Geology and mineralogy of the Sarfartôq carbonatite complex, southern West Greenland

Geology and mineralogy of the Sarfart6q carbonatite complex, southern West Greenland

KARSTEN SECHER & LOTTE MELCHIOR LARSEN

LITHOS Seeher, K. & Larsen, L. M. 1980: Geology and mineralogy of the Sarfart6q earbonatite complex, southern West Greenland. Lithos 13, 199-212. Oflo. ISSN 0)24-4937.

The Sarfart6q carbonatite complex was emplaeed in lower Palaeozoic time in a ~eakness zone within the Precambrian shield. Dolomitic magma int.ntded in two major stages ofactiwty. In the first stage a steeply dipping conical body of concentric sheets of rauhaugite was formect, while in the second stage several hatches of magma were emplaeed into the surrounding marginal ghock-zone as concentric and radial beforsite dykes and agglomerates. Hydrothermal activity gave rise to several phases of mineralisntion in veins and shear zones. The accompanying ~renitisation was of the Na-type. The whole complex covers about 90 km 2. The main rack-forming minerals are dolomite-ankerite, apatite, orange reversely pleoehroie phlogopite, richterite-arfvedsonite and magnetite. Important accessories are pyrochlore, zircon and niobian futile. A complete minerai list is given, together with microprobe data on mineral chemistry. The dolomitie magmas were poor in SiO2, AI203 and K20 in relation to other earbonatites. Nb, U and LREEs are strongly enriched in pyrochlore- mineralised zones where the Nb content may be up to 40~. Some shear zones are strongly enriched in Th and HREEs (specifically Eu) and lesser Pb and Zn. Niobium, uranium, rare earth elements and phosphorus occur in economically interesting concentrations.

Karsten Secher and Lotte Melchior Larsen, 6ronlands Geologiske Undersogeise, Oster Voldgade 10, DK-1350 Kobenham K, Denmark.

In recent years it ha,,; been realised that the Pre- cambrian basement rocks in southern West Green- land from Frederiksh~b to Egedesminde are cut by strongly alkaline rocks of such amount and extent that they define an alkaline province. At the moment this province is known to include numerous widespread kimberlitic ~nd lamprophyric dykes, two large carbonafite complexes and one smaller alkaline cornplex. The spatial extent of the province is not well-established, but activity seems to be

• concentrated in the Sukkertoppen-Helsteinsborg area. Radiometric ages from this area are ti~w; dyke ages are: 12.06+_18 m.y. (Brooks et al. 1978); 5844-18 m.y. (Bridgwater 1971) and 570 m.y. (Scott 1977). Fission track dating of apatites from the two earbonatite complexes ha- yielded minimum ages of arotmd 500 m.y. (A. J. Gloadow, pars. comm. 1978). They can thus be regarded as part of the widespread North Atlantic earbonatitic activity 500-600 m.y. ago (Doig 1970; Vartiainen & Woolley 1974).

The kimberlitic and lamprophyrie dykes have been described by several authors, as reviewed by Andrews & Emeleus (1976). The Qaqarssuk carbonatite complex (F~g. 1) was found in 19o5 by Kryolitselskabet Oresund and is described by Gothenborg & Pedersen (1975). The Sarfa~-t6q carbonatite complex (Fig. 1) was found by airborne

radiometrie survey 1975-76 by the Geological Survey of Greenland (sceher 1976; Secher& Larsen 1978). The two complexes are approx. 125 km apart. This paper reports the general geology, mineralogy and geochemistry of the Sarfart6q complex as presently known, realizing that several specific subjects need future detailed treatment:

Geology of the complex General

The Sarfart6q carbonatite complex is situat~L at 66°30'N, 51°15'W, in the transition zone between the Archaean grar,lulite facies gneiss complex to the south and the Pro terozoic Nagssugtoqidian mobile belt to the north 01ridgwater et al. 19761. Strike and dip of the Archaean gneisses are qu~te variable, while the orientation of the Nagssugtoqid~'n gneisses i:; parallel to the NE-SW strike of the transition zone, with gentle NW dips (Fig. 2). The gneisses are granodioritic with scme amphibolite layers and contain a swarm of abundant basic dykes (Kang~rfiut dykes, Bridgwater et ~.1. 1975).

The carbonatite complex comprises a core a~rea (15 km 2) mantled by a margina~ zone of gnei~;ses with earbonatite dykes (75 kmZ), see Fig. 2. Oa~ a

200 K. Secher & L. M. Larsen LITHOS ~3 (1980)

/;ig. 1. The location of the Sarfitrt6q and Qaqars.,uk car- bonattte complexes in southern ~ est Greenland.

large scale the core outline is discordant ~.o the gneisses, but in a 50-200 m wide zone adjacent to the core the foliation of the gneh:ses is turned to concordance with the core outline. The carbonatite rocks of the core ~ c u r as concentric sheets con- cordant with the core outline, defining the core as a NE-SW flattened conical body with a steeply N W~dipping central axis. It tbus seems to possess a very close structural dependence on the Nagssug- toqidian transition zone.

Aeromagnetic survey (L. Thorning, pers. comm.) has revealed a slight magnetic minimum ( - 200 ~,) at the NW side of the core, probably indicating the extension below surface of the core rocks. A rather strong magnetic max~num (+ 800 ?) is situ~tted at the northern rim of the marginal zone. This ~u ld indicate that associated intrusions of silicate rocks, e.g. ijolites and pyroxenite~, may be present at depth. At the surface, associated intrusions of silicate magmas are represented by thin lamprophyric and kimberlitic dykes occurring mainly

outside the ebmplex, and forming extensive oon- centric cone sheets dipping 30-50 ° towards the center (Larsen 1980).

The nomenclature used here is based on Bragger (1921) supplemented with the term beforsite (Ecker- mann 1948) for dolomitic earbonatite dyke rocks, which in Sarfart6q are easily distinguished from the main carbonatites.

The core

The core can be divided into three zones based on the proportion of carbonatite to fenite. The inner core (> 50% carbonatite) ~s only approx. 1 km z in area, the outer core (< 50~/~ carbonatite) forms a 1-3 krn broad ring, occupying around 9 kmL This is surrounded by a narrow rim of fenite (approx. 5 km 2, Fig. 2). The transition to the marginal zone is most often gradual over 50-21)0 m.

