GEOLOGICA CARPATHICA, 51, 4, BRATISLAVA, AUGUST 2000
SHALLOW LEVEL LOW-SULPHIDATION TYPE EPITHERMAL
SYSTEMS IN THE REGÉC CALDERA, CENTRAL TOKAJ MTS.,
, FERENC MOLNÁR
, KATSUHIKO MAEDA
and EIJI IZAWA
Department of Mineralogy, Eötvös University, Múzeum krt. 4/a, 1088 Budapest, Hungary
Department of Earth Resources System Engineering, Graduate School of Engineering, Kyushu University,
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
(Manuscript received September 28, 1999; accepted in revised form June 20, 2000)
Abstract: Detailed investigation of the Regéc caldera in the Central Tokaj Mts. revealed several types of hydrother-
mal centers of low-sulphidation type formed at different paleolevels. At the paleosurface, a hydrothermal eruption
breccia and layered siliceous deposit of opal-C and -CT material with cinnabar and anomalous enrichments of Hg and
Sb were formed from a hot spring. Silicified tuff horizons with alunite-kaolinite alteration indicate steam-heated
zones. This type of alteration was formed in the near surface zone, probably above the paleowater table. Stable
isotope data for large-sized alunite crystals in brecciated tuff may also indicate a magmatic steam origin. In the deeper
zone, around 90140 m minimal paleodepth, adularia-sericite alteration with quartz veining in andesite formed with
anomalous Sb, As and Ba concentrations. Two stages of K-feldspar formation can be recognized: 1. metasomatic K-
feldspar, replacing plagioclase phenocrysts, and 2. late adularia in quartz-pyrite-veinlets. Intensive brecciation and
adularia formation suggest a pressure drop and boiling of the mineralizing fluids. Late stage quartz crystals show
frequent homogenization temperatures between 170 and 190 °C and a maximum salinity of 3 wt. % NaCl equiv. Stable
isotope data for quartz crystals suggest that the dominant mineralizing fluid was meteoric water that underwent ex-
change reaction with the host rock and/or mixed with magmatic water.
Key words: Tokaj Mountains, epithermal systems, low-sulphidation type, fluid inclusions, stable isotopes.
In the last decade several studies have been carried out on the
epithermal systems of the Tokaj Mts. located in the central
part of the Tertiary-Quaternary Carpathian calc-alkaline vol-
canic belt. The low-sulphidation type features of hydrother-
mal alteration have been discussed in several papers (Molnár
1988, 1992, 1993, 1994a,b,c, 1997; Molnár & Zelenka 1995;
Csongrádi & Zelenka 1995; Csongrádi et al. 1996; Horváth
& Zelenka 1997; Molnár et al. 1999). The earlier studies
were mostly focused on areas with adularia-sericite alter-
ation due to the correlation between this type alteration and
precious-metal enrichment. The area of the Regéc caldera lo-
cated in the central part of the Tokaj Mts. offers a unique op-
portunity for studies because not only those relatively deep
adularia-sericite zones, but different and shallower alteration
zones are still preserved. Although there are no proofs for the
contemporaneous formation of various alteration zones in the
Regéc caldera, the results of their comparative mineralogi-
cal, geochemical, fluid inclusion and stable isotope studies
highlight features of shallow levels of low-sulfidation type
epithermal systems and these data may be useful for explora-
tion of other less eroded areas in the Tokaj Mts. and else-
where in the Carpathians.
The Regéc caldera is located in the central-western part of
the Tokaj Mountains around the villages of Regéc and Mogyo-
róska (Fig. 1). The present shape of the caldera with about 7
km diameter resulted from both volcanic and erosional pro-
cesses. Erosion has reached the deep levels of the pre-caldera
stratovolcano at the bottom of the depression (340 m to 400
m) and the hills forming the margins of the caldera (500 to
700 m) are mostly composed of Late Miocene (Sarmatian-
Pannonian) andesitic lava and tuffaceous rocks. In the center
of the volcanic structure a rhyodacite dome is well preserved.
The total thickness of the volcanic rocks of the caldera is not
known exactly (Ilkey-Perlaki 1968), though geophysical data
indicate at least 2000 m depth to the basement (Zentai 1991).
The lower part of the Baskó-3 drilling (1172 m) south of the
caldera (Fig. 1) consists of submarine dacitic and andesitic
lava and tuff of Badenian age. The Sarmatian-Pannonian stra-
tovolcanic sequence with about 900 m thickness is mainly
composed of pyroxene andesite and andesitic tuff with a lesser
amount of amphibole-pyroxene andesite. The Sarmatian
andesitic rocks are divided into three units (Ilkey-Perlaki
1968; Gyarmati 1977). The Lower Andesite Unit exposed at
the bottom of the caldera is of subvolcanic origin. The Middle
Andesite Unit of the caldera rim corresponds to the thick lava
and intercalated tuff products of the volcanism found in the
Baskó-3 drilling (Gyarmati 1977). The K/Ar ages of the Mid-
dle Andesite Unit are 12.412.1 Ma (Pécskay et al. 1987).
