Staging Chamber Research Articles

Petrogenesis and Ni–Cu–(PGE) prospectivity of the Mount Ayliff Complex in the Karoo Igneous Province: new insights from the Ingeli and Horseshoe lobes

The Mount Ayliff Complex comprises five cognate mafic–ultramafic bodies emplaced along the southern margin of the Kaapvaal Craton. This study examines the Ingeli and Horseshoe lobes and assesses their magmatic sulfide prospectivity with respect to the Insizwa lobe, which hosts massive sulfides at its Waterfall Gorge occurrence. The Ingeli lobe consists of 465 m of olivine–chromite cumulates overlain by 340 m of (olivine-)gabbronorite, while the Horseshoe lobe consists of 45 m of olivine gabbronorite overlain by 5 m of gabbro. The Ingeli lobe possesses sparsely disseminated sulfides at its mafic–ultramafic transition, whereas disseminated sulfides are present throughout the Horseshoe lobe. A petrogenetic model is proposed, where magma accumulated and fractionated nickeliferous olivine and chromite in an upper-crustal staging chamber hosted by Proterozoic basement rocks. Magma then ascended and deposited olivine–chromite cumulates at the level of the complex. Prolonged magma flux in the staging chamber facilitated the assimilation of basement rocks, triggering sulfide saturation and the crystallization of Ni-poor olivine. Contaminated magma then ascended and interacted with pre-existing cumulates, depositing sulfide melt that may have backflowed as magmatic activity waned. Basaltic magma then flowed over the ultramafic cumulates, depositing disseminated sulfides whilst undergoing closed-system fractionation. Basal depressions and underlying feeder structures are the most prospective locations for magmatic sulfide mineralization.

Physiological audience synchrony in classical concerts linked with listeners’ experiences and attitudes

A series of eleven public concerts (staging chamber music by Ludwig van Beethoven, Brett Dean, Johannes Brahms) was organized with the goal to analyze physiological synchronies within the audiences and associations of synchrony with psychological variables. We hypothesized that the music would induce synchronized physiology, which would be linked to participants’ aesthetic experiences, affect, and personality traits. Physiological measures (cardiac, electrodermal, respiration) of 695 participants were recorded during presentations. Before and after concerts, questionnaires provided self-report scales and standardized measures of participants’ affectivity, personality traits, aesthetic experiences and listening modes. Synchrony was computed by a cross-correlational algorithm to obtain, for each participant and physiological variable (heart rate, heart-rate variability, respiration rate, respiration, skin-conductance response), how much each individual participant contributed to overall audience synchrony. In hierarchical models, such synchrony contribution was used as the dependent and the various self-report scales as predictor variables. We found that physiology throughout audiences was significantly synchronized, as expected with the exception of breathing behavior. There were links between synchrony and affectivity. Personality moderated the synchrony levels: Openness was positively associated, Extraversion and Neuroticism negatively. Several factors of experiences and listening modes predicted synchrony. Emotional listening was associated with reduced, whereas both structual and sound-focused listening was associated with increased synchrony. We concluded with an updated, nuanced understanding of synchrony on the timescale of whole concerts, inviting elaboration by synchony studies on shorter timescales of music passages.

Petrogenesis of the Marginal Sill Phase and Lower Zone in the Eastern Limb of the Bushveld Complex: Incremental development of igneous layering and syn-magmatic emplacement of peridotite intrusions and sills

Abstract The petrogenesis of the mafic-ultramafic layered rocks of the Bushveld Complex, i.e., the Rustenburg Layered Suite (RLS) remains uncertain despite more than a century of intensive research. An investigation of a 100 km strike section in the Eastern Limb has led us to propose that igneous layering in the lowermost part of the RLS developed from a succession of intrusive events. Zonal boundaries and geochemical discontinuities are explained by radical switches in the composition of parental magmas. Magmas were sourced from deep staging chambers, and we find no evidence for the existence of a large magma reservoir in the shallow crust. At the base of the RLS is the Marginal Sill Phase (MSP), a stacked sequence of noritic-gabbronoritic and pyroxenitic sills that form a continuum with the syn-Bushveld sills in the floor rocks. The MSP does not constitute the chilled carapace of a magma chamber and the postulated so-called “B1” (tholeiitic) parental magmas have little direct bearing on the overlying components of the RLS. The Lower Zone (LZ) reflects a switch to ultramafic parental magmas. Magmas were fed into isolated subchambers, each of which has a unique igneous stratigraphy. The primary component of the LZ is an adcumulate comprised of >98.5 modal % orthopyroxene (En87-83) and from which Cr-spinel is conspicuously absent. Chemical equilibrium was maintained as the orthopyroxene-saturated magmas were intruded, not as sills, but as thin magma laminae. The increments of magma laminae matched the rate of batch crystallisation, and there is no evidence of a fractionated residue having been ejected. In a later phase of magmatism, peridotite magmas were injected into the floor rocks, where they formed complexly layered peridotite intrusions with their anastomosing groups of sills. Formation of peridotite bodies such as the Aapiesdoorndraai Intrusion with distinct layers of dunite, harzburgite and olivine orthopyroxenite (Fo92-83) (+Cr-spinel) result from near-equilibrium crystallisation of the olivine- and Cr-spinel-saturated magmas. Some peridotite sills stepped up through the thermal aureole and intruded the earlier-formed MSP and LZ, crystallising as syn-magmatic sills with broadly similar compositions. Development of the trough-like subchambers with thick sequences of the MSP and LZ triggered extensive deformation and thermal metamorphism of the floor rocks. Sinking of subchambers was compensated by intervening domes and upwarps, the crests of which reveal tectonically thinned sequences of the MSP and from which the LZ is absent. This early phase of syn-Bushveld tectonism has resulted in the Lower Critical Zone (LCZ) discordantly overlying either the MSP (on the crests of domes and upwarps) or the LZ (in subchambers). Field relationships thus demonstrate that the LCZ developed entirely independently of the MSP and LZ, from a new series of intrusive events and a new lineage of parental magma.

