Ever since it was identified as a speleogenetic process in the Guadalupe Mountains of New Mexico, USA, hypogene speleogenesis has become the focus of numerous research projects aimed at discerning between classical epigene caves and sulfuric acid or thermal caves. The first distinguishing characteristics that were recognized for hypogene caves were passage and cave morphology. The following step was the identification of rare minerals, specific for processes associated to hypogene speleogenesis. One other important step was the recognition of the importance of stable isotopes - mainly of S - in tracing the source of S and the chemical processes affecting it. Many of the caves now labeled as hypogene are fossil caves, in which presently the hypogene activity has long died off. Studies comparing stable isotopes from coexisting cave minerals and the waters that generate the cave are rarer. This extensive study encompasses a description of cave and passage morphologies, cave mineral assemblages, as well as hydrogeochemistry of thermomineral waters in a peculiar region of Romania. Băile Herculane (Cerna River Valley, SW Romania) is a spa town known since Roman times for its numerous thermal springs that were considered to have healing powers. These springs, along with wells drilled in the past century, are still being used for curative purposes in several treatment centers in Băile Herculane. The present study is important not only for the scientific data it produced, but also for economic purposes, as mixing of the thermomineral waters with meteoric sources is a major concern, due to the dilution it causes. The data presented here is based on multiple investigation methods, each specific to the analyzed material: powder X-ray diffractions, scanning electron microscope, electron microprobe (for mineral samples), sedimentological investigations (for cave sediments), stable isotope mass spectrometry (for water and mineral samples), field measurements (for water samples). The results presented here help to clarify the source of dissolved S species in the thermomineral water, the source of the water itself, as well as establish a connection between caves along the Cerna Valley and the thermomineral aquifers.

Baile Herculane, located in southwestern Romania, is a unique city that exploits its thermal waters. The geology consists of a granitic basement covered by 200 meters of limestone, marl, and flysch deposits. Extreme faulting carries heat ascending from the mantle, which intercepts percolating meteoric waters. Local springs have high concentrations of dissolved sulfide gas (H2S) and dissolved sulfate (SO4²-).These dissolved species indicate the progression of sulfate reduction in the aquifer. Water samples were collected in polyethylene syringes to prevent oxidation of sulfide. Then, sulfide and sulfate were quantitatively reacted for stable isotope analysis. Total sulfur isotopic composition was calculated to determine the source of the dissolved sulfur. The source of the sulfur is a sulfate of marine origin (𝛅34S≅20%0), which I found to come from impurities in the limestone since the Cerna Valley does not possess marine evaporites. The limestones of the Cerna Valley are host to a number of caves, which possess relatively large deposits of sulfates and exotic morphologic features that suggest speleogenesis by sulfuric acid. 𝛅34S of the sulfates relates to sulfide isotopic values from the springs, showing that the dissolved sulfide (upon oxidation) forms sulfuric acid s that reacts with limestone to produce sulfate minerals. A wide range of cave sulfate 𝛅34S values exist indicating that isotopic values of these deposits depend on several factors such as sulfur source, extent of sulfate reduction, and completeness of sulfide oxidation. This also implies that a single, narrow range of sulfur isotopic values does not represent sulfuric acid speleogenesis.