observations
plays a key role in understanding regional tectonics and related
metallogenesis (Keith, 2001; Lobkovsky and Kotelkin, 2015). Seismic
parameters observed in the surface, such as P- and S-wave velocities and
the Vp/Vs ratio, are indicators that reflect the bulk physical
properties of Earth’s interior generated by convection (Fukao et al.,
2009).
In the NCC, tomography and receiver function analysis have been used to
define a vestige of the upwelling mantle plume (He et al., 2015; He,
2020) (for example, Fig. 2). Tomography clearly shows a low-velocity
anomaly with a mushroom-like shape originating from the lower mantle
beneath the Eastern Block of the NCC (Fig. 3) (He, 2020), which is
consistent with the basic features of the upwelling mantle plume, i.e.,
a large head followed by a relatively narrow tail (Campbell, 2005;
Cloetingh et al., 2013; Weis et al., 2011).
Upwelling
mantle plumes can underplate in areas with lithosphere thinning (Begg et
al., 2010; Griffin et al., 2013) and intrude the lower crust (e.g.,
Pirajno, 2007; Saunders et al., 2018) or surface (Fig. 4), causing
ultramafic/mafic lower crust and increasing Vp/Vs ratios. H-k stacking
of receiver function analysis indicates ultramafic/mafic lower crust
(Vp/Vs>1.76) in the northern part of the NCC (red oval
region) (Fig. 5) (He et al., 2015), which corresponds well to the
location of the upwelling mantle plume (Fig. 3b). Meanwhile, the
distribution of gold deposits (Fig. 6) corresponds well with the
location of lower crustal underplating and the upwelling mantle plume in
the NCC (Fig. 2 and Fig. 5).
Geologists have also suggested an upwelling Mesozoic mantle plume
beneath the NCC based on indirect geochemical and isotopic evidence and
speculative tectonic models; this plume may be linked with major gold
mineralization in the NCC (Zhai et al., 2007). Mineralogical studies
have indicated that underplating or ultramafic/mafic magma may be
related to metallic or gold deposits (e.g., Begg et al., 2010; McInnes
et al., 1999).
Therefore, it is suggested that the upwelling mantle plume may have
generated lower crustal underplating and large-scale magmatism, which
led to gold mineralization in the eastern part of the NCC.
Mantle upwelling and metallogenesis in the South China Block
Since the Mesozoic, the Pacific Plate has subducted beneath the Eurasian
continent (Sun et al., 2007; Hall, 2012), which has considerably
influenced the tectonic and geological evolution of East China (Sun et
al., 2007), especially the Cathaysia Block. Subduction also induced
convective circulation (or mantle upwelling) (Zhao and Ohtani, 2009).
Tomography has been used to define a large-scale low-velocity anomaly
beneath the Cathaysia Block (Fig. 7), which might be linked to a vestige
of Mesozoic mantle upwelling (He and Santosh, 2016) and crustal
extension during the Jurassic to early Cenozoic in the SCB (Yan et al.,
2011). He et al. (2013) performed a receiver function analysis and
revealed two high Vp/Vs ratio regions in the Cathaysia Block (Fig. 8),
which may have been generated by magmatic intrusions from mantle
upwelling. The metallic deposits in the SCB show that the Nanling metal
deposit zone (Fig. 9a, blue oval) corresponds to the region with high
Vp/Vs ratio (blue oval) (Fig. 9b), whereas the Cretaceous
Sn-W-Pb-Zn-Au-U system along the continental margin (Fig. 9a, rectangle)
corresponds to another high Vp/Vs ratio zone (Fig. 9c, blue rectangle)
(Fig. 9c).
Earth’s geological history includes the cyclic dispersal and assembly of
continental blocks or fragments within supercontinents (e.g., Meert,
2012; Nance et al., 2014). The assembly of these continental blocks or
fragments into supercontinents involves the closure of intervening ocean
basins and the subduction of oceanic lithosphere (Maruyama et al.,
2007). The subducted slabs may extend to the lower mantle and accumulate
at the core-mantle boundary (e.g., Peacock, 1996; Zhao, 2004), forming
slab graveyards. Due to multiple prolonged periods of subduction and
heating from the core, slab graveyards are eventually transformed into
superplumes and eventually break supercontinents apart (Pirajno and
Santosh, 2015; Nance et al., 2014). The process may result in a
precursor stage of the metal mantle source refertilization in the SCLM
derived from upwelling superplumes, which might have played in a key
role in forming the large metallogenic provinces in the Earth’s crust
(Begg et al., 2010; Tassara et al., 2017).
