wyllieite reaction coronas on scorzalite in...

WYLLIEITE REACTION CORONAS ON SCORZALITE IN PEGMATITE DYKES - SERRA DE ARGA (PORTUGAL)

P. A. DIAS 1, C. LEAL GOMES 1, F. GUIMARÃES 2, F. HATERT 3

1 CIG-R, Univ. Minho, Portugal; patriciasdias@gmail.com; caal.gomes@gmail.com

2 LNEG, S. Mamede de Infesta, Portugal; Fernanda.Guimaraes@lneg.pt

3 Laboratory of Mineralogy, Univ. Liège, Belgium; fhatert@ulg.ac.be

In the region of Serra de Arga (Northern Portugal) pegmatite dykes with approximately 50 cm thick and 2 m long, affected by Variscan deformation, contain scorzalite that is partially replaced by wyllieite reaction coronas.

Mineral composition of the dykes consists of quartz, albite, potassium feldspar and muscovite. Accessory minerals include andalusite, Mn-rich fluorapatite, columbite-(Fe), gahnite, uraninite, montebrasite and brazilianite (Dias, 2012).

Scorzalite occur as disseminated bluish to greenish single crystals up to 3 mm in size. Inclusions of muscovite, gahnite and montebrasite (?) were identified. Scorzalite often displays complex alteration patterns corresponding to the development

of brownish to black Al-Fe-Mn rich products (gormanite or childrenite-eosphorite?). Other breakdown products include associations of crandallite-goyazite and variscite. Scorzalite electron-microprobe analysis showed the following average

composition: (Fe 0.90Mg0.05-0.07Mn0.02Zn0.0-0.01)=0.95-1.01Al 2.0-2.1(PO4)2(OH)2.

Wyllieite forms light blue corona-like overgrowths around primary scorzalite and also penetrate along fracture fillings of the scorzalite crystals, as revealed by transmitted light microscopy and EMP study. Electron-microprobe analysis provided

P2O5 = 45.5-47.2; Al2O3 = 8-8.6, MnO = 15.2-16.3, FeO = 23.5-24.6, MgO = 0.44-0.54; Na2O = 4.2-5.3 wt. %. The resulting formula, calculated on the basis of 12 O, is (Na0.64-0.79Ca0.02-0.03Mn0.30-0.39)=1.01-1.22 (Mn0.60-0.71 Fe 0.29-

0.40)=1(Fe 0.27-0.61Fe 0.34-0.67Mg0.05-0.06)=1(Al0.72-0.77Fe 0.23-0.28)=1(PO4)3. Some of these compositions correspond to wyllieite, while oxidized grains correspond to rosemaryite (Hatert , 2006).

Such unusual previously undescribed scorzalite breakdown was caused by post-magmatic, Na bearing fluids interacting with the pegmatite. Na could have become available by feldspar breakdown. Both albite and K-feldspar occur in the matrix

and reflect distinct high phosphorous contents. K-feldspar contains up to 3.6 wt% of P2O5 and coexisting albite up to 1.98 wt%. Distribution of P between Fk and Ab (PFk/Ab ) is 1.8. Textural relationships indicate albitization of the K-feldspar.

According to (Hatert , 2006), wyllieite could have formed at temperatures lower than 400ºC, considering a pressure of 0.1Kbar. These estimates are within the considered field for scorzalite collapse (475-560ºC, 1-3Kbar) (Schmid-Beurmann

, 2000).

2+

2+

2+ 3+ 3+et al.

et al.

et al.

3. SCORZALITE-WYLLIEITE INTERGROWTHS

2. SCORZALITE-BEARING PEGMATITES

5. CO-EXISTENT FELDSPARS - TEXTURES AND COMPOSITIONS

100 μm0.5 cm

Qz

Scz

Scz

Wyl

100 mμ

Abr Qz Scz Wyl. - quartz; - scorzalite; - wyllieite.

4. SELECTED COMPOSITIONS - ELECTRON MICROPROBE ANALYSIS

Mineral chemical analyses were performed on

representative sections using a five-channel

wavelength dispersion JEOL JXA 8500F

microprobe, routinely operated at an accelerating

voltage of 15 kV and a beam current intensity of 10

nA.The standards used were (K lines): Orthoclase

(Al, K), Albite (Na), MgO (Mg), Apatite (P, Ca),

Volastonite (Ca, Si), Fe2O3 (Fe), MnTiO3 (Mn, Ti),

Barite (Ba), Fluorite (F), Cr2O3 (Cr), SrTi O3 (Sr),

Sphalerite (Zn), FeS2 (S), AsGa (As), Vanadinite

(Cl), Yttrium Al Garnet (Y).

