Geological and geochemical investigation of ore bearing deposits in the Börzsöny mountains, N-Hungary

The Börzsöny mountain is part of the neogene Inner Carpathian volcanic belt and located in north of Hungary, close to the Central Slovakian Volcanic Field. The andesitic-dioritic rocks of the mountain host ore deposits of a porphyry hydrothermal system, but most of these ore bodies are barely known.

The complex volcanic history of the Börzsöny took place in the middle miocene. According to the K/Ar dating of the host and overlapping volcanic rocks, the ore formation is related to the first, mostly explosive and finally intrusive stage of the volcanic activity (15.2 – 14.8 Ma) (Korpás et. al, 1998). The elevated position of the crystalline basement in the central part of the Börzsöny showed by different geophysical measurements and endomagmatic rock inclusions suggest the presence of a large dioritic intrusive body at a depth of about 2.5 km from the surface (Csillagné Teplánszky et. al., 1980). The ore deposits discovered until now fall on a ring-shaped structure left over from the uplift (Korpás et. al, 1998).

The best investigated part of the ore complex is the central epithermal ore deposit called Rózsabánya. During geological research of two decades, a weak porphyry copper deposit was discovered below Rózsabánya at a depth of about 1.2 km (Csillagné Teplánszky et. al., 1980).

Based on ore petrography and SEM measurements, the multistage paragenesis of Rózsabánya started with the mineralization of arsenopyrite with some cobalt content, native gold and bismuth. This first stage was followed by a Zn-Cu-Pb-Ag-Sn-In mineral association mostly related to galena, sphalerite and chalcopyrite, that could be found as remnants in later minerals in almost all cases. In the third stage, the former minerals were partly consumed by a large amounts of pyrrhotite, which altered to different degrees to pyrite±marcasite. The next stage is characterised by galena, chalcopyrite and sphalerite mineralization, without Sn and In. The mentioned minerals are often replaced by pyrite±marcasite, Bi-sulphides and –sulphosalts, occasionaly arsenopyrite (with some cobalt and nikkel content) and finally by siderite. In some cases, where the alteration of pyrrhotite to pyrite and marcasite was intensive, the mineralization is associated with chloritization.

Based on the observed textures and paragenesis of Rózsabánya, some of the important thermodynamic parameters of the system could be estimated. After the mineralization of a low sulphidation state epithermal mineral assemblage, increasing temperature (around 400 – 450 °C according to the mol fraction of FeS of the pyrrhotite) and/or decreasing sulphur and oxigene fugacity caused the pyrrhotite mineralization. With cooling and elevation of sulphur and oxigene fugacity, the system crossed the pyrrhotite-pyrite reaction line, the sulphidation state of the system changed to intermediate and native gold and bismuth were partly dissolved during this process. While the bismuth re-precipitated as native bismuth, bismuth-sulphide and –sulphosalts (at slightly acidic-neutral pH), gold was carried to longer distances.

The presence of geochemical barriers where gold could be precipitated, the cause of the mentioned changes in the hydrothermal system and the relation of these processes to the deep level and the surrounding epithermal deposits are the subjects of further research.