Metallogenic fingerprint of a metasomatized lithospheric mantle feeding gold endowment in the western Mediterranean basin

We thank the Associate Editor W.U. Reimold, and the referees T. Jalowitzki and M.L. Fioren-tini for their constructive reviews of the submitted version of the manuscript. This research was sup-ported by the BES-2017-079949 Ph.D. fellowship to ES. The Spanish projects PID2019-111715GB-I00/AEI/10.13039/501100011033 and RTI2018-099157-A-I00 provided funding for field emission gun-environmental scanning electron microscopy (FEG-ESEM) and electron microprobe microanaly-ses (EMPA) /laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of sulfides, respectively, while the Junta de Andalucia project FUMESA B-RNM-189-UGR18 financed LA-ICP-MS analyses of silicates. Research grants, infrastructures, and human resources leading to this research have benefited from funding by the European Social Fund and the European Regional Development Fund. We thank Jesus Montes Rueda (Universidad de Granada) , Isabel Sanchez Almazo (Centro de Instru-mentacion Cientifica [CIC] , Universidad de Granada) , Xavier Llovet (Centres Cientifics i Tecnolgics, Uni-versitat de Barcelona) , Miguel Angel Hidalgo La-guna (CIC, Universidad de Granada) , and Manuel Jesus Roman Alpiste (Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Cientificas-Universidad de Granada) for their careful technical assistance during sample preparation, FEG-ESEM, electron microprobe analyzes, and LA-ICP-MS analyses, respectively.Spinel peridotite xenoliths (one plagioclase- bearing) hosted in alkaline basalts from Tallante (southeast Spain) record the mineralogical and geochemical fingerprint of the subcontinental lithospheric mantle (SCLM) evolution beneath the southern Iberian margin. Mantle metasomatism in fertile lherzolites caused the crystallization of clinopyroxene + orthopyroxene + spinel clusters through the percolation of Miocene subalkaline melts during the westward migration of the subduction front in the western Mediterranean. In the Pliocene, heat and volatiles provided by alkaline host-magmas triggered very low melting degrees of metasomatic pyroxene-spinel assemblages, producing melt quenched to silicate glass and reactive spongy coronae around clinopyroxene and spinel. Refertilization of the Tallante peridotites induced the precipitation of base-metal sulfides (BMS) included in metasomatic clino- and orthopyroxene. These sulfides consist of pentlandite ± chalcopyrite ± bornite aggregates with homogeneous composition in terms of major elements (Ni, Fe, Cu) and semi-metals (Se, As, Te, Sb, Bi), but with wide variability of platinum-group elements (PGE) fractionation (0.14 < PdN/IrN < 30.74). Heterogeneous PGE signatures, as well as the presence of euhedral Pt-Pd-Sn-rich platinum-group minerals (PGM) and/or Auparticles within BMS, cannot be explained by conventional models of chalcophile partitioning from sulfide melt. Alternatively, we suggest that they reflect the incorporation of distinct populations of BMS, PGM, and metal nanoparticles (especially of Pt, Pd, and Au) during mantle melting and/or melt percolation. Therefore, we conclude that Miocene subalkaline melts released by asthenosphere upwelling upon slab tearing of the Iberian continental margin effectively stored metals in metasomatized domains of this sector of the SCLM. Remarkably high Au concentrations in Tallante BMS (median 1.78 ppm) support that these metasomatized domains provided a fertile source of metals, especially gold, for the ore-productive Miocene magmatism of the westernmost Mediterranean.Junta de Andalucia B-RNM-189-UGR18European Social Fund (ESF)European Commission BES-2017-079949 PID2019-111715GB-I00/AEI/10.13039/501100011033 RTI2018-099157-A-I0

Platinum-group elements and gold in sulfide melts from modern arc basalt (Tolbachik volcano, Kamchatka)

