GeolCarp_Vol66_No3_173

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, JUNE 2015, 66, 3, 173—179 doi: 10.1515/geoca-2015-0018

Molybdenite Re-Os dating of Mo-Th-Nb-REE rich marbles:

pre-Variscan processes in Moldanubian Variegated Group

(Czech Republic)

MILAN DRÁBEK

and HOLLY STEIN

2,3

Czech Geological Survey, Geologická 6, 152 00, Prague 5, Czech Republic; milan.drabek@alumni.uni-heidelberg.de

AIRIE Program, Colorado State University, Fort Collins, CO 80523—1482, USA

Centre for Earth Evolution and Dynamics, P.O. Box 1028, University of Oslo, 0316 Oslo, Norway; holly.stein@colostate.edu

(Manuscript received July 27, 2014; accepted in revised form March 12, 2015)

Abstract: In an effort to contribute to the discussion concerning the age of rocks of the Moldanubian Variegated Group,
we have undertaken Re-Os dating of molybdenite of banded carbonatite-like marbles (CLM) from the graphite mine
Václav at Bližná (Southern Bohemia), which belong to the metamorphic sequence of this group. The Re-Os model ages for
the molybdenites range between 493 and 497 Ma and apparently correspond to the early stages of metamorphism con-
nected with pre-Variscan rift-related tectono-metamorphic events, which affected and recrystallized sedimentary CLM
material rich in Mo-Th-Nb-REE. The molybdenite bearing carbonatite like marbles situated in the footwall of Bližná
graphite mine have been interpreted as carbonates with a large share of volcano-detritic material derived from contempo-
raneous primitive alkaline (carbonatite-like) volcanism deposited in a shallow marine lagoonal environment. There is no
geological evidence for the participation of fluids mobilized from host rocks in the formation of the CLM. Because the
Re-Os chronometer in molybdenite is demonstrably stable through later Variscan facies metamorphism, the molybdenite
chronometer has not been affected by subsequent thermal overprints associated with the Variscan orogeny.

Key words: Re-Os dating, molybdenite, Moldanubicum, marble, Variegated group, carbonatite-like marbles, Bohemian
Massif, Czech Republic.

Introduction

The Bohemian Massif comprises several metamorphic units
among  which  the  Moldanubian  Zone  is  the  southernmost.
The  Moldanubian  Zone  is  a  very  complicated  tectonic  mé-
lange  of  high-grade  to  medium-grade  metamorphic  rocks
with  a  complex  polyphase  deformation  history  beginning
with  the  Cadomian  orogeny  (Late  Proterozoic—Early  Paleo-
zoic) and terminating with widespread Variscan tectonother-
mal  activity  (Chaloupský  1989;  Cháb  et  al.  2010).  The
Moldanubian  Zone  consists  of  the  Moldanubian  Variegated
Group, Moldanubian Monotonous Group and the Gföhl Unit
(Cháb et al. 2010). The presently studied marbles belong to
the Moldanubian Variegated Group (Fig. 1).

The age of individual tectonic slices varies from 2050 Ma

(Wendt et al. 1992) to Paleozoic. The timing and stratigraphic
division of the Moldanubian Zone is complicated by a strong
Variscan overprint. Nevertheless, orthogneisses from Hluboká
nad  Vltavou  dated  by  Vrána  &  Kröner  (1995)  using  single
grain evaporation yielded an age of 508 ± 7 Ma, which was in-
terpreted  as  the  time  of  granite  emplacement.  Whole  rock
Rb-Sr dating of the Choustník orthogneisses gave an isochron
age  of  459 ± 10 Ma  (Rajlich  et  al.  1992).  Pre-Variscan  ages
of 475—514 Ma for monazite were reported by Procházka et al.
(2010) from the Sudoměřice leucogranite. In situ U-Pb dates
of  columbite  and  tantalite  by  La-SF-ICP-MS  from  Li-bear-
ing  Moldanubian  pegmatites  yielded  emplacement  ages  of
~

333 ± 3 to 325 ± 4 Ma (Melleton et al. 2012). In an effort to

contribute to the discussion concerning the age of the Mol-

danubian rocks, we have undertaken Re-Os dating of molyb-
denite from carbonatite-like marbles (CLM) from the Bližná
graphite Václav Mine (Southern Bohemia, Czech Republic)
which record the metamorphic sequence of this group.

