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Miner Deposita (2014) 49:809–819
DOI 10.1007/s00126-014-0536-1
LETTER
Cooling and exhumation of the mid-Jurassic porphyry copper
systems in Dexing City, SE China: insights
from geo- and thermochronology
Xuan Liu & Hong-rui Fan & Noreen J. Evans &
Geoffrey E. Batt & Brent I. A. McInnes & Kui-feng Yang &
Ke-zhang Qin
Received: 10 April 2014 / Accepted: 23 June 2014 / Published online: 14 August 2014
# Springer-Verlag Berlin Heidelberg 2014
Abstract The Dexing porphyry copper and Yinshan
polymetallic deposits in Dexing City, southeastern China are
both giant porphyry ore systems. Located 15 km apart, they
formed synchronously and share a similar magma source and
metallogenic evolution, but their metal endowment, dominant
rock types, and alteration assemblages differ significantly. In
this contribution, we investigate the cause of these distinctions
through new molybdenite Re–Os ages and zircon and apatite
(U–Th)/He thermochronology data. Dexing has a molybdenite Re–Os age of ~170.3 Ma, zircon (U–Th)/He (ZHe) ages
of 110 to 120 Ma and apatite (U–Th)/He (AHe) ages of 7 to
9 Ma. In contrast, Yinshan has older ZHe ages of 128 to
140 Ma and an AHe age of ~30 Ma. Viewed in combination
with previously published data, we conclude that the apparently slow cooling experienced by these bodies is primarily a
reflection of their experiencing multiple episodes of thermal
disturbance. We tentatively infer that both deposits were exposed in the Late Miocene or more recent time, with the
Dexing deposit more deeply exhumed than Yinshan. Our
study has exploration implications for deeper porphyry-style
ores at Yinshan and for porphyry deposits in non-arc
(intraplate) settings in general.
Editorial handling: T. Bissig
X. Liu : H.<r. Fan (*) : K.<f. Yang : K.<z. Qin
Key Laboratory of Mineral Resources, Institute of Geology and
Geophysics, Chinese Academy of Sciences, Beijing 100029, China
e-mail: fanhr@mail.iggcas.ac.cn
N. J. Evans : B. I. A. McInnes
John de Laeter Centre for Isotope Research, Department of Applied
Geology, Curtin University, Perth 6845, Australia
G. E. Batt
Center for Exploration Targeting, School of Earth and Environment,
University of Western Australia, Crawley 6009, Australia
Keywords Dexing porphyry Cu deposit . Yinshan
polymetallic deposit . (U–Th)/He . Thermochronology .
Cooling . Exhumation
Introduction
Porphyry deposits are normally formed by late magmatic and
hydrothermal processes several kilometers (typically 1–5 km)
deep in the upper crust (Seedorff et al. 2005), and their
presence at economically accessible levels demonstrates an
additional history of tectonic and/or erosional exhumation
(Kesler and Wilkinson 2006). Examining the record of postmineralization cooling and exhumation for a prospective region thus represents a potentially valuable contribution to
prediction of the presence and character of porphyry-style
mineral endowment.
The closure temperature (Tc) of helium is approximately
~200 °C for zircon and ~75 °C for apatite, respectively (Wolf
et al. 1996; Reiners and Farley 1999; Farley 2000; Farley and
Stöckli 2002; Reiners 2005). It follows that a zircon (U–Th)/
He age for porphyry mineralization probably records the
timing of the latest hydrothermal event to have affected the
system, whereas an apatite (U–Th)/He age is more likely
related to thermal collapse and/or unroofing. A combination
of these two ages can thus provide valuable information
regarding the post-mineralization uplift and exhumation history of ore deposits, with significant implications for ore
preservation (McInnes et al. 2005a). More importantly, if
these two constraints are combined with other chronometers
such as zircon U/Pb (Tc >900 °C; Cherniak and Watson 2000)
and Re–Os (Tc =440–500 °C; Suzuki et al. 1996; Selby and
Creaser 2001), a thermal history spanning over 800 °C can be
elucidated.
810
The Dexing and Yinshan deposits are nearly identical in
terms of magma genesis (Wang et al. 2012a; Liu et al. 2013)
and formation age (Li et al. 2007; Zhou et al. 2011), and were
emplaced into the same country rocks. However, their metal
endowment, rock character, and alteration assemblages differ
significantly. Dexing is dominated by porphyry Cu–Au–Mo
ores hosted by porphyritic rocks whereas Yinshan comprises
epithermal Pb–Zn–Ag veins and coeval volcanic rocks. We
here re-evaluate the cooling history of the two ore deposits in
order to determine the degree to which differential exhumation
may be a factor in their contrasting preservation and to establish
preliminary thermal history relationships between the two.
Miner Deposita (2014) 49:809–819
0.011 g/t Au; Huang et al. 2001) is hosted by three granodiorite porphyry stocks (Zhushahong, Tongchang, and Fujiawu)
emplaced into the phyllite and slate of the Shuangqiaoshan
Group. Early hydrothermal alteration resulted in an inner zone
of potassic assemblages and an outer zone of propylitic rocks,
which is partially overprinted by later intense phyllic alteration along the contact zones between the porphyries and their
wallrocks. Mineralization is dominated by sulfide veinlets,
stockworks, and disseminated sulfides, with primary ore minerals consisting of pyrite, chalcopyrite, and molybdenite, together with minor bornite, tennantite, and bismuth-bearing
copper sulfide. Ore bodies (Fig. 1e) are primarily contained
in the phyllic alteration zone, the top of which has been
exhumed by tectonic unroofing (Hou et al. 2013).
Regional geology
Previous chronological work on the Dexing deposit
Dexing City in Jiangxi Province is one of the most important
Cu–Pb–Zn and Au mining districts in SE China (Hua et al.
