Chusi Li

Re-Os isotope systematics of Ni-Cu sulfide ores, Sudbury Igneous Complex, Ontario: evidence for a major crustal component

Sudbury Igneous Complex sublayer ores from the Levack West, Falconbridge and Strathcona mines were analyzed for their Re and Os concentrations and Os isotopic compositions. The Re-Os isotope systematics of three ores from the different mines give isochron ages of1 840 ± 60 Ma, 1770 ± 60 Ma and 1780 ± 110 Ma, suggesting that the Re-Os system became closed at the time of, or soon after the 1850 ± 1 Ma crystallization age of the complex. The Os isotopic compositions of different portions of the complex at the time of crystallization varied considerably, with initial187Os/186Os1850 ranging from 4.64 at Levack West to 7.55 at Strathcona. These heterogeneities require that the Os, and probably also the other platinum-group elements contained in the ores, were derived from at least two sources. In addition, the high initial187Os/186Os ratios indicate that the Os was derived predominantly from ancient crust. Previous studies have suggested that the complex either crystallized from a mixture of mantle-derived basaltic melt and ancient continental crust, or was derived exclusively from the fusion of ancient continental crust resulting from a meteorite impact. Results of modelling suggest that if a contemporaneous mantle-derived basaltic melt was involved in the origin of the SIC, it likely contributed < 50% of the Os to all three ores. The large percentage of ancient crust involved in the production of the ores is most consistent with an interpretation of substantial crustal fusion resulting from meteorite impact.

Copper solubility in a basaltic melt and sulfide liquid/silicate melt partition coefficients of Cu and Fe

The solubility of copper in a sulfur-saturated basaltic melt has been determined at 1245°C as a function of fO2 and fS2. Copper solubilities at log fO2 values between −8 and −11 fall into two distinct populations as a function of fS2. At log fS2 values < −1.65, sulfide liquid that coexists with the basaltic glass quenches to sulfur-poor bornite solid solution. At log fS2 values in excess of −1.65, the sulfide liquid quenches to a complex intergrowth of sulfur-rich bornite and intermediate solid solution. Copper solubilities in the low-fS2 population range from 594 to 1550 ppm, whereas those in the high-fS2 population range from 80 to 768 ppm. Sulfide liquid/silicate liquid partition coefficients (D) for Cu and Fe range from 480 to 1303 and 0.7 to 13.6, respectively. Metal-sulfur complexing in the silicate liquid is shown to be insignificant relative to metal-oxide complexing for Fe but permissible for Cu at high fS2 values. On log DFe (sulfide-silicate) and log DCu (sulfide-silicate) vs. (log fS2 − log fO2) diagrams, both fS2 populations show distinct but parallel trends. The observation of two D values for any fS2/fO2 ratio indicates nonideal mixing of species involved in the exchange reaction. The two distinct trends observed for both Cu and Fe are thought to be due to variations in activity coefficient ratios (e.g., γFeO/γFeS and γCuO0.5/γCuS0.5). Results of the experiments suggest that accurate assessments of fS2/fO2 ratios are required for the successful numerical modeling of processes such as the partial melting of sulfide-bearing mantle and the crystallization of sulfide-bearing magmas, as well as the interpretation of sulfide mineralogical zoning. In addition, the experiments provide evidence for oxide or oxy-sulfide complexing for Cu in silicate magmas and suggest that the introduction of externally derived sulfur to mafic magma may be an important process for the formation of Cu-rich disseminated magmatic sulfide ore deposits.

Sulfur and oxygen isotopic evidence of country rock contamination in the Voisey's Bay Ni–Cu–Co deposit, Labrador, Canada

