Resolving the source and ore-forming processes of the Takab Iranian BIF using Fe and O isotope pairs in magnetite.

Abstract

Iron ore deposits from Iran are spatially related to the main suture zones of the Iranian continental fragmented block. In western Iran, the Sanandaj-Sirjan structural zone (SSZ) hosts several iron ore deposits interpreted being of volcano sedimentary, hydrothermal or mixed volcano sedimentary-skarn origin. In the northern part of the SSZ the early Cambrian (~530 Ma) Takab iron ore deposit consists of disseminated, layered and nodular magnetite mainly hosted in folded micaschists, and also in calcschists or metavolcanics. Quartz may show grain boundary migration and feldspar is partly altered. Accessory minerals are Mn-Ba-oxides, barite, monazite ± uraninite and Mn-carbonate (in calschists) in the matrix or in cross-cutting veins.The low concentrations of Cr (<30 ppm) and Ni (10 ppm), low Ti (15-200 ppm) and V< 100 ppm) and high Mn (1800-2600 ppm) are consistent with a hydrothermal origin. Nodular magnetite shows distinct higher Mn (9400 ppm) and disseminated magnetite higher Ti (1400 ppm).All magnetite types show positive Eu anomalies, stronger in nodular and disseminated magnetite, and strong positive Y anomalies, with Y/Ho ratios (25-40) similar to that of MOR- hydrothermal fluids.  The (La/Yb)PAAS ratios are >1in disseminated (1.2) and nodular magnetite (2.2), but <1 in layered magnetite (0.5-0.7). Nodular magnetite shows a negative Ce anomaly, similar to that of the calcschists. These results indicate mixing of hot hydrothermal fluid and seawater during the precipitation of the Takab BIF.In nodular magnetite the average ∂56Fe of -0.3 ‰ is typical of low T-hydrothermal environment, while the heavier ∂56Fe (1.4 ‰) in disseminated magnetite points to magmatic or magmatic-hydrothermal fluid. ∂56Fe data in the layered magnetite are variable (-0.2 to +1.12 ‰) but mostly in the magmatic-hydrothermal box of discrimination diagrams. ∂18O values are positive in disseminated and nodular magnetite (+2.15‰ and +5.30 ‰ respectively on average), and vary from -2.52 ‰ to +1.22 ‰ in layered magnetite.Based on the trace elements and REE data it can be concluded that primary layered magnetite ore crystallized statically from a Fe-Si rich mixed seawater and hot hydrothermal fluid. Regional deformation induced dynamic recrystallization of quartz, and disruption of magnetite bands.The chemical and isotopic signature of the disseminated magnetite points to a predominant imprint of an ortho-magmatic fluid. However, post primary mineralization hydrothermal alterations complicate the signal recorded by magnetite and evidence a complex story: for example, the lighter ∂18O of layered magnetite suggests re-equilibration with low temperature fluid. Similarly, the low ∂56Fe of nodular ore results likely from the precipitation of magnetite from a light hydrothermal fluid that may have dissolved a primary magnetite with heavy iron isotope signature. Moreover, re-equilibration with carbonated rocks likely results in the observed negative Ce anomaly and higher ∂18O (up to 6.30 ‰ on average).