Ni Arsenides Research Articles

Controls on hydrothermal Fe oxide–Cu–Au–Co mineralization at the Guelb Moghrein deposit, Akjoujt area, Mauritania

The Guelb Moghrein Fe oxide–Cu–Au–Co deposit, with a total resource of 23.6 Mt at 1.88% Cu, 1.41 g/t Au, and 143 g/t Co, is hosted by an extensive metacarbonate body. However, it is restricted to up to 30-m wide tabular breccia zones developed parallel to discrete shear zones that transect the host metacarbonates. The Fe–Mg clinoamphibole–chlorite schists represent up to 1-m thick interlayer metasediments and localized viscous shearing in these shear zones. Siderite of the metacarbonate body was deformed into a breccia and was replaced by an ore and alteration assemblage comprised of Fe–Mg clinoamphibole, magnetite, pyrrhotite, chalcopyrite, graphite, Fe–Co–Ni arsenides, arsenopyrite, cobaltite, uraninite, and Bi–Au–Ag–Te minerals. In contact with wall rock amphibolites, the metacarbonate body is enveloped by an alteration halo up to 40 m wide, consisting of biotite, actinolite, grunerite, chlorite, calcite, albite, and quartz. The Guelb Moghrein ore body is structurally controlled by shear zones that developed in the footwall of a regional thrust zone. This thrust separates greenschist facies quartz–sericite schists and biotite–garnet–quartz schists of the Sainte Barbe volcanic unit in the hanging wall from amphibolite facies metavolcanic rocks, metacarbonates, and the Guelb Moghrein ore body of the Akjoujt metabasalt unit in the footwall. Peak temperatures of the latter unit are estimated by hornblende–plagioclase thermometry at 580±40°C. Thrusting was retrograde for the Akjoujt metabasalt unit, but prograde for the Sainte Barbe volcanic unit at P–T conditions of about 410±30°C and 2–3 kbar (garnet–biotite thermometry). Structural and petrological evidences suggest that the ore fluids migrated along the shear zones and reacted with the siderite in the metacarbonate. This evolution and the setting of Guelb Moghrein in the fold-and-thrust belt of the Pan-African to Variscan Mauritanides (Mauritania, West Africa) resemble Proterozoic Fe oxide–Cu–Au–Co deposits such as examples from the Tennant Creek and Mount Isa Inliers, Australia.

New data on base metal mineralogy and platinum-group minerals in the Ojen chromitites (Serrania de Ronda, Betic Cordillera, southern Spain)

Three small Cr-rich chromitites occurrences (62 < Cr# < 84) from the Ojen lherzolitic massif (south Spain) were studied for base metal mineralogy, bulk rock PGE (platinum group elements) abundances and platinum-group minerals (PGM). A great variety of base metal sulfides (BMS), Ni arsenides, Ni sulfarsenides and Ni-sulfobismuthides are reported. Bulk-rock PGE contents span more than two orders of magnitude. Two occurrences display the typical PGE pattern of ophiolitic chromitites (high contents of Os, Ir, Ru and Rh and depletion in Pt and Pd; ΣIPGE = 114.1-628.3 ppb: IPGE = Os, Ir, Ru). Another chromitite, the most chromiferous, is very poor in IPGE and Rh, while displaying a positively-slopping Primitive Mantle-normalized pattern from Pt to Pd and Au. PGM inclusions are abundant in the PGE-rich chromitites, consisting in laurite-erlichmanite, Ir-Pt-Rh sulfarsenides, Ru-Os sulfarsenides, Pt arsenides (sperrylite), Pd antimonide (stibiopalladinite) and Pt-Fe and Os-Pt alloys. The igneous crystallization sequence starts with laurite (containing appreciable amount of sulfarsenide in solid solution), erlichmanite, Ru-Os sulfarsenides and Ir-Rh sulfarsenides, followed by Ir-Pt-Rh sulfarsenides, Pt-sulfarsenide and the other Pt-minerals (sperrylite, Pt 3 Fe, Pt-Os alloys). The high As activity during the crystallization of chromite prevented the formation of Os-Ir-Ru alloys. Stibiopalladinite is a late magmatic phase. Pd and Au in the PGE-poor chromitite are accounted for the high modal abundance of intergranular Ni arsenides. PGM-BMS textures indicate late-segregation for the BMS, probably from late-magmatic fluids. The abundance of As minerals and the presence of Bi- and Sb-bearing phases in the Ojen chromitites confirm that volatile-rich, small-melt fractions have migrated through the Ojen Iherzolitic massif just before its tectonic emplacement into the continental crust.

