Multiple Sulfur and Iron Isotope Composition of Magmatic Ni-Cu-(PGE) Sulfide Mineralization from Eastern Botswana
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We report here high-precision multiple sulfur and iron isotope compositions for a series of mineralized samples from Ni-Cu-(PGE) sulfide deposits in the Archean Tati greenstone belt and the Phikwe Complex of eastern Botswana. Mineralized samples from the Phoenix and Selkirk Ni-Cu-(PGE) deposits in the Tati greenstone belt display slightly positive δ 34S isotope values, ranging from 0.2 to 0.8‰ V-CDT. Δ33S values of sulfides at Phoenix and Selkirk are −0.01 to −0.08‰ V-CDT, suggesting either a dominantly mantle sulfur source or effective eradication of a crustal Δ33S anomaly through equilibration with large amounts of silicate melt. In the Selebi-Phikwe belt, a granite-gneiss terrane with abundant amphibolite lenses of either volcanic and/or intrusive nature, mineralized lower grade samples from the Phikwe, Phokoje, and Dikoloti Ni-Cu-(PGE) deposits have more variable δ 34S values ranging from −3.1 to +0.3‰ and display significant mass independent anomalies (Δ33S values ranging from −0.89 to −0.27‰), suggesting that barren sulfides associated with distal or low-temperature sea-floor hydrothermal activity contributed sulfur to these deposits. Iron isotopes of sulfides from these deposits show a relatively small range of negative Δ56Fe values (−0.29 to −0.04‰), consistent with high-temperature fractionations in magmatic systems, with the exception of one sample from the Dikoloti Ni-Cu-(PGE) deposit of the Selebi-Phikwe greenstone belt, which shows a more negative δ 56Fe value of −0.61‰, consistent with assimilation of sedimentary or hydrothermal sulfides rather than fractionations in high-temperature magmatic systems. Data from this study highlight the complexity and variability that characterize ore-forming processes in magmatic systems. We suggest that the presence of sulfur-bearing lithologic units in host rocks of mafic and ultramafic intrusions may not be essential toward the assessment of the prospectivity of a province to host orthomagmatic nickel sulfides. Geologic settings without any or little sulfur in the stratigraphy, which have been traditionally neglected in terms of their prospectivity, should thus be revisited and possibly reassessed considering the potential importance of external source of sulfur to generate ore deposits.Keywords:
δ34S
Greenstone belt
Sulfide Minerals
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 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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