Petrogenesis of the Dar al Gani (DaG) 1.1 Ma ejection‐paired olivine‐phyric shergottites and implications for ~470 Ma Martian volcanism

AbstractThe Dar al Gani (DaG) olivine‐phyric shergottites share mineralogical and geochemical characteristics, which confirm that these meteorites are derived from a single source. Bulk trace elements (La/Yb—0.12), in situ maskelynite 87Sr/86Sr (~0.7014) and redox estimates (FMQ ~ −2) indicate derivation from a depleted, reduced mantle reservoir; identical to all ~470 Ma shergottites ejected at 1.1 Ma. The DaG shergottites have been variably affected by terrestrial alteration, which precipitated carbonate along fractures and modified bulk‐rock fluid mobile (e.g., Ba) elements. Nonetheless, sufficient data are available to construct a multi‐stage formation model for the DaG shergottites and other 1.1 Ma ejection‐paired shergottites that erupted at ~470 Ma. First, partial melting of a depleted mantle source occurred at 1540 ± 20°C and 1.2 ± 0.1 GPa, equivalent to > ~100 km depth. Then, initial crystallization in a staging chamber at ~85 km depth at the crust–mantle boundary took place, followed by magma evolution and variable incorporation of antecrystic olivine ± orthopyroxene. Subsequently, crystallization of olivine phenocrysts and re‐equilibration of olivine antecrysts occurred within an ascending magma. Finally, magmas with variable crystal loads erupted at the surface, where varied cooling rates produced a range of groundmass textures. This model is similar to picritic flood basalt magmas erupted on Earth.

Neodymium isotope variations in the Flatreef on Macalacaskop, northern limb, Bushveld Complex

The origin of the recently discovered Flatreef remains debated due to the pronounced interaction of the magmatic rocks with sedimentary floor rocks, resulting in a complex intrusive stratigraphy. In this study, we report new Nd isotopic compositions of Flatreef lithologies intersected by borehole UMT-393 on the farm Macalacaskop in order to improve our understanding of the magmatic history of the deposit and to further test the putative correlation between the Flatreef/Platreef and the Upper Critical Zone of the remainder of the Bushveld Complex. The initial epsilon Nd (εNdi) values for the Flatreef range between −5.2 and −7.6, overlapping with εNdi values of the Upper Critical Zone from the eastern (ranging between − 4.8 and − 8.5) and the Upper Critical Zone and Main Zone from the western limb (−6.3 and −7.6, and −6.3 and −7.4 respectively) of the Bushveld Complex. The Flatreef εNdi values also overlap with those of the Platreef; however, due to the varying footwall lithologies of the Platreef along strike, Platreef rocks display a wider variation in isotopic composition. Our findings support the correlation of the Flatreef with the Upper Critical Zone — Main Zone transition interval in the remainder of the Bushveld Complex, which includes the Merensky and Bastard reefs. Due to significant overlap between the εNdi values of the Flatreef and local potential contaminants occurring at the base of the Northern Limb, we propose that the Sr–Nd isotopic composition of the magmas that gave rise to the Flatreef are most likely attributable to the interaction of mantle-derived magma with upper and lower crustal rocks of the Kaapvaal Craton within a sub-Bushveld staging chamber, with possible syn- to post-emplacement modification as a result of interaction with dolomitic footwall rocks.

OrganoidChip facilitates hydrogel-free immobilization for fast and blur-free imaging of organoids

Organoids are three-dimensional structures of self-assembled cell aggregates that mimic anatomical features of in vivo organs and can serve as in vitro miniaturized organ models for drug testing. The most efficient way of studying drug toxicity and efficacy requires high-resolution imaging of a large number of organoids acquired in the least amount of time. Currently missing are suitable platforms capable of fast-paced high-content imaging of organoids. To address this knowledge gap, we present the OrganoidChip, a microfluidic imaging platform that incorporates a unique design to immobilize organoids for endpoint, fast imaging. The chip contains six parallel trapping areas, each having a staging and immobilization chamber, that receives organoids transferred from their native culture plates and anchors them, respectively. We first demonstrate that the OrganoidChip can efficiently immobilize intestinal and cardiac organoids without compromising their viability and functionality. Next, we show the capability of our device in assessing the dose-dependent responses of organoids’ viability and spontaneous contraction properties to Doxorubicin treatment and obtaining results that are similar to off-chip experiments. Importantly, the chip enables organoid imaging at speeds that are an order of magnitude faster than conventional imaging platforms and prevents the acquisition of blurry images caused by organoid drifting, swimming, and fast stage movements. Taken together, the OrganoidChip is a promising microfluidic platform that can serve as a building block for a multiwell plate format that can provide high-throughput and high-resolution imaging of organoids in the future.

