Kozák J., Koděra P., Lexa J., Bakos F., Molnár L. & Wälle M. , 2017: Porphyry gold system of Beluj in the mantle of the Štiavnica stratovolcano (Slovakia). Acta Geologica Slovaca, 9, 1, 45–61.

Porphyry gold system of Beluj in the mantle of the Štiavnica stratovolcano (Slovakia)

Jaroslav Kozák¹, Peter Koděra¹, Jaroslav Lexa², František Bakos³, Levente Molnár⁴ & Markus Wälle⁵

¹Department of Geology of Mineral Deposits, Faculty of Natural Sciences, Comenius University in Bratislava, Ilkovičova 6, 842 15 Bratislava, Slovakia; kozakminerals@gmail.com, kodera@fns.uniba.sk
²Earth Science Institute of the Slovak Academy of Sciences – Geological Division, Dúbravská cesta 9, 840 05 Bratislava, Slovakia; jaroslav.lexa@gmail.com
³GREEN VIEW s.r.o., Nevädzová 5, 821 01, Bratislava, Slovakia; fbakoss@yahoo.com
⁴Nám. Priateľstva 2172/33, 929 01 Dunajská Streda, Slovakia; luismoloar@gmail.com
₅Department of Earth Sciences, ETH Zürich 8092, Zürich, Switzerland; mwalle@mun.ca

Abstract

Beluj magmatic-hydrothermal centre is a small porphyry gold occurrence related to an andesite/diorite porphyry intrusion, located in the mantle of the Štiavnica stratovolcano in the Central Slovakia Volcanic Field. The upper part of the intrusion is affected by K-silicate alteration (biotite, magnetite, K-feldspar) that is in deeper parts accompanied by Ca-silicate alteration (actinolite). Intermediate argillic alteration overprint (smectite, illite, chlorite ± calcite, chabasite) is locally more intensive and can be geochemically modeled by decreased Na/Al ratio. The youngest advanced argillic alteration (kaolinite, dickite, pyrophyllite) occurs at margins of the intrusion. Gold mineralization is spatially associated with a quartz stockwork, including magnetite-biotite (EB-type), A- and B-type quartz veinlets. Native gold grains of high fineness (891–951) were found in smectite in cavities of a banded quartz veinlet. Rare sulphide minerals (mostly pyrite and rare chalcopyrite) usually occur in cavities in quartz. Quartz vein hosts abundant vapor inclusions, some CO₂- and Cu-enriched and rare Au-bearing Fe-K-Na-rich salt melt inclusions with various daughter minerals and homogenization temperatures >600°C. Oxygen isotope composition of vein quartz and magnetite indicates equilibrium with hot fluids (570/650°C) of magmatic origin. Alteration patterns, mineralogy and fluid properties at the Beluj occurrence are similar to porphyry gold systems in the Javorie stratovolcano. However, the hydrothermal system was quite small, which had a negative consequence on economic value of this locality.

Key words: porphyry, gold, Beluj occurrence, Štiavnica stratovolcano, salt melt, Raman spectroscopy, LA-ICPMS, fluid inclusion

Manuscript received: 2016-09-05

Revised version accepted: 2017-06-13