Ni Depletion Research Articles

Anomalous Sulfur-Poor Platinum Group Element Mineralization in Komatiitic Cumulates, Mount Clifford, Western Australia

An unusual occurrence of sulfur-poor platinum group element (PGE) mineralization has been discovered at Mount Clifford in the Agnew-Wiluna belt, Western Australia, within the lower part of a 1,000-m-thick komatiitic dunite body. The host body consists primarily of strongly serpentinized olivine adcumulates with accessory chromite. High-precision whole-rock ICP-MS data show that the PGE mineralization extends over tens of meters, whereas the lateral extent is unknown. Palladium and Pt abundances are closely associated with base metal sulfides but show offset distribution patterns with maximum concentrations associated with the relatively sulfide poor base of a disseminated sulfide-bearing layer. Whole-rock concentrations range up to 4,100 ppb Pd and 920 ppb Pt. Iridium, Rh, and Ru follow the trends in Pd and Pt but are strongly depleted (Ir ~0.4 ppb, Ru ~0.5 ppb, Rh ~0.1 ppb). Platinum group minerals (PGM) were located in essentially sulfide free serpentine 2 m below the peak in sulfide abundance and can be categorized into three groups: Pd antimonides, Pt-dominated PGM, and Pt-bearing Ni antimonides. The PGE mineralization zone is underlain by an interval of strong Ni depletion within olivine mesocumulates that is characterized by an unusual negative correlation between Ni and the Mg number of the rock. This relationship is interpreted to be the result of an equilibration of olivine of variable forsterite content with a relatively large mass of magmatic sulfide. However, there is no direct evidence of this sulfide mass anywhere in the entire Mount Clifford dunite body. The Ni depletion is accompanied by strong depletion in IPGE (Ir, Ru, and Rh). The association of the PGE zone with a reversal in the Mg/Fe ratio of the host cumulates and a distinctive offset relationship of multiple Pt, Pd, and Cu-S peaks is suggestive of a primary magmatic origin. However, this is inconsistent with current models for offset-reef PGE concentrations which involve the onset of sulfide liquid saturation from a previously sulfur undersaturated, PGE-undepleted magma. The association of enriched Pt and Pd with highly depleted Ni and IPGE in immediately underlying cumulates conflicts with this model. We suggest a combination of processes: early sulfide segregation at low R values producing Ni and IPGE depleted olivine, partial redissolution and removal of most of this sulfide component during a major replenishment event, and subsequent overprint by a migrating Pt-Pd-Cu–rich fluid.

Reevaluation of the Role of External Sulfur Addition in the Genesis of Ni-Cu-PGE Deposits: Evidence from the Nebo-Babel Ni-Cu-PGE Deposit, West Musgrave, Western Australia

The Nebo-Babel Ni-Cu-PGE sulfide deposit, located in the West Musgrave Block, Western Australia, is hosted within a tube-like, concentrically zoned, gabbronorite magma conduit emplaced at 1068 Ma. The Nebo-Babel deposit is associated with the ca. 1078 Ma Giles Complex which is part of the Warakurna large igneous province. Previous geologic and geochemical studies have shown that the Nebo-Babel consists of chemically related but temporally distinct magma pulses. The initial magma pulses, which were the most primitive and the most contaminated, formed the marginal units of the conduit. The later pulses were emplaced into the core of the conduit. The chilled margin rocks contain sulfide globules. Mineralization is confined to the early magma pulses and consists of massive sulfide breccias and stringers and disseminated gabbronorite-hosted sulfides. The Ni tenor of the massive sulfides, which have undergone sulfide liquid fractionation, and the disseminated sulfides are almost the same, indicating that both formed at similar R -factors. The whole-rock massive and disseminated sulfides contain ~650 ppb and 60 to 500 ppb total PGE, respectively. All of the Nebo-Babel rock units are slightly PGE depleted with Cu/Pd ratios higher than primitive mantle, indicating early saturation and segregation of a sulfide and/or PGM. Rocks with trace cumulus sulfides have the lowest Cu/Zr and highest Cu/Pd ratios and the most fractionated metal patterns, indicating that these units have undergone the highest degrees of Ni, Cu, and PGE depletion. The average δ 34S value of all Nebo-Babel disseminated and massive sulfides is 0.4 ± 0.2 per mil, which is consistent with the S originating in the mantle. Sulfur saturation did not occur in situ and is inferred to have been triggered by assimilation of felsic orthogneiss, perhaps in the parental magma chamber. This initial magma pulse with entrained sulfides formed marginal units of the Nebo-Babel intrusion. Some of the entrained sulfides accumulated within physical traps on the way into the conduit and/or at the exit from the chamber. These sulfides were later emplaced along the footwall contact as massive and/or marginal fragment-laden sulfide liquid. Distribution of the disseminated sulfides was influenced by the conduit geometry and magma flow dynamics. The final and volumetrically dominant barren magma pulse underwent crystal fractionation and oxide precipitation in the core of the Nebo-Babel intrusion, which in turn caused sulfide saturation and formed trace but PGE-depleted sulfides in these units. The Nebo-Babel deposit has a number of features in common with other Ni-Cu-PGE deposits hosted in dynamic magma conduits (e.g., Voisey’s Bay), such as multiple magma pulses and sulfide entrainment from depth, rather than in situ sulfide segregation. However, Si addition was the primary cause of sulfide saturation and external S addition did not play a role in the genesis of the Nebo-Babel deposit.

