Stratiform Iron Research Articles

Typochemistry of magnetite from massive sulfide and iron deposits of the Urals according to LA-ISP-MS data

The aim of the study is the determination of mineralogical and geochemical features of magnetite from massive sulfde and iron deposits to develop forecasting criteria of ore objects. The layered magnetitolites of sulfde (Mauk, Letnee, Sibai and Molodezhnoe) and stratiform iron (Kachar, Sarbai, Estyuninskoe, Osokino-Aleksandrovskoe and Techa) deposits of volcanosedimentary association are the objects of study. It is established using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) that, in contrast to magnetite of iron deposits, magnetite of ore-controlling volcanosedimentary horizons of the Urals massive sulfde deposits is characterized by a higher content and stable associations of Bi, Te, Co, As, Cd, Zn, Cu and Mo due to relict inclusions of sulfde clasts and products of their halmyrolysis. The elevated Ti, V and Zr contents, as well as the mixed associations of lithophile, siderophile and chalcophile elements, are more typical of magnetite of iron deposits after hyaloclastites. Typochemistry of magnetite from volcanosedimentary horizons can be a criterion for searching the massive sulfde deposits.

Digital mapping and GIS-based spatial analyses of the Pur-Banera Group in Rajasthan, India, with special reference to the structural control on base-metal mineralization

GIS-based ‘digital’ maps help to store geological field data in a georeferenced framework that can be easily displayed at map-scale to outcrop-scale, without the need to map an area separately at different scales. In the iron and base-metal (Pb–Zn ± Cu) prospect of Pur-Banera belt in Rajasthan, India, multiple phases of folding and shearing of the host rocks have led to a complex structural pattern. The relationship between deformation and ore (especially the base-metal ore) localization is not well understood, although it is very important for mineral prospecting and mining. To facilitate exploration targeting, a large part of the Pur-Banera belt is mapped by us at different scales, using the ArcGIS™ platform. The resulting map is ‘intelligent’ in nature, as it can display geological field data at different scales by just zooming in and out, and it is possible to retrieve any type of geological data from any outcrop/sector through a specific GIS-query. Photographs and virtual 3D models of an outcrop can be opened through hyperlinks. Geospatially referenced mapping of the Pur-Banera belt demonstrates four phases of folding, of which the F3 folds define the regional structural pattern. Detailed mapping of mineralized bands and gossans indicate that the originally stratiform iron ore is best mined at the F3 hinge zones where these layers are thickest due to repeated folding. In contrast, the primarily stratabound Pb–Zn ± Cu ore has largely remobilized during F2 folding, becoming concentrated in the hinge zone and/or along the axial plane of F2 folds, which are consequently now the best prospect sites for base metals.

Isotopic Composition (δ13C and δ18O) and Genesis of Mn-bearing Sediments in the Ushkatyn-III Deposit, Central Kazakhstan

The Ushkatyn-III deposit is located 300 km west of Karaganda (Central Kazakhstan). It is classified as a weakly metamorphosed Atasu-type hydrothermal-sedimentary ore deposit. Stratiform iron, manganese, and barite–lead orebodies occur in the Upper Devonian (D3fm2) carbonate sequence of this deposit. The Fe- and Mn-bearing rocks were studied. Iron ores are composed of hematite, calcite, and quartz. Manganese ores are divided into two (braunite and hausmannite) types. Braunite ores are composed of braunite, calcite, quartz, and albite; hausmannite ores are composed of hausmannite, rhodochrosite, Mn-calcite, tephroite (sometimes with sonolite and alleghanyite), and friedelite. The mineral composition of ores was formed during the burial of metal-bearing sediments containing oxides of Fe3+ and Mn3+/Mn4+. The nature and intensity of the postdepositional processes was controlled by the content of organic matter (OM) in initial rocks. Braunite ores formed under oxidizing conditions; hausmannite ores, under reducing conditions. The initial presence of OM is indicated by the carbon isotope composition in carbonates. High δ13Ccarb values (from –0.3 to 2.1‰) in hematite ores, braunite ores, and host limestones are close to those in the seawater bicarbonate precipitated as a part of biogenic calcite (e.g., shell fragments). At the same time, low δ13Ccarb values (average –12.3‰) in rhodochrosite and Mn-calcite from hausmannite ores indicate the influence of carbon from the OM buried in sediments. The general oxygen isotope composition in iron ores, manganese ores, and host rocks was determined by variations of the isotope ratio in the initial precipitates of 18O-rich calcite and 16O-rich Fe/Mn oxides. Conversion of the primary sedimentary Fe3+ and Mn3+/Mn4+ oxides to hematite, braunite, and hausmannite slightly changes the oxygen isotope composition of Fe–Mn minerals. Average values of δ18Otot determined in the hematite (1.7‰), braunite (14.8‰), and hausmannite (12.7‰) ores are close to those in the sedimentogenic iron (0‰) and manganese oxides (11.5‰). The oxygen isotope composition suggests that initial manganese oxides did not participate in the formation of carbonates in braunite ores. Average oxygen isotope composition of carbonates in these ores (δ18Ocarb 19.1‰) is close to that in host limestones (20.8‰). During the formation of hausmannite ores, in contrast, manganese oxides reacted with OM leading to the formation of rhodochrosite and silicates. Consequently, the light isotope oxygen in the starting oxides was fixed in carbonates (δ18Ocarb 15.9‰) and rocks in general (δ18Ototal 12.7‰). The oxygen isotope composition was not averaged during the formation of hausmannite ores even in the mineralogically similar aggregates in the adjacent rock layers.

