Geosciences and Engineering, Vol. 1, No. 1 (2012), pp. 261–269.

GEOLOGICAL OBSERVATIONS IN THE CSEREHÁT, BETWEEN IROTA AND GADNA

NORBERT NÉMETH

University of Miskolc, Institute of Mineralogy and Geology 3515 Miskolc-Egyetemváros

foldnn@uni-miskolc.hu

Abstract The easternmost outcrops of the Paleozoic succession of the Szendrő Mts are situated on the hills between Irota and Gadna. Previous explorations have found shale (phyllite) and metasandstone with high organic matter content, both suffered epizonal metamorphism; sandstone with jarosite cement in the Tertiary cover; a geomagnetic anomaly and pyrrhotite-dominated sulfide mineralization. The area is prospected for base and precious metal mineralization. In the initial stage of the prospecting geological mapping, localization and documentation of the outcrops were carried out. The results show that the area is dissected by Neogene faults with significant downthrows, also controlling the mineralization processes. 1. Introduction

The metalliferous black shale formations have a considerable potential to host economic grade mineralizations of base and precious metals, including platinum group elements [1]. ̒Black shale’ is a collective name for fine-grained clastic sedimentary rocks formed in restricted, reductive environments. Most of these deposits are thought to be of sedimentary-exhalative origin; the presence of organic matter and lack of oxygen enhances the precipitation of sulfide minerals [2].

One of the Paleozoic and Mezozoic formations of Northeastern Hungary corresponding to the definition of the black shale is the Irota Formation of the Szendrő Unit. In this an airborne geomagnetic survey [3] and subsequent uranium ore prospecting boreholes explored significant enrichment of pyrrhotite with traces of chalcopyrite, sphalerite and galena between Irota and Gadna at the Cserehát [4]. A new precious and base metal prospecting was initiated by the Institute of Mineralogy and Geology in cooperation with the Rotaqua Ltd. in 2010 based on these results. As a first step, the geology of the prospecting area was revised with mapping and documentation of the presently accessible outcrops, concentrated on the rock material affected by the alteration. This paper outlines the results of this mapping (Figure 1). 2. Geological setting

The stratigraphic structure of the Cserehát comprises an epimetamorphic Paleozoic basement (Szendrő Unit) and a sedimentary–volcanoclastic Tertiary (mainly Neogene) cover. The basement crops out in the SW part of the area, named Szendrő Mts in the geological literature (geographically a hilly country of 200–300 m a. s. l. high crests like any other parts of the Cserehát). On the eastern part the thickness of the cover exceeds 1000 m.

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Figure 1: (opposite page): Mapped prospection area. Legends: 1. village and church; 2. contour line; 3. major road; 4. EOV grid line; 5. debris of the Irota F.; 6. dip of the first

cleavage; 7. dip of the second cleavage; 8. dip of the bedding; 9. exploration borehole; 10. abandoned quarry or pit; 11. outcrops, other exposures (cliffs, cuts, gullies);

12. exploration trench; 13. supposed fault trace

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The oldest known formation of the Szendrő Unit is the Irota Formation. This formation is known from surface outcrops between Irota and Gadna and some exploration boreholes at Irota (Ir-1–8), Gadna (Gad-1, B-1), Felsővadász (Fv-1) and Alsóvadász (Av-1). Its assumed age comes from the Middle Devonian fauna of the conformly overlying Szendrőlád Limestone Formation in the borehole Fv-1. It comprises gray and dark gray shale (phyllite), siltstone and pelitic sandstone with organic matter and pyrite, sometimes calcareous, containing quartz veins and silicified rock bodies [5]. The silicification, the paragenesis of quartz, siderite, montmorillonite and pyrite, the fading of the dark color of the matrix and anomalous enrichments of Ti, Mn and Ba were interpreted as indicators of a low temperature hydrothermal alteration [6]. The organic matter is often described as ‘graphite’, but it is rather anthracite or semianthracite. Coal seams cannot be expected in this offshore facies.

