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]
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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 Coxheath porphyry known constituent of medium- to high-grade pelitic Cu-Mo-Au depositwithin the Avalon Zone of the Cana- schists(Henry & Guidotti 1985),and is common as a dian Appalachians. primaxyigneous phase in pegmatites(e.g., Iollff et aL
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1986) and as a late-magmaticto hydrothermalphase mineralized breccias at El Teniente in Chile record within orbicules in granites (Sinclair & Richardson hypersatinefluids (34-44 wtTo NaCl equivalent),unda 1992).At its upperstability limit near9@'C, hydrother- boiling conditionsat 300"-4@'C (Skewe.s1992). mal tourmalinebreaks down to form spinel + cordierite (Fuh 1965), or kornerupine+ sapphirine@obbins & Gsot,octcAr-SsrTrNG Yoder 1962). Other experimental results (Vorbach 1989) suggesta more restricted field of stability for The Avalon Zone of the CanadianAppalachians hydrothermaltourmaline, approximately250'-:7 50'C. (Frg. l) is characterizedby a vast Late Proterozoic Pressureapparently does not exercisea stong control volcano-plutonicarco constructed on a Gondwanabase- on the stability of tourmaline(Fuh 1965),but solutions ment, with correlativeunits extendingto otherportions of intermediatepH with low to high solute concentra- of theNorfh Atlantic, Africa, and South America (Willians tions favor its precipitation (Smith 1949, Morgan & 1984).On CapeBreton Island the Avalon Zonecontains London 1989).And fluid inclusionsin tourmalinefrom five volcanic belts @an et al. L988), ranging in age
z', "... Sydney 16 '+ km ----_\tEl-_ _-_-{ i"" \... N---{-:n uarDoruIerous ')r .: ffiF_:-, . rocks ./ Carboniferous rocks
<' ;
.-..- I
.x volcanic .rt. - r('t'i/'.x' .rocks----" ..J. - .i . X--a Argillic Alteration Qtz-rl(ln-rPrl=Py
ffi Phyllic Alteration titri{ I\4s-rQtz+Py +Ca.l*Qtz
l.l Propylitic Alteration I I Ep-rlVs-rCtrl *Cal:tQtz diorite Sodic Alteration granite Ab-r.Ttrr*Py*Ccp:tEt n
Mine shaft Potassic Alteration Road I(fs-rAb+Qtz-rEp =ActtTtntAp Ccp+Bn+Mag+Mo=Py
Frc. 2. Geology and alterationmap ofthe Coxheathdeposit Mineral abbreviationsare as follows: quartz (Qtz), calcite (Cal), epidote @p), chlorite (Chl), actinolite (Act), apatite (Ap), titanite CItn), tourmaline (Iu), albite (Ab), K-feldspar (Kfs), kaolinite (KIn), pyrophyllite (Prl), sericite(Ser), muscovite (Ms), pynte (pY), chalcopyrite(Ccp), bornite @n), molybdenite (Mo), magnetite(NIag).
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from 575 to 680 Ma (Bevier et al. 1993). However, yielded 40ArFeArages of 621.9t 4.2 Ma and 620.6t unconformably overlying Cambrian-Ordovician, as 4.7 Mu obtainedfrom hornblendeinterpreted to be of well as extensive Devonian{arboniferous sedimen- igneous origin (Keppie et al. 1990). Within the error tary rocks, cover much of the region. Metamorphism limits of the dating methods,the pluton and volcanic typically is only weakly developedwithin the hotero- rocks thus may be coeval. Bedding in the volcan:ic zoic units, being of subgreenschistto lower-greenschist successiontypically is poorly defiaed. Carboniferous grade.Rocks of the Coxheathbelt in the vicinity of the normal faults boundmuch of the Proterozoicunits, with CoxheathCu-Mo-Au depositconsis of flows andpyro- the most pronouncedset striking north-northeastand clastic deposits rangrng in composition from basalt, having a subverticaldip. andesite,and dacite, to rhyolile. Geochemicalstudies have idenffied a calc-alkaline trend (Ihicke 1987, HyDRoTmRMALAr,rmanon emr Mnurral ZoNnvc Dostal & McCutcheon1990, Ba:r 1993),with tace- element patterns indicating a continental-marginarc Hjdrothermal alteration and stockwork veining of setting (Dostal & McCutcheon 1990). The volcanic varying intensityhave been documented across an area rocks are cut by a large compositepluton comprisinga of approximately30 km2, measuring7 by 4 h. Five core of granite to granodioriteand a broad mmgin of principal zones @ig. 2) comprisethe alteration types dioriteto monzonitecontaining an abundanceofvolcanic typical of porphyry Cu-Mo-Au and epithermalsysterns xenoliths. The diorite, typically fine- to medium- (e.9.,Lowell & Guilbert 1970,Sillitoe 1993).From the grainedand equigranular,grades inward to porphyritic plutonic core to the outer overlying volcanic edifice, monzonitethat displays scafered very coarsephenocrysts theseconsist of potassic,tourmaline, propylitic, phyllic, of plagioclase.The granitic coreis medium-grainedand and argillic zones.Mineral paragenesesofthe principal equigranular,and locally containsxenoliths ofdiorite, vein-types and alteration facies are summarized in indicaring a cross-cuttingrelationship with the border Figure 3, basedon cross-cuttingrelations established by phase. However, compositionalvariations within the field and petrographicobservations. pluton are thougbt to haveresulted ftomin sin differ- entiation o1 169 crystalliz:ing magma (Chatterjee & Potassiczone Oldale1980). Rhyolite within the Coxheathbelt has beendated at Two setsof stockwork veinle6 occur in eachof the 613 t 15 Ma by the U-Pb zircon method(Beier et aI. monzonite and diorite border phasesof the Coxheath 1993). The compositepluton describedabove has pluton. Within the central part of the monzonitebody,
EARLY LATE FOTASSTC
PROPYLXTTC ARGILIJC K.FELDSPAR - ALBITE ffi EPIDOTE TOURMALINE - ACTINOLITE w TITANITE l$ffitliffil APATITE I CHLORITE MAGNETITE w HEMATITE t:,P.il,e.EilnRt MOLYBDENITE r CHALCOPYRITE ffi DORNITE @l QUARTZ PYRITE MUSCO1TTE KAOLINITE - PYROPHYLLITE ffi Frc. 3. Paragenesisdiagram ofprincipal vein and alterationminerals. Designation of early and late is an approximationbased on cross-cuttingrelations and overlappingmineral assemblages.
