Formation of non-expandable 7 A halloysite during Eocene-Miocene continental weathering at Djebel Debbagh, Algeria: A geochemical and stable-isotope study

摘要

In north-eastern Algeria, autochthonous limestone units at Djebel Debbagh varying in age from Aptian to Cenomanian underwent burial diagenesis followed by deformation during the Alpine orogeny. Karst windows formed during Oligocene to Miocene times, and are now filled with clay minerals, which currently help to sustain a local mineral industry. The largest karst window at Djebel Debbagh contains substantial volumes of 7 A halloysite ore. Mineralogical investigations on the 7 A halloysite indicate a non-expandable material, with measured water contents varying from 14.1 to 15.5 wt.% H_2O. Non-expandable 7 A halloysite showed limited hydrogen-exchange capacity during a 24-month laboratory experiment, suggesting that hydrogen and oxygen isotopes had not exchanged since equilibrium at the time of halloysite formation. Therefore, stable-isotope data were used to calculate reliable temperature profiles and to determine fluid composition. It is suggested on the basis of geochemical and mineralogical data that 7 A halloysite formed between 25 ℃ and 40 ℃ in equilibrium with meteoric water during Eocene to Miocene times. Estimates of supergene temperatures and calculated stable-isotope values for rainfall correspond to continental climate conditions during Miocene times, coeval with the closure of the Liguro oceanic basin.