Fe Oxide Deposits Research Articles

Genesis of Devonian volcanic-associated Lahn-Dill-type iron ores — part I: iron mobilisation and mineralisation style

Fe-oxide deposits of the Lahn-Dill-type in the eastern Rhenish Massif comprise haematite and quartz with minor siderite, magnetite, and calcite. The deposits are located in the hanging wall of thick volcaniclastic rock sequences and mark the Middle to Late Devonian boundary. Varying ore types with accompanying footwall rocks were sampled from two formerly important ore deposits, the Fortuna mine (Lahn syncline) and the Briloner Eisenberg mine (East Sauerland anticline), in order to elucidate the interplay of processes leading to ore formation. Deposit geology, petrography, and whole-rock geochemistry suggest that the ores formed by iron mobilisation from deeply altered footwall volcaniclastic rocks, subsequent venting of a modified H2O-CO2-Fe-rich and H2S-poor fluid, and precipitation on the seafloor (sedimentary-type), or locally by metasomatic replacement of wall rocks (replacement-type). Petrographic analysis to the sub-micron scale revealed that the sedimentary-type ores most likely formed from a Fe-Si-rich gel and accompanying maturation. Early gel textures include the presence of spherules, aggregates, tubes, and filamentous stalks consisting of nanocrystalline haematite dispersed in a matrix of microcrystalline quartz. Local diagenetic Fe3+ reduction within the gel is indicated by siderite replacement of haematite. Replacement-type ores formed due to a two-step process including coprecipitation of (precursor) haematite and carbonates and subsequent metasomatic replacement by haematite. These ore-forming processes took place during a time when several restricted shallow marine basins in the north-eastern Rheic Ocean were influenced by extensive volcanism and associated hydrothermal fluid flux. Examples of similar volcanic-associated Fe-oxide occurrences of Silurian to Carboniferous age can be categorised as being of Lahn-Dill-type ores as well.

Role of Fe oxide in the underdeposit corrosion of pipeline steel in oilfield produced water containing SRB

AbstractThe corrosion behavior of C20 pipeline steel was studied under Fe oxide and sand deposits in oilfield produced water containing sulfate‐reducing bacteria (SRB) via electrochemical techniques and surface analyses. A galvanic current of about 2 μAcm−2 was detected when the Fe oxide deposited sample and the bare steel were coupled in zero resistance ammeter (ZRA) mode, with the former being the anodic member of the couple. The Fe oxide deposited steel recorded severe localized corrosion after 14 days of testing in the SRB inoculated oilfield produced water. Corrosion severity decreased on the sand‐deposited steel, with general corrosion as the dominant morphology of surface attack. The development of product film was a vital factor in steel corrosion under the Fe oxide deposit.

Genesis of the 1.45 Ga Kratz Spring Iron Oxide-Apatite Deposit Complex in Southeast Missouri, USA: Constraints from Oxide Mineral Chemistry

Abstract Seven major and numerous lesser Fe oxide occurrences within the 1.47 Ga St. Francois Mountains terrane in Missouri (USA) have previously been described as iron oxide-apatite (IOA) and iron oxide-copper-gold (IOCG) deposits. Researchers speculate that these contain significant amounts of critical minerals, most notably rare earth elements and cobalt. One of the less-studied deposits in the region is the 1.455 Ga Kratz Spring deposit. The deposit consists of two steeply dipping magnetite bodies beneath 450 m of sedimentary cover. The genesis of the Kratz Spring deposit and its relationship to nearby IOA-IOCG deposits remains poorly constrained. To better understand the formation of the Kratz Spring deposit, the authors integrated stratigraphic, petrographic, and bulk rock studies with in situ trace element and Fe isotope chemistry of magnetite and hematite. These data show that the Kratz Spring deposit is hydrothermal in origin but is divided into two subdeposits according to different fluid sources and formation conditions: (1) a deep but cooler hydrothermal Kratz Spring South deposit with a juvenile fluid source and (2) a shallow but hotter magmatic-hydrothermal Kratz Spring North deposit with variable fluid sources. Our genetic model suggests the two Kratz Spring deposits are local expressions of the same mineralization system; i.e., the Kratz Spring South deposit is a distal, lower-temperature offshoot of the feeder system that formed the Kratz Spring North deposit. Understanding the magmatic-hydrothermal plumbing system that formed Missouri’s IOA-IOCG deposits is important to guiding critical mineral exploration efforts in the region.

