Cumulus Olivine Research Articles

Geochemical Thermometry of Ore-Bearing Gabbronorites from the Apophisis of the Yoko-Dovyren Massif: Composition, Amount of Olivine, and Conditions of Sulphide Saturation in the Parental Magma

The temperature and compositional parameters of the parental magma of the ore-bearing apophysis DV10 from the Yoko-Dovyren massif are estimated based on the results of thermodynamic modeling the equilibrium crystallization of melts of 24 samples, following the method of geochemical thermometry. Thermometric calculations were carried out using the COMAGMAT-5.3 program with a step of 0.5 mol. % to a maximum degree of crystallization 75–85% under oxygen fugacity controlled by the QFM buffer. The model order of mineral crystallization corresponds to the sequence: olivine (Ol) + + Cr-Al spinel (Spl) → plagioclase (Pl) → high-Ca pyroxene (Cpx) → orthopyroxene (Opx). Silicate-sulfide immiscibility was modeled to occur mostlybefore the onset of plagioclase crystallization, being consistent with sulfide saturation of the parental magma. The results of calculations demonstrate the convergence and intersection of the model liquid lines of descent at temperatures of about 1185°C. When applied to the average composition of the DV10 apophysis, this temperature indicates the existence of a suspension of the original crystals (52.1 wt. % cumulus olivine (Fo83.6), 2.3 wt. % plagioclase (An79.7), 0.24% clinopyroxene (Mg# 88.8), 1 wt. % aluminochromite (Cr# 0.62)) and about 0.2% sulfide liquid in a moderately magnesian melt (53.6 wt. % SiO2, 7.4 wt. % MgO). At that the solubility of sulfide sulfur (SCSS) was estimated to be 0.083 wt. %. This heterogeneous system had a viscosity of 4.71 log. units (Pa · s) and an integral density of 2929 kg/m3. Such rheological properties do not contradict the possibility of the migration and emplacement of the protocumulus mush from the main Dovyren chamber. However, a more probable scenario includes a localized accumulation of olivine in the trough-like part of the DV10 subchamber, which preceded or occurred in parallel to the accumulation of segregated sulfides.

Crustal Growth Identified by High-δ18O Zircon and Olivine: A Perspective from Ultramafic Arc Cumulates in Southern Tibet

Abstract In recent studies of crustal growth using global zircon Hf–O isotopic datasets, high-δ18O zircons are typically attributed to intra-crustal reworking during which very little juvenile mantle-derived magmas were added to the crust. Although arc magmas may originate from a high-δ18O mantle wedge, it has been difficult to decipher the contribution of high-δ18O mantle to zircon-saturated felsic magma due to superimposed intra-crustal processes. We address this issue by combining the data from high-δ18O zircon-bearing ultramafic cumulates and coeval lavas from a Cretaceous magmatic arc in southern Tibet. The cumulates mainly consist of different proportions of cumulus olivine and intercumulus amphibole. Amphibole analyses show a transition from increasing to decreasing Zr with increasing SiO2 (50–74 wt.%) contents in the intercumulus melts, indicating zircon saturation in late-stage interstitial melts. The εNd(t) values (2.4 ± 1.4) of the apatite grains crystallized before and after zircon remain almost constant. Interstitial zircons have δ18O (6.1–7.2‰) values similar to the earliest crystallized olivine (δ18O = 6.3–7.1‰) in the cumulates. The coeval lavas may represent the intercumulus melts extracted from amphibole-rich cumulates at different depths. Both the lavas and cumulates were ultimately derived from high-δ18O arc mantle modified by small amounts (<12%) of subducted sediments, and crystallized zircon during intra-crustal magma evolution without involving crustal contamination or melting. These high-δ18O zircons therefore are not products of crustal reworking, but record crustal growth during their crystallization (110 ± 2 Ma). Our study shows that the combination of zircon and olivine oxygen isotopes for ultramafic to felsic rocks is more effective than zircon data alone in evaluating the role of crustal growth vs. reworking in an arc system. The implication is that global zircon-based crustal evolution models that attribute all high-δ18O zircons to crustal reworking may conceal recent crustal growth.

Loveringite from the Khamal Layered Mafic Intrusion: The First Occurrence in the Arabian Shield, Northwest Saudi Arabia

Loveringite, a rare member of the crichtonite group with nominal formula (Ca,Ce)(Ti,Fe,Cr,Mg)21O38, was found in the Khamal layered mafic intrusion, the first known locality for this mineral in the Arabian Shield. The Khamal intrusion, a large post-collisional mafic complex, is lithologically zoned, bottom to top, from olivine gabbro through gabbronorite, hornblende gabbro, anorthosite, and diorite to quartz diorite. Loveringite is found near the base of the complex, as an intercumulus phase in olivine gabbro. Most loveringite grains are homogeneous, although a few grains are zoned from cores rich in TiO2, Al2O3, Cr2O3, and CaO towards rims rich in FeO*, ZrO2, V2O3, Y2O3, and rare earth elements (REE). Petrographic relations indicate that loveringite formed after crystallization of cumulus olivine, pyroxenes, and plagioclase. Anhedral and corroded crystals of loveringite are surrounded by reaction rims of Mn-bearing ilmenite and baddeleyite, suggesting that the residual liquid evolved into and subsequently out of the stability field of loveringite. The budget of incompatible elements (Zr, Hf, REE, U, and Th) hosted in loveringite is anomalous for a primitive mafic liquid. Saturation in loveringite is likely the result of early contamination of the primary melt by anatexis of country rock, followed by isolation of evolving liquid in intercumulus space that restricted communication with the overlying magma chamber. The zoned crystals likely reflect diffusive equilibration between residual loveringite grains and their reaction rims of ilmenite.

