Journal of Geosciences, 65 (2020), 229–247

DOI: 10.3190/jgeosci.314

Original paper

Geochronological constraints on the carbonate-sulfarsenide veinsin Dobšiná, Slovakia: U/Pb ages of hydrothermal carbonates, Re/Os
age of gersdorffite, and K/Ar ages of fuchsite
Stefan KIEFER1*, Martin ŠTEVKO2,3, Rastislav VOJTKO4, Daniel OZDÍN5, Axel GERDES6,7,
Robert A. CREASER8, Marek SZCZERBA9, Juraj MAJZLAN1
Institute of Geosciences, Friedrich-Schiller University, Burgweg 11, D–07749 Jena, Germany; email: stefan.kiefer@uni-jena.de
Earth Science Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovakia
3
Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, CZ-19300 Praha 9, Czech Republic
4
Department of Geology and Palaeontology, Comenius University, Ilkovičova 6, SK-842 15 Bratislava, Slovakia
5
Department of Mineralogy and Petrology, Comenius University, Ilkovičova 6, SK-842 15 Bratislava, Slovakia
6
Department of Geosciences, Goethe University Frankfurt, Altenhöferallee 1, D-60438 Frankfurt, Germany
7Frankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, Germany
8
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
9Institute of Geological Sciences, Polish Academy of Sciences, ul. Senacka 1, 31-002 Krakow, Poland*
Corresponding author
1
2

The abandoned deposit Dobšiná in eastern Slovakia used to be exploited for Ni–Fe–Co sulfarsenides in siderite-ankeritehydrothermal veins. These veins are locally accompanied by alteration zones of fuchsite-quartz-carbonate rocks. The
Ni–Fe–Co sulfarsenides are postdated by chalcopyrite and tetrahedrite. This work aims to determine the age of thesemineralizations utilizing U/Pb, Re/Os, and K/Ar dating of selected minerals. U/Pb dating of hydrothermal siderite andankerite gave ages from 145 ± 5 Ma to 114 ± 24 Ma. They broadly coincide with the metamorphic peak in the Gemeric
Unit and the formation of the Gemeric cleavage fan (CGF) during partial exhumation of this unit. Hydrothermal gersdorffite was dated (Re/Os) to 93.6 ± 0.9 Ma, the time of exhumation of the Gemeric unit. No data were extracted for the
younger Cu sulfides. By correlation with other units in the Western Carpathians, they could be assigned Late Cretaceousage, corresponding to welding of the Gemeric and Veporic units and the formation of the Trans-Gemeric shear zone.
The fuchsite rocks were dated (K/Ar) to 110 Ma, but this datum conflicts with textural evidence, which shows that theserocks are older than the hydrothermal carbonates. The age of 110 Ma is therefore considered to be cooling age after the
reset of the geochronometer by the Alpine metamorphism, in agreement with earlier data. The geochronological results
are supported by structural measurements and field observations. The main mineralized zone, investigated in one of theaccessible adits, conforms with the Variscan – ?Alpine metamorphic foliation. There are also younger, less significant
structures whose relevance to the ore mineralization is not yet clear.
Keywords: hydrothermal mineralization, geochronology, ore mineralogy, isotopic dating
Received: 14 May 2020; accepted: 21 October 2020; handling editor: P. Jeřábek

1. Introduction
The ore mineralizations around the small township
Dobšiná in eastern Slovakia include many hydrothermalveins that contain siderite, quartz, arsenides, and sulfidesof Ni, Co, Cu, Sb (Halahyjová-Andrusovová 1961, 1964;
Grecula et al. 1995; Mesarčík et al. 2001; Kiefer et al.
2017). Additionally, metasomatic bodies of siderite/
ankerite were mined here in the past. The Ni–Fe–Co
mineralization in Dobšiná is hosted by the Gemeric
Unit, one of the major thick-skinned tectonic units of
the Central Western Carpathians (Fig. 1). Beside the ores
in Dobšiná, the Gemeric Unit hosts more than 1200 ore
bodies (Fig. 1; Grecula et al. 1995) that can be roughly
divided into siderite-quartz-sulphide and quartz-stibnite

assemblages. Many of the ore bodies were of significant
economic importance in the past.
An excellent summary of the development of the ideas
and perception of the hydrothermal Ni–Fe–Co mineralization in Dobšiná was written in the Slovak language by
Mesarčík et al. (2001) and a digest thereof will be given
here. The early studies of Böckh (1908) and Woldřich
(1912) link the hydrothermal mineralization to the gneissamphibolite complex, granites, or both. Woldřich (1912,
1927) anticipated Early Carboniferous age of the veinsbecause he assumed that the post-mineralization tectonics
was of Upper Carboniferous age. Ahlburg (1913) foundlimonite-rich pebbles in Carboniferous conglomerates,
assumed that these conglomerates represent weatheredsiderite ores and postulated their pre-Carboniferous age.
www.jgeosci.org

Stefan Kiefer, Martin Števko, Rastislav Vojtko, Daniel Ozdín, Axel Gerdes, Robert A. Creaser, Marek Szczerba, Juraj Majzlan

0

ore vein
Alpine reverse faults
Alpine thick-skinned thrust
Alpine superﬁcial thrust
Neogene and Quaternary
sedimentary deposits
Eocene–Oligocene
sedimentary deposits
Upper Cretaceous–Paleocene
sedimentary deposits
Silicic Unit (Upper Permian–Middle Jurassic
sedimentary deposits
Turnaic Unit (Upper Carboniferous–Upper Triassic
sedimentary deposits
Meliatic Unit s.s. and Bôrka Unit (Permian–Jurassic)
sedimentary deposits, ophiolites
Gemeric Unit Permian to Triassic
sedimentary deposits with acidic volcanites
Gemeric Unit – Ochtiná and Črmeľ nappes(Carboniferous sedimentary deposits with magnezites)
Gemeric Unit – Štós nappe(Lower Devonian metasandstone ans phyllite)

4

8

12

16

20 km

Gemeric Unit – Rakovec Group (back-arc basin, Middle–Upper
gabbrodiorite, metabasalt, metasandstone, and phyllite)
Gemeric Unit – Klátov Group (oceanic crust, Devonian?)Gemeric Unit – Gelnica Group (Upper Cambrian–Devonian
metasediments with acid-intermediate volcanics)
Gemeric Unit – two-mica granite mostly Permian
Hronic Unit – mostly Triassic shelf limestone
Hronic Unit – Upper Paleozoic sedimentary deposits
with tholeiitic basalt
Southern Veporic Unit – mostly metamorphosed Triassic
sedimentary deposits
Southern Veporic Unit – Upper Carboniferous
metapsamite to metapeliteSouthern Veporic Unit – Late Variscan
porpfyric granitoids and orthogneiss
Northern Veporic Unit – Mesozoic
sedimentary formations
Northern Veporic Unit – Upper Paleozoic
sedimentary formations
Northern Veporic Unit – Late Variscan
porpfyric granitoids, orthogneiss, and migmatite

Fig. 1 Geological map of the Gemeric unit and adjoining areas.

Papp (1919) linked the ores to the quartz porphyries andpresumed their Permian age. The first views that there may
be two separate stages of mineralization can be attributed
to Ulrich (1928). The older stage could be connected tothe basic rocks of the gneiss-amphibolite complex; theyounger one is a result of remobilization. Other authors
seek the source of the mineralization in the granitic rocks(Kettner 1921; Hynie 1922), but their ideas about the ageof these granites diverge. Zoubek (1937), in contrast to allthese studies, identified the Alpine metamorphism as thesource of the mineralization. Hence, the dichotomy of the
Variscan versus Alpine age of the veins dates back to theearly years, although many details of geology and geochronology of the host rocks were unknown at that time.
230

The Alpine age of the veins was proposed and defended by Máška (1957), Varček (1962, 1985), and Bernard (1963). Some of them linked the mineralization to
the granites, assuming their Cretaceous age. Rozložník
(1989) insisted on the Cretaceous (or even younger) age
even though at that time, the Permian age of the granites
was already known. On the other hand, Ilavský (1986)
maintained that the mineralizations are Variscan, connected to the voluminous granitic magmatic activity inthose times.
The dichotomy persisted until today. Currently, there
are two competing models that address the origin andtiming of the ore mineralizations in the Gemeric Unit.
Radvanec et al. (2004) linked the siderite-quartz-sulfide

