Venezuela kimberlites

Abstract

Recently discovered kimberlitic dykes and sills in the Guaniamo area of Bolivar Province (Guyana Craton) are the source of nearby extensive alluvial diamonds occurring in Recent gravels and in the Roraima sedimentary group (ca. 1.7 Ga) to the south. The intrusives are altered to nontronite and saponite but texturally are identified as Group I macrocrystic olivine rich kimberlites with identifiable monticellite and phlogopite. The REE show steep patterns and high (La/Yb) n up to 480; Cr, Ni and Nb are enriched, but LILE and Ti, Zr depleted with respect to average kimberlite, partly as a result of weathering. Leached samples yield a Rb-Sr isochron age of 1732 ± 82 Ma making them the oldest kimberlites on record. Sr and Nd isotope data are more consistent with a Group II, rather than a Group I kimberlite paragenesis. Discrete nodules (megacrysts) which typify Group I kimberlites have not been identified. There are no mantle xenoliths but mantle xenocrysts of chromium-rich pyropes and spinels, similar to those from cratonic harzburgite xenoliths of diamond facies, are abundant. The spinels show metasomatic alteration to yimengite. The kimberlites are intruded by micaceous dykes enriched in K, Ba, Sr, Zr and F indicative of a second phase of alkaline igneous activity. These igneous and metasomatic events are discussed in the context of the evolution of the Guyana Craton. Archaean metamorphism associated with the Imataca Complex spans the harzburgite chromium pyrope model Nd age (2.6 Ga). The widespread Trans-Amazonian metamorphic event (ca. 2.1 Ga) was possibly contemporaneous with mantle metasomatism indicated by yimengite formation. The kimberlites were emplaced thereafter (1.7 Ga). The micaceous dykes (0.85 Ga, 39 40 Ar ) were generated in a mantle enriched in elements similar to those found in the earlier formed yimengite (K, Ba, Sr).