Ultrasonic Characterization of Aging Behavior in M250 Grade Maraging Steel

The increase in sound velocities during the aging of M250 maraging steel reported by Rajkumar et al. was analyzed. The present article provides a new perspective on why and to what extent the sound velocities change during aging. The main parameter that affects the elastic moduli and the sound velocities in the early and intermediate stages of aging in maraging steel is the depletion of Ni from the martensitic matrix due to Ni3Ti and Ni3Mo formation. The issue of sound velocity increase with aging time was addressed here in a few different ways to support the validity of Ni depletion.

Comparison of cytology and distribution of nickel in roots of Ni-hyperaccumulating and non-hyperaccumulating genotypes of Senecio coronatus

Two genotypes of Senecio coronatus (Thunb.) Harv. (Asteraceae) growing on ultramafic outcrops were identified previously: a Ni hyperaccumulator and a non-hyperaccumulator. The aim of the present study was to investigate the cytology of the roots of both genotypes, their Ni content and tissue distribution, and to ascertain whether there was a cytological basis for the differential uptake of Ni. Light and fluorescence microscopy together with histochemical methods were used to study root cytology. X-ray microanalysis by means of a nuclear microprobe—particle-induced X-ray emission (PIXE) and proton backscattering (BS) techniques—was utilized to determine the concentration and distribution of Ni and other elements. Average concentration of Ni and distribution in roots differed significantly between hyperaccumulating and non-hyperaccumulating genotypes. Ni amount in the hyperaccumulating genotype was ca. 60 times higher in the older part of the root (1,760 μg g−1) and ca. 10 times higher (314 μg g−1) in the younger root hair region in comparison with the equivalent parts of the non-accumulating genotype where Ni amounts were 30 μg g−1. Ni distribution pattern was also different in both cases. Cytological differences were observed in the inner cortical region and exodermis of the roots. Distinct groups of specialized cells with an organelle-rich cytoplasm that produced copious numbers of spherical bodies occurred in the inner cortical region of the hyperaccumulator. Such distinct cell groups were absent from the inner cortex of the non-hyperaccumulator. Cortical cells here had a thin parietal cytoplasmic layer and produced fewer spherical bodies. In both genotypes the spherical structures were extruded from the cytoplasm into air spaces between the cells where they coalesced to form amorphous deposits, significantly larger and more abundant in the hyperaccumulator. Histochemical tests identified these deposits as a mixture of lipids, alkaloids and terpenoids. Specialized cells present in the inner cortex of the hyperaccumulating genotype demonstrated significant relative Ni depletion in comparison with the adjacent inner cortex and phloem. Casparian bands were identified in exodermal cell walls of both genotypes but the bands fluoresced more intensely in the non-accumulator suggesting differences in chemical composition and probably also a more efficient apoplastic barrier. This feature was correlated with the observed Ni distribution pattern. The highest Ni enrichment in the hyperaccumulating genotype occurred in the outer cortex; 20 times more than in the adjacent epidermis/exodermis in older portions of roots and 3 times more than in the epidermis/exodermis in younger root hair regions. In contrast, in the non-hyperaccumulating genotype, a higher concentration of nickel was found in the epidermis/exodermis compared to the outer cortex. The Ni ratio between the outer cortex and epidermis/exodermis was about 0.4 in the non-hyperaccumulator. Different cytological features exhibited by the genotypes may represent adaptive responses to the presence of high concentrations of Ni in the soil and subsequent differential uptake of Ni. Basic characteristics and elemental content of soil collected from the close vicinity of roots of two S. coronatus genotypes are reported.

Effect of trivalent rare earth dopants in nanocrystalline ceria coatings for high-temperature oxidation resistance

Nanocrystalline ceria (NC) and La 3+-doped nanocrystalline ceria (LDN) particles synthesized by the micro-emulsion method were coated onto AISI 304 stainless steel (SS) in order to study their high-temperature oxidation resistance properties at 1243 K in dry air for 24 h. Results were compared with those for micro-ceria (MC) coatings. The coated samples showed 90% improvement in oxidation resistance compared to uncoated and MC coated steels as seen from SEM cross-sectional studies. X-ray diffraction (XRD) analysis showed the presence of chromia in both NC and 20 LDN (NC doped with 20 at.% La 3+) samples which is absent in uncoated steels. From secondary ion mass spectrometry (SIMS) depth profiles, Fe, Ni depletion zones were observed in LDN-coated samples, indicating diffusion through the oxide layer. The role of oxygen vacancies in the nanoceria coatings on the formation of protective chromia layers is discussed.

