Inversion of AEM Data With Strong IP Effects Over Metallic Sulfide Dissemination Within Banded Iron Formations

Tuesday, 11 June 2019
Davie West Lobby (Florida Atlantic University)
Marco Antonio Couto Jr., CPRM Geological Survey of Brazil, Belo Horizonte, Brazil, Gianluca Fiandaca, Aarhus University, Aarhus C, Denmark, Anders Vest Christiansen, Aarhus University, Aarhus, Denmark, Jorge L Porsani, USP University of Sao Paulo, São Paulo, Brazil and Esben Auken, University of Aarhus, Aarhus, Denmark
Abstract:
Induced Polarization (IP) has gained a lot of attention in connection to airborne Time Domain Electromagnetic (ATEM) data for mapping subsurface chargeability together with conductivity. Accounting for IP in AEM can have two purposes, the first one is recovering the true conductivity section without paying much attention to the magnitude of the IP parameters, the second one is to recover the IP parameters and correlate their magnitude to geology. In this study we focus on the second purpose.

In the context of mineral exploration, the chargeability information helps to identify mineralized zones, often associated with metallic sulfide dissemination. In geological mapping, the resistivity model can help to identify lithological units and its structural control. In both cases, an accurate resistivity model is crucial to define good targets or to understand the geological environment.

In this paper, we inverted data from an AeroTEMHD survey affected by IP. The area of study is in Quadrilátero Ferrífero region, Brazil. There is gold mineralization associated with metallic sulfide dissemination within banded iron formations (BIFs). The survey presents steep dB/dt decays, with the occurrence of negative transients in the late times in some spots, which some coincide with BIFs bodies. The BIFs are also well marked by the magnetic airborne data.

The IP inversions were conducted using the Maximum Phase Angle (MPA) re-parameterization of the Cole-Cole model and its results compared to the resistivity-only (RO) inversions. They were carried out as Laterally Constrained Inversions and integrated with magnetic data. The RO inversions indicate the occurrence of shallow and strongly resistive (> 2000 Ω.m) layers in the resistivity model. In contrast, MPA defined these resistive layers thinner, less resistive, in better agreement with geological data and with smaller data residual. The maximum phase sections for MPA identified a shallow polarizable layer and some deeper polarizable anomalies in the BIFs areas, which may be associated with the alteration of mafic unities and the sulfide zones, respectively.

The inclusion of IP in the MPA inversion suggests that we have retrieved a more accurate resolution of the subsurface compared to a standard RO inversion and possibly discriminated sulfide zones associated with the BIFs bodies.

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