Les skarns à wollastonite et grenat de Demir Tepe, Tahtaköprü (Villayet de Bursa, Turquie

0 Acadbmie des sciences / Elsevier, Paris GBomat&iaux / Geomaferials (M&allog6nie / of8 deposits)

Wollastonite-garnet skarns of Demir Tepe, Tahtakiiprii (province of Bursa, Turkey)

Les skarns A wollastonite et grenat de Demir Tepe, Tahtakijprii (Villa yet de Bursa, Turquie)

Michel DEIVIANGE’, Frarqois BERSON’, Michel FONTEILLE?, Marie-Lola PASCAL~, Sinan &GEN~ et Marie- Claude FORETTE~

] Centre d’injormattquegt’ologique, hole a’es mines. 35 rue Saint-Honow-, 72305 Fontuinebleau cedex, France

’ Ihoratotre de p&rolq@, universit6 Pierre-et-Mane-Curie, tour 26, 3” &age, 4, phce Jwieu, 75252 Park cedex 05, France ’ Centre de recherche sur la splh&e et la cbimie des minkraux, CNRS, lu, rue de la Hroilerie, 45100 Ork?am, France

4 Istanbul Univasity, Fact&y of Engineairag Institute of Geolrogical Sciences, .$4%50 Aucilar, Istambui, Turkey

ABSTRACT

Wollastonite-garnet skarns of Demir Tepe (Turkey) are developed in graphitic marbles, forming roof-pendants in the granodioritic Tertiary pluton of Gdyni.ikbelen, at the contact of stocks and veins of diorite-monzodiorites that intrude the main pluton. Skarn formation includes several superposed stages

-main stage: diopside-andesine and anorthite-diopside-grossufar rich garnet (or anonhite-diopside- -wollastonite) endoskams and exoskarns made of massive wollastonite; -anorthite-diopside-andradite rich garnet veins;

-scapolitisation; -development of massive garnet; -several later stages including copper mineralization, alteration into zeolites.,

0 Academic des sciences / Elsevier. Paris.

Keywords: Skarn, Wollastonite, Garnet, Pyroxene, Scapolite, Zeolites

Les skarns li wotlastonite et grenat de Demir Ttpe (Turquie) se dheloppent dans des calcaires graphiteux en roof-pendant dans la granodzorite tertiaire de GOyniikbekrt, au contact deJilons de

diorite-monzodiorite intnrsifs darts leplutonprinc@al. Laformation de ces skarns est unph&omtine polJ$ha.s8 : - stade principal avec formation d’endoskarns d diopside-andcsine et d anorthite-diopside-grenat riche en grossulaire ou d anortbitt+diopsid~wolCastonite et d’exoskarns d wollastonite massive; - veines ci anorthite-diopside-grenat ricbe en andradite ; - scapolitisation ;

- dheloppement extensifde grenat ; - d@%nts stades tardgs : min&alisation en cuivre, alt&ation en zkolites .,

0 Acadhnie des sciences / Elsevier, Paris

Mots cl& : Skarn, Wollastonite. Grenat, Pyroxhe. Scapolite, Z&olites

Version abrCg6e (voir p. 777)

Kate prksentke par Jean Dercourt

Note remise le 26 novembre 1997, acceptke apr& @vision le 2 avril 1998. .._~__

C. R. Acad. Sci. Paris, Sciences de lo terre et des planbtes / Earth & Planetary Sciences 1998. 326,771-778

M Demange et al.

1. Introduction. Geology of the skarns

Located in NW Anatolia, near the village of Tahtakdpru,

80 km ESE of Bursa, the skarns of Demir Tepe, belong to

the province of wollastonite skarns that streches to Can

and Cannakkale 200 km westwards &pen, 1992) and even may be extended to the region of Xanthi-Drama in

NE Greece.

The skarns of this province are associated to Oligocene-

Lower Miocene granodioritic intrusions (Bingo1 et al.,

1982) related to the subduction of the Tethys below the

Eurasian plate (actually the Sakarya microcontinent, Sen-

gor and Yilmaz, 1981). Ophiolites of the Izmir-Ankara

suture outcrop about 10 km south of Demir Tepe. Around Demir Tepe, the Eurasian crust is made of pelitic

paragneisses (transformed into cordierite-andalusite horn- fels by contact metamorphism) and marbles, intruded by

the Goynukbelen pluton. Two magmatic suites are recog-

nized: - the main Gijynukbelen suite contains amphibole-

biotite-(augite) granodiorites (a mesocratic central facies

and a more leucocratic main facies) with minor biotite-

amphibole granite, microgranites and aplites; - the Demir Tepe suite form a smaller stock and veins

intruding the main pluton, made of hornblende-biotite-

(augite) (micro)-diorites-(micro)-monzodiorites with very

minor biotite-ferrohornblende-augite banded syenites; the

diorites contain inclusions of norites.

