GeolCarp_Vol67_No1_105

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, FEBRUARY 2016, 67, 1, 105—115 doi: 10.1515/geoca-2016-

0006

Neotethyan rifting-related ore occurrences: study

of an accretionary mélange complex (Darnó Unit, NE Hungary)

GABRIELLA B. KISS

, ERIKA OLÁH

, FEDERICA ZACCARINI

and SÁNDOR SZAKÁLL

Eötvös Loránd University, Faculty of Science, Department of Mineralogy, 1117 Budapest, Pázmány P. stny. 1/c, Hungary;

gabriella.b.kiss@ttk.elte.hu

University of Leoben, Department of Applied Geosciences and Geophysics, 8700 Leoben, Peter Tunner str. 5, Austria

University of Miskolc, Faculty of Earth Science and Engineering, Department of Mineralogy and Petrology, 3515 Miskolc-

Egyetemváros, Hungary

(Manuscript received April 4, 2015; accepted in revised form December 8, 2015)

Abstract: The geology of the NE Hungarian Darnó Unit is rather complicated, as it is composed mostly of a Jurassic
accretionary mélange complex, according to the most recent investigations. The magmatic and sedimentary rock blocks
of the mélange represent products of different evolutionary stages of the Neotethys; including Permian and Triassic
sedimentary rocks of marine rifting related origin, Triassic pillow basalt of advanced rifting related origin and Jurassic
pillow basalt originated in back-arc-basin environment. This small unit contains a copper-gold occurrence in the Per-
mian marly-clayey limestone, an iron enrichment in the Triassic sedimentary succession, a copper-silver ore occurrence
in Triassic pillow basalts and a copper ore indication, occurring both in the Triassic and Jurassic pillow basalts.
The present study deals with the Cu(-Ag) occurrence in the Triassic basalt and the Fe occurrence in the Triassic sedi-
mentary succession. The former shows significant similarities with the Michigan-type mineralizations, while the latter
has typical characteristics of the Fe-SEDEX deposits. All the above localities fit well into the new geological model
of the investigated area. The mineralizations represent the different evolutionary stages of the Neotethyan rifting and
an epigenetic, Alpine metamorphism-related process and their recent, spatially close position is the result of the accre-
tionary mélange formation. Thus, the Darnó Unit represents a perfect natural laboratory for studying and understanding
the characteristic features of several different rifting related ore forming processes.

Keywords: hydrothermal processes, submarine basalt, pelagic sedimentary rocks, Michigan-type copper, SEDEX iron,
fluid inclusion study, EPMA analyses.

Introduction

Understanding  and  interpretation  of  the  complex  geology
of  the  NE  Hungarian  Darnó  Unit  (Fig. 1.)  was  constantly
changing according to the prevailing structural theories until
the past decade. According to the recent investigations, it is
composed  mostly  of  a  Jurassic  accretionary  mélange  com-
plex, with a great variety of rock types in complex geologi-
cal  structures.  The  magmatic  and  sedimentary  rock  blocks
of  the  mélange  represent  products  of  different  evolutionary
stages of the Neotethys; including Permian and Triassic ma-
rine sedimentary rocks of rifting related origin, Triassic pil-
low  basalt  of  advanced  rifting  related  origin  and  Jurassic
pillow  basalt  of  back-arc-basin  opening  related  origin  (see
e.g.  Aigner-Torres  &  Koller  1999;  Dimitrijević  et  al.  2003;
Haas  &  Kovács  2001;  Harangi  et  al.  1996;  Kovács  et  al.
2008; Kiss et al. 2010, 2012). Several ore indications occur
in this small (about 10 km

) structural unit, though until re-

cent times, they were poorly investigated, due to the uncer-
tainties about the geological background.

A copper -(Au) occurrence in the Permian marly-clayey

limestone,  an  iron  enrichment  in  a  sedimentary  succession,
a  copper -(Ag)  mineralization  in  pillow  basalt  series  and  an
epigenetic copper ore indication, occurring both in the Trias-
sic  and  Jurassic  pillow  basalt  series  were  also  recognized
in  this  unit  (Papp  1938;  Mezősi  &  Grasselly  1949;  Kiss

1958;  Baksa  et  al.  1981),  besides  a  few  ore  mineralogical
specialities (e.g. Co minerals in a drillcore sample, É. Hartai
pers. comm.). Two of the mineralizations were recently stu-
died in details (Kiss & Zaccarini 2013; Molnár et al. 2015),
while the copper -(Ag) occurrence in the pillow basalt series
and the iron occurrence in the sedimentary succession have
not  been  investigated  with  state-of-the-art  methodology.
Therefore,  the  relationship  of  the  different  ore  occurrences,
as  well  as  their  association  with  the  complex  geology
of the region was never studied before. Hence, this study has
double aims; one is the detailed characterization of previous-
ly neglected occurrences, while the other is their placement
into the regional geological context.