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Frc. 4. Photomicrographsof (A) vein chalcopyrite(light grey, cp) rimmedby a layer of bomite (dark grey,bn); @) hematiterim (ltght grey, ht) on magnetite(dark gey, mt). Both samFlesare fiom the potassiczone. Field of view is 1 -- along basefor (A) and 0.7 -m for (B).
quartzveinlets have developeda strong alteration-halo componentsare interpreted to reflect alterationof igne- dominated by K-feldspar accompaniedby varying ous plagioclasein the plutonic rocks, whereastitanium amountsof albite, epidote,apatite, as well as veinlet or for the hydrothermaltitanite waslikely derivedftomthe disseminatedchalcopyrite, and pyrite. Veins in the second breakdownof homblendeduring hydrothenral alteration. set containquartz with actinolite,surrounded by a halo On the basis of the abundanceof K-feldspar in this of K-feldspar,titanite, andapatite, witl disseminatedor hydrothermalzone, we infer that potassiumwas intro- veinlet chalcopyrite, bornite, pyrite, and magnetite. duced during alteration.Also, molybdeniteis concen- Bornite typically occurs as a rim on chalcopyrite tated outward, away from the monzoniteand toward @ig. aA), associatedwith varying proportionsof pyrite the diorite borderphase. or magretite.Outward, to the diorite margin,the potas- sic alterationfacies lacks pyrite, but containsmolybde- To urmaline--albit e zone nite and magnetite.Within the diorite, quartz veinlets display a halo of K-feldspar with variable amountsof The tourmaline zone is dennedby the presenceof albite, epidote,chlorite, titanite, and apatite,as well as tourmaline.and is limited to diorite nearits nortleastern vein or disseminatedchalcopyrite, bomite, molybde- contact with the volcanic units. Here, quartz-tourma- nite, and magnetite.Magnetite commonly has either a line veins form complex stockworks that overprint (Fig. hematite rim 4B), or hematite developedalong rocks previously affected by potassic alteration. In planes. cleavageand fracture Coarsespecular hematite some outcrops, steeply dipping veins of tourmaline alsois observe4and occurs as radial aggegatesdissemi- strike predominantly north-northeast. Where dense naf€din tle alteredrock Bomite forms a rim on chal- clustersof stockwork veinlets occur, the host may be copyrite, or is intergrown with chalcopyrite and completelyblack owing to replacementby tourmaline. molybdenite. A late set of quartz-molybdeniteveins Tourmaline selvagesalong the vein margins typically associatedwith a K-feldspar halo cuts earlier stock- are 1-2 cm wide, and progressoutward to solid pink works of potassicalteration in the diorite, and comprises albite selvages,also 1-2 cm wide, in a small-scale thicker veinsthat may be up to 20 cm thick. Late-stage, zonedconfiguration. ArarE, tourmaline,and albite occur fine-grainedmuscovite alters hydrothermalfeldspar in with accessorymuscoviteo chlorite, and epidote. The the potassiczone, and quartz-muscoviteveins locally veins are strongly zoned with altematingencrustations cut K-feldsparveinlets. of layers composedof either quartz + toumaline, or Although secondaryK-feldspar is the characteristic pyrite with accompanyingchalcopyrite and bornite. A mineralof the potassicalteration facies, secondary albite sharpsegregation of the layeredassemblages is common. definesa clearsodic component to the alterationassem- Single layers within a vein are generally less than blage; actinolite, titanite, apatite,and epidotecompose 0.5 cm thick; however, a vein may be as wide as a distinctive calcic component.The sodic and calcic 3-5 cm. Tourmaline occurs as subhedralto euhedral