Geochronological constraints on the carbonate-sulfarsenide veins in Dobšináores to Permian regional fluid flow, initiated by the upliftof the Variscan metamorphic core complex. This assignment is based on ‘available indirect data’ and Radvanec
et al. (2004) also stated that ‘more geochronological data
both on metamorphic rocks and individual mineralization
stages are needed to clarify all relationships unambiguously’. On the other hand, Hurai et al. (2006, 2015) tiedthe mineralizations to the deeply buried rocks complexes
under the stack of nappes thrust over the Gemeric Unit
in Cretaceous times. The deep formation environments
were confirmed also by the finds of superdense CO2 (Urban et al. 2006) and N2 inclusions (Hurai et al. 2008),
which are incompatible with the regional fluid flow of arock complex that is being exhumed. The Alpine greenschist facies metamorphic condition is also indicated byzircon fission-track data, which yielded ages of 115–75
Ma. Most probably, these data refer to two stages of the
Gemeric Unit evolution during the Alpine orogeny. The
older ages can be related to thrusting or final emplacement of the Gemeric Unit onto the Veporic Unit (115–100Ma) and the younger ages are related to final exhumationof the Gemeric Unit during the orogenic collapse (90–75
Ma) (cf. Plašienka et al. 2007; Králiková et al. 2016). The
Alpine (mostly Cretaceous) ages were also confirmed
by chemical electron-microprobe (CHIME) and laserablation ICP-MS dating of accessory monazite associatedwith the gangue minerals of the quartz-tourmaline stage.
The relevance of its age, however, to the main sideriteor sulfide stages may be difficult to establish. Some early
K/Ar dating of the shear zones, showing their particular
Cretaceous thermal and dynamic activity (Mesarčík etal. 2001, p. 68) was indirectly questioned by Radvanec
et al. (2004) who concluded that ”Alpine metamorphism... frequently reset Ar isotope systems to Alpine ages”.
One way of addressing this problem is the direct dating of major minerals of the ore veins. Even though notevery mineral can be easily dated, the rapid advance inmulti-collector (MC) mass spectrometers (MS) linked toinductively-coupled plasma (ICP) systems allow datingof matrices, which was unthinkable only a decade ago.
Connected with a superior spatial resolution of a laserablation (LA) system, the LA-MC-ICP-MS analyses of
the U/Pb isotopic system can provide new insights into
geological and metallogenetic questions and resolve, atleast to some extent, the temporal dichotomy of the Gemeric ore veins. There was significant progress in otherisotope systems used in geochronology, for example, theRe/Os system. Apart from molybdenite, used commonly
for Re/Os dating, other minerals, such as arsenopyrite(e.g., Morelli et al. 2005), have been subjected to thistype of analysis. This contribution makes use of thesubstantial progress in geochronological techniques andaddresses the question of the age of the Gemeric mineralization systems in a case study in Dobšiná. We arepresenting LA-MC-ICP-MS U/Pb dating of hydrothermal
siderite and ankerite from Dobšiná, K/Ar ages of the Crmica (fuchsite) closely associated with the ore veins, andRe/Os dating of gersdorffite, a major ore mineral of thehydrothermal veins studied (Halahyjová-Andrusovová
1961, 1964; Kiefer et al. 2017).

2. Geological evolution of the studied
area
The area around Dobšiná is known for its complicatedgeological settings located in the internal zones of theCentral Western Carpathians (CWC). The CWC forms
the core of the Carpathian Orogen, which underwent acomplex Variscan and Alpine structural and metamorphic evolution and it contains several nappe units ofvarious orders and ages. In the study area, there are twomain types of tectonic units (Fig. 1): (i) thick-skinned
thrust sheets, which comprise the pre-Alpine crystalline
basement along with its Upper Palaeozoic to Mesozoic
sedimentary cover (the Veporic and Gemeric units); and(ii) detached cover nappe systems containing Upper Palaeozoic to Mesozoic sedimentary formations with rare
volcanic rocks (the Meliatic, Turnaic, and Silicic units).
The Veporic Unit was overridden by the Gemeric Unit,
which consists of amphibolite-facies metamorphic rocksof the Klátov Group (Spišiak et al. 1985), greenschist facies metabasites and phyllites (ophiolitic) of the Rakovec
Group, and the greenschist facies volcano-sedimentary
sequences of the Gelnica Group. These Gemeric crystalline complexes are covered by Upper Carboniferous
to Middle Triassic volcano-sedimentary succession ofVariscan molasse, affected by subsequent Alpine anchimetamorphism. Younger formations than the Middle
Triassic are unknown in the Gemeric cover sequence. The
main phase of crustal shortening and nappe formation
took place during the late Early Cretaceous (≈137–115
Ma) immediately after the closure of the Triassic–Jurassic Meliata Ocean (≈155–145 Ma), followed by theformation of an accretionary complex (Kozur and Mock
1973; Maluski et al. 1993; Faryad and Henjes-Kunst
1997; Árkai et al. 2003; Dallmeyer et al. 2008; Hurai etal. 2008, 2015; Vozárová et al. 2014). This earliest stageof the Cretaceous N–S convergence in CWC orogenic
wedge is recorded in the Gemeric Unit by the formationof Alpine metamorphic foliation mainly in incompetent
lithologies partly to disjunctive foliations developed incompetent rocks of the unit and probably represent a
gradual transition from bedding or Variscan to Alpine
structure (cf. Hrouda and Faryad 2017).
Later, the tectono-metamorphic overprint is associatedwith the evolution of a large-scale cleavage fan structure
across the entire unit (Lexa et al. 2003). The evolution of231

Stefan Kiefer, Martin Števko, Rastislav Vojtko, Daniel Ozdín, Axel Gerdes, Robert A. Creaser, Marek Szczerba, Juraj Majzlan

Samples and Localities
DBK1 Zemberg-Terézia vein system
DN1 Zemberg-Terézia vein system
DN2 Zemberg-Terézia vein system
DN3 Zemberg-Terézia vein system
DN4 Zemberg-Terézia vein system
DR1 Martini vein system
DR2 Martini vein system
DR3 Martini vein system
DR4 Martini vein system
DE1 Martini vein system

Metabasite andtuﬃtestuﬃtesBasal conglomerateand sandstone

Amphibolite

Grey limestone or
ankerite and siderite

Biotite-amphibolite
gneiss

Sandstone andschists

Fuchsite quartzites,siderite, ankerite –Deposits of the
Georgi-type

Faultsa

Lenses of massive
Fe–Co–Ni arsenides
and sulfarsenides
in metasomatic
siderite-ankerite

Terézia I.
Terézia II.
KruhováKornélia
SevernáVein 2
Hlavná
Vein 1

Co–Ni bearing veins

Mária-Terézia
Vein 3

Zemberg

TešnárkyNJužná
Včelínec
CoburgBaniskáGeorgi

Ezechiel

MartiniFig. 2 Geological profile of the southern slopes of Kruhová hill near the town of Dobšiná (profile simplified after Grecula et al. 1995).

the Gemeric cleavage fan is associated with overthrustingof the Gemeric Unit over the underlying Veporic Unit in
the north during the late Early Cretaceous (Lexa et al.
2003; Jeřábek et al. 2012; Novotná et al. 2015; Vojtko et
al. 2016). The Gemeric Unit is overlain by a subductionrelated melange of sediments and relics of oceanic mantle
rocks of the Jurassic Meliata Ocean. The metamorphic
conditions of Jurassic accretionary wedge composed of
the Meliata, Turňa related rocks correspond to blueschist
or greenschist facies for Alpine regional metamorphism(e.g., Faryad 1995, 1997). The Jurassic accretionary
wedge and the underlying Gemeric Unit are tectonicallyoverlain by extensive horizontally lying unmetamorphosed Silica nappe system derived most probably fromthe Apulian shelf (Fig. 1).
The system of siderite-type hydrothermal carbonatequartz veins with Ni–Fe–Co-Cu ore mineralization near

232

Dobšiná is hosted in Early Paleozoic gneiss-amphibolitecomplex of the Klátov group belonging to the Gemeric
Unit. Minor occurrences of Ni–Fe–Co ores are known
from the metasomatic bodies of siderite hosted in Carboniferous rocks (Fig. 2). The two principal vein systems
were distinguished. The Zemberg-Terézia vein system islocated at the northern margin of the gneiss-amphibolitecomplex and the Martini vein system, which is situatedat the southern margin of the same complex (Rozložník
1959, 1965; Halahyjová-Andrusovová 1964; Grecula etal. 1995; Mesarčík et al. 2001). The Zemberg-Terézia
vein system is subdivided by the N–S trending fault towestern (Zemberg) and eastern (Terézia) segments. The
Zemberg segment generally consists of three (Severná,
Hlavná and Južná vein) and Terézia segment of two(Terézia I., Terézia II.) veins. The Martini vein system consists of four major (Jacobi, Ezechiel, Martini,

Geochronological constraints on the carbonate-sulfarsenide veins in Dobšiná

Tešnárky) and several minor veins (Grecula et al. 1995;
Mesarčík et al. 2001).

