Sampling and methods of analysis Petrography and mineralogy

Fig. 2. Outcrop of ferruginous facies Braemar ironstone facies and associated Sturtian glaciogenic rocks Pualco Tillite in the eastern part of the Adelaide Geosyncline modified from Rogers, 1978; Forbes, 1991.

3. Sampling and methods of analysis

Thirty-nine samples BR1 – 12, BR29 – 47, BR49 – 56 were taken from surface outcrop and included laminated and diamictic ironstones, silt- stones, diamictites and carbonate-rich rocks which were representative of the rock types and occurrences in the Olary-Yunta region. In addi- tion, 17 samples BR13 – 28, BR48 were collected from the exploration adit dump at Razorback Ridge. Thin and polished thin sections and blocks were prepared and subsequently investigated by optical microscopy. Twenty-seven laminated iron- stone, clastic and carbonate sediment samples were crushed and pulverised in a chrome steel ring mill. Major and trace elements were analysed by X-ray fluorescence on duplicate fused discs and pressed powder pellets at the Division of Earth Sciences, University of New England UNE. Se- lected rare earth elements REE, La to Lu; LREE: light REE, La to Sm; HREE: heavy REE, Tb to Lu and additional elements As, Au, Hf, Sb, Sc, Ta, Th, U, W were determined on nine samples by instrumental thermal neutron activa- tion analysis at Becquerel Laboratories, Sydney. REE concentrations exceeded the detection limits by several orders of magnitude. In addition, data on geochemical reference materials were within 10 of the accepted values. Oxygen and carbon isotope mass spectrometry on 15 rock samples was conducted at the Centre for Isotope Studies, CSIRO, Sydney, following conventional CO 2 gen- eration using phosphoric acid. Electron mi- croprobe analyses were performed on garnets, carbonates and chlorites of ironstone and siltstone samples at UNE.