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randomly orientedcrystals, with a dark brown colora- quartz, with minor associatedclay and pyrite. Minute tion and no apparentinternal sening. veinlets of quartz are found in much of the rock, and locally, veins up to 20 cm wide contain very fine- Propylitic, plr.yllic, and argillic zones gained quarE or chalcedonicquartz. The host rock in this zoneis predominantlyrhyolite, which canbe iden- Disseminatedor veinlet calcite,epidote, chlorite, and tifled by the presenceof squarish paramorphsafter accessorypyrite characterizethe propylitic assemblage. B-quartz. Propylitic alterationoccurred in the volcanicunits adja- cent to the margins of the pluton, as well as within TounMeLNs CHEMISTRY westemoutsrops of the granite;it also completelyover- printed the potassicand tourmaline-bearingalteration Tourmalineis a chemicallycomplex mineral with a zones, as distinct cross-cuttingsets of late propylitic large number of possible substitutions;it has the fol- veins. The intensity of the propylitic alterationis vari- lowing generalizedformula: able, and in portions of the zone, it is quite weak. In conftast, phyllic alteration is intensetbrough most of (N4Ca)(Mg,Fe,Mn,Li,Al)3A16 the phyllic zone,where pervasive sericitization, silicifi- [Si6O1 s] (BO3)3(OH,Da (l) cation, and pyritization (5-I0Vo pyrite) replacedmuch of the rock and destoyed primary textures.The phyllic zonecovers a wide areawithin the volcanicrocks away A site-reference scheme may be representedby from the pluton and centralzones of alteration Fig.2). (R1XR2)3(R3)6(BO3)3Si6O1s(OH,D+,with R1 repre- Very thin veinletsof quartzform subtlestockworks. In sentingNa, Ca, K, ot vacant;R2 representingF**, Mg, outcrop, the phyllic alteration is sniking owing to a Mn2+, and Fe3*, Cr3*, W+, Ti4r, or Al (where Rl is white colorationand abundanceofrust staining. vacant),or Li (wherecoupled with Al substitution),and The argillic zone occurs at the easlem end of the R3 representingAl, Fe3*, Cr3t, V3*oand Fd* or Mg hydrothermalsystem" adjacent to the phyllic zone,and (wherecoupled by Ca substitutionin R1), or 1.33 Ti4* is unconformably covered by Carboniferous clastic (Burt 1989,London ft \danning 1995).Also, it has sedimentaryrocksto the east.This zoneis characterized beenshown that Al may substitutefor Si in the tetahe- by fine-grainedclay mineralsand an intensesilicifica- dral site (Hawthome 1996). The numerouspossible tion of the host rock. Both kaolinite and pyrophyllite substitutionswithin tourmaline,as well asits wide field were identified by X-ray diffraction. The degree of of stability, render it a usefrrl petrogeneticindicator silicification is greaterin this zonerelative to the other mineralfor discriminatingmineralizing regimes (Henry alterationfacies. Large exposuresofrock occur that are & Guidotti 1985, Povondra& Nov6k 1986, Slack abnost completely converted to very fine-grained 1996). As many as eleven end-membershave been defined. Resultsof thirty-one analysesof tourmalinefrom the Coxheathdeposit are presentedin Table 1. The chemi- .l!@ z@ n.F n.rc 2t8 233 Um 6.* U.U Xg LtO Ag llgo 18 1,6 W &€ 7g 7.q Lq 124 7,17 7,@ 6& cal analyseswere obtainedat the GeologicalSurvey of w 0,D oa 0$ o.1l 0.a o2 0a os ox5 oJl o.n K l?3 r02 lo.s t2& u31 t3J5 l4 ld6r l4.J' 81 6J! Canadain Ottawa, using a CamecaSX-50 electron j@ l.& ry zD l.@ 1.3 al9 z\5 1.9! 16 t.t 1.s m \9 0n tx Ls ao I.l4 l.3l t.?l r.49 t.s !6 microprobe.Quantitative wavelength-dispersion analy- w 0.01 0.6 q@ 0g 0@ 0.0 o oil 0.6 o.@ o M& 6.4 U.1t U.14 gJ3 S.6 S.fl S,0l 6.9 gD s.lE u.s ses(WDA) were done at 15 kV acceleratingpotential, M@SAJAmGOT and 10-20 s counting time. Standardsused include s J.g 66 '.5? '.S CN 6.@ tg 6m ,.a !.s Jglo A 4X4 !.S J.€t ttol 1.ffi 5& 5.& 1l3t !g 46A J.@ NaCl (Na), KBr (K), wollastonite(Ca), corundum(Al), Mt l.9l !.s 116 m l.s 1.919 r.9 lm l.s L@ l.