Abstract
<title>Abstract</title> <p>The reduced porphyry mineralization of the central Börzsöny Mountains (Hungary) formed during Neogene calc-alkaline volcanism. Based on the paragenetic sequence, mineralogical and geochemical characteristics of sulfides, physicochemical parameters (e.g. temperature, redox conditions, sulfur fugacity) of the system fluctuated during the ore formation. To constrain the processes controlling ore mineral precipitation, two complementary models were developed using PHREEQC 3.0 geochemical modeling software. Model 1 simulates the reduced stages formed at higher, fluctuating temperatures (400–150°C); that are responsible for the formation of the primary sulfide assemblage. Model 2 represents the subsequent lower-temperature (< 300°C) oxidative overprint. The evaluated models support the observed paragenesis. Initially, arsenopyrite precipitated from the primary, reduced fluid at 400°C. It was followed by the formation of galena-I and sphalerite-I during cooling to 150°C, whereas minor chalcopyrite-I crystallized at 150°C. A subsequent, Fe-rich fluid pulse with lower temperature (250°C) and lower sulfur fugacity favored the precipitation of pyrrhotite. A high-temperature (400°C), sulfur-rich fluid pulse caused the mineralization of galena-II and sphalerite-II. During progressive oxidation and gradual depletion of sulfur, chalcopyrite-II and arsenopyrite-II precipitated between 300 and 200°C, while the existing pyrrhotite was replaced by pyrite and hematite. Marcasite could only precipitate under metastable conditions, which highlights a limitation of the available thermodynamic parameters in the studied environment. Our results demonstrate that the formation of the ore mineral assemblage found in the central Börzsöny requires multiple fluid pulses accompanied by fluctuations in temperature, redox conditions, sulfur fugacity, and fluid composition.</p>