Late Archean Lake Harris Komatiite, Central Gawler Craton, South Australia:Geologic Setting and Geochemistry

The Lake Harris Komatiite in the central Gawler craton of South Australia is the first documented komatiite outside the West Australian craton and the easternmost occurrence of such primitive ultramafic rocks in Australia. A U-Pb zircon age of ca. 2520 Ga for the komatiitic sequence indicates a previously unknown period of mantle-plume activity in the Late Archean. An integrated program of airborne magnetic surveys, gravity surveys, and core drilling was successful in defining the distribution and volcanic architecture of the komatiitic flows and associated greenstones through an extensive thin cover of Cenozoic alluvial sediments. Surface exposure of the komatiitic rocks is restricted to one small outcrop near Lake Harris. The greenstones form a series of subparallel east-northeast–trending sinuous magnetic highs flanked by large ovoid to elongate magnetic highs and lows that correlate with Archean-Proterozoic granitic bodies associated with province-wide shear systems, similar to the Archean greenstone terranes in the Yilgarn craton of Western Australia. The steeply dipping greenstone sequence was metamorphosed to middle amphibolite facies during the ca. 2440 Ma Sleafordian orogeny and sheared during the ca. 1700 Ma Kimban and ca. 1540 Ma Kararan orogenies. The greenstones consist of komatiite cumulates (43–32% MgO, anhydrous), high to low Mg komatiite (32–18% MgO), komatiitic and tholeiitic basalt (<18% MgO), pyroxenite cumulates, felsic volcanic rocks, minor metasedimentary rocks, pyroclastic rocks, and rare banded iron formation. They extend over 300 km in three subparallel belts that appear to be isoclinally folded around east-northeast axes and tectonically dismembered to the south by the Yerda shear zone. Komatiitic rocks have been confirmed by drilling in all three belts, but the absence of outcrop and structural complexities prevent detailed stratigraphic correlations within and between the belts. The komatiitic rocks display a range of quenched and cumulus textures defined by the different habits of olivine and its alteration products. Trace sulfides (pyrrhotite, chalcopyrite, pentlandite, pyrite, marcasite, polydymite violarite, heazelwoodite, millerite) form very small (0.01–0.2 mm) single-phase disseminated grains and coarser disaggregated grains. Their distribution largely reflects metamorphic and serpentinization processes, with high Ni/S ratios and probable sulfur loss from the more magnesian parts of the flows. Rare composite pyrrhotite-pentlandite-chalcopyrite blebs (0.1–0.5 mm) characterize some low Mg flows. Locally, there is supergene pyrite-marcasite and native copper-bornite-chalcocite(?) assemblage infilling of late low-temperature serpentine-chlorite veinlets. Thick ponded lava lake and distal composite sheet flow facies have been identified from different parts of the komatiitic sequences. Systematic whole-rock and mineral chemical trends indicate that despite the effects of recrystallization and reequilibration during amphibolite-facies metamorphism, the original magmatic geochemical profiles are largely preserved. The whole-rock data for the Lake Harris Komatiite does not show any obvious Ni depletion during fractionation but indicate a strong olivine control in dominantly sulfur undersaturated environments. Low sulfur (100–600 ppm S) and high Pd + Pt (5–30 ppb) contents, and Ti/Pd ratios of 2 to 4 × 10 5 for the komatiitic rocks are similar to sulfur-undersaturated Archean komatiites hosting Ni-Cu-PGE deposits, i.e., there is little evidence for sulfur saturation in the sampled komatiites. Identification of a pre-2.5 Ga source of sulfur in the substrate would be a positive indicator of potential sulfur saturation of the lavas elsewhere in the greenstone belt and a possible target for mineralization. The Lake Harris Komatiite has chemical (parent magma composition of 29% MgO, Al 2 O 3 /TiO 2 = 16, depleted light (L)REE and initial Nd isotope ( ϵ Nd = 2.8–3.0 at 2520 Ma) characteristics similar to typical Al-depleted Archean komatiites, and there is no clear evidence of chemical modification by processes associated with contemporary subduction. Coherent patterns of trace elements (Th, Nb, REE, Ti, Y, Zr, and P) and typical initial ϵ Nd signatures indicate a Late Archean komatiitic system involving a depleted mantle source and no obvious crustal contamination. This system probably exploited a lithosphere that was stretched and thinned by extension and/or thermal erosion in an intraplate environment. Dynamic tectonic environments involving mantle-plume activity, long-lived active plate margins, and widespread arc volcanism during the Late Archean are now favored for the evolution of the western Gawler craton.

Influence of Cu addition to interface microstructure between Sn-Ag solder and Au/Ni-6P plating

The formation and growth of intermetallics at the interface between Sn-Ag-(Cu) alloy balls and Au/Ni-6P plating were experimentally examined as a function of soldering period. Joint strengths were also evaluated by a ball pull test. For the joint with Sn-3.5Ag, the primary reaction product of Ni3Sn4 exhibits growth and shrinkage in thickness repeatedly with a passage of reaction time up to 30 min, while the Ni3SnP reaction layer monotonously increases its thickness without fluctuation. In the cases of the joints with Cu bearing solder, Sn-3Ag-0.5Cu and Sn-3.5Ag-0.8Cu, a single η-(Cu,Ni)6Sn5 interface layer grows by fast Cu segregation from liquid solder to the interface layer on soldering. For all the soldered joints, a P-rich layer appears at the surface region of a Ni-6P plating layer by Ni depletion to form those intermetallic compounds at interfaces. The growth rate of a P-rich layer for Sn-3.5Ag is faster by about 4–8 times than those of the Sn-Ag-Cu. The presence of Cu in solder enhances the formation of the Cu6Sn5 intermetallic layer at the interface resulting in prevention of Ni diffusion to liquid solder. For all the soldered joints, coarsened reaction interfaces decrease the joint strengths.

