K/Ar mica/illite and Rb/Sr sphalerite dating ofLate Variscan and post-Variscan sulphide and uranium

mineralizations of the Bohemian Massif (Czech Republic)B. Kříbek

Czech Geological Survey, Prague, Czech RepublicINTRODUCTION

The Bohemian Massif is a part of the Variscan belt of CentralEurope (Fig. 1). The formation of numerous vein and shear-zonehosted deposits of gold, base metal and uranium is linked to theVariscan and post-Variscan stages of development of theMassif. By now, the gold and uranium mineralizations have beendated with precision (Anderson 1987; Zachariá and Stein2001), whereas the ages of base metal deposits are unknown.Therefore, the K-Ar analysis of various grain-size fractions ofillite and muscovite from hydrothermal alteration zones and Rb-Sr isotope analysis of sphalerite and carbonates were used inthis study to determine the age of the base metal deposits. Theresults obtained are discussed with respect to the temporalrelations of hydrothermal events and structural development ofthe Bohemian Massif.

š

RESULTS

Anderson EB, Ivanov PA, Komínek J (1988) Intra-ore metasomatism at the uranium veins of the Ro ná deposit:Geology and Hydrometallurgy of Uranium 12: 70-88 (in Czech with English summary)

Arthaud F, Matté P (1977): Late Paleozoic strike-slip faulting in Southern Europe and North Africa: results of aright-lateral shear between theAppalachians and Urals: Geological Society ofAmerica Bulletin 88: 1305-1320

Franke W (2006) The Variscan orogen in Central Europe: construction and collapse. In: Gee DG, Stephenson RA(eds) European Lithosphere Dynamics. Geological Society, London, Memoirs 32: 333-343

Pitra P, Burg J-P, Guirraud M (1999) Late Variscan strike-slip tectonics between the Teplá-Barrandian andMoldanubian terranes (Czech Bohemian Massif): petrostructural evidence: Journal of the Geological Society,London 156: 1003-1020ntral Bohemian metallogenic zone, Czech Republic: In: Piestrzynski et al (eds) MineralDeposits at the beginning of the 21 century. Balkema, Rotterdam, pp 851-854

ák K, Dobe P (1991): Stable isotopes and fluid inclusions in hydrothermal systems: The Příbram ore region:Rozpravy eskoslovenskéAkademie V d 101:3

ž

Ž š

st

Zachari J, Stein HJ (2001) Re-Os ages of Variscan hydrothermal gold mineralization. Central Bohemianmetallogenic zone, Czech Republic: In: Piestrzynski et al (eds) Mineral Deposits at the beginning of the 21century. Balkema, Rotterdam, pp 851-854

š

š Ž

Ž š

st

Zachariá J, Pudilová M, ák K, Morávek P, Litochleb J., Vá a T., Pertold Z. (1997) P-T conditions, fluidinclusions and O, C, S isotope characteristics of gold-bearing mineralization within the Central Bohemianmetallogenic zone: Acta Universitatis Carolinae Geologica 41: 167-178.

ák K, Dobe P, Sztacho P (1996) Vein-type hydrothermal deposits of the Bohemian Massif: Evolution ofhydrothermal fluid sources and relation to extension events in the crust: Global Tectonics and Metallogeny 5:175-178

ň

ěČ

Fig. 1. Simplified geological map of the Bohemian Massif, locationof studied deposits and location of deposits discussed in text.

A

THE DEPOSITS STUDIEDHydrothermal alteration products were studied in the KutnáHora, Příbram and Ro ná districts (Fig. 1).

(1) The mineralized system of the Kutná Hora districtconsists of Fe-Zn-Pb-Ag sulphide veins and zones ofmineralization. The major minerals include pyrite, pyrrhotite,arsenopyrite, Fe-rich sphalerite and galena. The deposit is oftencompared with the Freiberg deposit in Germany.