& ! 016 0.@ 0.@? 0.@ 0.w 0.@ q@ qm q@ os 0.tq quartz (Si), MgO (Mg), and pure metal standardsfor ro AI l@ lS l.& 2616 1.S l.!3 2.W 2,tt2 n\ Z43l M 06a q?6 0.6 q@ 05n 0.8 o1t6 o.a2 o.@ q6l8 o.a5 eachof Mn, Ti, Cr, V, and Fe. However, Mn, Cr, and G 0.6 ql4 031 u' 031 0rlo o:42 os om q@ 0Jl0 K 0.@ oou qw o@ 06 0.@ 0 0@ 0.6 0.019 o@r mro TAHFI COWM Ta4 TS TA 14 T-28 T-32 T43 T4 733 T4 so2 y.69 33K 35.U 355 y3 35.14 33 3'X 3J.42 35.S MO3 4.4 Lt n.6 %.45 %.r9 w x.u g.o 3539yn g.gffi &6 '.35 30.I Nn b.61 .|W n,24 6,9 n,% AA U.% 6Xt X.% E& Xfi %,tt ',A McO 1i0 6.& 7.S 7.9 1.Y 7i4 1.D 1,n 8.79 7.n MrO 7.9 7,q 7,@ 1K 7t1 1il 79 ?.18 557 7.42 132 r(t2 0.38 0,?0 0.4 034 0.45 0.u 0 0.m La $4 re qr oa 0g o.a q4t o.t2 0J3 0i3 0.16 06 0A F@ 16.m 17.4J ll.S L4 l3.B llj2 10.?6 &33 9.S 9.33 a@ ll, 116 t0.71 \5fr B% tz6 116 13.6 t3.T I4g l0z Na2O 2.4 L8l ll9 m lg 2.6 138 110 1.94 23 wo & u m ls 2.\3 2.10 ar N L13 z.to U m 0.J5 LU l.u tA l.tr t.& l.l9 0J3 1.31 l.0l @ 0i l.lE o.m t?t t.4 ls ls lg t.4! l$ 0.s K20 0.9 0.03 0.02 0.@ 0.03 0.01 06 0.0 0 0.9 rc 0.q q@ 0.u 0.6 0s 0.q 0$ 0.q 0.6 o.s o@ TqT[ S.4 S.S S.l9 S.' S.O S.S S.A &.S S.4o &.59 M& &.1 g.$ S.A S.S S.t9 g.q g' s9 &31 s.! g.& t@@AM@(re M(BS2drMOW@) $ 5.m 6.0$ 5.S 6.W 5.933 5.S3 '.$3 6.m J.S J.S2 s 6.@ i.sl 6,013 6.@ 6!14 J.w 6010 661 C@ !.m 6.m A 4.S3 4Xy 3.m Jg J3S 5.@ 3.S8 3.S 5.9 { 5g JS u9 4S 5% a-61 5.33\ !241 !39 5.1@ t6l3 J.ffi \4a \w L6 1,91 1.92 Lg lS9 I.S 1.6 l.& l.g l.9l! MC 2.017 Ln3 1.968 1.927 1.893 Lffi l.Tl 1.94 221 l.W ! oil9 0.@, 0.q q@ 0.@ 0.m u@ 0s9 o.@ o.@ 0.@, T 061 0.8 0.0$ 0.93 0.038 0.016 0 0.0q3 0.131 0.92 R l.@ 1.W lfr 2.6 l.9ll l& I.8l? l.9l z0!0 a6 Ls Fo L39 2fi3 LA 1.ru Ltr lffi 1.519 l.lg l.& l.N M 0.4 o.78 018 0.9 0.711 0.t05 qru o,@ o,u of,\z 0149 Nr 0.t$ 03n 0.718 o,nl 0.6{8 0.6?t 0.615 0.682 0.69 0J6! & 0.139 0rl9 0.16? 039 06 0& oe 0a 0-l 0s 0.1& & 0.lg 03% on orj o.ffi 0s 02lJ o.ffi 0185 0.180 K 0@ o6 0.@ 0.@ 0.@ o@ om 0.@ 0.w 0.01t o.& ( o0l0 o.ffi 0.m 0.G 0.@ 0.@3 0.013 0.@5 0 0.m m& 16.10 ttl81 1665 l6s l6.lrt 16.6 16149 !6.t6 1614l t6& 16.6 m[ 16376 t6.319 16.6 15.u0 Kl69 l6.ml B.q3 r.K 16.019 F.ry
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3.0 1.6
E sB t.4 2.5 8, E F1 / g t.2 "*" Els ! *\€,/ 2.4 'sdE r 1.0 E+, '.t 4e ./ .t' g) E*E l-, T 0 0.8 _ sE I 1.5 !'gwE - E Los Bronces 0.6 tt 1.0 0.4 ns @ ElTeniente 0.2 Los Pelambres 0.5 r- 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 AI 3.0 B EdB 2.5 *"""*:1'@ I
s I I Il B. I r.o q- Uvrte - fi 4-? E "F o Los Bronces qs" schorl I rO t ll I E s' 1.5 6 r t al,os E E AI inR2 N- ElTbniente t.0 1.0 t.2 t.4 1.6 1.8 2.0 2.2 2.4 1.0 l.l 1.2 1.3 t.4 1.5 1.6 Mg Fe* Flc.5. Plotsof tourmalinecomposition. (A) Feversas A1, and FIc.6. Plots of tourmalinecomposition (A) TotalFe tn R3ver- @) Fe r;erszsMg, in atomsper formulaumt (apfu)for sas C4 and @) calculatedFe3'versus Fe4, in atomsper tourrnalinefrom the Coxheathdeposit. For comparison formula unit for tourmaline from the Coxheathdeposit. with otherporphyry Cu cleposits,average values for tour- The amountof ferric iron in the R3 site was calculatedas malinefrom the Los Bronces, El Teniente,and Los Pelam- the total amountof Fe in ft3 in excessof the numberof Ca bres depositsin Chile also are shown(Skewes 1992). atomsin Rl Be+ = Fe - (3 - Mg) - Cal. Star symbolis Ore Arrowsin @) emphasizetrends toward end-member com- averagevalue for Coxheathsamples. Note tlat the ratio of positions,as well as fields for Al-rich (Al in R2) and ferrousto ferric iron in (A) refersto proportionswithin fhe Al-poor(upper righrhand field) tourmaline. Star symbol is R3 site on1y,whereas the ratios in @) are for total iron averagevalue for Coxheathsamples. including both R2 andR3 sites.
