Geological Survey of Finland, Guide 51b, FIELD TRIP GUIDEBOOK - Mutanen Tapani
FIELD TRIP GUIDEBOOK
THE AKANVAARA INTRUSION AND THE KEIVITSA – SATOVAARA COMPLEX, WITH STOPS AT KAIKKIVALTIAANLEHTO AND RANTAVAARA INTRUSIONS
By Tapani Mutanen
PREPARED FOR THE 10 th PLATINUM SYMPOSIUM IN OULU FINLAND 2005 Geological Survey of Finland, Guide 51b
FIELD TRIP GUIDEBOOK
THE AKANVAARA INTRUSION AND THE KEIVITSA – SATOVAARA COMPLEX, WITH STOPS AT KAIKKIVALTIAANLEHTO AND RANTAVAARA INTRUSIONS
By Tapani Mutanen
PREPARED FOR THE 10 th PLATINUM SYMPOSIUM IN OULU FINLAND 2005
GEOLOGIAN TUTKIMUSKESKUS
GEOLOGICAL SURVEY OF FINLAND
ESPOO 2005 Cover photo: Diamond drilling in Akanvaara Mutanen, Tapani 2005. The Akanvaara intrusion and the Keivitsa – Satovaara Complex, with stops at Kaikkivaltiaanlehto and Särkivaara intrusions. Geological Survey of Finland, Guide 51b. 124 pages, 72 figures and 6 tables. The Akanvaara intrusion (zircon U-Pb age 2.435 Ga) belongs to a group of early Proterozoic cratonic mafic layered intrusions (radiometric ages ca 2.5 to 2.44 Ga) of the Fennoscandian Shield (Baltic Shield). During the exploration (1990 through 1998) by the Geological Survey of Finland (GSF) 112 diamond drill holes (DDH) totalling 17 370 meters, were drilled. The intrusion has an area of ca. 50 km2, with an explored strike length of ca 10 km. The intrusion is a block-faulted monocline dipping SE. During the amphibolite-facies regional metamorphism the cumulates and dyke rocks were hydrated and recrystallized to a variable degree. The cumulates above the basal chilled margin consist, ascending, of pyroxene cumulate layers of the Lower Zone (LZ) that host numerous PGE-bearing chromitite layers (LLC and LC chromitites; these are overlain by gabbroic cumulates with the group of thin MC chromitites. The Main Zone (MZ) begins with the ULC chromitite layer, overlain by an olivine-bearing ultramafic unit and pyroxenitic cumulates. The rest of the MZ consists of gabbroic rocks and occasional non- cotectic plagioclase-rich cumulates. The Upper Zone (UZ) is set to begin from the base of the UC succession (UC chromitite layer, associated anorthositic and gabbroic cumulates and microgabbros). Henceforth, monotonous gabbros take most of the interval up to the thick magnetite gabbro (MTGB) unit, underlain by a 100 meters-thick succession of anorthositic cumulates. A layer rich in Au and three PGE-enriched intervals were encountered beneath the MTGB. The lowermost 6 to 13 meters of the MTGB unit contain a significant reserve of vanadium. In the uppermost ferrodiorites, at 97.6 PCS, apatite enters as a cumulus phase. Chilled margins are lacking at the roof; the ferrodiorites have a sharp compositional boundary against the overlying granophyre unit. The evolution of the residual melt (magma) was calculated by incremental summation for 10 oxides and trace elements, and for Mg#, down from the ferrodiorite/granophyre contact using 924 whole-rock analyses from a gapless combined profile of 23 DDHs. The calculated parent magma is a typical tholeiite (in wt%):
SiO2 51.24, Al2O3 14.87, MgO 7.45, FeO(tot) 9.97, TiO2 0.778; P2O5 0.089, S 0.0345; V 305 ppm, Cu 118 ppm; Mg# 0.597). The far too high Cr (without UC 1212 ppm, with UC included 1266 ppm) corroborates other evidence that the chromium in the UC layer, and probably a great proportion in other chromitites, is exotic. Besides, the “doubling heights” and “halving heights” of components reveal unexpected anomalies in the order of compatibility. Akanvaara is very similar to the Koitelainen intrusion, and both have characteristics in common with other large Archaean and lower Proterozoic layered intrusions (Fiskenaesset, Burakovka, Bushveld, a.o.). The cumulate stratigraphy 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. The magma filled the chamber essentially as a single pulse, before any significant crystal accumulation, and formed a chilled lining against the country rocks. The magma was convective due to heat loss through the roof (thermal convection) and spatial differences in bulk densities of the melt-crystal suspension (two-phase, or multi-phase, convection). There were intermittent accelerated bursts of convection and convective overturns of part or all of the magma chamber. The mingling- mixing zone between the mafic main magma and overlying buoyant anatectic melt was the prime setting of phase separation (crystallization of cumulus silicates and oxides, separation of immiscible sulphide liquid, degassing). Crystallization near the roof created a density inversion, which resulted in downdrafts of density flows. Contaminants were dragged along in crystal-laden density flows and trapped in the cumulus pile by settling crystals. Samples of contaminants are found in cumulates as fossil melt and magma inclusions in olivine, chromite, orthopyroxene and plagioclase, and as “granitic” intercumulus material. The composition of intercumulus deviates strongly from “normal” mafic main magma. Unusual minerals (loveringite, chlorapatite, zircon, baddeleyite, thorite, galena, a.o.) are common in the intercumulus of ultramafic cumulates; ilmenite, chlorapatite and zirconolite occur as daughter and occluded minerals in melt and magma inclusions in olivines and pyroxenes. Chlorapatite, ilmenite and loveringite are the most common Doppelgänger phases, of which only apatite (as fluorapatite) appears as a late cumulus mineral. The crustal rocks were melted mainly due to the latent heat released by crystallizing cumulus minerals. The melting occurred in two stages, early and late in the crystallization history, corresponding to fractional water-saturated and dry melting of the granitic constituents of the crustal rocks. Practically no melting took place during the interval between the two stages of fractional melting. At melting, chilled linings were peeled off and they foundered in the magma. New generations of temporary chilled crusts formed from the evolving magma against the walls and even against the salic roof melt. The magma was contaminated by selective diffusion, wholesale mixing (hybridisation) and refractory residual phases after partial melting of the country rocks. The added contaminants shifted the liquidus phase boundaries of saturated and potential phases, resulting in excessive separation first of olivine and, subsequently, of Ca-poor pyroxene. Refractory aluminous phases after partial melting of pelitic rocks were responsible for the anorthositic cumulates. The residual xenocryst phases equilibrated their compositions or, if not part of the set of phases separating from the mafic magma, reacted with it, or dissolved. Thus, the effects of contamination were mainly catalytic, visible only in trace element and isotope systems, and in the intercumulus. The portion of the main magma that was not drawn into the density flows evolved independently towards iron enrichment. The two stages of fractional melting are reflected in two main contamination stages in intrusions. Towards the end of the fractionation, magma mixing became increasingly sluggish as the compositions of the mafic and salic silicate liquids approached the stable immiscibility solvus. The character of contamination depends on the water content and chemical composition of the wall rocks, the size and surface/volume ratio of the magma chamber and on the type of melting (equilibrium/fractional). In the intrusions of Koitelainen and Akanvaara the contamination was determined by fractional two-stage melting, and by salic and refractory aluminous residue materials.
In the Keivitsa intrusion contamination was effected by selective contamination (H2O, Cl, K), water- saturated partial melting of sulphide-rich pelitic and carbonaceous sediments, and by refractory olivine xenocrysts from disaggregated komatiitic rocks; the assimilation of carbonaceous material caused significant reduction of the oxidation state of the magma. The reversals can readily be explained by the effects of contaminants transmitted by two-phase convection. Thus, the “new magma pulses” were internal events in magma chambers. At Akanvaara chromitite layers occur from the base of the intrusions to the top of the MZ. The layers have variable but always elevated concentrations of PGE. The gangue is generally rich in biotite-phlogopite and in chlorine. All chromites are very low in MgO (from 0.0% to 1.2%). These features imply that chromite crystallized and equilibrated in a melt poor in magnesium and rich in potassium. In the melt evolution curve for Cr the UC appears as a conspicuous, unnatural “scarp”. All these features suggest, like at Koitelainen, that the chromium of the uppermost chromitite (UC layer) was exotic; exotic chromium may have contributed to the chromitites in the lower parts, as well as to other anorthosite-associated chromitites (e.g., in Fiskenaesset, Sittampundi, Soutpansberg, Bushveld, Stillwater). The PGE show little respect for sulphides, and are typically associated with oxides (chromite, ilmenomagnetite, ilmenite). Evidently tiny PGM nucleated easily and were early liquidus phases. The Keivitsa intrusion (2.057 Ga) is located south of the Koitelainen intrusion. Isotopic (Sm- Nd, S-isotopes, Re-Os), geochemical (Ni/Co, Ni/Cu, Ni/S, PGE/S, S/Se, S/Te, Re/PGE, a. o.) and mineralogical data all point to significant contamination by surrounding carbonaceous, sulphide-rich sediments and by solid refractory debris from disaggregated komatiitic rocks. The addition of sulphur (as anatectic immiscible sulphide melt) and olivine debris was decisive for the genesis of the low- grade Keivitsansarvi Cu-Ni-PGE-Au deposit, located far above the basal contact. Other contaminants acquired from surrounding rocks were water, reduced carbonaceous material and chlorine, all of which affected the crystallization of the magma. Mineralogical evidence from Keivitsa, as well as from other PGE-bearing magmatic sulphide deposits, and our experimental work (Mutanen et al., 1996) suggest that immiscible sulphide melt acted as a phase boundary collector for platinum-group mineral (PGM) particles already crystallized from melt, or that the PGM nucleated on sulphide droplets. Assimilation of carbonaceous material lowered the oxygen fugacity of magma, thereby radically decreasing solubilities of PGE. In Koitelainen and Akanvaara magmas oxides (chromite and titanomagnetite) were the prime collectors for crystallized PGM phases.
Key words (GeoRef Thesaurus, AGI): layered intrusions, ultramafics, chromitite, gabbros, geochemistry, copper ores, nickel ores, platinum ores, gold ores, genesis, 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 951-690-932-9 ISSN 0781-643X
Oy Sevenprint Ltd 2005
CONTENTS
Abstract ...... 3 Contents ...... 7 INTRODUCTION ...... 9 Locations ...... 9 Preglacial weathering, glacial geology and bedrock exposure ...... 9 Previous work ...... 11 Materials and methods ...... 11 GENERAL GEOLOGICAL SETTING ...... 13 Late Archaean to early Proterozoic volcanism ...... 13 MAFIC LAYERED INTRUSIONS OF THE FENNOSCANDIAN SHIELD ...... 15 THE AKANVAARA INTRUSION ...... 19 General geology ...... 19 Igneous stratigraphy and petrography ...... 21 Chemistry and fractionation ...... 24 The vanadium deposit in magnetite gabbro ...... 32 Chromitite layers in the Akanvaara intrusion ...... 32 The UC layer ...... 38 The ULC layer ...... 42 The MC layers ...... 43 The LC and LLC layers ...... 43 Behaviour of PGE in the Akanvaara intrusion ...... 48 Some petrological aspects ...... 51 THE KEIVITSA – SATOVAARA COMPLEX ...... 58 Location and exploration history ...... 58 General geology ...... 59 THE KEIVITSA INTRUSION ...... 62 Age, form and internal structure ...... 62 Emplacement, contact effects and xenoliths ...... 66 Contamination and crystal fractionation ...... 67 Dyke rocks and mineral veins ...... 71 Mineral deposits of the Keivitsa intrusion ...... 71 Petrography of ultramafic cumulates ...... 72 THE KEIVITSANSARVI Cu-Ni-PGE-Au DEPOSIT ...... 81 General structure ...... 81 Ore types ...... 81 Mineralogy of ore types ...... 92 ORE PETROLOGY OF THE KEIVITSA INTRUSION ...... 99 ACKNOWLEDGEMENTS ...... 103 FIELD TRIP PROGRAMME, AKANVAARA, KAIKKIVALTIAANLEHTO, RANTAVAARA AND KEIVITSA ...... 104 LITERATURE ON AKANVAARA ...... 113 LITERATURE ON KEIVITSA – SATOVAARA ...... 114 REFERENCES ...... 116
FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 9 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
INTRODUCTION
Locations
The locations of the intrusions are shown in escarpment is composed, in descending order, Fig. 1. Akanvaara is located 140 km northeast of metahornfelses, sulphide-rich granophyres of Rovaniemi and 80 km southeast of the and a talus apron of metagabbros of the Keivitsa Koitelainen intrusion; the Keivitsa-Satovaara intrusion. The Akanvaara hill (actually a row of area lies 140 km north-northeast of Rovaniemi. hills) is composed of the acid volcanic rocks, The terrain consists of interconnected aapa-mires. the roof rocks of the intrusion. The intrusions Geomorphologically the area is part of an ancient (Koitelainen, Akanvaara and Keivitsa-Satovaara) peneplain, with a general southern slope. The least are named after the commanding hills in the fractured and chemically the most resistant rocks respective areas. From the many other forms of the stand up from the flats as smoothed ridges and name Keivitsa (Kevitsa, Keevitsa, Keevitsä) we asymmetric cuesta-like hills (e.g., Koitelainen, have used Keivitsa, which is probably the original Keivitsa). The prominent Keivitsa hill is a remnant Sámi form (Samuli Aikio, pers. comm., 1993). of a cuesta ridge, its asymmetric shape best seen Kaikkivaltiaanlehto is located 20 km south- from the west. The gentle dip slope consists southeast of Keivitsa. The outcrop of Rantavaara, of resistant pelitic metahornfelses, roof rocks 5 km north-northwest of Kaikkivaltiaanlehto, of the Keivitsa intrusion; the northern stepped belongs to the 30 km-long Särkivaara intrusion.
Preglacial weathering, glacial geology and bedrock exposure
During much of the Phanerozoic time the grussification weathering stage are found deep topographic, geotectonic and climatic conditions in hard rocks; these include maghemitization of favoured strong chemical weathering and formation magnetite and formation of solid, primary-looking of thick weathered regolith profiles (Afanasev, secondary sulphides (violarite-siegenite, marcasite 1968, 1971, 1977). The most widespread type of and pyrite). The thickness of the grussification formation of "rapakallio" - weathered bedrock gives a good measure of the reach of preglacial - in Finnish Lapland and Kola peninsula is what weathering. In Koitelainen it is mostly ca 14 Afanasev (1968) calls hydromica weathering; meters, about the same as in Kola Peninsula (5- I have used the perhaps more universal term 20 meters, Afanasev 1977). In micaceous, grussification (Mutanen, 1979b, and references fractured rocks of the Bonanza copper prospect, therein). The process of grussification involves Koitelainen, grussification extends to at least 60 essentially alteration of micas (hydration, removal meters. In the Kejv area, central Kola Peninsula, of potassium and oxidation of iron) which resulted sulphides are lacking in the uppermost 10-12 m in soft, spadable rock. In grussified layered of the bedrock; from that on supergene pyrite sedimentary rocks poor in mica the sulphides and is found to a depth of 50-80 meters where it is carbonates have been dissolved, leaving a hardy superseded by primary pyrrhotite (Belkov, 1963). residue of macadam-like chips. Grussified rocks At Bonanza maghemitization reaches to a depth are developed and preserved typically on gentle of 120 m, more than the max. 70 meters found in slopes, and the weathering profile roughly conforms Bushveld (van Rensburg, 1965). A phenomenon the present surface (Afanasev, 1968). Afanasev of "hematite inoculation effect" was noticed in V- (1971, 1977) ascribed this type of weathering to rich magnetite of the Koitelainen intrusion: where temperate, humid climate during late Tertiary time magnetite had suffered, however slightly, from (Miocene to Pliocene). More subtle changes of the metamorphic oxidation to hematite ("martite"), 10 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
27o 00'
Keivitsa Rantavaara Kaikkivaltiaanlehto Sodankylä Savukoski
Pelkosenniemi Akanvaara
66o 30' Kemijärvi
ROVANIEMI
road
railway
100 km
Fig. 1. Locations of the intrusions: Akanvaara, Kaikkivaltiaanlehto, Rantavaara and Keivitsa. it did not "catch" maghemitization (see also van and layers and stratigraphic intervals associated Rensburg, 1965). with ore layers (pyroxenites, anorthosites). During Due to grussification the rocks containing diamond drilling the grussified rocks go with the biotite-phlogopite are deeply grussified. flushing water. On the other hand, grussified rocks Unfortunately, such rocks include chromitites provide welcome bedrock samples in large bog FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 11 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... areas, as they are easily sampled by percussion southern part of the intrusion Hirvas and Tynni (Cobra) drilling. (1976) have described a possibly allochthonous During the Pleistocene glaciations Central Tertiary deposit. Due to preglacial weathering Lapland was the stage of ice accumulation, and glacial erosion the bedrock, particularly in with very little basal ice movement and, hence, Akanvaara, is unevenly, rock-selectively and, glacial erosion (Penttilä, 1961). In certain leading on the whole, poorly exposed. Apart from solid, zones (see Gillberg, 1965), e.g., in the eastern glacially polished outcrops alongside streams, the part of the Keivitsa-Satovaara area, erosion and exposures are frost-shattered outcrops and boulder transport were surprisingly strong, as indicated fields, most often only groups of solitary, frost- by long transport distances of indicator erratics, heaved boulders. roches moutonnées, grooves, striations and Throughout postglacial time the area has crescentic fractures. The strongest ice advance, been supra-aquatic, but only nominally so in and the dominant effective (apparent) transport Akanvaara, where vestiges of the big Salla glacial in the Koitelainen-Keivitsa area was from north- lake (Johansson, 2005) are seen as boulder banks, northwest. While in Akanvaara glacial erosion cobble reefs and reworked shores across the whole seems to have been stronger and more uniform, southern flank of the Akanvaara hill. A washed there are preserves of grussified regoliths on steep gully (excursion stop 1) in the southwestern corner northwestern (proximal) slopes, where movement of the intrusion served as an overflow outlet for the at the base of the ice sheet was stagnated. In the glacial lake.
Previous work
The Exploration Department of the Geological the Keivitsa intrusion (op. cit.). The geological Survey of Finland (GSF) started geological mapping of the sheet 3714 was completed in the mapping and exploration of the mafic layered early 1980s (Tyrväinen, 1980, 1983). The northern intrusions of Finnish Lapland in 1969. The part of the Akanvaara intrusion is vaguely outlined Koitelainen intrusion had been known since in the general geological map by Mikkola (1937). the general geological mapping of the 1920s Later work has produced more detailed maps (Mikkola, 1937, 1941). Erkki Mikkola also made (Manninen, 1981; Räsänen, 1983; Juopperi, keen observations on the ultramafic rocks of 1986).
Materials and methods
The mapping of the contiguous Koitelainen and altitude and low-altitude (39 m) aerogeophysical Keivitsa-Satovaara area covered 770 km2; in line flights. Ground-geophysical surveys (mainly Akanvaara I revised ca 40 km2. In areas of poor magnetic, routine electromagnetic and gravity) exposure the maps in the Koitelainen-Keivitsa cover 90 km2 in Keivitsa-Satovaara and 102.5 km2 area are based to a large extent on scrutiny of in Akanvaara. samples and analyses of percussion drilling Diamond-core drilling has been indispensable (14991 samples of till, 3785 of weathered in these poorly exposed areas. During 1993-1998 bedrock). A large part of the samples were studied 112 holes (17369.6 m) were drilled at Akanvaara. under binocular microscope, and 96 polished thin In the Keivitsa-Satovaara area, from 1984 through sections, 89 covered thin sections and 10 polished 1995, 403 holes (43 139 m) were drilled. The latter sections were made and examined. The geological figures do not include the 175 shallow DDHs mapping was supported by geophysical surveys. (total 5121.85 m) drilled by the project "The All these areas have been covered by both high- geology of the Keivitsa area" (Manninen et al., 12 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
1996). The total count of polished thin sections Keivitsa-Satovaara: whole-rock XRF 1824; Se-Te- studied is 2446 from Keivitsa-Satovaara and 362 Sb-Bi 409, Te 263, trace-element ICP-MS 44, Pd- from Akanvaara: besides, there are 96 polished Au 15 366; PGE-Au-Re 4627 (of which 233 by the thin sections from percussion drilling samples and companies NAS and XRAL); ICP 16 151; AAS mineral processing test products. base metals 2800; S (by LECO) 682; H2O(tot) 79; Unless said otherwise, all analyses were made C 696; Akanvaara: whole-rock XRF 2271, Au-Pd in the Otaniemi and Rovaniemi laboratories of the 5207, PGE-Au-Re 585, ICP 371, ICP-MS (REE- GSF. The analyzing methods depend on practical Co-Nb-Rb-Sc-Ta-Th-U-V-Y-Zr) 75. For details of needs and reflect the availability of facilities electron microprobe analyses and XRD work, see and development of analysing equipment and Mutanen (1997). automation techniques during the long span of Extensive mineral processing and metallurgical investigations. The CN values for PGE and Au tests of Akanvaara and Keivitsa samples have been were calculated using the following C1 chondrite made by Rautaruukki Co (flotation tests of the values: Os 761 ppb (Tredoux et al., 1989), Ir graphite gabbro, Keivitsa), VTT (Keivitsansarvi 710 ppb, Ru 1071 ppb, Pt 1430 ppb, Pd 836 ppb Cu-Ni-PGE-Au deposit; serpentinite of the big (Taylor & McLennan, 1985) and Au 152 ppb xenolith in the lap of the Keivitsa intrusion; (Krähenbühl et al., 1973). In the following the Akanvaara UC and ULC chromitites and V-rich number of analyses from the study areas are listed: magnetite gabbro). FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 13 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
GENERAL GEOLOGICAL SETTING
The eastern part of the Fennoscandian Shield older zones and cores of inherited zircons aged (Shield in the following) is composed mainly 3.15-3.65 Ga (Bibikova et al., 1989; Sergeev et of Archaean tonalitic gneisses and greenstone- al., 1989, 1990; Kröner & Compston, 1990). The schist belts of late Archaean to early Proterozoic oldest crust found so far, the trondhjemitic gneiss age It seems inevitable that the magmas that fed of Siurua, Pudasjärvi, has a U-Pb age of ca 3.5 Ga the layered intrusions on the Shield interacted for zircon, with a still older (3.73 Ga) inherited with crustal rocks along the feeder walls, and so core (Mutanen & Huhma, 2003). The calculated the knowledge of the geochemistry and isotope Sm-Nd model ages T(CHUR) for rocks containing geochemistry of the crust, where accessible, is >3.1 Ga zircon generally fall on this same range, necessary when trying to assess the geochemical between 3.2 - 3.8 Ga (Jahn et al., 1984; Huhma characteristics, extent and effects of crustal et al., 1995). This old crust is possibly to be seen contamination to the intrusive magmas. in the Sm-Nd memory of contaminated layered Most of zircons of the basement rocks formed or intrusions (see Iljina, 1994). Occurrences of zircon reformed between 2.6 and 2.8 Ga. That time, called > 3.1 Ga old in acid volcanic rocks (Shurkin et the Saamian orogeny in Russian Karelia and Kola al., 1979; Kapusta et al., 1985) possibly indicate
Peninsula, was an intense crust-forming epoch newborn crust. The εNd(3.1 Ga) value for the old in the Shield. Apparently of global extent, it is basement of the Koitelainen area is -3.7±1.8 (Jahn represented in the Canadian Shield by the Kenoran et al., 1984), one of the lowest (most negative) of event (e.g., Kouvo, 1976). In many cases the crust Archaean crustal rocks. was evidently reborn, rather than newborn, as Zircon ages falling between the 3.1 Ga event and suggested by the ever growing number of findings the Kenoran event are interpreted to represent high of zircons, or cores of zircons older than 2.6 – 2.8 grade metamorphism (e.g., the 2.96 Ga zircon of Ga in the Shield and other old cratons in northern acid granulites of the Pudasjärvi Granulite Belt, Eurasia. According to zircon data an older turmoil Mutanen & Huhma, 2003; the 2.86 Ga zircon occurred at about 3.1 Ga ago (e.g., Shurkin et in the Vodlozero block, Sergeev et al., 1990). al., 1979; Kröner et al., 1981; Mints et al., 1982; However, zircon from acid volcanic rocks of the Hyppönen, 1983; Lobach-Zhuchenko et al., 1986; Koikar structure, northwest of the Lake Onega, Paavola, 1986; Bibikova et al., 1989; Sergeev et has been dated at 2.935 Ga (Bibikova & Krylov, al., 1990). This event was evidently of relatively 1983); thus, the zircon age of the acid granulites of short duration and not very thorough as indicated Pudasjärvi may represent primary crystallization by the common preservation of whole zircons, and from acid lavas.
Late Archaean to early Proterozoic volcanism
The late Archaean to early Proterozoic post- or sequential extrusion of contrasting magmas Kenoran greenstone belts were laid upon older such as komatiitic and acid lavas, alkali granites, sialic crust (Goodwin, 1977; Gorman et al., 1978; carbonatites and tholeiites (Mutanen, 1989, 1996; Kröner et al., 1981; Lazko et al., 1982; Lowe, Balashov, 1996). In general the "Kenoran crust" 1987). The history of the pre-Svecokarelian in the Shield bears no evidence of older zircons. cratonic magmatism included several phases of Thick sequences of acid volcanic rocks and related high magmatic activity, and there seem to have orthogneisses (e.g., the Lebyazhinsk series, 2.62- been no lulls of any major length (see Balashov, 2.78 Ga, Pushkarev et al., 1978; Mints et al., 1992; 1996; Huhma et al., 1996). The magmatism Kuhmo greenstone belt, 2.798 Ga, Hyppönen, was also compositionally more variegated than 1983; Kiviaapa basement gneiss, Koitelainen, previously thought, consisting of simultaneous 2.765-2.820 Ga, Mutanen, 1989) of this age group 14 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen suggest formation of new crust. event. In Kola Peninsula there are gabbro-norites The 2.44-2.49 Ga intrusion event was preceded with a Sm-Nd age of 2.555±65 Ga. Relicts of a by bimodal mafic-felsic volcanism, where pre-Koitelainen intrusion have been found below komatiitic lavas, basalts and siliceous high- the Koitelainen intrusion (Mutanen, 1989). magnesian basalts (SHMB) represent the mafic, The mafic layered intrusions represented the and rhyolites the felsic member. Acid volcanic finale of the APB eruptive activity; no external rocks of the Salla Formation (Manninen, 1991) ultramafic, mafic or felsic dykes are found cutting form the present floor and roof of the Akanvaara the freshly consolidated cumulates. The quartz intrusion, but occur in lesser amounts in the monzonite diapirs in the Koitelainen Lower Porttivaara intrusion and the Koitelainen intrusion Zone, felsic dykes in the Koitelainen magnetite (Mutanen, 1996). Acid volcanic rocks of the gabbro, and dioritic and felsitic dykes in Keivitsa Seidorechka Suite occur in the roof of the Imandra are "inside" phenomena of the intrusive systems intrusion (see e.g., Kozlov et al., 1967), although, (see later). As reliable ages of metasediments and drawing on published descriptions and radiometric metavolcanics in Finnish Lapland are lacking, the ages (Mitrofanov et al., 1995) I consider that intrusion ages become handy in fixing the minimum many of the "imandrites" of the area represent the age for the floor rocks and immediate roof rocks. granophyre cap of the underlying intrusion. Acid Refractory volcanic rocks are found as xenoliths igneous rocks (lavas, dykes and intrusions), as well among the cumulates. Xenoliths of basalts, as basaltic lavas and komatiites of the Archaean/ basaltic tuffs, komatiites and pre-intrusive gabbros Proterozoic boundary (APB) event, which are not occur in Koitelainen, komatiite xenoliths in the spatially associated with the layered intrusions, are Koillismaa Group intrusions (my interpretation of widespread in the Shield (see Mutanen, 1997a, and the case reported by Isohanni, 1976), and basaltic a long list of references therein). xenoliths in Akanvaara intrusion. Keivitsa is There is evidence in the Shield of mafic intrusive conspicuous for the abundance and wide variety activity that clearly predates the 2.44-2.49 Ga of xenoliths. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 15 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
MAFIC LAYERED INTRUSIONS OF THE FENNOSCANDIAN SHIELD
Mafic layered intrusions and dykes occur as a string of gabbro outcrops for 7 km east of the throughout the Shield. A very incomplete base of the intrusion (see Perttunen, 1971). presentation of their areal distribution is shown There are various opinions as regards the in Fig. 2. The data on the map is collected from composition of the parent magmas of the 2.44-2.49 published regional reviews, descriptions of Ma intrusions. This is partly due to lack of chilled individual intrusions, new unpublished reports rocks, representative of the parent magma before and published U-Pb zircon and baddeleyite ages fractionation and untainted by contamination or and Sm-Nd mineral isochron ages (the long list added cumulus crystals (see e.g., Hoover, 1989), of references in Mutanen, 1997a). Tolstik and partly because bulk intrusions analyses have been other intrusion of the NW Belomorian Belt (2.434 made from sections that do not represent of the –2.443 Ga) are added to the figure. It is possible contents of all of the cumulate mass. From many that many of the named intrusions were originally intrusions only the roots are exposed on the present parts of a single intrusion. erosion surface (Burakovka, Näränkävaara). The The first "respectable" ages from the 2.44-2.49 Ga Tornio-Kemi-Penikat intrusions were tilted and group of intrusions are the 2.45-2.46 Ga common beheaded by pre-Jatulian erosion and hence only lead model ages of the massive vein sulphides of remnants of the granophyre caps are preserved the Monchegorsk intrusion (Vinogradov et al., (Perttunen, 1991). The most completely analysed 1959). The first conventional isotopic ages on of the best preserved intrusions is Akanvaara. Its zircon of the ultramafic pegmatoids were obtained initial liquid composition corresponds to a fairly in 1974 of the Koitelainen intrusion (Mutanen, normal tholeiite, Cr being the only exception (Figs.
