Extreme fractionation of selenium isotopes and possible deep biospheric origin of platinum nuggets from Minas Gerais, Brazil
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https://doi.org/10.1130/G49088.1
Manuscript received 25 March 2021
Revised manuscript received 20 May 2021
Manuscript accepted 22 May 2021
© 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license.
Extreme fractionation of selenium isotopes and possible deep
biospheric origin of platinum nuggets from Minas Gerais, Brazil
Alexandre Raphael Cabral1,2*, Stephan König3,4*, Benjamin Eickmann3,5, Michael Brauns6, Miguel Tupinambá7,
Bernd Lehmann8 and María Isabel Varas-Reus3
1
entro de Pesquisa Professor Manoel Teixeira da Costa, Instituto de Geociências, Universidade Federal de Minas Gerais (UFMG),
C
Belo Horizonte-MG, 31270-901, Brazil
2
Centro de Desenvolvimento da Tecnologia Nuclear (CDTN), Belo Horizonte-MG, 31270-901, Brazil
3
Isotope Geochemistry, Department of Geosciences, University of Tübingen, 72076 Tübingen, Germany
4
Instituto Andaluz de Ciencias de la Tierra (IACT), Consejo Superior de Investigaciones Científicas and Universidad de Granada,
Avenida las Palmeras 4, Armilla, 18100 Granada, Spain
5
Departamento de Geociencias, Universidad de los Andes, Carrera 1#18A-12, 111711 Bogotá, Colombia
6
Curt-Engelhorn-Zentrum Archäometrie (CEZA), 68159 Mannheim, Germany
7
Tektos–Geotectonic Research Group, Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rio de Janeiro-RJ,
20550-050, Brazil
8
Mineral Resources, Technische Universität Clausthal, Adolph-Roemer-Strasse 2a, 38678 Clausthal-Zellerfeld, Germany
ABSTRACT 2000; Schilling et al., 2020). Elemental Se has
Platinum-rich nuggets offer an opportunity for understanding how precious metals ac- a large Eh-pH (Eh—activity of electrons) stabil-
cumulate. We analyzed the selenium (Se) isotopic composition of Se-rich (102–103 µg g−1) ity field and coexists with metallic Pt, whereas
platinum-palladium (Pt-Pd) nuggets from a recent placer deposit in Minas Gerais, Brazil, sulfur (S) occurs as soluble sulfate (Fig. S1 in
for which a biogenic origin has been inferred. We obtained Se isotopic values with a relatively the Supplemental Material1). It is thus expected
narrow range (δ82/76SeNIST3149 = −17.4‰ to −15.4‰ ± 0.2‰, two standard deviations [2 SD]). that supergene Pt and other native metals, such
The Pt-Os age of the nuggets is 181 ± 6 Ma (2 SD). The data indicate that the nuggets did not as silver and copper, incorporate Se0. We report
form in the recent placer deposit, but by replacement of hydrothermal vein minerals at ∼70 Se isotopic compositions and Pt-Os ages for Pt-
°C and at least 800 m below the surface. The high abundance and extreme isotopic composi- rich nuggets recovered from a placer deposit
tion of Se as well as the presence of other biophilic elements like iodine, organic carbon, and in Brazil. Our data provide evidence that the
nitrogen within the nugget matrix are consistent with a microbial origin. Although abiogenic neoformation of Pt-rich nuggets did not occur
reduction of Se oxyanions cannot be ruled out, the nuggets plausibly record Se-supported in the surface environment. Instead, they likely
microbial activity in the deep biosphere. record biogenic metal fixation at depth, ∼800 m
below the surface or even deeper, in response to
INTRODUCTION covering Pt-rich nuggets could indicate biogen- groundwater dissolution of Se-bearing, Pt-rich
The origin of platinum (Pt) nuggets in placer ic supergene formation (Reith et al., 2016), but vein minerals.