The origin of alluvial gold in the Leadhills area of Scotland: evidence from interpretation of internal chemical characteristics

A detailed chemical study has been carried out of 1794 gold grains from 55 sites within an area approximately 7 km by 18 km around Leadhills in the turbidite-facies, wacke-dominant Southern Uplands terrane of Scotland, formed as an accretionary thrust complex at the Laurentian continental margin during Ordovician and Silurian subduction of the Iapetus Ocean. The Leadhills area has had a long history of alluvial gold working, since at least 1510, and probably much earlier, but few bedrock sources of mineralisation have been located. Quantitative electron microprobe point analyses were obtained of sectioned and polished gold grains and of the associated microscopic inclusions, which in total comprised 35 types of opaque minerals. Microchemical X-ray maps were also obtained using a fully automated electron microprobe to show internal chemical compositional variation, to resolve the components of multiple and composite inclusions and to help in the identification of the smallest examples. Silver is present within most grains but Cu, Hg and Pd were only detected in about 3% of the grains. The shapes of cumulative frequency plots of the Ag contents of grains from different sites allow recognition of populations from different sources and enables comparison and differentiation between sites. In one of the main rivers, the Shortcleugh Water, the nature of the gold is generally similar, both in terms of Ag content and inclusion type, but a further type of gold, relatively rich in Ag and containing distinctive Ni and Sb-rich inclusions, is also present in 4 adjacent samples in one sector of the watercourse. This component of the samples reflects downstream dispersion, of up to 400 m, of gold grains from a mineralised structure cutting the river almost perpendicularly. Elsewhere abrupt changes in the nature of the alluvial gold within streams reflect a series of different sources and indicate limited glacial and alluvial dispersion. Four main types of gold which are considered to be derived from different varieties of source mineralisation can be recognised on the basis of composition and inclusion assemblage. The commonest type (ca. 70%) contains 8.4–13.1% Ag and inclusions, mostly of arsenopyrite, pyrrhotite, pyrite, chalcopyrite, galena, sphalerite and cobaltite, characteristics typical of mesothermal gold from shear-zones within a turbidite environment, suggesting sources within the local Lower Palaeozoic sedimentary rocks. The second type is richer in Ag (15.8–31.3%) with a basic igneous signature indicated from inclusions of Sb-rich gersdorffite, pyrite, Ni arsenide and other Ni-rich minerals, and shows some spatial association with discordant Tertiary basaltic dykes which cut the sedimentary rocks. The third type, mostly found in an area mantled by glacial drift, is poor in Ag (<1.2%) but frequently Cu-bearing and with inclusions of platinoids and Cu oxide. Associated with this type of gold are grains of the intermetallic compound Au 3Cu and platinoids such as isoferroplatinum and a complex Ir-rich phase. Its source is problematical and could partly reflect detrital material associated with ophiolitic debris, which is common in the northernmost turbidites, and partly a local source of lode gold. The fourth gold type is also Ag-poor (<3.2%), but frequently contains Pd, up to 6.3%, while the inclusions comprise selenides of Pb, Hg and Cu. In nearby areas this type of gold is associated with Permian red beds and associated basaltic volcanics and also structures in the Lower Palaeozoic into which oxidising solutions derived from the red bed sequence have penetrated.

Mineralogía y comportamiento geoquímico del oro en los macizos lherzoliticos Betico-Rifeños (España y Marruecos)

The Lherzolite massifs from the Serrania de Ronda and Beni Bousera are pieces of the upper mantle emplaced in the continental crust. Before their crustal emplacement, the Lherzolite suffered several partial melting events, giving rise to depleted peridotites (harzburgites and dunites) as well as to pyroxenites and magmatic Cr, Cr-Ni and (Fe-Ni-Cu) sulfide ores. The average Au content are strongly concentrated in the mineralizations, especially in the Cr-Ni ores, in contrast with the usual behaviour of the noble metals in magmatic ores related to mafic-ultramafic rocks. Au, Pt and Pd are concentrated in the Ni arsenides where most of the former elements are hidden in solid solution in the niccolite. The alteration of niccolite produces the exsolution of native Au and alloys of Au-Cu and Au-Cu-Ag. In most (Fe-Ni-Cu) sulfide ores, the Au content is very low. However, the bodies with the highest Cu/Ni ratio and with hydrothermally altered wall rocks are enriched in Au. The geochemical behaviour of noble metals suggests that the parental partial melts of the «ore magma» were generated from a depleted mantle source and originally, they were S- and As-undersaturated and enriched in noble metals. During ore formation, As played an important role as collector of Au (as well as of Pt and Pd); therefore, the earlier As saturation of the liquids and the subsequent crystallization of the Ni arsenides produced Au-, Pt- and Pd-depleted (Fe-Ni-Cu) sulfide ores.

