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Evidence of petrogenetic links to the formation of Oligocene Pb-Zn-Ag ore deposits of the Rogozna Mts. (Central Balkan Peninsula)

The study focuses on age and evolution of the Oligocene quartzlatite of the Rogozna Mts. (Central Balkan Peninsula), in order to better understand the link between magmatism and formation of Pb-Zn±Ag mineralization. New Ar/Ar biotite and amphibole plateau ages suggest that Rogozna Mts. quartzlatite originated through a continuous volcanic episode from 27.3±0.1 to 29.5±0.1 Ma which was immediately followed by a hydrothermal phase. The quartzlatites are hypocrystalline porphyritic with phenocrysts and microphenocrysts (~60 %vol.) of plagioclase (An37-49), biotite Mg# [100 × Mg/(Mg + Fetot)]<50, calcic amphibole, quartz, sanidine clinopyroxene and phlogopite (Mg#=79 to 84). The rocks display numerous disequilibrium textures, such as: sieved plagioclase phenocrysts, dissolution effects on quartz, phlogopitized biotite and amphibole crystals, and phlogopite microphenocrysts showing effects of incomplete growth (or dissolution?) and biotitization. The Rogozna Mts. quartzlatites are shoshonitic in character having Na2O/K2O<1, high LILE/HFSE ratios, strong depletions at Nb and Ti and K, Pb and U peaks on primitive mantle-normalized diagrams. They are similar to other potassic/ultrapotassic rocks in this region, in particular to those of Veliki Majdan and Rudnik (West Serbia), which are also related to Pb-Zn deposits. The evolution of the Rogozna Mts. quartzlatite is modeled using a trace element binary mixing model adopting a lamproite magma and a dacite-like calc-alkaline melt as end-members. Acid magma was emplaced in a magma chamber at depths of ~4.5–9.5 km and underwent cooling from temperatures of >860ºC to temperatures of ~720ºC. The crystallization sequence started at 850–795ºC, which is recorded by the cores of zoned amphiboles. The crystallization continued to the rims of zoned amphiboles at 760ºC to 820ºC and biotite at temperatures from 720ºC to 760ºC. The above described crystallization sequence most probably produced a rough stratification of the chamber in volatile- and melt-rich upper parts and volatile-poor and crystal-rich lower chamber sections. An ultrapotassic, most probably lamproite-like melt, injected the lower parts of the evolving and presumably stratified acid magma chamber. The increase in temperature is recorded by phlogopite microphenocrysts (760 to 800ºC), which apparently crystallized after biotite, whereas the presence of boxy cellular plagioclase phenocrysts, phlogopite overgrowths on biotite and amphibole and various partially dissolved phases suggest disequilibrium conditions. The involvement of lamproite melts provided additional fluids to the existing magma chamber system. The new fluids likely decreased viscosity of the resulting hybrid magma and facilitated its upwelling. Such an event might have triggered pyroclastic eruptions. Pyroclastic rocks are widespread in the Rogozna Mts. occurring as precursors of quartz latite volcanic and shallow intrusive rocks. The large explosive events were immediately followed by the emplacement of quartz latite effusions, extrusions and shallow subvolcanic bodies, between 27.3±0.1 and 29.5±0.1 Ma, whereas the addition of new volatiles by lamproitic melts established conditions for the hydrothermal phase in a magma chamber. The final cooling of the Rogozna Mts. magmatic system was related to hydrothermal circulation that gave rise to Pb-Zn-Ag epithermal mineralization. 40Ar/39Ar ages recorded by hydrothermal minerals vary between 29.1±1.3 Ma and 30.0±0.4 Ma, and they overlap with the 40Ar/39Ar ages revealed by minerals from fresh quartz latites.

Oligocene Pb-Zn-Ag ; Rogozna Mts. ; Central Balkan Peninsula

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