Thermobarometry of the Rajmahal Continental Flood Basalts and Their Primary Magmas: Implications for the Magmatic Plumbing System

The Late Aptian Rajmahal Traps originated through Kerguelen-Plume-related volcanism at the eastern margin of the Indian Shield. Clinopyroxene and whole-rock thermobarometry reveals that the Rajmahal magmas crystallized at P-T conditions of ≤~5 kbar/~1100–1200 °C. These pressures correspond to upper crustal depths (≤~19 km). Modeling shows that the Rajmahal primary magmas were last in equilibrium with mantle at P-T conditions of ~9 kbar/~1280 °C. The corresponding depths (~33 km) are consistent with gravity data that indicate a high-density layer at lower crustal depths below an upwarped Moho. Thus, the high-density layer probably represents anomalous mantle. It is likely that the mantle-derived magmas accumulated below the upwarped Moho and were subsequently transported via trans-crustal faults/fractures to the upper crust where they evolved by fractional crystallization in small staging chambers before eruption. In the lower part of the Rajmahal plumbing system, buoyant melts from the Kerguelen Plume may have moved laterally and upward along the base of the lithosphere to accumulate and erode the eastern Indian lithospheric root. The Rajmahal plumbing system was probably shaped by tectonic forces related to the breakup of Gondwana.

Reworking of aged mafic crust in a Palaeoarchaean layered complex inferred from coupled Sm-Nd and Lu-Hf isotope systematics, Stolzburg Complex, Barberton Greenstone Belt, South Africa

The Stolzburg Complex is a prominent and well-preserved Palaeo- to Mesoarchaean layered ultramafic-mafic complex in the vicinity of the Barberton Greenstone Belt. Whole rock Sm-Nd, Lu-Hf and 142Nd isotopic data obtained from a variety of lithologies, and augmented by major and trace element geochemistry, are presented to examine petrogenesis, magma source composition, and mantle differentiation as well as the geodynamic setting of emplacement. Geochemically, all samples are characterized by unfractionated normalized trace element patterns (La/Luchond. = 0.7–2.3, La/Gdchond. = 0.6–1.7, Sm/Luchond. = 0.9–1.4) associated with weak HFSE anomalies (Nb/[0.5Th + 0.5La]PRIMA = 0.5–2.3). Trace element concentrations vary as a function of mineralogy, but differences in trace element compositions are unsystematic across lithologies. The coherence of trace element characteristics is consistent with a common mantle source that underwent moderately large degrees of melting at mantle pressures within the spinel stability field. Whole rock Sm-Nd and Lu-Hf data yield well-defined apparent isochrons corresponding to ages of 3367 ± 62 (n = 12, MSWD = 1.5) and 3396 ± 36 (n = 11, MSWD = 2.1), respectively. Initial Hf and Nd isotopic compositions vary from +3.3 to +5.7 and + 0.9 to +1.7, respectively, indicative of derivation from slightly to moderately depleted mantle source(s). Isochron ages and a regressed initial Hf isotopic composition of +4.0 ± 0.9 (at 3.4 Ga) contrast with an established U-Pb emplacement age (zircon, titanite) of 3.25 Ga and εHf values for zircon of −2.5 to +3.0. Hydrothermal alteration or weathering, incorporation of evolved pre-existing (continental) crust and fractionation by high-pressure phases prior to melt extraction can all be ruled out to have significantly affected isotope systematics. Protracted magma chamber activity and non-synchronous emplacement of unrelated gabbroic magmas also fail to fully account for the discrepancy between whole rock and mineral age and isotopic data. Instead, the coherence in apparent isochron ages and variability of initial Hf and Nd isotopic composition is adequately explained by reworking (partial or near-complete remelting) of aged mafic-ultramafic (likely oceanic) crust in staging chambers with compositions indistinguishable from newly formed magmas. Thus, apparent isochrons represent mixing lines and do not record timing of crystallization. A subset of samples was also analyzed for 142Nd isotopic compositions to trace preserved mantle heterogeneities caused by early Archaean crust-mantle differentiation. The results (μ142Nd = −2.3 ± 2.2 to +1.7 ± 2.1) are not resolvable from the modern mantle value, indicating that 142Nd-enriched or depleted mantle reservoirs had not remained isolated from convective homogenization in the asthenosphere, or the volume of mantle underlying the layered complex was too small to sample mantle heterogeneities. The Stolzburg Complex was possibly emplaced in ancient oceanic lithosphere, followed by pervasive but inhomogeneous Ca-metasomatism as documented by exposed rodingites.

