Mineralization processes in the central zone of the Banska Stiavnica stratovolcano (Slovakia) associated with a subvolcanic granodiorite pluton
The central zone of the Stiavnica stratovolcano (Western Carpathians) hosts an extensive subvolcanic intrusive complex, dominated by a granodiorite pluton (16.3- 15.5 Ma) (Lexa et al., 1999). The granodiorite intrusion locally forms apophyses at the margins associated with Fe-skarns, while in the central, apical part it contains an irregular network of fractures, hosting base metal mineralisation. In order to develop a complex genetic model for the mineralization processes, detailed geochemical, fluid inclusion and stable isotope research were undertaken on the intrusion and on related mineralisation & alteration. The granodiorite intrusion, especially the marginal facies, is altered.Geochemical studies show that subsolidus alteration, intimately related to the skarns, resulted in an apparent depletion in Fe[sub]total, MnO, Zn and an increase in CaO, MgO, Na[sub]2 O (± K[sub]2 O), Cu. Hydrothermal alteration, closely related to stockwork mineralisation, is characterised by an enrichment in K[sub]2 O, MnO, Cu, Pb, Zn and depletion in Na[sub]2 O and Sr. Studies of fluid inclusion in skarn minerals (from Vyhne-Klokoč deposit) showed a large variation in salinity (4-23 wt% NaCl eq.) and Th (271°-371°C), reflecting a mixing process. In retrograde skarn minerals these fluids become progressively more dilute and cooler with indications of boiling at shallow depth. Fluid inclusions from the stockwork minerals (from B1 drill hole) also showed progressively more dilute and cool fluids (0.5-5.3 wt% NaCl eq., 191-328°C) resulting from mixing. Fluids related to acid leaching alteration in andesite (from Rozália mine) were of moderate temperature (-300°C). Secondary fluid inclusions in granodiorite were of variable vapour/liquid/solid ratios, Th values, salinities, and contained high proportions of Ca, Fe, K in addition to Na. The admixing effect of these components substantially decreased the solubility of NaCl, which significantly influenced the interpretation of PT evolution. The interpretation of data from the skarn-related granodiorite favoured here is that early immiscibility of an exsolved magmatic fluid (at -650°C and -600 bars) was followed by autonomous evolution of the saline liquid toward Fe-enriched NaCl saturated brine (650°-400°C). Subsequent cooling was followed by penetration of Ca¬rich fluids, late hydrothermal boiling and mixing with dilute fluids (400°-200°C). Studies on the stockwork-related granodiorite suggest broadly similar fluid properties in the apical part of the intrusion, but at depth the Th data suggest that there was no accumulation of magmatic fluids below -400°C.The isotopic composition of magmatic minerals indicates open magmatic degassing during granodiorite crystallisation, influenced by re-equilibration to variable degrees. Fluids in equilibrium with skarn and stockwork-related minerals showed a clear progressive mixing trend. Heavy, possibly [delta][sup]18 O-shifted, meteoric water seems to be most probable source of external waters that diluted the magmatically derived fluids. According to the fluid evolution model proposed here, magmatic brine accumulated only along the margins of the pluton. Chemical re-equilibration with the rock caused subsolidus reactions, while the liquid was further enriched in iron. The magmatic fluid was able to penetrate through the apophyses of the granodiorite into carbonates. In this hydrodynamic regime it mixed with circulating meteoric waters, forming magnetite and skarn lenses. These were later overprinted by retrograde mineralisation, dominated by heated groundwaters. In the apical part of the intrusion the magmatic vapour phase (the counterpart of saline liquid), escaped through several vents into the over-lying andesites, forming acid leaching alteration upon condensing into groundwater. Rapid cooling of this part of the intrusion caused extensive fracturing during the transition from ductile to brittle deformation (-400°C). Increased permeability enabled a large convective hydrothermal system and related K-rich alteration to form. Simultaneously, a low salinity supercritical magmatic fluid, exsolved from the intrusion at larger depths, probably contributed metals for the accompanying Pb-Zn mineralisation.