The Paasivaara PGE reef in the Penikat layered intrusion,northern Finland

Summary Three major PGE-bearing mineralized zones have been found in the layered series of the early Proterozoic Penikat layered intrusion. These are designated as the Sompujärvi (SJ), Ala-Penikka (AP) and Paasivaara (PV) Reefs according to the site of their initial discovery.The uppermost of these, the PV Reef, has the highest Pt/Pd ratio. It is located in the transition zone between the fourth and the fifth megacyclic units. The main host rock is the uppermost anorthosite, disseminated sulphides and associated PGM being concentrated in the interstices of this plagioclase orthocumulate. The Reef has also been encountered in other parts of the transition zone, however, and sometimes even in the lowermost parts of the fifth megacyclic unit. The dominant sulphide paragenesis is chalcopyrite-pyrrhotite-pentlandite, whereas the PGM identified are represented by sperrylite (PtAs₂), kotulskite (PdTe), merenskyite (PdTe₂), isomertieite (Pd₁₁Sb₂As₂), stibiopalladinite (Pd₅Sb₂), cooperite (PtS) and braggite ((Pt, Pd, Ni)S).It is suggested that the PV Reef was formed in the mixing process when the fifth magma pulse intruded into the magma chamber. Mixing of the new magma with the older residual magma in the chamber accounted for the sulphide precipitation. Mixing and convection were probably turbulent at first and the sulphides were thus able to "scavenge" PGE from a large amount of silicate melt. The metal ratios in the mineralization point to a close genetic relationship with the fifth magma pulse.

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Das Paasivaara PGE Reef in der Penikat-Intrusion, Nord-Finnland

Zusammenfassung In den geschichteten Serien der frühproterozoischen Intrusion von Penikat kommen drei grössere PGE-führende Zonen vor. Diese werden als die Sompujärvi (SJ), Ala-Penikka (AP) und Paasivaara (PV) Reefs bezeichnet, entsprechend den Lokalitäten der Entdeckung.Das am höchsten gelegene PV Reef hat die höchsten Pt/Pd Verhältnisse. Es liegt in der Übergangszone zwischen der vierten und der fünften megazyklischen Einheit. Das wichtigste Wirtsgestein ist der oberste Anorthosit, wo disseminierte Sulfide und assoziierte PGM in den Zwischenräumen dieses Plagioklas-Orthokumulates vorkommen. Das Reef wurde auch in anderen Teilen der Überganszone beobachtet und manchmal sogar in den untersten Partien der fünften megazyklischen Einheit. Die dominierende Sulfidparagenese ist Kupferkies-Magnetkies-Pentlandit; PGM sind Sperrylith (PtAs₂), Kotulskit (PdTe), Merenskyit (PdTe₂), Isomertieit (Pd₁₁Sb₂As₂), Stibiopalladinit (Pd₅Sb₂), Cooperite (PtS) und Braggit ((Pt, Pd, Ni)S).Es wird angeregt, dass das PV Reef während der Mischungsvorgänge bei der Intrusion des fünften Magma Pulses in die Magmenkammer entstanden ist. Mischung des neuen Magmas mit dem alten Residual-Magma in der Kammer war für die Ausfällung der Sulfide verantwortlich. Mischung und Konvektion dürften anfangs turbulent gewesen sein, und so konnten die Sulfide die PGE aus einem beträchtlichen Anteil der Silikatschmelze entfernen. Die Metallverhältnisse dieser Vererzung lassen eine enge genetische Verbindung mit dem fünften Magmapuls erkennen.