The 0.5-20 m wide concentric carbonatite sheets are strongly foliated or layered. In the outer part of the core they locally show folding, sometimes develo~,ing small-scale crenulation foliation. The interlayered fenites are without preferred orientation, bt~t toward the core rim the fenites gradua.~ly become foliated in a way resembling rheomorphic deformation. Also, signs of brittle deformation are numerous, normally as scattered joints and fractures, but sometimes even as breceiation of the carbonatites, still with carbonatite matrix.

The predominating carbonatite type in Ihe core is rauhaugite. Sovite occurs only sporadically in schlieren. The foliation and layering is acxentuated by the streaky occurrence of dark minerals such as phlogopite and magnetite:, sometimes concentrating in lenses ofglimmerite and bands in which magnetite constitutes up to 80 per cent of the rock volume.

The fe.nites are rather heterogeneous rocks where gneiss and basic dyke relies are still discernible. The most frequently found fenite is a light grey- coloured aegirine-bearing type, while more marie fenites occur as irregular patches in the fenitised areas, as a rule associated with pyrochlore concentrations.

The marginal zone

This zone consists mainly of gneiss es, frequently altered due to hydrotherm~ l activity and cataclastic deformation of varying intensity. The overall colour of this zone is red-brown due to widespread hematifisation of the gneis~;es. Locally large radio- ~ctive shear zones, 50-200 m wide, are found orientated tangentially to the core. They comprise strongly limonitised, hemat~fised,jointe,~ and crush-

LITHOS 13 (1980)

SARFART60 - Preliminary geologic~ mop ,~, / / " h

f

/ / /

.¢

..¢-~.~ i ~

s . s "¢~ ~.~

A \ i t _ . - /

/+ " '111/47.- 2;" \ ",,,,..~ % i s "

,y" \\/ ...--.~a \ I I \\

d N

i~r~ic~al xone

~ F e n i t e

~ Cofbonotito, outer core

¢orbon,~tito+ inner core

~ KdngamJu t dyke

. . . . . Mojor joint

. . . . Bou .n~, ory 9! Nagss~toqidian q11Oelle o~fll[

L - - Tfendlino, strike ~, dip

Fig. 2. Preliminary pologiml map of the Sarfart¢3¢l ¢axbonatite complex. The ~mp f~ based on ~ mal[~h~t'in the field, supplemented with 8eophysic~ data and photogeologi~! ob~ervatk~as made on a ¢omlmter-equipped Kern PG-2 photogrammetric instrument (Secher 1979).

Table !, Sequence of events in the Sarfart6q carbonatite complex. Age decreases upward3 in table.

Magmatic activity Associated rnetasomatic and hydrothermal activity

Veins with Calcite-black quartz-purple fluorite and Calcite-~aryte-yellow fluorite

Tangential beforsite dykes (margin) Radial beforsite dykes, breccias and agglomerates. (margin)

Tangential beforsRe dykes (margin)

Reactivation of radioactive shear z o n e s

Calcte-siderRe-K-feldspar veins and Calcite-dolomite-pyrite veins and Radioactive shear zones (margin) and Pyrochlore zones (margin)

Unplaceable: Lamprophyre ,dykes

Closely spaced concentric sheets Na-fenitis-'~tion of country rocks of rauhaugite (core)

S~vite lenses in rauhaugite

Country rocks: Granodioritic gneis~es, Kang~miut dykes

202 K. Secher & L. M. Larsen 1i.ITHOS 13 (1980)

Table 2. Minerals found in the Sarfart6q carbonatite complex.

0 Abundant Core Marginal + Widegpread accessory zone zone

Locally concentrated (3 Rare

Late mineralised veins

ELEMENTS Graphite

SULPHIDES Pyrite Marcasite Sphalerite Galena

OXIDES Magnetite llmenite Hematite Nb-rutile Pyrochlore

HYDROXIDES Limonite

HALIDES Fluorite

CARBONATES Calcite Dolomite-ankerite Siderite Monohydrocalcite

SULFHATES Baryte Hzxahydrite Gypsum

PHOSPHATES Apalite Uniden|ified

SILICATES Zircon Alkali pyroxene Alkali amphibole Phlogopite-biotite Chlorite Taeniolite? Quartz ,Chalcedony K-feldspar Albite Zeolite

O

(3 O (3 © (3

0 + + 0 0 @ 0 +

Q

0 ® @

0

• Q • • •

0

-t-

• + O

+

@ @ + O +

+

O @

O @

O O

+ O

Surface coatings

@

@

@ O

ed gneisses as well as carbonate material. Narrow I 5 m wide pyrochlore-rich zones are found, also ~triking tangentially tc. the core. They are almost manomineralic and consist of weakly foliated pyrochlore, suggesting a completely recrystallised, pyrochlore-mineralised ultracataclastic rock.

The marginal zone is penetrated by discordant 5 -100 cm wide carbanatite dykes, various types of carbonatite breccias and agglomerates, lampro-

phyre dykes and carbonate veins. The majority of the dykes are structurally groupad in two patterns, either radiating fi'om the cevtre wi~,h steep dips or arranged tangentially with vat t ing ciips. The radiating dykes are predominantly b~reo:ias and agglomerates, while most of the tangential dykes are homogeneous carbonatite ('fable 1';.

Beforsite, with or without flow ~;ttucture, is the typical carbonatite dyke reck. Breccias and agglo-

LITHOS 13 (1980) Sarfart6q carbonmfte complex 203

merates have a similar composition and enclose fragments of all other rock types in the area, the majority of the fragments being gneisses. Size and shape of the fragmenLs varies, even ~Lhin the same sample, from small to large, and from rounded to angular, suggesting a veryviolent kind of emplace, ment probably involving fluidisation of the dyke magma. Only very few lamprophyre dykes occur inside the complex; they are always strongly altered. Some contain rounded ultramafic inclusions.

The dykes occupy less than 1% of the rock vohwn¢ in the marginal zone, and their abundance decreases outwards from the core. Only a few dyke?, cut the core. Carbonate veins 0.5-10 cm wide are observed throughout the complex (including the core) in at least four generations, often pegmatitic and showing conspicuous zoning. Calcite-fluorite veins are the youngest expression of activity in the complex.

Relative age relations as indicated by fieRd evidence are apparent from Table 1.

