Evaluation of uptake mechanisms of phosphate by Fe(III)(oxyhydr)oxides in Early Proterozoic oceanic conditions

Banded iron formations (BIF) are proxies of global dissolved inorganic phosphate (DIP) content in Precambrian marine waters. Estimates of Precambrian DIP rely on constraining the mechanisms by which Fe(III)(oxyhydr)oxides scavenge DIP in NaCl solutions mimicking elevated Precambrian marine Si and Fe(II) concentrations. The two DIP binding modes suggested for Early Proterozoic marine waters are 1) surface attachment on pre-formed Fe(III)(oxyhydr)oxides (adsorption), and 2) incorporation of P into actively growing Fe(III)(oxyhydr)oxides (coprecipitation) during the oxidation of Fe(II) to Fe(III)(oxyhydr)oxides in the presence of DIP. It has been suggested that elevated Si concentrations such as those suggested for Precambrian seawater, strongly inhibit adsorption of DIP in Fe(III)(oxyhydr)oxides, however recent coprecipitation experiments show that DIP is scavenged by Fe(III)(oxyhydr)oxides in the presence of Si, seawater cations and hydrothermal As. In this study we show that the DIP uptake onto Fe(III)(oxyhydr)oxides by adsorption is less than 5% of that by coprecipitation. Differences in surface attachment and the possibility of structural capture within the Fe(III)(oxyhydr)oxides are inferred from the robust influence Si has on DIP binding during adsorption, but inhibited for coprecipitation when As(III) and As(V) are present. The data imply that in the Early Proterozoic open oceans, Fe(III)(oxyhydr)oxides precipitated when deep anoxic Fe(II)-rich waters rose and mixed with the first permanently oxygenated ocean surface waters, caused DIP removal from surface waters through coprecipitation rather than adsorption. Local variations in DIP and perhaps even stratification of DIP in the oceans were likely created from the continuous removal of DIP from surface waters by Fe(III)(oxyhydr)oxides, and its partial release into the anoxic bottom waters and in buried sediments. In addition to a DIP famine, the selectivity for DIP over As(V) may have led to As enrichment in surface waters both of which would have most likely decreased the productivity of cyanobacteria and O2 production

Carbonate alteration of ophiolitic rocks in the Arabian–Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits

Ultramafic portions of ophiolitic fragments in the Arabian–Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2 or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between −8.1‰ and −6.8‰ for δ13C, +6.4‰ and +10.5‰ for δ18O, and 87Sr/86Sr of 0.7028–0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2±CH4±N2) and aqueous-carbonic (H2O-NaCl-CO2±CH4±N2) low salinity fluid, with trapping conditions of 270–300°C and 0.7–1.1 kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite–antigorite transition

A morphological and size-based study of the changes of iron sulfides in the Caples and Torlesse Terranes (Otago Schist, New zealand) during prograde metamorphic evolution

It is widely accepted that metamorphism induces a remobilization of iron sulfides, sweeping away original ones while creating new ones. This paper analyzes size distributions of iron sulfides in several samples from the Caples and Torlesse terranes from the Otago Schist (New Zealand) using high-resolution X-ray computed tomography, which allows all iron sulfides larger than the resolution at which X-ray scans were performed to be characterized. Framboids and clusters of framboids are common in unmetamorphosed samples, but disappear in greenschist/amphibolite facies samples, where iron sulfides have anhedral habits. By contrast, the size and standard deviation of the new iron sulfides both remain within the same range. The results illuminate the evolution of iron sulfides throughout metamorphism, proposing boundaries for the metamorphic processes based on the shape of these iron sulfides

A Morphological and Size-Based Study of the Changes of Iron Sulfides in the Caples and Torlesse Terranes (Otago Schist, New Zealand) during Prograde Metamorphic Evolution

It is widely accepted that metamorphism induces a remobilization of iron sulfides, sweeping away original ones while creating new ones. This paper analyzes size distributions of iron sulfides in several samples from the Caples and Torlesse terranes from the Otago Schist (New Zealand) using high-resolution X-ray computed tomography, which allows all iron sulfides larger than the resolution at which X-ray scans were performed to be characterized. Framboids and clusters of framboids are common in unmetamorphosed samples, but disappear in greenschist/amphibolite facies samples, where iron sulfides have anhedral habits. By contrast, the size and standard deviation of the new iron sulfides both remain within the same range. The results illuminate the evolution of iron sulfides throughout metamorphism, proposing boundaries for the metamorphic processes based on the shape of these iron sulfides

Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand

C.D.M. was supported by NERC CASE PhD studentship award NE/G524160/1 (GNS Science, NZ, CASE partner). D.A.H.T. acknowledges support from research grants NE/H012842/1 and NE/J022128/1 and a Royal Society Wolfson Research Merit Award (WM130051). S.C.C. was funded under GNS Science's “Impacts of Global Plate Tectonics in and around New Zealand Programme” (PGST Contract CO5X0203). J.C.A. was supported by NSF OCE1334758. We also thank Matthew Cooper, Andy Milton, Darryl Green and Lora Wingate for laboratory assistance. We thank Mike Bickle for editorial advice and comments, and reviews from two anonymous reviewers that improved this manuscript.Peer reviewedPublisher PD