Geological background

Regional and local geology

The ca. 10 km

area of the Darnó Hill is found in NE Hun-

gary, about 15 km to the NW from the city of Eger (Fig. 1).
The hill forms a part of the Darnó Unit, which is interpreted
as a part of the Bükk Unit, within the Pelso Unit of the
ALCAPA Block (ALpine, CArpathian, PAnnonian; Csontos
1995; Schmid et al. 2008). The Bükk Unit is composed
of four nappes, of which the Darnó Unit, containing Triassic

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and Jurassic submarine basalts as well as sedimentary rocks,
is located in the uppermost position. Below it, the Szarvaskő
Unit is made up by a Jurassic incomplete ophiolitic sequence
and  related  sediments.  The  Mónosbél  Unit  is  found  below
the Szarvaskő Unit and is composed of Jurassic, redeposited
slope  sediments,  while  the  lowermost  positioned  Bükk
Parautochthon  contains  Paleozoic  to  Jurassic  formations.
Based  on  the  recent  tectonic  models,  the  Bükk  Unit  can  be
correlated  with  the  NW  Dinarides,  thus  its  magmatic  rocks
can be interpreted as dismembered fragments of the Dinari-
dic  Ophiolites,  which  suffered  300—400 km  displacement
along the Mid-Hungarian Lineament (Aigner-Torres & Kol-
ler 1999; Csontos 1995, 1999; Dimitrijević et al. 2003; Haas
&  Kovács  2001;  Harangi  et  al.  1996;  Kovács  et  al.  2008;
Kiss et al. 2010, 2012).

The origin of the Triassic and Jurassic magmatic rocks of

the region was a question of debate in the past few decades.
Uncertainty had been caused partly by contradictory petro-
logical and geochemical evidences and partly by the scarcity
of good outcrops in the Darnó Hill. In the case of the Triassic

magmatites,  origin  was  related  to  rifting  and  MOR  events
and  geology  correlated  with  the  Inner-Western  Carpathian
(Meliata) and Northwestern Dinaridic units (see e.g. Balla et
al.  1980;  Downes  et  al.  1990;  Dosztály  &  Józsa  1992;  Ha-
rangi et al. 1996). While the Jurassic magmatites, however,
were referred to back arc basin or marginal sea opening and
correlated with the Dinaridic Ophiolite Zone as well as with
the Vardar Zone (see e.g. Downes et al. 1990; Harangi et al.
1996;  Aigner-Torres  &  Koller  1999).  Extended  research
with  up  to  date  methodology  has  taken  place  in  the  region
from the 2000s. According to the latest models, the Triassic
submarine  rocks  are  of  Neotethyan  advanced  rifting  origin.
The  formation  of  the  Jurassic  submarine  succession  is  rela-
ted most likely to marginal basin opening, while their com-
mon  occurrence  in  the  Darnó  Unit  is  interpreted  as  a  result
of  accretionary  mélange  formation  during  the  evolution
of the Neotethys (Kovács et al. 2008, 2010; Kiss et al. 2008,
2010, 2012; Haas et al. 2011).

Ore indications of the Darnó Hill

The unresolved geology of the Darnó Hill also led to

the  insecure  interpretation  of  the  genesis  of  the  mineraliza-
tions.  The  afore  mentioned  deposits  were  already  described
and  partly  interpreted  by  a  number  of  researchers  (Papp
1938;  Mezősi  &  Grasselly  1949;  Kiss  1958;  Baksa  et  al.
1981;  Kiss  &  Zaccarini  2013;  Molnár  et  al.  2015  and  the
references cited therein).

Native copper found south of the hill drew attention to

the region in the middle of the 19

century. Discovery was

followed by the preparation of a 76 m long adit. The ore was
found  in  calcite-laumontite  veins  of  the  basalt,  but  mining
was  never  successful  due  to  the  irregular  occurrence  of  the
native  copper  grains  and  aggregates  (Haidinger  1850;  Lőw
1925;  Papp  1938;  Mezősi  &  Grasselly  1949).  Kiss  (1958)
ruled  out  the  earlier  proposed  Keweenawan-type  origin  of
the ore and related the ore formation to epigenetic alteration
processes.  According  to  his  study,  the  above  mentioned
quartz-prehnite-chalcopyrite  veins  found  on  the  central  part
of the Darnó Hill represent the deeper, while the calcite-lau-
montite-native  copper  veins  represent  the  shallower,  more
altered  parts  of  the  same  hydrothermal  vein  system.  In  this
study,  Kiss  (1958)  also  shed  light  on  the  hematite-bearing
iron ore occurrence developed in continuum on the host, red
radiolarite, found on the northern slopes of the hill and con-
cluded on its exhalative origin. Recent studies by Kiss et al.
(2013)  proved  the  presence  of  weakly  mineralized  Kupfer-
schiefer-type  occurrence  in  the  Permian  marly-clayey  lime-
stone,  while  Molnár  et  al.  (2015)  ascertained  epigenetic
origin,  related  to  the  low-grade  Alpine  metamorphism,  for
the quartz-prehnite-chalcopyrite veins found in both the Trias-
sic  and  Jurassic  basalt  blocks.  The  present  research  brings
some novelty regarding the formation of the calcite-laumon-
tite-native copper veins and the quartz-hematite occurrence.
Therefore, this study contributes to our mineralogical, petro-
graphical  and  ore  geological  knowledge  on  these  localities,
as well as to the better understanding of the connection be-
tween  the  ore  occurrences  and  the  local  geology  and  draws
the possible metallogenical conclusions.

