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G'or,Ocrc Descrrprron of Rhe Serres Il American Mineralogist, Volume 74, pages 852-358, 1989 Vanadian garnetsin calcareousmetapelites and skarns at Coat-an-Noz,Belle-Isle-en-Terre (C0tes du Nord), France CoNsrulcr BrNrnnnou Centre de G6ologie G6n6raleet Minidre, Ecole des Mines, 60, Boulevard Saint-Michel, 75006, Paris Cedex 06, France Mrcrrnr, Foxrprr-r-ns Laboratoire de P6trologieet M6tallog6nie and CNRS, URA 0736, Tour 26, E54, place Jussieu75252, Paris Cedex 05, France Ansrrucr V-rich garnetsbelonging to the grossular-goldmanite(CarVrSirO'r) series are found with- in metamorphosed calcareousmetapelites. These metapelites are derived from euxinic sediments,and their composition is similar to that of unmetamorphosed Silurian black shales.All garnet compositions in the range Golo to Golu, are represented,and the solid- solution seriesis a continuous one. Complex sector zoning is observed.Associated clino- pyroxenesand actinolites show significant V contents. In skarns developed from similar metapelites, goldmanite-rich garnets form the cores of crystals surrounded by grossular overgrowths.The crystallization of goldmanite seemsto be controlled by the bulk chemical composition of the original sediments. INrnooucrroN 1988). It includes a lower unit of white orthoquartzites Calcic garnets belonging to the gol k) and a middle unit of black pelitic series have been foundin the course ( ck). The Coat Losquet formation itself study of paleozoic metasedimentary ries of tholeiitic metabasaltsand meta- scheelitemineralization. These formal ratron)' small paleozoic basin situated 50 km tr ric units described above have been large Chdteaulin basin in Brittany, France.The borehole, transformed by regional metamorphism and by thermal were made through tungsten-mineralizedskarns near the metamorphism from Hercynian granites. The regional Belle-Isle-en-Terrelocality, Coat-an-Noz (Fig. l), a few metamorphism is attested to by the development of an- kilometers west of the cityof Guingamp (Cdtesdu Nord). dalusite,-staurolite, and almandine within pelitic rocks, The vanadiferous garnets are found in calcareousmeta- of hornblendeand plagioclase(andesine) in amphibolites, pelites and scheelile-bearingskarns. The object of the of cummingtonite in Fe-rich or manganiferousmetape- present paper is to describe these V-rich gamets as well lites, and.of andalusite and calcic plagioclase(bytownite as other coexisting vanadian silicates. to anorthite) within calcareousmetapelites. Contact metamorphism in the northern part of the area G'or,ocrcDEscRrprroN oFrHE sERrEs il,:',:i?t?jltt"*::J*1,:Til111tr#?1"?:,L,i"iJl: V-rich garnetsare found in the upper levels of the Coat per;, arsJnopyrite-gold, and uraniuim mineralization, Losquet formation. This stratigraphic unit is approxi- whereas tungsten has not been reported. The wide zone mately 200 m thick and comprisescentimeter- to meter- known to the south is attributed to a hidden granite cu- thick beds of alternating black shales (infrequently cal- pola, with the Quintin granite cropping out l0 km south careous), sandstones,and subordinate limestones. Be- of the Coat-an-Noz area. More specifically, its evolved causeof metamorphic recrystallization and the absence facies are associatedwith cassiterite,wolframite, schee- of fossils, the age of these euxinic and calcareousforma- lite, and bismuth showings, similar to the Coat-an-Noz tions can be determined only by comparison with sedi- association in which, however, tin remains unknown mentary seriesfrom the nearbyChdteaulin basin (Sagon, (Chauris,1983, 1986). 1976). Two similar series are present in the Ch6teaulin V-rich garnets(Fig. 2) and clinopyroxene occur as un- basin, namely the Upper Silurian (Ludlow) and the Low- deformed porphyroblasts in some of the Coat-an-Noz er Devonian (Coblencian). Hirbec (1979), however, has metamorphic calcareousmetapelites. Textural evidence given a Coblencian age for the Coat-an-Noz formations, suggeststhat they are the product of contact metamor- an attribution confirmed by chemical analyses.The Coat- phism. Nonoriented biotites are also widely developedat an-Noz formations rest unconformably on the Cambrian the same stagein various rock types such as impure mar- Loc-Envel orthogneiss(Andriamarofahatra and La Boisse, bles, calc-schists,and some metapelites. 0003404x/89/0708-o852$02.00 852 BENKERROU AND FONTEILLES: VANADIAN GARNETS 853 Crrntlrrc,c.L coMpiosrrroN oF THE METAMORPHIC ROCKS Bulk chemical analysesincluding trace elements were performed using an automated 1400 Philips X-ray fluo- rescencespectrometer. Fe2+/Fe3+ratios and weight loss on ignition (L.O.I.) were determined by wet-chemical methods. Three of the four black calcareousmetapelites analyzed(Table l), contain V-rich garnets,and their anal- ysesshow between894 and 2021ppm V (CAN8, CANI6, CANI47). The fourth specimen(CANI3) closely asso- ciated with the three others, does not contain any garnet. Though it contains a significant amount of V (450 ppm), its low CaO content (0.56 wto/oas opposedto 5-13 wto/o) appearsto be a reasonableexplanation for the absenceof garnet. The V-rich metapelites also have a remarkable Mo (l0G-200 ppm) and high Ni (100-200ppm) contents. The high V content of these rocks is similar to that of unmetamorphosedSilurian black shalesin Brittany (Da- bard and Paris. 