The magnetisation of the Elura orebody, Cobar, NSW

Abstract

Petrophysical sampling of the steeply plunging, roughly pipe-like Elura Zn?Pb?Ag orebody demonstrates a dominantly remanent magnetisation with steep negative inclination; the magnetisation is carried by central monoclinic pyrrhotite lenses. The remanence data suggests a mid-Cretaceous low grade thermal overprint possibly related to final cooling associated with minor uplift. Susceptibilities are high, averaging about 4200 x 10-6 (about 52800 x 10-6SI), and markedly anisotropic with k1:k2:k3 = 1.6:1.2:1.0. The anisotropy reflects a preferred crystallographic orientation of pyrrhotite grains ? the basal planes are vertically oriented suggesting plastic flow or crystal growth in a vertical direction. The high susceptibilities coupled with a Koenigsberger Ratio of 9.4 produce a ground magnetic anomaly 140 gammas (nT) in magnitude. This anomaly can be simply and correctly modelled by vertically plunging ellipsoids using equations that incorporate remanence, anisotropic susceptibility, and demagnetisation.