The skarns are developed in roof-pendants of graphitic

limestones in the Demir-Tepe diorites-monzodiorites. The contacts of the main Goyniikbelen granodiorite and gran-

ites are barren. The skarns appear at the contact of veins of

diorite-monzodiorite and form metric: to plurimetric veins

in the limestones in the continuation of the intrusive veins;

they reach their main development (up to 30 m) in the

embayments of the intrusive stock.

2. Formation of the wollastonite-garnet

skarns

It does not result from the direct interaction of the marbles

and the adjacent intrusives but appears due to the circu-

lation of fluids along this contact that have reacted with

both marbles and intrusive rocks. The limit of the endosk-

arn and exoskarn may be located within the present garnet

zone; most of this garnet zone is actually an endoskarn

and only its outer part, rich in wollastonite inclusions, is an exoskarn.

Formation of the skarns is the result of several super-

posed stages (figure 1): - main stage of formation of the skarns; - anorthite-andraditic garnet veins; - scapolitisation stage; - extensive development of garnet; - several later low temperature stages.

Study of the skarn is somewhat complicated because of

such superposition of phenomena. Garnet and scapolite are widely superposed to former parageneses and obliter-

ate the earlier associations and their contacts. So, some

parts of the original zonation of the main skarn stage

remain speculative.

3. Main stage of skarnification

The exoskams are developed from almost pure,

graphitic marbles, containing some levels of impure

dolomitic marbles (calcite-dolomite-phlogopite with

X,, = 0.99, forsterite-spinel).

Around the skarns, the black-coloured original marble

is transformed into a white marble due to oxydation of

graphite. The dolomitic phlogopite-bearing marble is

transformed into a diopside (X,, = 0.9, AI,O, up to

4.5 wt %) and garnet (XAnd = 0.2, TiO, up to 2.8 wt %,

figure 2) -bearing marble, sometimes with vesuvianite. A second zone, a few decimetres to 30 m thick, is made

of massive wollastonite with very minor clinopyroxene

(X,, = 0.9, AI,O, up to 2 wt %).

The most common endoskarns are developed on

diorites-monzodiorits. These rocks are made of major

plagioclase (both in phenocrysts and in the groundmass,

with complex twinning and zonation: mostly normal re-

current zoning from An45-50 to 35-30 with a rather calcic zone An53-55 short before the rim) and hornblende

(in phenocrysts and in the groundmass), some minor po-

tassic feldspar, quartz and biotite and ilmenitemagnetite-

titanite and zircon; augite may be present either as ar-

moured relicts in the phenocrysts of amphibole or in the

groundmass.

The change of these rocks into fine grained green en-

doskarns results mostly from the transformation of the

ferromagnesian minerals into diopside. The composition

of the diopside varies with the grade of re-equilibration

from X,, = 0.6 (similar to XMg of the initial amphibole) to

XMp 0.85; Al,O, and Na,O contents decrease from 1 % to

0.1-0.2 wt %, and from 0.3-0.4 too.1 wt %, respectively

(figure 3). In some cases the initial hornblende is directly

replaced as ‘a whole by pyroxene, in some others the

transformation is progressive with an intermediate stage of,

actinolitic amphibole surrounded by a diopsidic corona. The plagioclase remains first unchanged thus preserving

the diorite texture.

A further transformation grade involves an irregular

plagioclase recrystallization which blurs its shape and

zoning but leaves its composition unchanged (An 40-45).

Quartz and K-feldspar vanish. The endoskarn is thus com-

posed of diopsidic pyroxene-plagioclase-titanite-apatite.

Most often the contact of the endoskarn with the wol-

lastonite zone is masked by the subsequent extensive

development of garnet: the most transformed zones of the endoskarns are not preserved. Some rare garnet-anorthite

veins may represent one of them: they contain very fine-

grained anorthite An85-99 and orange-coloured

grossular-rich garnet, Xc, = 0.70-0.85, with compara- tively high contents in TiO, (up to 1.8 wt %), MnO (up to

1 wt %) and Fe’+ (up to 0.4 wt %, X,,, up to 0.07) (figure 2).

