Iron Formations as the Source of the West African Magnetic Crustal Anomaly

Abstract

The geological sources of major magnetic field anomalies are still poorly constrained, in terms of nature, geometry, and vertical position. A common feature of several anomalies is their spatial correlation with cratonic shields and, for the largest anomalies, with Banded Iron Formations (BIF). This study first unveils the magnetic properties of some BIF samples from Mauritania, where the main part of the West African magnetic anomaly is observed. It shows how strong the magnetic susceptibility and natural remanent magnetization for such rocks are. High Koenigsberger ratios imply that the remanent magnetization should be taken into account to explain the anomaly. A numerical modeling of the crust beneath this anomaly is performed using these constraints and both gravity and magnetic field data. A forward approach is used, investigating the depth, thickness and magnetization intensity of all possible crustal lithologies. Our results show that BIF slices can be the only magnetized crustal sources needed to explain the anomaly, and that they could be buried several kilometers deep. The results of this study provide a new perspective to address the investigation of magnetic field anomaly sources in other cratonic regions with BIF outcrops.

Understanding the crustal architecture beneath the Bangui magnetic anomaly and its interactions with central African tectonic megastructures based gravity and magnetic analysis

“…Thus, analysing the contribution of the mantle as a source of BMA should be consider. According with the present results (Figs.11A-I) and the developments over the West African magnetic Anomaly(Launay et al, 2018) the latest issue(Ferre et al, 2020) seems to be a good perspective.…”

“…2C). Launay et al (2018) indicated that the BIFs are the source of a similarly sized west African magnetic anomaly, which occurs at the edges of the West African Craton. The greenstone belts which are exposed at northern edge of the Congo Craton in Cameroon (Fig.…”

“…According toOuabego et al (2013) BIFs would be a likely issue to explain the Anomaly. RecentlyLaunay et al (2018) showed the BIFs as a good candidate to explain the West African Magnetic Anomaly. Another limitation is the lack of constraints on the magnetic composition of the lower crust and upper mantle.…”

Widespread and Persistent Deposition of Iron Formations for Two Billion Years

“…1400 Ma Xiamaling Formation to 520 Gt. Similarly, Launay et al (2018) inferred from a large magnetic anomaly that the total volume of the ca. The Belozyorsky-Konsky IF in Ukraine, with an estimated age of 3267 ± 26 Ma (Isley, 1995), may have originally contained 1000 Gt of IF (Huston & Logan, 2004;Isley, 1995).…”

Joint modeling of potential-field data and geodynamic interpretation for northeast Algeria

“…Concerning the effect of eventual remanence, in the absence of magnetization measurements on samples from cores of deep drillings, one cannot really investigate it. A recent study has however shown that it should be taken into account in the middle and lower crusts (Launay et al, 2017(Launay et al, , 2018. Fig.…”