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Multi-stage structural controls and hypogene alteration of high-grade iron mineralisation at Matthew Ridge prospect, Jack Hills greenstone belt, Yilgarn craton

Jack Hills greenstone belt, Yilgarn craton, Western Australia

This honours project deals with the characterization of structures and iron ore types ate the Matthew Ridge BIF-hosted high grade (>55% Fe) iron ore prospect in the Jack Hills greenstone belt of the southeast Narryer terrane, Yilgarn Craton.  

This ~3000 Ma Algoma type BIF is host to magnetite-hematite ore zones that have undergone a series of pervasive hydrothermal alteration events, with a later destructive talc alteration stage that is unlike any other documented BIF-hosted deposit in Western Australia. The prospect includes Archean gneiss, metasedimentary rocks, and BIF that are interlayered with dolerite. All rocks are metamorphosed to peak amphibolite facies conditions and have experienced a retrograde greenschist facies metamorphic overprint.  Hypogene iron ore can be directly correlated to three deformation and five main hydrothermal alteration events; (1) Initial replacement of silica-rich bands in the BIF by hypogene magnesite and magnetite; (2) removal of hypogene magnesite to form magnetite-rich residual high-grade ore zones. These pods trend NE and correspond to the hinge zones of parasitic, Z-shaped tight folds that formed in response to NW-SE directed shortening prior to high-grade ore formation.

Their chemistry is commonly characterised by enrichments in Fe and depletions in SiO2, with only minor changes in other major oxides and trace elements compared with least-altered BIF; (3) shear zone-hosted, talc–magnetite alteration; (4) talc-microplaty hematite alteration. The second deformation event coincides with both stages of talc alteration involving NNE-SSW directed shortening. This resulted in open, N-S-trending F2 folds and shear zones that cut the hypogene ore zones. Followed by a later stage, third deformation event that resulted in at least two generations of NE- to NW-trending extensional veins and strike-slip faults; Finally (5) near-surface, supergene alteration exploited pre-existing structures involving the replacement of silica-rich and carbonate minerals by goethite, and the oxidation of hypogene magnetite and hematite. The Matthew Ridge prospect represents an Archean Algoma-type BIF-hosted deposit whereby structural and alteration process have been crucial for the formation and localisation of ore bodies.