79 The Canadian Mine ralo gi s t Vol. 35, pp.79-94(1997) HYDROTHERMALALTERATION ANID TOURMALINE.ALBITE EOUILIBRIA AT THE COXHEATHPORPHYRY Gu-Mo-Au DEPOSIT,NOVA SCOTIA* GREGORYLYNCHI aNo JORGE ORTEGA GeologicalSurvey of Caru.da CentreGdoscientifiqac de Qulbec, 2535 boulevardLaarier, C.P. 7500,Sainte-Foy, Qu.dbec GIV 4C7 ABSIRACT The Coxheathdeposil is a zonedCu-Mo-Au porphyry systemrelared to Hadryniancalc-alkaline volcanic andplutonic rocks of the Avalon Zpne n northeasternNova Scotia. Alteration minerals are disftibuted over 30 km2, and cross the contact of a conposite porphyritic granitic pluton and overlying volcanic units. In the core of the pluton, a zone of potassicalteration comprisesquartz-feldspm stockwork veinlets that contain varying proportions of chalcopyrite+ bornite + molybdeniteand accessorymagnetite + hematite. Hydrothermal titanite, apatite, and actinolite also occur. Peripheral to the potassic zone, propylitic alterationassemblages contain cblorite + epidote+ calcite+ pyrite in veins or aspatchy replacements. Farther outward, volcanic units havebeen affected by widespreadand intensephyllic alterationand sericitization,and by the formation of quartz stockworksand up to 2-57a disseminatedpydte. The outermostzones were overprintedby argillic alteration,characterized by kaolinite, and local chalcedonicquaxtz. Toumaline occursin slockworkveinlets with albite, defining a reshicted zone of sodic alterationthat overlapsthe cenfial potassiczone and inner margin of the propylitic zone. Compositionsof the tourmalinetend toward dravite,with significantdravite-povondraite solid solutionmarked by Fe-Al exchangein the octahedral site,A minor proportionof the uvite componentand charge balance according to stoichiometiyindicate Fe3+(F#+ + Fe3+)values of 0.4 to 0.9. Fluid-mineral e4uilibria at the hematite-magnetitebuffer suggestthat tourmaline precipitation is strongly influenced by cooling, and by the activity of boric aci4 B(OII!. Boric acid is tle dominant aqueousspecies of boron in hydrothermalsystems, and is stableacross a wide rangein pH encompassingalkaline to acidic mineralizing conditions.As a relatively non-volatilecomponen! modeling suggests that boric acidbecomes concentral€d in the liquid fraction of boiling fluids, contributing to the precipitation of tourmaline within the residual hydrothermal component,and enhancingthe zoning of hydrothermalminerals. Keywords: boric aci{ tourmaline,povondraite, Coxhearh porphyry deposigboiling, Avalon Zone, Nova Scotia" Somaens Le gisementde Coxheathest un exempled'un gisementzon6 de type "porphne i Cu-Mo-Au" li6 b un cortlge de roches hadryniemescalco-alcalines volcaniques et plutoniquesde la zone avalonienne,da:rs le nord-estde la Nouvelle-Ecosse.Ics assemblagesde min6rauxd'alt6ration sont distribu6s sur une superficiede 30 knz, traversantle contactentre un pluton composite de graniteporphyrique et un cortbged'unit6s volcaniquessus-jacents. Dans le centredu pluton, une zonee alt&ation potassique comprendunr€seaudeveinules iquartz + feldspafl contenantdesproportions vmiables de chalcopyrite+ bomite + molybd6nite, ainsi que magn6tite + hdmatite accessoires.La titanite, I'apatite et l'actinolite, toutes d'origine hydrothermale,sont aussi pr6sentes.Disposds de fagon p€riph6riqueautour de la zone potassiqre,les assemblagesd'alt6ration propylitique contiennent chlorite + dpidote+ calcite + pyrite en veinesou en remplacementssous forne de taches.Encore plus loin du centre,le,s unit6s volcaniquesont subi les effets d'une alt6rationphyllique et d'une sdricitisationr6pandues et intenses,et de la formation de stockworksi quartzavec jusqu'd2-SVo de pyrite diss6min6e.Les zonesexternes montrent les effets d'une alt6rationargillique, qui a produit kaolinite, pyrorphylliteet, ici et li, du quartz,vari6t6 chalc6doine. Ia tourmalinese pr6sente en r6seauxde veinules avecde l'albite, et definit ainsi une zonerestreinte d'altdra.tion sodique qui chevauchela zonecentrale d alt6rationpotassiEre et la bordure interne de la zone d'alt6ration propylitique. Les conpositions de tourmaline sont dravitiques,mais tendentau $le povondrafte,signalant un remplacementde Al par Fe appr&iable dansla position ocla6drique.Une composantemineure d'uvite et un bilan balanc6des chargespour das compositionsstoechiomdfiiques indiquent des valeursdu rapport Fd+(Fdr + Fel) entre 0.4 et 0.9. l,as €quilibresentre min6rauxet phasefluide aux conditionsimpos6es par le tamponhdmatite - magndtitefont penserque la pr6cipitationde la tourmalineserait fortement favoris6e par un refroidissement,et par I'activitd de I'acide borique, B(OfDr. L'acide borique serait I'es$ce dominantedu bore en milieu aqueuxdans un systime hydrothernal et stablesur un grandintervalle de pH, allant de conditionsfavorisantla