Petrographic and geochemical characterization of coal from Santa Susana Basin, Portugal

Abstract

The current and future importance of coal as a geological resource is related with the production of coke, carbon materials, carbon derivatives and other chemical products as well as a promising alternative source of critical trace elements. This study aims the comprehensive characterization of coals from the Santa Susana Basin (SSB), SW of Portugal. The SSB is a Pennsylvanian continental basin located along the Santa Susana Shear Zone that separates two tectonostratigraphic zones of Iberian Massif. Samples of coal and coaly silt-claystone from the main outcrops, Jongeis and Vale de Figueira, were collected for this study. The methodologies used for the petrographic and geochemical characterization included: optical microscopy, scanning electron microscopy with energy dispersive X-ray spectrometry, proximate and elemental analysis and inductively coupled plasma mass spectrometry (ICP-MS). The results demonstrate that the organic matter of coal from SSB is essentially composed of vitrinite and small amounts of inertinite. The mineral matter includes detrital minerals (mainly quartz and clay minerals), iron oxides and oxidized and non-oxidized framboidal pyrite. Epigenetic carbonates (dolomite and ankerite) were only observed in samples from Jongeis outcrop. Vitrinite random reflectance of the samples ranges between 0.90% and 1.25%, indicating a bituminous coal rank. The slightly higher vitrinite reflectance and the occurrence of epigenetic carbonates in samples from Jongeis can be related with differentiated burial history of the basin and post-depositional processes including fluids circulation that may have promoted the enhancement of the thermal maturity. When compared with the geochemical composition of worldwide hard coals, samples from Vale de Figueira are significantly enriched in Cs and In, while samples from Jongeis are significantly enriched in In and Mn. Considering the elements' mode of occurrence, the majority of elements are preferentially associated with mineral matter. The elements S, Cd, Mo, Pb and Se have organic affinity. The strong positive correlation between Ca and Mg, Mn and Te indicates their association with carbonate phases, which is in accordance with the carbonate composition.