Sulfide melt inclusions entrapped in primitive olivine phenocrysts can be used to understand the compositions of early sulfide melts that may ultimately contribute to magmatic sulfide ore deposits. Sulfide globules hosted in olivine (86–92 mol% Fo) from the Tolbachik basalt (the 1941 eruption) are characterized in terms of their major and trace element abundances using electron microscopy and LA–ICP–MS analysis. Distribution of major elements within individual sulfide globules varies from homogeneous to heterogeneous. Phases include monosulfide solid solution (MSS) and intermediate solid solution (ISS) intergrowths and exsolved low-temperature minerals such as pyrrhotite, pentlandite, chalcopyrite and cubanite. Trace elements (platinum-group elements — PGE, Ag, Te, Au, Pb and Bi) are also present in solid solution in sulfide phases and as micron-sized particles (“nuggets”). Such nuggets of dominantly Au, Pt, Au–Pd and Pd–Te are contained randomly within sulfide matrices or, more commonly, at phase boundaries. Nuggets are also attached to outer surfaces of sulfide globules. Concentrations of PGE in sulfides follow a log normal distribution over four orders of magnitude. The highest measured noble metal concentrations in the analyzed globules (436 ppm Au + PGE) are 13.3 ppm Au, 115 ppm Pt and 299 ppm Pd, whereas 40% of globules have < 15 ppm of noble metals. Gold and PGE concentrations correlate, suggesting these elements were concentrated by the same process(es). We propose that a number of anomalous concentrations of one or several noble metals in the analyzed globules can be best explained by entrapment of Au–PGE-rich particles (solid or liquid) from the silicate melt. Although the individual Tolbachik sulfide globules have variable PGE abundances, their mean composition resembles those of major PGE-sulfide ore deposits (e.g., Norilsk, Sudbury, Platreef and Merensky Reef).This study was supported by the Russian Science Foundation grant #16-17-1014

Mineralogy of Noble Metals and “Invisible” Speciations of These Elements in Natural Systems

This Special Issue covers a broad range of topics related to the mineralogy of noble metals (Au, Ag, Pt, Pd, Rh, and Ru) and the occurrence, formation, and distribution of these elements in natural ore-forming systems. This collection of eleven research articles discusses various problems related to these topics. I hope this Special Issue will contribute to a better understanding of the genesis of gold, silver, and other noble metal deposits as well as the behavior of these elements in endogenic and supergene environments, and suggest ways forward to solving the problem of their full extraction from ores

Extreme enrichment of Se, Te, PGE and Au in Cu sulfide microdroplets: evidence from LA-ICP-MS analysis of sulfides in the Skaergaard Intrusion, east Greenland

The Platinova Reef, in the Skaergaard Intrusion, east Greenland, is an example of a magmatic Cu–PGE–Au sulfide deposit formed in the latter stages of magmatic differentiation. As is characteristic with such deposits, it contains a low volume of sulfide, displays peak metal offsets and is Cu rich but Ni poor. However, even for such deposits, the Platinova Reef contains extremely low volumes of sulfide and the highest Pd and Au tenor sulfides of any magmatic ore deposit. Here, we present the first LA-ICP-MS analyses of sulfide microdroplets from the Platinova Reef, which show that they have the highest Se concentrations (up to 1200 ppm) and lowest S/Se ratios (190–700) of any known magmatic sulfide deposit and have significant Te enrichment. In addition, where sulfide volume increases, there is a change from high Pd-tenor microdroplets trapped in situ to larger, low tenor sulfides. The transition between these two sulfide regimes is marked by sharp peaks in Au, and then Te concentration, followed by a wider peak in Se, which gradually decreases with height. Mineralogical evidence implies that there is no significant post-magmatic hydrothermal S loss and that the metal profiles are essentially a function of magmatic processes. We propose that to generate these extreme precious and semimetal contents, the sulfides must have formed from an anomalously metal-rich package of magma, possibly formed via the dissolution of a previously PGE-enriched sulfide. Other processes such as kinetic diffusion may have also occurred alongside this to produce the ultra-high tenors. The characteristic metal offset pattern observed is largely controlled by partitioning effects, producing offset peaks in the order Pt+Pd>Au>Te>Se>Cu that are entirely consistent with published D values. This study confirms that extreme enrichment in sulfide droplets can occur in closed-system layered intrusions in situ, but this will characteristically form ore deposits that are so low in sulfide that they do not conform to conventional deposit models for Cu–Ni–PGE sulfides which require very high R factors, and settling of sulfide liquids