An in-situ neutron diffraction study of the ageing of CaNi5Dx at 80$deg;C and 9bar

The intrinsic ageing of the CaNi 5 D x system has been studied at 80 °C and 9 bar over a period of 13 days. The system displays a distinct two-phase mixture of the intermediate α′ and β phases, whose phase proportions approach 40 and 50 wt.%, respectively at the end of the ageing period. The change in proportion of each intermediate phase and free fcc Ni suggests two periods of Ni depletion. The amount of free Ni in the system is doubled through the ageing period. The orthorhombic symmetry of the α′ and β phase is preserved, albeit with a strong depletion of three of the four Ni positions from the β phase during the second half of the ageing period. That one half of the hexagonally equivalent 2 c Ni position does not release Ni indicates that low symmetry geometrical ordering in orthorhombic symmetry is responsible for the rapid loss of recoverable hydrogen capacity in this system.

An in-situ neutron diffraction study of the ageing of CaNi 5D x at 80 °C and 9 bar

The intrinsic ageing of the CaNi 5D x system has been studied at 80 °C and 9 bar over a period of 13 days. The system displays a distinct two-phase mixture of the intermediate α′ and β phases, whose phase proportions approach 40 and 50 wt.%, respectively at the end of the ageing period. The change in proportion of each intermediate phase and free fcc Ni suggests two periods of Ni depletion. The amount of free Ni in the system is doubled through the ageing period. The orthorhombic symmetry of the α′ and β phase is preserved, albeit with a strong depletion of three of the four Ni positions from the β phase during the second half of the ageing period. That one half of the hexagonally equivalent 2 c Ni position does not release Ni indicates that low symmetry geometrical ordering in orthorhombic symmetry is responsible for the rapid loss of recoverable hydrogen capacity in this system.

Geochemistry and Origin of the Intrusive Hosts of the Noril'sk-TalnakhCu-Ni-PGE Sulfide Deposits

The world-class deposits of Cu, Ni, and platinum-group elements (PGE) of the Noril’sk-Talnakh district in Russia are hosted by relatively small, complex mafic-ultramafic bodies that intrude Permian sedimentary rocks and the lowermost suites of the Siberian continental flood-volcanic sequence. The Noril’sk-type, ore-bearing intrusions immediately overlie, and are probably penecontemporaneous with, weakly mineralized intrusions of the Lower Talnakh type, which have similar petrological features but contrasting and much more variable Nd, Os, and Sr isotopic compositions. Several types of spatially associated intrusions were emplaced after the ore-bearing intrusions and the flood-volcanic sequence. To clarify relations between the various types of igneous bodies (flood-volcanic rocks, Noril’sk-type intrusions, Lower Talnakh-type intrusions, and later intrusions) and to help understand the formation of the ore deposits, we undertook a detailed geochemical and Nd, Os, and Sr isotopic study of eight of the more important intrusions. The data indicate that these intrusions formed by the input of a series of magma pulses, each of which produced rocks with distinctive petrology and chemical compositions. These compositions are interpreted to have formed from mixtures of mantle-derived magmas and different types of crustal rock, which were encountered and assimilated during passage to the surface. Some of these variations may be superimposed on heterogeneities inherited from the mantle source. The magmas that formed the Lower Talnakh-type intrusions became contaminated with large amounts of crustal rock in a deep staging chamber. The chemical and isotopic characteristics of the resultant hybrid magmas are broadly similar to those of basalts of the lower and middle Nadezhdinsky lava subsuites (Nd 1-2 ), which are located about one-third of the way through the overlying lava sequence. These magmas became saturated in sulfur during contamination, and segregation of sulfides resulted in the depletion of chalcophile elements recorded in the Nd 1-2 lavas. Magmas feeding the ore-bearing intrusions had trace element and isotopic compositions similar to those of the basalts in the upper two-thirds of the volcanic sequence. These magmas underwent lower degrees of contamination in a deep staging chamber and sulfur saturation was not reached. These magmas therefore contained a full compliment of chalcophile elements when they encountered anhydrite-rich sediments during passage toward the surface. Assimilation of these sediments led to the segregation of sulfides to form the ores. Two aspects of our data impose important constraints on the ore-forming process. The first is the Os isotopic composition of the ore sulfides (avg γ Os ~7), which is similar to that of mantle rocks and very different from those of sulfides that segregate from highly contaminated magma. The second is the absence of Ni depletion in olivine in rocks from the ore-bearing intrusions, most notably in the ore-rich, picritic gabbrodolerites. The simplest explanation is that the sulfides segregated from magmas that were not highly contaminated and were transported into the intrusions by magmas different from those that yielded the sulfide. If this explanation is correct, it imposes some modifications to earlier models of sulfide ore deposition in the conduits of flood basalts.