772 C. R. Acad. Sci. Paris, Sciences de la terre et des planhtes / Earth & Planetmy Sciences 1998.326,77 l-778

Wollastonite-garnet skarns

green j endoskarn

garnet wollastonite i white black tone zane :

Figure 1. Organisation and evolution of the skarns. a idrnm!

Organisation et Cvolution des skarns.

The endoskarns developed from the banded syenites are

fairly rare. These rocks contain K-feldspar with minor

plagioclase (An29-32), quartz, biotite, ferrohornblende,

augite (X,, = 0.63-0.70), minor titanite, apatite, ilmenite and magnetite. The first endoskarn zone is K-feldspar +

pyroxene (XMg = 0.83-0.91, Al,O, up to 0.83 wt %) (+

titanite + apatite). Wollastonite appears as tiny and scarce

crystals in the second zone. In our samples the contact

with the next zone is blurred by extensive development of

scapolite. This zone, made of anorthite An73-95 +

wollastonite t pyroxene (X,, = 0.86-0.98, AI,O, up to

2.56 wt %) (+ titanite + apatite), is somewhat similar to the

anorthite + orange garnet veins developed from the diorite: the presence of wollastonite instead of grossular-rich

garnet may be explained by the difference in chemical

compositions between the host rocks (figure 4). The more

transformed zones and the contact with massive wollasto-

nite are overprinted by later massive garnetite. The temperature and oxygen fugacity conditions at this

stage have been estimated from the stability of the assemblage {anorthite-wollastonite-pyroxene (X,, = o.V)} in

the syenite-derived skarns, whereas garnet (&. = 0.75) is

observed coexisting with the same X,, = 0.9 pyroxene in

the {garnet-anorthitelveins of the skarnized monzodiorite. Assuming the same temperature and oxygen fugacity con-

ditions for these two assemblages, this temperature is higher than the minimum temperature for the stability

of (anotthite + wollastonite + garnet + CPXI assemblage,

written as follows:

2 CaAI,Si,O, + 3 CaSiO, + [Ca,Fe,Si,O,,]““’

= 2 [Ca,~AI,Si,O,,]ga’ + 3 [CaFeSi,OJCP” + l/2 0, (1)

(the brackets indicate the end-member component in a solid solution).

This temperature is dependent on the garnet composi-

tion which itself depends, at equilibrium with wollasto-

nite, and pyroxene, both on the oxygen fugacity and the

silica activity, as shown by the following equilibrium:

2 [CaFeSi,OJCPX + CaSiO, + l/2 0,

= [Ca,Fe,Si,O,J+’ + SiO, (2)

The lowest temperatures for Igarnet-wollastonite-CPX-

anorthitel equilibrium correspond to silica-saturated

conditions. This equilibrium is represented in figure 5,

where the garnet compositions along the curve are indi- cated by the solid dots. The calculations are performed

C. R. Acad. Sci Paris. Sciences de la terre et des plan&es / Earfh &Planetary Sciences 1998.326,771-778

M. Demange et al

Demir Tepe garnet

andradite grossularite

Figure 2. Garnets: Al/TiO, diagram. Garnets CR’+, R3’,Si,0,,) have been calculated in the base Rae + R3+ = 10; Fe3+ was calculated by balance of the oxygens. Demir Tepe garnets are calcic garnets varying between the grossularite and the andradite end members. Ti, Fe’+ and Mn are very minor constituants, but allow us to distinguish the various types of garnets; the latter elements vary in accordance. 31

Les grenafs : diagramme A//770,. Les grenats R2’b R3’,Si,0,, ant &ft+ calcul& sur une base Rzc + R3’ = 10, le fer ferrique &ant calcult; en fixanf /es oxyghtes correspondant d ces cations B 12. Les grenats de Demir Tepe sent des grenats calciques holuant entre le pale grossulaire et le p81e andradite. Ti, F&’ et Mn demeurent h& mineurs, mais permettent de distinguer les diff&en& types de grenats ; ces trois CMments varient parallP/ement.

using thermochemical data from Holland and Powell

(1990), and assuming ideal site mixing for both heden-

bergite-diopside and andradite-grossularite solid solu-

tions.

ity is thus slightly (about 1 log unit or less) below

hematite-magnetite buffer, and the minimum temperature

range is 620-650 “C for this early stage.

