Iron-oxides constrain BIF evolution in terranes with protracted geological histories: the Iron Count prospect, Middleback Ranges, South Australia

Abstract

When studied at appropriate scales of observation and using complementary methods, the intrinsic textural and geochemical complexity of iron-oxides can provide unparalleled insights into the evolution of iron ore resources from banded iron formation deposition to ore formation. Iron Count is one of >20 deposits and prospects within the Archean Middleback Ranges iron ore belt, South Australia, in which iron-oxides from BIFs and ores display variable textural and trace element signatures. Integrated petrography, iron-oxide laser-ablation mapping and geochemistry, and in-situ dating of BIF-derived ores was undertaken to assess whether features observed in samples from this representative prospect might be attributed to localized and superimposed overprint processes. The prospect consists of a West and East ridge, each characterized by a sequence of overprints expressed as interconversions between iron-(hydr)oxides and accompanying variation in trace element concentrations. Pseudomorphic replacement of early magnetite by hematite (martite), followed by replacement by iron-hydroxides is recognized in samples from the East Ridge. Further overprints are expressed as veining, brecciation and crystallization of (micro)platy hematite rich in granitophile elements (Sn, Mo, W, U). Martitization of early magnetite in the West Ridge is accompanied by enrichment in Zn, Co and Ni in martite. Subsequent recrystallization of martite/iron-hydroxides by granoblastic hematite is marked by enrichment in Ti, Ta, Nb, REE and granitophile elements, and co-crystallization of REE-minerals. This is followed by crystallization of (micro)platy hematite, similarly rich in these elements. Enrichment in Ti and granitophile elements in granoblastic hematite is interpreted to infer crystallization from highly saline fluids generated by interaction between evaporite-bearing sedimentary rocks and granite-derived fluids. Post-Archean Australian Shale-normalized REY fractionation trends of iron-oxides from the East Ridge are consistent with precipitation of iron-rich minerals from a mixture of anoxic seawater and hydrothermal vent fluids. Analogous trends from the West Ridge lack a seawater signature. Contrasts in REY fractionation trends between the two ridges is interpreted as a combination of the loss of primary seawater signature due to severe overprinting in the West Ridge, and to lateral variation in the marine depositional environment. Results validate the hypothesis that variation in iron-oxide trace element signatures seen throughout the Middleback Ranges iron ore belt results from overprinting due to superimposed tectono-magmatic events that have locally affected each deposit. Despite undergoing different sequences of ore-forming stages, as well as possibly distinct depositional environments, the two ridges share a common overprint, dated here at ~1790 Ma using in-situ U–Pb geochronology of co-existing hematite, monazite and xenotime. This age is considered to represent the timing of interaction between granite-derived fluids with ore-hosting rock. Such interpretation is supported by the geochemical signatures of the dated iron-oxides showing enrichment in granitophile elements and coincident ages for the Wertigo Granite and Myola Volcanics, which occur proximal to the prospect and were emplaced/erupted during intracontinental rifting. The approach undertaken here, combining petrography, iron-oxide geochemistry and in-situ dating of BIF-derived ores is generically applicable to other iron resources within the Middleback Ranges or analogous terranes elsewhere.