Search

Using electron probe microanalyzer (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), we analyzed major and trace element compositions of iron oxides from Ni-Cu-PGE sulfide deposits hosted by mafic-ultramafic rocks in northern Fennoscandia, mostly focusing on Finland. The main research targets were the Archean Ruossakero Ni-(Cu) deposit; Tulppio dunite and related Ni-PGE mineralization; Hietaharju, Vaara, and Tainiovaara Ni-(Cu-PGE) deposits; and Paleoproterozoic Lomalampi PGE-(Ni-Cu) deposit. In addition, some reference samples from the Pechenga (Russia), Jinchuan (China), and Kevitsa (Finland) Ni-Cu-PGE sulfide deposits, and a barren komatiite sequence in the Kovero area (Finland) were studied. Magnetite and Cr-magnetite show a wide range of trace element compositions as a result of the variation of silicate and sulfide melt compositions and their post-magmatic modification history. Most importantly, the Ni content in oxide shows a positive correlation with the Ni tenor of the sulfide phase in equilibrium with magnetite, regardless of whether the sulfide assemblage is magmatic or post-magmatic in origin. The massive sulfide samples contain an oxide phase varying in composition from Cr-magnetite to magnetite, indicating that Cr-magnetite can crystallize directly from sulfide liquid. The Mg concentration of magnetites in massive sulfide samples is lowest among the samples analyzed, and this can be regarded as a diagnostic feature of an oxide phase crystallized together with primitive Fe-rich MSS (monosulfide solid solution). Our results show that magnetite geochemistry, plotted in appropriate discrimination diagrams, together with petrographical observations could be used as an indicator of potential Ni-(Cu-PGE) mineralization. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Re-Os isotopic data and isochron ages from Finnish komatiite-hosted Ni-Cu-PGE deposits. • Chromites show mostly a near-chondritic initial Os signature, indicating a mantle Os source. • Sulfides record open-system behavior and resetting during Svecofennian orogeny. Abstract We have studied the Re-Os isotope systematics of komatiite-related Ni-Cu-PGE sulfide deposits in Finland, including the Archean Ruossakero Ni-(Cu), Vaara Ni-(Cu-PGE), Tainiovaara Ni-(Cu-PGE) deposits and Ni-PGE sulfide mineralization related to a dunitic body at Tulppio, and the Paleoproterozoic Lomalampi PGE-(Ni-Cu) deposit. We analyzed chromite separates from both mineralized and non-mineralized komatiitic rocks, bulk-sulfide separates, and country-rock black schist from one deposit, and for comparison, chromite separates from non-mineralized Archean komatiitic rocks from the Kovero and Kuhmo greenstone belts. We have observed open-system behavior of sulfides in three deposits, with their Re-Os isotope compositions indicating resetting at ca. 1.83–1.87 Ga by metamorphic events related to the Svecofennian orogeny. Most of the chromite separates, especially those with >10 ppb Os, retain their primary isotope compositions with near-chondritic initial 187Os/188Os ratios, indicating that their Os was derived from a mantle source with a chondritic Re-Os isotope evolution. These include the Vaara deposit, which yielded a chromite Re-Os isochron age of 2719 ± 140 Ma and low initial γOs of −1.2 ± 0.02, in spite of the evidence for the presence of mass-independent S isotope fractionation in this deposit, suggesting a low-Os, sulfur-bearing contaminant. In the case of the Lomalampi deposit, chromites yield an isochron with an age of 2058 ± 93 Ma and initial γOs of +0.9 ± 0.02, consistent with earlier isotope data from Central Lapland komatiites. However, one chromite from a mineralized cumulate gave a γOs(2050 Ma) value of +24 ± 18, indicating a minor/moderate degree of crustal contamination involving material from country rock black schists. This is also supported by earlier S isotope data. In the Archean Tainiovaara deposit, chromite also suggests crustal contamination (γOs + 13 ± 1), but country rocks are Os-poor Archean gneisses, leaving the source of radiogenic Os unclear, though it is possible that contamination occurred at a greater depth. Our results also provide evidence that the Re-Os age of the Ruossakero deposit is ca. 2.9 Ga, with the deposit likely representing komatiitic magmatism that is older than the komatiitic magmatism elsewhere in Finland. Re-Os results also confirmed that the mineralized Tulppio Dunite is Archean in age. [ABSTRACT FROM AUTHOR]

