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.
- [Show abstract] [Hide abstract]
ABSTRACT: A mathematical model of a squirrel-cage induction motor is presented. It considers skin effect in rotor bars for frequencies up to about 1500 Hz. The main idea is to represent the rotor bars as a lumped network of resistances and inductances. The model is part of a complete simulation program for a full-power main line locomotive. It is used for a more accurate investigation of copper losses and torque pulsations in the motor. Theoretical results are compared to data calculated with a finite-element-method (FEM) program in order to get the parameters for the special shape of the motor which is applied in a locomotive. The limitations of the derived formulas and the advantages of FEM calculations are shown.< >Industry Applications Society Annual Meeting, 1989., Conference Record of the 1989 IEEE; 11/1989
- Metallurgical and Materials Transactions B 11/2004; 35(6):1203-1207. · 1.32 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: The major and trace element characteristics of black shales from the Lower Cretaceous Paja Formation of Colombia are broadly comparable with those of the average upper continental crust. Among the exceptions are marked enrichments in V, Cr, and Ni. These enrichments are associated with high organic carbon contents. CaO and Na2O are strongly depleted, leading to high values for both the Chemical Index of Alteration (77–96) and the Plagioclase Index of Alteration (86–99), which indicates derivation from a stable, intensely weathered felsic source terrane. The REE abundances and patterns vary considerably but can be divided into three main groups according to their characteristics and stratigraphic position. Four samples from the lower part of the Paja Formation (Group 1) are characterized by LREE-enriched chondrite-normalized patterns (average LaN/YbN = 8.41) and significant negative Eu anomalies (average Eu/Eu∗ = 0.63). A second group of five samples (Group 2), also from the lower part, have relatively flat REE patterns (average LaN/YbN = 1.84) and only slightly smaller Eu anomalies (average Eu/Eu∗ = 0.69). Six samples from the middle and upper parts (Group 3) have highly fractionated patterns (average LaN/YbN = 15.35), resembling those of Group 1, and an identical average Eu/Eu∗ of 0.63. The fractionated REE patterns and significant negative Eu anomalies in Groups 1 and 3 are consistent with derivation from an evolved felsic source. The flatter patterns of Group 2 shale and strongly concave MREE-depleted patterns in two additional shales likely were produced during diagenesis, rather than reflecting more mafic detrital inputs. An analysis of a single sandstone suggests diagenetic modification of the REE, because its REE pattern is identical to that of the upper continental crust except for the presence of a significant positive Eu anomaly (Eu/Eu∗ = 1.15). Felsic provenance for all samples is suggested by the clustering on the Th/Sc–Zr/Sc and GdN/YbN–Eu/Eu∗ diagrams. Averages of unmodified Groups 1 and 3 REE patterns compare well with cratonic sediments from the Roraima Formation in the Guyana Shield, suggesting derivation from a continental source of similar composition. In comparison with modern sediments, the geochemical parameters (K2O/Na2O, LaN/YbN, LaN/SmN, Eu/Eu∗, La/Sc, La/Y, Ce/Sc) suggest the Paja Formation was deposited at a passive margin. The Paja shales thus represent highly mature sediments recycled from deeply weathered, older, sedimentary/metasedimentary rocks, possibly in the Guyana Shield, though Na-rich volcanic/granitic rocks may have contributed to some extent.Journal of South American Earth Sciences. 01/2007;