Papp Gábor szerk.: A dunabogdányi Csódi-hegy ásványai (Topographia Mineralogica Hungariae 6. Miskolc, 1999)

A Csódi-hegy kalcitja (Fekete Judit, Weiszburg Tamás és Gatter István)

Topographia Mineralogica Hungáriáé Vol. VI. 161-177. Miskolc, 1999 A CSODI-HEGY KALCITJA Calcite from Csódi Hill (Dunabogdány, Visegrád Mts., Hungary) FEKETE Judit, WEISZBURG Tamás és GATTER István Abstract: A characteristic member of the mineral paragenesis of Csódi Hill is calcite overgrowing fissures and cavities in the dacite (Harangi, 1999). The Csódi Hill laccolith was formed in the early stage of the Middle Miocene volcanism of the Visegrád Mountains (Korpás, 1997). The 15.2-14.8 Ma intrusion (Korpás & Lang, 1993) formed a dome below the Oligocene-Lower Miocene pelite and sandstone, while broke through them. This magmatism and the associated hydrothermal processes formed the zeolite and calcite mineralisation. Szabó (1871) was the first to recognise calcite here, and Reichert & Erdélyi (1934, 1935) were the last to describe it. This study attempts to contribute to the knowledge of the morphology and formation of the Csódi Hill calcite. 31 samples were studied by stereomicroscope and reflection goniometer (Table I), and four generations were recognised as related to zeolites. Generation "0". Massive, white calcite without zeolites - probably predates zeolites, suggested by stable isotope composition. Generation I and II. Specimens are usually lamellar, tabular according to (0001) (Fig. 1 ). Crystallisation of Generation I calcite preceded zeolite, and is always associated with late zeolite mineralisation (Fig, 4; Fig. VIII on the cover). Generation II calcite is contemporaneous with zeolites. Thin parallel lamellae - 0.X mm distance apart are grown between analcime or chabazite (Fig. 5). Generation III. Elongated to acute rhombohedral, scalenohedral crystals. Crystallisation follows zeolites. Variable morphology allowed further subdivision: Group Ill/a. ochre yellow, always steep, approx. {0221} and (0772) rhombohedra, isolated or as core of "octahedral" (Group Ill/b) crysta2s (Fig. I on the cover; Figs. 6-7). Group Ill/b. Colourless, transparent crystals, 5-10 mm in length. {0221} rhombohedron is always combined with the basal planes. A few specimens bear narrow prismatic faces. Those crystals with well-developed basal planes look like regular octahedra. (Figs. 8, 10-11). Group III/c. {0772} rhombohedron is combined with prism faces (Figs. 12-13). Group Hl/d. Larger, yellow or grey rhombohedra, 5-20 mm in size. Group Ill/e^iny, acute, clear crystals of 1-5 mm length. {0221} or {0772} rhombohedra, combined with {11.9.20.2} and even steeper scalenohedra (Figs. 14-15). X-ray powder diffraction study of two samples (AT/3, R/4) revealed no impurities or deviation from usual calcite cell values. Seven samples were studied by optical emission spectrophotometry. OES results also support the low trace element content of the samples: 0.1-1% Mn, 0.X% Fe and Mn, 6-8 ppm Be and 1-10 ppm Sr was found. 0.X% Si, Al, and Na is probably from accompanying zeolites. Stable isotope studies were made on 16 calcite specimens from Csódi Hill and on a two-generation calcite specimen from Keszeg to understand the genetics of calcite and the origin of the carbonate. The Keszeg specimen derived from a sedimentary environment, from the Mesozoic basement blocks in the Danube Bend. These formations extend subsurface as far as Csódi Hill. Four fields are recognised on the Ô"C-5'80 plot (Table II, Fig. 16). The Keszeg specimen (Field 4) is definitely far away from the Csódi Hill fields; its carbonate was mobilised from Mesozoic carbonate rocks (Demény et ai, 1994, Demény & Harangi, 1996). Carbonate component of the Csódi Hill calcite clearly indicates igneous origin. The first calcite precipitates (Generations 0,1, and Ill/a) are closely related to the early stage of laccolith formation, when the magma domed up the overburden of organic-rich Oligocène strata. The departing C02 made carbonate precipitation possible. Initially hydrothermal fluids were heavily mixed with intraformational water containing organic matter oxidized in the igneous contact zone (Fig. 16, Field 1; Generations 0 and I, Trend I). Later the mixing ceased and terminal calcite displays only hydrothermal influence (Fig. 16, Field 2; Generation Ill/a). The post-zeolite calcite - i.e. most of the