Mineralogy A list of minerals identified to date in the Saffart6q complex is presenteA in Table 2. The rauhaugites of the core consist predominantly of dolomite with subordinate amoumts of apatite, phlogop:,te, magnetite arid ric~,terite-arfvedsonite, and accessory zircon, ilmenite, pyrochlore and pyrite. This relatively simple mineralogy contrasts with that of the marginal zone beforsi~es, which in addition to these minerals contain K-feldspar, chlorite, quartz, baryte, pyrite, hematite and Nb-rutile. The fenites contain alkali t~ldspar, aegiri~te, biotite and amphibole together with minor calcite and pyrochlore, '.:~ut no feldspathoids. Widespread surface coatings of a white material consist of secondary mono- hydrocaleite and hexahydrite. Gypsum has been found as small n~mdle-sha~'d crystals in cawt:es.

Table3. l*,qicroprobe analy.,es ¢f carbonates from Sarfart6q.

1 2 3 4 5

Fen 0.00 !.97 4.29 10.27 12.68 MnO 0.00 0.22 0,.37 0.4! 0.34 MgO 0.60 1 9 . 2 7 18.40 1 4 . 6 1 13.39 Can 53.08 30.16 28.99 28.32 27.83 SrO 0.35 0.20 0.35 0.I0 0.10 sum 54.03 51.82 52.39 53,61 54.34 CO, (calc.) 42.46 46.13 45.83 44.76 44.48 sum 96.60 97.95 98.55 98,37 98.82

Ca 98.4 51.5 50.0 50.0 49.4 Mg 1.6 45.9 44,2 35,9 33.0 Fe 0.0 2.6 5.8 14.1 17.6

(1-2) Coexisting calcite and dolomite from core rauhaugite GGU 225265; (3) Dolomite (only carbonate) from core rauhaugite GGU 225264; (4) Ankerite from marginal dyke beforsite GGU 225214; (5) At~kerite from marginal dyke beforsite GGU 223876.

analysed minerals while interesting mineral groups like pyrochlore, sulphides and phosphates await a more detailed treatment.

Carbonate: Microprobe analyses are shown in Table 3 and Fig. 3. The carbonates were analysed with the electron beam defocussed to approx. 20 microns" diameter. This, in conjunction with tbe low beam current (5 nA), allowed the carbonates to be stable under the electron beam for several minutes. The analytical results for the dolomites fall exactly in the rather narrow dolomite-ankerite field oatlined by Rosenberg (1967) and Goldsmith & Newton (1969). The carbonates of the rauhaugites are magnesio-dolomi|es with < 10% FeCO3 component, while those of the beforsites are more iron-rich, with up to 18% FeCO3. These straddle the compositional limit at 10% FeCOa between dolomite and ankerite (Deer et al. 1962). The most iron-rich compositions are found in euhedral crystals projecting into late quartz+calcite-filled ~ugs. The compositional interval can possibly be tmlarged b~/analysing more material.

The strontium contents ~tary from 0.2-0.5% SrO in dolomite from the c'~ve zone, to <0.1% SrO in dolomite- ankerite from the marginat zone. Coexisting dolomile and calcite have partition coefficients SrdodSrc,=0.5, in accordance with Jacobson & Usdowski (1976).

Mineral ehemist.~y

Data on ~the chemistry of minerals from Sarfart6q are based on a reconnaissance microprobe study of ten selected samples, four rauhaugites, four beforsite dyke,,~, one fenite and one pyrochlore-aegir~ne rock. The probe was a TPD probe with ener:~- dispersive analytical system, and the procedure u~ed was as described by Reed & Ware (1975). Fully quaxititative detelminations were made of Na, M~:, A1, Si, K, Ca, Ti, Cr, Mn and Fe while other elements were only determined semiquantitatively. This presentation therefore centers on the fully

Mica: Phlogopite is the mos.l commonly occurring silicate mineral in the carbonatites. It: the rauhaugites (and glimmer- ites) of the core the mica is ~usually the tetraferriphiogopite ~ariety with distinct reverse pleochroism (~ = bright orange, [~ = ¥ = pale yellowish), Core-and-rim-zoning may be present, and both 'reverse' cores w~:h "normal' (¢=pale orange, [3=7=greenish orange) rirns and '~ormal' cores with 'reverse' rims have been fom~d. The lass-mentioned texture is similar to textures described from other cz~rbonatites, kimberlites and lamprophyres by e.g. Nash t1972), gcntt (1977) and Vartiainen et ai. (~978).

In the fenites and in the beforsite dykes the micas are variable and range from brown titaniferous biotite in b.a~ement inclusions through brownish and greenish types wi~:h normal pleochroism in fenites to yellow reverscly- ph:ochroic crystals in relatively unmodified beforsite.

204 K. Secher & L. M. Larsen LITHOS 13 (1980)

F¢

1 / Ca ~g

Co ~,- ., -, 10 20 /30 40 50 50 50 50

\ . \ \ -

C a ~ ¢ ' _ ~ ~ . ~ _ _ _ ~ ~ ~ M g I0 50 50 50 50 $0

Fig. 3. Microlarobe analyses of carbonates from Sarfartbq plotted in the CaCO~- MgCO3-FeCO3 (mol%) triangle. Filled circles: rauhaugites; filled squares: beforsites;

crosse.s: pyrochlore-aegirine rock; x'es: fenite.

Microprobe analyses of the micas are shown in Table 4 and Figs. 4 and 5. The reversely pleochroic tetraferri- phlogopites ha~e very low Al-eontents and high cation sums indicating high Fe ~ +-contents. The substitution seems to be AP v Fe a+~v, in accordance with previous invesiiga-

Al/ /A

30 A+A B

iW

/ M I C A S

fenites ere: corbonatite$: I ~ 0 ® Sarfartoq

+ a a ~ r~ ; Hil l

~ -'- & Aln6 x7 Others

~L~ = V ,, Y..=. Mg+Fe 10 2o -e~ Ti .~o

.Fig. 4. Analyses of micas plotted in the 1Vtg+Fe-Ti-AI (atomic,;,) triangle. Open symbols: reversely ph,,,caroic micas; closed symbols: NoramUy pleochroic ~aieas. The two lines denoted A and B are for two different Ti-mica substit~,- tions as discussed by Veide 11975). In this context the lines are mo~t~y drawn fer reference when comparing with analogous diagrams elsewhere. Data for other complexes are from ~ash ~972 (I~'on Hill), v. Eckr:rraann 1974 (Alni~), Faye & Hogarth 1969, and Puustinen ~973.

