Palaeomagnetism, Magnetic Petrophysics and Magnetic Signature of the Porgera Intrusive Complex, Papua New Guinea

Investigation of magnetic properties and magnetic modelling of the Porgera Intrusive Complex, Papua New Guinea, has established representative magnetic properties and the palaeomagnetic signature of the major rock types of the Complex.

Relatively unaltered intrusive rocks of the Porgera Intrusive Complex are moderately to strongly magnetic, whereas strongly altered intrusions, mineralised zones and country rocks are very weakly magnetic. For most intrusions, Koenigsberger ratios are substantially less than unity, indicating that induced magnetisation is predominantly responsible for anomalies associated with most of the Complex. Hornblende diorite and hornblende diorite porphyry tend to be the most magnetic rock types in the Complex. However, all fresh intrusive rocks sampled, from very mafic to intermediate compositions, have rather high magnetic susceptibilities.

Primary thermoremanent magnetisations are retained by most of the intrusive rocks, unless they are highly altered. Both normal and reverse polarity remanences are preserved in the Complex. Primary magnetisation is carried by multidomain (titano)magnetite. In addition, the remanence carried by the intrusions is weakly to heavily overprinted. Primary remanence directions have been rotated and are steeper than the Miocene reference field directions of corresponding polarity, thus demonstrating tilting of intrusions in the Complex since emplacement. The sense of tilting is reasonably consistent, although the amount varies up to 50° or 60°. Prior to tilting, there appears to have been variable rotations about the vertical, mainly in a clockwise sense.

Magnetic modelling incorporating measured magnetic properties and known geometry of the shallow intrusions has led to interpretation of a deeper mass of the Complex. Modelled tilts of the shallow bodies agree with the sense of tilting from palaeomagnetic data, but are less extreme. This suggests that rotations at two different scales are being detected.

The tectonic rotations are evidently in response to thin-skinned tectonic processes which have accompanied the rapid uplift of the Complex. Palaeomagnetism is a powerful tool for detecting such rotations and could be particularly useful for elucidating the structural history of young deposits in tectonically active areas. The present study has detected hitherto unsuspected structural complications, which may have implications for locating further mineralised zones.