2nd FIELD TRIP, 2002

  Bainska Stiavnica (Selmecbánya), Kremnica (Körmöcbánya), Central Slovakian volcanic fields, Slovakia

Introduction

The Banská Stiavnica (Selmecbánya) precious and base metal ore district is one of the largest in the Carpathian arc. It is well known since the Middle Ages, particularly for its long-lasting gold and silver mining history. Mining flourished spectacularly in the 18th century, when in the year 1740 the annual production reached its peak level of 680 Kg of gold and 25 835 Kg of silver. Gold and silver mining continued until the year 1947. An estimate of the total output of mines based on mining archive data stands at 80 tons of gold and 4 000 tons of silver. Base metal production increased gradually during the 19th century from deeper parts of veins and dominated during the 20th century. Roughly 70 000 t of Zn, 55 000 t of Pb and 8 000 t of Cu has been produced. Almost 1000 years of mining in the district seems likely to end in the near future. Underground mining of base metal ores could not continue under present economic conditions and stopped in the year 1992. Thanks to newly discovered resources of a limited extent, gold mining continues at the Rozália Mine. Recent exploration activities are oriented towards reopening of gold mining at other localities. Many distinguished scientists and geologists have worked in Banská Stiavnica and taught in its Mining Academy established in the year 1763. Mining and geological problems of this ore district have been reported extensively also by G. Agricola during the first half of the 16th century, including his famous book "De re metalica libris XII".
Figure 1. Volcanis structures around the Banska Stiavnica Caldera.


Figure 2.: Cross section across the Stiavnica strato volcano showing the distribution of vein types.

The ore district is situated in the central zone of the Stiavnica stratovolcano (Fig. 1, Fig. 2), the biggest volcano of the Central Slovakia Neogene Volcanic Field. This volcanic field, formed by products of areal andesite volcanic activity with subordinate rhyolites, is situated at the inner side of the Carpathian arc. Volcanic rocks are of the Badenian through Pannonian age (16.5 - 8.5 Ma) and their petrogenesis is closely related to subduction of flysch belt oceanic and/or suboceanic basement underneath the advancing Carpathian arc and to back-arc extension processes (Lexa et al. 1993). Volcanic rocks are similar to medium/high-K andesites of continental margins or evolved island arcs involving older continental crust. Petrography and geochemistry indicate mantle source magmas with variable crustal contamination and differentiation in a shallow magma chamber. The Stiavnica stratovolcano is the largest of the volcanoes in this field, as well as the largest in the Carpatho-Pannonian area. With its diameter almost 50 km it is a large volcano even in the world-wide comparison. Unique in the Carpatho-Pannonian area there is also a large caldera 20 km in diameter and a late stage resurgent horst in its center, exposing basement and extensive subvolcanic intrusive complex. These phenomena created conditions for extensive and long lived hydrothermal systems, giving rise to one of the richest mining districts in Europe. The extent of the ore district corresponds to the extent of the resurgent horst.

Metallogeny of the ore district The following types of ore deposits (mineralizations) have been identified so far:

-barren quartz-pyrophyllite-pyrite system at Sobov
-magnetite skarn deposits and occurrences stockwork disseminated base metal deposit
-porphyry/skarn copper deposits and occurrences
-intrusion related (?) gold mineralization
-hot spring type advanced argillic systems
-vein type epithermal base/precious metal mineralization
-metasomatic base metal mineralization

Vein type epithermal precious/base metal mineralization

Epithermal veins occupy the dominant position in multistage metallogeny of the ore district. Generally, they might be regarded as an epithermal gold-silver deposit despite the fact that, during the last fifty years, base metals were the main objective of mining. The vein system is closely related to post-caldera resurgent horst uplift (fig. 2) and covers almost 100 km . The complexities and depositional history the epithermal veins are a result of interaction among the structural evolution of the resurgent horst and evolving hydrothermal system with increasing meteoric water component. The epithermal veins and their zonality correspond roughly to the extent of granodionite/dionite bell-jar pluton. This may be an argument to interpret the cooling pluton as a source of thermal energy for the hydrothermal system. On the other hand, uplift of the resurgent horst was accompanied by the emplacement of rhyolite intrusions and extrusions along its marginal faults, indicating the presence of siliceous magma in deeper levels. Siliceous magma responsible for the resurgent horst uplift can be considered as an alternative and/or additional source of heat and fluids for the post-caldera epithermal vein system.

