Ultramafic Host Research Articles

The Role of Mafic Intrusions in Producing Alkaline Vent Fluids at the Lost City Hydrothermal Field: Evidence from Stable Potassium Isotopes

The alkaline, H2-rich vent fluids of the Lost City Hydrothermal Field (LCHF) have generally been attributed to serpentinization of tectonically uplifted mantle peridotite. However, elevated concentrations of highly incompatible and fluid mobile trace alkali metals, Rb and Cs, indicate reaction with relatively unaltered mafic components (Seyfried et al., 2015). Here, we present high-precision stable-K isotope analyses of LCHF vent fluids, which provide new constraints on source-rock lithology and support more quantitative estimates of fluid:rock ratio, providing a new constraint on heat and mass transfer processes. We find that δ41K values of LCHF vent fluids (0.03–0.07 ‰) are distinct from local seawater (0.13 ± 0.02 ‰). In combination with near-seawater concentrations of K (10.4 ± 0.1 mmol/kg), these data are incompatible with hydrothermal alteration of peridotite alone, which is highly depleted in K. Instead, K-isotope and concentration data are consistent with hydrothermal alteration of mafic rocks (e.g., gabbro or diabase) at a fluid:rock ratio of 6–26, a range that agrees well with Rb, Cs, and 87Sr/86Sr-based estimates of fluid:rock ratio, updated here to reflect derivation from mafic source rocks.Geochemical modeling of hydrothermal fluid-rock reactions indicates that LCHF vent fluids can be effectively reproduced by a two-stage reaction in which seawater initially reacts at 200–300 °C with mafic rocks at in a fluid:rock ratio of ∼ 10 —as constrained by the above isotopic and trace-element systematics— and subsequently reacts with peridotite at a fluid:rock ratio of ∼ 30 at roughly the same temperature, pressure conditions. We also note that LCHF vent fluids are ∼ 2 % depleted in Cl compared to seawater, which may reflect admixture of a vapor component derived from ongoing phase separation at higher (> 450 °C) temperatures. We thus propose that the LCHF represents waning-stage hydrothermal activity initiated by a discrete off-axis mafic intrusion that has subsequently cooled to intermediate temperatures conducive to olivine hydrolysis and production of alkaline vent fluids. In this interpretation, alkaline H2-rich vent fields occupy a narrow thermal window in the expected evolution of hydrothermal activity associated with episodic off-axis mafic intrusions into tectonically exposed ultramafic host rocks and are thus likely to be rare on the modern seafloor.

FRACTURED TECTONICS OF CHROMITITES OF THE MAIN SARANOVSKOYE DEPOSIT (PERM KRAI)

The Main Saranovskoe deposit is one of the largest chrome deposits in Russia. The chrome ore bodies are localized in the ultramafites of the Northern Saranovsky massif which has been studied since 1907. The research revealed that the massif is similar to the Bushveld stratiform complex. Unlike other layered plutons, the Northern Saranovsky massif has a large amount of chrome ore bodies as compared to ultramafic host rocks. As many studies have shown, the chromitites of the Main Saranovskoe deposit are severely tectonized. The strike and dip of the ore bodies is complicated by numerous faults. These observations imply that the massif is a fragment of a large stratified pluton intruded into the Proterozoic metasedimentary unit. The fracturing reflects the massif intrusion conditions and provides new petrogenetic information on the relevance of our study of the tectonic fractures of the Saranovsky massif. Studies have been made on the fracture and furrow displacement orientations along the planes in the main ore bodies: western, central and eastern, within the two parts of the deposit. The local stress tensor has been reconstructed using the Angelier method in the Win-Tensor program. It has been stated that the chromitites studied contain 3–4 main fracture systems whose orientation varies together with the orientation of the ore bodies. The main compressive stress axis is oriented subvertically both in the central and southern parts of the deposit, and the main tensile stress axis forms an acute angle with the ore body contacts in the center of the deposit and is oriented subperpendicular to them in the southern part. It has been concluded that fracturing of the chrome ore bodies in the Main Saranskoe deposit took place during the massif motion into the upper crustal horizons with the subvertical orientation of the ore bodies remained unchanged. This casts doubts on the probability of formation of the Northern Saransky massif as a result of gravity differentiation of magmatic melt, since in this case the ore bodies should have undergone a vertical turn which is not exhibited by their fracturing.

