Abstract

The Agnew (formerly Perseverance) nickel deposit, in the Agnew-Wiluna greenstone belt of the Yilgarn block, has previously been assigned to the category of intrusive dunite-associated deposits. On the basis of detailed reinterpretation of the geology of the deposit, we conclude that the nickel sulfide mineralization is hosted by komatiite flows which predate the associated dunite body, which is itself extrusive.Nickel sulfide mineralization is hosted by a suite of metamorphosed olivine-rich ultramafic rocks containing widely varying proportions of tremolite and chlorite. Chemical analyses of these rocks show them to be metamorphosed komatiites, in which original asymmetric differentiation profiles can be recognized. These provide facing directions which form the basis for structural and stratigraphic interpretation.Much of the massive sulfide has been structurally remobilized into fold noses and planar fractures, but some retains primary igneous contacts with underlying flow tops. Primary contacts are characterized by a zone of skeletal chromite developed across the interface between massive sulfide and flow top. The primary massive ore postdates a thick komatiite flow which hosts the matrix ore.The dunite body shows a discordant lower contact which truncates stratigraphic marker units in the country rock. At the northern limit of the mine area, the base of the dunite is separated from the top of the main mineralized flow by more than 150 m of felsic volcanic rocks. At the southern limit of mine development, the dunite directly overlies the matrix ore zone of the mineralized flow. This discordant contact is interpreted as the base of a thermal erosion channel formed by an extensive komatiite lava river. The geometry of this contact, and the competence contrast between the dunite body and the country rocks, exerted a dominant influence on subsequent deformation and sulfide remobilization.

First Page Preview

First page PDF preview
You do not currently have access to this article.