Platinum-group element mineralisation in the Unst ophiolite, Shetland

The ophiolitic basic and ultrabasic rocks of the island of Unst, Shetland comprise a sequence of harzburgites, dunites, clinopyroxene-rich cumulates, and gabbro, within tectonic blocks that have been thrust over a migmatite complex during the Laxer Palaeozoic. Concentrations of chromite are found in the harzburgite and dunite, and to a small extent in the pyroxene cumulate rocks. They occur as disseminations, sometimes forming millimetre scale layers, and as more massive schlieren and pods of chromitite. Five alteration or hydrothermal events have been recognised in the ultrabasic rocks. These comprise early pervasive serpentinisation, later fracture controlled serpentinisation, veining and pervasive carbonation, minor late serpentine veining and talc-carbonate alteration controlled by fault zones. Exploration for platinum group element (PGE) mineralisation uas carried out using a combination of drainage, overburden and rock sampling. Analyses of PGE were obtained by fire assay followed by either neutron activation analysis or flameless atomic absorption spectrometry, and up to 20 other elements Here determined by X-ray fluoresence analysis. Panned concentrate samples were taken from 73 drainage sites distributed throughout the complex. Ir, the only PGE determined in all samples, showed a greater concentration in samples derived from the harzburgite unit than those from other units. Lox amplitude anomalies are present in three discrete areas in the harzburgite but the maximum level of 210 ppb Ir is associated with a sample derived from a prominent N-S zone of faulting and hydrothermal activity markedly discordant to the regional trend of layering in the harzburgite and dunite. This discordant zone, which extends for at least 7 km, is also marked by samples containing enrichments in Fe, Co, Ni, Cu and As. The highest Cr levels are associated with an area in the north of the harzburgite with no previous history of chromite working but where many locally derived pieces of chromitite float have been discovered. Relatively high Cr levels are also associated nith the area of dunite containing the greatest concentration of visible chromite and old norkings. A technique of collecting panned heavy mineral concentrates from overburden samples was adopted as a reconnaissance exploration technique after orientation sampling in the harzburgite unit at Cliff, an area with high PGE levels in chromitite and associated dunite. Systematic sampling in the Cliff area outlined a zone of coincident Pd, Pt and Rh enrichment near to but separate from the chromite workings knorrn to be enriched in PGE. In contrast the distribution of Ru was entirely different with scattered lon amplitude anomalous zones and a maximum anomaly 300m from the chromite-rich zone. Reconnaissance lines were sampled at other locations within the harzburgite, dunite and cumulate units. Lore amplitude Pd and Pt anomalies were detected xithin the dunite unit, especially in 'a traverse across the trace of the prominent N-S fault zone at Helliers Uater, adjacent to the outcrop of the cumulate unit. In general the overburden data suggest some association between PGE enrichment and enhanced levels of Ni relative to typical silicate levels apparent when expressed as the ratio Ni/MgC. Rock samples Here collected from all parts of the complex, including most of the main chromitite workings. Very high levels of all PGE occur in samples of chromitite, chromite-rich dunite and dunite from the Cliff area, with a strong positive intercorrelation between all PGE. The proportions of the various PGE are very similar to those present in deposits in major layered basic/ultrabasic complexes like Bushveld and Stillwater, irith strong relative enrichment in Pd and Pt. These PGE proportions are completely different from the Ru-Ir-0s dominant assemblage typical of ophiolitic rocks. Associated with high levels of PGE are enrichments in Ni, Cu, As, Sb and Te. There is no correlation rrith Cr and some samples of chromitite from the Cliff area contain only background levels of PGE. High to moderate levels of PGE with the same proportions of elements as the Cliff samples also occur in samples of chromitite and serpentinised dunite from the dunite unit and in samples of pyroxenite from the cumulate unit, In contrast PGE-rich samples of chromitite from the harzburgite unit near Harold's Grave have entirely different proportions of PGE with Ru and Ir in greatest abundance. This PGE distribution is similar to that in some background samples of harzburgite and closely resembles the pattern found in typical ophiolites. The PGE in the Harold's Grave samples do not exhibit the Ni enhancement noted in the Cliff PGE mineralisation. In samples from the Cliff area the platinum-group minerals (PGM) sperrylite, stibiopalladinite, hollingnorthite, laurite and possibly irarsite have been identified, mostly as grains less than 10 microns in size. In chromite-rich rocks these minerals occur Rithin chlorite haloes around chromite, in the blackened altered rims of chromite grains and in interstitial Ni-rich serpentine/carbonate intergroxths in association with pentlandite, orcellite and other Ni sulphides and arsenides, sometimes spatially related to chlorite-carbonate-magnetite veins. They also occur as fine grains Rithin magnetite rims around chromite and in magnetite or carbonate veins in dunite. The Ni sulphide/arsenide assemblage associated Rith the PGH is characteristic of serpentinisation at temperatures less than 500'C, Rell belox the range of magmatic conditions. A hydrothermal origin for the PGE mineralisation is proposed, probably related to the second phase of serpentinisation. This involved the redistribution of Ni accompanied by the introduction of As, Sb and Te probably with a StrUCtUral Control. Pre-existing concentrations of chromite may have acted as a precipitation barrier causing rich PGH deposition in the alteration haloes around chromite grains. Continuous borehole or trench sections through mineralised zones are required to assess the economic significance of the PGE mineralisation. Nevertheless the high levels of PGE attained and the evidence of xidespread occurrence of the Cliff-type PGE enrichment are favourable indications. The PGE enrichments found in the cumulate complex are of potential interest as they may originally have been of magmatic origin. Larger tonnage targets may therefore be present in this unit compared Rith the likely size of structurally-controlled mineralisation elswhere in the complex