3D modeling of the Esker intrusive complex, Ring of Fire intrusive suite, McFaulds Lake greenstone belt, Superior Province: Implications for mineral exploration

A 3D geological model of the Archean Esker ultramafic–mafic intrusive complex of the Ring of Fire intrusive suite of northwestern Ontario (Canada) is presented, providing insight into the nature of its magmatic plumbing system and allowing for speculation on the formation of its valuable Ni-Cu-(PGE) and chromitite deposits. The 3D model was constrained by exploration drillholes and high-resolution aeromagnetic data, which allowed the interpretation of the broad internal structure of the intrusive complex and two shear zones, which define for a major part of their strike extent the contacts between ultramafic and overlying mafic intrusive rocks. Restoring the post-emplacement component of strike-slip displacement and subsequently rotating the intrusive complex back from its tilted to sub-horizontal orientation of emplacement, provides a three-dimensional rendition of a 14 km-long dominantly tabular-shaped intrusive complex. The complex is defined by intact basal intrusive contacts with localized keel-shaped ultramafic promontories parallel to a nearby blade-shaped magmatic conduit. The keels extend in a subparallel orientation to depths of up to 1 km into the underlying tonalitic rocks and coincide with significant offsets of the basal intrusive contact, leading to speculation that their intrusion may have been guided by pre-existing normal faults. Two of these magmatic conduits host Ni-Cu-(PGE) deposits/prospects that formed from gravitational segregation of sulfides. The chromitite deposits, hosted in ultramafic rocks at higher stratigraphic levels, were modeled based on drill hole assay data. Zones of dominantly massive to semi-massive chromitite were defined by the ≥ 35% Cr2O3 threshold, while intercalated sequences of chromitite in dunite/peridotite were defined by the ≥ 15% to ≤ 35% Cr2O3 percentage range. The 3D modeled surfaces fitted to these constraints exhibit ore zones with lateral extents from several hundreds of meters for massive bodies up to a maximum of 2 km for intercalated chromitite ore that are dominantly conformable to the layering of the ultramafic host rocks. The lack of lateral continuity of chromitite ore zones beyond this scale, suggests that the chromitite layers are not continuous along the entire strike extent of the Esker intrusive complex, which is consistent with bifurcations of the 3D modeled ore shells signifying pinch-outs of both the massive chromitite layers and the dunitic interlayers. This discontinuous distribution of chromitite ore resembles that in the stratiform deposits of the Stillwater Complex and may support recent models of mechanical segregation of chromitite from cotectic chromite-olivine slurries. Based on our 3D model, we propose that the Esker intrusive complex evolved as a series of individual intrusions that were emplaced during multiple magmatic pulses that eventually coalesced to form a composite ultramafic–mafic complex prior to being dissected and partly dismembered by post-ore shear zone deformation.

Temperature-Controlled Ore Evolution in Orogenic Gold Systems Related to Synchronous Granitic Magmatism: An Example from the Iron Quadrangle Province, Brazil

Abstract Against the background of an ongoing debate on the genetic relationship between orogenic gold and granitic magmatism, we studied the evolution of a gold-mineralizing system in shear zone-hosted veins that are spatially associated with a syn- to late tectonic 2.69 Ga granite at the Satinoco deposit in the Archean Pitangui greenstone belt (Iron Quadrangle, Brazil). Detailed underground mine mapping, petrography, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and elemental mapping of sulfide grains revealed a complex polyphase history, with at least three gold-bearing stages: (1) A syntectonic arsenopyrite (Apy1)-löllingite-pyrrhotite assemblage I that formed during prograde metamorphism from ca. 475° to 650°C at 3 to 4 kbars; (2) further fluid circulation during syn- to late tectonic retrograde metamorphism from ca. 510° to 445°C, which led to a pyrrhotite-arsenopyrite (Apy2) ± löllingite ± galena assemblage II; and (3) a pyrite-pyrrhotite-galena ± chalcopyrite ± sphalerite ± ullmannite assemblage III formed during a later post-tectonic brittle deformation event from ca. 350° to 300°C at 1 to 2 kbars. Ascending granitic magma acted as a heat engine for peak metamorphic temperatures locally, that is at the immediate contact, reaching as much as 730°C and, together with the dehydration of mafic and ultramafic host rocks, generated the sulfide assemblage I. Crosscutting field relationships, together with existing S isotope data, indicate a metamorphic origin for all the sulfide stages with a contribution from granite-derived magmatogenic fluids to the mineralizing system at stage II. New X-ray element maps together with LA-ICP-MS spot analyses document oscillatory zoning of As, Fe, and Ni in Apy1 and enrichment in Ni, Mn, Zn, and V. The large amount of pyrrhotite in assemblage II is explained by granite-related Fe metasomatism. This stage led to an enrichment in Co, Sb, and Pb and the redistribution of Au. Assemblage III involved the formation of pyrite veins and precipitation of free gold and Pb-Sb-Cu-Zn sulfides in microfractures of this pyrite. This stage was likely responsible for the remobilization of Ni, Cu, Zn, and Bi-Te minerals. Based on existing geochronological data, at least most of the gold formed between ~2.72 and 2.68 Ga. This study revealed that gold mineralization took place over a wide range of temperatures—a finding that might be not only specific to the Satinoco deposit but possibly to other orogenic-type gold deposits elsewhere as well.

