The magnetic signature of komatiitic peridotite-hosted nickel sulphide deposits in the Widgiemooltha area, Western Australia

Abstract

Wannaway, Mariners and Mt. Edwards are three of many type 1A komatiitic peridotite-hosted nickel sulphide deposits clustered in the Widgiemooltha area, approximately 560 km east of Perth. Magnetic susceptibility measurements show that the susceptibility of the ore is generally high but is extremely variable, from 4.5 ´ 10-3 SI units to 269 ´ 10-3 SI units in serpentinised environments, and 14.9 ´ 10-3 SI units in talc-carbonate altered environments, hangingwall ultramafic rocks have a generally high but variable susceptibility depending on the style of alteration. Serpentinised ultramafic rocks have a high susceptibility of 70 ´ 10-3 SI units, while talc-carbonate altered ultramafic rocks have a low susceptibility of 4.4 ´ 10-3 SI units due to the respective formation or destruction of magnetite. Footwall basaltic rocks have a consistently low susceptiblity average of 0.8 ´ 10-3 SI units. Susceptibility-temperature data indicate that ores with a high susceptibility contain monoclinic pyrrhotite and magnetite, and ores with a low susceptibility contain only small amounts of monoclinic pyrrhotite, minor magnetite and abundant, non-magnetic, hexagonal pyrrhotite. Components of remanent magnetisation in the ores and hangingwall ultramafic rocks lie in two dominant orientations. Remanent magnetisations held by pyrrhotite and magnetite at Wannaway and by pyrrhotite at Mt. Edwards are upwardly inclined toward the northwest. Vectors held by pyrrhotite at Mariners and magnetite at Wannaway are upwardly inclined toward the southeast. Two dimensional magnetic modelling of the Wannaway ore environment indicates that the anomaly over the Wannaway deposit is caused predominantly by the hangingwall ultramafic unit, with a small contribution from the near-surface hangingwall ore. Contact ore makes only a minor contribution to the anomaly due to the depth of the ore below the surface. Although the ore is significantly magnetic, it is difficult to directly detect on magnetic data due to the highly magnetic nature of the surrounding ore environment.