Fe-Si-gel like injections in banded iron formations of the Musselwhite Mine (Canada)

A sample of banded iron formation from a drill core from the Musselwhite Gold Mine, Ontario, Canada, shows millimeter-wide opaque veins at a low angle to the foliation with perpendicular offshoots. The veins are non-crystalline, structureless and chemically homogeneous, with oxide weight percentages of 35 % FeO, 40 % SiO2, 5 % MnO, 5 % MgO and 1 % CaO. The remaining ca. 15 % might be attributed to water content. Despite the water, the composition is very similar to the bulk rock composition, consisting mainly of grunerite and quartz. The overall geometry of the veins resembles the appearance of a pseudotachylyte with injection veins, partly sharp boundaries, and clasts of the host rock. However, no other indicative observations such as flow banding, quenched margins or corrosion of clasts can be made. Instead, the vein is overgrown by euhedral quartz, nucleating from wall rock quartz crystals. Some of the material is emplaced in a calcite vein where the calcite seems to be fragmented. EBSD orientation data show that the fragments have the same orientation as calcite crystals remaining attached to the wall rock. Calcite crystals are therefore not displaced, but rather dissolved or replaced by the vein filling material, but without leaving a chemical signature. We therefore favor an Fe-Si rich gel-like injection origin of the veins. It remains unclear, how the gel was produced and how it is mechanically possible to inject such a gel into a solid rock and replace calcite.