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Geological Survey of Finland, Bulletin 395 Geological Survey of Finland Geological Survey of Finland, Bulletin 395 Geological Survey of Finland Bulletin 395 - MUTANEN, TAPANI Tätä julkaisua myy Denna publikation säljes av This publication can be obtained from GEOLOGIAN GEOLOGISKA GEOLOGICAL SURVEY TUTKIMUSKESKUS (GTK) FORSKNINGSCENTRALEN (GFC) OF FINLAND (GSF) Julkaisumyynti Publikationsförsäljning Publication sales PL 96 PB 96 P.O. Box 96 02151 Espoo 02151 Esbo FIN-02151 Espoo, Finland 0205 50 11 0205 50 11 +358 205 50 11 Geology and ore petrology of the Akanvaara Telekopio: 0205 50 12 Telefax: 0205 50 12 Telefax: +358 205 50 12 and Koitelainen mafic layered intrusions and the GTK, Väli-Suomen GFC, Distriktsbyrån för GSF, Regional office for Keivitsa-Satovaara layered complex, northern Finland aluetoimisto Mellersta Finland Mid-Finland Kirjasto Biblioteket Library PL 1237 PB 1237 P.O. Box 1237 70211 Kuopio 70211 Kuopio FIN-70211 Kuopio, Finland by Tapani Mutanen 0205 50 11 0205 50 11 +358 205 50 11 Telekopio: 0205 50 13 Telefax: 0205 50 13 Telefax: +358 205 50 13 GTK, Pohjois-Suomen GFC, Distriktsbyrån för GSF, Regional office for aluetoimisto Norra Finland Northern Finland Kirjasto Biblioteket Library PL 77 PB 77 P.O. Box 77 96101 Rovaniemi 96101 Rovaniemi FIN-96101 Rovaniemi, Finland 0205 50 11 0205 50 11 +358 205 50 11 Telekopio: 0205 50 14 Telefax: 0205 50 14 Telefax: +358 205 50 14 Geological Survey of Finland Espoo 1997 ISBN 951-690-652-4 ISSN 0367-522X E-mail: [email protected] WWW-address: http://www.gsf.fi 9 X Geological Survey of Finland, Bulletin 395 GEOLOGY AND ORE PETROLOGY OF THE AKANVAARA AND KOITELAINEN MAFIC LAYERED INTRUSIONS AND THE KEIVITSA-SATOVAARA LAYERED COMPLEX, NORTHERN FINLAND by TAPANI MUTANEN with 233 pages, 88 figures, 11 tables in the text and 5 appended maps ACADEMIC DISSERTATION in Geology and Mineralogy To be presented, with permission of the Faculty of Science, University of Helsinki, for public criticism in Lecture Room 1 of the Department of Geology, on May 23rd, 1997, at 12 noon GEOLOGIAN TUTKIMUSKESKUS ESPOO 1997 Mutanen, Tapani., 1997. Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara layered com- plex, northern Finland. Geological Survey of Finland, Bulletin 395, – pages, 88 figures, 11 tables and 5 appended maps. The Akanvaara and Koitelainen intrusions, ca 2440 Ma old, represent a group of cratonic mafic layered intrusions (aged from 2500 to 2440 Ma) of the Fennoscandian (Baltic) Shield. These intrusions, which show all the stand- ard types of igneous layering, host deposits of chromite, vanadium, titanium, platinum-group elements (PGE) and gold. The cumulate stratigraphy in Akanvaara and Koitelainen reflects both normal tholeiitic fractionation and superimposed intermittent selective and wholesale contamination by anatectic salic melt (now granophyre) from the floor and the roof, and by refractory residual phases after anatexis of the roof rocks. Cumulus phases separated mainly in a hybrid melt between the mafic main magma and the overlying buoyant acid melt. The contaminants are found in cumulates as fossil melt and magma inclusions in olivine, chromite, or- thopyroxene and plagioclase, and as “granitic” intercumulus material. The composition of the intercumulus material in the most heavily contaminated cumulates deviates strongly from what one would expect of the mafic main magma. Unusual minerals (loveringite, chlorapatite. zircon, zirconolite, bad- deleyite, thorite, allanite, galena, perrierite, a Ti-Th-REE-P silicate) are com- mon in the intercumulus of the ultramafic cumulates; ilmenite, chlorapatite and zirconolite occur as daughter and occluded minerals in melt and magma inclusions in olivines and orthopyroxene. Chlorapatite, ilmenite and loverin- gite are the most common Doppelgänger phases (Mutanen, 1992), of which only the apatite phase (as a fluorapatite) appears as a late cumulus mineral. The melting occurred both early and late in the crystallization history, corresponding to the water-saturated and dry melting of the granitic constit- uents of the surrounding crustal rocks. There is no evidence for, nor need of, multiple magma pulses; the reversals can readily be explained by contami- nants transmitted by two-phase convection. Contamination of the magma by crustal material shifted the liquidus phase boundaries, resulting in excessive separation first of olivine and, subsequently of orthopyroxene. Refractory aluminous phases after partial melting of pelitic roof rocks were responsible for the anorthositic cumulates. Chromitite layers occur from near the base of the intrusions to the 86 PCS level. Almost all the chromitites have elevated concentrations of PGE. The gangue is generally rich in biotite-phlogopite. All chromites are very low in MgO (from 0.0% to 