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Hydrothermal and metamorphic fluid-rock interaction associated with hypogene “hard” iron ore mineralisation in the Quadrilátero Ferrífero, Brazil: implications from in-situ laser ablation ICP-MS iron oxide chemistry

journal contribution
posted on 2023-05-18, 10:04 authored by Hensler, A-S, Hagemann, SG, Rosiere, CA, Angerer, T, Gilbert, S

In-situ laser ablation ICP-MS analyses on iron oxides in itabirite and iron ore from the Quadrilátero Ferrífero (Brazil) reveal a wide range in trace element abundances (e.g., average concentrations in hematite: Al = 40–2200 ppm, Mg = 1–930 ppm, Mn = 5–540 ppm, Ti = 3–500 ppm, V = 2–390 ppm, Cr = 1–98 ppm, As = 0.5–60 ppm). The chemistry of early hematite stages is mostly inherited from host rock and precursor magnetite, e.g., Mn concentrations correlate with bulk Mn content in itabirite. With progressive iron ore formation and modification, external fluids play a more prominent role. This is reflected by REE-Y switching from seawater-like Y/Ho ratios (> 44) in early-, to more chondrite-like Y/Ho ratios (< 34), in late-hematite stages, likely due to fluid–rock reactions with country rocks (e.g., phyllites) or exchange with magmatic hydrothermal fluids.

The following ore formation stages and key processes, supported by mineral scale mass balance calculations, are constrained: (1) martitisation, cogenetic with gangue leaching, is driven by large volumes of oxidising, Si-undersaturated fluids resulting in an absolute depletion of Mg, Mn, Al, Ti, Ni and Zn, and enrichment of Pb, As, LREE and Y; (2) the formation of granoblastic hematite and locally microplaty hematite represents a largely isochemical recrystallisation of magnetite and/or martite accompanied by a depletion of Mg and Y and an elevated Ti mobility at the mineral scale; and (3) precipitation of schistose and vein-hosted specular hematite along shear and fracture zones is driven by an external Fe–Si-rich hydrothermal fluid likely under high fluid/rock ratios.

History

Publication title

Ore Geology Reviews

Volume

69

Pagination

325-351

ISSN

0169-1368

Department/School

School of Natural Sciences

Publisher

Elsevier Science Bv

Place of publication

Po Box 211, Amsterdam, Netherlands, 1000 Ae

Rights statement

© 2015 Elsevier

Repository Status

  • Restricted

Socio-economic Objectives

Iron ore exploration

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