tions on tetraferriphlogopite by for example Faye & Hogart (1969), Nash (1972) and Puustinen (1973). The. com- parative diagram (Fig. 4) shows that the micas seem to achieve the reverse pleochroism at AI/AI + Fe + Mg ratios below approx. 0.22. Ti is very low in phlogopites from carbonatite (0.0-1.0% TiO2) and somewhat higher (0.5- 4.7% TiO2) in those from fenites. Frola core to margin of each grain Ti increases while the Mg/Mg + Fe ratio decreases

Ti Sar far toq [

0..5 micas

0.4

0.3 x

4- 0.2 + / a / / ~

0.9 0.8 0.7 0.6 Mg/rMgeFe

Fig. 5. Atomic Ti against Mg/Mg+ Fe for analysed micas from Sarfartfq. Open symbols: reversely pleachroic miens; closed symbols: normally pleochroic micas. Circles: rauhaugites; squares: beforsites; crosses: pyrochlore,aeg/riee r(~k; x'es: fenites.

LITHOS 13 (19g0) ~u~fart~qcarbonatitecomplex 205

Table 4. Microprobe analyses of phlogopite and amphibole from Saffart6q.

I 2 3 4 5 6 7 8 9 10 11 ~

SiO2 40.65 39.42 40.54 3 8 . 1 1 39.75 38.18 55.13 55.04 55.10 54.36 54.68 52.~5 Tie2 0.00 0.85 0.00 0.46 1.79 1.27 0.00 0.00 0.00 0.45 0.95 0.4,1 Nb2Os 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 AI~O3 6.46 9.97 9.45 1 2 . 9 2 10.63 13.16 0.00 0.00 0.00 0.38 0.21 1.99 Fe20~ . . . . . . 2.28 6.42 9.92 9.01 5.38 8.0~ F e e 13.56 15.73 5.87 1 1 . 1 3 !!.37 12.15 2.34 2.93 0.60 3.90 8.32 7.07 MnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0. i 6 MgO 22.43 17.28 25.70 1 9 . 7 5 18.94 1 9 . 0 4 21.50 18.80 1 8 . 3 9 15.37 13.44 14.0[ CaO 0.34 0.79 0.38 0.26 0.20 0.1 ~ 6.36 3.34 1.82 0.82 0.10 2.28 Na20 0.00 0,00 0.00 0.56 0.46 0.29 6.17 7.70 8.66 8.35 9.09 8.1 i KzO 9.85 9.99 10.15 9.24 9.72 10.02 1.71 1.98 2.22 3.52 2.55 2.10 sum 93.29 94.03 92.09 92.43 93.06 94.22 95.49 96.21 96.72 96.15 94.72 96.4~ F e e tot. _ . . . . . 4.39 8.71 9.53 12.00 13.16 14.34

Cations: Base: 22 oxygens Base-- 16 cations and 23 oxygens

Si 6.204 6.028 6.032 5.773 5.999 5.719 7.890 7.907 7.868 7.932 8.~26 7.685 Ti 0.000 0.098 0.000 0.052 0.203 0.143 0.000 0.000 0.000 0.049 0.106 0.049 Nb 0.0 0.0 0.0 0.0 0.015 0.0 0.0 0.0 0.0 0.0 0.0 0.0 AI 1.161 1.796 1 . 6 5 6 2.307 1 . 8 9 1 2.324 0.000 0.000 0.000 0.065 0.037 0.345 Fe3+ . . . . . . 0.245 0.694 1 . 0 6 6 0.989 0.601 0.894 Fe 2+ 1.731 2.01 ! 0.731 1.410 1.435 1 . 5 2 2 0.280 0.352 0.072 0.475 1.034 0.870 Mn 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 +3.000 0.020 Mg 5.104 3.938 5.698 4.460 4.260 4.250 4.585 4.025 3.914 3.343 2.977 3.071 Ca 0.056 0.130 0.060 0.042 0.032 0.018 0.975 0.514 0.278 0.128 0.016 0.359 N~ 0.000 0.000 0.000 0.164 0.135 0.084 !.712 2.145 2.398 2.362 2.619 2.313 K 1.918 1.949 1 . 9 2 5 !.787 1 . 8 7 1 L915 0.312 0.363 0.404 0.655 0.483 e.394 sum 16.174 15.950 16.102 15.995 [5.841 15.975 16.000 16.000 16.000 16.000 16.000 16.000 Ca + Na+ K 1.974 2.079 1 . 9 8 5 !.993 2.038 2.017 2.999 3.022 3.080 3.145 3.118 3.066

100 MS/ M~I+Fe 74.7 66.2 88.6 76.0 74.8 73.6 89.7 79.4 77.5 69.5 64.5 63.5

(I) Orange to yellow, reversely pleochroic phlogopite, center of grain. Core rauhaugite GGU 225264, (2) Light brownish, slightly normally pleochroic phlogopite, rim of grain. Core rauhauf, ite GGU 225264; (3) Orange to yellow, reversely pleochroic unzoned phlogopite. Marginal dyke beforsite GGU 223896; (4) Light g~een, normally pleochroie ph|ogopite. Marginal dyke beforsite GGU 223896; (5) Brown, normally pleochroic phlogopite, enclosed in pyrochlore- Pyrochlore- aegirine rock GGU 229146; (6) Brown, normally pleochroic phlognpite. Marie fenite GGU 225236; (7) Richteri~e, ~:olourless center in zoned prismatic crystal. Core rauhaugite GGU 225264; (8) Calcian magnesio-arfvedsonite, faintly btmsh intermediate rim in zoned crystal. Same as previous; (9) Magnesio-affvedsonite, greenish blue outer rim in zoned crystal. Same as previo~s; (10) Potassium magnesio-affvedsonite, very pale lavender-blue fibrous crystals infilling ocellus-like areas. Marginal beforsite dike 223896; (! 1) Magnesio-aff,,edsonite, same appearance and occurrence as previous. Marginal beforsite dyke 225214; (12) Magaesio-affvedsonite, small blue-green crystals together with aegirine (Table 6) in strongly modified basement

inclusion.

(Fig. 5). The uncommon zoning from +reverse" to "normal' in one sample is ~scribed to late-s~ge wall-rock contamination of the ¢arbonatite.

