First occurrence of Paleoproterozoic oceanic plateau in the Guiana Shield: The gold-bearing El Callao Formation, Venezuela

Abstract

The El Callao Formation is part of the Guasipati-El Callao Greenstone Belt, located in the northernmost part of the Guiana Shield, Venezuela. It consists of volcanic rocks, which have been analyzed for major and trace elements, REE concentrations and Sr–Nd isotopic composition. Based on these data, they have been characterized as Fe-rich tholeiitic basalts. On a primitive mantle-normalized diagram, all basalt samples display fairly flat patterns for the REE (La/SmCN=0.77–1.13 and La/YbCN=0.87–1.42) as well as for other trace elements, without negative anomalies for either Eu (Eu/Eu*=0.88–1.19) or Nb. The basalts have low initial Sr isotopic ratios (∼0.7014 to ∼0.7019) and low positive ɛNdi values (+0.7 to +2.1). Based on these geochemical data we propose that the tholeiitic basalts were formed in an oceanic plateau tectonic setting from a mantle plume source, possibly from the plume-head peripheral part, and that they have a primitive mantle source. Samples show isotopically uniform juvenile features, which rule out the possibility of significant contamination of the protolith magmas by ancient continental crust. These results are similar to those obtained from other greenstone belts from the Guiana Shield and Birimian domains of the West African Craton. As noted recently by Bierlein and Craw (2009), metabasalts derived from an oceanic plateau, when accreted into a collisional orogen, may result in significant gold enrichment; metamorphic terranes hosting large volumes of these rocks, such as the Guasipati-El Callao Greenstone Belt, are consequently prospective for gold mineralization.