Dating Early Archean partial melting events: insights from Re-Os dating of micrometric Os-minerals from Kalahari Craton mantle xenoliths

Our understanding on the formation, mechanism and timing of the formation of the Earth’s earliest continents hinges on obtaining robust and accurate ages of melt depletion and lithosphere stabilisation. The Re-Os geochronometer has been used to obtain such information. However, recent advances indicate that metasomatism can impact on the signatures obtained. This thesis utilises the combination of Re-Os and highly siderophile element (HSE) systematics to accurately assess the impact secondary mineralisation has on the Re-Os ages determined. A suite of samples from the Kalahari craton (Kaapvaal and Zimbabwe cratons) are investigated in terms of their metasomatic overprint and Re-Os systematics. The sub-suite of peridotites from the Kaapvaal craton are first analysed for whole-rock major and trace elements, Re-Os and HSE systematics. The peridotites were also analysed for the 187Os/188Os compositions at the single grain base metal sulphide (BMS) and platinum-group mineral (PGM) scale. These analyses indicate that the Kaapvaal peridotites have experienced high degrees of melt depletion followed by variable and significant enrichment in the incompatible trace elements and HSE. The enrichment of HSE led to the precipitation of metasomatic BMS which impact on the Re-Os ages determined. As such, the whole-rock and single grain BMS TRD (rhenium depletion model age) indicate that the mantle was pervasively metasomatised as early as 3.2 Ga. Nano-particle PGM (Pt-alloys) with radiogenic 187Os/188Os (0.1294-0.1342) were found included within unradiogenic BMS (187Os/188Os 0.1066-0.1084). This signifies that the PGM formed in the presence of Re and evolved to high 187Os/188Os compositions over a long time scale. The Os composition of the Pt-alloys, combined with their nano-particle nature and the Os dichotomy with the host BMS signifies that the Pt-alloys formed in a HSE-Si-rich melt. This provides further evidence for the metasomatic overprinting of the Kaapvaal peridotites. Despite the high degree of metasomatism experienced by the Letlhakane peridotites (Zimbabwe craton), as evidenced by their re-enriched HSE-Se-Te systematics, single grain BMS preserve evidence of partial melting events. The 187Os/188Os analyses of the BMS provide ages >2.5 Ga older than the whole-rock. The oldest BMS TRD age of 3.7 Ga is preserved in a metasomatic BMS associated with secondary clinopyroxene and phlogopite. The attainment of an Eoarchean age from a metasomatic BMS suggests that the metasomatic fluid is able to entrain or nucleate on residual BMS. As such, the obtained ages reflect a mixing between the two different Os signatures. Whereas older ages reflect the dominance of the residual PGM on the bulk Os composition, younger ages are due to the control from the metasomatic melt. The 3.7 Ga TRD obtained age also pushes the age of initial stabilisation of the Zimbabwe lithosphere to within the age of the oldest crustal rocks in this region. The combined results of the Letlhakane and Kaapvaal peridotites indicate that whilst the Re-Os system can be affected by the metasomatic addition of BMS, the combination of HSE and Re-Os at the whole-rock and micro-scale can still resolve geologically significant ages

Les rôles de Te, As, Bi, Sb et Sn (TABS) lors de la formation des gisements d'éléments du groupe du platine

