REE BUDGET IN A NEOPROTEROZOIC IRON FORMATION DOMINATED BY ACCESSORY APATITE: WADI KARIM BIF, EASTERN DESERT, EGYPT

Banded iron formations (BIFs) are sedimentary rocks composed mainly of silica and iron-bearing minerals formed from the accumulation of Si and Fe in anoxic environments sustained in Archean-Paleoproterozoic and isolated Neoproterozic oceans. The Fe and Si sources are recognized to have been a combination of hydrothermal, seawater, and detrital inputs, but the relative contribution from each source was variable (Klein, 2005). For individual BIFs, shale-normalized (PAAS) whole-rock REE patterns are traditionally reproduced by mixing proposed sources to quantify chemical inputs into the Precambrian ocean. This assumes that current REE patterns match the primary depositional chemical signatures. However, REEs are highly incompatible in Fe-oxides and chert, leading to a REE budget conundrum and uncertainty over where REEs are predominantly hosted in oxide-facies BIFs.

We present whole rock geochemistry and in situ analyses of REEs in magnetite and apatite from the Wadi Karim BIF (WKB), Eastern Desert, Egypt. This Algoma-type Neoproterozoic BIF (~750 Ma) is intercalated with arc meta-volcanics and thought to have formed due to high hydrothermal activity in a proto-oceanic basin during the breakup of Rodinia (Basta et. al., 2011). Though Fe-oxides constitute 52-80% of the rock, these phases have relatively low REE concentrations (average ΣREEs 10.5 ppm compared to 38 ppm in the whole rock), accounting for only 17-35% of the total REE budget. Chert is unlikely to be a significant host of REEs. Substantial amounts of accessory apatite (0.7-3 modal %), however, contain ΣREEs up to 2800 ppm, on average 70 times more concentrated than magnetite. Including apatite closes the budget in most samples, resolving the disparity between Fe-oxide and whole-rock REE content. PAAS-normalized REE patterns for magnetite, apatite, and whole rock are nearly parallel and characterized by small positive Eu anomalies and HREE enrichment, implying precipitation from a common hydrothermal-seawater solution and insignificant alteration of REE patterns post-diagenesis. Although the apatite REE budget dominates that of WKB, the similarity of its REE pattern with that of magnetite indicates that whole-rock REE patterns can be used to infer the source of BIF components.