A drill core consisting of unaltered banded iron-formation from Paraburdoo, Western Australia, representing the entire Dales Gorge Member of the Brockman Iron Formation, has been analyzed in short units of about 300 mm or in groups of units of up to 1 m. These analyses have been combined to give average compositions for each of the 33 macrobands of the member. The average compositions for the 17 oxide "facies" BIF macrobands and the 16 intercalated chert-carbonate and silicate "facies" S macrobands are, respectively, 43.15 and 45.78 percent SiO2, 0.09 and 2.04 percent A12Oa, 46.37 and 27.77 percent Fe•Oa (total iron), 2.69 and 5.21 percent MgO, 1.65 and 2.85 percent CaO, 0.04 and 0.02 percent Na•O, 0.01 and 0.09 percent TiO2, 0.21 and 0.16 percent P2Os, 0.05 and 0.02 percent MnO, and 0.03 and 0.32 percent S. The analyses are similar to those reported by Trendall and Pepper (1977) for a part of the Dales Gorge Member at Wittenoom, 130 km away. These data, together with documented evidence of lateral continuity of both major and minor banding across the Hamersley Iron Province, support an assumption that the primary deposits were chemically similar throughout the area. Mineralogical variations between sites indicate low but variable metamorphism in the area, and magnetite-hematite ratios and textural relations suggest increasing grade from Paraburdoo to Wittenoom to Tom Price.A short length of drill core from Tom Price, representing two complete macrobands and parts of two others, was also analyzed, and the results were found to vary significantly from the Paraburdoo and Wittenoom data. These differences are attributed to metasomatism of the rocks at Tom Price. Primary precipitation of iron in BIF macrobands was caused by oxidation and was dominated by gels containing iron(Ill) hydroxyoxides and silica in varying proportions, depending both on seasonal conditions and on supply. These variations were responsible for the wide range in composition and in thickness of banding, from tens of micrometers (submicrobanding), to millimeters (microbanding), to tens of millimeters (mesobanding). Preservation of micro-and submicrobanding within specific mesobands is due to very early consolidation and accentuation of primary depositional layering during diagenesis. In other mesobands, crystallization was evidently delayed and, under deeper burial conditions, tended to destroy primary lamination. In S macrobands, iron was precipitated as siderite and silicates. Mineralogical and chemical data show that the apparently sharp lithological boundaries between BIF and S macrobands are in fact gradational. Such contacts are attributed to a more or less continuous precipitation of silica and iron, interrupted periodically and changed in character by increase in pH and/or fco•. These changes were initiated by additions of volcanic ash. S macrobands are characterized by increases in the overall content of CO•, Ca, Mg, Mn, A1, Ti, K, and Zr.Podding, and other localized distortion, of mesobands is attributed mainly to lateral flow of silica...

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A section through Archeart nickel-iron sulfide mineralization of Western Mining Corporation's Lunnon Shoot, Kambalda, consists of a footwall of pillow basalt, a narrow contact zone comprising magnetite-rimmed chromite enhedra in silicates, massive ore with 95 percent sulfide, matrix (disseminated) ore with 60-20 percent sulfide, and hydrated and carbonated ultramafic rocks in the hanging wall. The sulfides were selectively dissolved for analysis using a bromine attack (see Appendix) giving an average composition of the sulfide portion of Ni = 9.37%, Fe -50.56%, Cu = 0.87%, Co = 0.20% and S = 39.01%. The major sulfides are pyrrhotite, pentlandite (with minor alteration to violarite), pyrite and chalcopyrite. The pyrite is concentrated near the top of the massive ore. Magnetite is irregularly distributed through the ore, and is more abundant in the matrix ore than in the massive ore.The data are consistent with the ore having been eraplaced at elevated temperatures as an ultramafic crystal mush containing olivine and chromite crystals, magnesia-rich silicate melt and immiscible sulfide-oxide liquid. Gravity segregation, probably in flow. has resulted in an ore zone comprising a quenched chromite-bearing zone, a sulfide-rich massive ore zone and a matrix ore in which olivines were set in continuous sulfide. The hanging wall represents the original silicate crystal mush with only minor sulfides.Solidification of sulfide commenced at about 1,140 ø C near the contact, resulting in magnetite-poor massive ore and oxide-enriched matrix ore. Pyrite was the first subsolidus sulfide mineral to crystallize at about 400 ø C and grew to give bands and subhedra near the top of the massive ore (with rejected Ni and Fe diffusing away). Cobalt was preferentially concentrated in this pyrite and depleted from the surrounding sulfide (Mss). Pentlandite exsolving at lower temperatures, however, competed favorably with lateforming pyrite for available cobalt. Nickel is low in the early-formed pyrite but concentrated in pyrite formed at low temperatures.

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