Development of Gd-Enriched Alloys for Spent Nuclear Fuel Applications - Part 1: Preliminary Characterization of Small Scale Gd-Enriched Stainless Steels

The influence of Gd additions on the microstructure and hardness of type 316L stainless steel was investigated by various microstructural characterization techniques. This work was conducted as a first step toward the development of Gd-enriched alloys for spent nuclear fuel applications. Small (∼10 g) gas tungsten arc melt buttons were prepared to produce 316L stainless steel with Gd levels from 0.1–10 wt.% Gd. Electron microprobe measurements showed that Gd is essentially insoluble in the austenite/ferrite matrix. All of the alloys formed an interdendritic (Fe, Ni, Cr)3Gd intermetallic, and the amount of the (Fe, Ni, Cr)3Gd phase increased with increasing Gd concentration. Depending on the P and O levels, various amounts of Gd phosphides and oxides were also observed. The relatively high Ni concentration (∼28 wt.% Ni) and low Cr concentration (∼3 wt.% Cr) of the (Fe, Ni, Cr)3Gd phase led to Ni depletion and Cr enrichment in the matrix which, in turn, affected the matrix stability. Alloys with 0.1−6 wt.% Gd exhibited a two-phase ferrite/austenite matrix. Alloys containing 8 and 10 wt.% Gd exhibited a fully ferritic matrix due to extensive Ni depletion/Cr enrichment and concomitant stabilization of ferrite. Hardness increased with increasing Gd concentration due to the formation of the (Fe, Ni, Cr)3Gd intermetallic and the change in matrix structure from austenite to ferrite. A mass balance procedure is presented that accounts for changes to the matrix composition with Gd concentration. This procedure can be used to determine the nominal alloy composition required to produce a 316L-type matrix composition for any Gd level.

Nickeliferous gabbroic intrusions of the Pants Lake area, Labrador, Canada: Implications for the development of magmatic sulfides in mafic systems

The Pants Lake intrusions are located some 80 kilometers south of the world-class Voisey's Bay Ni-Cu-Co sulfide deposit in Labrador, Canada. They are dominated by olivine gabbro, with subordinate troctolite, melagabbro, peridotite and leucogabbro. They form several sheet-like bodies that were emplaced into metasedimentary gneisses that locally contain sulfides. The largest of these, termed the North and South intrusions, have been dated by other workers at 1322 ′ 2 Ma and 1337 ′ 2 Ma respectively, indicating that they formed in two discrete events. The North intrusion is more varied petrologically, and its three units show ambiguous contact relationships implying that they partly coexisted as liquids. Disseminated sulfide mineralization is widespread near the basal contacts of both intrusions, but massive sulfide mineralization is rare. In the older South intrusion, sulfides are hosted by melagabbro and peridotite of cumulate origin, and probably represent a gravitational accumulation. In the younger North intrusion, they are hosted by a complex mineralized sequence, interpreted to represent two or more influxes of magma charged with sulfides and reacted country-rock fragments. The older South intrusion is geochemically distinct from the younger North intrusion, and more closely resembles the Voisey's Bay Intrusion. Mineralized rocks within both Pants Lake intrusions are geochemically similar to the associated unmineralized rocks, and must be closer related to them. The mafic rocks almost all have low Ni contents and low Cu/Zr ratios that imply extraction of metals by sulfide liquids. This pervasive depletion signature, coupled with consistent Ni/Cu and Ni/Co, and clustering of sulfide metal contents, suggests that sulfide liquids were developed on an intrusion-wide scale, probably at depth, rather than by local processes. Sulfides at Pants Lake typically contain less than 2 percent Ni and 2 percent Cu. These values agree with predictions from the low Ni contents of silicate rocks, and imply a low mass ratio (R factor) of silicate magma to sulfide liquid (R 1000). The pervasive Ni and Cu depletion contrasts with the more localized metal depletion seen at Voisey's Bay. This may indicate that the Pants Lake sulfide liquids were not as effectively upgraded by later batches of undepleted magma. From the perspective of metallogenesis, data from the Pants Lake intrusions support several key concepts proposed m models for the formation of the Voisey's Bay deposit, suggesting that these may apply more widely as controls on the formation of magmatic sulfide deposits in broadly gabbroic magma systems.

Magmatic Ni-Cu versus PGE deposits: Contrasting genetic controls and exploration implications

Both Ni-Cu and PGE deposits are associated with mafic and ultramafic rocks, but the former often occur in dynamic magmatic systems such as lava channels and magma conduits whereas the later occur in larger layered intrusions. The contrasting modes of occurrence appear to be related to different degrees of preferential partitioning of the metals (PGE versus Cu and Ni) into segregating sulfide liquid and different concentration processes of the sulfide liquid during the formation of the ores. Primary magmas derived from partial melting of the upper mantle are fertile in PGE, Ni and Cu only when the molten sulfides are completely dissolved in the magmas. This requires a high degree of partial melting of the mantle, perhaps higher than 30 percent. The partial melts usually have basaltic to komatiitic compositions, and are unlikely to achieve sulfide saturation during ascent to the crust. Fractional crystallization and crustal contamination are the most important processes known to have induced sulfide saturation in magma upon intrusion. Sulfide liquid segregated in a large chamber may achieve high R-factors required for the formation of reef-type PGE deposits. Crustal assimilation appears to be most effective in dynamic lava channels and magma conduits. A large volume of sulfide liquid segregated from magma in such dynamic environments often has a lower R-factor, and relatively lower metal concentrations. Such metal-poor sulfide liquid needs to be deposited in restricted localities, and/or to be upgraded in metals by reacting with new surges of magma, to become a Ni-Cu sulfide deposit. Because of their extremely high partition coefficient between sulfide liquid and magma, PGE will become significantly depleted in magma even at small amounts of sulfide segregation. Copper, on the other hand, will be much less depleted because of its relatively low partition coefficient. Thus, Pd/Cu ratios in whole rocks can be used to detect PGE mineralization associated with small sulfide segregation in large layered intrusion. The whole rock Cu/Zr ratio is sensitive to large amounts of sulfide liquid segregation from magma and thus is a useful tool for Ni-Cu exploration. Nickel contents of olivine can also be used to identify Ni depletion in magma associated with large amounts of sulfide segregation.

