Cooling and exhumation of the oldest Sanqiliu uranium ore ... · Province, China, consists of...

Cooling and exhumation of the oldest Sanqiliu uranium oresystem in Motianling district, South China Block

Liang Qiu,1,2 Dan-Ping Yan,1 Shuang-Li Tang,1 Nicholas T. Arndt,3 Li-Ting Fan,4 Qing-Yin Guo5 andJian-Yong Cui61State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geo-

sciences, Beijing 100083, China; 2Department of Geoscience, University of Nevada, Las Vegas NV 89154, USA; 3ISTerre, Universit�e de

Grenoble, Grenoble, France; 4No.230 Research Institute of Nuclear Industry, Changsha 410011, China; 5China Nuclear Geology, Beijing

100013, China; 6Beijing Research Institute of Uranium Geology, Beijing 100029, China

ABSTRACT

The Sanqiliu uranium deposit belongs to a uranium ore sys-

tem in Motianling district. It is the oldest uranium deposit in

South China. Primary uranium mineralization occurred almost

simultaneously with the emplacement of the host granites

and subsequent dykes, and it has a relatively high grade of

uranium (0.421%). We clarify the age of mineralization and

investigate the cooling history through new pitchblende U–Pband apatite fission-track thermochronology. The pitchblende

U–Pb results indicate that uranium mineralization occurred at

~801–759 Ma. Fractionation of uranium and lead at ~374–295 Ma is interpreted as remobilization and resetting of the

original uranium. The Motianling area has apatite fission-track

ages of 57 to 18 Ma. By combining our results with previous

work, we conclude that the deposit cooled slowly and was

exposed at the surface during the Cenozoic. The timing and

depth of exhumation helped to preserve and avoid erosion of

the uranium deposit, and highlight the potential for regional

uranium exploration.

Terra Nova, 27, 449–457, 2015

Introduction

Uranium deposits exhibit extremediversity because they form innumerous geological environments,such as superficial settings, duringdiagenesis, or in plutonic, volcanic,metasomatic, hydrothermal or high-grade metamorphic conditions (e.g.Dahlkamp, 1993; Cuney, 2009).Vein-type uranium deposits, whichcontain the second biggest uraniumresources, are mainly formed byhydrothermal processes during defor-mation at depths of c. 1–10 km inthe upper crust (Cuney, 2009). Theirpresence at the surface implies anadditional tectonic evolution orexhumation process (Kesler andWilkinson, 2006). Dating uraniummineralization and tracing the post-mineralization exhumation of aprospective uranium deposit thuscontribute to our knowledge of howvein-type uranium deposits form.The closure temperature (Tc) of the

U–Pb system in pitchblende is ~250–300 °C (Zhao et al., 2004), whereas

for apatite fission tracks, it is ~110–60 °C (Laslett et al., 1987). A pitch-blende U–Pb age therefore recordsthe timing of uranium mineralizationand associated hydrothermal circula-tion, whereas an apatite fission-trackage relates to thermal collapse andunroofing and/or to a later hydrother-mal event. It follows that integratedtectono- and thermo-chronologicaldata can constrain the timing of bothuranium mineralization and post-min-eralization exhumation (McInneset al., 2005; Liu et al., 2014). More-over, in combination with otherchronometers such as zircon U–Pb(Tc = ~900 °C for igneous zircon,Cherniak and Watson, 2001;Tc = ~120–200 °C for hydrothermalzircon, Hoskin, 2005) and mica andfeldspar 40Ar/39Ar (biotite Tc = ~250–350 °C; muscovite Tc = ~350–450 °C;K-feldspar Tc = ~350–450 °C; Hackeret al., 2009), these thermochronome-ters enable a temperature and timepath of the ore system to beconstructed.The vein-type hydrothermal San-

qiliu deposit in northern GuangxiProvince, China, consists of pitch-blende ores hosted by granitoids andmetasedimentary rocks (Li, 1986; Huet al., 2008). This deposit (1) con-tains the oldest uranium ore in SouthChina (Zhang et al., 1984), (2) has

similar ages for the host rocks andore bodies (Lai, 1982; Zhang et al.,1984; Xue, 1988) and (3) has a rela-tively high grade of uranium(0.421%; Hu et al., 2008). Here, wepresent pitchblende U–Pb and apa-tite fission-track results to constructthe cooling and exhumation historyof the uranium deposit.