Desulphurisation, chromite alteration, and bulk rock PGE redistribution in massive chromitite due to hydrothermal overprint of the Panton Intrusion, east Kimberley, Western Australia

The Central and Western Zones of the Halls Creek Orogen (HCO) of the East Kimberley region of Western Australia host various Paleoproterozoic layered mafic–ultramafic intrusions. The 1856 ± 2 Ma layered mafic–ultramafic Panton Intrusion of the Central Zone contains a reef-type platinum-group element (PGE) mineralisation associated with metamorphosed chromitite horizons. Here we investigate the compositional variation of chromite within the A chromitite layer, or Main Chromitite Layer (MCL), base metal sulphides (BMS), and the associated PGE distribution. Electron-microprobe analysis (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data reveal complex chemical zoning of the chromite reflecting post-magmatic sub-solidus re-equilibration and fluid–rock interaction during metamorphism of the host rocks. The sulphur isotopic composition of the BMS (δ34S: −1.67 to 2.89‰) suggests negligible contamination of the parental melt by crustal sulphur and thus favours BMS formation due to fractional crystallisation. Furthermore, pentlandite compositions indicate the highest ƒS2 in the centre of the MCL, which coincides with a considerably heavier sulphur isotope composition compared to pentlandite in the surrounding ultramafic host rock. Alteration of the BMS during amphibolite facies hydration of the Panton Intrusion resulted in the formation of secondary bornite and digenite from the chalcopyrite as well as desulphurisation of pyrrhotite and pentlandite to secondary magnetite. The bulk rock PGE concentrations are controlled by the distribution of platinum-group minerals (PGM) due to the low PGE concentrations in BMS. PGM are present as inclusions in chromite, suggesting that they formed under sulphur-undersaturated conditions. Additional PGM are present within the interstitial serpentine-chlorite matrix suggesting that they precipitated during fluid-rock interaction and hydration of the silicate matrix. Likely sources for the remobilised PGE include PGM inclusions in altered rim zones of magmatic chromite and PGE released due to desulphurisation of BMS despite their low PGE concentrations.

Mg isotope fractionation during continental weathering and low temperature carbonation of ultramafic rocks

Abstract The Mg-isotope systematics of peridotite weathering and low-temperature carbonation have not yet been thoroughly investigated, despite their potential to provide insights into reaction pathways and mechanisms of lithosphere-hydrosphere transfer of Mg and sequestration of CO2 in carbonate minerals. Here, we present new observations of the evolution of Mg isotope ratios during subtropical ultramafic rock weathering and associated magnesite formation, including the lowest δ26Mg of magnesite reported so far. At the investigated field sites in eastern Australia, the proximity of the ultramafic Mg source rocks and associated magnesite deposits provides boundary conditions that constrain Mg isotope fractionation during low-temperature alteration. Saprolite samples from Attunga, New South Wales, show that weathering of serpentinite is accompanied by Mg loss and formation of secondary Mg-bearing clay minerals. Furthermore, Mg isotope ratios increase systematically with weathering intensity, indicating that incorporation of 26Mg into clay mineral structures controls Mg isotope fractionation during ultramafic rock weathering. The Mg-bearing clay formed by decomposition of serpentine minerals has a δ26Mg value of ∼0.35‰, which is up to ∼0.6‰ heavier than the ultramafic precursor. In contrast, nodular magnesite hosted in ultramafic rock shows δ26Mg values between −3.26‰ and −2.55‰ that are significantly lower than those of magnesite and dolomite formed by hydrothermal alteration of peridotite at higher temperature (δ26Mg = −0.69‰ and −0.62‰). The strong enrichment of 24Mg in nodular magnesite does not reconcile with simple fractionation during direct precipitation from ultramafic host rock buffered meteoric fluids and instead suggests multiple formation steps involving dissolution and re-precipitation of pre-existing carbonate accompanied by fractionation between species of dissolved Mg. Our data highlight the potential of Mg isotope studies for distinguishing the formation pathways of low temperature magnesite and for tracing Mg in low temperature alteration processes based on the distinct signatures of secondary silicate and carbonate minerals.

