INTRODUCTION The traditional resistivity method maps the electrical properties of the earth by measuring differences in potential at the earth's surface caused by galvanic current flow between two current electrodes. The magnetometric resistivity (MMR) method differs from the traditional method in that the potential electrodes are replaced by a highly sensitive coil or magnetometer and one or more components of the magnetic field are recorded.

Abstract The Yilgarn craton is one of the world’s major nickel provinces, containing 31.5 million tonnes (Mt) of Ni metal with an in situ value of about $350 billion on a pre-mining basis, amounting to 13.6 percent of the world’s currently known Ni resources. This entire resource inventory has been discovered since 1966. This chapter presents an analysis of the 40-year discovery history, which is ideal as a province-scale case study in mineral exploration. The province experienced a major peak in exploration activity between 1966 and 1971, the “nickel boom,” which accounted for more than half of all NiS deposits, and all of the giant (>1 Mt Ni) NiS deposits so far discovered. Almost 70 percent of those discoveries were related to direct surface prospecting methods, commonly based on recognition of magnetic ultramafic rocks as favorable hosts. Since the end of the nickel boom, the dominant discovery method has been follow-up exploration around significant known mineralization. From about the mid-1990s, electromagnetic (EM) surveying, which had been considered ineffective during the nickel boom phase, became a demonstrably successful technique for the detection of sulfide deposits. An improved understanding of geologic processes and controls has played an important role in sustaining exploration success since the end of the early, surface-prospecting phase of the nickel boom. Most nickel laterite deposits were first found during the nickel boom but not considered at that time to be economically significant. A large surge in exploration activity, much of it focused on resource delineation rather than true green-fields exploration, occurred between 1996 and 2001, triggered by the advent of the Pressure Acid Leach (PAL) technology in the mid-1990s. The discovery record of the Yilgarn province exhibits many patterns typical of an exploration province: early discovery of both the largest deposits and most of the metal and generally increasing discovery costs as the province matures. The average discovery costs for nickel in the Yilgarn have been 5.2 c/lb for sulfide nickel and 0. 6 c/lb for laterite nickel. The Yilgarn province offers two examples of exploration expenditure booms arising from the coincidence of an upturn in commodity price with the opening up of new exploration parameter spaces. These are the initial discovery of komatiite-hosted nickel sulfide at Kambalda in 1966, and the mid-1990s recognition of the potential of PAL technology to treat laterite ore deposits.

Abstract Most of the komatiite-hosted sulfide deposits in the Yilgarn craton exemplify the two major types: type 1, sulfide-rich accumulations at the base of magma pathways, and type 2, disseminated sulfides in the center of very olivine rich cumulate bodies. The Yilgarn komatiite resource consists of small, high-grade type 1 deposits, along with a number of much larger but lower grade type 1 and type 2 deposits. The largest deposits, Perseverance and Mount Keith, and the Kambalda Camp as a whole, are genuinely world-class deposits comparable in metal content to giant deposits elsewhere in the world. Type 1 deposits are almost exclusively hosted by bodies of olivine cumulate at least several tens of meters thick, and in some cases hundreds of meters thick. Immediately adjacent host rocks range from olivine orthocumulates to olivine adcumulates. Spinifex-textured rocks are only rarely found in contact with ores, but are more common in flanking rocks tens to hundreds of meters away. There is a close association between mineralization and the compound cumulate-rich flow facies. Komatiite-hosted ores in general have high Ni tenors and low Cu tenors, bulk ore compositions being controlled largely by parent magma compositions and magma channel dynamics. Superimposed variations are due to internal magmatic differentiation and hydrothermal effects. The ores of the Black Swan area illustrate primary magmatic features, including partially molten footwall inclusions in ores and host rocks. The Kambalda deposits show a wide spectrum of superimposed effects due to deformation, and there is an unresolved controversy surrounding the origin of linear troughs which evidently host the ores. Orebodies at Perseverance, Honeymoon Well, and elsewhere also demonstrate substantial structural modification but retain important magmatic signatures. The body of evidence supports the substrate erosion model, whereby sulfide is derived by thermomechanical erosion of sulfidic rocks in the floors of major magma pathways or channels. To some degree the geochemistry of the host rocks reflects this process, but the signal varies greatly according to the immediate environment. All the deposits are invariably deformed to some degree, and massive sulfide orebodies have had a profound influence on localizing the deformation. There is a wide spectrum from almost intact deposits to those which have undergone intense deformation and remobilization into entirely shear hosted type 5 deposits.

The magnetic induced polarization (MIP) method determines the variation of the induced polarization and resistivity of the earth through measurements of the magnetic field associated with galvanic current flow in the earth, rather than the electric field, as in the traditional induced polarization (IP) or electrical induced polarization (EIP) method. Important differences between the MIP and EIP methods are evident in field practice, mathematical theory, and field results. For example, the MIP method is insensitive to horizontal layering in the earth and reflects only lateral variations in its electrical properties. MIP also provides an enhanced ability to detect the presence of bodies of anomalous electrical properties even through a highly conducting surface layer. For this reason the MIP methods primary application is in regions of highly conducting (e.g., saline) overburden or weathered rock, such as in Australia. MIP responses tend to be more complex and varied in pattern than responses normally encountered in EIP measurements. For example, polarity reversals are the rule in MIP but are rarely encountered in EIP. MIP employs high-sensitivity component magnetometers as basic sensors. These are small relative to the length of the electric dipole sensors normally employed in EIP, and, therefore, provide relatively higher geometric resolving power.