Experimental petrology constraints on the recycling of mafic cumulate:a focus on Cr-spinel from the Rum Eastern Layered Intrusion, Scotland

Reactive liquid flow is a common process in layered intrusions and more generally in episodically refilled magma chambers. Interaction between newly injected melt and cumulates, or crystal mushes, perturbs the liquid line of descent of the melt and modifies mineral chemistry and texture. We present insights into the effects of assimilation of mafic cumulate rocks (gabbro, troctolite) by cogenetic Mg-rich basalt liquid using one-atmosphere, controlled fO2 phase equilibrium experiments on picritic parental liquid to the Rum layered intrusion, Scotland. For picrite-only experiments at fO2 = QFM, Cr-spinel (Cr# = Cr/[Cr + Al + Fe3+] = 0.43; Fe# = Fe2+/[Mg + Fe2+] = 0.32) saturates at 1320 °C, olivine (Fo88) at ~1290 °C, plagioclase (An77) at 1200 °C, and clinopyroxene (Mg#: 0.81) at 1180 °C. In melting experiments on picrite + gabbro mixtures, plagioclase (1230 °C, An80) and clinopyroxene (1200 °C, Mg#: 0.85) saturation temperature and mode are increased significantly. Cr-spinel in these experiments has a distinctive, low Fe#. In melting experiments on picrite + troctolite mixtures, plagioclase (An86) saturates at 1240 °C and clinopyroxene (Mg#: 0.81) at 1170 °C. Al-rich spinel crystallizes at high temperature (>1220 °C) and becomes more Cr-rich upon cooling, reaching the highest Cr# = 0.47 at 1180 °C (0.54 at QFM-1.2). The experimental results confirm that plagioclase and clinopyroxene stability plays a major role in determining the composition of coexisting spinel. Comparing our experimental results to the Rum Eastern Layered Intrusion, we propose a model for the precipitation of spinel from picrite–troctolite hybrid melt that is compatible with the observed olivine, plagioclase, and clinopyroxene chemistry.ISSN:0010-7999ISSN:1432-096

Platinum-group element signatures in the North Atlantic Igneous Province: Implications for mantle controls on metal budgets during continental breakup