2000). It is located in the eastern segment of a Neoproterozoic
orogen (commonly referred to as the “Jiangnan Orogen”;
Fig. 1a; Wang et al. 2012b) formed during amalgamation of
the Yangtze and Cathaysia terranes to form the South China
Block. The basement rocks of this area are separated by the
Northeast Jiangxi Fault Belt and consist of two rock units: the
early Neoproterozoic greenschist facies volcano-sedimentary
Shuangqiaoshan Group (Wang et al. 2008) and a tectonically
complex terrane of disrupted turbidities and early
Neoproterozoic arc-related rocks assigned to the Qigong
Group (Li et al. 2009). Locally, the basement rocks are unconformably overlain by Cambrian to Cenozoic strata.
The oldest structures in the Dexing and surrounding areas
are ENE-trending folds developed within the Shuangqiaoshan
Group and associated E-trending faults (Fig. 1b). This fabric
was overprinted during the Paleozoic and Mesozoic by NEtrending faults and subordinate fold structures, with the region
then disrupted by Jurassic and Cretaceous extension and the
development of several significant fault-bounded basins (JGB
1980). These late graben include such significant depocenters
as the Leping–Dexing basin, which accommodates basal Upper Jurassic volcanic tuff and breccias (~270 m in thickness)
overlain by greater than 1,800 m of Lower Cretaceous lacustrine facies sediments (Fig. 1c). The region was later affected
by a further phase of NNE-trending faulting and by a final
phase of extension producing a suite of NW-trending normal
faults cross-cutting all earlier structures.
Deposit geology
General geology of the Dexing deposit
The Dexing porphyry copper deposit (Fig. 1e; with a resource
estimate of 1,700 Mt of ore at 0.5 % Cu, 0.02 % Mo, and
Magmatic rocks at Dexing consist of granodiorite porphyries,
minor granitic aplite, and quartz diorite dykes. Zircon U–Pb
dating by laser ablation inductively coupled mass spectrometry
(LA-ICPMS; Wang et al. 2006; Zhou et al. 2011; Liu et al.
2012) indicates an intrusion age for the granodiorite porphyries
of 170–173 Ma. Initial mineralization is contemporaneous with
this emplacement episode, with molybdenite from molybdenite–pyrite–quartz veins yielding a well-defined Re–Os age of
~170 Ma (Zhou et al. 2011). Cross-cutting relationships demonstrate that the granitic aplite formed after emplacement of the
porphyries but before potassic alteration (Zhu et al. 1983).
Zhou et al. (2012a) reported a SHRIMP zircon U–Pb age of
~153.5 Ma for a quartz diorite dyke, with Zhu et al. (1983)
obtaining K–Ar ages of 152–157 Ma from quartz–K-feldspar
veins and a K–Ar age of ~112 Ma for hydrothermal illite of the
altered granodiorite porphyry. These K–Ar ages are not considered good estimates of the timing of mineralization, but they
are potential thermo-chronometers, recording the time at which
their host minerals passed through the relevant closure temperatures of ~230 °C for K-feldspar (Suzuki et al. 1996) and
~150 °C for illite (Hamilton et al. 1989). Zhou et al. (2012b)
reported SHRIMP U–Pb ages of 101–107 Ma for hydrothermal
zircon from a structurally late pyrite–chalcopyrite–quartz vein,
demonstrating the occurrence of a distinct later episode of
hydrothermal activity at Dexing.
General geology of the Yinshan deposit
The Yinshan deposit (Fig. 1d) hosts estimated ore resources of 83 Mt at 0.5 % Cu, 0.8 g/t Au, 1.3 % Pb,
1.0 % Zn, and 33.3 g/t Ag (Wang et al. 2013). Exposed
lithologies include subvolcanic rocks (dacite porphyries,
quartz porphyries, quartz diorite porphyries, and andesite
porphyries), voluminous volcanic and volcaniclastic materials, breccias, and Shuangqiaoshan metamorphic rocks
(primarily phyllite). Hydrothermal alteration is dominated
Miner Deposita (2014) 49:809–819
811
Fig. 1 Geological map of the Dexing area, Dexing deposit and Yinshan
deposit (a is modified after Liu et al. (2012); b is modified after JGB
Jiangxi Geological Bureau (1980); c is a stratigraphic column of the
Dexing region and its surrounding area, shortened since the Jurassic; d
is geological map of the Yinshan deposit (modified after JGEB 1996);
and e is geological map of the Dexing deposit (modified after Zhu et al.
1983))
by phyllic and propylitic assemblages which show
systematic zonation centered on the #3 dacite porphyry.
Epithermal Pb–Zn–Ag veins (quartz–galena–sphalerite)
are associated with propylitic alteration in the medial
and distal zones (JGEB 1996). Cu–Au ores, predominantly
in wide veins (quartz–pyrite–chalcopyrite, commonly 0.5–
1 cm wide, with some examples up to 15 cm), are associated
with phyllic alteration in the proximal zone (Fig. 1d; JGEB
1996). Pb–Zn–Ag veins occur primarily at shallow levels, and
Cu–Au veins have a large vertical extent, gradually
812
Miner Deposita (2014) 49:809–819
transitioning into small veins and disseminated ore at depth
(JGEB 1996).
Previous chronological work on the Yinshan deposit
Li et al. (2007) reported a SHRIMP zircon U–Pb age of 183±
3 Ma for the main dacite porphyry. Recent LA-ICPMS zircon
U–Pb dating (Wang et al. 2012a; Liu et al. 2013) gives ages of
176±1 Ma for the principal hornblende rhyolite, and of 170–
176 Ma for the early intrusives (dacite porphyry, quartz porphyry, and quartz diorite porphyry) at Yinshan. Wang et al.
(2012a) also defined an age of 166±1 Ma for late andesite
porphyry at the site. These radiometric ages demonstrate
significant diachroneiety of intrusive events at Yinshan. Li
et al. (2007) also obtained muscovite 40Ar–39Ar ages of ~175–
178 Ma for the hydrothermally altered porphyry and
interpreted this as the mineralization age for the deposit. Li
et al. (2005) reported younger illite K–Ar ages of 122–136 Ma
for the altered porphyries, which arguably date the postemplacement exhumation and cooling of the system.
