The evolution of a W, Au-Ag-Te and Au-Ag ...

The evolution of a W, Au-Ag-Te and Au-Ag hydrothermal sys­ tem, Tinos Island, Cyclades, Greece S.F. T om bros, K . St. Seym our, D . Z ou zias, & N . M astrakas

Department o f Geology, Laboratory o f Ore Deposits and Volcanology, University o f Patras, 265.00, Patras, Hellas, Department o f Geography, Concordia University, Montreal, Canada. P.G . Spry

Department o f Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3212, U.S.A.. A .E . W illiam s-Jon es

Department o f Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A 7, Canada.

A B S T R A C T : A t T inos Island, G reece, the p h ysicoch em ical evolu tion o f the hydrotherm al system and the a ssociated stages o f m etallic m ineralization that are d ev elo p ed in the v icin ity o f the T in os granodiorite-leucogranite has b een studied, b ased on fluid in clu sio n studies and ch em ical reactions m od elin g. Early tungsten m ineraliozation w a s related to the m etasom atic stage o f contact m etam or­ phism and precipitated at ~ 3 5 0 oC, from m oderate saline (1 0 .4 to 14.8 wt% N aC l eq.), C O 2efferv escin g flu ids, that contained variable am ounts o f C aC l2 and M g C l2. Panorm os B a y A u -A g -T e m ineralization, located 16 km aw ay from the intrusive site, w a s dep osited from cooler 20 0 ° to 3 0 0 oC, and lo w to m oderate saline (0 .2 to 13.2 wt% N aC l eq .) m in eralizin g fluids. A u -A g m inerali­ zation at A p igan ia B ay, w h ich represents a late evolutionary phase, w a s d ep osited from even cooler (1 2 5 o to 2 3 5 oC) and dilute (0 .2 to 6.8 w t % N aC l eq .) fluids. In all, the m ineralization stage p recip i­ tation w a s controlled by tw o principal factors: the exsolu tion o f g a seo u s phase and an increase in pH from 3.3 to 7.6 K E Y W O R D S : T ungsten, G old-silver-tellurium , G old-silver-b earin g m ineralization, Skarn, P anorm os B ay, A p igan ia B ay, T inos, G reece

1 IN T R O D U C T IO N U n ex p lo ited tu n gsten -m olyb denu m m iner­ alization occurs w ith in the contact aureole o f the syn tectonic calc-alk alin e T inos intrusion, and m ay b e gen etically related to g o ld -silv er telluride m ineralization at P anorm os B ay, and g o ld -silv er m ineralization at A p igan ia B ay, Tin os Island, G reece. In this study, d etailed in v e s­ tigation s o f paragenetic and fluid in clu sion s characteristics have b een conducted to co n ­ strain the ev o lu tion and p rovenance o f the m in ­ eralizin g flu ids at the three localities.

th o leiitic m afic to fe lsic v o lca n ic rocks and a s­ sociated m etased im en ts m etam orphosed to b lu esch ist fa cies (4 0 -5 0 M a), and subsequently retrograded to green sch ist fa cies (c a . ~ 25 M a) (A n d riessen e t al ., 1987). O verlyin g this are the klippen o f the U pper U nit, w h ich represent an­ cien t ocean ic crust m etam orphosed to greensch ist fa c ie s that w a s em placed at ~ 18 M a (A n driessen e t al., 1987). T he T inos pluton w as intruded syn tecton ically, and d isp lays therm altecton ic contacts w ith the B lu e sc h ist and U pper U n its (M astrakas, 2 0 0 7 ).

2

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G E O L O G IC A L SE T T IN G

T he tecto n ic stratigraphy at T inos con sists o f a series o f stacked nappes cut by a M io ce n e pluton (M elid on is, 1980). The lo w e st o f th ese nappes, the B asal U nit, con sists o f a platform o f late T riassic to late C retaceous neritic carbonate rocks. T his unit is overlain by the Interm ediate nappe or B lu e sc h ist U nit, a package o f

T IN O S P L U T O N IC R O C K S, C O N T A C T AUR EO LE A N D SKARNS

T he M io ce n e T inos pluton con sists o f a cen ­ tral b od y o f I-type b iotite-hornblende granodiorite that w a s em placed at c a . 17 M a (Altherr e t al., 1982) under com p ression (M astrakas and St. Seym our, 2 0 0 0 ). A n S-type leucogranite w ith the assem b lage b iotite-m u scovite-garn et-