Magmatic ores in high-temperature alpine-type lherzolite massifs (Ronda, Spain, and Beni Bousera, Morocco)

The alpine lherzolite massifs of southern Spain (Serrania de Ronda) and northern Morocco (Beni Bousera) are mantle intrusions emplaced at high temperature in the continental crust. Three types of mineralization occur in the lherzolites: (1) chromite-Ni arsenide ores (Cr-Ni ores) with orthopyroxene and/or cordieritc, (2) chromite ores (Cr ores) with clinopyroxene and/or orthopyroxene, (3) Ni-Fe-(Cu) sulfide-graphite ores (S-G ores) with chromite, plagio-clase, and phlogopite. The chromite ores are located in the plagioclase lherzolite core of lherzolite massifs, whereas the two other ore types are distributed in the outer spinel and garnet lherzolite zones.Chromite and Cr-Ni ores show primary magmatic textures, which are mostly overprinted by secondary high-temperature plastic deformation, and a compositional trend of chromite and orthopyroxene that corresponds fairly well to a magmatic Mg-Fe fractionation. Chromite also displays an unusual Zn and V enrichment. High gold contents (3-35 ppm) and up to chondritic platinum-group element values are found in the Ni arsenide-rich ores. S-G ores comprise a pyrrhotite-pentlandite-(chalcopyrite-cubanite)-graphite association which also includes Fe-rich chromites with high Zn and V contents; their platinum-group element and Au contents are much lower than those of Cr and Cr-Ni ores (<0.5 ppm platinum-group elements and <4 ppm Au).The proposed metallogenic model comprises (1) crystallization of chromite and pyroxene between 1,500 degrees and 950 degrees C; (2) segregation of an immiscible As-S-rich liquid (following the hypothesis of Oen, 1973) that accumulated downward and started crystallization between 800 degrees and 400 degrees C, with the formation of an arsenide matrix among chromite grains; and (3) later crystallization of an S-rich residual liquid in graphite-rich assemblages. During this magmatic evolution Os-Ir-Ru minerals were included in chromite, whereas Au and Rh-Pt-Pd were concentrated in the As-S-rich liquid and precipitated with the Ni arsenides. The residual S-rich liquid was therefore depleted in platinum-group elements and Au. This model implies a lherzolite source for the ore-forming elements and the fractionation of a magmatic liquid migrating at moderate- to low-pressure conditions from the core to the borders of the rising lherzolite body.

Relaciones entre la zonalidad petrológica y metalogénica de los macizos lerzolíticos de las cadenas alpinas del Mediterráneo Occidental (Cordillera Bético-Rifeña y Kabylias)

Gamet-lherzolite facies, Ariegite and Seiland subfacies of the spinel-lherzolite facies and plagioclase-lherzolite facies have been identified in the Serrania de Ronda, Beni Bousera and Collo lherzolitic massifs. These petrological facies occur in a zonal arrangement: gamet lherzolite are in contact with the overlaying metapelitic sequence and spinel then plagioclase lherzolite occur inwards, representing the innermost zones in the former mantle body. The various occurrences of mineralizations can be classified in two main groups: one of them is essentialIy made up of chromite and Ni arsenides with pyroxene and/or cordierite as gangue minerals; the other consists Qf Fe-Ni-Cu sulfides and graphite, with pyroxene, plagioclase and phlogopite as gangue minerals. In both groups, the ores with the more refractory composition were the first to crystallize in the hotter core of the diapir while those other with a more differentiated composition were formed later within the external zones. The close correlation between petrological and metallogenical roning supports a magmatic origin of the mineralizations related to the petrologic evolution of the mantle bodies. The mineralizing liquids• originated from magmas generated by partial melting of the peridotites. The former migrated outwards, from the internal part of the bodies, during which marked interaction with the enclosing rocks and complex fractionation processes took place under moderate to low pressure conditions.

