1. AU-PORPHYRY MINERALIZATION IN BELUJ (ŠTIAVNICA STRATOVOLCANO,
SLOVAKIA)
MOLNÁR, L., KODĚRA, P. (Comenius University, Faculty of Natural Sciences, Department of Geology of
Mineral Deposits, Mlynská dolina, 842 15 Bratislava, Slovakia), BAKOS, F. (EMED – Slovakia, s.r.o. Nám.
SNP 466/1, Detva, Slovakia)
luismoloar@gmail.com
The studied area is located in the Central Slovakia Volcanic Field, in the peripheral part of the Neogene
Štiavnica stratovolcano, near the villages Beluj and Prenčov. Au-porphyry systems in the peripheral part of the
stratovolcano are connected to intrusive andesite and diorite porphyry bodies, related to early stage of evolution
of the stratovolcano. This porphyry system was recently explored by EMED – Slovakia, s.r.o. including two
inclined drillholes down to 254 m depth, discovering uneconomic potential of 58.4 Mt at 0.3 ppm Au ore (Bakos
et al., 2010). The host rock of the mineralisation is a medium grained andesite porphyry body. The intrusion is
affected by K and Na silicate alteration which is overprinted by intermediate argilization. A post mineralization
advanced argillic alteration occurs in elongated NW-SE trending zones up to several tens of meters thick, formed
by argillised and silicified breccias with vuggy texture. The phyllic alteration locally appears on the margins of
the intrusive centre, while the broad surrounding is propylitized.
The rock forming minerals are feldspar, amphibole, and sporadically quartz. Feldspar is replaced by alkaline
plagioclase albite and anortite and locally secondary K-feldspar. Plagioclase is altered to illite and smectite;
mafic minerals are altered to clusters of secondary biotite and actinolite. The matrix is sericitized, chloritized and
smectitized. According to XRD and microprobe analysis biotite, chlorite and clay minerals (illite, smecktite) are
the main alteration products. Chlorite is enriched in Fe (clinochlore) and is present in the matrix, in thin veins
and in cavities, together with clay minerals (illite and smectite). Chlorite temperatures were calculated
(according to Caritat et al., 1993) and show an average of 420 °C. The gold mineralisation is spatially connected
to stockwork of quartz veinlets. Some of the veinlets are banded, while banding results from high content of
vapour rich fluid inclusions and tiny grains of magnetite and ilmenite. Relatively younger are veins with quartz,
calcite and Fe-rich smecktite (nontronite, saponite). Veinlets with cubic shaped zeolite (chabasite?) and clay
minerals are dominant in the middle section of the studied drill core. Relatively the youngest are chalcopyrite
veins rarely with sphalerite. Locally a fine grained breccia pebble-dyke cuts through the host rock. The breccia -
dyke is affected by silicification and biotitisation and contains older fragments of quartz veins. Brecciacion also
occur between 218 and 230 meters. The main ore mineral is pyrite disseminated in veins and as grains in the host
rock. Magnetite and ilmenite forms solid solutions and are disseminated in the rock and sporadically in veins.
Magnetite and ilmenite is replaced by pyrite. Galena, sphalerite and molybdenite are less frequent. Sphalerite
and galena is replaced by chalcopyrite. Apatite and titanite are common accessory minerals connected to
actinolitization of mafic minerals and are also present in veins with pyrite, chlorite, less frequently with epidote.
Garnet (andradite) is rare, but was found in the host rock, locally is zoned, where the younger rim is enriched in
grossular. Tourmaline was found in the matrix with biotite, plagioclase and K-feldspar. Other accessory minerals
are monazite, allanite, thorite, xenotime, and zircon.
According to geochemical analyses of drill core (provided by EMED), K shows positive while Na and Ca
negative anomalies in the sections of banded quartz veinlets and in the brecciated zone from 218 to 230 meters.
Au and Cu values are increased in the sections of banded quartz veinlets, veinlets with zeolite, and in the
brecciated zone, with a maximum of 1.2 ppm for Au and an average of 245 ppm for Cu. Pb and Zn are enriched
in the zones of positive K anomalies, with an average of 30 ppm for Pb and 176 ppm for Zn. Mo anomaly is in
the zone of a breccia - dyke from 118 to 140 meters with an average of 7 ppm. Ag is constantly under detection
limit except of an interval from 122 to 130 m where is slightly increased in the range 0,6-0,9 ppm.
Fluid inclusions occur in quartz crystals from veins. Dominant are secondary vapour rich inclusions. Primary
vapour rich inclusions coexisting with fluid rich inclusions, and vapour rich inclusions with solid phases of salts
are rare. The rim of plagioclase crystals also contains vapour rich fluid inclusions. Oxygen isotopes were
measured on quartz from veinlets. Isotopic composition showed a narrow range of data from 9.0 to 9.6 ‰ δ18
O,
and show similar values measured at the Biely vrch Au porphyry deposit (7.6 to 12.4 ‰ δ18
O; Koděra et al.,
2011) in the Javorie stratovolcano.
Acknowledgements: Support by APVV grant 0537-10 is acknowledged.
References
BAKOS, F. et al., (2010): Mineralia Slovaca, 42, 1-14.
KODĚRA, P., LEXA, J., HEINRICH, C. A., WÄLLE, M. & FALLICK, A. E. (2011): Proc. 11th
SGA Meeting,
417-419.
CARITAT, D. P., HUTCHEON, i., & WALSHE, L. J. (1993): Clays and Clay minerals, 41, 2, 219-239.