Banded Iron Formations Units Research Articles

Thermal magnetic analysis on iron ores and banded iron formations (BIFs) in the Hamersley Province: Implications of origins of magnetic minerals and iron ores

<abstract> <p>The genesis models of the iron-ores hosted in banded iron formations (BIFs) in the Hamersley Province of Western Australia have been debated since the iron-ore deposits were discovered in the 1960s. The existing models considered the few physicochemical conditions for the iron-ore enrichment from BIFs. This study incorporates the latest research outcomes in conversions among the major magnetic minerals under different physicochemical conditions with the thermal magnetic analysis for BIFs and iron-ores collected from the Hamersley Province to fill the gap in knowledge highlighted by existing studies of the iron ores and BIFs. The results indicate that the high-grade hematite ores might have been undergone a physicochemical process under hydrothermal conditions between 120 ℃ and 220 ℃ during the major stage of enrichment from the original BIFs in the Brockman Iron Formation. Such physicochemical conditions would require either that the BIF units were buried 4000–5000 m underground with tilted broad channels formed by large-scale deformation in the region that facilitates hydrothermal reactions and leaching by the fluids flowing down deep to 4000–5000 m, somehow similar to the deep-seated supergene model proposed in previous works, or that the BIF units were still buried but the hydrothermal fluids coming up from deeper sources spread widely over the broad channels to ensure the high-grade hematite ores are consistently uniform over the entire deposit. The large-scale martite-goethite deposits in the Marra Mamba Iron Formation might be derived from multiple supergene phases from hematite-martite ores below 100 ℃ in the natural process of oxidization near surface, somewhat similar to the existing model for the channel iron deposits. Magnetite contained within current BIFs and iron ores was least likely derived from primary hematite in BIFs.</p> </abstract>

Depositional Setting of Archean BIFs from Congo: New Insight into Under-Investigated Occurrences

Archean banded iron formations (BIF) represent a major contributor to better constraining and assessing the paleogeography and evolution of Archean cratons. In this context, we conducted an exhaustive sampling and analysis campaign of BIF units in the Congo Craton, covering several greenstone belts within the Ivindo, Kelle-Mbomo, and Chaillu blocks. The REE + Y patterns suggest: (1) Interaction of seawater with Fe-oxyhydroxides, as illustrated by strong REE enrichment coupled with La and Y enrichment; (2) contributions from high-temperature (>250 °C) hydrothermal fluids, illustrated by positive Eu anomalies; and (3) detrital input as suggested by relatively consistent REE concentrations and a chondritic Y/Ho ratio. These observations suggest a typical environment of Algoma-type BIF deposition. Moreover, assessment of the Ce anomalies in a combination of HREE enrichment indicates that some basins in the Chaillu and Ivindo blocks may have known potential oxygen-rich episodes in the early Archean during the deposition of these BIFs.

An integrated use of induced polarization and electrical resistivity imaging methods to delineate zones of potential gold mineralization in the Phitshane Molopo area, Southeast Botswana

We used 2D and pseudo-3D induced polarization (IP) in conjunction with electrical resistivity imaging (ERI) data inversion to identify and characterize potential zones of gold mineralization in Phitshane Molopo area. Reports on historic exploration suggest that gold mineralization is associated with quartz and/or carbonate veins that cut across the banded iron formations. The 2D IP/ERI measurements were acquired along ten 300 m long profiles trending northeast southwest, with 15 m line separation and 7.5 m electrode spacing. Thereafter, we inverted the data using the standard Gauss-Newton optimization method to produce 2D and 3D models. Based on a combination of IP and resistivity results, we identified distinct lithological units comprising the mineralized and non-mineralized zones. High chargeability (≥6.86 mV/V) and high resistivity (≥641 Ωm) anomalies were attributed to the presence of disseminated sulfides or gold-bearing quartz and/or carbonate veins that obliquely crosscut (east west trend) the banded iron formation (BIF) units and the intensely silicified BIF units, respectively. Zones exhibiting a combination of low to intermediate chargeability (1.0–6.86 mV/V) and low to intermediate resistivity (1.4–641 Ωm) are likely non-sulfidized zones with little/no gold mineralization in the study area. The integrated use of IP and ERI methods was successful in delineating zones that are likely to be hosting disseminated sulphides and/or potentially gold-rich zones in the Phitshane Molopo area, Kraaipan Greenstone Belt.