Fig. 1.  Simplified  geological  map  of  the  studied  area  (based  on
Földessy  1975)  and  its  location  in  Hungary.  The  numbers  refer  to
the  studied  locations,  the  ore  occurrences  observed  on  the  Darnó
Hill  (1  –  copper  and  gold  ore  occurrence  in  the  Permian  marly-
clayey limestone; 2 – iron ore occurrence in the Triassic sedimen-
tary  succession;  3  –  copper  and  silver  ore  occurrence  in  the
Triassic pillow basalt series; 4 – copper ore indication in both the
Triassic and the Jurassic pillow basalt series).

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Methods

Outcrops on the northern part of the Darnó Hill, and the

ore  exposed  in  an  old  adit  on  the  southern  part  of  the  hill,
were  studied  in  detail.  Sampling  paid  attention  on  the
gangue and ore minerals as well as on their host rock and its
alteration.  Stereomicroscopic  observations  were  carried  out
with  an  SM  Lab2  type  equipment,  while  polarized  micro-
scopic observations on polished standard thin and block sec-
tions were done using a Zeiss Axioplan microscope working
with  transmitted  and  reflected  light.  Fluid  inclusion  petro-
graphy was carried out on double polished, 100

µm thick sec-

tions  with  an  Olympus-BH2  type  microscope,  while
microthermometry  measurements  were  performed  with
a Linkam FT-IR 600 type heating-freezing stage mounted on
an  Olympus-BX51  type  microscope,  providing  magnifica-
tion up to 1000

×. Calibration of the stage was done by syn-

thetic CO

and H

O inclusions, allowing an accuracy of

0.1 °C below and 1 °C above 0 °C. The X-Ray Powder Dif-
fraction (XRD) method was used to determine the clay mine-
rals  on  oriented,  air  dried  and  ethylene  glycol  solvation
samples,  using  a  Siemens  D-5000  type,  Bragg—Brentano
geometric  diffractometer  emission  (

Θ−Θ working method,

Cu K

α (λ=0.154178 nm), secondary graphite crystal mono-

chromator  and  scintillation  detector).  The  interpretation  of
the  data  was  done  using  EVA  software.  A  PC  controlled
thermoanalytical instrument was used to determine the clay
mineral and zeolite content more precisely. MOM Derivato-
graph Q 1500 D was used, with a heating rate of 10 °C/min,
until  1000 °C,  the  sample  was  put  in  a  corundum  crucible.
All  of  these  measurements  were  completed  at  the  Depart-
ment  of  Mineralogy,  Eötvös  Loránd  University.  Raman
spectroscopy analyses were performed on fluid inclusions in
calcite  at  the  Eötvös  Loránd  University  Faculty  of  Science
Research  Instrument  Core  Facility,  using  a  Horiba  Yvon
Jobin LabRAM HR 800 edge filter based confocal dispersive
Raman  spectrometer,  with  800 mm  focal  length,  coupled
with  an  Olympus  BXFM  type  microscope.  During  the
2

×5 min long measurements, 784 nm emission of a frequen-

cy doubled Nd:YAG laser, 600 grooves/mm grating, 50

µm

confocal aperture and 50

× and 100× long working distance

objectives were used. ICP OES analyses was carried out on
a  Jobin  Yvon  Ultima  2C  type  spectrometer,  equipped  with
a monochromator at the Geological and Geophysical Institute
of  Hungary.  Lithium  borate  was  used  to  fuse  the  samples,
while the detection limits are shown in Table 2. SEM+EDS
analyses of the Cu-oxide bearing veins was carried out at the
Department of Petrology and Geochemistry (Eötvös Loránd
University)  with  an  Amray  1830  scanning  electron  micro-
scope,  equipped  with  an  EDAX  PV9800  energy  dispersive
detector,  operating  at  20 kV  accelerating  voltage,  10 nA
beam  current  and  50 nm  beam  diameter.  Electron  micro-
probe analyses of the native Cu were carried out at the Uni-
versity  of  Miskolc  using  a  Jeol  JXA-8600,  equipped  with
a wavelength dispersive detector, operated at 20 kV accelera-
ting  voltage,  20 nA  beam  current  and  1

µm beam diameter.

The  detection  limits  were  the  following  for  the  studied  ele-
ments:  0.049 wt. %  for  Cu,  0.277 wt. %  for  As  and
0.085 wt. %  for  Sb.  Electron  microprobe  analyses  of  hema-

tite was performed at the Eugen F. Stumpfl Microprobe
Laboratory (University of Leoben) using a Jeol Superprobe
JXA-8200, equipped with a wavelength dispersive detector,
operated at 15 kV accelerating voltage, 10 nA beam current,
~1

µm beam diameter. The counting time was 15 sec for the

peaks and 5-5 sec for the background (except in the case of
the  Zn,  where  20 sec  and  10 sec  were  used,  respectively).
The detection limits are shown in Table 1. Determination of
some  fine-grained  minerals  found  at  the  studied  localities
was  also  carried  out  with  the  above  mentioned  instruments
in  EDS  mode.  Calibration  of  both  instruments  was  carried
out with the help of natural and synthetic standards.

