Algoma-type Banded Iron Formation Research Articles

Chronology and geochemistry of Neoarchean BIF-type iron deposits in the Yinshan Block, North China Craton: Implications for oceanic ridge subduction

The Dongwufenzi iron deposit is a typical Algoma-type Banded Iron Formation (BIF) in the Yinshan block of the North China Craton. The iron orebodies occur as layers interlayered with plagioclase amphibolite. LA–ICP-MS U–Pb dating of zircons from the plagioclase amphibolite yielded protolith crystallization age of 2538±9Ma (MSWD=1.01), followed by metamorphism at 2452±7Ma (MSWD=0.68). The geochemical characteristics of the ores including low Al2O3, TiO2 and HFSE contents, pronounced positive Eu anomalies, and the Y/Ho ratios (26–37) suggest that the BIFs precipitated from hydrothermal fluids discharged on the seafloor. The geochemical features of the plagioclase amphibolite suggest the protolith magma to be high-Fe basalt. Sm–Nd analyses of the plagioclase amphibolite yielded εNdt (t=2.54Ga) and single-stage depleted mantle Nd model ages (TDM) in the range of ca. +1.8 to +3.0 and 2.68 to 2.81Ga, respectively, suggesting an enriched mantle source. We link the source of Fe in the BIFs to the komatiitic complex in the same greenstone belt, and the source of Si with the high-Fe basalt. Integrated geochronological and geochemical data from the other Archean rocks in this area suggest that the Dongwufenzi BIF formed at 2.5Ga, coeval with the other BIFs in the North China Craton (NCC) as well as similar occurrences in other parts of the world. We propose a geodynamic model involving ridge subduction and slab window for the formation of the BIFs and the associated rock suites in the greenstone belt.

Hydrothermal alteration events controlling magnetite-rich iron ore at the Matthew Ridge prospect, Jack Hills greenstone belt, Yilgarn Craton

Archean Banded Iron Formation (BIF) hosts high-grade (>55% Fe) iron ore at the Matthew Ridge prospect, in the Jack Hills greenstone belt, of the Narryer terrane, Yilgarn Craton. The ca 3000 Ma (SHRIMP U–Pb on zircons) Algoma-type BIF contains magnetite–hematite ore zones that are a product of successive overprinting hydrothermal alteration events. Rock types in the prospect area include Archean gneiss, metasedimentary rocks and BIF that are interlayered with dolerite. All rocks record peak amphibolite metamorphic mineral assemblages that are variably replaced by greenschist facies metamorphic minerals. Hypogene iron ore formed owing to: (1) the replacement of primary silica-rich bands in the quartz–magnetite BIF by Stage 1 hypogene magnesite and magnetite alteration; and (2) subsequent removal of Stage 1 magnesite to concentrate residual Stage 1 magnetite. These magnetite-rich ore bodies define <50 m-long by 20 m-wide lenses that trend NE and coincide with the hinge zones of Z-shaped, tight F1 folds, which are most likely parasitic folds to the regional NE-trending, steeply NE-plunging anticline in the Jack Hills greenstone belt. Magnetite-rich ore zones are enriched in Fe and depleted in SiO2, with only minor changes in other major oxides and trace elements, compared with least-altered BIF. Magnetite-rich ore zones are locally cut by shear zone-hosted, talc–magnetite hydrothermal alteration overprinted by talc–microplaty hematite alteration. The shear zones and both talc mineral assemblages formed during a second deformation event that was characterised by NNE–SSE shortening. The third deformation event resulted in at least two generations of NE- to NW-trending extensional veins and strike-slip faults that locally cut the talc-rich shear zones. Supergene goethite–hematite replaces magnetite-rich ore bodies within 80 m of the present surface. The strong structural control and distinct hydrothermal alteration assemblages of the Matthew Ridge prospect are the best exploration indices for high-grade magnetite-rich ore in the region.