This is the final version of the article. Available from the publisher via the DOI in this record.The North Atlantic Igneous Province (NAIP) is a large igneous province (LIP) that includes a series of lava suites erupted from the earliest manifestations of the (proto)-Icelandic plume, through continental rifting and ultimate ocean opening. The lavas of one of these sub-provinces, the British Palaeogene Igneous Province (BPIP), were some of the first lavas to be erupted in the NAIP and overlie a thick crustal basement and sedimentary succession with abundant S-rich mudrocks. We present the first platinum-group element (PGE) and Au analyses of BPIP flood basalts from the main lava fields of the Isle of Mull and Morvern and the Isle of Skye, in addition to a suite of shallow crustal dolerite volcanic plugs on Mull, and other minor lavas suites. BPIP lavas display both Ssaturated and S-undersaturated trends which, coupled with elevated PGE abundances (NMORB), suggest that the BPIP is one of the most prospective areas of the NAIP to host Ni–Cu–PGE–(Au) mineralisation in conduit systems. Platinum-group element, Au and chalcophile element abundances in lavas from West and East Greenland, and Iceland, are directly comparable to BPIP lavas, but the relative abundances of Pt and Pd vary systematically between lavas suites of different ages. The oldest lavas (BPIP and West Greenland) have a broadly chondritic Pt/Pd ratio (~1.9). Lavas from East Greenland have a lower Pt/Pd ratio (~0.8) and the youngest lavas from Iceland have the lowest Pt/Pd ratio of the NAIP (~0.4). Hence, Pt/Pd ratio of otherwise equivalent flood basalt lavas varies temporally across the NAIP and appears to be coincident with the changing geodynamic environment of the (proto)-Icelandic plume through time. We assess the possible causes for such systematic Pt/Pd variation in light of mantle plume and lithospheric controls, and suggest that this reflects a change in the availability of lithospheric mantle Pt-rich sulphides for entrainment in ascending plume magmas. Hence the precious metal systematics and potential prospectivity of a LIP may be affected by contamination of plume-derived magmas by subcontinent.HSRH acknowledges the financial support of the Natural Environment Research Council (NERC) for funding this work (studentship NE/ J50029X/1) and open access publication

A mineral-scale investigation of the origin of the 2.6 Ma Füzes-tó basalt, Bakony-Balaton Highland Volcanic Field (Pannonian Basin, Hungary)

Abstract The alkaline basalt of the Füzes-tó scoria cone is the youngest volcanic product of the Bakony-Balaton Highland Volcanic Field. The bombs and massive lava fragments are rich in various crystals, such as mantle-derived xenocrysts (olivine, orthopyroxene, clinopyroxene, spinel), high-pressure mineral phases (clinopyroxene) and phenocrysts (olivine, clinopyroxene). Peridotite xenoliths are also common. Ratios of incompatible trace elements (Zr/Nb and Nb/Y) suggest that the primary magma was formed in the transitional spinel-garnet stability field, at the uppermost part of the asthenosphere. Magmatic spinel inclusions with low-Cr# (22–35) in olivine phenocrysts can reflect a fertile peridotite source. The olivine, orthopyroxene, colourless clinopyroxene and spinel xenocrysts are derived from different depths of the subcontinental lithospheric mantle and their compositions resemble the mineral phases of the ultramafic xenoliths found in this region. The rarer green clinopyroxene cores of clinopyroxene phenocrysts could represent high-pressure products of crystallization from a more evolved melt than the host magma, or they could be derived from mafic lower crustal rocks. Crystallization of the basaltic magma resulted in olivine and clinopyroxene phenocrysts. Their compositions reflect polybaric crystallization with a final, strongly oxidized stage. The Füzes-tó basalt does not represent a certain magma composition, but a mixture of mineral phases having various origin and mantle-derived basaltic melt

Platinum-group element mineralisation in the Shetland ophiolite complex

The Shetland ophiolite complex exhibits the lower part of the definitive Penrose sequence, but chromitites from the Cliff locality contain levels of Pt and Pd which are unusual in ophiolites. Aspects of this mineralisation suggest both magmatic and hydrothermal concentration processes, although neither individually explain the observed paragenesis. This study examines the distribution of PGE within a representative fresh vertical section of the ophiolite stratigraphy, to assess the relative roles of such processes in the concentration and fractionation of PGE. In the field area around Balta Sound, tectonised mantle harzburgites are overlain by a layered ultramafic cumulate sequence. Accessory Ni-Cu sulphides occur throughout these cumulates. Two stratigraphically controlled zones of chromite enrichment have been identified which mark the bases of cyclic repetitions. Such chromite enrichments lack progressive changes in mineral chemistry so repetitions within the cumulate sequence are attributed to influxes of primitive magma during open system fractionation. The distribution of PGE and other chalcophiles show primary lithological associations, indicating a magmatic origin. Os-Ir-Ru concentrations occur in cbromitites, whereas Pt-PdRh concentrations occur in sulphide-bearing dunites, chromite-rich dunites, or basal pyroxenites. Stratigraphically controlled zones of PGE-enriched sulphide-bearing cumulate dunites occur close to the base of each cyclic repetition and are continuous over a strikelength of 2 km. The fractionation between PGE and other cha1cophiles is consistent with magmatic processes. The distribution and fractionation of PGE can be explained by an open system model. Negative slope patterns are produced by the precipitation of PGM from PGE saturated magmas during chromite crystallisation. This initial process buffers the PGE concentrations in each primitive magma pulse. On entering the axial chamber and mixing with the evolving resident magma, sulphide saturation occurs and the remaining PGE are partitioned into immiscible sulphide liquids. Multiple inputs result in repeated PGE enriched zones. This model explains why successive stratigraphic PGE-enriched horizons have similar positive slope patterns but progressively fractionated host silicate and sulphide compositions