Coupled Li-P Zoning and Trace Elements of Olivine from Magmatic Ni-Cu Deposits: Implications for Postcumulus Re-Equilibration in Olivine

Abstract Olivine is an important mineral in mafic–ultramafic rocks and records various crustal and mantle processes in basaltic magma. Generally, phosphorus (P) is immobile in olivine and its zoning tends to record magmatic processes. In contrast, lithium (Li) is one of the most mobile elements in olivine and tends to be re-distributed by postcumulus to hydrothermal processes. This study reports coupled Li-P oscillatory and sector zoning in cumulus olivine (Fo > 80 mol%) from magmatic Ni-Cu deposits in northwest China. The close to 1:1 relationship between Li + Na cation and P cation in olivine suggests P substitutes for slowly diffusing Si at the tetrahedral site and Li (Na) substitutes for Mg at the octahedral metal site. The preservation of such coupled Li-P zoning suggests that the charge-balanced–controlled substitution of Li+P5+ for Mg2+Si4+, producing a member of the Li(Fe)PO4 structure, may cause sluggish Li diffusion in olivine. The Li-P zoning is uncorrelated to other elements (Fe, Mg, Cr, Al, Ca, Ti, V, Ni, Zn, Mn, Co, and Sc). The Ca abundances in cumulus olivine are significantly depleted relative to those in volcanic olivine and modeled for olivine using the rhyolite–MELTS program, whereas the Mg, Fe, Mn, Zn, Ni, and Co contents show no signs of depletion. The Cr and Al contents in high Fo olivine are lower than those in volcanic olivine with a similar Fo value. The depletions in Ca, Cr, and Al (both divalent and trivalent cations) have been attributed to post-crystallization re-equilibration processes. We suggest that the selective Ca-Cr depletion in olivine resulted from re-equilibration between olivine and clinopyroxene–orthopyroxene–spinel–melts at the postcumulus stage, whereas the Al content variation in olivine is likely controlled by olivine–spinel (pyroxene) re-equilibration. Olivine fractionation modeling results reveal that Co content in both olivine and sulfides changes slightly during evolution, whereas the olivine Ni/Co ratio decreases dramatically from 30 (at Fo90) to 4 (Fo80). These are consistent with the variation of Ni and Co contents in olivine from sulfide-barren rocks but cannot explain the strong positive Ni-Co correlation observed in olivine from the sulfide-bearing rocks. The considerable increase in the Ni/Co ratio in olivine coexisting with the high Ni tenor sulfide compared with the slight increase of the Ni/Co ratio in olivine coexisting with moderate-low Ni tenor sulfide strongly suggests that the Ni and Co contents and Ni/Co ratio in olivine from the mineralized rocks were the results of olivine–sulfide interaction. The olivine Fe/Zn and Mn/Zn ratios show little difference between sulfide-barren and sulfide-rich rocks, but these ratios decrease considerably with the decreasing Fo values. Overall, the study suggests that elements (Ca, Cr, Al, Mg, Fe, Ni, Co, Zn, etc.) diffuse faster than P have been re-distributed in cumulus olivine by re-equilibration processes, modifying the elemental content and inter-element ratios in olivine. The characteristics of the crystallization, particularly the source recorded in cumulus olivine, may be obscured and overprinted by postcumulus processes.

Mantle and Crustal Contributions to the Mount Girnar Alkaline Plutonic Complex and the Circum-Girnar Mafic-Silicic Intrusions of Saurashtra, Northwestern Deccan Traps

