Lithostratigraphy, geochronology and gold metallogeny in the northern Guiana Shield, South America: a review

With a surface area of nearly 900,000 km(2), the Guiana Shield represents t he northern segment of the Amazonian Craton in South America, lying for the most part between the Amazon and Orinoco river basins. Most of the Guiana Shield formed during protracted periods of intense magmatism, metamorphism and deformation, culminating with the Trans-Amazonian tectono-thermal event , bracketed between 2.1 and 1.9 Ga. The Guiana Shield is among the least known Precambrian terranes because it is relatively inaccessible, lacks bedrock exposure due to intense weatherin g and is poorly documented in the international geological literature. This situation has significantly improved during the last 20 years, when shallo w in situ gold occurrences attracted exploration and mining companies to in itiate geological programs aimed at better understanding the geology and th e mineral deposits of the Shield. The only Archean terrane (ca. 3400 Ma) known to date in the Guiana Shield i s the Imataca Complex in Venezuela. The Paleoproterozoic, low-grade volcano -sedimentary greenstone sequences and associated granitoid intrusions have yielded ages between 2.25 and 2.08 Ga. Recent U-Pb age determinations of th e granitoid-green stone belts suggest protracted magmatic cycles from pre- to post-peak regional metamorphism. The younger terranes comprise anorogeni c sedimentary sequences of the Roraima Formation, as well as felsic volcani c rocks and associated intrusions of the Uatuma Formation, mafic dikes of t he Avanavero Suite and Rapakivi-type and alkali intrusions. Several large-scale ductile shear zones have been documented in the Guiana Shield. In northcentral Venezuela, the most outstanding structure documente d to date, the NE-SW trending Guri Fault, juxtaposes the Archean Imataca co mplex against Paleoproterozoic, terranes. The Central Guiana Shear Zone (CG SZ) extends from French Guiana westerly towards central Suriname and furthe r west towards northcentral Guyana, where it matches with the Makapa-Kuribr ong shear zone (MKSZ). In French Guiana, the North Guiana Trough (NGT) is i nterpreted as a sinistral strike-slip formed during the Trans-Amazonian oro geny. Most gold deposits and occurrences discovered to date in the Guiana Shield are sited in close proximity to major structures. In addition. they are lin ked with low- to medium metamorphic-grade granitoid-greenstone belts, simil ar to other better-explored Precambrian terranes. At a local scale, the gol d deposits are hosted within, or in close proximity to, quartz veins that a re syn- to late-tectonic, and to a lesser extent, in stockworks, breccias, and lenses. They are commonly located in units that behaved in a more britt le manner than the country rocks. Available information suggests that gold deposits are mainly epigenetic, although some are associated with specific lithostratigraphic units. Pyrite, pyrrhotite, chalcopyrite, galena, sphaler ite, scheelite, molybdenite and tellurides are the main metallic minerals a ssociated with gold. Non-metallic minerals are mainly quartz and carbonates (ankerite, calcite, siderite), associated with minor chlorite, epidote, al bite, muscovite and fuchsite. Silica. carbonate, propylitic and potassic al teration is common. High erosion rates expected after the creation of an orogenic belt did not occur in the northern Guiana Shield. Shallow-level deposits preserved in ma ny settings suggest that the granitoid-greenstone belts represent first-ord er exploration targets for large tonnage/low-grade gold deposits. (C) 2001 Elsevier Science B.V. All rights reserved.