Magmatic evolution of the Stiavnica volcano

The Stiavnica stratovolcano evolved in five stages: (1) construction of the extensive andesite stratovolcano during the interval 15.0-13.6 Ma, including diorite and diorite porphyry intrusions; (2) emplacement of subvolcanic intrusive rocks associated with a resurgent uplift, sector collapse and denudation of the volcano - with the granodiorite pluton showing the age around 13.6 Ma, quartz-diorite porphyry sills and dykes dated at 13.77-13.0 Ma and granodiorite porphyry stocks showing the age around 12.95 Ma, (3) subsidence of the caldera and its filling by differentiated andesites around 12.9 Ma, (4) renewed volcanic activity marked by post-caldera andesites between 12.8 and 12.2 Ma; (5) uplift of the resurgent horst in the central part of the caldera accompanied by rhyolite volcanic/intrusive activity between 12.2 and 11.4 Ma. Volcanic and intrusive rocks of the Stiavnica volcano were sourced from an upper crustal magmatic reservoir stored at a pressure between ∼1 and ∼3 kbar. The different phenocrysts (plagioclase, orthopyroxene, clinopyroxene, Mg-hornblende, and quartz) have crystallized from dacitic to rhyolitic melts at a temperature between ∼960 to ∼700 grad C. A clear change in mineralogy from plagioclase- orthopyroxene-clinopyroxene, plagioclase-orthopyroxene, plagioclase-orthopyroxene-hornblende, plagioclase-hornblende-biotite, and then plagioclase-hornblende-biotite-quartz is observed with the decrease of temperature of the melts. Crystal mush in the upper-crustal magma chamber was repeatedly fed by magmas having an andesitic composition resulting from differentiation and crustal contamination of primitive mantle magmas in a lower and/or middle crustal reservoir. Eruption of volcanic rocks and emplacement of subvolcanic intrusions resulted from a mixing between the evolved magmas stored in the upper crustal reservoir and newly injected primitive magmas and/or due to fluid saturation and exsolution of the magmas. Late stage rhyolites represent a segregation of the interstitial silicic melt from the crystals mush in the upper crustal reservoir during its cooling at temperature <750 grad C. The porphyry-skarn Cu-Au, Fe-skarn, disseminated base metal and epithermal mineralizations of the Stiavnica volcano associate genetically with those magmatic rocks that resulted from a prolonged cooling of magma in the upper crustal reservoir - cooling and perhaps magma mixing that provoked fluid exsolution from the long-lived crystal mush were the key factors in formation of ore deposits. (authors)