(2) Base metal and uranium mineralization in the Příbramdistrict are confined to NESW-striking 1 order tectonic lines. Thesulphide mineral assemblage consists of Ag-rich galena, Fe-poor sphalerite, tetrahedrite, chalcopyrite, Ag minerals andbarite. Slightly younger vein-type uranium mineralization atPříbram consists mostly of uraninite in carbonate and quartzgangue.

(3) At Ro ná, the shear zone-hosted uranium (uraninite andcoffinite) mineral assemblage forms the main economicmineralization but a pre-uranium Fe-Zn-Pb-Ag sulphide and apost-uranium Pb-Zn sulphide mineralization also occurs at thedeposit.

ž

ž

st

Fig. 2. Radiometric age of different grain-size fractions of illiteand nuscovite from hydrothermal alteration zones in thePříbram, Kutná Hora and Rožná deposits.

MORAVOSILESIC

UM

LUGICUM

MOLDANUBICUM

MOLDANUBICUM

SAXOTHURINGICUM

BOHEMICUM

PříbramPb-Zn-Cudeposit

Rožnáuraniumdeposit

Kutná HoraPb-Zn-Cudeposit

Příbramuraniumdeposit

Čelina,Mokrskogold deposits

Kašperské Horygold deposit

FreibergPb-Zn-Cudeposit

Studiedore deposits

Other ore depositsdiscussed in text

REFERENCES

A: VARISCAN POSTOROGENICEXTENTION

B: LATE VARISCAN TRANSCURRENTTECTONICS, FURROWS FORMATION

C: EARLY ALPINE TRANSTENSION

Propagation of ductile and laterbrittle shear zonesRetrograde-metamorphicalteration: white mica, chlorite,biotite, albitePRE-URANIUM MINERALIZATION:Quartz-sulphidic,Carbonate (siderite)-sulphidic

Ectraction of uranium from crystallinerocks by Permian basins formation water(300-280 Ma)Hydrothermal alteration: hematitization,albitization, chloritization, argillitizationMINERALIZATION (280-260 Ma):uraninite-coffinite, selenides

Brittle shear zonesAlteration: silicification, argillitizationpyritizationMINERALIZATION:Barite-fluorite-sulphides,Remobilized coffinite

307 - 300 Ma

300 - 280 Ma, 280 - 260 Ma

240 - 220 Ma

METAMORPHICWATER

CO2

PERMIANFORMATIONWATER

MIXTURE OFMETEORICAND FORMATIONWATER CO2

Moldanubikum BrunovistulikumMoravikum

Fig. 6. A model of mineralization at the Rožná deposit in relation to thetectonic evolution of the Bohemian Massif (after Kříbek et al., 2009).

K/Ar ILLITE/MUSCOVITE DATING

Rb/Sr DATING OF SPHALERITEAT THE PŘÍBRAM DEPOSIT

SideriteSphalerite I

Sampling point

A

Fig. 3. Types of sphaleriteused form Rb/Sr agedetermination:

A: Sphalerite I in association with siderite, B: sphalerite II inassociation with calcite/ankerite.

B

Sphalerite II CalciteHydrothermallyaltered diorite

Sampling point

B

1 3 5< < <�m �m �m

Ra

dio

me

tric

ag

e(M

a)

290

280

270

260

250

240

230

220

300

310

320

330

Rožná depositpost-uranium,base metalsmineralization

Rožná deposit,uraniummineralization

Příbram deposit,base metalsmineralization

Kutná Hora deposit,base metalsmineralization

Grain size

Fig. 7. Timing of vein- and shear zone-type hydrothermal mineralization ofthe Moldanubian and Bohemian domains of the Bohemian Massif, fluid

0.70

0.71

0.72

0.73

0.74

0.75

0 2 4 6 8 10

87Rb/

86Sr

87Sr

86Sr

#338, T = 276.4 ± 2.8 Ma

Initial 87Sr/86Sr =0.70694 ± 7

#337, T = 223.8 ± 2.4 Ma

Initial 87Sr/86Sr =0.70698 ± 7

#339, T = 330.0 ± 4.3 Ma

Initial 87Sr/86Sr =0.70771 ± 15Sphal

Sphal

Sphal

Carb

Carb

#339

#338

#337

Izochron 339Sphalerite I-Siderite

Izochron

#

# 338Sphalerite II-Ankerite

Izochron # 337Sphalerite II-Ankerite

Fig. 4. Rb-Sr isochron diagram for sphalerite and carbonatesfrom the Příbram deposit.