Caldera formation was associated with emplacement of the
rhyodacite dome (11.6 ± 0.3 Ma; Pécskay et al. 1987) in the
central-eastern part of the structure. The occurrences of the
Upper Andesite Unit probably represent late stage parasitic
cones and fissure volcanoes with local lava flows (Fig. 1). The
age of the Upper Andesite Unit north of the caldera is 10.7
218 BAJNÓCZI, MOLNÁR, MAEDA and IZAWA
Fig. 1. Geological scheme of the Regéc caldera with locations and numbers of studied outcrops and the section of Baskó-3 drilling (mod-
ified after Ilkey-Perlaki 1967; Gyarmati 1977).
± 0.6 Ma and in the uppermost part of the Baskó-3 drilling
south of the caldera 10.4 ± 0.5 Ma (Pécskay et al. 1987).
West and north of the caldera different types of rock accu-
mulated at the time of the andesitic volcanic activity. Mixed
tuff, redeposited rhyolitic tuff, lacustrine silica and clastic
sediments were deposited in a basin and pyroxene-amphibole
dacite intruded the formations.
Pervasive propylitic (chlorite-smectite) alteration is typical
of the Lower Andesite Unit at the bottom of the caldera (Il-
key-Perlaki 1968). Most intense quartz veining together with
SHALLOW LEVEL LOW-SULPHIDATION TYPE EPITHERMAL SYSTEMS 219
adularia-sericite alteration is situated in the eastern side of
the structure (Fig. 1). Hydrothermal eruption breccia, layered
siliceous deposit, as well as intensely silicified tuff levels
can also be found in the various parts of the caldera. Accord-
ing to the K/Ar dating of alunite and adularia-bearing rocks
the most probable period of hydrothermal activity is between
11.8 ± 0.5 and 12.3 ± 0.5 Ma (Molnár et al. 1999), thus pre-
dating the emplacement of the rhyodacite dome. In agree-
ment with this, the rhyodacite dome and the Upper Andesite
Unit are unaltered.
Methods of study
The mineralogy of the altered rocks and hydrothermal prod-
ucts was examined by conventional microscopic, X-ray pow-
der diffraction (Dept. of Mineralogy, Eötvös University,
Budapest) and IR spectroscopic methods (Dept. of Organic
Chemistry, Eötvös University, Budapest). Opal phases were
determined by measuring the d-value of the maximal intensity
reflection and the superstructure reflections of low-cristo-
balite, and by measuring the intensity ratio of the two strongest
peaks (Flörke et al. 1991; Elzea et al. 1994; Graetsch 1994;
Graetsch et al. 1994; Guthrie et al. 1995; Elzea & Rice 1996).
In most samples, quartz reflections were used as references.
The trace element geochemistry of 32 samples was deter-
mined by neutron activation analysis (NAA) at the Training
Reactor, Institute of Nuclear Technology, Technical University
of Budapest. A comparator method using an Au-standard
was employed (Balla et al. 1998). Measurements were carried
out in three steps at 5 minutes, 1 week and 1 month cooling
time using about 0.05 g of homogenized samples. NBS 1633a
Coal Fly Ash was used as a routine standard. Detection limits
are as follows: As 1 ppm, Sb 0.1 ppm, Hg 1 ppm, Au
0.005 ppm, Mn 0.5 ppm, Zn 10 ppm, Ba 60 ppm,
Mo 2 ppm, Se 0.5 ppm, K 0.01 %.
During the fluid inclusion studies (Dept. of Mineralogy,
Eötvös University, Budapest) the homogenization tempera-
), eutectic temperatures (T
) and final ice melting
) were determined on a Chaixmeca-type
microthermometry apparatus (Poty et al. 1976) using double-
polished, 0.11 mm thick sections of hydrothermal quartz
crystals. Accuracy of the measurements was ±0.1 °C during
freezing and ±2 °C during heating.
Oxygen, hydrogen and sulphur isotope analyses were car-
ried out at the Institute for Study of the Earths Interior,
Okayama University, Japan. Oxygen from silicate and oxide
samples was extracted by a conventional BrF
converted to CO
gas by graphite (Clayton & Mayeda 1963;
Matsuhisa et al. 1971). Oxygen isotopic compositions mea-
sured on the extracted CO
gases are reported in the
tion relative to SMOW. H
O from whole rocks samples was
released according to the method of Godfrey (1962) and re-
duced to H
gas by uranium. The H
O content of fluid inclu-
sions in quartz samples was extracted using the ball-milling
method of Kazahaya & Matsuo (1985). Hydrogen isotopic
compositions are also expressed in the
notation relative to
SMOW. The uncertainty of measurements is ±0.2 for oxy-
gen and ±2 for hydrogen. The pulverized whole rock sam-
ples for S isotope measurements were made according to the
description of Ueda & Sakai (1983). The sulphur isotopic ra-
tio is presented in the
notation relative to CDT standard,
and the analytical reproducibility is generally ±0.3 . Isoto-
pic analyses of the prepared alunite sample were carried out
according to the techniques described by Wasserman et al.