Les processus requis pour former une gisement magmatique a sulfures de Ni-Cu et elements du groupe du platine (EGP) comprennent: la saturation d'un magma mafique ou ultramafique avec un liquide sulfure de metal de base; interaction du liquide sulfure avec le magma mafique; cristallisation d'une solution solide de monosulfure (MSS), d'une solution solide intermediaire (ISS) et eventuellement de mineraux du groupe du platine (MGP) a partir du liquide sulfure; exsolution de MSS et ISS pour former des sulfures de metaux de base (SMB) et MGP; et modification des SMB par des fluides magmatiques tardifs ou metamorphiques. Independamment des processus impliques au cours de la genese, dans la plupart des depots de sulfures magmatiques, les EGP sont trouves generalement dans la structure des SMB, ou sous la forme de MGP. Ces MGP consistent principalement en la combination des EGP avec au moins un des elements suivants: Te, As, Bi, Sb et Sn (TABS). Par consequent, l'association frequente des EGP avec les TABS amene a la question principale de ce projet: quel est le role de TABS lors de la formation des gisements de EGP? Les roles suggeres pour TABS lors de la formation de depots de sulfures magmatiques sont: i) La presence des concentrations elevees de TABS provoque la cristallisation directe des MGP a partir d’un magma silicate; ii) Le liquide sulfure pourrait devenir sature dans un liquide immiscible riche en TABS, qui collecte des EGP, et les MGP pourrait cristalliser a partir de ce liquide; iii) Des concentrations progressivement plus elevees de TABS dans le liquide sulfure pourraient amener a la cristallisation directe des MGP. iv) Pendant le refroidissement, les EGP peuvent se combiner avec les TABS et se exsolve sous la forme des MGP a partir des SMB. v) Les TABS peuvent soit etre remobilises avec le EGP pendant l'alteration postmagmatique, ou soit agir comme des agents de fixation des EGP pendant la remobilisation tardive. Une partie du probleme dans l'etude des TABS est que ces elements ne sont pas systematiquement determines dans les analyses de roche en totale ou dans les mineraux. La tres faible concentration de TABS, combinee a leur volatilite, a conduit a les negliger au cours des dernieres decennies. Par consequent, la premiere etape du projet a ete de mettre en place une routine analytique pour determiner les TABS dans le roche totale a de faibles concentrations, en utilisant la spectroscopie de fluorescence atomique couplee a un generateur d’hydrure (HG-AFS). A la suite du developpement analytique, les concentrations de TABS dans les roche totale, les SMB et les mineraux silicates ont ete mesurees dans des echantillons provenant: i) du district minier de Noril’sk-Talnakh; i) les gisements de type PGE-reef des complexes du Bushveld et Stillwater (Merensky Reef, J-M Reef et Picket Pin); iii) des horizons steriles en EGP localise a l'exterieur des intervalles mineralise des complexes du Bushveld et Stillwater et; iv) la Marginal Zone du complexe du Bushveld. Les echantillons du district minier de Noril’sk-Talnakh comprennent des sulfures massifs riches en Ni-Cu-EGP, ce qui a permis d’etudier le comportement des TABS lors de la cristallisation fractionnee du liquide sulfure. Par contre, les echantillons des Reefs de EGP des complexes du Bushveld et Stillwater contiennent des sulfures dissemines, ce qui a permis d'etudier la distribution de TABS dans les minerais qui ont subi une cristallisation a l'equilibre. Les echantillons de la Marginal Zone du complexe du Bushveld ont permis de determiner la concentration de TABS dans les liquides initiaux qui ont cristallise cette intrusion, ainsi que les processus qui ont affecte la distribution de TABS dans ces liquides. Pendant la cristallisation fractionnee du liquide sulfure, les TABS (avec Pt et Pd) sont incompatibles dans le MSS et ISS, et restent dans le dernier liquide sulfure, plus fractionne. Bien que les concentrations de TABS n'atteignent pas des niveaux suffisamment eleves pour qu'un liquide imiscible riche en TABS se forme, elles se combinent avec Pd et Pt et cristallisent sous la forme des MGP, directement a partir du liquide sulfure fractionne. D'un autre cote, les minerais formes par cristallisation a l'equilibre, tels que les Reefs a EGP des complexes du Bushveld et Stillwater, enregistrent l'effet de l'exsolution des MGP a partir des SMB. Dans ces cas, des concentrations elevees des EGP dans les SMB ont cree un potentiel chimique pour l'exsolution des MGP. Par consequent, les EGP ont combine avec les TABS et ont exsolve sous la forme des MGP. Par consequent, le SMB trouve dans les Reefs sont epuise dans TABS par rapport au SMB trouve dans des echantillons provenant de l'exterieur des Reefs. Les resultats obtenus pour les TABS dans les liquides initiaux qui ont cristallise le complexe du Bushveld (Marginal Zone) ont montre que leur distribution depend en grande partie de leur comportement chalcophile. Le Te et le Se sont des elements fortement chalcophiles, et leur distribution est principalement controlee par les sulfures. Par contre, As