Geology, Mineralogy and Geochemistry of the Oligocene Oolitic Iron Ore of the Continental Terminal Formation, Kandi Basin, North-East Benin

The Oligocene Continental Terminal Formation of the Kandi Basin contains high grades of iron mineralization (~56.72% Total Fe). The microscopic study under the polarized and reflected light showed that the iron ore consists of silicate minerals (quartz 50% and zircon 1%) and non-silicate minerals (goethite 30%, hematite 7%, magnetite 3%, pyrite 1%, chalcopyrite 1%, blende 3%, galena 3%, scheelite 1% and gold 2%). The X-rays fluorescence shows that the iron ore is characterized by various elements, such as Fe2O3 (57.91% to 91.33%), SiO2 (3.07% to 33.19%), aluminum (2.94% to 7.74%), vanadium (0.04% to 0.11%), phosphorus (0.79% to 2.29%) and sulfur (<0.3%). The deleterious elements grade is above the permissible limit in metallurgy (0.05% - 0.07% for phosphorus and 0.1% for sulfur). Their high grades indicate that the Kandi Basin iron ore characteristics are not favorable for steel manufacturing despite its good vanadium contents (0.04% to 0.11%). However, it could be used for the cast iron manufacture. Spectrometric analysis by atomic absorption confirms the presence of low-grade gold associated to the iron ore (from 0.006 to 0.015 ppm). The comparative study of discontinuous stratiform iron ore of the Kandi Basin with other oolitic iron ores in exploitation from other countries such as Brazil, Australia, China, Russia, Uganda and the United States shows that iron ore of the Kandi Basin can be mined despite its high silica content.

Pre-Sturtian (800–730 Ma) depositional age of carbonates in sedimentary sequences hosting stratiform iron ores in the Uppermost Allochthon of the Norwegian Caledonides: A chemostratigraphic approach

Carbon and strontium isotope chemostratigraphy (52 δ13Ccarb and δ18O, and 50 87Sr/86Sr analyses of carbonate components in whole-rock samples) was applied for constraining an apparent depositional age of the carbonate protolith to amphibolite-grade, calcite marbles occurring in siliciclastic sedimentary sequences hosting iron formations (the Dunderlandsdalen type iron ores) of previously unknown age in the Rödingsfjället Nappe Complex of the Rana region, Nordland, Norway. The least altered 87Sr/86Sr (0.70676) and δ13C (+2.5 to +5.6‰) values of the marbles (Dunderland Marble 2a) in the hanging wall sequence of the Stensundtjern iron formation in the Rana region are consistent with seawater composition in the time interval 800–730Ma, hence the Middle Cryogenian (pre-Sturtian). The least altered Sr- and C-isotopic values obtained from two other marbles units (Marble 3 and 4) of the Rödningsfjället Nappe Complex are consistent with the Late Cryogenian (c. 660 and 670–700Ma, respectively). The ages obtained provide the first insight into the depositional time of sediment-hosted iron formations of the Uppermost Allochthon in the North-Central Norwegian Caledonides. The Middle Cryogenian-age of marbles associated with iron formations in the Rana region and those located c. 250km to the north (the Håfjellet iron ore horizon) share similar 87Sr/86Sr and δ13C ratios and hence similar chemostratigraphic ages. These iron formations were originally accumulated outside of Baltica, on a glacially influenced carbonate-siliciclastic shelf, apparently on a margin of an unknown microcontinent. The Scandinavian Dunderlandsdalen and the Håfjellet iron ores of Middle Cryogenian age (800–730Ma) were accumulated in an open marine environment distant from volcanic centres, and hence represent an outstanding exception to other reported Neoproterozoic iron formations which were all accumulated in volcanically active continental rift settings.