ANALYTICAL PROCEDURES

FORMULAS

Wyllieite-rosemaryite

Scorzalite

(Na Ca Mn ) (Mn

Fe ) (Fe Fe Mg

) (Al Fe ) (PO ) ;

(Fe Mg Mn Zn ) Al

(PO ) (OH) ; .

0.64-0.79 0.02-0.03 0.30-0.39 =1.01-1.22 0.60-

0.71 0.29-0.40 =1 0.27-0.61 0.34-0.67 0.05-

0.06 =1 0.72-0.77 0.23-0.28 =1 4 3

0.90 0.05-0.07 0.02 0.0-0.01 =0.95-1.01 =2.0-2.1

4 2 2

2+ 2+ 3+

3+

2+

N=5.

N=5

SiO2 Na2O Al2O3 K2O P2O5 MnO SrO CaO BaO Total

Fk 61,36 0,58 21,29 12,48 3,60 0,02 0,23 - 0,07 99,64

Ab 67,07 8,43 21,68 0,08 1,98 0,01 0,30 0,13 0,09 99,77

3Sta.

Cristina

1. GEOLOGICAL SETTING OF SERRA DE ARGA PEGMATITES

CUMIEIRA

COUTO DE OURO

CABEÇO DO MEIO DIA

300

40

0

500

200

100

600

700

10

0

400

200

300

600

Coura

Arga de Cima

56

7063

80

70

84

46

70

80

80 82

82

56

7654

18 84

80

15

8478

78

75

70

74

7630

66

80 70

84

62

8186

82

6840

74

76

20

28

7030

72

70 74

5070

38

63

52

30

30

12

1826 18 26

24

8030

38

2122

32

10

38

36

30

7070

1220

60

6220

50

45

84

3074

44

35

35

6010

10

6242

48

26

28

72

78

80

28

4450

50

7451

45

30

28

30

60

525000 526000 527000 528000 529000 530000 531000

4632000

4633000

4634000

4635000

4636000

D E T A I L E D G E O L O G I C A L M A P1C sheet - revision proposal (CIG-R, 2008; Dias, 2012)

coarse grained granite (Arga)

medium to fine grained gneissic granite (Romarigães - Sabariz)

medium grained granite (Covas E)

coarse grained granite (Covas W)

Syn-tectonic two mica granites

porphyritic granodiorite

Syn- to late-tectonic biotite granites

VARISCAN GRANITOIDS

�´´1

�g

�m

�mz

�g

Sapardos a Gandrachão - metapsammopelites interstratified withampelites, quartzites and lidites

Formigoso - metapsammopelites interstratified with black-shists, lidites,quartzites and tourmalinites

Minho Central Unit - Silurian (Wenlock, Landovery)

PARAUTOCHTHONOUS AND ALLOCHTHONOUS UNITS

UMC

UMC

Rib. Pombas

polygenic metapsammopelites (mica-schists and quartzphyllites)interstratified with quartzites, black-schists, lidites and exotic psammiticlithologies with volcanogenic-exhalative affiliation (+), volcanogenic-carbonated-exhalative affinity (++) and metassomatic derived products(+++).

CumieiraCerdeirinha

Domo de Covas Unit - Silurian (Landovery)

AC

++ +

Peraluminous anatectic pegmatites (abyssal and rare-elementmuscovite classes?) and metamorphic segregation veins(see )Dias, 2012

Hydrothermal quartz veins and breccias

Rare-element aplite-pegmatites and pegmatites - REL-Li LCT typespredominate: beryl, spodumene (after petalite), petalite, lepidolite andelbaite subtypes (see ).Leal Gomes, 1994

INTRUSIVE DYKES

+++

Scorzalite bearingpegmatites1

1

2

2

3

3Fisga Lousado

Sta.Cristina

Orbacém Thrust

Argagranite

Sto. Ovídeogranite

Vigo-Régua Shear Zone

FDC

�

�

�

�

�

�

�

��

�

�

�

�

�

�

FDC

a

a

aVM

VM

UMC

UMC

UMC

UMC

UMC

UMC

UMC

UMC

Domode

Covas

Serro

Porto Morais

Bragança

CIZ

GTMZ

SERRADE ARGA

1

2

2

1

Recent sedimentary deposits - Pliocenic to Quaternary

GTMZ

CIZ

Galiza-Trás-os-Montes Zone

Central Iberian Zone ( Terrain).Autocthonous

( andTerrains).Parautocthonous

Allochthonous

Variscan syn- to late-tectonic granitoids

Cambrian and Ordovician Autocthonous Formations

Intrusive dykes: pegmatites and hydrothermal quartzSilurian Parautochthonous Formations

