This coexistence may be due to changes in redox conditions during a process of diagenesis or metamorphism, for example. In the image, it appears that the haematite (rectangles) is invading the red matrix (carbonate?) containing the pyrite. You would need to increase the magnification to see the contact (circles) between the two iron phases. You could then determine whether the pyrite is destabilised by the haematite (under the effect of an oxidising fluid), which would indicate that the latter is secondary and the former the precursor. In this case, we would be talking about two parageneses that follow one another in time, without predicting the time interval between them.

Youd have simply two discrete types of mineralisation, a reducing ankerite/Fe carbonate > hematite. These have only a relation in space but not in time.

HGD

Thank you for the explanation, Claude Fontaine & Harald G. Dill . The pyrites occurring within the carbonate bands—or those situated near the hematite bands—consistently show signs of oxidation and tend to develop a spongy texture. Owing to their fine, disseminated nature, I initially considered them to be sedimentary pyrites. However, under SEM, they appear well-crystallized and lack the framboidal textures typically associated with a sedimentary origin. This makes me wonder: is there a way to ascertain whether these can be reliably considered as primary iron minerals for geochemical analysis?

Please do not overinterpret on a microscopic scale these system changes. I see only a small part of what the mineralization looks like. BIF is a geological body as any other mineral deposits.

HGD