Trace element and isotopic characterization of Neoarchean and Paleoproterozoic iron formations in the Black Hills (South Dakota, USA): Assessment of chemical change during 2.9–1.9 Ga deposition bracketing the 2.4–2.2 Ga first rise of atmospheric oxyge05/01/2008
Banded iron formations (BIFs) were deposited in the Black Hills, South Dakota, prior to and after the first main oxygenation of the atmosphere at ∼2.4–2.2Ga. The oldest, Neoarchean horizon (Nemo Iron Formation) is an oxide-facies, detritus-free BIF deposited at ∼2.9–2.6Ga, in association with mafic phyllite. These BIFs show rare earth and yttrium elemental (REY) signatures typical of ambient seawater, including Pr/Yb < 1 and positive La, Eu, and Y anomalies in Post-Archean-Australian-Shale (PAAS)-normalized REE diagrams, but they lack clear Ce anomalies. Deposition of an Early Paleoproterozoic horizon (Benchmark Iron Formation) is constrained to the interval between 2.56 and 2.48 Ga. These BIFs also show the REE distribution patterns typical of ambient seawater. Benchmark BIFs also exhibit slightly negative Ce anomalies, but BIFs included within the younger, 2.10–2.02 Ga Estes metaconglomerate exhibit mostly positive Ce anomalies, which are probably the result of oxidative supergene remobilization of Ce during transport. Mixed, silicate–carbonate–sulfide–oxide–facies BIFs with slightly elevated detrital inputs were deposited in Middle Paleoproterozoic time, between 2.02 and 1.97 Ga (Homestake Iron Formation and equivalents) and between 1.89 and 1.87Ga (Rochford Iron Formation). These BIFs are characterized by variably fractionated, PAAS-normalized REY patterns, which include: Pr/Yb ratios of 0.2–4.1; absence of La anomalies; and presence of positive Eu, positive and negative Ce, and mostly negative Y anomalies.
Sm–Nd isotopes from individual microbands of the Neoarchean BIF define Eo- to Mesoarchean, Nd TDM ages of 3.6–2.9 Ga, which are generally compatible with crustal residence ages inferred from elsewhere in the Wyoming craton (i.e., in the 2.72–2.67 Ga Atlantic City BIF, Wind River Range). Model ages of microbands from the Early and Middle Paleoproterozoic BIFs are highly variable, ranging from 3.8 to 2.15 Ga. The youngest components in the Neoarchean and Middle Paleoproterozoic BIFs, inferred from the model ages, correspond to associated, mafic metatuffs and metabasalts, which yield ∼2.9 and ∼2.15 Ga model ages, respectively. Further, the oldest (i.e., ≤3.9–3.6Ga) components indicate that Eo- to Mesoarchean crust was exposed during the entire BIF depositional time frame, between ∼2.9 and 1.88 Ga.
Pb isotopes distinguish the Neoarchean from Early and Middle Paleoproterozoic BIFs. Whereas all BIFs reflect the generally high-μ (238U/204Pb) values typical of the Wyoming craton, thorogenic-versus-uranogenic Pb-isotope data arrays constrain sources of Pb in the various iron formations, particularly those deposited in Early and Middle Paleoproterozoic time. Early Paleoproterozoic BIFs exhibit low 208Pb/204Pb relative to 206Pb/204Pb ratios, implying a highly U/Th-fractionated source. This source is compatible with the peraluminous, ∼2.60 Ga Bear Mountain granite exposed in the W-central Black Hills, which itself was derived from even older crust. In contrast, the Middle Paleoproterozoic BIFs define a Pb-isotopic array that is compatible with a moderately fractionated source resembling the metaluminous, ∼2.56 Ga Little Elk granite exposed in the NE Black Hills. The Neoarchean BIFs are generally less radiogenic than their younger counterparts, and plot near the intersection of the two Paleoproterozoic arrays on the thorogenic diagram.
Although the ∼2.9–1.9 Ga Black Hills BIFs bracket the worldwide, 2.4–2.0 Ga BIF depositional gap, which curiously coincides with the coupled, first atmospheric–biospheric oxygenation and subsequent, positive δ13C excursion, we are unable to delineate major changes in BIF geochemical compositions that might reflect concomitant changes in atmospheric oxygenation. However, slightly negative Ce anomalies recorded in the 2.56–2.48 Ga, oxide–facies BIFs, and an overall trend of decreasing, positive Eu anomalies between ∼2.9 and 1.9 Ga among Black Hills BIFs generally (in agreement with the trend observed worldwide), might argue for the onset of bottom-water oxidation at the Archean-Proterozoic boundary and a decrease of high-temperature hydrothermal activity over the worldwide, BIF depositional gap. In the Black Hills, the 2.4–2.0 Ga depositional gap reflects either a ∼400 Myr absence of seawater between incipient, intracratonic rifting of supercraton Superia (or supercontinent Kenorland) at ∼2.5–2.4 Ga and its final breakup at ∼2.1Ga, a ∼400 Myr missing rock record across a mapped unconformity, or both.