1976b, 1989; Mutanen & Huhma, 2001). Since 5 and 7). Thus, in regard to SiO2, TiO2, MgO and then a great number of intrusions have been dated Al2O3, the magma was not particularly "primitive", by U-Pb/zircon, baddeleyite and (or) Sm-Nd but rather similar to the bulk composition of the methods. Bushveld layered series (see von Gruenewaldt, The large mafic differentiated "plutons" in 1979). It is also clear, from what we know of the southern Finland (the Ylivieska, Parikkala and fractionation of the Koitelainen and Akanvaara
Hyvinkää intrusions shown in Fig. 2) are not magmas (low H2O) and the bulk intrusion generally recognized as layered intrusions, compositions (MgO and SiO2), that the magmas which they undoubtedly are. They present fine were not boninitic (see Crawford et al., 1989; examples of layering in outcrops and strong crystal Lobach-Zhuchenko et al., 1998, made the same fractionation down to cumulus ilmenite, magnetite point). The untainted chill compositions have a Cr and apatite. content appropriate to tholeiitic basalts. The REE The magmas of the 2.44-2.49 Ga intrusions are low and the REE(CN) pattern is flat or gently emplaced mainly along the unconformity contact sloping (down from LREE). Mafic volcanism between the basement and late Archaean or early with some characteristics of SHMB magmas (see Proterozoic metasediments and greenstones (e.g., Sun et al., 1989) preceded the intrusion of the Mutanen, 1981, 1989; Dokuchaeva et al., 1992). Akanvaara magma, but the intrusion itself is not of The upper feeder section of Näränkävaara, the the SHMB type. The parent magmas of the 2.44- connecting dyke (see Alapieti, 1982) as well 2.49 Ga intrusions were rather low in Ti and very as the feeder dykes southeast of the Kemi and low in P, and the Fe3+/Fe2+ was low. Thus, ilmenite Penikat intrusion trend northwest. The supposed never crystallized as a "legal" cumulus mineral, feeders of the Kemi and the Penikat intrusions cumulus apatite appeared only in the uppermost (the Loljunmaa dyke, see Alapieti et. al., 1990) ferrodiorites (in Akanvaara at ca 97 PCS), and were checked by gravity measurements and found cumulus magnetite (original titanomagnetite) to agree with the prediction (Elo, 1979). The entered late (in Koitelainen at ca 94 PCS). The Loljunmaa (also called Lolju) dyke is traceable magmas were also undersaturated in sulphur 16 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
2,45 2,45 2,50 2,50? 2,44
2,46
TO
Ages, Ga
1,90 2,05 - 2,20 2,44 - 2,49 2,45?
Fig 2. Mafic layered intrusions of the Fennoscandian Shield. KOI - Koitelainen, KEI - Keivitsa-Satovaara Complex, AKA -Akanvaara, T - Tsohkkoaivi-Kaamajoki, TKP - Tornio- Kukkola-Kemi-Penikat group, KG - Koillismaa group, KOU - Koulumaoiva, PE - Peuratunturi, G - General (Luostari), F - Fedorovo-Pana, OL - Olanga group, K - Kolvitsa-Kandalaksha, TO – Tolstik and other intrusions in the NW Belomorian Belt, B - Burakovka, IM - Imandra, MO - Monchegorsk, MLV - Monche-(Chuna)-Volche-Losevykh Tundr, PY - Pyrshin, HA - Haikola, Y - Ylivieska, H - Hyvinkää, P - Parikkala, dy – 2,45 Ga age group dyke complex. CAR = Kortejärvi and Laivajoki carbonatites. Modified from Mutanen, 1997. Tolstik and other intrusions of the NW Belomorian Belt from Lobach-Zhuchenko et al., 1998). FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 17 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
(sulphide liquid), so that, aside from transient from Tsohkkoaivi-Kaamajoki in the northwest sulphide saturation due to contamination, terminal to Kivakka, Tsipringa and Lukkulaisvaara (the saturation of sulphide liquid (with high Fe and Cu/ Olanga group) in Russia. Small intrusions and Ni) occurred at or above the magnetite-in phase dykes of the general magma type of 2.44 Ga contact. In this respect the magmas were quite intrusions are met with southeast of Olanga similar to other tholeiitic intrusions, e.g., Bushveld (see Fig. 2); the belt may extend still farther (Liebenberg, 1970) and Kiglapait (Morse, 1969, southeastwards, to Burakovka in the Lake Onega 1979b). region, Russia. Solitary dykes of the 2.45 Ga age
The initial εNd(T) values for cumulates of the group cutting the Archaean basement occur in 2.45 Ga intrusions are variable but generally wide areas (Vuollo et al., 2005). negative (Huhma et al., 1990, 2005; Amelin & Relicts of a pre-Koitelainen intrusion were Semenov, 1996; Mitrofanov et al., 1991). The intersected below the Koitelainen intrusion (see ε-values in the Burakovka intrusion show a clear Mutanen, 1989, 1997a). The Koitelainen event was negative trend upwards; the lower microgabbros, preceded by extrusion of rhyolitic and komatiitic however, have positive (up to +2.9) values. The lavas and minor basalts (see Mutanen, 1989). At total range of ε-values in this group of intrusion is Koitelainen, komatiites are more common, but at very large (around 6 units; see Amelin & Semenov, Akanvaara the intrusion was preceded mainly by
1996; Lobach-Zhuchenko et al., 1998). Cases of rhyolitic and basaltic volcanism (in basalts SiO2 gross isotopic heterogeneities between cumulus 52-53 %, MgO 6.5 %, TiO2 0.85 %). Komatiites minerals have been found (e.g., Vuollo et al., and rare basalts occur in Koitelainen among mafic 2005). Considering the various mineralogical cumulates as xenoliths, in "horizons" marking evidence of crustal contamination from the onset of anatexis of the roof rocks. Naturally, Koitelainen, Akanvaara and Keivitsa (Mutanen, acid rocks immersed in hot mafic magma did 1997a), findings of isotopic heterogeneity (Nd, not generally survive. However, partially melted Sr, Os, Pb) across layers, within one cumulate ("granophyrized") amygdaloidal acid lavas have layer, between cumulus minerals (as at Merensky been found in DDHs among granophyres in the Reef; see Prevec et al., 2004), and even within southeastern part of the Koitelainen intrusion. one cumulus crystal or phenocryst probably will Xenoliths of acid porphyries also occur in the prove to be a rule, rather than exceptions (see e.g., Akanvaara granophyre. Duffield & Ruiz. 1998; Davidson et al., 2002, Mafic dykes cut the rocks of the Koitelainen and 2005; Bryce & DePaolo, 2004). Akanvaara intrusions. Their chilled contacts and The mafic layered intrusions in northern Finland general hypabyssal petrography suggest that they and their counterparts in Russian Karelia and the were channels for lavas that extruded not far above Kola Peninsula host a variety of ore deposits of them. Because dykes like these have not been PGE, Ni-Cu, Ti-V and chromite (see a long list of found cutting the Keivitsa-Satovaara complex, the references in Mutanen, 1997a). Taken together, post-Koitelainen dykes may represent pre-Keivitsa the chromitite layers of Koitelainen, Akanvaara, basaltic magmatism (see Huhma et al., 1996). Monchegorsk, Imandra and Burakovka already The intrusions were later metamorphosed, constitute the largest known undeveloped resource faulted and folded, sometimes into upright and of chromite in the world. even overturned positions. Faulting, folding and A belt of layered intrusions runs in a general emplacement of younger granitoids dismembered southeasterly direction through Finnish Lapland the original intrusions. It seems that despite the and on into Russia (Fig. 2). The name Lapland superimposed folding, the LIB and the associated Intrusion Belt (LIB) is used here for the belt. cratonic supracrustal rocks are still in their The LIB contains intrusions, sills and dykes with original site, that is, the crustal rocks of Central ages from 2.49 Ga to 2.0 Ga and younger. The Lapland were never hauled around as tectonic intrusions of the 2.44 Ga age group occur in the plates, eventually to be stitched together like a axial part, and throughout the length, of the LIB, jigsaw puzzle along healed seams of collided 18 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen plates. The prime tectonic mover in Central with Keivitsa produced the gabbro-anorthosite Lapland was density inversion between the heavy complex of Otanmäki (2.06 Ga U-Pb/zircon, superstructure (greenstones and mafic intrusions) Talvitie & Paarma, 1980); the acid and basic and the underlying buoyant sialic crust. This lavas of the Kuopio district (2.06 Ga U-Pb/zircon, caused diapiric rise of granitoids as brachyantiform Heikki Lukkarinen & Olavi Kouvo; Heikki structures. Lukkarinen, personal comm.), acid porphyry of The Keivitsa event, like that of Koitelainen, was Ylitornio (2.05 Ga U-Pb/zircon, Perttunen & preceded by bimodal and often explosive rhyolitic- Vaasjoki, 2001) and the basaltic metalava of the komatiitic volcanism. Evidence of acid volcanism Jouttiaapa volcanics (Sm-Nd wr 2.09±0.07 Ga, first appears among the pelitic sediments below Huhma et al., 1990), the two last mentioned from the Keivitsa intrusion; acid volcanism was the Peräpohja schist belt. At Rautuvaara, western accompanied and followed by komatiitic lavas, Lapland, a differentiated dyke has been dated by agglomerates and tuffs, and by the intrusion of U-Pb/zircon at 2.027±0.033 Ga (Hiltunen, 1982). komatiitic sills. The U-Pb zircon age of 2.048 Ga The carbonatites of Laivajoki and Kortejärvi (Mutanen & Huhma, 2001) for the Matarakoski yield a U-Pb/zircon age of 2.02 Ga (Vartiainen & quartz porphyry (metarhyolite), 10 km southwest Woolley, 1974). of Keivitsa, is indistinguishable from the age of the The peculiar mafic dykes (Mutanen, 1997a; Sm- Keivitsa intrusion, 2.057 Ga (Huhma et al., 1996a; Nd mineral isochron age 1.916±0.067 Ga, Huhma Mutanen & Huhma, 2001). However, recent et al., 2005) that cut the solidified and cooled findings from the Rantavaara intrusion suggest Keivitsa intrusion have no known compositional that the pelitic metasediments and black schists, counterparts among the dyke rocks of the Shield. below komatiites and acid volcanics, are actually They may be contemporaneous with 1.97 Ga older than 2.15 Ga (see Huhma et al., 2005). tholeiitic dykes (Vuollo et al., 1992) and ophiolites In Finland, magmatism roughly contemporaneous (Kontinen, 1987) in eastern Finland. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 19 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
THE AKANVAARA INTRUSION
General geology
The Akanvaara intrusion is located in the southeast Here the floor and roof contacts of the intrusion, of the LIB (Fig. 2). Zircon from a crescumulate where known, have been preserved, but in the gabbro just above the Upper Chromitite layer northern wing long stretches of the western (floor) gives a U-Pb age of 2.43 Ga (Mutanen, 1997a, and eastern (roof) contacts have been faulted. The Mutanen & Huhma, 2001). general strike of the magmatic layering in the Owing to the lack of outcrops the existence of northern wing seems to be northeasterly, the same the northern wing of the intrusion was not known as in the southern part of the intrusion. at the time the Savukoski map sheet was printed Young faults and other "zones of weakness" (Juopperi, 1986; Heikki Juopperi oral comm., consist of crumbled material. No displacement 1991). The geology of this wing is still poorly along young faults has been recognized. Old known. The new geological map (Fig. 3) is based healed faults and fault zones are formed before and on revisionary mapping of the area, low-altitude (or) during regional metamorphism. Apart from aeromagnetic maps, ground geophysical surveys normal fracturing they are solid rocks. The broad (magnetic, gravity and electromagnetic) and fault zones are blastomylonites, often with mineral extensive diamond drilling. veins (quartz, carbonate). Thin fault traces (width ≤ The intrusion is a block-faulted monocline 0.5 m) are often filled with meta-pseudotachylytes dipping southeast, with a surface area of 50-55 km2. (metamorphosed pseudotachylytes). In Tuorelehto Its length from north to south is ca 15 km; its width a braided network of faults, shear zones and in the southern part, along the strike of layering, is metapseudotachylytes is visible in the Upper ca 7 km, and across the northern wing 2-3 km. In Chromitite (UC) outcrop (Stop 4, Fig. 16). The the east and west the intrusion is bounded by zones pseudotachylytes formed through cataclastic of steep NW-NNW faults. The eastern fault zone comminution, frictional melting and post- separates the intrusion from older acid volcanics. frictional annealing (see Reitan, 1968; Sibson, In the west, beyond the western fault zone, the 1975, 1978; Fleitout & Froidevaux, 1980). The terrigeneous psephitic and psammitic sediments colour, and composition, in pseudotachylyte veins of the Ritaselkä formation are considered younger reflect those of the nearest wall rocks. Pockets of than the Akanvaara intrusion (Jorma Räsänen, oral coarse plagioclase (with minor chlorite) occur in comm.). Thus, the southern part of the intrusion is meta-pseudotachylyte veins; they may represent an erosional section of a fault staircase descending segregated and slowly crystallized frictional neo- to the west, the parts west of the boundary fault melt. being buried beneath younger metasediments. The chromitite layers of the Lower Zone This general structure of the intrusion is shown frequently have contact faults, with displacement by the gravity field (increases westwards, towards roughly parallel to layering. Such faults may the buried part of the intrusion and away from already have formed during consolidation of the the lower-density acid rocks) and magnetic cumulate pile, between stiff chromitite layers field (increases eastwards, towards the strongly and less competent host pyroxene cumulates magnetic acid volcanics). (possibly still containing interstitial melt). Layer- The southern part of the intrusion consists of contact faults may also have formed during three large fault blocks and numerous sub-blocks. metamorphic hydration as a result of dissimilar The general direction of these faults is between NW volume expansion of chromitite and pyroxenite. and NNW, the same as that of the boundary faults. Similar contact faults are common in Bushveld The dip of igneous layering in the southern part (Lea, 1996). varies from 25° in the westernmost block to 40° The magma intruded supracrustal rocks among in the east, indicating relative rotation of blocks. which acid volcanics were dominant. Basalt 20 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
AKANVAARA LAYERED INTRUSION
7459
2 km
AKANVAARA INTRUSION 6 (MAP AREA SHADED)
40
5
35
35
7455 30 4 30
40
35
25
2 3
1 km 7451 1
3550 3555
Faults, shear zones Mafic dykes
Post-intrusion rocks of the Ritaselkä Formation Combined DDH profile
AKANVAARA INTRUSION Olivine-pyroxene (-chromite) cumulate and Granophyre ULC - chromitite Ferrogabbro, ferrodiorite (A = cumulus apatite); Pyroxene cumulates, chromitite layers layers of mottled anorthosite and ultramafic cumulates Magnetite gabbro Lower marginal chill Gabbro (Upper Zone, Main Zone, Lower Zone); anorthosite layers Pre-intrusion rocks: acid volcanic rocks, in MZ; pyroxene cumulate layers in LZ and in lower MZ blastomylonites
Anorthosite Metahornfelses from acid volcanic rocks
UC chromitite and associated anorthosites 40 Igneous layering
2 Excursion stop
Fig. 3. Geological map of the Akanvaara layered intrusion. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 21 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... xenoliths occur in granophyre. The acid volcanics distinct magma types: one is rather primitive (MgO of the roof, well exposed on the Akanvaara hill, 11 %, FeO(tot) 10 %, TiO2 0.85 %, Cr 950 ppm, Ni typically contain large silicic amygdules. All 300 ppm, Cu 100 ppm, SiO2 50.5 %), another is the acid volcanites intersected by DDHs below more evolved (MgO 7 %, FeO(tot) 13 %, TiO2 1 the intrusion are hornfelses (later altered to %, Cr 130 ppm, Ni 90 ppm, Cu 180 ppm, SiO2 48 metahornfelses). Porphyritic types are dominant %). Some dykes are komatiitic in appearance. here, but amygdaloidal rocks also occur. The During the regional metamorphism the rocks of hornfelses intersected and analysed so far are the intrusion suffered hydration but, apart from compositionally affected by the intrusion and that, metamorphism appears to have been almost hence resemble the ferrogranophyres of the roof isochemical. The granophyre and gabbros down to (see Mutanen, 1996, p. 107). The hornfelses are the Main Zone are usually heavily recrystallized. more "primitive" (higher MgO, Mg# and V, lower The former cumulus pyroxenes have not been Mn, Pb and Zn). replaced by the usual, tidy uralite pseudomorphs Acid volcanic rocks heated by the crystallizing but by a sheaf of long amphibole prisms that have magma of the intrusion melted relatively easily, grown far beyond the original grains. Plagioclase particularly if they contained glass. The nature and is whitish, having lost its characteristic dark amount of the digested roof rocks are not known, pigment during recrystallization, and has altered but the presence of basaltic xenoliths indicates a into clinozoisite and a more albitic plagioclase, lithology more variegated than that of the present sometimes into scapolite. However, in many cases roof. At Purkkivaara, 5 km south of the intrusion, the plagioclase retained the pigment even when high-aluminous pelitic schists and komatiites all the pyroxenes had been uralitized. With the occur at about the level of the "missing" lithology general decrease in alteration downwards, the dark- of Akanvaara (Jorma Räsänen, 1983, and personal pigmented plagioclase becomes more common, comm.). They may correspond to the high- and in the lower Main Zone pyroxenes and olivine aluminous pelitic roof rocks of the Koitelainen have often survived in gabbro, troctolite and intrusion. peridotite. Preserved pyroxene is common in the Numerous mafic dykes with a general strike of pyroxene cumulates of the Lower Zone. Unless NNW cut the Akanvaara intrusion and country stated otherwise, the terminology of the cumulates rocks. The two dykes analysed represent two is based on the original magmatic minerals.
Igneous stratigraphy and petrography
There are very few outcrops in the Main and (non-cumulate) gabbros, the latter often with Lower Zones. However, overlapping drill cores dendritic plumose pseudomorphs of former provide a continuous profile from near the top of pyroxene ("crescumulate microgabbro"). Most of the granophyre down to the floor rocks. The cores the microgabbros show clear geochemical signs of were analysed gaplessly for Pd and Au, and XRF contamination. whole rock analyses were made on all cores, in The Lower Zone (LZ; Fig. 18) consists of a homogeneous stretches (Main and Upper Zones) lower orthopyroxene cumulate unit and an upper at regular intervals. gabbroic unit; there are several chromitite layers. The total thickness of the cumulate succession In orthopyroxene cumulates cumulus chromite plus granophyre as exposed on the present surface is ubiquitous. These cumulates also contain, is 3144 m (Fig. 4). The layered succession above particularly when chromite is abundant, cumulus the lower chilled margin is formally divided into clinopyroxene as big, sparse cumulus crystals, zones. more rarely as small euhedra. Olivine (now The lower marginal chill is composed of altered) is a visiting cumulus mineral. It joins fine-grained microgabbros and non-laminated the pyroxene-chromite cumulus assemblage 1.4 22 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
orthopyroxene (see Mutanen, 1997a). The olivine abundance below the lowermost LC layer is very low. Olivine is not associated with the Lowermost Lower Chromitite (LLC) layers. Plagioclase is the main intercumulus mineral in LZ. Brown primary hornblende also occurs in intercumulus. The amount of postcumulus plagioclase, and also of quartz, is quite high (27- 39 vol% pl+q) near the base, to the extent that some pyroxene cumulates are modally gabbros and quartz gabbros. Euhedral zoned crystal cores in plagioclase oikocrysts indicate that plagioclase began to grow as a cumulus phase. Evidently these cumulates contain a considerable amount of acid contaminant magma from melted floor rocks. Layers of true gabbroic cumulates occur in the lower part of the unit. Fluorapatite and zircon are typical accessory minerals. The amount of intercumulus plagioclase decreases upwards. Zircon is present, and loveringite (Table 2; see also Mutanen, 1997, Fig. 4b) is common in pyroxene cumulates. I have found crystals of loveringite included in orthopyroxene; thus, it is apparently a cumulus phase, as in Koitelainen (Tarkian & Mutanen, 1987). Apatite is chlorapatite (Cl 5.03- 6.84 %, F 0.00-0.08 %) in pyroxene cumulates (Table 5), but in the heavily contaminated lowermost pyroxene cumulates and LLC layers the apatite is fluorapatite. As a “true”, cotectic cumulus phase fluorapatite enters at the 97 PCS level. Interlayers of gabbro appear in the upper part of PGE = ENRICHMENT OF PGE UC = UPPER CHROMITITE LAYER the unit. ULC = UPPERMOST LOWER CHROMITITE LAYER The lowermost part of the gabbro unit contains LC = LOWER CHROMITITE LAYERS pyroxenite interlayers. Otherwise the unit mainly CR = CHROMITITE LAYERS (THIN) consists of monotonous gabbroic cumulates. Chr = CHROMITE MA = MOTTLED ANORTHOSITE Discontinuous layers of pyroxene (-chromite) cumulates and a group of thin (≤ 5 cm) chromitite Fig. 4. Stratigraphy of the Akanvaara intrusion. layers (MC chromitites) occur in the upper part. Immediately below the base of the MZ is a 3 m-thick succession of alternating layers of - 3.1 m below each of the Lower Chromitite (LC) pyroxenite, gabbro and mottled anorthosite. layers, occurs between closely-spaced chromitite The Main Zone (MZ) comprises cumulates layers and in one case continues for 2 m above the from the base of the Uppermost Lower Chromitite chromitite. Sometimes euhedral olivine occurs, (ULC) up to the base of the Upper Chromitite without reaction, with euhedral orthopyroxene. (UC) succession (Fig. 17). As in Koitelainen and Keivitsa, I think this does An olivine-pyroxene cumulate unit about 50 m not really mean co-crystallization of olivine and thick overlies the ULC layer. This unit shows up FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 23 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... well on the magnetic map. Disseminated cumulus gabbro interlayers. Ilmenite may have crystallized chromite is always present; in the lower part of the as a cumulus mineral, possibly due to silicic- unit chromite occurs as thin massive bands (up aluminous contamination. The sequence contains a to 9 cm thick) and as concentrated net-textured 40-m-thick bipartite PGE-enriched interval (Figs. pockets between orthopyroxene cumulus crystals. 9, 14). Higher up, from 12 to 25 m below the Black oikocrysts of amphibolized clinopyroxene, base of the magnetite gabbro, is a gold-enriched enclosing olivine, give the rock its characteristic succession, with up to 1.94 ppm Au/1.0 m at its mottled look. Oikocrysts of orthopyroxene are base (Figs. 9, 13). This same interval also contains preserved in unaltered rocks. Chlorapatite occurs 99.5 ppb Pt. as big cumulus crystals (Table 5; see also Mutanen, The sudden arrival of cumulus titanomagnetite 1997a, Fig. 4a). marks the sharp basal contact of the magnetite In the upper part of the unit there are interlayers gabbro (MTGB). At this phase contact there of pyroxene cumulates (with chromite) and are marked anomalies of P2O5 (up to 0.3 %), rare olivine-plagioclase-chromite cumulates Zr (210 ppm), Cr (376 ppm), La (45 ppm) and (troctolites, with poikilitic pyroxenes). These Ce (76 ppm), which suggest that the arrival of are overlain by a succession, ca 35 m thick, of magnetite was promoted by a convective overturn alternating layers of gabbro and pyroxenite. Most and associated contamination of the magma. of the MZ rocks are cotectic plagioclase-pyroxene The REE(CN) diagram (Fig. 12) shows that the cumulates. In the lower gabbros, orthopyroxene anorthosites immediately below the MTGB are typically occurs as small (≤ 1-2 cm) oikocrysts. the most rich in REE; going upwards the REE Solitary layers rich in cumulus plagioclase concentration decreases. This, and the lack of (anorthosite gabbros) and two thick successions positive Eu-anomalies, infers that the plagioclase of alternating layers of gabbro, anorthosite gabbro, here was not the usual excess cumulus plagioclase gabbro-anorthosite and mottled anorthosite occur of the mafic residual magma; more probably it in the upper part of the Main Zone. crystallized in a contaminated, REE-enriched The coarse gabbros on top of the MZ contain uppermost part of the magma, oxidized to such plagioclase in excess to its due cotectic proportion. a degree that no Eu2+ remained. The high Cr These rocks are also rich in intercumulus quartz reflects the high D(Cr)spinel/liq , but the source of Cr and contain big fluorapatite crystals. may be Cr-enriched refractory residue matter of The Upper Zone (UZ) is set to begin from the melted crustal rocks. Anomalies of Zr, P and REE top of the homogeneous gabbros underlying the seem to be characteristic of reversals analysed in Upper Chromitite (UC) layer. It starts with the UC detail (see Lambert & Simmons, 1988; Halkoaho, sequence (Figs. 15, 16), which will be described 1994), suggesting, together with many kinds of later in more detail. isotopic data, that reversals were connected with Leucocratic layers become thinner and more contaminant-carrying density flows. sparse upwards from 10 - 15 m above the UC, The MTGB unit (Fig. 9) consists of two vanishing altogether about 40 m above it. The magnetite-rich subunits (with V > 0.1 %) separated interval between the UC succession and magnetite by a layer poor in magnetite. The lowermost 6 m gabbro consists mainly of cotectic pyroxene- consists of plagioclase-magnetite cumulates with plagioclase cumulates that are macroscopically 0.4 % V. Magnetite was partly altered into silicates monotonous except for size layering. In this (hornblende, biotite) in metamorphic reactions. innocuous gabbro a 14-m-thick interval enriched Titanomagnetite was the sole cumulus Fe-Ti in PGE was revealed by systematic Pd assays (Fig. oxide in magnetite gabbro. Ilmenite exsolved 14). The overlying cumulates up to the magnetite from titanomagnetite first by granule oxidation- gabbro are variably anomalous in PGE. exsolution, later by lamellar exsolution Near the top of the UZ is a 100-m-thick sequence (ilmenomagnetite). Titanomagnetite crystallized of plagioclase-rich cumulates (mottled anorthosite, as a cumulus phase up to the end with plagioclase gabbro-anorthosite, and anorthosite gabbro), with and pyroxenes. These were probably later joined 24 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen by fayalite. Apatite joins the cumulus assemblage known. A big xenolith of acid porphyritic rock was at ca 97 PCS, just below the granophyre. intersected by drilling; its composition is distinct Thin (1-4 cm) anorthosite layers occur in the from the surrounding granophyre (higher Si and lower part of the unit. In two DDHs a layer of Na, lower K, Ba, P and REE). anorthosite (0.7 – 1 m thick) occurs 5-10 m Microgabbros are encountered from the base above the basal contact of the MTGB. In the of the intrusion up to the UZ. They include magnetite-poor upper part there are seven layers of rocks of the lower chilled margin, layers (?) plagioclase-rich cumulates (mottled anorthosite, and autoliths in the basal LZ, and discontinuous anorthosite gabbro), 1 - 7.5 m thick, and more layers and autoliths in the upper MZ and in the than ten ultramafic layers (now hornblendite), UC succession. Microgabbros in the cumulate 0.2 - 1.7 m thick. In these rocks there is a positive succession are almost always associated with correlation between plagioclase-content and P-Zr- anorthositic cumulates. In the UC sequence Ce-La. microgabbros occur as irregular layers below, The uppermost mafic cumulates are ferrogabbros in and immediately above the UC layer, and as
(SiO2 < 55 %, FeO(tot) 17-25 %, CaO/Na2O < 2), autoliths above the UC (Figs. 15, 16). There are two apatite-ferrogabbros (apatite 2-5 %) and apatite- main types, a fine-grained and a medium-grained ferrodiorites. Two big basaltic xenoliths have been microgabbro, both commonly present in one intesected in ferrodiorite. sample (Fig. 21a). The fine-grained microgabbro Granophyre is the uppermost magmatic unit of seems to have been consolidated earlier and is the intrusion. The basal 20 m is ferrogranophyre often embedded (as autoliths) in medium-grained
(FeO 11 - 12.5 %, SiO2 60 – 65 %), which grades microgabbro. Both are non-cumulate rocks, into overlying acid granophyre (SiO2 70 – 75 representing mafic magma frozen at contact with %). The total thickness of the granophyre is > colder country rocks, an acid anatectic roof melt 260 m. Plagioclase was the first major silicate to or a "cold" density flow. Hybrid microgabbro may crystallize from the melt. The contact relations have formed by mixing of acid-hybrid magma and with the older acid volcanites of the roof are not mafic magma.
