Origin of Zn-Pb ores in the Olkusz and Chrzanów districts: A model based on fluid inclusions

Abstract

Fluid inclusion studies in sphalerite from early-stage Zn-Pb mineralization, hosted by Devonian, Triassic and Jurassic carbonate beds in the Olkusz and Chrzanów ore districts, Silesian-Cracow region (southern Poland), yielded homogenization temperatures (Th) from 80 to 158°C with the upper range higher in the south of the region and lower in the north. Vertical thermal gradient of the parent fluids was 6 to 10°C, and the ore crystallization temperature ranges varied from < 10°C at deep levels to 25°C at shallow levels. The peculiarities of formation of primary and secondary fluid inclusions from organic-matter-bearing water-dominated medium, position of the inclusions in crystals, features of secondary inclusions, the inclusion refilling phenomena, their bearing on recrystallization of ores, and Th distribution in single fissure fillings were considered. The ore-forming fluids were Iiquid-hydrocarbon-bearing aqueous solutions of Na-Ca-CI type with lower Ca contents in the south and higher Ca contents in the north of the region. The ore-forming fluids had salinities from nil to c. 23 wt. % of NaCl equivalent. Three types of fluids were recognized: (1) low-to-moderate salinity versus high Th, (2) high salinity versus moderate Th, and (3) low salinity versus low-to-moderate Th, attributed to ascending fluids, formation brines, and meteoric descending waters, respectively. The three fluids mixed during ore precipitation. Mobilization of the fluids (ascending and partly formation brines) is linked with folding and uplift of the Carpathians to the south of the ore region. A comparison of the Silesian-Cracow deposits with the Mississippi Valley-type ones is presented.