Biogenic non-crystalline U(IV) revealed as major component in uranium ore deposits

Abstract

Historically, it is believed that crystalline uraninite, produced via the abiotic reduction of hexavalent uranium (U(VI)) is the dominant reduced U species formed in low-temperature uranium roll-front ore deposits. Here we show that non-crystalline U(IV) generated through biologically mediated U(VI) reduction is the predominant U(IV) species in an undisturbed U roll-front ore deposit in Wyoming, USA. Characterization of U species revealed that the majority (∼58-89%) of U is bound as U(IV) to C-containing organic functional groups or inorganic carbonate, while uraninite and U(VI) represent only minor components. The uranium deposit exhibited mostly 238U-enriched isotope signatures, consistent with largely biotic reduction of U(VI) to U(IV). This finding implies that biogenic processes are more important to uranium ore genesis than previously understood. The predominance of a relatively labile form of U(IV) also provides an opportunity for a more economical and environmentally benign mining process, as well as the design of more effective post-mining restoration strategies and human health-risk assessment.

The Role of Organic Matter on Uranium Precipitation in Zoovch Ovoo, Mongolia

“…Amorphous UO 2 may be the result of such processes. Indeed, besides complexation and sorption, a third mechanism was recently proposed by [52]: the formation of U(IV) biofilms that evolve into biogenic non-crystalline U(IV) compounds. Thus, using EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy data, these authors revealed the role of microorganisms in the reduction of U(VI) and the formation of disseminated U(IV) in organic matter, acting both as nutrient and as trap.…”

A New Frontier for Palaeobiology: Earth's Vast Deep Biosphere

“…To answer these questions, substantial effort is now required to locate, verify, analyze, and integrate geological records of the deep biosphere in deep time. Recent work has begun to reveal geologically ancient chemical and isotopic signatures of microbial metabolism in the subsurface; fluctuations in these signals through sedimentary rock successions can in principle reveal changes in deep microbial activity over timescales of at least millions of years . Here, we focus instead on the traditional subject matter of palaeobiology: body fossils—the mineralized or organically preserved remnants of organismal structures.…”

Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere

“…Few reported cellular or molecular fossils pass all three tests unequivocally, and most candidates represent subseafloor environments 7 – 10 . Microbially mediated diagenetic phenomena on palaeo-redox fronts offer a potentially powerful alternative record that could extend to continental settings 11 , 12 . Of longstanding interest in this connection are reduction spheroids, which are very common mm–dm-scale bleached spots found most commonly (but not exclusively) in Proterozoic–Phanerozoic red beds, i.e., sedimentary rocks deposited in oxidising terrestrial environments and rich in early diagenetic haematite 13 , 14 .…”

“… 19 ). Thus, uranium phases (both mineralised and non-mineralised) in reduction spheroids and analogous low-temperature redox-front uranium deposits have been shown 12 , 19 to contain predominantly U(IV). Uranium reduction can occur via many pathways 18 , 25 , both abiotic (coupled to the oxidation of various aqueous, mineral, and organic species) and biotic (i.e., enzymatic catalysis by chemolithotrophic microorganisms capable of facultatively utilising U(VI) as an electron acceptor, including iron- and sulphate-reducers).…”

“…Several experimental studies have shown that bacterially reduced and precipitated uranium is isotopically heavier (i.e., records higher δ 238 U) than the dissolved precursor phase 28 – 30 . Bhattacharyya et al 12 recently determined a bacterial origin for isotopically heavy authigenic uranium phases in roll front ore deposits, which resemble reduction spheroids inasmuch as they are mineralised paleo-redox fronts formed at low temperatures in subsurface aquifers. Field studies confirm that modern groundwaters inoculated with metal reducing bacteria become isotopically lighter in uranium as the heavier isotope is preferentially precipitated 31 , 32 .…”