Cuestionario empleado para el estudio empírico CUESTIONARIO ACERCA DE LOS FACTORES PARA LA

1_{Štefan Ferenc}#

, 2Peter Koděra,

3

Rastislav Demko

1 _{Dept. of Geography, Geology and Landscape Ecology, Fac. of Natural Sciences, University of}

Matej Bel, Tajovského 40, 974 01 Banská Bystrica, Slovak Republic, # [email protected]

2 _{Dept. of Geology of Mineral Deposits, Fac. of Natural Sciences, Comenius University, Mlynská}

dolina G, 842 15 Bratislava 4, Slovak Republic

3 _{State Geological Institute of Dionýz Štúr, Mlynská dolina 1, 817 04 Bratislava 11, Slovak Republic}

Key words: silver, gold, mineralization, epithermal, rhyolite, supergene zone, Neogene, Nová Baňa

INTRODUCTION

Nová Baňa is one of the most important silver-gold historical deposits in the Central Slovakia Volcanic Field. Mining activities started probably in the 13th Century, and continued with pauses until the year 1887 (Bergfest 1955). Later, unsuccessful exploration was carried out in the second half of 20th and at beginning of 21st centuries. A lot of mineral phases are known from the Nová Baňa deposit. Majority of mineral descriptions originated in the 19th. and beginning of 20th Century, in contrast with a lack of modern analytical tools. This contribution is dedicated to the results of new mineralogical and geochemical research of epithermal precious metals veins at Gupňa occurrence (SE part of the Nová Baňa ore field).

GEOLOGICAL SETTING

Nová Baňa ore field is located within W part of the Banská Štiavnica stratovolcano. In upper parts of the deposit ore veins are

developed in propylitized andesites (Badenian), and rhyolitic volcanic rocks of the Jastrabá Formation (11.9-12.2 Ma, Lexa and Pecskay 2013). Footwall of Miocene volcanics is formed by the Late Paleozoic clastic sediments of the Hronicum Unit, overthrusting the Mesozoic carbonates of the Veporicum Unit. Paleozoic and Mesozoic rocks are intruded by Miocene quartz-dioritic porphyre. Ore veins have N-S to NE-SW direction, with variable dip direction. Mineralization disappears with increasing depth (Lexa et al. 2000).

RESULTS

In the past at Gupňa the Reissenschuch vein system was exploited, located predominantly in the adularized rhyolites. Vein system is about 1.7 km long, and consists of 80-400 m long partial veins.

Sampling was undertaken on three locations: I) mouth of the Hohen Klotzer adit, II) crest of the Gupňa Mt., III) Červená skala open cut. Vertical range of

35 samples from vein system is 170 m. One of the vein structures (dip direction 296°/65°, thickness 1.5 m) is well observable in outcrop at the mouth of Hohen Klotzer adit. Central vein part (15 cm thick) consists of limonitized fine-grained quartz with ore minerals disseminations. Fine- grained quartz zone is symetrically bordered by reddish brown jasper. Central vein part is accompanied by swarm of quartz veinlets (up to 20 cm long, about 1- 2 cm thick) without ore mineralization. Au content in various vein structure parts ranges from 0.05 to 50 ppm, Ag content varies in the range 42-570 ppm. Mineralization at Gupňa originated in the following steps:

I. magmatic (?) Fe-Sn mineralization represented by local cassiterite and hematite disseminations in altered rhyolite. II. ore mineralization stage:

a) base metals substage (quartz, pyrite, galena, sphalerite, gold)

b) precious metals substage (quartz II, pearceite, Sb pearceite, As polybasite, polybasite, uytenbogaardtite, acanthite?) c) chalcedony substage (quartz II, chalcedony)

III. supergene stage, including products of the cementation (acanthite) and oxidation (goethite, corkite) processes. Wall rocks of ore veins are affected by hydrothermal alterations: silicification, pyritization and adularization. Fluid inclusions hosted in vein quartz are mainly liquid-rich, but vapor-rich inclusions are also sometimes present, indicating boiling of fluids. Analyses of fluid inclusions showed low salinities (0-1.6, mean 1.1 wt. % NaCl eq.), which is typical for low sulfidation epithermal systems. Eutectic temperatures ranged from -42 °C to -34 °C indicative of the system NaCl-FeCl2-H2O. Total

homogenization temperatures ranged from 183 °C to 246 °C (average 226 °C). Boiling of fluids was the likely reason for precipitation of gold. Assuming hydrostatic pressures, boiling occurred from 17 to 28 bars, corresponding to paleodepth of boiling (today erosion level) 180 to 310 m.

Isotopic composition of vein quartz ranged from 6.4 to 7.7 ‰ δ18O, while illite- smectite from wall rock alteration had 1.9 to 6.9 ‰ δ18O and -66 to -98 ‰ δD. Composition of fluids in equilibrium with quartz (-4.1 to -3.0 ‰ δ18O; -80 to -46 ‰ δD) indicate mostly meteoric origin of fluids affected by isotopic reequilibration with country rocks. The isotopic composition of fluids was calculated using temperatures based on fluid inclusions microthermometry and illite-smectite expandability (130–196 °C, Šucha et al. 1992; Harvey and Browne 2000).

Acknowledgments: This study was supported by Project of the Ministry of Environment of the Slovak Republic (No. 1506) and funds APVV-0081-10, VEGA- 1/0744/11.

REFERENCES

BERGFEST, A. (1955): Mining in Nová Baňa. Manuscript, Geofond archive, Bratislava, 130 p. (in Slovak)

HARVEY, C.C, BROWNE, P.R.L. (2000): Mixed layer clays in geothermal systems and their effectiveness as mineral geothermometers. Proceedings of the

World Geothermal congress, Kyushu,

Japan, 1201-1205.

LEXA, J., ŽÁKOVÁ, E., HOJSTRIČOVÁ, V, ROJKOVIČOVÁ, Ľ. (2000): Gold deposit Nová Baňa. Manuscript, Geofond archive Bratislava, 11 p. (in Slovak)

LEXA,J.,PÉCSKAY,Z. (2013): K-Ar dating of the Jastrabá formation rhyolites and related mineralization in the Central Slovakia Volcanic Field. In Geological evolution of the Western Carpathians: new ideas in the field of inter-regional correlations (I. Broska and A Tomašových, eds.). Geological Institute of the Slovak

Academy of Sciences, Bratislava, 80-81.

ŠUCHA, V., KRAUS, I., GERTHOFFEROVÁ, H., PETEŠ, J., SEREKOVÁ, M. (1992): Smectite to illite conversion in bentonites and shales of the East Slovak basin. Clay

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GEMMOLOGICAL AND SPECTROSCOPIC STUDY