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Early Proterozoic layered intrusions in the northeastern part of the Fennoscandian Shield

Summary Early Proterozoic layered intrusions, about 2440 Ma in age, are widespread over a large area of the northeastern Fennoscandian Shield in Finland, Sweden and the Soviet Union. Only one intrusion, the Kukkola intrusion, is encountered in Sweden whereas in Finland, their number exceeds twenty. These are concentrated principally in two areas, the dicontinuous Tornio-Närdnkävaara intrusion belt which crosses northern Finland and the Koitelainen intrusion with its satellites located in central Finnish Lapland. The intrusions in the Soviet Union are concentrated in three areas: (i) on the Kola Peninsula, (ii) in the Paanajärvi area close to the Finnish border and (iii) northeast of Lake Onega.Examples of all the ore types characteristic of layered intrusions have been found in these intrusions. Chromitite layers are encountered in the Kukkola/Tornio, Kemi, Penikat, Koitelainen and Burakovsky intrusions, but only one, the Kemi chromitite, has so far been mined. The Portimo, Koillismaa, Monchegorsk and Fedorova intrusions are characterized by PGE-bearing Cu-Ni-deposits in their marginal series. Mineralized zones enriched in PGE are also encountered in the layered series. Those in the Penikat intrusion and in the Portimo intrusions are the most remarkable and the best known to date. Vanadium-bearing Fe-Ti-oxide layers are encountered in several intrusions, but only one, the Mustavaara deposit, is presently being exploited.Two types of parental magma have tentatively been proposed for these intrusions. The first type is represented by a magma which was relatively rich in magnesium and chromium and was as a whole boninitic in composition, whereas the plagioclase-rich intrusions and megacyclic units are interpreted as having crystallized from a magma which was greatly depleted in these elements, especially Cr, and had melted crustal material incorporated in it.The emplacement of the early Proterozoic layered intrusions in Fennoscandia was part of the world-wide igneous activity indicated by other layered intrusions and mafic dyke swarms of similar age in other ancient cratons, i.e. the Jimberlana intrusion in Australia, the Great Dyke in Zimbabwe, the Scouric picrite suite in Scotland, the Hearst-Matachewan dyke swarm, Copper Cliff Formation and East Bull Lake intrusion in Ontario, Canada, and the Vestfold Hills and Napier Complex dyke swarms in Antarctica. This almost contemporaneous occurrence in different parts of the world would suggest a more intimate relationship between the Fennoscandian Shield, northwest Scotland, Canadian Shield, Yilgarn Block, Zimbabwe Craton and East Antarctic Shield at the beginning of the Proterozoic than at present.

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Früh-Proterozoische geschichtete Intrusionen im nordöstlichen Teil des Fennoskandischen Schildes

Zusammenfassung Im Nordost-Teil des Fennoskandischen Schildes in Finnland, Schweden und der Sowjetunion kommen fast vierzig frühproterozoische geschichtete Intrusionen, die ungefähr 2440 Mio J. alt sind, vor. Nur eine davon, die Kukkola Intrusion, liegt in Schweden, während in Finnland mehr als zwanzig Intrusionen vorkommen. Diese sind hauptsächlich in zwei Gebieten konzentriert, nämlich in dem nicht-zusammenhängenden Tornio-Näränkävaara Gürtel, der das nördliche Finnland durchzieht, und die Koitelainen-Intrusion mit ihren Satelliten im zentralen Finnischen Lapland. Die Intrusionen in der Sowjetunion sind in drei Gebieten konzentriert: (i) auf der Kola Halbinsel (ii) im Paanajärvi Gebiet nahe der Finnischen Grenze und (iii) östlich vom Onega-See.Beispiele aller für geschichtete Intrusionen charakteristischen Erztypen kommen vor. Chromititlagen sind in den Intrusionen von Kukkola/Tornio, Kemi, Penikat, Koitelainen und Burakovsky zu finden, aber nur eine davon, der Kemi Chromitit, ist bisher in Abbau genommen worden.Die Portimo-, Koillismaa-, Monchegorsk- und Fedorova-Intrusionen werden durch PGE-führende Kupfer-Nickel-Lagerstätten in ihren randlichen Bereichen charakterisiert. Mineralisierte Zonen die an PGE angereichert sind kommen auch in den geschich teten Serien vor. Die bemerkenswertesten sind die PGE-Vererzungen der Penikat- und der Portimo-Intrusionen. Vanadium-führende Fe-Ti-Oxidlagen kommen in verschiedenen Intrusionen vor, aber nur eine davon, die Mustavaara-Lagerstätte, ist bisher abgebaut worden.Diese Intrusionen werden auf zwei verschiedene Magmentypen zurückgeführt. Ersteres ist ein Magma das relativ reich an Magnesium und Chrom war und eine boninitische Zusammensetzung hatte, während die Plagioklas-reichen Intrusionen, und die megazyklischen Einheiten auf ein Magma das an diesen Elementen (besonders Cr) verarmt war, und das Krustenmaterial aufgeschmolzen hat, zurückgehen.Die Platznahme der frühproterozoischen geschichteten Intrusionen in Fennoskandien stellt einen Teil weltweiter magmatischer Aktivität dar, die durch andere geschichtete Intrusionen und mafische Gänge von fast identischem Alter in anderen alten Kratonen repräsentiert wird. Hier ist die Jimberlana-Intrusion in Australien, der Great Dyke in Zimbabwe, die Pikrit-Suite von Scourie in Schottland, die Gänge von Hearst-Matachewan, die Copper Cliff Formation und die East Bull Lake Intrusion in Ontario, Kanada ebenso wie die Gangsysteme der Vesthold Hills und des Napier Komplexes in Antarctica zu nennen. Diese fast gleichaltrigen Vorkommen in verschiedenen Teilen der Welt weisen auf eine engere Beziehung zwischen dem Fennoskandischen Schild, Nordwest-Schottland, dem Kanadischen Schild, dem Yilgarn Block, dem Zimbabwe-Craton und dem Ostantarktischen Schild zum Beginn des Proterozoikums hin.