Geochemical Constraints on the Genesis of the Marquesado Iron Ore Deposits, Betic Cordillera, Spain: REE, C, O, and Sr Isotope Data

The Marquesado area of the Betic Cordillera, southeastern Spain, contains two iron deposits (Alquife and Las Piletas) located within a metamorphosed Permian to Triassic carbonate sequence. The main mineralization at Las Piletas forms stratiform lenses of specular hematite, whereas the mineralization at Alquife consists of irregular bodies of sideritic marble, transformed to goethite and hematite by weathering processes. REE patterns of the host rocks show trends either typical of marine carbonates or similar to those of the specular hematite and siderite, the latter two characterized by upward-convex REE trends and a positive Eu anomaly. Specular hematite and siderite have 87Sr/86Sr values from 0.7095 to 0.7104 and from 0.7103 to 0.7117, respectively, whereas the host carbonates are less radiogenic (87Sr/86Sr values from 0.7077–0.7085). Siderite has δ13C and δ18O values of − 7.6 to − 7.9 per mil (PDB) and 18.68 to 20.00 per mil (SMOW), respectively, whereas the host carbonates have δ13C and δ18O values of −1.40 to +2.02 per mil (PDB) and +19.32 to +25.11 per mil (SMOW). Both the mineralization and host rocks show a broad negative correlation between δ13C and 87Sr/86Sr and between δ18O and 87Sr/86Sr. Field and petrographic data, together with geochemical and isotopic data, indicate that formation of the deposits resulted from both exhalative sedimentary and replacement processes at temperatures between 150° and 200°C from originally hot (> 200°–250°C) crustal hydrothermal fluids enriched in radiogenic Sr and isotopically light C and O. The hydrothermal system acquired abundant Fe by leaching graphite-bearing metapelites under acid conditions within the underlying Paleozoic basement. A stratiform iron oxide facies (precursor of the specular hematite mineralization) formed by precipitation in a Triassic depositional basin under oxidizing conditions at Las Piletas. In contrast, irregular siderite bodies formed by metasomatic replacement of the Triassic carbonates under reducing conditions in the Alquife area.

Geochemistry of the Furnace Magnetite Bed, Franklin, New Jersey, and the Relationship between Stratiform Iron Oxide Ores and Stratiform Zinc Oxide-Silicate Ores in the New Jersey Highlands

Geochemistry of the Furnace Magnetite Bed, Franklin, New Jersey, and the Relationship between Stratiform Iron Oxide Ores and Stratiform Zinc Oxide-Silicate Ores in the New Jersey Highlands

Geochemistry of the Furnace Magnetite Bed, Franklin, NewJersey, and the Relationship between Stratiform Iron Oxide Ores and StratiformZinc Oxide-Silicate Ores in the New Jersey Highlands