The following succession comprises dark grey, laminar, fine grained limestone and marl (Szendrőlád Limestone F), coarse grained limestone with dark and light gray bands (Rakaca Marble F), yellowish gray, fine grained limestone with sericite and chlorite layers (‘cipollino’, Abod Limestone F) and an assemblage very much like the Irota F, dominated by shale (Szendrő Phyllite F). This flysch-type rock lies unconformly on the limestone formations with Devonian fauna, formed probably in the Carboniferous. South Alpine and Dinaridic analogies may link this succession to the Bükk Unit, occurring S from the Cserehát, which practically starts with the closing formation here (Szilvásvárad Siltstone F. being also a flysch-type assemblage with low grade metamorphic alteration) [5].

The Irota Formation was regarded previously as the youngest member of the succession [6]. In the opinion of Koroknai [7] its lithology differs in the alteration from the Szendrő Phyllite Formation only, and the age determining limestone and marl section in the Fv-1 cores may represent an olistolith analogous to other Szendrő Mts occurrences, so the assemblage can be classified as Szendrő Phyllite.

The structural boundaries of the Cserehát are two basins, the Borsod Basin in the S and the Kanyapta (or Komarovce) Basin in the N and two strike-slip fault zones, the Darnó Zone in the NW and the Hernád Zone in the SE. Both zones were active during the Tertiary up to the recent times, although in changing sense. The outcrop areas of the Szendrő Unit formations are situated in parallel strips, striking SW–NE in the SW part, bending to an E–W strike on the E side (probably a sequence of S–SE-vergent folds) [8]. This pattern is similar to that of the Central Bükk formations in the SW continuation of the Darnó Zone, and may reflect the deformations along this structural discontinuity. On the other side the downthrow of the basement SE from Gadna is linked to the trench development along the Hernád Zone.

In the cover Miocene shallow marine, littoral, fluvial and terrestrial clastic sediments include Upper Badenian and Sarmatian rhyolite tuff horizons. Sarmatian clay, marl, silt and the ‘upper tuff’ closing the sequence can exceed 400 m thickness altogether [9], [10]. Similar maximal, but very variable thickness characterizes the unconformly overlying Pannonian brackish, lacustrine and fluvial sediments. A special formation of the area is the sandstone cemented with limonite and jarosite with quartzite pebbles [11].

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Figure 2: Stereogram of the measured dips of cleavage, equal area projection, lower hemisphere. Number of data: 38. First (slaty) cleavage: continuous lines,

triangles; second (crenulation) cleavage: dashed bold lines, filled triangles 3. Mapped outcrops

Due to the low strength of the rock material and the hilly morphology, exposures are restricted to a few sites. Small outcrops of the Irota Formation were found on the steep SE side of the Vinyicska Hill only (outcrop group 5 on figure 1). Gullies, mainly in a valley descending to south on the eastern side of the Cigány Hill provide further natural exposu-res. Beyond these 5 small quarries were opened in this formation, three of which (in groups 6, 8 and 9) still have accessible rock faces. Cleavage orientations were measured on every appropriate outcrop (Figure 2). The debris of the material was found around these and in other areas too; steep slopes indicate usually that the formation is covered with 1–2 m thick debris only. Remnants of exploration trenches were also found on Cinceg Hill. Exposure group 1. The small quarries on the W side of Irota (1a) and the road cuts to the cemetery at the church (1b) expose laminar limestone and marl with folded slaty cleavage. Cleavage surfaces are usually shiny because of mica enrichments. As the prospection was concentrated on the Irota Formation, these outcrops were not documented. Exposure group 2. On the western peak of the Cinceg Hill a pit of cca. 10 m diameter exposes cross-bedded sandstone with pebbles. Grain size and the matrix vary with bedding; the pebbles are not well rounded. The beds are 2–3 dm thick on average. The matrix is bright yellow or yellowish brown (weathered) in fine-grained sandstone beds, but lacking from coarse-grained, pebbly beds. Yellow color may come from jarosite.

Remnants of exploration trenches and abundant debris of gray phyllite, metasandstone and quartzite were found north and east from the pit. This material is rich in limonite patches and veins, also containing hollow limonite concretions of 1–2 cm diameter.