(1992). A Fusion Prism mass spectrometer was used for
Petrography and mineralogy of hydrothermal zones
Hydrothermal breccia and layered siliceous deposit (Out-
crop 1, Gombás; Fig. 1, Table 1) with significant petrological-
mineralogical zonation occur on the western margin of the
caldera. In the center of the outcrop, a red-coloured, poorly
sorted and grain-supported breccia with hematite-opal matrix
covers an area of approximately 40 m
. The well rounded,
5 mm to 5 cm sized fragments are composed of opal material
and altered (opalized) andesite. The white breccia enclosing
the red breccia is restricted to an area of 80 m
and is a poorly
Table 1: Classification of hydrothermal centers of the Regéc caldera.
Hydrothermal breccia, layered siliceous deposit,
tuff with argillite and alunite alteration,
opal-C, opal-CT, chalcedony, cinnabar, hematite
opal-CT, kaolinite, smectite, alunite, hematite
2 and 3
4 (Kun Hill) and 5
Silicified tuff with alunite-kaolinite alteration
quartz, alunite, kaolinite
opal-C, opal-CT, alunite, kaolinite, smectite
6 (Serfõzõ Ridge)
Andesite with adularia-sericite alteration and
metasomatic K-feldspar, sericite, quartz,
adularia, pyrite, hematite
metasomatic K-feldspar, sericite, quartz,
chalcedony, pyrite, berthierite, smectite
8 (road cut near Regéc)
9 (Soltész Valley)
chlorite, quartz, smectite, pyrite
220 BAJNÓCZI, MOLNÁR, MAEDA and IZAWA
sorted and matrix-supported rock with 1 mm to 1 m sized,
weakly rounded, and altered andesite fragments. The porous
rock comprises an opal matrix with a concentrically laminated
texture enclosing irregular or round, formerly open spaces
mostly filled with patches of unlaminated opal. The cavities in
the opal matrix have alternating chalcedony and opal fillings
or crusts. The andesite fragments in the opal matrix have fi-
brous chalcedony or opal filling in the places of phenocrysts,
and their matrix is also replaced by opal. Toward the margins
of the outcrop, the size and number of fragments decreases
and the breccia gradually develops into a macroscopically
layered appearance. The layered siliceous deposit has cinna-
bar encrustments and disseminations. According to micro-
scopic, XRD and IR spectroscopic examinations, white and
red breccias and layered siliceous deposit are composed of
opal-C, opal-CT and chalcedony.
The layered siliceous rock is surrounded semicircularly by
a poorly sorted and grain-supported, redeposited tuff with 0.1
to 3 mm sized, weakly rounded pumice-, andesite-, quartz-
and glass fragments sometimes mixed with the eroded frag-
ments of the layered siliceous deposit. The matrix of the tuff
has siliceous and kaolinite-smectite alteration with occur-
rences of fine alunite.
In the surroundings of the hydrothermal center the Middle
Andesite is slightly altered and the degree of alteration de-
creases away from the hydrothermal center. The alteration
products are smectite after plagioclase and hematite-limonite
after mafic minerals.
Tuff with alunite-kaolinite alteration occurs in the central
and southern part of the caldera (Outcrops 2 to 5; Fig. 1, Table
1). The original rock of this group was acid and mixed (acid-
neutral) tuff. These outcrops are almost the only exposed rep-
resentatives of the tuff in the caldera. The preservation of the
tuff is probably due to the intensive silicification.
In Outcrop 2, the layered, silicified lapilli tuff contains an-
gular quartz, glass and pumice fragments 0.2 to 4 cm in size.
Some rounded tuffpellets or accretionary lapilli 0.2 to 2 cm
in size also appear with kaolinite core and silica rim. Beside
siliceous and kaolinite alteration, small amount of alunite (1
2 %) was detected by XRD from the kaolinite core of the
lapilli. In Outcrop 4 (Kun Hill) and 5, the original rock is
layered lapilli tuff and agglomerate with 1 mm to 6 cm sized,
angular andesite, pumice and quartz fragments. Alunite, ka-
olinite, illite, smectite and hematite alteration occurred dur-
ing hydrothermal processes. Andesitic rock fragments be-
came totally opalized and the matrix of the tuff was also
transformed into opal. The hydrothermal alteration was ac-
companied by brecciation and synsedimentary faulting.
Along the southern margin of caldera, in Outcrop 3, the tuff
comprises angular silicified rock and quartz fragments 0.5 to
1 mm in size in the silicified matrix. Alunite appears in sev-
eral forms and generations, scattered in the matrix of the tuff
(5 to 30
m) and as 1 to 6 mm sized, euhedral, comb-like or
bladed crystals in veinlets cutting the host rock. The kaolin-
ite content of the altered tuff is neglectable (12 %).