Studies on the geometries of various mineral deposit types and the
evolution of metallogenesis through Earth’s geological history indicate
that metallogenesis is broadly related to supercontinent cycles (Khomich
et al., 2014; Pirajno and Santosh, 2015). The SCB, which is one of the
potential polymetallic provinces in the world (Hu et al., 2010;
Shellnutt, 2014), is considered to have been involved in the cycles of
ancient supercontinents, including Columbia, Rodinia, Gondwana and
Pangea (Cawood et al., 2013; Yin et al., 2013). From ca. 850 Ma to 730
Ma, South China rifted from the Rodinia supercontinent (e.g., Yao et
al., 2011), and this rifting process may have been contemporaneous with
a major period of metallic ores ascending into the shallow mantle and
the crust. The metallogenesis in the crust may have been connected with
three major factors in space and time: enriched metal in the upper
mantle and lower crust, transient remobilization or mantle upwelling,
and favourable lithospheric plumbing structures or faults (Tassara et
al., 2017).
Large-scale magmatism (or mantle upwelling) in the SCB occurred during
the Mesozoic (Zhou et al., 2006) and may have induced the remobilization
of metallic ore material retained in the SCLM or lower crust, which
ultimately led to fluid circulation, deep-seated heat flow and
crust-mantle reactivation (Goldfarb et al., 2014). This process
preserved favourable ore deposits (Fig. 9a) in the shallowing crust or
surface (Begg et al., 2010; Tassara et al., 2017) and may be a major
cause of metallogenesis in the SCB.
In other regions of the world, similar mineral systems have been
reported (Begg et al., 2010; Herzberg and O’Hara, 2002), such as the
Au-bearing metasomatic vein in the Cerro Redondo peridotite xenolith,
which represents re-melting from mantle domains (Tassara et al., 2017).
Studies also indicated that magmatic Ni-Cu-platinum group elements and
platinum group element deposits are genetically associated with
metal-rich mafic or ultramafic magmas in the mantle (Begg et al., 2010),
which can be produced by high-degree melting and transport of deep
mantle rocks with temperatures greater than 1,500 °C at depths of 100 km
or less (Herzberg and O’Hara, 2002).
The difference in dynamic processes between the NCC and the
SCB
The upwelling mantle plume (a mushroom-like low-velocity anomaly)
beneath the NCC originated from the lower mantle or core-mantle
boundary, and its material might have included metallic ores or mafic
material. The upwelling of a mantle plume can lead to lower crustal
underplating (or ultramafic/mafic lower crust), where the Vp/Vs ratio is
greater than 1.76 (Fig. 5). In contrast, the upwelling mantle (not a
mushroom-like low-velocity anomaly) beneath the SCB was rooted from the
upper mantle rather than the lower mantle, and its upwelling material
might have been different from that of the upwelling mantle plume.
Upwelling mantle can intrude into the crust and result in magmatism,
which induces relatively high Vp/Vs ratios; however, its Vp/Vs ratio is
less than 1.76 (Fig. 8b). Geological studies also suggested that a large
volume of Yanshanian granitoid rocks and lesser mafic intrusion volcanic
rocks formed during the Jurassic-Cretaceous in the SCB (Sun, 2006; Deng
et al., 2014), which is different from the products of the upwelling
mantle plume. Accordingly, there is a great difference between the gold
mineralization in the NCC and the metallogenesis in the SCB due to the
difference in the dynamic processes.
Conclusion
Credible seismic evidence indicates that a vestige of a Mesozoic mantle
plume remains beneath the Eastern Block of the NCC, and it corresponds
well to the distribution of gold deposits in the NCC. Credible seismic
evidence also corroborates that a vestige of Mesozoic mantle upwelling
occurs beneath the Cathaysia Block that is related to the Mesozoic
metallogenesis in the SCB. Therefore, mantle plume upwelling might have
been a major cause for the gold deposits in the NCC, whereas the
metallogenesis in the SCB may have been associated with mantle
upwelling. Based on these results, it is suggested that mantle dynamics
might have played a key role in the gold mineralization and metallogeny.
These findings are very important for understanding metallogeny around
the world, which need to be studied in more detail in the future.