Publication: Petrography and geochemistry of the magnesites and dolostones of the Ediacaran
Ibor Group (635 to 542 Ma), Western Spain: Evidences of their hydrothermal origin
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Publication Date
2011
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Elsevier
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Abstract
The Ediacaran deposits (between 635 and 542 Ma) of the Central Zone of the Iberian Massif consist of alternating
siliciclastic and carbonate beds. These carbonates are dolostones and magnesites which are interpreted
to have been formed by the replacement of primary peritidal limestones. Through petrographic and
geochemical analyses, we recognize different types of dolomites (D1 to D4) and magnesites (M1 and M2).
Despite distinct petrographic features of the four types of dolomite, their oxygen and carbon isotopes overlap
with δ18O values ranging from +15.45 to +17.51‰ (SMOW) and δ13C from −0.13 to 3.21‰ PDB. Sr isotope
values for D1 and D2 range from 0.7028 to 0.7091. Magnesites (M1 and M2) show oxygen values higher than
+17.87.0‰, and δ13C values show the same variability as for the dolomites. D3 and D4 oxygen isotope values
are between +18.91 and 19.61, and the carbon isotope values range are similar to the other diagenetic
phases. Sr isotope values for the magnesites and late dolomites (D3 and D4) are 0.7095 to 0.7104, being
higher than those of the D1 and D2 dolomites. D1 is a relatively early dolomite phase formed by the replacement
of fine grained peritidal limestones. The coarser crystal size of D2, which shows similar geochemical
features as D1, suggests formation by dolomitization of coarser grained limestones. The replacement of D1
and D2 by M1 and M2 advanced along stylolites, fractures and bedding planes. This replacement is interpreted
to have occurred by hydrothermal fluids, which is suggested by the presence of talc and forsterite.
D3, a coarse dolomite, completely destroyed any previous texture and D4 (dolomite cement) post-dates magnesite
formation. Interactions of hydrothermal fluids with the prior carbonates reset the oxygen isotopes of
the earlier dolomite.
The study of these magnesites and related dolostones may offer new insights into the model of formation of
sparry magnesites hosted by mixed siliciclastic–carbonate platform deposits. The establishment of the factors
and mechanism that control the diagenetic evolution of these carbonates has a great importance in order to
understand and predicts porosity and permeability variations of rocks formed under similar geological
conditions.