Adularia-sericite alteration with quartz vein in the Lower
Andesite Unit is controlled by north striking faults in the east-
ern and central parts of the caldera (Outcrops 6 and 7; Fig. 1,
Table 1). The altered rock contains no mafic minerals, only hy-
drothermal K-feldspars and microcrystalline quartz. This type
of hydrothermal alteration is characterized by multiple hydro-
thermal processes and several episodes of brecciation. In Out-
crop 6 (Serfõzõ Ridge), hydrothermal processes resulted in
two generations of potassium-feldspar in andesite with differ-
ent morphologies and compositions. First the plagioclase phe-
nocrysts of andesite were replaced by metasomatic potassium-
feldspar pseudomorphs, which has been then altered to
sericite. Under SEM this type of feldspar shows enrichment of
barium in patchy arrangement. Later andesite underwent frac-
turing and silicification. Banded quartz veins and 12 mm
wide quartz veinlets with pyrite, hematite and 2040
fresh adularia crystals of pseudorhombohedral shape and pure
K-feldspar composition were formed. In Outcrop 7 (Csonkás)
the early K-feldspar replaced plagioclase phenocrysts of the
andesitic rock and later transformed to sericite. Minor smectite
and illite alteration in the groundmass is also present. The ear-
ly alteration was followed by intensive silicification and brec-
ciation of the host rock. Quartz veins and stockworks and
quartz-chalcedony veinlets with pyrite and berthierite were
Propylitic alteration is regionally characteristic for the
Lower Andesitic Unit (Outcrops 8 and 9; Fig. 1, Table 1). It
includes siliceous alteration of matrix, chlorite alteration of
pyroxene phenocrysts and smectite alteration of plagioclase
phenocrysts with some quartz veinlets. Pyrite disseminations
in siliceous veinlets and in the groundmass of rock are also
typical of these zones.
Different types of hydrothermal products were measured to
determine their trace element compositions. For evaluation
of data we used the anomaly thresholds of trace and main el-
ements earlier determined by Hartikainen et al. (1992) during
the regional geochemical study of the Tokaj Mts. (As 136
ppm, Sb 16 ppm, Au 12 ppb, Ag 1 ppm, K 5.8
%). They used not only fresh, but also altered rocks to deter-
mine the threshold value. Therefore these values are higher
than the average trace element content of fresh volcanic
rocks. In the case of Hg we considered the Clark Value (0.08
ppm), which is below the detection limit of the NAA analy-
sis. Regarding Ba, the average concentration of andesite in
the Tokaj Mts. is 400500 ppm (Gyarmati 1977). The results
of analyses are summarized in Table 2.
The red and white breccia, layered siliceous deposit and
altered tuff in Outcrop 1 (Gombás) generally have anoma-
lous Sb and Hg contents. However, these rocks show relative
enrichment in certain element(s) compared to each other and
to the smectitic-hematitic andesite from the outer zone of the
outcrop. The red breccia is relatively enriched in Sb, the
white breccia and layered siliceous deposit in Hg and Sb and
altered tuff in As. The latter one does not show As-concen-
trations above the threshold values. Considering white brec-
cia samples, the Hg-content increases towards the edge of
the outcrop where it appears as cinnabar in the layered de-
posit. In some samples the Au-content of the redeposited tuff
and smectitic-hematitic andesite exceeds the anomaly thresh-
old of 12 ppb.
SHALLOW LEVEL LOW-SULPHIDATION TYPE EPITHERMAL SYSTEMS 221
Tuff with alunite-kaolinite alteration (Outcrops 2 to 5) has
no Hg anomalies comparing to the Clark Value, whereas As
content is around or 1.5 times higher than the Clark Value.
Sb content increases with the increasing As concentration up
to 4 ppm, but neither As, nor Sb reaches the threshold values
determined by Hartikainen et al. (1992). This group contains
216996 ppm Ba.
Andesite with adularia-sericite alteration (Outcrops 6 and
7) is characterized by K-enrichment of 6.27.8 % in the
whole rock. Regarding trace elements Ba, As and partly Sb
anomalies occur in the altered rock. Slight Ba-anomaly of
672691 ppm can be connected mineralogically to the meta-
somatic K-feldspars, because these are the only Ba-bearing
minerals in the rock. Quartz veins cutting the host rock have
elevated Sb concentrations (174432 ppm).
In comparison to the relatively fresh andesite (Outcrop 1)
the propylitic andesite does not show any kind of anomalous
element content. The pyrite-quartz veinlets of Outcrop 8
have higher As- and Sb-concentrations than the host rock,
but they do not exceed the threshold values.
Fluid inclusions were studied in hydrothermal quartz crys-
tals from quartz veinlets hosted by the andesitic rocks showing
adularia-sericite alteration (Outcrops 6 and 7). In the case of
Outcrop 6 (Serfõzõ), measurements were made on inclusions
from late-stage euhedral quartz crystals formed after the
quartz-adularia-pyrite-hematite-bearing veinlets. For Outcrop
7 (Csonkás) the observations were made on inclusions from
comb quartz from veinlets cutting andesite with pronounced
adularia-sericite alteration and from quartz cement from a
breccia containing altered andesite fragments.
