Karst-bauxite formation during the Great Oxidation Event indicated by dating of authigenic rutile and its thorium content

Aluminium (Al)-rich palaeosols—i.e., palaeobauxite deposits—should have formed in karst depressions in carbonate sequences as a result of acidic solutions from oxidative weathering of

sulfide minerals during the Great Oxidation Event (GOE), but no GOE-related karst-palaeobauxite deposits have so far been recorded. Here, we report results of in situ uranium–lead (U–Pb)

dating of detrital zircon and spatially associated rutile from a metamorphosed Al-rich rock within a dolomite sequence in the Quadrilátero Ferrífero (QF) of Minas Gerais, Brazil, known as

the Gandarela Formation. Rutile grains are highly enriched in thorium (Th = 3–46 ppm; Th/U ratio = 0.3–3.7) and yielded an isochron, lower-intercept age of ca. 2.12 Ga, which coincides with

the final phase of the GOE—i.e., the Lomagundi event. The rutile age represents either authigenic growth of TiO2 enriched in Th, U and Pb during bauxite formation, or subsequent rutile

crystallisation during metamorphic overprint. Both cases require an authigenic origin for the rutile. Its high Th contents can be used as a palaeoenvironmental indicator for decreased soil

pH during the GOE. Our results also have implications for iron (Fe)-ore genesis in the QF. This study demonstrates that in situ U–Th–Pb-isotope analyses of rutile can place tight constraints

on the age and nature of palaeosols.

Rutile is a common accessory mineral of detrital origin in sedimentary and metasedimentary rocks. Detrital rutile has characteristically low contents of Th, generally less than 0.5 ppm Th1.

The detrital nature of rutile reflects the very low solubility of rutile in H2O2, which in turn explains much of the Ti content as detrital rutile in residual products of deep weathering,

such as bauxite deposits. Titanium has been demonstrated to be immobile in bauxite deposits, despite the formation of authigenic TiO2 as anatase during bauxitisation3.

Bauxitisation requires the removal of Fe from soils as Fe+2, at low Eh conditions that are compatible with pre-2.4-Ga levels of atmospheric oxygen, before the GOE4, recently redefined as the

‘Great Oxidation Episode’5. While bauxitisation is possible under non-oxidative weathering, some Archaean examples of which have been recorded as metamorphosed rocks rich in Al6, the

formation of authigenic anatase in bauxitic profiles implies oxidative weathering of ilmenite (FeTiO3), a major detrital source of Ti in present-day soils7. Authigenic anatase, as observed

in present-day soils, should also have existed in palaeosols, where it is expected to be subsequently converted to rutile during prograde metamorphism8. Authigenic rutile has nonetheless

gone unrecognised in metamorphosed bauxitic palaeosols.

Bauxitic palaeosols formed in karst depressions have not been documented during the GOE6. The apparent absence of karst bauxite is at odds with the GOE oxidative weathering, whereby the

oxidation of sulfide minerals would have generated acidic terrestrial waters9. The latter would have produced karst depressions in carbonate sequences, where karst bauxites could have

developed with relative Ti enrichment due to the formation of authigenic TiO2. Here, we describe for the first time the occurrence of authigenic TiO2 as rutile with high contents of Th in a

metamorphosed karst bauxite from the Gandarela Formation of the QF. We demonstrate that authigenic rutile can be recognised by conventional reflected-light microscopy, and used as a

timepiece of oxidative weathering during the GOE by applying in situ LA–ICP–SF–MS (laser ablation–inductively coupled plasma–sector field–mass spectrometry) for U–Th–Pb isotopes. We further

indicate implications for the Fe-ore genesis in the QF of Minas Gerais, a world-class Fe-ore district in Brazil.

The study area is a decommissioned dolomite quarry on the outskirts of Belo Horizonte (Fig. 1). The quarry, known as Acaba Mundo, exposes dolomitic rocks of the Gandarela Formation, below

which is an itabirite sequence, the Cauê Itabirite of Dorr10, likely deposited at 2.65 Ga11. Itabirite is a metamorphosed rock of alternating bands rich in either hematite or magnetite, and

bands rich in gangue minerals, mostly quartz. The Cauê Itabirite and the Gandarela Formation, which lies with gradational contact on the former, comprise the Itabira Group of Dorr10, or the

Itabira iron formation of Harder and Chamberlin12. The Cauê Itabirite forms ridges that host Fe-ore deposits in the QF. One of such ridges is Serra do Curral, on the southern flank of which

is Águas Claras, an itabirite-hosted world-class Fe-ore deposit13. In its vicinity, on the northern flank of the ridge, is the Acaba Mundo dolomite quarry.

