Rare-metal ore occurrences, related to the Late Archean A...

1 2Dmitry Zozulya , Nelson Eby1Geological Institute, Kola Science Centre, Apatity, Russia

2University of Massachusetts Lowell, USA

Rare-metal ore occurrences, related to the Late Archean A-type alkali granites from the Keivy zone (NE Fennoscandian shield)

Typology, geology, mineralogy and geochemical specialization of Keivy alkali granite rare-metal occurrences

syn-COLG

VAG ORG

WPG

Y+Nb

Rb

1 10 100 1000

10

100

1000

Y1 10 100 1000

ORG

VAG +syn-COLG

WPG

Nb

10

100

1000

Nb

A

MI

S

Y

10

100

M

M

A

Zr

IS

1000

100

A

SI

M

100

1010

1 10

10000Ga/Al20

0,1

1

10

100

0,1 1 10

Y/Nb

Yb

/Ta

graniteanorthosite

OIB

IAB

nepheline syenite

0,1

1

10

100

0,1 1 10

Y/Nb

Ce/

Nb

graniteanorthosite

IAB

OIB

nepheline syenite

Geochemistry of the rocks from the Keivy complex and tectonic discriminant diagrams

The rocks of the Keivy complex are extremely enriched in Zr (300-5000 ppm), Y (40-500 ppm), Nb (20-600 ppm), Rb (160-900 ppm), REE (100-1000 times chondrites), have associated Zr-REE ore occurrences, are very low in Sc (0.3-1.3 ppm) and Sr (10-30 ppm), and high Ga/Al ratios. On standard trace element discriminant diagrams of Whalen et al. (1987), Pearce et al. (1984), Eby (1990) the Keivy peralkaline granites plot as within-plate or post-collisional A-type granitoids. The low Y/Nb and Yb/Ta ratios for the associated nepheline syenite indicate an OIB affinity. The anorthosites show high compatible element (Sc, 25-40 ppm and Sr, 460-670 ppm) abundances and very low incompatible elements (Zr, 60-100 ppm, Y, 4-15 ppm, Nb, 8-12 ppm, Rb, 15-30 ppm), but the same Y/Nb, Ce/Nb, Yb/Nb ratios as the granites. The enriched source for the Keivy anorthosites has low eNd (-0.15 to -0.24) and low Y/Nb ratios (0.6-1.3). The least evolved syenogranites plot in the EM2-field on the eSr - e Nd diagram.

1

2

3

4

57

0 500 1000 1500 2000 2500 3000

R1= 4Si - 11(Na+K) - 2(Fe+Ti)

500

1000

1500

2000

1 - mantle fractionates2 - pre-plate collision3 - post collision4 - late orogenic5 - anorogenic6 - syn-collision7 - post orogenic

R2=

6C

a +

2M

g +

Al

6

A suite of peralkaline granites and associated syenite and of massif-type anorthosites is found in the Archean Keivy terrane of the NE Baltic shield. The Keivy peralkaline granite complex consists of 2650-2660 Ma aegirine-arfvedsonite granites (six massifs of a few hundred meters thicknesses and with the total exposure area of ca.

22500 km ), 2670 Ma lepidomelane-ferrohastingsite syenogranites in margins of some massifs, and 2680 Ma lepidomelane-ferrohastingsite syenite dykes cutting the TTG basement. Small dike-like bodies of 2610 Ma nepheline syenite cut the West Keivy peralkaline granite massif. The Keivy anorthosite complex consists

2of several large (up to the 170 km ) lopoliths composed mainly of anorthosite and gabbro-anorthosite and spatially confined to granite

White Tundra West Keivy

343+/-342 Ma 1

4

5

3 2

2560

2640

2600

2680

2674+/-6 MaMSWD=0.94

Pike&Bear Lake

2

1

3

4

2663+/-7 MaMSWD=0.49

793+/-354 Ma

2600

2660

2640

2620

Tsaga

3

2

1

18-2

18-42640

2620

2600

2580

2668+/-10 MaMSWD=0.045

0

18-3

18-5

0

2659+/-3 MaMSWD=0.90

4

3

2

1

466+/-28 Ma

2654+/-5 MaMSWD=0.32

2000

2600

2500

2400

2300

2200

2100

Sakharjok

1400

1600

1800

2000

2200

2400

2600

42

1

3

2613+/-35 MaMSWD=1.1

1467+/-22 Ma

3

42

1

1700

1900

2100

2300

2500

1709+/-49 Ma

2682+/-10 MaMSWD=0.44

Sakharjok

A B

F

C D

E18-1

206 Pb238 U A -

207Pb 235 Pb D - alkaline syenite; E - F - gabbro-anorthosites

peralkaline granite; B - syenogranite; C - nepheline syenite;

040038036034

032030

068

066

KANDALAKSHA BAY

B A R E N T S S E A

0 40 km

F i n

l a

n

d K e i v y

Khibiny & Lovozeromassifs

0 10 km

Gabbro-anorthosite(Tsaga & Pike Bear Lake)

Nepheline syenites(Sakharyok intrusion)

Metasediments(Chervurt Fm.)

Metavolcanics(Lebyazha Fm.)

Amphibolite

Gabbro-norite(Fyodorov-Pana intr.)

Peralkaline granite

Latite - monzonite(Mar'yok River suite)

TTG gneisses

Imandra-Varzuga Belt

A

A

B

CD

E

Peralkaline granite massifs: A - West Keivy; B - White Tundra. Gabbro-anorthosite massifs: C - Tsaga; D - Pike and Bear. E - Sakharjok nepheline syenite massif.

