Abstract
The Douro Carboniferous Basin (DCB), aged from Gzhelian, is an important coal-bearing basin occurring in Northern Portugal. While the coals and the sedimentary sequence of the DCB have been deeply studied, the inorganic geochemical data are scarce. This study intends to provide major and trace element contents and discuss their modes of occurrence and origins using a set of twenty-four coal samples from the São Pedro da Cova Coalfield taken from different sectors/outcrops. Thus, an integrated approach using petrographic, geochemical, both organic and inorganic, and mineralogical data was used to achieve these purposes. The main results demonstrated that these coals are anthracite A and vitrinite is the main organic component. Most of the elements have inorganic affinities and are associated with aluminosilicates, while the other elements have affinities with sulfides. Illite and muscovite are the main phyllosilicates occurring in these coals and pyrite is the most common sulfide. However, cinnabar, together with phosphates (fluorapatite, monazite, xenotime and gorceixite), were also identified. The enrichment of most elements as well as a heterogenous rare earth elements (REE) distribution pattern in the tectono-sedimentary unit (TSU) samples are related to magmatic fluids. On the other hand, on the Eastern Outcrop (EO), a tectonic slice, the subparallel trend of the REE distribution patterns, and a depletion of all the elements are related to the sedimentary contribution. The occurrence of cinnabar and gorceixite epigenetic mineralizations is interpreted as the action of a porphyry intrusion identified in this area of the DCB, between the TSU B1 and TSU D1.
Highlights
The dendrogram obtained for the tectono-sedimentary unit (TSU) D2 (Figure 16) is divided into two main groups, the first one corresponding to the elements with organic and intermediate affinities, and a second one corresponding to the aluminosilicates where the rare earth elements (REE) present in the phosphates are dispersed in the clay minerals
The occurrence of both cinnabar and gorceixite in the studied samples reveal a volcanic input and take into account that a vein of porphyry intrusion was identified between the TSU B1 and TSU D1; it is suggested that an epigenetic mineralization was responsible for the enrichment of the majority of the elements quantified in the samples of this sedimentary sequence as well as the different patterns of the REE
In this study the maceral composition and vitrinite reflectance were determined and the major and trace element compositions were analyzed to characterize the organic matter of coals from the Douro Carboniferous Basin (DCB), and to identify the mode of occurrence of the elements and their enrichment/depletion considering the values established for the worldwide hard coals
Summary
The São Pedro da Cova Coalfield is part of the Douro Carboniferous Basin (DCB), the most important coal-bearing basin in Portugal, and has been, since the early 1990s, the focus of several geological studies with emphasis on tectono-stratigraphic development, e.g., [1,2,3,4,5], paleobotany, e.g., [6,7,8,9,10,11,12], paleozoology, e.g., [6,7,13,14,15,16], organic petrology and physical parameters of the coals, e.g., [17,18,19,20,21,22,23,24,25], and coal exploration and its environmental impacts (e.g., [26,27,28,29,30,31,32,33,34,35,36]). The occurrence of igneous intrusions into coal-bearing sedimentary sequences is described in many basins of the world, e.g., [39,40,41,42,43,44,45,46,47], and is one of the events that can affect the properties of the organic and inorganic components of the coals These magmatic events may be responsible for the coalification of coals, producing changes to its petrography, mineralogy and inorganic and organic geochemical composition. In this study a multi-element geochemical study of the coal-bearing units of the DCB in the São Pedro da Cova Coalfield is presented and multi-proxy approach is taken: (i) to assess the affinities and mode of occurrence of the trace elements and the geological factors that may have controlled their origin and enrichment/depletion; and (ii) to identify the influence of the magmatic fluids on the inorganic fractions of the coal-bearing strata
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