Rock Magnetism Of Pyrrhotite in the Stawell Gold Field, Victoria, Australia: Relating Aeromagnetic Anomalies to Mineralizing Fluid Flow

Gold in the Stawell gold field, a fault-bounded Cambrian slice at the base of the Lachlan Orogen in southeastern Australia, is hosted by a sequence of sulfide-mineralized metasediments, including the gold-bearing Magdala Facies, overlying basalts. Aeromagnetic anomalies in the Stawell corridor display an intense axial high flanked by equally intense paired negative flanks. Demagnetization analysis confirms that remanence dominates the magnetization in pyrrhotite-bearing basalts and metasediments, and indicates both normal and reversed characteristic remanence, with directions strongly controlled by the schistosity. Most samples were unoriented; samples from a small oriented subset were overprinted during core recovery, precluding direct measurement of remanence. Hysteresis analysis, a technique that previously has had very little application to aeromagnetic interpretation, provides an alternative approach to understanding remanence in the Stawell corridor. Samples group in two populations, which define two parallel domain-state mixing curves on a Day plot. Population A comprises samples from the pyrrhotite-bearing basalts, and population B the Magdala Facies. 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 metasediments and lower fluid flow in the basalts. Fluid-flow modeling of the Stawell corridor has emphasized the influence of high fluid flow in the metasediments draping the flanks of a basalt dome on the location of gold mineralization; these high fluid flow regions underlie the negative lobes of the aeromagnetic anomaly, and are characterized by samples that fall in hysteresis population B. From this we infer that the fluid flow regime has a direct control on remanence polarity, either by resetting remanence at the time of mineralization, or by changing the response of the high fluid flow regions to later demagnetization processes.