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Cryptic variation of augite in the Penikat Layered Intrusion,Northern Finland,with reference to megacyclic units and PGE-enriched zones

Summary The Penikat Layered Intrusion is located in northern Finland, about 70 km to the south of the Arctic Circle. It belongs to a discontinuous layered intrusion belt about 300 km in length and about 2440 m.y. old which stretches from the Swedish-Finnish border to Russian Karelia. The Penikat Intrusion itself is 23 km long and 1.5 to 3.5 km wide. Its magmatic stratigraphy may be divided into a thin marginal series and a 2 to 3 km thick layered series. The latter is made up to five megacyclic units (MCU) which are attributed to repeated influxes of new magma into the Penikat chamber during solidification.The intrusion hosts at least seven PGE-enriched zones, the most remarkable of which are the SJ, AP and PV PGE Reefs. The SJ Reef is located at the boundary between MCU's III and IV, about 600 m above the base of the intrusion; the AP Reef in the lower part of MCU IV, about 300 m above the SJ Reef; and the PV Reef in the transition zone between MCU's IV and V, about 800 m above the AP Reef.The Penikat Intrusion underwent complex deformation and metamorphism during the Svecokarelian Orogeny, resulting in variable alteration of most of the magmatic minerals. Augite is the least altered magmatic mineral and was, therefore, selected to indicate changing composition during fractionation and the manner in which its composition reflects the existence of megacyclic units.Augite occurs as an intercumulus mineral in the ultramafic lower part and as a cumulus mineral in the gabbronoritic and anorthositic upper part of each megacyclic unit. The 100 Mg/(Mg + Fe² + Mn) ratio shows decreasing trends in the three lower-most megacyclic units, excluding the lower part of MCU II, where a considerable reversal occurs, i.e. this ratio indicates progressive increase upward in the stratigraphy, providing evidence of magma mixing. The ratio decreases abruptly at the base of MCU and remains relatively constant in MCU's IV and V. The variation in the Cr₂O₃ content resembles that of the 100 Mg/(Mg + Fe²⁺ + Mn), and clearly indicates differences between the three lowermost chromium-rich and the two uppermost chromium-poor megacyclic units. The variations in TiO₂ and MnO content form roughly mirror images with chromium, being lower in the lower part and higher in the upper part of the intrusion. Na₂O correlates positively with chromium, since Na^– provides a charge balance to Cr³⁺ in augite.Besides the megacyclic units, the Penikat Intrusion is characterized by an unusually great number of PGE-enriched zones, even though the total thickness of the intrusion is only about three kilometers unlike the Bushveld and Stillwater Complexes which are much thicker. It is suggested that the presence of these kinds of PGE zones in layered intrusions, which comprise similar megacyclic units, is linked to the multistage processes which are also related to the development of these units and take place within them. Thus the identification of these kinds of megacyclic units may serve as a useful tool in prospecting for PGE ores both here and in other layered intrusions. This investigation reveals that the well-known method of studying cryptic variation in ferromagnesian silicates, such as augite, clearly indicates the existence of megacyclic units even in fairly altered layered intrusions like the Penikat.