Amphibole: Amphibole is almost as frequently encountered as phlo$opite but is not so conspicuous. In the carbonatites it is pa[e b l u e - ~ n to co':apletely colourless and occurs in slender prismatic crystals often showin$ parallel arrange- ment, or in fibrous aggregates. Analyses are shown in Table 4 and Figs. 6 and 7. It is magnesioarfvedsonite; some larger prismatic grains contain cores o f richteritic composition. The fenites and basement inclusions contain more varied amphiboles, from green hornblende to blue o~r colourless magnesioarfvedsoni~e. Th~ amphibole diagram in Fig. 7 is analogous to the mica diagram in Fill. 5: the+ amphiboles from the carbonatiles have higher Mg/Mg + Fe r~tios and lower Ti contents than those from the fenitv.~.

14 - - Lithos 2/80

Pyroxene: Pyroxene is not ~ound in the carbonatites them- selves but only in fenites and in massive pyrochlore-aegirine rocks, it occurs both as irregular spongy masses and euhedral prismatic crystal~, it is mostly aegirine with Ac%> ~0, but ~alite and aegirine-augite have been found. The ae~irine int.ergrown with pyrochlore contains up to i % Nb2Os (Table 6).

Magne,+iw and ilmenite: Magnetite only occurs sporadically in the marginal zorn. ~, while it is very frequent in the core zone c~rbonatites where the grains often have a cataclastic texture. Concentrations in bands and in clumps of up to ! 5 cm diameter have been noted. The magnetite grains are Pesh, without any martitisation; they usually contain a few e~solved iamellae and blabs, of ilmenitp, especially along the margins. |)menite also occurs as scaltered larger discrete grains +De seemingly primary origin.

206 K. Secher & L. M. Larsen

? Sarfartoq amphiboles KT~ ~ |l

Ca bin

Fig. 6. Microprobe analyses of amphiboles from Sarfart6q plotted in the Ca-Na-K (morale %) triangle. Lines connect points within individual crystals zoned from richterite to magnesioarfvedsonite. Filled circles: rauhaugites; filled squares: beforsites; Yes: fenites.

Ti

0.2

0.1

Sarfartoq amphiboles

0.9 0.8 0,7 M g / M g + Fe

m x

@

I¢

I 0.6

F:g. 7. Atomic Ti against Mg/Mg + Fe for analysed amphibole.; from Sarfart6q. Symbols as in Fig. 6. For equal Ti contents the amphiboles from the fenites have. lower Mg/Mg+ Fe ratios than those from the carbonadtes.

LITHOS 13 (1980)

Table 5. Microprobe analyses of magnetite and ilmenite.

1 2 3 4 5

TiC 2 6.37 1.47 46.16 48.64 49.80 A1203 0.42 0.47 0.00 0.25 0.12 Fe~O3 58.23 66.59 131.21 8.39 6.25 FeO 35.06 3 0 , 6 3 3 9 . 3 9 41.09 37.87 MnO 0.27 0.00 0.75 0.65 1.86 MgO 1.34 0.9I Ct.69 0.81 2.49 CaP 0.18 0.53 0.10 0.43 0.46 sum 101.85 100.72 10~J.30 100.26 98.86 FeO~,t 87.46 90.55 5 1 . 2 8 48.64 43.50

Formulas*

Ti 1.422 0.335 1 . 7 4 9 1 . 8 3 4 i.878 AI 0.147 0.168 0.000 0.015 0.007 Fe 3÷ 13.009 15.163 (~1.501 0.317 0.236 Fe 2 + 8.704 7.752 i .660 !.723 1.589 Mn 0.068 0.000 13..032 0.028 0.079 Mg 0.593 0.410 13.052 0 . 0 6 1 0.186 Ca 0.057 0.172 0 . 0 0 5 0.023 0.025

usp% min. 13.3 0.6 usp% max. ~7.8 4.2 hm% rain. 12.53 7.6 5.7 hm% max. 13.11 7.9 5.9

* Magnetites based on 24 cations and 32 oxygens, and il- menites based on 4 cations and 6 oxygens. (!-2) Center and rim, respeetiively, of a magnetite crystal in core rauhaugite GGU 225264'; (3) Discrete ilmenite crystal in core rauhaugite GGU 22522117; (4) Discrete ilmenite crystal in core rauhaugite GGU 225264; (5) Ilmenite granule in magnetite. Core rauhaugite GGU 225265.

Chemically (Table 5) the magnet!tes conform very closely to the magnetites from carbonatites analysed by Prins (1972). Like these the Sarfart6q magnetites are low in TiP2 and zoned with decreasing Ti, Mn and Mg from core to rim. Ca may either decrease or increase, and AI stays very constant. The most Ti-rich magnetite analysed from Sarfartfq has 6.4% TIP2; this relatively high value ts in accordance with the Ti.-c~ntents in o~her carbonatite- magnetites coexisting with ilmenite, a rather uncommon feature. The ilmenite is enr!ched in Mn and Nb relative to the magnetite. The magnetite contains appro×. 0.1% Nb, while the ilmenite contains 0.2-0.3% Nb. In very Ti-poor magnetites approx. 0.5% V2()~ can b~; measured; this element is normally masked by titanium.

Niobian rutile: This phase is a common accessory in the beforsite dykes, occurring in fine-grained! spongy aggregates which sometimes have a distinct appearance of being pseudomorphs, and much o~:" the futile probably formed by breakdown of Fe-Ti-oxide grains from wall-rock inclusions. The analyses indicate up to 6% Nb2Os, often around 3%. Nb-rutile also occurs as a late phase in crack-fili:angs.

CMorite. This mineral occurs in the marginal zone rocks as small greenish~ felly aggregates or rosettes, both in fenitised wall-rock fragments and directly embedded in the carbonate. Analyses (Table 6) show that, despite substantial differenceg

in AI, Fe and Mg, both types classify as diabantite according to Deer et al. (1962, p. 13'?). They are thus a Si-rich, AI-poor type.

Alkali feldspar: Potassium ~eldspar is invariably present in the marginal zone carbc,aatite dykes, mostly as diffuse hematite-stained masses intimately mixed with carbonate, but also as euhedral crystals and as clear rounded xeno- crystie grains, it is a very pure K-feldspar; however~ 3 4 per cent ElaO has been found in one seemingly xenoerystic grain. Albite occurs in fenitised rocks and fragments in both core and marginal zorae.