Microthermometric analyses were carried out on primary
and secondary fluid inclusions containing liquid and approx.
1015 vol. % vapour phase at room temperature. In addition
to this type of inclusion, apparently liquid-absent vapour and
vapour rich (at least 6080 vol. % vapour phase) liquid + va-
pour inclusions were also detected. This fluid inclusion as-
semblage indicates boiling of parent fluids (Roedder 1984)
and the variable vapour to liquid ratios can be attributed to
Table 2: Trace element range (in ppm) and K-content (in %) of the hydrothermal altered rocks, hydrothermal products and rhyodacite
from the Regéc caldera. Number in parenthesis shows the numbers of measured samples of each rock type.
(road cut near Regéc)
of the caldera)
tuff with argillite
lapilli tuff with
tuff with siliceous,
lapilli tuff and agglome-
rate with opaline,
alunite, hematite, illite
and smectite alteration
<0.005 0.010 <0.005 0.020
222 BAJNÓCZI, MOLNÁR, MAEDA and IZAWA
inhomogeneous trapping (Bodnar et al. 1985). In addition to
fluid inclusions rare occurrences of rhombohedral calcite in-
clusions were also detected.
Microthermometric data for fluid inclusions are summa-
rized in Table 3. Total homogenization temperatures (T
of liquid-rich inclusions for the late stage euhedral quartz
crystals from veinlets in Outcrop 6 (Serfõzõ) are 140220 °C
with most values in the range 170185 °C (Fig. 2a). In the
case of Outcrop 7 (Csonkás) the distribution of homogeniza-
Fig. 2. Distributions of microthermometric data for fluid inclusions
of late-stage quartz crystals from andesite with adularia-sericite al-
teration. a Distribution diagram of homogenization temperatures
) measured in fluid inclusions of euhedral quartz from Serfõzõ
(Outcrop 6). b Distribution diagram of homogenization tempera-
) measured in fluid inclusions of quartz crystals from
Csonkás (Outcrop 7), n number of measurements.
(NaCl equiv. wt%)
Outcrop 6 (Serfõzõ)
Outcrop 7 (Csonkás)
Quartz from breccia
Outcrop 7 (Csonkás)
All sample from Outcrop 7
Table 3: Summary of fluid inclusion microthermometric data and calculated fluid properties of quartz crystals of Outcrop 6 (Serfõzõ) and
Outcrop 7 (Csonkás). Data from Molnár (1992) are also included. P: primary inclusion; S: secondary inclusion; T
: homogenization tem-
perature of vapour phase (°C); T
: eutectic temperature of ice phase (°C); T
: final melting temperature of ice phase (°C); salinity of fluid:
calculated from the melting temperature of ice (T
); density of fluid: calculation based on measured homogenization temperature (T
fluid inclusion. Numbers in parenthesis () show the most frequent ranges of data. Numbers in paranthesis  show the numbers of measure-
Serfõzõ, euhedral quartz
n = 55
Csonkás, quartz from breccia
Csonkás, comb quartz
n = 42
tion temperatures for the comb quartz is very wide and rang-
es from 130 °C to 260 °C (Fig. 2b) with the most frequent
values between 180 and 210 °C. The distribution of homoge-
nization temperatures is quite narrow and ranges from
150 °C to 190 °C (mode 170190 °C for primary inclusions
and 150170 °C for secondary inclusions) in the quartz ce-
ment from a breccia. The most frequent range for homogeni-
zation temperatures of all quartz samples from Csonkás is
between 175 and 190 °C.
Eutectic temperatures (T
) around 20 °C indicate that the
composition of fluid inclusions can be modelled by the NaCl
O binary system (Crawford 1985). The final ice melting
) are between 0.1 and 1.8 °C, clustering
around 1.0 °C. From the final ice melting data, the calculated
apparent salinities of inclusions (Bodnar 1993) are between
0.2 and 3.1 NaCl equiv. wt. % clustering at 1.41.7 wt. %. The
corresponding fluid densities exhibit a very narrow range be-
tween 0.87 and 0.92 g/cm
(calculated after Haas 1976).
Light stable isotope studies
Stable isotope analyses for H, O and S were carried out on
fresh and strongly altered volcanic rocks, and hydrothermal
minerals (Table 4). A total of 12 and 11 samples were analysed
O and for
D, respectively, and 5 samples for
With the exception of rhyodacite and propylitic andesite, the
O values of the whole rock samples cluster around +10 .
By comparison, the
D values show a wide range. Altered tuff
from Outcrop 1 has the highest
D value (55 ), whereas val-
ues of altered andesite are between 64 and 75 . Fresh
rhyodacite has rather low
D value (83 ). Two andesite sam-
ples with adularia-sericite alteration show a strong depletion in
D = 96 and 102 , Fig. 3) compared to other
whole rock samples.
