Magnetic Anomaly of the Bramfield Iron Formation, South Australia

Summary

Lynch Mining holds numerous exploration licences on the Eyre Peninsula where there is very little outcrop although the average cover is only a few tens of metres. Geophysical methods are crucial to exploration in these circumstances. Early drilling of a large (7000 nT) ~2500-3000 m  350 m Magnetic anomaly, intersected east dipping (60°-80°) iron formation and associated forsterite-magnetite marbles and calcsilicates. A zone between 87 m and 125 m of BLDD06 assayed at 40% Fe with low (<900 ppm) phosphorus and ~ 3000 ppm manganese. From the size and area of the magnetic anomaly, a large iron ore resource was inferred. Initial magnetic modelling indicated an elongated broad vertically dipping body. Subsequent drilling, while intersecting significant (up to 70 m downhole) interpreted lens-like bodies, failed to intersect a thick uniform ironrich body commensurate with the initial magnetic modelling.

Magnetic property measurements (k >1.0 SI) suggest that the magnetic susceptibility value used in the initial modelling (0.5 SI) was too low by a factor of two. Moreover, the remanence is viscous and probably in the same direction as the induced magnetisation so the effective susceptibility may be >2.0 SI. Selfdemagnetisation constrains the magnetisation to be aligned along the bodies, deflected from the geomagnetic field direction, so that no matter how the bodies dip the anomaly will be symmetric. The causative bodies appear to be dispersed magnetite-rich lenticular pods giving a combined anomaly that simulates a uniform body.

Key words::

• magnetic anomaly
• iron formation
• demagnetisation
• non-uniqueness