Geochemistry of a komatiitic, boninitic, and tholeiitic basalt association in the Mesoarchean Koolyanobbing greenstone belt, Southern Cross Domain, Yilgarn craton: Implications for mantle sources and geodynamic setting of banded iron formation

Komatiite–basalt magmatism and Algoma-type banded iron formation (BIF) of Archean greenstone terranes have common time series, but there are few constraints on the geodynamic setting in which BIF were precipitated. The ∼3.0Ga Koolyanobbing greenstone belt (KGB), Southern Cross Domain, Yilgarn craton, exhibits an greenschist to lower amphibolite facies metamorphosed lithostratigraphic series, including four predominantly mafic to ultramafic volcanic sequences (S1–S4) with prominent BIF horizons (and local basal pyrite-dominated massive sulfide lenses) in between. Petrography, XRD, high-precision major, trace high field strength, and REE element chemistry of representative ultramafic and mafic rocks from S1, S2, and S3 are consistent with a komatiitic, boninitic, and tholeiitic basalt association. Komatiites are Al-depleted and Al-undepleted (Al2O3/TiO2 12–28 and (Gd/Yb)N, ≥1) endorsing a zoned plume and/or melting over a range of depths. The boninite-series rocks (Al2O3/TiO2 28–45 and (Gd/Yb)N, <1) preserve characteristic U-shaped REE patterns and positive Zr–Hf/MREE anomalies. Komatiites include uncontaminated members and counterparts contaminated by metasomatized continental mantle lithosphere, suggesting eruption proximal to a rifted arc setting, whereas boninite-series were erupted at an intraoceanic arc, collectively consistent with plume eruption into a convergent margin. Al-depleted and Al-undepleted komatiites plot close together to the mantle array on the Th/Yb versus Nb/Yb discrimination plot of Pearce (2008), implying a depleted, mildly heterogeneous, source for the mantle plume from which komatiites erupted. Boninites lie parallel to and above the mantle array, in the field of intraoceanic arcs, in keeping with their inferred oceanic convergent margin setting. Ratios of Nb/Yb extend to lower and higher values than komatiites implying a more heterogeneous upper mantle source of the subarc wedge.The KGB exhibits the first reported boninite-suite rocks in the Southern Cross Domain, Yilgarn craton. Conclusively, the komatiitic, boninitic, and tholeiitic basalt association suggests that BIF and basal massive pyrite lenses of the Koolyanobbing greenstone belt were precipitated from low-temperature submarine discharge of a seawater-dominated hydrothermal system driven by magmatic systems of a mantle plume erupting in a forearc convergent margin suprasubduction ophiolite.

Archean cherts in banded iron formation: Insight into Neoarchean ocean chemistry and depositional processes

This study reports new REE+Y and 3-isotope sulfur data for Archean banded iron formation and volcaniclastic rocks with cherty bed tops in the Neoarchean Abitibi greenstone belt of Canada. The data were analyzed with a view to better constrain Neoarchean ocean chemistry, atmospheric conditions prevalent during weathering and transport, the development of Algoma-type banded iron formation and the overall process of stratigraphic development of greenstone belts. The Abitibi greenstone belt consists of 7 mafic to felsic volcanic cycles, each capped by a sedimentary interface zone consisting of chemical and minor clastic metasediments. We concentrated sampling on the iron formation capping the ca. 2730Ma Deloro assemblage as it occurs over a wide area (300km×600km) and because there is a substantial depositional gap prior to deposition of the overlying volcanic rocks. Volcaniclastic rocks within the ca. 2710Ma Tisdale assemblage were also sampled.Chemical analyses focussed on the SiO2-rich portion of the samples and were conducted by laser ablation ICP-MS. In situ analysis of S isotopes was obtained for pyrite by ion probe. REE data display four types of patterns: (1) hydrothermally influenced marine hydrogenous sediment, (2) contaminated, hydrothermally influenced marine hydrogenous sediment, (3) hydrothermally dominated patterns, and (4) replacement patterns indicating silicification of precursor volcanic units. Contamination and/or the presence of non-chert components were documented with Th, U and Zr content. Non-chert components were defined as: (1) phosphates that led to elevated Th/U, (2) clastic detritus leading to flat shale normalized REE patterns, and (3) volcanic detritus leading to elevated values for Zr. No meaningful difference in REE+Y geochemistry as a function of elevated Th/U was found implying that phosphates have the same REE patterns as the host chert. The cherts within banded iron formation exhibited stratigraphic variation in several localities, progressing from replacement chemistry (flat REE profile) at the base, hydrogenous sediment geochemistry (positive La, Gd, Y/Ho anomalies) in the middle part and hydrothermal patterns (depleted LREE, elevated positive Eu anomalies) in the upper part. The upper parts of some units also display +Ce anomalies possibly reflective of more oxygenated water also supported by S isotope data. A consistent increase in Pr/Yb in the upper parts of units is postulated to reflect shallowing upward of depositional depth to an unknown extent but not above storm wave base. A number of samples with flat REE patterns lacking La and Gd anomalies represent hydrothermal deposition with the largest Eu/Eu* values recorded for Archean iron formation. The main contribution of the Abitibi banded iron formations is that they provide a deeper water perspective on Archean ocean chemistry. The resulting picture is that of slow BIF accumulation, a generally strong hydrothermal input of REE and complex oceanic cycling, possibly involving a chemocline above the sampled water depth.The new sulfur isotope data show a greater extent of mass independent fractionation than previously recorded for the Neoarchean, with Δ33S ranging from −1.4 to +4.2‰. The strongly MIF positive source (Δ33S=+4‰) was apparently similar to Paleoarchean values.

