Fluid movements along major north-south-trendingseismotectonic zones were responsible for the development of hydrothermal caves in limestones of the Bohemiankarst and elsewhere in the Bohemian Massif, Czech Republic. Many caves and caverns are closelylinked to hydrothermal calcite veins and reveal characteristiccupola-form cavities and exotic internal precipitates.A possible role of sulphuric acid speleogenesis in the origin of the caves is also discussed

 Discovered in 1994, Ogof Draenen is currently the longest cave in Britain and among the thirty longest caves in the World, with a surveyed length in excess of 70km. Like other great caves, Ogof Draenen has had a complex multiphase history. This interpretation of the genesis of the cave is based on speleo-morphological observations throughout the system. Evidence of at least four phases of cave development can be identified, associated with major shifts in resurgence location and changes in flow direction of up to 180°. Joints have had a dominant influence on passage genesis. In particular joints have facilitated the development of maze networks and remarkably shallow horizontal phreatic conduits. The amplitude of these conduits is much shallower than predicted by models based on flow path length and stratal dip. Here, we suggest that presence of laterally extensive open joints, orientated perpendicular to the regional neo-tectonic principal stress field, determines the depth of flow in the aquifer, rather than fissure frequency per se as suggested in Ford’s Four State Model. We argue that the rate of base-level lowering, coupled with the depth of karstification determines whether a cave responds by phreatic capture or vadose incision. Maze cave networks within Ogof Draenen were probably initiated by bedrock-hosted sulphide oxidation and sulphuric acid speleogenesis.

(Note: Welsh terms used in this paper: Ogof = Cave; Afon = River; Cwm = Valley; Mynydd = Mountain).

Montecchio cave (Grosseto province, Tuscany, Italy) opens at 320 m asl, in a small outcrop of Jurassic limestone (Calcare Massiccio Fm.), close to the Albegna river. This area is characterised by the presence of several thermal springs and the outcropping of travertine deposits at different altitudes. The Montecchio cave, with passage length development of over 1700 m, is characterised by the presence of several sub-horizontal passages and many medium- and small-scale morphologies indicative of sulphuric acid speleogenesis (SAS). The thermal aquifer is intercepted at a depth of about 100 m below the entrance: the water temperature exceeds 30 °C and sulphate content is over 1300mg l−1. The cave hosts large gypsumdeposits from40 to 100mbelowthe entrance that are by-products of the reaction between sulphuric acid and the carbonate host rock. The lower part of the cave hosts over 1 m thick calcite cave raft deposits, which are evidence of long-standing, probably thermal, water in an evaporative environment related to significant air currents. Sulphur isotopes of gypsumhave negative δ34S values (from−28.3 to−24.2‰), typical of SAS. Calcite cave rafts and speleogenetic gypsumboth yield young U/Th ages varying from68.5 ka to 2 ka BP, indicating a rapid phase of dewatering followed by gypsumprecipitation in aerate environment. This fastwater table lowering is related to a rapid incision of the nearby Albegna river, andwas followed by a 20–30mfluctuation of the thermalwater table, as recorded in the calcite raft deposits and gypsum crusts.

Montecchio cave (Grosseto province, Tuscany, Italy) opens at 320 m asl, in a small outcrop of Jurassic limestone (Calcare Massiccio Fm.), close to the Albegna river. This area is characterised by the presence of several thermal springs and the outcropping of travertine deposits at different altitudes. The Montecchio cave, with passage length development of over 1700 m, is characterised by the presence of several sub-horizontal passages and many medium- and small-scale morphologies indicative of sulphuric acid speleogenesis (SAS). The thermal aquifer is intercepted at a depth of about 100 m below the entrance: the water temperature exceeds 30 °C and sulphate content is over 1300 mg l−1. The cave hosts large gypsumdeposits from40 to 100mbelowthe entrance that are by-products of the reaction between sulphuric acid and the carbonate host rock. The lower part of the cave hosts over 1 m thick calcite cave raft deposits, which are evidence of long-standing, probably thermal, water in an evaporative environment related to significant air currents.

Sulphur isotopes of gypsum have negative δ34S values (from−28.3 to−24.2‰), typical of SAS. Calcite cave rafts and speleogenetic gypsumboth yield young U/Th ages varying from68.5 ka to 2 ka BP, indicating a rapid phase of dewatering followed by gypsum precipitation in aerate environment. This fastwater table lowering is related to a rapid incision of the nearby Albegna river, and was followed by a 20–30 m fluctuation of the thermal water table, as recorded in the calcite raft deposits and gypsum crusts.