Meso–Neoarchaean crustal evolution of the Bundelkhand Craton, Indian Shield: new data from greenstone belts

ABSTRACTThe Mauranipur and Babina greenstone belts of the Bundelkhand Craton are formed of the Central Bundelkhand greenstone complex (CBGC). This complex represents tectonic collage which has not been previously studied in depth. The purpose of this study is to contribute to the understanding of the main features of the Archaean crustal evolution of the Bundelkhand Craton. The CBGC consists of two assemblages: (1) the early assemblage, which is composed of basic-ultramafic, rhyolitic–dacitic, and banded iron formation units, and (2) the late assemblage, which is a felsic volcanic unit. The units and assemblages are tectonically unified with epidote–quartz–plagioclase metasomatic rocks formed locally in these tectonic zones.The early assemblage of the Mauranipur greenstone belt is estimated at 2810 ± 13 Ma, from the U–Pb dating (SHRIMP, zircon) of the felsic volcanics. Also, there are inherited 3242 ± 65 Ma zircons in this rock. It is deduced that this assemblage is related to early felsic subduction volcanism during the Mesoarchaean that occurred in the Bundelkhand Craton.Zircons extracted from metasomatic rocks in the early assemblage’s high-Mg basalts show a concordant age of 2687 ± 11 Ma. This age is interpreted as a time of metamorphism that occurred simultaneously with an early accretion stage in the evolution of the Mauranipur greenstone belt.The felsic volcanism, appearing as subvolcanic bodies in the late assemblage of the Mauranipur greenstone belt, is estimated to be 2557 ± 33 Ma from the U–Pb dating (SHRIMP, zircon) of the felsic volcanic rocks. This rock also contains inherited 2864 ± 46 Ma zircons. The late assemblage of the Mauranipur greenstone belt corresponds with a geodynamic setting of active subduction along the continental margin during Neoarchaean.The late assemblage Neoarchaean felsic volcanic rocks from the Mauranipur and Babina greenstone belts are comparable in age and geochemical characteristics. The Neoarchaean rocks are more enriched in Sr and Ba and are more depleted in Cr and Ni than the Mesoarchaean felsic volcanic rocks of the early assemblage.Through isotopic dating and the geochemical analysis of the volcanic and metasomatic rocks of the CBGC, this study has revealed two subduction–accretion events, the Meso–Neoarchaean (2.81–2.7 Ga) and Neoarchaean (2.56–2.53 Ga), during the crustal evolution of the Bundelkhand Craton (Indian Shield).

Petrography, lithostratigraphy and major element geochemistry of Mesoarchean metamorphosed banded iron formation-hosted Nkout iron ore deposit, north western Congo craton, Central West Africa

The Nkout banded iron formation (BIF) or iron deposit is located in the northwestern margin of the Congo Craton (CC) and is hosted within high-grade metamorphic Ntem complex. It is mainly made up of metamorphosed banded iron formations with resource estimation of 1.8 Bt @ 32.6% Fe indicated, and 0.9 Bt @ 30.3% Fe inferred. Nkout iron deposit is subdivided into three prospects namely East, Centre and West deposits. Results from lithostratigraphic studies allow detailed correlations between units in the Nkout succession. The litho-package from the top to the base stratigraphy comprises iron-rich member and the gneissic member. The first member forms the upper and middle units while the second member forms the lower unit and represents the footwall of the deposit. The upper unit is ∼200 m deep and consists of oxidized (hematite) and un-oxidized (fresh metamorphosed BIF) ore types. The middle unit is approximately 150 m thick and consists of fresh metamorphosed BIF with minor intercalation of biotite gneiss. The lower unit is made up of garnet-magnetite micaschist, biotite gneiss and pyroxene-biotite gneiss.The Nkout banded iron formation belongs to oxide facies magnetite-rich type which has suffered greenschist to amphibolite facies metamorphism, leading to recrystallization of chlorite, amphibole and garnet minerals. Its overall major oxide bulk chemistry is very similar to that of most Archaean iron-formations worldwide, with high SiO2 and Fe2O3 contents. However, the garnet-rich BIF has higher contents of Al2O3 (8.73 wt%), TiO2 (0.27 wt%), K2O (2.37 wt%) relative to magnetite-rich BIF, suggesting clastic contamination, thus a depositional environment closer to the continent. Furthermore, the absence of carbonate facies BIF in the entire Nkout deposit suggests deep water distal depositional setting, in an area of little or no organic carbon supplies.The lithostratigraphic correlation of metamorphosed BIF unit reveals both thicknesses and depth variations from West to the East, inferring a folded structure for the Nkout deposit. However the occurrence of BIF units in the entire deposit supports the fact that BIF deposition took place on a basin-wide scale. Considering the extent of the Ntem basin, it is suggested that a huge amount of Fe2+ in sea water was required. Thus, the deposition of Nkout BIF probably involved an iron rich environment of reduced Fe2+ rich hydrothermal fluid in chemically stratified sea water, similar to the depositional model proposed for the Minas BIF Supergroup of Quadrilátero Ferrífero in Brazil.