Abstract The emplacement of basaltic magma into sulfide-bearing country rocks provides a favorable geologic environment for magmatic sulfide ore formation related either directly to assimilation of country rock sulfur or indirectly to a depression of sulfide solubility caused by assimilation-induced changes in magma composition. Pelitic country rocks of the Proterozoic Tasiuyak Gneiss in the area of the Voiseys Bay Ni–Cu–Co deposit contain sulfidic layers that may have provided sulfur to basaltic magmas during emplacement of the Voiseys Bay intrusion. Sulfur isotopic compositions of the Tasiuyak Gneiss range from −0.9 to −17.0‰, values typical for sulfides produced via bacterial sulfate reduction in an open marine environment. Archean gneisses in the area contain low amounts of sulfide and are less likely to have served as a source of externally-derived sulfur. Sulfur isotopic compositions of the sulfide minerals from the Voiseys Bay deposit show consistent variations, both spatially and with rock types. Disseminated and massive sulfides show a decrease in δ 34 S to the west, with values typically between 0 and −2‰ in the Eastern Deeps, Ovoid, and Discovery Hill zone, and between −2 and −4‰ in the Reid Brook zone. δ 34 S values of the Mushua intrusion to the north and the Normal Troctolite in the Eastern Deeps are more positive, ranging between −0.5 and 1.8‰. This range is taken to represent the isotopic composition of primary mantle-derived sulfur in the area because the Mushua intrusion and Normal Troctolite show the least geochemical evidence for contamination by country rocks. Sulfur isotopic data from the Reid Brook zone are consistent with up to a 50% sulfur contribution from the Tasiuyak Gneiss. Correspondingly lower proportions are indicated for the eastern portion of the deposit where country rocks are predominantly low-sulfide enderbitic and quartzofeldspathic gneisses. Oxygen isotopic values of gneiss fragments in the Basal Breccia Sequence and Feeder Breccia suggest that the assimilation process involved a greater proportion of high− 18 O contaminant to the west. δ 18 O values of the Tasiuyak Gneiss (5.9 to 14.0‰), enderbitic gneiss (6.4 to 8.7‰), and Archean quartzofeldspathic gneiss (9.5 to 9.7‰) are consistent with an increased proportion of Tasiuyak Gneiss contaminant to the west. Isotopic data strongly indicate that sulfur from the Tasiuyak Gneiss has been involved in ore deposition at the Voiseys Bay deposit. However, sulfur and oxygen isotopic data also strongly suggest that the addition of externally derived sulfur was not the sole process responsible for mineralization, and that assimilation of both Proterozoic and Archean country rocks played a key role in depressing sulfide solubility prior to sulfide localization via dynamic, physical mechanisms.

Sulfur isotopic studies of continental flood basalts in the Noril’sk region: implications for the association between lavas and ore-bearing intrusions

Previous studies of both ore and non-ore-bearing intrusives in the Permo-Triassic flood basalts of the Siberian platform in the Noril’sk area have shown that high-grade Ni–Cu–platinum group elements (PGE) mineralization is associated with anomalously high δ34S values of ∼8 to 12‰. In addition, several researchers have proposed that observed depletions in the Cu, Ni, and PGE content of basaltic lavas of the Nadezhdinsky (Nd) Formation are related to diffusional exchange with, and upgrading in metal tenor of, sulfides in the volcanic conduit system. Sulfur isotopic studies of the lavas at Noril’sk were initiated to determine if interaction with crustally derived sulfur in the conduit system was evident, and if the Nd lavas in particular were characterized by an anomalous isotopic signature. δ34S values of the lavas range from −4.5 to 8.7‰ Vienna Canon Diablo Troilite (VCDT), with S concentrations from <40 to 1373 ppm. The majority of δ34S values range from 0 to 4‰, and are similar to those from S-poor intrusions in the Noril’sk area. Although textural data are not supportive of early sulfide saturation and the presence of immiscible sulfide droplets in the lavas, recrystallization may have erased expected mineralogical and textural evidence. Mineralogical data indicate that hydrothermal alteration of the lavas has occurred, but S redistribution has been restricted to localized areas and δ34S values have not been affected. The relatively low S concentrations of the lavas are thought to be due in large part to degassing of the lavas in the shallow conduit system and during eruption. Our calculations are consistent with the premise that degassing of basaltic magmas at temperatures in excess of ∼900°C at QFM leads to only minor 34S-depletion of sulfur remaining in the melt, and decreases in δ34S values of less than 2‰ at 90% degassing. For this reason all lavas with δ34S values in excess of ∼ 2‰ require a contribution of 34S-enriched country rock sulfur. Because of the high S content and δ34S value (∼ 16–20‰) of evaporites in the country rocks at Noril’sk, contamination of less than 0.5% is required to explain the most 34S-enriched lavas. The Nd lavas have an average δ34S of 2.9‰, but show no difference in S isotopic composition relative to the other lavas, suggesting that metal depletion involved only limited S transfer, or that exchange between mantle-derived S and S of crustal origin buffered δ34S values to less than ∼5‰. Anomalously positive δ34S values, similar to those of the ore-bearing intrusives in the Noril’sk region, are not consistently found in low-S rocks, either lavas or intrusives. Although the mechanism for the derivation of sulfide in the ore-bearing intrusions remain speculative, it is clear that the formation of sulfide ores characterized by high metal tenors proceeded only in the presence of sulfur of crustal origin.