Constraints on martian depleted shergottite volcanism from the petrogenesis of olivine-phyric shergottites NWA 2046 and NWA 4925

The 1.1Ma ejection-paired depleted shergottites are sourced from a long-lived volcanic centre, likely located on the Tharsis plateau. Here we present bulk-rock geochemistry, mineral major- and trace-element concentrations, and in-situ maskelynite and pyroxene 87Sr/86Sr data for two depleted olivine-phyric shergottites, Northwest Africa (NWA) 2046 and NWA 4925, from this 1.1 Ma ejection event. The earliest formed olivine megacryst cores in NWA 2046 (Fo83) and NWA 4925 (Fo81) are among the most Mg-rich recorded in martian meteorites and are also in Mg–Fe equilibrium with their respective bulk-rock compositions. We infer the olivine megacrysts in NWA 2046 and NWA 4925 are phenocrysts and suggest that both samples represent melt compositions. CI chondrite-normalised rare earth element patterns for melts calculated to be in equilibrium with pyroxene in NWA 2046 and NWA 4925 are parallel to those of melts in equilibrium with maskelynite and merrillite, which suggests that both samples crystallised from continuously fractionating melts unaffected by crustal contamination. Oxygen fugacity estimates for NWA 2046 (QFM -2.7) and NWA 4925 (QFM -2.9), derived from the V-in-olivine oxybarometer, are comparable to other depleted olivine-phyric shergottites, including Tissint, Larkman Nunatak (LAR) 12095/12240, Dar Al Gani (DaG) 476 and Dhofar (Dho) 019.Effects of terrestrial alteration are observed in bulk-rock and mineral compositions from NWA 2046 and NWA 4925. The degree of terrestrial alteration is most pronounced in NWA 4925, consistent with an older reported exposure age relative to NWA 2046. The low Ni-contents of bulk-rock and olivine megacrysts in NWA 2046 and NWA 4925 suggests that both samples are not primary mantle-derived melt, but rather have undergone 8–10% and 10–13% fractional crystallisation, respectively, in a staging chamber at depth. Pyroxenes Ti/Al for NWA 2046 and NWA 4925, suggest initial crystallisation at ∼ 10 kbar, consistent with other ejection-paired olivine-phyric shergottites, indicating either a common or multiple staging chambers near the base of the martian crust. The 87Sr/86Sr compositions for unaltered maskelynite grains in NWA 2046 (0.70130 ± 7) and NWA 4925 (0.70128 ± 10) suggest that both samples originate from a similar mantle source to other depleted olivine-phyric shergottites, particularly DaG 476, and Sayh al Uhaymir (SaU) 005 and Yamato 980459 (Y-980459).

Constraints on the Nd-isotopic composition and nature of the last major influx of magma into the Bushveld Complex

The Pyroxenite Marker, a thin, orthopyroxene-dominated marker horizon, is observed towards the top of the Main Zone of the Bushveld Complex, where the last voluminous influx of magma into the Bushveld Complex is thought to have occurred. In an attempt to constrain the Nd-isotopic composition of the magma added at the level of the Pyroxenite Marker, a total of 21 whole-rock samples from a borehole (BH7771) drilled on the Central Sector of the Eastern Limb of the Bushveld Complex were analysed for their Sr–Nd isotopic ratios. Modelling suggests that the added magma had a unique Sr (87Sr/86Sri = 0.7063–0.7067) and Nd (ƐNdi on the order of − 5.9) isotopic composition, distinct from any of the rocks constituting the layered sequence below the Pyroxenite Marker. Dispersion of data points around the modelled isotopic (melt–melt) mixing curves is interpreted to reflect the incorporation of minerals derived from either the incoming or resident magmas into individual rock layers occurring across the Pyroxenite Marker interval, either in response to the mixing of minerals settling through a stratified magma column, or potentially through the intrusion and mixing of crystal-laden magmas with unique isotopic compositions from a sub-Bushveld staging chamber.

Compositions and Ni-Cu-PGE Tenors of Nova-Bollinger Ores with Implications for the Origin of Pt Anomalies in PGE-Poor Massive Sulfides

Abstract The Nova-Bollinger Ni-Cu-platinum group element (PGE) deposit in the Fraser zone of the Albany-Fraser orogen consists of two main orebodies, Nova and Bollinger, hosted by the same tube-shaped intrusion but having distinctly different Ni tenors of around 6.5 and 4.8 wt %, respectively. Nova is also higher in Pd, but Cu and Pt tenors are similar. Both deposits have very low PGE tenors, with average Pd concentrations of 110 ppb in massive sulfide at Bollinger and 136 ppb at Nova. The Nova and Bollinger orebodies show relatively little internal differentiation overall on deposit scale but show strong differentiation into chalcopyrite-rich and chalcopyrite-poor regions at a meter scale. This differentiation is more prevalent at Nova, where massive sulfide-filled vein arrays are more extensively developed, and in massive ores, particularly veins, than in net-textured ores. Net-textured and disseminated ores have on average Ni and Cu grades and tenors similar to those of massive, semimassive, and breccia ores in the same orebody but a smaller range of variation, largely due to a more limited extent of sulfide liquid fractionation and higher average concentrations of Pt and Pd than adjacent massive ores. Unusually for differentiated magmatic sulfides, there is no systematic positive correlation between Pt, Pd, and Cu. A partial explanation for the lack of a Pd-Cu correlation is that Pd was partitioned into peritectic pentlandite in the middle stages of sulfide liquid solidification. This explanation is not applicable to Pt, as Pt characteristically forms its own phases rather than residing in base metal sulfides. PGE tenors are very low in both orebodies, very similar to those observed in other Ni-Cu-Co sulfide ores in orogenic settings, notably the Savannah and Savannah North orebodies. This depletion is attributed to sulfide retention in the mantle source of the parent magmas rather than to previous fractional extraction of sulfide liquid in staging chambers or feeder networks. The higher Ni and Pd tenors at Nova are attributed to reworking and upgrading of precursor sulfide liquid originally deposited upstream at the Bollinger site. Replicate analyses of multiple jaw-crusher splits returned highly variable Pt and Au assays but much smaller relative errors in the other PGEs. The poor Pt and Au reproducibilities are attributed to nugget effects, explicable by much of the Pt and Au in the samples being present in sparse Pt- and Au-rich grains. This is principally true for Pt in massive rather than disseminated ores, accounting for a strong contrast in the distribution of Pt/Pd ratios between the two ore types. Numerical simulation suggests that Pt is predominantly resident in Pt-rich platinum group minerals with grain diameters of 100 µm or more and that at the low (<100 ppb) concentrations in these ores, this results in most assays significantly underreporting Pt. This is likely to be true in other low-PGE ores, such that apparent negative Pt anomalies in massive ores may in such cases be attributable to sampling artifacts.