Geological setting

The Bližná graphite mine Václav is situated in the south-

western part of the Moldanubian Variegated Group, correla-
tive  with  the  Český  Krumlov  unit  (ČKU)  (Zoubek  1979),
35 km SW of České Budějovice, Southern Bohemia, Czech
Republic.  The  ČKU  consists  of  biotite  and  amphibole—bio-
tite  paragneisses  with  frequent  intercalations  of  quartzites,
graphitic  gneisses,  calc-silicate  rocks,  calcite  and  dolomite
marbles,  and  amphibolites.  The  abundance  of  scapolite  in
some marbles of the ČKU (Kříbek et al. 1997) has been used
as  an  argument  for  their  evaporitic  origin.  The  rocks  were
metamorphosed  under  amphibolite  facies  conditions  during
regional Variscan metamorphism. The Bližná graphite deposit
is  confined  to  an  intercalation  of  carbonate  rocks  in  biotite
paragneiss (Fig. 2) (Drábek et al. 1986, 1999;  Veselovský  et
al.  1987).  The  carbonate  rocks  represent  dolomite—calcitic
and  calcite—dolomitic  marbles  (metacarbonates).  Graphitic
marbles and calc-silicate rocks are represented to a lesser ex-
tent. Foliation is NE—SW with a steep NW dip, and in some
parts of the deposit a SE dip. In addition to ordinary marble
(OM), typical of the whole ČKU on the basis of mineralogy,
chemistry and composition of radiogenic (Sr—Nd) and stable

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Fig. 3. Markedly layered CLM from the 3

level of the Václav

graphite mine. The dark layers are silica-rich.

Fig. 4. Transmitted light microphotograph of CLM. Edenite replac-
ing diopside in CLM. Plane polarized light. Ed – edenite, Di – dio-
pside, Cal – calcite, Po – pyrrhotite.

Fig. 5. Photomicrograph of euhedral molybdenite platelets. SEM.

(C-O)  isotopes,  Drábek  et  al.  (1999)  distinguished  in  the
Bližná  graphite  mine  two  other  types  of  marbles:  carbon-
atite-like  marble  (CLM)  and  transitional  marble  (TM).  The
TM is less enriched in elements typical of CLM, however re-
lations  between  the  elements  are  preserved  (Veselovský  et
al. 1987). The CLM differs markedly from other marbles of
the ČKU, and chemically and isotopically resemble carbon-
atite compositions. CLM and TM occur only in the footwall
of  the  graphite  bed,  where  they  form  a  single  continuous
stratabound layer. Only CLM are molybdenite bearing.

Banded carbonatite-like marble

The banded carbonatite-like marble (CLM) forms medium

grained  and  silicate—rich  intervals  that  are  distinctly  layered
(Fig. 3).  The  darker  layers  are  particularly  enriched  in  sili-
cates. The lighter layers consist primarily of calcite. Insoluble
residues of CLM vary between 1.5 and 65.4 wt. %. The miner-
alogy of the insoluble residue of CLM is quite complex. The
mineralogy  of  the  CLM  has  been  described  by  Drábek  et  al.
(1986), Veselovský et al. (1987) and Drábek et al. (1999). The
mineral  assemblage  of  the  CLM  includes  calcite  diopside,
edenite, phlogopite, forsterite, antigorite, talc, chlorite and ser-
pentinized  forsterite.  Edenite  (subcalcic  sodium  edenite  ac-
cording  to  the  IMA  nomenclature  Leake,  B.E.  1978)  is  the
most  abundant  silicate  in  the  CLM.  Edenite  forms  idiomor-
phic columnar crystals up to several mm in size (Fig. 4).