ABSTRACT Continental flood basalt (CFB) provinces, while dominated by tholeiitic basalts and basaltic andesites, often also contain alkaline mafic to felsic lavas and intrusions. The tholeiitic and alkaline magmas may reflect different degrees of partial melting of the same mantle source, or the alkaline magmas may be derived from metasomatised, incompatible element-enriched mantle sources. The tholeiitic and alkaline suites, even if closely associated spatially or temporally, require independent magmatic plumbing systems. In the Saurashtra region of the northwestern Deccan Traps CFB province, India, tholeiitic lavas have been intruded by the ~66 Ma Mount Girnar plutonic complex, which comprises olivine gabbros (often with cumulate textures), diorites, and monzonites, profusely intruded by dykes and veins of foid-bearing syenites and lamprophyres. In the region surrounding the complex the tholeiitic lavas have been intruded by a large (12 km-diameter) silicic ring dyke, as well as tholeiitic dykes and sills. The region thus provides an excellent opportunity to study potential petrogenetic relationships between tholeiitic, alkaline, and silicic magmatism in a CFB province, evaluated here using field, petrographic, mineral chemical, and whole-rock geochemical (including Sr–Nd–Pb isotopic) data. Initial (at 65 Ma) Sr–Nd–Pb isotopic ratios of an olivine gabbro and diorites of the Girnar plutonic suite are in the ranges (87Sr/86Sr)t = 0.70499 to 0.70584, (143Nd/144Nd)t = 0.512675 to 0.512484 (εNdt = +2.4 to −1.4) and (206Pb/204Pb)t = 18.270–18.679. Foid-bearing syenites and lamprophyres have broadly similar isotopic ratios and marked enrichments in the most incompatible elements. Thermobarometric calculations indicate crystallisation of mineral phases in the Girnar plutonic suite at varied crustal pressures (0.02–0.9 GPa). Small but significant Sr–Nd–Pb isotopic variations within the plutonic suite rule out closed-system fractional crystallisation as a viable process, whereas a lack of correlation between isotopic ratio and degree of magmatic evolution (rock type) also negates any simple scheme of combined assimilation–fractional crystallisation. The circum-Girnar tholeiitic intrusions, hitherto practically unstudied, are low-Ti and moderately to fairly evolved (MgO = 8.0–3.9 wt %); olivine gabbro and picrite dykes with cumulus olivine show higher MgO (10.1–15.7 wt %), Ni (360–700 ppm), and Cr (410–1710 ppm) contents. The circum-Girnar tholeiitic intrusions have a large range of Sr–Nd–Pb isotopic ratios (e.g. εNdt = +4.2 to −18.7) indicating open-system processes. We infer that magmas of the alkaline Girnar plutonic suite were derived from enriched mantle, with only minor crustal residence or material input, possibly reflecting a very thin basement crust under the complex. In contrast, magmas forming the circum-Girnar tholeiitic intrusions were derived from depleted mantle (εNdt > +4.2) by high degrees of melting, and they experienced olivine fractionation or accumulation in crustal chambers and significant contamination by ancient granitic basement crust. These features probably reflect a much thicker crust surrounding the plutonic complex than directly under it. The circum-Girnar silicic ring dyke has Sr–Nd–Pb isotopic ratios suggesting an origin by anatexis of the basement crust. Based on a range of evidence, the tholeiitic and silicic circum-Girnar dykes and sills are petrogenetically and structurally unrelated to the alkaline Girnar plutonic suite.

Nova-Bollinger Ni-Cu Sulfide Ore Deposits, Fraser Zone, Western Australia: Petrogenesis of the Host Intrusions

Abstract The Nova-Bollinger Ni-Cu sulfide ore deposit is the first economic Ni-Cu-Co sulfide deposit to have been discovered in the Albany-Fraser orogen in Western Australia. The host rocks are mafic-ultramafic intrusive cumulates subdivided into two connected intrusions, designated Upper and Lower. The Upper Intrusion is bowl-shaped and modally layered with alternating peridotite and norite mesocumulate layers, with a Basal Series of dominantly orthocumulate mafic lithologies. The Lower Intrusion is a much thinner, semiconformable chonolith (flattened tube-shaped intrusion) consisting of mostly unlayered mafic to ultramafic orthocumulates. The Lower Intrusion hosts all the high-grade mineralization and most of the disseminated ores. A distinctive plagioclase-bearing lherzolite containing both orthopyroxene and olivine as cumulus phases is a characteristic of the Lower Intrusion and the Basal Series of the Upper. The intrusions differ slightly in olivine and spinel chemistry, the differences being largely attributable to the more orthocumulate character of the Lower Intrusion. Sector zoning in Cr content of pyroxenes is observed in the Lower Intrusion and in the lower marginal zone of the Upper and is attributed to crystallization under supercooled conditions. Symplectite pyroxene-spinel-amphibole coronas at olivine-plagioclase contacts are ubiquitous and are attributed to near-solidus peritectic reaction between olivine, plagioclase, and liquid during and after high-pressure emplacement, consistent with high Al contents in igneous pyroxenes and estimates of the peak regional metamorphism. Original cumulus olivines had compositions around Fo86 and were variably Ni depleted, interpreted as the result of preintrusion equilibration with sulfide liquid. The Upper and Lower Intrusion rocks represent cumulates from a similar parental magma, a high-Al tholeiite with MgO between 10 and 12%, low TiO2 (0.5–0.6%), and high Al2O3 (14–17%). Modeling using alphaMELTS indicates a primary water content of around 2 wt %. The cumulates of both intrusions were derived via multiple magma pulses of liquid-olivine-sulfide slurries with variable amounts of orthopyroxene emplaced into the deep crust at pressures of around 0.7 GPa during the peak of regional metamorphism. The intrusions developed initially as a bifurcating sill, the lower arm developing into the ore-bearing Lower Intrusion chonolith and the upper arm inflating into the cyclically layered Upper Intrusion.

Ultramagnesian Olivine in the Monchepluton (Fo96) and Pados-Tundra (Fo93) Layered Intrusions (Kola Peninsula)