Polygenic Silurian Allochthonous (?) Formations

Minho Central Unit (W of the Vigo-Régua shear)

Vilar de Mouros

Domo de Covas Unit

Major thrusts

FDC

Minho Central Unit (E of the Vigo-Régua shear)Continuous shear structures

�

a

UMC

UMC

VM

S I M P L I F I E D G E O L O G I C A L M A P

SPAIN

AT

LA

NT

IC O

CE

AN

AT

LA

NT

IC O

CE

AN

PORTUG

AL

Geotectonic Units:

Dias & Leal Gomes (2013)

The photomicrographs were made from a thin

section of the sample illustrated above and show

PPL, XPL and BSE views of scorzalite crystals.

: a section of irregularly shaped scorzalite

which has been replaced by gormanite or

childrenite-eosphorite (brownish to black in PPL)

show inclusions of muscovite, gahnite (Zn Fe

Mn ) Al O ) and montebrasite (?). The

brownish yellow color around the margin of

scorzalite is a narrow mantle of rosemaryite which

is also clearly identified in the BSE view. : another

scorzalite crystal less altered to gormanite showing

a deep blue absorption color and a corona of

wyllieite and rosemaryite (developed

indiscriminately between scorzalite and the various

groundmass minerals).

A-C

D

0.85 0.08

0.002 =0.94 2.04 4

E-G: High magnification views of parts of the field

of view shown in fig. D. In the first view (E) the

brownish yellow color of rosemaryite serve to

distinguish it from the wyllieite which shows a light

blue absorption color and high relief. In the second

view (F) wyllieite appears to form an inner mantle

surrounded by rosemaryite. Photopgraph G shows

scorzalite relics and a predominant slightly more

hydrated and Mn-rich scorzalite blue to greenish in

color (Scz-OH). The dark patches which are lighter

in the BSE view are mainly of gormanite/childrenite-

eosphorite. The narrow brown rim is of rosemaryite,

with anomalous interference colors in the XPL view.

H-K: scorzalite crystals altered to aggregates of

variscite and crandallite. Scorzalite relics can be

recognized and wyllieite-rosemaryite coronas are

absent.

The photomicrographs show the equigranular

groundmass consisting of quartz, plagioclase,

potassium feldspar and muscovite ( ). Plagioclase

feldspar and potassium feldspar are mostly subhedral

and have slightly interdigitating boundaries. It

appears that two generations of plagioclase are

present: small subhedral to euhedral crystals showing

multiple twinning (Pl1) and large subhedral crystals

most easily identified by a turbid brownish

appearance (Ab) resulting from very small inclusions

of mica. The strong alteration of the plagioclase ( ) is

the first characteristic that enables to distinguish it

from potassium feldspar, recognized here by the lack

of alteration and sometimes the presence of simple

twins ( ). Evidence for potassium feldspar dissolution

and replacement by albite + muscovite is shown in .

Apatite microinclusions are widely distributed within

the albite and are clearly seen in the BSE view ( ). The

compositions of both albite and potassium feldspar

are exceptionally rich in phosphorous.

A

B

A

C

D

The polished specimen is of a fine-grained portion of a pegmatite dyke from Santa Crisitina type locality (see Fig.2), consisting essentially of quartz, plagioclase, potassium feldspar and muscovite in a subhedral granular texture.

Scorzalite crystals ranging in size from 0.1-3mm are scattered throughout the rock and may be recognized by their deep blue color. The photomicrographs show PPL views of one scorzalite crystal surrounded by a 0.02-0.05 mm

wide rim of wyllieite.

