Petrography, biomarker composition, mineralogy, inorganic geochemistry and paleodepositional environment of coals from La Ballesta mine, Peñarroya Basin, Spain

Abstract

The purpose of this work is to study coal samples from various exposed seams of the La Ballesta mine, at the southeastern edge of the Penarroya-Belmez-Espiel coalfield, southwestern Spain. Shaly seat- and caprocks were also investigated. Petrographic, palynological, and biomarker analyses were carried out. The major elements were determined by X-ray fluorescence (XRF). The trace elements were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). The Pennsylvanian (Westphalian)-age, high volatile bituminous coal studied is a humic coal with vitrinite concentrations of over 86% on a minerals-free basis and low amounts of liptinite and inertinite. The coals have a relatively higher volatile matter yields than the bituminous coals in western areas of the basin with similar vitrinite reflectance values and present both a dark and a light variety of vitrinite. The dark variety is more abundant in the low ash coals considered to have been formed in nearby lacustrine settings. The mineral assemblage of the coals and non-coal (roof and floor) rocks from the La Ballesta mine is dominated by quartz, kaolinite and illite, with minor to trace proportions of epigenetic carbonates, anatase, and K-feldspar, among others. Concentrations of B and B/Be ratios, along with other data, suggest that the La Ballesta coals may have originated from topogenous mires that developed next to lacustrine settings. Coal seams 9, 9bis, and 14 were probably more restricted systems than coaly shale 15, the latter corresponding to swamps associated with alluvial and floodplain environments. Abnormally high concentrations of trace elements such as Li, Rb, Cs, Sc, V and Cr might be explained by the input of detrital minerals into the Penarroya Basin from the outcropping Precambrian and Lower Palaeozoic rocks to the northeast. In this regard, elemental ratios such as Al2O3/TiO2 for the samples suggest that the modes of occurrence of the trace elements, and the minerals, in La Ballesta coals and non-coal rocks are to be mainly attributed to detrital input from felsic to intermediate metamorphic rocks.