Trace metal mobility during magnetite to hematite transformation in iron ores

Iron oxides are ubiquitous minerals in the earth’s crust and earth scientist are utilizing their chemical complexity to characterize rock/ore forming conditions. Compared to magnetite, only few studies exist on hematite, although alpha-Fe2O3 is an abundant alteration/weathering/metamorphic product in primarily magnetite-bearing rocks and ores – under certain conditions hematite represents the main iron oxide mineral. Hematitization of magnetite (including pseudomorphism after magnetite, i.e., martitization) take place by coupled dissolution-reprecipitation process (CDR) under redox and non-redox fluid-rock reactions in static or dynamic structural state. These mechanisms have impact on the iron oxide trace elemental budget. In order to understand better the chemistry of metamorphic- and alteration-induced hematitization in various ore settings (iron oxide apatite IOA, Fe-skarns, iron formation and related hematite ore), we report in-situ LA-ICP-MS data from hematite and precursor magnetite. The aims are to test the use of hematite in magnetite discrimination diagrams, and to define element proxies to characterize certain oxide transformation conditions.

Results show that hematitization significantly modify trace metal budgets in iron oxides. Metamorphic hematitization in various domains shows rather consistent depletion of low-valent and enrichment of high-valent elements. Alteration-induced martitization show contrasting mobility, allowing discrimination of hydrothermal from weathering-related condition, even within the same deposit. Elements commonly used in magnetite discrimination diagrams (particularly Al, Ti, Mn, Ni, Co, but also HFSE or REE) are variously modified by up to several orders of magnitudes, meaning that hematite analyses should be avoided in those diagrams. Further examination will aim to explain characteristic element mobility and “fingerprint” ore-forming/-alterating processes, including fluid-rock ratios.