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.
Article: Dr. Rajan Sankaran: “Things happen”Revista Medica de Homeopatia 09/2010; 3(3):120-124.
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ABSTRACT: A high-pressure Raman study of SrWO4 reveals a pressure induced phase transition starting at 11.5 GPa. Several Raman lines exhibit a nonlinear behaviour in the pressure range of 11.5–15 GPa, which can be attributed to either stabilization of the high-pressure phase or an intermediate phase. Using a theoretical lattice dynamical calculation, based on an empirical potential model, we have obtained the Raman active mode eigenvectors giving us an insight into the phase transition mechanism. The lowest-frequency mode exhibits a negative pressure slope in the scheelite phase and involves a motion of the WO4 tetrahedron as a whole according to the theoretical results. The experimental evidence suggests that the structure of the high-pressure phase is closely related to the scheelite structure, being formed by closely lying distorted WO4 tetrahedra rather than involving an octahedrally coordinated W ion.Journal of Physics Condensed Matter 11/2002; 14(47):12641. · 2.22 Impact Factor
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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