Iodargyrite from Zalas (Cracow area, Poland) as an indicator of Oligocene–Miocene aridity in Central Europe

Abstract

Iodargyrite, the hexagonal silver iodide, was found in the Permian rhyodacite quarry at Zalas (near Cracow, Southern Poland) in a fault zone cutting Middle Jurassic sandy limestone. It occurs within a tectonic breccia encrusted by chalcopyrite, pyrite, Ag-containing chalcocite, covellite, galena, native bismuth, malachite, cuprite, Fe- and Mn-dominant oxides, Cu sulfates and barite. The hydrothermal encrustacion is most likely connected with rejuvenation of Old–Paleozoic faults in the Sava phase of the Alpine orogeny on the Oligocene–Miocene boundary. The periods of aridity when iodargyrite precipitated, determined by evaporation of salt lakes or the inland Miocene sea that delivered halide ions to the groundwater oxidation system, or leaching of weathered Ag-containing mineralization by saline, I −-rich deep formation waters derived from Paleogene sediments during folding and formation of the Carpathians nappe structure, are connected rather with the Low to Middle Miocene period. At this time, steppe areas with a dry climate existed on the north foreland of the Carpathians in southern Poland as a result of a displacement to the north of a dry subtropical zone because of uplift of the Carpathians and regression of the inland Miocene sea.