The geophysical characteristics of the Trilogy massive sulphide deposit, Ravensthorpe, Western Australia

Abstract

The Trilogy deposit, located near Ravensthorpe in southern Western Australia, is a polymetallic massive sulphide deposit hosted within the conductive graphitic phyllites of the Proterozoic Mount Barren metasediments. There are two different styles of mineralisation, Pb-Zn massive-style and Cu-Au stringer-style mineralisation, contained within a silicified envelope that hosts the mineralisation. Petrophysical samples were taken and geophysical surveys conducted to characterise the response of the massive sulphide system and to help define an exploration model. Gravity, ultra-detailed airborne magnetic and radiometric, airborne time domain electromagnetic (EM) and ground time domain EM surveys have all been conducted over the deposit. The gravity survey outlined the position of the massive Pb-Zn mineralisation. A radiometric low around the surface projection of the silicified zone was evident in the airborne radiometric data. There was no response from the deposit recorded in the airborne magnetic survey. There is a subtle airborne EM anomaly over the known mineralisation, but the ground EM better defines its location and EM response. The airborne EM anomaly produced by the deposit was small compared with the response of the conductive phyllites elsewhere in the region. Geophysical exploration may aid in the search for additional sulphide mineralisation in the area. The combination of airborne EM with followup detailed gravity and ground EM has proven to be the most effective geophysical strategy in locating Trilogy style mineralisation in the Mount Barren basin.