The Serra de Arga pegmatite field consists of a swarm of granite-related aplite-pegmatite sills and dykes (Leal Gomes, 1994) and earlier highly peraluminous anatectic

pegmatites (Dias, 2012), mostly emplaced in metassedimentary and metavolcanosedimentary-exhalative Silurian series (Minho Central and Domo de Covas Units). The

first group, developed around the Arga granite plutonite (S-type peraluminous granite, ± 318 Ma), comprises evolved Li-bearing pegmatites with a layered structure,

belonging to the beryl, petalite, lepidolite and elbaite subtypes of the rare-element class; the pegmatites are mineralized with cassiterite and Nb-Ta oxides. The anatectic

pegmatites consist of thin stroma and vein-like irregular bodies, derived from low-degree hydrated partial melts in conditions of intermediate P-T (2.9-4.2 kbar, 650-710 º

C). The composition is significantly enriched in muscovite and andalusite (or albite) and depleted in potassium feldspar. They are characterized by a more or less simple

structure although an internal zonation is commonly observed and inward fractionation is noticeable. A classification as abyssal or muscovite types is proposed, although a

remarkable feature is the occurrence of tantalian rutile, ferrocolumbite and tapiolite in some of the vein deposits.

Abr Scz Wyl Ros Gm/Ch Scz (OH) Cra/GoyVar Qz Ab Ms Fk Mbr Ghn

. - scorzalite; - wyllieite; - Rosemaryite, - gormanite or childrenite-eosphorite; - more hydrated and Mn-rich scorzalite; -crandallite- goyazite; - variscite; - quartz; - albite; - muscovite; - potassium feldspar; - montebrasite; - gahnite.

S3

S3

S2

S2, S3 , L2, L3

39

c/s53º

43º

40º

70º

50º

53º

28º35º

L3

S3

S2

S2

D2

D3

D3/D´3

S3

49º

50º

50º

L3

L2

L2

Qz

Qz

30 cm

N

N

10 cm

S2

S3

1cm

2 mm

1cm

Fisga type

(*1)

- Thin, highly

deformed, fine-grained pegmatite

veins hosted in metaquartzphyllite

and lidite layers, possibly anatectic

in origin. Although scorzalite is

disseminated across the veins, is

more persistent along a 3 cm thick

border zone. Sillimanite is a

widespread mineral mostly

replacing scorzalite, and is related

to a late-tectonic increase of

regional P conditions.

Lousado type - Thicker (up to 1 m),

deformed, fine-grained pegmatite

lenses with simple structure hosted

in a poly-deformed micaschist. The

lenses are subconcordant and the

contact with the host rock is not

easily traced. Scorzalite occur as

millimeter disseminated grains.

(*2)

1

2

Fisga

Lousado

quartzphyllite

50 cm

N

B

B

A A

A

1

123

ScorzaliteMuscovite fractionPotassium feldspar with

fractures filled with scorzalite 32

3

Sta.Cristina

80

andalusitemicaschist

Sorzalite salband

micaschist

Pegmatite contour

Peg. contour

Metamorphic foliation

B

20cm

2.5 cm

20cm

Metamorphicfoliations andlineations

Shear structures

Legend

Legend

Legend

A B

Aplitic facies

Pegmatiticfacies

W12

G11

S17

Ms

Ghn

Scz(OH)

Mbr?

Scz

Scz

Gm/Ch

Gm/Ch

Gm/Ch

Cra/GoyVar

Var

Scz

Ghn

Ros

hole

Ros

R4

R9

S´8

G1

S6

S23

C25

Wyl

Wyl

Ros

Ros

200 m� 200 m�

100 m�

200 m�

200 m�

QzQz

Qz

Ms

Fk

Fk

Pl

Pl(I)

Qz

Qz

Qz

Qz

QzPl

Fk

Pl

Ms

MsQzQz

MsMs

Ms

Ms

Ms

Fk

200 m�

100 m�

Ros

10 m�

Qz Qz

Qz

Qz

Qz

Ab

Ab

Ab

Ab

Ab

Ab

Pl1Ms

Ms

Fk

Fk

Fk

Fk

Ap

Fk

200 m�200 m� 200 m� BSEXPLXPLXPL

PPL

A B C D

Poster Session: Pegmatites, and pegmatite mineralogy

ABSTRACT

A B C D

E F

H I J K

BSE

BSE

BSE PPL

PPL PPLPPL XPL

PPL XPL BSE BSE

G

BSE images in Fig. 3 (B, C, G, J and K) indicate the spots chosen to perform the analyses.

- Below detection limit.

