The petrophysical and petrologic characteristics of the magnetic igneous intrusion at Mt Derriwong, N.S.W.

Abstract

An isolated, complex, 3000 gamma magnetic anomaly, occurs on the Condobolin-Fifield road about 25 km from Condobolin. It lies in the Girilambone-Wagga Anticlinorial zone of the Lachlan Geosyncline. Poorly outcropping, mostly basic, rocks were sampled for laboratory determinations of susceptibility, density and magnetic remanence. The rocks associated with the anomaly are hornblende rich diorites and pyroxenites with a very variable magnetite content; non-titaniferous magnetite is the dominant iron oxide with minor hematite. The intrusion appears to occur over a 1000 ? 1000 metre area; it is heterogeneous in a magnetic density and petrologic sense; the country rock is non magnetic Ordovician metasediment. The physical properties of the three main rock types recognised are: diorite with a density of about 2.80 and a susceptibility range of 0.001 to 0.004 cgs; pyroxene hornblendite with a density of about 3.14 and a susceptibility range of 0.0002 to 0.005 and hornblende pyroxenite with density of about 3.24 and a susceptibility of about 0.0065; the average Koenigsberger ratio is about 0.4. The data indicate a dense, variably magnetic body in which induction contributes the major part of the magnetic anomaly. It is considered that the body is a mafic pod having the form roughly of a cube with a side of about 1000 m. The body is thought to be the remnant of a dioritic stock.