The flemington intrusion fifield N.S.W.?petrophysical and petrological notes

Abstract

The Flemington intrusion outcrops over an area of 65 sq. km about 10 km northwest of Fifield. It is part of the Tout Complex of Silurian age; complex and extensive aeromagnetic anomalies, over 4000 gammas, occur in this region. Rock types were sampled near Flemington homestead. (Refer to Figure 1 in other paper by E.E. & C. this issue for regional location). In Table 1 are presented the results of chemical and transmitted light microscopy studies on typicsl samples. These rocks are monzonites (FIB is strictly a quartz monzonite). The sample FM5B is typical of an outcrop 750 m east of Flemington homestead; sample FIB was collected 1.4 km east of FM 5B ? it is typical of a large outcrop that lies in the centre of the intrusion 10 km north west of Fifield. The rocks have a medium grain size (+1 mm). It should be noted that there are other rock types in the intrusion ? these have not yet been sampled. Laboratory tests on the monzonite rocks (see Table 2) showed the rocks to be characterised by medium density, high susceptibility and a very pronounced susceptibility anisotropy. Natural magnetic remanence is high with Koenigsberger ratios in the range 1.6 to 65.4 indicating that induction makes only a minor contribution to the magnetic anomalies. Any quantitative interpretation and modelling of causative bodies must take into account the magnitude and direction of remanence and the effect of susceptibility anisotropy. Reflected light microscopy studies on the iron oxide minerals showed them to be mainly titaniferous magnetites with some prominent lamellae of ilmenite and exsolved lamellae of iron spinel ? hercynite. The titanomagnetites contain abundant small inclusions of hercynite and ulvospinel. The titanomagnetites constitute about 3% of the monzonites, they are randomly distributed in the form of equant, anhedral grains with an average size of 0.15 mm with some grains up to 0.5 mm. Small inclusions (few microns size) of pyrite, chalcopyrite and gold were noted in association with the non opaque (pyroxene?) phases. Figure 1 illustrates the character and type of magnetite in the monzonites.