deposits and lateritic profiles worldwide remains biofilms are surface coatings that postdate the
debated, with two contrasting origins being pro- nugget formation and, therefore, may not nec- SAMPLES AND GEOLOGICAL
posed: (1) magmatic, formed in ultramafic rocks essarily reflect the nugget-forming process. The SETTING
(Hattori and Cabri, 1992; Oberthür et al., 2017); isotopic composition of within-nugget elements, Platinum-palladium (Pt-Pd) nuggets were
and (2) neoformation under supergene condi- such as selenium (Se), may help in discriminat- sampled from the placer deposit of Córrego
tions (Cousins and Kinloch, 1976; Bowles and ing between abiogenic versus biogenic origins. Bom Sucesso, Diamantina-Serro region, in the
Suárez, 2021). The former is based on the me- Selenium is an essential nutrient for micro- platiniferous Au-Pd belt of Minas Gerais, Brazil
chanical resistance of Pt-rich alloys and their organisms, and Se isotopes have been used to (Cabral et al., 2009). Cambrian quartz veins,
low solubility, essential in causing residual con- reconstruct redox conditions and to identify hosted in Paleoproterozoic quartzite (Fig. 1A),
centrations in placer deposits, whereas the latter microbial processes in ancient and modern contain specular hematite, palladiferous gold,
requires dissolution and precipitation processes, environments (Stüeken, 2017; Schilling et al., and Pt-Pd minerals such as arseno-antimonides
with or without biogenic interactions. Biofilms 2020). Selenate- and selenite-reducing bacte- and selenides (e.g., jacutingaite, Pt2HgSe3). The
ria produce Se0 that is isotopically lighter than Bom Sucesso deposit is the locality where Pd
*E-mails: arcab@ufmg.br; skonig@gmx.de the starting Se-oxyanion pool (Herbel et al., was originally identified (Wollaston, 1809;
1
Supplemental Material. Methods and data tables. Please visit https://doi.org/10.1130/GEOL.S.14842731 to access the supplemental material, and contact editing@
geosociety.org with any questions.
CITATION: Cabral, A.R., et al., 2021, Extreme fractionation of selenium isotopes and possible deep biospheric origin of platinum nuggets from Minas Gerais,
Brazil: Geology, v. 49, p. XXX–XXX, https://doi.org/10.1130/G49088.1
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A D
E
B
Figure 1. (A) Geological map of the study area, Córrego Bom Sucesso, Minas Gerais, Brazil (by A.R. Cabral and M. Tupinambá; UTM coordinates,
datum Córrego Alegre). (B) Photograph of the site where Pt-Pd nuggets occur; i.e., underneath boulders fallen from a precipice. Standpoint and
view direction are shown on the map in A. (C) Sketch of rockfall shown in B. (D,E) Backscattered-electron images of Pt-Pd nuggets, showing
typically botryoidal aggregate (D), the surface of which is Pt-rich, Pd-poor rim, and biofilm coating (E, light gray, center) on pitted, Pt-rich rim.
Cassedanne and Alves, 1992). The platiniferous just east of the placer deposit (Fig. 1A) has the of the Diamantina region were fractured. Af-
deposit is located between a potholed bedrock preferential direction of the Triassic–Jurassic ter the Jurassic, denudation exhumed veins and
of quartzite and boulders fallen from a quartzite Transminas mafic swarm, which marks the onset their quartzite host rock (Barreto et al., 2013;
precipice along which the stream runs (Figs. 1B of the breakup of West Gondwana (Chaves and Amaral-Santos et al., 2019). Following the col-
and 1C; Cassedanne et al., 1996; Cabral et al., Correia Neves, 2005). During the emplacement lapse of the precipice in the Holocene, the Pt-Pd
2011a). A north-northwest–trending mafic dike of the dike swarm, veins and supracrustal rocks nuggets that had formed within the Cambrian
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quartz veins in the Jurassic, as determined in this METHODS AND RESULTS positive covariations with Se (115–1278 µg
study, were released to the placer deposit. Min- Given the Se distribution as an alloying com- g−1), Pd (5–24 wt%), and thallium (Tl,1–33 µg
eralized veins have not been found in the study ponent of the Pt-Pd matrix (Cabral et al., 2019), g−1) concentrations and a negative covariation
area, and consequently no vein selenide miner- we investigated eight nuggets for their Se isoto- with Os/Ir ratios, but are unrelated to size and
als have been sampled. That such a mineralized pic compositions using a 74Se-77Se double-spike, mass of the nuggets. Measured Se isotopic com-
vein system existed is deduced from abundant hydride-generation, multicollector–inductively positions are even lighter than the lightest Se
specular hematite and, particularly, arborescent coupled plasma–mass spectrometry (MC-ICP- isotopes reported from weathered seleniferous
palladiferous gold in the placer deposit (Cabral MS) analytical method at the Isotope Geochem- and organic-rich shales of the Permian Maok-
et al., 2008a). istry laboratory of the University of Tübingen ou Formation in China (δ82/76Se = −14.2‰ to
The Pt-Pd nuggets typically have a Pt-rich (Germany; Kurzawa et al., 2017; Yierpan et al., +11.4‰; Zhu et al., 2014; Fig. 2), which have
and Pd-depleted rim (Cassedanne et al., 1996), 2018; see the Methods section of the Supple- hitherto been regarded as the lowest δ82/76Se val-
and are botryoidal (Fig. 1D; Hussak, 1904; mental Material). Another sample set of five ues in natural deposits and the largest δ82/76Se
Cassedanne and Alves, 1992; Cabral et al., Pt-Pd nuggets, which are from the same col- range. Such a large range, or an isotopically
2009). The Pt-Pd nugget matrix is seleniferous lection used for Se isotopes, were individually heavier complementary reservoir, is entirely
(102–103 µg g−1 Se) and contains minor mer- measured to calculate 190Pt-186Os ages (Table S1 absent in our nuggets.
