Magnetic susceptibilities of rocks associated with some Archaean gold deposits in Western Australia

Abstract

Magnetic susceptibility measurements of rocks obtained from a number of Archaean mesothermal gold deposits have aided the interpretation of their geological and geophysical settings. Five deposits within the Yilgarn Craton were chosen on the basis of their magnetic host rocks: Youanmi (tholeiitic basalts), Greenfields (layered differentiated gabbro), Mount Martin (strongly sheared komatiitic sequence), Queen Margaret (serpentinised komatiitic peridotite) and Bounty (banded iron-formation). Iron-rich minerals associated with the gold ores include: weakly magnetic pyrrhotite (Greenfields and Bounty), magnetite (Mount Martin and Bounty) and non-magnetic pyrite (Youanmi and Queen Margaret). The existence of two styles of mineralisation, whose apparent susceptibilities are either less than, or greater than, the host rocks, is a consequence of the geochemical interaction between the hydrothermal fluids and the wallrocks. This can have important implications for exploration since the target magnetic anomalies will be different in each case. Where apparent susceptibility values in the ore are greater than the host rocks, the mineralisation would be expected to be represented by a secondary positive anomaly on the flanks of a larger regional anomaly; where they are less, any local magnetic minima could be highly significant in terms of targeting drillholes. A correlation between gold grades and apparent susceptibility at Bounty and Mount Martin suggests that, in some cases, susceptibility measurements can be used as a guide to ore. In deposits containing komatiitic volcanic rocks, such as Queen Margaret and Mount Martin, the susceptibility meter can be used as an effective mapping tool, particularly where facing evidence is required to interpret complex structures.