Significance of remanence in Stawell goldfield aeromagnetic anomalies

Abstract

Gold in the Stawell goldfield is hosted by a sequence of sulfide-mineralised metasediments, the Stawell Facies and Albion Formation, overlying chloritised basalts. Aeromagnetic anomalies in the northwestern part of the Stawell corridor display intense negative and positive lobes: at the Kewell prospect, these take the form of an axial high flanked by paired lows. Such a pattern requires that remanence dominates magnetisation, and that the net magnetisation must vary in direction across strike. Remanence demagnetisation analysis confirms the dominance of remanence over induction in the pyrrhotite-dominated basalt, Stawell Facies and Albion Formation sequence, and indicates the presence of both normal and reversed remanence, with directions strongly controlled by the dominant S₂ foliation. A minor proportion of the Stawell Facies and basalts that display coarse-grained, secondary magnetite are dominated by magnetic induction. Oriented samples from the Kewell prospect were overprinted during core recovery, precluding direct measurement of the directions of remanence. Hysteresis analysis, a well-established rock-magnetic technique that previously has had very little application to aeromagnetic interpretation, provides a model to understanding remanence at Kewell. Samples from throughout the Stawell corridor group in two populations, which define two parallel curves on a Day plot of hysteresis parameters. Each curve follows the form expected for mixtures of single- and multi-domain ferrimagnets, and the offset between the two curves resembles that reported in other research to arise from variations in the Fe:S ratio in pyrrhotite. Population A comprises samples from the Stawell Facies, and population B is dominantly drawn from the pyrrhotite-bearing basalts. We speculate that the distinction between the two populations reflects differences in the pyrrhotite arising from contrast between high fluid flow in the high-permeability Stawell Facies and lower fluid flow in the basalts. Fluid-flow modelling at Kewell has emphasised the influence of high fluid flow on the flanks of the basalt structure on the location of gold mineralisation: these high fluid flow regions underlie the negative lobes of the aeromagnetic anomaly and are characterised by samples that fall in hysteresis population A. Population B samples, by contrast, are distributed in the lower flow regime near the crest of the structure. Magnetic modelling that assigns a reversed remanence in population A material to the flanks of the structure closely matches the observed aeromagnetic profile. From this, we infer that the fluid-flow regime has a direct control on remanence polarity at Kewell, either by resetting remanence at the time of mineralisation or by changing the response of the high fluid flow regions to later remagnetisation processes.

Key words:

• Albion Formation
• fluid flow
• magnetic hysteresis
• pyrrhotite
• remanence
• Stawell goldfield
• Stawell Facies

Acknowledgements

We wish to thank Leviathan Resources Limited for access to their core library, and Jon and Allison Dugdale for their assistance in sampling. Constructive criticism and enthusiastic interest from Bob Smith and Chris Wilson helped frame the research. Reviews by Shanti Rajagopolan and Phil Schmidt added much polish to this paper.