A Budai Termálkarszt (BTK) fluidum-fejlődéstörténetét az ásványparagenezisek tükrében a késő-miocén fedett karbonátos állapottól vizsgáltuk. Ekkor a termikus felhajtóerő vezérelte a felszínalatti vízáramlást. Később, a Budai-hegység kiemelkedésével és a csapadékvíz beszivárgásával megkezdődött a vízszint különbségek által vezérelt vízáramlási rendszerek kialakulása. További változást okozott a Gödöllői-dombság kiemelkedése, mely a Budai-hegység kiemelt területei mellett a keleti medencerész felől is domborzati hajtóerőt biztosított. A meginduló beszivárgás lehetővé tette a fedő sziliciklasztos képződmények nátrium-kloridos vizeinek karbonátos víztartóba történő lejutását. Jelenleg a Budai-hegység és tágabb környezete a BTK áramlási rendszerének nyugati félig fedett részét, míg a Gödöllői-dombságtól a Dunáig tartó terület a rendszer keleti, fedett részét képezi. A BTK összefüggő áramlási rendszerében hidrosztatikus közeli nyomásviszonyok uralkodnak. A rendszer regionális és köztes megcsapolódási területei a Duna mentén húzódnak. A helyi, köztes és regionális áramlási rendszerek különbségeit a nyugati, félig fedett terület forrásai jelzik. A jelenkori vízáramlás domináns hajtóereje a vízszintkülönbségekből fakad, asszimmetrikus jellege a hidrosztratigráfiai helyzet, valamint a keleti és nyugati medencerészek közötti beszivárgáskülönbség következménye. A Duna alatt Ny­–K irányú regionális átáramlás zajlik. A regionális feláramlás hidrotermális komponense NaCl-os medenceeredetű vízzel egészül ki. A meteorikus eredetű, langyos köztes, ill. a hideg helyi vízáramlásokban Mg2+ és SO42- gazdag vizek jellemzőek, a medenceeredtű fluidomok jelenléte itt nem bizonyított. Az eredmények rámutatnak a rendszer hőmérsékleti eloszlását meghatározó folyamatokra, valamint a fedőüledékek hő felhalmozódásában betöltött szerepére. A rózsadombi és a Gellért-hegy előterében található források fizikai-kémiai tulajdonságok szerinti elkülönülését szerkezeti és a két áramlási rendszer különbségei okozhatják. A Gellért-hegynél csak termálvíz lép felszínre, míg a Rózsadombnál langyos és termálvíz megcsapolódás egyaránt zajlik. A különbség a hidrotermális vizek összetételében, eltérő forrásterületre és vízkémiai folyamatokra utal. A déli rendszer szulfát-többlete az evaporit összletekkel, a központi rendszer szulfátja medenceeredetű kén-hidrogénnel hozható összefüggésbe. A rózsadombi langyos meteorikus vizek szulfát forrása a fedő képződménybek piritje lehet. A fluidum-fejlődéstörténeti tanulmány eredményei hozzájárulnak a barlangképződési folyamatok értelmezéséhez.

In the Transdanubian Range (Hungary), dolostone and dolomitic limestone appear in a number of sedimentary successions formed from the Late Permian to the Late Triassic in various depositional settings and under various diagenetic conditions, whereas only a negligible amount of dolomite was detected in the post-Triassic formations. Seven dolomite-bearing units representing ramp, small and large carbonate platforms, and intraplatform basin settings are presented in this synopsis. In most cases, multi-stage and polygenetic dolomitization was inferred. The main mass of the dolostones was formed via near-surface diagenetic processes, which were commonly preceded by the formation of synsedimentary dolomite. Accordingly, surficial conditions that prevailed during sediment deposition controlled the dolomite-forming processes and thus the lateral extension and the time span of dolomitization. The area of episodic subaerial exposure was a critical controlling factor of the lateral extension of the near-surface dolomite genesis, whereas its temporal extension was mostly governed by climate. Burial diagenesis usually resulted in only moderate dolomitization, either in connection with compactional fluid flow or via thermal convection. The Triassic fault zones provided conduits for fluid flow that led to both replacive dolomitization and dolomite cement precipitation. In the Late Triassic extensional basins, synsedimentary fault-controlled dolomitization of basinal deposits was reconstructed.