Epitherrnal vein mineralization at the present surface level is distributed in four zones (StohI in Bunian et al. 1985-fig. 6):
1. Cu-(Bi)
2. Pb-Zn-Cu
3. Ag-Au
4. Au-Ag-(Sb-Bi?).
Three vein types are distinguished:
1. base metal veins with increasing Cu-content in depth
2. silver sulphosalts veins with minor gold and base metals
3. gold veins with minor base metals in depth
According to classification of Sillitoe (1993) low sulphidation epithermal veins in the ore district belong to the sulphide and base metal poor and sulphide and base metal rich types associated with calc-alkaline rocks.

A common zoning and succession of mineral stages on individual veins favor a genetic continuum. Sulphide and base metal poor gold mineralization represents the youngest event, often separated spatially from preceding stages. Structural analysis indicate a change in stress field and activation of a new set of faults. Mineralization occupies mostly marginal faults of the resurgent horst, associating closely in space and time with emplacement of rhyolites. Rhyolites occupy eventually an intermineralization position, separating sulphide and base metal rich and poor epithermal systems (Stohl et aI. 1967) or the first and second cycle in the sense of Kovalenker et aI. (1991). A possible genetic link of sulphide and base metal poor gold mineralization to siliceous magma has to be considered. Mineralogy of veins is not known equally over the district. Marginal precious metal veins are not accessible for recent studies, being abandoned long time ago. MineralogicaI reconstruction of these veins is based on historical mining records and rare samples in museum. Mineralogical description of base metal and silver veins has been carried out by Kodèra (1956, 1959, 1960, 1963). In a recent study of the Michael ore shoot at the Terézia vein (Kovalenker et aI. 1991) five mineralization stages have been recognized:

I. Hematite-quartz including rhodonite, rhodochrosite)
II. Sphalenite (galena, chalcopynite, pynite, quartz)
III. Rhodonite, quartz, carbonates
IV. Galena-chalcopynite (sphalente, hematite, Bi-sulphosalts, scheelite)
V. Sulphosalts-baryte (quartz, iron-poor sphalenite, carbonates, chalcopynite, pyrite, electrum, Ag-sulphosalts, acanthite, fluorite)

Kovalenker et al. (1991) have carried out also an extensive study of physical-chemical aspects of mineral deposition. Fluid inclusions in minerals of the second, fourth and fifth mineral stages indicate that minerals precipitated from diluted Mg-Ca-Na chloride brines. Homogenization temperatures of the second stage vary from 380 to 240 0C, salinity of fluids varies between 0.5 and 11.5 wt% NaCI equiv. (TE = -30 to -39C), a periodic fluid heterogenization (boiling) has been observed. Homogenization temperatures of the fourth stage vary from 310 to 190 C, salinity of fluids varies between 0.5 and 9 wt% NaCI equiv. (TE = -29 to -37 C), in two sporadic inclusions increased Ca content is indicated by TE = -43 to -48 0C, fluid heterogenization (boiling) has been observed. Homogenization temperatures of the fifth stage vary from 225 to 125 C, salinity of fluids varies between 0.5 and 11 wt% NaCI equiv., values above 7 wt% NaCl equiv. being rare (TE = -27 to -37 C). Vertical extent of sampled mineralization from the surface is 800 m. Restoration of the boiling curve indicates its original depth as 800 - 1600 m during the second stage and 400 - 1200 m during the fourth stage, indicating syngenetic erosion and uplift.

Kovalenker et al. (1991) also modelled physical-chemical conditions of ore deposition and came to the conclusion that the epithermal system developed in two cycles divided by temperature and composition inversion between the stages III and IV. The first cycle is connected with formation of pollymetalic assemblages enriched in zinc, the second one with formation of relatively lead-, copper-, bismuth- and antimony-rich ores including assemblages of gold and silver minerals. Oxygen and hydrogen isotopes studies indicate an increasing proportion of meteoric water in fluids towards younger mineralization stages (Kovalenker et al. 1991, Kantor et al. 1990).