Results

The native copper bearing calcite-laumontite veins

Though several natural pillow basalt outcrops were stu-

died in the valley of the Báj Brook (southern part of the Darnó
Hill), the calcite-laumontite veins were found only in the old
shaft  and  the  adjacent,  partly  collapsed  adit.  The  shaft  is
about 8 m deep and the connecting N-S running adit is about
15 m long, ending in a room, from which E—W running, ca.
10-10 metres  long  crosscuts  (collapsed  at  the  ends)  are
found. The NNW—SSE trending, 10—20 cm thick, nearly ver-
tical dipping veins are traceable in the wall of the shaft, on
the  roof  of  the  N—S  adit  and  on  the  wall  of  the  room
(Fig. 2A).  Coarse  grained  laumontite  is  placed  along  the
walls  of  the  vein,  together  with  calcite  and  finer  grained
quartz.  Coarse  grained  calcite  and  quartz  with  fine  grained
laumontite is found in the middle of the veins. Native copper
(Fig. 2B) is most commonly found in this coarse grained cal-
cite, while secondary Cu minerals are abundant in every part
of the vein as well as in the host rock. The host rock is red,
amygdaloidal,  strongly  altered  pillow  basalt  with  abundant
jig-saw-fit and epigenetic veinlets and a high amount of inter-
pillow  hyaloclastite  breccia.  The  younging  direction  of  the
pillows shows the effects of later tectonic movements.

Based on the microscopic observations, the host basalt is

composed  of  glassy,  microcrystalline,  strongly  hematitized
ground  mass  (up  to  50—70 %).  Skeletal  euhedral  crystals  of
plagioclase  (up  to  30—40 %)  and  disseminated  calcite,
quartz,  chlorite  and  clay  minerals  filled  0.2—1.2 mm  sized
pseudomorphs after olivine (Fig. 2C) occur in the rock. The
size of the plagioclase crystals is up to 0.3 mm, the crystals
are  partly  altered  to  clay  minerals  and  their  position  high-
lights  the  sphaerolitic-variolitic  texture  of  the  basalt.  The
rock is rich in 0.15—0.7 mm sized calcite and rarely greenish
smectite  filled  amygdales  and  cooling  cracks.  Jig-saw-fit
veins, filled up by calcite, quartz and greenish smectite with
a thickness up to 1.5 mm are present in places. The contact
of  the  host  rock  and  the  10—20 cm  thick  calcite-laumontite
(epigenetic)  veins  is  sharp,  marked  with  strong  hematitiza-
tion and the presence of 1—3 mm thick auxiliary veins.

Two types of calcite occur in the veins, filling up to

70 vol. % of it. The 1—10 mm sized subhedral grains formed
earlier, or together with euhedral coarse grained laumontite,
while the 0.1—0.6 mm sized anhedral grains formed later,

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together and even after the zeolite and quartz formation. Up to
10 % of the veins is filled up by quartz, forming subhedral,
1—2 mm  sized  grains  as  well  as  later,  fine  grained  anhedral
crystals.  Laumontite  (up  to  20 %)  is  generally  strongly  al-
tered  (to  clay  minerals  and  limonite),  but  phillipsite  (up  to
5 %) is more fresh and forms anhedral and subhedral grains
up  to  0.8 mm.  Barite  forms  fine  grained  disseminated  crys-
tals  occurring  together  with  calcite  and  zeolites.  Fine
grained, disseminated hematite is a common opaque mineral,
but  up  to  10

µm sized native silver in zeolite, Ag

(acanthite,  based  on  optical  characteristics)  as  inclusions
within the quartz and as small grains among the quartz and
calcite crystals and clay minerals, as well as galena in calcite
and  together  with  native  copper  were  also  identified  by
SEM+EDS. Up to 3 cm sized native copper aggregates were
found in calcite, while copper oxides and carbonates, formed
obviously as its alteration products, were observed dissemi-
nated  together  with  all  the  veinfilling  minerals.  The  native

Th(LV-L)=81±14 °C  (n=33),  while  the  calculated  salinity
was  0.5±0.24  NaCl  equiv.  wt.  %  (n=11)  (Fig. 3).  Based  on
eutectic temperature observed at —21.8 °C and totally frozen
inclusions at —45 °C, the thermodynamics of the system were
modelled  as  a  binary  NaCl-H

O system. No traceable gas

content was found in the vapour phase of the inclusions by
Raman spectroscopy observations.

The quartz-hematite occurrence

This mineralization was found in natural outcrops on the

northeastern slopes of the Darnó Hill. Besides the quartz-he-
matite bearing samples, blocks of red radiolarite and red ba-
salt (together with pinkish limestone) were identified in the
vicinity. Due to the bad exposure and the mélange nature of
the studied formation, the connection with the basalt cannot
be clarified based on the surface mapping. The studied rock
is composed of up to 5 mm sized grains of quartz and fine

Fig. 2. Textural features of the studied Cu-Ag bearing samples (Báj Brook, S Darnó Hill). A – texture of a vein, found at the end of the
adit. The host basalt is strongly altered. B – native copper, found in one of the veins. C – typical textural features of the host basalt, with
calcite filled amygdales, calcite and clay minerals filled pseudomorph after olivine and sphaerolitic-variolitic plagioclase (microphoto-
graph, 1N). D – acanthite grains in quartz (BSE image). E – domeykite is surrounding the partly altered (to cuprite) native copper grain
(BSE image). F – late, cavity filling malachite (BSE image).

