Generalized Stratigraphy
Guyana lies in the northern part of the Guiana Shield which includes Guyana, Suriname and French Guiana, and parts of Brazil and Venezuela. The rocks and sediments of Guyana can be subdivided into four groups on the basis of their ages.
The Precambrian rocks (GSC) north of the Takutu Basin (TB) include folded metasedimentary rocks and metavolcanic rocks as well as coarse- and fine-grained sedimentary rocks with intercalations of volcanic rocks. Intrusive bodies occur within the folded strata.
Intrusive rocks, volcanic rocks and folded metasedimentary rocks comprise the Guiana Shield Complex south of the Takutu Basin. Mylonitized zones within high grade metamorphic rocks in the region have been related to an Upper Proterozoic tectono-thermal event.
The intrusive (granitoid) Precambrian rocks north and south of the Takutu Basin can be differentiated on the basis of their mineral assemblages. The granitoid rocks south of the basin are petrologically similar whereas those north of the basin are rather variable in composition.
The Roraima Supergroup (RSG), consisting of mainly continental sedimentary rocks with interbedded volcanic rocks, unconformably overlies Precambrian rocks and is intruded by sills and dykes. The rocks of the Roraima Supergroup were previously assigned to the Paleozoic Era. However, recent studies have shown that these deposits are Precambrian in age.
Basic dykes of Mesozoic age intrude the Precambrian bedrock north and south of the Takutu Basin. These rocks are not shown on the accompanying map because of their limited size and areal extent.
The Takutu Basin (TB) is part of a graben that has been infilled mainly with sedimentary rocks and minor volcanic rocks of Jurassic to Cretaceous age in the lower part of the sequence. The thickness of the infill sequence has been estimated at about 7,000 m. Evaporite deposits and hydrocarbons have been reported from the basin.
The Cretaceous to Recent Coastal Deposits (CD) consist of alternating sands and clays of freshwater and marine origns. In some areas, these sediments are underlain by alluvial and residual bauxite and kaolin that, in turn, are underlain by Precambrian bedrock. Thin peat layers are present in the upper part of the succession.
Selected Mineral Deposits
Recent geological investigations on the Guiana Shield have identified significant deposits of gold in Guyana (Omai deposit) and in Venezuela (Las Cristinas deposit). The auriferous deposits occur in Precambrian greenstone belts similar to the Archean greenstone belts of the Canadian Shield.
In Guyana, primary gold deposits are commonly found near the margins of granitic to dioritic stocks that intruded the greenstone belts. Fault zones, shear zones and tectonic lineaments within the greenstone belts are also potential areas of gold deposits.
The mining of gold and diamond associated with placer deposits is common in the Essequibo region. The source of the alluvial diamonds has not been established.
Bauxite, associated with the Coastal Deposits, is mined at Linden, Aroaima and Kwakwani. The bauxite was formed from the underlying Precambrian bedrock during a mid-Tertiary weathering episode. |