Protracted mafic magmatism and two-stage mineralization of the Kalatongke Cu-Ni sulfide deposit in the Central Asian Orogenic Belt

The Kalatongke Cu-Ni sulfide deposit in the Central Asian Orogenic Belt (CAOB) is hosted by the early Permian mafic intrusions. After conducting a detailed field survey, we found that the massive sulfide ores and network olivine gabbro were emplaced separately into the dominant sulfide-barren mafic suite. Secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were performed for zircon U-Pb dating to determine the ages of rocks derived from the sulfide-rich suites and adjacent sulfide-barren suites. The results revealed that the U-Pb ages of olivine gabbro with network ore, gabbro with globular ore and breccia ore are 276.0 ± 1.1, 272.4 ± 0.9 and 271.9 ± 1.3 Ma, respectively. Biotite diorite in the Y1 intrusion, biotite gabbro and biotite norite in the Y2 intrusion presented with ages of 287.4 ± 1.1, 281.7 ± 3.0 and 286.7 ± 1.1 Ma, respectively. Combined with the published data, our findings demonstrated that the Kalatongke sulfide deposit experienced two stages of magma replenishment, lasting at least 16 Myr (287–271 Ma). The sulfide-rich magmas were most likely transported upward from the staging chamber via magma overpressure attributable to considerable tectonic compressive stresses. It could be proposed that the sulfide-rich magmas were derived from a strongly evolved magma chamber, whereas the sulfide-barren mafic suite was related to the emplacement of a weakly evolved magma in the same or a related plumbing system.

A Comparative Study of Sulfur Isotope Variations within the Flatreef and Merensky Reef of the Bushveld Complex, South Africa

ABSTRACT The Flatreef, a down-dip, sub-horizontal extension of the Platreef, which underlies the Turfspruit and Macalacaskop farms, represents the future of platinum mining in South Africa. The stratigraphic connection between the Platreef, located at the base of the northern limb of the Bushveld Complex, and the Merensky Reef in the western and eastern limbs of the complex, was disputed for many years due to the heterogeneous nature of the Platreef along strike. However, the discovery of the Flatreef led to a new perspective on the Platreef as the former allowed for the study of a magmatic stratigraphy less affected by footwall interaction. Here, we report whole-rock S isotope (δ34S) compositions across the stratigraphic units of the Flatreef and its footwall and hanging wall as intersected by boreholes UMT-276 and UMT-393, as well as stratigraphic units of the Merensky Reef at Two Rivers Platinum mine in the eastern limb. The units of the Flatreef containing platinum group element mineralization, namely the Main Reef and Upper Reef, have δ34S values that overlap with the range recorded for the Merensky Reef in the western and eastern limbs. In UMT-393, Main Reef δ34S values range between 0.2 and 1.5‰ (with the exception of three outliers, 9.7‰, 11.1‰, and 7.9‰), and 0.52‰ and 11.2‰ for two Upper Reef samples. However, in UMT-276, Main Reef δ34S values range between –0.96 and 2.24‰ and 3.19‰ was recorded for an Upper Reef sample. The S isotope compositions recorded for the Merensky Reef pyroxenite at Two Rivers Platinum mine are relatively higher with δ34S values ranging between 1.24 and 4.83‰. The top unit of the Flatreef, which is a transition zone below the Main Zone, as well as the Footwall Cyclic Unit have heavier S isotope compositions with δ34S values ranging between 6 and 17‰ for the former and 0.7 and 18.6‰ for the latter. At Two Rivers Platinum mine, the hanging-wall anorthosite has a δ34S value of 2.9‰ in contrast to the 5.7‰ measured for the footwall anorthosite and 3.27‰ for the footwall feldspathic pyroxenite. The consistent near-mantle S isotope signature and accompanying metal enrichment in the Main Reef of the Flatreef may be explained by extensive interaction of sulfide minerals in a Lower Zone conduit/pre-Platreef staging chamber with large volumes of uncontaminated magma. The δ34S values of the Merensky Reef at Two Rivers Platinum mine are slightly higher compared to that of the Main Reef at Turfspruit and Macalacaskop possibly due to interaction with underlying carbonate rocks.