METHODSK-Ar age data were obtained from the <1μm, <3μm and <5 μmseparates of eight illite/white mica-rich samples. K-Ar dating wasperformed by Actlabs Laboratories, Canada. Data were storedand processed by a Hewlett-Packard MX-21-E data system. ForRb-Sr studies, rock powders were dissolved in a mixture of HF,HNO and HClO . Before decomposition, all samples werespiked with Rb- Sr mixed solution. Rb and Sr were separatedusing conventional cation-exchange techniques. Total blankswere 0.010.05 ng for Rb and 0.30.7 ng for Sr. Accuracy of themeasurements of Rb and Sr contents was ± 0.5%, Rb/ Sr1.0%, and Sr Sr 0.007% (2s). Isotope ratios were determinedon a Finnigan MAT 261 thermal ionisation mass spectrometer. Sr

3 485 84

87 86

87 86

isotopic ratios were normalized to Sr/ Sr = 8.37521. Rb-Sr datingwas performed byActlabs Laboratories, Canada.

88 86

The K-Ar age data on illite/muscovite in hydrothermal alterationsgroup into three distinct intervals:(a) 314 ± 18 to 298 ± 8 Ma: The Kutná Hora base metal depositand pre-uranium base metal mineralization at the Ro ná deposit(3 samples).(b) 274 ± to 268 ± 7 Ma: The Příbram base metal deposit (2samples) and 277 ± 5 to 264 ± 4 Ma (uranium mineralization,Ro ná deposit (2 samples).(c) 233 ± 5 to 227 ± 5 Ma: Post-uranium base metal mineralizationat the Ro ná deposit (2 samples).At Kutná Hora and Příbram, differences in age determination indifferent grain size fractions (< 1

pectra are flat. Differences in agedetermination in hydrothermally altered rocks associated with theuranium mineralization at Ro ná are slightly higher (11 m.y.) andK-Ar data of different size fractions distinctly decrease withdecreasing size. The inclined spectra are suggestive ofincomplete homogeneity of the K-Ar system.

ž

ž

ž

ž

7

μm, <3 μm, <5 μm) are relativelysmall (68 m.y.) and age s

Three samples of sphalerite and carbonates (siderite andankerite) from various paragenetic stages in the Příbramdeposit were analysed. The Rb/ Sr values for sphaleriteand carbonates differ markedly from one another (Fig. 2).The isochron #339 of sphalerite I associated with sideriteyielded the age of 330 ± 4.3 Ma. Isochrons #338 and #337 ofsphalerite II in association with ankerite yielded the ages of276.4 ± 2.8 Ma and 223.8 ± 2.8 Ma. The initial Rb/ Sr ratiosfor the older sphalerite I and the younger sphalerite II differand could indicate either a different source of hydrothermalfluids or an interaction and local re-equilibration andequilibration with wall rocks.

87 86

87 86

DISCUSSION:

Hydrothermal vein and shear zone-hosted deposits of theBohemian Massif can be divided into four groups:

(1) Gold deposits (not discussed in detail in this extendedabstract) dated at 342.9 ± 4 Ma (molybdenite, Re-Os) and339338 Ma (Ar-Ar, Zachariá and Stein, 2001). conditions,fluid inclusions composition and O, C, S isotope characteristicsindicate the metamorphic origin of gold mineralization(Zachariá et al. 1997). These orogenic-type gold deposits arebelieved to have been formed during the period of Variscansynorogenic extension (340 and 325 Ma; Franke 2006).