The New Jersey Highlands terrane, which is an exposure of the Middle Proterozoic Grenville orogenic belt located in northeastern United States, contains stratiform zinc oxide-silicate deposits at Franklin and Sterling Hill and numerous massive magnetite deposits. The origins of the zinc and magnetite deposits have rarely been considered together, but a genetic link is suggested by the occurrence of the Furnace magnetite bed and small magnetite lenses immediately beneath the Franklin zinc deposit. The Furnace bed was metamorphosed and deformed along with its enclosing rocks during the Grenvillian orogeny, obscuring the original mineralogy and obliterating the original rock fabrics. The present mineralogy is manganiferous magnetite plus calcite. Trace hydrous silicates, some coexisting with fluorite, have fluorine contents that are among the highest ever observed in natural assemblages. Furnace bed calcite has δ 13 C values of –5 ± 1 per mil relative to Peedee belemnite (PDB) and δ 18 O values of 11 to 20 per mil relative to Vienna-standard mean ocean water (VSMOW). The isotopic compositions do not vary as expected for an original siderite layer that decarbonated during metamorphism, but they are consistent with nearly isochemical metamorphism of an iron oxide + calcite protolith that is chemically and mineralogically similar to iron-rich sediments found near the Red Sea brine pools and isotopically similar to Superior-type banded iron formations. Other manganiferous magnetite + calcite bodies occur at approximately the same stratigraphic position as far as 50 km from the zinc deposits. A model is presented in which the iron and zinc deposits formed along the western edge of a Middle Proterozoic marine basin. Zinc was transported by sulfate-stable brines and was precipitated under sulfate-stable conditions as zincian carbonates and Fe-Mn-Zn oxides and silicates. Whether the zincian assemblages settled from the water column or formed by replacement reactions in shallowly buried sediments is uncertain. The iron deposits formed at interfaces between anoxic and oxygenated waters. The Furnace magnetite bed resulted from seawater oxidation of hydrothermally transported iron near a brine conduit. Iron deposits also formed regionally on the basin floor at the interface between anoxic deep waters and oxygenated shallower waters. These deposits include not only manganiferous magnetite + calcite bodies similar to the Furnace magnetite bed but also silicate-facies deposits that formed by iron oxide accumulation where detrital sediment was abundant. A basin margin model can be extended to Grenvillian stratiform deposits in the northwest Adirondacks of New York and the Mont Laurier basin of Quebec. In these areas iron deposits (pyrite or magnetite) are found basinward of marble-hosted sphalerite deposits, such as those in the Balmat-Edwards district. Whether the iron and zinc precipitated as sulfide assemblages or carbonate-oxide-silicate assemblages depended on whether sufficient organic matter or other reductants were available in local sediments or bottom waters to stabilize H2S.

Origin and depositional environment of Ordovician stratiform iron mineralization from Zamora (NW Iberian Peninsula)

The Ordovician stratiform iron deposits at Zamora (NW Iberia) are arranged in several levels ranging between 0.2 and 1.5 m in thickness, which are interstratified in the upper member of the “Pielgo” Quartzites Formation (Arenig). The sandy nature, and trace-fossils corresponding to the ichnogenus Cruziana and Daedalus, together with major and trace element contents of this formation suggest an inter and subtidal, shallow marine depositional environment, which on a global scale formed part of a broad shelf situated in the northern margin of the Gondwana continent. The iron mineralization displays a foliated and banded structure due to the alternation of quartzitic, phosphatic (apatite), chamositic, chamositic-biotitic and ferriferous (magnetite and hematite) beds. They have high TiO2, Ta, Sc, V, Nb, Co, Zn and Y contents. The magnetite contains unusually large amounts of TiO2, V, Cr and Ni; there is also a clear depletion in Eu and the (Eu/Sm)CN ratio is 1. The chamosite contains high concentrations of Cr and V. These results suggest that iron was supplied from the weathering of a continental source, in combination with volcanic activity, such as within-basin basic volcanism or the presence of basic volcanic rocks in the exposed land. The physicochemical conditions of iron mineral crystallization calculated from chamosite compositions are the following: log f O2: −38.8 to −30.7, log f S2: −13.2 to −9.5 and T: 200 to 330 °C. These results together with the δ18O value (∼2‰) of the magnetite suggest that chamosite and magnetite were crystallized during later diagenesis and early low-grade metamorphism under redox conditions below the magnetite-hematite buffer.

