lazulite-quartzvein

alteration zonemonazite --> florencite

alteration zonemonazite stable

unalteredcountry rocks

0

0.05

0.3

0.10

0.4

0.15

0.5

0.20

0.6

0.25

0.7

0.8

Mn

O

XM

g

0

0.05

0

0.10

10

0.15

20

0.20

30

40

50

P2

O5

(w

t%)

Ca

O (

wt%

)

Sr

(pp

m) 0

.60

0.7

7

168

me

dia

n c

om

po

siti

on

of m

eta

psa

mm

itic c

ou

ntr

y r

ocks

with

95

% c

on

fide

nce

lim

its

Fl

Fl

Fl

Mz

10 µm

Replacement of Monazite, CePO , by Florencite, CeAl (PO ) (OH) during Hydrothermal Alteration3 4 2 64Franz BERNHARD Institute of Engineering Geology and Applied Mineralogy, Graz University of Technology, Rechbauerstraße 12, A-8010 Graz, Austriag

10 µm

2 µm

50 µm

50 µm

50 µm

20 µm

20 µm

Fl

Fl

Fl

Fl

Fl

FlRt

Rt

Ms

Ms

Qtz

Rt

Fl

Mz

Mz

Mz

Cltd

Cltd

Qtz

Qtz

Ms

Ms

Chl

Qtz

Cltd

Mz

Mz

Mz

X

Ms, Chl

Ms, Chl

Rhabdophane?(Th-poor)

Rhabdophane?(Th-rich)

50 µm

FlMs

Chl Qtz

Qtz

Fl

X

Fl

Cltd

Cltd

Rhabdophane?

Mz

Mz

Ms

ChlChl

Ep

Ap, Ep

50 µm

Ep

Mz

Ms, Qtz

10 µm

Mz

Mz

Fl

Ms, Qtz

Ms, Qtz

Fl

Mz

Mz

Mz

Mz

Ms

Chl

Qtz

Qtz

Mz

Rt

Mz

20 µm

Ms, Qtz

Ms, Qtz

Mz

Mz

Mz

Fl

Fl

Fl

Fl

Mz

Fl

FlFl

Fl

A

B

A

A

BB

C

A

C

(1)

(0)

(2) (3)

Typical patchily zoned florencite in a lazulite-rich domain of the lazulite-quartz vein. BSE image.

lazu

lite

qu

art

za

pa

tite

Monazite partly replaced by

florencite.BSE-image.

Monazite with concentric

zoning in Th,Ca-content is

replaced by florencite.

Note the sharply cut growth

zones of monazite without

signs of significant

compositional modification.BSE-image.

Monazite is partly replaced by florencite. Florencite is broken, cracks are filledwith muscovite.BSE-image.

Strongly zoned, non-corrodedmonazite crystal. Th-content range from 0.5 to 13 wt.%.BSE-image.

Fresh monazitecrystals inassociation withquartz, muscoviteand Mg-rich chlorite.BSE-image.

Monazite is partly replaced by florencite. Differences in BSE-contrast of florencite are due to variable Sr-, Ca- and REE-content. Darker areas are enriched in Sr and Ca.BSE-image.

Typical replacement texture of of monazite in unalteredmetapsamitic-metapelitic country rocks. Monazite is partlyreplaced by apatite and REE-bearing epidote. BSE-image.

Typical replacement texture of of monazite in unalteredmetapsammitic-metapelitic country rocks. Monazite is sligthly corroded by apatite. BSE-image.

Eo-Alpine chloritoid is overgrowing

florencite and fresh monazite as well.

BSE-image.

Eo-Alpine chloritoid is overgrowing large,

unaltered monazite grains.

BSE-image.

Intimate intergrowth of florencite with Eo-Alpine rutile. Agerelationships between florencite and rutile are not entirely clear. Note small fresh monazite grains enclosed in muscovite. BSE image.

Fractured florencite with monazite relics and muscoviteinclusions. Fractures in florencite are filled with muscovite. Differences in BSE contrast of florencite are due to vari-able Th-, Ca- and REE-content. Rims are enriched in Ca and Th compared to interior parts.