Lower silica activity would strongly shift this line towards higher temperatures. The {wollastonite-anorth-

ite-CPXI association thus lies on the high-temperature

side of this curve. The mineralogical difference between

the skarnized syenite tanorthite + wollastonite) and the (anorthite + garnet + CPX) veins in the skarnized monzo-

diorite is most probably due to a difference in asiod, which

must be lower in the skarnized monzodiorite thus favour-

ing the garnet stability. The anorthite-pyroxene-garnet equilibrium:

4. Anorthite-andradite-rich garnet veins

The endoskarns developed on diorite/monzodiorites are

crosscut by centimetric to decimetric veins of anorthite +

orange garnet + pyroxene similar to those described as the

most external zone of the endoskarn, except that the

garnet is richer in iron: XAnd = 0.58-0.80,

X Cro - - 0.26-0.10. It is also rich in TiO, (up to 2.46 wt %),

MnO (up to 1.31 wt %I and Fe0 (X,,, up to 0.13). These veins are sometimes independent of the anorthite-

grossular-rich garnet veins, as most often the garnet compositions are fairly homogeneous within a single vein.

However in some cases the fluids clearly used the same

pathways during both stages, as some garnets exhibit a

zonation from Al-rich composition in the core to Fe-rich

composition at the rim; the central parts of the veins contain Fe-rich garnets and the edges Al-rich ones.

CaAI,Si,O, + 2 [Ca,Fe,Si,O,,]“a’ + 3 SiO;q

= [Ca,AI,Si,O,Jga’ + 4 [CaFeSi,O,lCP” + 0, (3)

is shown in figure 5 for three different aSi values not far

from silica saturation, as expected in thesk rocks where pyroxenes have a fairly low AI,O, content. Oxygen fugac-

774 C. R. Acad. Sci. Paris, Sciences de la terre et des plan&es / farm & PlanetorySciences 1998. 326,771-778

Wollastonite-garnet skarns

Den-k Tepe pyroxenes

amptub& + CPX endoskarn

CPX + plag1wlrse endoskarn

orange garnet + anorthite

marble

50 60 711 80 90 100

X mg B

Figure 3. Pyroxenes: X&AI,O, diagram. Pyroxenes of the initial eruptive rocks are augites; their compositions vary according to the grade of differentiation: X and A&O3 contents decrease and Na,O content increases from norites to syenites. In the skarns, pyroxenes are diipsides, the composition o which varies from an end member similar to the one of the diorites to an end member re-equilibrated with the fluid ? (XMs = 0.85-90, A&O, 0.1-0.2 wt %, Na,O0.142 wt 46) in the endoskarn and the garnet zone. Pyroxenes of the exoskarns have X@and Na,O contents similar to the ones of the endoskarns, but their Ai,O, content is far higher: introduction of Tschermak’s component is the consequence of the deficiency in silica of these rocks.

Les pyroxPnes : diagramme X,,,,.JAl~O,. Les pyroxPnes des roches Pruptives initiales sont des augites dont la composifion refi&e /‘&o/&ion magmatique de ces roches : XMs et AI,O, d&roissenf, Na,O augmente avec la diff&enciation. Les pyrox&nes des skarns sont des diopsides qui montrent des variations depuis un pale voisin de /a composition des pyroxt’nes des diorites vers un terme r&quilibr& avec les solutions IX,, = 0,8.5-90, AI,O, O, l-0,2 %, Na,O 0, J-O,2 %) dans /es endoskarns ef /a zone du grenat. Les pyroxPnes des exoskams ont des XhlK et teneurs en Na,O simiiaires d celles de ces derniers, mais des teneurs en A/,0, beaucoup plus C/e&es : /‘introduction du composanf tschermakitique dans le r&eau du pyrox.&ne esf /iG au d&i& en silice de ce milieu.

Although the conditions of formation of these veins

cannot be precisely determined, they are clearly incom- patible with the anorthite-grossular-rich garnet veins (fig-

ure 4). According to equation (3), the andraditic composi-

tion of garnet might be explained by more silica-

undersaturated fluids which could have been equilibrated with the exoskarns prior to entering the endoskarns.

so, this transformation registers mostly a fixing of volatile

elements. Scapolite contains about 0.60 wt % of SO,; the

average calculated composition is [Na,,,Ca,,,], [Si,AIO,l,

Kl0.s, (SO,),,,,(CO,),,,,,l. The first pyrite and chalcopyrite appear at this stage.