Spinel group minerals in metamorphosed ultramafic rocks from Río de Las Tunas Belt, Central Andes, Argentina

In the Río de Las Tunas belt, Central Andes of Argentina, spinel group minerals occur in metaperidotites and in reaction zones developed at the boundary between metaperidotite bodies and their country-rocks. They comprise two types: i) Reddish-brown crystals with compositional zonation characterized by a ferritchromite core surrounded by an inner rim of Cr-magnetite and an outer rim of almost pure magnetite. ii) Green crystals chemically homogeneous with spinel (s.s.) and/or pleonaste compositions. The mineral paragenesis Fo+Srp+Cln+Tr+Fe-Chr and Fo+Cln+Tr+Tlc±Ath+Fe-Chr observed in the samples indicate lower and middle grade amphibolite facies metamorphic conditions. Nonetheless, the paragenesis (green)Spl+En+Fo±Di indicates that granulite facies conditions were also reached at a few localities. Cr-magnetite and magnetite rims in zoned reddish-brown crystals and magnetite rims around green-spinel/pleonaste grains are attributed to a later serpentinization process during retrograde metamorphism. The chemical composition of spinel group minerals in the ultramafic reaction zones is determined by the mineral zone where they crystallize. Green pleonaste occurs in the chlorite zone, ferritchromite predominates in the amphibole zone, whereas Cr-magnetite and magnetite are more common in the carbonate zone. The mineral paragenesis of the Río de Las Tunas metaperidotites together with the chemical characteristics of the spinel group minerals support a clockwise P-T path evolution for the ultramafic protoliths during the Paleozoic regional metamorphic cycle of this area.Fil: Gargiulo, Maria Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Geológico del Sur; Argentina. Universidad Nacional del Sur; ArgentinaFil: Bjerg, Ernesto Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Geológico del Sur; Argentina. Universidad Nacional del Sur; ArgentinaFil: Mogessie, A.. Karl-Franzens Universität Graz; Austri

Linking in situ Crystallisation and Magma Replenishment via Sill Intrusion in the Rum Western Layered Intrusion, NW Scotland

The construction of layered mafic-ultramafic intrusions has traditionally been attributed to gravity driven accumulation, involving the mechanical settling of crystals onto the magma chamber floor, at the interface between the crystal mush at the base and overlying replenishing magma, such that the layered sequence of cumulates (i.e., the crystal mush) at the floor aggrades upwards. The Rum Western Layered Intrusion (WLI) is a ~250 m sequence of layered peridotite cumulates comprising the structurally lowest portion of the Rum Layered Suite (RLS). As such, it is taken to represent the oldest sequence in the RLS and has been assumed to young upwards. The WLI hosts the largest proportion of harrisite, a cumulate composed of skeletal olivine that formed by in situ crystallisation, in the Rum layered intrusion. Harrisite layers in the WLI ubiquitously exhibit extremely irregular upward-oriented apophyses, up to several metres high and metres across, alongside laterally extensive dome-like structures; features consistent with intrusive, sill-like emplacement of harrisite. The distribution and abundance of harrisite therefore points to chaotic sill-like emplacement of the magmas that produced at least half of the WLI cumulate. This probably occurred various ambient crystal mush temperatures and punctuated intervals during cumulate formation. The harrisite layers are associated with numerous Cr-spinel seams occurring along the tops, bases, and interiors of these layers, suggesting they formed in situ alongside harrisite sills within the crystal mush. Detailed quantitative textural and mineral chemical analysis of Cr-spinel seams support a simple in situ crystallisation process for their formation. It is suggested the Cr-spinel seams form within melt channels that develop along the same hot tears that allowed the harrisite parental melts to enter the crystal mush. The chemistry and texture of Cr-spinel is controlled by the volume of through-flow of melt through the melt channel. Where melt flux through channels was high, sulphide and platinumgroup minerals are more abundant, highlighting the key economic implications of this model for the platinum-group element enrichment of chromitite horizons in layered intrusions. We also highlight the role of infiltration metasomatism at multiple levels of the WLI, where porous percolation of interstitial melt and reactive liquid flow played a key role in cumulate formation, supporting the notion of layered intrusion growth by incremental sill emplacement

