Unusual nickel and copper to noble-metal ratios from the Råna Layered Intrusion, northern Norway

Rocks from the Rana Layered Intrusion usually have low concentrations of noble metals but have normal Ni and Cu concentrations. Consequently the rocks have higher Cu/Ir, Ni/Ir, Cu/Pb and Ni/Pd ratios than extrusive rocks from the literature. If a small amount of sulphide was removed from the magma prior to its emplacement at Rana the magma would be depleted in noble metals relative to Ni and Cu, and rocks forming from it would have high Ni and Cu to noble-metal ratios

Compositional variations in Cu-Ni-PGE sulfides of the Dunka Road deposit, Duluth Complex, Minnesota : the importance of combined assimilation and magmatic processes

The Dunka Road deposit is one of ten occurences of Cu-Ni sulfides bearing platinum-group elements (PGE) on the northwestern margin of the Duluth Complex, in Minnesota. Mineralization has been linked to contamination of the host troctolitic magma through assimilation of argillaceous rocks from the Virginia Formation. On the basis of texture and composition, the sulfide mineralization is divided into five types: 1) norite-hosted disseminated sulfides, 2) troctolite-hosted disseminated sulfides, 3) PGE-rich disseminated sulfide horizons, 4) pyrrhotite-rich massive sulfides, and 5) chalcopyrite-rich disseminated sulfides. The norite-hosted sulfides exhibit featues suggestive of the magma's substantial contamination, such as high proportions of pyrrhotite and arsenide minerals, and high mean values of S/Se (9,700) and δ34S (ll.2%₀). They are also generally metal-poor, implying that the sulfides interacted with a relatively low volume of silicate melt (i.e., low R factor). The troctolite-hosted sulfides formed at moderate degrees of contamination, as indicated by their intermediate mean values of S/Se (4,600) and δ34S (7.8%₀). The PGE-rich sulfide horizons show little sign of contamination, and have mantle-like mean values of S/Se (2,600) and δ34S (2.1%o). Their very high PGE contents suggest that they formed at elevated R factors. The pyrrhotite-rich massive sulfides and associated chalcopyrite-rich disseminated sulfides have relatively high mean values of S/Se (8,000) and δ34S (10.2%o), indicative of significant contamination. The former are interpreted to represent a cumulate of monosulfide solid-solution (mss), whereas the chalcopyrite-rich sulfides represent the fractionated sulfide liquid. A general increase in the degree of contamination is observed toward the base of the intrusion, associated with a decrease in R factor and metal concentration of the sulfides. This likely results from the introduction of partial melt from the metasedimentary country-rocks, which was cooler than the mafic magma and led to the early crystallization of the sulfide liquid

Unusually high concentrations of magnetite at Caraiba and other Cu-sulfide deposits in the Curaca Valley, Bahia, Brazil

The Curaça Valley, in Bahia, Brazil, contains several pyroxenite-hosted Cu-sulfide deposits. They have unusually high Cu and low Ni contents (Cu/Ni up to 2500), with chalcopyrite and bornite being the dominant sulfides (Cu/S up to 3.5). Oxides (magnetite and lesser ilmenite) constitute approximately 50% of the opaque phases. Most other magmatic Cu-Ni-sulfide deposits contain between 2 and 15% oxides. The only other known magmatic sulfide deposits with comparable Cu/Ni values, at Okiep, South Africa. also have elevated contents of magnetite. There, the oxides have been interpreted as a result of oxidation and desulfurization of primary sulfides, resulting in a high abundance of pure magnetite. In the Curaça Valley, reaction-replacement of sulfides to form magnetite is not apparent: the magnetite contains up to 14 wt% Cr₂0₃, 2% V₂0₃, 7% Ti0₂, and 1% ZnO, and is generally replaced by chalcopyrite and bomite. However, the composition of the oxides seems to have been modified by equilibration with associated orthopyroxene, in response to high-grade metamorphism. The metamorphism could also have led to significant S-loss of the sulfides, reflected by relatively low S/Se values of the rocks (200-1500). As at Okiep, a large proportion of the oxides secms lo have becn a product of replacement of primary sulfides. La vallée de Curaça. dans l'état de Bahia, au Brésil, contient plusieurs gisements de sulfures de cuivre dans un hôte pyroxénitique. Le minerai est fortement enrichi en Cu et remarquablement appauvri en Ni (le rapport Cu/Ni atteignant 2500); chalcopyrite et bornite sont les sulfures dominants (Cu/S jusqu'à 3.5). Les oxydes (magnétite surtout, avec ilménite accessoire) constituent environ 50% des phases opaques. La plupart des autres gisements de sulfures de Cu-Ni contiennent entre 2 et 15% d'oxydes. Les seuls gisements de sulfures magmatiques connus ayant une valeur Cu/Ni comparable se trouvent à Okiep, en Afrique du Sud; ils sont aussi enrichis en magnétite. Dans ce cas, on a interprété la proportion élevée en magnétite pure en termes d'oxydation et de perte de soufre des sulfures primaires. Dans la vallée de Curaça, un remplacement des sulfures par la magnétite ne semble pas évident. La magnétite contient jusqu'à 14% (poids) de Cr₂0₃, 2% de V₂0₃, 7% de Ti0₂ et 1% de ZnO, et semble plutôt remplacé par la chalcopyrite et la bomite. Toutefois, la composition des oxydes semble avoir été modifiée par équilibrage avec l'orthopyroxène associé, en réponse à un métamorphisme de forte intensité. Cet épisode de métamorphisme aurait pu déstabiliser les sulfures et causer une perte en soufre, comme l'indique le faible rapport S/Se des roches (200-1500). Tout comme à Okiep. une proportion importante des oxydes semble s'être formée par remplacement des sulfures primaires