Abstract —The paper focuses on compositional variations of olivine and chromian spinel in the Monchepluton and Pados-Tundra layered intrusions, which host significant chromitite mineralization. Ore-bearing dunite (with up to 25–30 vol.% Mcr) in the Sopcheozerskoe chromite deposit from the Monchepluton complex, Kola Peninsula, Russia, bears an assemblage of phases with exceptionally high magnesium contents: Fo96 + augite (Mg# = 94) + magnesiochromite, Mcr (Mg# ≈ 65); Mg# = 100·Mg/(Mg + Fe2+ + Mn). However, olivine in the host dunite has normal maximum values of Mg# comparable to those in cumulus olivine from layered intrusions worldwide (Fo≤91–92). The Fo96 phase in the Sopcheozerskoe deposit shows the most primitive composition ever reported from any layered intrusion. Magnesiochromite occurs as unzoned homogeneous euhedral crystals unaffected by subsolidus exchange or metasomatic effects. Olivine in ore-bearing dunite (20–25 vol.% magnesian chromite) from the Pados-Tundra complex attains Fo93, with the Mg# value notably higher than the range (Fo85.5–90.6) in olivine from orthopyroxenite, harzburgite, and dunite within the intrusion. Olivine and chromian spinel in the two complexes behave coherently, with covarying patterns of Mg# and Ni contents in olivine at R = 0.75 (n = 160) and positive correlation between Mg# in coexisting chromian spinel and olivine grains at R = 0.8 (n = 150). This behavior indicates that the two phases attained equilibrium during crystallization. It appears unlikely that the extremely high Mg enrichment in olivine (Fo96), as well as in all associated phases of the Monchepluton complex, would result from a subsolidus reaction between olivine and chromian spinel or low-temperature alteration of olivine. We suggest a more realistic explanation that the olivine (+ high-Mg augite)–chromian spinel assemblage crystallized from komatiitic magma under the conditions of progressively increasing oxygen fugacity (fO2). The high Mg# in the Mcr-chromite-enriched system, above the maximum values common in cumulus olivine from layered intrusions (up to Fo96 against Fo≤91–92), may be caused by shortage of ferrous iron.

Compositional variations, thermometry, and probable parental magmas of Archean chromite from the Sargur greenstone belt, Western Dharwar Craton (India)

The chromite and magnesite deposits from Talur, Sindhuvalli, and Doddakanya in the layered ultramafic-mafic plutonic sequence are a part of the Archean Sargur greenstone belt within the Western Dharwar Craton of the Indian shield. The ultramafic bodies in these three areas are highly sheared and comprise of serpentinized dunite with chromitite bodies along with harzburgite and pyroxenite. Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~ 50 vol% of chromite) and serpentinite (~ 2 vol% of chromite) throughout the entire ultramafic sequence. In Talur, the chromites in massive chromitites (~ 60–70 vol% of chromite) are characterized by relatively lower Mg# (0.28–0.39) compared to chromites from massive chromitites in Sindhuvalli (Mg# = 0.63–0.69). The accessory chromite in dunite from Talur shows higher Cr#, Fe2O3, TiO2, and MnO content than other localities. The range of Fo content of cumulus olivine in dunites from Talur (Fo91-92) is relatively lower than the dunite from Doddakanya (Fo92-93) and net-textured chromitite from Sindhuvalli (Fo95-96). The range of Mg# in the intercumulus clinopyroxene in Talur net-textured chromitite varies between 94 and 99, whereas, in the Sindhuvalli net-textured chromitite, the intercumulus clinopyroxene shows a lower range of Mg# (= 96–97). Chromites in massive and silicate-rich chromitites, and from dunite contain several rounded to subhedral inclusions of silicate minerals of various sizes (~ 4–20 μm). Spherical inclusions of olivine (Fo94) are present in accessory chromites of dunite (Fo87). The Mg-rich clinopyroxene inclusions have Cr2O3 content from 1.5 to 2.3 wt%. The clinopyroxene inclusions in the Talur schlieren-banded chromitite are more magnesian (Mg# = 95–98) compared to the inclusions in chromites of the Sindhuvalli net-textured chromitite (Mg# = 96–97). The euhedral phlogopite inclusions in chromites from the Sindhuvalli net-textured chromitite are Mg-rich (~ 31.09 wt% MgO) with a significant amount of K2O (~ 9.28 wt%). The chlorite inclusion in chromites from the Talur schlieren-banded chromitite is also Mg-rich (~ 39.32 wt% MgO) with Al2O3 ~ 21.01 wt% and a significant amount of Cr2O3 (~ 3.51 wt%). Clinopyroxene (along with phlogopite) inclusions always occur as independent grain testifying their primary nature and might be genetically linked with the chromite crystallization from a primary high-Mg silicate magma. Chlorite and dolomite inclusions may be related to secondary hydrothermal alteration events. The average re-equilibration temperature of the olivine-chromite pairs in the Sindhuvalli net-textured chromitite and Doddaknaya dunite of around 545 °C represents the first stage of metamorphism under reducing conditions. Subsequently, the second hydrothermal episode involved fluids evolving to an average temperature of about 327 °C calculated from the accessory chromite and olivine pairs from Talur dunite, consistent with the abundance of magnesite veins throughout the Talur mines. The computed values of Al2O3, TiO2, and FeO/MgO ratio of the melt show that the parental melt associated with the chromitites in the Sargur greenstone belt has a close resemblance with the parental melt of the chromitites in the Nuggihalli schist belt, which is low-Al komatiitic. In the tectonic discrimination diagrams, most of the primary chromite compositions from Sargur plot within the field of chromites from komatiites from the Archean greenstone belts. Parental komatiitic magma fractionated in the subvolcanic conduits within the thin lithospheric crust and formed the sill-like layered ultramafic-mafic bodies with chromite deposits. These conduits fed the overlying extrusive system that developed the ultramafic-mafic volcanic sequence of the Sargur greenstone belt in the Western Dharwar Craton.