Chemistry and fractionation
The general course of crystal fractionation is seen in UC included, the other (broken line) for melt in diagrams of cumulate compositions (Figs. 5, with Cr in UC excluded (supposing that the Cr 6). The evolution of liquid composition (liquid in UC was exotic). Incontestably, the latter curve line of descent) was calculated for 10 components looks more natural, the former quite unnatural. and Mg# (FeO was calculated as 0.9x FeO(tot)) However, towards the end of the summation the by incremental summation, down from the curves almost merge – in unnatural levels of Cr! granophyre-ferrodiorite contact to the basal chilled Crystal fractionation produced melts that margin (Fig. 7). The calculation is based on 924 evolved duly according to “Fennerian” trend, whole-rock XRF analyses from a combined profile with decreasing SiO2 and increasing FeO(tot). of 22 overlapping DDHs. In the final calculation The decrease of compatibles (MgO, Cr, Ni) 330 internally weighted increments were used. The and the build-up of incompatibles (Ti, V, S, Cu, thickness of the increment (from 0.2 to 100 m) was P2O5, Zr) in residual liquid are clearly visible in chosen according to the homogeneity of rocks; it rock diagrams (Figs. 5 and 6) and melt evolution was shortest where compositional changes were diagrams (Fig. 7). In the melt diagrams the phase- rapid (for compatible elements near the floor, for in contacts (arrivals of titanomagnetite, sulphide incompatibles approaching the roof). Downwards liquid, apatite) appear as distinct inflections that from the UC layer two curves were calculated for were followed by permanent saturation levels. Cr: the one (solid line) for melt with the Cr content However, the arrival of sulphide liquid and FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 25 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... 0 a Fig. 5. Profiles of some major, minor and trace elements of rocks of the Akanvaara intrusion. Data combined and matched from
several diamond drill holes. 5a - SiO2 and Al2O3, FeO(tot), MgO and Mg#, 5b - Ni, Cr , TiO2, V, P2O5, Zr and Cu. 26 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
Fig. 6. DDH R322 intersection, lower part of LZ (Cr, Ni, TiO2, Mg#). titanomagnetite depleted the liquid of Cu and V, Thus it seems that practically all Cr contained in respectively. chromitite layers is of exotic origin. The thick anorthositic cumulates and, to a Until now Kiglapait (Nain Anorthosite Suite) lesser extent, the arrival of titanomagnetite, cause was the only mafic layered intrusion whose melt stagnation and even slight reversals in the melt evolution is satisfactorily known (Morse, 1979a, evolution trends of MgO and Mg#. It is well known b, 1981; Nolan & Morse, 1986; Morse, 2001; in experimental systems that arrival of magnetite see also Scoates & Mitchell, 2000). The initial may increase the MgO in liquid. The thick magma composition of Akanvaara is surprisingly anorthositic and other non-cotectic plagioclase- similar to that of Kiglapait. The best fit is with rich layers, without more mafic “counterlayers”, Kiglapait “bulk-summated” composition minus imply that alumina was added to the magma 10 wt% FeMgAl spinel (Morse, 1981, Table 12; system. Far more dramatic is the “scarp” in Cr KIGSUM-SPI in the following). The summated at the UC chromitite. This supports the model initial Akanvaara magma (AKA), compared of exotic origin of Cr (see Mutanen, 1981,1992, with the range of variously corrected summated 1997). But subtracting the Cr in UC still leaves magmas and marginal rocks of Kiglapait, is as
“too much” Cr in magma. Only subtracting the follows (in wt%, trace elements in ppm): SiO2 Cr contained in all chromitite layers leaves an 51.243 (KIG 47.46 – 49.83, KIGSUM-SPI 52.81), initial magma with Cr concentration of 581 ppm, a TiO2 0.778 (KIG 0.3 – 0.81, KIGSUM-SPI 0.90), reasonable figure for an aluminous mafic magma. Al2O3 14.871 (KIG 18.67 – 19.58, KIGSUM- FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 27 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... ; c – V, Cu, S. V, ; c – 5 O 2 , P 2 , FeO(tot), MgO, Mg#, Mg# in cumulates; b – TiO , FeO(tot), MgO, Mg#, Mg# in cumulates; b – 3 O 2 , Al , 2 Fig. 7. Evolution of the Akanvaara liquid. a – SiO Fig. 7. Evolution of the 28 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen 2 2 5 P O 0,0889 % TiO 0,778 % Base contact 3000 2500 2000 1500 b Depth (m) 1000 Magnetite-in 2 5 PO 500 Apatite-in 2 5 0.6 0.2 PO% 0.8 0 0.4 0.2 1.0 0 1.2 2.4 2.0 1.0 1.6 0.7 Roof contact 2 TiO % FERROGRANOPHYRE Fig. 7. b FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 29 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... V 304,5 ppm S 345,1 ppm Cu 117,9 ppm Base contact 3000 2500 2000 V S Cu 1500 c Depth (m) 1000 Sulphide-in ? Magnetite-in 500 2500 2000 1500 1000 500 S, ppm 0 13 Roof contact 800 500 300 100 ppm Cu, V, 5m Fig. 7. c FERROGRANOPHYRE 30 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
SPI 14.45), FeO(tot) 9.970 (KIG 8.91 – 11.08, same as in Kiglapait (21.3 wt%, see Morse, 1981). KIGSUM-SPI 9.78), MgO 7.445 (KIG 7.78 The last Fe-rich liquid was probably immiscible
– 8.17, KIGSUM-SPI 7.51), P2O5 0.0889 (KIG with any salic liquid, formed either by anatectic 0.04 – 0.11, KIGSUM-SPI 0.10), Mg# 0.597 (KIG melting of the roof rocks or by spontaneous 0.60 – 0.64, KIGSUM-SPI 0.63), Cr 1212 – 1266 splitting of the residual liquid. (KIG 132), V 305 (KIG 52). The only differences The melt attained sulphide liquid saturation at are in Al2O3 (4-5 wt% higher in Kiglapait), Cr the arrival of titanomagnetite (72.5 PCS). As seen (much higher in Akanvaara) and V (much higher in Fig. 8, the concentration of total S (soluble in Akanvaara). The low-Al marginal gabbros of sulphide plus sulphide liquid) is proportional to the Harp Lake intrusion and KIGSUM-SPI have FeO(tot) and inversely proportional to Al2O3. the same Al2O3 (14.43 and 14.45, respectively) as There is no correlation with SiO2 (not shown). The Akanvaara. behaviour shown in Fig. 8 does not imply, though, The “halving height” (HH) and the “doubling that increasing total iron in melt increases the height” (DH), expressed as PCS, is a handy measure sulphide solubility. of the compatibility/incompatibility of elements The summated Akanvaara initial magma during crystal fractionation. In Akanvaara the resembles the fine grained, uncontaminated basal HH-values are: Cr 6.65 PCS, MgO 70.7 PCS; DH- chilled margin in DDH R322/390.1-390.5 m (in values: P2O5 58.8 PCS, Cu 60.6 PCS, S 62.5 PCS, wt%): SiO2 52.28 (summated 51.243), Al2O3 14.47
TiO2 63.7 PCS, V 69.4 PCS. The “wrong” order of (summated 14.871), FeO(tot) 8.45 (summated incompatibility of P2O5, TiO2 and V is interesting, 9.970), and P2O5 0.069 (summated 0.0899). considering that in Akanvaara magma pyroxenes However, the chill has higher MgO (12.08 – 9.92; crystallized all through the magma evolution. The summated 7.445) and lower TiO2 (0.40, summated DH-values in Kiglapait (calculated from data by 0.778), S (0.000, summated 0.0345), Cr (476 Morse, 1981) are in the “right” order: 62 PCS for ppm, summated 1266/1212 ppm), V (234 ppm,
TiO2 and 51 PCS for P2O5. The DH-values of TiO2 summated 305 ppm) and Cu (29 ppm, summated for Akanvaara and Kiglapait are about the same. 118 ppm). The compositions of the basaltic But as the rocks in Kiglapait are troctolitic that xenoliths in ferrodiorite (DDH R305) are in some up to 82.5 PCS level contain only 0.9 - 6.8 wt% respect similar to the summated composition (SiO2 of normative pyroxenes, Ti should behave much 50.10 - 50.58, TiO2 0.81 - 0.84, MgO 6.18 - 9.38, more incompatibly than in Akanvaara. As for P in P2O5 0.078 - 0.094, Cr 929 - 959 ppm, V 210 - 232
Akanvaara, its paradoxical behaviour (apparently ppm), but they have lower Al2O3 (10.64 - 10.70), more compatible than Ti) has a simple explanation: S (0.002 - 0.008) and Cu (11 - 57 ppm) and higher
While pyroxenes are dilute phases with regard to FeO(tot) (11.08 - 16.38 wt%). The high K2O (2.41
Ti, apatite is a concentrated P2O5 phase. It was to 3.26) in the xenoliths suggests that some of continuously winnowed from the mafic magma the differences between summated and xenolith in the contaminated mixing zone near the roof composition (Al2O3, FeO, Cu, S) may be due to (where solubility and the cotectic ratio of apatite metasomatic processes operating during the late- was low), was carried down in density flows with magmatic stage and regional metamorphism. silicate cumulus crystals (and some salic melt) The lower marginal chill and the basaltic and finally settled in the cumulate pile. Apatite xenoliths have very similar REE pattern (not is found in acid intercumulus material throughout shown), with a smooth negative slope from the intrusion. Thus, apatite was actually a minor La(CN) 30 to Yb(CN) 5.5 for the chill and from cumulus phase. Likewise, loveringite, baddeleyite, La(CN) 43 toYb(CN) 8.5 for the basaltic xenolith. zircon and chlorapatite (often as big crystals) in These have no Eu-anomalies. The REE pattern of ultramafic cumulates (see Mutanen, 1997a) should the contaminated chill has about the same form be counted as true cumulus minerals. but it has a little higher LREE level and a small The last liquid in Akanvaara was very rich in negative Eu-anomaly. iron, with FeO(tot) 20.6 wt%. This is about the The undisturbed route from the parent magma FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 31 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
2500
2000
1500
Al2 O 3
S ppm FeO Sulphide liq. saturated
1000
liquid evolution 500
liquid evolution
10 12 14 16 18 20 22 %
FeO, Al2 O 3
Fig. 8. Evolution of the Akanvaara liquid. S vs. Al2O3 and FeO.
(of the basal chill composition) would have the reverse fractionation from chilled and other been: pyroxene(s) + plagioclase (+ chromite?) non-cumulate rocks to orthopyroxene(-chromite) -> pyroxenes + plagioclase -> pyroxenes + cumulates. The reverse fractionation continues up plagioclase + magnetite -> pyroxene + plagioclase to the lowermost LC layers, as seen in the Mg# + fayalite(?) + magnetite -> pyroxene + plagioclase curve for rocks (Fig. 6), and is quite similar to + fayalite + magnetite + apatite, and for the most the reverse fractionation of the Basal zone of the part the magma did indeed evolve along this Stillwater intrusion (Page, 1979). cotectic line. The basal part of the LZ shows Crystal fractionation produced rocks and melts 32 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen with decreasing Cr, Ni, Mg, and Mg#. The build- at the base of the MZ; 9) plagioclase (out), up of incompatible elements (Cu, S, V, Ti) and pyroxenite layers in the lower MZ; 10) pyroxene iron in residual liquid is clearly visible in silicate (out), anorthosite layers in the upper MZ; 11) cumulates only above the magnetite phase contact. chromite (in) / plagioclase and pyroxene (out), The crystallization of magnetite effectively UC chromitite; 12) pyroxene (out), anorthosite depleted the vanadium in the residual liquid. layers of the UZ; 13) pyroxene (out), magnetite Towards the top of the magma chamber there is (in), base of magnetite gabbro; 14) Cr peak, basal a dramatic rise in Cu, Cl, P, Zr and S (Figs. 5 and contact of the magnetite gabbro; 15) pyroxene and 7). magnetite(?) (out), anorthosite layers in magnetite The most remarkable reversals and phase gabbro; 16) plagioclase (out), ultramafic layers in boundaries are: magnetite gabbro. 1) chromite (in) / plagioclase (out), above the The reversals, which are associated with non- lower marginal chill; 2) orthopyroxene (out), each cotectic cumulus assemblages, imply that the LC chromitite layer in the LZ; 3) chromite (in) / composition of magma (melt) went astray from plagioclase (out), pyroxene-chromite cumulates, cotectic surfaces. The cause of the reversals was upper LZ gabbro; 4) olivine (in/out), before and the addition of exotic material from heated and after each LC layer; 5) pyroxene (out), anorthosite melted country rocks (Mutanen, 1997). This layers in the upper LZ; 6) chromite (in) / pyroxene- addition changed the composition and structure of plagioclase (out), chromitite layers in the LZ the melt, and shifted the liquidus phase boundaries. gabbros; 7) chromite (in) / pyroxene-plagioclase As at Koitelainen, there is no evidence of, nor need (out), ULC chromitite at the base of the MZ; 8) for, new magma pulses of any kind to explain the olivine (in), olivine-pyroxene-chromite cumulate evolution of the magma system.
The vanadium deposit in magnetite gabbro
The basal part of the MTGB unit is rich in V, reaches 0.44 %. The magnetic concentrate (10- originally contained in ilmenomagnetite. In 12 wt% of the feed) contains 1.55 – 1.59 % V. A regional metamorphism magnetite was partly Cu-sulphide concentrate obtained in a preliminary or totally altered to hornblende and biotite test showed 8-14 % Cu, 1.4 ppm Au, 53-57 ppm ("silicatization"). Another V-rich layer with a Ag, 0.073 ppm Rh and 0.068 ppm Pt (Mutanen, thickness of 27 m is located 300 m above this layer 1998). In three DDH intersections there was an As (Figs. 5, 9-11). The 6-13 m-thick basal part, with hike (5 – 42 ppm As). In the CN-normalised PGE V-rich magnetite (1.93 – 1.99 % V), is estimated diagrams (Fig. 29) the base of the MTGB is similar to contain an indicated and potential reserve to the Au-enriched layer in anorthosites lower (resource) of 20 Mmt down to 100 m vertical down, with sharp peaks at Pt and Au and very depth, with 0.34 % V, 0.10 % Cu, and 2.4 ppm Ag. low Pd. Note that Ir is detectable in Au-enriched Near the base of the basal layer V-concentration MTGB.
Chromitite layers in the Akanvaara intrusion
To date, 23 chromitite layers have been counted in Chromitite (ULC) at the base of the MZ and the the Akanvaara intrusion (Fig. 19): the Lowermost Upper Chromitite (UC) at the base of the UZ. In Lower Chromitite (LLC) group at the base of the the total count; closely-spaced layers are counted layered sequence, the Lower Chromitite (LC) as one. In addition, there are numerous chromitite layers among LZ pyroxene cumulates, the MC bands of uncertain continuity, particularly in LZ chromitites of the upper LZ, the Uppermost Lower pyroxene cumulates and in the ultramafic unit FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 33 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
AKANVAARA MAGNETITE GABBRO AND Au - PGE - ENRICHED LAYERS (DDH 304 - DDH 308)
SE DDH304 DDH303 NW DDH307 DDH309 DDH308
50 100 m
50
100 m
till uralite gabbro anorthosite gabbro
magnetite gabbro uralite gabbro, interlayer layered sequence: mottled anorthosite, gabbro - anorthosites, anorthosite gabbros and uralite gabbros V>1000 ppm (1), 1 2 mottled anorthosite Au - rich layer >2000 ppm (2) V-Cu (Au-Ag-Pt) deposit mottled anorthosite, PGE - enriched layers interlayer ultramafic, olivine-bearing gabbro - anorthosite compositional layer boundary
Fig. 9. Akanvaara intrusion. DDH profile of the magnetite gabbro unit and underlying rocks.
above the ULC layer. rich layers and discontinuous layers of mottled In the following I shall describe some general anorthosite and pyroxenite. The ULC layer is features of the chromitite layers and look at the underlain by alternating layers of gabbro, mottled individual layers (UC, ULC, MC, LC and LLC) anorthosite and pyroxenite, and overlain by in more detail. olivine-pyroxene(-chromite) cumulates. Chromitites occur from the base of the LZ to the The UC is the most regular and tectonically least base of the UZ. Chromitite autoliths are met with disturbed of the chromitite layers. The stratigraphic in the lower chill rocks (Fig. 21a). This implies positions of the LC layers seem to be regular; that chromite had accumulated and consolidated however, because individual layers pinch and into solid cumulate somewhere, and had then been swell, and are often badly disturbed by faults, their detached, before the last chill rocks had crystallized. connections between drill holes can not always On the other hand, there are microgabbro autoliths be established. In this respect the Akanvaara in chromitite. Obviously, chill-formation was a chromitites are similar to the UC layer and LC process of alternating formation and peeling-off of layers of the Koitelainen intrusion. Load cast temporary chilled linings. structures were found in a DDH at the base contact The lowermost LC chromitites are associated of a LC layer, where a lightweight feldspathic layer with gabbroic and pyroxenitic cumulates, rich is overlain by a massive chromitite. Load casts are in quartz, feldspar and biotite-phlogopite. The reported below the Steelport Seam (Cameron, other LC layers occur in pyroxene(-chromite) 1980) and below magnetite gabbro in Bushveld cumulates, underlain, intercalated and sometimes (Willemse, 1969) overlain by olivine-pyroxene-chromite cumulates. As a rule, the chromitites are monocumulate Thin chromitites (MC layers) between the LC layers of small (0.1 - 0.5 mm) euhedral chromite and ULC are associated with thin plagioclase- crystals embedded in silicate matrix. Fossil 34 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
Fig. 10. Akanvaara. Compositional profiles across the basal part of the magnetite gabbro unit (DDHs R303 and R307). FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 35 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
AKANVAARA DDH404 ICP - AAS AKANVAARA, DDH405 BASAL PART OF MAGNETITE GABBRO BASAL PART OF MAGNETITE GABBRO
Depth (m) Ag (ppm) V (ppm) Au (ppb) Depth (m) Ag (ppm) V (ppm) 2 4 1000 4000 0 10 20 30 2 4 0 1000 2000 3000 4000 5000 47 47 38 38
50 50 40 40
V Cu 55 55 45 45 Au V Cu Ag
As Au Ag
60 60 50 50 ANS
65 65 55 55
0 400 800 1200 1600 0 20 40 Cu (ppm) As (ppm)
0 20 40 0 400 600 800 1000 60 60 Au (ppb) Cu (ppm)
Fig. 11. Akanvaara. Variations of Ag, Cu, V, Au and As across the basal part of the magnetite gabbro unit (DDHs R404 and R405).
melt inclusions and embayments are common; Os-Ir-Ru alloy (Mutanen, 1997a; Gornostayev & coarse chromite often has skeletal outlines. The Mutanen, 2003). base contacts are usually sharp, even under the The majority of the chromitites are rich in microscope; the upper contacts are sharp to rapid. biotite-phlogopite, which occurs as oikocrysts, In the chromitite layers the chromite crystals tend secondary filling of intercumulus space and in to coarsen towards the base. fossil melt inclusions. However, the ULC and The LC layers typically grade upwards, the chromitite below the uppermost LC layer are and sometimes downwards, through a thin extremely poor in K2O. This is a clear indication chromite-orthopyroxene cumulate to the ordinary that the K-enrichment is a primary compositional orthopyroxene(-chromite) cumulate. The pyroxene feature. These two potassium-poor layers are also cumulates and microgabbros associated with the the layers poorest in PGE. UC layer contain a variable amount of chromite as Interestingly, biotite-phlogopite is always dissemination, concentrated bands and schlieren. present, even in abundance, in chromitites of In some chromitites there are rare occurrences various types (see Mutanen, 1997a and references of interstitial sulphides (pyrite with relics therein). The chromitite layers of the Koitelainen of pyrrhotite, pentlandite and chalcopyrite, intrusion are enriched in K2O, particularly the sometimes molybdenite), but they show no extra UC layer (Mutanen, 1979a, 1981, 1989). Some enrichment in PGE. chromitites near the base of the Kemi intrusion
The PGM grains in chromitites are small are rich in K2O and LREE (Alapieti et al., 1989). (generally 3 - 10 μm ); they occur as euhedral Similar features have been described from Entire inclusions in chromite, on chromite surfaces anorthosite gneiss, where P, Zr and Nb are strongly and in silicates. The most common are laurite, concentrated in chromitite (see Sivell et al., 1985). sperrylite, and hollingworthite that often occur These features demonstrate the presence of crustal as composite grains (Fig. 30). Other PGMs contaminant melt in chromite-charged density identified by EPM are ruarsite, irarsite, platarsite, flow. osarsite, tulameenite, anduoite, bowieite, Cumulate petrography shows that the chromitite isoferroplatinum(?), kotulskite, moncheite and an layers were formed by the sudden disappearance of 36 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen cumulus silicates and the accumulation of chromite zircon and fluorapatite. Near the base of this layer as a sole cumulus mineral; equally suddenly, the is an autolith of fine-grained microgabbro. re-entry of orthopyroxene (in LC layers) and of The compositions of chromite from the pyroxene and plagioclase (at the UC) terminated chromitite layers are listed in Table 1. There are the process. Banded contacts, internal banding and no systematic upward trends in the LC layers, other signatures of co-cumulation of significant although there are clear differences (e.g., in Al, amounts of silicates along with chromite are rare; Mn, Fe, Cr and Mg) between the layers, intra- they do occur, however, in the very lowermost layer variation (Mg, Fe, Al, Cr) and even slight but chromite-rich layer 9 m above the top of the distinct variation between grains in one sample. lower chilled margin. The layer, which occurs in There is no correlation between Fe and Mn, Fe and cumulate gabbros, is 0.51 m thick and consists of V, but a positive one between Cr and V. There is a alternating chromite-rich and pyroxene-rich layers. general, but not tight, negative correlation between Quartz, plagioclase and biotite are abundant in the Al and Fe, indicating Al3+/Fe3+ substitution. In LLC intercumulus, which also contains loveringite(?), layers the substitution is between Cr3+ and Fe3+.
AKANVAARA REE in magnetite gabbros and associated rocks, DDH R405
100
upper (38.2 - 44.3 m)
middle (44.3 - 52.0 m) Magnetite gabbro 47.8 - 48.3 m
lowermost (52.00 - 55.76 m)
Gabbro-anorthosite 50.00 - 51.15 m Anorthosites, below magnetite gabbros 55.76 - 59.00 m
10
Rock/Chondrite (N = 35)
1
Er
Pr
Lu
La
Tb
Eu
Yb
Nd
Dy
Ce
Ho
Gd
Tm
Sm
Fig. 12. Akanvaara. REE(CN) patterns of the basal part of the magnetite gabbro unit and underlying rocks. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 37 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
AKANVAARA, DDH303, 307 (107 - 130m) AKANVAARA DDH303, AU - RICH LAYER IN ANORTHOSITE CUMULATES DETAIL OF AU - RICH LAYER
Au (ppb) Au, Pd, PGE (ppb) 0 20 40 50 100 0 20 40 50 100 Depth Depth S (ppm) (m) Depth (m) 100 400 DDH303 (m) DDH307 400 Pd 395 110
405 400 Cu S 115
TOT.PGE 410 405 Au Au 120
Au Pd => 1490 ppb => Au (FA) = 230 ppb 415 410 125 Pd
1490 ppb
420 TOT.PGE => 174 ppb 415 130 uralite gabbro
anorthosite gabbro gabbro-anorthosite 425 420 mottled anorthosite 135
418 0 300 0 50 100 Cu (ppm) Au (ppb)
Fig. 13. Akanvaara. Au-enriched layers in the anorthositic sequence below the magnetite gabbro unit. Variations of Cu, S and Au in DDH R303; Au in R303 and Au, Pd and PGE(tot) in R307.
The composition of the chromite from a pyroxene- minerals - sulphide, plagioclase and carbonate chromite cumulate between two chromitite layers - showed that the compositional differences differs from that of the chromitites (Table 1). between these chromites are negligible. Chromite Upwards from the LC to the ULC there is a in plagioclase is slightly higher in Al, whereas slight decrease in Mg and increase in Zn and Mn, chromite in sulphide is higher in Fe and Ti and but chromites with similar Mn values occur in the lower in Mg, Cr and Al. lowermost LC layers. The UC is distinct from the Other evidence for the lack of significant post- ULC and LC. Its chromite is even lower in Mg and cumulus changes come from the UC layer, where higher in Ti, V and Zn; Mn is high, being about the the Mg# in bulk silicates is even lower than that same as in ULC chromite. The individual layers in underlying and overlying silicate cumulates have characteristic V/Cr ratios (Fig. 22). This ratio (Fig. 31). This indicates that the low Mg in is also a kind of measure of fractionation; thus, the UC chromite is an original feature and was not most “evolved” is UC, followed by LLC, MC and markedly lowered by postcumulus (magmatic plus LC. Surprisingly, the ULC is apparently the least metamorphic) re-equilibration. chr/liq evolved. As the DMg in all imaginable magma Post-cumulus re-equilibration was probably not compositions is > 1 (see Maurel & Maurel, 1982, very effective. Analyses of LC chromite on one 1984), the low Mg in chromite implies that the sample (DDH 322/258.27 m) in which chromite chromites of all chromitite layers crystallized in an is enclosed by three different poikilitic matrix environment very poor in Mg. 38 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
AKANVAARA, DDH307, 309, 404, 407 AKANVAARA DDH308 PGE - ENRICHED LAYERS BELOW MAGNETITE GABBRO PGE - ENRICHED LAYER
PGE (ppb) Pd (ppb) Pd, PGE (ppb) Depth Depth (m) 0 200 400 (m) 0 100 200 300 400 500 90 20 Pd (ppb) Pd (ppb) Depth Depth (m) 0 200 (m) 0 200 400 TOT, PGE PGE (ppb) Pd Pd (ppb) TOT.PGE 110 123 Depth Pd (m) 0200 400 600 30 95 130 130 120
40 100 Au 140 TOT. PGE 140 Pd 130
50 105 150 DDH307 DDH309 DDH404 DDH407 DDH308 150 140
110 => 1258 60 160
150
115 70 170
160
120 80 180
125 90
Fig. 14. Akanvaara. PGE-enriched layers in UZ gabbros, in DDHs R307, R309, R404 and R407.
The UC layer
The UC layer (Figs. 15, 16) is similar to the UC thickness varies from 0.64 to 1.79 m (average 1.25 layer of the Koitelainen intrusion. It was found m), the average thickness of the massive ore is in an outcrop in 1991 and was later (1995-1996) 1.04 m. The weighted average Cr2O3 is 19.08 %, traced by diamond drilling in the southern part of the massive ore 22.79 %. In the massive ore V of the intrusion. Between the eastern and western varies from 0.3 to 0.52 % (weighted average 0.403 fault borders it is continuous over a strike length of %). The average PGE in massive ore is 0.915 ppm, 8 km. From east to west the relative stratigraphic Pt/Pd 4.76 (Fig. 23a). The order of abundance is height of UC succession decreases several hundred Pt>Ru>Os>Rh>Ir>Pd (or Pd>Ir). Au is very low, meters in relation to the base of the magnetite always below detection limits (0.1 or 1.0 ppb). gabbro, whereas the combined thickness of the The relative abundances of PGEs vary very little MZ and UZ remains approximately constant. (Fig. 28b). - The estimates resources down to 300 The UC have been intersected by 30 DDHs. The vertical meters are 18.1 Mmt (Mutanen, 1998). FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 39 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Figure 15 shows the general stratigraphy altogether. The irregularity of the sub-UC unit is of the UC succession, and details of DDH ascribed, as at Koitelainen, to strong magmatic intersections. The homogeneous MZ gabbro has erosion (see Mutanen, 1997a). a sharp boundary with the overlying UZ. This Closely associated with the UC are microgabbro, begins with a 0.5 - 4.6 m thick unit of alternating medium-grained non-cumulate gabbro, pyroxenite, layers of mottled anorthosite, gabbro-anorthosite, pyroxene pegmatoid and pyroxene-plagioclase anorthosite gabbro and medium-grained non- pegmatoid, all of which contain chromite as cumulate gabbro and microgabbro. The layers are dissemination, schlieren and thin layers. The UC is irregular and do not always persist laterally. Below overlain by layers of mottled anorthosite and other the UC the mottled anorthosite is from 0.2 to 1.2 plagioclase-rich cumulates, gabbro and layers/ m thick, but in five DDH intersections it is lacking autoliths of noncumulate gabbro and microgabbro.
