Poisson magnetization-to-density-ratio and magnetization inclination properties of banded iron formations of the Carajás mineral province from processing airborne gravity and magnetic data

Abstract

According to the Poisson theorem, gravity and magnetic fields arising from geologic bodies that share common sources, with a uniform magnetization-to-density ratio (MDR) and a uniform magnetization direction, are related by a linear transformation that allows each field to be calculated from the other. Provided that these conditions on the sources are met, when the gravity and magnetic data are available over an area, the Poisson theorem can be used to infer the MDRs and magnetization directions of sources from their associated gravity and magnetic anomalies. These conditions are partially met in many geologic structures but are expected in iron ore deposits, usually associated with strongly magnetic and highly dense formations. Due to the importance of iron ore as a global commodity, most mineral provinces of the world have been investigated by accurate gravity and magnetic sensors, providing a reliable database, but they have not yet been explored with joint interpretation based on Poisson’s relationships. We have interpreted a gravity-magnetic survey covering the Serra Sul of the Carajás Mineral Province, Brazil, where world-class iron deposits are found. We have adapted a formulation formerly developed to estimate the MDR and the magnetization inclination (MI) from profile data to process gridded data sets. Due to faulting and folding, the same density and magnetic structure may assume different strike directions, requiring corrections to improve MDR and MI estimates. Because the geomagnetic field inclination in the studied area is very low (−6.7°), a procedure for stable computation of the components of the anomalous magnetic field vector is applied. The inferences for Serra Sul MDR suggest minor variations for the entire 30 km long formation containing the mineralized bodies, the strong remanent magnetization showing reverse polarity for banded iron formation segments of the Carajás Serra Sul.