Geochronology of the Voisey's Bay intrusion, Labrador, Canada, by precise U–Pb dating of coexisting baddeleyite, zircon, and apatite

Abstract The emplacement history of the Voiseys Bay troctolite intrusion, that hosts the major Ni–Cu–Co sulfide deposit of the same name, has been studied using precise U–Pb geochronology of baddeleyite, zircon and apatite. The baddeleyite U–Pb ages of multiple drill core samples of troctolite and gabbro indicate that all of the mafic rocks studied from different components of the Voiseys Bay intrusion: Eastern Deeps, Discovery Hill Zone and Reid Brook Zone, and from the adjacent Red Dog area, were emplaced at 1332.7±1.0 Ma. On the basis of combined geological and geochronological evidence, it is suggested that the Voiseys Bay Ni–Cu–Co deposit was formed during the same period. The zircons coexisting with the ca. 1333 Ma baddeleyite show a diversity of ages. The zircons from normal troctolite and some of the olivine gabbro samples are coeval with the baddeleyite, while zircon from the varied textured troctolite and feeder olivine gabbro are much younger at 1305.0±0.8 Ma. The identical ages of the younger zircon population and the Voiseys Bay syenite that cuts the mafic rocks indicate a link between zircon growth in the mafic rocks and contact metamorphism, related to the emplacement of the syenite. Various mechanisms of zircon growth were probably involved, including reaction of the 1333 Ma baddeleyite with a silica-enriched fluid with formation of a secondary polycrystalline zircon, and zircon crystallization from syenite micro-veins in the mafic rocks. The mean 207 Pb / 206 Pb age of 1303.5±2.6 Ma of the Voiseys Bay apatites is similar to the age of the younger zircon population. The apatite age may either be a result of resetting the U–Pb system in response to the syenite intrusion, or may reflect the closure of the system during regional cooling and cessation of fluid circulation. The presence of xenocrystic zircon in a Discovery Hill Zone feeder olivine gabbro indicates that the Voiseys Bay magmas were contaminated with 1.90 Ga crustal rocks.

Olivine and sulfur isotopic compositions of the Uitkomst Ni–Cu sulfide ore-bearing complex, South Africa: evidence for sulfur contamination and multiple magma emplacements

