Trace-element characteristics of different pyrite types in Mesoarchaean to Palaeoproterozoic placer deposits |
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Authors: | Nikola Koglin Hartwig E Frimmel W E Lawrie Minter Helene Brätz |
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Institution: | 1. Geodynamics and Geomaterials Research Division, Institute of Geography, University of Würzburg, Am Hubland, 97074, Würzburg, Germany 2. Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africa 3. Lithosphere Dynamics Section, Geo-Center of Northern Bavaria, Schlossgarten 5a, 91054, Erlangen, Germany
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Abstract: | A comparative in situ LA-ICP MS trace-element study on pyrite from three different, variably auriferous, Archaean to Palaeoproterozoic
palaeoplacer deposits in the Ouro Fino Syncline (Quadrilátero Ferrífero; Brazil), the Elliot Lake area north of Lake Huron
(SE Canada) and several deposits within the Witwatersrand Basin (South Africa) revealed systematic differences between morphologically
different pyrite types and between the various palaeoplacer deposits. Especially the Ni and Au concentrations as well as Co/Ni
and Mo/Ni ratios were found to be systematically different in detrital compact, detrital porous and post-sedimentary/hydrothermal
pyrite grains from different source areas. High Co/Ni ratios and low Au concentrations are typical of post-sedimentary pyrite,
which is hydrothermal in origin. In contrast, relatively low Co/Ni ratios and high Au contents characterise detrital porous
banded and concentric pyrite grains (Au > 1 ppm), which are syn-sedimentary in origin. In the Elliot Lake area and the Witwatersrand
Basin, detrital compact rounded pyrite is characterised by high Co/Ni ratios, which is in agreement with derivation from a
hydrothermal source. Low Au concentrations in this pyrite type support the contention of the gold and the pyrite in these
deposits coming from different source rocks. In contrast, derivation from an originally diagenetic pyrite is suggested for
the detrital compact pyrite in the Ouro Fino Syncline because of low to intermediate Co/Ni ratios. High Au contents may indicate
a genetic relationship between pyrite and gold there. Systematic differences exist between the three areas with respect to
Au, Ni, Co, Mo and Cu distributions in detrital pyrite, which reflects differences in the provenance. A predominantly mafic/ultramafic
source is indicated for the Ouro Fino, a felsic source for the Elliot Lake, and a mixed felsic–mafic provenance for the Witwatersrand
pyrite populations. Independently of pyrite type, the higher Au endowment of the studied Witwatersrand and Ouro Fino conglomerates
are also reflected by an overall higher Au concentration in the respective pyrite grains compared to the relatively Au-poor
samples from Elliot Lake. In general, a strong positive correlation between Au and Pb levels in the various pyrite grains
is noted. Analogous to Pb, which is well known for not being easily accommodated in the pyrite crystal lattice but occurring
as discrete PbS phases, Au is considered to be present mainly in the form of discrete Au phases in minute pores and interstices
of the pyrite grains rather than within the pyrite lattice. |
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