A Hydrothermal Origin for the Jaspilite-Hosted, Giant Serra Norte Iron Ore Deposits in the Carajás Mineral Province, Pará State, Brazil
E Silva Rosaline Cristina Figueiredo, Lydia Maria Lobato, Carlos Alberto Rosière, Steffen Hagemann, Márcia Zucchetti, Franciscus Jacobus Baars, Roberta Morais, Ivan Andrade, 2008. "A Hydrothermal Origin for the Jaspilite-Hosted, Giant Serra Norte Iron Ore Deposits in the Carajás Mineral Province, Pará State, Brazil", Banded Iron Formation-Related High-Grade Iron Ore, Steffen Hagemann, Carlos Alberto Rosière, Jens Gutzmer, Nicolas J. Beukes
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The Carajás iron ore deposits located in the southern part of the state of Pará in Brazil were discovered in 1967 and have produced about 70 million metric tons (Mt) of iron ore annually. The deposits are hosted by the Neoarchean metavolcano-sedimentary sequence of the Grão Pará Group, Itacaiúnas Supergroup. The protoliths to high-grade iron ore in the Serra Norte deposits are jaspilites, which are under- and overlain by basalts. The major Serra Norte N1, N4E, N4W, N5E, and N5S iron ore deposits of the Carajás mineral province are distributed along, and structurally controlled by, the northern flank of the Carajás fold. High-grade iron mineralization (>65% Fe) is made up of hard and soft ores. The hard ores can be banded, massive and/or brecciated, and are characterized by hematite-martite and hematite types. The soft ores are very porous, discontinuous and tabular, friable and banded. The basal contact of high-grade iron ore is defined by a hydrothermally altered basaltic rock mainly composed of chlorite and microplaty hematite.
Varying degrees of hydrothermal alteration have affected jaspilites to form iron ores. The study of variably altered jaspilites and hard ores indicates that the distal alteration zone represents an early alteration stage. It is mainly characterized by the recrystallization of jasper and the removal of its iron, and the formation of magnetite (commonly martitized), overgrowing original microcrystalline hematite and associated with quartz and calcite veins. Two vein breccia types characterize the distal alteration zone: V1a (quartz ± sulfide breccias) and V1b carbonate ± sulfide breccia veins). Sulfides are pyrite and chalcopyrite. The intermediate alteration zone, synchronous with the main iron ore-forming event, is characterized by (1) progressive leaching of chert and quartz, leaving oxides and vugs; (2) presence of martite as the dominant oxide along altered jaspilite layers; and (3) partial filling of open spaces with microplaty and/or platy hematite. The intermediate alteration zone also contains the V2a (quartz ± hematite bedding-discordant veins), V2b (vug-textured quartz + hematite discordant vertical veins), and V3 (hematite ± quartz veins crosscutting and/or parallel to the jaspilite bedding). The proximal alteration zone, also synchronous with the iron ore-forming event, represents an advanced alteration stage (i.e., the high-grade iron ore) and is characterized by progressive martitization, forming anhedral hematite, continued space filling by comb-textured euhedral and tabular hematite in veinlets and along banding. The proximal alteration zone contains intense carbonate alteration associated with the high-grade ores, resulting in the formation of ore breccias cemented by dolomite. Vein breccias are classified as V4 (carbonate (iron cloud)-quartz breccia), and V5 (quartz ± microplaty hematite breccia), both located in high-grade ore.
The distribution of the rare earth elements in variably altered jaspilites and hard ores follows two main distinctive patterns. Jaspilites from the N4W, N5E, and N5S deposits, and hard ores from N1 and N4E have a low ΣREE content, are enriched in light REE, and exhibit positive europium anomalies (Eu/Eu* >1), which is typical of Archean banded iron formations. The REE pattern defined by N5E ores is nearly flat and displays an increase in ΣREE and absence of the positive Eu anomaly. The increase in LREE was accentuated during the formation of magnetite and microplaty hematite and the advance of martitization to form anhedral hematite. This may have favored the relative increase of HREE in the residual fluid, resulting in an increase in HREE in advanced-stage precipitates and almost flat REE patterns associated with the advanced stage of mineralization. It is during this hydrothermal stage that euhedral and tabular hematite are dominant. The REE increase in N4E and N5E ore samples further suggests the presence of significant amounts of Fe in the mineralizing fluid.