Cryogenian ophiolite tectonics and metallogeny of the Central Eastern Desert of Egypt

The Central Eastern Desert (CED) is characterized by the widespread distribution of Neoproterozoic intra-oceanic island arc ophiolitic assemblages. The ophiolitic units have both back-arc and forearc geochemical signatures. The forearc ophiolitic units lie to the west of the back-arc related ones, indicating formation of an intra-oceanic island arc system above an east-dipping subducted slab (present coordinates). Following final accretion of the Neoproterozoic island arc into the western Saharan Metacraton, cordilleran margin magmatism started above a new W-dipping subduction zone due to a plate polarity reversal. We identify two belts in the CED representing ancient arc–forearc and arc–back-arc assemblages. The western arc–forearc belt is delineated by major serpentinite bodies running ∼NNW–SSE, marking a suture zone. Ophiolitic units in the back-arc belt to the east show an increase in the subduction geochemical signature from north to south, culminating in the occurrence of bimodal volcanic rocks farther south. This progression in subduction magmatism resulted from diachronous opening of a back-arc basin from north to south, with a bimodal volcanic arc evolving farther to the south. The intra-oceanic island arc units in the CED include coeval Algoma-type banded iron formations (BIFs) and volcanogenic massive sulphide (VMS) deposits. Formation of the BIFs was related to opening of an ocean basin to the north, whereas development of the VMS was related to rifting of the island arc in the south. Gold occurs as vein-type mineral deposits, concentrated along the NNW–SSE arc–forearc belt. The formation of these vein-type gold ore bodies was controlled by the circulation of hydrothermal fluids through serpentinites that resulted in Au mobilization, as constrained by the close spatial association of auriferous quartz veins with serpentinites along the western arc–forearc belt.

Petrology and geochemistry of the banded iron formation (BIF) of Wadi Karim and Um Anab, Eastern Desert, Egypt: Implications for the origin of Neoproterozoic BIF

Banded iron formation (BIF) is exposed among the Precambrian rocks in the Wadi Karim and Um Anab areas in the Eastern Desert of Egypt. The BIF conformably alternates with Neoproterozoic arc metavolcanic rocks, which comprise metabasalts and mafic to intermediate metapyroclastic rocks. The BIF of Wadi Karim belongs to the oxide and mixed carbonate-oxide facies, while that of Um Anab belongs to the oxide facies only. Iron bands are generally composed of iron-rich mesobands rhythmically alternating with jasper, chert or carbonate mesobands as in the Wadi Karim area, or chert mesobands as in the Um Anab area. The BIF of Wadi Karim is composed essentially of magnetite, hematite and microcrystalline quartz, in addition to ankerite in the carbonate-bearing bands, while that of Um Anab is composed of magnetite and microcrystalline quartz. The positive correlation between Al 2O 3 and TiO 2 in the studied BIFs indicates that these chemical sediments incorporate minor detrital components. The limited areal extent of the studied BIF and its association with metavolcanic rocks are features of Algoma-type BIF of Archean greenstone belts. The spatial and temporal association between the BIF and the arc metavolcanic rocks indicates a genetic relationship between volcanic activity and BIF. The REY patterns of the studied BIFs suggest their precipitation after mixing of reduced bottom water carrying hydrothermal component with oxidized seawater in variable proportions. The absence of strong positive Eu anomaly, which characterizes BIF of Archean greenstone belts, in the BIF of the Eastern Desert may be attributed to contribution of Fe and Si through low-temperature hydrothermal solutions and to a high oxygen level of the atmosphere in the Neoproterozoic. It is proposed that the deposition occurred in an oceanic island arc setting, probably intra-arc or back-arc basins. In contrast with the majority of the Neoproterozoic iron formations (IFs), the studied BIFs are intimately associated with volcanic rocks and show no direct evidence of forming in response to glaciation. Consequently we attribute their formation to volcanic activity accompanying the break up of Rodinia rather than to snowball Earth condition. Thus, the Neoproterozoic IFs can be subdivided into Algoma and Rapitan-types. The Algoma-type, of volcanic association, is represented by African and Arabian Shield IFs, whereas the Rapitan-type, of glacial association, comprises deposits from all continents. The Wadi Karim and Um Anab BIFs constitute an additional evidence for the widespread return of IFs in the Cryogenian and Ediacaran after disappearance for ∼1 Ga reflecting the recurrence of anoxic ferruginous conditions in the Neoproterozoic deep sea.