Constraints of Fe isotopes on the origin of volcanic-hosted Fe-oxide deposits: A case study of the Late Cambrian 7918 Fe deposit, North Altun orogenic belt, Xinjiang, NW China

• The 7918 iron deposit is hydrothermal in origin. • Volcanic rocks hosting orebodies act as a main Fe source in the 7918 deposit. • A three-stage model has been proposed to account for genesis of 7918 iron deposit. The 7918 Fe deposit in the North Altun orogenic belt , NW China, is composed of strata-bound orebodies between Late Cambrian basalts and marble, associated with intensive skarn-type alteration. Basalts close to the orebodies are altered, with gradational contacts to skarns. Variations of major and trace elements and δ 57 Fe values of low-Ti and high-Ti basalts reflect the role of both magmatic evolution and hydrothermal alteration . Negative correlations between δ 57 Fe and TFeO/TiO 2 and between Na 2 O and TFeO/TiO 2 of high-Ti basalts are explained by lighter Fe isotopes leached from basalts during water–rock reaction. Hence, we considered basalts as the main source of Fe. Magnetite precipitation was related to a retrograde skarn stage. Early-precipitated magnetite has lower incompatible trace elements (Lu, Zr, Hf, Nb, Ta, and Yb), higher compatible trace elements (Co and Ni), and lower δ 57 Fe values than late-precipitated magnetite. Hydrothermal fluids equilibrated with early-precipitated magnetite (−0.11‰ to 0.16‰) and late-precipitated magnetite (0.22‰ to 0.27‰) have δ 57 Fe values of −0.38‰ to −0.11‰ and −0.05‰ to 0.00‰, respectively. The isotopically heavier late-phase fluids indicate an increase of oxygen fugacity, in which Fe 2+ was mostly oxidized to Fe 3+ . This is also consistent with reduced V concentrations from early-precipitated to late-precipitated magnetite. Our new data and previous geochronological results suggested that the 7918 Fe deposit records a continuous process of volcanic eruption , hydrothermal alteration, and Fe mineralization , which was related to a deep “magma chamber” formed at a subduction-related setting.

Chapter 6 Paleoproterozoic (1.9–1.8 Ga) syn-orogenic magmatism, sedimentation and mineralization in the Bergslagen lithotectonic unit, Svecokarelian orogen

Abstract Felsic volcanic rocks ( c. 1.91–1.89 Ga) and interlayered limestone, hosting Zn–Pb–Ag ± Cu ± Au ± Fe sulphide and Fe oxide deposits, characterize the Bergslagen lithotectonic unit, Svecokarelian orogen, south-central Sweden. Three sulphide mines are currently in operation. Siliciclastic sedimentary rocks stratigraphically envelop this volcanic succession and all the rocks are intruded by a dominant calc-alkaline, c. 1.91–1.87 Ga plutonic suite. Fabric development associated with folding and localized shear deformation followed at c. 1.87–1.86 Ga (D 1 ) and was succeeded by strongly partitioned strain (D 2 ). Dextral transpression along steeply dipping, WNW–ESE or NW–SE shear zones prevailed in the northern and southern domains, whereas major folding with east to northeasterly axial surface traces and shearing along limbs occurred in the central domain. Open folding (D 3 ) subsequently affected the western areas. Polyphase metamorphism under low-pressure and variable temperature conditions included anatexis at c. 1.86 Ga (M 1 ) and 1.84–1.80 Ga (M 2 ). More alkali–calcic magmatic activity, combined with the emplacement of anatectic granite and pegmatite, overlapped and succeeded the M 1 and M 2 migmatization events at c. 1.87–1.83 Ga and c. 1.82–1.75 Ga, respectively. The younger granites are genetically linked in part to W skarn deposits and host Mo sulphide mineralization. Switching between retreating and advancing subduction systems during three separate tectonic cycles along a convergent, active continental plate margin is inferred.

