Integrative geophysical approach for assessing the prospectivity of the Idlewilde intrusion, NSW

Abstract

The Idlewilde Intrusion (IWI) sits below ~150 m-thick sediment overburden and consists of a Palaeozoic basement of presumed Ordovician Macquarie Arc volcanics and the Nyngan Intrusive Complex. The geological setting is similar to the gold-rich Cadia and Northparkes porphyry-style deposits. Geophysical data include ground gravity, airborne magnetic and electromagnetic.Gravity data define a negative ovoid with an elevated gravity inner zone. Magnetic data show a central high surrounded by a circular low zone as part of a broader regional low. Electromagnetic data outline: (a) ?150 m- thick dual conductor corresponding to the overburden; (b) a medium conductor associated with the volcanics; (c) a resistor linked to the intrusion and (d) a slightly conductive annulus around the resistor interpreted as alteration, but may instead reflect the lack of sensitivity below the conductive overburden. The central part of the intrusion associated to gravity and magnetic highs could conceivably correspond to: (1) a magnetite-altered potassic core characteristic of gold-rich porphyry, (2) eroded granite with volcanics close to the surface, (3) roof pendant volcanics above a barely-eroded granite, and (4) a late monzonite pulse.From an exploration point of view, gravity data delineate the extent of the IWI. The regional low magnetisation suggests that the intrusion may be a later intrusive phase. Electromagnetic data identify the footprint of a granitic body which cannot be clearly associated with an alteration system because of conductive overburden. A mineralised system is yet to be identified, and one of the first company priorities is now to resolve the inner part of the IWI.