3. Samples and Methods
Most of the samples used in this study were collectedin the old mines with a precise localization (Details areincluded in Tab. 1). Samples of fresh hydrothermal carbonates (siderite and dolomite-ankerite series) were collected in-situ in the abandoned adits located both at the
Zemberg-Terézia vein system (Karol adit-DBK1, Lower
Jórémeny adit-DN1 to DN4) and Martini vein system(Remény adit – DR1 to DR4). Fuchsite samples were collected from the Martini vein system (DR3 – undergroundfrom the Remény adit and DE-1 from the dump of theLower Ezechiel adit). Samples rich in the hydrothermal
Ni-Co arsenides were collected by Halahyjová-Andrusovová (1961, 1964) from both vein systems and recently
re-investigated by Kiefer et al. (2017).
All samples have been examined in the form of polished sections with a Zeiss-Axio-Imager.M2m optical
microscope equipped with a CCD-camera system. The
chemical composition of the hydrothermal carbonates andthe ore minerals was determined by electron microprobe
analyses using a JEOL JXA-8230 instrument. For the
carbonate minerals, the operating conditions were set toan accelerating voltage of 15 kV, a beam current of 15 nA
and a beam diameter of 1 µm. The wavelength-dispersive
X-ray spectrometers were used to measure the elements
and X-ray lines of Mg (Kα), Si (Kα), Sr (Lα), Ba (Mα), Ca(Kα), Fe (Kα), and Mn (Kα). To improve the count-ratestatistics, the counting times were 40 s. The standard
specimens used for calibration were MgO for Mg, rhodonite for Si and Mn, celestine for Sr, baryte for Ba,
apatite for Ca, and hematite for Fe. The detection limits,
calculated from the peak and background counts, themeasurement time and the standard material concentration, are 0.03 wt. % for Si, 0.02 wt. % for Mg, 0.09 wt. %for Ba, 0.09 wt. % for Sr, 0.02 wt. % for Ca, 0.05 wt. %for Fe and 0.05 wt. % for Mn. The ore minerals, mainly
sulfarsenides, were analyzed with an accelerating voltage
of 20 kV, a beam current of 50 nA, a beam diameter of
1 µm and counting times of 40 s. The measured elements
and lines were As (Kα), S (Kα), Sb (Lα), Pb(Mα), Ni (Kα),
Co (Kα), and Fe (Kα). The standard specimens used were:arsenopyrite for As, pyrite for S and Fe, pure metal standards for Ni and Co, galena for Pb and stibnite for Sb.
The detection limits, calculated after the same procedureas the above mentioned, were: 0.04 wt. % for Sb, 0.08
wt. % for Pb, 0.07 wt. % for As, and 0.02 wt. % for S,
Co, Cu, Fe, and Ni. The elements analyzed but not listed
in the corresponding tables (Pb, Cu) were always below
the detection limit.The rocks at the vast majority of exposures possess particular planar structure. In metamorphic rocks, a planar structure is known as metamorphic
or tectonic foliation (cleavage, schistosity). In this paper,
we use foliation as a general term for all pervasively
developed planar structures found in rocks. The second
type of planar structure were veins with their typicaltabular shape, which were deformed during the tectonogenesis. Structures in rocks with linear, as opposed toplanar, the character also occur in many forms. Observed
lineations are both mineral lineations (linear alignmentsof minerals in metamorphic tectonites) and stretching
lineations defined by strained objects with elongated,cigar-like shapes. These lineations were measured on veinsurfaces to provide information on the direction of movement during or after the vein formation. All oriented data
were processed in the stereographic projection (Lambertnet), lower hemisphere using the FaultKin 7.5 (Marrett
and Allmendinger 1990; Allmendinger et al. 2013).Uranium-Pb ages were acquired in situ in thick polishedsection by laser ablation-sector field-inductively coupled

Tab. 1 Localities and general mineralogy of the samples used in this work
SampleDR1

Location
Zemberg–Terézia vein system,
Karol adit–Južná vein
Zemberg–Terézia vein system,
Lower Jórémeny adit–Severná vein
Zemberg–Terézia vein system,
Lower Jórémeny adit–Severná vein
Zemberg–Terézia vein system,
Lower Jórémeny adit–Severná vein
Zemberg–Terézia vein system,
Lower Jórémeny adit–Severná vein
Martini vein system, Remény adit–Jacobi vein

DR2

Martini vein system, Remény adit–Jacobi vein

DR3
DR4
DE1

Martini vein system, Remény adit–Jacobi vein
Martini vein system, Remény adit–Jacobi vein
Martini vein system, Lower Ezechiel adit–dump

DBK1
DN1
DN2
DN3
DN4

Mineral

Associated minerals

Host rockdolomite–ankerite series

quartz, nickeline, gersdorffite

dolomite–ankerite series

quartz, arsenopyrite, gersdorffite gneiss–amphibolite

dolomite–ankerite series

quartz, arsenopyrite, gersdorffite gneiss–amphibolite

dolomite–ankerite series

quartz, arsenopyrite, gersdorffite gneiss–amphibolite

dolomite–ankerite series

quartz, arsenopyrite, gersdorffite gneiss–amphibolite

sideritesiderite, dolomite–ankerite
seriesfuchsite
dolomite–ankerite series
fuchsite

gersdorffite

gneissgersdorffite

gneissquartz, siderite
arsenopyrite
siderite, gersdorffite

gneissgneissgneissgneiss–amphibolite

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Stefan Kiefer, Martin Števko, Rastislav Vojtko, Daniel Ozdín, Axel Gerdes, Robert A. Creaser, Marek Szczerba, Juraj Majzlan

plasma-mass spectrometry (LA-SF-ICPMS) at the Goethe
University Frankfurt (GUF). At GUF a ThermoScientific
Element 2 sector field ICP-MS is coupled to a
RESOLution S-155 (Resonetics) 193 nm ArF Excimer
laser (CompexPro 102, Coherent) equipped with a twovolume ablation cell (Laurin Technic, Australia). Static
ablation in a helium atmosphere (0.3 l/min) used a spot
size of 213 µm and a fluence of ca. 1 J cm-2 at 6 Hz.
Soda-lime glass SRM-NIST614 was used as a reference
glass together with 3 carbonate standards to bracket
sample analysis. Raw data were corrected offline using amacro-based in-house MS Excel© spread­sheet program(Gerdes and Zeh 2009). The 207Pb/206Pb ratio was corrected for mass bias (0.6 %/amu) and the 206Pb/238U ratio
for inter-element fraction (ca. 5 %). Due to the carbonate
matrix, additional correction of 9 % has been applied on
the 206Pb/238U, which was determined using WC-1 carbonate reference material dated by TIMS (254 ± 6 Ma; Roberts et al. 2017). All reported uncertainties are the 2σ
level. For the Re/Os geochronological analyses, acquiredat the University of Alberta, ten samples of Ni–Fe–Co
arsenides were selected for preliminary analyses, andonly three of them contained sufficient Re to be considered further. These three samples were analyzed by themethods described in detail by Selby and Creaser (2004)
and Morelli et al. (2010). Initially, the Re abundance ineach of 3 mineral separates was tested by isotope dilution
analysis and found to be suitable for one sample only
(sample 436: 10.0 ppb Re, 12d: 0.2 ppb Re, 25b: 2.0 ppb
Re). An initial full Re-Os analysis revealed little common
Os was present, with 187Os* comprising > 99.7 % of the
Os measured. As such, for this work, a mixed double
spike containing known amounts of isotopically enriched
185
Re, 190Os, and 188Os analysis was then used (Markey etal. 2007). The 187Re and 187Os concentrations were determined by isotope dilution mass spectrometry using
Carius-tube, solvent extraction, anion chromatography,