1.2%). These features imply that chromite crystallized and was equilibrated in a melt poor in magnesium and rich in potassium. It is suggested that the chromium of the uppermost chromitites (UC layers) was exotic: it was liberated from melted high-aluminous schists averaging 500 ppm Cr. Exotic chromium may well have contributed to the genesis of some other anorthosite-associated chromitites, too (e.g., Fiskenaesset, Sittampundi, Soutpansberg, Bushveld, Stillwater). In the Akanvaara and Koitelainen intrusions the PGE show little respect for sulphides, and are typically associated with oxides (chromite, ilmenomagnet- ite, ilmenite). Evidently the PGM phases crystallized from the silicate liquid direct. The Keivitsa intrusion (age 2057 Ma) is located south of the Koitelainen intrusion. Isotopic (Sm-Nd, S-isotopes, Re-Os), geochemical (Ni/Co, Ni/S, PGE/S, S/Se, S/Te, Re/PGE, among others) and mineralogical data all point to significant exotic contamination by surrounding carbonaceous, sulphide- rich sediments and by solid refractory debris from disaggregated komatiitic rocks. The addition of exotic sulphur (as anatectic immiscible sulphide melt) and olivine debris was decisive for the genesis of the low-grade Keivitsansarvi Cu-Ni-PGE-Au deposit, which is located far above the basal contact of the intrusion. Other contaminants acquired from surrounding rocks were water, reduced carbonaceous material and chlorine, all of which affected the crys- tallization of the magma. Mineralogical evidence from Keivitsa and from other PGE-bearing magmatic sulphide deposits together with our experimental work (Mutanen et al., 1996) suggest strongly that immiscible sulphide melt either acted as a phase boundary collector for platinum-group mineral (PGM) particles already crystallized from melt or that the supersaturated PGM nucle- ated on sulphide droplets. Key words (GeoRef Thesaurus, AGI): layered intrusions, contamination, de- gassing, petrology, chlorapatite, Cl, ultramafics, chromitite, gabbros, geo- chemistry, copper ores, vanadium ores, nickel ores, platinum ores, gold ores, Proterozoic, Finland, Lappi Province, Savukoski, Akanvaara, Sodankylä, Koitelainen, Keivitsa, Keivitsansarvi, Satovaara Tapani Mutanen Geological Survey of Finland P.O. Box 77 FIN-96101 ROVANIEMI, FINLAND ISBN 951-690-652-4 ISSN 0367-522X Vammalan Kirjapaino Oy 1997 4 Geological Survey of Finland, Bulletin 395 Tapani Mutanen ABBREVIATIONS AND DEFINITIONS ab – albite an – anorthite APB – Archaean-Proterozoic boundary bi – biotite CC model – convection-contamination model CN – chondrite-normalized cp – chalcopyrite cpx – clinopyroxene (augite, diopside, hedenbergite) crt – chromite CuEQ – copper equivalent, wt% (includes Cu, Ni and part of Pd and Au) cumulus crystal – settled (or floated) crystal DDH – diamond (core) drill hole D-value – simple ratio; distribution coefficient of an element between phases Doppelgänger – a premature magmatic phase, opposite to reversed phase (in phase reversal; (see (doubleganger) Mutanen, 1992, p. 26) economic – pertaining to the production, distribution, and use of income, wealth, and commodi- ties — pertaining to an economy, or system of organization or operation, esp. of the process of production — pertaining to one’s personal resources of money (Webster’s Encyclopedic Unabridged Dictionary of the English Language. Portland House, New York, 1989). ‘Economic’ is misleadingly used in Finland as a synonym with ‘profitable’. false ore – Ore type in Keivitsa. Analogous to false money: rock with massive or rich dissemi- nated sulphides but low in Ni. Metal analysis (as with false coins) is needed to distinguish false from genuine (viable) ore. No economic connotation! Fe(tot) – Total iron calculated as FeO GSF – Geological Survey of Finland hb – hornblende ilm – ilmenite ilmenomagnetite – magnetite with exsolved ilmenite KEI – type intrusion – Keivitsa-type intrusion KOI – type intrusion – Koitelainen-type intrusion KSC – Keivitsa-Satovaara Complex LC – Lower Chromitite LIB – Lapland Intrusion Belt LLC – Lowermost Lower Chromitite LZ – Lower Zone mg# – atomic Mg/Mg+Fe ratio MZ – Main Zone Ni(100S), Ni(S) – Ni concentration (in wt%) in calculated 100% sulphides ol – olivine opx – orthopyroxene (Ca-poor pyroxene) P(fl) – fluid pressure P(lit) – lithostatic pressure PCS – per cent solidified PGE – platinum-group elements PGE(100S), PGE(S) – PGE concentration (in ppb or ppm) in 100% sulphides (similarly, Pd(S), Au(S) etc.) PGM – platinum-group minerals phl – phlogopite pl – plagioclase pn – pentlandite po – pyrrhotite px – pyroxene py – pyrite q – quartz RONF – Regional Office for Northern Finland SHMB – siliceous high-Mg basalt SLI – silicate liquid immiscibility sp – spinel (denotes the spinel group, unless stated otherwise) titanomagnetite
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