Constraints on the Archean atmospheric oxygen and sulfur cycle from mass-independent sulfur records from Anshan-Benxi BIFs, Liaoning Province, China

The Archean atmospheric oxygen concentration and sulfur cycle was long debated. The banded iron formation (BIF) is a special type of the sedimentary formation, which has truly recorded the atmospheric and oceanic conditions at that time. In this study, the composition of multiple sulfur isotope (δ 34S/δ 33S/δ 32S) for sulfides bedded in the Archean (∼2.7 Ga) BIFs, in Anshan-Benxi area of Liaoning Province has been measured. The value of Δ33S varies from −0.89‰ to +1.21‰, which shows very obvious mass-independent fractionation (MIF) signatures. These non-zero Δ33S values indicate that the Archean sulfur cycles are different from what it is today, which have been deeply influenced by gas phase photochemical reactions. Algoma-type BIFs which are closely related to the volcanic activity have negative Δ33S value, however, Superior-type BIFs which are far away from the volcanic center have positive Δ33S value. The δ 34S varies in a large range from −22.0‰ to +11.8‰, which indicates that the bacteria reduction activity has already existed at that time, and that the oceanic sulfate concentration has at least reached 1 mmol/L in local areas. Combined with the contemporaneous existence of the hematite, magnetite and the occurrence and preservation of the sulfur MIF, it can be inferred that the Archean atmospheric oxygen level must be at 10−2–10−3 of the present atmospheric level (PAL).

Stability of iron oxides and their role in the formation of rock magnetism

Thermodynamic conditions (first of all, temperature) are the main dynamic factors in the transformation process of ferrous to ferric iron (TFFI). TFFI usually takes place within a temperature range of 473–843 K (most active at temperatures above 673 K) and does not require presence of the oxidizing agents above 673 K. Analysis of the chemical composition of different rocks and minerals indicates that only for some sedimentary rocks is the relative content of ferrous iron oxide less than its value in magnetite, and this value is minimal for oceanic sediments. The relative content of ferrous iron oxide in oceanic magmatic rocks exceeds this value in continental magmatic rocks and depends on the rate of rock cooling. An investigation of the role of the titanium oxide content of different rocks on stability of ferrous iron oxide against its transformation to ferric iron oxide shows that a significant correlation (r = 0.79) does exist between the relative content of ferrous iron oxide and ratio of TiO2/Fe2O3. Temperature within the solar nebula at location of the Earth was within the temperature range of the TFFI. During the Earth accretion and its early evolution, ferric iron oxide was unstable and most likely did not exist. The first magnetic minerals containing ferric iron could have appeared only after the Earth’s surface had cooled below ∼843 K. The formation of the first Algoma-type banded iron formations could be used as a marker of the Earth’s surface cooling below ∼843 K.

