Stable carbon isotope and organic carbon characteristics of Um Ghamis and Um Nar banded iron formations, Central Eastern Desert, Egypt: are Precambrian biosignatures?

Ibrahim A. Salem, Mohamed M. Hamdy

Geology Department, Faculty of Science, Tanta University, Egypt

Banded iron formations (BIFs) from Um Ghamis and Um Nar occur within mélange matrix of metavolcanosediment and mica schist rocks, respectively, where accumulation of the Fe-rich sediments was supposed to be related to the submarine island arc volcanic activity. Stable carbon isotopes, organic carbon (OC) content and mineralogy of carbonates and carbon in these BIFs were studied in an attempt to find biosignatures which probably refer to a Precambrian life.

Carbon in the studied BIFs can be tentatively separated into three groups. The first group is constrained by δ13C values between −1.6 and −9.3‰ for the Um Nar BIF. This range supposed a deep-seated or magmatic to post magmatic fluid source for a CO2 carbon. The second group includes δ13C from −10.8 to −15.5‰, which is dominated by most samples from the Um Ghamis BIF, is consistent with the δ13C values of the metamorphic carbon. The last group includes δ13C results from −17.5 to −25.5‰ are consistent with the δ13C range for the sedimentary organic carbon.

Um Ghamis BIF, presence of graphite associating siderite and not included within apatite concludes that the graphite mostly was not formed by thermal decomposition of carbonates but its origin is mostly biogenic. The predominant δ13C values of the metamorphic carbon range and the presence of graphite constrain the role of metamorphism in the generation BIFs. Low δ13C values in the range of organic carbon in some samples point to the isotopic fractionation between carbonaceous materials with the metamorphic carbonate.

In Um Nar BIF, the δ13C values of −1.6 to −9.3‰ and the presence of calcite interstitial particularly within Si-rich bands indicating that iron bands were subjected to carbonate alteration due to magmatic or hydrothermal intrusion. The OC content is inversely proportional to the CO2 content, concluding that microorganisms (might be bacteria) mostly lived in BIFs by reduction of carbonate to organic carbon.