cury (103–104 µg g−1 Hg), but lacks iron (Fe), in the Supplemental Material). The ages are,
which identifies magmatic Pt-Fe alloys (Cabral within error, indistinguishable from each other DISCUSSION
et al., 2019). A within-placer origin for the Pt- at 181 ± 6 Ma (2 SD). Their contents of radio- Combined with the 190Pt-186Os ages, the
Pd nuggets has been proposed (Hussak, 1904; genic Os are in the range from 23 to 26 ng g−1, δ82/76Se values of the Pt-Pd nuggets point to a
Cabral et al., 2009), given that their botryoidal while their 187Os/188Os ratios are between 0.90 unidirectional formation process during which
and arborescent morphologies would not have and 1.30 (Table S1). Se, derived from Se-bearing vein minerals, was
resisted alluvial abrasion. In addition, high abun- The Pt-Pd nuggets yielded a narrow distribu- incorporated in the Jurassic. Assuming that the
dances of iodine in the Pt-Pd matrix (102 µg g−1 tion of negative δ82/76Se values—i.e., deviation initial aqueous fluid had a δ82/76Se value of 0‰,
I; Cabral et al., 2011b), where organic carbon of sample 82Se/76Se relative to reference solution reflecting water equilibrated with the upper
and nitrogen are heterogeneously distributed NIST3149 (U.S. National Institute of Science continental crust (Fig. 2A; Stüeken, 2017; and
(Reith et al., 2016), as well as nugget-covering and Technology; see the Supplemental Mate- crustal 187Os/188Os ratios between 0.90 and 1.30;
biofilms (Fig. 1E; Reith et al., 2016), suggest a rial)—between −17.4‰ and −15.4‰ (±0.2‰, Table S1), the highly negative δ82/76Se values in
biogenic origin. 2 SD; Fig. 2; Table S2). The δ82/76Se values show the nuggets require a large isotopic fractionation
process and a small extent of Se-oxyanion re-
duction—e.g.,a biogenic origin can thus be further discussed where sulfate-reducing bacteria were isolated
without additional information. A from heated groundwater at 50 °C (Moser et al.,
The heterogeneous distribution of organic 2003). Supporting evidence for the presence of
carbon and nitrogen in some Pt-Pd nuggets microbial activity in a terrestrial deep biosphere,
(Reith et al., 2016), together with high iodine related to fracture-controlled groundwater flow,
abundances (Cabral et al., 2011b) and the close has been found at 2400 m depth in 2.7 Ga rocks
association of Se and phosphorus (Reith et al., of the Canadian Shield (Li et al., 2016). It is
2019), all indicate that the δ82/76Se values reflect therefore reasonable that metal-tolerant, Se-oxy-
a biogenic pathway. Consequently, we infer that anion-reducing bacteria were thriving in metal-
the formation of the Pt-Pd nuggets was microbi- liferous solutions in deep-seated groundwater.
ally mediated. Such a process is analogous to The covariation between nugget δ82/76Se values
the formation of framboidal pyrite, which has and concentrations of Tl (Fig. 2C), a highly toxic
highly negative δ34S values and organic carbon B element, further suggests that the Tl concentra-
and nitrogen in the pyrite matrix, identifying tions were critical and eventually reached a le-
microbial sulfate reduction (Wacey et al., 2015). thal level for the Se-oxyanion-reducing bacteria
The Pt-Pd nuggets show a δ82/76Se variability in this niche environment.