Differentiated Mafic–Ultramafic Intrusions of the Zubovsky Type in the Norilsk Area: Petrochemistry, Geochemistry, and Ore Potential

Intrusions of the Zubovsky type (1) are commonly dominated by troctolite in their ultramafic parts and have an insignificant number (if any) of ultramafic units; (2) occur among olivine and olivine-bearing gabbro-dolerites and often show evidence of a number of accumulation of cumulus olivine, (3) have weakly pronounced (if any) taxitic gabbrodolerite units; (4) include widespread leucocratic hybrid-metasomatic rocks and hybrid quartz- and cordierite-bearing norites and gabbronorites at contacts with the host aluminosilicate rocks; and (5) generally have a low ore content, with the prevalence of disseminated ore units among troctolites and ultramafic rocks, and almost complete absence of massive sulfide ores. The paper is the first to present data on the composition and Nd–Sr–Cu–S isotope-geochemical characteristics of rocks and sulfide ores from the Zubovsky-type intrusions in the Norilsk area. The silicate material of the Zub-Marksheider and Vologochan intrusions is characterized by similar Nd–Sr isotopic parameters: eNd = –1.2 ± 0.5, (87Sr/86Sri) = 0.70685 ± 0.00127, and eNd = 1.2 ± 0.8, (87Sr/86Sri) = 0.70634 ± 0.00068, respectively. These parameters are close to those of intrusions accompanied by economic ore mineralization in the Norilsk area. The whole-rock samples generally have a more ‘radiogenic’ Nd isotopic composition than that of the pyroxene and plagioclase and less ‘radiogenic’ than that of the olivine. The highest eNd value (ca. 5.3) of the olivine indicates that depleted mantle (DM) material was involved in the magma-generating processes. Copper–sulfur isotope data for sulfide ores of the Zub-Marksheider and Vologochan intrusions are remarkably different: δ34S = 1.3 ± 2.3‰, δ65Cu = –0.1 ± 0.15‰ and δ34S = 7.0 ± 1.2‰, δ65Cu = –0.7 ± 0.3‰, respectively. The Cu isotope composition of the sulfide mineralization of both intrusions is close to that of massive and disseminated sulfide ores from the Talnakh deposit in the Norilsk area. The sulfur isotope data suggest that the host rocks had little effect on sulfide mineralization. The identified Nd–Sr–S–Cu isotope-geochemical parameters of the rocks and ores are an important indicator of the potential for rich PGE–Cu–Ni sulfide ores.

Magmatic structure and petrology of the Vermelho Complex, Carajás Mineral Province, Brazil: Evidence for magmatic processes at the lower portion of a mafic-ultramafic intrusion

Magmatic structure and petrology of the Vermelho Complex, Carajás Mineral Province, Brazil: Evidence for magmatic processes at the lower portion of a mafic-ultramafic intrusion

Formation of banded chromitites and associated sulphides in the Neoproterozoic subarc deep-crustal magma inferred from the Alaskan-type complex, Egypt

The concentrically zoned Dahanib Alaskan-type complex in the Southern Eastern Desert (SED) of Egypt contains cumulate dunites associated with banded chromitites in the core, through harzburgites, wehrlites, and gabbroic rocks to the rim. Primary mineral inclusions hosted in chromian spinels of the banded chromitites are usually pargasite, orthopyroxene (Opx), aspidolite, phlogopite, edenite, olivine and PGE-bearing sulphides. The chromian spinel in Dahanib chromitites has a narrow range of Cr# (0.53–0.63) along with high Ti and Fe+3, like that in the host clinopyroxene-rich cumulate dunites. It resembles in chemistry to spinels in the Alaskan-type complex formed in an island arc setting. The early cumulus olivine in disseminated chromitites and host dunites (Fo88-91) shows a typical fractional crystallization trend. The interstitial clinopyroxene (Cpx) in chromitites is similar in chemical properties to Cpx crystallized from tholeiitic basalt melts, and traces a cumulate trend of arc-related Alaskan complexes. The peridotitic and gabbroic rocks contain primary amphiboles, which are pargasite and edenite with high Ti, Na and Al. The sulphide minerals are usually chalcopyrite, pentlandite and horomanite with subordinate Cu-pentlandite, cubanite, samaniite, heazlewoodite and pyrite. Pentlandite, horomanite and cubanite at temperatures between 900 and 1040 °C are rich in some platinum group elements (PGE: Os, Pt, Ir). They are classified into primary and secondary origins, which were crystallized from segregated sulphide droplets and from hydrothermal fluids at temperature < 760 °C during postmagmatic stage, respectively. The Dahanib banded chromitites and their mineral inclusions crystallized from hybrid melts, consisting of hydrous tholeiitic basalt melts mixed with slab-derived melts/fluids. They possibly formed by gravitational settling of semi-consolidated crystal mushes in a sub-arc lower-crustal magma chamber. Chemistry of primary minerals suggests that the Dahanib subarc deep-crustal magma is rich in Mg, Cr, Al, Ti, V, Ca, Sc and Na during the early magmatic stage, followed by increasing Fe, Mn, Ni, Co, Cu, S, Cs, B, Sr, and Pb with a progress of fractional crystallization.