Mz

Mz

10 µm

Ab

Qtz

Qtz

Ms

Ap

Lower Austroalpine units

Wechsel complex

Waldbach complex

Grobgneis complex

Phyllites, phyllonites

Micaschists, paragneisses

Metagranitoids, coarse grained

Metagranitoids, fine grained

Permomesozoic cover

Phyllites, Arkoses, Quartzites

Metacarbonates

Porphyroids

Middle Austroalpine units

Upper Austroalpine units

Grauwackenzone,Paleozoic of Graz

Northern Calcareous Alps

Tertiary and Quaternary cover

Lazulite-quartz veins

a

Geological map of the northwestern part of the Lower Austroalpine Grobgneis Complex with locations of lazulite-quartz veins. Source: Cornelius (1936), modified.

Krieglach

Mürzzuschlag

KindbergHK

GR

Introduction

Petrography of Monazite Replacement by Florencite

Geological Setting

Monazite, ideally CePO , is a wide-spread 4

accessory mineral in igneous, metamorphic and sedimentary rocks. It is highly stable during weathering and also often during metamorphism up to granulite facies con-ditions and migmatization.

The stability of monazite during metamor-phism, however, appears to depend on rock type and associated fluids. Monazitegrows in pelitic rocks from ca. 450° C on-wards, whereas in metamorphosed granitoids, primary magmatic monazite can be partly or totally replaced by apatite, allanite and epidote at the same metamorphic conditions. This kind of monazite alteration requires some addition of Ca through metamorphic fluids.

A different type of fluid-assisted monazitereplacement was observed in alteration zones adjacent to Permo-Triassic lazulite (MgAl (PO ) (OH) )-quartz veins in polymeta-2 4 2 2

morphic metapelites and metapsammites of the Lower Austroalpine Grobgneis Complex, northeastern Styria, Austria. Here, monazite is largely or totaly replaced by florencite, ideally CeAl (PO ) (OH) . Florencite is a3 4 2 6

member of the crandallite group, belongingto the alunite supergroup.

The Grobgneis Complex is part of the Austroalpine nappe pile at the eastern end of the Alps. It belongs to the Lower

2Austroalpine units and covers an area of ca. 2000 km , east and west of the underlying Wechsel- and Waldbach Complex.The Grobgneis Complex consists of a polymetamorphic basement, comprising phyllites, phyllonites, mica schists, paragneisses and voluminous Carboniferous to Permian granitoid intrusions, and a Permo-Triassic, parautochthonous cover sequence, comprising phyllites, metaarcoses ("Verrucano",Permian), porphyroids, quartzites ("Semmeringquarzit", Scythian) and various metacarbonates (Triassic-Jurrasic).

Three metamorphic events can be distinguished, based on petrological and geochronological data:- A Variscan (340-310 Ma) amphibolite facies metamorphism (garnet + staurolite).- An only locally observed Permian (280-240 Ma) HT-LP metamorphism (andalusite + sillimanite + biotite).- An Eo-Alpine (100-80 Ma) greenschist to lower amphibolite facies overprint (chloritoid + chlorite + garnet staurolite kyanite). For the Permian and Eo-Alpine event, metamorphic conditions seem to increase from north to south.

A conspicuous feature of the northern part of Grobgneis Complex is the occurrence of abundant hydrothermal lazulite-quartz

3veins. In most places, they are found as up to several m large, loose blocks, and only a few outcrops are known, eg. locality Höllkogel, HK and Granegg, GR. These outcrops were studied in more detail, because the relationships beween veins, host rocks, and alteration can be readily obsered.

±±

50

100

150

200

300

350

Tertiary

Cre

tace

ou

sJu

rass

ic

250

Tria

ssic

Pe

rmia

nC

arb

on

ifero

us

Devonian

ign

eo

us a

ctiv

ity

me

tam

orp

his

m

hyd

roth

erm

al e

ve

nts

loca

lize

d d

efo

rma

tion

Granitoid intrusions

Lazulite-quartz veins

upper greenschist -amphibolite faciesmetamorphism

shearzones("leucophyllites")

Granitoid and pegmatite intrusions

High-temperatureLow-pressure metamorphism

amphibolite facies metamorphism

V

ari

sc

an

oro

ge

ne

sis

P

erm

o-T

ria

ss

ic

cru

sta

l e

xte

ns

ion

Eo

-Alp

ine

o

rog

en

es

is

Endogenic geological events recorded in the Grobgneis Complex, based on petrological and geochronological data.