Regional geology

The South China Block consists ofthe Cathaysian Block to the south-east and the Yangtze Block to thenorthwest, separated by the Jiang-nan Fold Belt (Fig. 1a; Yan et al.,2003; Wang and Zhou, 2012). Thebasement rocks of the southernJiangnan Fold Belt are dominatedby two units: (1) the Sibao Group,which consists of early Neoprotero-zoic epimetamorphic sandy-argilla-ceous detrital flysch and volcanicintercalations (BGMRGX, 1985;Wang et al., 2006; Zhao and Asi-mow, 2014); and (2) the DanzhouGroup, a sequence of metamor-phosed late Neoproterozoic pelitic-silty flysch (BGMRGX, 1985; Wangand Zhou, 2012). Locally, the SibaoGroup is intruded by early Neopro-terozoic granitoid plutons andunconformably overlain by lateNeoproterozoic to Carboniferous

Correspondence: Dan-Ping Yan, State

Key Laboratory of Geological Processes

and Mineral Resources, China University

of Geosciences, Beijing 100083, China.

Tel.: +86 13910571865; e-mail: yandp@-

cugb.edu.cn

© 2015 John Wiley & Sons Ltd 449

doi: 10.1111/ter.12179

strata (Zhao et al., 2013). TheMotianling uranium district islocated in the southernmost part ofthe Jiangnan Fold Belt (Fig. 1a;Zhao et al., 2011).

The oldest structures in theMotianling district and adjacentregions are NNE-trending foldsdeveloped in the Sibao Group; thesepre-date the intrusion of the elon-

gated Sanfang pluton (BGMRGX,1985; Fig. 1b). The folds were over-printed by ESE-dipping shear zonesand WNW-dipping normal faults,produced by Carboniferous and

(b) (a)

(c)

(d)

Fig. 1 (a) Simplified sketch of the South China Block showing the Yangtze Block, the Cathaysia Block and the Jiangnan FoldBelt. (b) Geologic map of the Motianling district in the southern part of the Jiangnan Fold Belt, showing the structures, rockunits and locations of the uranium deposits. (c) Detailed geologic map and (d) cross section of the Sanqiliu uranium deposit,illustrating structural controls on the vein-type ore bodies observed and showing the locations of samples used for U–Pb dating.

450 © 2015 John Wiley & Sons Ltd

Cooling and exhumation of uranium ore system • L. Qiu et al. Terra Nova, Vol 27, No. 6, 449–457

.............................................................................................................................................................

Cretaceous extensions respectively. Aseries of top-to-the-SE thrust faultscuts all earlier structures (BGMRGX, 1985).

Deposit geology

The Sanqiliu deposit is hosted byepimetamorphic basement rocks andthe fine- to medium-grained biotitegranite of the southwestern Sanfangpluton (Fig. 1). The deposit containsmore than 100 individual orebodies,which occur as stringers, veins andlenses. The orebodies are mainly inthe north-east-striking fault zone, butalso occur in the surroundingmetasedimentary rocks of the SibaoGroup. Most are less than one metrethick; they extend 40–80 m alongstrike and continue to depths of 60–200 m (Fig. 1d).The uranium minerals include

early-stage botryoid and kidney-shaped pitchblende and late-stageconcentric spherulitic pitchblende,which form in conjunction with chlo-rite and pyrite (Zhang et al., 1984).The alteration assemblage is quartz +chlorite + pyrite + haematite + seri-cite + chalcedony. Accessory quartz,pyrite and sericite occur with allstages of mineralization and alter-ation. Uranium mineralization andalteration are distributed withingranitic mylonites of the NNE-trend-ing shear zone (Fig. 1c, d).