Dolomite most commonly forms via replacement of precursor carbonate minerals. For this reason, diagnosing primarily precipitated organogenic dolomite in microbial mat deposits from the rock record is not straightforward, even though the deposits exhibit microbial fabric. Single and multiple dolomite crusts exhibiting microbial fabric occur in a pervasively dolomitized Middle Triassic platform succession. Two sections were studied in the Transdanubian Range. In both sections, two fabric types occur in the upper part of the metre-scale cycles. One of that is microbial boundstone (fabric type 1)—characterised by clusters of dolomite microcrystals which display diagnostic microbial features, such as calcimicrobes, clotted–spherular aggregates and globules. The other one is different in the two sections. In Section 1, it is micritic dolomite (fabric type 2) that is characterised by predominantly fine crystals and contains obscured microbial components. In Section 2, it is bioclastic dolomite (fabric type 3) that is rich in reworked dasycladalean alga fragments and consists of dolomite crystals of wide size-range from fine to coarse. The precipitation of the microcrystalline dolomite phase is interpreted as being facilitated by mats and biofilms favouring/tolerating an increasing frequency of subaerial conditions in the upper intertidal setting. Petrographic analyses revealed that organogenic calcite was also precipitated, especially in mat deposits rich in bioclasts. Synsedimentary dolomitization, resulting in fine crystals, was coupled with aragonite dissolution and it postdated the organogenic precipitation. It took place only in the peritidal caps of the shallowing-upward depositional units. Petrographic analyses provide circumstantial evidence constraining that microcrystalline dolomite did not form via mimetic replacement. Accordingly the microcrystalline dolomite, which shows microbial microfabrics in the studied samples, is interpreted as an organogenic primary precipitate. Both peritidal processes, dolomite precipitation and replacement, were likely controlled by the environmental factors in a semi-arid climate. Those components of the platform succession that were not dolomitized in the peritidal environment were replaced and cemented by medium and coarsely crystalline dolomite during further burial at elevated temperature, as shown by fluid inclusion homogenisation temperature (62 to 83 °C) and negative stable oxygen isotope values. Thus, the majority of the studied formation consists of fabric-destructive dolomite (fabric type 4).

Disintegration of dolostones to dolomite powder (powderization) was a widespread phenomenon in Triassic dolostones of the Buda Hills, where the areal extent of powdered dolostones is large compared to similar occurrences elsewhere in the world. In the Buda Hills, dolostone disintegration proceeded in four stages that correspond to a gradual decrease in particle size, that is, from the parent dolostone to (1) crackle breccia; via (2) mosaic breccia (diameter <2 cm); via (3) mosaic breccia blocks ‘floating’ in dolomite powder; to (4) dolomite powder (diameter 100–300 μm). Stable isotope ratios and trace element compositions of dolomite remained constant throughout these stages, and there are no indications of dissolution in most locations, suggesting that disintegration was predominantly a mechanical process. Combining these findings with the geological history of the region, and supported by a simple freezing/thawing experiment and pertinent experimental studies on weathering of building stones, it appears that powderization in the Buda Hills was caused by repeated freeze–thaw cycles during and/or after the Pleistocene glaciations. Subaerial exposure under cold climate conditions involves multiple freeze–thaw cycles that create mechanical stresses in the rock framework related to the opposing thermal expansion of rock and water that freezes and of ice that liquefies. This process is herewith called ‘cryogenic powderization’. Our data further suggest that the synergy of four factors promoted dolostone powderization in the Buda Hills: (1) tectonics, which created a pervasive fracture network; (2) intercrystalline porosity of the dolostone; (3) relatively high water saturation; and (4) subaerial exposure under cold climate conditions.

Calcite veins and related sulphate–sulphide mineralisation are common in the Buda Hills. Also, abundant hypogenic caves are found along fractures filled with these minerals pointing to the fact that young cave-forming fluids migrated along the same fractures as the older mineralising fluids did. The studied vein-filling paragenesis consists of calcite, barite, fluorite and sulphides. The strike of fractures is consistent—NNW–SSE—concluding a latest Early Miocene maximum age for the formation of fracture-filling minerals. Calcite crystals contain coeval primary, hydrocarbon-bearing- and aqueous inclusions indicating that also hydrocarbons have migrated together with the mineralising fluids. Hydrocarbon inclusions are described here for the first time from the Buda Hills. Mixed inclusions, i.e., petroleum with ‘water-tail’, were also detected, indicating that transcrystalline water migration took place. The coexistence of aqueous and petroleum inclusions permitted to establish the entrapment temperature (80°C) and pressure (85 bar) of the fluid and thus also the thickness of sediments, having been eroded since latest Early Miocene times, was calculated (800 m). Low salinity of the fluids (<1.7 NaCl eq. wt%) implies that hydrocarbon-bearing fluids were diluted by regional karst water. FT-IR investigations revealed that CO2 and CH4 are associated with hydrocarbons. Groundwater also contains small amounts of HC and related gases on the basin side even today. Based on the location of the paleo- and recent hydrocarbon indications, identical migration pathways were reconstructed for both systems. Hydrocarbon-bearing fluids are supposed to have migrated north-westward from the basin east to the Buda Hills from the Miocene on.