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Geology of the Venezuelan Guayana Shield and its relation to the entire Guayana Shield

1991; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr91141

2332-4899

G.B. Sidder, Vicente Mendoza S.,

Geochemistry and Geologic Mapping

The Guayana Shield in Venezuela is composed of five lithotectonic provinces: 1) an Archean amphibolite-to granulite-facies gneiss terrane; 2) an Early Proterozoic greenstone-granite terrane(s); 3) an Early Proterozoic unmetamorphosed volcanoplutonic complex; 4) Early to Middle Proterozoic continental sedimentary rocks; and 5) Middle Proterozoic anorogenic rapakivi-type granite.Early Proterozoic rocks in the Amazon Federal Territory of western Venezuela are undivided and their relation to other rocks of the Venezuelan Guayana Shield is unclear.Early to Middle Proterozoic continental tholeiitic dikes, sills, and small irregular intrusive bodies and Mesozoic dikes emplaced during the opening of the Atlantic Ocean cut all of the lithotectonic provinces.Major mineral deposits of the Venezuelan Guayana Shield include gold, iron, bauxite, and diamonds.The Archean Imataca Complex, the oldest unit, consists of gneiss and granulite with minor dolomite and banded iron-formation (BIF).Large isoclinal folds, which have been refolded into relatively open folds, are common in the Imataca Complex.Metamorphic grade ranges from granulite facies in the northeast part of the belt to amphibolite facies in the southwest.Deposits of enriched BIF in the Imataca Complex contain more than 2 billion metric tons of iron ore.During the Pre-Trans-Amazonian tectonomagmatic event, between about 2,800 and 2,700 Ma, granitic rocks intruded the Imataca Complex, and injection gneisses and migmatite were developed.The Early Proterozoic greenstone belts, which formed between about 2,250 and 2,100 Ma, consist of a submarine sequence of tholeiitic mafic volcanic rocks, a sequence of tholeiitic to calc-alkaline basalt to rhyolite, and an interval of turbiditic graywacke, volcaniclastic rocks, and chemical sedimentary rocks that characterize the basal, middle, and upper parts, respectively.Layered mafic complexes also occur in the greenstone belts.The greenstone-belt rocks range in metamorphic grade from greenschist to amphibolite facies.Low-sulfide gold-quartz veins are hosted by Early Proterozoic greenstone-belt rocks cut by major shear zones.of the Cuchivero Group.Undivided Proterozoic rocks in the Amazon Federal Territory of western Venezuela include granitic rocks, gneiss, and migmatite.Peak metamorphism and magmatism occurred between about 1,860 and 1,730 Ma.Unmetamorphosed, post-tectonic sedimentary rocks such as quartz arenite, conglomerate, arkose, siltstone, and shale of the Roraima Group were deposited in fluvial, deltaic, shallow marine, and lacustrine or epicontinental environments.The Roraima Group is at least 1,670 Ma in age and possibly as old as about 1,900 Ma and as young as about 1,500 Ma.Paleo-placer deposits of gold and diamonds in the lower part of the Roraima Group are the source for modern placers.Continental tholeiitic dikes, sills, and irregular intrusive bodies of the Avanavero Suite cut all older rocks of the Guayana Shield.These intrusions are about 1,650 Ma in age, and possibly as old as about 1,850 Ma.Middle Proterozoic, about 1,545 Ma, undeformed granite with rapakivi texture is characteristic of the Parguaza province.Tin in quartz veins, pegmatite, and greisen are mined elsewhere in the Guayana Shield; in Venezuela, moderate potential exists for discovery of tin associated with the Parguaza granite.Similarities in age, composition, and tectonic environment indicate that Olympic Dam-type Fe-Cu-U-Au-REE deposits are possible in the Parguaza granite and its associated volcanic rocks.Continental collision in the westernmost part of the Guayana Shield during the Nickerie orogeny reset many potassium-argon and rubidium-strontium mineral ages of Archean and Early Proterozoic rocks in the central and eastern parts to about 1,200 Ma.Tholeiitic diabase dikes intruded the Guayana Shield during the opening of the Atlantic Ocean from about 210 to 200 Ma.Erosion of the Precambrian terranes and uplift during the Mesozoic and Cenozoic Eras produced at least six erosional surfaces at distinct elevations between about 2,900 and 50 m above sea level in the Guayana Shield.Tropical weathering of the diverse lithologies of the Guayana Shield has formed numerous occurrences of bauxite and lateritic bauxite.The largest bauxite deposit is Los Pijiguaos, which developed on the Parguaza granite.Deposits of bauxite and enriched BIF were formed on the Imataca-Nuria erosional surface.Placer diamond and gold deposits are mined in modern channels of the major rivers and in colluvial-alluvial deposits in low-order drainages.