Geology and age of the Keivy anorogenic complex in the Kola Peninsula, NW Russia

Apatity

1951

SC RK I ASG

Genetic type Geological position

Geological structure Main rock-forming minerals Rare-metal mineralization Geochemical specialisation and type by Cherny (1991)

Occurrence' symbol on the map

Mineralised granites-I

Apical parts of massifs

Lens-like bodies of few tens m thickness and up to 5 km length

Quartz, albite, microcline, magnetite, aegirine

Zircon, thorite, chevkinite, allanite,fergusonite, britholite-(Y), bastnäsite

Zr, Y, HREE, Th, Nb (Nb/Ta>15) NYF-type

Mineralised granites-II

Bottom parts of massifs

Bodies of irregular shape (10-2100km ), confined to tectonically

fractured zones

Quartz, microcline, magnetite, chlorite, ±hematite

Zircon, thorite, fluorite, cassiterite, pyrochlore

Zr, REE, Th, Nb (Nb/Ta=8-10), Sn, Li, FNYF-LCT-type

Albitites Exocontact zones

Isometric and irregular-shaped 2bodies (tens m ) with gradual

transition to country rocks; zonal structure: quartz core and albite outer zone

Quartz, albite, aegirine, arfvedsonite

Zircon, thorite, titanite, aeshynite-(Y), thalenite, britholite-(Y)

Zr, Y, HREE, Th, Nb (Nb/Ta=20), Ti NYF-type

Microclinites Bottom and inner parts of massifs

2Irregular-shaped bodies (tens m ) with gradual transition to country rocks with rare quartz cores

Microcline, quartz, protolithionite

Zircon, thorite, pyrochlore-(W), cassiterite

Zr, REE, Th, Nb (Nb/Ta=8), Sn, W, LiNYF-LCT-type

Apobasic metasomatite

Contact zones of granites with basic rocks

Small irregular-shaped bodies of inhomogeneous structure; bear restites of mafic rocks

Quartz, albite, microcline, aegirine, arfvedsonite, ferrohastingsite, lepidomelane

Zircon, thorite, epidote-(REE), fergusonite, chevkinite, gadolinite, pyrochlore, fluorite, cassiterite, danalite, monacite

Zr, REE, Th, Nb (Nb/Ta=10 -24), Sn, F, Be, W NYF-LCT or NYF type

Silexites-I Exocontact zones

Small irregular-shaped bodies and veins (1-2 m); sharp contact to country rock; taxitic structure

Quartz, arfvedsonite, aegirine, ilmenite, lepidomelane, magnetite, albite

Zircon, fergusonite, britholite-(Y), chevkinite, bastnäsite, thorite, fluorite

Zr, Y, HREE, Nb (Nb/Ta=15 -20), Th, Ti, FNYF-type

Silexites-II Endocontact zones

Small isometric and irregular-shaped bodies (up to 1 m); gradual transition to country granite; taxitic structure

Quartz, aegirine, arfvedsonite, magnetite, ilmenite, ±microcline

Zircon, fergusonite, britholite-(Y), chevkinite, yttrialite, thorite,

Zr, Y, HREE, Nb (Nb/Ta> 20), Th, TiNYF-type

Quartz-feldspar-astrophylite pegmatites

Apical and inner parts of massifs

Vein-like bodies up to 10 m length with chambers; zonal pegmatitic structure

Quartz, microcline, arfvedsonite, ±albite

Astrophyllite, gadolinite Y, HREE, Ti, BeNYF-type

Quartz-microcline pegmatites

Contact zones Vein-like bodies up to 30x10 m length of zonal pegmatitic structure

Quartz, microcline, magnetite, ±albite, ±aegirine

Fergusonite, chevkinite, britholite-(Y), aeshynite-(Y), gadolinite

Y, Nb (HREE, Be)NYF-type

Amazonite pegmatites

Roof country rocks (gneiss of Keivy complex)

Large vein-like bodies up to few 100 m length of zonal pegmatitic structure

Amazonite, quartz, albite, lepidomelane

Microlite, betafite, pyrochlore-(W), keyviite, fluorite-(Y), xenotime, gadolinite, genthelvite, polylithionite, kainosite, cassiterite, galena, molybdenite

Y, Yb, Nb, Ta, F, Li, Sn (Pb, W, Mo)NYF-LCT-type

Hydrothermal veins

Apical parts of massifs and roof country rocks

Veins of up to several tens m length and 0.5 m thickness; sharp contact to country rocks

Quartz, riebeckite, ±microcline Zircon, ilmenite, fluorite, astrophyllite (Zr, Ti, F)NYF-type

Mineralised nepheline syenites

Linear zones in magmatic body

Several steeply dipping linear zones of 200-1350 m length and of 3-30 m thickness

Nepheline, aegirine, microcline, albite, lepidomelane

Zircon, britholite-(Y), pyrochlore, fluorite, meliphanite, behoite

Zr, Y, REE, Nb, F, Be

Zircon in mineralised granite-I (image width 0.4 cm)

Aeshyinite-(Y) and zircon in albitite(image width 0.5 cm)

Britholite-(Y) in silexite-I(image width 1.0 cm)

Fergusonite and zircon in silexite-I(image width 0.8 cm)

Chevkinite and zircon in silexite-II(image width 0.5 cm)

Britholite-(Y) in silexite-I(image width 1.2 cm)