V arebelow detectionlimits or presentin traceamounts and ferric iron. The amount of Fe3+in the R3 site may and are not includedin Table 1. be approximated by the total amount of Fe in R3 in Oxide totals for most analysesare near 85Vo.T\e excessof the number of Ca atoms in Rl (i.e.,FE+ = Fe SiO2contents are quiteuniform at 34-35 wt-Vo,apprort- - (3 - Mg) - Ca; Fig. 6A), required for charge balance maring6 atornsper formula luulrrt(apIu) Si. Fe and Al according to the following stoichiomety: show the geatest amountof variability (Fig. 5A); Mg is remarkablyuniform at close to 2 apfu 5B). Fe @g. (Naca)Mgtre) g(FeAl)olSi6O rsl@O3)3(OHD4. variesby 1,.5apfu, from 1.2 to 2.7, whereasAl varies by 1..3apfu,from4.7 to 6.0 (Fig. 5A',Table 1). The Fe versusN plot in Figure 5A showsa linear correlation The Fe2+(Fez++ Fel) value calculated in this manner defining a slope of approximatelyminug e1s, demon- ranges from 0.4 to 0.9, with an average value of stratingthat Fe substitutesdirectly for Al in the R3 site, 0.7 (Fig. 68). Tourmaline at Coxheath shows only a sinceAl is below 6 apfu. Consequently,with Mg filling limited uvite component, with Ca at 0.4 apfu or less, two thirds of the R2 sile, Fe occupiesa portion of both which allows for up to I atom of Fe3* to reside in ft3. R2 andR3, requiring the likely presenceofboth ferrous The presence of ferric iron within Al-depleted R3 and
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^ Tourmalinecompositions Tourmaline composltions from A from the Coxhealh deposit AI selected Chilean porphyry copper deposits (Skewes 1992)
a Mean value of Coxheath (q Mean tourmalfue composifon tJ tourmaline composition (n=36) from Chilean deposits Mean tourmaline composition from the Guangxi tin deposit, China o (Mao 1995)
Mean tounnallne composition from o hydrothermal veins in Cornwall, SW England (London and Mannlng 199s)
SCHORL DRAVTTE n=4 oa a*.., o $^-Errtmentt
FtroAlro MS.oAlro
Frc. 7. Plot of tourmalinecompositions on an FeropAl-Mg triangulardiagram. The samplesfrom Coxheathdefine a linear trend characterizedby Fe-Al exchangein the field of Fe$-bearingtourmalines. Average values for samplesfrom tlree other porphyry-Cudeposits fall close to this trend whereastourmaline from granite-relatedSn deposits(diamond symbol) are typically Al-emiched and Mg-poor, tendingmore toward the schorl end-member.
the Mg dominanceof. K2 characterizethis tourmalineas contents,with Al typically exceeding6 apfu, ftJJtllg dravite with a povondraitecomponent (cf. Gice et al. R3 and part of the R2 sites. 1989).The averagecomposition for Coxheathtourma- line is representedby the following formula: Acrryrry DIAGRAMS
(Nao.eaCao:e)O{g r.srFe?i7 ) Activity diagramsare commonly used when portray- (Als.afe33zFeEiu xsiuo,rl ing equilibrium assemblagesof alterationminerals and mineral reactionsin porphyry systems.Mineralogical (BO3)3(OH,D4 Q) changesthat occur acrossalteration zoneshave been extensivelystudied by a numberof investigators,with Figure 7 is an Al-Fe-Mg plot of tourmaline from emphasisplaced on silicafesophyllosilicates, clay miner- Coxheathand other porphyry-stockworksystems. This als,sulfides, and oxides.Although tourmalineis locally plot displaysthe Al-Fe exchange,at relatively constant an important vein and alteration phase in poryhyry Mg, in the Coxheathtourmaline, as well as its distribu- systems,tourmaline-producing reactions have not been 6ea pithin the domainof ferric tourmaline.Tourmaline investigatedor illustratedwith activity diagrams.Aside compositionsfrom threeChilean porphyry-Cu deposits from the inherent chemical complexity of tourmaline, are plotted for comparison,and are close to the frend one of the problemsencountered is that it most often is defined by the Coxheathtourmaline, although one has the only boron-bearingphase present in which case distinctly higher Al/Fe. Tourmaline from a number of reactionsthat attemptto explain tourmalineprecipita- othermineraldeposits, mostly Sn+W stockworksystems, tion must be balancedusing aqueousboron species.On are characterizedby a stronger tendency toward the the onehand boronin graniticmagrna,s pafiialy disrupts schorl end-memberGig. 7). These other tourmalhe the aluminosilicatenetworko has a low solubility, andis samplesare for the most part distinguishedby high dl largely expelled from magma as an incompatible
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I 6 I e(oH). m a: ct c) c >-2 o (J o o0c'l c't pK 10.9 o 3000c I ht 5OO bor -u""i"it" -2 koolinite 2 7 100 200 300 pH T("C)
Ftc. 8. Calculatedconcenfration - pH diagramfor boronunder Frc.9. Comparisonof volatilitiesof majordissolved gases in hydrothermal conditions demonstratesthat boric acid hydrothermatsystems (Giggenbach 1980) with thatof bo- (Ellis Cv representsthe is the dominantspecies at pH levels (e.9.,2 3 pH ric acid & Mahon1977), where [B(O[I):] andCl representsthe concentration principal concenfiationin vapor 3 7) that encompass zones of alteration within partitioningdemonsrafes that boric acid is a porphyry pres- in water.The systems.Equations for temperature-and species.However, data are not availablefor constantsfor B(OII| and B(O[I)+ non-volatile sure-dependentacidity hightemperatues, where partition coeffrcients con- are from Gieskes(1974). An activity of 0.05 is very speciation verge. usedfor K' in estimatingthe stability limits of silicatesin tenns of pH.