Chapter 8 Outboard-migrating accretionary orogeny at 1.9–1.8 Ga (Svecokarelian) along a margin to the continent Fennoscandia

Abstract An intimate lithostratigraphic and lithodemic connection between syn-orogenic rock masses inside the different lithotectonic units of the 2.0–1.8 Ga (Svecokarelian) orogen, Sweden, is proposed. A repetitive cyclic tectonic evolution occurred during the time period c. 1.91–1.75 Ga, each cycle lasting about 50–55 million years. Volcanic rocks ( c. 1.91–1.88 Ga) belonging to the earliest cycle are host to most of the base metal sulphide and Fe oxide deposits inside the orogen. Preservation of relict trails of continental magmatic arcs and intra-arc basins is inferred, with differences in the depth of basin deposition controlling, for example, contrasting types of base metal sulphide deposits along different trails. The segmented geometry of these continental magmatic arcs and intra-arc basins is related to strike-slip movement along ductile shear zones during transpressive events around and after 1.88 Ga; late orogenic folding also disturbed their orientation on a regional scale. A linear northwesterly orogenic trend is suggested prior to this structural overprint, the strike-slip movement being mainly parallel to the orogen. A solely accretionary orogenic model along an active margin to the continent Fennoscandia, without any trace of a terminal continent–continent collision, is preferred. Alternating retreating and advancing subduction modes that migrated progressively outboard and southwestwards in time account for the tectonic cycles.

Iron oxide geochemistry in the Great Lakes Region (North America): Implications for ochre provenance studies

Evidence for ochre use has been identified at archaeological sites in the eastern Great Lakes Region (North America) dating from the Paleoindian through to the Late Woodland periods. Yet, little is known regarding its procurement practices or if it was a component of established exchange networks. Addressing archaeological evidence for ochre provenance requires an assessment of the geochemical variability of Fe-oxide bearing deposits to determine if they can be differentiated on the basis of their chemical compositions, and if their chemical signatures satisfy the provenance postulate. We present the results of a geochemical characterization and multivariate statistical analysis of Fe-oxide deposits and ochre artifacts. Using Neutron Activation Analysis (NAA) and X-ray Diffraction (XRD), we determined the elemental and mineralogical characteristics of ten Fe-oxide sources in the Great Lakes Region. In addition, we analyzed and compared twenty-three ochre nodule artifacts from the Haney-Cook villages (BcHb-27) and the Ball village (BdGv-3) to determine if they could be geochemically linked to any of the source deposits. The results presented here demonstrate that the Fe-oxide bearing sources each possess unique geochemical signatures, and, that ochre artifacts recovered from the archaeological sites can be linked to source deposits or source zones. Inhabitants of Haney-Cook were most likely engaged in direct procurement of local Fe-oxides, while those at the Ball site used ochres that were acquired either by mid-distance (<100 km) direct procurement, or intra-group exchange. Our findings illustrate the potential for ochre-based provenance studies in the Great Lakes Region.

Fe–O stable isotope pairs elucidate a high-temperature origin of Chilean iron oxide-apatite deposits

