Supergene marcasite, replacing pyrrhotite.
2, 3, 4

Most occurrences of marcasite seem to be of supergene origin. An interesting mode of occurrence is in connection with pyrrhotitebearing ores at the border between the oxidation- and cementation zones, the marcasite here representing the first stage in the process of secondary enrichment. Secondary iron sulfides, deposited under these conditions, are not very common. It is generally only pyrrhotite, which undergoes this sort of replacement, because it is most easily soluble among the ordinary sulfides.