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Genesis of hypogene iron ore mineralization in the Serra Norte deposits

Carajas, Brazil

This PhD project investigated the mineralogy, zonation and fluid chemistry of the hypogene high-grade iron ore bodies at the Serra Norte deposits at Carajas Brazil which represents the world’s largest high-grade (>65% Fe) iron ore resource. These investigations were the first undertaken at Carajas and thus instrumental in defining the P-T-X conditions of hypogene ore formation at this giant iron ore system. A three stage genetic model was postulated which has subsequently been applied in the exploration for deep extensions or concealed iron ore bodies.  

The Serra Norte deposits are operating since 1985 and have produced over 1.5 billion tones of jaspilite-hosted iron ore. The paragenetic sequence of microcrystalline hematite magnetite martitised magnetite → micro-platy hematite → anhedral hematite → euhedral, tabular hematite reflects the mineralogical changes from early, to intermediate to advanced hydrothermal alteration of the unaltered jaspilite rocks. Bulk oxide samples from the advanced stages of mineralization are characterized by near-horizontal REE patterns; anhedral, lamellar and tabular hematites are characterized by a small increase in HREE also resulting in a flat REE pattern, when compared to paragenetically earlier magnetite-martite oxides. The advanced alteration zone which also constitutes the high grade hypogene iron ore is characterized by low salinity Na-K-Mg fluids trapped between 245 and 295° C. These fluids as well as the associated iron oxides display very low oxygen isotope values (up to -10 permil). In combination, the low salinity fluid and low oxygen isotope values of oxides and fluids in the advanced stage alteration, ie., the high grade ore bodies are interpreted to reflect gargantuan downward flow of Paleoproterozoic meteoric water into the already hydrothermally altered and mineralized jaspilte host rocks, thereby forming the Earths largest iron ore resource.