1986). These trace-elementconcentra- tions attest to the euxinic characterofthese sedimentary rocks. The high IGO content and L.O.I. of CANl47 and CANI3, respectively,are due to alterationphenomena- Fig. 1. Simplified geologicrnap and location of the Coat Los- late-stageK-feldspathization for the former specimen,and quet formation. 1, granite; 2, microgranite; 3, Cambrian Loc intense sericitization for the latter one. Envel gneiss;4, Brioverian basement;5, undifferentiatedPaleo- Two different types of skarn have also been analyzed zoic formations; 6, Coat-an-Nozformations (after Hirbec, 1979); (Table l). Sample CAN58 (type l) contains numerous 6a, Foz formation; 6b, Coat Losquet formation; 6c, Loch Du garnets with V-rich cores, scheelite,and sulfides. Speci- formation; 6d, Coat-an-Hay formation; 7, limit of contact meta- men CANI08B (type 2) contains homogeneousV-free morphism; W, tungsten mineralization; solid star, borehole lo- garnetsand no sulfides.Both developed from calcareous catron. TABLE1. Chemicalanalyses (wt%) of Coat-an-Nozmetapelites and skarns cANl 47 CAN16 CANS CAN13 CANlOSB sio, 55.25 c4-ov 43.28 47.18 37.31 42.94 Tio, o.77 0.s6 0.92 093 0.58 0.46 Atro3 19.86 16.09 25.41 25 44 16.55 14.92 FerO" 2.79 4.95 z-50 2.96 7.30 2.99 FeO 2.17 454 2.52 4.84 7.77 3.16 MnO 0.08 0.08 0.08 0.07 0.84 0.26 Mgo 2.19 1.86 3.03 4.49 1.84 2.05 CaO c.z1 8.45 13.22 u.co 20.92 26.34 Naro 0.41 0.11 0.65 0.13 tr. tr. K.o 5.60 3.15 2.81 D,VC 0.03 o.02 DA 0.21 o.24 0.15 0.13 0.24 0.18 L.O.r. 4.51 c.+c 4.58 7.67 o.co 5.09 Sum 99.06 100.17 99.21 100.35 99.91 98.41 6a 831 104 227 1491 <15 <15 UU 289 475 176 125 661 12 Rb 426 143 146 439 <10 <10 5r 446 336 Iaz 82 62 20 30 oz 22 zo 39 '12 Nb <10 11 15 24 10 Zr 192 94 vc 131 143 81 Co 15 19 11 zc 11 10 Ni 120 234 100 79 211 153 Cr' 147 133 140 145 166 112 2021 1745 894 446 2433 1283 Zn 433 1981 34 124 246 180 Note; CAN 147: C261,level208.8 m; garnet + KJeldspar+ sphene+ pyrite + sericite.CAN16: C220, level163.3 m; diopside+ garnet a quartz + sphene + pyrite + arsenopyrite+ epidote + chlorite + prehnite + sericite CANS: CZ20 level 153.8m; plagioclase(Ane)+ clinopyroxene+ garnet +sphene+pyrite+arsenopyrite+actinolite+epidote+chlorite+prehnite.CANI3:CZ20,level157.4m; muscovite+chlorite(afterbiotite)+ ilmenite+pyrite+sericite.CANSS:CZ20 level 166.1 m; garnet+epidotetcalcite+siderite+chlorites+quartz+scheelite+sulfides.CAN10SB: CZ45 level 241.3 m; garnet a epidote + calcite + siderite. * rcp determination(Geology Department of St. EtienneSchool of Mines). 854 BENKERROUAND FONTEILLES:VANADIAN GARNETS Fig.2. Idiomorphic zoned goldmanite-rich garnet showing anisotropic sectors.Plane-polarized light (left) and crossedNicols (right). Contiguous sectorshave different optical orientation, whereasopposed sectors have identical optical properties.The profiles in Fig. 5 were obtained on this particular garnet, following the longestcrystal diameter from top left to bottom right. kngth of bar is 0.2 mm. V-rich metapelites. Their V content is 2433 and 1283 (24 s); Y, vanadium metal (24 s); and Y, yttrium metal ppm, respectively. (24 s). CrrO, was corrected for VK, interference. When skarn compositions are compared to those of the The metapelites are composed of plagioclase(Anro to calcareousmetapelites, it appearsthat Al, Ti, P, Zr, and Anrr), pyrite, arsenopyrite,sphene, very conspicuousgar- probably Mg behaved as inert components during skarn nets (5 volo/o),and uncommon clinopyroxene.Retrograde development. Small amounts of Mn and larger amounts minerals include actinolite with finely prismatic to fi- of Fe (CAN58, Table l) were probably added while con- brous habit, chlorite commonly replacing clinopyroxene, spicuousamounts of Si, K, Ba, Rb, Sr, and Na were sub- and epidote as rounded porphyroblasts or forming a rim tracted by percolating fluids. The CaO increase from around the sulfide grains. Prehnite or zeolites may also metapelites(5-13 wto/o)to skarns(21-26 wto/o)suggests a be found. Carbonaceousmatter is ubiquitous. substantial addition of Ca directly related to skarn for- mation. Garnets The garnets (Table 2) with their yellow (V-poor) to Mnnnll AssocrATIoNs rN THE green (V-rich) coloring and beautiful sector zoning are CALCAREOUS METAPELITES very conspicuous in thin section. Their morphology is Quantitative analyseswere performed using a cAMEcA similar (Fig. 2) to that reported by Filippovskaya et al. cAMEBAXelectron microprobe (GeologyDepartment, Paris (1972). They show large variations in birefringence,and Schoolof Mines, Fontainebleau).Accelerating voltage was contiguous sectors have different optical orientation 20 kV, and specimen current was 15 nA.
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