Fig. 3. Results of the fluid inclusion study performed on primary fluid inclu-
sions of calcite of the Cu-Ag bearing samples (Báj Brook, S Darnó Hill).

copper is composed almost exclusively of Cu
(97.60—98.89 wt. %), no traceable As or Sb con-
tent was observed. Domeykite forms up to 50

µm

grains, rimming the native copper aggregates
(Fig. 2D, E, F).

More precise identification of the clay minerals

was  performed  with  derivatography  and  XRD
analyses.  The  results  supported  the  presence  of
laumontite and phillispite as well as revealing the
occurrence  of  a  dioctahedral,  Ca  and  Al-bearing
smectite, i.e. beidellite and montmorillonite.

Fluid inclusion study was carried out in order

to  determine  the  minimum  formation  tempera-
ture as well as the salinity of the mineral forming
fluid. Both the coarser grained calcite and the fi-
ner  grained  anhedral  calcite  and  quartz  contain
a high amount of secondary fluid inclusion planes,
which makes the observations difficult. Only the
early,  coarse  grained,  subhedral  calcite  contains
measurable  primary  fluid  inclusions,  occurring
generally  close  to  the  rim  of  the  crystals.  The
coarser grained quartz was not sufficiently trans-
parent  to  identify  the  inclusions.  The  observed,
generally 4—7

µm sized (rarely up to 15 µm) pri-

mary  fluid  inclusions  were  found  independently
from the secondary planes and they were charac-
terized by a constant phase ratio of 5—10 area %
vapour phase and 95—90 area % liquid phase, in-
dicating  a  homogenous  entrapment  from  a  ho-
mogenous  parent  fluid.  The  phenomenon  of
metastability  often  hindered  the  observation  of
the  final  melting  temperature,  thus,  the  calcula-
tion  of  the  salinity.  The  homogenization  tempe-
rature (i.e. minimum formation temperature) was

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grained  hematite.  The  latter  may  form  up  to  4—5 mm  sized
patches,  composed  of  0.05-0.1 mm  sized  specular  hematite
flakes  (Fig. 4A).  A  continuous  change  is  observable  from
this quartz-hematite rock through hematitized siliceous sedi-
ment  to  the  red  radiolarite.  Both  rocks  may  be  cut  by  later
quartz veins.

Based on the microscopic observations, the host siliceous

sediment  is  composed  of  fine  grained  quartz  (up  to  80 %)
and  hematite  (<3 %)  and  up  to  20 %  bioclasts  (0.1—0.2 mm
recrystallized radiolarians and similar sized siliceous sponge
spicules). This rock contains quartz filled cavities of 1—3 cm
in  diameter  and  it  is  cut  and  at  some  places  even  slightly
brecciated by later quartz veins, which are 1—5 mm thick and
contain exclusively up to 2 mm sized quartz crystals.

The hydrothermal quartz-hematite samples are composed

of 5—20 % hematite, 5—20 % prehnite and up to 90 % quartz
(Fig. 4B, C). The distribution of the different minerals is in-
homogenous. The quartz forms anhedral to subhedral, gene-
rally  0.1—0.3 mm  sized  grains,  though  bigger  crystals  up  to
5 mm may occur in cavities, too. The subhedral quartz may
be characterized with growth zones, in which hematite inclu-
sions  may  also  occur.  The  anhedral  to  subhedral  hematite
forms  up  to  0.1 mm  sized  grains,  often  forming  crystal  ag-
gregates. This hematite is often surrounded by anhedral preh-
nite  grains  of  up  to  0.05 mm  size  and  eu-  to  subhedral
elongated  pumpellyite  crystals  of  up  to  0.05 mm  size,  both
occurring together with the quartz. The SEM+EDS observa-
tions  revealed  the  rare  presence  of  Fe-Ti-oxides  and  Mn-
oxides in the studied sections and the occurrence of 5—10

µm

thin barite veinlets. The EPMA analyses of the hematite
grains show that besides the 99.52—101.42 mass % Fe

, it

contains generally 0.018—0.077 mass % MgO, 0.688—
3.583 mass % Al

, 0.058—0.361 mass % MnO and 0.046—

0.192 mass % V

, while rare measurements of Cr

up to

0.050 mass %, ZnO up to 0.155 mass % and TiO

up to

0.199 mass % were also determined (Table 1).

In contrast with the findings of Kiss (1958), who described

rocks with up to 87 mass % Fe

content from the area, the

whole rock ICP OES analyses of the studied samples re-
vealed, that they contain 8.95—11.4 mass % Fe

and 87.8—

90.2 mass % SiO

. Traces of Al

, MnO, CaO, Na

O, SrO

and a total metal content of 190—235 ppm (Co, Cr, Cu, Ni,
Pb, V and Zn) were also observed (Table 2). The Fe

-SiO

(hematite-quartz) ratio was also proven by the XRD study.

