O dolomicie z Imielina na Górnym Śląsku

Abstract

On the dolomite of Imielin (Upper Silesia, Poland) The dolomites of midle-triassic age (»Muschelkalk«) are called in part »ore-bearing dolomites«, on account of important deposits od lead- and zinc-ores they often contain in the Uppersilesian district. At Imielin, however, they are wholly ore-free and they represent a well-stratified series of yellow-greyish compact rocks of rather uniform appearance. They are usually more or less lime-bearing and their average content of free calcite can be estimated, according to chemical analyses, as 24 % CaCO(3). The microscopic structure of typical lime-bearing dolomites of Imielin consists of a framework of fine dolomitic rhombohedrons (0,02 -0,3 mm. in diameter), cemented in a poikilitic manner by much bigger but xenomorphic crystals of pure calcite. The repartition of both mineral constituents-dolomite and calcite is quite uneven: here isolated, somewhat bigger and more regular rhombohedrons of dolomite are imbedded in an abundant calcitic matrix, and there they are smaller, less regular and so densely joined with one another, that they hardly leave any place to the calcitic cement. The dolomitic rhombohedrons reveal sometimes a sort of zonal structure: their core, more or less dusty from minute indeterminable inclusions, is mantled with pure and limpid dolomite. By the microchemical method of tinting the rockslides with silver-chromate it could be ascertained that the cores consist of calcite. Owing to subsequent action of phreatic water the cores may be in part dissolved and tinted by rusty points of iron hydroxydes. Some dolomitic banks contain numerous white flints; in their microscopic slides many inclusions of corroded dolomitic rhombohedrons may be detected amidst the cryptocrystalline groundmass of chalcedony, but no trace , of any limestone-remains. It can hardly be any doubt therefore that the silification of the rock is subsequent to its dolomitisation. Upper horizons of the dolomitic complex show another mode of silification: The dolomitic rhombohedrons are there cemented with a microgranular quartz-aggregate, which was certainly introduced in place of the calcitic matrix. Nothing positive can be said about the source of that migratory silica, but it must be supposed somewhere out of the rock itself.