Sulphide mineralisation in the Olary Block, South Australia

The South Australian portion of the Willyama Inliers hosts a diversity of small sulphide and uranium deposits and numerous outcropping gossans. This fact, together with geological similarities to the adjacent Broken Hill Block has led to extensive exploration. A broad classification distinguishes two main types of sulphide mineralisation: 1) stratiform iron sulphide-dominated (±Cu, Zn, Co) deposits which occur widespread within specific stratigraphic intervals, and stratabound occurrences of syn-depositional to diagenetic origin which show some structural control; 2) syn-tectonic to post-peak metamorphic replacement and vein-type deposits (Fe-Cu-Au and Cu-Zn-Pb), which are hosted by fractures and within faults and shear zones. These occurrences show no stratigraphic control and are not spatially related to type 1 mineralisation. Late-stage deposits also differ from stratiform/stratabound mineralisation in their texture, mineral assemblage and geochemical composition. Much of the sulphide mineralisation in the Olary Block has been interpreted as resulting from rift-associated syn- to diagenetic processes, such as hot spring exhalations and base metal precipitation along reduction-oxidation interfaces. Subsequent granitic intrusive, high grade metamorphic and multiphase deformation events would have induced remobilisation and redeposition of sulphides in a variety of epigenetic modes. However, a detailed petrographic and geochemical study of sulphide mineralisation in the Olary Block demonstrates that due to the lack of abundant pervasive fluids, translocation and modification of preexisting sulphides were restricted to less than a few centimetres. Instead, widespread syn-tectonic to epigenetic (i.e., post-peak metamorphic) mobilisation of ore constituents occurred to form retrograde sulphide mineralisation as well as multiple generations of late-stage vein deposits. These epigenetic deposits are genetically unrelated to synsedimentary and diagenetic occurrences, an aspect of significance for exploration in the Olary Block. Temporal separation of peak metamorphism in deeper crustal levels from its occurrence in shallow levels, periodic tectonic disturbances and repeated seismic pumping are processes believed to have resulted in intermittent mobilisation of ore constituents from a deep-seated metasedimentary reservoir.

Magnetite lava flows in the Pleito-Melon District of the Chilean iron belt

(1982) concluded that this sequence was deposited in a volcanic island-arc setting. In the Pleito-Mel(n district the Bandurrias Group is composed principally ofandesitic lavas and volcanic breccias with intercalated rhyolitic tuffs and stratiform iron ores; the Bandurrias volcanics continues to the east of the area shown in Figure 2. An intrusive complex with tonalitic, granodioritic, and dioritic units crops out to the west and east of the volcanic sequence (Fig. 2). This complex has up to now been regarded as Tertiary in age (Moscoso et al., 1982), but our stratigraphic data indicate that parts of it may be Cretaceous and constitute a basement for the Bandurrias sequence. In the Berenguela area (Fig. 3) the diorite is younger than the Bandurrias andesites, which are recrystallized. A large number of rhyolitic dikes and a few andesitic ones cut extrusive as well as intrusive rocks, and some of the dikes may be feeders of the extrusions. Structures There are three fault systems in the district, all of them part of the Atacama megafault (cf. Thiele and Pincheira, 1987). The most important systems follow north-northeast and northwest to north-northwest directions and the third one consists of secondary east to east-northeast faults. The north-northeast system is longitudinal and seems to be the oldest. It has produced downfaulted blocks, for example in the E1 Mel(n area (Fig. 3), and many of the iron ores in the district are associated with it. The north

Sulfur and lead isotope studies of stratiform Zn-Pb-Ag deposits, Anvil Range, Yukon; basinal brine exhalation and anoxic bottom-water mixing

Five stratiform Zn-Pb-Ag deposits are known in Early Cambrian metapelitic rocks along a curvilinear trend in the Anvil Range, central Yukon. The Anvil Range deposits occur along the southwestern boundary of the Selwyn basin in the stratigraphic transition zone between metapelites of the Mt. Mye unit and calcareous phyllites of the overlying Vangorda unit. The massive sulfides are associated closely with anomalously thick graphitic phyllites, apparently related to a second-order basin. A typical Anvil cycle of mineralization begins with a ribbon-banded graphitic-quartzitic-pyritic unit. This grades upward into sulfide-bearing quartzite, quartzitic massive sulfide, massive sulfide, and finally a baritic massive sulfide horizon. Sericitic alteration envelopes irregularly encompass each deposit and locally are developed best in footwall rocks.Detailed sulfur isotope studies have been carried out on the DY and Grum deposits and on one representative drill hole from the Faro deposit. The delta 34 S values of sulfide minerals generally range from 10 to 22 per mil and are similar in all three deposits. The delta 34 S values of pyrite in unmineralized samples from the district exhibit a wider range, from 6 to 34 per mil, and show distinct upward stratigraphic increase due to a stagnation cycle in the basin.The delta 34 S values of barite samples are strongly dependent on bottom-water conditions and mode of mixing during brine exhalation. They range from 22 to 26 per mil in the Faro deposit to 36 to 42 per mil in the DY deposit. These variations are due to mixing of isotopically light sulfate (18-20ppm) in Ba-bearing ore fluid and isotopically heavy residual sulfate (30-60ppm) in anoxic seawater.Thirty-eight samples of galena from the DY, Grum, Faro, SB, and Swim deposits have been analyzed for lead isotope ratios. In general, the lead isotope ratio data indicate an upper crustal lead source, with the Proterozoic Grit unit which is inferred to underlie the district being the most likely source rock. A small component of mantle leads from mafic igneous rocks or due to source rock inhomogeneity is also indicated.Graphitic host lithologies, lack of stratiform iron oxides, delta 34 S values of sedimentary pyrite, and sulfide sulfur-organic carbon variations in unmineralized cores indicate formation of the Anvil deposits in strongly reduced bottom water related to a previously unknown Early Cambrian anoxic event.

