Geochemical and isotopic variations in paleoproterozoic BIF and BMF deposits: Implications for the chemistry of Precambrian oceans

Major element geochemistry demonstrates that the Trommald Formation formed as a marine metalliferous deposit, with small contributions of terrigenous material, during the Paleoproterozoic. Two groups of REE patterns are observed from the metalliferous zones of the Trommald Formation. When normalized against localized crustal abundances, one group exhibits significant MREE enrichment and negative cerium anomalies, whereas another group displays flat REE patterns with negligible cerium anomalies. The REE-normalized patterns are geochemical artifacts related to different episodes that produced the protolith and the altered metalliferous zones. The presence of prominent negative cerium anomalies, instead of positive europium anomalies, appears to be a more reliable indication of whether ancient metalliferous deposits were derived from hydrothermal sources. Localized REE crustal-abundances, rather than REE chondritic abundances, should be used as normalizing standards for inferring seawater chemistry because they allow for the identification of processes that affected REE distribution and fractionation in Archean and Proterozoic BIF and BMF deposits.; Multivariate statistics techniques provide a set of tools that can identify geologic processes, identify elements that were geochemically coherent, and quantify mass-balance changes during protolith alteration. Utilizing results provided by statistical mass balance calculations, and combining them with petrologic observations, a tectonic model is proposed that explains the deposition of the Cuyuna North Range (Minnesota, U.S.A.) Trommald Formation banded-iron-formation (BIF) protolith and its subsequent alteration into oregrade deposits. Alteration of the Trommald Formation BIF protolith resulted in a mass increase of 51 ± 37 percent in the altered ore-grade deposits.; A 207Pb/204Pb-206Pb/ 204Pb isochron of Trommald-Formation BIF samples corresponds to an age of 1597 ± 120 Ma. A Pb-Pb isochron based on samples from the Ghanaian Birimian Successions banded-manganese-formation (BMF) produced an age of 2096 ± 56 Ma. In both cases, these ages agree with other radiometric ages in these regions and hence appear to date actual geologic events. The Trommald Formation age records the alteration of the deposits during the post-Penokean ∼1630 Ma Mazatzal Orogeny, whereas the 2096 ± 56 Ma age records the depositional age for the Birimian BMF. 207Pb/204Pb- 206Pb/204Pb isochrons appear to be useful tools for dating BIF and BMF deposits.