The first evidence for hydrothermal fluids that infiltrated the jaspilites is the vein breccia type 1, which contains Ca-Fe-rich, high-salinity (up to 29.3 wt % CaCl2 equiv) fluid inclusions in quartz and carbonate with Ttapping of 209° to 285°C. The next stage of hydrothermal fluid infiltration is characterized by vein type 2, which contains medium-to high-salinity Na-Fe-Mg-rich (13.6–21.2 wt % CaCl2 equiv) and Ca-rich fluid inclusions (6.8–18.4 wt % CaCl2 equiv) with Ttrapping of 225° to 275°C and 190° to 295°C, respectively. Vein type V3 is characterized by low- and medium-salinity Ca-(Mg)-Fe-Na-rich inclusions (1.2–19.2 wt % CaCl2 equiv) with Ttrapping of 195° to 255°C and medium-salinity Na-Mg-rich fluid inclusions (8.9–14.4 wt % CaCl2 equiv) with Ttrapping of 240° to 277°C. Brecciated vein types V4 and V5 have Ca-rich, medium- to high-salinity fluid inclusions in quartz and high-salinity inclusions in carbonate (9.7–24.5 and 19. 2–30.1 wt % CaCl2 equiv, respectively), both trapped at 237° to 314°C, and low-salinity Na-K-Mg fluid inclusions (0.2–7.3 wt % NaCl equiv) trapped at 245° to 316°C.
Oxygen isotope analyses on quartz from V1 to V3 veins range from +10 to +18 per mil, respectively, and –1.0 per mil on martite to –10.0 per mil for the paragenetically latest euhedral-tabular hematite in the high-grade ores. This shift in δ18O values of oxides may reflect influx of meteoric water during the advanced hydrothermal alteration stage and/or represents a result of intense fluid fluxes (i.e., high fluid/rock ratios). Sulfur isotope analyses of pyrite within distal V1 veins display a range of δ34S from +2.5 to +10.8 per mil, with lighter δ34S values (-5 to +5‰) in sulfides from the intermediate alteration zone of the wall-rock basalts; these latter values are compatible with juvenile magmatic fluids. Carbon and oxygen isotopes on carbonates (i.e., calcite, kutnahorite, and dolomite) from V1 and V4 vein types revealed a restricted range of δ13C from –6.0 to –2.0 per mil and a wider range of δ18O from +8.0 to +20.0 per mil, suggesting variable oxygen sources due to interaction with more than one fluid type, or significant changes in fluid-rock ratios during interaction with a heavy δ18O fluid, possibly magmatic. Strontium isotope (87Sr/86Sr) ratios of calcite ± kutnahorite (V1 vein type) in equilibrium with magnetite, and kutnahorite-dolomite (V4 vein type) range from 0.712 to 0.750. The extremely radiogenic 87Sr/86Sr values from V1 vein-type carbonates are probably only compatible with a granitic source. The mineralogical, geochemical, and isotopic changes from jaspilites to high-grade iron ores suggests a hydrothermal origin for hard ore via interaction with an early-stage medium- to high-salinity Ca- and Ca-Fe-rich, relatively reduced magmatic fluid, which leached silica and formed magnetite. This fluid evolved to more oxidizing conditions, with the advance of martitization, increase in the REE concentration, and microplaty hematite precipitation in veins and martite borders. Low δ18O values of oxides suggest mixing with meteoric water during this intermediate hydrothermal alteration stage. The predominance of oxidized phases such as anhedral and euhedral and/or tabular hematites, low-salinity Na-rich fluid inclusions, and decreasing oxygen isotope values toward late hematite types, indicate that the advanced alteration stage is dominated by the meteoric fluids.
The proposed magmatic-meteoric hydrothermal mineralization model for the Carajás hard ores is substantially different from models for the Hamersley or Iron Quadrangle iron ores but may have a genetic link to the numerous Proterozoic magmatic hydrothermal deposits in the Carajás mineral province. The new hydrothermal model has also significant implications for iron ore exploration under cover sequences and/or the exploration for deep extensions of existing shallow orebodies. New exploration parameters include the distinct structural control of ore zones by faults and folds, widespread hydrothermal alteration zones, and related pathfinder minerals and chemical pathfinder elements such as REE, Ca, Na, Fe, and S.