Other silicates: A zeolite was found with the microprobe in the clouded center of a plag~ioclase remnant in a fenite. It is

iow-Si type containing both Ca and K but no appreciable Na (Table 6). It resembles a gismondine analysis given by Deer et al. (1963, p. 403) aldltough it has higher K20.

A mineral resembling taeniolite (K2Li2M~Si~OzoF4) both optically and chemical!y ('fable 6) was found in a carbonatite dyke together with Nb-rutile aggregates, possibly in a former basement inclusion, it has, however, significant amounts of Na.

Apatite: The X-ray investigatian of this mineral showed it to be a carbonate-fluor-apatite. This type of apatite is that usually found in carbonatJites {Prins 1973). Trace element

LITHOS 13(1980) Sarfart~q carbonatite complex 207

Table 6. Microprobe analyses of vadous silicates.

1 2 3 4 5 6

SiO z 50.87 52.54 33.17 33.13 37.53 54.48 TiO 2 0.66 1.95 0.0(-' 0.00 1.03 0.76 NbaOs 1. t 0.0 0.0 0.0 0.0 0.0 AI20.~ 1.65 1.04 26.02 16.25 7.64 0.20 FezOa 27.82 28.00 0.51 - - - FeO - - - - 25.94 19.20 1.45 MnO 0.00 0.00 0.00 0.OO 0.00 0.00 MgO 1.67 1.87 0.0C ~ 11.26 18.33 19.08 CaO 3.78 0.44 14.08 0.54 0.53 0.15 Na20 12.49 13.27 0.32 0.23 0.83 4.83 KzO 0.00 0.00 8.20 0.55 1.63 10.08 sum 100.04 99.11 82.30 88._00 86.72 91.03

Formulas*

Si 1.929 i.996 17.882 6.883 7.730 7.918 Ti 0.019 0.056 0.000 0.000 0.160 0.083 Nb 0.0i9 0.000 0,000 0.000 0.000 0.000 AI 0.074 0.047 16,537 3.979 1,856 0.034 Fe a + 0.794 0.800 0,207 - - - Fe 2 + - - - 4.507 3.307 O. 176 Mn 0.000 0.000 0.000 0.000 0.000 0,04)0 Mg 0.094 0.106 0.000 3.489 5.628 4.133 Ca 0.154 0.018 8.133 0.121 0,116 0.023 Na 0.918 0.978 0.334 0.094 0.341 1.361 K 0.000 0.000 5.640 0.146 0.429 ! .869 sum 4.000 4.000 48.733 1 9 . 2 3 7 1 9 . 5 6 8 15.597

* Based on: 4 cations (!-2), 72 oxygens (3), 28 oxygens (4-5) and 22 oxygens (6). (i) Aegirine from aegirine-pyrochlore rock GGU 229146; (2) Aegirine, together with magnesio-arfvedsonite (Tab|e 4) in strongly modified basement inclusion. GGU 225214; (3) Zeolite with gismondine-like chemistry. From center of recrystallised plagioclase grain in marie fimite GGU 225236; (4) Diabantitic chlorite, small greenish feity crystals in carbonatite matrix. Beforsite dyke GGU 223886; (5) Diabantitic chlorite, small brownish-green rosettes with blb-ruffle and K-feldspar in presumed basement inclusion remnant. Beforsite dyke GGU 225214; (6) Colourless micaceous mineral with taeniolite- like chemistry, together with Nb-rutile in the same inclusion remnant as the preceding chlorite. GGU 225214.

data on the apatites (Table 7) are largely in accordance with the data of Prins (1973) although the Sarfart6q apatites are iligher in Mn and Y than those analysed by Prins.

Pyrochlore: This mineral has only been qualitatively in- vestigated. Besides Na, Ca and Nb it contains significant amounts of Ti and U, and minor amounts orAl, Fe, REE aad C1. U-contents vary from 5% in darker, i.e. more melamict, central parts of crystals to <1% in lighter margins of crystals. In some crystals oscillatory zoning with alternating dark and light bands may be seen.

Discussion of mineralogy

A characteristic feature of the Sarfart6q carbona~ rites is the absen~ of silicate minerals like olivine, diopsidic pyroxene, garnet a~d spheric. This is probably connected with the low SiOz-content in the carbonatit¢ magmas. The chemistrie'~ of the

analysed minerals are in accordano~ with other published results as far as comparati'Je material exists. The Sarfart6q minerals are thus typical for carbonatites, and the existing peculiarities mostly arise from usual minerals being either absent (as ~ e above-lnentioned silicates) or present in unusual

Table 7. Trac~ elements in apatite from rauhaugite.

GGU No. La Y Sr Mn

225325 1050 210 0.74% 750 225328 870 195 0.64% 440 225271 800 190 0.5% 210 225319 480 155 0.41% 280

Data in ppm unless otherwise st~.ted. Optical emission spectrographic analyses by H. Bollingberg.

208 K. Secher & L. M. Larsen

Table8. Chemical analyses of Sarfar t6q ¢arbonatites (wt %).

LITHOS 13 (1980)

! 2 3 4 5 6

SiO 2 0.52- 3.27 2.(~5 5.08-31.07 17.43 12.10 2.22 TiO2 0.01- 0.22 0.08 0.11- 0.60 0.31 0.80 0.15 A1203 0.02- 0.31 0.13 0.78- 7.23 3.18 3.55 2.01 Fe203 0.39- 5.06 1.86 0.85- 5.07 2.53 3.12 1.99 FeO 3.66- 6.42 4.68 3.06- 7.87 4.70 3.78 6.23 MnO 0.30- 0.62 0.41 0.22- 0.49 0.36 0.61 0.90 MgO 14.35-16.78 15.53 8.24--15.54 11.92 5.64 9.40 Cad 27.53-30.24 28.97 14.96-26.86 20.81 35.12 30.24 Na20 0.03- 0.57 0.26 0.03- 1.54 0.76 0.42 0.26 K20 0.02- 0.15 0.08 0.59- 5.11 2.35 1.49 0.31 P205 2.30- 5.10 3.59 0.07- 1.74 0.86 2.06 1.0,9 vol. 36.23-42.41 39.30 21.37-39.98 31.80 28.73 35.96

t o t a l 96.30-97.97 96.41-98. ! 3

Analyses carried out at GGU's chemical laboratory. Method: XRF analysis on fused glass dr:sos except for MgO and NazO which are by AAS. (1) Range of rauhaugites (15), core zone; (2) Average of 1; (3) Range of beforsite dykes ~4), marginal zone; (4) Average of 3; (5) Average of carbonatites (Gold 1963); (6) Average of rauhaugites from Fen (S~ether !957).

quantities (e.g. pyrochlore), and unusual minerals being pres~mt, e.g. taeniolite? and an unidentified Th-REE-substance.