Wyl Wyl Wyl Ros Ros Scz Scz Scz(OH) Grm/Ch Cra/Goy

W7 W12 W13 R9 R4 S6 S23 S´8 G11 C25

TiO2 0,02 - - - 0,02 - -0,008 0,0057 -

Al2O3 8,07 8,59 8,41 8,29 8,43 30,61 30,63 28,10 25,72 31,99

FeO 24,64 23,88 23,49 23,81 24,12 19,38 19,04 21,51 22,86 1,11

MnO 16,16 16,43 16,27 15,92 15,20 0,43 0,37 2,80 5,42 0,15

MgO 0,52 0,47 0,51 0,54 0,44 0,57 0,80 0,42 0,24 0,008

CaO 0,22 0,28 0,34 0,28 0,35 - 0,03 0,07 0,09 7,99

Na2O 4,98 5,20 5,31 4,69 4,24 0,008 - 0,04 0,21 0,01

K2O 0,02 0,02 0,00 0,01 0,03 0,061 - 0,01 0,03 0,06

BaO 0,08 0,12 0,01 0,11 - 0,061 0,13 0,02 0,77

ZnO - - - - 0,09 0,04 0,33 0,06 0,06 -

F 0,15 0,08 - 0,15 0,07 - - 0,04 - 0,35

Cl 0,01 0,00 - - - - 0,01 0,00 0,01 0,01

Cr2O3 0,07 - 0,02 0,06 - - 0,01 0,04 0,08 0,01

P2O5 46,56 46,30 45,93 47,18 45,46 41,16 41,7 39,15 33,4 30,26

Y2O3 - - - 0,04 0,02 - - -

SrO 0,10 - - 0,04 0,03 - 0,006 0,03 0,03 6,97

SO3 - 0,04 0,05 - - - 0,05 0,06 0,002 -

As2O5 - 0,04 - 0,04 - - - 0,01 -

Total 101,59 101,44 100,35 101,17 98,47 92,33 93,11 92,32 88,18 79,53

-

-

-

OTHER PHOSPHATESWYLLIEITE - ROSEMARYITE SCORZALITE

Electron microprobe analysis; Rb O not quantified; - below detection limit.2

CIG-R (2008)Dias, P.A.; Leal Gomes, C. (2013).

Dias, P. A. (2012)

Hatert, F.; Hermann, R.; Fransolet, A.; Long, G.; Grandjean, F. (2006)Leal Gomes, C. (1994)

. Carta Geológica de Portugal 1/50 000.

. In: Actas da

9ª Conferência anual do GGET: Geodinâmica e Tectónica global; a Importância da Cartografia Geológica, pp. 117-121.

.

. Ph.D Thesis, Univ. Minho, 464 p.

. . Eur. J. Mineral., 18, 775-785.

. . Ph.D Thesis, Univ. Minho, 695 p.

Minutas de revisão da folha 1C – Caminha.

Considerações sobre a natureza vulcanogénica a exalativa de algumas formações metamórficas da Serra de Arga – Minho – Portugal – consequências para a cartografia

Análise estrutural e paragenética de produtos litológicos e mineralizações de segregação metamórfica – Estudo de veios hiperaluminosos e protólitos poligénicos Silúricos da região da Serra de Arga

(Minho)

A structural, infrared, and Mossbauer spectral study of rosemaryite, NaMnFe Al(PO )

Estudo estrutural e paragenético de um sistema pegmatóide granítico - O campo filoniano de Arga - Minho (Portugal)

3+

4 3

REFERENCES

Three types of scorzalite bearing pegmatites were identified:

*1 and 2

*3

- The composition of the veins is 45% quartz, 26% muscovite, 13% andalusite+sillimanite, 6% albite, accessory minerals (scorzalite, apatite, monazite,

chrysoberyl, columbite-tantalite (?) and gahnite).

- The composition of the aplitic facies is: 41% quartz, 7.5% muscovite, 15.2% potassium feldspar, 29.6% albite, accessory minerals (scorzalite, wyllieite,

rosemaryite, crandallite group phosphates, variscite, Mn-rich fluorapatite, montebrasite, brasilianite, andalusite, columbite-Fe, gahnite, uraninite).

Sta. Cristina type

(*3)

- Dikes up to 50

cm thick, ranging from aplite (A) to

pegmatite (B), concordant to the

metamorphic foliation of the

andalusite micaschist host. The

pegmatitic part display a muscovite

enriched border zone followed

inward by an intermediate zone

composed of quartz, potassium

feldspar, muscovite, albite and

scorzalite. In the core zone

potassium feldspar exhibits thin

fractures filled with scorzalite.

wt.%