The structure of individual veins is complex, branching and "en echelon" patterns are frequent. Rich parts of mineralization are bounded to ore shoots localized in bents and transtensioflal segments. Their vertical extent is over 700 m. Striations indicate dip-slip movement with variable strike-slip component. Preliminary results of structural analysis show that while the pattern of sulphide and base metal rich veins corresponds to the stress field with minimum stress axis oriented E-W to ESE-WNW, the pattern of younger sulphide and base metal poor veins corresponds to the stress field with minimum stress axis oriented NW-SE. The reactivation of former veins was accompanied by a new set of veins around margins of the resurgent hoist, and in the central part of the resurgent horst in relation to a second order graben-like extension subsidence (Onacila et al. 1993).

Most of the veins have banded texture, brecciation is frequent. Whilst some of the breccias are tectonic, owing to continuous displacement on veins during their evolution, most of the breccias have formed by hydrothermal explosion mechanism indicating the state of boiling. No related breccia pipes have been observed. Veins are hosted by massive andesites, andesite porphyry, quartz-diorite porphyry, granodionte, diorite and in the western part also, by basement rocks. Host rocks are generally affected by propylitic alteration. Veins, especially the sulphide and base metal poor ones, are accompanied by zones of silicification, adularization and sericitization. Epidote, calcite and pyrite dominate along sulphide and base metal rich veins in deeper levels. Alteration minerals indicate a low sulphidation environment.


Most of the veins have banded texture, brecciation is frequent. Whilst some of the breccias are tectonic, owing to continuous displacement on veins during their evolution, most of the breccias have formed by hydrothermal explosion mechanism indicating the state of boiling. No related breccia pipes have been observed. Veins are hosted by massive andesites, andesite porphyry, quartz-diorite porphyry, granodionte, diorite and in the western part also, by basement rocks. Host rocks are generally affected by propylitic alteration. Veins, especially the sulphide and base metal poor ones, are accompanied by zones of silicification, adularization and sericitization. Epidote, calcite and pyrite dominate along sulphide and base metal rich veins in deeper levels. Alteration minerals indicate a low sulphidation environment.

STOP-1 All Saints Adit, Hodrusa-Hámre

The vein system represents an Ag-Au type base metal poor epihermal stockwork network with some major veins. It situated at the eastern marginal part of the caldera srtucture. The typical minerals are Ag-sulphoslats, acantite, electrum, gold, rare sphalerite and pyrite. Alteration minerals are quartz, carbonate, barite, fluorite.

Picture 1.: Geology cartoon of the All Saint stockwork Ag-Au deposit.


Picture 2.: Medieval adits were following the metal rich horisons of the veins.

Picture 3.: Strongly brecciated vein wall.


Picture 4.: Oxydized stockwork network exposed on the walls of the entrance adit.


Picture 5.: One of the main veins, strongly oxydizied. The 1 metre thick vein dips towards the observer.

STOP-2 Spitaler base and precious metal vein

The Spitaler vein is one of the major NE-SW trending base melal-precious metal rich vein system in the central Banska Stiavnica area. The vein belongs to a group of veins that contain significant Pb-Zn-Cu in addition of precious metals. The vein was mined from srface in the early times giving rise a good open exposure of minerals. Banded, brecciated, colloform textures appear in diferent speciment. Major ore mineral include sphalerite, galena, chalcopyrite, Ag-sulphoslats.

Picture 6.: Surface geology plan of Spitaler vein surroundings.


Picture 7.: Surface expopsure of the Spitaler vein. Note mined out mass of the vein. The outcrop provides excellent specimens of vein materials and wallrock alteration.


Picture 8.: Macro photograph of Spitaler vein sample. Note the brecciated and collomorph stucture on the vein and also the ore minerals; chalcopyrite, gelena, sphalerite.


Picture 9.: Macro photograph of Spitaler vein sample. Quartz-galena vein in propillitic andesite.


Picture 10.: Barren comb quartz vein with later filling of Mn-oxides.




STOP-3 Sobov quarry




Picture 11.: Geological section through the Sobov occurence.


Picture 12.: The quarry. Note large mass of pyrite rich rock both fresh and oxydized.