min6ralisation eh milieux alcalinsi acides.Comme il s'agit d'une e.spCce " GeologicalSurvey of Canadacontribution number 43295. I E-mail address:[email protected] Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/35/1/79/3435789/79.pdf by guest on 24 September 2021 80 THE CANADI,AN MINERALOGIST relativementpeu volatile, notre modblefait penserque I'acide borique devient concentr6dam la fraction liquide d'un systdme en 6bullition, contribuantainsi i la pr6cipitationde la tourmalinei partir du systdmehydrothermal r6siduel, et ajoutantau schdma de zonationdes min6raux hydrothermaux. (Iraduit par la R6daction) Mots-clAs:acide borique, tourmaline, dravite, povondralte, gisement de type "porphne" de Coxheatl, dbullition, zone avalonienne.Nouvelle-Ecosse. INTR.oDUcffoN recognizedassociafion with the higher-gradeparts of the deposit,and because it hasconsiderable potential as The Coxheath deposig on Cape Breton Island in a petrogeneticindicator mineral (e. g.,Hemy & Guidotti northeastemNova Scotia (Fig. 1), is a vein and stock- 1985,Povondra & Novak 1986,Clarke et al. L989). work Cu-Mo-Au system.Since its discoveryin 1875, Tourmalineis an importantvein andalteration mineral it has undergonevarious phases of exploration and that occursin a wide variety of hydrothermaldeposits, development including extensivedrilling, shaft sink- including different types of granite-relatedporyhyry ing, andcross-cut driving (Oldale 1967).It was initially systems(Sillitoe I973,Lynch 1989,Koval et al. 1991, developedas a high-grade,low-tonnage prospct (Beafon London & Manning 1995), mesothermalAu veins & Sugden1930), but eventualrecognition of the (King & Kerrich 1986, Robert & Brown 1986, King stockwork style of mineralizationchanged exploration 1990),and massivesulfide ores (Slack 1982,Taylor & strategiesto that of a low-grade,high-tonnage prospect Slack1984, Palmer & Slack1989), among others (e.9., (Oldale 1967).Reported grades include valuesofup to Clarke et al. 1989, Slack 1996). Qsmpositionally, 2Vo Clt, lVo MoS2, and 8.6 ppm Au nsrqsslimit€d hydrothermal tourmaline spansa wide rangebetween widths (Oldale 1967).However, to date, only the core the schorland dravite end-members, and in somedeposis potassiczone, tonnnaline veining, and propylitic zone has a substantialFe3t-rich or uvite component(Slack of alteration within the deposit have been described 1996).In zonedporphyry-stockwork Sn t W deposits, (Oldale 1967,Hollister et al. 1974,Clwkeet al. 1989). tourmalinemay be concentratedin parts of the potassic In thisstudy, we ctablish anddocume,nt al theCoxhealh core and phyllic margin @orsythe& Higgins 1990), depositall of the zonesthat chancteriz,etypical porphyry where selective tourmalin2ation of host-rock shales systems,including the potassic core, phyllic margin, can be corrmon (Hsu 1943, Ahlfeld 1945, Lynch and outer argillic and epithermalassemblages, as well 1989). Tourmaline also is an abundantmatrix mineral as propylitic (calcic alteration) and albite-tourmaline in brecciapipes spatially associated witb, or cross-cu$ing, (sodic alteration) facies. Identification of these new porphyry Cu depositsin volcanic teranes (Sillitoe & zoneshas resultedin significant expansionofthe area Sawkins 1971, Sillitoe 1973, Wamaarset al. 1985, affectedby the mineralizedhydrothermal syste,m. Surface Lubis et al. L994').Ir somecase,s, tourmaline occurs in mappingwas carriedout at variousscales, and included high-level epithermalsystems (Shelnutt & Noble 1985, detailed descriptionsof ftenches,excavated sites, and Foit et al. 1989,Meldrum et al. 1994,Fuchs & Maury drill core.Petrographic examination was supplemented 1995).Tourmaline @currences appear to be volumetri- by X-ray diffraction and electron-microprobeanalysis. cally most abundantin regionsunderlain by thick accu- Our study has emphasizedtourmaline becauseof its mulations of clastic marine sediments (Ethier & Qampbell1977), by virtue of tle high boron contentof the argillaceous componentsof such sediments (Goldschmidt& Peters1932,1-arderyren 1945, Harder 1959), or in volcanic arcs where boron is recycled during volcanic activity into upper crustal reservoirs from subductedsediments and altered oceanic crust (e.g., St.Lo,u.rence Palmer 1991a"Bebout et al. 1993,Ishikawa & etufoundla,rtd Nakamura1994, l"eeman et al. 1994,Youet al. 1995). Althoughfield relafionsand experimental data demon- stratethat tourmaline has a wide field of stability,parame- tersthat control the mobilization andtransport of boron, and the precipitationof tourmaline,are not well under- stood. Low-temperature authigenic tourmaline from limestone and sandstonehas been reported by many investigators(e.9., Krynine 1946, Robbins & Yoder 1962, Henry & Dutrow 1992); tourmaline is a well- FIc. 1. Map showing location of the