The gradual shift from forearc basalt-like to boninite-like magmatism during intra-oceanic subduction-initiation recorded by ophiolitic chromite deposits from Cuba

Cuba contains the largest number of ophiolitic chromite deposits throughout the Americas. Most of these deposits are found within the mantle section of the Eastern Cuba and Camagüey ophiolitic massifs, which contain four different chromite mining districts (Camagüey, Mayarí, Sagua de Tánamo, and Moa-Baracoa). In addition to their potential as economic resources, chromite deposits are also excellent petrogenetic indicators to interpret the nature of ancient upper mantle, processes of melt formation in the mantle, and large-scale geodynamic processes. In this sense, major and trace elements of unaltered Cr-spinel cores together with chromitite whole-rock PGE composition reveal that high-Al Camagüey and Moa-Baracoa chromite districts were formed in equilibrium with forearc basalts (FAB)-like magmas during the incipient intra-oceanic subduction of the proto-Caribbean lithosphere underneath the Caribbean lithosphere, in a subduction-initiation process. Conversely, the high-Cr Mayarí chromite district was formed in equilibrium with more hydrated melts of boninitic affinity, typical of a more advanced stage of the subduction-initiation process. Nonetheless, the shift from FAB-like to boninite-like magmatism in an intra-oceanic subduction is gradual. This progressive change is well-recorded in the Sagua de Tánamo district that contains both high-Al and high-Cr chromitites. Thus, the studied ophiolitic chromitites allow tracing the complete magmatic evolution of an intra-oceanic subduction-initiation process. Furthermore, our data exhibits that accessory Cr-spinel composition of peridotites surrounding chromitites can be used as a prospecting indicator to anticipate the composition of ophiolitic chromitite bodies. Systematically, Cr-spinel from dunites associated with high-Al chromite deposits have lower Cr# values compared to the accessory Cr-spinel from the associated harzburgites. On the contrary, Cr-spinel from dunites of high-Cr chromite deposits show higher Cr# compared to the accessory Cr-spinel from the host harzburgites

Geochemistry of mantle peridotites and chromitites from Upper Cretaceous ophiolites in NW Türkiye: Insights into abyssal to forearc mantle settings

This paper presents new data on whole-rock and mineral chemistry, trace elements of clinopyroxene (Cpx) and chromite, along with detailed petrographic observations of mantle peridotites and chromitites from the NW Anatolian (Harmancık-Orhaneli, Bursa) ophiolites to better understand contrasting geochemical signatures and different stages of lithospheric evolution along the İzmir-Ankara-Erzincan Ocean (İAEO) in northern Türkiye. The mantle peridotites, characterized by abundant high-grade podiform chromitites in Harmancık and banded chromitites in Orhaneli, are particularly notable. The mantle rocks include depleted- lherzolites, Cpx-rich harzburgites, harzburgites, Cpx-poor harzburgites, and dunites. Compositional variations in the mantle peridotites from both Harmancık and Orhaneli reflect continuous evolution driven by melt depletion and metasomatic enrichment occurring in both mid-ocean ridge (MOR) and supra-subduction zone (SSZ) environments. Our modeling suggests that the depleted lherzolites and Cpx-rich harzburgites in the Harmancık area are residues of anhydrous MOR-type melting of a fertile mid-ocean ridge mantle (FMM), with partial melting degrees of approximately 12–17% and 15–22%, respectively. In contrast, the harzburgites, Cpx-poor harzburgites, and dunites in the Harmancık area represent residues after 20–25% partial melting of a depleted MORB-type mantle, which itself was a residue following around 22% partial melting of an FMM during subduction initiation in the Late Cretaceous. The mantle peridotites in the Orhaneli area are comparatively more depleted, comprising Cpx-rich harzburgites, harzburgites, and dunites. This is supported by highly depleted rare earth element (REE) contents, similar to SSZ-type peridotites, indicating residues after 20–35% partial melting of a depleted MORB-type mantle (DMM). Furthermore, the compositions of parental magmas, as well as the trace element contents of chromites from the high-Cr chromitites and associated dunites in both Harmancık and Orhaneli, suggest that they were generated from boninitic melts in a supra-subduction zone (SSZ) environment. All the data suggests that the peridotites with high-grade podiform chromitites in the Harmancık region form the deeper mantle levels, whereas the banded chromitites within extensive dunite bodies in the Orhaneli region suggests proximity to the Moho transition zone, derived from the İAE Ocean in Late Cretaceous