Genesis history of iron ore from Mesoarchean BIF at the Wodgina mine, Western Australia

ABSTRACTSeveral iron-ore deposits hosted within Mesoarchean banded iron formations (BIFs) are mined throughout the North Pilbara Craton, Western Australia. Among these, significant goethite±martite deposits (total resources >50 Mt at 55.8 wt% Fe) are distributed in the Wodgina district within 2 km of the world-class pegmatite-hosted, tantalum Wodgina deposits. In this study, we investigate the dominant controls on iron mineralisation at Wodgina and test the potential role of felsic magma-derived fluids in early alteration and upgrade of nearby BIF units. Camp-scale distribution and geochemistry of iron ore at Wodgina argue against any significant influence of identified felsic intrusions in the upgrade of BIF. Whereas, the formation of BIF-hosted goethite±martite iron ore at Wodgina involves: (i) early (ca 2950 Ma) metamorphism of BIF causing camp-scale recrystallisation of pre-existing iron oxides to form euhedral magnetite, with local enrichment to sub-economic grades (∼40 wt% Fe) within or proximal to metre-wide, bedding-parallel shear zones, and (ii) later supergene lateritic enrichment of the magnetite-bearing BIF and shear zones, forming near-surface goethite±martite ore. The supergene alteration sequence includes: (i) downward progression of the oxidation front and replacement of magnetite by martite, (ii) local development of silcrete at ∼40 m below the modern surface caused by the lowering of the water-table, (iii) intensive replacement of quartz by goethite, resulting in the goethite±martite ore bodies at Wodgina, and (iv) late formation of ferricrete and ochreous goethite. Goethitisation most likely took place within the hot and very wet climate that prevailed from the Paleocene to the mid-Eocene. Goethite precipitation was accompanied by the incorporation of trace elements P, Zn, As, Ni and Co, which were likely derived from supergene fluid interaction with nearby shales. Enrichment of these elements in goethite-rich ore indicates that they are potentially useful pathfinder elements for concealed ore bodies covered by trace element-depleted pedogenic silcrete and siliciclastic rocks located throughout the Wodgina mine.

The world-class Musselwhite BIF-hosted gold deposit, Superior Province, Canada: New high-precision U–Pb geochronology and implications for the geological setting of the deposit and gold exploration

New high-precision U–Pb ages from the host succession of the Musselwhite banded iron formation (BIF)-hosted epigenetic gold deposit (5.7Moz Au) in the North Caribou greenstone belt of the Superior Province provide timing constraints on the setting of the deposit, with implications for geological and exploration models. The deposit is mainly hosted by a folded garnet-grunerite-chert silicate facies of the Northern BIF, part of the Opapimiskan-Markop assemblage. Dating of a volcaniclastic sedimentary rock, a garnet–biotite±staurolite schist, and a cross-cutting foliated felsic dike indicate that the age of the Northern BIF is between 2967 and 2909Ma. A felsic tuff from the South Rim volcanic assemblage structurally overlies the ore-bearing Northern BIF, but yields an older age of 2979Ma. Reappraisal of stratigraphic and polyphase structural relationships, in conjunction with the U–Pb ages, indicate that the mine stratigraphy is inverted as it is located on the overturned limb of a kilometre-scale refolded F1 syncline, greatly influencing the distribution and geometry of the BIF units hosting the bulk of the gold. Felsic ash tuff and a fine-grained siliciclastic sedimentary rock from the “Lower Sediments” unit (Zeemel/Eyapamikama assemblage) have much younger ages of <2850Ma and <2846Ma, respectively and provide a maximum age for D2 deformation. The ∼60–120Ma time gap between the Zeemel/Eyapamikama assemblage and the structurally overlying Opapimiskan-Markop assemblage is coincident with the location of a high-strain zone within iron-carbonatized mafic–ultramafic rocks. This unconformity/thrust boundary and a polymictic conglomerate in the upper stratigraphic sequence may indicate the presence of a major structure and an important first-order gold exploration target.