(2) The Kutná Hora base metal deposit (Freiberg type) isrelated to the short period of post-collisional collapse andextension of the Variscan belt. Its origin must have beencontrolled by a high heat flow (thermal doming) in theneighbourhood of “younger granitoids” (320-290 Ma) which arewidespread in Western and Central Europe. Fluid inclusion,paragenetic, and isotope data suggest that pre-uraniummineralization formed from reduced low-salinity metamorphicor magmatic fluids at temperatures close to 300 C ( ák andDobeš, 1991).

š

š

Ž

P-T

o

This type of mineralization was formed withoutobvious control by distal field tectonics and coincides in timewith the 300 15 Ma metalliferous event in the French MassifCentral.

P-T CONDIIONS OF MINRALIZATIONIN RELATION TO THE AGE OF THEIR FORMATION

(3) The age of base metal deposits of the Příbram type and uraniummineralization (Příbram and Ro ná deposits) overlaps with, or is slightlyyounger than, the period of the Stephanian-Early Permian wrench-inducedcollapse of the Variscan belt (305-280 Ma). Continental dextral shears, suchas the Tornquist-Teisseyre fracture zone, were linked by secondary sinistraland dextral shear zones in the territory of the Bohemian Massif (Pitra et al.1999). This deformation probably reflects a change in the Condwana-Laurussia convergence from oblique collision to a dextral translation(Arthaud and Matté 1977). High salinity of hydrothermal fluids together withPb and Sr isotope evidence show that the base metal deposits of thePříbram type were formed during very deep crustal circulation of high- Ofluids possibly from the lower crustal reservoirs ( ák et al. 1991) whereasthe formation of uranium mineralization is associated with the shallowinfiltration of oxidized basinal brines of the Upper Stephanian and LowerPermian basins, mixed with meteoric water, into the crystalline basement

(4) Small base metal deposits and remobilization of uranium are thought tohave taken place during the Triassic and Jurassic onset of a new rifting cyclethat preceded and accompanied the stepwise break-up of Pangea.Mineralization is probably associated with several periods of reactivation ofthe older Stephanian to Early Permian brittle structures. Wide range ofsalinities and O, C, S isotope characteristics of carbonates and sulphidesindicate mixing of residual brines (“shield brines”) with meteoric waterduring the formation of this mineralization type.

ž

Žδ18

250

240

230

220

CA

RB

ON

IFE

RO

US

Tou

rna

isia

nV

ise

an

Na

mu

ria

nW

sp.

Ste

.P

ER

MIA

NM

idd

leT

RIA

SS

ICE

arly

Ea

rly

Mid

dle

La

teL

ate

Syn-collisionalextension

Post-colissionalcollapse and extension

Late Stephanian-Permiantranscurrenttectonics

Early Alpinereactivation

TECTONIC SETTINGAge(Ma) MOLDANUBICUM BOHEMICUM

Intramonate basins formation

Transtensional furrows opening

Early Triassic Unconformity

260

270

280

290

300

310

320

330

340

350

360

RožnáUranium

PříbramUranium

PříbramPb-Zn-Cu

RožnáPb-Zn

Kutná HoraFe-Zn-Pb-Ag

JílovéČelinaMokrskoGold Kašperské Hory

Gold

FLUIDS

RožnáPb-Zn

Th = 190-340 CNaCl eq. = 2-8 wt. %

o

Th = 100-280 CNaCl eq = 14-25 wt. %1 Ofluid =

+6 to +11 per mil

o

8�

Th = 80-200 CNaCl eq = 5-25 wt. %

O fluid =+3 to +6 per mil

o

18�

Th = 60-200 CNaCl eq = 3 - 25 wt. %

O fluid =+12 to +15 per mill

o

18�

Th = 200-330 CNaCl eq. 1-4 wt. %

O =+4 to -2 per mil (SMOW)

o

18�

Gra

nito

ids

Gra

nito

ids

RoudnýGold ?