Corundum Anorthosites-Kyshtymites from the South Urals, Russia: A Combined Mineralogical, Geochemical, and U-Pb Zircon Geochronological Study

Kyshtymites are the unique corundum-blue sapphire-bearing variety of anorthosites of debatable geological origin found in the Ilmenogorsky-Vishnevogorsky complex (IVC) in the South Urals, Russia. Their mineral association includes corundum-sapphire, plagioclase (An61–93), muscovite, clinochlore, and clinozoisite. Zircon, churchite-(Y), monazite-(Ce), and apatite group minerals are found as accessory phases. Besides, churchite-(Y) and zircon are also identified as syngenetic solid inclusions within the sapphires. In situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb zircon geochronology showed the ages at about 290–330 Ma linked to the Hercynian orogeny in IVC. These ages are close to those of the syenitic and carbonatitic magmas of the IVC, pointing to their syngenetic origin, which is in agreement with the trace element geochemistry of the zircons demonstrating clear magmatic signature. However, the trace element composition of sapphires shows mostly metamorphic signature with metasomatic overprints in contrast to the geochemistry of zircons. The reason for this discrepancy can be the fact that the discrimination diagrams for sapphires are not as universal as assumed. Hence, they cannot provide an unambiguous determination of sapphire origin. If it is true and zircons can be used as traces of anorthosite genesis, then it can be suggested that kyshtymites are formed in a magmatic process at 440–420 Ma ago, most probably as plagioclase cumulates in a magma chamber. This cumulate rock was affected by a second magmatic event at 290–330 Ma as recorded in zircon and sapphire zoning. On the other hand, Ti-in-zircon thermometer indicates that processes operated at relatively lower temperature (<900 °C), which is not enough to re-melt the anorthosites. Hence, zircons in kyshtymites can be magmatic but inherited from another rock, which was re-worked during metamorphism. The most probable candidate for the anorthosite protolith is carbonatites assuming that metamorphic fluids could likely leave Al- and Si-rich residue, but removed Ca and CO2. Further, Si is consumed by the silicification of ultramafic host rocks. However, kyshtymites do not show clear evidence of pronounced metasomatic zonation and evidence for large volume changes due to metamorphic alteration of carbonatites. Thus, the obtained data still do not allow for univocal reconstruction of the kyshtymite origin and further investigations are required.

Metasomatism and the crystallization of zircon megacrysts in Archaean peridotites from the Lewisian complex, NW Scotland

Zircon megacrysts are locally abundant in 1–40 cm-thick orthopyroxenite veins within peridotite host rocks in the Archaean Lewisian gneiss complex from NW Scotland. The veins formed by metasomatic interaction between the ultramafic host and Si-rich melts are derived from partial melting of the adjacent granulite-facies orthogneisses. The interaction produced abundant orthopyroxene and, within the thicker veins, phlogopite, pargasite and feldspathic bearing assemblages. Two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins. Patterns of zoning, rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts, whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host. Both zircon types have U–Pb ages of ca. 2464 Ma, broadly contemporaneous with granulite-facies events in the adjacent gneisses. Zircon megacrysts locally form > 10% of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts. Their unusual size is linked to the suppression of zircon nucleation and increased Zr solubility in the Si-undersaturated melts. The metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs. As such, the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas.

Local Seismicity of the Rainbow Massif on the Mid‐Atlantic Ridge

AbstractThe Rainbow massif, an oceanic core complex located in a nontransform discontinuity on the Mid‐Atlantic Ridge (36°N), is notable for hosting high‐temperature hydrothermal discharge through ultramafic rocks. Here we report results from a 9 month microearthquake survey conducted with a network of 13 ocean bottom seismometers deployed on and around the Rainbow massif as part of the MARINER experiment in 2013–2014. High rates (~300 per day) of low‐magnitude (average ML ~ 0.5) microearthquakes were detected beneath the massif. The hypocenters do not cluster along deeply penetrating fault surfaces and do not exhibit mainshock/aftershock sequences, supporting the hypothesis that the faulting associated with the exhumation of the massif is currently inactive. Instead, the hypocenters demarcate a diffuse zone of continuous, low‐magnitude deformation at relatively shallow (< ~3 km) depths beneath the massif, sandwiched in between the seafloor and seismic reflectors interpreted to be magmatic sills driving hydrothermal convection. Most of the seismicity is located in regions where seismic refraction data indicate serpentinized ultramafic host rock, and although the seismic network we deployed was not capable of constraining the focal mechanism of most events, our analysis suggests that serpentinization may play an important role in microearthquake generation at the Rainbow massif.