The Tasiast deposit, Mauritania

The Tasiast gold deposits are hosted within Mesoarchean rocks of the Aouéouat greenstone belt, Mauritania. The Tasiast Mine consists of two deposits hosted within distinctly different rock types, both situated within the hanging wall of the west-vergent Tasiast thrust. The Piment deposits are hosted within metasedimentary rocks including metaturbidites and banded iron formation where the main mineral association consists of magnetite-quartz-pyrrhotite±actinolite±garnet±biotite. Gold is associated with silica flooding and sulphide replacement of magnetite in the turbidites and in the banded iron formation units. The West Branch deposit is hosted within meta-igneous rocks, mainly diorites and quartz diorites that lie stratigraphically below host rocks of the Piment deposits. Most of the gold mineralisation at West Branch is hosted by quartz–carbonate veins within the sheared and hydrothermally altered meta-diorites that constitute the Greenschist Zone. At Tasiast, gold mineralisation has been defined over a strike length>10km and to vertical depths of 740m. All of the significant mineralised bodies defined to date dip moderately to steeply (45° to 70°) to the east and have a south–southeasterly plunge. Gold deposits on the Tasiast trend are associated with second order shear zones that are splays cutting the hanging wall block of the Tasiast thrust. An age of 2839±36Ma obtained from the hydrothermal overgrowth on zircons from a quartz vein is interpreted to represent the age of mineralisation.

Biologically recycled continental iron is a major component in banded iron formations

Banded iron formations (BIFs) record a time of extensive Fe deposition in the Precambrian oceans, but the sources and pathways for metals in BIFs remain controversial. Here, we present Fe- and Nd-isotope data that indicate two sources of Fe for the large BIF units deposited 2.5 billion y ago. High-εNd and -δ(56)Fe signatures in some BIF samples record a hydrothermal component, but correlated decreases in εNd- and δ(56)Fe values reflect contributions from a continental component. The continental Fe source is best explained by Fe mobilization on the continental margin by microbial dissimilatory iron reduction (DIR) and confirms for the first time, to our knowledge, a microbially driven Fe shuttle for the largest BIFs on Earth. Detailed sampling at various scales shows that the proportions of hydrothermal and continental Fe sources were invariant over periods of 10(0)-10(3) y, indicating that there was no seasonal control, although Fe sources varied on longer timescales of 10(5)-10(6) y, suggesting a control by marine basin circulation. These results show that Fe sources and pathways for BIFs reflect the interplay between abiologic (hydrothermal) and biologic processes, where the latter reflects DIR that operated on a basin-wide scale in the Archean.

Interpretations and implications of LA ICP-MS analysis of chert for the origin of geochemical signatures in banded iron formations (BIFs) from the Meadowbank gold deposit, Western Churchill Province, Nunavut

Among the many types of mineral deposits within Archean cratons, gold mineralization is an important economic commodity with over 20,000metric tons of gold produced from greenstone belts in 2001. Of the Archean–early Paleoproterozoic gold deposits, several different types of mineralization are known, which includes the important Algoma-type banded iron formation (BIF) where gold is locally associated with sulfide-facies zones within regionally extensive oxide-facies. It is commonly accepted that the shale-normalized chemical signature of REE+Y of chert bands in Algoma-type BIFs may reflect one of the three processes, each of which may be relevant to the nature and origin of the gold mineralization: (1) direct seawater precipitation; (2) involvement of and contribution from hydrothermal fluids; and (3) replacement of precursor volcanic units due to silicification. An essential question in regard to the mineralization is, therefore, whether the gold mineralizing fluids have a preference for one geochemical type of iron formation versus another. In order to assess the relevance of these competing models, we report herein the results of a LA ICP-MS study of chert samples within different Algoma-type BIFs from the Meadowbank deposit (24.5Mt proven/probable ore reserves grading 2.8g/t (2011)) hosted in the Neoarchean Woodburn Lake Group of the Rae Domain of the western Churchill Province, Canada. This study used 39 carefully selected and characterized (i.e., petrography and SEM-EDS imaging) chert samples from the main deposit, the Central BIF, and four additional BIFs, the Far West, West, East and Grizzly zones, with data collected using line traverses along the chert bands. The geochemical data indicate that an ambient seawater signature (i.e., enrichment in HREE relative to LREE associated with positive La and Y anomalies) dominates the samples with a lesser hydrothermal component (characterized by a positive Eu anomaly) and the influence of detrital contamination can also be detected. These initial results indicate that the methodology and protocol employed provides a reliable means to assess and interpret the chemical signature of BIFs hosting gold mineralization. In the present case, the results for the Meadowbank deposit suggest that chert from mineralized BIF units does not record a typical chemical signature that may be used as a vector for potential gold mineralization.