Results
Re–Os geochronology
Six molybdenite samples were analyzed in this study (Table 1
in Appendix II). They show variable Re contents (50.07–
604.7 ppm) and low common Os contents (0.0889–
3.378 ppm), yielding consistent individual calculated model
ages between 165.1 and 170.3 Ma. Combined synthesis of all
six analyses yields an isochron age of 170.3±2.5 Ma (Fig. 2).
Fig. 2 Molybdenite Re-Os isochron for the Dexing deposit
yielded (U–Th)/He (ZHe) ages ranging from 110 to 120 Ma
with a weighted mean of 114.4±4.8 Ma (Fig. 3a and Table 3 in
Appendix II). Three apatite grains yielded (U–Th)/He (AHe)
ages ranging from 7 to 9 Ma with a weighted average age of
8.0±1.6 Ma (Fig. 3b).
Sample 10YS10 is a dacite porphyry sample with strong
phyllic alteration, collected from the #3 porphyry of the
Yinshan mine (Table 2 in Appendix II). Three zircon grains
were dated, yielding ZHe ages ranging from 128 to 140 Ma
with a weighted mean of 132.6±6.7 Ma (Fig. 3a). Three
apatite grains yielded a consistent AHe age of 29.6±1.6 Ma
(Fig. 3b).
Discussion
(U–Th)/He thermochronology
Magmatic to hydrothermal cooling
Sample 10DX89 is a slightly K-altered granodiorite porphyry,
collected from the Tongchang open pit of the Dexing mine
(Table 2 in Appendix II). Four zircon grains from this sample
Cooling from magmatic through hydrothermal conditions in a
porphyry deposit encompasses a history that begins with
Table 1 Molybdenite Re–Os isotopic results for the Dexing deposit
Sample name
Weight (g)
w(Re)/μg.g−1
w(common Os)/ng.g−1
ω(187Re)/μg.g−1
ω(187Os)/ng.g−1
Model age (Ma)
Value
Uncertainty
(2σ)
Value
Uncertainty
(2σ)
Value
Uncertainty
(2σ)
Value
Uncertainty
(2σ)
Value
Uncertainty
(2σ)
498.3
4
0.1015
0.1609
313.2
2.5
6.9
170.3
2.3
0.76
168.4
2.7
9
169.7
2.4
10DX99
0.0052
10DX160
0.00518
0.54
0.1188
0.3996
10DX100
0.00564
604.7
5.3
0.0889
0.3984
10DX157
0.00532
143.1
1.1
0.1117
0.7512
10DX181
0.00524
137.1
1.2
0.1131
10DX190
0.00528
121.8
1
3.378
50.07
31.47
380.1
0.34
889.6
88.43
3.3
1076
89.94
0.7
250
2.5
166.6
2.5
0.3803
86.17
0.73
237.3
2.2
165.1
2.5
0.376
76.56
0.62
214.7
2.1
168.1
2.5
Miner Deposita (2014) 49:809–819
813
Table 2 Sample location and petrologic descriptions
Sample No.
Locality
Elevation (above the sea level)
Petrology
10DX89
Tongchang open pit, Dexing deposit
80 m
Porphyritic texture; greenish; phenocrysts contains plagioclase,
quartz, hornblende, biotite; matrix consists of the same
mineral assemblage; feldspar phenocryst and matrix
were sometimes replaced by secondary K-feldspar and biotite
10YS10
Jiuqu open pit of the Yinshan deposit
48 m
Porphyritic texture; gray in color; primary phenocrysts including
feldspar, biotite and minor hornblende were strongly replaced
by hydrothermal sericite
intrusion of porphyry stocks and effectively continues until
the intrusion and country rocks have reached thermal equilibrium. Adopting 750 °C as the magmatic temperature (with the
timing of crystallization provided by the zircon U–Pb closure
temperature) and 200 °C for the ZHe closure temperature, the
Fig. 3 Graphs of weighted
average zircon (a) and apatite (b)
U–He ages for the Dexing and
Yinshan deposits
cooling rates of typical shallow level porphyry deposits may
range from several hundred to several thousand °C per million
years (°C/Ma; McInnes et al. 2005a; Harris et al. 2008; Li
et al. 2012, 2014). Using these temperature points for the
Dexing and Yinshan deposits, however, yields respective
814
Miner Deposita (2014) 49:809–819
Table 3 (U–Th)/He thermochronology results
Apatite
Zircon
Apatite
Zircon
232
Th
(ng)
Th
error
(%)
238
U
(ng)
U
error
(%)
10YS10-1
0.33
4.00
0.08
4.12
19.4
10YS10-2
0.30
4.00
0.14
4.17
24.5
10YS10-3
0.10
4.01
0.02
4.01
U
(ppm)
7.04
1
sigma
Th
(ppm)
1
sigma
0.8
77.6
3.1
He
(ncc)
He
error
(%)
TAU
(%)
Th/U
0.43
2.83
4.0
4.0
Unc.
