Stratigraphy And Geochemistry Of Neoproterozoic Iron Formation, South Australia

In the southern portion of the Adelaide geosyncline, the Neoproterozoic Yudnamutana Subgroup unconformably overlies older Neoproterozoic sedimentary rocks of the Adelaidean succession. The Braemar and Holowilena iron formations occur locally in the glacigenic Yudnamutana Subgroup at the transitional contact between the basal Pualco Tillite and overlying Benda Siltstone formations, or stratigraphic equivalents elsewhere.;Magnetite and/or hematite and absence of chert typify these iron formations which are associated with diamictite, subarkosic wacke, siltstone and minor carbonate. Diamictites are typically unstratified and have iron-poor ({dollar}{dollar}30 wt.% Fe{dollar}\\sb2{dollar}O{dollar}\\sb3{dollar}) matrices. The former commonly occur below the iron formation-bearing intervals whereas the latter are intercalated with them. Low Fe{dollar}\\sb2{dollar}O{dollar}\\sb3{dollar} diamictites are typified by plutonic and/or extra-basinal clasts and are considered to represent glacimarine deposition from iceberg melt-out during waning glacial conditions. The high Fe{dollar}\\sb2{dollar}O{dollar}\\sb3{dollar} diamictites locally exhibit evidence of ice grounding (glacial striae) and typically contain intraformational clasts incorporated by sediment gravity processes. Subarkosic Fe wackes and Fe siltstones generally contain 5-20% iron oxide minerals. Sharp-based, coarse-tail graded beds of subarkosic Fe wacke formed due to rapid deposition from high-concentration turbidity currents, whereas intervals of unstratified, ungraded and structureless subarkosic Fe wacke are considered to reflect deposition from density-modified grain flows.;Iron formation exhibits similar major, trace and rare earth element distributions to recent submarine hydrothermal deposits. The chemical composition of associated clastic rocks suggests that iron formation represents deposition of chemically precipitated iron oxides during periods of lessened clastic input. The iron-rich fluids probably emanated from extensional, fault-controlled conduits related to rifting during evolution of the Adelaide geosyncline.