Late Neoproterozoic banded iron formation (BIF) in the central Eastern Desert of Egypt: Mineralogical and geochemical implications for the origin of the Gebel El Hadid iron ore deposit

The banded iron formation (BIF) in the Gebel El Hadid area is part of the Late Neoproterozoic island-arc/ophiolitic assemblage of the central Eastern Desert of Egypt, where it alternates concordantly with low-grade metavolcaniclastics. The BIF units are mainly of oxide facies consisting of iron oxide mesobands with intercalations of quartz–carbonate±chlorite±magnetite mesobands in the form of layers and pods. Geochemically, the Gebel El Hadid BIF is characterized by heavy REE-enriched NASC-normalized patterns, rarely with a weak positive Eu anomaly. The Neoproterozoic age of the Egyptian BIF suggests that they are of Rapitan type, but their size, petrological and geochemical characteristics suggest that they are of Algoma type having formed before the final collision between major fragments of East and West Gondwana. The nature and composition of the BIF and associated metavolcaniclastics indicate their formation as a result of submarine hydrothermal activity concomitant with arc volcanism. The Gebel El Hadid BIF units were deposited in pulses during periods of quiescent arc activity at ca. 717±8Ma (U–Pb age on volcaniclastic zircon). Submarine diagenesis under suboxic conditions produced an early generation of magnetite, hematite and quartz, whereas regional metamorphism led to the formation of coarse-grained and porphyroblastic magnetite and quartz. Post-metamorphic oxidation led to martitization of magnetite, and formation of hematite and specularite.

Magnetic petrophysical results from the Hamersley Basin and their implications for interpretation of magnetic surveys

An extensive magnetic petrophysical investigation was carried out in the Hamersley Basin in Western Australia; which hosts large high-grade iron-ore deposits derived from banded iron formations (BIFs). Based on our new results, as well as those from previous studies, we demonstrate that the bulk susceptibility of BIF units has a bimodal distribution caused by the presence of chert-rich and magnetite-rich bands. The mean bulk susceptibility (MBS) of magnetite-rich bands is more than 10 times higher than that of chert-rich bands. Measurements of anisotropy of magnetic susceptibility (AMS) in BIF units show this to vary between 1.06 and 2.76. The natural remanent magnetisation (NRM) carried by iron ores varies with different deposits, and even within individual deposits. NRM measurements on non-ore units defined a northwesterly upward regional magnetic overprint (310°/–27°) present in the Fortescue and Hamersley Groups, and also in the lower Wyloo Group. Modelling of aeromagnetic data over the Hamersley Range demonstrates that the effects of AMS and NRM are significant and must be accounted for. The effects of AMS and NRM on magnetic anomalies due to BIFs depend on the volume fraction of magnetite-rich members in a BIF unit. Forward modelling of magnetic and gravity responses, using petrophysical results summarised in this study, over a mined hematite ore deposit in Tom Price indicates that TMI variations are expected to be significantly reduced in areas of mineralisation. However, magnetic data alone are not a reliable indicator of the presence of mineralisation because of other causes of reduced magnetism in BIFs.

Banded Iron Formation to Blue Dust: mineralogical and geochemical constraints from the Precambrian Jilling-Langalata Deposits, Eastern Indian Craton