Magmatic Ni-Cu versus PGE deposits: Contrasting genetic controls and exploration implications

Research Article| December 01, 2001 Magmatic Ni-Cu versus PGE deposits: Contrasting genetic controls and exploration implications Chusi Li; Chusi Li Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA. e-mail: cli@indiana.edu Search for other works by this author on: GSW Google Scholar W. D. Maier; W. D. Maier Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa. e-mail: wdmaier@scientia.up.ac.za Search for other works by this author on: GSW Google Scholar S.A. de Waal S.A. de Waal Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa. e-mail: sadw@scientia.up.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information Chusi Li Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA. e-mail: cli@indiana.edu W. D. Maier Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa. e-mail: wdmaier@scientia.up.ac.za S.A. de Waal Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa. e-mail: sadw@scientia.up.ac.za Publisher: Geological Society of South Africa First Online: 07 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2001 Geological Society of South Africa South African Journal of Geology (2001) 104 (4): 309–318. https://doi.org/10.2113/gssajg.104.4.309 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Chusi Li, W. D. Maier, S.A. de Waal; Magmatic Ni-Cu versus PGE deposits: Contrasting genetic controls and exploration implications. South African Journal of Geology 2001;; 104 (4): 309–318. doi: https://doi.org/10.2113/gssajg.104.4.309 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Both Ni-Cu and PGE deposits are associated with mafic and ultramafic rocks, but the former often occur in dynamic magmatic systems such as lava channels and magma conduits whereas the later occur in larger layered intrusions. The contrasting modes of occurrence appear to be related to different degrees of preferential partitioning of the metals (PGE versus Cu and Ni) into segregating sulfide liquid and different concentration processes of the sulfide liquid during the formation of the ores.Primary magmas derived from partial melting of the upper mantle are fertile in PGE, Ni and Cu only when the molten sulfides are completely dissolved in the magmas. This requires a high degree of partial melting of the mantle, perhaps higher than 30 percent. The partial melts usually have basaltic to komatiitic compositions, and are unlikely to achieve sulfide saturation during ascent to the crust. Fractional crystallization and crustal contamination are the most important processes known to have induced sulfide saturation in magma upon intrusion. Sulfide liquid segregated in a large chamber may achieve high R-factors required for the formation of reef-type PGE deposits. Crustal assimilation appears to be most effective in dynamic lava channels and magma conduits. A large volume of sulfide liquid segregated from magma in such dynamic environments often has a lower R-factor, and relatively lower metal concentrations. Such metal-poor sulfide liquid needs to be deposited in restricted localities, and/or to be upgraded in metals by reacting with new surges of magma, to become a Ni-Cu sulfide deposit.Because of their extremely high partition coefficient between sulfide liquid and magma, PGE will become significantly depleted in magma even at small amounts of sulfide segregation. Copper, on the other hand, will be much less depleted because of its relatively low partition coefficient. Thus, Pd/Cu ratios in whole rocks can be used to detect PGE mineralization associated with small sulfide segregation in large layered intrusion. The whole rock Cu/Zr ratio is sensitive to large amounts of sulfide liquid segregation from magma and thus is a useful tool for Ni-Cu exploration. Nickel contents of olivine can also be used to identify Ni depletion in magma associated with large amounts of sulfide segregation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Unusual Depletions toward the SMC Star S[CLC]k[/CLC] 155—Differences in Dust Composition in the SMC Interstellar Medium?

We report initial results from an analysis of Hubble Space Telescope/Space Telescope Imaging Spectrograph echelle spectra of interstellar absorption toward Sk 155, located in the wing region of the SMC. There are at least 12 Milky Way (disk and halo) and at least 25 SMC components discernible in the profiles of various neutral and singly ionized species. Fits to the line profiles yield column densities and relative elemental abundances for the individual components. In the SMC components, the depletions of Fe and Ni range from mild ([Fe, Ni/Zn] ~ -0.3 to -0.8 dex) to severe ([Fe, Ni/Zn] ~ -1.7 dex); Mg and Si, however, appear to be essentially undepleted ([Mg, Si/Zn] ~ -0.1 to +0.2 dex) throughout. The combination of severe depletion for Cr, Mn, Fe, and Ni and negligible depletion for Mg and Si has not been seen in any Galactic sight line. If Si is generally undepleted in the SMC, then current models of the SMC dust—which rely heavily on silicates—will have to be reconsidered; oxides and/or metallic grains may dominate. We briefly discuss possible the implications of these SMC depletions for understanding the elemental abundances in QSO absorption-line systems.