age
22.26
±1σ
Ft
0.90
0.73
Corrected
age (Ma)
30.39
±1σ
1.37
1.0
52.3
2.1
0.57
2.64
4.1
2.1
22.35
0.91
0.76
29.47
1.33
0.28
30.0
1.2
0.12
3.86
4.8
4.3
20.58
0.99
0.71
28.96
1.50
10YS10-1
0.93
4.58
2.35
3.04
782
24
307
14
33.0
2.5
3.8
0.39
104.71
3.95
0.75
140.29
5.99
10YS10-2
1.33
4.58
3.40
3.00
942
28
366
17
45.1
2.5
3.8
0.39
99.10
3.72
0.76
129.91
5.53
1358
47
671
31
34.4
10YS10-3
1.37
4.58
2.75
3.49
2.6
4.1
0.49
91.37
3.74
0.71
128.14
5.84
10DX89-1
0.86
4.00
0.39
4.30
34.1
1.5
74.6
3.0
0.43
2.84
4.2
2.2
5.96
0.25
0.80
7.42
0.32
10DX89-2
0.33
4.00
0.13
4.04
19.6
0.8
50.7
2.0
0.15
2.96
4.2
2.6
6.07
0.25
0.75
8.09
0.37
10DX89-3
0.50
4.00
0.21
4.26
3.00
4.3
2.4
6.92
0.30
0.79
8.73
0.41
10DX89-1
1.60
4.58
2.50
3.05
768
23
487
22
31.8
2.5
3.7
0.64
90.46
3.34
0.75
120.50
5.06
10DX89-2
1.96
4.58
2.93
3.08
759
23
504
23
37.3
2.5
3.7
0.66
89.63
3.32
0.76
117.34
4.94
10DX89-3
1.44
4.58
1.83
3.02
697
21
544
25
20.9
2.6
3.7
0.78
78.99
2.92
0.72
109.31
4.60
10DX89-4
1.33
4.58
1.71
3.02
1031
31
795
36
19.1
2.5
3.6
0.77
76.94
2.81
0.69
111.18
5.25
22.4
1.0
53.4
2.1
0.28
Note: U and Th ppm are approximate. They are calculated from microscopy grain measurements and assumed mineral densities (3.16 and 4.65 g/cc for apatite and
zircon, respectively)
cooling rates of 10 °C/Ma and ~15 °C/Ma, which are 1 to 2
orders of magnitude lower than these typical figures.
This apparent discrepancy can be resolved in either of
two ways—the original helium ages defining postemplacement cooling could have been reset by later reheating of the system, or the currently exposed porphyry
systems may have developed deeper than typical for economic porphyry systems, at ambient conditions hotter than
the ZHe closure temperature. The geological context of
the Dexing system seems to favor the first of these alternatives—with evidence pointing to multiple episodes of
significant post-emplacement heating of the porphyry
body. Intrusion of a suite of quartz diorite dykes at
~154 Ma and K–Ar ages of K-feldspar (152–157 Ma)
from quartz–K-feldspar veins support the introduction of
significant heat at this time (Fig. 4a; Table 4 in Appendix
II). For lower temperature constraints, however (ca.
200 °C for ZHe and 150 °C for Ar in illite—Hamilton
et al. 1989), this event is overprinted at approximately
110 Ma, pointing to partial to complete loss of radiogenic
He and Ar in the late Cretaceous. At Yinshan too, a small
but distinctive volume of andesite porphyry formed after
the main body at ~166 Ma. As at Dexing, these intrusions
may have introduced significant heat, overprinting the
primary cooling ages of the system (Fig. 4b; Table 4 in
Appendix II). The thermal bounds of such reheating are
highly under-constrained, but we suggest the temperatures
experienced may have been sufficient to reset ZHe ages
for the porphyry.
The alternative of emplacement at ambient temperatures
near or above the blocking temperature for helium in zircon
(as at the Ciemas Cu deposit, Indonesia; McInnes et al. 2005a,
2005b) is considered unlikely for these systems—particularly
Yinshan. Although high homogenization pressures (corresponding to minimum paleo-depths of ~6 km) were obtained
from fluid inclusions within early A type veins for Dexing
(Pan et al. 2009; our unpublished data), this still corresponds
to background conditions of only ca. 150 °C (assuming a
geothermal gradient of 25 °C/km)—high enough to perhaps
slow post-emplacement cooling somewhat, but still nowhere
near the levels required to maintain open system ZHe behavior
for the 40 to 50 million years indicated. Such a mechanism is
even less tenable for the Yinshan system. Geological and fluid
inclusion evidence point to the Yinshan deposit as the shallow
manifestation of a deeper porphyry system, with formation of
the exposed elements of mineralization limited to <4 km depth
(Wang et al. 2013)—far too shallow to account for delayed
ZHe closure.
Exhumation
Late stage cooling that cannot be associated with intrusive
emplacement, hydrothermal veining, or other mechanism of
significant heat flux is most likely to represent progressive
solid state cooling during unroofing and exhumation
(McInnes et al. 2005a). AHe dating provides the clearest guide
to the timing of such late exhumation, with its closure at low
nominal temperatures of around 70 °C (Farley 2000) affording
the greatest possible distinction from typical hydrothermal and
other near-surface activity (McInnes et al. 1999, 2005a;
Arehart et al. 2003; Chakurian et al. 2003; Harris et al.
2008; Li et al. 2012; Zeng et al. 2013).
Miner Deposita (2014) 49:809–819
815
Fig. 4 Graphs of possible magmatic to hydrothermal cooling paths for
the Dexing (a) and Yinshan (b) deposits; and possible exhumation
cooling path for both of Dexing and Yinshan (c). Inset is adapted from
the Fig. 2 of Wolf et al. (1998) illustrating an apatite helium partial
retention zone (HePRZ) by isothermal holding for 50 Ma. Note that the
cooling paths are constructed in a qualitative sense, and thus, cannot be
used quantitatively to determine temperatures at a given time. Age data
are from Table 4. GdP granodiorite porphyry, QdD quartz diorite porphyry dyke, QP quartz porphyry, DP dacite porphyry, AP andesite
porphyry, zir. zircon, moly. molybdenite, Kfs. K-feldspar, ill. illite, bio.
biotite, MHC magmatic to hydrothermal cooling, ZHe zircon helium age,
AHe apatite helium age
Perhaps the most significant distinction between the
Yinshan and Dexing geo-thermochronometry data sets is
the ~20-million-year difference between the AHe ages of
the two deposits. Assuming that the Dexing region has an
average continental geothermal gradient of around 25 °C/km
and a mean annual surface temperature of 20 °C, these
apatite (U–Th)/He ages define the time when each deposit
was eroded to within 2 km of the paleo-surface (McInnes
et al. 1999, 2005a, b). The older AHe age of the Yinshan
deposit is consistent with some combination of earlier,
slower, and/or shallower exhumation relative to the Dexing
deposit. The relative contributions of these three variables
are impossible to directly assess, but the comparative geology of the two deposits favors the third factor—differential
exhumation—as a significant contribution.