The high-sulphidation system at the locality Sobov north of Banská Stiavnica is represented by an East-West oriented body of metasomatic quartzite, which serves as a source of silica for refractory bricks (fig. 10). 1.3 m. tons of silica has been produced since the year 1951. The quartzite body is 1000 m long and up to 200 m wide. Its central core consists of pure massive quartzite with accessory rutile. Hydrothermal explosion breccias have been observed in places. Pure quartzite passes outward into the zone of gray, often vuggy quartzite with disseminated pyrite. Metasomatic quartzites are surrounded by argillites, composed of pyrophyllite, pyrite/marcasite and minor illite and diaspore. Hydrothermal explosion breccias are present alsoJn this zone. Argillites are formed at the expense of pyroxene andesites. Downward the hydrothermal system grades into the east-west oriented zone of intense silicification, disseminated base metal sulphides, minor Agsulphosalts and pervasive pyritization in diorite intrusion, which is the most probable source of the hydrothermal system. The hydrothermal system at Sobov affects pyroxene andesites of the first stage of the volcano and its activity could continue until caldera subsidence. Caldera lake sediments next to Sobov are altered and/or silicified and include rare fragments of geysirite-Iike rocks. However, quartzdiorite porphyry dykes and sills, as well as resurgent hoist faults, postdate the system. Quartz-diorite porphyry contain rare quartzite xenoliths, show a chilled intrusive contact with quartzite and are not affected by silicification. Resurgent hoist faults and related epithermal veins segment the quartzite body into a number of blocks with relative displacement.

STOP-4 Kremnica Ag-Au veins


Picture 13.: The open area behind the group was caved-inn in the 17th sentury during a major earthquake, because the hillside was weekend by the very dense network of mining adits.

The Kremnica Mts. are predominantly constituted by the Kremnica Graben of N-S direction. On the western part, the graben is bordered by NE-SW fault system and it is limited by the N-S striking Ihrács fault zone. In the graben central part, an about 4 km wide tectonic block has been upthrown. The Kremnica Graben began to have formed in Lower Sarmatian only, in time of the activity of NW-SE and NE-SW tectonic strikes crossing the area of the uplifted Kremnica block. This tectonic system was presumably responsible for an extensive pyroxene andesite strato-volcanic formation. By the end of the Badenian, the stratovolcano has undergone intense denudation up to subvolcanic levels. The younger volcanic formations are directly emplaced on the propylitized intrusions of the central zone. After having been originated, the central part of the graben filling was dissected by N-S and N NNE-SSW faults and uplifted as a local horst in the Kremnica area. In later stages, tectonic lines were apparently renewed and served as passageways for rhyolite dike and mineralization solution ascending. By the end of Sarmatian and at the beginning of Pannonian time, rhyolite volcanism took there place. In the Kremnica horst, the hyohites are of dike nature being 10 to 100 m thick only. They are considered to be of N-S orientation accompanying in dip and strike the younger Au-Ag vein structures. After rhyolite origination, the volcanic activity was of limited nature in the Krcrmnica Mts. In the northern part, a small basaltoid andesite stratovolcano formed. In the southern part, the some younger aphanitic basaltic andesite intrusions, dikes, knobs and flows have been created.

Metallogenetic Evolution of the Kremnica Mts. Ore District

The evolution of melallogenesis in two individual mineralization stages took there place being well distinguishable from temporal paint of view.

The Pb-Zn-Cu skarn mineralization stage is associated to subvolcanic levels of the Upper Badenian andesite stratovolcano central zone referred to the Zlatá Studina Formation.

The Kremnica Ore District veins are characterized predominantly by Au-Ag mineralization, while the Sb and Hg one is of subordinate nature only.

