Abstract

The Serra Norte Carajás banded iron-formation (BIF)-hosted iron ore deposits are located in the Carajás mineral province. The deposits are hosted in the ca. 2.7 Ga Grão Pará Group, a metamorphosed volcanic-sedimentary sequence where jaspilites are under- and overlain by basalts, both at greenschist facies conditions. They represent one of the largest high-grade (>60 wt % Fe) BIF iron ore deposits and resources in the world, with hypogene iron mineralization considered to be Paleoproterozoic. Four main open pits have, to date, produced about 1.2 billion metric tons (Bt) of high-grade iron ore with additional resources of 10 Bt. Ore types at the Serra Norte deposits include soft and hard ore; the latter consists of banded, massive and/or brecciated ores and is mainly localized along the contact with the surrounding hydrothermally altered basalts.

Distinct hydrothermal alteration zones consist of veins and breccias that surround the hard ores, including: (1) an early alteration zone (distal portion of orebodies), characterized by recrystallization of jasper, formation of magnetite (± martite), and the local introduction of quartz and carbonate-sulfide (±quartz) veins; (2) intermediate alteration, synchronous with the main iron ore-forming event, which is accompanied by widespread development of martite, quartz-hematite and hematite-quartz veins, and dissolution of carbonate; and (3) proximal alteration zone having various types of hard and hard-porous hematite ores containing microplaty, anhedral, euhedral, and tabular hematite species. Locally, high-grade breccia ores contain dolomite and kutnahorite matrices indicating carbonate introduction. High-grade ore zones contain quartz ± carbonate-hematite veins and breccias.

Combined microthermometry, iron chromatography, and in situ laser ablation ICP-MS analyses on fluid inclusion assemblages from five vein types reveal that (1) early alteration vein-breccia quartz-carbonate contains high-salinity (up to 30 equiv wt % NaCl) fluid inclusions, with Ca, besides Na, K, and Mg, which were trapped at temperatures of 220° to 320°C. The quartz-hosted fluid inclusions have a wide range of Cl/Br ratios, presence of Li, base metals Cu-Pb-Zn, and Fe; (2) intermediate alteration vein quartz contains both low-salinity (Na-Fe-Mg-rich) and high-salinity (Ca-Mg-Fe-rich) fluid inclusions, with trapping temperatures of 210° to 290°C; (3) advanced alteration vein and breccia quartz-carbonate has low- to high-salinity fluid inclusions and trapping temperatures between 240° to 310°C, with the low-salinity inclusions being much more abundant in quartz. There is a gradual dilution of the metals signature in fluid inclusions from early to late- and/or advanced-stage veins and breccias.

The large amount of Ca in the fluid inclusions is compatible with extensive exchange of the hydrothermal fluids with the surrounding chloritized-hematitized metabasaltic wall rock.

Oxygen isotope analyses on different oxide species reveal that the heaviest δ18 OSMOW values, up to 15.2‰, are recorded for jaspilites, followed by magnetite, between −0.4 to +4.3‰, and then by different hematite species such as microplaty, anhedral and tabular, which fall in the range of −9.5 to −2.4‰. These results show a progressive depletion in δ18O values from the earliest introduced hydrothermal oxide magnetite toward the latest tabular hematite. The advanced alteration stage in high-grade ore displays the most depleted 18O values and represents the highest fluid/rock ratio during hydrothermal alteration. This depletion is interpreted to result from the progressive mixture of descending, heated meteoric water with ascending modified magmatic fluids. Sulfides from the distal zone of metabasaltic rocks have δ34S values close to 0‰, consistent with a magmatic origin for the sulfur. Heavier δ34S values, of up to 10.8‰, in vein sulfides hosted in jaspilite, may reflect interaction with meteoric waters or, alternatively, variations in fO2 and pH conditions during evolution of the hydrothermal fluid.

Calcite-kutnahorite δ13C and δ18O values from the distal alteration zones show a large δ13C range of −5.5 to −2.4‰ and a relatively narrow δ18O range of 9.3 to 11.7‰. However, dolomite matrix breccias from the advanced hydrothermal zone, i.e., ore, exhibit a wider δ18O range from 15.1 to 21.8‰ and a more restricted δ13C range from −5.0 to −3.9‰. This latter range points to a single carbon source, of possible magmatic nature, whereas the larger δ18O range suggests multiple carbon and oxygen sources. The 87Sr/86Sr ratios for carbonates from the distal and advanced hydrothermal zones range between 0.7116 to 0.7460, suggesting incorporation of strontium from multiple crustal sources, including magmatic-hydrothermal fluids.

A dual magmatic-meteoric hydrothermal fluid-flow model is proposed for the hematite ores in which an early, low Cl/Br ratio, saline, ascending modified magmatic fluid, caused widespread oxidation of magnetite to hematite. Progressive influx of light δ18O meteoric water, mixing with the ascending magmatic fluids, is interpreted to have been initiated during the intermediate stage of alteration. The advanced and final hydrothermal stage was dominated by a massive influx of low-salinity meteoric water, which maintained intermediate temperatures of 240° to 310°C, and concomitant formation of the paragenetically latest tabular hematite.

The giant Carajás iron deposits are unique in their setting within an Archean granite-greenstone belt and their modified magmatic-meteoric hydrothermal system, compared to the other two end-member BIF iron deposit types, namely the basin-related Hamersley type and the metamorphosed metasedimentary- basin-related Iron-Quadrangle-type. The distinct hydrothermal alteration signature present in both wall-rock basalts and jaspilites, in combination with distinct fluid chemistry signatures, particularly the low δ18O values of paragenetically late oxides indicative of massive influx of meteoric water into the high-grade orebodies, provide distinctive parameters for defining the Carajás end-member type BIF deposit class.

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