Lithostratigraphy, geochronology and gold metallogeny in the northern Guiana Shield, South America: a review
ABSTRACT With a surface area of nearly 900,000 km2, the Guiana Shield represents the 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 weathering and is poorly documented in the international geological literature. This situation has significantly improved during the last 20 years, when shallow in situ gold occurrences attracted exploration and mining companies to initiate geological programs aimed at better understanding the geology and the mineral deposits of the Shield.The only Archean terrane (ca. 3400 Ma) known to date in the Guiana Shield is 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 the granitoid–greenstone belts suggest protracted magmatic cycles from pre- to post-peak regional metamorphism. The younger terranes comprise anorogenic sedimentary sequences of the Roraima Formation, as well as felsic volcanic rocks and associated intrusions of the Uatuma Formation, mafic dikes of the 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 documented to date, the NE–SW trending Guri Fault, juxtaposes the Archean Imataca complex against Paleoproterozoic terranes. The Central Guiana Shear Zone (CGSZ) extends from French Guiana westerly towards central Suriname and further west towards northcentral Guyana, where it matches with the Makapa–Kuribrong shear zone (MKSZ). In French Guiana, the North Guiana Trough (NGT) is interpreted as a sinistral strike-slip formed during the Trans-Amazonian orogeny.Most gold deposits and occurrences discovered to date in the Guiana Shield are sited in close proximity to major structures. In addition, they are linked with low- to medium metamorphic-grade granitoid–greenstone belts, similar to other better-explored Precambrian terranes. At a local scale, the gold deposits are hosted within, or in close proximity to, quartz veins that are 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 brittle 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, sphalerite, scheelite, molybdenite and tellurides are the main metallic minerals associated with gold. Non-metallic minerals are mainly quartz and carbonates (ankerite, calcite, siderite), associated with minor chlorite, epidote, albite, muscovite and fuchsite. Silica, carbonate, propylitic and potassic alteration 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 many settings suggest that the granitoid–greenstone belts represent first-order exploration targets for large tonnage/low-grade gold deposits.
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ABSTRACT: Located in the El Callao Mining District (Bolivar State), Choco 10 is one of the most important gold deposits in Venezuela. The deposit occurs in the greenstone belt of the Pastora Supergroup, a lithostratigraphic Province of the South American Guyana Shield. The Choco 10 lithostratigraphy is composed of two different formations. The lower El Callao Formation is composed of a Metabasalt Unit, mostly made of tholeiitic basalts, overlain by a Mafic Volcano-Sedimentary Unit, which represents its primary cover. The tholeiitic rocks show a flat, unfractionated REE pattern which is typical of greenstone belt basalts worldwide; the incompatible elements and REE distributions suggest a mid-ocean ridge or back-arc basin setting. Within the tholeiitic series, a transition to more evolved terms is suggested by the occurrence of rocks of andesitic composition mostly in the upper part of the same formation. The overlying Felsic Volcano-Sedimentary Unit, probably pertaining to the Cicapra Formation, is a relatively heterogeneous rock series largely composed of volcaniclastic rocks deriving from an acidic calc-alkaline source, as confirmed by the geochemical data. SHRIMP analyses on zircons from these rocks show consistent magmatic ages of 2143 ± 6 to 2145 ± 5 Ma. This unit contains a gabbroic sill-like body (Metagabbro Unit), composed of medium-grained amphibole gabbro, which yields an age (2142 ± 2 Ma) that is undistinguishable from that of the main body. Both formations are affected by a greenschist-facies metamorphic overprint and different deformation phases. At 2117 ± 3 Ma, the formations have been intruded by a trondhjemite that displays fractionated REE and incompatible elements patterns typical of calc-alkaline rocks. Gold has been introduced into the rock sequence in different stages; the main mineralization is related to highly focused flow of H2O-CO2, low salinity fluids to give quartz + ankerite ± albite ± pyrite veins, surrounded by strong sericite + carbonate + pyrite + quartz alteration. Gold concentrations are controlled by both structural setting and fluid-rock interaction processes. The gold metallogeny at El Callao is related to the world-scale metallogenic event of Paleoproterozoic age, which is responsible of important orogenic gold ores formation in the once adjacent Amazonian and West Africa cratons. Even in absence of direct dating of the hydrothermal phases, our precise geochronological dating–together with the geological and petrographic data–constrains the onset of deformation (i.e., the beginning of the Trans-Amazonian deformation) and the subsequent main mineralization phase to the time span comprised between 2142-2144 Ma (age of gabbro and calc-alkaline tuffs) and 2117 Ma (emplacement of the trondhjemite). On a regional scale these data suggest eastward younging of deformation and mineralization within the context of the Trans-Amazonian tectonism.Precambrian Research 10/2014; · 6.02 Impact Factor
Article: Dr. Rajan Sankaran: “Things happen”Revista Medica de Homeopatia 09/2010; 3(3):120-124.
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ABSTRACT: The El Callao Formation is part of the Guasipati-El Callao Greenstone Belt, located in the northernmost part of the Guiana Shield, Venezuela. It consists of volcanic rocks, which have been analyzed for major and trace elements, REE concentrations and Sr–Nd isotopic composition. Based on these data, they have been characterized as Fe-rich tholeiitic basalts. On a primitive mantle-normalized diagram, all basalt samples display fairly flat patterns for the REE (La/SmCN=0.77–1.13 and La/YbCN=0.87–1.42) as well as for other trace elements, without negative anomalies for either Eu (Eu/Eu*=0.88–1.19) or Nb. The basalts have low initial Sr isotopic ratios (∼0.7014 to ∼0.7019) and low positive ɛNdi values (+0.7 to +2.1). Based on these geochemical data we propose that the tholeiitic basalts were formed in an oceanic plateau tectonic setting from a mantle plume source, possibly from the plume-head peripheral part, and that they have a primitive mantle source. Samples show isotopically uniform juvenile features, which rule out the possibility of significant contamination of the protolith magmas by ancient continental crust. These results are similar to those obtained from other greenstone belts from the Guiana Shield and Birimian domains of the West African Craton. As noted recently by Bierlein and Craw (2009), metabasalts derived from an oceanic plateau, when accreted into a collisional orogen, may result in significant gold enrichment; metamorphic terranes hosting large volumes of these rocks, such as the Guasipati-El Callao Greenstone Belt, are consequently prospective for gold mineralization.Precambrian Research 04/2011; 186(1):181-192. · 6.02 Impact Factor