Ground Magnetics, Kimberlite Exploration, South-Central Zimbabwe

Objective

Planning, QC, process and interpret 23 line km of ground magnetics data collected in south-central Zimbabwe. 

The anomaly was thought to possibly represent a kimberlite or carbonatite pipe, and is 1300m x 700m in size.

 Resulting total field magnetics from the survey

Resulting total field magnetics from the survey

 1VD magnetics with interpretation overlaid. The black line outlines the  main body, the green line shows the prominent dyke-like body. The WNW dual-structure that defines the southern edge of the body is the dash-black line.

1VD magnetics with interpretation overlaid. The black line outlines the  main body, the green line shows the prominent dyke-like body. The WNW dual-structure that defines the southern edge of the body is the dash-black line.

 A cross-section model of the source

A cross-section model of the source

Survey Details

The survey line spacing was planned at 100m as the anomaly is large. North-south lines with 20m station spacing were planned. The survey was designed to go well into background in case a large associated magnetic low was present.

Equipment Used

GEM’s GSM-19 (Overhauser) Magnetometer was used. The derivatives and analytical signal were calculated in Oasis Montaj. 3D modelling in MGinv3D 

Data Quality and Processing

Good quality data was collected and the diurnal variation was properly removed. The data were gridded as total field data and the derivatives and analytical signal calculated in Oasis Montaj.

Interpretational Work

The total field magnetics showed four main features:

  1. A linear dyke-like anomaly in the east
  2. A busy, high-frequency (shallow) magnetic response, approximately 250-300 nT in amplitude, over the anomaly.
  3. Half of a large, sub-circular magnetic high developing in the south.
  4. A prominent WNW-trending dual-structure separating the latter two.

Point 2, the central body, was the focus of this study. The amplitude of this anomaly was too low for it to be readily visible in the regional magnetics as there are a number of strong anomalies in the area.  The outline of the body is about 1350m x 650m and agrees reasonably closely with the spectral anomaly. A wide, highly magnetic dyke is believed to be the source.

3D Modelling

In order to try and try and confirm the nature of the source, a 3D inversion model was created using MGinv3D software. This software created magnetic susceptibilities for a matrix of cells in the ‘ground’ below the anomaly and then adjusted these values until they generated a magnetic field as similar as possible to what was observed in the survey. The distribution of magnetic susceptibilities is then displayed as a coherent body where possible.

The 3D inversion shows a shallow body at about 25m depth extending to depth in a pipe-like manner. However, a thick sill is still a possibility. The only way to accurately determine the source would be to undertake drill the anomaly. 

Conclusions

The survey was completed successfully and confirmed the possibility of a large, shallow pipe-like body. The survey could not determine the source and a thick sill is as possible a source as a kimberlite pipe. It was recommended that further exploration be carried out in the form of trenching, drilling and soil sampling.