The Hietaharju Ni–Cu–PGE deposit occurs in the Archean Suomussalmi greenstone belt of eastern Finland and is associated with high-Mg basalts and low-Mg komatiitic rocks. It represents a PGE-enriched type among komatiite-related Ni–Cu deposits. The Hietaharju deposit consists of several mineralized lenses in highly altered olivine–pyroxene cumulates (i.e., talc–carbonate–chlorite rocks) and contains disseminated, massive to semi-massive and brecciated sulfides. Post-magmatic processes have not significantly changed the primary magmatic sulfide assemblage (i.e., pyrrhotite–pentlandite–chalcopyrite), but has modified sulfide textures and possibly remobilized and reconcentrated the sulfides. Though the deposit is currently subeconomic, it has high PGE contents in the sulfide fraction, averaging 5.7 ppm Pd and 2 ppm Pt (average of all ore types), being highest in disseminated sulfides. The parental magma represents an intermediate type between Al-undepleted and Al-depleted komatiites and differs from most other komatiitic rocks in the Tipasjärvi–Kuhmo–Suomussalmi greenstone belt complex, which are normally of the Al-undepleted type. The relatively fractionated nature of the komatiitic basalt parental magma of the Hietaharju deposit is consistent with the low Ni/Cu (< 4) and Ni/Co (< 18) and high Pd/Ir (< 25) ratios of the sulfide fraction, similar to the values in the Paleoproterozoic Raglan belt deposits, Canada. Use of REE for constraining crustal contamination is hampered by the highly altered nature of komatiitic rocks in the Hietaharju area. Sulfides in the country rock metasediments that host the Hietaharju Ni–Cu–PGE deposit show clear mass-independent sulfur isotope fractionation (MIF-S) consistent with their age and published S isotope data from the Suomussalmi belt. In contrast, sulfides of the Hietaharju Ni–Cu–PGE deposit only possess a small degree of MIF-S, providing limited direct evidence for the role of local metasedimentary sulfur in ore formation. Archean base metal sulfide mineralization is among the potential sources of sulfur, however, and, as shown by the epithermal Taivaljärvi Au–Ag–Zn deposit, can have mass-independent sulfur isotope fractionation that is similar in magnitude and sign to that of the Hietaharju Ni–Cu–PGE deposit. [ABSTRACT FROM AUTHOR]

The Mesoarchean chromitites in the Singhbhum craton provide the main source of chromium in India. We report here results of dating zircon grains that were found in the chromitite of ultramafic rocks belonging to the southern part of the Archean Baula Complex. The chromite-bearing ultramafic rocks are cut by a gabbronorite intrusion, which had earlier been dated by SHRIMP at 3.12Ga. We have performed isotopic analysis of the zircon grains by Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) and obtained an age of 2.84Ga, which is inconsistent with the previous age reported for the gabbronorite. We have also determined the Pb isotopic composition of galena found in the chromitite and found out it was highly radiogenic given the Archean age of its host rock. We discuss the potential reasons for these results, such as later Neoarchean or even Proterozoic tectono-magmatic/hydrothermal events, and stress the importance of future detailed petrographic, mineralogical and isotopic studies to solve the dilemmas. [ABSTRACT FROM AUTHOR]

Archean komatiites host important resources of Ni, Cu, Co, and PGE, particularly in Western Australia and Canada. In Finland, several small, low-grade sulfide deposits have been found in komatiites, including the ca. 2.8 Ga Vaara deposit in the Archean Suomussalmi greenstone belt. It occurs in the central part of the serpentinized olivine cumulate zone of a komatiitic extrusive body and is composed of disseminated interstitial sulfides consisting of pyrite, pentlandite, millerite, violarite, and chalcopyrite accompanied by abundant magnetite. Although currently subeconomic, the mineralization is interesting due to the very high chalcophile element contents of the sulfide fraction (38 wt% Ni, 3.4 wt% Cu, 0.7 wt% Co, 22.4 ppm Pd, and 9.5 ppm Pt). The sulfides occur in relatively Cr-poor olivine cumulates suggesting involvement of a chromite-undersaturated