et Sb ne sont que des elements legerement chalcophiles, et leur distribution est principalement controlee par le degre de cristallisation fractionnee et l'assimilation crustale des magmas. Le Bi est moderement chalcophile et sa distribution resulte d'une combinaison des processus susmentionnes. Processes required to form magmatic sulfide Ni-Cu and platinum-group element (PGE) deposits include: saturation of a mafic or ultramafic magma with a base metal sulfide liquid; interaction of the sulfide liquid with the mafic magma; crystallization of monosulfide solid solution (MSS), intermediate solid solution (ISS) and possibly platinum-group minerals (PGM) from the sulfide liquid; exsolution of MSS and ISS to form base metal sulfides and PGM; and modification of the magmatic BMS by late magmatic or metamorphic fluids. However, regardless of the processes involved during the genesis, in most magmatic sulfide deposits the PGE generally occur within BMS structure, or as discrete PGM. These PGM mainly consist of the binding of PGE with at least one of the elements Te, As, Bi, Sb and Sn (TABS). Therefore, the frequent association of PGE with TABS leads to the main question of this project: what is the role of TABS during the formation of PGE deposits? Suggested roles for TABS during the formation of magmatic sulfide deposits are: i) The presence of high concentrations of TABS leads to direct crystallization of PGM from silicate magma; ii) The sulfide liquid could become saturated in an immiscible TABS-rich liquid, which collects PGE, and PGM could further crystallize from this liquid; iii) progressively higher concentrations of TABS in the sulfide liquid could lead to the direct crystallization of PGM. iv) Upon cooling PGE may combine with TABS and exsolve as PGM from BMS. v) TABS may either be remobilized together with PGE during post-magmatic alteration, or alternatively, act as fixing agents for PGE during late remobilization. Part of the problem of studying TABS is that these elements are not routinely determined in whole rock or mineral analyses. The very low concentration of TABS, combined with their volatility, has resulted in them being neglected by researchers over the past decades. Therefore, the first step of the project was to implement an analytical routine to determine TABS in wholerock at low concentrations, using hydride generation-atomic fluorescence spectrometry (HGAFS). Following the analytical development, concentrations of TABS in whole-rock, BMS and silicate minerals were measured in samples from: i) the Noril’sk-Talnakh mining district; i) the PGE-reef type deposits of the Bushveld and Stillwater Complexes (Merensky Reef, J-M Reef and Picket Pin deposit); iii) PGE-barren horizons from outside the reef intervals of the Bushveld and Stillwater Complexes and; iv) the Marginal Zone of the Bushveld Complex. The samples from the Noril’sk-Talnakh mining district comprise massive sulfide ores, which allowed investigating the behaviour of TABS during fractional crystallization of the sulfide liquid. In contrast, samples from the PGE-reefs of the Bushveld and Stillwater Complexes contain disseminated sulfide minerals, which allowed investigating the distribution of TABS in ores than underwent predominantly equilibrium crystallization. Finally, the samples from the Marginal Zone of the Bushveld Complex allowed constraining the concentration of TABS in the initial liquids that crystallized this intrusion, and also which processes affected the distribution of TABS in initial liquids. During fractional crystallization of the sulfide liquid, TABS (together with Pt and Pd) are incompatible into the crystallizing MSS and ISS, and remain in the lattermost fractionated sulfide liquid. Although concentrations of TABS do not reach sufficiently high levels for an immiscible TABS-rich liquid to segregate, they combine with Pd and Pt and crystallize as composite PGM, directly from the fractionated sulfide liquid which is enriched in TABS. On the other hand, ores formed by equilibrium crystallization, such as the PGE reefs of the Bushveld and Stillwater Complexes, record the effect of PGM exsolution from BMS. In these cases, high PGE concentrations in BMS created a chemical potential for the exsolution of PGM. Therefore, PGE combined with TABS and exsolved as PGM. Consequently, the BMS found within the PGE reefs are depleted in TABS relative to the BMS found in samples from outside the reef intervals. The results for TABS in the initial liquids that crystallized the Bushveld Complex (Marginal Zone) revealed that their distribution largely relies on their chalcophile behaviour. Tellurium and Selenium are strongly to highly chalcophile elements, and their distribution is mainly controlled by sulfide minerals. In contrast, As and Sb are only slightly chalcophile elements, and their distribution is mainly controlled by the degree of fractional crystallization, and crustal assimilation of the magmas. Bismuth is moderately chalcophile, and its distribution results from a combination of the aforementioned processes