Estimates of temperature and oxygen fug~lcif~y have been obtained for the rav_haugiFtes. Magnetite- ilmenite temperatures (cak'ulated after Powe21 & Powell 1977) vary due to compositional zoning of the oxides, and range from 6~10-770°C for cevtres in discrete crystals to 500-550°C for exsolved ilmenite blebs in magnetite. Calcite-dolomite temperatures are much lower, the highest being 380°C (ca]culated using the equation of Rice 1977). Equally low calcite temperatures were found for Fen (Jennings& Mitchell 1969) and Iron Hill (Nash 1972), but as discussed by Gittins 0979) these temperatures only represent the point at which re-equilibration of the solid carbonates effectively stops. The oxide temperatures of 600-770°C, on the other hand, seemingly reflect magmatic conditions. Textural relations depict the oxides as early-formed phases, but due to the compositional zoning present, only the highest calculated temperatures could relate to the liquidus temperature of the magma. The temperature interval of 500-550°C for exso~u- tion of flmenite in magnetite is the same as found by Pfins (1972) for this process.

The calculated oxygen fugacities for Sarfart6q all place this intrusion somewhat above the NNO buffer curve in the log fo2, T diagram. This oxidation state is similar to that obtained for Iron Hill (Nash 1972) hut higher than found in the Tanz~.nian carbonatites (Prins 1972) and the Oka carbonatite

(Friel& Ulmer 1974), both these occurrences having oxidation states around the FMQ buffer level.

Analogous estimates cannot at present be made for the marginal zor~e ~drbonatites at Saffart6q. However the widespread ocxx~rrenee of hematite in these rocks indicates that the oxygen fugacity rose above the HM-level in the closivg stage of crystallisation.

Geochemistry Chemical analyses are shown in Table 8. Compared to the earbonatite average given by Go~d (19613) and the average of the Fen raahaugites given by Saether (1957) the Sarfartbq car bonatites have very low contents of SiO2, AI20 3 and K20. P205 is above average in the rauhaugites and below average in the beforsite dykes.

Of the trace elements (Table 9) Nb is remarkably low in both rauhaugites and beforsites. On the other hand this element increases to 10% in marie fenite and up to 40% in the pyrochlore-rieh zones in the margin. Zr and Y are around average in the carbonatites. Sr is around average in the rauhaugites and low in the beforsites- this is also reflected m the dolomi~te analyses. Cu is enriched in all groups.

Fenites are especially enritehed in Ga, Zr, Nb, Pb and U. The radioactive sharer zones are enriciaed in Pb and Zn and have rer~a~rkable concentrations of REEs (Table 10) and T h

U and Th contents are moderate ~o low in both

L I T H O S 13 (1980)

Table 9. Trace elements in Sarfart6q rocks (ppm).

Sarfart~q carbonatite complex 209

l 2 3 4 5 6 7 8

Cu 30 30 30 20 40 40 40 2.5 Zn 60 100 80 60 40 7300 - - Ga 5 10 5 10 30 10 69 Rb 80 120 50 90 nd 70 nd - Sr 3510" ! 661Y ! 390 870 1180 1440 640 3400 Y 50 50 110 30 nd 100 nd 96 Zr 60 140 - 210 nd 20 nd 83 Nb 80 310 380 160 > 1% 230 > 15 % 1600 Pb 30 30 40 20 60 680 80 - Th 4 73 259 11 > 50 > 200C 70~ - U 15 8 7 2 > 500 5 7066 -

Trace element analyses by giso National Laboratory: Cu to Pb by energy dispersive X-ray fluorescence; Th by gamma- spectromet~$; U by delayed neutron activation. nd = not detected; x--X-ray fluorescence by GGU's chemical laboratory. (l) Average of rauhaugites (15), core zone; (2) Average of beforsite dykes (12), marginal zone; (3) Average of dolomite- pyrite-calcite veins (9), marginal zone; (4) Average of light-grey fenites (7), core zone; (5) Average of marie fenites (4), core zone; (6) Average of radioactive shear zone-rocks (7), marginal zone; (7) Average of pyrochlore-rich rocks (9), marginal zone; (8) Average of carbonatites (Gold 1963).

Table 10. Rare earth elements in rocks of the Sarfartbq carbonatite complex (ppm).

GGU No. La Ce Nd Sm Eu Tb Yb Lu

Radioactive shear zone rocks:

229150 1200 225291 1860

Pyrochlore-rich rocks:

1680 - - 236.0 43.8 t 26.0 3300 - - 200.0 | 8.4 3.15

229148 3400 8690 4380 253679 2870 7210 -

Beforsites:

225356 212 631 279 225365 180 499 341

Rauhaugites:

225253 346 684 303 223895 398 858 -

31.2 24.4

0.32 1.24

37.9 9.38 ~ .05 0.72 0.13 30.9 6.56 - 0.5C 0.23

94.5 17.20 - 2.43 0.31 43.1 8.66 - 1.71 -

Analyses by INAA at Institute for Petrology, University of Copenhagen.

rauhaugites and beforsites. IncEeased Th coqte~,ts are seen in the dolomite-pyrite-calcite veins. The Th/U ratio increases outwards in the complex, from 0.3 in the center to 550 i~:l tile outer margin, mainly due to increasing Th. U is sele~:tively enriched in the pyroehlore-rich zones and Th in the radioactive shear zones. Generally there ~s a negative correlation between U and Th; U is positively correlated with Nb and LREE, and Th with Zn and HREE.