and negative thermal ionization mass spectrometry techniques. The isotopic analysis used a ThermoScientific
Triton mass spectrometer by Faraday collector. Total
procedural blanks for Re and Os are less than <1 picograms and 0.1 picograms, respectively, which are insignificant in comparison to the Re and Os concentrations
present. The Reference Material 8599 Henderson molybdenite (Markey et al. 2007) is routinely analyzed as astandard, and during the past five years returned an average Re-Os date of 27.78 ± 0.07 Ma (n = 15), indistinguishable from the Reference Age Value of 27.66 ± 0.1
Ma (Wise and Watters 2011). The 187Re decay constantused is 1.666 × 10–11 a–1 (Smoliar et al 1996).For the K/
Ar dating, two portions of each sample (8–16 mg and≈30 mg) were weighed. A small quantity of CuO (approximately 0.1 mg) was added to the lighter portion.
This sample was then wrapped in an Al-foil and proceeded to radiogenic argon measurement. The heavier
portion was weighted directly on platinum evaporator andused for potassium measurement.Radiogenic argon measurements were performed on Nu Instruments Noblesse
multicollector noble-gas spectrometer (NG 039). The
Al-foil wrapped portions was melted in a double-vacuum
resistant crucible at ≈1200 °C. Pure 38Ar produced by theInstitute for Inorganic and Physical Chemistry of Bern
University was used as the spike. CuO added to thesamples enhances the oxidation of organic matter during
this step. Titanium sponge acted as a sublimation getter
and was always put to the place of the reactor where
samples are heated. This was the first cleaning level ofgases extracted from the samples. The final purificationof argon was carried out in an isolated section of the line
by a warm getter pump (D-100, SAES Getters), monitored not to release additional argon atoms (after installation it was activated for 12 h). Small portions of gas
aliquot released from the sample (ca. 1 %) were thenmeasured at least three times by opening and closing

Fig. 3a – Fuchsite-rich rocks with veinlets of siderite. b – Coarse-grained siderite (brownish) and ankerite (white) with gersdorffite (metallic grey)mineralisation. Jóremény adit. Both photographs courtesy of D. Peterec.

234

Geochronological constraints on the carbonate-sulfarsenide veins in Dobšiná

valves of the preparatory line in a certain sequence. Blank
measurements were performed after every sample andconfirmed that released radiogenic 40Ar for every sample
was above 99.5 %. Every day, the 40Ar/36Ar and 40Ar/38Ar
ratios are measured for air sample aliquots, deliveredfrom a calibrated air pipette. Based on this results
40
Ar/36Ar and 40Ar/38Ar ratios were corrected for instrument mass fractionation and detector efficiencies assuming atmospheric ratios of ( 40Ar/36Ar)air = 298.57 and(40Ar/38Ar)air = 1583.52 (Lee et al. 2006). The amount of
the original aliquot of 38Ar spike was determined bymeasuring international standard GL‑O glauconite, performed every day. For this series of samples 1σ for GLO
was 0.7 % (13 samples). Standard age errors were calculated from the law of error propagation, also taking thesevalues into account.The potassium contents were measured using Sherwood Model 420 flame photometer on
HF dissolved samples. The maximum error of this measurement was estimated to be equal 0.03 % K. Based on
six measurements, K content in GL-O glauconite wasslightly higher: 6.61 ± 0.02 %, than that of the standard:
6.56 ± 0.06 % (Odin 1982). LP-6 biotite has a slightly
lower amount of K: 8.27 ± 0.022 % (4 measurements)
comparing to reference value: 8.33 ± 0.03 % (Odin 1982).
Both measured standards are within reference ranges assuming 0.03 % K error.

4. Results

4.1. Laboratory and field observations
4.1.1. Macroscopic observations
Samples collected for this work are either fuchsite-richcountry rocks or hand specimens from ore veins, collectedin old mines. Samples from both old dumps and debris
were avoided as much as possible because their origin andassociation with the ore veins are difficult to ascertain. Various kinds of metapelites (phyllite) and, less frequently, also
metasandstone, with intercalations of metamorphosed mafic
volcanic to volcanoclastic rocks of the Northen Gemeric
Unit, are predominant host rocks of the mineralized veins.
The fuchsite-rich rocks are massive, fine-grained,
distinctly green rocks that form thick (tens of meters)
alteration haloes. They are not necessarily directly associated with the ore veins. If intermingled with hydrothermal
siderite or ankerite, the carbonates are clearly younger
than the fuchsite-rich rocks (Fig. 3a).The ore veins form
tabular bodies and lenses, with a thickness of up to 1 m.
Over a distance of a few meters, however, the thickness may change drastically, sometimes to only a few
millimetres filled by greyish clay. The veins are usuallyaccompanied by a splintery rim of black, carbonaceous

Fig. 4 Sawn slabs of samples from Dobšiná. The boxes show positionsof the sections used for dating. a – Siderite (brownish, Sid I) cut by anankerite (white, Ank I) veinlet. The metallic grey mineralisation consistsof Ni–Fe–Co sulfarsenides. b, c – Ankerite (Ank I) with dispersed ore
mineralization.

matter. The main minerals of the veins are carbonates,
either siderite or ankerite. Siderite is especially common
in the lower parts of the mineralized zone (Remény adit)
and occurs there together with ankerite. In the higher
parts (Zemberg, Jóremény), siderite is scarce. If they arefound together, siderite is older than ankerite (Fig. 4a).
The Ni–Fe–Co sulfarsenides are usually dispersed in
the carbonates (Figs. 4a–c), localized in small veinlets
or zones near the contact of the veins with the hostrocks. The Ni–Fe–Co sulfarsenides are younger than themain carbonate mass (Fig. 3b). They are hosted by bothtypes of carbonates but, it some cases, whitish anker235

Stefan Kiefer, Martin Števko, Rastislav Vojtko, Daniel Ozdín, Axel Gerdes, Robert A. Creaser, Marek Szczerba, Juraj Majzlan

itic carbonate appears to be even younger than the ore
minerals.A common, although a not volumetrically abundant component of the veins is quartz with tourmaline,
chlorite, and albite. These minerals form pockets in the
carbonates, but a precise temporal relationship to siderite,
ankerite, and ore minerals was challenging to determine.
The uppermost portions of the veins at the Kruhová(Gugl) hill contain siderite, baryte, and cinnabar. The
relationship of these minerals to the main mineralization
is not clear because they can be observed only as hand
specimens in shallow pits.

4.1.2. Structural observations
Structural observations were carried out only in the abandoned Remény adit close to Dobšiná town because of its
accessibility at the time of this study. Data for geometricaland structural analysis were obtained from the main mineralized zone emplaced in the Lower Palaeozoic metapelites
in the adit. In this site, the geometry of the mineralizedzone is different from other mineralized localities close
to Dobšiná. In the broader area, the mineralized veins
generally strike W–E with southward inclination. In the
Remény adit, the mineralized structures can be roughly
divided into two geometrically different groups. The mainzone has the same strike as the other zone in the broader
area of Dobšiná, but the dip is northwards (Fig. 5a). This
relatively older structure is intersected by most probablya)slightly younger vein system with the NW–SE direction
with the dip south-westwards (Fig. 5b).
The thickness of the main mineralized zone varies fromseveral centimetres to several decimetres. It is a mylonite
zone with shallow to medium inclination, with uniform
both strike and dip in the entire accessible and observable
length in the Reméy adit. The measured structured along
the mylonite zone refer to an extensional tectonic regime
with the orientation of the principal minimal stress axis
(σ3 = 185/04°) in the NNE–SSW direction and subverticalprincipal maximal stress axis (σ1 = 066/83°). The mineralized mylonite zone generally conforms to the Variscan–?Alpine metamorphic foliation and the slight variationsin its dip are caused by younger folding. It is possible toassume that the mylonite zone was formed as a heterogeneous structure. However, a further detailed structural investigation is needed. Firstly, the mylonite zone could also
be formed by the compressional tectonic regime (Early
Cretaceous) and later (Mid- to Late Cretaceous) was overprinted by a younger low-angle normal fault mechanism.
Unfortunately, limited access to the structure did not allow
us more detailed structural research.
The younger mineralized veins are generally thinnerand less significant. The veins are steeper than the older
mineralized mylonite zone and were formed during an extensional tectonic regime with the orientation of the principal minimal stress axis (σ3 = 207/02°) in the NE–SW
direction and subvertical principal minimal stress axisb)Fig. 5 Stereograms of observed mineralized structures with movement directions in the Remény adit near Dobšiná. a – great circles and movement
trajectories in the main mineralized zone; b – great circles and movement trajectories of the younger structures than the main mineralized zone.
Note: the planar structures are drawn as the great circles, and the movement trajectories (lineation on that planes) are represented by arrows. The
compressional area with calculated principal compressional stress axes (σ1) is depicted by red contours and dots. The tensional area with calculated
principal extensional axes (σ3) is shown by blue. Stereograms use Lambert projection, lower hemisphere.