Fig. 15. Akanvaara. The general layered sequence of the UC layer, and selected details of DDH intersections. Distance between intersections shown between columns. 40 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
Fig. 16. Akanvaara, Tuorelehto. Upper Chromitite (UC) outcrop. The layer of mottled anorthosite overlying the UC acid, and would make an ideal high-Cr, but still is generally from 1.9 to 2.6 m thick, but is lacking reactive, chemical grade raw material, with the in some DDH intersections. added benefit of vanadium. Dendritic (crescumulate) plagioclase occurs in The matrix is very rich in biotite (with 1.44- a coarse leucocratic gabbro a few meters above 1.50 % Cr2O3) and in K2O; the silicate tailings the UC. This rock contains zircon (it is the definite of a big sample from the ore outcrop, used for
“age sample” of Akanvaara). The layer seems to mineral processing tests, contained 7.5 % K2O. be laterally irregular. The plagioclase evidently Biotite is abundant in fossil melt inclusions of crystallized from a supercooled aluminous magma chromite. The other main intercumulus minerals (see Poldervaart & Taubeneck, 1959; Taubeneck & are oikocrystic green chromian hornblende (with
Poldervaart, 1960; Mutanen, 1974). 0.77-2.04 % Cr2O3) and light-green actinolite. A persistent marker layer of mottled anorthosite, Sodic plagioclase comprises a small portion from 1 to 2 m thick, occurs 10-15 m above the of the intercumulus matrix. The amounts of UC. intercumulus ilmenite and secondary titanite and Besides Cr, V and PGE, the massive ore contains rutile are relatively high. Protoclastic fractures in on average 2.4 % TiO2, 5 % MgO, 1.5 % K2O and chromite were filled by postcumulus ilmenite and 320 ppm Ni. pyroxene (now amphibole). Ilmenite often occurs The euhedral chromite (0.1-0.2 mm) is the sole as large oikocrysts that have euhedral outlines cumulus mineral. It is rich in Fe, Ti, V, Mn and Zn ("idiocrysts"), enclosing chromite. Ilmenite and poor in Mg (Table 1). Fossil melt inclusions oikocrysts seem to be lacking in the lower part are common in chromite and skeletal chromites of the UC. Chlorite, quartz, chromian clinozoisite have been found. The Ti in chromite resides in ("tawmawite", with up to 3.47 % Cr2O3), exsolved ilmenite rods; occasionally the exsolved fluorapatite and sulphides (pyrrhotite, pentlandite, ilmenite occurs as bigger euhedral laths. pyrite, violarite, millerite, marcasite, bornite and The UC chromite (as well as other Akanvaara chalcocite) occur in small to very small amounts. chromites) are amenable to reduction into Like ilmenite, fluorapatite sometimes occurs as big ferrochrome in shaft furnace conditions (see, e.g., oikocrysts enclosing chromite. Vatanen, 1998; Hiltunen, 2001). Low in MgO The UC is high in Cl (average 1800 ppm Cl). and high in FeO, the chromite is easily soluble in In REE(CN) diagram (not shown) the UC has a FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 41 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Table 1. Akanvaara. Compositions of chromite from chromitite layers. Electron microprobe analyses by Bo Johanson and Lassi Pakkanen (GSF, Espoo). Number of analyses = n. LC samples: 1 - accessory chromite between chromitite layers. Samples 2 - 9 are from chromitite layers, in descending order: 3 a/b - upper/lower part of a layer; 5 a-f, samples across a layer; 6c1 - chromite in sulphide matrix, 6c2 - chromite plagioclase matrix, 6c3 - chromite in carbonate matrix; 7a1-7a3 - compositionally heterogeneous chromites in one sample; 8a-e - samples across a layer; 9a-e - samples across a layer, where 9c1 and 9c2 are from compositionally heterogeneous chromites in one sample; Samples 10 - 15 are from LLC chromitites, in descending order: 10a/b - upper/lower part of a layer; 11a/b - upper/lower part of a layer; 12 - middle part of a layer; 13a/b - upper/middle part of a layer; 14 - analyses from one thin section: a1 - in plagioclase matrix, a2 and a3 - in uralite-plagioclase matrix, a4 - in uralite-biotite matrix, a5 - in quartz or granophyre matrix; 15 - chromitite autolith in microgabbro (Fig. 21a). Analyses of this sample include silicate impurities. 42 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
Table 2. Compositions of loveringite and ilmenite, Akanvaara and Koitelainen intrusions. Electron microprobe analyses by Lassi Pakkanen. Analysis no. 5 from Tarkian and Mutanen (1987). straight negative slope with La(CN) 20-40. The The microgabbros have LREE similar to UC´s; REE(CN) curves for associated rocks below the HREE end approaches silicate rocks. Two UC (uppermost MZ gabbro) and above (gabbro) run samples show small negative Eu-anomalies; all parallel to UC curves at a slightly higher REE silicate rocks and one UC sample are “straight”, level, with La(CN) 33-43 and La/Yb(CN) 5.4. without any kind of Eu-anomalies.
The ULC layer
This layer, intersected in 13 DDHs (Fig. 17), has hosted chromitites is more coarse (0.5 – 1 mm) been traced for 8.2 km, but it certainly extends than in other chromitite layers and the grain size longer. It has been excavated in a trench (Stop. varies only slightly. Chromite is often surrounded 6). In the overlying peridotite cumulate there are by a rim of secondary magnetite. The intercumulus massive chromitite bands (from a few mm to 9 cm consists mainly of chlorite, with smaller amounts thick) and net-textured chromite-rich rocks with of tremolite, talc, serpentine, fine-grained
9-12 % Cr2O3. The chromite in these peridotite- secondary magnetite, biotite-phlogopite, ilmenite, FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 43 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... ilmenorutile, carbonate and apatite. amounts of ilmenite (and its alteration products The massive ULC ore contains 22.30 % rutile and titanite), clinozoisite, carbonate,
Cr2O3 (weighted). As compared with most other big crystals of fluorapatite and oikocryst-like Akanvaara chromitites the ULC is poor in PGE tourmaline. Chalcopyrite, pyrite and millerite (0.15 ppm); Pt/Pd varies widely (from 1.83 to 30). occur in very small amounts. Big zoned zircon The order of abundance is: Pt>Ru>Os>Rh>Ir>Pd. crystals were found in a coarse footwall layer of Au is very low, generally below the detection limit the ULC chromitite.
(0.1 or 1.0 ppb). The ULC is very low in K2O (typically 0.0 The chromite is very fine-grained (<0.2 mm), %). This is a primary feature. The ULC is very but there are occasional more coarse parts. The heterogeneous with regard to REE: the abundances chromite is rich in TiO2 (0.61 %; Table 1), but and REE(CN) patterns (not shown) are very most of the exsolved ilmenite is submicroscopic. variable: La(CN) varies from 1.25 to 190, La/ The chromite is very poor in MgO and rich Yb(CN) ratios from 130 to ca 1; the Eu-anomalies in Al2O3 (12.71 %). The intercumulus matrix vary from positive to very deep negative. The consists mainly of chlorite, which is often purplish overlying peridotite represents the average ULC,
“kämmererite” (Cr2O3 up to 1.84 %). Present in with La(CN) 22, La/Yb(CN) 8.8 and a moderately lesser amounts are hornblende, widely variable negative Eu-anomaly.
The MC layers
This succession of thin (0.5 – 10 cm) chromitite and metapyroxenite are associated with the layers has been intersected in one DDH. The layers chromitite layers. One massive chromitite assayed were first found in a frost-shattered outcrop (Stop 28.17 % Cr2O3, 0.15 % V and 0.296 ppm PGE. The 6). The succession is located ca 60 m beneath the order of abundance is: Pt>Rh>Ru>Ir,Pd>Os. Au is ULC among homogeneous gabbroic cumulates, and below detection limit (1.0 ppb). is about 2 m thick. Layers of mottled anorthosite
The LC and LLC layers
The two uppermost LC layers are located in the There is ample room for even thicker swells at the upper part of the thick pyroxenite unit of the base of the intrusion (Mutanen, 1998). basal LZ, 45-55 m below the upper boundary In massive LC and LLC layers the Cr2O3 varies of the unit (Fig. 18). Another group is located in from 18 to 34 % (weighted average 24.55 % the middle of the unit. It consist of 4-6 thicker Cr2O3), the weighted average for PGE is 0.639 layers. The LLC layers are located between the ppm, Pt/Pd 5.63 (Fig. 23b,c). The maximum PGE pyroxenites and the chilled margin. This sequence values are ca 1.6 ppm. The orders of abundances consists of pyroxenitic and gabbroic cumulates in the LC layers are: Ru>Pt>Ir, Rh>Pd>Os or and microgabbros rich in plagioclase and often in Pt>Ru>Os>Rh>Ir>Pd. Au is very low, almost quartz. The thicknesses of layers varies laterally always below 1.0 ppb. In the LLC layers the order impending reliable connection of layers between is: Pd>Ru>Pt>Rh>Ir, Os. Au is low, 1 – 14 ppb widely-spaced DDHs. In the swells the thickness (for more details see Mutanen, 1998). of layers may reach 4.3 – 5 m (Fig. 20). The LLC The LC layers occasionally contain spots of layers are very irregular: in one DDH (R340) talc(-tremolite), pseudomorphs after scattered chromitite is present only as autoliths (Fig. 21a), cumulus crystals of orthopyroxene(?). Oikocrysts another hole (R376) 600 m northeast intersected of hornblende (former pyroxene) are common; two thick (4.48 m and 2.25 m; Fig. 20b) chromitites. plagioclase oikocrysts occur in the salic 44 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
AKANVAARA, DDH319 > DDH330, ULC SEQUENCE SE NW
319 338 337 336 335 334 333 332 331 330 m 0
x 10 x x x 20
30
x x x x 40
50
60
peat, till
olivine gabbro, troctolite (above ULC) gabbro, uralite gabbro (below ULC)
peridotite x x x x x
x pyroxenite, olivine pyroxenite x x x
x x pyroxenite interlayer
40 m ULC chromitite layer
mottled anorthosite
Fig. 17. Akanvaara. DDH profile of the lower MZ and upper LZ.
Fig. 18. Akanvaara. DDH profile of the basal part of the intrusion. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 45 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Fig. 19. Thicknesses and grades of Akanvaara chromitite layers. UC - Upper Chromitite, ULC - Uppermost Lower Chromitite, LC - Lower Chromitite Layers, LLC - Lowermost Lower Chromitite layers.
AKANVAARA DDH 322 AKANVAARA, DDH376 SEQUENCE OF CHROMITITE LAYERS SELECTED GROUPS OF CHROMITITE LAYERS
Depth Cr2 O 3 (%) Depth Cr2 O 3 (%) (m) (m) 10 20 30 35 10 20 30 108 150 26.2 151 a 30.4 b 110 29.6 162 26.4 weighted 16.3 163 5.06 m / 23.7 % 32.4 172 Cr2 O 3 173 26.6 32.3 180 27.1 30.2
238 115 239 20.1 240 6.3 25.3 241 weighted 211 26.0 LC Group 242 23.4 4.3m / 18.5% 23.7 (ca. 130 m thick) 243 Cr2 O 3 6.2 11.8
257 20.7 258 weighted 19.2 19.0 4.48 m / 16.0 % 259 215 Cr2 O 3 260 21.7
261 24.0 7.4 BASAL ACCUMULATIONS 266 2.7 267 27.9 = LLC LAYERS 7.1 282 31.4 283 32.5 220 24.1 weighted 2.25 m / 15.1 % 12.1 Cr2 O 3 368 222 16.3 369 BASAL CHILL 23.0 370 LLC Group 371 10.5 21.7 372 17.5 14.3 373 4.00 Fig. 37420. Akanvaara. LC and LLC chromitite layers, a – in DDH R322, b – thick chromitite intersections in DDH R376. 46 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
Fig. 21. a - Chromitite autoliths in heterogeneous non-cumulate microgabbro (including fine-grained chilled microgabbro), near the base of the Akanvaara intrusion, DDH 340/138.24-138.30. Bar = 2 cm. Photo by Reijo Lampela; b - Diascanner photo of a thin section, by Reijo Lampela; b - tourmaline band ? (TO B?) and abundant tourmaline oikocrysts (some marked by T-O); inset in b - tourmaline oikocryst (black area in the centre) enclosing euhedral chromite. DDH341/81.03 m. Bar = 1 cm.
intercumulus matrix of the lowermost LC layers. ilmenite occurs as a daughter mineral in fossil The oikocrysts have grown around cumulus cores. melt inclusions in chromites. Euhedral cumulus The mutual modal proportions of secondary crystals of Mn-rich ilmenite (Fig. 30b) imply intercumulus minerals – biotite-phlogopite, talc, that chromite and ilmenite saturated together, chlorite, amphibole and carbonate – vary in but this could not have happened in the main different layers. Ilmenite, which is ubiquitous magma body. Apatite and tourmaline have been as exsolved rods in chromite and as poikilitic observed. Tourmaline (Cr2O3 up to 3.65 %) grains enclosing chromite, is commonly altered occurs as poikilitic, oikocryst-like grains and to rutile and "ilmenorutile" (Fig. 30a). Euhedral oikocrystic bands reminiscent of layers (Fig. 21b). FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 47 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Fig. 21. b
Possibly boron was a primary contaminant (from layers. The patterns of LLC and lowermost LC melted sediments), and the poikilitic tourmaline layers are similar to each other. They conform grains grew as true postcumulus oikocrysts from to chill patterns but have distinct negative Eu- boron-enriched intercumulus melt. Fluorapatite anomalies. Higher up the LC layers have much and zircon occur in pegmatoid rock immediately lower total REE concentrations and negative underlying a LC chromitite layer. slopes towards HREE, so that from the elements The REE(CN) patterns (not shown) are very between Gb and Lu only Yb rises above the variable between layers and, possibly, within detection limit. 48 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
AKANVAARA V vs. Cr in chromitites 5000
UC ULC 4000 LC LLC MC = thin layers betw. LC and ULC 3000 UC
V ppm
2000 LLC MC
1000 LC ULC
0 0 5 10 15 20 Cr %
Fig. 22. Akanvaara. V vs Cr in massive chromitites and chromite-disseminated rocks.
Behaviour of PGE in the Akanvaara intrusion
Anomalous PGE values occur from the base of the Mutanen, 2003). There are no sulphide-associated intrusion up to the lowermost part of the magnetite concentrations of PGE. Au seems to be the only gabbro, mostly in chromitites, but also in other truly chalcophile noble metal. Its first (and only) rocks without any visual clues. Geochemically the marked enrichment occurred when the first tiny most important PGE enrichment is in gabbros and premature fraction of sulphide liquid separated anorthosites in the upper part of the intrusion. from residual liquid 40 m below the magnetite The PGE do not keep company with sulphide phase contact. Here, Cu jumps from the low concentrations. Apparently, PGE behaved as background values of ca 40 ppm (Fig. 13) to incompatible elements throughout most of the 130-250 ppm, and at the same time S rises from crystallization. However, at times (and sites) of very low values (generally below 100 ppm, the contamination, the PGM crystallized as tiny free detection limit of the XRF method) to 100-400 crystals, remaining in suspension or nucleating ppm. A strong Au anomaly (1.5-1.94 ppm/1 m) on chromite. It is also possible that the PGM coincides with this Cu-S inflection, with a tail (20- crystals were already in suspension in the original 50 ppb) extending for 15-20 m upwards. There is magma at the time of the emplacement. The PGE- no associated increase in PGE. Terminal saturation enriched intervals in the upper zone may represent of sulphide liquid took place with the arrival of an episode of true saturation of PGM phases in cumulus magnetite (see Fig. 7a-c). The sulphide residual liquid. PGM nucleated homogeneously, liquid, quite naturally, was rich in Cu. The Cu- or heterogeneously on any substrate suitable S-enriched basal part of the magnetite gabbro is and available, such as cumulus silicates and slightly enriched in Au (25-45 ppb) and Pt (10-40 (or) ilmenite. The PGM paragenesis has been ppb), but Pd is below the detection limit of 3 ppb studied only in the UC layer (Gornostayev & (see Fig. 29h). FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 49 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
PGE-Au(CN) diagrams for chromitites are AKANVAARA UC CHROMITITES presented in Figs. 28a-h and for silicate-associated WEIGHTED PGE PGE-anomalies in Figs. 29 a-h. The Cr-PGE relationships are illustrated in Figs. 27a-d. The m LLC layers (Fig. 28a) differ from other chromitites in having a M-shaped CN profile, with peaks at a chromitites (Cr O > 10 %) Rh and Pd; the shape resembles those of the PGE- 2 3 anomalous silicate rocks of the MZ and of the
UC association (Figs. 28d and 28g), the footwall 5 disseminated rocks of the PGE-anomalous anorthosite in the UZ (Fig. 29), and the chromitites of the Koitelainen intrusion (Fig. 28h). Other chromitites have a domed shape, sloping from Rh towards Au and Ir. Os(CN) is higher than Ir(CN), the only exception being the ULC layer. Silicate cumulates, whether 0 separated from or associated with chromitites, 0 400 800 1200 ppb show M-shaped CN profiles, with peaks at Rh and m Pd. Au(CN) is always low. From UC upwards, Os, AKANVAARA LC AND LLC CHROMITITES Pt/Pd Ir and Ru are low, as if depleted by accumulated chromite. In the PGE-enriched anorthosites and 10 gabbros below the magnetite gabbro (Fig. 14), LLC the PGE element ratios vary over a wide range and often very rapidly (Figs. 29a-g). Here the CN LC (average 7.34) diagrams have many shapes: M (peaks at Rh and b 5 Pd, Rh(CN) < or > Pd(CN)); flat domes, with a Pt-Pd top; peaked, with varying slopes (peaks at Rh, Pt or Pd). Upwards from the anorthosites the diagram is M-shaped, but now the peaks are at Pt and Au. The stratiform chromitites of New Caledonia 0 4 8 12 16 20 24 ophiolites (see Augé & Maurizot, 1995) have LC, LLC dome-shaped PGE(CN) patterns, with the peak weighted avaverage 596 ppb at Rh (at Pt in Pirogues); the Os to Ru stretch is horizontal, like in the chromitites of Akanvaara and m AKANVAARAAKA LC AND LLC 10 Koitelainen and the Ni-PGE-type ore in Keivitsa. PGEP CONCENTRATIONS The lowermost LG chromitites in Bushveld also have a domed PGE(CN) pattern, with the peak at c Ru-Rh (von Gruenewaldt & Merkle, 1995). Cr O > 10 % Fig. 24 shows the ratio of Pd to total PGE. The 2 3 5 Cr O < 10 % silicate rocks and the LLC chromitites are the most 2 3 enriched in Pd. The ULC chromitite is poorest in PGE, and its Pd/PGE is among the lowest for chromitites.
In chromitites a portion of the PGM are enclosed 0 400 800 1200 1600 in chromite (Fig. 28f); Gornostayev & Mutanen, PGE (ppb) 2003). Fig. 25 presents Pd values determined by Fig. 23. Akanvaara. Histograms of PGE; a – PGE in UC two different methods (acid digestion + AAS chromitite, b – Pt/Pd in LC and LLC chromitites, c – PGE in and NiS fire assay) from samples of chromitites, LC and LLC chromitites. 50 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
AKANVAARA chromite-disseminated rocks and chromite-free Pd vs. tot.PGE 1:1 silicate rocks. The Pd(NiS) values (about total 1000
LLC values) are only slightly higher than the values for 1:10 the acid-soluble (AAS) Pd. The difference is due to PGM grains still armoured by chromite after 100 Silicate rocks 1:50 comminution of the samples for acid digestion (see and cf. Fig. 30).
Pd ppb Chromitites There is a general but very loose positive 10 correlation between Cr and PGE (Fig. 27a). Below chromitite chrt dissem ULC about 1 % Cr (corresponding a chromite content silicate rocks LLC of 3-4 %) the total PGE is indifferent to Cr. In silicate rocks 1 ass. with chromitites chromitites and in rocks with Cr concentrations 1 10 100 1000 E PGE ppb down to ca 1000 ppm Ir and Ru correlate well Fig. 24. Akanvaara. Pd vs. PGE(tot) in chromitites, associated with Cr (Fig. 27b-d). The diagrams suggest that rocks and other PGE-enriched rocks. chromite is the main collector for Ir and Ru (and, supposedly, for Os). Direct positive correlation between Cr and PGE has been reported from the 1000 AKANVAARA 2:1 1:1 1:2 Pd (AAS) / Pd (NiS) New Caledonia stratiform chromitites (Augé & Maurizot, 1995) and from the LG6 chromitite of Bushveld (Teigler, 1999). Besides these 100 occurrences, the connection of PGE with oxides (chromite, magnetite, ilmenite) is very common.
Pd (AAS) To the long list presented before (Mutanen, 1997a, 10 p. 93) might be added new cases of the chromite- PGE connection: the Greenhills Complex, New chromitite chrt. dissemination Zealand (Spandler et al., 2000), Rum, Scotland no chromite 1 (Butcher et al., 1999), new data from Merensky Reef (Cawthorn, 1999). Evidently, the first-born 0.1 1 10 100 1000 Pd (NiS) PGM were early liquidus phases that nucleated Fig. 25. Akanvaara. Pd assays by GAAS vs Pd assays by NiS on early spinels or even served as sites of fire assays. heterogeneous nucleation for chromite (Augé, 1986, Capobianco & Drake, 1990; Mutanen, 1992,
AKANVAARA 1997a; see also, e.g., Sattari et al., 1999; Zaccarini Pd vs. Cu et al., 2002).
1000 While Pt, Rh and Pd are clearly enriched in chromitites, there are other collectors and collection mechanisms for these elements in magmas. Sulphide liquid took no part with the 100 concentration of PGE the Akanvaara magma UC LC, ULC system. Cu and Pd, which are generally regarded as MZ
Pd ppb Lowerm. LC chalcophile elements, show no mutual correlation 10 MTGB in Akanvaara cumulates (Fig. 26). PGE-horiz. (66 m thick) The general order of depletion (i.e., of decreasing compatibility) in the evolution of the 1 Akanvaara magma system seems to be: Ir > Os 1 10 100 1000 Cu ppm > Ru > Rh > Pd > Pt > Au. This order may hold Fig. 26. Pd vs Cu in Akanvaara chromitites, associated rocks, in general for S-undersaturated magmas, with and PGE-anomalous rocks recurrent deposition of chromite monocumulates. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 51 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
1
Fig. 27. PGE vs Cr, Akanvaara chromitites and associated rocks. a - PGE vs Cr; b - Ru vs Cr; c - Ir vs Cr; d - Ir vs Cr in UC and LC chromitites.
As regards the end triplet, the Akanvaara case is Barnes and others (1985). somewhat at variance with the order presented by
Some petrological aspects
Most of what I have conjectured about the was dry. Consequently, primary hornblende Koitelainen intrusion (Mutanen, 1989, 1992, never crystallized as a stable phase from the 1997a) applies to Akanvaara as well. The fractionating main magma. At the beginning and parent magma was a silica-saturated low-Ti throughout most of the crystallization the magma basalt, low in P2O5 and sulphur. The magma held on or aspired to the cotectic of augite, low-Ca 52 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
Fig. 28. Diagrams of chondrite-normalized values of PGE and Au from Akanvaara chromitite layers and associated rocks. Arrow top lines = analytical detection limits. a - LLC chromitites and associated rocks, b - LC and associated orthopyroxene-chromite cumulates; c - ULC layer and thin chromitite layers (MC) between LC and ULC; d - PGE-anomalous layers between ULC and UC; e - UC layer; f - UC ore, chromite concentrate and silicate reject; g - silicate rocks (with or without accessory chromite) associated with the UC layer; h - Koitelainen and Akanvaara chromitites. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 53 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Fig. 28. e - h pyroxene and plagioclase. Iron became enriched models for Bushveld, see Rice 1997, 1998 and in the residual liquid, along with Ti, V, Zn, Na, McCandless et al., 1999). The evidence for and the truly incompatible elements. Fractional multiple magma pulses is ambiguous or lacking crystallization of pyroxenes rapidly depleted the (see, e.g., McCarthy et al., 1984; Mitchell, 1990; liquid of Cr and Ni. An unknown part of Cr was Scoon & Teigler, 1994). exotic (see Mutanen, 1997a, p. 114-119). The cotectic routine was interrupted by episodic The magma filled the chamber as a single melting of country rocks (see Mutanen, 1992, cast, before any significant crystal accumulation 1997a).The roof portion of the magma chamber occurred (see also: Cameron & Emerson, 1959; was stirred by sinking peeled-off chill autoliths, Hess, 1960; Jackson, 1961; Ferguson & Botha, xenoliths and heavy residue minerals from partial 1963; Ferguson, 1969; Bow et al., 1981; Loferski fractional melting of pelites. The mixing of the et al., 1990; Higgins et al., 1997; for single input mafic melt and the anatectic roof melt augmented 54 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
Fig. 29. Diagrams of chondrite-normalized values of PGE and Au from Akanvaara Upper Zone PGE-enriched anorthosites and gabbros. a - PGE-anomalous gabbro cumulates between UC and magnetite gabbro, footwall rocks; b - PGE-anomalous zone; c to g are for PGE-anomalous anorthosite sequence below magnetite gabbro: c - footwall rocks; d - lower part of the PGE sequence; e - low-PGE middle part of the sequence; f - upper part of the sequence; g - immediate hanging wall rocks; h - basal part of the magnetite gabbro. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 55 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Fig 30. BE images of Akanvaara chromitites. a - euhedral chromite and poikilitic "ilmenorutile". LC, DDH320/283.10 m. Bar = 20 µm; b - euhedral chromite and Mn-rich ilmenite (MnO 9.53 %), the latter altered to "ilmenorutile". LC, DDH320/271.05 m. Bar = 100 µm; c - laurite crystal in chromite (laurite contains Ru 47.1 %, Pd 1.8 %, Os 6.5 %, Ir 5.5 %, Rh 0.36 % and As 2.1 %). UC, DDH363/131.19 m. Bar = 20 µm; d - laurite (L), hollingworthite (H) and sperrylite (white), lower left, a zoned laurite crystal with an Os-rich rim (erlichmanite). UC, sample TM-91-15.10. Bar = 2 µm; e - a zoned laurite crystal (L), hollingworthite (H) and sperrylite (white). UC, sample TM-91-15.3. Bar = 5 µm. Photos by Lassi Pakkanen (a-c) and Bo Johanson (d-e).