Iron oxide–apatite (IOA) ore deposits occur globally and can host millions to billions of tons of Fe in addition to economic reserves of other metals such as rare earth elements, which are critical for the expected growth of technology and renewable energy resources. In this study, we pair the stable Fe and O isotope compositions of magnetite samples from several IOA deposits to constrain the source reservoir of these elements in IOAs. Since magnetite constitutes up to 90 modal% of many IOAs, identifying the source of Fe and O within the magnetite may elucidate high-temperature and/or lower-temperature processes responsible for their formation. Here, we focus on the world-class Los Colorados IOA in the Chilean iron belt (CIB), and present data for magnetite from other Fe oxide deposits in the CIB (El Laco, Mariela). We also report Fe and O isotopic values for other IOA deposits, including Mineville, New York (USA) and the type locale, Kiruna (Sweden). The ranges of Fe isotopic composition (δ56Fe, 56Fe/54Fe relative to IRMM-14) of magnetite from the Chilean deposits are: Los Colorados, δ56Fe (±2σ)=0.08±0.03‰ to 0.24±0.08‰; El Laco, δ56Fe=0.20±0.03‰ to 0.53±0.03‰; Mariela, δ56Fe=0.13±0.03‰. The O isotopic composition (δ18O, 18O/16O relative to VSMOW) of the same Chilean magnetite samples are: Los Colorados, δ18O (±2σ) = 1.92±0.08‰ to 3.17±0.03‰; El Laco, δ18O=4.00±0.10‰ to 4.34±0.10‰; Mariela, δ18O=(1.48±0.04‰). The δ18O and δ56Fe values for Kiruna magnetite yield an average of 1.76±0.25‰ and 0.16±0.07‰, respectively. The Fe and O isotope data from the Chilean IOAs fit unequivocally within the range of magnetite formed by high-temperature magmatic or magmatic–hydrothermal processes (i.e., δ56Fe 0.06–0.49‰ and δ18O=1.0–4.5‰), consistent with a high-temperature origin for Chilean IOA deposits. Additionally, minimum formation temperatures calculated by using the measured Δ18O values of coexisting Los Colorados magnetite and actinolite separates (630°C) as well as Fe numbers of actinolite grains (610–820°C) are consistent with this interpretation. We also present Fe isotope data from magmatic magnetite of the Bushveld Complex, South Africa, where δ56Fe ranges from 0.28±0.04‰ to 0.86±0.07‰. Based on these data and comparison to published Fe and O stable isotope values of igneous magnetite, we propose extending the magmatic/high-temperature δ56Fe range to 0.86‰. Considering that the Chilean IOAs and Kiruna deposit are representative of IOA deposits worldwide, the Fe and O stable isotope data indicate that IOAs are formed by high-temperature (magmatic) processes.

Iron isotope composition of aqueous phases of a lowland environment

Environmental context Iron (Fe) isotope analysis is a powerful tool to understand the transport of Fe within and from soils to rivers. We determined Fe isotopes and Fe concentrations of soil solutions at different depths and found that the Fe isotope compositions are modified owing to adsorption onto Fe oxides, especially in the subsoil. Hence Fe-rich capillary rising groundwater or seeping Fe-rich surface water are depleted in Fe and potentially other metals in Fe oxide-rich soil horizons. Abstract The mobility of iron (Fe) in soils is strongly affected by redox conditions, which also affect Fe input into groundwater and rivers. Stable Fe isotope analyses allow further investigation of Fe translocation processes within, into and out of soils. Soil solutions taken from a Gleysol in a lowland area (NW Germany) at different depths revealed that Fe concentration and isotope ratios strongly varied with abundance of solid Fe oxides. Low δ56Fe values of –1.7 ‰ and minimum Fe concentrations of ~0.2mgL–1 were recorded in soil solutions of Fe-rich horizons. Soil solutions of a Fe-poor horizon, however, yielded higher δ56Fe values (–0.39 ‰) and Fe concentrations of up to 68mgL–1. The water of an adjacent drainage ditch featured δ56Fe values of –1.1 ‰, in strong contrast to +0.60 ‰ of short-range ordered Fe oxide deposits in the ditch bed. We attribute the coupled low δ56Fe values and Fe concentrations to combined adsorption and atom exchange between dissolved Fe and Fe oxides. Consequently Fe oxide-poor horizons had higher δ56Fe values and dissolved Fe concentrations. Outflow of Fe-rich groundwater and surface water during rainfall into rivers is responsible for high δ56Fe for Fe-oxide precipitates and low riverine δ56Fe values.