236

Geochronological constraints on the carbonate-sulfarsenide veins in Dobšiná

Tab. 2 Representative chemical compositions of carbonates from Dobšiná (in wt. %) in the samples used in this work.
Sid I

Sid II

FeO
48.18
MnO
2.18
MgO
4.52
CaO
0.19
BaO
<D.L.
SrO
<D.L.
Total
55.20
<D.L. – below detection limit

42.71
0.59
9.65
1.41
<D.L
<D.L.
54.46

Siderite(disseminated)
46.63
3.16
5.31
0.35
<D.L.
<D.L.
55.46

(σ1 = 109/73°). These younger veins disrupt the Variscan–?Alpine metamorphic foliation and Alpine mineralized
aforementioned mylonite zone. According to the crosscut
criteria, the faulting and mineralization process have tobe younger than the Gemeric cleavage fan. Conditionally,
it could be linked with the exhumation process in the Gemeric Unit. However, it is still not entirely clear whether
the movement in these two zones is directly related to themineralization process or it is the subsequent process offaulting in the Gemeric Unit or both.

4.1.3. Observations in optical and electron
microscope

Ank I

Ank II

Ank III

17.10
2.63
6.91
26.63
<D.L.
<D.L.
53.42

12.48
0.95
10.71
28.41
<D.L.
<D.L.
52.68

16.06
2.05
7.66
27.61
<D.L.
<D.L.
53.51

positional range of Ank III is 19.8–20.8 mol. % MgO,
52.2–53.0 mol. % CaO, 23.5–24.4 mol. % FeO, and 2.7–
3.7 mol. % MnO. This carbonate is homogeneous whencompared to Ank I and II. It is comparable to carbonate
chemistry of Ank I (more Fe and less Mg than Ank II).
Siderite in the investigated samples can be found as massive coarse-grained siderite in carbonate-sulfarsenide veins(Figs. 3, 4a, 6c) or as siderite in the quartz-fuchsite rocks(Figs. 3a, 6e). The main mass of vein siderite is denoted as
Sid I and has a compositional range of 11.0–16.4 mol. %MgO, 0.1–1.0 mol. % CaO, 78.4–83.7 mol. % FeO, and2.8–5.6 mol. % MnO. Siderite from the quartz-fuchsite
rocks has a similar composition of 12.0–24.1 mol. % MgO,
0.2–1.4 mol. % CaO, 70.7–82.8 mol. % FeO, and 4.2–6.1
mol. % MnO. We assume that this siderite is also Sid I and
was brought into these rocks during the siderite sub-stage.
Later siderite, denoted as Sid II, is found is thinveinlets populated by grains and crystals of Ni–Fe–Co
sulfarsenides or as rims around these minerals (Fig. 6c).
The compositional range of Sid II is 23.7–29.7 mol. %MgO, 0.8–6.4 mol. % CaO, 65.5–72.5 mol. % FeO,
and 0.5–1.8 mol. % MnO. The results of all WDX spot
analyses on carbonates can be seen in the electronic
supplementary table S1.1.

Back-scattered electron (BSE) images (Fig. 6) and WDS
analyses (Tab. 2, Fig.7a) distinguished multiple generations of ankerite (Ank I–III) and siderite (Sid I and II).
Ankerite I (Ank I) is the dominant carbonate phase in all
samples (Figs. 3b, 4) and shows a compositional rangeof 15.6–21.2 mol. % MgO, 47.0–53.3 mol. % CaO,
21.5–29.1 mol. % FeO and 3.0–5.0 mol. % MnO.
Ankerite II (Ank II) forms veinlets (hundreds ofmicrometres, Fig. 6a) in Ank I. The compositional
range of Ank II is 25.1–29.1 mol. % MgO, 51.3–55.2
mol. % CaO, 15.0–20.2 mol. % FeO and 1.1–1.7 mol. %MnO. In general, Ank II has less Fe than Ank I (mean
4.1.4. Ore mineralization
concentration Ank IFe = 26.8 mol. % vs Ank IIFe = 18.1
Optical microscopy and BSE images suggest that themol. %) and more Mg (mean concentration Ank IMg =ore mineralization, represented mostly by Ni–Fe–Co
18.6 mol. % vs Ank IIMg = 27.7 mol. %). Some Mg/Carich compositions might violatecompositional rules for carbon- Tab. 3 Representative chemical compositions of sulfarsenides and arsenides from Dobšiná (in wt. %)ate-nomenclature and therefore from the samples used for geochronological dating in this work. Mineral abbreviations: gdf = gersdorcould be labelled as dolomite ffite, aspy = arsenopyrite
or Fe-dolomite. However, those
DR2
DN3
DN3
DN2
436
compositions only represent
gdf
aspy
gdf
gdf
gdf
minor portions and are textur- Sb
0.14
<D.L.
0.09
0.09
0.09
13.94
19.18
15.56
11.37
15.74
ally not isolated and therefore S
51.67
45.51
49.43
54.83
49.87
are part of the corresponding As
Co
0.60
<D.L.
7.09
4.11
2.48
ankerite stage.
Ni
32.17
0.09
20.82
26.13
29.47
Ankerite III (Ank III) can
Fe
1.35
34.62
7.12
2.99
1.86
be found as veins and fractures
Total
99.87
99.40
100.1199.52
99.50
cutting through Ank I, Ank II,
<D.L.
–
below
detection
limit
and siderite (Fig. 6b). The com237

Stefan Kiefer, Martin Števko, Rastislav Vojtko, Daniel Ozdín, Axel Gerdes, Robert A. Creaser, Marek Szczerba, Juraj Majzlan

a

ank I

b

ank III
ank I
gdf

ank II200 μm

100 μm

c

d

gdf
sid I

200 μmsid II200 μm

e
fu
sid
qtz

chr

100 μmsulfarsenides, is hosted by all carbonates except for thelate Ank III. The ore minerals found are arsenopyrite andNi-rich sulfarsenides, mainly gersdorffite and membersof the gersdorffite-arsenopyrite solid-solution (Tab. 3,
Fig. 7b). Chemical composition of ore minerals in thesystem Ni–Fe–Co–As–S is defined by a great chemicalvariability and abundant polymorphism. Such variability
is also observed in the samples from Dobšiná (Kiefer et
238

Fig. 6 Back-scattered electron (BSE) images of the samples from Dobšiná. The square holes in some images are traces after laser ablation.
a – Early ankerite I with thin veinlets of ankerite II. Note that the ore
mineralization is localized in Ank II. b – Early ankerite I with veinlets
of the remobilized ankerite III. c – Sideritic carbonates with ore mineralization. d – Gersdorffite with oscillatory zonality. e – Intergrowth offuchsite (fu), siderite (sid), quartz (qtz), and corroded relics of chromite(chr) in fuchsitic rocks.

al. 2017), but it is not bound to a particular mineralization
stage but rather a general observation for all samples.
Gersdorffite occurs as small disseminated crystals
in Ank II (Fig. 6a), as large strongly zoned and often
highly fractured assemblages in Ank I and siderite(Figs. 3b, 6c, d) and it is the most common ore mineral in the investigated samples.The compositional
range of gersdorffite is 0–0.5 wt. % Sb, 10.8–19.8 wt. %Geochronological constraints on the carbonate-sulfarsenide veins in Dobšiná

Ni

FeO+MnO

siderite in fuchsite-rich rocks

a)

b)

sid I
sid IIat.%at.%

ank I, ank III
ank II

CaO

MgO

Co

Fe

Fig. 7 Triangular diagrams showing a – the composition of the generations of hydrothermal carbonates and b – composition of the Ni–Fe–Co
sulfarsenides in the samples used for dating.