Dexing contains typical porphyry mineralization consistent
with emplacement at depths of 2–5 km, whereas the veining
and alteration present at Yinshan argues for a mineral endowment developed at shallower crustal levels. The shallower
Yinshan deposit may have sat within the partial retention
zone for helium in apatite (Fig. 4c inset; the range of
crustal depths corresponding to temperatures between ca.
40 °C and 70 °C, under which conditions diffusion of
helium is just fast enough that a portion of that daughter
product created by radioactive decay of U and Th is
retained on geological timescales) for an extended period
leading up to final exhumation. On the contrast, the
deeper Dexing deposit may have remained below this
depth and experiencing complete loss of its radiogenic
helium. Therefore, the variable depth alone could account
for the difference in AHe age (Fig. 4c). Subsequent exhumation and cooling of the two deposits at or around
8 Ma would be recorded relatively faithfully as an event
in the Dexing Deposit—the AHe age of which was held
at zero prior to this final phase of cooling—but the helium
built up within apatite at Yinshan during its residence in
the PRZ would result in older apparent ages, despite
experiencing final uplift and cooling at the same time.
This paper
29.6±1.6
Li et al. (2005)
114.4±4.8
8.0±1.6
230
200
150
75
K-feldspar K–Ar
ZHe
Illite K–Ar
AHe
Reiners (2005)
112
300
Illite Ar–Ar
Hamilton et al. (1989)
This paper
Zhu et al.(1983)
154.5±10
Quartz–K-feldspar vein
Granodioritic porphyry with
slight potassic alteration
Granodioritic porphyry with
phyllic alteration
Granodioritic porphyry with
slight potassic alteration
500
Molybdenite Re–Os
Reiners (2005)
Zhu et al.(1983)
This paper
This paper
Zhou et al. (2012b)
101–107
170.3±2.6
Pyrite–chalcopyrite–quartz vein
Suzuki et al. (1996)
Molybdenite–pyrite–quartz vein
Dacitic porphyry with
phyllic alteration
Dacitic porphyry with
phyllic alteration
Dacitic porphyry with
phyllic alteration
Porphyry stocks
with phyllic alteration
122–125
175–178
132.6±6.7
This paper
Li et al. (2007)
Wang et al. (2012a)
166+/−1
Zhou et al. (2012a)
153.5±2.4
Quartz dioritic dyke
Andesitic porphyry
170–183
Reiners (2005)
750
Zircon U–Pb
Reference
Liu et al.(2012)
172.5±0.5
Fresh granodioritic porphyry
Porphyry stocks with
phyllic alteration
Age (Ma)
Host rock
Host rock
Value
Reference
Age (Ma)
Dexing
Yinshan
Exploration implications
Tc used (°C)
Chronometer
Table 4 Summary of geochronological and thermochronological data from the Dexing and Yinshan deposits
Li et al. (2007);
Wang et al. (2012a);
Liu et al. (2013)
Miner Deposita (2014) 49:809–819
Reference
816
Diamond drilling has revealed that the wide Cu–Au bearing
sulfide veins at Yinshan gradually transition into smaller veins
and disseminated ore at depth (JGEB Jiangxi Geological
Exploration Bureau 1996). Recent geochemical, geochronological, and isotopic studies have demonstrated that the dacite
porphyries at Yinshan and granodiorite porphyries at Dexing
were emplaced contemporaneously, and probably, originated
from a similar source (Liu et al. 2012, 2013; Wang et al.
2012a). These characteristics indicate that the ore-forming
magmas at Yinshan have a similar potential for forming
porphyry-style copper mineralization to that of Dexing. The
differential exhumation between the two ore deposits in the
Late Cenozoic indicated by our (U–Th)/He data led to the
exposure of the Dexing porphyry ore bodies, while equivalent
mineralization at Yinshan would remain concealed at depth,
supporting the hypothesis that a porphyry Cu–Au deposit may
exist below the current Yinshan mine.
In addition, our study indicates that the Middle Jurassic
porphyry copper systems of the Dexing region may have been
preserved under relatively stable crustal conditions for a
protracted period. Such favorable preservation conditions
may arise from the post-Jurassic tectonically quiescent nonarc or intraplate tectonic setting of the Dexing region (Wang
et al. 2006; Liu et al. 2012). Such intraplate regions could be
prospective for porphyry Cu systems like Dexing and
Yinshan, if post-subduction remelting of the pre-fertilized
lithosphere has occurred (Richards 2011).
Conclusions
The cooling ages of the Dexing copper deposit post-date
primary crystallization of the major associated porphyry system. In light of the relatively deep primary intrusion of this
body, recorded zircon (U–Th)/He ages may reflect post emplacement exhumation, but independent evidence for later
intrusive activity and late mineralization at ~154 and
~105 Ma indicates these ZHe ages most likely reflect thermal
re-setting of the system. The recorded cooling ages of the
Yinshan polymetallic deposit may similarly reflect disturbance and resetting of early ingrowth of helium by an intrusive event at ~166 Ma and/or by burial heating in the Early
Cretaceous.
Subsequent to these Jurassic–Cretaceous thermal excursions, both the Dexing and Yinshan deposits appear to have
experienced relative stasis within or immediately below the
apatite helium partial retention zone (ca. 40–70 °C). Similarities in timing and mineralogy between the two deposits
suggest that their differences in style and tenor may arise
largely from differential exhumation—with the Yinshan
Miner Deposita (2014) 49:809–819
deposit a less-exhumed equivalent of the Dexing deposit. It
follows that porphyry-style mineralization similar to that at
Dexing may exist at depth under the current Yinshan mine.