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Kryptische Variation in Augit der Penikat-Intrusion, Nord-Finnland, und ihre Beziehung zu megazyklischen Einheiten und PGE-Zonen

Zusammenfassung Die geschichtete Intrusion von Penikat liegt in Nord-Finnland, rund 70 km südlich vom Polarkreis. Sie gehört zu einem diskontinuierlichen Gürtel aus geschichteten Intrusionen mit einer Länge von über 300 km und einem Alter von 2440 ma, der sich von der Grenze zwischen Schweden and Finnland bis nach Russisch-Karelien erstreckt. Die Intrusion von Penikat ist 23 km lang und 1.5 bis 3.5 km breit. Ihre magmatische Abfolge kann in eine geringmächtige randliche Serie und eine 2 bis 3 km mächtige geschichtete Serie unterteilt werden. Letztere wird von fünf megazyklischen Einheiten (MCU) aufgebaut, die auf den wiederholten Zufluß von neuem Magma in die Penikat-Magmenkammer während der Erstarrung zurückgeführt werden.Die Intrusion enthält zumindest sieben an PGE angereicherte Zonen, von denen die SJ-, AP- und PV-Reefs besonders erwähnenswert sind. Das SJ-Reef befindet sich an der Grenze zwischen MCU III und IV, 600m über der Basis der Intrusion; das AP-Reef im unteren Teil der MCU IV, rund 300 m über der SJ-Anreicherung; und das PV-Reef in der Übergangszone zwischen MCU IV und V, rund 800 m über dem AP-Reef.Die Penikat-Intrusion erfuhr während der svekokarelischen Orogenese eine komplexe Deformation mit Metamorphose, was eine unterschiedliche Umwandlung der meisten magmatischen Minerale zur Folge hatte. Da Augit das am wenigsten umgewandelte magmatische Mineral ist, wurde es als Indikator dafür ausgewählt, wie sich die Zusammensetzung während der Fraktionierung änderte und wie seine Zusammen- setzung die Existenz von megazyklischen Einheiten wiederspiegelt.Augit tritt im ultramafischen unteren Teil einer jeden megazyklischen Einheit als Interkumulus-Mineral auf, während er in den gabbronoritischen und anorthositischen oberen Bereichen als Kumulus-Mineral vorkommt. Das Verhältnis 100 Mg/(Mg + Fe²⁺ + Mn) zeigt mit Ausnahme des unteren Teils von MCU II, wo eine beträchtliche Umkehr eintritt, in den drei untersten megazyklischen Einheiten abnehmende Trends. Dieses Verhältnisnimmt gegen das Hangende der Abfolge hin zu, was auf Magma-Mixing hinweist. Das Verhältnis nimmt an der Basis der MCU plötzlich ab und und bleibt in MCU IV und V relativ konstant. Die Änderung im Cr₂O₃-Gehalt gleicht der von 100 Mg/(Mg + Fe²⁺ + Mn) und zeigt klar die Unterschiede zwischen den drei unteren Cr-reichen und den zwei oberen Cr-armen megazyklischen Einheiten. Die Änderungen im TiO₂- und MnO-Gehalt verhalten sich ungefähr spiegelbildlich, während die Chromgehalte im unteren Teil der Intrusion niedriger, im oberen Teil höher sind. Na₂O korreliert positiv mit Chrom, da Na⁺ den Ladungsausgleich zu Cr³⁺ im Augit besorgt.Neben den megazyklischen Einheiten wird die Penikat-Intrusion durch eine ungewöhnlich hohe Anzahl von mit PGE angereicherten Zonen charakterisiert, obwohl die Gesamtmächtigkeit der Intrusion im Gegensatz zu den viel mächtigeren Bushveld- und Stillwater-Komplexen nur rund drei Kilometer beträgt. Es wird angenommen, daß das Auftreten dieser Arten von PGE-Zonen in geschichteten Intrusionen, welche ähnliche megazyklische Einheiten umfassen, an mehrstufige Prozesse gebunden ist. Diese Prozesse stehen auch im Zusammenhang mit der Entwicklung dieser Einheiten und finden innerhalb dieser Einheiten statt. Daher kann die Identifizierung solcher megazyklischen Einheiten sowohl hier als auch in anderen geschichteten Intrusionen zur Prospektion von PGE-Erzen herangezogen werden. Die vorliegende Untersuchung zeigt, daß geringste Veränderungen in Augit die Existenz von megazyklischen Einheiten selbst in stark umgewandelten geschichteten Intrusionen wie Penikat er kennen lassen.

