Canodian Mineralogist Yol.22, pp. 43742 (1984) IRON.RICHAMESITE FROM THE LAKE ASBESTOS MINE. BLACKLAKE. OUEBEC MEHMET YEYZT TANER,* AND ROGER LAURENT DAporternentde Gdologie,Universitd Loval, Qudbec,Qudbec GIK 7P4 ABSTRACT o 90.02(1l)', P W.42(12)',1 89.96(8)'.A notreconnais- sance,c'est la premibrefois qu'on ddcritune am6site riche Iron-rich amesite is found in a metasomatically altered enfer. Elles'ct form€ependant l'altdration hydrothermale granite sheet20 to 40 cm thick emplacedin serpentinite of du granitedans la serpentinite,dans les m€mes conditions the Thetford Mi[es ophiolite complex at the Lake Asbestos debasses pression et temperaturequi ont prdsid6d la for- mine (z16o01'N,11"22' W) ntheQuebec Appalachians.The mation de la rodingite dansle granite et de I'amiante- amesiteis associatedsdth 4lodingife 6semblage(grossu- chrysotiledans la serpentinite. lar + calcite t diopside t clinozoisite) that has replaced the primary minerals of the granite. The Quebec amesite Mots-clds:am6site, rodingite, granite, complexeophio- occurs as subhedral grains 2@ to 6@ pm.in diameter that litique, Thetford Mines, Qu6bec. have a tabular habit. It is optically positive with a small 2V, a 1.612,1 1.630,(t -'o = 0.018).Its structuralfor- INTRoDUc"iloN mula, calculated from electron-microprobe data, is: (Mg1.1Fe6.eA1s.e)(Alo.esil.df Os(OH)r.2. X-ray powder- Amesite is a raxehydrated aluminosilicate of mag- diffraction yield data dvalues that are systematicallygreater nesium in which some ferrous iron usually is found than those of amesitefrom Chester, Massachusetts,prob- replacingmapesium. The extent of this replacement ably becauseof the partial replacement of Mg by Fe. The calculatedunit-ceu parameters are: a 5.385(Q,b9.2gl(5), can be appreciable in some cases,so that the ideal composition of arnesiteis given in terms of AlrO3, c 14.124(16)A and o 90.02(ll)", B %.AQ2). afi 7 89.96(8)'. To our knowledge, this is the fhst specimenof (MgO, FeO), SiO2and HrO. Amesite, with chamo- iron-rich amesite to be described. It formed during site, greenalite and cronstedtite, is related chemical- hydrotiermal alteration of granite in serpentinite at the ly to the chlorite goup aud structurally to the samelow P-T conditions that prevailed during the forma- serpentineand kandite gfoup of minerals. The name tion of the rodingite suite and the chrysotile asbestosin the septechlorite has been proposed for these minerals serpentinite. (Nelson & Roy 1958)as they are structurally charap- terized by serpentine-like layers with d@r - 7 L. Keywords: amesite, lgdingrte, ophiolitic complex, Thet- Largely on the basisof thermal ford Mines, Quebec. studies, Orcel(1927) and Orcel et al. (1950)were the first to suggestthat Souuarnn amesitemust be regarded as distinct from the main group of chlorites. According to the AIPEA (Associ- On trouve une nm6site riche en fer dans une dcaille de ation Internationale Pour I'Etude des Argiles) 20 i 40 cm d'dpaisseurde granite mdtasomatiquementalt€r6 Nomenclature Committee, ,tmesitebelongs to the d la mine du lac d'Amiante (46'01'N, 71o22'W). Ce gra- kaolinite-serpentine group (Bailey 1980a);this com- place nite a 6td mis en dans la serpentinite du complexe mittee recommendedthat the nameskandite andsep ophiolitique de Thetford Mines. L'am€site est associ€ei techlorite not be used for kaolinite- and un assemblagersdingitigue Grossulaire + calcite * diop- side t clinozoisite) qui remplace les min&1ux primaires serpentine-group minerals. du granite. L'am6site qudbdcoiseapparait en grains sub- We found an iron-rich variety of amesiteassociat- idiomorphes i facies tabulaire de 200 d 600 m de diam&- ed with gxossular,calcite and, more rarely, diopside tre. EIle est optiquement positive av*, faible2V et a 1.612, and clinozoisite in a rodingitized body of granite en- - 1 1.630 (r o : 0.018). Sa formule structurale esl closedwithin the serpentinizedperidotite of the Lake partir calcul6e i des donndes de la microsonde: Asbestos mine, at Black Lake in the Ap- (Mg1.1Fe0.eAb.t(Ab.eSi1.dO5(OH)r.u. par Quebec Les donndes palachians. We report here the description of this diffraction des rayons X (mdthode des poudres) indiquenr que les espacementsd sont systdmatiquementplus grands occurTence. pour l'am€site du lac d'Amiante que pour celle de Ches- ter, Massachusetts,ce qui est probablement dOau rempla- GEoLocIcAL SETrING cementpartiel de Mg par Fe. La maille 6l6mentairea pogr The Lake Asbestosmine is located at 46'01 'N, param&trescalcut6s: a 5.385(6),b9.291(5), c 14.12-4lAL, 71"22'W rnthe Black Lake areanear Thetford Mine (Frg. l). The serpentinized peridotite bearing the *Present address:Mineral Exploration ResearchInstitute, chrysotile-asbestosore forms the lower strustural Casepostale @79, SuccursaleA, Montrdal, eudbec H3C unit of the Thetford Mines ophiolite complex (Lau- 3N rent l95a). This ophiolite was tectonically emplaced 437 438 THE CANADIAN MINERALOGIST :=-:-:€loct t =_:-:_:-_-,---------=l ./I EXPLANATION ffi cotcrellGroup f] Porldoilirlcclonllc @ Grcnlrr nr Thrult toult C? Opcnplt lo.l8ea.