The Uitkomst Complex in northern South Africa is host to a Ni–Cu deposit containing a minerable reserve of 2.9 Mt massive ore with grades of 2% Ni, 1% Cu and 6 ppm Pt+Pd and an indicated plus inferred resource of 98 Mt disseminated ore with grades of 0.6% Ni, 0.2% Cu and 1 ppm Pt+Pd. Based on similar age and composition to that of the Bushveld Complex, the Uitkomst Complex is believed by some investigators to be a satellite body of the Bushveld Complex. It has a tubular shape with an exposed surface area of about 0.8×8 km and a thickness of up to 1000 m, apparently exploiting the bedding planes of gently dipping footwall sediments that include shale, quartzite and dolomite. The Uikomst Complex comprised of stratiform mafic and ultramafic rock units that from base to top include gabbro, sulfide-mineralized harzburgite, unmineralized harzburgite, pyroxenite, gabbronorite and gabbro. The compositions of olivine from different rock units are significantly different. Olivines from the gabbronorite unit are highly fractionated, containing <30 mol% Fo (forsterite, Mg2SiO4) and <300 ppm Ni. Olivines from the underlying pyroxenite and harzburgite units are much more primitive, containing 86–91 mol% Fo. Abrupt changes in the contents of Ni in olivine occur within the pyroxenite unit, and between the mineralized and unmineralized harzburgite units. The contents of Ni in olivine from the unmineralized harzburgite unit are between 2900 and 3600 ppm. The contents of Ni in olivine from the underlying mineralized harzburgite units are ∼1500 ppm lower. Olivines from the lower part of the pyroxenite unit are similar to olivines from the underlying harzburgite unit, whereas olivines from the upper part of the pyroxenite unit contain much lower Ni (<600 ppm). Numerical modelling suggests that the strikingly different compositions of olivine are related to different parental magmas with different MgO/FeO ratios and/or Ni contents. The δ34S values of the basal gabbro and unmineralized rock units range from −0.9‰ to 2.6‰, which are similar to typical mantle values (∼0±3‰). In contrast, the δ34S values of the sulfide ores in the harzburgite units are significantly lower, varying between −2.6‰ and −7.1‰. The low δ34S values of the sulfide ores indicate addition of 34S-depleted crustal sulfur. The obvious sources of such crustal sulfur are the Malmani dolomite and the Timebal Hill shale that contain pyrite with negative δ34S values up to −11‰ and −18‰, respectively. Sulfide saturation in the magma of the mineralized harzburgite units is thought to have resulted from addition of the crustal sulfur. The resulting immiscible sulfide liquid droplets were then concentrated in the base of the magma channel. Some of the sulfide liquid was expelled into the basal gabbro that separated the active magma channel from the footwall quartzite. New magma then entered the channel, displacing most of the resident magma to form the unmineralized harzburgite. The overlying pyroxenite, gabbronorite and gabbro units formed either by in situ differentiation of the same magma giving rise to the underlying unmineralized harzburgite or by subsequent emplacement of a more evolved magma.

Petrogenesis of the Pt–Pd mineralized Jinbaoshan ultramafic intrusion in the Permian Emeishan Large Igneous Province, SW China

The Jinbaoshan ultramafic intrusion is a sheet-like body with a thick wehrlite unit in the center and thin pyroxenite units at the margins. PGE are enriched in several disseminated sulfide zones in the intrusion. Olivine from the intrusion has low Fo and depleted Ni contents compared to olivine from coeval Emeishan picrites. Whole rock major and trace element concentrations suggest that the Jinbaoshan wehrlites originally contained <30% trapped liquid. The total amount of sulfide in the rocks exceeds that which could have been dissolved in the trapped liquid. The Jinbaoshan wehrlites are interpreted to represent residual assemblages formed by dissolution of plagioclase by passing magma. No clear evidence of crustal contamination is indicated by S, Nd and Os isotopes. We envision that sulfide saturation occurred at depth due to olivine and chromite crystallization. Immiscible sulfide droplets were transported to the Jinbaoshan conduit where they accumulated and reacted with magma successively passing through the conduit to achieve high PGE concentrations.

Magmatic anhydrite-sulfide assemblages in the plumbing system of the Siberian Traps

We report the first discovery of magmatic anhydrite-sulfide assemblages in a subvolcanic intrusion associated with the Siberian Traps. The δ34S values of anhydrite and coexisting sulfide crystals analyzed by ion probe are 18‰–22‰ and 9‰–11‰, respectively. More than 50% of the total sulfur in the intrusion is estimated to derive from marine evaporites in the footwall strata. The contaminated magma was highly oxidized and able to dissolve up to one order of magnitude more sulfur than pure mantle-derived basaltic magma. Such contaminated magma, if erupted, would have released far more SO2 into the atmosphere than is generally appreciated.