The Mo-Nb-Th-REE mineralization in CLM is represented

by  the  following  sulphides  and  oxides:  molybdenite,  pyrite,
pyrrhotite,  galena,  chalcopyrite  sphalerite,  pyrochlore,  il-
menite, magnetite, rutile, euxenite and uranothorite. Barite is
the  only  sulphate  mineral,  and  apatite  is  the  only  phosphate
mineral.

Molybdenite (MoS

) is quite common in the CLM. Whole

rock molybdenum contents vary from 7 to 1930 ppm (aver-
age, 754 ppm). Molybdenite flakes occur in both the darker
and lighter layers, but in the darker layers the percentage of
molybdenite is higher. Molybdenite forms perfect euhedral,
isolated  hexagonal  thin  platelet  crystals,  most  commonly
1—3 mm but up to 7 mm in size (Fig. 5). Stacked molybdenite
platelets  comprise  molybdenite  crystals  (Fig. 6).  X-ray  dif-
fraction pattern for molybdenite correspond to the 2H poly-
type.  Very  sharp  diffraction  profiles  attest  to  excellent
crystallinity  of  molybdenite.  Drábek  et  al.  (1993)  reported
the  following  trace  element  compositions  in  molybdenite:
10 ppm Re,  20 ppm W,  51 ppm Se,  and  4 ppm Te.  These
contents  are  low  compared  to  molybdenites  from  other  oc-
currences in the Bohemian Massif (Drábek et al. 1993). The
isotopic  composition  of

S in molybdenite is + 4.2 ‰

(Drábek  &  Hladíková  1990).  Pyrite,  pyrrhotite  and  chal-
copyrite occur together (Fig. 7). Pyrite forms small euhedral
cubic  crystals  containing  0.2 ppm Te  (AAS).  Pyrrhotite
forms small irregular grains and chalcopyrite forms small in-
clusions in other sulphides. Pyrochlore occurs together with
ilmenite and magnetite. Pyrochlore (according to Atencio et
al. (2010) classification, calciopyrochlore) forms grains up to
3 cm  in  size  or  small  euhedral  crystals  with  rounded  edges
typically 2 mm across. Pyrochlore is strongly enriched in Th

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Fig. 8. Ilmenite grain. Photomicrograph of bubble foam structure of
ilmenite (Ilm). Reflected light.

Table 1: Representative chemical compositions of carbonatite-like
marble (CLM) and ordinary marble (OM) from the 3

level of the

Václav mine, Bližná.

b.d. – below detection, n.d. – not determined

Fig. 7. Photomicrograph of molybdenite (Mlb), pyrite (Py), pyrrho-
tite (Po), chalcopyrite (Cp), and calcite (Cal) intergrowths in CLM.
Reflected light.

Fig. 6. Staked molybdenite plates, show sequential growth of the
crystal. SEM.

and  REE.  Ilmenite  is  most  commonly  found  as  individual
grains  commonly  showing  bubble  foam  structure  (Craig  &
Vaughan 1981) indicating a recrystallization during thermal
metamorphism  (Fig. 8)  or  prismatic  exsolution  lamellae  in
magnetite. Magnetite forms rounded aggregates up to 10 mm
across.  Galena,  rutile,  euxenite  and  uranothorite  were  identi-
fied only as small (5—10 µm) inclusions in pyrochlore. In con-
trast  to  the  OM,  the  CLM  is  graphite  free.  Representative
chemical compositions of CLM and OM are given in Table 1.

The MgO content in CLM is low and ranges from 5 to

21 wt. %. The CLM are enriched in elements typical for car-
bonatites: Y, Th, Nb, Zr, Sr, Mo and the CLM is markedly en-
riched in REEs (e.g. Hoernle et al. 2002). Compared with OM,

Sample No.

