Magnetic properties and metastability of greigite-smythite mineralization in brown-coal basins of the Krušné hory Piedmont, Bohemia

Abstract

Finely dispersed forms of greigite or greigite-smythite mineralization were found in layers a hundred and more metres thick in the Miocene strata of the Krušné hory (Erzgebirge) Piedmont brown-coal basins. Under laboratory thermal treatment, a pronounced instability of greigite or greigite-smythite mineralization was revealed by the magnetic parameters, conditioned by mineralogical metastability. Thermal treatment in oxidation conditions caused the most pronounced changes in magnetic parameters in a temperature range of 320–380°C, in which a laboratory process of self-reversal of remanence was observed. The products obtained during thermal treatment were identified with the aid of X-ray diffractographs (Co-radiation, Fe-filter). When they were heated to 250°C, no substantial changes were found, while at 300°C the intensities of greigite became weaker, and pyrite and marcasite originated to its detriment. Hexagonal pyrrhotite was generated in addition to pyrite and marcasite. A total decomposition of bisulphides took place at temperatures above 400°C, accompanied by the formation of various modifications of Fe 2O 3, until finally at higher temperatures only α-Fe 2O 3 was formed. Laboratory tests suggested self-reversal of remanence in relation to the formation of pyrrhotite. So far, greigite or greigite-smythite mineralization has been proven to exist in the Bohemian Massif solely in connection with Miocene rocks containing fossil micro-organic matter. Products of thermal alteration, pyrite, marcasite, pyrrhotite, γ-, η-, and α-Fe 2O 3 (or Fe 3O 4 depending on redox conditions) may be expected in rocks of similar genesis, but partly or completely carbonified as a result of a process of pyrolysis of the micro-organic matter.