Hydrothermal Fluid Processes and Evolution of the Giant Serra Norte Jaspilite-Hosted Iron Ore Deposits, Carajas Mineral Province, Brazil

摘要

The Serra Norte Carajas banded iron-formation (BIF)-hosted iron ore deposits are located in the Carajas mineral province. The deposits are hosted in the ca. 2.7 Ga Grao Para 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. 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; (3) advanced alteration vein and breccia quartz-carbonate has low- to high-salinity fluid inclusions and trapping temperatures between 240 to 310, 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 ~(18)O_(SMOW) 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 ~(18)O 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 ~(18)O 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 ~(34)S values close to 0, consistent with a magmatic origin for the sulfur. Heavier ~(34)S values, of up to 10.8, in vein sulfides hosted in jaspilite, may reflect interaction with meteoric waters or, alternatively, variations in f_(O_2) and pH