Economic Geology Vol. 71, 1976, pp. 1214-1228 Palladium, Iridium, andGoldin theOres andHostRocks of Nickel Sulfide Deposits in Western Australia REID R. KEAYS AND R. M. DAVISON Abstract Neutron activationanalysis has been usedto determine palladium,iridium, and gold and, occasionally, rutheniumand osmiumin the ores and ultramarie host rocks of 14 nickel sulfide depositsin Western Australia. The depositsrange from low-grade dis- seminated sulfides(typically pentlanditc, pyrite, millerite, heazlewoodite) in intrusive ultramarie bodies to high-grade massive or matrix-type pyrrhotite and pentlanditc at the baseof volcanic-ultramafic units. The total precious metal tenor of typical Western Australian nickel sulfide ore is similar to that of average Sudbury ore although gold, ruthenium,iridium, and osmiumare higher (and platinmn may be lower) in the West- crn Australianores. Both palladium and platinum are distinctly lower than in average ore from the MerenskyReef. Palladium rangesfrom 5,000 ppb in high-grademassive oreswith 20 weight percent nickelto 0.2 ppb in unmineralized ultramarie rocks; iridium variesfrom 1,000 to 3 ppb in the same rocks. Palladium and iridium correlate strongly with the nickel sulfidecontentof the rocks. Deviations from perfect correlations are explained by fractionation of the metals during solidification of the parent sulfide melts or by redistribution during metamorphism or secondary alteration of the sulfides. The distribution of palladium and iridium strongly supports a magmatic origin for the nickelsulfide ores and permits assessment of the effectof exotic processes such as talc-carbonate alteration in the upgrading of low-grade nickel ores. The evidence in handindicates that although such a process may produce local redistribution of nickel, it is not responsible for large-scale mobilization of nickel. However, it may release significant quantities of goldand sulfur. The ultramarie host rocks of the nickel sulfide ores havevery low Pd/Ir ratiosrela- tive to the nickel ores and also to "normal" ultramarie rocks that are not known to be associated with major amounts of nickel sulfides. This relation may be a valuable ex- plorationguide. Introduction S•CE the discovery of nickel sulfide ores in 1966at Kambaldain Western Australia, Australia has be- come a major producer of nickel, supplying 6 per- centof the world total in 1973 (Miller, 1975). The ores occuras massive, matrix, and disseminated sul- fidesat the baseof stratiform(extrusive?)ultra- mafic units and as disseminations within intrusive dunites or as segregations neartheir margins. The present study was undertaken to document the dis- tributionand absolute abundances of the precious metals in the ores and host rocks of the Western Australian nickel sulfidedeposits. Such informa- tion has inherent economic importance because nickeliferous sulfides in mafic and ultramaficrocks, togetherwith native alloys of the precious metals disseminated in ultramafic rocks, constitute the major type of lode deposit of the platinum metals else- where (Mertie, 1969). At Sud'bury the precious metalscontributea significant amount to the value of the mined ore (Allen, 1960). Specific problems to which studies of the pre- cious metals in Western Australian ultramafic rocks may be applied include (1) discrimination of nickel gossans from ferruginous and siliceous material of similarappearance produced by the deepoxidation of rocks with a minor nickel sulfide component (Travis et al., 1976); (2) the modes of origin of segregated nickelsulfide deposits; (3) evaluation of the effects of modifying processes (e.g., talc-carbon- ate alteration) on the distribution of nickel within ultramafic units, especially as this relates to the formationof low-grade nickel sulfide ores; (4) the recognition of ultramaficrocks which might contain nickel sulfide deposits; and (5) the role of chromite in controlling the distribution of the precious metals. GeologicalSetting and Opaque Mineralogy The nickel sulfide deposits examined in this study are in the Archean greenstone belts of t.heYilgarn Block of the Precambrian shield of Western Australia (Fig. 1). Most of the .deposits are in stratiform vol- canic ultramafic assemblages (which may be entirely extrusive or partlyintrusive in origin) or in intrusive dunitic bodies. According to Gemuts and Theron (1976), the deposits in the stratiform association are 1214