Fe-Zn-Cu-Pb mineralization in the Salton Sea geothermal system, Imperial Valley, California

The Salton Sea geothermal system is an area of active metamorphism in a Plio-Pleistocene deltaic sedimentary sequence. Within it, hypersaline brines containing up to 250,000 ppm total dissolved solids and temperatures up to 365 degrees C are causing incipient sulfide and oxide mineralization. We have made a detailed analysis of ore mineralization in two boreholes drilled to depths of up to 2,500 m in this system. Major ore minerals observed are, in order of decreasing abundance, pyrite, hematite, sphalerite, chalcopyrite, pyrrhotite, marcasite, and galena. The associated nonore minerals are mainly calcite, quartz, epidote, anhydrite, adularia, and chlorite. Ore mineralization can be divided into three main types. Diagenetic sulfide mineralization, occurring at depths less than 760 m and temperatures less than 250 degrees C, consists of fine-grained stratiform iron sulfides occurring as cement in sandstone and as disseminations and bands in shale. Metamorphic sulfide mineralization, occurring at depths greater than 760 m and temperatures greater than 250 degrees C, includes the progressive development of porphyroblastic pyrite with increasing depth and temperature, its replacement by other Fe-Cu-Zn-Pb sulfides at depths greater than 1,220 m, and its decomposition into skeletal aggregates at depths greater than 1,525 m, concurrent with hornfelsic recrystallization of the host sediments. Vein and related pore-filling ore mineralization is of two contrasting types: hematite-silicate-sulfide + or - sulfate and sulfide-carbonate + or - silicate. The first type occurs in open porous veins in single vertical zones at least 220 m thick in each borehole. The second type occurs in veins that are generally sealed and occur in relatively thin, scattered vertical intervals in both boreholes. Thermodynamic analysis indicates that the modern reservoir brines are near equilibrium with the hematite-silicate-sulfide + or - sulfate vein assemblage at 300 degrees C, with pH = 5.4 and log a (sub O 2 ) = -30. Reactions between hematite, pyrite, and iron-bearing silicates (chlorite, epidote) probably control the redox state of the present brines. Calculated metal-chloride complex solubilities for Fe, Zn, Pb, and Cu agree well with brine analyses. Hematite, pyrite, and chalcopyrite are supersaturated; galena and sphalerite are undersaturated in the modern brines.The phase relations of the presently sealed sulfide-carbonate + or - silicate veins require that earlier fluids, if formed at the same temperature and pH, had to be more reduced and sulfur rich than the present slightly oxidized, sulfur-poor brines. Contrasting fluid redox states may be caused by a late lateral or fault-controlled influx of more oxidized surficial waters. Lack of acid alteration implies boiling has not occurred to produce the oxidized fluids. Early formed diagenetic iron sulfides are commonly replaced by later Cu-Pb-Zn sulfides and iron oxides and enveloped and resorbed by veins, acting as nuclei and sources of S and Fe for later mineralization. Precipitation probably occurs when seismic or hydraulic fracturing allows the metal-rich, sulfur-poor brines to interact with earlier pyrite sulfur in the host rocks. Thus the geothermal system as presently explored is best classified as an incipient stratabound sulfide deposit that is being overprinted by hydrothermal and metamorphic processes.