Fluid inclusion study was carried out in order to determine

the minimum formation temperature and the composition of
the hydrothermal fluid. Quartz syngenetic with the hematite
flakes indicated by the tiny hematite inclusions in its growth
zones was examined. The crystals contain several secondary
generations, but among those planes, rarely independent pri-
mary inclusions as well as primary inclusions in 3D clouds
occur with a size of 4—10

µm. The growth zones of the

quartz generally contain ~1

µm sized fluid inclusions, too,

but in rare cases measurable sized (up to 10

µm) primary in-

clusions also occurred. All primary inclusions were trapped
homogenously from a homogenous parent fluid, which is
represented by a constant phase ratio of 5—10 area % vapour
and 95—90 area % liquid phase. The phenomenon of metasta-
bility often hindered the observation of final melting tempe-

Fig. 4.  Textural  features  of  the  studied  Fe  bearing  samples  (NE
Darnó  Hill).  A  –  hematite  flakes  with  quartz  and  prehnite.
B – textural relation of the hematite, quartz and prehnite (micro-
photograph,  1N).  C  –  inhomogenous  distribution  of  the  hematite
flakes and the quartz, prehnite grains (BSE image).

ratures, thus, only a few salinity data are available. The mea-
sured homogenization temperature was Th(LV-L)=70—155 °C
(n=6) while the calculated salinity (based on the final mel-

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ting temperature of ice) was 3.87—4.34 NaCl equiv. wt. %
(n=2). Though no eutectic temperature was observed, the
system was modelled in a NaCl-H

O system, as all the inclu-

sions were totally frozen by —45 °C.

Discussion

The native copper bearing calcite-laumontite veins

In the early study of Haidinger (1850), diorite was identi-

fied as the host rock of this occurrence. However, a later
study by Lőw (1925) has found, that the veins occur in dia-
base, though the age was still not determined. Modern re-

search  by  Kiss  et  al.  (2010)  revealed  that  both  Triassic  and
Jurassic basaltic occurrences are known on the surface in the
Darnó Hill and collected their distinguishing features. Com-
paring  the  typical  textural  features  written  above,  with  the
typical  characteristics  described  by  Kiss  et  al.  (2010),  the
host  rock  is  most  likely  a  Triassic  basalt  of  rifting-related
origin.  Kiss  et  al.  (2012)  have  also  shown,  that  this  basalt
bears  a  within-plate  basalt  geochemical  signature  and
formed in shallow-marine conditions.

The observed mineral paragenesis (calcite, laumontite,

phillipsite, quartz, barite, native copper and silver, acanthite,
domeykite  and  galena  and  their  alteration  products)  is  not
the  typical  mineral  assemblage  of  volcanogenic  massive
sulphide  deposits  formed  by  submarine  hydrothermal  pro-
cesses (see e.g. Shanks & Thurston 2012). Furthermore, the
measured  fluid  characteristics  are  significantly  different
from  those  found  in  the  primary  hydrothermal  minerals  of
the  Darnó  Unit  (Kiss  et  al.  2008,  2012;  Fig. 5).  These  fea-
tures draw attention to the fact, that the studied mineraliza-
tion  was  most  likely  not  related  to  the  primary  submarine
processes.  The  occurrence  of  domeykite  and  some  other
mineralogical and textural features corroborates origin from
some  kind  of  hydrothermal  processes  (Ramdohr  1969).The
alteration  characteristics  of  the  host  rock  (e.g.  intensive  he-
matitization)  suggest  an  oxidative  and  slightly  acidic  envi-
ronment for the beginning of the ore forming processes. This
allows  Cu,  Ag  and  Pb  to  form  chlorocomplexes,  which
means that they are collected from the host basalt, dissolved
in a hydrothermal fluid. The mineral precipitation sequence
of the veins (Fig. 6) shows that the conditions became more
and more reducing. This allowed the precipitation of native
copper  and  silver  and  the  early  gangue  minerals.  Later,
reaching  neutral  pH  and  redox  conditions,  the  precipitation
of  sulphides  and  arsenides  could  happen,  together  with
gangue minerals (Fig. 7). Cooling of the hydrothermal fluid
resulted  in  the  precipitation  of  late  calcite  and  phillipsite
(forms generally between 65—85 °C), followed by the forma-
tion  of  different  alteration  products  (clay  minerals,  Cu-
oxides,  Fe-oxides,  -hydroxides  and  Cu-carbonates)  (Barton
&  Skinner  1967;  Brown  2006;  Hanor  2000;  Deer  &  Howie
2004; Pirajno 2009).

Table 1: Results of the EPMA analyses of the studied hematite grains (NE Darnó Hill)

Table 2: Results of the ICP OES analyses of the quartz-hematite
samples (NE Darnó Hill)

Detection limit

MgO Al

MnO ZnO TiO

Total

0.0189

0.0169

0.0361

0.0444

0.0624

0.0455

0.0374

0.0434

0.048

3.583

b.d.l.

0.109

101.135

104.875

0.035

0.838

0.05

0.058

0.07

b.d.l.

0.12

101.521

102.692

0.021 1.136 b.d.l. b.d.l. b.d.l. b.d.l. 0.162

99.588 100.907

0.031 1.026 b.d.l. 0.106 b.d.l. b.d.l. 0.071

99.420 100.654

b.d.l. 0.688 b.d.l. 0.361

0.148 0.083 0.072 101.991 103.343

0.028

1.119

b.d.l.

0.119

b.d.l.

0.108

0.056

100.187

101.617

0.049 1.126 b.d.l. 0.222 0.089 0.074 0.046 100.514 102.120

b.d.l.

0.746

b.d.l.

0.19

0.155

0.151

0.033

100.418

101.693

0.027 0.979 b.d.l. 0.147 b.d.l. 0.199 0.04

99.869 101.261

0.077 0.077 b.d.l. 0.335 b.d.l. b.d.l. b.d.l. 100.746 101.235

b.d.l.

