Economic Geology Vol. 83, 1988, pp. 524-536 The Agnew Nickel Deposit, Western Australia: Part I. Structure and Stratigraphy STEPHEN J. BARNES, MARTIN J. GOLE, AND ROBIN E. T. HILL CSIRODivisionof Mineralsand Geochemistry, PrivateBag, Wembley, Western Australia 6014, Australia Abstract The Agnew(formerly Perseverance) nickeldeposit, in the Agnew-Wiluna greenstone belt of the Yilgarnblock, has previously been assigned to the category of intrusive dunite-asso- ciateddeposits. On the basis of detailedreinterpretation of the geology of the deposit, we conclude that the nickel sulfide mineralization is hosted by komatiiteflows which predate the associated dunitc body, which is itself extrusive. Nickel sulfide mineralization is hosted by a suiteof metamorphosed olivine-rich ultrama- fic rocks containing widelyvarying proportions of tremoliteandchlorite.Chemical analyses of these rocks show them to be metamorphosed komatiites, in which original asymmetric differentiation profiles canbe recognized. Theseprovidefacing 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 retainsprimary igneous contacts with underlyingflow tops. Primary contacts are characterized by a zone of skeletalchromitedeveloped across the interface between massive sulfide and flow top. The primary massive ore postdates a thick komatiite flow which hosts the matrix ore. The dunitc body shows a discordant lower contact which truncates stratigraphic marker units in the countryrock. At the northernlimit of the mine area, the baseof the dunite is separated from the top of the mainmineralized flow by morethan 150 m of felsic volcanic rocks. At the southern limit of mine development, the dunite directly overlies the matrixore zone of the mineralizedflow. This discordant contact is interpreted asthe base of a thermal erosion channel formedby an extensive komatiitelavariver. The geometry of this contact, and the competence contrast between the dunitc body and the countryrocks,exerted a dominant influence on subsequent deformation and sulfide remobilization. Introduction THE Agnew (formerly known as Perseverance) nickel deposit is the largest knownaccumulation of nickel sulfides associated with komatiites and one of the largest nickelsulfide deposits of anyclass in the world, with published reserves of 45 million metric tonsat 2.05 percent Ni (Marston et al., 1981). Pre- viously,the Agnew deposithasbeen regardedas the foremostexample of the class of intrusive du- nite-associated deposits (Marston et al., 1981; Ross and Travis, 1981). More recently, on the basis of detailed field relationships and particularly the presence of overlying, underlying, andflanking ko- matiiteflows, manyof the intrusive dunitclenses of Western Australia, including that at Agnew, have been reinterpreted as being integral parts of the extrusive komatiite succession (Donaldsonet al., 1986; Barneset al., 1988a; Hill et al., 1985, 1988). Genetic models for this classof ore depositsare therefore in need of reevaluation. This paper presents some of the results of a comprehensive geologic, petrographic, and geochemical study of the Agnew deposit,basedprincipally on detailed relogging and sampling of over 200 surface andun- derground diamond drill holes. GeologicSetting The Agnewdeposit occurs within the Agnew-Wi- lunagreenstone belt of the Archean Yilgarn blockin Western Australia. This greenstonebelt (Fig. 1) contains over 100 km of continuous strikelengthof komatiitic rocks andhas been described by Naldrett and Turner (1977), Groves et al. (1982), and Mar- ston (1985). Previous accounts of the geology of the Agnewdeposit are given by Martin andAllchurch (1975) andBillington(1984). The area has been subjected to strong penetrative deformation. In a studyof the Lawlersarea 20 km southwest of the Agnew deposit, Platt et al. (1978) identified an earlyepisode of isoclinal folding (D;), refolded about a series of large amplitude(several kin) north-northwest trendingupright folds (D2). The Agnewdeposit occurs within the overturned east limb (average dip about 80øto the west) of a D2 anticline. The prominent structural feature recog- nizable in the mine area is a set of 1- to 70-m-am- plitudeopenfolds with axes plunging at about 20ø to the north. These structures fold a strongpene- trative bedding-parallel foliation and are probably related to the regional D• anticline. A second folia- tion or crenulation cleavage is locallydeveloped at o36 I-O 1 o8/88/8oo./5o.4-13 $o.. 5o 5 2 4