4. Petrography and mineralogy

The Braemar ironstone facies consists of lentic- ular laminated and diamictic ironstones interbed- ded in calcareous or dolomitic siltstone including several thin quartzite and dolostone units Fig. 3. Fig. 3. Stratigraphic succession of the Adelaide Geosyncline in the southeastern Nackara Arc a, b modified from Preiss, 1987; Preiss et al., 1998 and c Braemar ironstone facies in the Razorback Ridge area modified from Whitten, 1970. Fig. 4. A Typical outcrop of the Braemar ironstone facies. Ironstone is intercalated with carbonate-bearing siltstone and minor diamictite and dolostone. Near Bimbowrie Hill, AMG: 420 700 mE, 6 455 650 mN. B Diamictic ironstone with recrystallised carbonate-rich siltstone, quartz and carbonate clasts sample BR24. Razorback Ridge, AMG: 379 740 mE, 6 352 770 mN. C Laminated ironstone. Darker laminae are rich in magnetite and hematite, and lighter laminae in siliciclastic and carbonate components sample BR6. Field of view approximately 30 mm long, note scale bar in millimetres. Iron Peak, AMG: 384 100 mE, 6 353 900 mN. D Laminated ironstone with interbedded lighter coloured siltstone displaying cross-laminations and soft-sediment deformation sample BR28. Razorback Ridge, AMG: 379 740 mE, 6 352 770 mN. E Laminated ironstone with interbedded lighter coloured siltstone displaying soft-sediment deformation sample BR16. Razorback Ridge, AMG: 379 740 mE, 6 352 770 mN. The Braemar ironstone facies is made up of two types, diamictic and laminated ironstones, which are substantially different in macroscopic appear- ance Fig. 4B – E but apart from the clasts, identi- cal compositionally. The mineralogy of the laminated ironstones and the matrix of diamictic ironstones is simple: com- mon facies are fine-grained typically B 0.05 mm and composed of magnetite, hematite and quartz with minor muscovite, chlorite, biotite, carbonate, apatite, plagioclase and tourmaline. Associated siltstones contain abundant quartz, biotite, car- Fig. 4. Continued Fig. 4. Continued bonate, plagioclase, muscovite, chlorite, variable amounts of magnetite and hematite and traces of clinozoisiteepidote, tourmaline, zircon and pyrite altered to goethite due to supergene oxidation. Detrital mineral grains and lithic clasts occur in laminated and diamictic ironstones and siltstones. They are angular to subrounded and include scat- tered detrital grains of quartz, carbonate, plagio- clase, K-feldspar, muscovite and tourmaline, foliated sediments, siltstones, quartzites, and quartzofeldspathic and quartz-carbonate rocks. Detrital feldspars have been variably replaced by carbonate, muscovite and traces of chlorite and biotite, and biotite has been retrogressed to chlor- ite and traces of rutile. Diamictic ironstones are massive and clasts range in size from 10 mm to 1.2 m but are most commonly between 25 mm and 150 cm Fig. 4B. A few striated boulders were noted by Whitten 1970. Angularity and nature of the detrital grains and lithic clasts are similar to those found in the associated laminated ironstones and siltstones. Laminated ironstones are usually inequigranu- lar with grain sizes ranging from B 0.1 to 5 mm. Lamination is generally well developed and ranges from B 0.5 mm to 1 cm in thickness Fig. 4C – E. The laminae are defined by the relative abundance of magnetite and hematite, ranging from : 80 to : 20. Magnetite grains display varying degrees of martitization and larger subhedra are up to 0.1 mm in diameter. Rare pressure shadows of chlor- ite andor biotite are well developed adjacent to magnetite – hematite porphyroblasts. However, much hematite is not weathering-related, because grains display a preferred orientation oblique to compositional laminations, and late dilational veins contain magnetite, quartz, carbonate, goethite and platy hematite. Therefore hematite is, in part, syn- or pre-tectonic. Locally, nearly pure layers of Fe-oxides : 80 are present, with magnetite, hematite and quartz forming a metamorphic granoblastic aggregate. There are large variations in modal proportions of the major rock-forming minerals quartz, Fe oxides, carbonate and silicates within single rock specimens thereby forming fine layers of iron- stones and siltstones. Fe oxide-rich laminae dis- play sharp or gradational bases with associated siltstone layers. The Fe-oxide beds are commonly graded with magnetite decreasing and abundances of quartz and silicates increasing. Other sedimen- tary structures include cross-laminations Fig. 4D, microfaulting and piercement of laminae, and micro- to meso-scale folding of laminae Fig. 4D – E which may be the result of soft-sediment deformation. The Braemar ironstone facies has undergone regional metamorphism and deformation. The rocks display interlocking aggregates of mineral grains and rare porphyroblastic Fe oxide and carbonate grains. Slaty cleavage is commonly defined by the preferred orientation of layer sili- cates and hematite plates. The subhedral shape of the magnetite crystals, the presence of rare por- phyroblastic magnetite grains, together with the occurrence of magnetitehematite-bearing veins and foliated hematite, indicate that the magnetite and some hematite are of metamorphic origin and not detrital. The Fe oxides are intergrown with silicates and carbonates, with the mineral assemblages indicative of greenschist facies bi- otite grade metamorphism. Carbonates in the ironstones and associated ferruginous siltstones are ferroan dolom- ite Fe 0.01 – 0.10 Mn 0.00 – 0.03 Ca 0.48 – 0.53 Mg 0.37 – 0.46 CO 3 and ferroan calcite Fe 0.01 – 0.06 Mn 0.00 – 0.01 Ca 0.92 – 0.99Mg 0.00 – 0.02 CO 3 in composition and chlorite is typically ripidolite Si 2.61 – 2.73 atoms per for- mula unit and atomic FeFe + Mg, 0.27 – 0.63. Calculations using chlorite compositions on the AlIV – T plot of Cathelineau 1988 indicate chlorite growth at : 360 – 400°C. In the Bim- bowrie Hill region Fig. 2, the Braemar iron- stone facies is associated with manganiferous siltstone units : 1 m thick. These are composed of variable amounts of fine-grained B 0.05mm granoblastic carbonate, garnet, magnetite, quartz, plagioclase, muscovite and phlogopite Holm, 1995. Garnet is typically spessartine py 2.6 – 3.2 alm 4.2 – 9.0 spess 82.l – 87.2 gross 1.4 – 2.2 uvar 0 – 0.1 a-ndra 3.5 – 11.4 in composition, with carbonates including calcite, ankerite and manganoan magnesian sider- ite.

5. Geochemistry