> *n-/:+ l63ellc-rich drkc ['"{W/ o I -.\ :-tlo.l898l -l- \gl I LAKEAsBEsTos MNEI / oLi-t' LocATroNtvrAp Ftc. l. Geological map of the Lake Asbestos mine at Black Lake and location of the amesite-bearingrodingrte. in the internal tectonic domain of the Quebec Ap- palachians during late Early or Early Middle Ordo- vician time (St-Julien & Hubert 1975).The ophiolite has been thrust over regionally metamorphosed Cambrian rocks known as the Caldwell Formation. The thrust fault dips at a high angleto the soutleast. The asbestosmines are located near this tectonic con- tact in the peridotitic lower unit of the ophiolite. The serpentinizedperidotite of the Lake Asbestos mine @ig. l) is a tectonite harzburgite grading lo- cally to dunite. The rock is strongly serpentinized along shear zones, which trend in a northeasterly direction parallel to the basaltectonic contact. Sheet- like and lenticular bodiesofgranite' from lessthan I m to more than 100 m thick, occur especiallywi- thin the shear zones. They are deformed and hydrothermallyaltered (Cooke 1937,Riordon 1953' De 1972,Laurent 1975b,Laurent & H|bett 1979, Laurent 1982, Laurent et al. 1984). is found 11 a l6dingife lo- Ftc.2. View ef tfus thin and lenticular body of amesite- The iron_rich amesite bearing rodingrte within the highly serpentinizedharz- catedinthe southeasterncorner of the LakeAsbestos burgite of the Lake Asbestosmine (ocation of outcrop mine at 3500N and 9250E (Fig. 1). This ro.lingite given in Figure l). oscupiesa northwest-striking fault plane (oblique to AMESITE FROM TIIE LAKE ASBESTOS MINE, QUEBEC 439 Fro.3. Samplela,l898l of iron-rich amesitein thin sectionunder crossednicols. a. Single crystal showing a perfect cleavageparallel to (@l) and sector twinning on (@l), surroundedby isotropic grossular.b. Group of amesitecrystals (A) with different optical orientations, showing cleavageand twinning, in associationwith calcite (C) and grossular (G), Bar represents0.1 mm. the main shear-zone)that dips subvertically within 2 m, hydrothermal alteration is limited to their mar- hiebly serpentinizedharzburgite. The body of roding- gins. This alteration id thought to be associatedwith ite is thin and lenticular (Fig. 2), between 20 and the episode of serpentinization that led to the de- 40 cm thick and at least l0 to 20 m long. The pro- velopment of chrysotile-asbestosin the enclosing tolith of the redingite was a calcalkatinebiotite-rich peridotite (Laurent & Hdbert 1979,Laurent 1980, eranite sirnilar in composition to the thick and well- Laurent et al. 1984). preservedsheets of granite tlat crop out in the mine; they have been describedrecently by Laurent et al. PBTROGRAPHY (1984).In thosegranite sheetsthat are thicker than Rodingite sample la. 18981containing the iron- 440 TIIE CANADIAN MINERALOGIST TABLE I. CHE}'IICAL @}TPOSITION OF A'1F.9ITE-BEARINC NODINGITE HABIT AND OPucel PnoPsnuEs AND OF CROSSI'LAR Samle la.l898l crosaular 10 contact Gruner (1944) described the macroscopic and samle zonei rlth [email protected] physical properties of amesite. Becauseof the small core @r91tr ;ize of the crystals, our observations of the sior 37.35 33.18 | 2 Quebec rro; o.o? 0.09 sto2 40.20 40.27 iron-rich amesite are limited to their microscopic Al2d1 23.3E 24.71 AIZ03 22.04 20.65 O.44 features. fer-oe- 0.33 0.14 Feo 0.28 re6 l.6I o.a7 Mno o.o9 o.l7 The iron-rich amesiteoccurs as groups of euhedral MgO l.2l 0.97 cao 37.A4 38.45 600 pm lino 0.19 O.ll to anhedral crystals oftabular habit, 200 to cao 34.24 34.5I T0TAL 100.45 99.97 wide and 50 to 100 1r.mthick, occupying the inter- Naro - O.3l No. of toc o ttE ba81sof 24(0) xr6 o.o9 o. 19 si 6.ot 6.07 sticesbetween clusters of grossular grains. The ane- H;o r.r5 4.lo Al 3.88 1.67 site crystalshave a perfect cleavageparallel to {001} c6r o.57 0.43 Fe 0.03 0.05 roiAl 16:ZZ ld6;ZT lh o.ol o.o2 6pical of phyllosilicats, and display sectortwinning Ca 6.07 6.21 on (01) as well as hourglasstexture @ig. 3a, b). The sattole la.1898l: Frossulat (-902)' dlopslde' aneslte' calclte' amesite is biaxial positive, with a 2V of about Z)o ctLnozoldtte, hlottte (pri@ry), chlortte' sPhene"!rcon (prtury), opaque phaae. and positive elongation. It has weak dispersion, r < Gro66u1ar amlyzed by electron idcroplobe' rodloglte aoal)€eg v. XRF AA. by and Indices of refraction were measlued with immer- sion oils calibrated with a refractometer:a 1.612, "y 1.630(t 0.002), cu- y 0.018. The small crystal- rich amesiteis a white, homogeneous,fine-grained size of our sampleshas allowed us to measureonly rock with a ganoblastic textue. A daxk margin ttrat the maximum and the minimum indices.