Re-Os geochronology of base metal sulfides from cratonic mantle xenoliths : Case study from Somerset Island (Canada) and method development

Robust and reliable time constraints are necessary to infer the formation and evolution of the cratonic mantle. Due to the large fractionation of Re from Os during mantle melting, the Re-Os decay system has been largely used for dating the melting event that led to the formation of the subcontinental lithospheric mantle (SCLM). In mantle rocks Re, Os and the other highly siderophile elements (HSE: Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) are controlled by base metal sulfides (BMS), which can be residual phases of partial melting processes or can be re-introduced in mantle rocks during metasomatism. The present study aimed at the improvement and enhancement of our ability to use the Re-Os system and the HSE to unravel geological processes recorded in BMS. The contribution of this work is twofold because it provides new data on natural samples as well as a novel analytical technique for future applications. In the first part of this dissertation (Chapter 1) some basic concepts are introduced to make the reader more familiar with the topics encountered in the next sections. This includes an overview of the HSE behavior in terrestrial reservoirs and the explanations of the geochemical tools that will be used in the following sections. The second part of this dissertation (Chapter 2) is focused on the investigation of partial melting and metasomatic processes recorded in four mantle xenoliths from Somerset Island (Rae craton, Canada). After textural and mineralogical investigations, individual BMS grains were micro-sampled and analyzed for 187Os/188Os. The two xenoliths with the most metasomatic HSE signature (e.g. suprachondritic Pd/Pt) are distinguished for the high BMS modal abundance, the occurrence of large interstitial BMS grains, and the extreme 187Os/188Os variation measured in BMS grains (187Os/188Os = 0.172-0.108). Archean Re-depletion model ages (TRD) are recorded in BMS grains from three different xenoliths, suggesting a main formation of the SCLM at 2.7-2.8 Ga, in association with the local Rae greenstone belts. A similar scenario was proposed for the nearby Slave craton, which confirms that different terrains of the Canadian Shield share a similar Neoarchean history. At the whole rock scale, the TRD age of 2.7-2.8 Ga is clearly recorded only in one xenolith with residual HSE signature (i.e. subchondritic Pt/Ir, Pd/Pt, and Re/Pd). This supports and further stresses that: 1) whole rock TRD ages should be used carefully in xenoliths with metasomatic HSE signature, and 2) single grain BMS can record the age of formation of the SCLM even in heavily metasomatized mantle xenoliths. Single BMS grains yielded two distinct Paleoproterozoic TRD ages (~1.9 and ~2.2 Ga) that are not resolvable at the whole rock scale. The two TRD ages are consistent with a scenario where metasomatic BMS were introduced in the SCLM during a first phase of rifting of the Slave from the Rae craton (2.2 Ga) and a later collision of the two cratons (1.9 Ga, Thelon-Talston orogeny). In the third part of this dissertation (Chapter 3) a novel analytical method is proposed to analyze 187Os/188Os along with Ru, Pd, Re, Os, Ir, and Pt concentrations in individual µg-weight BMS grains. To set up this method, two Fe-Ni sulfides were synthetized and independently characterized for HSE content and 187Os/188Os. Fragments of the two sulfides were used to test different digestion and separation methods. It is here shown that a simultaneous digestion and Os extraction yields inaccurate Os concentrations. The improved procedure proposed in this study includes BMS digestion in HBr + HCl, Os micro-distillation, and cation resin separation of Ru, Pd, Re, Ir and Pt. The 187Os/188Os ratio and the HSE concentrations are measured by mass spectrometry (N-TIMS and SF-ICP-MS). The independently determined HSE concentrations are reproduced by this technique with differences 187Os/188Os ratio is indistinguishable within the analytical precision (2SD ~ 0.1%). Owing to the chemical separation of the analytes, the proposed procedure overcomes many of the analytical issues encountered during LA-ICP-MS analyses (e.g. 187Re isobaric interference on 187Os and matrix effects). Moreover, the analysis of the entire grain, avoid any sampling bias related to the complex mineralogical assemblage typically observed in natural BMS. As shown in this dissertation, BMS grains record a multitude of magmatic and metasomatic processes that cannot be individually discriminated at the whole rock scale. The comprehension of these processes represents an exciting challenge as it will improve our ability of using the Re-Os system and, ultimately, to constrain the timing of mantle dynamics. Coupled HSE and 187Os/188Os investigations in individual BMS grains will provide an essential tool towards this goal