5. Scapolitisation stage

6. Development of garnet

Scapolite develops extensively on the plagioclases of the

endoskarns, including the anorthite of the anorthite +

orange garnet (both grossular-rich and andradite-rich) veins. Scapolite appears first in small patches on the

plagioclases and the texture of the rock is preserved. Then scapolite forms large xenomorphic crystals invading the

whole rock. The AI/Si ratio is the same in the scapolite as in the plagioclase; it comes from (Ca/Ca + Na = 57-63 %):

Garnet develops extensively in the central part o’f the

skarns, around a drain which is probably the previous limit

between endo- and exoskams. The thickness of this massive garnet zone ranges from a few decimetres to 5 m.

Towards the endoskarn, garnet replaces plagioclase and

scapolite; pyroxenes, allanite, titanite and apatite are pre-

served; the pattern of the pyroxene is also sometimes

preserved and the former porphyroid texture remains rec- ognizable. In the exoskarns, garnet replaces wollastonite;

C. R. Acad. Sci. Paris, Sciences de la terre et des plan&es I Earth & Planetaty Sciences 1998 326,771-778

775

M. Demange et al.

Si

k banded syenifes lied IO pymxene

monzodiorites

Fe Al

Figure 4. Relations between chemical composition and mineralogical associations in the skams: parageneses of the skarns formed on banded syenites (anorthite + CPX + wollastonite) reflect a chemical compositiin different from those of the skarns formed on diorite-monzodimrite (anorthite + CPX + garnet). In the latter skarns the parageneses containing a grossular rich garnet are not compat- ible with those containing an andraditic garnet and thus have been formed under different conditiins. ~~~~~~~~1,~

Relations entre /a composition chimique et la composition minera-

logique dans les skarns : /es associations des skarns sur syenite

rubanee lanorthite + CPX + wollastonitel ref/etent des compositions

chimiques differentes de celles des skarns Wvelopp& sur diorite-

monzodiorite lanorthite + CPX + grenat). Par contre pour ces derniers

les associations d grenat riche en grossulaire et celles a grenat riche

en andradite sont incompatibles et reftetent done des conditions de

formation differentes.

isolated garnets and veins of garnet in wollastonite pre- cede the front of massive garnet which contains inclusions

of wollastonite.

Garnet exhibits a complex zoning: most times the core

is isotropic and has the composition of the earlier garnets,

both andradite-rich and grossular-rich and is often rich in

titanium; then comes an anisotropic garnet poor in tita-

nium, with a complex and recurrent zoning with compo-

sitions intermediate between those of the core and the

final garnet; this final garnet is isotropic and may appear as

the external zone of the zoned garnets (and sometimes

crosscuts the zoning of the anisotropic part) or as veins

crosscutting earlier structures of the garnet zones or even

of the endo- and exo-skarns. The compositions of the outer zones of the anisotropic garnets and this final isotropic

garnet are fairly homogeneous throughout the deposit:

X - 0.45, very poor in Ti, Mn and Fe”. And -

Pyroxenes are quite similar to those of the endoskarns

wlMg = 0.83-94, low AI,O,, and Na,O contents) and are

also rather homogeneous throughout the deposit.

Formation of garnet implies the deposition of iron and

silica, such as expected from fluids undergoing a tempera-

ture decrease. Transformation of wollastonite into garnet

also demonstrates that alumina becomes then a mobile

element, while it was inert during earlier stages.

7. late stages

Copper mineralization, mined during Ottoman times, de-

velops in the wollastonite zone and to a lesser extent, in

1 I I 1 Kbar

0 -.. _ _s”nzt X,,=0.75-anorthite-cpx

logas,&- - - - - - -

Figure 5. Temperature-f,, (relative to hematite-magnetite buffer) conditions for the main stage skarn formation: the solid line indicates the (garnet-wollastonit+CPX (XMs = 0.9O)-anorthite-SiO,) equilibrium corresponding to silica saturation (the garnet composition is shown by the solid circles on this line); the (anorthite-wollastonite-CPX] assemblage of the syenite-derived endoskams lies on the high temperature side of this curve. The dashed tines indicate {garnet (A&, = 0.75)-pyroxene (XMs Data from Holland and Powell (1990).

= 0.9O)-anorthite) equilibria corresponding to different values of asio2.