substance@ichavant 1 987, London er aL l 988). On the otherhand boric acid B(OIDr is the dominantB species in the hydrothermalregime, on the basis of studiesof 300'c active geothermalsystems, seafloor systemso fumaroles *4 uofeanicemanations (e.9., Shuvalov 1974, Cbio- drnieta\.1988, Palmer 1991b, Shaw & Strchio 1992), and on high-temperatureexperimental determinations and theoreticalconsiderations (Mesmer et al. L972).A number of minor polyboratesmay also exist (Mesmer et al. 1972).Boric acid hasa wide rangeof stability and convertsto borate only under conditionsof very high nlkalinity (Frg. 8). B(OH)3 remains as the dominant speciesacross a pH range (Chiodini et al. 1988, Shaw & Sturchio 1992) that encompassesall of the minera- 8z logical zoneswithin porphyry-alterationhaloes, includ- C) ing potassic,phyllic, and argillic (Fig. 8). Comparedto B(oH): the dominant neutral and gaseousspecies dissolved within hydrothermalsystems, boric acid is a relatively non-volatilecomponent (Ellis & Mahon L977)(Fig. 9), that concentratesin the residualhydrothermal fluid in boiling hydrothermal systerns.Boiling consequently providesan effective mechanismforincreasing B contenb in hydrothermalfluids associafedwith porphyrysyst€ms o6 (Frg. 10). Thus for many reasons,boric acid is an ideal 20 40 60 choicefor balancingchemical reactions involving tour- %boiling maline, and for use as a principal componenton activ- ity-activity diagrams. FIc. 10. Plot of an open-system(Rayleigh, partition coeffi- Petrographicobservations in the tourmalinezone of boiling-distillation model that dem- cientsas in Figgre 9) the Coxheathdeposit demonstratea close paragenetic onstratessegregation of t}te volafile speciesH2S, whicb link and albite: tourmalinemay be becomesstrongly depletedin concentrationwithin the re- betweentourmaline sidual fluid (C1)relative to its initial concentration(Co). directly intergrownwith albite,or occurin veinletswith Conversely,B(OII! is progressivelyemiched in the resid- an albite hato.Tourmaline and albite arethe two domi- ual fluid with increasedboilins. nant Na-bearingminerals in the deposit,making up the
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mineral assemblagethat defines sodic alteration. To The following reaction models the coexistenceof investigate the chemical changesthat occurred, and tourmalineand albite: thosethat may haveconholled the precipitationof tour- maline and albite" chemicalreactions were wrifien for L2 albite+ 6 magnetite+ 6 boric acidr*t the systemH-B-O-Na-A1-Si-Fe under quartz satura- + l0H* tion, with hydrogen activity defined by the magnet- ite-hematite buffer, at T and P encompassing = 2 schorl + 24 qualz+ 6 hematite mineralization in porphyry systems. For simplicity, + 10 H2O+ 10 Na+ (3) only schorl was considered,and as a startingpoint for balancingthe equations,Al was treatedas an immobile This reaction requires the presenceof imporant element.The aboveassumptions were made because of quantitiesof iron oxides, which are, along with other the following observations:(1) quartz is ubiquitous iron-bearingminerats such as pyrite, disprsd throughout the Coxheathdeposit in veinlets and as a in the Coxheathdeposit, whereas tourmaline and albite replacementmineral (2) althoughourmaline aJ Coxheath are closely intergrown at a microscopic scale. Both contains approximately equal proportions of Fe and magnetiteand hematite occur in the potassic assem- Mg, compositionsof tourmaline from other poryhyry blage that was overprinted by the tourmaline stock- systemsare mainly schorl-rich,and (3) magnetiteand work, suggestingthat the magnetite-hematitebuffer hematite occur together in the core of the Coxheath provided a useful meansof balancingiron and fixing depositand in the tourmalinezone. the hydrogenfugacity in the system.Also, this buffer is
-15 4000c 5000c 3 kbor -20 Tur rt - o (n -25 o c'l And Ab o -30
-35 468 02468 o2468 lo9(oNo+/oH+) lo9(oNo+/oH+) log(oNo+/oH+) log(oNo*/oH*) -15 Tur -20 1 kbor Tur |Il :tr o -25 (Il Tur o c'l And Ab And Ab o -30
-?trI | | lr I "-02468 o246802468 log(oNo+/oH+) log(oNo+/oH+) log(oNo+/oH+) log(oNo+/oH+) Ftc. 11.Activity diagramsgenerated at 1 and3 kbar pressureand from 3@oto 6@oCfor tourmalineCfur), albite (Ab), paragonite (hg), andalusite(And), sillimanite (Sil), pyrophyllite (hl), andkaolinite (Kh), in presenceof quartz- magnetite- hematite - water. Diagramsindicate an expandingfield of stability of tourmalineupon cooling, and show that the Na*/H* ion ratio only sipificantly affects tourmaline stability relative to albite. Thermodynamicdata are taken Kuyunko et aL (1984) for tourmaline, from Shock et al, (1989) for boric acid, and from Helgesonet aL (1978) for other silicates. Adjustmentsto equilibrium constantsto high temperatureswere nade using the Maier & Kelley (1932) power-seriesequation for heat capacities;the effects of pressureon equitbrium constantswere estimatedby determiningvolumetric changesassuming incompressiblereactants.
