Hydrothermal events in the Hárshegy Sandstone Formation and their relationships to regional geological processes, Buda Hills, Hungary

Abstract

The distribution of the transgressive, coastal Hárshegy Sandstone Formation of the Middle Oligocene is strongly
defined by the NNE–SSW striking Buda Line which forms the eastern boundary of its extension. The Buda Line was a
palaeogeographic boundary in the Late Palaeogene and the location of intensive post-volcanic activity as well. (FODOR et
al.1994). The sandstone is strongly silicified in the so-called Buda Zone which is a 5-20 km belt along the western side of
the Buda Line (BÁLDI & NAGYMAROSI 1976). Stratigraphic and tectonic evidence suggests Late Kiscellian age for the
silicification (BÁLDI & NAGYMAROSI 1976).
Hydrothermal formations in the typical facies of the Hárshegy Sandstone were studied in two reference areas: in the
surroundings of Pilisborosjenő village (Köves Ridge and Ezüst Hill) and in the vicinity of Csobánka village (Majdán
Saddle). In both areas, hydrothermal mineralization consists of chalcedony and barite veins as a product of two distinct
hydrothermal events. Most of these veins are usually rather thin (1-5 cm thickness) and appear to be simple extensional
fractures. However, occasional displacement can also be observed along the veins. The density of the veins is uneven. In
the vicinity of Pilisborosjenő and especially on the Köves Ridge, siliceous veinlets form a dense stockwork, whereas the
barite veins are more common on the Majdán Saddle where the frequency of the chalcedony veins is subordinate. The
orientation of the chalcedony veins is dominantly WNW–ESE, and the orientation of the barite veins is NNW–SSE. The
barite veins always cut through the chalcedony veins, clearly indicating their younger age. Considering the most simple
extensional nature of the veins and their relative age relationships, their orientation fits with the model of stress-field
variation during the Oligocene–Miocene (BADA et al. 1996, MÁRTON & FODOR 2003). Based on stratigraphic and
structural evidence the age of the first phase (i.e. chalcedony veins) is late Early Oligocene, while the younger phase (i.e.
barite veins) is Middle Miocene. These hydrothermal phases can be related to the Palaeogene and the Neogene volcanism
in the Carpathian–Pannonian region.
The chalcedony veins often have argillic alteration selvage mainly consisting of kaolinite with a small amount of illite.
Kaolinite is also present in the unmineralized sandstone and considered to be detrital in origin. Illite occurs only along the
chalcedony veins suggests its hydrothermal origin. Limonite is also present in the alteration zone which is usually not wider
than a few centimetres. In association with the chalcedony veins, two sulphide phases are present: pyrite and chalcopyrite.
Pyrite forms euhedral crystals as inclusions in the quartz of the sandstone (cogenetic with the quartz) and amorphous masses
in the intergranular spaces (cogenetic with the hydrothermal chalcedony veins). Chalcopyrite only appears in the
intergranular spaces and can be considered as a hydrothermal mineral associated with the chalcedony veins.
Veins with barite do not contain other minerals and have sharp contact with the sandstone without an alteration halo.
The barite veins have open spaces and therefore the crystals usually have an euhedral appearance. The barite crystals most
commonly have simple orthorhombic-tabular morphology in most of the thin veins. However, a definite zoning in the
distribution of the various habits of barite was observed in the major and thickest vein (approximately 2 m thick zone) on
the Majdán Saddle. Variation of the crystal habit as a function of distance from the central hydrothermal zone probably
reflects the variation of temperature and the saturation of the solution for barium and sulphate. This observation can be
used in predictions with respect to the occurrence of major fluid flow zones which precipitated the barite in the sandstone.
Fluid inclusion data suggest the barite was formed by the mixing of a saline fossil water (with high Ba2+-concentration)
and hot (up to 250 °C) ascending water with moderate salinity and higher sulphate-content, driven by magmatic heatflow. This mixing resulted in cooling and a rise in salinity, which finally caused barite precipitation.