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The bedding of the sandstone dips to W on the southern side and steeply to SW on the northern side of the pit (Figure 3). This post-sedimentation tilting, possibly in consequence of faulting was not equal, producing differently dipping blocks. Back-tilted cross-bedding indicates WNW transport direction, so the deposited clasts originate from the east. Exposure group 3. These locations are slope breakdowns (steep sections) without outcrops, but with abundant debris of phyllite and quartzite indicate a thin cover, while the surroundings are covered with reddish brown clay without any debris. The material is gray phyllite, metasandstone and quartzite with reddish brown limonite. The phyllite contains light and dark gray stripes, sometimes showing pencil cleavage at location 3b. The quartzite of 3c contains cm-sized vesicles with mm-sized grown-up crystals on the walls. 3a and 3b slopes follow similar NW-SE strike, 3c is a nearly N-S trending crest.

Figure 3: Measured dips of bedding and cross-bedding and dips tilted to subhorizontal bedding around 354 /00 axis by 25 . The cross indicates the intersection of bedding and

cross-bedding. The data dipping to SW come from a more tilted block Exposure 4. A shallow gully formed on a dirt road exposes the depositional base of the fluvial sand with pebbles on the phyllite with quartz veins on the 215 m level. Orientation data are uncertain because of ambiguous stability of the exposed blocks. Exposure group 5. The steep southeastern slope of the Vinyicska Hill is covered by the debris of the Irota Formation containing abundant limonite. On a side crest there is an exploitation pit of cca. 20 m diameter, without retained rock faces. On the NE side of a small valley, however, there are small natural rocks of dark red silicified phyllite. The slaty cleavage of this rock is folded in cm-scale, forming an anastomosing secondary crenulation cleavage (similar folding of mm-scale wavelength), and a 0.5–1 dm spaced joint set was observed along these cleavage planes. The rock was strongly fractured along a 258 /80 fault plane, cemented with limonite. Green, cm-sized stains of fuchsite (Cr-bearing mica, determined by Szakáll, S.) and some mm-sized acicular gypsum were found here.

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The slope angle changes suddenly above this outcrop at cca. 250 m a.s.l. On this level quartzite pebbles can be found all along the top of the phyllite-covered slope. Two soil samples were taken in 20 m distance measured across this change by hand-driven auger drilling. The sample from the lower point contained compact reddish brown clay with phyllite debris down to 80 cm, where the drilling got stuck, reaching the loose weathered top zone of the phyllite. In the upper hole the compact clay was 2.5 m thick with no phyllite debris, only sand. Below that quartzite pebbles and limonite stains were observed with variegated clay to 2.8 m, and then yellow rhyolite tuff was found down to 3.7 m, where the sampling was stopped. The compact clay forms a general cover in this area. At the crest of Vinyicska Hill, 500 m to SE from this location and 272 m. a.s.l, however yellow marl was found on the surface.

Figure 4: Stereogram of the measured orientation data from the quarry of outcrop 6, equal area projection, lower hemisphere. First (slaty) cleavage: continuous lines,

triangles; second (crenulation) cleavage: dashed bold lines, triangles; faults: bold lines with arrows indicating the sense of movement; quartzite vein: dot line

Exposure 6. The quarry on Vinyicska Hill with a cca. 10 m high face exposes several faults and quartzite veins in the gray silicified phyllite. Both slaty and crenulation cleavage can be observed. Open folds of dm-scale wavelength were also observed; the axis of the folding is at the crossing of major circles on the stereogram of Figure 4, and it is practically the same for rotated fold limbs and crenulation cleavage surfaces. A dm-sized quartzite vein follows the general trend of the slaty cleavage, and crosscuts the short limb of a shear band fold, indicating subsequent development. The fault planes also follow roughly the slaty cleavage dipping to N, but in an undulating pattern. Poorly preserved slickenlines indicate left-oblique shear sense. Exposure 7. A pit of similar size like the quarry of outcrop 6 is situated on the S part of the Cigány Hill. It provides no free rock surfaces, only debris and blocks. The rock material is the same as in outcrop 6.

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Exposure group 8. On the lower part (below 230 m a.s.l.) of the eastern slope of Cigány Hill several gullies, a quarry with 8 m high rock face and the creek of the valley expose the gray phyllite, metasandstone and the direct covering red limonite and yellow jarosite cemented sandstone with quartzite pebbles. Globular limonite concretions are typical on the southern side of the area.