SIMS U-Pb zircon geochronology at the Kuså Ni-Cu deposit, south-central Sweden

The Kuså orthomagmatic Ni-Cu sulphide deposit is situated c. 13 km west of Falun in Bergslagen, south-central Sweden. Ion probe data on zircon from the mafic to ultra-mafic host rocks yield a 207Pb/206Pb weighted average age of 1798 ± 4 Ma, suggesting a genetic connection to intrusive activity forming the Transscandinavian Igneous Belt (TIB-1 phase). TIB-1 ages have recently been reported also for the Kleva deposit of similar type in Southern Sweden. The presence of these two occurrences suggests prospectivity potential for Ni-Cu mineralisation in mafic to ultramafic members of the areally extensive TIB.

Approaches for Extractive Hydrometallurgy of Niobium and Tantalum from Ethiopian Kenticha Ores

A Review Approaches for Extractive hydrometallurgy of niobium and tantalum from Ethiopian Kenticha Ores The Kenticha rare-element pegmatite, a globally important tantalite source in the Neoproterozoic of southern Ethiopia, is a highly fractionated, huge, subhorizontal, sheet-like body, discordantly emplaced in ultramafic host rock. It corresponds to the spodumene subtype of the rare element pegmatite class and belongs to the lithium–cesium–tantalum petrogenetic family. The Kenticha pegmatite is asymmetrically zoned from bottom to top into granitic lower zone, spodumene-free intermediate zone, and spodumene-bearing upper zone. The Kenticha pegmatite is rich in Ta-mineralized rare-element pegmatites. Thus, need to conduct research in enrichment of Ta2O5 and Nb2O5 and extraction of Nb and Ta into metallic form. This paper reviews the successful extraction of Nb and Ta from tanatlite ores. Its success normally depends on the physiochemical properties of the ore, choice of leaching extraction solvent. Currently, HF and methyl-isobutyl Ketone has been used for decomposition and separation of Nb and Ta from tantalite ore. Therefore Kenticha atantalite ore containe vaiaty compositions of elements and higher concentration of Li and U. Thus needs further investigation in extraction process in the sence of removing the use of HF for leaching and replacement of traditional organic solvents by “Green Solvents”. This review gives directions for future development of extraction of Kenticha tantalite ore. Keywords: Kenticha; leaching; magnetic separation; pegmatite, rare-element; solvent extraction, sopdumene; tantalite.

Geochemistry and origin of the ophiolite hosted magnesite deposit at Derakht-Senjed, NE Iran

The Derakht-Senjed magnesite deposit, hosted by Torbat-e-Heydarieh ophiolite in NE Iran, is developed as veins, veinlets and stockwork type mineralization. While the veins and veinlets only contain magnesite, the stockwork mineralization in addition contains sparry dolomite interlayered with magnesite. Magnesite and dolomite are both poor in FeO and SiO2. The carbon and oxygen isotope compositions of magnesite (δ13CV-PDB = −3.9 ± 0.1 to −5.0 ± 0.1‰; δ18OV-SMOW = +25.2 ± 0.1 to +26.5 ± 0.1 ‰) can be explained by contribution of atmospheric CO2 and/or an involvement by organic carbon. Dolomite typically shows slightly lower values of δ13C V-PDB -5.2 ± 0.1 to −5.5 ± 0.1‰ and δ18OV-SMOW + 23.8 ± 0.1 to +24.8 ± 0.1‰ compared to the magnesite. The formation of magnesite at Derakht-Senjed was structurally controlled by a fracture network in the ultramafic host rocks, which provided suitable fluid pathways for leaching of Mg from the host rocks and subsequent precipitation of magnesite from carbonated solutions. It is likely that dolomite formed due to precipitation from a fluid having lower XCO2 and higher Ca2+/Mg2+ activity ratio, rather than by replacement of magnesite.

Enviromental contamination of heavy metals and chrysotile asbestos in the Munzur and Pulumur streams (Tunceli, Turkey)

The Munzur and Pulumur streams are essential water supplies of the Tunceli Province (Turkey). Chromite bearing ultramafic rocks and Zn-Cu mineral- ization products are possible causes of the toxic elements enrichment in the streams. Sediment and soil samples were collected along these streams to deter- mine the level of toxic element enrichment. Multi-element normalized patterns of the samples with respect to the upper crust display enrichments in Cr, Ni, Co, Cs, W, Pb, As, Sb, Au, Hg and Cr, Ni, Co, W, for the Munzur and Pulumur streams, respectively. Also, the calculated Igeo index values show the pres- ence of contaminations of Cr, Ni, As, Hg, Cd and Cr, Ni, As in the Munzur and Pulumur streams, respectively. Elemental grouping in the samples indicates two different effects related to the ultramafic host rocks and mineralization products in the study area. Additionally, mineralogical XRD analyses exhibit the presence of chrysotile, plagioclase, quartz and calcite. The fibrous chrysotile and heavy metal contaminations must be considered as a threat for public health.