Petrologic evolution of gneissic xenoliths in the Voisey's Bay Intrusion, Labrador, Canada: Mineralogy, reactions, partial melting, and mechanisms of mass transfer

Nickel-copper-cobalt sulfide ores in the Voiseys Bay Intrusion are closely associated with troctolitic to gabbroic rocks that contain abundant country rock xenoliths. Potential sources of the xenoliths include pelitic paragneiss, enderbitic orthogneiss, and mafic to quartzofeldspathic gneisses that form immediate country rocks to different parts of the intrusion. Regardless of location, all xenoliths have a similar refractory mineral assemblage composed of hercynite, magnetite, Ca-rich plagioclase, and corundum. The refractory mineral assemblage formed via partial melting immediately after xenoliths were engulfed by magma. Rapid thermal equilibration allowed the xenoliths to survive prolonged interaction with magma. Corundum was formed by the incongruent melting of Na-rich plagioclase in pelitic and quartzofeldspathic gneisses. Corundum and Ca-rich plagioclase assemblages are aluminous; their origin involved either multistage melting of protoliths where the production of a granitic minimum melt was followed by the liberation of a more silica-rich and Al-poor melt or one-stage disequilibrium melting. Density differences between the xenoliths, restite assemblages, and enclosing mafic magma facilitated the separation of partial melt from restite. No evidence for the melts in the form of channels or interstitial glass is observed in the restite. Flow of mafic magma in the conduit system is thought to have dispersed the Si- and alkali-rich melts derived from the xenoliths. The only record of the xenolith-derived melts is in the form of concentric rims of plagioclase and biotite which crystallized from a hybrid melt in boundary layers around most xenoliths. Hercynite in the restitic assemblage was produced either by partial melting involving Fe- and Mg-bearing minerals such as garnet and pyroxene or by replacement of corundum. FeO and MgO that were excluded from the boundary layer diffused inward and reacted with corundum to form hercynite. The thickness of the hercynite bands suggests formation times between 3,000 and 23,000 years. Where the insulating rims of plagioclase and biotite were not present, diffusion of FeO and MgO from the surrounding crystal mush continued, resulting in near complete conversion of corundum to hercynite. The highly refractory mineral assemblages which characterize xenoliths present in the Voiseys Bay Intrusion provide evidence for a complex history of magma–country rock interaction. The transfer of xenomelts and sulfur to flowing magma may have been essential for the formation of the magmatic sulfide ores.

Sr-Nd-Os-S isotope and PGE geochemistry of the Xiarihamu magmatic sulfide deposit in the Qinghai–Tibet plateau, China

The newly discovered Xiarihamu Ni-Cu deposit is located in the Eastern Kunlun orogenic belt in the northern part of the Qinghai–Tibet plateau, western China. It is the largest magmatic Ni-Cu sulfide deposit found thus far in an arc setting worldwide and ranks second in China in terms of total Ni resources. Fe-Ni-Cu sulfide mineralization occurs in a small ultramafic body that is part of a larger mafic-ultramafic complex formed by protracted Silurian-Early Devonian basaltic magmatism. The mineralized ultramafic body is composed predominantly of lherzolite and olivine websterite, with minor dunite, websterite and orthopyroxenite. Here we report new PGE (platinum group element) data and the results of a new, integrated Sr-Nd-Os-S isotope study. The initial concentrations of Rh and Pd in the parental magma are estimated to be 0.014 ppb and 0.24 ppb, respectively, which are more than one order of magnitude lower than those in undepleted mantle-derived magmas such as many continental picrites. The observed PGE depletions in the Xiarihamu parental magma are attributed to sulfide retention in the source mantle, because the degree of partial melting required to generate the Xiarihamu primary magma was not high enough for a magma of that composition to dissolve all sulfides in the source. The (87Sr/86Sr)i ratios and εNd (t) of the Xiarihamu host rocks range from 0.7062 to 0.7105 and from −1.97 to −5.74, respectively, indicating 5–30 wt% crustal contamination in the Xiarihamu magma. These data also reveal that the source mantle for the Xiarihamu magma is isotopically (Sr-Nd) more enriched than that for the average Cenozoic arc basalt. The γOs(t) and δ34S values of sulfide ores from the Xiarihamu deposit range from 78 to 1393 and from 2 to 6‰, respectively. These values clearly indicate addition of crustal Os and S to the Xiarihamu parental magma. Metal tenors such as Ni and Rh are inversely correlated with γOs(t) and δ34S values. This indicates that mixing between different pulses of magma with different isotope compositions took place during magma emplacement at Xiarihamu, and that external S played a critical role in the formation of the Xiarihamu magmatic Ni-Cu sulfide deposit.