Ground Magnetics, Gold Exploration, Kadoma, Zimbabwe

Objective

QC, process and interpret 168 line km of ground magnetics within the Kadoma area, Zimbabwe. 

The area is a gold mining area, so detailed structural analysis of the ground magnetics, and targeting, would be required.

 Ground magnetics data at 200m line spacing with the central area at 50m in-fill spacing

Ground magnetics data at 200m line spacing with the central area at 50m in-fill spacing

 Structural interpretation with shears in black, splays in green and short brittle fractures in red.

Structural interpretation with shears in black, splays in green and short brittle fractures in red.

 Structural interpretation with primary targets circled in red and secondary in blue.

Structural interpretation with primary targets circled in red and secondary in blue.

Survey Details

The survey line spacing was theoretically 50m in a central core area, with 200m line spacing on the two sides. The line orientation was ENE in order to cross the geology at right angles to the strike. In practise, because a Walkmag was used with GPS navigation in real time, with no pre-prepared lines, the 50m lines were often skewed and crossed-over each other as the surveyor traversed around obstacles in his path. For this reason small sections of data were filled-in with short lines wherever the spacing between lines became too great. In effect then, a 50m line spacing was achieved. The station spacing was generally between 9m and 12m, aiming for 10m, which made for a highly detailed survey.

Equipment Used

Survey completed using GEM’s GSM-19 (Overhauser) Magnetometer. Derivatives and analytical signal calculated in Oasis Montaj.

Data Quality and Processing

The base station data for each day were checked and all were unaffected by magnetic storms and spiking. On two days the data were slightly noisier than normal, and the field data were checked for evidence of this. The effect was found to be negligible, and did not degrade the overall data quality. The field data are highly variable and very busy in certain areas, which is in-keeping with greenstone gold areas. Because of this, processing the ground magnetics image so that it could be more easily be interpreted often involved smoothing the data to a degree, and the calculation of vertical and horizontal derivatives which highlight the areas of greatest change in the field. Once the data were lightly smoothed with a 3x3 convolution filter to remove spottiness, the structures of interest mainly showed up in the higher resolution central areas as small-scale magnetic anomalies. The outlying 200m areas were thus much harder to interpret, and horizontal gradient processing perpendicular to strike was essential in order to infer the continuity of structures.

Interpretational Work

Magnetism is in general highly heterogeneous in shear- and vein-hosted gold areas, with certain mafic to ultramafic rock-types and shear zones hosting far more magnetite that the surrounding sediments. The idea of ground magnetics was to try and isolate the structures of interest which either hosted the gold, or acted as fluid pathways. In the latter case particularly, hydrothermal magnetite and pyrrhotite have often been introduced, causing structures to have a magnetic signature. Other structures have to be inferred or interpreted by the offsets which they create in other magnetic units or structures. Sometimes these more subtle structures are the more important ones.
In terms of targeting, either structural trap-sites, or areas where open volume were believed to have been structurally created, were highlighted. The importance of open volume once again relates to the movement of gold-bearing fluids, the idea being that these fluids will move from high pressure to low-pressure zones. Any shear or fault intersection may therefore release rock stress and create a low-pressure zone into which fluids may preferentially move. Thus, for this project, the main shears are assumed to be the focal point of targeting, with ductile splay intersections then being highlighted as targets. Likewise, any later brittle fractures, which crosscut and offset the shears or splays, are highlighted in terms of re-mobilised gold potential.

Conclusions

The survey was successful and extremely useful in a geologically complex area
Using horizontal gradient processing to enhance the continuity of structures and interpretation showing complex shear and splay structures was presented.

The targets were split into primary and secondary depending on their potential and should all be followed up with further ground exploration such as soil sampling.