S values for propylitic andesites and rhyodacite are
34 . The andesite sample with adularia-sericite alteration
from outcrop 7 (Csonkás) has a higher value; probably due to
the presence of secondary jarosite. Alunite shows the highest
S among samples from the caldera
S = 8.9 ).
SHALLOW LEVEL LOW-SULPHIDATION TYPE EPITHERMAL SYSTEMS 223
Regarding the euhedral alunite from Outcrop 3 the differ-
ence between the directly measured
value and the
value does not exceed 4 . The
(34 ) of alunite strongly differs from that of the rocks and
is between the values for isotopically heavy volcanic vapour
and for late-stage magmatic water (Hedenquist & Lowen-
O values for hydrothermal quartz crystals range from
10.2 to 13.6 , while
D data for their fluid inclusions
have a relatively large spread between 79 and 107 .
The textural-structural characteristics of the red and white
breccias in the central part of Outcrop 1 (Gombás) are con-
sistent with those of hydrothermal eruption breccias of recent
geothermal fields (e.g. New Zealand, Hedenquist & Henley
1985a). The formation of the eruption breccia can be attribut-
ed to overpressure in the fluid flow zone, which in the case
of Gombás was probably the result of silica sealing. The dep-
osition of a hydrothermal eruption breccia takes place on the
surface, thus the hydrothermal enviroment of rocks of Out-
crop 1 represent a paleosurface in the caldera.
Rocks consisting of opaline silica (e.g. silica sinters, sili-
ceous hot-spring precipitates) are typical of the surface zones
of recent geothermal areas (Fournier 1985). With time, the
amorphous material transforms to opal-C and opal-CT, then
finally to chalcedony. The transformation is time dependent
but is strongly influenced by burial diagenesis in higher tem-
perature and pressure environments (Fournier 1985). The sil-
ica material of Outcop 1 is opal-C and opal-CT, thus mineral-
ogically it is analogous to the recrystallized hot-spring
precipitates, but there is no evidence for deep burial. The
most intense XRD reflection of the microcrystalline opal
phases of Gombás shifts to higher values towards the margin
of the outcrop (from 0.405 nm to 0.412 nm). This tendency
refers to the more disordered structure of opal phases and
may be attributed to temperature decrease. Towards the mar-
gin of the outcrop, precipitation of opal was probably quicker
because of the lower temperature, and hence a less ordered
The siliceous deposit adjacent to the red and white brec-
cias comprises layers of silica. However, it shows no pres-
ence of columnar structure perpendicular to and between
laminations, which is unequivocal evidence for a sinter ori-
gin (White et al. 1989), thus it is not reasonable to name it as
silica sinter. The layered appearance suggests underwater ac-
S isotopic compositions (in ) of fresh and altered rocks and hydrothermal minerals of the Regéc caldera, pre-
cipitation temperatures and calculated isotopic compositions (in ) of the parent hydrothermal fluids.
Whole rock samples
Tuff with argillite and alunite alteration
Outcrop 1 (Gombás)
Outcrop 1 (Csonkás)
Tuff with siliceous, alunite
(and kaolinite) alteration
Outcrop 3 (south margin of the caldera)
Andesite with adularia-sericite alteration
Outcrop 6 (Serfõzõ)
Andesite with adularia-sericite alteration
Outcrop 7 (Csonkás)
Outcrop 8 (road cut, Regéc)
Outcrop 9 (Soltész Valley)
Rhyodacite (Castle Hill)
Outcrop 3 (south margin of the caldera)
Outcrop 6 (Serfõzõ)
Outcrop 6 (Serfõzõ)
Outcrop 7 (Csonkás)
Quartz from stockworks
Outcrop 7 (Csonkás)
224 BAJNÓCZI, MOLNÁR, MAEDA and IZAWA
andesite with adularia-sericite alteration (Outcrops 6 & 7)
tuff with argillite and alunite alteration (Outcrop 1)
smectitic-hematitic andesite (Outcrop 1)
propylitic andesite (Outcrops 8 & 9)
rhyodacite (Castle Hill)
for parent fluid of alunite (Outcrop 3) between 100°C
(open diamond) and 274°C (filled diamond)
for parent fluid of alunite (Outcrop 3) between 100°C
(open diamond) and 274°C (filled diamond)
parent fluid of quartz crystals of Serfõzõ (Outcrop 6) with the error of
the estimated range of precipitation temperatures
parent fluid of quartz crystals of Csonkás (Outcrop 7) with the error of
the estimated range of precipitation temperatures
composition of present-
day meteoric water
cumulation probably as a fine silica mud in a shallow pool
fed by a hot spring.