Previous chronologic work

The Neoproterozoic felsic complexcomprises numerous intrusive bodies.The Sanfang pluton is the largest,and hosts the Sanqiliu deposit (Li,1999; Zhao et al., 2013). Zircon U–Pb dating of the Sanfang granitoidsyields crystallization ages from794.2 � 8.1 Ma to 835.8 � 2.5 Ma(e.g., Li, 1999; Wang et al., 2006;Zhao et al., 2013). The veins anddykes that include greisen, fine-grained granite, albitite and maficrocks in the pluton produce U–Pbzircon ages of 782.5 � 2.5 Ma and741.8 � 1.9 Ma, and younger agesof 306.7 � 7.1 Ma, 234.2 � 2.0 Maand 85 � 11 Ma (Yu, 2012). Pitch-blendes from the Sanqiliu mineraliza-tion yield a U–Pb age of 293–378 Ma with an isochron age of329 Ma (Zhang et al., 1984). A bio-tite K–Ar age of 393–398 Ma, a Table

1U-Pbisotopic

agedeterminationforpitchblendefrom

theSanqiliu

deposit,South

China

Sample

number

Quan

tity

(mg)

U(lg)

Pb(lg)

206Pb

Measuredlead

isotopic

ratio

Isotopic

ratio

208Pb/206Pb

1r

207Pb/206Pb

1r

204Pb/206Pb

1r

206Pb*/238U

1r

207Pb*/235U

1r

207Pb*/206Pb*

1r

D2

1.2

887

5189.932

0.039362

0.00002

0.070801

0.000014

0.001045

0.000300

0.0587

0.00008

0.4542

0.0070

0.0561

0.00086

D3

1.1

234

2874.346

0.204344

0.00001

0.133104

0.000013

0.004957

0.000070

0.0945

0.00014

0.8183

0.0390

0.0628

0.00300

D5

2.7

2133

103

94.04

0.006249

0.00005

0.056617

0.000015

0.00016

0.0010

0.0527

0.00007

0.3964

0.0040

0.0545

0.00051

D6

2.4

1655

8593.259

0.012223

0.00005

0.059318

0.000017

0.000326

0.00100

0.0556

0.00008

0.4208

0.0050

0.0549

0.00068

D7

1.0

218

9579.242

0.140996

0.00001

0.115399

0.000037

0.003606

0.00010

0.3745

0.00055

3.3296

0.2300

0.0645

0.00450

D9

1.0

634

5289.931

0.038502

0.00001

0.071767

0.000017

0.000964

0.00050

0.0841

0.00012

0.6757

0.0110

0.0583

0.00095

D11

0.6

323

4681.083

0.121005

0.00002

0.107480

0.000010

0.003098

0.00009

0.1273

0.00018

1.1186

0.0360

0.0637

0.00210

D12

2.9

2149

117

93.959

0.006367

0.00005

0.057415

0.000015

0.00016

0.0010

0.0592

0.00008

0.4517

0.0040

0.0554

0.00051

D14

2.2

789

4390.611

0.033712

0.00002

0.068390

0.000013

0.000848

0.00050

0.0570

0.00008

0.4438

0.0060

0.0565

0.00076

D15

1.2

1003

114

73.372

0.214007

0.00001

0.140526

0.000009

0.005598

0.00007

0.0872

0.00012

0.7328

0.0280

0.0609

0.00230

D16

0.5

151

1191.852

0.023214

0.00003

0.064606

0.000013

0.000333

0.00060

0.0748

0.00011

0.6206

0.0070

0.0602

0.00065

Age

calculationconstant:t 0

=4430

Ma,

a0=9.307,

b0=10.294,k 2

35=0.984859

10�9a�

1,k 2

38=0.155125

910

�9a�

1.Samplequantityisan

estim

ated

valueforreference,

butcontents

ofUandPb

aremeasured

values.Instrumenttype:isoprobe-t.