Numerous economic deposits of high-grade iron ores occur in the Singhbhum-Orissa Craton, in Eastern India. The deposits are mainly located in the Jilling-Langalata, Noamundi and Joda areas which are part of the eastern limb of the regional horseshoe shaped synclinorium, where millimetre to centimetre scale Archaean Banded Iron Formation units have been converted to steel grey, iron rich fine grained powder, known as Blue Dust. Field observations and subsequent laboratory investigations indicate that in this region, the Blue Dust deposits occur as pockets or lenses of varying dimensions and are randomly oriented. However, in most cases the Blue Dust deposits are found above the Fe-rich primary host rock known as Banded Haematite Jasper. Mineralogical observations indicate that the Blue Dust is mainly composed of haematite, martite and goethite while quartz and kaolinite are the gangue minerals. Silica removal is the primary control of iron enrichment. Geochemical and field observations indicate that the Blue Dust in these deposits is regarded to be of supergene-modified hydrothermal origin. In the first stage, the early hydrothermal process affects the primary unaltered Banded Iron Formation by simultaneously oxidising magnetite to martite and replacing quartz with hydrous iron oxides. In the second stage, the supergene processes upgrade the hydrous iron oxides to fine grain microplaty haematite. The supergene process causes the leaching of remnant silica from hydrothermally upgraded iron ore under a suitable Eh and pH condition and leads to the formation of Blue Dust.

Archaean lode gold mineralisation in banded iron formation at the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO2 (LOI), MgO, Ba, K and Rb, but significant depletion of SiO2 and, to a lesser extent, Fe2O3. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO2- and H2S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.

Wavelet analysis of detailed drillhole magnetic susceptibility data, Brockman Iron Formation, Hamersley Basin, Western Australia

Closely spaced drillcore susceptibility measurements for the complete Dales Gorge Member of the Brockman Iron Formation were made as part of a study of oxide mineralogy and rock magnetic properties of unmineralized banded iron formations (BIFs) in the Hamersley Basin, Western Australia. Magnetic properties of BIFs are dominated by magnetite because of its much higher intrinsic magnetization even when the opaque oxides are volumetrically dominated by hematite. Bulk magnetic susceptibility varies considerably throughout the BIF units due to variations in thickness and composition of oxide/silicate mesobands, variations in the proportion of microbands, and changes in the hematite/magnetite ratio. This local-scale variability and the presence of layering effects at several scales means that the drill-hole susceptibility data are non-stationary, and conventional tools for analysis such as autocorrelation, spectral analysis, and semivariogram techniques have limited value. Both continuous wavelet (CWT) and discrete wavelet (DWT) techniques proved to be very powerful for analysis of these non-stationary data. CWT analysis using a Morlet wavelet helped to quantify the periodicity of the layering and DWT analysis using a Haar wavelet provided an effective means of 'blocking' the log so that susceptibilities of individual BIF macrobands can be compared. The results suggest that wavelet analysis can provide definitive information on the scaling properties of magnetic susceptibility needed to determine 'formation response' or facies variation within the major BIF units. Rapid changes in magnetic susceptibility in BIFs are mirrored by the oxide mineralogy. Opaque-oxide petrography and rock magnetism results show that replacement of depositional or diagenetic hematite by magnetite within and between sedimentary band structures by reduction during prograde metamorphism was incomplete, and equilibrium in the magnetite stability field was not reached even at the mesoband scale.

Wavelet analysis of detailed drillhole magnetic susceptibility data, Brockman Iron Formation, Hamersley Basin, Western Australia

Closely spaced drillcore susceptibility measurements for the complete Dales Gorge Member of the Brockman Iron Formation were made as part of a study of oxide mineralogy and rock magnetic properties of unmineralized banded iron formations (BIFs) in the Hamersley Basin, Western Australia. Magnetic properties of BIFs are dominated by magnetite because of its much higher intrinsic magnetization even when the opaque oxides are volumetrically dominated by hematite. Bulk magnetic susceptibility varies considerably throughout the BIF units due to variations in thickness and composition of oxide/silicate mesobands, variations in the proportion of microbands, and changes in the hematite/magnetite ratio. This local-scale variability and the presence of layering effects at several scales means that the drill-hole susceptibility data are non-stationary, and conventional tools for analysis such as autocorrelation, spectral analysis, and semivariogram techniques have limited value.Both continuous wavelet (CWT) and discrete wavelet (DWT) techniques proved to be very powerful for analysis of these non-stationary data. CWT analysis using a Morlet wavelet helped to quantify the periodicity of the layering and DWT analysis using a Haar wavelet provided an effective means of ‘blocking’ the log so that susceptibilities of individual BIF macrobands can be compared. The results suggest that wavelet analysis can provide definitive information on the scaling properties of magnetic susceptibility needed to determine ‘formation response’ or facies variation within the major BIF units. Rapid changes in magnetic susceptibility in BIFs are mirrored by the oxide mineralogy. Opaque-oxide petrography and rock magnetism results show that replacement of depositional or diagenetic hematite by magnetite within and between sedimentary band structures by reduction during prograde metamorphism was incomplete, and equilibrium in the magnetite stability field was not reached even at the mesoband scale.