magma. The parental magma was an Al-undepleted komatiite with an estimated MgO content of at least 24 wt%. In contrast to the common komatiite types in the eastern Finland greenstone belts, the Vaara rocks are moderately enriched in LREE relative to MREE, suggesting that crustal contamination played an important role in the genesis of the Vaara deposit. Multiple sulfur isotope data reveal considerable mass-independent sulfur isotope fractionation both in country rock sedimentary sulfides (Δ 33S ranges from −0.50 to +2.37 ‰) and in the Vaara mineralization (Δ 33S ranges from +0.53 to +0.66 ‰), which provides strong evidence for incorporation of crustal sulfur. Extensive replacement of interstitial sulfides by magnetite and the presence of millerite- and violarite-bearing, pyrrhotite-free sulfide assemblages indicate significant post-magmatic, low-temperature hydrothermal oxidation of the primary magmatic pyrrhotite-pentlandite-chalcopyrite assemblages and associated sulfur loss that led to a significant upgrading of the original metal tenors of the Vaara deposit. [ABSTRACT FROM AUTHOR]

Abstract: The Paleoproterozoic ca. 2.05Ga ultramafic volcanic rocks occurring in the Central Lapland Greenstone Belt are classified as Ti-enriched komatiites and picrites. Some of the rocks are high in Cr (ca. 4000ppm) and contain fresh chrome spinel grains hosting melt inclusions up to 50μm in size. These inclusions enabled us to study the compositional features of the magmas at the time of melt entrapment. Chrome spinels are divided into the low-Ti and high-Ti types reflecting crystallization from komatiitic and picritic parental magmas, respectively. There is a good correlation between the major element compositions of the melt inclusions and their host spinels, with picritic melts being found in high-Ti spinels and komatiitic melts in low-Ti spinels. MgO contents measured in the inclusions range between 10 and 22wt.%. Trace element analyses performed by LA-ICP-MS and SIMS revealed two contrasting magma compositions, which are either depleted or highly enriched in incompatible elements and produce convex-upwards or LREE-enriched chondrite-normalized REE patterns, respectively. Due to the shielding effect of the host spinel, melt inclusions provide information on the original mobile trace element contents (Li, K, Ba, Sr, Be) and LILE/HFSE ratios of the magmas. Interestingly, spinel grains carrying both depleted and enriched melt inclusions were discovered in a single komatiitic sample, demonstrating the coexistence and mixing of two different primitive magma types. The whole-rock and melt-inclusion geochemical data together with previous Nd and Os isotopic results indicate an asthenospheric origin of the magmas with insignificant crustal contamination or interaction with the subcontinental lithospheric mantle. We suggest that adiabatic upwelling of a Paleoproterozoic mantle plume was able to simultaneously produce komatiitic and picritic magmas from an originally depleted source via dynamic melting involving re-fertilization and re-melting processes and re-melting processes, though the details of these processes are difficult to constrain in a quantitative manner. These processes should be taken into account when using the geochemistry of Precambrian primitive magmas for assessing the temporal evolution of mantle plume reservoirs. [Copyright &y& Elsevier]

The large range of chemical variation within intimately associated highly magnesian volcanic rocks in the Palaeoproterozoic Central Lapland Greenstone Belt prompted the construction of a new classification scheme for MgO-rich volcanic rocks, based on an [Al2O3] vs [TiO2] diagram where the axes are the Al2O3 and TiO2 contents (in mole proportions) of the rocks projected from the olivine composition. This diagram places the Lapland rocks in the fields of Ti-enriched komatiites and picrites. Komatiitic rocks are depleted in both light and heavy rare earth elements (LREE and HREE) relative to middle REE (MREE) and possess relatively high TiO2 even in the most LREE-depleted varieties, whereas picritic rocks approach geochemically Hawaiian picrites. Seven clinopyroxene and whole-rock pairs analysed for Sm–Nd isotopes yield an average age of 2056 ± 25 Ma for the komatiites. Uncontaminated komatiites and picrites have similar positive εNd values (+4) indicating