Terra Nova, Vol 27, No. 6, 449–457 L. Qiu et al. • Cooling and exhumation of uranium ore system

............................................................................................................................................................

feldspar K–Ar age of 377–407 Maand a muscovite 40Ar/39Ar age of415–418 Ma from the Sanfang plu-ton record a Caledonian metamor-phic event (Shi, 1976; Wang et al.,2013; Tang et al., 2014). These agesroughly constrain the timing of min-eralization of the uranium ore systemand also the passage through the rel-evant closure temperatures.

Analytical results

Pitchblende U–Pb geochronology

Eleven samples of ore bodies fromthree drill holes in the Sanqiliu depositwere analysed in this study (Table 1;Fig. 1). Analytical procedures aredescribed in the supplementary mate-rial. The samples exhibit variable U(21.3–83.6 wt%) and Pb contents(2.0–9.5 wt%) of pitchblende. Pitch-blende grains show lower and upperintercepts with discordance (Fig. 2).Upper intercepts are at 801 � 57 Maand 759 � 100 Ma, whereas thelower intercepts of eight samples areat 304 � 15 Ma and 295 � 44 Ma(Fig. 2a,b). In addition, three samplesyield an isochron age of 374 � 59 Ma(Fig. 2c).

Apatite fission-trackthermochronology

Eight samples from different altitudes(246–1426 m) in the area have clearlydefined Early Palaeogene ages (be-tween 18 � 1 Ma and 57 � 4 Ma)and similar mean track lengths(12.0–12.6 lm), suggesting that theyexperienced similar uplift histories(Table 2; Fig. 3). The fission-trackcentral ages have a weighted mean of33.5 � 2.8 Ma, and show a positivecorrelation between age and eleva-tion, with a slope, which reflects thelong-term exhumation rate, of~0.02 km Ma�1 between 57 Ma and18 Ma. This estimate is based on theexclusion of two samples (MT23 andMT30) that do not fall on the trend-line, probably because of local ther-mal disturbance or differentialuplifting (Fig. 3).Time–temperature thermal history

modelling results are available for allapatite fission-track samples becausethe K–S test (Kolmogorov–Smirnov)and Age GOF (goodness-of-fit) areeach greater than 0.5 (Fig. 4). The

(a)

(b)

(c)

Fig. 2 U–Pb concordia diagrams for pitchblende samples from the Sanqiliu deposit.(a) and (b) are the concordia ages of the samples from two drill holes. (c) is an iso-chron age of samples from one drill hole. Analytical results are listed in Table 1.The locations of some samples are shown in Fig. 1d.



.............................................................................................................................................................

modelling paths suggest that thesamples experienced: (1) a quiescentphase from ~100 to ~66 Ma and (2)final cooling and exhumation to thesurface in the last 65 Ma. Addition-ally, the samples entered the apatitepartial annealing zone (110–60 °C;Laslett et al., 1987) at ~80–40 Ma,and crossed this zone at mean tem-peratures at ~50–5 Ma.

Discussion

Timing of uranium mineralization

The discordance of the pitchblendeU–Pb age is attributed to (1) the lossof radiogenic lead, which is crystallo-graphically incompatible with pitch-blende, and/or (2) the gain ofradioactive daughters of uranium(Min et al., 1999, 2001). The twoupper U–Pb intercepts of801 � 57 Ma and 759 � 100 Ma areinterpreted as the ages of uraniummineralization of the Sanqiliudeposit; the two lower intercepts of304 � 15 Ma and 295 � 44 Ma arethought to date the Late Carbonifer-ous to Early Permian fractionationof uranium and lead (Fig. 2a,b). Thepitchblendes probably evolved in U–Pb closed systems from c. 800 Ma toc. 300 Ma. At c. 300 Ma, the ura-nium deposit experienced uranium–lead fractionation, and then returnedto a closed system from c. 300 Ma