The observed geochemical anomalies and the occurrence
of cinnabar also support the assumption that the hydrother-
mal rocks of Outcrop 1 represent a shallow hydrothermal
zone. Sb, As and Hg are typical of hot-spring type systems
(Weissberg 1969; Berger & Silberman 1985; Krupp &
Seward 1987). Besides the anomalous concentration of Sb in
the central red breccia, the white breccia and layered sili-
ceous deposit also has an Hg-anomaly, increasing towards
the margins. In hot-spring hydrothermal systems, mercury
transports such as (bi)sulfide-complexes and precipitation of
cinnabar may be triggered by cooling and/or pH decrease of
the mineralizing fluid (Hedenquist & Henley 1985a). Thus
the presence of cinnabar in the outer zones of the hydrother-
mal center correlates with the structural characteristics of
opal varieties, which may suggest decrease of temperature
from the red breccia zone towards the marginal zones with
layered siliceous deposit.
The alunite-kaolinite bearing tuff of the Regéc caldera (Out-
crops 2 to 5) corresponds to an acid and oxidative (sulphate)
type alteration zone (Hemley et al. 1969). Acid-sulphate,
steam-heated fluids may have originated from the condensa-
tion and oxidation of H
S gas derived from an underlying boil-
ing horizon and are typical above the groundwater table (Silli-
toe 1993). In contrast to the hot spring environment of Outcrop
1 that may have formed at the paleogroundwater table, these
acidic alteration zones were formed most probably above that
level. In accordance with this, the acid alteration zones occur
at a relatively high level within the Regéc caldera (Fig. 1), al-
though there are no proofs that this corresponds to their origi-
nal position. Above the paleogroundwater table descending
acid fluids dominate therefore this is not an accumulation
zone, but rather an acid-leaching zone of elements (Heden-
quist & Arribas 1999). This explains the lack of pronounced
geochemical anomalies in these horizons around Regéc.
The marked difference (56 ) between the
S values of
euhedral, coarse-grained alunite from Outcrop 3 and propylitic
andesite from Outcrops 8 and 9 also supports the assumption
that alunite has not been derived from the supergene oxidation.
From the measured
values of alunite crys-
tals, the calculated temperature 274 °C (Stoffregen et al. 1994)
of precipitation is higher than the most typical 90160 °C tem-
perature range of steam-heated alteration (Rye et al. 1992).
Usually the O-isotope in crystallographic SO
sites is in equi-
librium with the fluid in a steam-heated acid-sulphate environ-
ment, but sometimes unreasonably high temperatures are cal-
culated, mostly because of post-depositional retrograde
isotope exchange in the OH site (Rye et al. 1992). For 100 °C
O composition of parent fluid is very close to
the meteoric water line (Fig. 3).
D value (27 ) of alunite is the highest among the
data from the caldera (Fig. 3). As the D-fractionation between
alunite and water is small (
= 6 +4 ) at temperatures be-
low 274 °C (Rye et al. 1992; Stoffregen et al. 1994), the original
alunite mineralizing fluid is not the same as the present meteoric
D = 66 to 68 ; Deák 1995) or the inclusion flu-
ids in hydrothermal quartz (
D = 79 to 107 ). Evaporation
or boiling of the parent fluids of quartz crystals cannot result in
such a huge shift in
D values (Fig. 3). Thus the isotopic com-
position of euhedral, coarse-grained alunite crystals of Outcrop
3 suggests a presence of magmatic component. The data can be
indicative of a magmatic steam (Rye et al. 1992) rather than a
Studies on recent geothermal deposits demonstrate the
close relation between precipitation of adularia and boiling
of fluids (e.g. Broadlands, New Zealand, Simmons et al.
1992). Thus, andesite with adularia-sericite alteration (Out-
crops 6 and 7) represents the boiling zone of hydrothermal
fluids in the caldera. This is in agreement with the fluid in-
clusion assemblage found in quartz crystals of these alter-
ation zones. Most common homogenization temperatures
and salinities correspond to 710.5 (Outcrop 6) and 812
bars (Outcrop 7) boiling pressures, respectively, which yield
90 to 120 m (Outcrop 6) and 90 to 140 m (Outcrop 7) depth
below the paleogroundwater table using the density values
for hydrostatic conditions (Haas 1971). Presence of fresh ad-
ularia formed after the sericitic alteration of metasomatic K-
feldspar reflects a pH increase of fluids during boiling. This
can be attributed to loss of volatiles during the initial stages
of boiling (Henley et al. 1984) and the gas content of fluids
should also be considered in the determination of paleodepth.
The existence of minor amount of CO
in the low pressure,
near-surface fluids is general and it is usually undetectable
by microthermometric methods in inclusions that trapped
these fluids (Hedenquist & Henley 1985b). As the CO
tent increases the vapour pressure, the paleodepth of Out-
crops 6 and 7 could be more than 90140 m.
Quartz veins crosscutting andesite with adularia-sericite
alteration exhibit anomalous enrichment in Sb. This feature
O isotopic compositions of fresh and altered
rocks and the calculated parent fluids of the hydrothermal miner-
als from the Regéc caldera.