Geochemistry of ultramafic–mafic rocks of Mesoarchean Sargur Group, western Dharwar craton, India: Implications for their petrogenesis and tectonic setting

The Nuggihalli and Holenarsipur greenstone belts of the western Dharwar craton expose ultramafic–mafic rocks of the Mesoarchean. The rocks in these belts are geochemically considered as komatiites and komatiitic basalts with minor occurrences of tholeiitic and calc-alkaline basalts. The dominant ultramafic rocks of the Nuggihalli greenstone belt are layered and indicate fractionation processes at relatively shallower crustal levels. The Al-undepleted and Al-depleted signatures obtained could be attributed to magmatic differentiation processes and might be due to fractional crystallization of minerals such as hornblende and plagioclase, in addition to cumulus olivine and pyroxene. The chemical heterogeneity in the rocks of these greenstone belts might have therefore developed during the intrusion of the parental melts and their differentiation into a layered igneous complex. The differences in the lithological characteristics of the Holenarsipur and Nuggihalli greenstone belts can be explained by their different crustal levels of exposure. Presence of spinifex-textured komatiites need not necessarily imply that the sources have to be ultramafic and therefore of a deeper origin. This study indicates that the parental melts for unambiguous layered intrusive ultramafic–mafic complexes could be high-Mg basalts originating from relatively shallower levels. The probable geodynamic setting for the emplacement of the rocks of the two greenstone belts could be in a plume-modified mid-ocean ridge that was too thick and buoyant to be subducted, and the decompression-melted magma chamber developed igneous layering as the magma stalled in the lithosphere.

Paleoproterozoic Mafic-Ultramafic Magmatism in the Northern Borborema Province, Northeast Brazil: Tectonic Setting and Potential for Deposits

Mafic-ultramafic intrusions form a 32-km-long NE-SW-trending array within Paleoproterozoic arc-related calc-alkaline plutonic rocks of the Rio Piranhas terrane in the Borborema Province, Northeast Brazil. The mafic-ultramafic intrusions consist of wehrlite, clinopyroxenite, and troctolite. They outcrop as small elongated bodies hosted by orthogneiss and migmatites associated with minor amphibolite and supracrustal rocks. Primary magmatic features predominate in the core of the intrusions, whereas metamorphic minerals and textures characterize their outer zones. The composition of cumulus olivine from different intrusions ranges from Fo80.6 to Fo67.6 (forsterite), indicating moderately primitive parental magmas. The compositional range of cumulus plagioclase coexisting with cumulus olivine in troctolites is characterized by high anorthite content (An79.9–An89.0). This feature, together with interstitial magmatic amphibole in troctolites, is characteristic of gabbroic intrusions in subduction-related magmatic arcs. U-Pb zircon ages indicate that mafic-ultramafic intrusions crystallized at ca. 2190 ± 5 Ma. U-Pb zircon ages of host orthogneiss indicate that they crystallized from calc-alkaline felsic magmas at ca. 2220–2223 Ma, thus providing only ∼30 My for the reported events of felsic and mafic plutonism. U-Pb zircon ages of host rocks include older ages (2.47 and 2.69 Ga) indicating that older crustal components are associated with the 2.23 Ga calc-alkaline magmatism. The presence of older crustal components is consistent with arc-related features, indicated by geochemical data and variable εNd(t) values in the mafic-ultramafic intrusions. The tholeiitic and calc-alkaline magmatism is correlated with the ca. 2.15–2.25 Ga magmatic arc reported in previous studies of the Rio Piranhas terrane. The mafic-ultramafic magmatism is interpreted as having originated in a magmatic arc, possibly resulting from partial melting of a mantle wedge above the subduction zone in late stages of the orogeny and suggests a new window of opportunity for exploration of Ni-Cu–platinum group element deposits.

X-ray computed tomography as a method of reconstruction of 3d-characteristics of disseminated sulfides and spinel in plagiodunites from the Yoko-Dovyren intrusion

The paper describes a methodology of applying X-ray computed tomography (CT) in studying textural–morphological characteristics of sulfide-bearing ultramafic rocks from the Yoko-Dovyren layered massif in the northern Baikal area, Buryatia, Russia. The dunites are used to illustrate the applicability of a reliable technique for distinguishing between grains of sulfides and spinel. The technique enables obtaining statistical characteristics of the 3D distribution and size of the mineral phases. The method of 3D reconstructions is demonstrated to be applicable at very low concentrations of sulfides: no than 0.1–0.2 vol %. Differences between 3D models are determined for sulfide segregations of different size, in some instances with features of their orientation suggesting the direction of percolation and accumulation of the sulfide liquids. These data are consistent with the morphology of the largest sulfide segregations, whose concave parts adjoin the surface of the cumulus olivine and simultaneously grow into grains of the poikilitic plagioclase. Detailed information of these features is useful to identify fingerprints of infiltration and concentration of protosulfide liquids in highly crystallized cumulate systems.

X-Ray Computed Tomography as a Method for Reproducing 3D Characteristics of Sulfides and Spinel Disseminated in Plagiodunites from the Yoko-Dovyren Intrusion

The paper describes a methodology of applying X-ray computed tomography (CT) in studying textural–morphological characteristics of sulfide-bearing ultramafic rocks from the Yoko-Dovyren layered massif in the northern Baikal area, Buryatia, Russia. The dunites are used to illustrate the applicability of a reliable technique for distinguishing between grains of sulfides and spinel. The technique enables obtaining statistical characteristics of the 3D distribution and size of the mineral phases. The method of 3D reconstructions is demonstrated to be applicable at very low concentrations of sulfides: no less than 0.1–0.2 vol %. Differences between 3D models are determined for sulfide segregations of different size, in some instances with features of their orientation suggesting the direction of percolation and accumulation of the sulfide liquids. These data are consistent with the morphology of the largest sulfide segregations, whose concave parts adjoin the surface of the cumulus olivine and simultaneously grow into grains of the poikilitic plagioclase. Detailed information of these features is useful to identify fingerprints of infiltration and concentration of protosulfide liquids in highly crystallized cumulate systems.