until the present day. The pitch-blendes defining the discordia eithergained uranium or lost lead at c.300 Ma. Since uranium gain cannotbe distinguished from lead loss byisotopic analyses alone, petrographicevidence is required to decipher thispuzzle. Because they did not considerlead loss, Zhang et al. (1984) treatedthe isochron age of c. 300 Ma as theage of uranium mineralization. Inaddition, since Zhang et al. (1984)neither tabulated the errors norshowed their sampling locations, it isdifficult to evaluate and discuss thedifferences between the two studiesin detail.The previously reported ages

mainly fall into four groups, whichcoincide with Neoproterozoic, Car-boniferous, Triassic and Palaeogenetectonothermal events (e.g. Wanget al., 2013). Neoproterozoic deposi-tion of the basement rocks, emplace-ment of the host granitoid pluton andthe crystallization of subsequentdykes and veins occurred shortlybefore or almost simultaneously withthe upper intercepts of the pitchblendeU–Pb ages: these are interpreted asthe ages of the uranium ore forma-tion. The coincidence of the agesimplies a possible genetic relationshipbetween magmatic and/or hydrother-mal fluids and uranium mineraliza-tion. The Late Devonian toCarboniferous metamorphic ages

from the Sanfang pluton correspondwith the lower intercepts age of leadloss or uranium gain. Moreover, thefault system that hosts the Sanqiliudeposit is known to have experiencedmetamorphism and reactivation dur-ing the Caledonian orogeny (Shi,1976; Wang et al., 2013). Thus, thisage can be interpreted as reworking,resetting, or remobilization of theoriginal uranium during a metamor-phic event related to Caledonian post-orogenic extension (McKerrow et al.,2000; Wang et al., 2013). The Triassicand Palaeogene ages are treated as themetamorphic records of the Indosi-nian orogeny and Late Cretaceousextension respectively (Wang et al.,2013). The cooling path of the ura-nium ore system can therefore bereconstructed using these ages and therelevant closure temperatures (Fig. 5).Hydrothermal zircons from the

dykes and veins yield younger ages of306.7 � 7.1 Ma, 234.2 � 2.0 Ma and85 � 11 Ma. Based on the differentclosure temperatures of the U–Pb sys-tem in pitchblende (~250–300 °C;Zhao et al., 2004) and in hydrother-mal zircon (~120–200 °C; Hoskin,2005), we infer that the temperatureof the ca. 300 Ma hydrothermal ormetamorphic event was higher thanthe ~300 °C required to reset thepitchblende U-Pb system, which isfurther supported by muscovite40Ar/39Ar dating (~350–450 °C;Hacker et al., 2009). In contrast, theTriassic and Late Cretaceoushydrothermal events had no impacton the U-Pb system in pitchblende,indicating that the temperature ofthese events was less than ~250 °C. Inaddition, the differences in the agesimply that the uranium mineralizationand the hydrothermal fluids in theinvestigated area were complex pro-cesses (Fig. 5).

Cooling and exhumation

The thermal modelling suggests con-tinuous fast cooling from 150–120 °Cto 20 °C during the last c. 65 Ma(Fig. 4). Given the absence of sourcesof significant heat flux, such ashydrothermal veining and graniteintrusions, the late-stage cooling isconcluded to be due to progressiveunroofing and exhumation (McInneset al., 2005; Liu et al., 2014). Becauseof the low closure temperature of

Fig. 3 Plot of central age versus elevation for samples from the Motianling area.



