Ground Magnetics, Kimberlite Exploration, Zimbabwe

Objective

To plan, process and interpret 83 line Km of ground magnetic data collected near Bulawayo in Zimbabwe. 

The goal was to map out the size and shape of any intrusive bodies present and recommend trench or drill-sites. A spectral anomaly is very close to a known kimberlite and is thus thought to possibly represent a kimberlite. However, there are numerous greenstone remnants in the area and the anomaly could also have been created by one of these.

 Total field magnetic data with the spectral anomaly overlaid.

Total field magnetic data with the spectral anomaly overlaid.

 1VD magnetics with interpretation overlaid.

1VD magnetics with interpretation overlaid.

 N-S cross section through the source body.

N-S cross section through the source body.

Survey Details

The survey area was 3.5km x 2.3km with 100m line spacings, due to the large size of the area. 20m stations were spaced on north-south lines. The survey was designed to go well into the background in case of a large associated magnetic low. 

Data Quality and Processing

The data was of good quality and the diurnal variation has been properly removed. The data were gridded as total field data and the derivatives and analytical signal calculated in Oasis Montaj.

Equipment Used

The survey used GEM’s GSM-19 (Overhauser) Magnetometer. Oasis Montaj software used to calculate the derivatives and analytical signal from the total field data
MGinv3D software for 3D modelling

Interpretational Work

The total field magnetics showed four main features:

  1. An elongate, busy, high-frequency dipolar magnetic response, approximately 700 nT in amplitude, beneath the spectral anomaly.
  2. A linear dyke-like anomaly running WNW north of the magnetic lithology. Possible a second dyke just south of it.
  3. A less well-defined northerly trending dyke in the west of the survey area.
  4. It is also apparent that there is a NNW-trending fault in the centre of the magnetic body of interest which also offsets the WNW-trending dyke.

The magnetic body of interest was 2.3km long by 300-400m wide, and appeared to be internally layered. The latter characteristic was consistent with both a sill or with a greenstone fragment so the magnetics did not shed light as to the true nature of the body. The high frequency of the magnetic response did however show that the body was very shallow, lying just below surface.

3D Modelling

The high-frequency of the magnetic response indicated that the source of this anomaly was shallow. 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 was then displayed as a coherent body where possible. The 3D inversion model showed a shallow body at about 10m below surface extending down to 200m depth- but depth extent is often the least reliable part of modelling. Only a borehole will be able to tell exactly what this body is. Layering is clear in the body. 

Conclusions

The survey supports the possibility of a large 2300 x350m layered sill-like body. The survey cannot say if the body is a kimberlite or a greenstone fragment. Only trenching and drilling can confirm the nature of the body. The survey recommended two locations or trenching and/or drilling.