generation from a mantle source with a long-term depletion in LREE relative to MREE. Geochemical characteristics of the komatiite–picrite association, including REE and Nb/Y–Zr/Y systematics, indicate chemical heterogeneities in the source region, which seem to have been created by complex depletion and enrichment processes shortly before or related to a dynamic melting process. The high MgO contents of the rocks coupled with chemical similarity between the Lapland and Hawaiian picrites supports a mantle plume model for their genesis. Nevertheless, the geotectonic evolution appears to have proceeded without significant regional uplift shortly before volcanism. [ABSTRACT FROM PUBLISHER]

The Palaeoproterozoic Ni–Cu sulphide deposits of the Pechenga Complex, Kola Peninsula, occur in the lower parts of ferropicritic intrusions emplaced into the phyllitic and tuffaceous sedimentary unit of the Pilgujärvi Zone. The intrusive rocks are comagmatic with extrusive ferropicrites of the overlying volcanic formation. Massive lavas and chilled margins from layered flows and intrusions contain <3–7 ng/g Pd and Pt and <0·02–2·0 ng/g Ir, Os and Ru with low Pd/Ir ratios of 5–11. The abundances of platinum group elements (PGE) correlate with each other and with chalcophile elements such as Cu and Ni, and indicate a compatible behaviour during crystallization of the parental magma. Compared with the PGE-depleted central zones of differentiated flows (spinifex and clinopyroxene cumulate zones) the olivine cumulate zones at the base contain elevated PGE abundances up to 10 ng/g Pd and Pt. A similar pattern is displayed in intrusive bodies, such as the Kammikivi sill and the Pilgujärvi intrusion. The olivine cumulates at the base of these bodies contain massive and disseminated Ni–Cu-sulphides with up to 2 μg/g Pd and Pt, but the PGE concentrations in the overlying clinopyroxenites and gabbroic rocks are in many cases below the detection limits. The metal distribution observed in samples closely representing liquid compositions suggests that the parental magma became sulphide saturated during the emplacement and depleted in chalcophile and siderophile metals as a result of fractional segregation of sulphide liquids. Relative sulphide liquid–silicate melt partition coefficients decrease in the order of Ir > Rh > Os > Ru > Pt = Pd > Cu. R-factors (silicate-sulphide mass ratio) are high and of the order of 104–105, and they indicate the segregation of only small amounts of sulphide liquid in the parental ferropicritic magma. In differentiated flows and intrusions the sulphide liquids segregated and accumulated at the base of these bodies, but because of a low silicate–sulphide mass ratio the sulphide liquids had a low PGE tenor and Pt/Ir and Cu/Ir ratios similar to the parental silicate melts. During cooling the sulphide liquid crystallized 40–50% of monosulphide solid solution (mss) and the residual sulphide liquid became enriched in Cu, Pt and Pd and depleted in Ir, Os and Ru. The Cu-rich sulphide liquid locally assimilated components of the surrounding S-rich sediments as suggested by the radiogenic Os isotopic composition of some sulphide ores (γOs > 100). Most of the massive and breccia ores represent mixtures of mss and residual Cu-rich sulphide liquid whereas chalcopyrite-rich veins formed when the Cu-rich sulphide liquids were squeezed out into the surrounding sediments. [ABSTRACT FROM PUBLISHER]

Initiation of large igneous provinces (LIPs) and temporal and spatial variations in their chemical compositions provide unique opportunities to understand mantle dynamics. It has been recently revealed that water‐rich reservoirs in the mantle sources have played a significant role in the production of LIPs. However, the dominant causes for the chemical variations in primitive magmas of LIPs remain debated. In the Emeishan LIP (ELIP), there are several places where picrites accompany more abundant flood basalts. These picrites span a large range of Ti/Y, Sm/Yb, and TiO2/Al2O3. In this work, we have determined the water contents of picrites with Ti/Y > 500 from the ELIP, based on the water content of clinopyroxene phenocrysts and water partitioning coefficients. We have also calculated the water content of the mantle sources by applying batch and fractional partial melting models. We show that the