B

Conditions de temperaturefo, (rapport&a au tampon hematite-magnetite) de formation du stade principal des skarns : en trait plein, equilibre

Igrenat-wollastonite-CPX(X Mrr = 0,90)-anorthite-SO,) a saturation de la silice ; la composition du grenat est indiquee par des points noirs sur le

trait plein. L’association (anorthite-woliastonite-CPX/ des endoskarns se situe d droite de cette courbe. Les 6quilibreslgrenat

CX,, _ 0,75)-pyroxene (X,, =

/I 990). 0,90)-anorthitej sont represent& par les tiretes pour ditferentes valeurs de a,,,,,. Donnees de Holland et Powell

776 C. R. Acad. Sci. Paris, Sciences de la terre et des plan&es / Earth & Planetary Sciences

1998. 326.771-778

Wollostonite-garnet skarns

the garnet zone and in the endoskarns. Bornite and minor

chaicopyrite and pyrite fill the interstices between the

laths of wotlastonite and form veins. This mineral association is only stable under 550 “C. In the endoskarns epidote

appears commonly as a reaction rim between the sulfides and the plagioclase. Ferrohastingsite and a late phlogopite

appear also in association with sulfides in the endoskams.

Bornite is transformed into chalcopyrite + digenite in

symplectic association; this reaction takes place at tem-

peratures lower than 335 “C (Craig and Scott, 1982). Su-

pergene alteration produces chalcosite and covellite. Zeolites appear as alteration products of plagioclase

and scapolite and in vugs in the endoskarns and the garnet

zone. A whole series of calcic zeolites demonstrates a

progressive enrichment in silica: scolecite, levine, yu-

gawaralite, epistilbite and heulandite. Yugawaralite is an

uncommon mineral which has been, but very rarely, re-

corded from skarn deposits. The association yugarawalite

+ quartz has a limited range of stability: 420-490 “C with

pressure lower than Cl.5 kb (Kisela et al., 1996).

Alteration products of wollastonite include veins of

apophyllite + quartz and vugs of thaumasite. The upper

stability limit of this rare mineral is 205 “C (Font-Alba,

1960).

Au nerd-ouest de I’Anatolie, les skarns de Demir Tepe appar- tiennent i une province de skarns i wollastonite, qui s’Ctend

sur 200 km depuis Demir Tepe, 2 80 km ;I I’ESE de Hursa, jusqu’a la region de Cannakkale (ongen, 1992). Ces gisements sont associCs aux intrusions oligo-mioctnes

(Bing61 et al., l!$!Q) qui se mettenl en place dans la plaque eurasiatique subductee pa les ophiolites de la ceinture Izmlr- Ankara (Seng6r et Yilmaz, 1981). Les skarns de Demir Tepe se localisent dans des marbres graphiteux en c’ roof pendant n dans le granite de Giiyniikbelen. C&i-ci comprend deux suiles

magma1iques : - la suite de Giiynfikbelen formCe de granodiorites 2 amphi-

bole/biotlte, avec, en plus faible quantitk, cles granites, microgranites et aplites; les contacts de ces roches avec les marbres sent s&riles : - la suite de Demur Tepo qui comprend (ICS

diorites-monzodiorites avec quelques rares syenites r&an&s. Les diorites-monzodiorires sont form&s de plagioclase (An50-301, hornblende, feldspath potassique et quartz mineurs, biotite, ilmenke, magnetite, sphene et zircon; de

l’augite apparait soit en inclusion dans l’amphibole, soit dans la matrice. Les skarns P wollastonire se d@veloppenl exclusive- merit au contact de ces roches. La formation de ces skarns est un ph&om?ne polyphase

comprenant six ou sept SkKkS successifs (figure 1). La structure spatiale lies differentes zones montre que le gisement ne resulte pas d’une interaction directe entrc le calcaire et les granitoi’des adjacents, mais de la circulation dr fluides guides

par ce conract et qui ant r&t@ des deux ci,tCs, aussi bien avec les granirciides qu’avec les calcaires. La limite emre endoskarn

et exoskarn :ie situe dans l’actuelle zone du grenat. L’existence de phCnomenes superposils complique l’ktude des skarns. puisyue les mineraux tardifs, en particulier la scapolite

et le grmat, se superposenl aux paragenkes an&rieures. L’organisation des stades les plus p&coces demeure done incompli-tement connue.

Stade de skamification principal

Les exo.&Zrr~~ se &veIoppent sur des rnarhres calciques noirs graphiteux, qui contiennent de rares niveaux i phlogopiteAo- lomite-ohvine el spinelle. Le marbre initial est d’ahord trans-

form6 en marbre blanc par oxydation du graphite. Les niveaux

impurs ri phlogopite se transforment en marhre & grenat (X,,,, = 0,2 et TiO, jusqu’P 2,8% en poids, figure 21, diopside (&, = 0,9, AlJO jusqu’g 4,5 %, figure 3), et parfois idocrase.