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tourm aline w a s em placed peripherally to the granodiorite, and also as sills at sh a llo w le v e ls in an exten sion al regim e (M astrakas & St. Seym our, 2 0 0 0 ). T he age o f em placem en t o f the leu cogran ite has b een determ ined as ca. 14 M a (K -A r ages, Altherr e t al., 1982). T he g e n e ­ sis o f the granodiorite m agm a is attributed to partial m eltin g o f m afic lo w er crustal rocks, w h ereas the leu cogran ite m elt w ith m elt contri­ bution from the granodiorite represents a hybrid partial m elt co n sistin g m ainly o f m etased im en ­ tary country rock. G eobarotherm om etric studies indicate that the granodiorite w a s em placed at a tem perature range o f 75 0 ° to 800°C and at pres­ sures o f ~ 4 .7 kbars, w h ereas the leucogranite w a s em placed ca. 6 8 0 °C and at ~ 2 kbars (M astrakas, 2 0 0 7 ). T he latter exp erien ced inten se retrograde b o ilin g as evid en ced by num erous m iarolitic cavities. T he T inos pluton caused contact m etam orphism at ca. 14 M a (B rocker et al., 1993). A d iscontin u ous scap olite zo n e is surrounded su cce ssiv e ly by p yroxen e-, horn­ b lend e-, albite- and ep idote-h orn fels zon es (M astrakas, 2 0 07). Skarn rocks are d evelop ed w ith in am phibo­ lite sch ists and m arbles o f the B lu e sc h ist U nit, and outcrop as podiform , len so id and v ein -lik e b o d ies and m ainly as an exoskarn near m arbleam phibolite sch ist contacts. In general, skarns are spatially related to leu cogran ite ap oph yses and aplitic stockw orks. Skarns co n sist pre­ dom inantly o f garnet and clin op yroxen e. H orn­ b len d e replaced clin op yroxen e and grew in open sp aces w ith calcite, quartz, feldspar, titanite and epidote. A c cesso ry m inerals includ e apatite, tourm aline, allanite and w ollaston ite, lo ca lly , in the m arbles. A p p lication o f the p y ­ roxene-garnet geotherm om eter o f Patti son & N e w to n (1 9 8 9 ) to pyroxen e-garn et skarn rocks g a v e non-equilibrium results, w ith frequency peaks occurring at 6 8 0 °, 550°C (attributed to an early contact m etam orphic th erm al-isoch em ical stage); and a num ber o f v a lu es b etw een 375° and 3 2 0 ° C (related to a late infiltration m e­ tasom atism event, w h ich occurred at pressures o f < 500bars; M astrakas, 2 007). 4

O R E S A S S O C IA T E D W IT H TH E T IN O S PLUTO N

con tin uou s zon es, up to 30 cm w id e, sub­ parallel to the sch istosity, particularly in silicified -carb onatized zo n es or as residual crusts on fractures. S ch eelite m ineralization, w h ich m akes up to 2 0 vol. % o f the skarn, occurs in garnet-pyroxene skarns (grossular-rich grandite co ex istin g w ith hedenbergite during the contact m etam orphic ep isod e ev o lv ed to andratite-rich oscillatory zon ed garnet co ex istin g w ith salited iop sid e) containing m agnetite, chalcopyrite, pyrrhotite, pyrite and lesser sphalerite. O pen sp aces in the skarn are lo ca lly filled w ith calcite, apatite, hornblende, chlorite, titanite, quartz, feldspar, m olyb d en ite and sch eelite. S ch eelite d ep osition w a s lik ely controlled by the reaction: CaCO3(s) + H2WO40(aq) = CaWO4(s) + H 2O(p + C O 2(g)

(1)