Petrographic and chemical characteristics of an alteration zone in metapsammitic country rocks.Outcrop at Höllkogel (HK).

This alteration zone is composed of quartz, muscovite, Mg-rich chlorite (XMg = 0.70-0.85), apatite, rutile, zircon, xenotime, and monazite/florencite. Replacement of monazite by florencite is restricted to the proximal part.

10 cm

Metapsammitic host rock Metapelitic host rock

Chemistry of Monazite Replacement by Florencite

Monazite

Florencite

(LREE,Th,Ca)(P,Si)O4

(LREE,Th,Ca,Sr)Al (PO ) (OH)3 4 2 6

a

GRMonazite freshMonazite relicticFlorencite

Th

/ S

um

RE

E (

mo

le r

atio

)

La / Nd (wt.% ratio)

0.30

0.20

0.25

0.15

1 2 3 4

0.05

0.10

0

UC

C

Th

/ S

um

RE

E (

mo

le r

atio

)

La / Nd (wt.% ratio)

0.30

0.20

0.25

0.15

1 2 3 4

0.05

0.10

0

UC

C

HKFlorencite vein (0)

Monazite relictic (1)Florencite (1)

Monazite relictic (2)Florencite (2)

Monazite fresh (3)

A-->C: within-grain variation

a

a

a

a

Variation of La/Nd ratio and Th/REE ratio in monazite and florencite in the alteration zone at Höllkogel (HK - metapsammitic host rock). Some florencite analyses show a significant shift to higher La/Nd ratios compared to precursor monazite. Th/REE ratios in florencite do not reach values as high as in precursor monazite. UCC: Upper Continental Crust.

Variation of La/Nd ratio and Th/REE ratio in monazite and florencite in the alteration zone at Grannegg (GR - metapelitic host rock). Both ratios are in the same range for florencite and precursormonazite. UCC: Upper Continental Crust.

Sr

(apfu

)

Ca (apfu)

0.20

0.25

0.15

0.05 0.1 0.15 0.2 0.25

0.05

0.10

0

HKFlorencite vein (0)Florencite (1)Florencite (2)

GRFlorencite

a

Content of divalent cations in florencite. Sr- and Ca-contents aredifferent for the alteration zones studied and florencite in lazulite-quartz veins itself.

Incorporation of Th in monazite and florencite in the alteration zoneat Höllkogel (HK - metapsammitic host rock). Note the highly variable Th-content within a single monazite grain. Monazites in sample (2) contain up to 1.5 wt.% SO . Th-content in florencite is 3

not correlated with any mono-, di- or tetravalent element.

Incorporation of Th in monazite and florencite in the alteration zoneat Grannegg (GR - metapelitic host rock). Th-content in florencite ispositively correlated with mono-, di- and tetravalent elements.

Th

(a

pfu

)

Ca+Sr+Ba+K+Si (apfu)

0.20

0.15

0.10

0.1 0.2 0.3 0.4 0.5

0.05

0

A

B

C

C

B

A

HKFlorencite vein (0)

Monazite relictic (1)Florencite (1)

Monazite relictic (2)Florencite (2)

Monazite fresh (3)

A-->C: within-grain variation

a

a

a

a

to T

hS

iO4

and

CaT

h(P

O4)

2

Th

(a

pfu

)

Ca+Sr+Ba+K+Si (apfu)

0.20

0.15

0.10

0.1 0.2 0.3 0.4 0.5

0.05

0

GRMonazite freshMonazite relicticFlorencite

to T

hS

iO4

and

CaT

h(P

O4)

