Banded iron formations (BIFs) are Precambrian chemical sedimentary rocks marked by a dichotomous layering of iron-rich minerals and chert. Determining the ages of iron-rich minerals is the first step in unraveling the geochemical and environmental conditions that prevailed at the time of their formation. To date, the age of BIFs is determined by dating bracketing stratigraphic units. This is due to the apparent dearth of minerals within BIFs that are suitable for geochronology. This indirect approach often yields imprecise ages arising from, for example, stratigraphic complexities. The present thesis addresses this problem by investigating the possible application of (U-Th)/He and (U-Th)/Ne dating methods in direct dating of sedimentary (or diagenetic?) hematites from the Brazilian Urucum BIF.
The stratigraphic position of this Neoproterozoic deposit is debatable because its bracketing stratigraphic units are poorly dated. Urucum is an attractive target for testing this new geochronologic approach because the deposit is well exposed by mining activities; it consists of very low-grade metamorphic assemblages; it is composed of iron solely in the form of hematite, which makes understanding and interpreting the (U-Th)/He-Ne system simpler; and it also contains interbedded manganese oxides that are amenable for dating by the more thoroughly tested 40Ar/39Ar method. The aim of this work is to refine methods that only have been tentatively applied to date iron oxides and hydroxides. In order to achieve this goal, the interbedded manganese oxides are used to impose limits on the age of the hematites. Dating of manganese oxides and hematites was done using 40Ar/39Ar and (U-Th)/He-Ne, respectively.
The application of 40Ar/39Ar geochronology on interbedded manganese oxides from the Urucum deposit sets a minimum depositional age for the chemical sediments (> 590 Ma). Combination of high resolution mineralogical tools with 40Ar/39Ar geochronology permits unravelling the paragenetic sequence recorded in the manganese oxides into three main episodes: 1) crystallization of a diagenetic to early metamorphic cryptomelane, followed by 2) structurally controlled hydrothermal braunite, pyrite and muscovite, and 3) late crystallization of supergene cryptomelane, lithiophorite, and some pyrolusite.
(U-Th)/He and (U-Th)/Ne geochronology of hematite reveals systematically younger He apparent ages in comparison with Ne apparent ages in the same phase, suggesting preferential He loss by diffusion. Concordant He and Ne apparent ages identify samples in which diffusive loss of both isotopes is either negligible or of identical magnitude. This suggests metamorphic thermal resetting or recrystallization of hematite by an event that overlaps with the hydrothermal event associated with braunite crystallization in the manganese oxide horizons. One possible problem that can hamper direct dating of BIF sequences is the presence of U- and Th-rich phases intimately intergrown with hematites. For the Urucum deposit, the influence on the radio-isotopic system by the presence of coexisting apatite was qualitatively tested and modeled. Unreasonably old (U-Th)/He apparent ages were proved to be caused by parentless He implanted into hematites from now leached apatites.
40Ar/39Ar, (U-Th)/He and (U-Th)/Ne dating of supergene cryptomelane implies that weathering and consequent iron ore enrichment at Urucum is a process that started before 70 Ma and continued throughout the Cenozoic. Sampling at different elevations permit evaluating the timing of precipitation of supergene minerals at progressively greater depths into the profile. This allows estimating the rate of propagation of the weathering front. Helium and neon apparent ages for cryptomelanes are systematically lower than the 40Ar/39Ar ages for the same samples, suggesting preferential loss of the lighter isotopes by diffusive loss.
Rare earth element signatures for the manganese oxides are consistent with the paragenetic sequence established by 40Ar/39Ar geochronology. They also imply that the chemical sediments precipitated in the Urucum proto sea may have had a strong component derived from continental sources with little, or even no hydrothermal input.
The results presented here suggest that combining noble gas geochronology and geochemistry is a potentially important approach that must be further explored to unravel the ages and origin of banded iron-formations through time.