STUDIUL INTEGRAT ROCK-MAGNETIC ŞI TERMO-MINERALOGIC AL UNOR PORŢELANITE DIN ZONA JILŢ (BAZINUL DACIC)

摘要

The Upper Pliocene coal bearing formations from the Jilt area (Dacic Basin, Romania) contain occurrences of baked clays (porcelanites) generated by natural spontaneous burning of lignite seams. The heat-affected rocks show gradual changes within their mineralogical composition, which can be correlated with the increasing temperature, in successive stages. Among these changes, the transformation and/or destruction of clay minerals, and hematite neoformation are the most important. Consequently, the magnetic signatures of clays - due to the magnetic susceptibility (MS) of component minerals, and to the remanent magnetisation (RM) printed in rocks while they are forming - have been modified from the original clays (low and very low MS and RM values) to the porcelanite-like clays and porcelanites (high and very high MS and RM values). The changes recorded by the polarity of total natural remanent magnetisation (NRM), and the enhancement of the anisotropy of magnetic susceptibility (AMS) parameters are also commented. The mineral changes induced in clays by heating during the natural burning of coal beds can be used as a geothermometer to control the RM transformation in these rocks. Due to the important petromagnetic contrast between the heat-affected clays and the fresh clays, and, at the same time, due to the shallow burial position, the porcelanite deposits are able to produce significant magnetic anomalies, which can be easily mapped with portable magnetometers.