2

500

100

10

1

0.01

0.1

5001001010.01 0.1

Alte

red

Me

tapsa

mm

ites

at H

K, n

= 7

(w

t.%

, p

pm

)

Fresh Metapsammites, n = 6 (wt.%, ppm)

10 2 1

0.5

0.1

TiO2

Na2O

MgO

MnO

CaO

P2O5

Al2O3

SiO2

Rb

Sr

Zr

Fe2O3K2O

LOI

Isocon-diagram for the alteration zone in metapsammitic rocks at Höllkogel (HK). Assuming Ti and Zr as immobilie elements, Na, Mn, and Fe are significantly depleted, whereas Mg is ca. 3-fold enriched in the alteration zone compared with unaltered metapsammitic country rocks. Volume change is not significant. Error bars represent 95 % confidence limits.

gain

loss

Chemical Characterization of Host Rocks and Alteration Zones Summaryand

Conclusions

Chemical comparison of alteration zones with host rocks. Lazulite-quartz veins with alteration zones occur both in metapsammites and metapelites. These rock types are clearly discernible by characteristic Zr/Ti-ratios. Ratios of typical "immobile" elements remained virtually unchanged during the alteration process.

lazulite-quartz veinsmetapelitesalteration zone in metapelites (GR)metapsammitesalteration zone in metapsammites (HK)metagranitoidsPermo-Scythian siliciclastic rocks("Verrucano")

TiO x 2002Zr

Al O x 102 3

l

metapelites

metapsammites

sedim

entary

fracti

onation

shale (Garcia et al.,1991)sandstone

P

PAl

Al

H O2

H O2

Conceptual Model of Monazite and FlorenciteFormation and Behavior in the Studied Region

Alteration zone -proximal parts

Alteration zone -distal parts and

metasedimentarycountry rocks

Crystallization ofstrongly zoned

monazite, locally recrystallization during HT-LP

metamorphism

No changes

Partial or total replacement of monaziteby florencite due to influx of P, Al, H O,2

(Sr). Preservation of primary growthzoning in relictic monazite. Conservationof REE would result in a three-fold volumeincrease.

Brittle deformation of florencite.Overgrowth by chloritoid.

Deformation and recrystallizationof monazite in shear zones("leucophyllites").

OR

Preservation of monazite orpartial to total replacement byapatite and REE-bearing epidote.

Greenschist to loweramphibolite facies

metamorphism.Ductile and brittle

deformation

80-100 Ma

.Formation of

lazulite-quartz veins

ca. 240 Ma

.Amphibolite facies

metamorphism

ca. 340 Ma

HT-LP metamorphism

ca. 270 Ma

s

Aluminium and phosphorus are major elements in lazulite-quartz veins, and breakdown of mona-zite to florencite in the alteration zones can be most probably related to the hydrothermal fluids associated with vein formation. This interpretation is supported by the occurrence of accessory floren-cite as the sole LREE-bearing mineral in the lazulite-quartz veins itself, suggesting instability ofmonazite, and stability of florencite, in the vein forming fluids.

Assuming constant REE, this type of monazitealteration would result in a ca. three-fold volumeincrease by addtion of Al, P and H O. However, 2

some fractionation and transport of REE and Thmay have occurred.

Distal parts of the alteration zones and unaltered country rocks contain monazites with total Pb ages between 360 and 250 Ma. These monazites are usually fresh or can be partly replaced by apa-tite and REE-bearing epidote. This replacement can be related to the Eo-Alpine greenschist to lower amphibolite facies overprint of the area. Florencite remained stable during this metamorphic event.

Mz

Mz

Mz

FlCltd

Ms

Fl

Mz

Mz

Mz

Mz

Ap Ep

Mz

quartz, muscovite, Mg-rich chlorite (XMg = 0.70-0.85) apatite, rutile, zircon, xenotime, monazite/florencite

quartz, muscovite, Mg-rich chlorite (XMg = 0.70-0.85) apatite, rutile, zircon, xenotime, monazite

quartz, muscovite, chlorite (XMg = 0.40-0.50), +/- albite, +/- garnetapatite, rutile, zircon, xenotime, monazite,epidote