Discriminant diagrams for iron oxide trace element fingerprinting of mineral deposit types

Mineralium Deposita (Impact Factor: 2.67). 04/2011; 46(4):319-335. DOI: 10.1007/s00126-011-0334-y

ABSTRACT Magnetite and hematite are common minerals in a range of mineral deposit types. These minerals form partial to complete solid
solutions with magnetite, chromite, and spinel series, and ulvospinel as a result of divalent, trivalent, and tetravalent
cation substitutions. Electron microprobe analyses of minor and trace elements in magnetite and hematite from a range of mineral
deposit types (iron oxide-copper-gold (IOCG), Kiruna apatite–magnetite, banded iron formation (BIF), porphyry Cu, Fe-Cu skarn,
Fe-Ti, V, Cr, Ni-Cu-PGE, Cu-Zn-Pb volcanogenic massive sulfide (VMS) and Archean Au-Cu porphyry and Opemiska Cu veins) show
compositional differences that can be related to deposit types, and are used to construct discriminant diagrams that separate
different styles of mineralization. The Ni + Cr vs. Si + Mg diagram can be used to isolate Ni-Cu-PGE, and Cr deposits from
other deposit types. Similarly, the Al/(Zn + Ca) vs. Cu/(Si + Ca) diagram can be used to separate Cu-Zn-Pb VMS deposits from
other deposit types. Samples plotting outside the Ni-Cu-PGE and Cu-Zn-Pb VMS fields are discriminated using the Ni/(Cr + Mn)
vs. Ti + V or Ca + Al + Mn vs. Ti + V diagrams that discriminate for IOCG, Kiruna, porphyry Cu, BIF, skarn, Fe-Ti, and V deposits.

KeywordsMagnetite–Hematite–Mineral deposit–Electron microprobe–Mineral chemistry–Discriminant diagram

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    ABSTRACT: Magnetite is a common mineral found in a wide range of mineral deposits and in different geological environments. The study of surface textures and morphology of magnetite can provide information that is useful to 1) discriminate different types of magnetite such as that attributed to magmatic, metamorphic and supergene environments, 2) identify host bedrocks, 3) sediment provenance, and 4) recognize chemical and mechanical processes affecting grains during erosion, transport, and after deposition in sedimentary environments. In this study, magnetite grains from the Izok Lake volcanogenic massive sulfide deposit (Nunavut, Canada) and from till covering the area have been investigated using scanning electron microscopy, mineral liberation analysis, and optical microscopy to document their mineral associations, surface textures, grain shape and size distribution. Evidences such as 1) contact relations between magnetite and sphalerite, 2) sphalerite and chalcopyrite inclusions in magnetite, and 3) intergrowths of magnetite with actinolite and gahnite suggest that in Izok Lake deposit and related gahnite-rich stringer zone, magnetite formed by replacement of sulfide minerals during regional, upper greenschist to amphibolite facies metamorphism. Magnetite from iron formation also formed as a result of oxidation-dissolution of almandine, or breakdown of Fe-bearing minerals during metamorphism. Euhedral, fine-grained magmatic magnetite in association with ilmenite, plagioclase and hornblende was identified in bedrock gabbro. Magnetite overgrowths on the surface of existing magnetite and other metamorphic minerals fingerprinted the supergene processes affecting bedrocks and sediments after metamorphism.
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