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Petrology and geochemistry of greywackes from Goa-Dharwar sector,western Dharwar Craton: Implications for volcanoclastic origin

Late Archaean Supracrustals of the Goa-Dharwar sector (GDS) are composed of a thick sequence of greywacke sequence with narrow intercalations of quartzite, BIF and carbonates. Mafic volcanics occupy the base of the sequence. The greywackes are predominantly tuffacious containing chlorite-sericite and hornblende. Arkosic variations containing biotite dominate the western part of the sector. Fine-grained variations occur as isolated narrow lenses within other types of greywackes. The conglomeratic greywackes are localized along the western and the eastern margins of the sector. All of the greywackes are all typically immature containing coarser clasts of mostly plagioclase (18–23%) and quartz (32–34%). Lithic fragments of felsic volcanic rocks are common. The matrix is dominated by mafic material. Biotite and amphibole are related to metamorphic recrystallization. Chlorite, sericite, epidote, carbonate and chert are products of the interplay of diagenesis and low-grade metamorphism. Fe-Ti oxide, sphene, apatite and zircon are usual accessories. But for slight enrichment in K₂O, the major element chemistry of the GDS greywackes is similar to the chemistry of Late Archaean greywackes. They also compare in respect of V, Co, Hf contents, K₂O/Na₂O, SiO₂/Al₂O₃, Na₂O/Al₂O₃, Ba/Rb, Th/U, La/Th, Sm/Nd ratios, steep REE patterns with distinct LREE enrichment and HREE depletion. The GDS greywackes however are distinctly enriched in Rb, Ba, Sr, Th, U, Cu, Zr, Ce/Ce* and depleted in Cr, Ni, and Zn. The conglomeritic and biotite bearing verities contain considerable proportions of clasts derived from the basement tonalitic/granitic terrain. The common tuffacious greywackes containing hornblende and biotite-sericite however include only volcanic clasts and bear evidence of derivation from submarine weathering of predominantly felsic volcanics erupted on a large scale to form a magmatic arc in the later stages of geosynclinal deposition. Geochemical data suggest that the GDS greywackes were laid down in progressively changing basin geometry from a passive to active continental margin and island arc setting.     

Post-Normal Science and the Global Climate Change Issue

Science has recently faced a new challenge in that it must now provide itsbest knowledge to support the urgent policy-making concerning, e.g., risks oftechnology, environmental pollution, or the climate change. However, thisknowledge unfortunately often can host high uncertainties as the naturalsystems are complex. How to proceed when the facts given by the scientists arediverging and uncertain, while the decision-making is urgent? Funtowicz andRavetz (1992, 1993) argue that in this case traditional `Normal' science(described by Kuhn (1970)) becomes inappropriate and that science shouldbecome `Post-Normal' in order to more effectively cope with these contemporaryproblems. The philosophy, or methodology, of Post-Normal Science is brieflyintroduced and its corelation with the climate change issue, specifically withthe compilation process and summary content of the Second Assessment Report(SAR) from the Working Group I of the Intergovernmental Panel on ClimateChange (IPCC, 1996a), is viewed. It seems that climate science around IPPCcan, to a relatively large extent, be characterized as `Post-Normal'.Moreover, results from some related studies indicate that the elements ofPost-Normal Science in the IPCC have enhanced the problem-solving in theclimate change issue.     

