ALBERT

Description: The enrichment of redox-sensitive trace metals in ancient marine sedimentary rocks has been used to determine the timing of the oxidation of the Earth's land surface. Chromium (Cr) is among the emerging proxies for tracking the effects of atmospheric oxygenation on continental weathering; this is because its supply to the oceans is dominated by terrestrial processes that can be recorded in the Cr isotope composition of Precambrian iron formations. However, the factors controlling past and present seawater Cr isotope composition are poorly understood. Here we provide an independent and complementary record of marine Cr supply, in the form of Cr concentrations and authigenic enrichment in iron-rich sedimentary rocks. Our data suggest that Cr was largely immobile on land until around 2.48 Gyr ago, but within the 160 Myr that followed--and synchronous with independent evidence for oxygenation associated with the Great Oxidation Event (see, for example, refs 4-6)--marked excursions in Cr content and Cr/Ti ratios indicate that Cr was solubilized at a scale unrivalled in history. As Cr isotope fractionations at that time were muted, Cr must have been mobilized predominantly in reduced, Cr(III), form. We demonstrate that only the oxidation of an abundant and previously stable crustal pyrite reservoir by aerobic-respiring, chemolithoautotrophic bacteria could have generated the degree of acidity required to solubilize Cr(III) from ultramafic source rocks and residual soils. This profound shift in weathering regimes beginning at 2.48 Gyr ago constitutes the earliest known geochemical evidence for acidophilic aerobes and the resulting acid rock drainage, and accounts for independent evidence of an increased supply of dissolved sulphate and sulphide-hosted trace elements to the oceans around that time. Our model adds to amassing evidence that the Archaean-Palaeoproterozoic boundary was marked by a substantial shift in terrestrial geochemistry and biology.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Konhauser, Kurt O -- Lalonde, Stefan V -- Planavsky, Noah J -- Pecoits, Ernesto -- Lyons, Timothy W -- Mojzsis, Stephen J -- Rouxel, Olivier J -- Barley, Mark E -- Rosiere, Carlos -- Fralick, Phillip W -- Kump, Lee R -- Bekker, Andrey -- England -- Nature. 2011 Oct 19;478(7369):369-73. doi: 10.1038/nature10511.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada. kurtk@ualberta.ca〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22012395" target="_blank"〉PubMed〈/a〉

Description: The Serra Pelada Au-Pd-Pt deposit is located within the Carajás mineral province, which also hosts the world-class iron ore and iron oxide-hosted copper-gold deposits of the Amazon craton in Brazil. The unusual low-temperature hydrothermal mineralization at Serra Pelada is epigenetic, hosted by metasedimentary rocks of the Águas Claras Formation and structurally localized in the Serra Pelada overturned syncline. The orebodies are controlled by the intersection of subvertical NE-trending fault zones with metasiltstones, mainly at the syncline’s hinge, with minor ore occurrences at the upper and lower limb. Intense tropical weathering over the last 70 m.y. has completely overprinted the shallow ore in and near the flooded open pit, but primary hydrothermal features are preserved in deeper drill core delineating the remaining resource. Gold, platinum, and palladium mineralization is associated with intense argillic alteration, hematite breccias, and silicification, with the highest grade ore hosted by brecciated metasiltstones that are highly enriched in amorphous carbon. Distal alteration zones comprise a reducing and oxidizing alteration front. The hydrothermal mineral paragenesis comprises kaolinite, quartz, sericite, amesite (Mg-rich Al-silicate), amorphous carbon, hematite, monazite, rutile, pyrite, and a complex assemblage of Bi-, Ag-, Pb-, Cu-, Co-, Ni-, Pt-, Pd-, and Au-bearing, chalcogenide (S, Se), and arsenide (As, Sb) minerals. Major element changes during hydrothermal alteration include C and Mg addition, K depletion, localized silica loss, and silicification with notable introduction of trace elements including light rare earth elements (LREE), Bi, Pb, U, V, Cu, Co, Ni, and As. The hydrothermal alteration and element association of the Serra Pelada deposit show geochemical similarities with unconformity-related uranium deposits, which may also be enriched in Au, Pd, and Pt and were formed by mixing of fluids that interacted with a highly oxidized cover sequence and highly reduced rock packages in structures of brittle-ductile strain.

Description: 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 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 conditions during evolution of the hydrothermal fluid. Calcite-kutnahorite 13 C and 18 O values from the distal alteration zones show a large 13 C range of –5.5 to –2.4 and a relatively narrow 18 O range of 9.3 to 11.7. However, dolomite matrix breccias from the advanced hydrothermal zone, i.e., ore, exhibit a wider 18 O range from 15.1 to 21.8 and a more restricted 13 C range from –5.0 to –3.9. This latter range points to a single carbon source, of possible magmatic nature, whereas the larger 18 O range suggests multiple carbon and oxygen sources. The 87 Sr/ 86 Sr 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 18 O 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 18 O 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.

Description: This study examines the whole-rock geochemistry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) iron oxide chemistry of itabirite and related iron ore, as well as underlying phyllitic wall rock, of the Pau Branco iron ore deposit, Quadrilátero Ferrífero, Brazil. The phyllite and iron ore of Pau Branco extend from hypogene deep-seated into supergene weathered zones. The phyllitic sequence that underlies the itabirite-hosted iron ore along a sheared contact includes (from the bottom to top) carbonaceous and carbonate-, chlorite-, and Fe-rich zones, which likely reflect distal to proximal alteration halos. The Fe-rich phyllite and iron ore comprise a complex iron oxide mineralogy: (1) magnetite-martite is hosted in itabirite and related medium-grade iron ore and present as disseminated minerals in phyllite; (2) microplaty hematite replaces chlorite and carbonate in the phyllite; (3) granoblastic hematite is a recrystallization/precipitation product and the dominant iron oxide of the hard iron ore; (4) schistose specular hematite overgrows earlier iron oxides; and (5) goethite replaces earlier hematite and amphibole within the weathering horizon. Whole-rock geochemistry and related mass balance calculations reveal that, unlike the itabirite-hosted iron ore, interpreted to be the result of a solely residual iron enrichment via depletion of most major oxides and trace elements, the iron enrichment of the phyllite is caused by addition of Fe 2 O 3 . The Fe 2 O 3 is likely sourced from the overlying itabirite/iron ore and is mineralogically reflected in the replacement of carbonate and chlorite by hematite. The rare earth elements (REE) abundances of the different ore and phyllite zones reveal seawater-like REE signatures and Y/Ho ratios of the itabirite, hypogene and supergene iron ore, shale-atypical REE patterns in the unaltered phyllite, and Eu fractionation trends. This suggests a distinct input of hydrothermal fluids during the Fe enrichment of the phyllite. Locally, restricted Ce fractionation is attributed to distinct (e.g., Mn-rich) lithostratigraphic intercalations within the itabirite. Laser ablation-ICP-MS mineral chemistry of phyllite-hosted martite and microplaty hematite suggests a strong chemical inheritance by the host rock and precursor mineral. Positive Ce anomalies are restricted to phyllite- and itabirite-hosted martite only and, therefore, may reflect the highly oxidative conditions during martitization. At Pau Branco, the fluid-rock interaction affected not only the itabirite, but also the underlying phyllite, resulting in distinct alteration halos that extend beyond the itabirite toward the footwall. The significant chemical influence and importance of country-rock lithologies as a possible elemental source, for example, must be taken into account when interpreting whole-rock geochemical data and investigating an itabirite-/banded iron formation-hosted iron ore system.