............................................................................................................................................................

apatite of ~110–60 °C (Laslett et al.,1987), as distinguished from typicalhigh-to-medium hydrothermal activ-ity (Donelick et al., 2005), the apatitefission-track results constrain the tim-ing of the latest exhumation of thewhole Motianling area.A notable aspect of the T–t path is

the increase in the exhumation/cool-ing rate during the Cenozoic from~65 Ma to the present. Assuming anaverage continental geothermal gradi-ent of c. 25 °C km�1 and a meanannual surface temperature of 20 °C,based on heat flow and the present-day depth of the Motianling district inSouth China, the cooling rate of thewhole area translates into an averageexhumation rate of 0.07 km Ma�1

since 65 Ma. The apatite fission-trackages record the time when the whole

Fig. 4 Diagrams of time–temperature history. Sampling locations are shown in Fig. 1b.

Fig. 5 Cooling path for the Motianling uranium district. Data sources: Shi, 1976;Zhang et al., 1984; Wang et al., 2006; Yu, 2012; Zhao et al.,2013; Tang et al.,2014; this study. References for closure temperatures are available in the text.



.............................................................................................................................................................

Motianling area including the San-qiliu uranium deposit was exhumedfrom ~4.6 km below the palaeo-sur-face (McInnes et al., 1999, 2005). Thisevent may have caused uplift and ero-sion of the Late NeoproterozoicDanzhou Group, and the formationof the 1.8 km high Motianling moun-tain range.

Implications for regional uraniummineralization

Important controls on uranium min-eralization in vein-type depositsinclude the availability of uraniumsources, the geometry of the faultsystems, and the mechanism of fluidtransport (e.g. Li et al., 2002; Huet al., 2008; Cuney, 2009). Vein-typeuranium deposits form during thecirculation of hydrothermal fluids inassociation with structures of thehost rocks (e.g. Baudemont andFedorowich, 1996; J�ebrak, 1997;Cuney, 2009; Cloutier et al., 2011;Doln�ı�cek et al., 2014), especially inregional extensional settings (e.g. Huet al., 2008; K�r�ıbek et al., 2009).After uranium is leached from grani-toid and metamorphic basementrocks (e.g., Marignac and Cuney,1999; Doln�ı�cek et al., 2009, 2014;Zhao et al., 2014), it is transportedthrough fault zones and fractures byhydrothermal fluids of meteoric, dia-genetic and/or metamorphic origin(e.g. Kerrich, 1986; Li et al., 2002;Cuney, 2009).The Motianling area is character-

ized by a large-scale fault and shearzone system. Uranium mineralizationis hosted in metasedimentary rocksand plutonic granitoids, and is closelyassociated with hydrothermal alter-ation. The geochronological and iso-topic data reported abovedemonstrate that the uranium wasprobably derived from magmatic flu-ids or hydrothermal fluids or was lea-ched from the host granitoids andmetasedimentary rocks (Zhang et al.,1984). Additionally, our results sug-gest that the Neoproterozoic uraniummineralization of the Sanqiliu depositwas preserved under relatively stablecrustal conditions for a long period(at least c. 500 Ma). More impor-tantly, the Cenozoic exhumation ofthe Sanqiliu deposit evidenced by theapatite fission-track thermochronologyled to the exposure of the Sanqiliu oreTa

ble

2Apatite

fission-track

data

from

theMotianlinguranium

district

Sample

Latitude(N),Longitude(E)

El.(m

)ng

q s(105cm

�2)

(Ns)

q i(105cm

�2)

(Ni)

q d(105cm

�2)

(Nd)

P(v

2)

(%)

Cen

tral

age

(Ma)

�1r

Pooledag

e

(Ma)

�1r

L(lm)(N

)K–S

test

AgeGOF

MT30

25.25230000,

108.82271667

246

282.758(637)

13.506

(3120)

10.392

(7463)

6.0

37�

337

�3

12.0

�2.1(107)

0.82

0.82

MT23

25.28220000,

108.69750000

401

172.578(369)

12.884

(1844)

10.399

(7463)

71.2

37�

337

�3

12.5

�2.1(56)

0.65

0.99

MT41

25.35521667,

108.90595000

528

280.64

(142)

5.425(1203)

10.406

(7463)

25.3

22�

322

�2

12.3

�2.2(50)