Transgressive nature and chilled margins of the Upper Zone in the western Bushveld Complex, South Africa

ABSTRACT The Upper Zone of the Bushveld Complex has long been known to have formed from a major influx of magma into the chamber that caused large-scale erosion of the chamber floor cumulates. The most dramatic manifestations of this process are two major gap areas (Northern and Southern) in the western Bushveld Complex in which the Upper Zone appears to have eroded away the underlying cumulates down to the very base of the Complex. However, due to almost complete lack of outcrops in the gap areas, no direct field observations have ever been reported to confirm the transgressive nature of the Upper Zone. Here, we present for the first time such observations from the Kameelhoek chromite mine located at the margin of the Northern Gap. In the open pit we have documented several transgressive depressions (up to 40 m in width) in the orthopyroxenite and chromitites of the Lower Critical Zone that are filled in with magnetite gabbro of the Upper Zone. The magnetite gabbro is chilled against the sidewalls of the depressions, forming glassy and fine-grained textured rocks with plagioclase laths arranged in radial clusters. Mineralogically and chemically, the magnetite gabbro correlates with cumulates from the lowermost part of the Upper Zone at its normal position in the complex. Three major points that have emerged from this study are: (1) the Critical Zone has been eroded away by magma that was parental to the Upper Zone, (2) this eroding magma was not the one that initiated formation of the Pyroxenite Marker, but rather the evolved melt that replenished the chamber at some later stage, and (3) the melt was phenocryst-free and likely derived from a deep-seated staging chamber. Our study thus supports a recent notion that even during the formation of the Upper Zone, the Bushveld chamber had still been operating as an open system that was replenished by melts from deeper magma sources.

Platinum mineralisation in the Owendale Uralian-Alaskan-type complex, New South Wales, Australia: The effects of serpentinization on Cu-PGE-Ni sulphides

Significant, albeit highly erractic, PGE mineralization is hosted in the ultramafic rocks of the Owendale Complex, a Uralian-Alaskan type intrusion in the Fifield region of central New South Wales, Australia. Three types of PGE mineralization have been identified in drill core samples of these rocks. The first to be recognized was PGE mineralization associated with “P-units” which are narrow (a few millimetre to 2 m wide) lenses of pegmatoidal clinopyroxenites within olivine clinopyroxenites; the best intersection of this type of mineralization averaged 13.19 g/t Pt and 0.90 g/t Pd over 1.57 m, has very low S contents and contains a primary magmatic PGM assemblage. Two types of PGE mineralization occur within dunites and wehrlites, a low-Cu Pt type and a high-Cu Pt type of mineralization. The low-Cu Pt type of mineralization consists mainly of disseminated Cu-Pt-(Fe) alloys that lie along the mesh lines of partially serpentinized olivine; this type of mineralization can have grades of up to 25 g/t Pt but very low amounts of Cu, S, Au and Pd. The high-Cu Pt type of mineralization consists of disseminated, partially oxidised PGE-rich Cu sulphides with Pd-bearing PGM; this type of mineralization has up to 25 g/t Pt and elevated levels of Cu, S, Se, Au and Pd. The Pt:Ir:Rh ratios of the low-Cu Pt and the high-Cu Pt types of mineralization are virtually identical attesting to a common source, but with variable removal of the more mobile Pd, Au, Cu and Se. Some of the high-Cu Pt type sulphides are associated with an unusual Cr-Al-Ti-Mg-rich magnetite that occurs in veins and pods that are up to 15 cm thick. Phlogopite is a common accessory mineral in the dunites and wehrlites, occurring as disseminated grains and pods of mica that are up to 4 cm wide, and as selvedges along the Cr-Al-Ti-Mg-rich magnetite veins. Owendale represents a staging chamber in which olivine cumulates, followed by clinopyroxenite cumulates, were formed and later infiltrated from above by immiscible Fe-rich and Cu-sulphide melts. It is suggested that the phlogopite was produced by K2O-rich fluids released from the monzonites and diorites that intruded the ultramafics and which also drove serpentinisation. Serpentinisation altered the dunite/wehrlites with small amounts of sulphides to remove all the S and most of the Cu, Au, and Pd to leave only Cu-Pt-(Ni-Fe) alloys, forming the low-Cu Pt type of mineralization, whereas the magmatic sulphides in dunites/wehrlites with larger amounts of sulphides were only partially altered, forming the high-CuPt type of mineralization. The Pt alloys produced during serpentinization of the PGE-rich Cu sulphides at Owendale are predominantly Cu-rich, unlike the Pt-Fe alloys found in the placer deposits associated with most Uralian-Alaskan complexes. However, the Owendale Complex was lateritized, as were the Uralian-Alaskan Complexes in the Urals. It is possible that during lateritization, the Cu-Pt-Fe alloys were converted into Pt-Fe alloys similar to those found in the Pt placers.

