Martitization is pre-depositional alteration. There are two types of martitization , namely normal and heating.
Normal martitization: Sometimes complete replacement by martite takes place. Martitization usually starts at the grain borders and proceeds inwardly either parallel to the octahedral planes of magnetite or grow irregularly along cracks.
Heating martitization: This type of martitization is of important genetic significance. Ramdohr (1955) suggested that this type of alteration indicates that magnetite belongs to extrusive lava and explosive volcanic fragments. In heating martitization the martite appears as broads lamellae or bands traversing the whole grain and forming uniform pattern of line network parallel to the (111) planes of original magnetite.
Martitization almost occurs along the weakness points of the original mineral (magnetite) such as; edges, cracks, fracture planes or cleavages.
https://www.researchgate.net/project/High-grade-iron-ores-as-paleoweathering-crusts-paleolaterites
In dikes, along 111 mineral planes martization and NOT on rims, variable oxidation is likely due to cooling and heating of a magmatic source as the magmatic is contaminated during ascent, recharged during differentiation. The issue in porphyry systems the process seems best related to buffering, particularly by polyvalent sulfur species...anhydrite-sulfide etc. Below see images from 16.4my Andean Miocene Milena Porphyry system showing residual grains of iron opaques hosted in quartz sericite matrix from an altered "quartz-flooded feldspar hornblende augite-bearing porphyry". Image shows non-rimming lamellar alteration parallel to 111 on one grain and perpendicular to 111 face on another. Likely it is residual magmatic not "secondary-alteration".
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