Formation and evolution of the chromitites of the Stillwater Complex : a trace element study

Large layered intrusions, such as the Stillwater Complex, contain cyclic units of chromite-rich layers (cm to m thick) having kilometre-scale lateral extension. Chromite cumulates are among the first to form after new primitive melt injections into the magma chamber. Therefore, chromite cumulates could be used to investigate the nature of the parental magma, given the fact that chromite preserves its primary original magmatic composition. The cooling and crystallization history of large layered intrusions is long, complex, and involves multiple injections of hot primitive magma into an evolving and fractionating magma chamber. Our study on Stillwater chromites shows that the early crystallized chromite experiences various post-cumulus processes with the interstitial silicate melt, such as the precipitation of chromite overgrowths on early formed cumulus chromite and/or the reaction - reequilibration of early formed cumulus chromite. These processes have modifed the primary magmatic composition of the chromite making it difficult to identify the parental magma. Moreover, mineralogical evidence for chromite - interstitial melt interactions have probably been obliterated during late post-magmatic textural maturation and recrystallization which tends to homogenize chromite grain size and composition

A U-Pb zircon age for the Råna intrusion, N. Norway : new evidence of basic magmatism in the Scandinavian Caledonides in Early Silurian time

Four morphologically distinctive zircon types from quartz-bearing gabbronorite in the Rana intrusion in the Narvik Nappe Complex yield coincident and concordant U-Pb analyses indicating a crystallization age of 437+1-2 Ma. The age of the Rana intrusion, together with other zircon ages from similar layered mafic intrusions and ophiolites in Norway, provide evidence for a distinct and possible short-lived period of back-arc spreading and mafic plutonism in the upper part of the Upper Allochthon and in the Uppermost Allochthon of the Scandinavian Caledonides in Early Silurian time. The intrusion was emplaced during a period of regional crust extension that post-dates the local D1 deformational phase and pre-dates phases D2-D6. The main period of regional metamorphism (and D2-6) in the Narvik Nappe Complex post-dates emplacement of the Rana intrusion, and it is concluded that these latest deformational phases are probably related to the medial Silurian Scandian orogeny and not the Cambrian-Early Ordovician (?) "Finnmarkian' orogeny, as previously supposed

Textural variations in MORB sulfide droplets due to differences in crystallization history

Sulfide droplets from fresh Mid-Ocean-Ridge Basalt (MORB) glasses show different textures. Some are fine-grained droplets consist of Monosulfide Solid Solution (Mss) and Intermediate Solid Solution (Iss) micrometric intergrowths with pentlandite at the Mss-Iss interface and disseminated Fe-oxide grains; other droplets display a characteristic “zoned” texture consisting of segregated massive grains of Mss and Iss, with euhedral Fe-oxides and pentlandite occuring as equant grains and as flame-shaped domains in the Mss formed by exsolutions. The difference in the textures implies a difference in the crystallization history of the sulfide droplets. These different textures are observed in droplets that are only millimeters apart in the same sample, and thus had an identical cooling history. Therefore, some other factors controlled the textural development. There is relationship between the size and the texture of the droplets. The larger sulfide droplets tend to have zoned textures and the smaller ones fine-grained textures. We propose that the latter have experienced greater undercooling before crystallization. The reason for the delay in crystallization could be that, in the small sulfide droplets, large stable grains with low surface to volume ratio cannot form, which results in higher effective solubility of the Mss. Due to the high degree of undercooling in the small droplets, there were numerous nucleation sites and the diffusion rates of the crystal components in the liquid were lower, leading to fine-grained Mss-Iss intergrowths. In contrast, larger droplets with lower effective solubility of Mss began to crystallize at higher temperature, and thus had fewer nucleation sites, higher diffusion rates, and more time for sulfide differentiation

Exploration for magmatic Ni-Cu-PGE sulphide deposits : a review of recent advances in the use of geochemical tools, and their application to some South African ores

