Ground Magnetics, Iron Ore Exploration, Zimbabwe
Planning, QC, process and interpret 233 line Km of ground magnetics data over an iron ore project in central Zimbabwe.
The survey area was 10km x 4.8km and the buried banded iron formation (BIF) shows up strongly in regional aeromagnetics. The main aim was to target not only the magnetite-rich BIF but also the hematite-rich ore that characteristically has a much weaker to negligible magnetic signature.
The survey was planned using the regional aeromagnetics as a guide. The line spacing was designed at 200m, considered the minimum resolution required. The lines were orientated at right angles to the strike of the BIF, the station spacing along lines was 20m. The survey was designed to go well into background in order to allow for accurate modelling of the dataset after processing.
The survey was completed using GEM’s GSM-19 (Overhauser) Magnetometer. Data were captured in WGS84 datum and re-projected as Arc 1950 Zimbabwe, using Oasis Montaj. 3D modelling using MGinv3D software
Data Quality and Processing
Good quality data was collected and the diurnal variation completely removed. There was no striping or corrugation visible and little noise speckling. The data set was gridded as total field then reduced to the magnetic pole. The derivatives and analytical signal were then calculated.
The interpretation was done using mainly the RTP image, the analytical signal and the first vertical derivative (1VD) of the RTP magnetics. The widths of the units were hard to gauge as the anomalies were so strong and had large associated magnetic lows, which may well overlie more BIF. A best estimate based mainly on the RTP magnetic high was used, but only drilling and trenching will confirm this. Weak, blurred magnetic anomalies were interpreted as potential hematite-rich ironstone in red. A number of sinuous, arcuate shear structures were also interpreted based on the magnetics, giving an indication of the structural framework which appears to have folded and offset several of the ironstone units.
A 3D model was created to map the magnetic units with depth, done on MGinv3D software, which created magnetic susceptibilities for a matric of cells in the “ground” below the survey and adjusted the values until they generated a magnetic field similar to what the survey measured.
Two separate 3D inversion models were created. The first to show only the highly magnetic material i.e. BIF, the second for the southern part of the survey where the weak (potentially hematite) anomalies occur. This was done on order to create separate depth slices and 3D bodies for these two different types of target.
One of the advantages of the 3D modelling was that rough volumes could be calculated. This was done in 50m slices for each of the 5 major anomalies on the first 3D model.
The ground survey successfully outlines a number of large magnetite zones. Additionally, hematite zones in the south were mapped. The data was successfully modelled in 3D with shells and depth slices provided in the relevant software for drill planning.