CLM

BL 4

CLM

BL 11

BL61

%
SiO

25.26

15.92

8.96

TiO

0.15

0.07

0.02

2.63

1.56

0.50

0.49

1.93

<0.10

FeO

0.42

2.62

0.27

MnO

0.13

0.41

0.21

MgO

7.24

7.21

20.00

CaO

38.12

39.42

29.61

SrO

0.02

0.15

0.01

BaO

0.02

0.16

<0.005

0.09

0.47

0.01

1.02

0.12

0.08

0.02

0.03

0.02

22.62

27.24

37.66

0.40

<0.01

1.38

0.05

0.03

0.08

0.61

0.13

99.33 99.21 99.21

307

409

146

105

159

176

1334

465

1930

222

b.d.

56.21

72.70

1.41

258.56

332.97

3.72

46.82

60.53

b.d.

239.45

322.49

1.73

80.53

111.31

0.30

28.01

41.08

0.08

72.24

117.20

b.d.

10.72

18.89

b.d.

66.22

113.43

0.23

11.81

20.70

b.d.

30.24

51.13

0.13

3.71

6.77

b.d

22.34

38.92

0.15

2.44

4.67

b.d

325.70

572.97

1.37

5.20

17.2

2.00

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CLM are further enriched in Sr, Ba, Cu, Pb, and Zn. Concen-
trations  of  Cr,  Ni  and  V  are  below  their  detection  limits
( < 7 ppm).  The  chondrite—normalized  pattern  of  REE  is
strongly enriched in Ce-Gd (Drábek et al. 1999). Compared
to CLM, the OM has higher MgO contents and elements typ-
ical  for  CLM  (Mo,  Nb,  Th)  are  below  the  detection  limit
( < 7 ppm).  The  Sr  and  REE  contents  are  also  significantly
lower  in  OM.  Furthermore,  the  CLM  have  significantly
lighter  isotopic  compositions  of  oxygen  compared  to  OM
from the ČKU. The isotopic composition of

O and

C in

CLM vary from —15 to —19 ‰ and from —3 to —6 ‰, respec-
tively. This isotopic composition falls into the range reported
for carbonatite rocks (e.g. Heinrich 1966). The isotopic com-
position of Sr in CLM also differs from OM. According to
Drábek et al. (1999) the Sr isotopic composition of the CLM
is relatively primitive with the

Sr/

450

~ 0.708. On the

other hand, the OM has a more evolved strontium ratio with
the

Sr/

450

~ 0.715.

The CLM have been interpreted by Drábek et al. (1999)

and  Drábek  &  Stein  (2003)  as  regionally  metamorphosed
carbonates  (metacarbonates)  with  a  large  share  of  volcano-
detritic material derived from contemporaneous primitive al-
kaline  (carbonatite-like)  volcanism  deposited  in  a  shallow
marine  lagoonal  environment  that  also  produced  evaporitic
sequences. There is no geological evidence for the participa-
tion of fluids mobilized from host rocks in the formation of
CLM.  Mineralization  in  CLM  is  strictly  stratiform  (Drábek
et  al.  1990),  and  the  surrounding  rocks  (graphite  and  para-
gneisses) contain only pyrite, pyrrhotite and rare sphalerite.
Molybdenum was not detected in these rocks (Jiřele 1984).

Sample description

The molybdenite sample used for dating comes from

banded carbonatite like marble within the 3

level of the

Václav graphite mine. The molybdenite used in this study
comes from two distinct mineral separates prepared at AIRIE
(Applied Isotope Research Program, Colorado State, Univer-
sity) from a large hand-size sample of the dark grey variety
of CLM. In this sample, molybdenite occurs as disseminated
flakes 1—2 mm in diameter (Fig. 6).

