A 3-D Geological and Structural Synthesis of the Leinster Area of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia, with Special Reference to the Volcanological Setting of Komatiite-Associated Nickel Sulfide Deposits

The Leinster area is situated toward the southern end of the Agnew-Wiluna Belt in the Yilgarn craton of Western Australia and hosts the giant Perseverance komatiite-associated nickel sulfide deposit. At in excess of 1.2 Mt contained Ni, this is the biggest type 1 (basal sulfide-rich accumulation) deposit in the world, and it is associated with a number of smaller deposits that extend over a strike length of 20 km adjacent to the Perseverance fault on the eastern margin of the belt. In broad terms, the greenstone sequence comprises an eastern, rift-proximal komatiite-dacite sequence and a western and more marginal basalt-dominated sequence with minor komatiite. The komatiites display a wide range of compositions and textures, from highly magnesian, olivine meso- to adcumulate rocks composed of olivine with forsterite contents of up to 94.5% to sequences of thin, spinifex-textured komatiite flow units. This compositional range indicates considerable variation in volcanic facies, from rift-proximal to distal. Loci of focused flow (termed pathways), indicated by thickened cord-like bodies of more olivine rich rock, occur at all scales and are intimately associated with the Ni sulfide mineralization. The pathways are linked by thinner sequences of lower-MgO rocks, either layered olivine orthocumulate rocks or sequences of thin spinifex-textured flow units. Some of the lower-MgO komatiite sequences appear to emanate from the tops of pathways, in the manner of breakout flows in tube-fed basaltic terrains. Variably sulfidic black shales and cherts and barren exhalative iron sulfide horizons are intimately interlayered with the komatiitic rocks. The overprinting effects of regional deformation and metamorphism, spanning at least 80 m.y. , are considerable. At least eight local deformation events have been recognized by previous workers. These have been used to constrain the 3-D model and can be linked to craton-wide structural schemes. Some of the major faults are clearly accretionary and reflect reactivation of underlying basin-forming structures. Early north-over-south thrusting was followed by E-W–directed basin closure and the development of NNW-trending open regional folds. Extension accompanied batholithic granitoid intrusion and reactivated rift-parallel NNW-trending faults, producing the major strike-slip fault systems that divide the greenstone succession into a series of structural and stratigraphic domains. The ensuing orogenic collapse resulted in overturning of the stratigraphy to the east. Subsequent brittle deformation events have produced fault sets in a variety of orientations that can be related to ongoing shortening with a variable principal stress orientation. There is a complex interplay of structural and volcanological controls on the formation and subsequent deformation of the Ni sulfide deposits in this ancient terrane. Three-dimensional modeling has furthered a detailed understanding of the geologic and structural evolution of the belt, allowed the identification of komatiite pathway positions and their plunges, and provided quantitative data on the magnitude and direction of fault offsets of different generations. The modeling exercise has shown that it is possible to strip away the effects of overprinting metamorphism and deformation to reveal some of the primary controls on komatiite volcanism and the location of Ni sulfide deposits, even in those portions of greenstone belts that are highly deformed and have undergone amphibolite facies metamorphism. This is illustrated with case studies of the Perseverance, Venus, Rocky’s Reward, Harmony, and Sir Lancelot deposits. Three-dimensional modeling has the capacity to integrate geologic, geochemical, and geophysical datasets in a way that fully utilizes the results of past exploration programs and produces a premium geologic product that can be used to guide further exploration. The recent discovery of the Venus deposit is, in part, a tribute to the power of this approach.