Re-equilibration processes in magnetite from an Iranian BIF deposit.

Magnetite is common in various iron ore deposits including BIF, Fe-skarn, IOGC (iron oxide-copper-gold), IOA (iron oxide-apatite) and porphyry Cu-Au deposits. Magnetite incorporates a number of trace elements in tetrahedral and octahedral site in its structure. Its trace element composition has been used in numerous studies to fingerprint deposit type and ore genesis and to provide a guide for mineral exploration, based on a series of discriminant diagrams (e.g. [1]). However, the applicability of such diagrams must be carefully evaluated as the composition of magnetite can be modified by various processes. In this study we present textural and compositional data for magnetite from a late Ediacaran BIF in NW Iran[2] to illustrate complex re-equilibration processes.

Three stages of magnetite (Mt) has been identified. Mt1 forms large (≤1 mm) inhomogeneous dark grey grains surrounded and locally invaded by a light grey porous Mt2. The Mt1/Mt2 boundary is irregular and sharp, consistent with replacement textures. Bright Mt3 forms needle-like bands (10-80 µm width) aligned along fracturing planes. Mt3 contains rare hematite relicts and is porous in close proximity to hematite.

Mt1 shows a variable trace element composition and contains the highest Si (average 1.14 wt.%), Al and Ca (0.28 wt.%), Mg (0.13 wt.%) and the lowest Fe (68 wt.%) contents. Both Mt2 and Mt3 show a restricted range of composition. Mt2 has lower Si (0.68 wt.%), Al (0.14 wt.%), Ca (0.15 wt.%) and Mg (0.08 wt.%) contents, while Mt3 is characterized by the lowest Si (0.16 wt.%), Al and Ca (0.05 wt.%) and Mg (0.01 wt.%) and the highest Fe (71.1 wt%) contents. The three magnetite have low Mn (≤0.03 wt.%), and Ti (≤0.02 wt.%), and Ni and Cr are mostly below the detection limit.

The silician dark Mt1 magnetite likely forms in a rather reduced Si-rich environment. The presence of structural silicon is supported by correlations / antithetic correlations with R²⁺/R³⁺ cations and the lack of inclusions. The incorporation of Si may cause lattice defects or deformation facilitating fluid alteration. A fluid-assisted coupled dissolution of Mt1 and precipitation of Mt2 (CDR process) is supported by close spatial relationship, sharp compositional boundaries, similar crystallographic structure of MT1 and Mt2 and abundant porosity in Mt2. The increase in porosity promotes the infiltration of hydrothermal fluids and further advances the CDR process. By removing trace elements from the early Mt1 this process increases the iron grade of Mt2.

Micro-fracturing allows the penetration of a more oxidized fluid along cleavage planes and formation of needle-like bands of hematite. Then porous mushketovite Mt3 formed after hematite under a more reduced fluid composition by a redox transformation supported by the volume decrease.

All these processes significantly modified the texture and composition of the magnetite and point to a predominant imprint of hydrothermal fluid, thus causing difficulties in using magnetite as a genetic indicator.

 [1] Dupuy and Beaudoin, 2011; [2] Honarmand et al, in press.