Application of chromium stable isotopes to the evaluation of Cr(VI) contamination in groundwater and rock leachates from central Euboea and the Assopos basin (Greece)

Major and trace elements (a) in groundwater, ultramafic rocks from natural outcrops and soil samples from cultivated sites of Central Euboea and Assopos basin, and (b) in experimentally produced laboratory water leachates of rocks and soils were investigated by SEM/EDS, XRD and ICP/MS. In addition, stable chromium isotopes (expressed as δ53Cr values) were measured in groundwater and leachates in order to identify potential sources for Cr-contamination.The higher Cr(VI) concentrations in soil leachates compared to those in the rock pulp leachates potentially can be explained by the presence of larger amounts of Fe (Fe(II)) and Mn (Mn-oxides acting as oxidizing catalysts). Assuming that redox processes produce significant Cr isotope fractionation (groundwater δ53Cr values range between 0.8 and 1.98‰), the compilation of the obtained analytical data suggests that the dominant cause of Cr isotope fractionation is post-mobilization reduction of Cr(VI). However, the lack of a very good negative relationship between Cr(VI) concentrations and δ53Cr values may reflect that sorption, precipitation and biological processes (fractionation during uptake by plants) complicate the interpretation of the Cr isotope signatures.The variation in δ53Cr values (0.84 to 1.98‰ in groundwater from Euboea, and from 0.98 to 1.03‰ in samples from the Assopos basin) imply initial oxidative mobilization of Cr(VI) from the ultramafic host rocks, followed by reductive processes that lead to immobilization of portions of Cr(III). Using a Rayleigh distillation model and different fractionation factors of Cr(VI) reduction valid for aqueous Fe(II) and Fe(II)-bearing minerals, we calculate that more than ~53%, but maximum ~94%, of the originally mobile Cr(VI) pool was reduced to immobile Cr(III) in the waters investigated. This indicates that efficient processes in the aquifers may facilitate natural attenuation of the toxic Cr(VI) to less harmful Cr(III).

Lithologically controlled invisible gold, Yukon, Canada

The newly discovered Cretaceous Coffee orogenic gold deposit (>4 Moz resource) consists of an extensive oxidised zone developed on primary sulphidic rock. The primary mineralised rock is characterised by invisible gold in arsenian pyrite that has replaced biotite in selected host rocks. The deposit has a cryptic surface expression and is an example of an extremely subtle exploration target. Hydrothermal emplacement was controlled by extensional fractures, with breccias, but most mineralisation was focused on biotite-bearing granitic gneiss, metasedimentary gneisses, and younger biotite granite. Fine-grained (<0.1 mm) arsenian pyrite replaced biotite along mineral cleavage planes and followed biotite-rich metamorphic and post-metamorphic structural fabrics. Arsenian pyrite also formed overgrowths on earlier coarse-grained (up to 2 mm) barren hydrothermal pyrite. Arsenian pyrite is concentrically zoned on the 1–10-μm scale with respect to As, Sb, and Au contents and typically contains ∼5 wt% As, ∼500 mg/kg Sb, and ∼500 mg/kg Au, in solid solution. Biotite replacement was accompanied by sericitisation, silicification, and ankerite impregnation. Hydrothermal alteration involved dilution and localised depletion of K, Na, and Al in silicified host rocks, but most Ca, Mg, and Fe concentrations remained broadly constant. Magnesium-rich ultramafic host rocks were only weakly mineralised with auriferous arsenian pyrite and have fuchsite and magnesite alteration. Near-surface oxidation has liberated nanoparticulate and microparticulate supergene gold, which remains essentially invisible. Varying degrees of oxidation extend as deep as 250 m below the present subdued topographic surface, well beyond the present vadose zone, and this deep oxidation may have occurred during post-mineralisation uplift and erosion in the Cretaceous. Oxidation has leached some As from the surficial mineralised rocks, decreasing the geochemical signal, which is also obscured by the localised presence of high background As (up to 100 mg/kg) in metasedimentary quartzites in the region. Antimony provides more reliable soil anomalies than As, but most Sb anomalies are <100 mg/kg. The persistence of invisible gold into the extensive supergene zone, with little gold particle size enhancement, has ensured that no placer deposits have formed in nearby streams, further restricting the surface footprint and Au dispersal halo of this subtle exploration target.