0.831

b.d.l.

0.197

b.d.l.

0.125

0.072

100.764

101.989

0.018 0.843 b.d.l. 0.268 b.d.l. 0.076 0.087 100.490 101.782

1.767

0.254

b.d.l.

1.721

0.202

23.477

0.236

71.187

98.844

Results are given in mass %. b.d.l. — below detection limit.

Detection limit

Sample 1

Sample 2

SiO

0.05 87.8 90.2

TiO

0.02 b.d.l.

b.d.l.

0.1

0.159

0.03 11.415 8.955

MnO

0.003 0.021 0.015

CaO

0.03 0.088

0.175

MgO

0.15

b.d.l.

0.03 b.d.l.

0.100

0.2 b.d.l.

b.d.l.

0.15 b.d.l.

b.d.l.

0.15

b.d.l.

BaO

0.005 b.d.l.

b.d.l.

SrO

0.0002 0.0003 0.0003

0.01 0.05 0.05

LOI

0.01

0.33

0.27

0.5 40.4 88.2

0.5 18.1 12.0

0.5 16.9 8.6

1.0 19.8 15.9

1.5 5.0 b.d.l.

2.5 97.4 12.8

0.25 37.2 51.8

Results are given in mass % for the major and minor elements (above
the thick line) and in ppm for the trace elements (below the thick
line). b.d.l. — below detection limit.

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Fig. 6. The observed mineral precipitation sequence in the studied Cu-Ag occurrence (Báj Brook, S Darnó Hill). Newly described minerals
from the area are marked with grey columns.

The epigenetic mineral paragenesis is characteristic of the

zeolite  facies  metamorphism,  which  happens  generally  at
200—270 °C temperature and 150—250 MPa pressure (Ivanov
&  Gurevich  1975).  Using  this  approximation,  pressure  cor-
rection  on  the  fluid  inclusion  microthermometry  data  sug-
gests  formation  temperature  of  140—200 °C  (Fig. 8).  This  is
in  good  agreement  with  the  presence  of  acanthite,  which
forms below 173 °C (Ramdohr 1969) and laumontite, which
forms  generally  between  90—190 °C  (Deer  &  Howie  2004).
Although  these  metamorphic-hydrothermal  conditions  are
presumed, no gas content was identified by Raman spectro-

scopy and the observed low salinity values assume the possi-
bility of mixing with meteoric water.

Similar typical Cu, Ag and As bearing mineral assemblage

in red amygdaloidal basalt was described by several authors
(e.g.  Butler  &  Burbank  1929;  Heinrich  1976;  Lefebure  &
Church  1996;  Brown  2006;  Bornhorst  &  Barron  2011  and
the  references  cited  therein)  on  the  Keweenaw  Peninsula,
Michigan, USA. There the mineralization is related to rifting
related  subaerial  to  shallow  marine  basalt,  which  suffered
sub-greenschist  (typically  zeolite)  facies  metamorphism.
According to the genetic model of Brown (2006), the basaltic

Fig. 5. Results of the fluid inclusion study of the calcite-laumontite veins (Báj Brook,
S Darnó Hill), compared to the submarine hydrothermal processes described in the Darnó
Unit by earlier studies (Kiss et al. 2008, 2012). The observed characteristics are signifi-
cantly different.

Michigan-type  copper  ore  formation  is
related to the evolution of an epigenetic,
mixed metamorphic and meteoric hydro-
thermal  fluid,  which  steadily  accomo-
dates  its  original  oxidative  and  acidic
characteristics  to  neutralizing  condi-
tions.  Although  originally  Precambrian
deposits  were  classified  as  Michigan-
type,  nowadays  Phanerozoic  examples
are  also  known  (e.g.  in  Central-Iran,
Nezafati et al. 2005).

In the case of the studied Darnó Hill

occurrence, not only the ore and gangue
mineral  paragenesis  shares  similarities
with  the  above  mentioned  deposit  type,
but  also  the  tectonic  setting  (rifting  re-
lated,  shallow  marine  conditions),  the
observed  metamorphic  facies  (zeolite),
the  formation  temperature  (within  the
typical  range  of  150—250 °C)  and  the
evolution  of  the  hydrothermal  fluid  are
also analogous. Therefore, based on the

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Fig. 7. The change in the redox and pH conditions during the evolu-
tion of the hydrothermal fluid, as mirrored by the precipitation se-
quence and the stability field of the studied minerals (Báj Brook,
S Darnó Hill).

Fig. 8. Pressure correction performed on the fluid inclusion data in order to obtain the real
formation conditions of the calcite-laumontite veins (Báj Brook, S Darnó Hill). Standard
deviation of the data was also taken into consideration, when the isochores were deter-
mined (see the thin lines).

present research, a genesis similar to the Michigan-type ore
formation is suggested for this native copper occurrence of
the Darnó Unit, as opposed to former opinions (Kiss 1958).
This finding fits well into the geology of the rocks found in
the studied mélange.

The quartz-hematite occurrence

The host rock of the quartz-hematite occurrence is most

likely the Ladinian chert and radiolarite, as suggested by

Kiss  (1958).  The  studied  samples  also  support  this  finding,
as  a  continuous  change  from  the  hematite-bearing  chert  to
the  quartz-hematite  rock  was  observed  in  the  different
samples.