Could bacteria have formed the Precambrian banded iron formations?

Banded iron formations (BIFs) are prominent sedimentary deposits of the Precambrian, but despite a century of endeavor, the mechanisms of their deposition are still unresolved. Interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation, in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy. On the basis of chemical analyses from BIF units of the 2.5 Ga Hamersley Group, Western Australia, we show here that even during periods of maximum iron precipitation, most, if not all, of the iron in BIFs could be precipitated by iron-oxidizing bacteria in cell densities considerably less than those found in modern Fe-rich aqueous environments. Those ancient microorganisms would also have been easily supported by the concentrations of nutrients (P) and trace metals (V, Mn, Co, Zn, and Mo) found within the same iron-rich bands. These calculations highlight the potential importance of early microbial activity on ancient metal cycling.

3800 to 2500 Ma crustal evolution in the Anshan area of Liaoning Province, northeastern China

SHRIMP UPb zircon geochronology has revealed several generations of granitoids and gneisses from the Anshan area of northeastern China: two samples of the Baijiafen ‘granite’ (sensu lato), 3804 ± 5 and 3812 ± 4 Ma; Chentaigou granite, 3306 ± 13 Ma; Lishan granite, 3142 ± 7 Ma; East and West Anshan granites, 3001 ± 4 and 2994 ± 8 Ma; Tiejiashan granite, 2962 ± 4 and 2964 ± 6 Ma; and Qidashan granite, ∼ 2475 Ma. The ∼ 3800 Ma Baijiafen ‘granite’ consists of banded, migmatitic trondhjemitic gneisses and forms only a small part of the area. Nonetheless, they are the oldest-known rocks in Asia and are one of very few locations in the world where rocks of this antiquity have been found. The younger suites are predominantly granite (sensu stricto) in composition, and although foliated, they have not been transformed into banded gneisses. Nd isotopic data suggest that the < 3800 Ma granites were produced at least in part by remelting of very ancient LREE-enriched sources, perhaps like the exposed remnants of the ∼ 3800 Ma rocks. The Chentaigou supracrustals contain layered mafic and ultramafic rocks, banded iron formation (BIF) and siliceous layered rocks of probable volcanogenic origin. A sample of the latter has yielded a concordant UPb zircon age of 3362 ± 5 Ma, interpreted as the age of deposition. A trondhjemitic sheet cutting these rocks has yielded a UPb zircon age of 3342 ± 6 Ma. The Chentaigou supracrustals are the oldest recognised in the area. Younger supracrustal sequences collectively known as the Anshan group contains the main economic BIF units in the area. BIF is intruded by the ∼ 2475 Ma Qidashan granite, indicating that the Anshan group is Archaean or earliest Proterozoic in age.

Petrophysical Characteristics of BIF – Hosted Gold Deposits and the Application of Downhole EM to Their Exploration, With Examples From Hill 50 Gold Mine, Mt Magnet, Western Australia

Hill 50 Gold Mine is located at Mt Magnet, 560 km NNE of Perth, in the Murchison Province of the Yilgarn Craton. The Mt Magnet greenstone belt consists of a sequence of mafic, ultramafic and felsic volcanic rocks, with interbedded volcanogenic sediments, predominantly banded iron formations (BIFs). The belt has been deformed into a principal structure, known as the Boogardie Synform. Major NNE ? NE striking faults, superimposed on the synform, are referred to as Boogardie Breaks and exhibit important control over many of the ore deposits. Historically, most of the gold production in the Mt Magnet district has come from BIF ? hosted deposits (e.g. Hill 50 and Boomer ? styles). Hill 50 ? style deposits are hosted by BIF bars of the Sirdar Formation, having a width generally greater than 20 m, and being structurally controlled by the Boogardie Breaks. Gold mineralisation is closely associated with pyrrhotite and to a lesser degree, pyrite. Hillcrest South and Brown Hill West prospects are currently being explored using a Hill 50 model. Petrophysical measurements carried out on samples from these areas demonstrate that pyrrhotite ? rich BlFs are characterised by lower magnetic susceptibilities and higher Koenigsberger Ratios, conductivities and densities. Use of downhole EM in locating the pyrrhotite ? rich BIF has been highly successful.Boomer ? style deposits are hosted by multiple NW ? trending BIF units, and breccias, and are distinguished by the close association of gold mineralisation with pyrite. Petrophysical testing on samples from the Boomer pit showed that the pyrite ? rich rocks are characterised by higher densities and magnetic susceptibilities than their pyrite ? poor counterparts. Although the conductivities are also slightly higher, Boomer ? style deposits are not detectable using downhole EM. The differing conductivities observed in unmineralised BlFs and BlFs with significant pyrite andlor pyrrhotite can be explained by varying textural and mineralogical characteristics of the metallic lustre minerals involved.