Possible sources for CH4-rich fluid inclusions in olivine from troctolitic rocks of the Bathtub intrusion, Duluth Complex, U.S.A.

Cumulus olivine from the Babbitt deposit in the troctolitic Bathtub intrusion (Duluth Complex) contains abundant CH4-rich fluid inclusions with common hydrous silicate and carbonate precipitates at their inclusion walls. Given the active rifting environment in which magma of the Duluth Complex (1.1 Ga) has intruded as repeated pulses, fluid inclusions in olivine are traceable throughout an entire stratigraphic drill core section with comparable textural features that enable a classification of three generations: (1) primary CH4-rich fluid inclusions oriented parallel to growth planes of olivine or arranged in clusters; (2) intragranular fluid inclusion planes of CH4-rich fluid inclusions extending towards grain boundaries and (3) transgranular fluid inclusion planes of CH4-rich fluid inclusions, closely associated with serpentinization. The hydrous minerals and carbonates contained in fluid inclusions of generations (1) and (2) indicate that original constituents of the early homogenous magmatic fluid must have been H2O and CO2. Upon cooling (below 700 °C), at fixed Pfluid (2 kbar) and fO2fluid at or below QFM conditions, an increase of XCH4 at the expense of CO2 results in the CH4-rich chemistry of the fluid. The formation of late secondary fluid inclusion planes of generation (3) is related to extensive magma-country rock interaction. These inclusions contain no solids and their composition reflects the reduction of CO2 to CH4 during serpentinization in an upper stability limit of 620 °C. This study emphasizes fluid inclusions as powerful indicators in remodeling chemical changes of the magmatic fluid during crystallization of cumulus phases, magma-country rock interaction and subsequent alteration.

Rapid orthopyroxene growth induced by silica assimilation: constraints from sector-zoned orthopyroxene, olivine oxygen isotopes and trace element variations in the Huangshanxi Ni–Cu deposit, Northwest China

The Permian Huangshanxi Ni–Cu-hosted intrusion, located in the southern Central Asian Orogenic Belt in NW China, is dominated by cumulus olivine and orthopyroxene. Various types of compositional zonings in cumulate phases were studied using high-resolution synchrotron X-Ray fluorescence imaging, electron microprobe, laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectrometry, to shed light on the relationship between the orthopyroxene-rich cumulate and sulfide accumulation. Normal, oscillatory, and sector zonings of Cr in orthopyroxene from both lherzolites and olivine websterites were noted, likely due to the different cut orientation of orthopyroxene crystals composed of sector-zoned cores and reverse-zoned rims. Significant Fe–Mg–Al–Si variations found in the exterior regions of the grains do not correspond to the Cr zoning. However, Cr content is positively correlated with Ti–Al contents in interior regions of orthopyroxene, reflecting coupled charge substitution during the development of sector zoning, and indicating diffusion-modified Fe–Mg–Al–Si variation in the exterior regions. Primary Cr-sector zoning in the core is likely the result of rapid in situ growth in a boundary layer at the cumulus stage, probably induced by Si addition from wall rocks and crystallization under supersaturated conditions. In addition, some olivine shows Mg–Fe–Mn–Ca–Cr variations in the rim but not in the core, and both core and rim of olivine show a uniform O isotope composition, illustrating that they have not been modified by post-cumulus processes. Given the distinct difference in O isotopic signatures between mantle-derived melts and crustal materials, olivine O isotope composition could be used to decipher the relative degree of Si addition at crystallization. The Fo value, O isotopic signature and trace element content in the cores of olivine grains from different depths show systematic variations, dividing the sulfide-bearing cumulate into two intervals; the lower and upper lherzolite zones with harzburgitic rocks at the contact. In the lower zone, the olivine δ18O values increase upward from ~ 6 to ~ 7‰ and the oxygen fugacity decreases from QFM to QFM-2, suggesting increasing contamination by more reduced crustal materials. Over the same interval, the modal proportion of orthopyroxene and modal orthopyroxene/olivine ratio increase with increasing depth with little variation in whole-rock Mg#, clearly illustrating that the non-cotectic orthopyroxene proportions resulted from Si addition from the wall rock. In the upper zone, the decrease in δ18O values, and increase in olivine nucleation density, Fo value and oxygen fugacity suggest there was a pulse of sulfide- and olivine-charged magma into the magma chamber, that mixed with resident magma to form a hybrid bottom layer in contact with the early-crystallized orthopyroxene-rich cumulate pile. The sulfide content decreases up-section in the lower zone, and then increases again towards the top of the cumulate sequence. This, combined with the compositional variations with depth, suggests that sulfides in the Huangshanxi Ni–Cu deposit settled downward during at least two sulfide-loaded magma pulses. The fast-growing orthopyroxene-rich cumulate may have acted as a permeability barrier, preventing sulfide percolation into the lower cumulate formed during the first magma pulse. Overall, sector zoning in orthopyroxene and compositional variations in olivine suggest that Si and reduced material addition during contamination is of great importance in forming magmatic Ni–Cu deposits hosted by olivine- and orthopyroxene-dominated intrusions.

