Origin of ultramafic-hosted magnesite on Margarita Island, Venezuela

Abstract

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 6~sO data indicate that magnesite precipitated on Margarita in near-surface environments at low P and T. ~13C ranges from -9 to -16%o PDB within the magnesite and -8 to -10%o PDB within some calcite and dolomite elsewhere on the island. The isotopically light dolomite fills karst and the calcite occurs as stockwork 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 rareearth 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.

Source of the mineralizing fluids in ultramafic related magnesite in the Eskişehir area, northwest Turkey, along the İzmir–Ankara Suture: a stable isotope study

“…Soil-derived decomposition of plant material and surface weathering (e.g., Zachmann and Johannes, 1989;Zedef et al, 2000), 5. Regional metamorphic reactions at >300 °C (e.g., Abu-Jaber and Kimberley, 1992;Zedef et al, 2000), 6. Volcanogenic sources (e.g., Illich, 1968Zedef et al, 2000), and 7.…”

“…Numerous studies have used the δ 18 O and δ 13 C isotope ratios of magnesite to suggest the source of the fluids responsible for magnesite formation in Australia, Yugoslavia, Greece, and Turkey (e.g., Gartzos, 1980Gartzos, , 1990Gartzos, , 2004Kralik et al, 1989;Jedrysek and Halas, 1990;Fallick et al, 1991;Abu Jaber andKimberley, 1992a, 1992b;Brydie et al, 1993;Zedef et al, 2000;Schroll, 2002;Ece et al, 2005;Kuşcu and Cengiz, 2005). In this study, the δ 13 C values of the Süleymaniye stockwork magnesite deposits have a range between -2.70‰ and -7.7‰ V-PDB.…”

Overview of radiometric ages in three allochthonous belts of northern Venezuela: Old ones, new ones, and their impact on regional geology

“…3), which consists of three metamorphic units as well as metaigneous and igneous lithologies (e.g., Maresch, 1975;Avé Lallemant and Guth, 1990;Guth, 1991;Stöckhert et al, 1995;. In addition, there are large serpentinite bodies (Abujaber and Kimberley, 1992). The metamorphic units include the Juan Griego Group, the La Rinconada Group, and the Los Robles Group.…”

Geochemistry and tectonic setting of igneous and metaigneous rocks of northern Venezuela

“…The largest is Margarita Island, which consists of three metamorphic units: the Juan Griego, La Rinconada, and Los Robles Group, as well as the Matasiete trondhjemite, Guayacan Gneiss, El Salado granite, and Agua de Vaca granite (e.g., Maresch, 1975;Guth, 1991;Stöck-hert et al, 1995) and a large serpentinite body (Abujaber and Kimberley, 1992). Paleogene to Quaternary sedimentary rocks such as the Paleogene Pampatar Formation and Eocene Punta Carnero Group overlie all these units (see Fig.…”