Genesis of itabirite-hosted Au–Pd–Pt-bearing hematite-(quartz) veins, Quadrilátero Ferrífero, Minas Gerais, Brazil: constraints from fluid inclusion infrared microthermometry, bulk crush-leach analysis and U–Pb systematics |
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Authors: | Volker Lüders Rolf L Romer Alexandre R Cabral Christian Schmidt David A Banks Jens Schneider |
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Institution: | 1. GeoForschungsZentrum Potsdam, Telegrafenberg, 14473, Potsdam, Germany 2. Institut für Mineralogie und Mineralische Rohstoffe, Technische Universit?t Clausthal, Adolph-Roemer-Strasse 2A, 38678, Clausthal-Zellerfeld, Germany 5. Département de géologie et de génie géologique, Université Laval, Pavillon Adrien-Pouliot, Québec, QC, Canada, G1K 7P4 3. School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK 4. Institut für Geowissenschaften und Lithosph?renforschung, Justus-Liebig-Universit?t Gie?en, Senckenbergstrasse 3, 35390, Giesen, Germany
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Abstract: | Fluid inclusions hosted in quartz and specular hematite from auriferous (jacutinga) and barren veins in the Quadrilátero Ferrífero (QF) have been studied using conventional and near infrared microscopy, respectively.
The mineralization consists of veins that cross-cut metamorphosed iron formation (itabirite) of the Paleoproterozoic Itabira
Group. The sample suite comprises hematite from veins from the low-strain domain in the W and SW of the study area, as well
as hematite samples from the eastern high-strain domain in the central and NE parts of the QF. Halogen ratios of fluid inclusions
in quartz and hematite from all studied deposits are consistent with a fluid evolved from dissolving and reprecipitating halite
that was subsequently diluted. Fluid inclusions hosted in quartz and hematite are characterized by consistent Na/K ratios
and considerable SO4 contents, and suggest similar formation conditions and, perhaps, fluid origin from a common source. Na/K and Na/Li fluid
mineral geothermometers indicate water–rock interaction at approximately 340±40°C. Hematites from the high-strain domain contain
fluid inclusion assemblages of high-temperature aqueous-carbonic and multiphase high-salinity, high-temperature aqueous inclusions
probably due to fluid immiscibility in the system H2O–NaCl–CO2. Fluid inclusions hosted in hematite from barren veins in the low-strain domain, as well as in hematite from jacutinga-type mineralization from the central part of the QF, only host multiphase aqueous fluid inclusions all showing narrow ranges
of salinity (7.2–11.7 wt.% NaCl equiv.) and homogenization temperatures (148 to 229°C). Lower homogenization temperatures
and the absence of CO2-rich inclusions in specular hematite from these occurrences are attributed to carbonate precipitation and/or CO2 escape due to cooling during fluid migration from the high- to the low-strain domain. Pb–Pb and U–Pb systematics of gold,
hematite and hematite-hosted fluid inclusions in combination with geochemical evidence indicate distinct sources for Pd, Au,
and Pb. The formation of specular hematite veins may be related to retrograde metamorphic fluids being released during the
Brazilian orogenic cycle (600–700 Ma). The Pb isotopic characteristics of all samples are readily reconciled in a simple model
that involves two different Paleoproterozoic or Archean source lithologies for lead and reflects contrasting depths of fluid
percolation during the Brasiliano orogeny. |
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Keywords: | Fluid inclusions Infrared microscopy U– Pb Jacutinga veins Au– Pd– Pt Minas Gerais |
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