The geology, geochemistry, and petrogenesis of the Huckleberry Cu-Ni-PGE prospect in the Labrador Trough, Canada: Perspectives for regional prospectivity

The Palaeoproterozoic Huckleberry Cu-Ni-(PGE) prospect in the Labrador Trough, northern Québec, represents a ~ 400-m-thick, out-of-sequence sill complex that comprises a ~200-m-thick glomeroporphyritic gabbro, intruded in its centre by a ~200-m-thick differentiated gabbro-peridotite sill and in its gabbroic footwall, several thinner (<30 m) ultramafic sills. Globular sulphides are present at the base of the sill complex, whereas disseminated to net-textured sulphides occur in the ultramafic units (Cu/Ni = 0.1–0.8) as well as their gabbroic footwall (Cu/Ni = 1–3). The glomeroporphyritic gabbro sill stack (MgO ~4 wt%, TiO2 ~0.6 wt%, Na2O + K2O ~2–3%, Eu/Eu* ~ 1.2, Anplg ~70–60) comprises several sills characterised by sharp changes in size and abundance of plagioclase glomerocrysts. We hypothesise that the glomerocrysts represent remobilised crystal mushes that were dislodged from a floatation cumulate in a staging chamber during episodic expulsion of magma. In the central gabbro-peridotite sill, mineral compositions (Foolv ~75–75, Mg#opx ≈ 78–68, Anplg ≈ 78–70) and whole-rock data (MgO ≈ 22 – 27%, TiO2 ≈ 0.4%) suggest that the parent magma was an olivine-saturated basalt containing 8–9 wt% MgO. Whole-rock geochemical data further suggests that the parent magmas did not undergo any significant contamination (La/SmN < 2, S/Se < 4000). The Cu/Pd values of ultramafic units (>10,000) suggest sulphide melt saturation was attained before their final emplacement in magma feeder conduits or staging chambers at relatively low R factors (1000–5000). Downward decreasing concentrations of chalcophile elements in the drill cores suggests that sulphide melt percolated downward from the ultramafic cumulate units into the glomeroporphyritic gabbro footwall. We propose that the footwall ultramafic sills represent downward injections of olivine and sulphide melt from the overlying gabbro-peridotite sill, which mechanically concentrated high volumes of sulphide in narrow sills.

Strontium isotope variations in the Flatreef on Macalacaskop, northern limb, Bushveld Complex: implications for the source of platinum-group elements in the Merensky Reef

The Platreef of the Bushveld Complex is one of three key stratigraphic units, along with the Merensky and UG-2 reefs, in which platinum-group elements are concentrated. The correlation between the Platreef and the stratigraphy from the western and eastern limbs of the BC has been challenging due to the heterogeneous and variable nature of the Platreef along strike. However, the discovery of the Flatreef, interpreted as the down dip, sub-horizontal extension of the Platreef, on the farms Turfspruit and Macalacaskop, opened new avenues for enquiry that allow the study of a magmatic stratigraphy less affected by footwall interaction. In this study, we report on the Sr isotope composition of plagioclase through an ~ 280-m-thick intersection of the Flatreef and its footwalls and hanging walls as intersected by borehole UMT-393 drilled on the farm Macalacaskop. Comparison of the Sr-isotopic composition and the anorthite content of plagioclase across this intersection with data from the Merensky and Bastard cyclic units elsewhere in the Bushveld Complex supports the contention that the Flatreef is a correlative of the Upper Critical Zone–Main Zone transition, including the Merensky and Bastard cyclic units. The available data suggest that the sulfides of the Flatreef and its correlatives in the remainder of the Bushveld Complex originally formed in a deeper staging chamber. During continued magma ascent some of the sulfides were entrained and deposited in the Flatreef.

In Situ Multiple Sulfur Isotope and S/Se Composition of Magmatic Sulfide Occurrences in the Labrador Trough, Northern Quebec

AbstractThe interaction between mafic-ultramafic magma and crustal sulfide is considered a key process in the formation of magmatic Ni-Cu-platinum group element (PGE) sulfide deposits. Integrated S/Se and multiple sulfur isotope studies are the most robust in constraining the role of crustal sulfur during ore genesis. In the present study, we report the first integrated S/Se and multiple sulfur isotope study of magmatic sulfide occurrences in the Labrador Trough, namely, on the recently discovered Idefix PGE-Cu and Huckleberry Cu-Ni-(PGE) prospects.Whole-rock and in situ S/Se values (~810–3115) of magmatic sulfides and their host rocks are consistent with S loss during postmagmatic hydrothermal alteration, negating their use in interpreting the origin of S. Values of ∆33S ~0 indicate no record of the assimilation of Archaean sulfur. Disseminated (–0.5 to +2.5‰) and globular (3.0–4.5‰) sulfides at Idefix as well as globular sulfides (2.1–9.6‰) at Huckleberry have δ34S values greater than the accepted mantle range, suggesting that crustal S played a role in the formation of these sulfides. In contrast, disseminated and net-textured sulfides at Huckleberry have variable δ34S values (–4.6 to +3.2‰) that are mostly within the accepted mantle range, excluding one anomalous sample that records relatively higher δ34S values (11.9–15.0‰). It is proposed that sulfide melt segregated in response to the addition of small proportions of crustal S prior to the final emplacement of the host intrusions, i.e., in a feeder conduit or staging chamber. Isotopic exchange between the sulfide melt and silicate magma has diluted and, in places, eradicated a crustal δ34S signature.

