The detrital zircon record of an Archaean convergent basin in the Southern São Francisco Craton, Brazil
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Keywords:
Detritus
Greenstone belt
Supercontinent
Greenstone belt
Lithophile
Ultramafic rock
Trace element
Sill
Rare-earth element
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The Taivaljaervi Ag-Zn deposit is associated with felsic metavolcanics in the Archean green-stone belt of eastern Finland. The deposit is stratiform and consists of four ore layers, the A, B, C, and D orebodies. The deposit contains 3.45 million metric tons of ore with 1.27 percent Zn, 0.57 percent Pb, 160 g/metric ton Ag, and 0.37 g/metric ton Au. The content of sulfide ore minerals is low, only 5 percent on average. The main ore minerals are sphalerite, galena, chalcopyrite, pyrite and pyrrhotite, dyscrasite, freibergite, electrum, and native Ag. The deposit displays disseminated and vein-type ore textures. The volcanic host rocks are deformed and metamorphosed in high amphibolite facies, and the metamorphic mineral assemblages reflect the variation in chemical compositions of the rock sequence. The intensive hydrothermal alteration associated with the mineralization predated the regional metamorphism, but it can still be recognized in the chemical compositions of the rock sequence. The variation in main and trace element compositions in a traverse of the volcanic formation is presented in tables and diagrams. The delta 34 S values fluctuate from 2.0 to 3.8 per mil, indicating volcanic origin of sulfur. The ore deposit is a product of hydrothermal activity associated with volcanoclastic felsic eruption, and the type of deposition is assumed to be epithermal in character.
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Abstract Many Archean cratons exhibit Paleoproterozoic rifted margins, implying they were pieces of some ancestral landmass(es). The idea that such an ancient continental assembly represents an Archean supercontinent has been proposed but remains to be justified. Starkly contrasting geological records between different clans of cratons have inspired an alternative hypothesis where cratons were clustered in multiple, separate “supercratons.” A new ca. 2.62 Ga paleomagnetic pole from the Yilgarn craton of Australia is compatible with either two successive but ephemeral supercontinents or two long-lived supercratons across the Archean-Proterozoic transition. Neither interpretation supports the existence of a single, long-lived supercontinent, suggesting that Archean geodynamics were fundamentally different from subsequent times (Proterozoic to present), which were influenced largely by supercontinent cycles.
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Yilgarn Craton
geodynamics
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Ephemeral key
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Abstract Major Archaean greenstone gold deposits are located in distinctive structural settings, and Zimbabwe is a good example with over 90% of its gold production being derived from structurally controlled veisns, shear zones and probably epigenetic replacement BIF hosted deposits. The structural framework of the Zimbabwe craton is dominated by the evolution of the c. 2700 Ma Limpopo belt. Strongly developed, major shear zones were initiated at this time. These major shear zones focused strain, which in the Midlands greenstone belt, evolved from pure N-S compression, to lateral extrusion of the main Rhodesdale gneiss block westwards. The structural features seen in the gold deposits of the Midlands greenstone belt are consistent with such a model of NNW-SSE compression linked to lateral extrusion of the Rhodesdale gneiss block westwards. Initiation of structures during NNW-SSE compression resulted in early folding such as the Kadoma anticline, together with initiation of the major mineralized shear systems as thrust faults. Simple shear evolved to transpressive shear with development of oblique dextral and sinistral mineralized shear systems. Rotation of the principal stress direction clockwise to an ENE-WSW orientation led to evolution of dominantly dextral transpressive shear on the major Lily, Munyati and Rhodesdale boundary shear systems, probably in response to the lateral extrusion of the Rhodesdale gneiss block westwards. This even is linked to waning gold mineralization and accounts for the major dextral offsets along the Lily fault and the Munyati shear close to Battlefields.
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