CuFeNiS mineral assemblages in upper-mantle peridotites from the Table Mountain and Blow-Me-Down Mountain ophiolite massifs (Bay of Islands area, Newfoundland): Their relationships with fluids and silicate melts

Plastically deformed ultramafic rocks in the Table Mountain and Blow-Me-Down Mountain ophiolites comprise a basal unit of slightly depleted Iherzolites, an intermediate sequence of strongly depleted harzburgites and an upper zone of dunites, also referred to as a transition zone intensively percolated by basaltic melts or magmatic fluids. Thirty-five samples including all of the above rock types have been investigated on 100 polished thin sections for CuFeNiS mineral assemblages. Most of them contain traces of CuFeNi sulfides, native metals and locally Ni arsenide. Compositional features of opaque assemblages as well as their textural sites in the rocks indicate that the present CuFeNiS minerals derive from an upper-mantle sulfide component through extensive subsolidus re-equilibration down to 100°C. The primitive component (predominant pentlandite, minor pyrrhotite and chalcopyrite) is preserved as sulfide inclusions in chromites of the transition zone, due to a subsolidus re-equilibration in a closed system. On the contrary, sulfide assemblages interstitial to silicates and spinel have extensively reacted with reducing serpentinizing fluids to produce sulfur-deficient sulfides such as heazlewoodite and mackinawite and native metals (native copper and awaruite). Microscale variations of redox conditions and the removal of Fe from the silicate during serpentinization may account for the peculiar “grain-by-grain” equilibrium state of intergranular assemblages. In spite of low-temperature alteration, a gradual depletion in sulfide component has been recognized from the basal lherzolites to the intermediate harzburgites while the sulfide content gradually increases in the transition zone (up to 0.2% by volume). The first pattern is consistent with the low-melting nature of the sulfide component in mantle melting processes. Microstructural criteria such as the absence of sulfide inclusions in olivine neoblasts demonstrate that the sulfide component postdates plastic deformation of the transition zone. The sulfide-enrichment pattern is thus ascribed to the percolation of a sulfur-saturated basaltic magma into residual dunites.

Co–Ni arsenide deposits, with accessory gold, in ultramafic rocks from Morocco

A type of Co–Ni arsenide deposit with accessory gold and no silver can be recognized in ultramafic massifs of upper mantle origin. The Bou Azzer deposits (50 000 t Co) are distributed along the borders of serpentinite massifs in an Upper Proterozoic ophiolite (Anti-Atlas, Morocco). They consist of quartz–carbonate lenses with Co (Ni–Fe) arsenides. Gold is related to the skutterudite (8–15 ppm average content). In the Alpine lherzolite massif of Beni Bousera (Rif, Morocco) are found small chromite–Ni arsenides veins (300 t Ni) with accessory gold. They are associated with late orthopyroxene–vermiculite dikes. Several occurrences of As, arsenides, and gold-bearing carbonate rocks are found in serpentinites from various ophiolites. It is proposed that gold was leached by As- and Co2-rich hydrothermal solutions related to the serpentinization of mantle peridotites.

Stable Isotope Geochemistry and the Origin of the Great Bear Lake Silver Deposits, Northwest Territories, Canada

Within the Bear Province, the Echo Bay Group of sediments and volcanics (~1800 m.y.) are intruded by Hudsonian granites and later diabases. Northeasterly trending fracture zones contain vein type U–Ag–Bi–Co–Ni–As–Cu deposits. Samples from the Terra, Norex, Silver Bay, and El Bonanza mines give a general paragenetic sequence of uraninite + hematite; Co–Ni arsenide minerals and silver; dolomite and sulfides; and sulfosalts with bismuth. The vein mineralogy is consistent with an ore fluid of high oxidation potential, low sulfur content (10−3 m) and low pH (~4).Dolomite δ 13C and δ 18O values of around −4‰ (PDB) and +15‰ (SMOW) respectively from all stages reflect a relatively constant temperature of deposition, which from mineralogical and other data is estimated at 200 °C. A δ 18O value of +2‰ (SMOW) is calculated for the ore fluid. The bulk of the sulfide δ 34S values are around +1‰ (CDT), which suggests a magmatic sulfur source. However, an equilibrium model is presented whereby the sulfur is derived from Precambrian sea-water sulfate and the carbon from limestones. The embryo ore fluid (connate sea water?) probably circulated both through the Echo Bay Group and the intrusives, and underwent chemical changes before moving into dilatant zones of mineral deposition.