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t)?ically used when calculating mineral equilibria in DISCUSSION AND CONCLUSIONS porphyry-Cusystems (e.9., Beane 1982). Inspection of equation(3) revealsthat iron obtainedfrom the break- Although mostporphyry depositsoccur in Mesozoic down of magnetiteis distributedbetween hematite and and Cenozoic volcano-plutonic arcs, an increasing tourmaline, which is consistentwith the presenceof number are being recognizedin Precambrianterranes. ferric iron in the Coxheathtourmaline. Alsoo relative to The Coxheath deposit is a good example of a albite, tourmalinebecomes stable with decreasingpH, Cu-Me-Au mineralized poryhyry from the hotero- whichis consistentwith theexperimental work of Morgan zoic, which displaysalteration and zoning pattems typi- & Iondon (1989).Figure 11 is anactivity-activity diagam cal of younger, well-characterizeddeposits. The displaying mineral equilibria for temperafuresranging presenceof clay altemtionat Coxheafhshows that argillic from 300' to 600'C and pressuresof 1-3 kilobars. alterationlinked to porphyry systemsmay alsooccur in Mineral equilibria were calculated using data from hecambrianexamples. Rrfhermore, the Coxheafhd€posit Kuyunko et al. (1984) for tourmaline, Shock et al. contains a number of featuresthal distinguish it as a (1989) for boric aci{ and Helgesonet al. (1978) for Au-bearingporphyry (Sillitoe 193), including(l) dissemi- othersilicates. Adjusfrnents to equilibrium constantsfor natedmineralization that yields assaysnear or above high temperatureswere calculatedusing a power-series 4 ppm Au, aswell as apositive correlationbetween Au equationfor heat capacities(Maier & Kelley 1932)and andCu (datafrom unpublishedassessment files in provin- assumingincompressible reactants in an evaluationof cial archives),(2) the p're.senceof maguetiteas an impor- the effectsof pressure.Under all conditions,tourmaline tant hydrothermal mineral in the Au zone, (3) is stableai a high activity of boric acid. Howeverothe ocqurenceof both calcic and sodic alterationsubtypes stability field for tourmaline is strongly affected by wirhin tle potassiczone, and (4) the predominanceof temperature,and expandsgreatly upon cooling. The propylytic alterationover phyllic alterationin the zone Na+/II+ value also affects relative mineral stabilities, immediafely next to the potassiccore. Consequently, but the steeperslope of the reactionboundary between the Coxheathdeposi! togetherwith a numberof other tourmalineand albite comparedto the slopesfor reac- porphyry deposits(Hollister et al. 1974)and epithermal tions involving mica, clay, and alkali-free aluminosili- depositssuch as Hope Brook in Newfoundland(Stewart cates,indicates that the ratio Na+/II+ is relativd less 1992,Dtft6 et aI. 1996),help define the Avalon Zone importantfor tle lafter equilibria. of the Appalachianorogen as a potentially significant
meteoricwater
-water tablT -{ propylitic
' \ $,-rIlif monzonite-fl 1+ L diorite , i*14 ot
FIc. 12. Generalized diagram illustrating volcano-plutonic setting for the Coxheath Cu-Mo-Au depositand the distribution of principal zonesof alteration (seetext for detaileddescription).
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metallogenic province for porphyry-epithermal tion is indicaledby texhres recordedin sulfides,oxides, sysrerns. and silicates:the developmentof a rim of bomite on Tourmalineis an accessorymineral associatedwith chalcopyrite,and of hematiteon magnetite(Fig. 13), as potassicand phyllic alterationzones and hydrothermal well ashydrothennal alteration of K-feldsparto musco- brecciasin numerousporphyry-Cu deposits (e.9., Silli- vite, are all featuresthat canbe indicative ofdecreasing toe & Sawkins1971. Camus 1975. Gustafson & Hunt temperature.Furthermore, it has been shown in the 1975,Gustafson & Quiroga 1995),and in somecases is activity diagramsthat the stability field of tourmaline linked directly to sodic-calcic alteration subtypes relative to thoseof albite, paragonite,and clay minerals (Carten 1986, Dilles & Einaudi 1992). Mineral zona- increasessignificantly during cooling. However, tour- tion and paragenesisat Coxheath,as at other deposits, malineoccupies a resticted areain the Coxheafhsystem; provide information regardinghydrothermal it separatesthe outer pyrite-enriched zone, charac- processesin porphyrysyst€ms @ig. 12).Typically, poas- terzrdby acidic alterationassemblages, from the pyrite- sic alterationin the core of porphyry systemsis directly depletedcore, characterizedby more strongly alkaline linked to late-magmatichydrothermal fluids in crystal- alterationassemblages. Investigations at a number of lizing plutons(e.9., Lowell & GuilbertI97U,Beaae & depositshave shown that boiling is an importanthydro- Titley 1981,Sillitoe 1993,Hedenquist & Lowenstern thermal processin the formation of porphyry deposits, 1994).This is likely fue at Coxheathas well, because and that the escapeand condensationof acid volatile of the direct link betweenthe potassic zone and the speciescontibute to the formation of argillic alteration intusivercds. Rrthemore, a link to magmatio-*rydrother-adjacentto or abovethe potassiccore, effectively seg- mal processescan also be inferred for the sodic altera- regating mineralogical domains (e.9., Hedenquist & tion because of the mineralogical association; Lowenstern 1994). Tourmaline precipitation can be experimentalwork by Pichavant(1981, 1987) has placedin a contextof boiling by consideringthe behav- shownthat with the addition of B to granitic melts,the ior of boric acid in the hydrothermal environment. aqueousvapor phasecoexisting with the meltsbecomes Fluid-mineralequilibria suggest that tourmaline precipita- emichedin Na relative to K, and that B also is parti- tion is stongly influencedby the activity ofboric acid tioned into the vapor phase relative to the melt (as B(O[D:, which is concentratedin the residualhydrother- NaBO2, among others).Thus the tendencyfor paired mal fluid during boiling. Thus for porphyry systems,B exsolutionof B and Na from a crystallizing magmais may be concentratedin two stages,fust asan incompat- possibly indicatedby the close associationoftourma- ible elementin granitic melts, and subsequentlyas a line and albite, togetheras an alterationassemblage at nonvolatilecomponent of boiling-water-dominatedhy- Coxheafh.Sfong temperature-gradientsare also expected drothermalsystems. Tourmaline at theCoxheath deposit is with magmatic-lydrothermalprocesses, and at Coxheafh, found adjacentto the core zone,wherg tourmaline pre- decreasingtemperature during the courseof mineraliza- cipitation may havebeen induced by boiling. If this was
0 ffiffiffiw& Effi@&g ffiwwe&ry$rry aN_5 b0 .9-ro ffi -15 ru$,nn', -4o- -35 -30 -25 -2o- -15 log aO 2 Flc. 13.Activity diagramillustrating stability fields of the principal opaquemineral phases from the Coxheathdeposit, at relatively high (550'C, dashedlines) and low (300'C) temperatures.Note sipificant expansionof the stability fields of bornite + pyrite and hematiGrelative to thoseof chalcopyriteand magnetiteupon cooling (arrows),which may promotethe formation of bornite andhematite rims in Coxheathsamples (Fig. 4). Modifredfrom Beane& Titley (1981).