0.91

0.97

MT43

25.35731667,

108.87885000

663

282.414(397)

16.448

(2705)

10.389

(7463)

21.0

27�

227

�2

12.0

�2.3(91)

0.91

0.95

MT74

25.36595000,

108.82925000

807

282.286(559)

22.946

(5612)

10.393

(7463)

64.6

18�

118

�1

12.4

�2(100)

0.54

0.77

MT16

25.21926667,

108.68028333

958

300.283(84)

1.632(485)

10.395

(7463)

93.2

32�

432

�4

12.3

�3.3(8)

0.87

0.89

MT58

25.41235000,

108.76765000

1270

283.619(873)

17.684

(4266)

10.404

(7463)

2.8

38�

338

�2

12.3

�1.9(103)

0.58

0.72

MT61

25.39980000,

108.78098333

1426

285.092(978)

16.238

(3119)

10.408

(7463)

43.2

57�

457

�4

12.6

�1.9(100)

0.61

0.83

ngisthenumberof

analysed

grains.q s,qiandqdarethedensities

ofspontaneous,inducedandstandard

tracks

inan

external

detector

(ED)irradiatedagainstadosimeter

glass(IRMM-540).qs,qiandqdareexpressed

as10

5tracks

cm�2.Ns,NiandNdarethenumbers

ofcountedspontaneous,inducedandstandard

tracks

intheED

(Ndisan

interpolated

value).P(v

2)isthechi-squared

probability

that

thedatedgrains

have

aconstant

q s/q

i-ratio.Age

GOFisthegoodness-of-fit

betweenthemodel

anddata

ages;K–S

testistheKolmogorov–Smirnov

statistic.Length

data

ofapatite

fission

tracks

arereported

asameantracklength

(L)with

standard

devi-

ation(r)(in

lm),obtained

from

themeasurementof

anumber(N

)of

natural,horizontalconfined

tracks.



............................................................................................................................................................

bodies. Similar hydrothermal vein-type uranium deposits hosted in gran-itoid plutons, such as the Huangao,Liueryiqi, Sanerlin and Dawan depos-its, have been found in abundance inSouth China during the past half-cen-tury (e.g., Min et al., 1999, 2005; Liet al., 2002; Zhao et al., 2014). On theother hand, some plutons share thecharacteristics of similar uraniumcontents, hydrothermal activity,deformation and exhumation pro-cesses, but no uranium deposits havebeen discovered. The timing anddepth of exhumation play a key rolein terminating favourable preserva-tion conditions or exposing depositsat the surface, which may explain theabsence of uranium deposits in theseother plutons.

Conclusions

Uranium mineralization occurred atc. 800 Ma in the Sanqiliu deposit,shortly after the crystallization of thehost granitoids and subsequentdykes. The pitchblende U–Pb age ofc. 300 Ma is interpreted as a time oflead loss or uranium gain duringreworking or remobilization of thedeposit. This age coincides withuplifting and hydrothermal alterationduring Caledonian post-orogenicextension. The whole Motianlingarea including the Sanqiliu depositexperienced a slow cooling path andwas exposed at the surface duringthe Cenozoic.

Acknowledgements

This study was supported by the NationalBasic Research Program of China (973Program) grant no. 2014CB440903, theNSFC (Grants 41172191 and 41372212),the State Key Laboratory of GeologicalProcesses and Mineral Resources(GPMR2011) and the Oversea FamousProfessor Program to Nicholas Arndt(MS2011ZGDZ (BJ) 019). We have bene-fited from helpful discussions with JuntingQiu, Junzhi Wang and Lixian Tian duringthe study. We appreciate discussions andconstructive comments on the manuscriptby Ganqing Jiang, Michael L. Wells,Georges Calas and four anonymousreviewers.

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Received 30 May 2015; revised version

accepted 15 September 2015

Supporting Information

Additional Supporting Informationmay be found in the online versionof this article:Data S1. Methodology.



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