S, 43.2–63.1 wt. % As, 0.3–12.7 wt. % Co, 0–0.03
meaningful regression lines with sufficient variation in
wt. % Cu, 13.8–34.2 wt. % Ni, and 0.4–11.7 wt. % Fe,
the U/Pb to calculate lower intercept ages. Representative
highlighting the variations from almost ideal stoichiTera-Wasserburg diagrams are shown in Fig. 8. Threeometries (e.g., Ni0.94Co0.04Fe0.01As1.00S1.01) to substantial
samples (DN1, DR2, and DN3) gave older and younger
deviations, therefore (e.g., Ni 0.54Co 0.21Fe 0.28As 1.04S 0.94,
ages and one sample (DN2) only one younger age. The
Ni 0.86 Co 0.07 Fe 0.08 As 1.18 S 0.82 or
Ni0.98Co0.03Fe0.02As1.52S0.45) includ- Tab. 4 Geochronological data for the carbonate-sulfarsenide veins from Dobšiná.
ing some high-As compositions sample
methodage (Ma)
mineral analyzed
145.3 ± 5.1
corresponding to rammelsberAnk I
U/Pb LA–SF–ICP–MS
20.7 ± 5.3
gite/krutovite-like stoichiom- DN1
Ank III
etries.Arsenopyrite is a minor DN2
U/Pb LA–SF–ICP–MS
27.4 ± 6.2
Ank III
phase and only occurs as small
114 ± 24
Ank I
disseminated crystals in Ank II DN3
U/Pb LA–SF–ICP–MS
29.5 ± 5.7
Ank III
together with gersdorffite. The
U/Pb LA–SF–ICP–MS
118 ± 10
Sid I
mineral chemistry of arsenopy- DR2
Pr1
K/Ar
109.5 ± 3.4
fuchsiterite is relatively constant and apK/Ar
108.6 ± 5.9
fuchsite
proaches the mean stoichiometry Pr2
95.7 ± 1.2
gdf
Re/Os
of Fe 1.02As 1.00S 0.98. The results 436
93.6 ± 0.9
gdf
of all WDX spot analyses on
sulfarsenides and arsenides can Tab. 5 Mineralogical composition and geochronological data for the fuchsite samples from Dobšiná
be seen in the electronic supplesample
Pr1 repetition
Pr1
Pr2
mentary table S1.2.

4.2. Geochronology
4.2.1. U/Pb LA-SF-ICP-MS
datingon carbonates
Nine samples (DR1–DR4, DN1–
DN4, DBK1) were selected for
the U/Pb dating, but only fourgave one or two statisticallymineralogical composition (all data in wt. %)quartzsieritekaolinite
mica 2M1 (fuchsite)
total
K/Ar isotopic age data
mass (mg)
K2O (%)K (%)
40
Ar* (%)age (Ma)
2σ (Ma)

45
11

59
2
2
37
100

44
100
16.18
4.82
4.00
43
109.5
5.8

8.19
4.82
4.00
66
109.6
3.9

8.85
4.68
3.88
42.3
108.6
5.9

239

Stefan Kiefer, Martin Števko, Rastislav Vojtko, Daniel Ozdín, Axel Gerdes, Robert A. Creaser, Marek Szczerba, Juraj Majzlan

sample DN1
ankerite

0.9

0.8

0.83
207Pb/206Pb

207Pb/206Pb

sample DN2
ankerite

0.85

20.7 ± 5.3 Ma on Ank III

0.7

0.6

0.81
27.4 ± 6.2 Ma
on Ank I

0.79

145.3 ± 5.1 Ma on Ank I
0.5

0.4

0.77

0

4

8

238

12

U/ 206Pb

16

20

0.75

24

sample DN3
ankerite

0.84

0

4

8

238

12

16

U/ 206Pb

20

sample DR2
siderite

0.84

0.80

29.5 ± 5.7 Ma
on Ank III

0.80

207Pb/206Pb

207Pb/206Pb

0.82

0.78
114 ± 24 Ma on Ank I

0.76

118 ± 10 Ma on Sid I

0.76

0.72
0.74
0.72

0

2

238

4

U/ 206Pb

6

8

0.68

0

2

4

238

6

U/ 206Pb

8

10

12

Fig. 8 Tera-Wasserburg diagrams for the carbonate samples dated in this study by LA–SF–ICP–MS. Note, that for display reasons, plots showing
young and old isochrones are merged from two plots.

older isochron ages vary between 145 ± 5 Ma and 114 ± 24
Ma. All old ages have been acquired on Ank I and Sid I(Tab. 4). The three younger isochron ages vary between
20.7 and 29.5 Ma and are assigned to Ank III (Tab. 4).
The U and Pb concentrations as well as the calculated
isotopic ratios of the four samples DN1, DN2, DN3 andDR2 can be seen in the electronic supplementary table
S2.K/Ar dating of fuchsite
Two samples from the wall-rock alteration werechosen for K/Ar-dating. A preliminary investigation by
XRD showed that both samples contained mostly quartz
and fuchsite and to a lesser amount siderite and kaolinite(Tab. 5). Both samples gave similar ages averaging at 109
Ma (Tabs 4 and 5).

4.2.1. Re/Os dating of gersdorffite
Nine representative gersdorffite samples were preanalyzed or analyzed but only the sample 436 yielded240

an age, with a value of 95.7 ± 1.2 Ma (Tab. 6). The age
uncertainty is quoted at 2σ level and includes all known
analytical uncertainty, including a ≈0.31 % uncertainty
in the decay constant of 187Re. The gersdorffite sample
analysis was replicated using a larger sample aliquot andyielded a more precise age of 93.6 ± 0.9 Ma, which lies
within the uncertainty of the first analysis. To our best
knowledge, this datum is the first Re/Os age obtained
from gersdorffite.

5. Discussion

5.1. Precipitation sequence of hydrothermal
minerals in Dobšiná
The precipitation sequence of hydrothermal minerals in
Dobšiná was established by Halahyjová-Andrusovová(1959) on many samples from the old mines that were

Geochronological constraints on the carbonate-sulfarsenide veins in Dobšiná

accessible during the explora- Tab. 6 Re/Os isotopic and age data for gersdorffite from Dobšiná (sample 436)
tion activities in the 1950s. The Sample
187
187
Re (ppb)
Re (ppb)
Os (ppb)
Model age (Ma)sequence published by Halahy- 436
12.14 ± 0.10
7.630 ± 0.063
0.01218 ± 0.00011
95.7 ± 1.2
jová-Andrusovová (1959) can 436 repetition
11.40 ± 0.09
7.165 ± 0.056
0.01118 ± 0.00004
93.6 ± 0.9
be augmented by observations
done in this work, the work of Kiefer et al. (2017), andconclusion also agrees with the earlier studies; Hurai
especially correlation with siderite-sulfide deposits in
et al. (2008) assert that the main carbonate associatedthe Gemeric Unit (Varček 1985; Chovan et al. 1994;
with the sulfide stage is ankerite (Fig. 3b). Ni–Fe–Co
Hurai et al. 2008). The sequence constructed here (Fig.
arsenides are scarce minerals in the ore deposits in the9) focuses mainly on the Ni–Fe–Co veins and neglects
Gemeric Unit, except Dobšiná and some veins at theother important ores found in the Gemeric Unit, e.g., theRudňany and Rožňava deposits (Koděra et al. 1990).
quartz-sulfide phase with stibnite (see Fig. 5 in Hurai et
If considered in the precipitation sequence, they are
al. 2008).
usually placed early in the quartz-sulfide stage (e.g.,
All these authors agree that fuchsite and the asRojkovič 1977; Chovan et al. 1994). Their chemicalsociated minerals represent the oldest–fuchsite stage,
and mineralogical evolution in Dobšiná was described
either in Rudňany (cf. Rojkovič 1977) or elsewhere.
in detail by Kiefer et al. (2017), but there is no reason,
In our samples from Dobšiná, siderite-ankerite veinlets
either from that or the current work, to assume that they
penetrate the fuchsitic rocks (Fig. 3a), thus confirming
were formed in several separate hydrothermal pulses or
the observations from Rudňany. The following siderite
events. The Ni–Fe–Co arsenides are often crushed andsub-stage (Hurai et al. 2008) is mineralogically simple,
healed by younger Cu sulfides, either chalcopyrite orwith dominant siderite and subordinate ankerite. The
tetrahedrite. A simple placement of the arsenides withbaryte sub-stage is prominently developed at somethe later Cu sulfides in one stage, without indication
deposits, for example in Rudňany, but not in Dobšiná.
of the tectonic damage, is therefore questionable. We,
Baryte, together with siderite and cinnabar, does occur
therefore, distinguish the Ni–Co and Cu–Sb (chalcopyin the highest portions of the veins in Dobšiná. The
rite-tetrahedrite) mineralizations as two separate subhand specimens, recovered from shallow surface pits atstages of the quartz-sulfide stage (Fig. 9).
the Kruhová hill do not allow
the placement of this baryte
phasesideriter*quartz-sulﬁdeinto the precipitation sequence.
quartzWe assume, however, that its
tourmalineﬆage
Fu
position in the precipitation
sub-ﬆage
Sid BaNi-Co
Cu-Sb
sequence is the same as in theRudňany deposit.
quartzThe quartz-tourmaline stage
fuchsiteis devoid of ore minerals andsericitedeveloped locally. Owing totourmaline
the common occurrence ofalbitesid I
sid II
monazite, this stage has beensideritea subject of previous geochroankerite
ank III
ank I
ank II
nological work (e.g., Hurai etbaryteal. 2015) and provides an impyrite
portant marker that separates,
arsenopyrite
in time, the older (fuchsite,
krutovite
siderite, baryte) and youngrammelsbergiteer (sulfide) stages.Numerous
gersdorﬃte
observations in the old mines
nickeline
suggest that ankerite is generskutterudite
ally younger than siderite. This
Fig. 9 Precipitation sequence of minerals
from Dobšiná. Compiled from the workof Halahyjová–Andrusovová (1959),
Varček 1985, Chovan et al. 1994, Huraiet al. 2008, Kiefer et al. 2017, and thiswork.