Sources of fluids and metals in orogenic deposits : the Otago Schists, New Zealand

The theme of this thesis is to investigate the source of ore forming elements enriched at the ore deposits in Otago, and the fluids responsible for their formation. A large suite of samples collected across the Otago and Alpine Schists has been analysed to investigate whether any specific rock types, metamorphic grades or structural settings has been depleted in ore farming elements relative to un-metamorphosed samples, and could consequently be the source of enrichments in the Otago ore deposits. A new method to analyse gold at ultra low concentrations (&lt; 10 ppt) in rocks has been developed, because the determination of Au in un-mineralised or leached rocks requires very sensitive analytical methods. As, Se, Sb and Hg have also been analysed by methods with ultra low detection limits. Many of the elements analysed show localised mobility, particularly in greenschist facies rocks, but most elements show no systematic mobility with lithological variation or crustal depth. However, As, Se, Mo, Ag, Cd, Sb, W, Au and Hg are all systematically depleted in metamorphosed rocks relative to un-metamorphosed rocks. Depletion often begins at different crustal levels; Hg and Cd are depleted from low-grade rocks, whereas depletion of As only occurs in high-grade rocks. This suite of elements is identical to that enriched in mineralised samples from Macraes gold mine. Mass balance calculations show that during metamorphism from protolith to amphibolite facies, 1 km3 of rock could yield over 23000 T of As, 50 T of Se, 1350 T of Mo, 100 T of Ag, 1100 T of Sb, 440 T of W, 3 T of Au, and 90 T of Hg. The enrichments observed at Macraes could have been sourced from a cube of amphibolite facies rock with side length 5 km. Elemental leaching at depth throughout the 30000 km2 exposure of the Otago schist provided sufficient ore forming elements to form more than 3000 Macraes-sized deposits.</p

Metabasalts as sources of metals in orogenic gold deposits

Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9?±?0.5 ppb) and upper greenschist facies (1.0?±?0.5 ppb), but decrease significantly in amphibolite facies samples (0.21?±?0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit

The Gold Conveyor Belt: Large-scale gold mobility in an active orogen

The Southern Alps of New Zealand are part of an active collisional orogen where metamorphism, hydrothermal fluid flow and the formation of orogenic gold deposits are ongoing. The Southern Alps are forming due to transpressional collision between continental crust fragments on the Pacific and Australian tectonic plates. The plate tectonic rates and geometries, the sources of fluid and broad-scale fluid pathways in the hydrogeological system, and the geochemical compositions of the Torlesse Terrane rock that is being advected through the orogen are well defined so that a mass balance of metal mobility during active orogenic processing in the Southern Alps of New Zealand can be calculated. Advection of a 10 km wide x 5 km deep section of Torlesse rock through the orogen at tectonic rates (0.01 m/yr) that is then metamorphosed up to amphibolite facies, causes mobilisation of over 1127 t Au, 10.1Mt As, 47000 t Hg, 560000 t Sb and 14000 Mt H2O in 1 Myrs. The masses of elements mobilised at the same rate along the length of the Southern Alps (&gt; 200 km) for 5 Myrs would be more than 100 times greater. The metals were mobilised by the metamorphic fluid produced during the orogenic processing of the Torlesse Terrane rocks and the concentrations of Au, As, Hg and Sb in this fluid are calculated to be 0.08, 711, 3, and 40 mg/kg respectively. The mobilised metals form the orogenic gold deposits that occur in the Southern Alps. Different styles of gold deposits form contemporaneously during the active orogenesis of the Southern Alps, including those with a fluid temperature &gt; rock temperature that may appear have formed after the peak of metamorphism but are instead just the product hydrothermal fluid mineralising rocks on their retrograde metamorphic path. The mass balance shows that there has been orders of magnitude more metal mobilised in the orogen than resides in the currently known deposits. There is clear potential for large gold deposits occurring in the yet to be uplifted parts of the Southern Alps if there have been efficient enough fluid focussing and metal precipitation mechanisms occurring under the Southern Alps

A lithological context for stratabound REE mineralisation at the birthplace of REE – Bastnäs, Riddarhyttan, Sweden

The Bastnäs ore field, in central Sweden, is the cradle of the rare earth elements (REE). It is the place of the discovery of several REE and important REE-minerals (e.g., Bastnäsite one of the primary REE-ore minerals). In recent years there has been an increased interest due to rising demand of REE for technological applications. Several recent studies have focused on the mineralogy and geochemistry but a lack of fresh in situ samples has meant that textural and stratigraphic relationships are not as well described. Recent exploration in the area has produced drill core traverses across the host stratigraphy of the Bastnäs deposit, allowing the collection of relatively fresh in situ samples which can be placed in lithological context. Here we present new mineralogical and textural information linked to the lithology indicating that the REE-mineralisation in Bastnäs is commonly associated with magnetite skarn and that it occurs over a wide range of stratigraphic levelsISBN for host publication: 978-2-8399-4046-7</p