The Early Permian mafic–ultramafic complexes in the Beishan Terrane, NW China: Alaskan-type intrusives or rift cumulates?

The petrogenesis and geodynamic setting of the Early Permian mafic–ultramafic complexes in the Beishan Terrane, NW China have important bearing on the prospects of Ni–Cu–PGE sulfide and Ti–Fe oxide deposits, as well as in understanding the history of evolution of the southern Central Asian Orogenic Belt (CAOB). Here we present results from a detailed study on a representative suit of a mafic–ultramafic rocks from the Bijiashan complex in the Beishan Terrane. The complex is composed of dunite, troctolite, olivine gabbro and gabbro without clinopyroxenite or hornblendite. In addition to olivine and clinopyroxene, orthopyroxene and plagioclase are also present in all these rock types, in the absence of chromite or primary hornblende. The minerals display marked compositional variations with the Fo content in olivines ranging from 66 to 83 and the clinopyroxenes ranging from diopside to augite. The whole-rock geochemistry shows good correlations among the major elements, and the trace elements are characterized by flat REE patterns with Eu positive anomalies, and HFSE depletion relative to LILE. These features are comparable to other mafic–ultramafic complexes in the Beishan and Eastern Tianshan terranes, but obviously deviate from the typical features of Alaskan-type intrusives. The present study suggests that the Beishan complexes were formed in a post-orogenic extensional environment with potential for Ni–Cu mineralization, rather than in arc-related setting with Alaskan-type PGE mineralization. Our study confirms that the Beishan area does not conform to an arc-related setting, but was located within a rift setting in Permian probably related to Permian mantle plume event, suggesting that the subduction of the Paleo-Asian ocean had culminated by this time, and the southern CAOB witnessed a post-orogenic extensional regime in late Paleozoic.

Composition and structure of nanocrystalline Fe and Mn oxide cave deposits: Implications for trace element mobility in karst systems

Fe and Mn oxides are ubiquitous, highly reactive minerals in the environment which often control the distribution and speciation of trace elements in soils and aquatic systems. Pautler Cave in Southwest Illinois contains Fe and Mn oxide deposits with properties that differ spatially and temporally. Two depositional environments exist in the cave: Fe and Mn precipitation where anoxic groundwater enters the oxic cave passage (seep deposition) and direct deposition from the cave stream. Seep waters have higher concentrations of Ca 2+, Fe 2+, and Mn 2+, lower pH, and lower O 2(aq) concentrations relative to stream waters. In addition, seep waters have nearly constant δ 18O values that are similar to the yearly average of local precipitation, whereas cave stream waters have more erratic δ 18O values and appear to be influenced by seasonal variations of δ 18O in rainfall. These differences suggest that the seep water source has a longer residence time in the subsurface than the cave stream water. Fe oxides are solely found as seep deposits whereas Mn oxides are deposited both by groundwater seepage and cave streams. Freshly precipitated Fe oxides are composed of ferrihydrite and filamentous material consistent with biomineralization structures of the microbial genera Gallionella and Leptothrix. Mn oxide minerals consist of 7 and 10 Å phyllomanganates having a range of three dimensional order. Mn oxide samples show a greater heavy metal and trace element content compared to Fe oxides, and Mn oxides deposited by cave streams have higher concentrations of metals than Mn oxide seep deposits. In addition, phyllomanganate particle morphologies and crystallinity display a compositional dependence, with platelet size and phyllomanganate sheet stacking-order decreasing with increasing Ba and trace element content and decreasing Ca content; these properties also appear to correlate with dissolved Mn(II) in the source water. These observations suggest that Fe and Mn oxide minerals in oxic cave systems affect trace element concentrations in carbonate-fed springs, and illustrate a previously unrecognized limitation on the transport of inorganic nutrients and contaminants in karst aquifers.

Nanotubes: Shaking Hands, Talking, or Sharing?