crystallization and accelerated two-phase similar to that presented earlier for Akanvaara convection. and Koitelainen (Mutanen, 1989 to 1997a) have The magma formed a chilled lining against the been presented by Rice (1997, 1998), Robinson & country rocks. The magma was convective due Miller, (1999) and Kruger (2003). to heat loss through the roof (thermal convection) The general autonomous iron-enrichment and spatial differences in bulk densities of the fractionation trend of the rest magma (Fig. 7) melt-crystal suspension (two-phase convection: suggests that the main magma reservoir was not Hess, 1960; Morse, 1969, 1988; Brandeis & much involved in, nor affected by, two-phase Jaupart, 1986; Mutanen, 1997a; Simakin et al., convection and associated contamination. The 1997; Philpotts & Dickson, 2000). There were “normal” cotectic crystallization occurred in intermittent accelerated bursts of convection. this “Sargasso Sea”, the calm region between Extreme cases of two-phase convection were convection toroids (Morse, 1979b, 2001; Simakin the sudden downdraft surges of chromite-laden et al., 1997). density flows (Mutanen, 1992, 1997a). Models The contamination was by: 1) selective diffusion 56 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
(Barker, 1975; Compston et al., 1968; Morse, loveringite, zircon and chlorapatite, fluorapatite 1969; Pankhurst, 1969; Willemse & Viljoen, (in the lower zone), the abundance of intercumulus
1970; Watson, 1982) of components (H2O, K, quartz, potassium feldspar and biotite-phlogopite, Na, Cl, P, REE, Zr) between mafic magma and and the presence of "granitic" intercumulus solid or molten country rocks (floor, roof and material and "granitic", Cl-rich melt inclusions immersed xenoliths), and across melt boundaries in cumulus olivine and chromite (see, e.g., Fig. in the acid/mafic emulsion in the mixing zone 31 and Table 5 in Mutanen, 1997a). The added of mafic magma and anatectic roof magma; 2) contaminants shifted the field boundaries of wholesale mixing (hybridization) of mafic magma saturated and potential phases. In Akanvaara, and acid contaminant magma; 3) by digestion of due to contamination by salic alkalic material (or reaction with) refractory residual or otherwise orthopyroxene crystallized in the basal part as insoluble phases (e.g., sulphide liquid) after partial the sole cumulus silicate, and olivine appeared at melting of country rocks. The cumulus phases the main zone boundary. Plagioclase cumulates, crystallized in the hybrid mingling-mixing zone without mafic counterlayers, are thought to between the mafic main magma and the buoyant represent the excess alumina left behind after anatectic acid roof melt (see Mutanen, 1989, 1992, partial melting of pelitic schists. The addition of 1997a). alkalies produced Fe/Mg compositional reversals Mineralogical and geochemical indicators of in mafic cumulus minerals (the alkalic – ferric contamination are the common occurrence of iron effect, Paul & Douglas, 1965). Contamination
Fig. 31. Profiles of Mg#, Al2O3, MgO and Cr through the UC sequence, Akanvaara intrusion. DDH 315. The Mg# of the UC silicate reject shown by arrow. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 57 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... and subsequent settling of cumulus crystals from MgO, Mg# and Cr of the cumulate decreased. In heterogeneous density flows could explain the the long run plagioclase would not have been able rapid isotopic changes across reversals, internal to keep crystallizing in non-cotectic proportion isotopic heterogeneities (e.g., Kruger & Marsh, without enriching the residual magma in MgO 1982; Eales et al., 1993), as well as isotopic and, as a result, restarting pyroxene crystallization. heterogeneities in cumulus assemblages (e.g., As there are no counterlayers compensating Prevec et al., 2004) and even inside single crystals the excessive plagioclase separation, it must be (e.g., Duffield & Ruiz, 1998). concluded that alumina was added to the magma The stratigraphic position of the UC high in the system, most probably as an aluminous residual layered sequence, the composition of the chromite, phase from melted sedimentary rocks. It is well and the petrography and geochemistry of the known that assimilation of aluminous sediments intercumulus and melt inclusions are all features augments the crystallization of more anorthitic not easily accommodated to standard petrogenetic plagioclase and of Ca-poor pyroxene at the models. Customarily, new primitive magma pulses expense of Ca-rich pyroxene (see e.g., Bowen, (rich in Mg and Cr) are invoked to explain even 1928; Kuno, 1950; Gribble & O`Hara, 1967; lesser problems in layered intrusions. The models Willemse & Viljoen, 1970). Suppression of the based on new magma pulses rely on circumstantial crystallization of the Ca-rich pyroxene and its evidence; transgressions of new magma through removal from the cumulus assemblage results in settled cumulates are hardly ever visible, even in radical decrease of Cr in cumulate. Accordingly, well exposed mafic intrusions. At Akanvaara the assimilation of aluminous sediments would result UC layer is well exposed in trenches and has been in compositional profiles seen in Fig. 31. intersected in 30 drill holes, but there is nothing in Let us then consider the possibility that the the field or in rock and mineral analyses to suggest chromium was derived from the main magma. The the entry of new magma in any amount, or of any uppermost MZ gabbro, a few meters below the UC density or composition. layer, contains 50 ppm Cr. With a px/pl ratio of 1, Likewise in liquid evolution diagrams (Figs. 7a- the original "bulk" pyroxene would have had 100 px/liq c), there is no evidence for new pulses of primitive ppm Cr; further, using DCr values from 10 to 20 magma into the Akanvaara intrusion. If anything, (a reasonable range for terrestrial basalts, see e.g. there are added inputs of alumina and, of course, of Huebner et al., 1976; Duke, 1976; Grove & Bence, Cr. The addition of primitive magma should have 1977; Henderson, 1979; Allègre et al., 1977), we left its signature in the mineralogy and chemistry of get a concentration of 5-10 ppm Cr for the last MZ the cumulate pile overlying the UC. But, as shown melt. The calculated liquid (Fig. 7a) at the level by Figs 5 and 31, the magma reassumed its former of the UC layer had 53 ppm Cr. This is probably fractionation trend after the UC event. As seen in somewhat too high, because in the UZ rocks the Fig. 31, there is no increase in Mg# below, at or concentration of Cr was generally near or below above the UC. In fact, there is a marked decrease the detection limit, 20 ppm; nevertheless, for those in Mg# at and above the UC (in chromitite the ratio rocks the concentration of 20 ppm Cr was used in is 0.233, in associated silicate rocks 0.438). The summation. very low Mg# in UC and plagioclasic cumulates Obviously, it was not possible to make the UC is, of course, partly due to the trapped liquid from the lean, old magma. shift effect as recognized by Barnes (1986) and The low Mg in chromite, the low Mg# of the Wadsworth (1986) and convincingly applied by UC intercumulus, the potassic composition of the Cawthorn (1995a,b, 1996). melt inclusions and of the intercumulus material It is seen in Fig. 31 that the curves of MgO, Mg# suggest that the chromite crystallized in an and Cr each represent a mirror image of the Al2O3 environment poor in Mg and rich in K (and Cl). curve. The latter reflects the amount of cumulus Thus, it seems that for the UC event to happen the plagioclase. Thus, when plagioclase crystallized only "primitive" additive was Cr (see Mutanen, (or settled) in excess of the cotectic pl/px ratio, the 1997a, p. 114-119). 58 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
THE KEIVITSA SATOVAARA COMPLEX
Location and exploration history
The Keivitsa - Satovaara complex (KSC) is situated intrusion (Fig. 32). Exploration has been 34 km north of Sodankylä, in the area of map concentrated in the western part of the complex, sheets 3414 and 3732, just south of the Koitelainen the Keivitsa intrusion. Of several prospects, the
Fig. 32. The location of the Koitelainen intrusion and the Keivitsa-Satovaara complex. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 59 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... main target has been the Keivitsansarvi Cu-Ni- deposit. The two discovery holes had probed a part PGE-Au deposit. of the deposit which was later known as the Upper The first observations of the ultramafic rocks Ore. More trenches and shallow pits were also dug were made by Erkki Mikkola during the 1920s in the ore subcrop area. Some more holes were and 1930s (Mikkola, 1937, 1941). Since the 1960s drilled in the PGE-Cr-V deposit and other targets several mapping and exploration programmes have in the Keivitsa area. been conducted in the area. However, the massive In Satovaara a disseminated Ni-Cu-PGE-Au and disseminated sulphides encountered were deposit, similar to the Upper Ore of Keivitsansarvi, always very low in Ni, of the type called "false was intersected in 1991. The results from the ore" during the last exploration phase by GSF. widely-spaced holes drilled in 1995 by GSF were The present exploration was prompted by glacial not encouraging. erratics of sulphide-disseminated ultramafic rocks In 1995 the deposit of Keivitsa was transferred found on the bank of the Luiro river (1973) to Outokumpu Co which gave it up in 1998 and in Satovaara (1983), southeast of Keivitsa. (Tahkolahti, 1998). In 2000 Scandinavian Gold A large area around Keivitsa and Satovaara Prospecting Ab (now Scandinavian Gold Limited) was covered by systematic ground geophysical was granted claims of the Keivitsansarvi area; (magnetic, gravity, electromagnetic) surveys and the company has done additional diamond core pedogeochemical line sampling of basal till in drilling and mineral processing tests. 1984 - 1987. Percussion soil sampling at a good The Keivitsansarvi deposit has features which electric conductor yielded massive sulphides with make it difficult to discover by routine sampling 1 % Ni, 2 % Cu and 0.33 % Co. This and other and surveys. First, the percentage of sulphides geochemical and geophysical indications were is too low, and their grains are not sufficiently investigated by diamond core drilling in 1987. All interconnected to give a perceptible response the good electric conductors proved to be false ore to the electromagnetic inductive method used sulphides. A peculiar PGE-Cr-V deposit in quartz- in the routine regional surveys, before the carbonate rocks ("chondritic" PGE mineralization; discovery. Second, the subcrop failed to show Mutanen, 1989) was found, as a Pd anomaly, by up in pedogeochemical sampling. Detailed post percussion sampling. Two of the last drill holes festum till sampling of trench walls revealed of the programme the twenty-third and twenty- that above the ore subcrop the secondary fourth holes drilled in Keivitsa intersected 22-24 (glaciogenic) dispersion aureole is very thin, that m of disseminated sulphides with 0.36 % Ni, 0.46 it lies at the till-bedrock contact and that it is not % Cu and 1 - 1.4 ppm PGE+Au. Small trenches traceable downstream. Also, the stony basal till is made in the vicinity indicated the continuity of the impenetrable to percussion sampling. Even with deposit between and beyond the discovery holes. hindsight, only one sample on the distal side of (The trenches were visited in August 1989 by the subcrop shows anomalous Cu, Ni, Pd and Au. participants at the pre-symposium field trip of the And finally, the Keivitsansarvi deposit, located as 5th International Platinum Symposium.). it is several hundreds of meters above the basal Subsequent drilling programmes in Keivitsa contact of the intrusion, has an unpredictable, or in 1990 and 1992-1995 delineated a large, low- “impossible, stratigraphic position. grade Cu-Ni-PGE-Au deposit, the Keivitsansarvi
General geology
The KSC is situated in the northeastern part of a Satovaara intrusion (in the east) probably represent faulted brachysynclinal structure on the southern blocks of a single intrusion, separated by a zone of limb of the Koitelainen brachyanticline (Fig northeasterly faults (Satojärvi fault zone). The 33). The Keivitsa intrusion (in the west) and the Satovaara block has been displaced downwards, 60 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen Excursion stop 4 4 KEIVITSA - SATOVAARA COMPLEX 3 6 5 9 8 2 7 1 Fig. 33. Geological map of the Keivitsa Satovaara Complex. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 61 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... perhaps 2-3 km, in relation to Keivitsa. high-carbonaceous beds (Sokolov, 1980; Karhu, With the exception of the short description that 1993; Karhu & Holland, 1996). The Ludian has follows I will not deal with the Satovaara here. The been bracketed between 2.113 Ga and 2.062 Ga by ultramafic cumulates in Satovaara have a higher Karhu & Holland (1996). The Ludian corresponds MgO content (higher olivine/pyroxene ratio) and to the sediments of the upper Transvaal sequence the proportion of gabbros is smaller than in the in South Africa (2.225 2.050 Ga; Walraven et al., Keivitsa intrusion. Unlike Keivitsa, the Satovaara 1990). It appears, however, that the pelite-black intrusion is concordant and intruded higher in the schist sequence in the Keivitsa-Satovaara area is at stratigraphy, well above the black schists. The least 2.15 Ga old (Huhma et al., 2005). hornfels aureole is considerably thinner than that The Ludian is underlain by Jatulian formations, around the Keivitsa intrusion. The thick basal which probably contained evaporite deposits olivine-rich cumulates are overlain, successively, (Shatskii & Zaitsevskaya, 1994). The high sulphur by pyroxenite (10 m) and gabbro (100 m). On top isotope δ34S values of the Keivitsa sulphides are quartz gabbros and sodic granophyres, whose (Hanski et al., 1996; Grinenko et al., 2003) may thickness ranges from a few meters to tens of ultimately be derived from the brines of the now meters. Near the top of the ultramafic zone, ca 100 vanished Jatulian evaporites (see Shatskii & m below the gabbro, is a discontinuous layer of Zaitsevskaya, 1994). The same circulating brines disseminated Fe-Ni-Cu sulphides, with PGE and may have been responsible for the elevated Cl in Au. The stratigraphic position of the Satovaara metasediments and serpentinite and the scapolite sulphide reef is comparable to the Upper Ore of porphyroblasts common in altered eruptive and the Keivitsansarvi deposit. metasedimentary rocks. The base of the Keivitsa intrusion is estimated The increase in sulphides and graphite in pelites to lie 500-600 m above the top of the Koitelainen coincides with the appearance of acid volcanic granophyre. Immediately above the Koitelainen dropstones (Fig. 32 in Mutanen, 1997a). Evidently, intrusion are (in ascending order) metasedimentary from then on acid volcanism contributed material hornfelses, micaceous arkoses, calcareous rocks, to the pelitic sediments. low-Ti basaltic volcanics (now plagioclase- Emplaced in the lower part of this succession are hornblende-chlorite rocks, possibly flows and two thick differentiated komatiitic sills composed tuffs), arkosic quartzites and a thick sequence of of thick olivine cumulates (+chromite) overlain pelites. The pelites, with MgO from 3.0 to 4.5 % by pyroxene cumulates and gabbro. At the top of and elevated Cr and Ni, are comparable to early the upper sill is a thin seam of granophyre, with Archaean high-Mg pelites (see e.g., Sheraton, dendritic hornblende. The peridotitic main part 1980). of the lowermost sill contains augite oikocrysts The lower pelites (anal. 20, Table 4) are very enclosing olivine crystals (Stop 8). These "spotted poor in sulphides and exhibit cm-scale graded komatiites" hug the roof contact of the Koitelainen bedding. Upwards the contents of sulphides and intrusion round its western half. The peridotites graphite increase. There are thick carbonaceous of the sills are lower in MgO than the dunitic to beds - black schists - very rich in graphite, with up peridotitic komatiites which occur as xenoliths in to 32wt% graphitic carbon/14 m. The uppermost the Keivitsa intrusion. pelites are rich in sulphides (see anal. 21 in Table Several bodies of peculiar albite-rich breccias 4) but poor in graphite. Komatiitic volcanic rocks occur in a wide area west and northwest of the (flows, agglomerates, pillow lavas, tuffs) occur intrusion. The Puilettilampi hydrothermal breccia above the black schists intercalated with sulphide- pipe west of Keivitsa (Fig. 33) contains abundant rich high-Mg pelites. carbonate, magnetite and pyrite. Originally (Mutanen, 1997a) I correlated the Younger dyke rocks will be described later. pelitic and carbonaceous rocks and associated The grade of regional metamorphism reached bimodal volcanism with the post-Jatulian Ludian the amphibolite facies. Metamorphism obliterated formations, which include the famous shungites, almost all the magmatic minerals of small igneous 62 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen bodies as well as most of the original hornfels During the subsequent folding the intrusions minerals around the KSC and the komatiitic and their hardened hornfels aureoles behaved as sills. Thermometamorphic minerals are locally competent blocks, and the deformation inside the preserved inside the Keivitsa intrusion. Large intrusions was largely elastic. Fold structures are portions of the ultramafic rocks of the Keivitsa large and the bedding in general dips at angles intrusion have undergone surprisingly little less than 45o. Subhorizontal bedding occurs in the jointing and are mineralogically fresh. There western part of the map area (Fig. 33). are no discernible large-scale later hydrothermal The brachysyncline is sliced by northeasterly impressions in the Keivitsa peridotites. There are faults and fault zones. The fault zone separating unaltered parts in the gabbro cumulates, but the the Keivitsa and Satovaara intrusion contains granophyres and uppermost gabbros are wholly numerous blocks of ultramafic rocks similar to the altered. olivine-pyroxene cumulates of the intrusions.
THE KEIVITSA INTRUSION
Age, form and internal structure
The U-Pb/zircon U-Pb age is 2.057±5 Ga (Huhma The contacts of the intrusion are discordant et al., 1996; Mutanen & Huhma, 2001). The to the strata of the supracrustal rocks. The basal pyroxene-plagioclase-whole rock Sm-Nd dating contact dips 45-50° south. Near the base contact, gives an age of 2.05 Gy (op. cit.). The zircons xenolithic rafts and tectonic slices of pelitic schists used for dating are euhedral, zoned crystals in are interfingered with ultramafic cumulates. olivine pyroxenite and they evidently crystallized The roof was intersected by one DDH; there the from magma. Primary pyroxenes from olivine contact between the granophyre and the pelite pyroxenite and ferrogabbro, intercumulus hornfels roof is sharp. plagioclase from olivine pyroxenite, and cumulus The igneous layering in the lower part is roughly plagioclase from ferrogabbro were used for the conformable to the base contact, but upwards the Sm-Nd dating (op. cit.). Zircon from a diorite dip of layering becomes more gentle, being 20-30° dyke cutting ultramafic cumulates, interpreted to in the upper part of the ultramafic zone (Fig. 46) represent “back-intrusion” from the granophyre and almost horizontal in the gabbro zone and the cap melt and to be coeval with the intrusion granophyre. gives an age of 2.053±7 Ga (see Mutanen, 1997a; The intrusion is formally divided into zones: Mutanen & Huhma, 2001). The basal marginal chill zone is 0 - 8 m thick, The intrusion is funnel-shaped and dips and consists of microgabbro or contaminated southwest (Fig. 33). The surface area is ca 16 quartz gabbros and quartz-rich pyroxenites. km2, of which ultramafic rocks make up 6.4 The contact rocks are massive but altered into km2, pelitic hornfels xenoliths 0.5 km2 and the uralite gabbros. The composition of the chilled big peridotite-serpentinite xenolith 0.5 km2; the margin (Table 3) was from microgabbros least remainder (8.6 km2) are gabbros and granophyres. affected by contamination, added cumulus The separate gabbro bodies south and southwest crystals or metamorphic alteration. The effect of of the intrusion may be apophyses from the main contamination (S, Cl) is evident; it is plausible, magma chamber. though, that the composition has some bearing Chemical analyses of the most common rock on the composition of the parent magma. A DDH types of the Keivitsa area are presented in Table profile through the marginal microgabbro (Fig. 34) 4. shows that with the increase in cumulus pyroxene FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 63 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... and olivine, the gabbro rapidly grades into olivine discontinuous layers (?) of pyroxenites and pyroxenite. The REE(CN) pattern (not shown) is gabbros and sharply-defined masses (autoliths?) very similar to the Akanvaara basaltic xenoliths of gabbros. Xenoliths of various komatiitic rocks and chilled margins, with a smooth negative slope abound, particularly in the host rocks of the from La(CN) 75 to Yb(CN) 10; there is no Eu- Keivitsansarvi deposit (Fig. 39). Pelitic xenoliths anomaly. are common near the base contact but rare in the The ultramafic zone thins and tapers out to main part of the zone. Komatiitic xenoliths are all the west; in the southeast it runs against the wall but lacking in these rocks. rocks. The zone is thickest in the northeast of the The rocks of the lower part of the zone, about intrusion, where the Keivitsansarvi deposit is also 500 m thick, are petrographically indistinguishable located. The maximum thickness is not known, from the overlying ore sequence. However, in the but it may be as much as 2 km or more. The Ni/Co plot (see Fig. 61 in Mutanen, 1997a) these zone consists of olivine-augite(-orthopyroxene)- rocks form a low-Ni group, separate from the magnetite cumulates (mesocumulates and rocks of the ore sequence. Moreover, the Au and poikilitic orthocumulates). Modally the ultramafic Pd contents are always low, generally below the cumulates are olivine wehrlites, but in the field detection limit of 1 ppb. (and here) they are called olivine pyroxenites In the upper ultramafic zone, immediately (for compositions, see anal. 1-9, Table 4). The overlying the upper ore layer, is a peculiar name metaperidotite is used for altered (hydrated) olivine pyroxenite called "pitted peridotite". The olivine pyroxenite. weathered parts (pits), roundish or amoeboid Among the olivine pyroxenites there are in outline, consist of olivine websterite that is relatively rich in orthopyroxene and contains biotite-phlogopite and disseminated sulphides. The knobs are richer in augite and carry neither sulphides nor mica. In a series of outcrops in the eastern part of the intrusion the size of the pits decreases up in stratigraphy and the contrast between knobs and pits diminishes. The gabbro zone consists mainly of pyroxene gabbro, ferrogabbro, with pigeonite and fayalite (anal. 10, Table 4); magnetite gabbro (with V- rich magnetite), and their uralitized equivalents. Graphite gabbro (Fig. 35) occurs in the southeastern part of the intrusion. Visible layering is rare; only indistinct ratio layering (plagioclase/pyroxene) has been seen in one outcrop. High up in the gabbro zone there are discontinuous units of olivine-pyroxene cumulates, which, compared with those of the ultramafic zone, are rich in Fe and poor in Mg, Cr and Ni. Ilmenite is a cumulus phase. These rocks often host concentrated sulphides of the false ore type.. Huge rafts of pelitic hornfelses and minor komatiites occur in the gabbro zone. Some of the hornfels rafts may actually be wedges projecting Fig. 34. Profile through the basal chilled contact rock and from the walls. Ultramafic cumulates seem to underlying pelitic metahornfelses, Keivitsa intrusion. have been deposited upon the hornfels rafts. The 64 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen thickness of the gabbro zone is not known, but Table 3. Chemical composition of Keivitsa chilled diamond drilling indicates that it is more than 500 margin, recalculated to 100 per cent (Sample m in the axial part of the intrusion. R800/650.0-652.0 m).
Magnetite gabbro grades very rapidly into the granophyre (anal. 11, Table 4), the uppermost % ppm ppb magmatic unit of the intrusion. It is well displayed SiO2 53.13 Cr 173 Au 3.58 on the magnetic map due to its magnetite content. TiO2 0.95 Ni 89 Ir <0.1 A vertical DDH drilled near the southeastern Al2O3 11.1 Cu 205 Os <1 FeO(tot) 10.4 Sc 54 Pd <1 wall of the intrusion intersected 22 m of the MgO 9.36 V 254 Pt 0.64 granophyre. Shallow holes drilled in the southern MnO 0.12 Zn 32 Rh 1.8 part of the intrusion indicated that the granophyre CaO 9.92 Ba 50 Ru <2 is considerably thicker there. Small xenoliths of Na2O 3.88 Sr 97 pelitic hornfels occur in the granophyre. In the K2O 0.23 Zr 74 P2O5 0.08 Y 25 overlying hornfelses there are salic pockets that S 0.52 evidently represent incipient melting. Cl 0.15 Superimposed on other structures are fractal Table 3. Chemical composition of the Keivitsa chilled margin, structures, seen in the size distribution of xenoliths recalculated to 100 %.
Fig. 35. Keivitsa. Euhedral cumulus graphite in augite-pigeonite gabbro. The longest graphite flakes are about 2 mm. Textures indicate that graphite crystallized together with plagioclase and pyroxene. Fine flakes of “intercumulus” graphite occur throughout the rock but are not visible in this picture. Reflected light. Photomicrograph by JariVäätäinen. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 65 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Table 4. Keivitsa – Satovaara Complex. Chemical compositions of rocks types. Analyses recalculated to 100 %. Analyses made by XRF, some trace elements (P, Sc) by ICP. 66 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
(from single olivine grains to huge xenoliths), in wire” structures (Fig. 62) are largely invisible the concentrations of disseminated sulphides (from circuitries; they are important in interpreting the size of a nail to homogeneous masses hundreds the electric conductivity and electromagnetic of meters across) and in the sulphide veins (from behaviour of disseminated sulphides. about 1 m down to 1 μm thick). These ”chicken
Emplacement, contact effects and xenoliths
The Keivitsa magma intruded into the sedimentary bulk composition to the interstitial sulphides in pile at about the level where the amounts of the surrounding cumulate matrix. The xenoliths sulphides and graphite increase. The intrusion is in barren olivine pyroxenites do not contain discordant to the surrounding rocks. Black schists sulphides. Thus, is seems that sulphides infiltrated are found both below and above the intrusion. The (as liquid) into the xenoliths from the surrounding base contact of the intrusion is slightly discordant magma. to the floor strata. Both flanks are discordant. The small graphite-rich xenoliths are telltale The western flank is composed of interfingered signs of incorporated graphite-rich black schist gabbros and hornfelses; the eastern flank is also material. Actually, graphite is not uncommon in interfingered and cuts steeply across the strata. mafic and ultramafic intrusive rocks. One ofthe The intrusion chamber was filled as a single many examples is the South Kawishiwi intrusion cast. No evidence of repetitive entries of magma of the Duluth complex, where semi-massive has been found in the Keivitsa intrusion. graphite occurs in the anorthosite gabbro unit The intrusion has a broad thermometamorphic (Severson, 1994). hornfels aureole. Hornfelses were altered into Undoubtedly, the small pelitic xenoliths were metahornfelses in later regional metamorphism. Of readily melted. At the partial (fractional) melting the original hornfels minerals, kyanite is preserved of bigger xenoliths alkalies and silica were lost in pelitic rocks in both the floor and the roof rocks. to the main magma, leaving a residue enriched in Its presence indicates high fluid pressure in rapidly pyroxene and sulphides. Sulphide liquid remained heated, wet sediments rather than high lithostatic a residual phase after total melting of pelites. pressure. Wall rocks hydrated to any degree prior to The magma swallowed and digested significant intrusion underwent contraction by dehydration, quantities of country rocks. Pelitic rocks, now which efficiently increased the surface area of pyroxene-plagioclase hornfels, are found as large the xenoliths swallowed by magma and thus xenoliths and as half-digested (rotten) xenoliths. accelerated their melting. The Keivitsa magma was Komatiitic rocks occur as solid layered, banded clearly not able to melt the komatiitic xenoliths. and massive ultramafic xenoliths (Fig. 39 or However, the 10-15 % dehydration-contraction of as mechanically disintegrated rotten xenoliths. the original komatiitic rocks (composed mainly Composite komatiitic-pelitic xenoliths are also of serpentine, tremolite and chlorite) resulted in present. Fresh komatiite xenoliths (komatiite mechanical disintegration of both the roof and hornfelses) are composed of olivine, chromite, xenoliths (Fig. 40). When the roof was rapidly augite and orthopyroxene, the mineral proportions exposed to magma, dehydration began with the depending on the original chemistry of the initial separation of a high-pressure fluid. This rocks. Komatiite xenoliths are often overlain "fluid explosion" shattered the rock and prepared by contaminated material rich in pyroxene the ground for further comminution. Heating and sulphides, presumably from the melted produced explosions in quartz-bearing rocks sedimentary part of the original composite due to significant expansion at the low- to high- xenoliths. The komatiite xenoliths contain fine- quartz inversion. Most of the fluid components grained disseminated sulphides corresponding in (H2O, Cl) were subsequently dissolved in magma. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 67 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Melting of sedimentary interlayers promoted the in DDHs can be traced for 300 m from north to demolition of komatiitic roof rocks and xenoliths. south. Typical of the moussé layer are big, thin- Clarke et al. (1997, p. 64) used the term “thermal layered xenoliths of komatiitic tuff. stress” for various processes leading to shattering The komatiite xenoliths exchanged components of xenoliths. with the surrounding magma, and solitary olivine Komatiitic xenoliths are characteristic of the xenocrysts reacted with, and adjusted their Keivitsa intrusion (Fig. 35). Even more significant composition to, the melt. In this re-equilibration is the fact that the xenoliths are most abundant the thermometamorphic komatiitic olivine became within the Keivitsansarvi deposit. On the other enriched in Fe. But still more important was that hand, rotten pelitic xenoliths are always associated it lost Ni to magma (see Fig. 36). This Ni addition with false ore sulphide. The komatiite xenoliths are markedly increased the Ni content of the sulphides of all sizes, from single olivine grains to the huge of the Keivitsansarvi deposit. serpentinite-peridotite xenolith in the middle of A peculiar quartz-carbonate rock occurs at the the intrusion (Fig. 33; for compositions, see anal. base of the biggest serpentinite-peridotite xenolith 15-17, Table 4). This mother xenolith has a low- (anal. 18-19, Table 4). Variable but generally high density serpentinite core and a rim of peridotite. PGE, Cr and V concentrations are characteristic; Its position on top of the ultramafic cumulate also, Ce and La are sometimes very high (up to pile could be explained as follows: The smaller 1132 ppm Ce and 643 ppm La). Besides quartz ultramafic fragments were dehydrated early and there are euhedral, zoned carbonate crystals, began to sink in magma soon after detachment. albite, magnetite, chromite, ilmenite and rutile Dehydration of the innards of the big xenolith, in varying amounts. The rocks are massive or however, was slow and, thus, sinking was delayed banded and hornfels-like. The colour ranges from until the xenolith reached the critical overall white and grey to dark grey and black, depending density; also, at the moment of sinking, the density on the abundance of Fe-Ti-Cr oxides. Structures of the magma was decreasing when plagioclase reminiscent of load-cast occur between ligh- entered (and olivine finished) as a cumulus phase. coloured and dark, oxide-rich layers (Fig. 45). The Near the top of the Main Ore is a layer of weathered parts are cavernous due to dissolved concentrated komatiite xenoliths, called the carbonate. moussé layer. It is usually 4 - 10 m thick, and
Contamination and crystal fractionation
As at Akanvaara and Koitelainen, the Keivitsa first, selective diffusion (“transvaporization”) of magma received contaminants from various rocks H2O, Cl, K, P and REE from wall rocks, xenoliths, and by various mechanisms. The contaminants and salic anatectic melt; second, bulk assimilation from komatiites, black schists and pelites swayed of salic neo-melts, and, third, incorporation of the crystallization path and all but obliterated residue phases of partially melted or mechanically the geochemical colour of the parent magma. disintegrated rocks (sulphide liquid from pelites, Contamination is seen in the occurrence of reduced sulphide liquid and insoluble graphite from black carbon (graphite) and a high content of volatiles schists, olivine and chromite from komatiites).