Sulphide and oxide mineralisation in the Uitkomst Complex, South Africa: origin in a magma conduit

In the magmatic system of the Uitkomst Complex, several factors may influence the occurrence of major sulphide segregation: (i) a mixing of compositionality contrasting magmas; (ii) a rapid decrease in temperature at the fast cooling margins of the intrusion; (iii) a decrease in FeO content as a result of chromite and magnetite precipitation; and (iv) the assimilation of dolomitic, or siliceous, sulphide-bearing country rocks. The original Uitkomst magma had a typical primary magmatic S isotope signature (0ℵ δ 34S) and on emplacement became contaminated to a variable degree with 32S-enriched sedimentary S (<− 10ℵ δ 34S). The sulphide mineralisation occurs as: (i) a massive sulphide body in the quartzite and granite basement; and (ii) disseminated sulphides within the BGAB and LHZBG Units. Geochemical variations throughout the complex and the combined thickness of the chromitite seams and the associated massive sulphide bodies strongly underline the argument for the conduit-type nature of the intrusion similar to Voisey's Bay and Noril'sk. The over large quantity of sulphides for the complex's thickness is mainly explained by separation from a larger magma pool at depth, entrapment by the ascending magma and deposition in the conduit. The fractionation of magma probably occurred at depth and involved olivine, as indicated by the distinct Ni depletion of the chills, at undepleted Cu contents. The Uitkomst ores probably represent a mss cumulate with Cu-Pt-Pd-enriched fractionated sulphide melt remaining unexposed. The origin of mineralisation due to country rock assimilation at Uitkomst is comparable with that of the Platreef portion of the Potgietersrus Limb of the Bushveld Complex. The S isotope signature (6.3–9.2ℵ δ 34S) of the Platreef mineralisation is also indicative of a sedimentary contribution, although by evaporitic layers. The similar Pt/Pd ratio of Uitkomst and the Platreef, however, implies compositional variations within the Bushveld magma rather than fundamentally different magma sources of both complexes.

Studies on modification of some first wall materials using 3–6.8 MeV He ions

Because very few results are reported in the MeV region, we have started a systematic investigation concerning the fluence and energy dependence of blistering induced in some useful materials for the first wall (stainless steels, Ni, Cu, Mo). They have been irradiated with 3.0, 4.7 and 6.8 MeV He ions. The irradiation effects have been investigated by means of a TEMSCAN-200-CX electron microscope and two metallographic microscopes. Irradiation phenomena as sponge- and wave-like structures, submicronic cracks, multilayer flaking, micro-conglomerates, secondary blisters, microcraters were observed. An interesting effect is the amorphous phase in the Ti-modified austenitic steel 12KH18N10T, phase formatted during irradiation with 6.8 MeV He + ions up to a dose of crater occurrence (7×10 18 ions per cm 2), the temperature of specimens (100 μm thick) being maintained during irradiation in the range 30–60°C. The X-ray microanalysis shows the presence of the major alloy elements in the amorphous area, whereas in that embedding a microcrystallite it is to be noticed the Ni depletion and the presence of Si and Mo. A potential explanation of the energy dependence of average blisters diameter d and blister critical dose could be stressed-induced model, because, in our cases (medium energy He ions), d– t m , where t is the blister skin thickness. For our high energy data, an m=1.8 value is relatively convenient, except Mo, where m=1.5 is more suitable.

Layering and Precious Metals Mineralization in the ;Rincón del Tigre Complex, Eastern Bolivia

Geologic investigations supporting recent, technically successful, platinum exploration of the Rincon del Tigre Complex—a ca. 4.6-km-thick layered sill—have provided significant new understandings of the layering, development, and precious metals mineralization of one of South America’s largest mafic-ultramafic intrusions. Three macrocyclic units are recognized—from the base up, Basal unit, Unit 1, and Unit 2—each most likely the product of a major replenishment with mafic magma of one compositional type. The ultramafic rocks of all three macrocyclic units largely comprise repeated minor cyclic units of poikilitic harzburgite → granular harzburgite (→ olivine bronzitite) each a few meters thick, with occasional development of bronzitite at the top. Mafic rocks, absent from Unit 1 and present only locally in the Basal unit, form a well-developed norite → gabbro → magnetite gabbro sequence in the upper half of Unit 2. Associated with the base of the magnetite gabbro is the Precious Metals zone, a persistent zone of low-grade (subeconomic) sulfide and precious metals mineralization, 80 to 185 m thick. The Precious Metals zone comprises an upper Cu sulfide-rich portion, its base (the sulfide phase boundary) about 23 to 70 m above the magnetite phase boundary, and a lower precious metals-rich portion, the lower part straddling the magnetite phase boundary. The individual precious metals are concentrated in separate, sulfide-poor subzones, each up to many tens of meters thick, in the stratigraphic order (Rh →)Pt → Pd → Au, with peak offsets at the same scale. Modal layering of silicates and magnetite is well developed in the mafic rocks as is modal and cryptic (precious metals) layering of the sulfides in the Precious Metals zone, all at three different superimposed scales from about 3 m to several tens of meters. A possible interpretive framework for the cryptic layering (and perhaps for much of the other layering, including the meter-scale layering of the ultramafic rocks) is provided by the likely compositional stratification of the resident magma initiated by the fluid dynamics of the replenishment and mixing processes and, in the magnetite gabbro, by repeated convective overturns caused by the onset of magnetite precipitation. The characteristics of the Precious Metals zone fit an orthomagmatic model. The composition, stratigraphic relations, and modal correlation (with magnetite) of the sulfides all imply that S saturation was induced by loss of FeO to magnetite precipitation. This took place after prolonged closed-system fractionation (following the last major magma injection at the base of Unit 2) during which crystallization of olivine, pyroxene, plagioclase, and (finally) magnetite precipitation led to overall S, Cu, and precious metals enrichment, and Ni and (finally) Fe depletion of the magma. The development of the mineralization and its complex layering was controlled by sulfide segregation, Rayleigh fractionation, the different but generally very high partition coefficients (Dsilicate/sulfide) for each precious metal, and repeated convective overturns at three different superimposed scales. The decoupled metals distribution is analogous and similar in origin to that of the Main Sulfide zone of the Great Dyke (Zimbabwe) and of the Main Sulfide layer (offset portion) of the Munni Munni Complex (Western Australia), but the stratigraphic order of metal enrichment peaks is different (Pt → Pd → Au vs. Pd → Pt → Au). This suggests a reversal in the relative magnitudes of the partition coefficients DPt and DPd that was possibly brought about by changes in the temperature and O, S, and Fe activities of the magmas between the middle and late stages of crystallization. Constancy of D values during fractionation of mafic magma should not be assumed. The Precious Metals zone is one of several Skaergaard-type strata-bound precious metals zones associated with magnetite gabbros in the upper levels of many layered intrusions, and the origin of the Skaergaard mineralized zone itself—including the potentially economic Platinova reefs—can be explained in terms of the Precious Metals zone model presented here. The very low grade, great thickness, and wide separation of individual metal subzones in the Precious Metals zone are similar to those of the subeconomic axial facies of the Main Sulfide zone in the wider parts of the Great Dyke (cf. the economic, more compact, marginal facies). On the basis of the lateral and vertical thermal model of the Great Dyke magma chamber, the subeconomic status of the Precious Metals zone may be in large part a function of the specific fluid dynamics of the Rincon del Tigre magma chamber.

