Influence of magmatic fluids on the organic and inorganic fractions of coals from the Peñarroya-Belmez-Espiel Basin (Spain)

The study of coals affected by magmatic intrusions is important from a geological point of view, in order to understand how heat and magmatic fluids could affect the organic and inorganic fractions as well as the properties of the coals. These fluids may lead to an enrichment of trace elements and heavy metals of environmental concern or serve as unconventional sources of critical elements. Therefore, the objective of this study is to investigate the organic and inorganic fractions of Peñarroya-Belmez-Espiel Basin (PBEB) (Spain) coals in order to verify whether changes of the petrography, chemical composition, mineralogy, and inorganic geochemistry have taken place. The PBEB corresponds to a Carboniferous intramontane basin filled with a Middle Mississippian to Lower-Middle Pennsylvanian sedimentary succession with coal-bearing units. Two main successive depocentres (extended from NW to SE) were recognized and in which bituminous and anthracite coals, as well as natural cokes occur. Petrographic analysis, ultimate and proximate analysis, scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDS) analysis, inductively coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) analysis were carried out for the purpose of this study. The sampling consisted of a selected set of nine samples of bituminous coals, ten samples of anthracites and three samples of natural cokes. The results show that the organic fraction of bituminous and anthracite coals is mainly made up of vitrinite. Although the minerals identified in the two depocentres vary, they mainly consist of quartz and clay minerals in both cases. Authigenic kaolinite and epigenetic mineralization related to thermal events that occurred in the basin were observed in the anthracites. Natural coke shows isotropic and fine mosaic textures, and signs of epigenetic mineralization. The trace element content of the coal samples was compared with the values established for world hard coals and an enrichment of Cs, in bituminous and anthracite coals, and of Hg in natural cokes, was observed. The occurrence of REY in bituminous coals is related to the detrital sedimentary source, whereas in anthracite the epigenetic and thermal effects may have also been involved. In natural cokes, the REY content is probably associated with the original coal. Mercury enrichment in natural cokes may be associated with magmatic fluids and explained by Hg volatilization, its capture in natural coke pores and subsequent precipitation.
The author H. Moura benefited from a doctoral scholarship financed by the FCT - Fundação para a Ciência e a Tecnologia (Ref: SFRH/BD/134109/2017). This work was financed by national funds through FCT - Fundação para a Ciência e a Tecnologia, I.P., under the project Ref. UIDB/04683/2020.