Location of the study area (Acaba Mundo) in the geological context of the Quadrilátero Ferrífero of Minas Gerais. The map is adapted from that presented in Rosière et al.33, following the

work of Dorr10 and Harder and Chamberlin12. Metamorphic zones, based on the distribution of amphibole minerals in itabirite42, are as follows: Gru grunerite, Cum cummingtonite, Act

actinolite, Tr–Act tremolite–actinolite.

The quarry dolomite rocks are delimited to the north by predominantly clastic rocks of the Piracicaba Group, which disconformably overlies the Itabira Group. The disconformable contact

between the two groups is an erosional surface10, the age of which has been constrained at 2141 ± 6 Ma by U–Pb dating of zircon grains from a deeply weathered sequence of pillow lavas14.

This Palaeoproterozoic age is interpreted as the subaqueous volcanism that covered the erosional unconformity between the Itabira and the Piracicaba groups. The subaqueous volcanism occurred

either shortly before or at the onset of the final phase of the Transamazonian orogeny, marked by gneiss-dome emplacement at 2095 ± 65 Ma15. Another orogenic event, the Brasiliano orogeny,

was superimposed between 0.62 and 0.50 Ga, this time span being determined in the south-eastern QF16.

The sample material is a dolomite-hosted body of non-foliated metamorphic rock. It has cm-long laths of a kyanite-like mineral. The mineral is extensively altered to a soft, fine-grained

mineral assemblage in shades of light green to grey. The rock was sampled from the dump of an exploratory adit that was driven into the quarry dolomite. The excavated material from the adit

was too aluminous for dolomite mining.

The aluminous rock within the dolomite sequence of the Gandarela Formation has pyrophyllite, diaspore, muscovite and kyanite as the main mineral components (Supplementary Information Fig. S1

and Table S1). Pyrophyllite and diaspore occur as overprints on kyanite (Fig. 2). Rutile, a widespread accessory mineral in the rock (Fig. 3a,b), exhibits its distinctive internal

reflections, abundant and very bright17, as well as its characteristic geniculated twin (Fig. 3c). This elbow twin may form elongated protrusions, reaching about 0.5 mm in length (Fig. 3d).

Raman spectra confirmed the reflected-light identification of rutile (Fig. 4). Another omnipresent accessory mineral is zircon (Fig. 3a,b), which is typically euhedral to round and shows no

outgrowths.

Backscattered-electron (BSE) image of an Al-rich rock. Pyrophyllite (py) and diaspore (dia) occur as overprint on kyanite (ky), with which rutile (rt) is spatially associated. Some rutile

grains have lamellae and patches of a Cr–Fe-oxide mineral (white).

Photomicrographs of rutile in a kyanite–muscovite–diaspore–pyrophyllite rock. (a) Rutile grains show brownish-red colour in plane-parallel transmitted light. (b) The same rutile (rt) grains

depicted in a appear grey in reflected light (air). Dashed circles in a and b denote a zircon (zrc) grain. (c) Twinned rutile (centre, reflected light, oil immersion). (d) Rutile and its

protrusion stemming from a twinned contact (centre, reflected light, air).

Raman spectra of three rutile grains, rutile 1, 2 and 3, which occur in the Al-rich rock of Figs. 2 and 3, in comparison with Raman spectra of TiO2 polymorphs—i.e., anatase, brookite and

rutile.

The thin section investigated has a total of 20 zircon grains that could be measured by LA–ICP–SF–MS, some of them twice (Supplementary Information Table S2). Most analyses yielded

discordant U–Pb ages, but seven grains returned concordant ages between ca. 3000 and ca. 2670 Ma (Fig. 5a). The youngest zircon has an age of 2666 ± 23 Ma. Measurements of 30 rutile grains

gave variable 238U/206Pb ratios, with 28 analyses defining an isochrone with a lower-intercept 206Pb/238U age of 2124 ± 100 Ma (Fig. 5b; Supplementary Information Table S3). All grains show

high Th contents (3–46 ppm) and elevated ratios of Th/U (0.33–3.75) and Pb/U (2–16).

Concordia and Tera-Wasserburg plots for zircon (a) and rutile (b), respectively.