Magnetite composition in Ni-Cu-PGE deposits worldwide: application to mineral exploration

The concentration of trace elements in magnetite is determined primarily by the geological environment at the time of its formation, which makes magnetite a useful indicator mineral in the exploration of ore deposits. Magnetite is a common accessory mineral in magmatic Ni–Cu–Platinum-Group Element (PGE) sulfide deposits forming primary grains in massive sulfide ore and/or secondary grains during alteration of sulfide ore and host rock. We report magnetite composition from massive and disseminated sulfide samples (n=94), representative of 13 major Ni–Cu–PGE deposits, worldwide, and from a range of geological environments and formation ages (Archean to Permo–Triassic). The samples are divided into 6 different types according to the composition of the parental host magma: (1) komatiite, (2) ferropicrite, (3) picrite–tholeiite, (4) anorthosite–troctolite, (5) flood basalt and (6) impact melt. The minor and trace element composition of magnetite was measured by electron probe micro-analysis and a subset of samples (n=61) by laser ablation-inductively coupled plasma-mass spectrometry.The composition of all primary magnetite, crystallized from an immiscible sulfide liquid, plots on a trend in Ti vs. Cr and V vs. Cr. However, primary magnetite trace element content is controlled by element partitioning during sulfide liquid differentiation, from early-forming Fe-rich monosulfide solid solution (MSS) to Cu-rich intermediate solid solution (ISS). The concentration of most lithophile elements (Cr, Ti, V, Al, Sc, Nb, Ga, Ta, Hf and Zr) is highest in the early-forming magnetite, which crystallized with Fe-rich MSS. The fractionated Cu-rich liquid was depleted in lithophile elements so that late-forming magnetite, which crystallized with the Cu-rich ISS, has a low concentration of these elements. Compatible chalcophile elements partitioning into magnetite depends largely on the co-crystallizing sulfide mineral. Elements such as Ni partition preferentially in Fe-rich sulfide minerals (pyrrhotite or pentlandite) such that coprecipitated magnetite is depleted in chalcophile elements compatible with the sulfides. In contrast, magnetite co-crystallized with Cu-rich ISS is enriched in Ni because Ni is not compatible with the co-crystallizing Cu sulfides. Magnetite composition is also dependent on parental magma composition. Magnetite in massive sulfides from ultramafic parental magmas is most similar in compositions to primitive magnetite from Fe-rich sulfides in fractionated intermediate to mafic hosted orebodies.Two types of secondary magnetite are distinguished from the komatiite-hosted Thompson Ni-deposit (Manitoba, Canada): (1) magnetite formed by replacement of pyrrhotite and (2) magnetite formed during the serpentinization of the ultramafic host rocks. Secondary magnetite is depleted in most trace elements (Ni, Mn, V, Ti, V, Al, Cr), with the exception of Si and Mg, which are enriched.To be useful as a tool for mineral exploration, primary and secondary magnetite should not have the same minor and trace element composition. Primary magnetite from all 13 major Ni–Cu deposits plot in the field for Ni–Cu sulfide deposits in the Ni+Cr vs. Si+Mg discrimination diagram. Secondary magnetite plot outside of the field for Ni–Cu deposits. Therefore using this diagram, we can discriminate between primary and secondary magnetite using the low Ni+Cr content of secondary magnetite. Magnetite is resistant to surficial weathering and destruction during mechanical transport, such that it is a useful indicator mineral in exploration that can be used to detect eroded Ni–Cu–PGE deposits in surficial sediments.

Mineralogy and geochemistry of hydrothermal precipitates from Kairei hydrothermal field, Central Indian Ridge