Palaeomagnetism and magnetic anisotropy of Proterozoic banded-iron formations and iron ores of the Hamersley Basin, Western Australia

Rock magnetic properties and palaeomagnetism of weakly metamorphosed banded-iron formations (BIFs) of the Palaeoproterozoic Hamersley Group, Western Australia, and Proterozoic BIF-derived iron ores have been investigated. The BIF units sampled here are slightly younger than 2500 Ma. At Paraburdoo, Mount Tom Price and Mount Newman iron ore formation was completed before 1850 Ma. Sampling was mainly from the Mount Tom Price and Paraburdoo mining areas and for the first time a palaeomagnetic fold test on fresh (unweathered and unaltered) BIF samples has allowed the nature of the remanence of the BIFs to be defined. The remanence of the BIFs is carried by late diagenetic/low-grade metamorphic magnetite after primary haematite. This remanence is pre-folding and is unlikely to be greatly affected by the high anisotropy because the palaeofield inclination was demonstrably low. Determination of palaeofield directions from measured remanence directions is complicated by self-demagnetization effects in strongly magnetic, highly anisotropic BIF specimens. We present a method for correcting measured directions for the effects of self-demagnetization and anisotropy. For typical BIFs, the effect of magnetic anisotropy on measured remanence inclinations and inferred palaeolatitudes is minor for low palaeolatitudes, but can lead to large errors in calculated palaeopoles for intermediate to moderately steep palaeolatitudes. Anisotropy also causes cones of confidence to be underestimated, due to compression of the range of inclinations. In principle, deflection of post-folding remanence towards the bedding plane by high magnetic anisotropy can produce an apparent syn-folding signature, with best agreement between directions from different fold limbs after partial unfolding. Thus high anisotropy cannot only bias estimated palaeofield directions and cause underestimation of errors, but can also mislead interpretation of the time of remanence acquisition. The anisotropy of anhysteretic remanent magnetization (ARM) probably yields an upper limit to the anisotropy of the chemical remanent magnetization (CRM) carried by the BIFs. Therefore, from the anisotropy of ARM, a maximum inclination deflection of 9° is suggested for the sampled BIFs. This corresponds to less than 5° change of palaeolatitude. The palaeomagnetic pole position calculated for BIFs at Paraburdoo is 40.9°S, 225.0°E (dp=2.9°, dm=5.8°) after tilt correction, but without correction for anisotropy. Other pole positions reported include that from flat-lying BIFs from Wittenoom at 36.4°S, 218.9°E (dp=4.6°, dm=9.1°), from Mount Tom Price iron ore at 37.4°S, 220.3°E (dp=5.7°, dm=11.3°) and from Paraburdoo ore at 36.4°S, 209.9°E (dp=4.7°, dm=8.8°). The poles from the BIFs, the Paraburdoo ore and the part of the Tom Price deposit that was sampled in this study are indistinguishable from each other and from the Mount Jope Volcanics overprint pole. The magnetization of the BIFs was probably acquired during burial metamorphism of the Hamersley Group, soon before the main folding and uplift event in the southern part of the Hamersley Province. This tectonic event exposed magnetite-rich BIFs to near-surface oxidizing conditions, producing extensive martite-goethite orebodies and also appears to have produced the syn-folding overprint magnetization recorded by the Mount Jope Volcanics of the underlying Fortescue Group. Ages of magnetization are tentatively interpreted as ∼ 2200±100 Ma for the BIFs, ∼ 2000±100 Ma for the supergene enrichment of BIF to martite-goethite ore, recorded by the Parabudoo and Mount Tom Price orebodies, and ∼ 1950±100 Ma for the metamorphic martite-microplaty haematite ore, recorded as an overprint by the Tom Price orebody and as the only surviving magnetization of the Mount Newman orebody.