The role of geophysics in the discovery and delineation of the Cosmos Nickel Sulphide Deposit, Leinster area, Western Australia

Abstract

Cosmos is a typical Kambalda-style, komatiite-hosted, highly conductive and magnetic, massive pyrrhotite-pentlandite sulphide deposit. It is located approximately 40 km south of the Mt. Keith nickel mine in the northern Yilgarn Craton of Western Australia. It was discovered in mid-1997 using a combination of geological, geochemical and geophysical exploration techniques during routine evaluation of a prospective ultramafic belt. The discovery hole was the initial test of a short strike-length, late-time transient electromagnetic (TEM) anomaly in an area that had previously been partially drill tested by other companies. The original moving-loop TEM anomaly, recognised on two 150 m spaced lines, was defined in detail using a large, fixed transmitter loop survey. Modelling of the late-time fixed-loop data indicated that the source was a steeply east dipping conductor at about 50?75 m depth. Ore-grade massive nickel sulphides were intersected in the predicted position in the first diamond hole drilled to test the conductor. The limited depth extent of the conductor suggested by the modelling was confirmed by subsequent drilling and downhole TEM surveys. Detailed ground and aeromagnetic data subsequently collected over the deposit defined the host ultramafic well, but did not clearly distinguish the magnetic massive sulphide zone. The significance of the Cosmos TEM surveys was their ability to quickly and accurately focus drilling on the small but high-grade massive sulphide lens within the much larger mineralised halo.