Original paper

Greenstone belts of the Venezuelan Guayana Shield - A stratigraphie revision

Menéndez, V. de V. Alfredo

Global Tectonics and Metallogeny Volume 7 Number 2 (1999), p. 83 - 84

2 references

published: Dec 1, 1999

BibTeX file

ArtNo. ESP136000702006, Price: 19.00 €

Download preview PDF Buy as PDF

Abstract

Early Proterozoic greenstone belts are exposed in the northern part of the Guiana Shield in Venezuela. They are characterized by isoclinally folded synforms of supracrustal volcano-sedimentary sequences surrounding soda granite domes (the Supamo Complex), as has been observed in other Precambrian Shields. The supracrustal sequence has been metamorphosed from graywacke-prehnite-pumpellyite facies of regional metamorphism distally to amphibolite facies close to the granitic domes intrusive contacts. There are four welldefined greenstone belts in the younger Pastora province exposed south of the older Imataca province (to 3.4 By); the latter contains three remnants of similar greenstone belts and marks the northern edge of the Guiana Shield in Venezuela. In the Pastora the pre-greenstone rocks are not exposed, while in the Imataca the Archeozoic gneisses it contains may represent the protobasement. The volcano-sedimentary cycle began with an essentially volcanic phase (the Pastora Supergroup and equivalent units), and ended with a largely sedimentary phase (The Botanamo Group and equivalent rocks). The initial volcanic sequences are subalkaline mafic to ultramafic in composition, with a komatiite to tholeite magmatic trend (The Carichapo Group and equivalent rocks), followed by a closely related dacitic volcanic sequence with minor associated sediments (The Yuruari Formation and equivalent rocks). The sedimentary phase began with flysh-like deposits and calc-alkaline volcanics (the Caballape Formation and equivalent rocks), and ended with a molasse sequence (the Los Caribes Formation and equivalent rocks). The sedimentary and volcanic phases are separated by an angular unconformity, possibly a product of the forcible emplacement of the pre-Caballape sodic granite domes. Two regional structural patterns have been identified; one is North-South to Northwest-Southeast, and the other is East-West to Northeast-Southwest; these are derived from the general orientations of the greenstone belts. The aforementioned structural patterns possibly reflect the orientation of the basins where the volcanic and sedimentary phases were deposited. The greenstone belts have been mineralized along shear zones where about ten gold deposits have been defined to date, like the Las Cristinas and Las Brisas deposits containing gold reserves above fifteen million ounces and the El Callao gold district with gold quartz veins under exploitation since the nineteen century.

Abstract

En el Escudo de Guayana Venezolano afloran Cinturones de Rocas Verdes (CRV) proterozóicos constituidos de una secuencia supracortical volcanico-sedimentaria que forman grandes cuerpos sinformales rodeados de cuerpos dómicos de granitos sódicos (Complejo de Supamo), al igual que en otros escudos precambricos. Las rocas supracorticales estan afectadas por metamorfismo desde la facies de la Anfibolita cerca de los contactos con granitos hasta la facies de la Grauvaca prehnito-pumpellyftica lejos de los mismos. Se encuentran CRV en la Provincia de Pastora y remanentes de cinturones en la Provincia de Imataca; en los primeros se desconoce el basamento, mientras que en los segundos los gneises de Imataca aparentemente representan el basamento. El cicio volcanico-sedimentaria se inicia con una fase esencialmente volcanica (Sgpo Pastora y equivalentes) y termina con una fase escencialmente sedimentaria (Gpo Botanamo y equivalentes). El conjunto volcanico inicial es de composicion mafica a ultramafica subalcalinas y con tendencia comatiitica a toleitica (Gpo Carichapo y equivalentes) y termina con un conjunto volcanico dacftico y sedimentarias menores asociadas (F. Yuruari y equivalentes). La fase sedimentaria se inicia con un depósito tipo flysh y de volcanicas calcoalcalinas (F. Caballape y equivalentes) y termina con un conjuncto molasico de capas rojas (f. Los Caribes y equivalentes). La fase sedimentaria esta separada de la volcanica por una discordancia angular, producto posiblemente de emplazamiento forzada de granitos s6dicos en tiempos pre Caballape. Se definen dos patrones estructurales primarios, uno de N-S a NO y otro de E-O a NE, a partir de la orientacion general de los CRV; dichos patrones posiblemente controlaron la orientación de las cuencas donde se depositaron la fase volcanica y la fase sedimentaria, respectiveamente.

Keywords

greenstone beltsVenezuelan Guayana Shieldstratigraphic revisioncinturones de rocas verdesescudo de guayanaVenezuelarevision estratigrafica