Au-Ag -/ Pb-Zn-Cu ±Sb± /-Ag Vein type mineralization

This mineralization stage is associated with rhyolite volcanism of the Jastrabá Formation (Upper Sarmatian-Pannonian) presumably including polymetallic mineralization with increased Ag content. The subject mineralization is younger than 11,7 m. y., which age the Stará Kremnicka rhyolite is dated to. The Kremnica ore district veins are characterized predominantly by Au-Ag mineralization while the Sb and hg on is of subordinate nature only. The veins are related to the Badenian volcanic center and it has tectonic connection to the pre-mineralization Kremnicka Horst. The Krenmnica ore deposit is underlain by a hypothetic granitoid body being considered as deep-seated equivalent of rhyolite extrusions and dikes which the genesis of Au-Ag mineralization seems to be associated with. It is now questionable, however, why just vein systems have been formed in the central zone of this older volcanic edifice. There are two reasons to be taken into consideration. Firstly, as constant thermal energy source, the central zone was an optimum environment favourable for thermodynamic hydrothermal regime generation. Secondly, the existing Kremnica Ore Field is only a tectonically restricted part of the Au-Ag Vein System in the frame of young post-rhyolite uparching of the Kiemnica Horst. The limitation of this coincides with spatial distribution of the ore-localizing structures. The vein zone and rhyolite dikes as well are considered as tectonic elements constituting the horst.

The Kremnica Ore District mineralogy was subject to study in detail by M. Böhmer (1965) having been completed by M. Korim (in Döhmer et al., 1976). On the basis of studies mentioned, mineralization in two evolution stages took there place:
1. Au-Ag Evolution Stage
2. Sb (Hg-As) Evolution Stage
An independent base metal rich ore formation period was also recognized from the district. As proved by drilling survey the base metal mineralization period is considerably widespread as northwards as in the depth. In the 1st vein system an apparently increased silver content (50 to 120 g/t) and decreased Au content has been by noticed Knésl (1984). At the same time, increased amounts in Pb, Zn and mainly in Cu were there observed. The vein filling composition may be characterized as complex precious metals-base metals.

The 1st Quartz Period is characterized by metasomatic replacement of the wall rock quartz. In less amount, auriferous pyrite, arsenopyrite and "cinopel" quartz are present. This period is considerably widespread. The 2nd Quartz Period is filling the joints in the 1st one. It is represented by pyrite, arsenopyrite and rare clusters of galena, sphalerite and chalcopyrite. In the termination stage of this period, some higher electrum concentrations have been noticed in the 2nd vein system (producing subperiod). In addition to it, silver minerals (polybasite. pyrargyrite, argentite) as well as tetrahedrite, bournonite and molybdenite were observed. In the vein filling (gangue), adularia was found. In the 1st vein system it is the Pyrite Introducing Period, which corresponds to the producing sub-period mentioned.nm It is formed by quartz with a greater amount of pyrite, less arsenopyrite. From among ore minerals, silver ones and slightly dispersed electrum are there present. In addition to it, barite, goethite and adularia have been observed as typical minerals.

The younger Sb (Hg, As) Evolution Siege begins with Quartz-Carbonate Introducing Period being separated from the older evolution stage by very intensive inter-mineralization tectonics of apparently dislocated nature. The mineral association is relatively simple composed of antimonite, pyrite and marcasite. From among gangue minerals, quartz and chalcedony are observed. K-metasomatism is a typical vein and wall rock alteration but other type of alteration such as sericitization, argillitization, zeolitization, alunitization, silicification are also present.

Picture 14. Typical macro texture of sulphide barren vein fragments. Quartz pseudomorphs after bladed calcite. Fine longer blades.


Picture 15. Typical macro texture of sulphide barren vein fragments. Quartz pseudomorphs after balaed calcite. Thicker shorter blades.


Picture 16. Balded layers vary with thick banded quartz.



EPILOGUE

Participant of the field trip: background; Gábor Kósa, Zsolt Benkó, István Pári, Viktor Jáger, Zsolt Hefner, center line; Emese Gáspár, Dóra Földeáki, Ágnes Nagy, Veronika Szilágyi, Rita Mohai, Anita Tóth, Zsófia Wáczek, Péter Molnár, Dr. Ferenc Molnár, Dr. Peter Kodera, foreground; Balázs Szinger, János Borsody, Krisztián Szentpéteri.


Dr. Peter Kodera (Geological Survey of Slovak Republic, Bratislava), left, is sincerely acknowledged for his excellent guidnece and supervision on this Field trip by ELUSCSEG president, right.

Prof. Géza Hámor (Hungarian Institute of Geology) is also acknowledeg for lending the van (background) for the time of our one day field trip.