Ore genesis of Pb-Zn deposits in the Nappe zone of Northern Tunisia: Constraints from Pb-S-C-O isotopic systems


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Ore genesis of Pb-Zn deposits in the Nappe zone of Northern Tunisia: Constraints from Pb-S-C-O isotopic systems
Ore Geology Reviews
Jemmali N., Souissi F., Villa I.M., Vennemann T.W.
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The Jalta and Jebel Ghozlane ore deposits are located in the extreme
North of Tunisia, within the Nappe zone. The mineralization of Jalta,
hosted in Triassic dolostones and the overlying Mio-Pliocene
conglomerates, consists of abundant galena, barite, and cerussite with
accessory sphalerite, pyrite, and jordanite. At Jebel Ghozlane, large
Pb-Zn concentrations occur in the Triassic dolostones and Eocene
limestones. The mineral association consists of galena, sphalerite,
barite, and celestite and their oxidation products (cerussite,
smithsonite, and anglesite). Lead isotope ratios in galena from both
districts are relatively homogeneous ((206)Pb/(204)Pb = 18.702-18.823,
(207)Pb/(204)Pb = 15.665-15.677, (208)Pb/(204)Pb = 38.725-38.875). The
delta(34)S values for sulfates from both areas (+12.2 to +16.2 parts per
thousand at Jalta and + 14.3 to + 19.4 parts per thousand at Jebel
Ghozlane) are compatible with a derivation of sulfur from marine
sulfates, possibly sourced from the Triassic evaporites. The delta(34)S
values of the sulfides have a range between -10 and +12.5 parts per
thousand at Jalta, and between -9.1 and +22.1 parts per thousand at
Jebel Ghozlane. The large range of values suggests reduction of the
sulfate by bacterial and/or thermochemical reduction of sulfate to
sulfur. The high delta(34)S values of sulfides require closed-system
reduction processes. The isotopically light carbon in late calcites
(-6.3 to -2.5 parts per thousand) and authigenic dolomite (-17.6 parts
per thousand) suggests an organic source of at least some of the carbon
in these samples, whereas the similarity of the delta(18)O values
between calcite (+24.8 parts per thousand) and the authigenic dolomite
(+24.7 parts per thousand) of Jalta and their respective host rocks
reflects oxygen isotope buffering of the mineralizing fluids by the host
rock carbonates. The secondary calcite isotope compositions of Jalta are
compatible with a hydrothermal fluid circulation at approximately 100 to
200 degrees C, but temperatures as low as 50 degrees C may be indicated
by the late calcite of Jebel Ghozlane (delta(18)O of +35.9 parts per
thousand). Given the geological events related to the Alpine orogeny in
the Nappe zone (nappe emplacement, bimodal volcanism, and reactivation
of major faults, such as Ghardimaou-Cap Serrat) and the Neogene age of
the host rocks in several localities, a Late-Miocene age is proposed for
the Pb-Zn ore deposits considered in this study. Remobilization of
deep-seated primary deposits in the Paleozoic sequence is the most
probable source for metals in both localities considered in this study
and probably in the Nappe zone as a whole. (C) 2011 Elsevier B.V. All
rights reserved.
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29/09/2012 16:22
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20/08/2019 16:29
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