The association of carbonate alteration of banded iron formation with the Carajás high-grade hematite deposits

Abstract

The Carajás iron ore deposits in the eastern part of the Amazonas Craton contain approximately 18 000 Mt of hematite ore with > 66%Fe. These deposits are hosted by jaspilitic banded iron formation (BIF) of the Carajás Formation which together with metavolcanic rocks comprise the ∼2·8 Ga Grao Para Group of the Itacaiunas Supergroup in the ∼1000 km long by 100 km wide, eastwest trending Carajás Synclinorium. The Carajás Formation comprises discontinuous layers and lenses of partly dolomitised jaspilitic BIF (17-43%Fe and 35-61%SiO₂), bodies of jaspilitic BIF and lenses of high-grade iron ore, cut by mafic sills and dykes. Dolomitisation of the BIF occurs mainly at the base of the Carajás Formation. With increasing amounts of dolomite, the BIF lamination (microbanding) and other fine primary structures are progressively obliterated resulting in irregular alternating carbonate and iron-oxide mesobands. The high-grade iron ores occur as tabular bodies of soft friable hematite with smaller lenses of hard (compact) massive hematite. The hard hematite lenses typically occur at the contact with the underlying metavolcanic rocks of the Parauapebas Formation surrounded by an aureole of hydrothermal carbonate alteration in both overlying and underlying rocks. The common association of carbonate altered and jaspilitic BIF with the Carajás hematite ore bodies is similar to the Mount Tom Price deposit where carbonate altered BIF has been recognised below massive hematite ore and a jaspilitic halo occurs above the ore body. This indicates that, as at Mount Tom Price, hydrothermal carbonate alteration and associated loss of silica from BIF may be an important step in the formation of the giant Carajás hematite deposits.