Most major magmatic Cu-Ni-PGE sulphide deposits are thought to have formed by segregation of an immiscible sulphide melt from a silicate host magma, in response to processes such as magma mixing, rapid cooling, differentiation, and contamination. The metal content of the sulphides is governed by the concentration of the metals in the silicate host magma, the sulphide melt/silicate melt partition coefficients (D values) of the metals, and the R-factor during sulphide segregation. Fractionation between the metals occurs during partial melting of the upper mantle source, crystallization of oxides, platinum-group minerals (PGM), and silicates (mainly olivine and less so orthopyroxene), segregation of sulphide melt, and crystallization of the sulphide melt. The latter process may yield zoned ore bodies consisting of Os, Ir, Ru, Rh, Fe, (Ni)-rich monosulphide solid solution (mss) cumulate ore and fractionated sulphide ore rich in Cu, Pt, and Pd. It is possible to model these processes and thereby to estimate the potential of a magmatic body to host economic Cu-Ni-PGE sulphide deposits. The location of Cu-Ni-PGE sulphide ores may be facilitated by applying a number of geochemical tools. PGE-rich horizons within layered intrusions are particularly difficult to locate because the ore zones are generally thin compared to the thickness of the intrusions. Variation in Cu/Pd ratios of the silicate rocks may delineate the position of some of these horizons, since the strongly chalcophile Pd is preferentially depleted during sulphide segregation, resulting in an increase in Cu/Pd of the subsequently crystallizing overlying cumulates. Cu/Pd ratios may also be applied in sill-like bodies such as the Uitkomst intrusion, Mpumalanga, to estimate the potential for conduit-type deposits, and in lavas where they may help to locate possible Noril'sk-type mineralized feeder zones to basalt flows. Other methods of determining whether igneous bodies have experienced magma mixing and sulphide segregation are based on Ni contents of olivines, Ti contents, and Cr/Fe ratios of spinels, and petrographic features such as the presence of plagioclase inclusions within ferromagnesian phases. Se/S ratios may help to determine whether magmatic sulphide ores underwent post-magmatic (metamorphic or hydrothermal) sulphur loss. They may also discriminate between a magmatic or sedimentary source of the S, and between a magmatic or hydrothermal origin of the metals. Hydrothermal sulphide ores may further be distinguished from magmatic ores by means of significantly higher Cu/Ni and Pd/Ir ratios of the former. Such distinction is important because it would be futile to investigate hydrothermal deposits for magmatic ore zonation

Platinum-group element contents of Karelian kimberlites: implications for the PGE budget of the sub-continental lithospheric mantle

We present high-precision isotope dilution data for Os, Ir, Ru, Pt, Pd and Re in Group I and Group II kimberlites from the Karelian craton, as well as 2 samples of the Premier Group I kimberlite pipe from the Kaapvaal craton. The samples have, on average, 1.38 ppb Pt and 1.33 ppb Pd, with Pt/Pd around unity. These PGE levels are markedly lower, by as much as 80%, than those reported previously for kimberlites from South Africa, Brazil and India, but overlap with PGE results reported recently from Canadian kimberlites. Primitive-mantle-normalised chalcophile element patterns are relatively flat from Os to Pt, but Cu, Ni and, somewhat less so, Au are enriched relative to the PGE (e.g., Cu/Pd > 25.000). Pd/Ir ratios are 3,6 on average, lower than in most other mantle melts. The PGE systematics can be largely explained by two components, (i) harzburgite/lherzolite detritus of the SCLM with relatively high IPGE (Os-Ir-Ru)/PPGE (Rh-Pt-Pd) ratios, and (ii) a melt component that has high PPGE/IPGE ratios. By using the concentrations of iridium in the kimberlites as a proxy for the proportion of mantle detritus in the magma, we estimate that the analysed kimberlites contain 3–27% entrained and partially dissolved detritus from the sub-continental lithospheric mantle, consistent with previous estimates of kimberlites elsewhere (Tappe S. et al., 2016, Chem. Geol. http://dx.doi.org/10.1016/j.chemgeo.2016.08.019). The other major component in the samples is melt, modelled to contain an average of 0.85 ppb Pt and 1.09 ppb Pd. Assuming that Group II kimberlites are derived from relatively metasomatised SCLM, our data suggest that the metasomatised Karelian SCLM is relatively poor in Pt and Pd. If our data are representative of other Group II kimberlites elsewhere, this result could imply that the PGE enrichment in certain continental large igneous provinces, including Bushveld, is not derived from melting of metasomatised SCLM

Petrogenesis of Cu-Ni sulphide ores from O’okiep and kliprand, Namaqualand, South Africa : constraints from chalcophile metal contents

The petrogenesis of sulphide ores in the O’okiep district has remained controversial. Based mainly on the concentration of chalcophile metals (PGE, Cu, Ni), it is proposed that the sulphides segregated from a basaltic magma generated during melting of sub-continental lithospheric mantle. Sulphide saturation of the magma was delayed due to relatively high fO2 until crustal contamination occurred during the advanced stages of differentiation. The immiscible sulphide melt was entrained and fractionated in dynamic magma conduits. Sulphides enriched in monosulphide solid solution (mss) component precipitated at depth in the Kliprand area of southern Namaqualand to form the Hondekloof deposits, whereas the O’okiep ores crystallised at shallower levels from highly fractionated residual sulphide liquids enriched in intermediate solid solution (iss). Sulphides of intermediate composition occur at Ezelsfontein. In the context of this model, the O’okiep intrusions could represent the proximal magmatic members of an IOCG suite of deposits, raising the prospect for additional IOCG deposits elsewhere in southern Africa. The model also predicts an enhanced potential at O’okiep for undiscovered Ni sulphide ores at depth