Puis I’ensemble se transformc en wollastonice massive, asso- cib ;i un peu de cfinopyroxke CX& = 0,9, Al& jusqu’P 2 %).

Les erzdoskatmd~velopp&s sur les ~jordtes-monzodiorioesont des roches vertes a gl-ain fin, clans lesquelles les minCraux

ferromagn6siens initiaux sent transform&e en salite (dent le

YdppOTt xMp, d’abord voisin de celui des amphiboles des

diorites (O,b), se r&Cquilibre vite vets XMK = 0,84). Les plagioclases demeurent d’abord inchang&s et la texture de la roche initiale est p&e&e. Puis les plagioclases recristallisent, tout

en gardanl des compositions identiques (An50-351.

Lrs zones plus externes de ces endoskarns sent g&Cralement

masqu$es par Ie d&eIoppement du grenat. Queiques rares veines 1 anorthite (An85+) + grenat orang6 riche en grossulaire (Xc;,,, = 0,65-0,84) ct titanif?re (l‘i0, jusqu’s 1,s 96) et diopside (semblable 2 celui de la zone prMdente1 en sont saris doute repr@sentatives.

Les evuioskarns d&elopp& sur de (rares) @-r&s rubarzkes (roche initiale formee de feldspath potassique majeur, quartz,

plagioclase, hiotite, ferrohornblende, augite) montrent aussi une zone verte oti les minkraux ferro-magn&iens sont transform& en diopside, mais 00 le feldspalh est un feidspath

polaasirpe (et non un plagjoclase) ; puis vient uric zone oti apparaissent de rares petits cristaux de wollastonite ; la zone

suivante, dont le contact est masque par le developpement de scapolite, est Formee d’anorthite + wollastonite + diopside. Les

zones plus externes sent masqu6es par le grenat massif plus tardif’. Les differences entre les endoskarns d&elopp& sur

&rite et ceux sur syPnite refl@tent dees diff&ences de composition et de mineralogie des roches initiales (figure 4).

Les conditions de formation de ces skams pcuvent Ctre esti- m6es a l’aide de la stabiN de l’assemblage wollastonite +

anorthirc + CPX (XMn = 09) qwi. impose une tempgrature minimaie de 620-650 *C (figure 5). 1.a Stihi[it6 de l’assemblage

grenat (q;n, = 0,751 + CPX + anorthite observe dans les veines, sous une faible sous-saturation cn silice, en accord avec la

C. R. Acod. SCI. Paris, Sciences de la terre et des plan&es i Earth & Planetary Sciences 1990.326.771-770

777

M. Demonge et al.

basse teneur en AI@, du pyroxtne, indique une fugacite en

oxygene un peu plus faible que celle du tampon hematite-magnetite (figure 5).

Veines ii anorthite-grenat orange Ache en anwte

Ces veines centimetriques P grain tres fin, developpees darts la diorite, sont t&s semblables aux veines precedentes P anorthite

+ grenat orange riche en grossulaire : anorthite, clinopyroxene (XM8 = 0,871, sphene, mais le grenat, toujours titanifere (jusqu’l 2,46 % de TiO,), est riche en andradite (Xnnd = 0,85-0,55, figure 2). Ces veines se developpent parfois independamment

des veines precedentes, mais empruntent parfois aussi les memes voies, puisqu’il a ete trouve des grenats P cceur riche en grossulaire et peripherie riche en andradite, et qu’il existe des

veines dont la partie centrale contient un grenat riche en andradite, alors que le grenat de leur peripherie est riche en grossulaire. Les fluides responsables de la formation de ces

veines a grenat orange andraditique sont clifferents des fluides qui ont forme les skarns precoces, peut-etre plus sous-satures en silice.

Stade de scapolitisation

La scapolite se developpe tres largement sur les plagioclases

des endoskarns, y compris sur ceux des veines 1 grenat orange (riche en andradite et riche en grossulaire). Le rapport ALSi de

la scapolite est identique P celui du plagioclase remplace (le rapport Ca/Ca + Na est de l’ordre de 0,57-0,63) : la scapolitisation resulte done pour l’essentiel de la fixation d’elements volatils. La scapolite est assez riche en soufre (0,6 % de SO,) ;

la formule structurale calculee est en rnoyenne [Na,,,Ca,,,], ~Si~AlO~l~~Cl,,,,~SO~)~~,,,(CO~),,~~l ; a ce stade apparaissent

d’aileurs les premieres pyrite et chalcopyrite.