SHALLOW LEVEL LOW-SULPHIDATION TYPE EPITHERMAL SYSTEMS 225
Fig. 4. Recent elevation of the studied outcrops in the Regéc caldera along a horizontally exaggerated W-E section with indication of the
seems to be characteristic for the upflowing zones of hydro-
thermal fluids in the Regéc caldera since the central red brec-
cia in Outcrop 1 also has an anomalous Sb content.
O values of the parent fluids for various
types of quartz from the adularia-sericite alteration zones
(Outcrops 6 and 7) are from 0.8 to 3.6 for average
precipitation temperatures of 170190 °C (Matsuhisa et al.
1979; Table 4). The calculated
O values and
D data mea-
sured in fluid inclusions show that the composition of hydro-
thermal fluids is between the meteoric water line and the val-
ues of the late-stage magmatic water (Hedenquist &
Lowenstern 1994; Fig. 3). The hydrothermal fluids are prob-
ably of meteoric origin and were shifted from their original
isotopic composition (from the meteoric water line) by an
isotope-exchange reaction with
O-rich magmatic rock and/
or a mixing with magmatic fluids (Field & Fifarek 1985).
This intensive water/rock interaction may also explain the
D values found in the rock samples of adular-
From the measured data we can assume that the lowest
value of former meteoric water was 107 because the evo-
lution of meteoric water through boiling, evaporation, ex-
change reaction with rocks or mixing with magmatic fluid re-
sults in only D-enrichment and not D-depletion (Field &
Fifarek 1985). Isotopic data from inclusion fluids from Ban-
ská tiavnica, Slovakia (values up to 113 , Kantor et al.
1983), also refer to similar negative
D value of meteoric
water during the Sarmatian. However, recent groundwater is
D = 66 to 68 and
O = 9.3
to 9.5 values (Deák 1995) and this indicates that impor-
tant changes have occurred in the isotopic composition of
meteoric water during the past 10 million years. The possible
cause of the difference in isotopic composition could be cli-
matic change and with less importance change in elevation.
Propylitic (chloritic, smectitic, silicic) andesite with pyrite
always appear in the outermost and/or deepest alteration zone
of an epithermal hydrothermal system (White & Hedenquist
1995). The regional extent of propylitic alteration in the Lower
Andesite Unit of the Regéc caldera also supports this fact.
Detailed mineralogical, geochemical, fluid inclusion and
stable isotope studies reveal different styles of mineralization
of low sulphidation type formed at various paleolevels in the
Regéc caldera (Fig. 4).
Hydrothermal eruption breccia incorporating altered
andesitic rock fragments and surrounding a layered siliceous
deposit made of opal-C and -CT precipitated from a hot
spring and thus represents a paleosurface environment along
the paleogroundwater table. Hg and Sb anomalies of the hy-
drothemal breccia and the layered siliceous deposit with oc-
currence of cinnabar confirm a hot-spring origin.
Acidic and mixed tuff with alunite-kaolinite alteration and
slightly anomalous Ba-concentrations indicates the existence
226 BAJNÓCZI, MOLNÁR, MAEDA and IZAWA
of acidic, sulphate-bearing fluids. The presence of fine-
grained alunite and kaolinite in the silicified tuff levels may
suggest extensive steam-heating origin. However, the high
D value of unusually coarse-grained alunite crystals from
an altered tuff horizon may also suggest incorporation of
magmatic steam into the hydrothermal fluids. Presence of
magmatic fluid may indicate a magmatic reservoir at shallow
depth in the area of the Regéc caldera.
At around 90140 m minimal paleodepth, andesite with ad-
ularia-sericite alteration formed with quartz veins and veinlets
and show multiple alterations and brecciation reflecting the al-
ternating chemical character of the mineralizing fluids. Meta-
somatic K-feldspar and a second generation of adularia are the
typical hydrothermal minerals. Enrichments in Sb, As and Ba
are characterictic for this type of alteration. Fluid inclusion
studies of quartz crystals generated after the precipitation of
hydrothermal feldspars show dilute NaClH
with less than 3 wt. % NaCl equiv. and most frequent homoge-
nization temperatures between 170190 °C. This result is con-
sistent with the stable isotope data for the same quartz crystals
indicating that the dominant mineralizing fluid was evolved
meteoric water with shifted isotopic composition due to fluid-
rock interaction and/or mixing with magmatic fluids. Data also
suggest that the Sarmatian meteoric water had depleted
values compared to the recent ones.
The regional alteration type in the caldera is propylitiza-
tion of andesite with quartz, smectite, chlorite and pyrite.
Acknowledgements: This work was carried out in the frame-
work of the Hungarian-Japanese Intergovernmental Science &
Technology Co-operation Programme for 19971998 support-
ed by OMFB (Budapest, Hungary) and its foreign partner, Sci-
ence & Technology Agency (Tokyo, Japan). Assistance with
IR spectroscopy measurements is acknowledged to E. Vass
(Dept. of Organic Chemistry, Eötvös Univ., Budapest, Hunga-
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