A new classification of the gold deposits of Egypt

Gold deposits and occurrences located in the Nubian Shield have been known in Egypt since Predynastic times. Despite the fact that these deposits were long under exploitation and investigated many times, they are still insufficiently classified in harmony with the crustal evolution models suggested for the evolution of the Nubian Shield. Several plate tectonic models were proposed for the development of the Nubian Shield and the present classification relies heavily on the model that implies collision of arc-inferred continent through subduction and obduction of oceanic lithosphere. A three-fold classification of gold deposits of Egypt is offered here in harmony with this evolutionary model. These are stratabound deposits and non-stratabound deposits hosted in igneous and metamorphic rocks, as well as placer gold deposits. The stratabound deposits are hosted in island arc volcanic and volcaniclastic rocks of comparable composition formed in ensimatic island arcs. They are thought to have formed by exhalative hydrothermal processes during the waning phases of sub-marine volcanic activity. Stratabound deposits are sub-divided into three main types: gold-bearing Algoma-type Banded Iron Formation, gold-bearing tuffaceous sediments and gold-bearing volcanogenic massive sulphide deposits. Non-stratabound deposits occur in a wide range of igneous and metamorphic rocks. They were formed during orogenic and post-cratonization periods by mineralizing fluids of different sources. Non-stratabound deposits are divided into vein-type mineralization, which constituted the main target for gold in Egypt since Pharaonic times, and disseminated-type mineralization hosted in hydrothermally altered rocks (alteration zones) which are taken recently into consideration as a new target for gold in Egypt. Placer gold deposits are divided into modern placers and lithified placers. The former are sub-divided into alluvial placers and beach placers. Conglomerates occurring on or near ancient eroded surfaces represent lithified placers.

Geological, mineralogical and geochemical aspects of Archean Banded Iron-Formation-hosted gold deposits: Some examples from Southern Africa

This paper reports on different styles of gold mineralization observed in Archean gold deposits hosted by Algoma-type Banded Iron-Formations (BIF) in southern Africa. Genetic aspects of various occurrences are discussed in the context of mineralogical as well as geochemical data of BIFs from the greenstone terranes of the Zimbabwe and Kaapvaal cratons. The study revealed that, in spite of their different provenance and age (3.5 to 2.6 Ga), the BIFs are geochemically similar, whereas observed mineralogical differences reflect various degrees of metamorphic overprint. Generally, the BIFs belong to mixed oxide-carbonate-(±sulfide)-facies. REE distribution patterns of the investigated Archean BIF samples exhibit positive Eu-anomalies, which suggest a strongly reducing nature of the solutions which also provided the distinctive element contents now present in the chemical sediments. Irrespective of their formation, gold enrichment in BIF only occurs if the S- and/or As-contents of the BIFs exceed specific threshold values, i.e. gold mineralization is always associated with increased contents of the iron-sulfides pyrite, arsenopyrite and pyrrhotite. The studies indicate that BIF-hosted gold occurrences are not products of a single universal metallogenic process, but may be explained by several different genetic processes such as primary syn-sedimentary formation, diagenetic changes, metamorphic remobilization, and epigenetic hydrothermal emplacement.

Geologic setting of volcanogenic base and precious metal deposits of the West Georgia Piedmont; a multiply deformated metavolcanic terrain

The northern Piedmont of Georgia contains numerous gold and base metal occurrences within a sequence of high-grade, multiply deformed metamorphic rocks. These rocks are interpreted as regionally correlative with portions of the Ashe Formation in North Carolina, and sulfide deposits are similar to those seen in the Ashe at Ore Knob, North Carolina, and Great Gossan Lead, Virginia. Gold and sulfide deposits in northern Georgia were sporadically worked prior to the Civil War until approximately 1920. These deposits were previously thought to have formed by replacement from hydrothermal solutions but are now recognized as syngenetic with surrounding volcanic rocks. Mineralization is mainly strata bound within a metamorphosed sequence of mafic to felsic volcanic rocks interlayered with metasedimentary rocks. Banded iron-formation commonly is associated with mineralized zones. Sulfide deposits, previously mined chiefly for pyrite for the production of sulfuric acid, also contain pyrrhotite, chalcopyrite, and sphalerite with trace amounts of gold. These deposits are generally of three types: pyrite + gold, pyrite + base metal sulfide, or barren pyrite. Sulfide ores are massive and disseminated and occur within metamorphosed felsic volcanic rocks, interlayered felsic and mafic volcanic rocks, and interlayered volcanic and sedimentary rocks. Gold occurs in placers and in vein deposits associated with banded iron-formation, siliceous alteration zones, and quartzose schists.Mineralized rocks are dominantly within the New Georgia Group and immediately overlie units of the Sandy Springs Group. New Georgia Group rocks are dominantly metavolcanic and metaplutonic rocks with interlayered metamorphosed volcaniclastic and sedimentary rocks. Algoma-type banded iron-formation, formed during periods of quiescence in the volcanic sequence, is interlayered with amphibolite. New Georgia Group rocks grade upward, through decreasing abundance of metavolcanics, into overlying metasediments of the Sandy Springs Group. Multiple folding has overturned the sequence and older volcanic rocks now structurally overlie metasedimentary rocks.