X-ray study of phase transformations in martensitic Ni-Al alloys

The formation of the Ni5Al3 and Ni2Al phases in Ni-Al alloys with L1o ↔ B2 thermoelastic martensitic transformation has been studied by X-ray analysis. Ni5Al3 can form both the L1o and B2 structures, but the kinetics of L1o → Ni5Al3 and B2 → Ni5Al3 reactions are significantly different. A homogeneous mechanism for the former reaction and a mechanism of precipitation and growth for the latter are proposed. Ni2Al forms from the B2 structure by the complex rearrangement of atoms. The initial stage of this reaction proceeds very rapidly and involves segregation of Ni atoms into Ni-rich zones leading to a Ni depletion in the surrounding regions. The nucleation of Ni2Al retards the Ni5Al3 formation, so preaging in the B2 region affects the kinetics of the L1o → Ni5Al3 reaction on further aging in the L1o region. The microstructural mechanism for this effect is suggested.

Geochemistry of mineralised and barren komatiites from the Perseverance nickel deposit, Western Australia

The Perseverance (formerly known as Agnew) and the neighbouring Rocky's Reward nickel deposits are associated with metamorphosed komatiite flows erupted onto a substrate of felsic crystal tuffs. The deposits are overlain by the Perseverance Ultramafic Complex, which consists of a central dunite lens, 700 m thick by 3 km wide, flanked by olivine orthocumulates and spinifex textured komatiites. A suite of samples of A-zones of komatiite flows from the area has been analysed for major and trace elements, rare earth elements and platinum-group elements. The samples fall into two distinct geochemical groups on the basis of major and rare earth elements. 1. (1) Samples from A-zones of spinifex-textured flows flanking the central dunite show well constrained, linear trends of major and trace elements typical of komatiite suites related by simple olivine fractionation. 2. (2) Samples from the Perseverance Mine (mineralised and basal flows), from Rocky's Reward and from unmineralised flows at the base of the Perseverance Ultramafic Complex all show evidence for LREE enrichment relative to the first group. They are also generally higher in SiO 2 and lower in FeO for a given MgO content. Geochemical features of the second group are consistent with the effects of crustal assimilation. This is in line with current models for nickel sulphide genesis, based largely on Kambalda, which emphasise the role of thermal erosion of sulphidic footwall sediments. The chalcophile element (Ni, Ir, Ru, Pt, Pd, Au and Cu) concentrations in A-zone samples from the study area are typical of komatiites. Komatiites from the Perseverance mineralised units and from Rocky's Reward show evidence of depletion of the more strongly chalcophile PGE relative to Ni and Cu. This feature is also evident in massive and matrix sulphide ore samples, although less evident in the main Perseverance cloud sulphides. Quantitative geochemical computer modelling indicates that the major and rare earth element chemistry of the samples analysed is consistent with derivation by combined assimilation of felsic material and crystallisation of olivine from a primitive komatiite magma. The parental composition is represented by the spinifex textured units flanking the dunite lens. Compositions of komatiite liquids, sulphides and cumulus olivines can be modelled successfully by varying degrees of batch equilibration between komatiite magma, olivine and assimilated sulphidic sediment. Models involving combined assimilation and fractional crystallisation, in which olivine and sulphide are fractionally segregated as they form, are much less successful at matching the data. In physical terms, this suggests that the mineralisation is due to wholesale assimilation of floor rocks close to the site of sulphide accumulation. Comparison with quantitative computer model trends suggests an average ratio of assimilation to olivine fractionation in the order of 0.5 to 1 in order to generate the observed degrees of LREE enrichment in flowtops, Ni depletion in olivine and PGE depletion in sulphides and flow tops.