of only 1.7‰, regardless of their size and mass Denudation since the Jurassic brought the Pt-
(4.33–18.38 mg). Subtle covariations of δ82/76Se Pd nuggets to the near surface where the precipice
values with selected trace metals, which are dis- collapsed. The collapse-triggering rupture fol-
seminated in our Pt-Pd nuggets (Figs. 2B–2D), lowed the fractured vein system in the country-
support a single incorporation mechanism rather rock quartzite. Subsequently, the Pt-Pd nuggets
than a two-step process. For example, the cor- were liberated from friable material, quartz vein
relation between δ82/76Se and Os/Ir ratios implies and immediate host-rock quartzite, and redistrib-
that Se-oxyanion reduction involved Os release C uted in the nearby placer deposit (Fig. 3C). Con-
to the fluid phase relative to less-mobile Ir in trary to supergene neoformation within alluvial
a unidirectional process (see also Reith et al., sediments (Cabral et al., 2011b; Reith et al., 2016,
2016). Moreover, preferential removal of 82Se by 2019), our data suggest that the Pt-Pd nuggets
oxidative weathering, as experimentally shown resulted from deep microbial activity.
by Wasserman et al. (2021), is inconsistent with
the observed negative covariation between Os/ CONCLUSION
Ir and δ82/76Se. Rather than complex, multiple- The Pt-Pd nuggets of Córrego Bom Sucesso
stage Se redox events in the weathering zone, were likely microbially mediated and not formed
as expected from the seasonal variation of the Figure 3. Sketch illustrations of: vein by supergene accretion within the placer deposit.
emplacement at ca. 500 Ma (Cabral et al., 2013)
near-surface water table, the 190Pt-186Os age of (A); vein fracturing and formation of Pt-Pd
In combination with 190Pt-86Os ages and trace
ca. 180 Ma obtained for the Pt-Pd nuggets in- nuggets at ca. 180 Ma in the deep biosphere, elements, Se isotopes can be reconciled with
dicates that they formed at ∼800 m below the after dissolution of vein Pd-Pt minerals and biogenic Se fixation. Taken together, our data
surface, assuming a constant average denudation precipitation either in situ or adjacent to vein can further be interpreted as indicative of Se-
rate of 4.4 m m.y.−1, as estimated by cosmogenic system (B); and host-rock quartzite and its sustaining microbial life in the deep biosphere.
vein system with Pt-Pd nuggets exhumed to
beryllium for the Diamantina region (Barreto the level of the precipice, which collapsed
et al., 2013). This assumption is supported by and locally redistributed nuggets in alluvial ACKNOWLEDGMENTS
material underneath rockfall boulders (shown The Se isotopic work was financially supported by
thermochronological modeling using apatite
in dark gray), with minimal transport from the European Research Council (ERC) Starting Grant
fission-track (AFT) data from basement granit- O2RIGIN (636808) to S. König. A. Cabral acknowl-
source to sink (C).
ic-gneissic rocks (Amaral-Santos et al., 2019). edges a Rudolf-Vogel-Preis (TU Clausthal, Germany),
used as a startup fund that was subsequently com-
Nugget Formation in the Deep Biosphere iron (Fe), metals dispersed in serpentinite, where plemented by a Deutsche Forschungsgemeinschaft
(DFG) project (LE 578/29-1) to B. Lehmann. M.
Microstructural and compositional evidence nanosized particles of Ni-Fe alloy are reported Varas-Reus also acknowledges DFG (project VA
from the southern part of the belt indicates that to occur with organic matter in the deep bio- 1568/1-1). Rogerio Kwitko-Ribeiro is thanked for
vein Pt-Pd minerals, including jacutingaite sphere (∼10,000 m below the seafloor; Plümper having generated the backscattered-electron images
(Pt2HgSe3) and hongshiite (PtCu), are the pre- et al., 2017). Relatively high Se-oxyanion abun- of the Pt-Pd nuggets. Nicole Lockhoff (CEZA,
Mannheim, Germany) determined Pt concentrations
cursors of Pt-Pd concentrations engendered by dances in a nutrient-poor, quartz-rich environ-
in sample digests. Ronny Schoenberg (University of
the removal of less-noble metals; e.g., Se and ment would provide a suitable source of energy Tübingen, Germany) is thanked for continuous sup-
Cu (Cabral et al., 2008b). It is thus likely that for microbial life (Schilling et al., 2020). This port and discussion. Critical appraisals by Eva Stüeken
the dissolution of precursor vein minerals pro- would have occurred at ∼70 °C, as estimated by and two anonymous reviewers have greatly improved
duced Pt-Pd nuggets within the veins at ∼800 m AFT thermal modeling for the basement rocks of the manuscript. Gerald Dickens’s diligent editorial
handling is appreciated.
or even deeper (Fig. 3A). Conditions of rela- the Diamantina region, at ca. 180 Ma (Amaral-
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