The textural features of the samples, as well as their che-

mical composition and fluid inclusion data suggest that these
rocks  were  not  formed  during  submarine  sedimentary  pro-
cesses (e.g. bacterial precipitation). According to Bonatti et
al.  (1971,  1972),  low  Mn  and  Ni+Co+Cu+Cr  contents
(<1 mass %)  are  not  typical  during  submarine  sedimentary
conditions, but can be characteristic in exhalative processes.
In our case, 0.015—0.021 mass % MnO and low, 95—125 ppm
Ni+Co+Cu+Cr  support  the  origin  by  an  exhalative  process.
The minimum formation temperature (70—155 °C) also sug-
gests  a  hydrothermal  origin  and  the  flake-like  (specular)
shape of hematite also supports this assumption. The precipi-
tation series is established by the microscopic observations.
The  hematite  was  followed  by  prehnite  and  quartz,  both
occurring  as  simultaneous  and  later  minerals.  Pumpellyite
and barite formed only as late minerals.

The composition of the hematite grains gives further re-

finement about the origin. Only a low amount of replace-
ment is allowed in the unit cell of hematite; about 1 mass %
of Al

and TiO

may occur, together with rare V and Cr

(Gaines  et  al.  1997).  On  the  contrary,  the  studied  hematite
crystals  are  richer  in  trace  elements  and  besides  the  above
mentioned  components,  ZnO,  MgO  and  MnO  were  also
found.  Comparing  these  results  with  the  ones  published  by
Dupuis  &  Beaudoin  (2011)  from  several  different  deposit
types, significant similarities with the trace element distribu-
tion patterns of the iron oxides from SEDEX deposits can be
observed (slight enrichment in Al, Mn, Zn and slight deple-
tion in Ti, V, Mg and Cr) (Fig. 9). This finding supports the
submarine exhalative origin, together with the few available
fluid   inclusion data, which resulted in slightly higher sali-
nity,  than  the  typical  seawater  (Kennett  1982).  This  slight

enrichment to 3.87—4.34 NaCl equiv.
mass % can be the result of the water-
rock interaction during the circulation of
the hydrothermal fluid.

According to Bonatti et al. (1972) and

Boström et al. (1979), the occurrence of
Fe-SEDEX is typical in the early stages
of  rifting,  therefore  it  fits  well  into  the
geology of the Darnó Hill. This finding
helps  in  understanding  the  relationship
of  the  different  ore  occurrences  of  the
studied mélange.

Conclusions

The NE Hungarian Darnó Unit is

composed of a Neo-tethyan accretionary
mélange  complex,  containing  blocks  of
different  age  and  origin.  Its  magmatic
and  sedimentary  rock  blocks  derive
from  different  evolutionary  stages  of
the  Neo-tethys;  including  Permian  and

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is most likely a Fe-SEDEX, which is a characteristic deposit
type during early rifting (Bonatti et al. 1972; Boström et al.
1979).  The  studied  Cu(-Ag)-bearing  calcite-zeolite-quartz
veins are found in Triassic pillow basalt and their genesis is
assumed  to  be  related  to  a  process,  which  is  similar  to  the
Michigan-type  deposit  formation.  This  deposit  type  forms
generally  during  epigenetic  (though  closely  related)  hydro-
thermal process, during the advanced rifting stage (Lefebure
&  Church  1996;  Brown  2006;  Bornhorst  &  Barron  2011).
Finally, Molnár et al. (2015) found that the origin of the Cu-
bearing quartz-prehnite veins, occurring in both Triassic and
Jurassic  basalt  blocks,  was  related  to  the  Alpine  low-grade
regional metamorphism, which is obviously a later (Jurassic
or younger), regional epigenetic process.

Different types of ore occurrences found on the Darnó Hill

are  closely  related  to  different  evolutionary  stages  of  the
Neotethys  in  space  and  time.  Their  recent,  spatially  close
position  is  the  result  of  the  accretionary  mélange  formation
(Dimitrijević et al. 2003), which brought blocks with various
origins  close  to  each  other.  Therefore,  the  direct  economic
importance  of  the  studied  occurrences  is  low,  as  the  extent
of  the  isolated  ore-bearing  blocks  in  the  mélange  is  not
always  known.  However,  the  region  has  a  metallogenic
importance, as the Darnó Unit is a perfect natural laboratory
to understand the rifting-related ore forming processes.

Acknowledgements:  Special  thanks  to  Ferenc  Molnár,
Ágnes Takács, Tibor Horváth, Tibor Németh, Ivett Kovács,
Zsolt Bendő, Giorgio Garuti and all the fellows at the Eötvös
Loránd  University  and  the  University  of  Leoben.  The  Uni-
versity  Centre  for  Applied  Geosciences  (University  of
Leoben)  is  thanked  for  the  access  to  the  Eugen  F.  Stumpfl
Electron  Microprobe  Laboratory  while  the  Research  Instru-
ment  Core  Facility  (Eötvös  Loránd  University)  is  thanked
for the access to the Raman laboratory. János Földessy and
Ladislav  A.  Palinkaš  are  thanked  for  the  constructive  re-

views, playing an essential role during improvement of the
original manuscript.

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