Oxygen and hydrogen isotopic composition of clay minerals of the Bahloul Formation in the region of the Bou Grine zinc-lead ore deposit (Tunisia): evidence for fluid-rock interaction in the vicinity of salt dome cap rock
A. Bechtel et al., Oxygen and hydrogen isotopic composition of clay minerals of the Bahloul Formation in the region of the Bou Grine zinc-lead ore deposit (Tunisia): evidence for fluid-rock interaction in the vicinity of salt dome cap rock, CHEM GEOL, 156(1-4), 1999, pp. 191-207
The siliciclastic Bahloul Formation of Tunisia is the host for Zn/Pb deposi
ts. A detailed study of O-18/O-16 and D/H ratios of kaolinite, quartz and c
alcite was done to elucidate the conditions and mechanism of deposition of
the diapir-related Bou Grine Zn/Pb ores in the formation. D/H and O-18/O-16
ratios of the minerals of the host rock have been affected by the minerali
zing process. The magnitudes of the isotopic shift in O-18/O-16 Of kaolinit
e as well as the intensity of base metal mineralization seem to be governed
by the permeability of the Bahloul strata, due to different water-rock rat
ios. In contrast, the variability in D/H ratios of kaolinite shows no corre
lation with the estimated permeability of the respective sedimentary sectio
n. Published microthermometric fluid inclusions data and organic geochemica
l maturation parameters both argue for a maximum temperature between 80 and
120 degrees C during ore formation and host rock alteration. The O-18/O-16
ratios of the clay minerals of samples taken over a range of distance from
the ore body suggest alteration by hot, saline, O-18-enriched brines. Thes
e could have evolved at the flanks of the diapir and would have been capabl
e of remobilizing metals from deeper sedimentary sequences, providing the m
etals now found in the Bou Grine deposit. The delta(18)O-value of the miner
alizing fluid is estimated to have been between + 8 and + 18 parts per thou
sand. The delta D-value was between - 30 and - 55 parts per thousand. The h
igh delta(18)O-values of these basinal brines provide evidence for a contro
l of the isotopic composition of the fluids by O-18-rich carbonate minerals
of the post-Triassic limestones. Hydrogen isotopic data argue for a meteor
ic origin of the brines. The variability of the GD-values might be explaine
d by a contribution of evaporated meteoric waters entering the groundwater
system, and mixing of these waters with the hot saline brines in the contac
t zone of the diapir. (C) 1999 Elsevier Science B.V. All rights reserved.