Reconnaissance analyses of REEs were made on two samples from each of the four groups: core

rauhaugites, marginal dyke beforsites, pyrochlore- rich zones and radioactive shear zones (Table 10). These groups are clearly distinguished in the REE spectra in Fig. 8. The carbonatites have relatively smooth and steep, LREE-enriched spectra, the beforsite dykes atteining somewhat higher values than the rauhaugites. These patterns are c~nsistent with those reported for African carbonatites (Lou- bet et al. 1972), and for rauhaugite from FerJ (Mitchell & Brunfelt 1975). Highest absolute values of REEs in Sarfart6q are found in the pyrochlore- rich zones (up to 1.6%) which are strongly LREE-

IO00O

1000

t00

10

• Rauhaugites

210 K. Secher & L. M. Larsen L1THOS 13 (1980)

k . . . ~ I I 1 I I I 1 I . I I I I I

LaCe Pr Nd SmEu Gd Tb C,y Ho Er TmY~ to

Fig. 8. Chonddte-normalised REE s[¢ctra for rocks from Sarfart6q. The four rock groups analy.,;ed are clearly seen as groups in these spectra. The extrevmly LREE-enriched spectra of the pyrochlore-rich rocks are reminiscent of that reported for the 'Rodberg' rock from Fen by Mitchell & E~runfeit (1975). Nole that, due to interferences on Sm and qb, some spectra are quite extensivel.~ interpolated.

enriched. The radioactive shear zones are notably different, with high contents of HREEs and a distinclive positive Eu attomaly (ap to 236 ppm Eu). One of these spectra is also rather fiat, with La/Yb=9.5. These zones contain pseudomorphs after a prismatic mineral with ~n appearance very much It ke ancylite; however, the microprobe showed that this material is REE-free, ind that the REEs together with Th are contained in semiopaque Ca-rich 'pigmentary' material around the pseudomorphs wl: ich are stiB believed ~o have originated as ancyfite.

. J

Discusshm of geochemistry

The data obtained on rc~:k geoch e:nistry show some interesting pa',terns with reganl to the late-stage d~str~botion of trace element,,;. I he pyroehlore-~ich zones .and the radio,tctive shear, zones both formed

by del~sition of material from hydrothermal solu- tions in earlier farmed cataclastic zones. All the same, two very differ~mt types of deposits resulted, Nb, U and LREEs beaing eoncentratedin one situa- tion, and Th, HREF',s, Pb and Zr~i in the other. There is probably a certain degree of mineralogical control of the copreeipitation of Nb, Uand LREEs, these elements being accomodated in the pyroehlore lattice and any other elements being carried on in solution. The Th-REE rich radioactive shear zones bear some striking similarities to thorium veins in the Unffed States described by Staatz (1974) arid Staatz et al. (1972). These veins usually have the same geological setting as in Sarfartrq, occurring in the periphery of carbonatite and alkaline rock complexes, along fractures with several periods of demonstrable movement. They are enriched in intermediate REEs (Nd-Dy), often with a peak at Eu (compare Fig. 8), and some of the REEs and Th occur in 'goeth~te'. The processes by which these mineralisations formed cannot at present be further elucidated but they appear to be a general and common part of carbonatite activity.

Economic geology Rare earth elements, uranium, niobium and phos- phorous oomr in economically interesting concentrations.

The pyrochlore-rich rocks have up ~o 1.6% total lanthanides, mostly La-Nd, occurring in pyro- cl~lore. The Th-rieh shear zones ha~e specifically high concentrations of Eu, of the order 200-3(20 ppm. The mineralogical host for this is not well- established.

High U concentrations are found within the rauhaugites, the marie fenites and in pyrochlore- rich rocks in the r~,arginal zone. Maximum values are in the order of 400 ppm, 0.5% and 1% U, respectively, always explained by high modal contents of pyrochlore.

Nb reaches va]ues of 10% in marie fenites and 409/0 in pyrochlore rocks. The values are strongly correlated with the U contents.

P is not observed as independent mi:nteralisations. However, broad zones of the core rauhaugites may hold as much as 30% modal apatite.

Conclusions

The Sarfart6q carbonatite complex w,s emplaced in lower Palaeozoic time into the we,~kness zone

LITHOS 13 0980) Sarfart~q carbonatite complex 211

constituted by the Nagssugtoqidian boundary zone in the Precambrian shield. It is situated 125 km north o f the other large carbonatite complex known from West Greenland, the Qaqarssuk complex. The major structural features controlling the location o f the complexes and the widespread lamprophyre-kimberlite dykes in the area are presently not well known. The carbonatites form part o f the 500-600 m.y. old N o a h Atlantic carbonatite activity.

The Saffart6q complex was formed m two major stages o f activity. In the first stage a steeply dipping conical body o f concentric rauhaugite sheets was formed, constituting the core. The major part o f the fenitisation took place at this stage. The em- placement o f the core generated a shock zone in the surrounding gneisses. This probably eased the way for the following intrusion o f several batches o f slightly more iron-rich magma, forming concentric and radial beforsite dykes in the marginal zone. Dyke breccias and agglomerates represent vent-like formations, indicating that the present erosional level in the complex is high. The accompanying hydrothermal activity gave rise to several phases o f mineralisation in veins and shear zones.

According to the views of Koster van Groos (1975) do!omitic carbonatites are primary magmas generated in the mantle at pressures above 10 kb. I f during ascent the dolomitic magma reacts with surrounding silicate rocks or magmas the Mg, Fe carbonates will dissociate and MgO and F e e combine with the silico.tes, giving rise to s~vitic carbonatite melts, ijolitic silicate melts and an alkali-enriched vapour phase Applying this scheme to Sarfart6q the rauhaugite magma could represent relatively little modified primary magma, and the paucity in sovites finds its counterpart in the paucity of associated silicate intrusions. The bcfor- site dykes following the rauhaugites may represent even more original magma compositions, despite their strong modification by highqevel wall rock contamination.

Acknowledgement. - Thanks are due to A. L GL-adow for the apatite fission track datings, to E. Leonards¢n for the X-ray identification of several minerals, and to H. Kunzen- doff, H. Bollingbt',rg and R. Gwozdz for trace element analyses. Kryolitse~skabet Oresund A/S is thanked for kind assistance on Qaqarssuk. The microprobe analyses were performed at the gesearch School of Earth Sciences, Can- bcrra, and N. G. Ware is t.hanked for technical assistance during the probe work. Publication of the paper is authorised by the Director of the Geological Survey of Greenland.

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212 K. Secher & L. M . Larsen LITHOS 13 (1980)

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Accepted for lmblication November 1979 Printed April J980

Documents

Geology and mineralogy of the Sarfartôq carbonatite complex, southern West Greenland