Analytical technique

Molybdenite presents a unique set-up for the Re-Os method

of dating in that it usually contains ppm level Re and essen-
tially  no  initial  or  common  Os,  making  it  a  single  mineral
chronometer.  General  principles  and  methodology  for  mo-
lybdenite  dating  are  outlined  in  Stein  et  al.  (1997,  2001).
Sample-to-sample reproducibility for molybdenites illustrat-
ing the robustness of the chronometer has been documented
(e.g. Watanabe & Stein 2000 – see summary in Stein 2014),
and the chronometer is demonstrably robust through granu-
lite facies metamorphism and ductile deformation (e.g. Stein
&  Bingen  2002;  Bingen  &  Stein  2003).  Sample  size  and
preparation  followed  the  procedures  to  obtain  meaningful
age results (e.g. Stein et al. 2006). The two molybdenite sep-

arates were 16 and 41 mg each. A Carius-tube digestion was
used, whereby molybdenite is dissolved and equilibrated with

185

Re and

190

Os spikes in HNO

—HCl (inverse aqua regia)

by  sealing  in  a  thick-walled  glass  ampoule  and  heating  for
12 hours at 230 °C. The Os is recovered by distilling directly
from the Carius tube aqua regia into HBr, and is subsequently
purified by micro-distillation. The Re is recovered by anion
exchange. The Re and Os are loaded onto Pt filaments, and
isotopic  compositions  were  determined  in  2002  using
NTIMS  on  NBS  12-inch  radius,  68°  and  90°  sector  mass
spectrometers at Colorado State University (now replaced by
two  Triton  TIMS  machines).  Two  in-house  molybdenite
standards, established and calibrated at AIRIE, are routinely
run as an internal check (Markey et al. 1998). Blanks are in-
significant  (Re < 10 pg,

187

Os < 3 pg). The Re-Os data and

ages are shown in Table 2 along with a footnote explanation
of uncertainties in measurements and calculations.

Chemical analyses were performed in the Central Labora-

tory of the Czech Geological Survey, Prague. Major-elements
were determined using wet chemical analysis; trace-elements
were determined by ICP.

Discussion

The Re-Os ages 497 ± 2 and 493 ± 2 Ma obtained from the

investigated  samples  are  contradictory  to  a  Precambrian  age
for the Moldanubian Variegated Group in Southern Bohemia
suggested,  for  example,  by  Kodym  (1966)  Chaloupský
(1978),  Zoubek  (1979),  Konzalová  M.  (1981),  Frank  et  al.
(1990) and Procházka (2007). On the contrary, this age is con-
sistent  with  that  of  Kröner  et  al.  (2000).  They  reported  two
concordant zircon ages of 469.3 ± 3.8 Ma (cores with Variscan
overgrowths) from regionally overlying granulite of the Gföhl
Unit. A maximum age limit T of 530 Ma of the betafite
(Drábek et al. 1999) from CLM is also in agreement with our
dating. The reported age is also in accordance with the sugges-
tion made by Janoušek et al. (1997, 2008). These authors in-
terpreted metabasic rocks from the ČKU as EMORB tholeiite
basalts  derived  by  Early  Paleozoic  melting  of  a  strongly  de-

pleted mantle source (

500

= + 8.6 to 9.4; T

DM
Nd

= 0.43—0.50 Ga).

Friedl  et  al.  (2004)  also  reported  pre-Variscan  geological
events  from  the  Austrian  part  of  the  Bohemian  massif.
Houzar  &  Novák  (2002)  distinguished  events  related  to  a
polyphase metamorphism in the variegated units of the Bo-
hemian  Massif.  According  to  these  authors,  diopside—bear-
ing  assemblages  in  marbles  correspond  to  temperatures  as

Table 2: Re-Os data for molybdenite from Carbonatite-like marbles
(CLM), Václav graphite mine, Bližná.