Nanotubes: Shaking Hands, Talking, or Sharing?

FLUID INCLUSION AND STABLE ISOTOPE STUDY OF THE ESFORDI APATITE-MAGNETITE DEPOSIT, CENTRAL IRAN--A REPLY

Sir: We are pleased that our recent paper in Economic Geology (Jami et al., 2007), presenting the geologic setting, a description of the mineralization, fluid inclusion, and O isotope analyses of the Esfordi P-Fe oxide deposit in central Iran, has been of interest to Aftabi and his colleagues (Aftabi et al., 2009). The opportunity to discuss further some aspects of our paper is appreciated, including support for a hydrothermal-dominated depositional model for Esfordi. Aftabi et al. (2009) focus their comments on the question of the affinity of the Esfordi P-Fe oxide deposit. In particular, they argue that Esfordi and, by inference, other diverse Fe oxide deposits within the Bafq district of central Iran should be considered Rapitan-type banded iron formations, i.e., BIF (Klein, 2005) rather than Kiruna-type (Hitzman, 2000). In discussing our results and conclusions in the light of previous work (particularly, Forster and Jafarzadeh, 1994), Aftabi et al. (2009) have misconstrued our interpretation of some of the textural and isotopic data as providing support for an orthomagmatic model for the Esfordi P-Fe oxide deposit. Based on the structure, relationships between mineralization and host rocks, fluid inclusion and isotopic data, we concluded that Esfordi exhibits strong evidence of hydrothermal origin, though we did not preclude some component of precursor orthomagmatic processes in formation of the P-Fe mineralisation. It is noted that some examples of the Kiruna end member of the continuum of Fe oxide deposits discussed by Hitzman (2000) are controversial, particularly in regard to their evolution (Sillitoe and Burrows, 2002; Henriquez et al., 2003). We have shown that Esfordi has a number of features typical of the Kiruna-type deposits and does not exhibit features consistent with an affinity to Rapitan-type Fe-oxide deposits. This evidence will be discussed briefly, with further data …

Fabric variability within layered Fe-oxide deposits in Mid-Late Miocene sedimentary formations, NW Borneo: Impact on facies architectural interpretations

Iron (Fe) can accumulate in various forms in sedimentary environments that experience alternating moisture conditions, hydration and dehydration processes and redox processes. It has been pointed out that there is a major gap in the understanding of the composition of mixed solid-phase minerals, their size, morphology and arrangement in the matrix and possible interactions with pore solutions and the environment of deposition. Variations in the fabric within concretions in some Mid-Late Miocene beds in NW Borneo suggest that there were more changes in the energy levels and processes involved than what has been perceived from routine fabric analyses of the sedimentary rocks. The sedimentary rocks are essentially carbonaceous claystones containing varying amounts of highly restricted marginal-marine fauna. The fauna probably accumulated in brackish waters, and comprise in-situ buried turritellid snails, fish teeth and bone tissue, fragments of crabs, very few forams and possibly ostracod shells. Evidence exists to support the concept of reworked cobble or pebble-sized fossiliferous mudstone clasts picking up contemporaneous shell fragments in these restricted conditions. Iron oxide content is seen as (i) micrite-size primary components forming part of a muddy matrix and (ii) diagenetic coatings. Coatings occur in a non-rhythmical pattern after the formation of mudstones, and the reworking of the material and subsequent coatings by iron. Iron coated pebbles/cobbles may experience re-cementing into large sheets giving the appearance of red beds. These occur under some very special redox conditions. These iron-concretion rich horizons appear then to represent extreme (climatic, etc) facies developments that were restricted to coastal areas during Mid-Late Miocene. Present-day iron deposition processes might be a key to understanding past processes. This study forms the basis for an enhanced understanding on facies architectural interpretations.