(H2O, Cl) and of certain trace elements (PGE-Au, Most affected by contamination was the S, Se, Te, Se, Ni, Co, V, REE, U). Isotope data uppermost part of the magma chamber. Moreover, (Sm-Nd, S isotopes, Sm-Nd, Re-Os; see Huhma as discussed before, it was also the principal et al., 1996; Hanski et al., 1996; Grinenko et nursery of cumulus crystals, which were promptly al., 2003) verified the heavy contamination by transported downwards by density flows and surrounding crustal rocks. deposited upon earlier cumulates. Contaminated The principal contamination mechanisms were, liquid was dragged along in the density flow and 68 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen eventually trapped between settled crystals. Study presumably the brine in the pores and cracks. of the gabbro zone cumulates and comparison The source of the brines may have been Jatulian with intercumulus material of the ultramafic evaporites (Shatskii & Zaitsevskaya, 1994) zone suggest that the main mafic magma was not underlying the Ludian formations. greatly stirred nor homogenized (the Sargasso Sea The magma had a considerable capacity to phenomenon). dissolve Cl (see Carroll & Webster, 1994). Mantle The ultimate reason for selective contamination magmas, being very poor in Cl (50-60 ppm; e.g., was that the mafic magma was undersaturated in Byers et al., 1984, 1986; or 10-40 ppm, Michael components (H2O, Cl, K, P, REE) transferred from & Schilling, 1989), are undersaturated in Cl and, country rocks and neo-melts. As regards quantity, thus, acted as a sponge for Cl. Evidently, the Cl petrological implications and petrographic of Cl-rich magmas came from sediments. The Cl visibility, H2O was undoubtedly the principal content in lavas can be very high, up to 1.62 % contaminant. The main effect of added H2O and (e.g., Johnston, 1980; Metrich, 1990; Kovalenko K (possibly augmented by Cl) was expansion of et al., 1992; Naumov et al., 1996). Selective the olivine liquidus field. The2 H O that entered contamination, as depicted above, is certainly a the upper part of the magma chamber was not feasible process. At Keivitsa the Cl was hitched dispensed in the main magma, but was carried to magmatic and high-T postmagmatic minerals down, along with cumulus crystals, by density (chlorapatite, dashkesanite, biotite-phlogopite); flow suspension and stored in the cumulus thus, at the pressure conditions of layered intrusions deposits that were rapidly piling up. Ultimately, Cl is not necessarily enriched proportional to at the prevailing total pressure, H2O was fixed in the amount of crystal fractionation as generally high-T postcumulus OH-minerals (hornblende, purported (see e.g., Metrich, 1990; Villemant & biotite-phlogopite). No degassing of H2O from Boodon, 1999). The Cl-concentration of the melt intercumulus space has been noted. The main is buffered by high-temperature concentrated magma body remained poor in H2O and, as far up and dilute crystal phases (e.g., chlorapatite, as can be seen, hornblende did not crystallize from hornblende and biotite, respectively) and a dense magma. magmatic brine (Webster et al., 1999, Webster, As a case example of selective contamination I 2002; Signorelli & Capaccioni, 1999). Finally, shall describe the behaviour of Cl, which has been even at low pressures Cl does not necessarily studied rather little in magmas and their environs. behave as a “volatile” component: In coal burning The selective transfer of Cl from the serpentinite and contact metamorphism, at 1000-1200ºC and xenoliths and from pelites can be followed in even higher temperatures, Cl can combine with several diamond drill holes (see Figs. 43 and Ca to produce OH-free crystal phases (Chesnokov, 44. The transfer of Cl into magma as opposite to 1995). In Akanvaara, Koitelainen and Keivitsa degassing was associated with selective, possibly intrusions there is no evidence for magmatic Cl coupled, diffusion of H2O and K. As serpentinites degassing, the idea suggested and elaborated by in general (see, e.g., Anselmi et al., 2000) the Boudreau and co-workers (Boudreau et al., 1986; Keivitsa serpentinite has a very high Cl content Boudreau & McCallum, 1986, 1988; Boudreau, (Figs. 42 and 43); the change from Cl-rich to Cl- 1988). Metrich (1990) estimates that degassing depleted serpentinite near the magma is sharp. In occurs only at pressures below 40 bar. Lacking any pelites the Cl is enriched in argillic beds (Fig. 53 in positive evidence from Keivitsa, I will not discuss Mutanen, 1997a); the Cl depletion goes smoothly further the idea of the transfer and deposition of towards magma. Note that Cl enrichment in the PGE by fluids exsolved from intercumulus liquid underlying magma is accompanied by enrichment (Boudreau, 1993). of K2O. Note also (Fig. 44) that the granophyre The Cl content of ultramafic cumulates varies magma evidently had a low capacity to dissolve but is generally high, commonly 1000-1500 ppm (or to keep) Cl. Besides the Cl fixed in minerals but occasionally up to > 3000 ppm (see Figs. 42, of the wall rocks, an important source of Cl was 43). As the Cl content does not correlate with the FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 69 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... total OH content (i.e., degree of metamorphic The granophyre and most of the cumulates seem hydration; Fig. 41) of the rock, Cl was not to have preserved the negative Eu(CN) anomaly introduced later by the metamorphic fluid. There inherited from sediments (Fig. 68). Only some late is local but no general correlation between Cl and gabbros with cumulus plagioclase have a flat or S contents. slightly positive Eu. The contaminated magma immediately below Strong convection caused local mixing of the the roof obviously had a very high Cl content anatectic acid melt and the mafic liquid, resulting (see Fig. 43). It is possible that the high Cl, either in the formation of a hybrid melt. The apparent alone or, particularly, in combination with other simultaneous crystallization of olivine and contaminants, expanded the apatite liquidus field orthopyroxene can be understood if orthopyroxene to the extent that chlorapatite crystallized from crystallized in a hybrid melt richer in silica, and the magma. Thus, the big chlorapatite crystals in two nursing environments were only incompletely ultramafic rocks would be true cumulus crystals, mixed. The pitted peridotite would then represent extended by postcumulus growth. Daughter this kind of heterogeneous cumulate (see Jackson, crystals of chlorapatite in melt inclusions in olivine 1961). The low-Cr type of olivine pyroxenite, the (Fig. 37b) indicate that chlorapatite crystallized host of some false ore sulphides, also indicates early from this hybrid melt. bulk mixing of salic and mafic magmas. The The studies of Watson (1982) show that in wide spread of the intercumulus plagioclase salic/mafic emulsion K is transferred from compositions (see Mutanen, 1997a, Fig. 45) the anatectic salic melt to the mafic magma; implies a heterogeneous intercumulus liquid with contrariwise, Na is enriched in the acid melt. variable amounts of salic contaminant melt. The This is what actually seems to have happened at hybrid melt is preserved as inclusions in olivine Keivitsa. The granophyre is rich in Na and the (Fig. 37). Na/K is very high, whereas the melt inclusions Melting and assimilation of pelitic schists and the intercumulus are rich in K. Like H2O and promoted the separation of sulphide liquid by the Cl, the K was compatible by association: included double effect of added sulphur and siliceous liquid. in the contaminated mafic melt, it was retained in Understandably, assimilation of ordinary pelites, the intercumulus melt and eventually formed (by as such, did not contribute anything of value to reaction with orthopyroxene) postcumulus biotite- the ores; quite the contrary, it diluted the magma phlogopite. with Ni-poor material. However, the relatively Comparison of REE distribution in the high Au in sulphides, when compared with other granophyre and sedimentary rocks (Fig. 68) shows Ni-Cu(-PGE) deposits may be explained by the that the granophyre is depleted of the LREE in addition of Au from assimilated rocks. In fact, in relation to sediments, which suggests that LREE the southern part of the intrusion several sulphide- were enriched in the contaminated melt, possibly bearing hornfels intersections have high Au values transferred as Cl complexes. Concentrated carriers (see Mutanen, 1997a). Thus, it may not be mere of REE are apatite and monazite (Fig. 37). It was coincidence that false ore sulphides in this same noted at an early stage that the host rocks of the Ni- area have relatively high Au and high Au/Pd. PGE ore type are rich in La and Ce (Fig. 68). The Olivine xenocrysts from disintegrated general high REE content reflects the high REE in komatiites were residue phases. Insoluble as they contaminated magma. were, they were not chemically inert, but reacted The Ni-PGE type reflects the geochemical and exchanged components with the magma they characteristics of the komatiitic dunite and the arrived in. The dunite-serpentinite, with 0.2 - 0.4 associated quartz rock in many ways. High % Ni, is the rock richest in Ni in the complex, the contents of REE (Ce up to 1130 ppm, La up to Keivitsansarvi deposit included. On the other hand, 640 ppm) in quartz-carbonate rock suggest that the Ni content of the parent magma was only about it may have been an important source of REE 90 ppm (Table 3) and, so, with the fractionation of contamination. olivine and pyroxenes, Ni was doomed to decline 70 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen in the residual liquid. With no evidence of added would have crossed this reducing float layer. The primitive magma, the only possibility that remains graphite gabbro may have crystallized from this is that a great part of the Ni in the Keivitsansarvi kind of carbon-saturated melt. deposit was supplied by Ni-rich komatiite debris. The fractional crystallization soon began The melting of pelites left a residue of sulphide to deviate from the path predestined by the liquid, most of it not soluble in the hybrid magma, composition of the parent magma as a result of which was charged with cumulus crystals. the contaminants acquired. The contaminants
Mixing and equilibration with the main magma (H2O, alkalies), leading to an expanding of the and komatiitic olivine xenocrysts produced a olivine liquidus field, had effects similar to those beautiful mixing line between pelitic sulphides at Koitelainen (see Mutanen, 1997) except that no and komatiitic dunites (see Figs 51 and 53). The olivine monocumulates were formed; olivine and main magma was undersaturated in sulphide and augite (+ Cr-magnetite) crystallized together in acquired little of the added sulphur. It evidently roughly fixed proportions. reached terminal sulphide saturation only at the Nowhere in the ultramafic zone are there clearly level of the uppermost ferrogabbros. defined geochemical trends. At times, and in Partial melting of black schists left a residue places, the increased role of acid contaminant of graphite. Under the P(tot) conditions of the melt is reflected in an increase in V, decrease in intrusion, part of the graphite were oxidized to Cr and the change in the sulphide liquid toward
CO and CO2 and part dissolved in melt as carbon sedimentary composition (low Ni/Co, Ni(100S), (see Mathez & Delaney, 1981). That graphite can Se/S, Te/S, etc). crystallize from magmas in layered intrusions has In the upper part of the ultramafic zone in been realized since the early 1980s (e.g., Elliott et general, in the Cu-Ni-PGE-Au deposit in particular al., 1981, 1982; Ulmer, 1983). and above all in the host rocks of the Ni-PGE At Keivitsa, graphite crystallized locally from ore type, the residual olivine from disintegrated carbon-saturated magma as euhedral crystals, komatiitic dunites significantly contributed to the and the local residual liquid remained saturated olivine content of the rocks. with graphite (Fig. 35). Intercumulus graphite is During accumulation of the ultramafic zone particularly common in the Ni-PGE ore type. the effect of carbon assimilation in decreasing Assimilation of carbonaceous sediments caused the ferric-ferrous ratio of melt (and, thus, of reduction of magma. The low ferric/ferrous ratio crystallizing silicates) was countered by the is reflected in the suppression of crystallization of alkalic-ferric iron effect (Paul & Douglas, 1963) cumulus magnetite, in the relatively low Mg/Fe of associated with selective transfer of K and general the equilibrated komatiitic olivine and in the early assimilation of salic material. appearance of primary pigeonitic pyroxene and Apart from general reduction, the main magma fayalitic olivine in gabbros. was only marginally affected by contaminants and The presence of intercumulus graphite indicates thus evolved autonomously. Cumulus plagioclase that the graphite buffer was not consumed but appeared soon after the disappearance of magnesian survived to subsolidus temperatures. The sulphide olivine, to be joined later by pigeonite, and locally paragenesis, with troilite, talnakhite and very rare by graphite, fayalitic olivine and magnetite. secondary magnetite, reflects reducing subsolidus Remarkably, the P content of the residual liquid conditions. remained very low, and so apatite saturation Graphite and graphite-rich xenoliths, having a failed by a wide margin (the last ferrogabbros density lower that that of the magma, were able to contain only 0.1 % P2O5). It is possible that float and stack at the roof. The trajectories of any the crystallization of chlorapatite continuously crystalline phases, whether cumulus or residual, skimmed P2O5 from the main magma. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 71 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Dyke rocks and mineral veins
Several kinds of dykes cut the Keivitsa intrusion. La/Yb(CN) ratios. One felsite has the REE(CN) The earliest are gabbroic veins, often porphyritic similar to some false ores and regular ores. with microgabbro matrix, with sharp contacts. The diabase and olivine gabbro-diabase dykes The mineralogy of these veins (e.g., chlorapatite (anal. 22-26, Table 4) are considerably younger is abundant) is what would be expected from than the Keivitsa intrusion (Sm-Nd mineral age intercumulus melt. I interpret these to represent 1.916±0.067 Ga, Huhma et al., 2005). They have local, evolved intercumulus liquid, which seeped fine-grained, chilled contacts against the Keivitsa into late contraction cracks. Small coarse-grained cumulates, and the magmas were geochemically pockets, sometimes with sulphides, occur sparsely quite different from that of Keivitsa. The dykes throughout the ultramafic cumulates; these I strike ENE, vertical or with a steep dip to south. interpret to represent pegmatoids similar to those The diabase dykes are thicker (up to 10 m); the described earlier from Ylivieska and Koitelainen thickness of the olivine gabbro-diabase dykes (see Mutanen, 1997). varies from about 0.1 to 3 - 4 m. They represent The diorite, felsite and composite diorite-felsite a remarkable case of flow differentiation, with dykes (anal. 12-14, Table 4) have sharp, winding a heavy concentration of olivine crystals in the contacts, and contact effects are often surprisingly centre. The composition of the diabase dykes slight. Felsite dykes are rare; most often felsite is similar to that of the margins of the gabbro- occurs in the middle of the diorite dykes diabase. It seems that the respective magmas came (composite dykes). These dykes are typically from the same source. For more data on these composed of coarse marginal zones containing dykes, see Mutanen (1997a). big, zoned plagioclase crystals and a medium- Massive sulphide veins are common but they grained inner part (with plagioclase, hornblende account for an insignificant proportion of the total and quartz). Felsites are leucocratic, fine-grained mass of sulphides. Geochemically they are of the rocks with occasional zoned phenocrysts of false ore type. The contacts of sulphide veins plagioclase. Some acid veins are composed of are sharp and tidy, and contact effects are often lava-like porphyritic rock, with phenocrysts of very faint. Evidently, sulphide veins that poor quartz, feldspar, pyroxene and magmatic brown in Ni cannot represent sulphides mechanically hornblende in a fine-grained matrix. These dyke mobilized from surrounding disseminations at rocks invariably contain disseminated and (or) any reasonable temperatures. They represent low- mobilized pyrrhotite and chalcopyrite. Both viscosity interstitial sulphide liquid of the false ore rock types have a high Na/K ratio. There is a type trickled down into contraction cracks from paragenetic and compositional continuum between the overlying crystal mush containing false ore diorite and felsite. Probably these magmas formed type sulphide liquid. by anatectic melting of the wall rocks and then Mineral veins (carbonate, albite and quartz and intruded into contraction cracks opened during combinations of them) are usually accompanied late stages of consolidation of the intrusion. The by appropriate wall rock alteration. Crystals of REE(CN) patterns (not shown) resemble those uraninite seem to be common in carbonate veins. of granophyres (Fig. 68) but have slightly lower
Mineral deposits of the Keivitsa intrusion
Besides the Keivitsansarvi deposit, to be shot and disseminated sulphides. Very low Ni, Cu described later, there are other metal and mineral and noble metals. occurrences: 2) Offset sulphide veins in footwall hornfelses. 1) Basal contact sulphides. Semimassive, buck- Rich in Cu (up to 20 %), anomalous Au, low Pd. 72 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
3) False ore sulphides (massive, breccia and coarse milky quartz. Near the surface the rocks are disseminated), frequently associated with pelitic often vuggy (from dissolved carbonate). The rocks hornfelses. The sulphides have a bulk composition show variable, often high V and Cr (up to 0.97 % similar to that of false ores in and around the V and 13 % Cr2O3), low Ni (generally 200-500 Keivitsansarvi deposit. In these deposits Cu is ppm), low Mg; anomalous to high Ce and La (see sometimes high, Au anomalous. Sulphides have above); anomalous PGE (up to 6 ppm). There is a low in Ni(100S) and PGE. slightly positive slope in the PGE(CN) diagram, 4) Graphite in gabbro. A high-grade graphite similar to that in overlying serpentinite (see Fig. concentrate was obtained in tests, but the 60). This peculiar rock may have been formed percentage of recoverable graphite is low. by hydrothermal(?) leaching of Mg and Ni from 5) PGE-Cr-V deposit in quartz(-carbonate) serpentinite (Mutanen, 1989). rocks (see Table 4). This is a discontinuous 6) Serpentinite xenolith. The magnetite is high deposit (up to 25-30 m thick) between the big in Ni. Mineral processing tests. serpentinite-peridotite xenolith and underlying 7) Chrysotile asbestos. Veins in the hornfelsed cumulates of the Keivitsa intrusion (gabbro, marginal part of the serpentinite-peridotite feldspathic metapyroxenite). These are black, xenolith. fine-grained, massive or banded rocks with 8) Vermiculite. Allochthonous filling in an open widely varying amounts of magnetite, chromite, fissure. rutile and ilmenite (Fig. 45). There are unevenly 9) Fresh olivine pyroxenite. A valuable building distributed carbonate rhombohedra, albite and stone.
Petrography of ultramafic cumulates
The basal microgabbro grades into a pyroxene (up to 5300 ppm U; see Fig. 37f) occur in this rock. cumulate and pyroxene-olivine cumulate rich in Sulphide-mantled euhedral orthopyroxene (Fig. intercumulus plagioclase. This transitional zone, 61a-b) is common in the basal ultramafic zone. with MgO 10.8 - 17.0 % and Al2O3 7.4 - 5.0 %, The main part of the ultramafic zone consists is ca 40 m thick. Chlorapatite and euhedral zircon of olivine-augite-orthopyroxene-magnetite cumu-
NiO, % KEIVITSA 1.8 OLIVINES
OLIVINE GABBRO-DIABASE 1.5 Ni-PGE ORE TYPE
TRANSITIONAL ORE TYPE
REGULAR ORE
FALSE ORE
1.0 MARGINAL ZONE
OLIVINE PYROXENITE
THERMOMETAMORPHIC OLIVINE IN KOMATIITE XENOLITHS
0.5
75 80 85 90 95 Ol Mg/Mg+Fe at-%
Fig. 36. Plot of NiO(wt%) vs Mg/Mg+Fe (at%) for olivines of the Keivitsa intrusion. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 73 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Fig. 37. Evidence of contamination in feldspathic olivine pyroxenites, Keivitsa. BE images by Ragnar Törnroos and Bo Johanson (a-c), SE image (d) by Ragnar Törnroos and BE images by Lassi Pakkanen (e-f). a - a large, crystallized acid melt inclusion in olivine. False ore-type rock. Glacial erratic, Satovaara, sample TM-83-3; Ol - host olivine, Opx - orthopyroxene reaction rim, Bi - biotite-phlogopite rim, a - albite, H - hornblende, M - magnetite, c - chlorapatite daughter crystal. Diameter of the inclusion ca 0.2 mm; b - close-up of the chlorapatite daughter crystal in Fig. 46a; c - distribution of Cl in area of Fig. 46b (a negative); d - melt inclusion (0.2 mm) in chlorapatite. a - host chlorapatite, f - La-rich fluorapatite rim, b - Phlogopite crystal (low Cl and Ba), d - Ba- Cl-rich phlogopite (light-coloured matrix crystals), c - monazite, g - galena, e - chalcopyrite; e - rim of dashkesanite (medium grey) with ilmenite lamellae (white) around biotite-phlogopite (B) in brown hornblende (H). Sample TM-84-116.3; f - skeletal zircon crystals (cross sections upper right and left of the long crystal. The zircon contains 2600-5300 ppm U. DDH 307/92.50 m. 74 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
Fig. 38. Keivitsa olivine pyroxenite. Dashkesanite (light green) with ilmenite lamellae (black), between primary hornblende (brownish green) and biotite-phlogopite (brown); white - Ca- clinopyroxene. Microphotograph by Jari Väätäinen. Parallel nicols, width of photo 3 mm.
Fig. 39. Xenoliths in Keivitsa intrusion. Photo (a) and diascanner photos by Reijo Lampela. a - dunite xenolith in olivine pyroxenite. DDH822/289.70 m. Core diameter 4.5 cm; b - xenolith of banded dunite in sulphide-disseminated olivine pyroxenite. The xenolith is devoid of sulphides. DDH360/262.00 m. Bar = 1 cm; c - finely-layered komatiite xenolith (granoblastic tremolite- chlorite-magnetite rock), magnetite veins. Xenoliths of this type are typical of the moussé layer. Bar = 1 cm; d - fine-grained granoblastic dunite in olivine pyroxenite. DDH332/39.75 m. Bar = 1 cm. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 75 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... lates. Chromite occurs only in clinopyroxene, as tiny euhedral to subhedral inclusions. The H2O- rich intercumulus liquid crystallized largely to pri- mary hornblende and biotite. H2O was unevenly distributed in the intercumulus liquid. Thus, the degree of replacement of clinopyroxene by brown hornblende commonly varies around a single clinopyroxene grain. The amount of olivine generally varies in the range 15 - 25 vol%. Unless encased in Fig. 40. Schematic presentation of the mechanical sulphides, orthopyroxene occurs as anhedral to decomposition of dehydrating serpentinite roof rocks and oikocrystic grains and contains resorbed olivine xenoliths in Keivitsa magma (see text). and augite inclusions. This indicates a reaction relation of olivine with orthopyroxene and calcic clinopyroxene. On the other hand, clinopyroxene commonly occurs as roundish inclusions in olivine, suggesting that the magma began crystallizing in the clinopyroxene field and turned, possibly due to added H2O, into the olivine field. Then, again, clinopyroxene contains olivine inclusions. All this can be translated into a cumulus path cpx → ol → cpx → (peritectic reaction of ol, and apparent reaction of cpx) → opx. Euhedral plagioclase inclusions are found in orthopyroxene oikocrysts and sulphide pockets (Fig. 61d), suggesting that plagioclase was actually an early cumulus phase.. Thus, before the intervention of contamination, plagioclase and pyroxenes crystallized together from the original basaltic parent magma (Table 3), Fig. 41. H2O vs Cl in Keivitsa ultramafic cumulates. as should be expected. The clinopyroxene/orthopyroxene ratio varies in ultramafic cumulates; false ores are normally richer in orthopyroxene, whereas in the Ni-PGE ore type the orthopyroxene content is very low. Chlorapatite often occurs as big grains, not unlike Ni-PGE ore type are remarkably high in Ni. Using cumulus crystals. the distribution of Ni between olivine and augite Cumulus magnetite occurs as solitary euhedral Häkli (1971) estimated model crystallization crystals (for more data on magnetite, see Mutanen, temperatures of 1150 1200° C for the ultramafic 1997). cumulates of Keivitsa. The compositions of olivine are presented Primary intercumulus minerals are plagioclase, in Fig. 36. From the base upwards pyroxenes biotite-phlogopite, brown hornblende, and olivine become more magnesian ("reverse dashkesanite, chlorapatite, monazite, graphite, fractionation") up to the level of the Ni-PGE ilmenite and sulphides (mainly pyrrhotite, ore; from there upwards data are lacking, but chalcopyrite, pentlandite, cubanite and evidently fractionation followed the normal trend mackinawite). Sulphides and other ore minerals of decreasing Mg/Fe, as inverted pigeonite and are described in association with ore types. The fayalitic olivine occur in gabbro not far above the ore minerals identified are listed in Mutanen big serpentinite-peridotite xenolith. Olivines of the 1997a, Tables 8 and 9. 76 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
There are cases of unholy co-habitation of and a high-density chloride brine (Fig. 38). It magnesian olivine and quartz (also in Koitelainen always contains rods of (exsolved?) ilmenite. and Merensky Reef; op. cit., in Uitkomst olivine Dashkesanite analyses are presented in Table gabbro-norite, Kenyon et al., 1986, and in the 6. Like most high-Cl hornblendes the Keivitsa Elan intrusion, Central Russia, Chernyshov, dashkesanites are potassium hastingsites, but they 1995). Intercumulus plagioclase contains anhedral are richer in MgO than are most of the analysed inclusions of potassium feldspar. chloramphiboles (see e.g., Krutov, 1936; Krutov & In the Keivitsa intrusion dashkesanite is common Vinogradova, 1966; Pavlov, 1968; Jacobson, 1975; in ultramafic cumulates; it also occurs in gabbroic Dick & Robinson, 1979; Sharma, 1981; Kamineni interlayers in the upper part of the ultramafic zone et al., 1982; Vanko, 1986; Volfinger et al., 1985; and in pigeonite-fayalite gabbro. Dashkesanite is Suwa et al., 1987; the new mineral potassium associated with biotite-phlogopite, and it seems chloropargasite is high in Mg; see Chukanov to be a product of reaction between primary et al., 2002). The presence of ilmenite rods brown hornblende, primary biotite-phlogopite suggests that the original high-T dashkesanite was
KEIVITSA Histograms of Cl concentrations in various rocks and ore types
N Granophyre & felsite dykes 50 N Strongly contaminated False ore 15 melagabbros 30 10
10 5
0 100 500 1100 2500 Cl ppm 0 1000 5000 8000 Cl ppm
N Serpentinites, N 30 dunites 100 20
10
Olivine pyroxenites 0 70 100 500 1100 1500 1900 Cl ppm
50 Ni - PGE & Transitional ore types
30 Regular ore
10
100 500 1000 1500 2100 2500 3100 4100 4900 5500 Cl ppm Fig. 42. Keivitsa. Histograms of Cl concentrations in various rocks and ore types. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 77 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... probably richer in Ti than the present hornblende %). The micas most rich in Ti are also rich in V (up matrix analysed by microprobe. It appears that to 1.81 % V2O3). Many electron microprobe spot dashkesanite crystallized direct from intercumulus analyses show high contents of Br (up to 3.67 %) melt; this is possible as Cl-rich amphiboles are and I (up to 0.18 %). stable at higher temperatures than OH-amphiboles Brown primary hornblende has growth (Xiao et al., 2005). extensions of green to colourless amphibole. The primary mica is mostly phlogopite, These amphiboles are always low in Cl (≤ 0.27 sometimes biotite. It contains up to 0.66 % Cl, and %), but occasionally high in Br (up to 2.5 %) and is rich in TiO2 (up to 4.18 %) and Na2O (up to 0.82 I (up to 0.26 %).
Fig. 44. Gradual downward depletion of Cl in pelitic metahornfelses, roof rocks of the Keivitsa intrusion. Note the low Cl and high S in the granophyre, and the lack of S depletion in roof rocks. Data from DDH 728.
Fig. 43. Depletion of chlorine in the big serpentinite xenolith near the magma contact. Keivitsa, DDH 334 and 335. The thinner Cl-depleted zone in DDH 334 is against a magma apophyse, the broader Cl-depleted zone in DDH 335 against the main magma chamber. Note the dramatic high in Cl (and K) in the underlying intrusion rocks. 78 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
Fig. 45. Load-cast (?) structure in quartz-albite rock, pushing through magnetite-rich layers. Carbonate idioblasts in quartz-albite rock. The sample contains 4 ppm PGE. Keivitsa, DDH323/10.70-13.60.1. Bar = 1 cm. Diascanner photo by Reijo Lampela.
Chlorapatite is actually quite common in susceptibility) by olivine, plagioclase and ultramafic intrusions (see Mutanen, 1997a). In clinopyroxene. The alteration generally proceeded postmagmatic appinitic intrusions crystals of along fractures, mineral veins and dyke contacts. chlorapatite are found along with fluorapatite The advancing alteration has a thin front of (Mutanen & Väänänen, 2004). Thus, chlorapatites serpentinized olivine, but with completion of (and other Cl-rich minerals) must be discredited as the process olivine was altered into tremolite. special signals from known or undiscovered PGE The completely hydrated olivine pyroxenites deposits; their occurrence only signifies trapped (metaperidotites) always contain mineral veins contaminant liquid rich in Cl. Electron microprobe (carbonate, amphibole, sulphides) and are more analyses of chlorapatites from Koitelainen and fractured than unaltered rocks. Partial hydration Keivitsa intrusions are presented in Table 5. sometimes pervades the rock without connection In metamorphic and retrograde processes to any visible fluid paths; a field name "pervasively primary mafic minerals were hydrated (into amphibolized olivine pyroxenite" is applied to serpentine, amphiboles, talc, chlorite, secondary such rocks. phlogopite) and plagioclase was altered into Rodingite alteration is characteristic of the chlorite, clinozoisite-epidote, carbonate and ultramafic cumulates of Keivitsa. Rodingite (talc, scapolite. Orthopyroxene was the most susceptible hydrogrossular, diopside, carbonate) typically to hydration, followed (in order of decreasing appears as small (1-2 mm) white spots. There are FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 79 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... also coarse, vein-like bodies of rodingite (former stretching of Ca pyroxene compositions toward pegmatoid pockets of intercumulus plagioclase ?), diopside. Evidently, rodingitization at Keivitsa with big, zoned hydrogrossular crystals. Rodingite did not involve true Ca metasomatism, but was alteration occurs in all olivine pyroxenites, essentially an autometasomatic alteration of calcic but is particularly common in the Ni-PGE ore intercumulus plagioclase. type. The effect of rodingitization is seen in the
Table 5. Compositions of chlorapatite, Akanvaara and Koitelainen intrusions and Keivitsa-Satovaara complex. Electron microprobe analyses by Lassi Pakkanen (1-5, 7), Ragnar Törnroos (6) and Bo Johanson. 80 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
Table 6. Compositions of dashkesanite and chlorine-bearing hornblende, Koitelainen intrusion and Keivitsa- Satovaara Complex. Electron microprobe analyses by Lassi Pakkanen (1-3) and Bo Johanson (4-6). FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 81 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
THE KEIVITSANSARVI Cu-Ni-PGE-Au DEPOSIT
General structure
The Keivitsansarvi deposit is a large, low-grade The eastern margin of the deposit has a deposit (in the following, ore, ore deposit or complicated structure, with fingers of false ores deposit for short) of disseminated sulphides in extending into the Main Ore. In fact, the structure olivine pyroxenites and their hydrated equivalents is deeply fractal (see Mutanen, 1997a). A big (metaperidotites). It consists of a few ore bodies finger of false ore has a separate subcrop east of containing several ore types. the main ore subcrop (Figs. 33 and 46). The subcrop of the deposit covers an area of Lenses of false ore, with very low Ni(100S), ca 13 hectares. The combined area of false ore occur high above the Upper Ore. subcrops in the vicinity is ca 12 hectares. Along From east down to west, false ores grade rapidly the northwest axis the length of the projected ore into the regular ore type of the Main Ore. Even is > 1200 m, the width > 500 m, and the maximum in the deceptively homogeneous Main Ore there vertical depth established so far is ca 800 m. The are strong lateral variations both across and along tonnage figure released is 263 Mmt calculated the long axis, e.g., in Pt/Rh and PGE(100S). It is down to 630 m (see Mutanen, 1997a). The thick, possibble that the west-sloping planes in Fig. 46 monotonous main orebody (Main Ore) is overlain mark the direction of a stationary system of density by a hanging deposit, the Upper Ore, a relatively flow downdraft, part of a central convectional thin layer, hanging above the Main Ore (see Figs. toroid, with very strong magmatic erosion near 46 and 48). All known Ni-PGE type ores are above the confluence of accelerating density currents the Main Ore. Ore types transitional between the downdip (Morse, 1988). The deposit represents regular and the Ni-PGE type (transitional type) a rapidly accumulated intra-intrusion delta of occur above the Main Ore but also in the lower unknown distal extensions. Flow movement was part of it. The distance between the Main Ore and from northeast to southwest (from east to west in the Upper Ore increases downdip. The Main Ore the profile, Fig. 46). In the following, terms such as grades upwards into barren rocks and eastward distal and proximal refer to this model. The Upper into false ores. The upper contact of the Main Ore Ore, with its more lateral spread, may represent the dips southwest. In the north this contact is rather waning deposition of ore on a shelf overlying the sharp; inferred from the common occurrence cumulate delta. of „hot“ mylonites the contact is controlled by The deposit is cut by ENE trending dykes of prototectonic faults. However, forerunner ores diabase and olivine gabbro diabase and faults occur far beneath the base of the Main Ore north running in the same direction. of it.