Le stade ii grenat massif

Le grenat se developpe a travers tout le gisement, a partir d’un

drain qui est sans doute l’ancienne limite endoskar-exoskarn Ce grenat se developpe aussi bien au depens de l’endoskarn

REFERENCES __-.--___ ---

Berson F. 1997. Les skarns a wollastonite et grenot de Demir Tepe, commune de Tohtakopti, villayet de Bursa, Turqule, &~pport de tfavail d’option. option Sciences de la Terre, Ecole des mines de Paris. 69 p,

Bingo1 E.. Delaloye M. and Ataman G. 1982. Granltic intrusions in western Anatolia: a contribution to the geodynamic study of this area, Eclogoe Geol. He/v., 7512.437-446

Craig J.R. and Scott SD. 1982. Sulfide phase equilibria, in: Ribb P. H. (Ed.). Sulfide mineralogy, Mineralogical Society of America, Reviews in Mineralogy

Font-Alba M. 1960. A thermal study of thaumasite, Mineralogical magazine, 32,567-572

qu’au d&pens de l’exoskam a wollastonite, formant des grena-

tites massives dont I’epaisseur peut atteindre 5 m. Les fronts

sont obliques sur les structures anterieures. Dans l’endoskarn, le grenat se developpe soit sur la scapolite, soit directement sur

l’endoskam precoce.

La zonation des grenats est complexe : cceurs analogues aux grenats precoces riches en andradite et/au en grossulaire, une pame anisotrope to&e (evolution globale des compositions

depuis celles des grenats initiaux jusqu’a une composition tres uniforme a &avers tout le gisement, XAnr, = 0,451 et grenats

ultimes isotropes (de meme composition).

Les grenatites renferment des inclusions de pyroxenes tres analogues a ceux des endoskarns precoces (X,, = 0,83-0,94) ;

les textures sont parfois p&e&es. Du c&e exoskam, les grenatites renferment des inclusions de wollastonite et se developpent manifestement aux depens de ce mineral ; la

transformation de la wollastonite en grenat suppose non seulement un apport de fer et de silice, mais aussi un apport

d’aluminium.

Ies stades tardifs

Une mineralisation en cuivre (bomite et a un titre moindre chalcopyrite et pyrite) se developpe en impregnations et veines, surtout dans la zone de la wollastonite. Cette associa-

tion minerale n’est stable qu’en dessous de 550 OC. Dans les endoskarns, epidote, phlogopite et ferrohastingsite s’associent

aux sulfures. La bornite se transforme ensuite en chalcopyrite + digenite (335 OC, Craig et Scott, 1982). L’alteration supergene

forme chalcosite et covellite.

Des zeolites apparaissent en alteration des plagioclases et de la scapolite et en geodes dans les endoskams et la zone du

grenat. Parmi toutes les zeolites calciques observees, la yugawaralite presente des conditions de stabilite (en presence de quartz) tres limides : 420-490 “C pour une pression inferieure P 0,5 kbar (Kisekd et al., 1996).

Les produits d’alteration de la wollastonite comprennent des veines P apophyllite + quartz et des geodes de thaumasite, tres

rare mineral dont la limite de stabilite ne depasse pas 205 ‘C (Font-Alba, 1960).

Ganguly J. 1976, The energetics of natural garnet solid solution. II, Mixing of the calcium silicate end-members, Contribution to mineralogy and petrology, 558 l-90

Holland T.J.B. and Powell R. 1990. An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: the system K,O-Na,O-CaO-MgO-MnO-FeO- Fe,O,-A&O,-TiO,-SiO,C-H,-O,. J. Mefamorph. Geol., 8.89-124

Kiseleva I,, Navrotsky A., Belitsky LA. and Fursenko B.A. 1996. Thermochemistry and phase equilibria in calcium zeolites, Ameri- can Mineralogist, 81,658-667

tjngen S. 1992. Les itchanges metosomatiques entre granitoTdes et encaissants partlculiers. Quelques exemples en Turquie du Nord-Ouest. Thbe, Universite Nancy I, 554 p. Sengor A.M.C. and Yilmaz Y. 198 1, Tethyan evolution of Turkey: a Plate tectonic approach, Tecfonophysics. 75, 181-241

C. R. Acad. Sci. Paris, Sciences de lo terre et des planetes / Earth & Plcrnefary Sciences 1998.326,771-778

Les skarns à wollastonite et grenat de Demir Tepe, Tahtaköprü (Villayet de Bursa, Turquie

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