Four-Hundred-and-Ninety-Million-Year Record of Bacteriogenic Iron Oxide Precipitation at Sea-Floor Hydrothermal Vents

Fe oxide deposits are commonly found at hydrothermal vent sites at mid-ocean ridge and back-arc sea floor spreading centers, seamounts associated with these spreading centers, and intra-plate seamounts, and can cover extensive areas of the seafloor. These deposits can be attributed to several abiogenic processes and commonly contain micron-scale filamentous textures. Some filaments are cylindrical casts of Fe oxyhydroxides formed around bacterial cells and are thus unquestionably biogenic. The filaments have distinctive morphologies very like structures formed by neutrophilic Fe oxidizing bacteria. It is becoming increasingly apparent that Fe oxidizing bacteria have a significant role in the formation of Fe oxide deposits at marine hydrothermal vents. The presence of Fe oxide filaments in Fe oxides is thus of great potential as a biomarker for Fe oxidizing bacteria in modern and ancient marine hydrothermal vent deposits. The ancient analogues of modern deep-sea hydrothermal Fe oxide deposits are jaspers. A number of jaspers, ranging in age from the early Ordovician to late Eocene, contain abundant Fe oxide filamentous textures with a wide variety of morphologies. Some of these filaments are like structures formed by modern Fe oxidizing bacteria. Together with new data from the modern TAG site, we show that there is direct evidence for bacteriogenic Fe oxide precipitation at marine hydrothermal vent sites for at least the last 490 Ma of the Phanerozoic.

Integrated cation–anion/volatile fluid inclusion analysis by gas and ion chromatography; methodology and examples

Combined gas and ion chromatographic analysis of well characterized, small (∼1 g) fluid inclusion-bearing samples is a powerful, but simple, means for obtaining integrated fluid concentrations of major and trace, volatile and ionic fluid constituents without using microthermometrically determined salinity for normalization. The methodology, which is described and assessed in detail, involves crushing a carefully cleaned sample at ∼105°C in a stainless steel crusher on-line to a gas chromatograph. After volatile analysis, the crushed sample is removed and leached with deionized water to produce a leachate solution, which is filtered and analyzed by ion chromatography, and other methods. For example, detailed procedures are given for I − analysis with a low detection limit of 0.5 ppb using a trace anion concentrator column and pulsed amperometric, rather than electrochemical, detection. The data are combined (calculation procedure given) to give whole fluid analyses in concentrations of mol% or mmol/l; this procedure removes a large systematic error of approximately a factor of 2, if separate volatile and hand crush cation/anion analyses are linked through sample weights. Results indicate a mean crushing efficiency of 82±6% for a mean sample mass of 0.97 g, based on 97 determinations. Tests on the efficiency of the leaching process showed that >90% of univalent ions are removed. Based on 145 analyses, charge balances are found to be close to 1.0 when all major positive and negative species are included, both analyzed and calculated (e.g., carbonate species). Analytical errors (coefficients of variation; percentage) for species in moles vary from 1.1 (Br −) to 17.9 (I −), from 0.02 (H 2O) to 0.21 (I −) for species in mol%, and from 13.1 (F −, Br −) to 22.1 (I −) for species in mmol/l. Application of the combined GC/IC technique to well characterized sample sets from diverse settings, including the Tanco Li–Cs–Ta pegmatite, Archean Au–quartz vein systems, the Polaris MVT Pb–Zn deposit, and the ∼3.2 Ga Barberton sea-floor Fe-oxide deposits (ironstone pods) has demonstrated its utility for constraining fluid sources, identifying fluid types, and determining the effects of processes such as H 2O–CO 2 phase separation and fluid–wall rock interaction.