AIRIE Run #

Sample Re,

ppm

187

Os, ppb

Age, Ma

CT-509

sep. #1

5.462 (6)

28.54 (2)

497 ± 2

CT-558

sep. #2

10.24 (1)

53.05 (5)

493 ± 2

Absolute uncertainties shown, all at 2-sigma level.
Decay constant used for

187

Re is 1.666 x 10

–11

–1

(Smoliar et al. 1996).
Ages calculated using

187

Os =

187

Re (e

λt

–1) include all analytical and

187

Re decay constant uncertainties.

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high as T > 630—660 °C. Detailed field observations show that
CLM  forms  a  single  continuous  stratabound  layer.  There  is
no  geological  evidence  for  the  participation  of  fluids  mobi-
lized  from  the  host  rocks  or  for  metasomatic  processes.
A possible role for the evaporites is unclear. The high crys-
tallinity  and  perfect  shape  of  molybdenite  crystals  supports
the concept of crystallization of molybdenite in a carbonate-
rich shale matrix of a Ca—rich layer with a large component
of  volcano-detrital  material  (insoluble  component  up  to
65.4 wt. %)  enriched  in  trace  elements  typical  of  carbon-
atites, such as REE, Y, Th, Nb, Zr Sr and Mo. On the basis of
trace  element  chemistry  and  the  isotopic  composition  of  C,
O  and  Sr  we  suggest  that  the  volcano—detrital  material  was
derived from contemporaneous relatively primitive (alkaline
carbonatite-like)  volcanism  which  was  deposited  in  a  shal-
low  marine  lagoonal  environment  which  also  produced
evaporitic  deposits,  and  later,  with  metamorphism,  graphite
sequences. These constraints support the suggestion that the
Re-Os  molybdenite  ages  date  geological  processes  that  ap-
pear to have followed volcanic activity with unusual alkaline
compositions captured in volcanic—detrital basinal sedimen-
tation as intercalations within the carbonate layers. The Re-Os
molybdenite ages presented in this paper correspond to post—
Cadomian rifting in the Moldanubian Zone accompanied by
intrusions  of  basic  and  acid  magmas  between  600  and
490 Ma  (e.g.  Gebauer  &  Grünenfelder  1982;  Teufel  1988;
Finger & Steyrer 1995; Fritz 1996; Friedl et al. 2004). Fiala
(1976) also reported Proterozoic volcanism with a calc-alca-
line trend in the Barrandian basin. Houzar & Novák (2002)
described  metacarbonates  with  a  carbonatite-like  signature
from lithologically similar metamorphic sequences assigned
to  different  Variegated  geological  units  of  the  Bohemian
Massif (Moravicum and Silesicum). Nevertheless, additional
geochronological  and  petrological  data  are  needed  particu-
larly for the ČKU.

Conclusions

Our Re-Os dating of molybdenite from the CLM provides

ages  for  the  formation  of  molybdenite  at  about  495 Ma
(497±2 and 493±2). As expected, the Re-Os ages for molyb-
denite  are  not  affected  by  high-grade  Variscan  metamor-
phism and the molybdenites preserve their pre-Variscan age
of formation. We interpret the ages presented in this paper as
a  record  of  pre-Variscan  metamorphism  of  limestones  with
an admixture of volcanic—detrital basinal sediments of alka-
line composition related to rifting and crustal thinning on the
Gondwana margin, during extensional tectonics.

Acknowledgments: This paper is dedicated to the late Pro-
fessor Zdeněk Pouba. The Re-Os dating was supported by a
U.S.  National  Science  Foundation  Grant  EAR—0087483  to
HS.  Previous  and  on-going  studies  at  the  Václav  graphite
mine are supported by Czech Grant Agency Grant 205/96/563
to  MD.  We  wish  to  thank  V.  Sixta  and  D.  Weiss,  (Czech
Geological Survey, Prague) for the chemical analyses. Com-
ments made by S. Vrána, M. Rieder and anonymous review-
ers are gratefully acknowledged. We appreciate the valuable

comments of handling editor Igor Broska, which significantly
improved our paper.

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