Non-Sequential Injection of PGE-rich Ultramafic Sills in the Platreef Unit at Akanani, Northern Limb of the Bushveld Complex: Evidence from Sr and Nd Isotopic Systematics

Abstract The Platreef Unit is a deceptively complex sequence of layered cumulates located in the northern limb of the 2·055 Ga-old Bushveld Complex. The unit contains the Platreef, a thick, richly mineralized stratabound PGE orebody which differs markedly from the comparatively thin, predominantly stratiform Merensky Reef found in the Upper Critical Zone of the eastern and western limbs. The Platreef Unit is interpreted as a localized facies of the Upper Critical Zone, despite layering being neither as systematic nor as clearly defined as in the equivalent stratigraphy found in the other limbs. The Platreef Unit in the Akanani project area includes well-defined layers of feldspathic harzburgite and norite, in addition to the ubiquitous feldspathic orthopyroxenite–melanorite that characterizes other sections. The paucity of floor-rock xenoliths is an additional feature. The relatively well-developed nature of the layering and paucity of xenoliths in the Platreef Unit at Akanani is explained by separation of the unit from the floor of the intrusion by a thick succession of ultramafics assigned to the Lower Critical Zone. We identify three lithological subgroups in the Platreef Unit at Akanani. They do not define an upward-younging stratigraphy. The primary stratigraphy, or PU1 subunit, is dominated by multiple layers of feldspathic orthopyroxenite, melanorite, and norite. This subunit built up from incremental addition of relatively small magma pulses. Repeated magma replenishment induced concomitant partial melting of earlier-formed layers. The PU1 subunit includes thin chromite stringers that contain Cr-spinels with unusual, amoeboidal textures consistent with several stages of growth and re-equilibration. The feldspathic harzburgite of the younger PU2 subunit was emplaced non-sequentially into the already complexly-layered PU1 subunit as a series of sinuous lenses or syn-intrusive sills. One of the PU2 sills contains the richest and most consistent of the mineralized sections at Akanani, i.e., the Main Mineralized Reef (MMR). The irregularly-developed pegmatoidal lithologies of the PU3 subunit are ascribed to recrystallization of earlier-formed cumulates (PU1 and PU2). Whole rock isotopic data for a section of the Platreef Unit, together with the overlying Lower Main Zone and underlying Lower Critical Zone, mostly from drill-hole ZF-1, demonstrate a complex pattern in both Sr87/Sr86 initial ratios and ϵNd values. These patterns are consistent with multiple lineages of parental magmas. The Lower Main Zone and the majority of the Platreef Unit are characterized by anomalously high Sr initial ratios (with a large degree of scatter) and low ϵNd values (relatively tightly constrained). Harzburgite layers from the Lower Critical Zone have a low Sr initial ratio and a relatively high ϵNd value. The new isotopic data suggest these sequences crystallized from multiple magma batches, broadly constrained within the U-type (ultramafic) and A-type (tholeiitic) lineages, derived from mantle sources and/or staging chambers which experienced varying degrees of crustal contamination. The MMR crystallized from a specific pulse of the U-type magma lineage characterized by a high Sr87/Sr86 initial ratio (average of 0·71113) and a markedly low ϵNd value (average of -11·35). The olivine-saturated magmas associated with the MMR were derived from a localized mantle source and yet underwent an unusually high degree of crustal contamination. Some layered PGE orebodies in the Bushveld Complex, including the Platreef and Merensky Reef, were emplaced as syn-magmatic sills which crystallized from anomalously PGE-rich parental magmas with an unique isotopic fingerprint.

Mantle source to near-surface emplacement of enriched and intermediate poikilitic shergottites in Mars

Poikilitic shergottites make up >20% of the current martian meteorite collection, with a total of 27 samples. These meteorites are intrusive gabbroic to lherzolitic rocks and represent igneous materials recording important processes in the martian crust. To further constrain petrogenetic relationships amongst enriched and intermediate poikilitic shergottites, we studied a comprehensive suite of poikilitic shergottites — including four newly recovered samples (Northwest Africa [NWA] 11065, NWA 11043, NWA 10961, NWA 10618) — using bulk rock major- and trace-element compositions, mineral major-element compositions, oxygen fugacity (ƒO2) values, crystallization temperatures, phosphorus maps of olivine grains, and quantitative textural analyses. The characteristic bimodal textures (poikilitic and non-poikilitic textures) of poikilitic shergottites record evolving magmatic conditions at different stages of crystallization. Higher temperatures and more reducing conditions during early-stage crystallization are recorded in the poikilitic textures, while lower temperature and more oxidizing conditions are recorded in the non-poikilitic textures during late-stage crystallization. Oxygen fugacity estimates relative to the quartz-fayalite-magnetite (QFM) buffer for early-stage olivine-pyroxene-spinel assemblages of enriched and intermediate poikilitic shergottites suggest decoupling of ƒO2 and the degree of light rare earth element (LREE)-enrichment (i.e., [La/Yb]CI). An increase in ƒO2 exceeding 1 log unit from poikilitic to non-poikilitic textures implies degassing, with possible auto-oxidation, and/or crustal contamination. Quantitative textural analyses support the emplacement of both enriched and intermediate poikilitic shergottites as various shallow intrusive bodies, as well as a potentially widespread emplacement mechanism responsible for a major lithology of the martian crust. In addition, early assemblages (i.e., pyroxene oikocrysts) of all the poikilitic shergottites likely formed close to the crust-mantle boundary, implying a possible widespread presence of magma staging chambers at these depths. Fractional crystallization and magma storage in these chambers could have possibly resulted in all of the different enriched and intermediate shergottites that have been analyzed from Mars.