Early proterozoic Cu(Au) and Fe ore deposits associated with regional NaCl metasomatism in northern Fennoscandia

Scapolite is widely distributed in 1.9–2.5 Ga volcano-sedimentary rocks and 1.77–2.2 Ga igneous rocks over several hundred square kilometres in northern Fennoscandia, comprising northern Sweden, northern Finland and adjacent parts of Norway and Russia. This region is one of the largest scapolite-bearing Precambrian terranes in the world. Albitization, and to a lesser extent carbonatization, phyllic and tourmaline alteration, are spatially associated with scapolite. A number of epigenetic Cu(Au) sulphide and Fe oxide deposits in northern Fennoscandia show a spatial and genetic relationship to this type of alteration, mainly scapolitization and albitization. The main metal occurrences are in 2.0–2.5 Ga mafic volcanics and sediments of the Lapponian Greenstone group and in 1.9 Ga intermediate-composition volcanic and volcaniclastic rocks of the Svecofennian Porphyry group. The scapolite is mainly a dipyre-mizzonite with Cl and CO 3 and small amounts of SO 4 and F, indicating high Na and Cl activity at the time of crystallization. Fluid inclusion data of the Lapponian Pahtohavare and similar CuAu deposits indicate formation temperatures of about 300°C and ore deposition from highly saline aqueous solutions. The deposition of copper and gold was in places regulated by a redox barrier; graphite in associated schists controlled the reduction reactions of the ore fluids and metals were precipitated. The Lapponian and Svecofennian sulphide deposits contain tourmaline of the schorl-dravite series. Aitik-Nautanen Cu(Au) style deposits and in particular some deposits with vein-style iron ore, contain dravite-schorl deficient in Al and enriched in Fe 3+, which is due to FeAl substitution in an oxidizing, relatively iron-rich environment. Scapolite and, probably also tourmaline, formed by a complex, multistage process. The source of the components in scapolite may have been evaporitic sequences or high salinity brines in Lapponian rift basins that contain 2.0–2.5 Ga mafic volcanics. During low to medium-grade (low P) regional metamorphism, the components that formed scapolite and tourmaline were mobilized and transported to their present positions in several metasomatic phases. Fault zones with fractures and breccias channeled the fluids, resulting in locally developed intense alteration. Gold and copper was transported by saline, high fO 2, high temperature solutions as metal-chloride complexes. The ultimate source of fluids and heat sources is uncertain, but deep-seated crustal magmatic processes seem prerequisite. The alteration occurred mainly around 1.9 Ga at the peak of the main regional metamorphism and the intrusion of granitoids through to around 1.8 Ga. Cuz.sbnd;(Au) sulphide and Fe oxide ore deposits associated with large-scale

Proterozoic low-Ti iron-oxide deposits in New York and New Jersey: Relation to Fe-oxide (Cu–U–Au–rare earth element) deposits and tectonic implications: Comment and Reply

Proterozoic low-Ti iron-oxide deposits in New York and New Jersey: Relation to Fe-oxide (Cu–U–Au–rare earth element) deposits and tectonic implications: Comment and Reply

Proterozoic low-Ti iron-oxide deposits in New York and New Jersey: Relation to Fe-oxide (Cu–U–Au–rare earth element) deposits and tectonic implications: Comment and Reply

Proterozoic low-Ti iron-oxide deposits in New York and New Jersey: Relation to Fe-oxide (Cu–U–Au–rare earth element) deposits and tectonic implications: Comment and Reply

Proterozoic low-Ti iron-oxide deposits in New York and New Jersey: Relation to Fe-oxide (Cu–U–Au–rare earth element) deposits and tectonic implications

Low-Ti iron-oxide deposits in exposed Grenville-age rocks of New York and New Jersey belong to a distinct class of iron-oxide (Cu-U-Au–rare earth element [REE]) deposits that includes similar iron deposits in southeastern Missouri and the Kiruna district of Sweden, the giant Olympic Dam U-Cu-Au-Ag deposit (Australia), and the Bayan Obo REE-Nb deposit (China). Most of the New York–New Jersey deposits exhibit features consistent with a hydrothermal origin and define a regionally significant metallogenic event that provides important clues to the evolution of this part of the Grenville orogen. In the Adirondacks, the tectonic setting of these deposits is consistent with postorogenic uplift and extensive crustal melting at 1070–1050 Ma that was accompanied by late tectonic to posttectonic deposition of iron.