Temporal, lithospheric and magmatic process Controls on ni, cu and platinum-group element (PGE) mineralisation: A case study from Scotland

A temporal and spatial relationship between plume magmatism, cratonic lithosphere and the occurrences of orthomagmatic Ni-Cu and platinum-group element (PGE) sulphide mineralisation has been documented in the literature. However the underlying causes for this correlation have yet to be resolved – is there an inherent feature of the cratonic lithosphere and its mantle ‘keel’ that controls mineralisation? Or is this correlation purely a preservational bias in the geological record? Scotland has experienced multiple tectono-magmatic events and provides an ideal testing ground, or ‘framework’, in which to assess the role of lithospheric mantle on chalcophile element (Ni and Cu) and precious metal (PGE and Au) abundances through time. Given the well-documented geological history of the region (including several suites of mantle xenoliths), coupled with exploration campaigns in Greenland (with which Scotland has comparable geology), this thesis aims to assess the contributions and influences of lithospheric mantle vs. asthenospheric mantle during melting and mineralisation. It also evaluates the Ni-Cu-PGE mineralisation potential for Scotland, particularly in a Noril’sk-type conduit-hosted setting within the British Palaeogene Igneous Province (BPIP). The earliest major tectono-magmatic event following cratonisation of the North Atlantic Craton (NAC) occurred c. 2.4 Ga during Palaeoproterozoic extension, forming the maficultramafic Scourie Dyke Swarm. Despite evidence for lithospheric mantle melting at this time, the subcontinental lithospheric mantle (SCLM) below the Scottish portion of the NAC did not become severely depleted in sulphides or PGE. Instead, spinel lherzolite mantle xenoliths from this region (e.g., Loch Roag) record an influx of carbonatite-associated sulphides at this time, enriched in PGE, and providing a deeper indication of continental extension that may be correlated to carbonatitic intrusions in Greenland. Subsequent collision and orogenesis of the NAC in the late Palaeoproterozoic (c. 1.9 to 1.7 Ga) represents a second significant tectonomagmatic event, recorded in the Scottish SCLM as sulphide (re-)melting and formation of discrete Pt-sulphide minerals (cooperite). Hence the lithospheric mantle here became appreciably enriched in precious metals during the Palaeoproterozoic, but crucially this preserved multiple co-existing populations of sulphides, distinct in their petrographic setting and geochemistry. Cratonic basement and associated mantle lithosphere are absent in the southern terranes of Scotland. This provides a direct comparison between lithospheric mantle geochemistry for Archaean-Palaeoproterozoic terranes north of the Great Glen Fault vs. Palaeozoic terranes south of the Great Glen Fault. Rifting of Rodinia and opening of the Iapetus Ocean in the late Neoproterozoic thus marks a significant change in geodynamic setting. This is especially apparent in the concentration of cobalt in lithospheric mantle sulphides, which appears to be inherently