Thomas Angerer

Banded iron formation to iron ore: A record of the evolution of Earth environments?

Banded iron formations (BIF) are the protolith to most of the world’s largest iron ore deposits. Previous hypogene genetic models for Paleoproterozoic “Lake Superior” BIF-hosted deposits invoke upwards, down-temperature fl ow of basinal brines via complex silica and carbonate precipitation/dissolution processes. Such models are challenged by the necessary SiO 2 removal. Thermodynamic and mass balance constraints are used to refi ne conceptual models of the formation of BIF-hosted iron ore. These constraints, plus existing isotope and halogen ratio evidence, are consistent with removal of silica by down- or up-directed infi ltration of high-pH hypersaline brines, with or without a contribution from basinal brines. The proposed link to surface environments suggest that Paleoproterozoic BIF-ore upgrade may provide a record of a critical time in the evolution of the Earth’s biosphere and hydrosphere.

A mineral system approach to iron ore in Archaean and Palaeoproterozoic BIF of Western Australia

Abstract This review paper examines banded iron formation-hosted higher-grade (>58 wt% Fe) iron ore types present in the two main metallogenic districts of Western Australia, the Yilgarn Craton and the Hamersley Province. The principal iron ore deposits from both districts exhibit variation in ore properties and genesis within and across districts, but also striking similarities. There are five critical elements involved in iron ore formation and preservation: (a) BIF iron fertility defined by stratigraphic and geodynamic setting; (b) Si-dissolving fluid flow; (c) high permeability at a range of scales; (d) exhumation and supergene modification; and (e) preservation of BIF-hosted iron ore bodies by surficial modification, cover or structures (downdrop, overthrust). Several subsidiary or constituent processes are important for the formation of distinct iron ore types and have expressions as (mappable) targeting elements. Deposits in the Hamersley Province record the presence of basinal brines and meteoric fluids, whereas deposits in the Yilgarn Craton, while less well constrained, suggest the influence of metamorphic/magmatic and meteoric fluids. A scheme for BIF alteration related to ore formation in a crustal depth continuum is presented, which integrates pressure-/temperature-dependency of assemblages, fluid–rock ratios and Si-dissolution capability and is a conceptual guide to prospective zones for iron ore.

Fabric evolution at basement–cover interfaces in a fold-and-thrust belt and implications for décollement tectonics (Autochthon, Lower Allochthon, central Scandinavian Caledonides)

Microstructural and magnetic investigations (anisotropy of magnetic susceptibility, AMS) on sections across basement–cover interfaces (BCI) revealed a complex evolution in the crystalline basement rocks beneath and in the basal units of the Caledonian fold-and-thrust belt: (1) Pre-Caledonian mylonitic fabrics in basement granite relate to steep shear zones. (2) Palaeoweathering formed smectite and illite at the expense of feldspar and mica. Secondary Fe-bearing clay minerals and the intensity of the chemical weathering control the bulk susceptibility. Changing susceptibility and AMS relate to a (time) sequence from primary magnetite to secondary paramagnetic clay to pyrite and ferrimagnetic pyrrhotite. (3) Burial compaction with BCI-parallel fabrics. (4) Caledonian cleavage, overprinted by décollement zones with S–C–C′ fabrics. Décollement cataclasis overprinted pre-existing magnetic fabrics and produced horizontal magnetic lineations and subhorizontal foliations defined by the S–C–C′ fabrics. Clay mineral enrichment, together with subsequent, BCI-parallel compaction fabrics, decreased the shear strength in the basement rocks beneath the BCI. Detachments initiated at such low-strength zones and produced allochthonous units with their footwall within crystalline basement rocks, an observation of general importance for orogenic fold-and-thrust belts.

The K Deeps magnetite mineralisation at Koolyanobbing, Western Australia

Abstract K Deposit (Dowds Hill), at Koolyanobbing, Western Australia, hosts enriched magnetite mineralisation below the presently mined goethite-hematite ore zone. Two highly magnetic bodies which are elongated NW–SE and dip to the NE, appear to reflect the geophysical signature of this deeper mineralisation. Magnetic modelling indicates significant remanent magnetisation with Koenigsberger ratios (Q ratios) around 1·5, directed shallow down. The NW anomaly is overlain by a ridge of uneconomic BIF and the present waste dump, whereas the SE body was partly exposed during ongoing mining of overlying hematite-goethite ore. Sections of the magnetite mineralisation (about 45–69 wt-%Fe) in this SE anomaly were intersected previously by diamond drill holes and further evaluated by reverse circulation (RC) drilling. Review of the historic drill cores identified potential magnetite enrichment, with grades and mineralogy that might be amenable to beneficiation. The following magnetite mineralisation assemblages are recognised from cores, RC drill cuttings and pit mapping of the SE magnetic body: higher-grade magnetite-hematite±goethite and magnetite±pyrite, and lower-grade, recrystallised magnetite-quartz (BIF), magnetite-carbonate, magnetite-talc, and minor magnetite-bearing mafic schist. Disseminated specular hematite is locally present in the assemblages. Talc-schist and massive pyrite occur along the footwall contact. Magnetic susceptibility measured from core and drill chips within the magnetite-enriched zones reveals an apparent susceptibility values range of 0·2 to 1·3 SI. Results from Satmagan and Davis Tube Recovery analysis show that the magnetite content in these mineralised zones correlates well with existing geophysical data and magnetic susceptibility measurements.