Koděra P., Bakos F., Kubač A., Lexa J. & Sýkora M., 2016: Epithermal Au-Ag mineralisation at the locality Prochot – Lazy in the mantle of the Štiavnica Stratovolcano. Acta Geologica Slovaca, 8, 2, 149–163.

Epithermal Au-Ag mineralisation at the locality Prochot – Lazy in the mantle of the Štiavnica Stratovolcano

Peter Koděra¹, František Bakos², Alexander Kubač¹, Jaroslav Lexa³ & Maroš Sýkora²Peter Koděra¹, František Bakos², Alexander Kubač¹, Jaroslav Lexa³ & Maroš Sýkora²

¹Department of Economic Geology, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská dolina, Ilkovičova 6, 842 15 Bratislava, Slovakia; kodera@fns.uniba.sk
²Green View s. r. o., Nevädzová 5, 821 01 Bratislava, Slovakia; fbakoss@yahoo.com, maros.sykora@gmail.com
³Earth Science Institute of the Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava; geoljalx@savba.sk

Abstract

The Prochot-Lazy area hosts a poorly known historical epithermal precious metal deposit hosted by andesites of the Lower structural unit of the Štiavnica Stratovolcano. According to current exploration, this locality consists of two roughly parallel mineralised zones. The Main zone is probably related to a regional fault and it was followed by old mine workings. The inclined bore hole PT-1 has determined here exploited spaces and remnants of quartz-carbonate veining (with 5.4 ppm Au, 10.5 ppm Ag) and native gold accompanied by tetrahedrite, pyrite, calcite, siderite and rhodochrosite. A zonal arrangement of alteration patterns includes uppermost steam-heated zone (kaolinite, alunite, crystobalite and increased Hg), base of steam-heated alteration (kaolinite, smectite, opal/chalcedony), adularia-rich zone in the hanging-wall of the vein structure and foot-wall alteration (smectite, calcite, kaolinite, adularia, chlorite). Alteration is accompanied by extensive brecciation and pyritisation, with increasing intensity towards the vein. Western continuation of the Main zone and parallel East zone were explored by channel sampling and showed alteration and geochemical features similar to the hanging-wall zone in the PT-1 hole with up to 1.3 ppm Au and 1.7 ppm Ag. The studied mineralisation has an intermediate position between low and intermediate sulfidation types. It evolved during the pre-caldera stage of the stratovolcano with the source of ore-bearing fluids related either to the magma chamber below the central zone of the stratovolcano (similar in source to the Rozália mine Au-deposit) or to a local magma chamber below the Prochot Intrusive Complex.

Key words: epithermal, exploration, Prochot-Lazy site, alteration, mineralogy, geochemistry, Štiavnica Stratovolcano

Manuscript received: 2016-06-16

Revised version accepted: 2016-09-30