Conditions of riebeckite formation in the iron-formation of the Dales Gorge Member, Hamersley Group, Western Australia

Quantitative evaluation of the stability relations of riebeckite, in the system Na-Fe-MgSi-C-GH at low temperature and pressure, is based upon estimated thermodynamic parameters for riebeckite and on chemical and petrologic data. This evaluation forms the basis for an assessment of the conditions of riebeckite formation in the Dales Gorge Member, Hamersley Group, Western Australia. Computed values of Sie6, Llfs,2sg, AG,zss for riebeckite are 657.3 J/mol K, 10058.30 kJ/mol, and -9365.2 kJimol, respectively. At constant temperature riebeckite is, in general, stable to lower/6, conditions than hematite and higher pH values than magnetite or hematite. Where riebeckite coexists with carbonates, its stability field is enlarged by a decrease of total carbonate (a1o1.1 ser) at constant pH. Riebeckite assemblages in the iron-formations of the Dales Gorge Member can be divided into two groups, carbonate-free and carbonate-bearing. The former riebeckite has a high XF5 (ranging from 0.65 to 0.80) and the latter a much lower range from 0.40 to 0.55. The log aNr.*/aH* of riebeckite with such compositions ranges from 3.54 (for low.X$I) to 3.69 (for high,t$5 ut 130'C and on the HM bufer. Because thefs,for these iron-formations is almost fixed at the HM bufer, the conditions of riebeckite formation appear to have been controlled mainly by aN"* and d61u1 cozat a given temperature. Introduction Several major Precambrian iron-formation sequences in the world contain abundant riebeckite and/or crocidolite which is of very low-grade metamorphic (or late diagenetic) origin. Major riebeckite and crocidolite (blue amphibole asbestos) deposits are well known in ironformations of the Hamersley Group of Western Australia (e.9., Trendall and Blockley, 1970) and the Transvaal Group of South Africa (e.9., Beukes, 1973). Small amounts of riebeckite have also been reported from the Sokoman Iron Formation, Knob Lake area, Labrador Trough (e.g., Zajac, 1974). In sedimentary rocks other than iron-formations, authigenic magnesio-arfvedsonite surrounding detrital hornblende was reported by Milton et al. (1974) in the Green River Formation. Similar authigenic textures also occur in the iron-formation of the Hamersley Group (Miyano, 1982). The mineralogy and petrology of riebeckite-bearing iron-formations have been described by Hall (1930), Du Toit (1945), Genis (1964), Cilliers (19f'4), Beukes (1973, 1980) for the Transvaal Group; by Miles (1942), Trendall I Permanent address: Institute of Geoscience, University of Tsukuba, Ibaraki 305, Japan. 0003-004x83/0506-0517$02.00 517 (1965,1966), Trendall and Blockley (1970), Grubb (1971), Ayres (1972), Miyano (1976a, 1982), Klein and Gole (1981), Miyano and Miyano (1982) for the Hamersley Group;and by Dimroth and Chauvel (1973), Klein (1974), and, Zajac (1974) for the Sokoman Iron Formation. Ernst (1962) experimentally determined the stability field of riebeckite, which at relatively high temperature and pressure is bounded by the lower part ofthe stability field of acmite, at the hematite-magnetite (HM) buffer. Grubb (1971) showed, using experimental gel-runs that riebeckite may form at temperatures as low as 100-150'C and prefers low Eh and relatively low pH conditions. The stability relations of riebeckite in very low-grade metamorphic (late diagenetic) iron-formations, however, have not been quantitatively assessed because ofthe lack of available thermodynamic data and on account of experimental difficulties. In this paper, we briefly describe some of the mineralogy, petrology, and chemistry of riebeckite within the Dales Gorge Member of the Hamersley Group, Western Australia, as an introduction to the evaluation of the stability relations of riebeckite using the estimated thermodynamic properties. We also discuss the physicochemical environment of riebeckite (crocidolite) formation during very low-grade metamorphism (or late diagenesis). 5 lE MIYANO AND KLEIN: CONDITIONS OF RIEBECKITE FORMATION Mineralogy and petrology The Dales Gorge Member of the Brockman Iron Formation is a rock unit characterized by the most abundant riebeckite (crocidolite) among the various rock types of the Hamersley Group. Riebeckite is also known in the Joffre Member of the same sequence (above the Dales Gorge Member) and in the Marra Mamba Iron Formation, the lowest member of the Hamersley Group (Trendall and Blockley, 1970; Klein and Gole, 1981). The mineralogy and petrology of the Dales Gorge Member rocks have been described by Miles (1942), Trendall (1965, 1966), Trendall and Blockley (1970), Grubb (1971), Ayres (1972), and Miyano (1976a). Recently Miyano (1982) and Miyano and Miyano (1982) reported the common occurrence of ferri-annite from riebeckitebearing rocks of the Dales Gorge Member. This mineral appears to have been misidentified as ferri-stilpnomelane in earlier publications because of the optical and textural similarities of the two minerals. Ferri-annite is included in the reported mineral assemblages for such rocks, as Table 1. List of riebeckite assemblages in banded ironformations of the Dales Gorge Member, Hamersley Group, Western Australia shown in Table l. It appears texturally that riebeckite and also ferri-annite formed later than other minerals (Trendall and Blockley, 1970; Ayres 1972;Miyano 1976a,19821' Klein and Gole, 1981). The mode of occurrence of riebeckite may be classified as follows: (1) Crocidolite (fibrous variety of riebeckite) bands (mesobands) ranging from 2-150 mm thick; (2) Massive riebeckite bands (massive aggregates of fibrous to acicular riebeckite), ranging from 2-150 mm thick; (3) Thin layers (inclusive of veinlets) or lenses composed of fibrous to acicular riebeckite, generally less than 2 mm thick; and (4) Needle-like to prismatic riebeckite laths, dispersed in various mesoband matrices. Most of the riebeckite is concentrated in finely fibrous and massive bands. The other occurrences represent insignificant amounts. The textural types listed above are described in the following paragraphs.