Origin of ultramafic-hosted magnesite on Margarita Island,Venezuela |
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| Authors: | N S Abu-Jaber M M Kimberley |
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| Institution: | (1) Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, 27695-8208 Raleigh, N.C., USA;(2) Present address: Department of Earth and Environmental Science, Faculty of Science, Yarmonk University, Irbid, Jordan |
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| Abstract: | Ultramafic-hosted deposits of magnesite (MgCO3) have been studied on Margarita Island, Venezuela, to elucidate the source of carbon and conditions of formation for this
type of ore. Petrographic, mineralogic, and δ18O data indicate that magnesite precipitated on Margarita in near-surface environments at low P and T. δ13C ranges from −9 to −16‰ PDB within the magnesite and −8 to −10‰ PDB within some calcite and dolomite elsewhere on the island.
The isotopically light dolomite fills karst and the calcite occurs as stock-work veins which resemble the magnesite deposits.
These carbon isotopic ratios are consistent with a deep-seated source rather than an overlying source from a zone of surficial
weathering. However, there is not much enrichment of precious metals and no enrichment of heavy rare-earth elements, as would
be expected if the carbon had migrated upward as aqueous carbonate ions. The carbon probably has risen as a gaseous mixture
of CO2 and CH4 which partially dissolved in near-surface water before leaching cations and precipitating as magnesite and other carbonates.
The process probably is ongoing, given regional exhalation of carbonaceous gases. |
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