817
(molybdenite from the Jinduicheng ore deposit, Du et al.
2004).
(U–Th)/He thermochronology
Acknowledgements We wish to thank Allen Thomas and Cameron
Scadding (TSWTM Analytical) for assistance with ICPMS analysis and
Celia Mayers and Brad McDonald for grain selection for
thermochronology. Kind help from Wenjun Qu and Chao Li during
molybdenite Re–Os analysis is gratefully acknowledged. Dr. Junxing
Zhao provided useful discussion and comments. Dr. Diego Villagomez,
an anonymous reviewer, and the Associate Editor Thomas Bissig are
thanked for their constructive and valuable comments which greatly
contributed to improvement of the manuscript. This research was financially supported by the Student Research Grant (Hugh E. McKinstry
Fund of 2012) of the Society of Economic Geologists granted to X.
Liu, and the Intellectual Innovation Project, Chinese Academy of Sciences (KZCX1-YW-15-3).
Appendix
Methodology
Re–Os geochronology
Molybdenite Re–Os isotopic analysis on samples from the
Dexing deposit was conducted at the National Research Center for Geoanalysis (NRCG), Chinese Academy of Geological
Sciences. Analytical conditions and analytical procedures are
the same as those described by Yang et al. (2005). Re and Os
concentrations were determined by isotope dilution. The
185
Re and 190Os spikes were obtained from Oak Ridge National Laboratory, USA, and calibrated at NRCG. Molybdenite was separated from six molybdenite–pyrite–quartz veins
with more than 1.0 g molybdenite selected from each vein.
Accurately weighed 0.2 g aliquots were selected from the
<200 mesh material, loaded into Carius tubes, together with
185
Re and 190Os spikes, and digested by reverse aqua regia. Os
was separated as OsO4 by distillation at 105–110 °C and
trapped by Milli-Q water. The residue solutions were diluted
with distilled water and dried. The solids were then dissolved
in a 5 M NaOH solution, from which Re was extracted by
acetone.
Re and Os isotopic compositions were determined using an
Inductively Coupled Plasma Mass-spectrometer (ICP-MS, PQ
Excell). The intensity of the 190Os signal was monitored to
correct for trace Os in Re, and 185Re was used to monitor trace
Re in Os. 5 % ammonia; and H2O2 were repeatedly used to
wash the Teflon injection tube in order to avoid crosscontamination. The total procedural blanks were 20 to 30 pg
for Re and 1 pg for Os, based on blanks analyzed with the
samples. The background 187Os/188Os was 0.25 based on
routine repeated blank measurement. The accuracy of the
analysis was controlled by laboratory reference sample JDC
Zircon and apatite (U–Th)/He thermochronology was performed in the John de Laeter Center for Isotope Research,
Curtin University. Euhedral zircon and apatite grains were
selected under a long working distance binocular microscope.
Individual grains were visually interrogated in plain and crosspolarized light in order to detect and remove material containing U- and/or Th-rich mineral or fluid inclusions that may
contribute excess helium. Measurements were recorded for
each grain for calculation of the alpha correction (Farley et al.
1996), and images of selected grains were recorded digitally
(Fig. 4). Grains with a diameter greater than 70 μm were
preferred where possible in order to ensure helium gas values
were optimal for measurement and to minimize the alpha
ejection correction. Characterized grains were loaded into
niobium (zircon) or platinum (apatite) microvials. Helium
was thermally extracted from each individual encapsulated
crystal using a 1,064-nm Nd-YAG laser. 4He abundances were
determined by isotope dilution method using a pure 3He spike,
calibrated daily against an independent 4He standard tank.
Uncertainty in the sample 4He measurement was <1 %.
For zircon, the U and Th content were determined using
isotope dilution ICP-MS. Samples were removed from the
laser chamber and transferred to Parr pressure dissolution
vessels where they were spiked with 235U and 230Th (25 μl
of a solution containing 15 ppb 235U and 15 ppb 230Th) and
digested at 240 °C for 40 h in 350 μl of HF. Standard solutions
were spiked and treated similarly, as were a series of unspiked
reagent blanks. After dissolution, samples were removed from
the pressure vessels and allowed to dry for 2 days. Three
hundred microliters of HCl was added to each vial, which
was then subjected to a second bombing for 24 h at 200 °C to
ensure dissolution of fluoride salts. Final solutions were diluted to 10 % acidity for analysis on an Agilent 7500CS mass
spectrometer (TSW™ Analytical). For single crystals digested
in small volumes (0.3–0.5 ml), U and Th isotope ratios were
measured to a precision of <2 % (Evans et al. 2005). Repeated
measurement of internal age standards by zircon (U–Th)/He
methods at Curtin has an estimated precision of <6 %.
For degassed apatite, the U and Th content were determined
by isotope dilution using 235U and 230Th spikes. Twenty-five
microliters of a 50 % (by volume; approximately 7 M) HNO3
solution containing approximately 15 ppb 235U and 5 ppb 230Th
was added to each sample. The apatite was digested in the spiked
acid for at least 12 hours to allow the spike and sample isotopes
to equilibrate. Standard solutions containing 27.6 ppb U and
28.4 ppb Th, were spiked and treated identically to samples, as
were a series of unspiked reagent blanks. Two hundred fifty
microliters of Milli-Q water was added prior to analysis on an
818
Agilent 7500CS mass spectrometer. U and Th isotope ratios
were measured to a precision of <2 %. Overall apatite (U–Th)/
He thermochronology analysis at Curtin has a precision of
2.5 %, based on multiple age determinations (n=26) of Durango
standard which produce an average age of 31.1±1.0 (2σ)Ma.