Mantle Potential Temperature Estimates and Primary Melt Compositions of the Low-Ti Emeishan Flood Basalt

The Late Permian Emeishan large igneous province (ELIP) is considered to be one of the best examples of a mantle plume derived large igneous province. One of the primary observations that favour a mantle plume regime is the presence of ultramafic volcanic rocks. The picrites suggest primary mantle melts erupted and that mantle potential temperatures (TP) of the ELIP were > 200oC above ambient mantle conditions. However, they may represent a mixture of liquid and cumulus olivine and pyroxene rather than primary liquids. Consequently, temperature estimates based on the picrite compositions may not be accurate. Here we calculate mantle potential temperature (TP) estimates and primary liquids compositions using PRIMELT3 for the low-Ti (Ti/Y < 500) Emeishan basalt as they represent definite liquid compositions. The calculated TP yield a range from ~1400oC to ~1550oC, which is consistent with variability across a mantle plume axis. The primary melt compositions of the basalts are mostly picritic. The results of this study indicate that the Emeishan basalt was produced by a high temperature regime and that a few of the ultramafic volcanic rocks may be indicative of primary liquids.

Geochemistry and PGE of the lower mineralized Zone of the Waterberg Project, South Africa

The Waterberg deposit is located north of the Northern Lobe of the Bushveld Complex in South Africa and represents a large, high-grade, new Platinum-Group-Element (PGE) discovery. The first comprehensive study of the lower ultramafic section of the area has been completed, and whole-rock chemical analyses and PGE data are presented.The section studied comprises mineralized harzburgite and marginal orthopyroxenite, overlain by troctolite grading into gabbroic rocks. Whole-rock analyses show geochemical variations typical of differentiated assemblages of cumulus olivine, plagioclase, and pyroxene. Normalized trace element data display HREE depletion, strong positive Eu and Sr anomalies and LREE enrichment. The negative anomalies for Th, Rb, Nb, and Ta are typical for rocks of the Bushveld Complex. Normalized PGE distributions are fractionated (Pd/Ir 177), Pd-enriched, and Au-poor.Emplacement of the magmas is believed to have commenced with west-east trending, finger-like intrusions, followed by lateral dilation and emplacement of sulfide droplet-bearing, ultramafic magmas. This was followed by a second phase of intrusions, characterized by sheet-like bodies of troctolite. Fractionation of these magmas led to the development of gabbroic rocks that make up the top of the succession. The Waterberg Project is located in the Southern Marginal Zone of the Limpopo Belt. This position in a structurally active area may have facilitated the creation of space for initial magmas.It is argued, that the mafic to ultramafic succession of the Waterberg Project does not represent a simple marginal extension of the Northern Lobe, nor does it directly correlate with the Platreef. It shares geological features but represents a separate magmatic basin.

Whole-rock and mineral compositional constraints on the magmatic evolution of the Ni-Cu-(PGE) sulfide ore-bearing Kevitsa intrusion, northern Finland

The ~2.06Ga mafic-ultramafic Kevitsa intrusion is located in the Central Lapland greenstone belt. The lower ultramafic part of the intrusion hosts a large disseminated Ni-Cu-(PGE) sulfide deposit with Ni tenors ranging widely from <4wt% (uneconomic false ore and contact mineralization) to 4–7wt% (regular ore) and up to 40wt% (Ni-PGE ore). The stratigraphy of the ultramafic cumulates is divided into the basal pyroxenite–gabbro (Basal series), olivine pyroxenite (OLPX), pyroxenite, and plagioclase-bearing (ol) websterite (pOLWB), of which the latter occurs together with minor microgabbros in the ore-bearing domain of the intrusion. Around the ore domain, the ultramafic cumulate succession records a simple lithological stratigraphy and modest and predictable variations in whole-rock and mineral compositions. The ore-bearing domain, in contrast, is characterized by a complex internal architecture, variations in whole-rock and mineral compositions, and the presence of numerous inclusions and xenoliths.The OLPXs are mainly composed of cumulus olivine (Fo77–89) and clinopyroxene (Mg#81–92) with variable amounts of oikocrystic orthopyroxene (Mg#79–84). They comprise the bulk of the ultramafic cumulates and are the dominant host rocks to the sulfide ore. The host rocks to the regular and false ore type are mineralogically and compositionally similar (Fo~80–83, mostly) and show mildly LREE-enriched REE patterns (CeN/YbN~2), characteristic for the bulk of the Kevitsa ultramafic cumulates. The abundance of orthopyroxene and magnetite is lowest in the host rocks to the Ni-PGE ore type, being in line with the mineral compositions of the silicates, which are the most primitive in the intrusion. However, it contrasts with the LREE-enriched nature of the ore type (CeN/YbN~7), indicating significant involvement of crustal material in the magma.The contrasting intrusive stratigraphy in the different parts of the intrusion likely reflects different emplacement histories. It is proposed that the Kevitsa magma chamber was initially filled by stable continuous flow (“single” input) of basaltic magma followed by differentiation in an at least nearly closed system. In the following stage, new magma pulses were repeatedly emplaced into the interior of the intrusion in a dynamic (open) system forming the sulfide ore bodies. To gain the peculiar compositional and mineralogical characteristics of the Ni-PGE ore type, the related magma probably interacted with different country rocks en route to the Kevitsa magma chamber.