linked with the formation and/or later destruction (subduction) of oceanic crust during the Grampian event of the Caledonian orogeny. The impingement of the proto-Icelandic mantle plume initiated in the Palaeogene at the base of the NAC lithospheric mantle keel of Scotland and Greenland. The earliest Palaeogene magmas are enriched in Pt (i.e., have a high Pt/Pd ratio), whilst subsequent magmas associated with the opening of the Atlantic Ocean have successively lower Pt/Pd ratios. High Pt/Pd ratios are therefore coincident with magmas that have intruded through cratonic lithosphere. The SCLM at the margin of this region is known to be Pt-enriched (with cooperite) and therefore the changing Pt/Pd ratio of North Atlantic Igneous Province magmas suggests a fundamental interaction between the mantle plume and pre-enriched SCLM. Thus, whilst the concentration of metals, particularly Ni and Cu, is largely based on the high degree of asthenospheric mantle melting associated with the plume itself, the ratio of precious metals, such as Pt/Pd, can be strongly influenced by SCLM geochemistry. Overall, the intricate subtleties of metasomatic signatures recorded by mantle xenolith sulphides (or populations of sulphides) could allow for metallogenic ‘mapping’ of the upper mantle. This may identify areas of geochemical and mineralogical ‘preconditioning’, and together with geophysical constraints such as major lithospheric lineaments, it may be possible to establish the craton-specific fertility of a region. Finally, in order for orthomagmatic sulphide mineralisation to occur, magmas must achieve sulphur saturation in the upper crust, forming an immiscible sulphide liquid and thereby collecting PGE and chalcophile elements, possibly to economic grades. Thus a crucial part of assessing the mineralisation potential of a region must entail an investigation into the causes and locations of S-saturation. Given that crustal sulphur contamination is a common trigger for magmatic S-saturation, this thesis establishes the first S-isotopic (δ34S) framework for western Scotland in order to identify areas of sulphur contamination in the BPIP. In Scotland, the most readily available and S-rich rocks occur in the Mesozoic Hebrides Basin. Sulphur contamination of BPIP rocks is widespread and both S-saturation and S-undersaturation can be observed, suggesting that the region may be extremely fertile for orthomagmatic Ni-Cu-PGE mineralisation. By reconstructing the Hebrides Basin stratigraphy we can assess locations of contamination, even if these are above the current level of exposure (and since removed by erosion), and in some situations sulphide liquid sinking may be demonstrated, suggesting further possibilities for mineralisation present ‘up-stream’ in magmatic conduits. In conclusion, the Scottish BPIP represents a new exploration frontier not yet identified by industry for orthomagmatic Ni-Cu-PGE mineralisation. The conclusions are based on approximately 500 rock samples from across Scotland, which have been analysed for major elements and over thirty trace elements (including PGE) and S-isotopes. All data are available on an accompanying CD