löllingite
chalcopyrite
tetrahedrite
aikinite
cinnabar* remobilization phase

241

Stefan Kiefer, Martin Števko, Rastislav Vojtko, Daniel Ozdín, Axel Gerdes, Robert A. Creaser, Marek Szczerba, Juraj Majzlan

5.2. Age constraints on the fuchsite stage
There is no doubt that fuchsite, together with quartz andcarbonates, constitute the oldest mineralization stage
in Dobšiná and Rudňany (cf. Rojkovič 1977; Hurai etal. 2008). Fuchsite-carbonate-quartz rocks have also
been described from several other sites in the Gemeric
Unit (Roztoky, Bindt, Stará Voda, Nálepkovo, Mlynky,
Mišíkova skala, Košické Hámre, Hnilec) and interpreted
as a final product of metasomatic alteration of ultrabasic
or basic rocks by CO2-rich hydrothermal fluids (Ivan1982).
Fuchsite from the Gemeric Unit has been subjected
to several K/Ar geochronological studies. The value
determined from Dobšiná (Augustín quarry, Malá Vlčia
dolina) is 190.8 Ma (0.83 % K2O) (Cambel et al. 1980).
This work reported K/Ar age determinations from metamorphic rocks in the broader area of Dobšiná, some ofthem with clear signs of retrograde metamorphism. Theseages scatter considerably and are 273, 258, 227, 154 Ma.
Fuchsite was also dated in Rudňany to 210 ± 2 Ma (1.11 %K2O) (Bagdasaryan et al. 1977), later revisited and revised to 205 Ma, with another datum of 144 Ma (Cambel
et al. 1990). The significance of these scattered values
in comparison with the newer geochronological datais questionable. Our K/Ar ages for fuchsite of 109 and
110 Ma (4.68–4.82 % K2O) (Tab. 5 and Fig. 10) are distinctly younger than the previously published values andalso contradict the U/Pb age of the carbonates (Tab. 4)
which are texturally younger than fuchsite (Fig. 3a). Low
K-amounts in previous measurements can also indicatethat these dates correspond to rocks containing low
amounts of fuchsite. Early on, it was noted that the K/Ar
ages in this area are scattered and contradictory (Hovorka
and Spišiak 1981) and may have been subject to resetting during the intensive Alpine reworking (Radvanec etal. 2004). Similar ages (clustered around 100 Ma) wereobtained by Rb/Sr (Kovách et al. 1986) or K/Ar (Kantor and Rybár 1979) dating of sheet silicates from theGemeric Unit. Given that fuchsite is a Cr-rich variety of
muscovite, the closure temperature of 350 ± 40 °C (Spearand Parrish 1996) also applies to this mineral. Fuchsite
from Dobšiná is a product of alteration, although not too
fine-grained, and its closure temperature may lie near the
lower end of the interval specified by Spear and Parrish(1996). Additionally, fluid-assisted recrystallization compared to pure temperature-controlled diffusion can be ofimportance, as was shown, e.g. by Tartèse et al. (2011). If
taken at its face value, the ≈110 Ma could be interpreted
as the age of the last cooling below ≈320 °C or lower,
due to the influence of fluids (e.g., Villa 2015). Thistemperature coincides with the last hydrothermal pulses
related to the crystallization of the hydrothermal carbonates (see below). This interpretation is also supported
242with the estimates of peak P-T conditions of the Alpine
metamorphism in the Gemeric Unit of 320–350 °C and
0.4–0.55 GPa (Faryad and Dianiška 1999). This event,correlated with the geochronological data of Dallmeyer
et al. (1996), was assigned Cretaceous age (120–86 Ma)
and the zircon fission-track data (Plašienka et al. 2007;
Králiková et al. 2016). The meaning of the previously
published, older ages remains questionable. Why, forexample, are the two ages of fuchsite from one locality,Rudňany, 60 Ma apart (Cambel et al. 1990)? They may
represent mixed ages of several minerals in the bulk
samples used at that time. Some of the minerals may havebeen formed earlier than fuchsite and experienced partial
Ar loss during the Alpine times (e.g., hornblende fromamphibolites). Argon loss could be locally enhanced by
the influence of hydrothermal fluids, promoting recrystallization, producing thus varying ages from a single
locality.

5.3. Age constraints on the carbonate minerals
The carbonate minerals found in Dobšiná, either siderite
or ankerite, formed mostly during the siderite sub-stage(Fig. 9). Among the massive vein carbonates, siderite(sid I) is usually older than ankerite (Ank I). The BSE
images (Fig. 6) document repeated remobilization ofcarbonates, thus suggesting that their mutual relationship and assignment to a particular stage may not be
straightforward.
There are no geochronological data for the hydrothermal carbonates in the Gemeric Unit. The two competingand conflicting models argue for Permian (Radvanec etal. 1994a, b, 2004; Radvanec and Grecula 2016) or Cretaceous (Hurai et al. 2002, 2008) age of these and associated minerals. They are based on geological arguments,
application of geothermobarometry, and estimation of hydrothermal fluid properties and chemistry.All our results
place the formation of the hydrothermal carbonates inDobšiná to Cretaceous. Two ankerite samples gave agesof 145 ± 5 and 114 ± 24 Ma and the only siderite sample
that returned an age gave 118±10 Ma. These ages markthrusting of the Gemeric Unit onto the Veporic crystalline basement from 140 to 115 Ma (Hurai et al. 2008,2015; Vozárová et al. 2014). This overthrust is further
constrained by the cooling of the uppermost Gemeric andoverlying rocks complexes of the Meliata accretionary
wedge documented by zircon (U–Th)/He ages and ZFT
ages (130–90 Ma; Plašienka et al. 2007; Putiš et al. 2014;
Králiková et al. 2016). Formation of the main mass of thehydrothermal siderite and ankerite appears to be confinedto this crustal-scale event. The subsequent exhumationof the Gemeric Unit (115–80 Ma) was accompanied bydifferent types of ore mineralizations, described below.
Thrusting of the Gemeric Unit onto the Veporic Unit

Geochronological constraints on the carbonate-sulfarsenide veins in Dobšiná

was also accompanied by the hydrothermal remobilization of U–Mo–Cu mineralization in the northern parts ofGemeric Unit at 130 ± 20 Ma (Rojkovič et al. 1993) or
124 ± 10 Ma (Rojkovič and Konečný 2005).

