Low temperature magnetic properties of the Late Archean Boolgeeda iron formation (Hamersley Group, Western Australia): environmental implications

Abstract

The origin of the iron oxides in Archean and Paleoproterozoic Banded Iron Formations (BIFs) is still a debated question. We report low and high temperature magnetic properties, susceptibility and saturation magnetization results joined with scanning microscope observations within a 35 m section of the Late Archean Boolgeeda Iron Formation of the Hamersley Group, Western Australia. With the exception of two volcanoclastic intervals characterized by low susceptibility and magnetization, nearly pure magnetite is identified as the main magnetic carrier in all iron-rich layers including hematite-rich jasper beds. Two populations of magnetically distinct magnetites are reported from a 2 m-thick interval within the section. Each population shows a specific Verwey transition temperature: one around 120-124 K and the other in the range of 105-110 K. This temperature difference is interpreted to reflect two distinct stoichiometry and likely two episodes of crystallization. The 120-124K transition is attributed to nearly pure stoichiometric magnetite, SEM and microprobe observations suggest that the lower temperature transition is related to chemically impure silician magnetite. Microbial-induced partial substitution of iron by silicon is suggested here. This is supported by an increase in Total Organic Carbon (TOC) in the same interval.

A New Tool for Separating the Magnetic Mineralogy of Complex Mineral Assemblages from Low Temperature Magnetic Behavior

“…As mineral magnetism aims to broaden its reach, contributing for example to the study of the formation of the solar system (e.g., Strauss et al, 2016;Wang et al, 2017), of deep-time terrestrial environments (e.g., Carlut et al, 2015;Slotznick and Fischer, 2016) or of extreme environments of hydrothermal vents (e.g., Toner et al, 2016), it is necessary to accompany these new frontiers with experimental tools adapted to the greater diversity of mineral compositions and greater complexity of mineral assemblages resulting from the superposition of geological and biogeochemical processes through time. Magnetic characterization presents the advantages of a low detection limit (order of 1 ppm) and in situ bulk sample analyses avoiding time consuming physical and chemical separation techniques shown to be biased (e.g., Lagroix et al, 2004;Wang et al, 2013).…”

Petrology of banded iron formation-related country rocks at the northern limit of the Mbalam iron ore district, southeastern Cameroon

“…In the Aguas Claras and Matuca BIF deposits of the Quadrilatero Ferrifero in Brazil the principal iron oxide is heamatite (Spier et al, 2008). This is in contrast to other Archean and Early Proterozoic BIFs with essentially magnetite as the main Fe-oxide mineral (Carlut et al, 2015).…”