References
Arehart GB, Chakurian AM, Tretbar DR, Christensen JN, McInnes BA,
Donelick AR (2003) Evaluation of radioisotope dating of Carlintype deposits in the Great Basin, Western North America, and
implications for deposit genesis. Econ Geol 98:235–248
Chakurian AM, Arehart GB, Donelick RA, Zhang X, Reiners PW (2003)
Timing constraints of gold mineralization along the Carlin Trend
utilizing apatite fission-track, 40Ar/39Ar, and apatite (U-Th)/He
methods. Econ Geol 98:1159–1171
Cherniak DJ, Watson EB (2000) Pb diffusion in zircon. Chem Geol 172:
5–24
Du AD, Wu SQ, Sun DZ, Wang SX, Qu WJ, Markey R, Stein H, Morgan
J, Malinovskiy D (2004) Preparation and certification of Re-Os
dating reference materials: molybdenite HLP and JDC. Geostand
Geoanal Res 28:41–52
Evans NJ, Wilson NSF, Cline JS, McInnes BIA, Byrne J (2005) Fluorite
(U-Th)/He thermochronology: constraints on the low temperature
history of Yucca Mountain, Nevada. Appl Geochem 20:1099–1105
Farley KA (2000) Helium diffusion from apatite: general behavior as
illustrated by Durango fluorapatite. J Geophys Res 105:2903–2914
Farley KA, Stöckli DF (2002) (U-Th)/He dating of phosphates: apatite,
monazite, and xenotime. Rev Mineral Geochem 48:559–577
Farley KA, Wolf RA, Silver LT (1996) The effects of long alpha-stopping
distances on (U-Th)/He ages. Geochim Cosmochim Ac 60:4223–4229
Hamilton PJ, Kelley S, Fallick AE (1989) K-Ar dating of illite in hydrocarbon reservoirs. Clay Miner 24:215–231
Harris A, Dunlap WJ, Reiners PW, Allen CM, Cooke DR, White NC,
Campbell IH, Golding SD (2008) Multimillion year thermal history
of a porphyry copper deposit: application of U–Pb, 40Ar/39Ar and
(U–Th)/He chronometers, Bajo de la Alumbrera copper–gold deposit, Argentina. Miner Deposita 43:295–314
Hou Z, Pan X, Li QY, Yang ZM, Song YC (2013) The giant Dexing
porphyry Cu-Mo-Au deposit in east China: product of melting
of juvenile lower crust in an intracontinental setting. Miner
Deposita 1–27
Hua RM, Li XF, Lu JJ, Chen PR, Qiu DT, Wang G (2000) Study on the
tectonic setting and ore-forming fluids of Dexing large oreconcentrating area, northeast Jiangxi province. Adv Earth Sci 15:
525–533, in Chinese with English abstract
Huang CK, Bai Y, Zhu YS, Wang HZ, Shang XZ (2001) Copper deposits
of China. Geological Publishing House, Beijing, pp 57–72 (in
Chinese with English abstract)
JGB (Jiangxi Geological Bureau) (1980) Report on regional geology of
the People’s Republic of China (Leping area of Jiangxi Province).
Geological Publishing House, Beijing, pp 104–123 (in Chinese with
English abstract)
JGEB (Jiangxi Geological Exploration Bureau) (1996) Yinshan
Cu-Pb-Zn-Au-Ag deposit in Jiangxi Province. Geological
Publishing House, Beijing, p 380 (in Chinese with English
abstract)
Kesler SE, Wilkinson BH (2006) The role of exhumation in the temporal
distribution of ore deposits. Econ Geol 101:919–922
Li XF, Wang C, Mao JW, Hua RM, Liu YN, Xu QH (2005) Kübler index
and K-Ar ages of illite in the Yinshan polymetallic deposit, Jiangxi
Miner Deposita (2014) 49:809–819
province, south China: Analyses and implications. Resour Geol 55:
397–404
Li XF, Watanabe Y, Mao JW, Liu SX, Yi XK (2007) Sensitive HighResolution Ion Microprobe U-Pb zircon and 40Ar-39Ar muscovite
ages of the Yinshan deposit in the northeast Jiangxi province, South
China. Resour Geol 57:325–337
Li XH, Li WX, Li ZX, Lo CH, Wang J, Ye MF, Yang YH (2009)
Amalgamation between the Yangtze and Cathaysia Blocks in
South China: constraints from SHRIMP U-Pb zircon ages, geochemistry and Nd-Hf isotopes of the Shuangxiwu volcanic rocks.
Precambrian Res 174:117–128
Li JX, Qin KZ, Li GM, Cao MJ, Xiao B, Chen L, Zhao JX, Evans NJ,
McInnes BIA (2012) Petrogenesis and thermal history of the Yulong
porphyry copper deposit, Eastern Tibet: insights from U-Pb and UTh/He dating, and zircon Hf isotope and trace element analysis.
