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Hydrothermal fluids influence on the thermal evolution of the Stephanian sequence, the Sabero Coalfield (NW Spain)

Botor D.

Geology, Geophysics and Environment

2012

Vol. 38, no. 4

369--393

In the present study, the thermal history of the Late Carboniferous (Stephanian) coal-bearing sediments of the Sabero Coalfield has been reconstructed in order to elucidate coal rank. The Sabero Coalfield is located in a small intramontane coal-bearing basin along the Sabero-Gordón fault zone, one of the major E-W trending strike-slip fault systems of the southern part of the Cantabrian Zone (NW Spain). The total thickness of the Stephanian succession is in excess of 2,000 m, and is composed of siliclastic rocks and intercalated coal seams with tonsteins. Mean vitrinite reflectance values in the Stephanian rocks in the Sabero Coalfield are in the range from 0.61 to 3.14% Rr, but most values are in the range from 0.8 to 1.5% Rr (based on 84 samples). Average vitrinite reflectance gradient is high (0.73% Rr/km), which suggests high value of average paleogeothermal gradient (52°C/km). The maximum paleotemperatures calculated from vitrinite reflectance values for the Stephanian rocks range between 89°C (top of the Stephanian) and 195°C (bottom of the Stephanian). Coalification of the organic matter in the Stephanian rocks was achieved in the Early Permian, and was most likely related to several almost simultaneous related to magmatic and hydrothermal activity during high subsidence period in the pull-apart basin. The primary, burial-related maturity pattern, was probably slightly overprinted by fluid migration event, which is supposed to have occurred in Early Permian time.

Trace element geochemistry of coals from the Southern Cantabrian Zone (NW Spain): preliminary results

Botor D.

Mineralogia

2011

Vol. 42, iss. 1

39--51

Bituminous to anthracite coals from three small Stephanian intramontane coal-bearing basins (La Magdalena, Cinera-Matallana and Sabero) located along the Sabero-Gordón fault line strike-slip systems of the Southern Cantabrian Zone (SCZ) were examined. Coal rank expressed as mean vitrinite reflectance values of these Stephanian coals is in the range 0.72-3.96%. The vitrinite maceral group exceeds 72 vol. % in all of the coals. The coals are characterized by relatively variable contents of mineral matter and coal-ash. The mineral matter comprises, in the main, clay minerals, carbonates, sulphides and quartz. The coals exhibit medium-high concentrations (see for comparison Ronov et al. 1990; Kabata-Pendias, Pendias 1999; Ketris, Yudovich 2009) of the following elements (in ppm): ΣREE (53-205), Ba (300-900), As (11-57), Zn (<50-150), Cr (10-160), Rb (50- 145), Co (5-26), Sc (2-24.6), Ce (17-99), Yb (1.3-4.5), Th (2.4-11.9) and U (1.1-8.1), Br (<1-14), Cs (<2-9), Eu (<0,3-1.5), Lu (0.11-0.85) and Sb (0.8-4.8), and relatively low concentrations of Sm (0.6-6.6) and Ta (<1-2). They are also characterised by relatively high Th/U values (1.31-2.29). LREE/HREE values fall in the range 24-44 (average - 30). In contrast, concentrations of Au, Ag, Hg, Ir, Ni, Se, Sn, Sr, and W are below detection limits for the applied INAA method. As the concentrations of elements are significantly higher in coal-ash, most are likely related to mineral matter in the coals.