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granite-relatedSn t W deposits,which typically con- orgillic tain more aluminum and liule or no ferric tron (e.g,, tondon & Manning 1995,Slack 1996).This is substan- tiatedby the commonpresence of magnetiteand hema- tite in porphyry-copperdeposits, and the scarcity of theseminerals in most Sn t W deposits. The presenceof tourmalinemay alsobe an indicator of material provenance.Typically, in volcanic arcs,B E rx#H+ isotopestudies have shown that B is principally derived lowl +. + from subductedsediments (e.9., Palmer 1991a). Conse- quently, the presenceof tourmalinein someporphyry /\-L K/H. Cu deposits,such as Coxheath,may be interpretedto Q9z' indicatethe incorporationinto the hydrothermalsystem of material from a subducted slab, subsequentto $i plutonic-volcanic recycling. However, for deposits fw/ that are surroundedby argillaceoussedimentary rocks, local derivation of B from the host rocks seemsmore likely.
B(oH)3 Acroowr-mcnrvrgr.trs
Supportfor this projectwas received from the Canada - B(oHljourmoline'J B(oH)3 NovaScotia Cooperation Agreement on MineralDevel- ---\ opment1992-95, and from the Geological Survey of /potossi> Canada.Guidance during tle electron-microprobe sessionsand data processingwas given by G. Pringle mognetite andJ.A.R. Stirling; )(RD analysisof clay mineralswas +, + performedby A. Chapon. Field assistancewas provided . INo/H' by N. Fagnanand D. Pankewich.We are gtatefirl to highl B.E.Taylol W.D. Sinclair,and K Schrijverforcomments K/n,,*/,,* on a preliminary draft of this paper.Extensive reviews, and close attentionto detail fromjournal refereesJ.F. Slack and J.T. O'Connor, resultedin a much improved Ftc. 14.Schematic model depicting context for tourmaliniza- tion within a boilingporphyry-related hydrothermal sys- manuscript. tem. Segregationof acidicand alkaline alteration zones basedon lossof acidicvolatiles (ag., HzS)from thecore Rmmrncrs ofthe systemresults in increasedboric acid content within the residualflui{ thuspromoting tourmalinization along ArtrFED,F. (1945):The Chicotetungsten deposil Bolivia. theboiling front, EconGeol.4A,39+407. Benn,S.M. (193): Geochemistryand tectonic setting of late the case,then the position of the tourmalinezone de- hecamb'rianvolcanic and plutonic rocks in southeastern lineates the boiling interface within the deposit CapeBreton Islan4 NovaScotia- Can J, Earth Sci.30, Gig. 1 ). This is consistentwith resultsof a numberof 11,47-tt54. strdies doneon fluid inclusionsin tourmaline(Skewes 1992)and within intergowu quartz(Sillitoe & Sawkins MACDoNATD,A.S. &WlilrE" C.E. (1988): The Four- 1971, Carlson & Sawkins 1980) from other porphyry chu Group and associaredgranitoid rocks, CoxheathHills, Cu deposits,which indicatethat tourmalinewas pnecipi- EastbayHills, andsouthwestern Stirling andCoastal Belts, CapeBreton Islan{ Nova Sconu GeoLSurv. tatedduring boiling from hypersalinefluids. southeastem Can, Open-FilcRep.1759. Tourrnalinecornpositions al Coxheathmay also provide informationon the mineralizingenvironmen| relatively BEAN4R,E. (1982):Hydrothermal alteration in silicate rocks, low aluminumandhigh iron contentssuggestthat ferric southwestemNorth America. /z Advancesin Geology of North Amer- iron is an important constituent Compositionaltrends the PorphyryCopper Deposits, Southwestern ica (S.R. Titley, ed.). The University of Arizona Press, characterizethe tourmaline as povondraite.The pres- Tucson,Arizona ( I 17-138). ence of ferric iron in tourmaline at Coxheathand in similar depositsmay be interpretedto indicate rela- & Tm-ev,S.R. (1981): Porphyry copper deposits. tr. tively oxidizing environments for porphyry-copper Hydrothermalalteration and mineralization.Econ GeoI., mineralization compared to tourmalines from most Sev enE - F ifrh Anniv. Vo 1., 235 -269.
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