The Kairei hydrothermal field was the first confirmed active submarine hydrothermal system on the Central Indian Ridge. It has been suggested to be related to mafic as well as ultramafic host rocks based on vent fluid composition and the presence of ultramafic rocks in its vicinity. In this study, detailed geochemical and mineralogical analyses have been carried out on the hydrothermal precipitates from the Kairei vent field in order to investigate the possible presence of indications for an ultramafic substrate at this vent site. The studied samples included fragments of sulfide chimneys, massive sulfides and talc-bearing and silicified breccias. Three mineralization stages were identified: (1) a high-temperature stage consisting largely of chalcopyrite, isocubanite, and pyrite; (2) a medium to low temperature stage characterized by the mineral assemblages of sphalerite and pyrite; and (3) a weathering stage characterized by secondary Cu-sulfides (bornite, digenite, covellite and idaite), Fe-oxihydroxides, Opal-A, and Cu-chloride (paratacamite and atacamite). The sulfide geochemistry is characterized by high concentrations of Cu and Zn (Cu + Zn up to 29.3 wt.%, n = 17) and Au (mean 5.28 ppm, n = 17), which is comparable to results from seafloor massive sulfides collected from ultramafic-hosted sites in the Atlantic Ocean, but differs from those of typical mafic-hosted deposits. The high concentrations of Cu and Au at the Kairei hydrothermal field could be an indication for the involvement of ultramafic rocks in the subseafloor. Ultramafic-hosted, Au-rich sulfide deposits may not be restricted to the Atlantic Ocean and may be common along all slow- and intermediate-spreading ridges.

Alteration minerals in impact-generated hydrothermal systems – Exploring host rock variability

Impact-generated hydrothermal systems have been previously linked to the alteration of Mars’ crust and the production of secondary mineral assemblages seen from orbit. The sensitivity of the resultant assemblages has not yet been evaluated as a function of precursor primary rock compositions. In this work, we use thermochemical modeling to explore the variety of minerals that could be produced by altering several known lithologies based on martian meteorite compositions. For a basaltic host rock lithology (Dhofar 378, Humphrey) the main alteration phases are feldspar, zeolite, pyroxene, chlorite, clay (nontronite, kaolinite), and hematite; for a lherzolithic host rock lithology (LEW 88516) the main alteration phases are amphibole, serpentine, chlorite, clay (nontronite, kaolinite), and hematite; and for an ultramafic host rock lithology (Chassigny) the main minerals are secondary olivine, serpentine, magnetite, quartz, and hematite. These assemblages and proportions of phases in each of those cases depend on W/R and temperature. Integrating geologic, hydrologic and alteration mineral evidence, we have developed a model to illustrate the distribution of alteration assemblages that occur in different levels of an impact structure. At the surface, hot, hydrous alteration affects the ejecta and melt sheet producing clay and chlorite. Deeper in the subsurface and depending on the permeability of the rock, a variety of minerals – smectite, chlorite, serpentine, amphiboles and hematite – are produced in a circulating hydrothermal system. These modeled mineral distributions should assist with interpretation of orbital observations and help guide surface exploration by rovers and sample return assets.

The West Jordan deposit, a newly-discovered type 2 dunite-hosted nickel sulphide system in the northern Agnew–Wiluna belt, Western Australia

The West Jordan nickel deposit, in the northern Agnew–Wiluna greenstone belt of Western Australia, is a newly-discovered Type 2 dunite-hosted, low-grade, large tonnage, disseminated sulphide system. Located in the core of a large dunite body, mineralisation is dominated by intercumulus sulphide blebs (20μm to 6mm across) in assemblages containing pentlandite, pyrrhotite, heazlewoodite and locally, native nickel, sphalerite and chalcocite. Mineralisation grades between 0.2 and 2wt.% Ni, with the majority of samples in the 0.35–0.7% Ni range, were consistent with most komatiitic Type 2 systems. Hypogene alteration of the ultramafic host rock is interpreted to have been effected by retrograde metamorphic fluids, and has resulted in extensive serpentinisation and localised, structurally-controlled, talc-magnesite alteration. This gangue alteration has resulted in modification of original magmatic sulphide assemblages, and localised remobilisation of the minor Cu and Zn components of the magmatic sulphides. The deposit is deeply weathered, and all samples utilised in this study were obtained from a series of 12 diamond drill holes which were comprehensively assayed. An igneous stratigraphy is presented which is interpreted to be west-younging, consistent with along-strike deposits to the south, such as the Mount Keith and Yakabindie Type 2 nickel deposits.

Granitic Pegmatites as Sources of Colored Gemstones

Pegmatites are sources of gem-quality crystals of beryl, tourmaline, topaz, spodumene, and spessartine. Historic localities are found in Brazil, Madagascar, Russia, and the United States, but important deposits have recently been discovered in Africa and Asia. Most high-quality gem minerals occur in miarolitic cavities found near the centers of pegmatite bodies or in reaction zones between pegmatites and ultramafic host rocks. The most important gem-bearing granitic pegmatites formed at shallow levels in the continental crust during the latest stages of collisional plate tectonic events. Single, spectacular miarolitic cavities in some pegmatites have produced tons of gem crystals valued in excess of $50 million.