5.4. Age constraints on the quartz-tourmaline
stage
This stage, regionally distributed in the Gemeric Unit,
has been dated in the past. Monazite, an integral component of this stage, was analyzed either by electron
microprobe or LA-ICP-MS. Monazite from Čučma gaveages of 120 ± 9 and 76 ± 12 Ma, with a weighted averageof 92 Ma (Hurai et al. 2006). Later, Hurai et al. (2015)
extracted a more precise age of 139 ± 1 Ma from monazite
from Rožňava. There are some data on monazite from thequartz-tourmaline stage from the Tatric and Veporic units,
where the results scatter broadly around 100 Ma (Majzlan
et al. 2020). Another temporal hint is the age of 131 Ma,
extracted from a zircon overgrowth in the Paleozoic siliciclastic metasediments (Vozárová et al. 2013).
The CHIME age of monazite from Jedľové Kostoľany
is 83 ± 9 Ma, that from Ľubietová (Veporic Unit) 92 ± 11
and 97 ± 9 Ma (Ozdín 2008, 2015; Ozdín et al. 2016).
At these localities, these authors consider the quartztourmaline stage with monazite to be contemporaneouswith the Ni–Co arsenide mineralization. If so, thesedates would place independent constraints on the Ni–Co
sub-stage, even though not in the Gemeric Unit.According to the position of this quartz-tourmaline stage in thegeneral precipitation sequence in the Gemeric Unit, it isyounger than the siderite phase and usually older than the
Ni–Co sub-stage of the quartz-sulfide phase. Siderite andankerite, perhaps components of the siderite stage, havebeen dated in this work to 145–114 Ma, with multiple
remobilization events visible in the BSE images andgeochronological data. Given that monazite could havealso been repeatedly remobilized (cf. Hurai et al. 2006),
we can only conclude that the available data fit roughly
together. Exact differentiation of the ages of these twostages is thus complicated by multiple remobilization
events.

5.5. Age constraints on the Ni–Fe–Co
sulfarsenides
The carbonates Sid II and Ank II are closely associated with the Ni–Fe–Co sulfarsenides and precede theircrystallization. We were not able to obtain their ages,
however, because of their low U content.
In the quartz-sulfide phase in the Gemeric Unit,
precipitation of the Ni–Fe–Co sulfarsenides precedes
the more abundant and voluminous Cu sulfides (Fig. 9)(Varček 1985; Chovan et al. 1994). Such a position

also agrees with the results from the Tatric and Veporic
Unit (Ozdín 2003, 2008; Ozdín et al. 2016) where the
Ni–Fe–Co sulfarsenides occur only locally and in small
quantities.
There are no exact geochronological data for any
opaque minerals of this phase. Our datum of 93.6 ± 0.9
Ma is the first numerical result that documents the age ofthese minerals. It places constraints not only on the Ni–
Fe–Co arsenides, but also on the older quartz-tourmaline
stage and the younger Cu sulfides. We should note that
we attempted U/Pb LA-ICP–MS dating of the gersdorffite and other Ni–Fe–Co sulfarsenides. All these attempts
failed because of the high Pb content of these minerals.
As mentioned above, the Cu sulfides postdate the Ni–
Fe–Co sulfarsenides. The age of the Cu sulfides remains
unknown; the only hint is the Ar/Ar dating of muscovite
from Jedľové Kostoľany (Ozdín 2010), with the ages of
75 ± 1 and 75 ± 2 Ma. This muscovite was assumed to
postdate the sulfide mineralization at this locality.

6. Conclusion
Geochronological data from this work place the sideritesulfide veins from the Gemeric Unit in Cretaceous (Fig.
10), in agreement with the results and interpretations in
Hurai et al. (2002, 2006, 2008). The arguments presented
there are based on P–T estimates, which suggest that themineralization formed at relatively high pressures, compatible with the overburden of crystalline rocks, PermoTriassic cover and the Jurassic (Meliata) accretionary
wedge (Hurai et al. 2008). The new geochronological
data disentangle the evolution of the complex, multi-stagehydrothermal systems more precisely.
The new data can be reconciled with the models of
the crustal evolution of the Gemeric Unit (Lexa et al.
2003) and its thermal history (Fig. 10). The formation of
the siderite stage broadly coincides with the metamorphic peak in the Gemeric Unit and the formation of the
Gemeric cleavage fan (CGF), a large arcuate fan whose
origin is linked to N–S compression of the Gemeric rocks
during the thrusting (Lexa et al. 2003). Continuation of
the Cretaceous shortening resulted later in the formationof the Trans-Gemeric shear zone, a zone that is developedin both Gemeric and the Veporic units. Thus, at this time,these two units must have already been amalgamated.
This time matches with the formation ages of some of themonazite in the quartz-tourmaline stage, the Ni–Fe–Co
sulfarsenides in Dobšiná, and marginally also with theformation of the Cu sulfides. The Cu sulfides, if given thetentative Late Cretaceous age of 75 Ma, postdate the unroofing and exhumation of the Veporic Unit from beneaththe Gemeric Unit. These mineralizations also date the ageof the Trans-Gemeric shear zone formation (cf. Lexa et243

Stefan Kiefer, Martin Števko, Rastislav Vojtko, Daniel Ozdín, Axel Gerdes, Robert A. Creaser, Marek Szczerba, Juraj Majzlan

220

Jurassic shortening,
beginning of Gemeric unit reworking

ank I

sid I

100

metamorphic maximum inGemeric Unit

ank I
formation of Gemeric cleavage fanpartial exhumation

Cretaceousage (Ma)

150

Jurassic

200

50

welding of Gemeric and Veporic units,
formation of trans-Gemeric shear zone

Neog.

ank III

Paleogene

exhumation of Gemericunit

U/Pb on carbonates, this work,
related to the siderite sub-ﬆage and remobilization phase
K/Ar on fuchsite, this work,
related to the fuchsite ﬆage
Re/Os on gersdorﬃte, this work,
related to the Ni–Co sub-ﬆage
K/Ar on fuchsite, Dobšiná, Rudňany (Cambel et al. 1980,
1990), related to the fuchsite ﬆage
U/Th/Pb on monazite, Čučma, Rožňava (Hurai et al. 2006,2015), Ľubietová, Jedľové Koﬆolany (Ozdín 2008, 2015,
Ozdín et al. 2016), related to the quartz-tourmaline ﬆage
Ar/Ar on muscovite, Jedľové Koﬆolany (Ozdín 2010)
perhaps related to the Cu–Sb sub-ﬆage
Fig. 10 Summary and review of the results of this work and earlier
studies in relation to the tectono-thermal evolution of the Gemericunit. For display reasons, the data was plotted with a horizontal offset.

al. 2003). The young, Paleogene-Neogene ages recordedby the carbonates are a product of remobilization. K/Ar
data on fuchsite provide scattered, perhaps mixed ages
and may indicate that the last cooling below ≈320 °C
occurred at ≈110 Ma. This datum broadly agrees withthe age of the Alpine exhumation of the Gemeric Unit at
110–90 Ma (Plašienka et al. 2007; Králiková et al. 2016)
and the K/Ar ages on muscovite and biotite, interpreted
as cooling ages between 110–95 Ma (Hurai et al. 2008).
At 87–75 Ma, the Gemeric Unit has been shown to coolbelow ≈240 °C, the annealing temperature of zircon fission tracks (Plašienka et al. 2007). This work did not
address several outstanding issues. First, no age was
retrieved for the metasomatic Fe-rich carbonates, which
occur in Dobšiná and were mined in much larger depositsNižná Slaná and Kobeliarovo. In Dobšiná, these bodies
are weathered so profoundly that no reasonable samples
could be collected at the surface. Hence, their temporalcoincidence with the vein siderite mineralization cannot
be confirmed or refuted. Models from the Eastern Alps
(Prochaska 2016) on similar metasomatic siderite bod244

ies argue for their older – Triassic age, with formation
conditions similar to those invoked by Radvanec et al.
(2004). The other issue touched upon is the age of the
fuchsite-carbonate-quartz alteration and the role of theserocks as suppliers of Ni and Co for the hydrothermal ores.
Because of the Alpine metamorphic overprint (Faryad
and Dianiška 1999), no formation ages were determined
for these rocks. Such ages would be needed, however,to understand the mobility of Ni and Co in temporally
distinct events, such as the fuchsite-carbonate-quartz
alteration and formation of hydrothermal veins with Ni–
Fe–Co sulfarsenides. One must ask why only the latterproduced ore minerals when Ni, Co, and Fe must havebeen mobile during alteration.
Acknowledgements. We are very thankful for the helpfulcomments of the handling editor Petr Jeřábek and thereviews of Jiří Sláma and one anonymous reviewer. Allreviews and comments helped to improve the manuscript.
We are most thankful to Mr. Š. Polgáry (Dobšiná) for
great help with accessing the old mines in Dobšiná. We
also thank Mr. D. Peterec for the photographs in Fig. 3and the permission to publish them. We appreciate thecooperation with S. Honig, the company European
Cobalt, and their permission to publish these data. This
work was financially supported by the Deutsche Forschungsgemeinschaft grant KI 2131/2-1 (S.K., J.M.), by
Slovak Research and Development Agency projects
APVV-0375-12 (D.O.), APVV-0315-12, APVV-17-0170(R.V). This is FIERCE contribution No. XX.

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