Geochemical peculiarities of the Lower–Middle Devonian transition in the Jurkowice block (Holy Cross Mountains, Poland) and the problem of the Devonian“ore-bearing horizon”

The deposits of the Emsian to the Eifelian period in the Holy Cross Mountains are of interest due to the occurrence of the “ore-bearing horizon” composed of clays with nested iron ores at their outcrops. In boreholes drilled in the vicinity of sites of ore occurrence no ore is found at depth but only pyrite-bearing clay-mudstones. Exposures of these deposits in the dolomite quarry in Jurkowice have allowed the authors to study their sedimentological features and geochemical properties. There are present here deposits of the upper part of the Winna Formation: red sandy mudstones and hematite-bearing clays, followed by grey-black tuffitic mudstone with disseminated iron sulphide, overlaid by laminated heterolithic clayey and sandy sediments with intercalations of quarzitic sandstone. They are strongly convolute-folded, in their uppermost part and covered by quartz-dolomitic sandstone, followed by the carbonate rock series. Chemical analyses reveal a considerably elevated rare-earth elements content in the grey heteroliths. The convex-upwards MREE distribution pattern suggests their supply by hydrothermal mineralizing fluids. The hematite and iron sulphide-bearing rocks with signs of hydrothermal activity allow us to assume that the “ore-bearing horizon” at the Emsian-Eifelian boundary encompasses the upper part of the Winna Formation. The siderite appearing in the neighbouring limestones was a product of their alteration by iron sulphites produced during the weathering of iron sulphides.