The gray phyllite and metasandstone contains here abundant quartzite veins and organic matter as in previously described outcrops. The crenulation cleavage was often observed as intersection lineation on slaty cleavage surfaces, sometimes causing pencil cleavage (Figure 5). Cm-scale zigzag folding is most intensive around quartzite nests in the hinges. Stretching lineation was also observed in these folds. Light gray and white bands were formed by argillitic alteration in the phyllite; the rock material was white and loose or disintegrated to powder on some sites.

Figure 5: Phyllite sample with slaty and crenulation cleavage. Left: profile wiev of the folded cleavage. Right: slaty cleavage plane with lineation caused by the crenulation

Exposure 9. A single quarry at the farm in Nagy Valley exposes laminar, dark gray phyllite which leaves black trace on white paper due to the organic matter content. This outcrop is extended by the cores of Gad-1 drillhole down to 100 m depth. Faded stripes and silicified zones with some secondary minerals indicating decomposed sulfides occur in the cores. Exposure group 10. On the W side of the Nagy Valley a quarry and some gullies expose the sand and conglomerate beds, which are horizontal here. Dark red, 1–5 cm thick horizons are prepared out due to the limonite cementation. Such beds can be observed also around Szakácsi and in a small roadside quarry below Irota, with limonite concretions, but no jarosite. Drillhole Fv-1. This 600 m deep hole was drilled and documented in 1988 [5] and resampled in the frames of this prospection. Paleozoic rocks were reached at 158 m below 3 m phyllite breccia with brown clay matrix (slope debris) and rhyolite tuff; the top 12 m was weathered with limonite bands along cleavage. Calcareous (marly) intervals occur down to 285 m alternating with gray phyllite with no sharp boundaries. Further downwards black phyllite, gray shale and light gray metasandstone with reddish matrix comprise the Irota Formation. A considerable part of the sulfide content was decomposed during storing of the

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cores, also smashing the rock material. Secondary sulfates were formed on the core surfaces, mainly in phyllite, while the sandstone rather contained preserved disseminated sulfide grains. Pyrrhotite was detected by a magnet. Sulfides (and subsequent sulfate coatings) are unevenly distributed, concentrated in some dm thick zones. Such intervals of silicification and sulfide enrichment affect the whole Paleozoic assemblage. 4. Conclusions

The previously known rock types and signs of hydrothermal mineralization were found on the prospection area during the mapping phase. The altered Paleozoic rocks and the limonitic weathering products in the Pannonian cover are also potential locations of metal enrichments coming either from the original metal content of the black shale or from an unknown deep source.

The Paleozoic rocks have twofold cleavage. The primary slaty cleavage dips generally steeply to NNE, the secondary crenulation cleavage is subvertical, striking NE–SW. The crenulation often causes strong lineation on primary cleavage planes, plunging to NE on average. The fissures and quartzite veins follow mostly the slaty cleavage planes, but a dm-spaced joint set along crenulation cleavage is also developed. Silicification and the quartzite veins overprint the cleavage, but it is possible, that a generation of these veins was formed during the crenulation.

The variable dip and base level of the Pannonian fluvial sediments indicates Pannonian or post-Pannonian activity of the faults with a downthrow of 10–100 m magnitude. The most important structural boundary (with respect to the mineralization) is the E–W striking fault zone at the crest of Cinceg Hill, the northern edge of the geomagnetic anomaly [3] [4].

The indicators of hydrothermal alteration are not bound to this anomaly. Abundance of limonite and faded stripes or patches are typical along the fault zone, although also occurring on every other part of the area. Fuchsite was found on Vinyicska Hill, jarosite cemented sandstone on Cigány Hill. Although this fault zone was active after the alteration and sulfide mineralization, and the pyrrhotite bearing body is displaced along this zone, there is a possibility that it existed before and controlled the mineralization together with other faults, which were not reactivated. “Acknowledgements: The present work was supported by the Rotaqua Ltd. and the TÁMOP-4.2.1.B-10/2/KONV-2010-0001 project in the framework of the New Hungarian Development Plan. The realization of this project is supported by the European Union, co-financed by the European Social Fund. The author wishes to thank to János Földessy for managing the prospection and revising this text.” REFERENCES [1] Vine, J. W.–Tourtelot, E. B.: Geochemistry of black shale deposits – a summary report.

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