New analytical developments have made radiogenic helium (He-4) applicable to archeological gold artifacts for age determinations. Here we report the application of the U/Th-He-4 method to the direct dating of gold from the historically important gold deposit in Diamantina, Minas Gerais, Brazil. The U/Th-He-4 age of 515 +/- 55 Ma for the Diamantina gold is corroborated by a new U/Pb age of 524 +/- 16 Ma for rutile recovered from auriferous pockets. These ages tie the Diamantina gold mineralization to the Brasiliano orogenic event, in the context of the Gondwana amalgamation. Our results indicate that U/Th-He-4 dating of gold is possible, opening new perspectives for the dating of gold deposits without assuming contemporaneity between gold and datable hydrothermal minerals.
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"Therefore, the regional mineralising event responsible for the gold-rush gold took place in the Late Cambrian . Our xenotime age is indistinguishable within error from the age obtained by direct dating of gold, 515 ± 55 Ma, from a quartz–hematitelode deposit in Diamantina (Cabral et al., 2013a), in the northernmost part of the approximately 240-km-long belt between Ouro Preto and Diamantina (Fig. 1). The common age for quartz–sulphide lodes (Passagem) and quartz–hematite lodes (Itabira) corroborates the idea of a common origin for fluids that interacted with different rocks at different redox conditions (Boiron et al., 1999; Cabral et al., 2013b). "
[Show abstract][Hide abstract]ABSTRACT: Zircon and xenotime occur in tourmaline-rich hydrothermal pockets in the auriferous lode of Passagem de Mariana, a world-class gold deposit. Zircon grains show pristine oscillatory zoning, but many of them are altered, exhibiting porous domains filled with graphite. Uranium–Pb dating of zircon, using in-situ laser ablation–inductively coupled plasma–mass spectrometry, yields ages between 3.2 and 2.65 Ga, which match those for detrital zircon of the footwall quartzite of the > 2.65-Ga-old Moeda Formation. Discordant analyses point to zircon-age resetting during the Brasiliano orogeny at ca. 500 Ma. This interpretation is supported by U–Pb dating of euhedral xenotime immediately adjacent to altered zircon within the same tourmaline pocket. The xenotime grains give a Concordia age of 496.3 ± 2.0 Ma, which is identical to that determined for monazite of a quartz–hematite vein-type deposit (i.e., jacutinga lode) in the region (Itabira), another important mineralisation style of gold. The occurrence of relatively abundant inherited detrital zircon, but absence of rock fragments in the tourmaline pocket investigated here, implies that detrital material was completely replaced by tourmaline. The graphite overprint on the altered detrital zircon attests to a reducing fluid, which was likely formed by fluid–rock interaction with carbonaceous phyllite of the Batatal Formation, the host rock of the Passagem lode.
"The age of 496 Ma for the jacutinga lode at Conceição, in the southern part of the platiniferous Au-Pd belt of Minas Gerais, is significantly younger than the rutile U-Pb age of 524 ± 16 Ma for auriferous quartz-hematite-rutile veins in Diamantina, in the northern extreme of the belt (Cabral et al., 2013). Gold from the Diamantina veins gave a U/Th-He age of 515 ± 55 Ma (Cabral et al., 2013). With respect to vein emplacement, the time span between the Diamantina rutile age and the Lico monazite age may be explained either by a protracted event of hydrothermal activity, or by episodic events along the belt. "
[Show abstract][Hide abstract]ABSTRACT: Bonanza-style mineralization of palladiferous gold in specular hematite-rich lodes engendered the most important gold rush in Brazilian history, which took place at the end of the seventeenth century in the province that became known as Minas Gerais. The timing of this itabirite-hosted vein mineralization, locally referred to as jacutinga, is controversial; different views have ascribed jacutinga to one of the two orogenic events recorded in the Quadrilátero Ferrífero of Minas Gerais, the ∼2.0 Ga Transamazonian event and the ∼0.6 Ga Brasiliano event. This controversy is due mainly to the absence of reliable age data for hydrothermal minerals. Here we report the first U-Pb age for a jacutinga lode, obtained by laser ablation-sector field-inductively coupled plasmamass spectrometry (LA-SF-ICP-MS) on monazite grains recovered from the Conceição Fe-ore deposit within the Itabira district. The monazite analyses yield a precise Late Cambrian age of 495.6 ± 2.2 Ma, which is well within the time period between ca. 524 and 444 Ma, suggested by recently published geochronologic data on regionally distributed similar hydrothermal systems. The age of 496 Ma for the jacutinga monazite corroborates an orogen-scale hydrothermal overprint at later stages of the Brasiliano event. This hydrothermal overprint also affected surrounding Neoarchean rocks of the Minas Supergroup, as well as younger rocks of the southern Serra do Espinhaço, along the platiniferous Au-Pd belt of Minas Gerais.
"The latter implies a continental source or, at least, fluids that interacted with continental rocks. Assuming a Neoproterozoic age for the regional fluid overprint , which is justified by the structural setting and some radiometric ages (Chauvet et al. 2001; Chaves et al. 2010; Cabral et al. 2013), the barite-δ 34 S values for Antônio Pereira are lighter than those expected for Neoproterozoic sea water (vide above). Instead of sea water, the relatively low bariteδ 34 S values could have been derived from continental evaporites . "
[Show abstract][Hide abstract]ABSTRACT: The distribution of mineral deposits, characterised as barite deposits, hematite-rich auriferous deposits and auriferous tourmaline–sulfide deposits, displays a regional sulfate–hematite–sulfide zoning along the thrust-delineated limbs of the Mariana anticline, in the south-eastern part of the Quadrilátero Ferrífero of Minas Gerais, Brazil. Cross-cut relationships of barite veins and sulfide lodes indicate that sulfidation occurred in a late-tectonic context, which is here attributed to the collapse of the ∼0.6-Ga Brasiliano thrust front. Reconnaissance S-isotopic data from barite and pyrite (Antônio Pereira barite deposit and its adjacent gold deposit, respectively), and arsenopyrite (Passagem de Mariana gold deposit), suggest a new interpretation for the hydrothermal fluid overprint in the Mariana anticline. The Antônio Pereira barite has Δ33S values that are near zero, constraining the sulfate source to rocks younger than 2.45 Ga. The barite-δ34S values are between +19.6 and +20.8 ‰. The Passagem arsenopyrite and tourmaline have Co/Ni ratios that define a positive linear trend with the Antônio Pereira pyrite. The latter has homogenous δ34S values, between +8.8 and +8.9 ‰, which are compatible with thermochemical reduction of aqueous sulfate with the S-isotopic composition of the Antônio Pereira barite.
Full-text · Article · Oct 2013 · Mineralium Deposita