chemical compositions of picrites in the ELIP closely correlate with the calculated water content in their mantle sources. Our model suggests that the variation in the chemical composition of the primary magmas of the ELIP both on a local and regional scale can be mainly explained by the complementary roles of relatively dry recycled pyroxenite and hydrated components in the mantle plume, without requiring catastrophic thinning of the lithospheric mantle. Our model thus implies that both the initiation and chemical variation of the ELIP are affected by water from the deep mantle. Plain Language Summary: Large igneous provinces (LIPs) represent a manifestation of the activity of a mantle plume. In most LIPs, the chemical compositions of basalts are heterogeneous and can usually be divided into high‐Ti and low‐Ti groups. This division has been used to infer differences in the mantle composition and/or partial melting conditions, which ultimately reflect the picture of the compositional and thermal structure of the deep mantle and their partial melting processes. In this study, we report that both the high‐Ti and low‐Ti type picrites of the Emeishan LIP were derived from hydrous mantle sources and such a compositional dichotomy is closely associated with the water content of the mantle sources. This case study provides a new perspective to connect the presence of volatiles in the deep mantle, the activity of deep‐seated mantle plumes, and widespread basaltic magmatism on the Earth's surface. Key Points: Both high‐Ti and low‐Ti picrites in the Emeishan large igneous province are hydrousThe major and trace element compositions of picrites are related to the water content in their mantle sourcesCompositional variations in a large igneous provinces are affected by volatiles in mantle plumes [ABSTRACT FROM AUTHOR]

Several komatiite-hosted Ni-Cu-PGE deposits occur in Archean and Paleoproterozoic Greenstone Belts in Finland. Some of these deposits are enriched in platinum-group elements, especially in Pd and Pt. The Lomalampi PGE-(Cu-Ni) deposit is associated with a peridotitic cumulate body of the Sattasvaara Formation in the Paleoproterozoic Central Lapland Greenstone Belt. The sulfides in the deposit occur in disseminated form. Whole rock sulfur contents are 0.4-2 wt%, and Ni contents are <0.5 wt% and Cu <0.4 wt%, while PGE contents exceed 500 ppb. The sulfides consist of magmatic pentlandite, pyrrhotite, and chalcopyrite and have not been substantially modified by metamorphic processes. Palladium minerals are associated with sulfides and silicates, but the only Pt-bearing phase, sperrylite, occurs mainly within silicates. The host rock of the deposit is a chromite undersaturated Al-undepleted high-Mg basalt or low-Mg komatiite. In contrast to most other komatiite-hosted Ni-Cu-PGE deposits world-wide that have Pt/Pd around 0.5, the Lomalampi deposit is enriched in Pt over Pd (Pt/Pd = 2). Only a weak contamination signal in the host-cumulate is evident in REE data, but a strong signal is evident in S-isotope ratios (δS + 10 ‰ to +15 ‰), which differ substantially from the mantle value (0 ± 2 ‰). Geochemical characteristics (e.g., PGE enrichment) and R-factor modeling indicate certain similarities between Lomalampi and the Raglan Ni-Cu-PGE deposits of Canada. The combined data suggest that Lomalampi formed through contamination of a PGE-rich magnesian magma with S rich country rocks. This suggests that the extensive Central Lapland Greenstone Belt is favorable for komatiite-hosted Ni-Cu-PGE deposits, which are substantially enriched in platinum and palladium. [ABSTRACT FROM AUTHOR]

Pillow lava rims and interpillow hyaloclastites from the upper part of the Pechenga Greenstone Belt, Kola Peninsula, N-Russia contain rare tubular textures 15-20 μm in diameter and up to several hundred μm long in prehnite-pumpellyite to lower greenschist facies meta-volcanic glass. The textures are septate with regular compartments 5-20 μm across and exhibit branching, stopping and no intersecting features. Synchrotron micro-energy dispersive X-ray was used to image elemental distributions; scanning transmission X-ray microscopy, Fe L-edge and C K-edge were used to identify iron and carbon speciation at interfaces between the tubular textures and the host rock. In situ U-Pb radiometric dating by LA-MC-ICP-MS (laser ablation multicollector inductively coupled plasma mass spectrometry) of titanite from pillow lavas yielded a metamorphic age of 1790 ± 89 Ma. Focused ion-beam milling combined with transmission electron microscopy was used to analyze the textures in three dimensions. Electron diffraction showed that the textures are mineralized by orientated pumpellyite. On the margins of the tubes, an interface between mica or chlorite and the pumpellyite shows evidence of dissolution reactions where the pumpellyite is replaced by mica/chlorite. A thin poorly crystalline Fe-phase, probably precipitated out of solution, occurs at the interface between pumpellyite and mica/chlorite. This sequence of phases leads to the hypothesis that the tubes were initially hollow, compartmentalized structures in volcanic glass that were mineralized by pumpellyite during low-grade metamorphism. Later, a Fe-bearing fluid mineralized the compartments between the pumpellyite and lastly the pumpellyite was partially dissolved and replaced by chlorite during greenschist metamorphism. The most plausible origin for a septate-tubular texture is a progressive etching of the host matrix by several generations of microbes and subsequently these tubes were filled by authigenic mineral precipitates. This preserves the textures in the rock record over geological time. The micro textures reported here thus represent a pumpellyite-mineralized trace fossil that records a Paleoproterozoic sub-seafloor biosphere. [ABSTRACT FROM AUTHOR]

Abstract: New in-situ trace element and hydrogen isotopic data of hydromagmatic amphiboles from ferropicritic gabbro–wehrlite intrusions and lava flows of the Palaeoproterozoic Pechenga Greenstone Belt (Russia) provide insights into the controversial source of volatiles that are implicated in Precambrian ultramafic magmatism. The intrusions mainly contain oikocrystic kaersutites and pargasites, whereas lava flows generally include idiomorphic Fe-kaersutites and Fe-pargasites. All orthomagmatic amphiboles display slightly LREE-enriched chondrite-normalised patterns. Primitive mantle-normalised patterns of amphiboles from the intrusions display positive Nb–Ta, moderate negative Th–U and distinctive negative Sr–Li anomalies. Conversely, Fe-kaersutites and Fe-pargasites from the lava flows display undepleted Th, U, Sr and Li signatures. Boron and Cl enrichment is primarily observed in secondary edenites, which locally rim orthomagmatic kaersutites. In-situ δD measurements of hydromagmatic amphiboles from the gabbro–wehrlite intrusions display values ranging between −70‰ and −170‰, and total water contents between 1 and 3 wt.%. Conversely, primary hydromagmatic amphiboles from the Pechenga flows display lighter δD compositions ranging between −100‰ and −270‰, and lower (<2 wt.%) total water contents. The Ti– and Nb–Ta-enriched nature of hydromagmatic amphiboles from the intrusions and lava flows indicates that the source of volatiles in the Pechenga ferropicrites could be either a metasomatised subcontinental mantle or a volatile-bearing plume. Positive Nb–Ta/La anomalies in amphiboles from the intrusions rule out crustal contamination with granitoid crust during petrogenesis and emplacement as the main source of the volatiles. Local B–Cl enrichment suggests that some samples may have interacted with hydrothermal fluids. The range of in-situ δD values in hydromagmatic amphiboles is consistent with the presence of magmatic water. The lighter hydrogen isotopic composition and lower water contents of the primary igneous amphiboles from the lava flows compared to amphiboles from the intrusions indicate that the lava flows degassed upon emplacement. Previous studies investigating the role of volatiles in Precambrian ultramafic magmatism preferentially collected data from thin lava flows, whereas thicker flows or intrusions have been generally neglected. We emphasise the necessity to re-evaluate the controversial role of volatiles in the petrogenesis of ultramafic magmas by investigating evidence from thicker flows or intrusions, which may have retained their full volatile budget. [Copyright &y& Elsevier]