Mineral Petrol 105:201–221
Li GM, Cao MJ, Qin KZ, Evans NJ, McInnes BIA, Liu YS (2014)
Thermal-tectonic history of the Baogutu porphyry Cu deposit,
West Junggar as constrained from zircon U–Pb, biotite Ar/Ar and
zircon/apatite (U–Th)/He dating. J Asian Earth Sci 79:741–758
Liu X, Fan HR, Santosh M, Hu FF, Yang KF, Li QL, Yang YH, Liu
YS (2012) Remelting of Neoproterozoic relict volcanic arcs in
the Middle Jurassic: implication for the formation of the
Dexing porphyry copper deposit, Southeastern China. Lithos
150:85–100
Liu X, Fan HR, Santosh M, Hu FF, Yang KF, Wen BJ, Yang YH, Liu YS
(2013) Origin of the Yinshan epithermal-porphyry Cu-Au-Pb-ZnAg deposit, SE China: insights from geochemistry, Sr-Nd and zircon
U-Pb-Hf-O isotopes. Int Geol Rev 55:1835–1864
McInnes BIA, Farley KA, Sillitoe RH, Kohn BP (1999) Application
of apatite (U-Th)/He thermochronometry to the determination
of the sense and amount of vertical fault displacement at the
Chuquicamata porphyry copper deposit, Chile. Econ Geol 94:
937–947
McInnes BIA, Evans NJ, Fu FQ, Garwin S (2005a) Application of
thermochronology to hydrothermal ore deposits. Rev Mineral
Geochem 58:467–498
McInnes BIA, Evans NJ, Fu FQ, Garwin S, Belousova E, Griffin WL,
Bertens A, Sukarna D, Permanadewi S, Andrew RL, Deckart K
(2005b) Thermal history analysis of select Chilean, Indonesian and
Iranian porphyry Cu-Mo-Au deposits. In: Porter TM (ed) Super
porphyry copper and gold deposits—a global perspective. PGC
Publishing, Adelaide, pp 27–42
Pan XF, Song YC, Wang SX, Li ZQ, Yang ZM, Hou ZQ (2009)
Evolution of hydrothermal fluid of Dexing Tongchang coppergold porphyry deposit. Acta Geol Sin 12:1929–1950 (in Chinese
with English abstract)
Reiners PW (2005) Zircon (U-Th)/He thermochronometry. Rev Miner
Geochem 58:151–179
Reiners PW, Farley KA (1999) He diffusion and (U-Th)/He
thermochronometry of titanite. Geochim Cosmochim Ac 63:3845–
3859
Richards JP (2011) Magmatic to hydrothermal metal fluxes in convergent
and collided margins. Ore Geol Rev 40:1–26
Seedorff E, Dilles JH, Proffett JM, Jr., Einaudi MR, Zurcher L,Stavast
WJA, Johnson DA, Barton MD (2005) Porphyry copper deposits:
characteristics and origin of hypogene features. Econ Geol
Economic Geology 100th Anniversary Volume: 251–298
Selby D, Creaser RA (2001) Re-Os geochronology and systematics in
molybdenite from the Endako porphyry molybdenum deposit,
British Columbia, Canada. Econ Geol 96:197–204
Suzuki K, Shimizu H, Masuada A (1996) Re-Os dating of molybdenites
from ore deposits in Japan: Implication for the closure temperature
of the Re-Os system for molybdenite and the cooling history of
molybdenum in ore deposits. Geochim Cosmochim Ac 60:3151–
3159
Miner Deposita (2014) 49:809–819
Wang Q, Xu JF, Jian P, Bao ZW, Zhao ZH, Li CF, Xiong XL, Ma JL
(2006) Petrogenesis of adakitic porphyries in an extensional tectonic
setting, Dexing, South China: implications for the genesis of porphyry copper mineralization. J Petro 47:119–144
Wang XL, Zhao GC, Zhou JC, Liu YS, Hu J (2008) Geochronology and
Hf isotopes of zircon from volcanic rocks of the Shuangqiaoshan
Group. South China: implications for the Neoproterozoic tectonic
evolution of the eastern Jiangnan orogen. Gondwana Res 14:355–367
Wang GG, Ni P, Zhao KD, Wang XL, Liu JQ, Jiang SY, Chen H (2012a)
Petrogenesis of the Middle Jurassic Yinshan volcanic-intrusive complex, SE China: implications for tectonic evolution and Cu-Au
mineralization. Lithos 150:135–154
Wang W, Zhou MF, Yan DP, Li JW (2012b) Depositional age, provenance, and tectonic setting of the Neoproterozoic Sibao Group,
southeastern Yangtze Block, South China. Precambrian Res 192–
195:107–124
Wang GG, Ni P, Wang RC, Zhao KD, Chen H, Ding JY, Zhao C, Cai YT
(2013) Geological, fluid inclusion and isotopic studies of the
Yinshan Cu-Au-Pb-Zn-Ag deposit, South China: Implications for
ore genesis and exploration. J Asian Earth Sci 74:343–360
Wolf RA, Farley KA, Silver LT (1996) Helium diffusion and lowtemperature thermochronometry of apatite. Geochim Cosmochim
Ac 60:4231–4240
819
Wolf RA, Farley KA, Kass DM (1998) A sensitivity analysis of the
apatite (U-Th)/He thermochronometer. Chem Geo 148:105–114
Yang G, Du AD, Lu JR, Qu WJ, Chen JF (2005) Re-Os (ICP-MS) dating
of the massive sulfide ores from the Jinchuan Ni-Cu-PGE deposit.
Sci China Ser D 48:1672–1677
Zeng Q, Evans NJ, McInnes BIA, Batt GE, McCuaig CT, Bagas L,
Tohver E (2013) Geological and thermochronological studies of
the Dashui gold deposit, West Qinling Orogen, Central China.
Miner Deposita 48:397–412
Zhou Q, Jiang YH, Zhao P, Liao SY, Jin GD (2011) Origin of the Dexing
Cu-bearing porphyries, SE China: elemental and Sr-Nd-Pb-Hf isotopic constraints. Int Geol Rev 54:572–592
Zhou Q, Jiang YH, Liao SY, Zhao P, Jin GD, Jia RY, Liu Z, Xu XS
(2012a) SHRIMP zircon U-Pb dating and Hf isotope studies of the
diorite porphyrite from the Dexing copper deposit. Acta Geol Sin
86:1726–1734 (in Chinese with English abstract)
Zhou Q, Jiang YH, Zhao P, Liao SY, Jin GD, Liu Z, Jia RY (2012b)
SHRIMP U-Pb dating on hydrothermal zircons: evidence for an early
Cretaceous epithermal event in the middle Jurassic Dexing porphyry
copper deposit, southeast China. Econ Geol 107:1507–1514
Zhu X, Hang CK, Rui ZY, Zhou YH, Zhu XJ, Hu CS, Mei ZK (1983) The
geology of Dexing porphyry copper ore field. Geological Publishing
House, Beijing, p 336, in Chinese with English abstract
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