Infiltration metasomatism in layered intrusions revisited: a reinterpretation of compositional reversals at the base of cyclic units

Summary Irvine (1980) proposed an elegant mechanism termed “infiltration metasomatism” to explain upward displacement of discontinuities in mineral compositional trends relative to contacts between cyclic units in the Muskox intrusion. It was concluded that the offsets of Mg/(Mg + Fe) discontinuities in olivine and chromite are a secondary postmagmatic feature that resulted from reaction between the cumulus minerals and intercumulus liquid that was frontally displaced upwards from the underlying crystal pile as a result of compaction. We reinterpret this feature in the Muskox and other layered intrusions as basal reversals that arise from a temperature gradient-driven flux of low melting point components from the hot magma parental to cyclic units towards a relatively cold cumulate floor. In this interpretation basal reversals are a primary magmatic feature that does not involve intercumulus liquid migration. Authors’ addresses: R. M. Latypov, S. Yu. Chistyakova, Kola Science Centre, Geological Institute, Fersman Str. 16, 184200, Apatity, Russia; Present address: Department of Geosciences, University of Oulu, Oulu, P.O. Box 3000, FIN-90014, Finland; T. T. Alapieti, Department of Geosciences, University of Oulu, Oulu, P.O. Box 3000, FIN-90014, Finland     

PGE mineralization in the late Archaean iron-rich mafic-ultramafic Hanumalapur Complex, Karnataka, India

Summary An unusually thick sulfur-poor mineralized zone enriched in platinum-group elements (PGE) is described in the Hanumalapur Complex, Shimoga District, Karnataka State, India. This promising occurrence was discovered in the early 1990s and the best samples at the time of writing have yielded Pt+Pd concentrations in excess of six ppm. The western part of the area concerned belongs to the late Archaean Dharwar Super Group (3000–2500 Ma), while the eastern part is occupied predominantly by a granite-gneiss terrain ∼3000 Ma in age. Ten mafic-ultramafic complexes which host interesting vanadium-bearing titanomagnetite occurrences are encountered in the western part, one of which is the Hanumalapur Complex. The PGE mineralized zone in this complex may be divided into four mineralogically distinctive types, which are, in descending order of PGE content: 1) a silicate-hosted Pd type, 2) a silicate-hosted Pt type, 3) a base-metal sulfide-hosted Pd type, and 4) an oxide-hosted PGE type. The genesis of the mineralization is somewhat unclear at this point of investigation, especially because of complete re-crystallization, but the evidence gathered so far suggests something different than a traditional orthomagmatic model requiring magma mixing processes and resulting in sulfide immiscibility. This is backed-up by the general lack of base metal sulfides in favor of chromite, although pure chlorite-amphibole and chlorite-albite-epidote-amphibole rocks may contain significant PGE concentrations regardless of the amount of chromite. The PGM textures show little evidence of hydrothermal alteration and remobilization, but the PGE mineralogy itself displays some characteristics of fluid action, as it seems that there are some OH-bearing Pt and Pd minerals present. The first author was Deceased Author’s address: R. J. Kaukonen, Department of Geosciences, University of Oulu, Oulu, P.O. Box 3000, FIN-90014 Finland     

Northward cooling and freshening of the warm core of the West Spitsbergen Current

The narrow slope-confined warm core of the West Spitsbergen Current (WSC) is a major pathway for warm and saline Atlantic Water to enter the Arctic Ocean. Long-term mean northward cooling and freshening rates in the warm core of the WSC are calculated with error ranges, and heat loss estimates are derived. The observed combination of cooling and freshening is not consistent with a hypothesis of pure isopycnal offshore mixing, but rather suggests influence from diapycnal mixing connected, for example, to exchange with shelf waters.