Ore types
The ore types can be classified according to their increase, and S/Se, S/Te, Re and Re/PGE decrease Ni(100S) and Ni/Co (Figs 49-52). The Ni/Cu (see Figs 53, 55, 57). Some of the ore types have ratio would also discriminate false and regular a characteristic REE signature (Fig. 68) and S- ores from the transitional and Ni-PGE types. As isotope composition (see Hanski et al., 1996; a matter of fact, ore types arranged this way have Grinenko et al., 2003). In general, there is a good distinguishing identities in other compositional correlation between Cu and Au (Fig. 54). diagrams. From false ores to Ni-PGE type ores Likewise, the various ore types have PGE, PGE(100S), PGE/(Ni+Cu), Pt/Rh and Pt/Au distinguishing mineralogical, paragenetical and 82 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen 3499.9 DDH no. 803 694 3499.5 693 692 691 690 689 688 KEIVITSA 687 100 m KEIVITSANSARVI 802 680 679678 3499 360 367 366 Transitional ore type (between regular and Ni-PGE type) Ni-PGE type False ore, Ni (100S) 2-4% False ore, Ni (100S) <2% Other ore types Ore types "Here be monsters" 810 Profile x = 7512.250 775 Cu-Ni-PGE-Au deposit 750 High grade Low grade 754 Regular ore 807 751 768 3498.5 0 200 -200 -400 -600 -800 Fig. 46. Keivitsa, Keivitsansarvi Cu-Ni-PGE-Au deposit. Grades in regular ore type; other types. DDH profile x = 7512.250. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 83 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... High-grade ore Projected boundary of high-grade ore Ore limit open Low-grade, unhomogeneous ore False ore Ni-PGE-ore Ni-PGE-ore, surface projection Transitional ore type (between Cu-Ni-PGE-Au and Ni-PGE ore) Transitional ore type projected from 50 m depth Transitional ore type projected from 500 m depth Section Keivitsansarvi Ore types Suboutcrop level Cu-Ni-PGE-Au-deposit KEIVITSA - 3499.30 3499.00 B A Upper Ore 3498.50 Section A - B along Ni/Cu Very high 100 m 7512.50 7512.00 Fig. 47. Keivitsa, Keivitsansarvi deposit. Ore types, subcrop level, with some projections from deeper levels. 84 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen 0.53 1 0.98/2 0.90 4.3 0.71 5 profile = / = == / / E E E E 490 m > lenght along 450 m 1.33 44 0.90 14 0.61 4 / = / / == / / 0.46 3 Cu 94 ppm Ni 0.44 Ni Cu 46 E Cu 32 ppm Ni 0.33 Ni Cu 103 E E = / E Till Diabase dyke DB 400 m DB 5.3 HORNF. XEN. 0.44 9.7 / = / Cu 0.35 Ni 0.15 Ni Cu 0.42 E 350 m metres 33.9 R746 R827 R332 R355 R333 R335 0.52 4.2 / = Cu 0.50 Ni 0.31 Ni Cu 0.63 E = PGE+Au, ppm 0.54 9 E Cu (%) Ni (%) Ni/Cu / = / Box Cu 0.05 Ni 0.34 Ni Cu 6.8 E 300 m 29 1.02 17 10 - 20 20 - 50 > 50 / = / Cu 0.42 Ni 0.32 Ni Cu 0.76 E 250 m Cu/Ni ratios (AB in Fig. 47) KEIVITSA 200 m R821 R327 / 23.75 UPPER ORE PROFILE R805 - R355 2.22 19 / = / Cu 0.24 Ni 0.45 Ni Cu 1.88 E in cl. 3.28 10 Transitional type Ni-PGE-type 150 m R708 50 m >9.9 26 50 m / = 0.63 1.02 Cu 0.26 Ni 0.21 Ni Cu 0.81 E / = Cu 0.43 Ni 0.33 Ni Cu 0.77 E 100 m False ore Regular type Upper orebody >22.6 0.51 / = Cu 0.15 Ni 0.18 Ni Cu 1.24 E Ore types in DDH intersections 50 m 16 6.1 1.03 1.25 / = / = Cu 0.46 Ni 0.30 Ni Cu 0.82 E Cu 0.026 Ni 0.29 Ni Cu 11.22 E 0 m R805 R804 R707 R326 Fig. 48. Keivitsa, Keivitsansarvi deposit. DDH profile (A B in Fig. 47) along Upper Ore, showing ore types, grades and Ni/Cu ratios intersections surrounding rocks. Fig. 48. Keivitsa, Keivitsansarvi deposit. DDH profile (A FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 85 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Fig. 49. Keivitsa intrusion. Ni histograms for various classes of S concentrations. Note the prominence of false ores in high-S samples.
Fig. 50. Keivitsa intrusion. Plot of Ni(100S) vs Ni/Co, all cases. Note the sharp lower boundary of the plot cluster; this seems to represent the mixing line between pelitic and komatiitic sulphides; the halo extending into high Ni(100S) values represents sulphide-olivine mixtures. 86 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen textural features. Of course there are transitional false ores occur both below and above the Upper cases and some odd natural and sampling mixtures, Ore (see Fig. 48). but these do not significantly affect the picture as a Without much exaggeration or oversimplification whole. It should be stressed that the classification it can be stated that the ore types represent mixtures presented below is geochemical and geological; it of komatiitic dunite and pelitic sulphide. As usual cannot be used for technical purposes direct. The with PGE deposits in layered intrusions, the parent Upper Ore in particular is heterogeneous: ore types magma was low in PGE (Sun et al., 1989). An from false ores to Ni-PGE type ores occur laterally exceptionally high PGE was not a prerequisite of in close proximity, and in individual DDH sections PGE deposits to form. The roles of the Keivitsa different ore types are intercalated. Thick lenses of intrusion and of the parent magma were hardly more (or less) than those of the pot and the cooking liquid in making a soup. In the west-east profile of the deposit (Fig. 46) the regular ore type is divided into four quality classes according to their Cu equivalent (CuEQ), and the false ore type into two classes according to the Ni(100S); the transitional type and the Ni-PGE type presented in the profile are geochemical types without strict regard for CuEQ values. Rocks of the false ore type have Ni(100S) < 4 % and Ni/Co < 10 (typically ca 4-5) (Figs 51-52). False ores are rich in sulphides and generally look better than the regular ores. The Ni content in the rock is low, generally < 0.1 %, the record low is 87 ppm. The meanest false ores, with Ni(100S) 0.3 - 0.5 % and Ni/Co 1-2, are identical to pelitic sulphides, as are their S/Se and S/Te ratios (Fig. 53). The total PGE-Au content in false ores is low, and directly proportional to Ni(100S) grade (Fig 60). Au and particularly Rh are enriched relative to other PGE. The Re content is distinctly higher than in other ore types, and the Re/PGE ratios are very high (Mutanen, 1997a). The Re is attributable to sedimentary contribution; shales and sapropelites sometimes contain very high concentrations of Re (see, e.g., Kalinin et al., 1985, Hulbert et al., 1992). It is significant that low-Cr false ores (400-700 ppm Cr in host olivine pyroxenite) are depleted of LREE relative to other ore types and sediments and enriched in HREE relative to other ore types (Fig. 68). This indicates that the acid contaminant was already depleted in REE in general and LREE in particular, by selective diffusion, before it was incorporated into the magma. The occurrence of sulphide-mantled Fig. 51. Keivitsa intrusion. Plots of Ni(100S) vs Ni/Co; a - samples with S > 0.5 %, b - S > 1 %, c - S > 2 %. Note the orthopyroxene crystals is diagnostic of false ores parting of the cluster into regular and false ore with the increase (Fig. 61a, b). They are occasionally present in in sulphur. regular ore types with a low sulphide content. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 87 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Fig. 52. Keivitsa. Histograms of Ni/Co; a - for various classes of S concentrations. Note the gradual parting of the histogram into regular and false ore with the increase in sulphur; b - samples with Ni+Cu > 0.8 %. The peak around 21 represents the regular ore. 88 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
The compositional mixing line is continuous between sediments and false ores, but there is a gap between false and regular ores (Fig. 51c). Spatially, too, the proximal false ores grade very rapidly into regular ores (Fig. 46). Evidently, the change from false to regular ore was caused by rapid and effective (turbulent?) mixing of "sedimentary" sulphide melt with magma enriched
Fig. 54. Keivitsa intrusion. Pt-Au relationships; a plot of Pt Fig. 53. Keivitsa-Satovaara Complex. S/Te vs S/Se in various vs Au; b - plot of Pt vs Au for samples with Pt<1000 ppb and rocks. Au<400 ppb; c - histogram of Pt/Au ratios.
Fig. 55. Keivitsa. General Pt-Rh-Au relationships in various ore types. Fig. 56. Keivitsa. Plot of (PGE+Au) vs S, all samples. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 89 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Fig. 57. Keivitsa. Schematic diagram of the relationships between PGE+Au and S, and geochemical trends between ore types.
Fig. 58. Keivitsa. Plot of PGE and Pd vs S; a - PGE vs S, Ni-PGE ore type; b - Pd vs S, Ni-PGE ore type; c - Pd vs S, transitional ore type. 90 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen
KEIVITSANSARVI Slope and extent between consecutive assays along drill holes (PGE + Au rise over Ni + Cu run)
2,8
2,5
2,0
(PGE + Au)ppm = 0.806 x (Ni + Cu)2 %
1,5
PGE + Au (ppm)
1,0
0,5
Ni-PGE type
transitional type
regular ore type
0 0,5 1,0 1,5 Ni + Cu (%) Fig. 59. Keivitsa. Slope of change between successive assays in various ore types. The curve that encloses most lines is of the form of the Stoke´s law. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 91 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Fig. 60. Keivitsa ore types. Diagrams of PGE-Au(CN); a - various ore types; b - komatiitic xenoliths and quartz-carbonate rocks associated with the dunite xenolith. in Ni and PGE-Au. The silicate part of most of the The postkinematic (1.79 Ga) appinitic intrusions false ores is compositionally and mineralogically (Mutanen & Väänänen, 2004) have PGE(CN) similar to that of the host olivine pyroxenites of patterns mutually identical and very similar to the regular ore; thus, mixing of silicate ingredients Keivitsa regular ore type. apparently preceded the mixing of the sulphide The transitional ore type occurs mainly in the liquid and the silicate magma. upper part of the deposit, between the regular ore Results of exploration drilling suggest that and the Ni-PGE-type ore (Fig. 47). Gradations sulphides of the false ore type make up the bulk of into both are common. A thick transitional ore was the total mass of sulphides in the intrusion. intersected in a deep DDH below regular ore in the The regular ore type forms the main mass of Main Ore (Fig. 46). Rare pegmatoid pockets with the Keivitsansarvi deposit. Its sulphide content massive to semimassive sulphides occur in this is 2-6 %, with Ni(100S) 4-7 %, Ni/Co 15-25 and type. The gradual contacts, presence of euhedral Ni/Cu 0.6-0.8. A typical ore contains 0.5-1 ppm olivine and augite in massive sulphide matrix and PGE+Au. The ore mass consists of homogeneous high Ni(100S) contents indicate that the pegmatoids dissemination over hundreds of meters along represent intercumulus sulphide-silicate emulsion, DDHs, almost without a break. There are, however, accumulated in contraction openings. All ores of invisible lateral trends in the Main Ore from the transitional type do not, however, occur in this proximal to the distal part. In the regular ore type north-trending belt; actually, one of the best PGE the PGE+Au sum generally correlates positively intersections (DDH R821, with PGE-Au 2.22 with Ni+Cu (Fig. 59), but among successive drill ppm/19m, including 3.28 ppm/10 m) is located core samples of the transitional and Ni-PGE-types west of the belt at the Upper Ore level between there are numerous aberrations (i.e., Ni+Cu is the discovery holes R326 and R327 (see Fig. 48). indifferent as regards changes in PGE, Fig. 59). In this DDH intersection dunitic xenoliths are 92 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen conspicuously abundant. Thick sections with very pentlandite). Because of the high Ni content in high Ni/Cu ratios in the west (DDH R805) suggest silicates (in olivine up to 1.74 % NiO, in augite up that unknown PGE-enrichments may occur west to 0.26 % NiO) all calculated Ni(100S) values are of the area delineated in Fig. 47. far too high. Occasional high As values (based on This type has a somewhat lower sulphide acid-soluble Ni) are characteristic. content, higher Ni/Cu (1.5-2.5 or more) and All the rocks surrounding the transitional and Ni(100S) (15-23 %), higher total PGE and higher Ni-PGE ore types have very high Ni/Cu ratios Pt/Au (6-10) than regular ore, but in appearance (Fig. 48), reaching up to 300. In this respects, too, it is much like regular ore. In contrast to regular they are similar to the dunites of the big xenolith. ore, there is no correlation whatsoever between Komatiitic xenoliths are rare in the host rocks PGE and S (see Fig. 58). Moreover, the Cu-S of most of the transitional type ores and in all Ni- correlation is weak, particularly in samples poor PGE ore types. in S (Figs. 74a,b in Mutanen, 1997a). Transitional Massive sulphide veins are common but ores in the lower part of the Main Ore differ from volumetrically insignificant. They are simple or the upper transitional ores only in their higher consist of branching stockworks or breccias. The Pt/Rh ratios. contacts are sharp. Contact effects in fresh olivine This type differs from the Ni-PGE type in peridotites (hydration, carbonate alteration) having a higher sulphide content, lower Ni/Cu and are conspicuously weak, although portions slightly higher S/Se and S/Te (Fig. 53). The Ni of metaperidotite usually contain networks content in olivine is equal to or lower than in the of sulphide veins. Compositionally massive Ni-PGE type Fig. 36). The REE(CN) patterns are sulphides are of the false ore type, poor in Ni but similar to those of the Ni-PGE type (Fig. 68). occasionally rich in Cu. The PGE-Au is low (Figs. The volume of the Ni-PGE type ores is small. 57, 60). The record low is a massive sulphide vein The ores occur in the upper part of the deposit, at with a mere 4 ppb Pt. about the level of the Upper Ore (Figs. 46 and 48), Ultramafic xenoliths within the ore contain and are characterized by a very low abundance of fine-grained disseminated sulphides of the same sulphides, high Ni/Cu (50-90) and Ni/Co (25-80), composition as sulphides in the cumulate matrix. high Ni(100S) (40-60 %), PGE(100S) (25-300 This suggests that sulphide liquid infiltrated ppm) and low Au (max. 0.13 ppm). In general, the into xenoliths from the surrounding matrix. The highest PGE grades are found in this type (Fig. 65). xenoliths, however, have lower total PGE, and There is no correlation between PGE and S (Fig. their PGE(CN) diagrams are similar to those of 63) and only very weak correlation between Cu the big serpentinite-peridotite xenolith (Fig. 60), and S, particularly when compared with other ore suggesting that the PGE were not dissolved in the types (Fig. 74, in Mutanen 1997a). The sulphides infiltrating sulphide liquid. are high in Ni (millerite, heazlewoodite, Ni-rich
Mineralogy of ore types
The petrographic description of the ultramafic presence of troilite, talnakhite (Fig. 64) and rocks presented before applies in general to the graphite and the scarcity of primary and secondary host rocks of the ores, too, and so is not repeated. magnetite, were predetermined by low-oxygen Here, only the most important ore minerals and fugacity magmatic and postmagmatic conditions. their textures are described. Findings of sulphide beads (droplets; Figs. In most of the ore types sulphides crystallized 61c,d) allow a window into the phase separation from the sulphide liquid and equilibrated to in the Keivitsa magma. Olivine, clinopyroxene low temperatures. However, the present ore and plagioclase crystallized in equilibrium with parageneses, which are characterized by the a silicate liquid and sulphide liquid. Postcumulus FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 93 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
Fig. 61. Keivitsa. Sulphide-mantled orthopyroxenes in olivine pyroxenites. Diascanner photos by Reijo Lampela. a - discontinuous sulphide mantles around orthopyroxene with resorbed clinopyroxene inclusions. Non-mantled orthopyroxene oikocrysts (middle left and lower right corner) contain resorbed olivine and clinopyroxene. Primary intercumulus biotite-phlogopite (brown) is common. DDH672/6.35 m. Width of photo = 2.5 cm; b - mantled orthopyroxene (6 mm) with resorbed olivine and clinopyroxene. Most orthopyroxene grains are unmantled oikocrysts. DDH 692/195.93 m; c - sulphide droplet with euhedral olivine in olivine pyroxenite (with 6 vol% olivine). Roundish inclusions of clinopyroxene occur in the cores of some olivines. DDH824/250.43A m. Bar = 1 cm; d - another section of the same droplet as in 61c. Euhedral olivine (top and right end of the droplet), euhedral plagioclase (white). Clinopyroxene is euhedral when against the big plagioclase crystal. Bar = 5 mm. 94 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen growth and reactions obscured the primary to nerve cells (Fig. 62). These sub-μm to μm-thin relationships, but it can be seen in sulphide sulphide veins represent the fractal ”ends” of the beads that the silicates crystallized in the order: sulphide vein system. clinopyroxene -> olivine -> plagioclase. The main sulphides are troilite, hexagonal In false and regular ores the sulphide grain pyrrhotite, chalcopyrite, pentlandite, talnakhite aggregates are interstitial to silicates. These primary and cubanite; pyrite and monoclinic pyrrhotite aggregates are joined by very thin “chicken wire” are very rare, and occur only in association with sulphide veins with a texture similar in appearance retrograde alteration, as in carbonate and albite-
Fig. 62. "Chicken wire" sulphide network in olivine pyroxenite, Keivitsa. BE images by Lassi Pakkanen. a - chicken-wire texture of sulphide nuclei and "axons"; b and c - details of axons, with thicknesses of 0.5 µm and less. DDH713/38.90 m. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 95 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ...
carbonate veins. Mackinawite and sphalerite occur regularly; molybdenite, galena and altaite occasionally. In the following description chalcopyrite denotes to chalcopyrite proper, talnakhite and putoranite. Obviously, one or more of the compositionally similar and paragenetically related Cu-Fe sulphides (mooihoekite, haycockite, Ni-putoranite) should occur in the ores, but no positive identifications have yet been made. Pentlandite exsolved from high-T monosulphide solid solution (Mss) first by granule exsolution Fig. 63. Keivitsa. Oxidized talnakhite in olivine pyroxenite. as equant grains; subsequent low-T exsolution Creamy white mineral is pyrrhotite. Reflected light microphotograph by Jari Väätäinen. DDH679/148.80 m. Width produced lamellar and dendritic flake pentlandite. of photo 0.7 mm. Flake pentlandite seems to be more common in false ores than in regular ores. Flake pentlandite also occurs in chalcopyrite. A thin seam of pentlandite is typically seen between pyrrhotite and chalcopyrite. Pentlandite in association with troilite and hexagonal pyrrhotite contains (in wt%) 27-36 % Ni (average ca 32 %); the pentlandite in the low-T paragenesis of the Ni-PGE type is very rich in Ni (40-42 %). Sulphides in the transitional ore type are rich in pentlandite; pyrrhotite is rare and sometimes absent; millerite occurs in pyrrhotite-free assemblages. Sulphides are interstitial to silicates, indicating that sulphide liquid was once present. The ore paragenesis of the Ni-PGE ore type is quite different from that of other ore types. Graphite is ubiquitous and magnetite is all but absent. The most unusual, and significant, feature is the low-T sulphide-arsenide-sulpharsenide paragenesis associated with a high-T silicate paragenesis (olivine, augite, plagioclase). Pyrrhotite is absent, and the sulphides are not interstitial to silicates. The composite sulphide grain aggregates are roundish, and the sulphides in them are concentrically arranged, with the metal/sulphur ratio decreasing from the centre to the margin (see Fig. 64). The core of millerite (±heazlewoodite) is surrounded by Ni-rich pentlandite. The outermost zone is composed of pyrite and a very delicate symplectite of pyrite and Ni-rich pentlandite. The As(-S) paragenesis Fig. 64. Keivitsa. Schematic drawing of the textures of the shows similar grain forms and a similar concentric paragenetic sequences of the S and AsS minerals of the Ni- arrangement, with the metal/As ratio decreasing PGE ore type. from the maucherite core to an intermediate zone 96 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen of nickeline and to the gersdorffite rim. The rare olivine and augite in a reducing, low-sulphur Cu minerals (bornite, chalcocite, chalcopyrite) magmatic environment. The concentric aggregates occur apart from the sulphide grain aggregates. of sulphides and arsenides-sulpharsenides formed The low S content of the rocks and the high at low temperatures when errant S and As reacted metal/S ratios of sulphides in this ore type indicate with alloy particles. that the local magma system was very poor in Sulphide-associated secondary magnetite is sulphur. Paragenetic and mineralogical features more abundant in false and regular ores than suggest that a Ni alloy crystallized together with in Ni-rich types. It is significant that, ample
Fig. 65. Keivitsa Satovaara Complex. PGM and tsumoite; BE images (a,b,f) and SE images (c-e) by Lassi Pakkanen. a - sperrylite in magnetite. DDH 330/106.35 m. Bar = 10 µm; b - Pt>Pd-Bi Fig. 66. Keivitsa. Pt-Pd relationships; a - plot of Pt vs Pd, all samples; b - histogram of Pt/Pd ratios, all samples. 98 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen sediment contamination notwithstanding, all (Ir -> Rh -> Co) AsS grains, similar to those of primary and secondary spinels are very poor in Hitura (Häkli et al., 1976) occur in the Satovaara Zn. The maximum Zn content in primary cumulus intrusion (Fig. 65e). magnetite is 0.37 %, in chromite 0.34 %, in The Pt-Pd PGMs are either Pt-rich or Pd-rich; secondary magnetite 0.08 % and in sulphide- only 11 % of the analyses fall within the range with associated magnetite 0.06 %. the proportion of Pt (of the Pt+Pt sum) between 7 Until 1997, 35 PGM phases were found in and 87 at.%. Despite the polarized mineralogy, the Keivitsa (Mutanen, 1997a). Later, Gervilla and average Pt/Pd ratio (wt%) of the analyzed PGM is Kojonen (2002) and Gervilla et al. (2003) studied 1.82, the same as in ores. Volborth and co-workers PGE-rich intersections of two DDHs, and found (1986) suggested that the Pt/Pd separation took PGMs new to Keivitsa: vysotskite, geversite, place very early with the participation of chloride- mertieiteII(?), keithconnite, laurite and some complexes. undetermined phases. It should be noted that these Of the Pt-mineral grains 55.3 % are included in intersections represent the transitional type and silicates (even in pyroxene), 12.8 % in sulphides the Ni-PGE-type, with up to 13 ppm PGE/2 m and 31.9 % are located at the silicate-sulphide (Mutanen, 1997a), and are not representative of contact; of Pd-minerals 25.0 % are included in the huge masses of regular ores. silicates, 29.5 % in sulphides and 45.5 % are at PGE minerals (PGM in the following) mostly the silicate-sulphide contact; these figures are occur as small grains (< 10 μm) at the margins about the same as those of Gervilla et al., 2003. of sulphide grain-aggregates, in silicates or in All this supports the idea that Pt and Pd parted secondary magnetite (Fig. 65). Most typical into separate phases in the magmatic stage (see are Te-Bi compounds of Pt and Pd (moncheite, Mutanen et al., 1996). kotulskite, merenskyite, michenerite, maslovite) Anomalous concentrations of PGE and Au and sperrylite; sulphides (cooperite, braggite) and have been detected in several ore minerals (see alloys (isoferroplatinum) are more rare. Zoned Mutanen, 1997a). Fig. 67. Keivitsa. Histogram of (Pt+Pd+Rh)/(Os+Ir+Ru) ratios. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 99 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... ORE PETROLOGY OF THE KEIVITSA INTRUSION The manifold ore types of the Keivitsansarvi exceptionally rich in Se, their S/Se ratios being deposit encompass an astounding range of element fairly close to mantle ratios. The relatively high ratios (see e.g., Figs 49-53, 55), probably greater Se and Te contents point to a restitic terrigeneous than found anywhere within a single magmatic component in pelites; alternatively, they may sulphide deposit (or, for that matter, in all such be associated with pre-Keivitsa volcanism and deposits combined). The variation, however, is degassing (see Greenland & Aruscavage, 1980; not haphazard nor caused or obscured by later Zoller et al., 1983). The very low S/Se and S/Te events, but reflects original magmatic conditions ratios (min. S/Se 21) of komatiitic dunites, the and processes. These involved contamination opposite end member, may be the property of by material from sulphide-rich, Mg-rich pelites, the source, a depleted mantle residue. As a result black schists and various komatiitic rocks. Due to of Se contamination the S/Se ratios in ores are carbonaceous contaminant from black schists the much lower than in ordinary magmatic sulphide magma became progressively reduced (Mutanen, deposits. 1994a, 1996, 1997a). The Ni-PGE type rocks have higher Mg#, In diagrams of chalcophile and siderophile Ni/Co, Ni/Cu, PGE, PGE/Au and As than other elements (e.g., Figs. 51, 53) all ore types are ore types. All these features are traceable to ordered neatly along an apparent mixing line from komatiite dunites, suggesting that the rocks of sedimentary sulphides to komatiitic dunite. Along the transitional and Ni-PGE ore types contain this line Ni(100S) and Ni/Co increase and S/Se material from disintegrated komatiite dunites, and S/Te decrease; there is also a regular decrease mainly refractory olivine debris. The debris sank in Au/PGE, Rh/Pt, Re/PGE, and an increase in through and interacted with contaminated roof (Os+Ir+Ru)/(Pt+Pd+Rh), PGE(100S), PGE/ magma enriched in Cl and float graphite. This Ni+Cu and total PGE (see Mutanen, 1997a). contaminated part of the magma chamber was also The mixing line most probably reflects true enriched in total REE and still more in LREE (see mixing of appropriate components of sedimentary Fig. 68c). The REE(CN) pattern of the Ni-PGE and komatiitic end members. Recent S isotope data type is quite different from that of the komatiitic (see Hanski et al., 1996; Grinenko et al., 2003) xenoliths (Fig. 68b); instead it resembles the explicitly affirm the important role of sedimentary pattern of sedimentary rocks, even showing the sulphur in the deposit and support the idea that the negative Eu anomaly. The metasomatic quartz- ore types contain component mixtures of exotic carbonate rocks may have contributed REE, as end members. suggested earlier. The contaminants (REE, Cl, The Pt/Pd ratio in all ore types is close to the H2O) are such that they could have been conveyed chondritic ratio (Mutanen, 1997a), suggesting from sediments to the roof magma by selective that all the ingredients (magma, komatiites and diffusion. The REE concentration in the local sediments) had this Pt/Pd ratio and that the magma system seems to have been high enough processes of crystallization-fractionation did not for monazite to crystallize. disturb it. However, the sedimentary components As suggested before, the low S in Ni-PGE may well have had a different Pt/Pd, but because of magma system, combined with the high Ni and their low total PGE contributions from sediments low oxygen pressure enabled crystallization of a was overwhelmed by other sources richer in PGE. Ni alloy from magma. The olivine that crystallized On the other hand, the effect of different PGE (or equilibrated) with the alloy became very rich in sources can be seen in the wide spread of the Ni (Mutanen, 1994, 1997a). Of course, olivine in (Pt+Pd+Rh)/(Os+Ir+Ru) ratio (Fig. 67) and of the general, and with such high Ni in particular, could Pd/Ir ratio (Fig. 85 in Mutanen, 1997a). Note also not be formed in greenschist-facies metamorphism, the odd slope of the histogram, in Fig. 67. as claimed by Gervilla et al. (2003). The sedimentary rocks around the KSC are In the regular ore there is a good correlation 100 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen 200 200 100 100 Mica schist Black schist Dunitic xenoliths 10 10 Sample/Chondrite a b 1 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Lu Yb La Ce Pr Nd SmEu Gd Tb Dy Ho Er Tm Yb Lu 200 200 100 Ni-PGE ore 100 Transitional ore Regular ore 10 10 Sample/Chondrite c d 1 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Lu Yb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 200 200 100 100 Low-Cr false ore High-Cr false ore 10 10 Sample/Chondrite e f 1 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Lu Yb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Lu Yb 200 200 100 Granophyre 100 Ferrogabbro 10 10 Sample/Chondrite Graphite gabbro g h 1 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Lu Yb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Lu Yb Fig. 68. Keivitsa. REE(CN) diagrams for: a - mica schists and black schists; b - komatiitic dunite xenoliths; c to f - various ore types; g granophyre; h ferrogabbro and graphite gabbro. Data and diagrams by Eero Hanski. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 101 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... between PGE and Ni+Cu, but in the deposit as a remove PGE from terrestrial magmas” (Naldrett whole there is no correlation between PGE and & Duke, 1980). sulphide (Figs. 57, 59). The only regularities are A spatial connection of Cl minerals with PGE that massive sulphides always have low total PGE, is commonly noted. It was suggested earlier and the highest PGE are found among rocks which (Mutanen et al., 1987, 1988) that halogens are very low in sulphides. acquired from sediments formed melt soluble There is no PGE-S correlation in Ni-PGE and complexes with PGE. Vatin & Perignon (2000) transitional ore types (Fig. 58). In regular ore, also suggested that the behaviour of Pt and Pd PGE correlate positively with Ni+Cu values. are partially controlled by melt-soluble chloride However, most of the PGM grains are not included complexes. These were able to enter the sulphide in sulphides but occur at the sulphide-silicate liquid only after breakdown of the complexes boundary or in silicates, as is common in PGE (Mutanen et al., op.cit.). Thus, the formation and deposits (see e.g., Bow et al., 1982; Viljoen et al., breakdown of PGE complexes would sometimes 1986a, b; Scoates et al., 1988; Harney & Merkle, govern the seemingly arbitrary, even irrational 1990; Rudashevskii et al., 1991; Bird et al., 1991; stratigraphic distribution of PGE, with little Arnason & Bird, 2000; Hoatson et al., 1992; respect for sulphides. There are some others Severson, 1994; Butcher et al., 1999). In general, who have pleaded for the melt-soluble halogen in sulphidic PGE deposits a great part (30-83 %) complexes of PGE (Anfilogov et al., 1984; Miller of PGM grains occur outside sulphides (Mutanen, et al., 1988; Gorbachev et al., 1994a; see also: 1997a). Ringwood, 1955). In contrast to the wide acceptance of the idea that The breakdown of PGE complexes could lead sulphide liquid is the universal solvent-collector of to the liberation of PGE and formation of alloys PGE it is surprising how common are the cases and other PGE compounds. Direct crystallization where there is no correlation between PGE and of PGM from magmas is often observed, indicated sulphides, as in the Koitelainen and Akanvaara or invoked (Hiemstra, 1979; Distler & Laputina, intrusions (more examples are listed in Mutanen, 1981; Augé, 1986; Augé & Maurizot, 1995; Distler 1997a; see also e.g. Brügmann et al., 1990; Saini- et al., 1986; Mutanen, 1989, 1992, 1997; Ertel et Eidukat et al., 1990; Cowden et al., 1990; Halkoaho al., 1999; Sylvester & Ballhaus, 1999; Borisov & et al., 1990; Severson & Hauck, 1990; Stone et Walker, 2000; Spandler et al., 2000; Peregoedova al., 1991; Teigler & Eales, 1993; Prendergast & Ohnenstetter, 2002). et al., 1998; Prendergast, 2000; Teigler, 1999). In the Keivitsa intrusion addition of exotic Two notable examples of the lacking correlation carbonaceous matter caused a dramatic lowering between PGE and S are Platreef (Kinnaird & Nex, of the oxygen fugacity of the magma. This in turn, 2003) and Insizwa (Maier et al., 2002). strongly reduced the PGE solubility (see, e.g., Ertel In many cases PGE seem to be chalcophobic et al., 1999; Borisov & Walker, 2000; Blaine et al., rather than chalcophile (see Stone et al., 1991; 2005), which may have been a decisive factor in Dillon-Leitch et al., 1986; Legendre & Augé, the precipitation of PGMs direct from melt. 1992; Hildreth et al., 2001). The general pattern That PGM grains are so often located at the of the PGE-S distribution in the Kap Edvard sulphide/silicate grain boundaries makes one Holm complex (see Arnason & Bird, 2000) is very wonder whether the PGE-sulphide bind is similar to that of Keivitsa. mechanical rather than chemical. It is tempting Perhaps it is too often forgotten that PGE to think that the sulphide liquid droplets acted as are, above all, siderophile elements (for rare phase boundary collectors for tiny PGM particles exceptions in this matter, see Hiemstra, 1979; that had already crystallized from silicate liquid Tredoux et al., 1995). It seems that too much is (Mutanen, 1992; see and cf. Hiemstra, 1979; see made of the chalcophile character of the PGE, as also Cawthorn, 1999). The process would thus in the statement “Equilibration with sulphide is be analogous to the old-time oil flotation. Our the only known mechanism that can effectively experimental tests showed that the idea is feasible 102 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen Fig. 69. Skeletal Pt-Cu alloy crystals on sulphide beads, BE and SE images by Bo Johanson. Top left - Pt-Cu crystals on a NiS bead; top right - NiS bead with a "crater"; middle left - detail of the arrow point area in top left image; middle right - detail of the crater rim in top right image, with a skeletal "harpoon" crystal of Pt-Cu; lower left - a feathery rosette of skeletal Pt-Cu crystals: lower right - BE image of the harpoon crystal in middle right image. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 103 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... (Fig. 69; see Mutanen, 1995b; Mutanen et al., 1996). it may explain why the D-values of PGE (sulphide Further experiments should confirm whether the liquid/silicate liquid) calculated for natural PGM (alloy particles in the experiment) nucleated systems are much lower than experimental values on sulphide droplets (heterogeneous nucleation) or (see Duke, 1990). It appears that the increase of were mopped along by sinking sulphide droplets. D-values with decreasing oxidation of magma In either case, the experiment indicated that the (Bezmen et al., 1991) is only apparent, perhaps PGM alloy was a true liquidus phase. In a later due to attached metallic particles on sulphide experiment (with Marja Lehtonen) we dissolved beads. the Ni sulphide bead and found that cuboid grains In a similar way PGM could preferentially of both Pt alloys and Pd alloys had crystallized nucleate on early spinels or even serve as sites of from the sulphide melt. These results also strongly heterogeneous nucleation for them (Augé, 1986; support the idea that the Pt/Pd separation, so Capobianco & Drake, 1990). We would then common in PGE deposits, may have originated have a simple explanation for the association of already in the magmatic stage. PGE with oxide cumulates in the Koitelainen and If PGE alloy particles can nucleate on sulphide, Akanvaara intrusions. ACKNOWLEDGEMENTS I thank the following persons who contributed to Seppo Aaltonen, Viena Arvola, Reijo Lampela and the preparation of this book, in alphabetical order: Helena Murtovaara. 104 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen FIELD TRIP PROGRAMME, AKANVAARA, KAIKKIVALTIAANLEHTO, RANTAVAARA AND KEIVITSA Akanvaara Stops 1-6, see Fig. 3. Stop 1. AKANVAARA. ALTERED FERROGABBRO The frost-shattered roadside outcrop is located on magnetite gabbro layer. West of the stop is a gully a ridge crest in the western part of the Akanvaara which served as an overflow outlet for the big hills, close to the big fault line which separates the Salla glacial lake. Typical granophyre vegetation, Akanvaara intrusion from the younger Ritaselkä with tall birch trees, is seen on the roadside south formation. The rocks are strongly altered, sheared of the outcrop. ferrogabbros immediately overlying the upper Stop 2. AKANVAARA. APATITE FERROGABBRO This is the uppermost mafic cumulate on the 11.27, FeO(tot) 17.18, MnO 0.26, MgO 1.52, southern slope of the Akanvaara hill. Acid CaO 6.69, Na2O 3.12, K2O 1.02, P2O5 1.05; trace granophyres are exposed south of the outcrop. elements (ppm): Ba 298, Ce 71, Cr 20, Cu 49, Ga The rock contains cumulus apatite. The silicate 18, La 34, Nb 6, Ni 0, Pb 25, Rb 21, S 200, Sr 263, minerals are hydrated and recrystallized. The rock Th 4, U 4, V 84, Y 31, Zn 171, Zr 116. contains (in wt%): SiO2 54.06, TiO2 1.65, Al2O3 Stop 3. AKANVAARA. FERROGRANOPHYRE Southern slope of the Akanvaara hill. typical ferrogabbro (DDH R305/256.00-257.00 Ferrogranophyre lies above the apatite m) has the following composition (in wt%): SiO2 ferrogabbros and apatite ferrodiorites. The outcrop 62.63, TiO2 0.90, Al2O3 11.46, FeO(tot) 12.96, consists of granophyre boulders that were moved MnO 0.24, MgO 0.27, CaO 4.57, Na2O 3.45, K2O downslope by solifluction during the waning stage 1.50, P2O5 0.20; trace elements (ppm): Ba 800, Ce of the Quaternary ice sheet. The original minerals 75, Cl 910, Cr 22 (below detection limit, bdl), Cu are altered and recrystallized. Plagioclase was an 19 (bdl), La 38, Ni 8 (bdl), Pb 36, S 360, Rb 50, Sc early phase to crystallize. The composition and 21 (bdl), Sr 245, Th 6 (bdl), V 35, Y 34, Zn 189, appearance of this granophyre are similar to those Zr 199. of the Koitelainen and Koillismaa intrusions. A FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 105 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... Stop 4. TUORELEHTO. UPPER CHROMITITE Looking south, the top of the Akanvaara hill dips the lateral shift is apparent only). Meta- consists of acid volcanic rocks; descending, pseudotachylytes (MPT) are often seen in faults. first granophyre, then ferrogabbro. The boulder Coarse feldspathic rock, probably representing fields and frost-shattered outcrops near the foot frictional neo-melt, are occasionally seen in of the hill contain material from the lowermost MPT. The local composition of MPT reflects the magnetite gabbro. These craggy banks are washed adjacent wall rocks, so that the MPT veins may and reworked shores of the ancient Salla glacial be asymmetric at places where different rocks are lake. A thick anorthositic sequence (with PGE- juxtaposed. In faults chromitites are converted into enriched layers) occupies the gully at the foot very fine-grained rocks. of the hill. In the south-sloping foreground the The UC layer is exposed here over tens of rocks are monotonous gabbroic cumulates of the meters along strike. Its contacts are generally UZ. Sankavaara, a hill in the east, with a steep rather sharp. The UC is immediately underlain northern slope, is composed of magnetite gabbro by dark microgabbros and other non-cumulate and ferrogabbro. The Ritaselkä ridge is seen in the gabbros which locally contain pockets and strings west. of chromite. Autoliths of microgabbro occur above Walk to the UC outcrop. East of the track is a the UC. A packet of UC and microgabbros is boulder reef of the glacial lake. sandwiched between layers of mottled anorthosite. Lunch. A few meters above the UC is a coarse feldspathic The UC sequence is well exposed in shallow crescumulate; zircon from this rock yielded a U-Pb trenches and outcrops. We approach the UC age of 2.43 Ga. descending in the stratigraphical sense. Well The boundary with the underlying homogeneous above the UC, thin plagioclasic layers occur in MZ gabbro is sharp. Three samples from this coarse uralite gabbro. Where the trench widens to gabbro contained 50 ppm Cr each. The MZ a cleaned outcrop (Fig. 16) a braided fault system gabbros continue northwards (= down) for ca 150 dissects the UC sequence. The faults here are m. At the end of the trench, stratigraphically ca 80 left-lateral (with only one exception); actually, of m below the UC, rocks of an anorthositic sequence course, the faults are nearly vertical (with gentle are exposed. Stop 5. SOMPIOVAARA. BASE OF THE MAIN ZONE The stop is on the northeastern flank of the 26, Cl 263, Cr 3300, Cu 0, Ni 647, S 100, Sr 53, Sompiovaara hill. The rocks of the basal MZ V, 84, Zn 44, Zr 36, Pd < 1 ppb, Au < 1 ppb. The run across the hill. On the roadside we see local rock is olivine(-chromite) cumulate, possibly with boulders of altered poikilitic olivine pyroxenite. a small amount of original cumulus augite. The This unit overlies the Uppermost Lower Chromitite black spots are amphibolized augite oikocrysts. (ULC) layer. A typical olivine pyroxenite (DDH In an outcrop 100 m southeast the contact of R333/16.00-17.00 m) contains (in wt%): SiO2 gabbro and metapyroxenite (pyroxene cumulate) 41.33, TiO2 0.22, Al2O3 5.33, FeO(tot) 10.45, will be examined. These occur above the olivine MnO 0.20, MgO 27.51, CaO 4.26, Na2O 0.00, pyroxenite. The rocks are strongly sheared and K2O 0.012, P2O5 0.044; trace elements (ppm): Ba altered. 106 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen Stop 6. UTSJOKI. ULC CHROMITITE Boulders containing chromitite seams were found uppermost LZ. The thickest chromitite seam (5 in situ in 1991. Exposed in roadside trenches are, cm) lies between a layer of mottled anorthosite from southeast to northwest: Strongly altered (below) and pyroxenite. Thin seams and strings of olivine pyroxenite, basal unit of the MZ (the same chromite occur in plagioclase-rich cumulates. as at Stop 5); ULC chromitite and underlying Drive to Savukoski. gabbros, with chromitites (MC layers) of the Stop Kaikkivaltiaanlehto, en route to Keivitsa This is an unexposed mafic layered intrusion gabbros underlying the granophyre typically (location, Fig. 1). There are no age determinations, contain (in wt%): SiO2 44.4, TiO2 2.59, Al2O3 yet. Stratigraphically it is located below the 12.1, MgO 5.64, FeO(tot) 20.6, Cl 0.491, S 0.346; 2.15 Ga-old Särkivaara intrusion (next stop at trace elements (ppm): V 559 (max. 800), Cu Rantavaara). The intrusion has been intersected 231 (max. 538). About 175 m below the base of by diamond core drilling in a profile across a the granophyre, in a coarse-grained anorthosite magnetic anomaly. The intrusion has a gentle dip gabbro unit is a 20 m-thick layer with anomalous to the north; its thickness in the drilling profile is PGE-Au concentrations. There is no apparent 400 500 m. The roof rocks are arkosic quartzites, correlation between sulphides and PGE-Au (Fig. with micaceous quartzitic interlayers; the floor 70a). Lower down, 50 m above the base contact of rocks are aluminous pelitic metahornfelses. The the intrusion is another layer with higher PGE-Au uppermost unit of the intrusion is granophyre with (Fig. 70b). With a thickness of 8 m, the PGE-Au a thickness of 7 m. A typical granophyre contains content in this layer is about the same order as in (in wt%) SiO2 64.7, FeO(tot) 6.37, MgO 2.04, the Merensky Reef. The PGE-Au(CN) patterns of CaO 4.83, Na2O 3.26, K2O 2.90. the two layers (Fig. 71) have steep positive slopes. The mafic succession shows a pronounced iron- The lower layer is more enriched in Pt-Pd and enrichment trend, with significant enrichment in (relatively) depleted in Au. chlorine (with up to 6800 ppm Cl). The magnetite Stop Rantavaara, en route to Keivitsa The outcrop at Rantavaara (location, Fig. 1) is part basal fine-grained chilled gabbros to plagioclase of the Särkivaara intrusion (U-Pb zircon age 2.15 peridotite, to peridotite and back to plagioclase Ga; Räsänen & Huhma, 2001). The intrusion winds peridotite and olivine gabbro. There is a thin for 30 km or more in a general east-west direction. layer with primary sulphides 50 m above the base The dip is about 45º north, the thickness up to contact. There is no increase of PGE-Au in this about 1 km. The lower part has been intersected by layer, or anywhere in the intrusion. diamond core drilling at Rantavaara; another drill A detail, Fig. 72c, is drawn to illuminate the hole at Metsävaara, north of Rantavaara, started in effects of selective diffusion (Watson, 1982) the overlying pelites and black schists, intersected across the basal contact. The basal part of the the roof contact and reached down to the middle intrusion above the chilled margin displays levels of the intrusion. pronounced enrichment of K, P and Cl, while the Beautiful cm-scale layering in the basal pelites immediately below the contact are strongly ultramafic cumulates are seen in the outcrop. Figs. enriched in Na. 72a-b show the reverse fractionation from the FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 107 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... KAIKKIVALTIAANLEHTO INTRUSION DDH R336 & R342 1% 0.6 0.2 R336 Pt+Pd+Au, ppb S Cu, ppm 300 100 10 20 30 40 ppb m m 200 200 a Magnetite Cu gabbro Uralite gabbro Albitite S Coarse anorthosite gabbro Magnetite gabbro 250 250 By GFAAS Pt/Pd, by GFAAS : 1,061 Pt/Pd, by NiS - ICPM-MS: 0,878 S, % 0.8 0.2 R342 PGE+Au, ppb Cu, ppm 400500 100m 100 200 250 100 100 b S Uralite gabbro Quartz gabbro Cu + magnetite Pyroxene gabbro + magnetite Uralite gabbro ± magnetite 140 140 Pt/Pd, by GFAAS : 0,583 Pt/Pd, by NiS - ICPM-MS: 0,854 172 m base of the intrusion Pt Pd Au Sum PGE+Au by NiS-ICPM-MS Fig. 70. Kaikkivaltiaanlehto intrusion. PGE-enriched layers in the upper part (DDH R336) and lower part (DDH R342) of the intrusion. 108 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen KAIKKIVALTIAANLEHTO INTRUSION 0,1 Lower PGE 0,01 Upper PGE dl. 0,001 0,0001 Fig. 71. Kaikkivaltiaanlehto intrusion. PGE(CN) diagrams for the PGE-enriched layers in the upper and lower parts of the intrusion.Os Ir Ru Rh Pt Pd Au FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 109 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... base 3 900 2 5 4 700 2 5 PO 3 1 2 2 KO Na O 500 2 2 2 5 K O, % Na O, % 1 2 P O , ppm 160,25 m m 110 120 140 130 150 100 depth DDH R339 20 1000 2 2 2 5 Na O, K O, P O , Cl, Zr RANTAVAARA INTRUSION Zr 60 3000 , MgO, Cr, V, Ni; b Cu and S; c detail c S; and Cu b Ni; V, Cr, MgO, , 2 , TiO , 100 5000 2 Cl base 8470 7000 C Zr, ppm Cl, ppm 1% 0.4% 3000 2000 1000 S 100 150 50 S, ppm Cu, S m DDH R339 depth Cu 100 200 RANTAVAARA INTRUSION 300 B 400 base 500 Cu, ppm Primary sulphides 2 2 TiO , % MgO, % SiO , % , Cl and Zr. 5 22 60 1.3 O 2 55 18 MgO 1.0 O, P 2 14 50 PERIDOTITE CHILL 2 2 45 TiO O, Na SiO 2 40 0.5 2 150 50 m DDH R339 100 depth 200 6 10 100 2 SiO , MgO, TiO , Cr, Ni, V V 200 RANTAVAARA INTRUSION Ni 300 300 OLIVINE GABBRO DIABASE DYKE 700 600 400 200 PLAGIOCLASE PERIDOTITE 400 Cr pelitic metahornfels base 400 A Ni, ppm V, ppm Cr, ppm Fig. 72. Rantavaara, Särkivaara intrusion. Compositional variations in a DDH (R339) crossing the lower part of the intrusion; a SiO a intrusion; the of part lower the crossing (R339) DDH a in variations Compositional intrusion. Särkivaara Rantavaara, 72. Fig. of the DDH, showing variations K 110 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen Keivitsa Stops 1-9, see Fig. 33 Stop 1. HANHILEHTO HILL. GABBRO AND ROOF HORNFELSES Strongly altered rocks of the gabbro zone of the are pelitic and black schist metahornfelses, the Keivitsa intrusion are visible on top of the low local roof rocks of the intrusion. Southwards hill of Hanhilehto. The gabbro is medium grained; (away from the roof contacts) the rocks become crescumulate textures are occasionally seen. more schistose. The dip of the layering is ca 15º Exposed in the roadside ditch south of the gabbros south. Stop 2. OLIVINE PYROXENITE NEAR THE BASE OF THE KEIVITSA INTRUSION Roadside outcrop. A shallow vertical DDH hornblende, big grains of chlorapatite (cumulus was drilled at the site. The rock is a very fresh ?) and zircon. The zircon is very rich in U (up to olivine pyroxenite, with cumulus olivine, augite, 5300 ppm). Clouds of disseminated sulphides and orthopyroxene and Cr-magnetite, and intercumulus sulphide-mantled euhedral orthopyroxene crystals plagioclase, primary biotite, primary brown are seen. Stop 3. KEIVITSANSARVI Cu-Ni-PGE-Au DEPOSIT 3a. The discovery hole (HHD R326) that the top show the ice movement from the NNW. intersected the Upper Ore in 1987. The next hole Walk across the barren window and the zone of (R327) 100 m east, intersected good Upper Ore transitional and Ni-PGE ore types (not exposed). from the beginning. The Upper Ore includes many 3d. The small test pit. Oxidized subcrop of the ore types (false ores, transitional and Ni-PGE type Main Ore is covered by a till layer 0.3 to 1.4 m ores), and thick false ores occur above and below thick. A knob (tor) of pervasively amphibolized the Upper Ore (see Fig. 48). Typical Upper Ore is olivine pyroxenite is seen. An inclined DDH due of the regular ore type, with concentrations (wt%): east from the western rim of the pit intersected Ni 0.36, Cu 0.47, Co 0.02; PGE-Au (ppb): Pt 760, good ore to the end (151 m). Pd 450, Au 220, Os 10, Ir 16, Ru 20, Rh 15. 11e. Walk south along a long trench across the 3b. A trench excavated by Outokumpu Co. in Main Ore subcrop and a parallel DDH profile. The 1969. This, and all other trenches were filled and moussé layer is traceable for hundreds of meters in the pits landscaped in 1995. The rock is pitted DDHs. At the moussé subcrop graphite xenoliths peridotite (olivine pyroxenite). A vein of massive were found. sulphide was found in the trench; this fully 11f. Tors of partially amphibolized olivine explained the electromagnetic anomaly. A shallow pyroxenite, about 50 m above the Main Ore. vertical DDH a few meters east of the southern end Turning west we walk over the Ni-PGE type ore of the trench intersected Upper Ore from 16.7 to (not exposed). 40 m. 11g. False ore outcrop. This lens (layer) of false 3c. The big test pit, oxidized subcrop of the ore lies 140 m above the Upper Ore layer. A typical Upper Ore. The ore grades eastwards into barren DDH assay of unoxidized false ore shows 2.43 % olivine pyroxenite. In the southern wall of the pit S, 233 ppm Ni, 358 ppm Cu, 135 ppm Co, 10 ppb is a steep-sided tor that consists of pervasively am- Au, 3 ppb Pd; Ni/Co is 1.73, Ni(100S) is only 0.35 phibolized olivine pyroxenite. Glacial striations on %. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 111 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... Stop 4. KEIVITSANSARVI. PITTED PERIDOTITE No hammers! primary biotite and disseminated sulphides are Picturesque outcrops in the southeastern part deeply weathered; the orthopyroxene content is of the intrusion. The rock is a heterogeneous higher than in the knobs, which lack biotite and feldspathic olivine websterite. Parts containing sulphides. Stop 5. KEIVITSA. THE BIG SERPENTINITE-DUNITE XENOLITH Northwest of the Keivitsa hill serpentinite (plagioclase, augite, inverted pigeonite, fayalitic (metadunite) is exposed in a small outcrop, olivine) is exposed 300 m southwest of this located in the eastern part of a big serpentinite outcrop. The MgO in serpentinite (recalculated to xenolith. Stratigraphically, the xenolith lies above 100 %) is up to 46 %. the ultramafic zone. An outcrop of ferrogabbro Stop 6. KEIVITSA HILL. PELITIC METAHORNFELS AND GABBRO Pelitic metahornfelses are well exposed in hilltop terrace, but it is not exposed here. At the foot of the outcrops. The hilltop vantage point offers a hill we cross a talus heap of gabbro. The gabbro panorama of Central Lapland, with geological zone is rather thin here. Komatiite peridotite was landmarks. Descending due north, the roof contact intersected in a DDH on the northeastern slope, of the Keivitsa intrusion is located just below below the eagle´s nest. the hilltop. Granophyre occupies the uppermost Stop 7. LAKE ISO VAISKONLAMPI. CALCAREOUS HORNFELSES AND A KOMATIITE SILL Outcrops of metasedimentary rocks baked between the upper sill are exposed above hornfelses. The two differentiated komatiite sills. Hornfelses of composition of this peridotite is identical to that of impure calcareous sediments are exposed in steep the spotted komatiitic peridotite at the next stop. lakeside cliffs. Basal peridotitic cumulates of Stop 8. PIKKU VAISKONSELKÄ. SPOTTED KOMATIITIC PERIDOTITE Peridotite, with characteristic spots, of the lower mostly altered to amphibole, enclose clusters of komatiite sill are seen alongside the road. Similar olivine crystals. It seems that the spots have been spotted komatiites occur all around the western half formed by the thermometamorphic effect of the of the Koitelainen intrusion, immediately above Koitelainen intrusion. The rock contains: 34.07 % the granophyre cap. Spots of poikilitic augite, MgO, 5940 ppm Cr and 1210 ppm Ni. 112 FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b Tapani Mutanen Stop 9. ISO VAISKONSELKÄ. GRANOPHYRE OF THE KOITELAINEN INTRUSION The road going north crosses a broad granophyre. are outcrops of altered magnetite gabbro, where To the north is a layer of strongly sheared and magnetite has been altered to hydrous mafic altered anorthosite between the granophyre and silicates. the magnetite gabbro below. Further to the north FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 113 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... LITERATURE ON AKANVAARA Franssila, L. 2001. Lapin kromiitin pelkistyskäyttäytyminen 400 000 selitys. Summary: Quaternary deposits of Northern (The reduction behaviour of Lapland chromite). Oulun Finland – Explanation to the maps of Quaternary deposits 1: yliopisto, Prosessi- ja ympäristötekniikan osasto. 51 p. 400 000, Espoo. Geological Survey of Finland, 127-142. Gornostayev, S. 2003. Mineralogical research on chromitites of Juopperi, H. 1986. Savukoski. Geological Map of Finland the Akanvaara Deposit and products of their processing with 1:100 000, Pre-Quaternary Rocks, Sheet 3723 + 4711. emphasis on PGM/PGE. University of Oulu, Department of Geological Survey of Finland. Process and Environmental Engineering. 30 p. Köykkä, M. 2001. 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Geological Association of Turku, Department of Geology. 98 p. Canada, Mineralogical Association of Canada & Society Mutanen, T. 1995. Mafic layered intrusions of Finnish of Economic Geologist joint annual meeting, Vancouver, Lapland. In: Petrology and metallogeny of volcanic B.C., Canada, May 25-28, 2003. GAC-MAC program with and intrusive rocks of the Midcontinent Rift System: abstracts 28, 1 p. proceedings of the international field conference and Gornostayev, S., Hiltunen, R., Mutanen, T. & Härkki, symposium, Duluth, Minnesota, August 19 – September 1, J. 2003b. Mineralogical research on chromitites of the 1995, Duluth, Minnesota, 135-136. Akanvaara deposit, northern Finland and products of their Mutanen, T. 1996. The Akanvaara and Koitelainen intrusions reduction. In: Applied Mineralogy `03, Helsinki, Finland, and the Keivitsa-Satovaara complex, northern Finland: March 17-18, 2003, 2 p. Guide to the pre-symposium field trip in Finland, August Grönlund, T. 2005. 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Zeist: European Association of Exploration Geophysicists Häkli, T. A. 1971. Silicate nickel and its application to the (EAEG), 2 p. exploration of nickel ores. Bulletin of the Geological Kojonen, K. , Johanson, B. & Pakkanen, L. 1997. Tuloksia Society of Finland 43, 247-263. mikroanalysaattorilla tehdyistä hivenaineanalyyseistä Hanski, E., Grinenko, L. N. & Mutanen, T. 1996. Sulfur Keivitsan ja Kumisevan PGE-metallien suhteen isotopes in the Keivitsa Cu-Ni-bearing intrusion and its anomaalisista malmiaiheista. Geological Survey of Finland, country rocks, northern Finland: evidence for crustal unpublished report M42.4/2343, 3714/-97/1, 4 p. sulfur contamination. In: IGCP Project 336 Symposium in Kojonen, K., Johanson, B. & Gervilla, F. 1998. Electron Rovaniemi, Finland, August 21-23, Program and abstracts. microprobe analysis of trace Pt and Pd: examples of Turun yliopiston geologian ja mineralogian osaston arsenides and sulfarsenides. 8th International Platinum julkaisuja 38, 15-16. Symposium, 28 June – 3 July; Abstracts. The Geological Hintikka, V., Ingerttilä, K., Laukkanen, J. & Leppinen, Society of South Africa and the South African Institute J. 1994. Keivitsan malminäytteiden lisärikastuskokeet of Mining and Metallurgy, Symposium series. The South ja alustavat liuotuskokeet. Technical Research Centre African Institute of Mining and Metallurgy, 175-178. of Finland (VTT), Laboratory of Mineral Processing. Kojonen, K., Pakkanen, L. & Johanson, B. 1996. Tuloksia Rep.KET4010/94. 38 p. mikroanalysaattorilla tehdyistä automaattisista TURBO Hirvas, H., Saarnisto, M., Hakala, M., Huhta, P., Johanson, SCAN-hauista eräistä suomalaisista platinaryhmän P. & Pulkkinen, E. 1994. Maaperän kerrosjärjestys ja metallien suhteen anomaalisista malmiaiheista. Geological geokemia Keivitsassa. Geological Survey of Finland, Survey of Finland, unpublished report M42.4/2343, 3112, unpublished report. P23.4.014. 44 p. 3142, 3714/-96/1. 4 p. Huhma, H., Mutanen, T., Hanski, E. & Walker, R. J. 1995b. Kojonen, K., Välimaa, J., Gervilla, F. & Parkkinen, J. 2004. Sm-Nd, U-Pb and Re-Os isotopic study of the Keivitsa Cu- Platinum-group element mineralization pipes of the early Ni-bearing intrusion, northern Finland. In: Kohonen, T. & Proterozoic Keivitsa mafic-ultramafic intrusion, Sodankylä, Lindberg, B. (eds.) The 22nd Nordic Geological Winter northern Finland. 32nd International Geological Congress, Meeting, 8-11 January 1996 in Turku – Åbo, Finland. Florence, Italy, August 20-28, 2004, abstracts, 1267-1268. Abstracts of oral and poster presentations. Turku – Åbo, Manninen, T., Hyvönen, E., Johansson, P., Kontio, M., Turun yliopisto, Åbo Akademi, 72. Pänttäjä, M. & Väisänen, U. 1995. The geology of the Huhma, H., Mutanen, T., Hanski, E., Räsänen, J., Keivitsa area, Central Finnish Lapland. In: The 22nd Nordic Manninen, T., Lehtonen, M., Rastas, P. & Juopperi, H. Geological Winter Meeting, Turku, Finland, January 8-11, 1996. Sm-Nd isotopic evidence for contrasting sources of 1996. Abstracts of oral and poster presentations. Turku, prolonged Palaeoproterozoic mafic-ultramafic magmatism Turun yliopisto, Åbo Akademi. FIELD TRIP GUIDEBOOK, Geological Survey of Finland, Guide 51b 115 The Akanvaara intrusion and Keivitsa – Satovaara complex, with stops at Kaikkivaltiaanlehto and ... Manninen, T., Hyvönen, E., Johansson, P., Kontio, M., Mutanen, T., Törnroos, R. & Johanson, B. 1988. The Pänttäjä, M. & Väisänen, U. 1996. Keivitsan alueen significance of cumulus chlorapatite and high-temperature geologiaa. Geological Survey of Finland, unpublished dashkesanite to the genesis of PGE mineralization in the report K/21.42/96/1, 28 p., 27 app. pages. Koitelainen and Keivitsa-Satovaara complexes, northern Mutanen, T. 1989. Koitelainen intrusion and Keivitsa- Finland. In: Prichard, H.M., Potts, P.J., Bowles, J.F.W. & Satovaara complex. Excursion guide, 5th International Cribb, S.J. (eds.) Geo-Platinum 87. 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