Abstract: The abundance of platinum group alloy (PGA) grains available from some alluvial deposits combined with their great chemical resistance to conventional acid attack generates the need for an alternative method of obtaining routine, rapid yet precise Os isotopic data. Laser ablation multi-collector ICP-MS (LA-MC-ICP-MS) is ideally suited for this purpose and has been applied, in a relatively limited extent, to Os-rich samples by previous workers (e.g., Hirata, T., Michinari Hattori, M., and Tsuyoshi Tanaka, T., 1998. In-situ osmium isotope ratio analyses of iridosmines by laser ablation-multiple collector-inductively coupled plasma mass spectrometry. Chem Geol., 144, 269–280.; Malitch, K.N., 2004. Osmium isotope constraints on contrasting sources and prolonged melting in the Proterozoic upper mantle: evidence from ophiolitic Ru–Os sulfides and Ru–Os–Ir alloys. Chem Geol. 208. 157–173.; Walker, R.J., Brandon, A.D., Bird, J.M., Piccolia, P.M., McDonougha, W.F., R.D., Asha, R.D., 2005. 187Os–186Os systematics of Os–Ir–Ru alloy grains from southwestern Oregon. Earth Planet. Sci. Lett. 230, 211–226.). The wide variety of PGA minerals yield a broad spectrum of Pt, Re and Os contents which translate into a wide spread in parent–daughter ratios that is an attractive proposition for geochronology using the Re–Os and Pt–Os decay schemes. Here we show that single PGA grains, can contain sufficient internal Re, Pt and Os elemental variability and coupled variations in parent/daughter and daughter isotope compositions that they present an opportunity to obtain ‘isochron’ ages from two different isotope decay systems. We present a rapid (40 s acquisition time), precise and accurate LA-MC-ICP-MS methodology suitable for applying Pt–Os and Re–Os geochronology approaches to single PGA grains, for use in dating chromitite deposits and identifying and dating multiple sources in alluvial PGA deposits. Because of isobaric interferences from Pt, PGA analyses by LA-MC-ICP-MS are best corrected for instrumental mass fractionation using the 189Os/188Os ratio. We demonstrate that, within our analytical uncertainties, mass bias effects for Os isotope ratios during LA-MC-ICP-MS are similar in nature and magnitude to mass bias variations observed during normal solution-mode analysis. While robust elemental interference corrections can be applied for Re and W on the Os mass spectrum during solution analyses (Nowell, G.M., Luguet, A., Pearson, D.A., Horstwood, M.S.A., 2008-this volume. Precise and accurate 186Os/188Os and 187Os/188Os measurements by Multi-Collector Plasma Ionisation Mass Spectrometry (MC-ICP-MS) part I: solution analyses. Chem Geol.) we show that the accurate correction for the Re interference on Os during laser ablation is problematic at 187Re/188Os ratios >∼0.5 which means that Re-corrected 187Os/188Os ratios for some PGAs and high Re/Os sulfides must be regarded as problematic. Our interfering element correction approach can also be used for the correction for Os on 190Pt for obtaining 190Pt/188Os ratios, removing the need for an independent Pt monitor isotope. Accurate and precise geochronology by LA-MC-ICP-MS can be further limited by poorly quantified fractionation of the parent/daughter elemental ratio during ablation. Although additional work is required to understand the nature and exact extent of inter-element fractionation of the Pt/Os and Re/Os ratios during laser ablation analysis of Pt- and Re-rich PGA grains, our best estimate is that fractionation of the Pt/Os during laser ablation is around 5% or less. We present multi-grain isochrons and single-grain internal ‘isochrons’ for the Re–Os and Pt–Os isotope systems in PGA grains from Lapland. In all cases, the Pt–Os isotope system gives significantly more precise ‘isochrons’ than the Re–Os system. We interpret this discordance between the two systems as being most likely due to problems inherent with Re interference corrections at elevated Re/Os ratios. We show that a suite of Pt-rich PGA grains from the Central Lapland greenstone belt yields a spectrum of internal Pt–Os isochron ages that coincides well with pulses of formation of chromitite-bearing ophiolitic material within this magmatic terrane and we take this to indicate the record of chromitite formation in different magmatic units. The single-grain Pt–Os and Re–Os geochronology technique shows great promise in the dating of chromitite deposits in ophiolites and in the discrimination of multiple source regions within alluvial PGA deposits. [Copyright &y& Elsevier]