The unusually high whole-rock content of Al, mineralogically represented by kyanite that was altered to pyrophyllite, diaspore and muscovite, and the paucity of quartz and Fe-rich minerals

indicate that Al was residually enriched in the protolith. Residual Al enrichment needs to be reconciled with Fe removal within the dolomite sequence of the Gandarela Formation. A likely

reconciliation scenario is bauxitisation during karst weathering3. The karst-bauxite scenario is corroborated by the restricted occurrence of Al-rich minerals in pockets within the dolomite,

and the detrital nature of zircon grains with a wide range of concordant U–Pb ages, between ca. 3000 and ca. 2660 Ma. These data imply that: (1) Archaean granite-gneiss basement rocks

provided detritus to karst depressions; and (2) part of the Gandarela dolomite sequence was exposed to karst weathering and subsequent bauxitisation.

Bauxitisation under oxidative weathering is expected to form authigenic TiO2, which is a residual product of ilmenite oxidation and progressive Fe removal7,18. Coarsening of authigenic TiO2

as nanocrystalline anatase particles takes place at higher temperatures and involves twinning and rutile growth, as modelled from kinetic experiments19. Twinning and rutile growth during

prograde metamorphism would then be expressed in the widespread occurrence of coarse-grained twinned rutile in the metamorphosed aluminous rock (Fig. 3), interpreted to have been a

karst-bauxite deposit. The rutile age of 2124 ± 100 Ma reflects the timing of either authigenic TiO2 crystallisation during karst-bauxite formation, or its subsequent metamorphic overprint.

The authigenic nature of the rutile grains can further be examined considering their high Th contents (3–46 ppm), resulting in elevated Th/U ratios (up to 3.7), far higher than those

obtained from detrital rutile grains supplied from magmatic or metamorphic rocks (mostly  1 ppm) in authigenic rutile might be an indicator of decreased soil pH, possibly involving organic

acids, during the GOE. It should be noted, however, that Th solubilities also increase in alkaline CaCl2 solutions by six orders of magnitude from pH 10 to pH 12 at temperatures between 17

and 25 °C31.

An upgrade in the Fe content of itabirite to form high-grade orebodies of massive, hard hematite has been attributed to the metamorphic overprint of the Transamazonian orogeny, an

interpretation supported by a U–Pb age of 2034 ± 11 Ma for monazite that coexists with granoblastic hematite after magnetite32. The Fe upgrade involved leaching of gangue minerals by

hydrothermal fluids33. Much of the leaching of gangue minerals could have occurred in itabirite domains exposed to weathering, concomitant with the ca. 2.14-Ga bauxitisation of terrigenous

material that filled karst depressions in the overlying dolomite sequence of the Gandarela Formation, as indicated by the results of this study.

Authigenic rutile from an Al-rich rock is characterised by coarse-grained twinned crystals that are considerably younger than spatially associated grains of detrital zircon. Recognising the

authigenic nature of rutile in extremely aluminous rocks enables its use as a palaeoenvironmental timepiece because: (1) authigenic TiO2 is formed from the oxidative weathering of ilmenite,

as recorded from modern weathering profiles; (2) high Th contents (> 1 ppm) attest to either acidic or alkaline soil conditions; (3) U–Pb dating of authigenic rutile and detrital zircon

places age constraints. These criteria define the extremely Al-rich rock as metamorphosed karst bauxite, originally developed during the GOE over the uplifted part of the Gandarela dolomite

sequence. High Th contents in authigenic rutile are consistent with decreased soil pH during the GOE.

One sample of the aluminous rock was cut for polished-thin-section preparation; an aliquot of it was milled for powder X-ray diffraction (XRD), performed using a PANalytical X’Pert Pro

instrument, with a CuKα source and a proportional point detector (PW 3011/20). The instrument, housed at the Instituto de Geociências, Universidade Federal de Minas Gerais, was operated at

40 kV and 45 mA. Data collection was in Bragg–Brentano geometry from 5° to 69° (2θ), with 0.02° (2θ) steps at 0.5 s per step. All XRD data are presented in Supplementary Information Table S1

and Fig. S1.

Raman spectra were collected on three representative rutile grains using a Bruker Senterra spectrometer, coupled to an Olympus BX51 light microscope, at the Institute of Applied Geosciences,

Karlsruhe Institute of Technology (KIT), Germany. A 532-nm laser, the power of which was 2 mW, with aperture of 50 µm and total time of 30 s (3 s times 10 coadditions), was focused onto

specimens with a 20×-microscope lens (OLYMPUS M-PLAN 20x), resulting in a spot diameter of approximately 5 µm on the sample surface.

Measurements for U–Th–Pb isotopes in zircon and rutile were performed in situ on a polished thin section, using a 193-nm ArF Excimer laser (Analyte Exite+, Teledyne Photon Machines), coupled

to a Thermo-Scientific Element XR instrument at the KIT. Twenty zircon grains,