Temperatures of Formation of a Metahalloysite Deposit at Djebbel, Debbagh, North Eastern-Algeria

ABSTRACT

Mount Debbagh (Djebel Debbagh) is part of the neritic Constantinois Formation located in northeastern Algeria. This Formation consists of autochthonous limestones units varying in age from Jurassic to Cretaceous that contain autochthonous Aptian to Cenomanian limestones deposited in seawater. During diagenesis, temperatures of limestones varied from 50 to 130°C as measured by fluid inclusions and determined by stable-isotopes (C, O). These temperatures and stable-isotope values are consistent with the diagenetic evolution of a sedimentary pile with small influxes of meteorichydrothermal fluids, mixtures of saline fluids within a neotectonic framework. Karsts filled with detrital and neoformed clay deposits were located below an unconformity that was dated at Lutetian times. The largest karst is mainly composed of metahalloysite, whereas other karsts contain kaolinite, illite and illitesmectite detritus from nearby clastic units. In the central part of the largest karst, metahalloysite is associated with minor sulphates due to dominant weathering below 40°C (presumably between 15 and 25°C) by coastline meteoric-fluids with a 18O value ca. –2‰ V-SMOW. These low temperatures do not correspond to previous interpretations that considered the thermal system to be at elevated temperatures (>75°C) associated with neotectonics and/or hot-spring activity. Isotope exchange experiments with different size fractions of pure metahalloysites, with no expandable capacity indicate that they do not exchange hydrogen isotopes over 24 months for a temperature of 23°C. These results used together with kaolinite oxygen and hydrogen fractionation suggest that halloysite to metahalloysite transformation occurred within a temperature range between 25 and 40°C.

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