The nature of sulfidic metasediments at Kambalda and their broad relationships with associated ultramafic rocks and nickel ores

Abstract

Sedimentary horizons at Kambalda occur at the base of, and at interflow boundaries within, the same lower one-third of the ultramafic sequence as do the major Ni-Cu orebodies. Although sediments occur on all the same stratigraphic horizons as the ores, they are almost entirely absent from within the volume containing the ores, being separated by zones free of both sediments and Ni-Cu sulfides. Generally, they are broadly lenticular, are not very extensive in area, and correlate poorly laterally.Mineralogically and chemically, the sediments are very variable, but much of this is attributable simply to a variable iron sulfide content. Geographic and stratigraphic location and lithology are other significant causes of these variations. Proximity to the Ni-Cu ores has, at best, an exceedingly subtle control on chemistry or mineralogy. The Kambalda sediments are not compositionally very different from other Archean sediments in broadly similar environments. However, their sulfur and chalcophile element levels are high, and Au is especially enriched. Sulfur isotopes have a narrow spread near zero (range: 1-5.5%), and the S/Se is around 15,000 to 20,000, indicating a little-modified magmatic source for the sulfur.Apart from transfer of sulfur (due to pyrite breakdown at peak metamorphism) to the ultramafics, there was negligible chemical exchange across sediment-ultramafic interfaces. Virtually no Ni was extracted from the ultramafic silicates during this sulfurization, and migration of Ni into the sediments was negligible. Nickel sulfide ore formation by sulfurization or sedimentary replacement is rejected.Ores intimately involved with sediments make up only 3 to 5 percent of the total Ni reserve. There is a strong inverse relationship between the quality of ore environments and the amount of their ore which is in contact with sediments: the four largest ore locations (accounting for about 70% of the reserve) each has considerably less than 1 percent of ore with sediments. Extensive ore-sediment contact occurs locally and can result from either (1) physical injection of preexisting Ni-Cu ore into previously barren sediments, (2) a synsedimentary intermixing of ores and sediments, or (3) a passive and largely coincidental relationship. The volcanic-exhalative model of Lusk (1976) for the Ni ores is rejected.The sediments are interpreted to be a mixture of: local (mafic-ultramafic) and distant (felsic-granitic) detritus; S, Pb, Zn, Fe, and Cu from ultramafic exhalations; S, Co, Ni, and Cu from local sea-floor leaching of ultramafics; a biological carbonaceous component; and probably minor chemical silica. Sea-floor alteration (affecting Na, K, and Mn), diagenesis, and metasomatic K, Rb, Ba, Sr, and CO 2 introductions are evident.