4.2. P a n o rm o s B a y m in e ra lized vein system The Panorm os B a y m ineralized v ein system is hosted in m arbles o f the B asal and B lu e sc h ist U n its and con sists o f 30 subparallel steep ly dip­ p in g banded, syntaxial stockw orks o f quartz vein s. M arble units are intruded by tw o genera­ tion s o f v ein s, an older, northeasterly-trending set o f m ilk y quartz v ein s and a you n ger group o f northw esterly trending clear quartz vein s. A lteration h aloes form ed as concentric sh ells and planar b o d ies that en v elo p e m ilk y and clear quartz vein s. B lu e sc h ist U n it m arbles d evelop a ssem b lages com p osed o f m u scovite, albite and tourm aline, w h ereas alteration h aloes in d olom itic m arbles o f the B asal unit co n sist o f tw o su cce ssiv e zones: an inner talc zone and an outer chlorite zone. T alc zo n es are character­ ized by an assem b lage o f m ilk y quartz, talc, calcite, brucite, m u scovite, and albite. Chlorite zo n es co n sist o f m ajor clear and m inor m ilk y quartz, chlorite, ep idote, m u scovite, albite and barite. Form ation o f talc-brucite-calcite is in ­ terpreted to h ave b een controlled by the reac­ tion: CaMg(CO3)2(s) + 4SiO2(aq) + 2M g+2 + 3H 2O(l) = Mg3Si4O10(OH)2(s) + CaCO3(s) + C O 2(g) + 4H+ (2)

4.1. Scheelite M in era liza tio n T ungsten m ineralization is h osted in skarns and hornfelses. S ch eelite occurs as d issem in a­ tion s and m ainly as m a ssiv e aggregates in d is­

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S even ty m etallic and gan gue m inerals, in ­ clu din g C u -cervelleite and an unnam ed A g -A u Cu sulphotelluride w ere identified. E igh t paragen etic stages o f h y p ogen e m ineralization

'Digging Deeper” C.J. Andrew et al (editors)

w ith the assem blages: pyrite, arsenopyrite, pyrrhotite (S tage I); tetrahedrite, tennantite, g old field ite (S ta g e II); bornite, chalcopyrite (S tage III); Sn sulphides, sulphosalts o f A s and Sb and arsenides o f N i or C o (S tage IV); tellurides (S ta g e V ); galena, betekhtinite, argentite (S tage V I ) ; w urtzite, green ock ite, sm ithsonite (S tage V II) ; n ative gold , silver, copper, arsenic, strom eyerite, pyrargyrite (S tage V III) w ere fo l­ lo w ed by supergene m ineralization (stage IX ) (T om bros e t al., 2 0 0 4 , 2 0 0 5 ). Precipitation o f A g -, and A u - tellurides w a s lik ely associated w ith reactions (3 ) and (4), w h ereas n ative pre­ cio u s m etals w ere d ep osited by reactions (5) and (6): 2 A g ( H S )-2 + H T e-(aq) = 4 H S -(aq) + Ag2Te(s) + H+(aq) (3) A u (H S )-2(aq) + 2 H T e-(aq) + 0.75O2(g) + H+(aq)= 2 H S -(aq) + A u T e 2(s) + 1.5H2O(l) (4) 0.5H2O(l) + A u (H S )-2(aq) = Au(s) + 2 H S -(aq) + H+(aq) + 0.25O2(g)

(5)

A g (H S )-2+aq) + 0.5H2O(l) = Ag(s) + H (aq) + 2 H S -(aq) + 0.25O2(g)

(6)

4.3. A p ig a n ia B a y m in e ra lized vein system T he A p igan ia B a y v ein system is h osted in m arbles, b lu esch ists, green sch ists, am phibolites and ultram afics o f the B lu e sc h ist U n it (T om bros, 2 0 0 1 ); and con sists o f fiv e h igh -an gle quartz vein s, w h ich bear considerable resem ­ b lan ce to v ein s at Panorm os B ay. A lteration ha­ lo e s com prise an inner epidote zone and an outer chlorite zone. T he ep idote zo n e is a sso c i­ ated w ith m ilk y quartz, albite, m u sco v ite and ankerite, w h ereas the chlorite zo n e contains clear and m inor m ilk y quartz, chlorite, calcite, albite and m u scovite. There are three paragenetic stages o f hyp o g en e m ineralization: pyrite, sphalerite, pyrrhotite, arsenopyrite, m agnetite, argentite, elec trum, stephanite, xan th ocon ite, proustite, can field ite (S tage I); tetrahedrite, fam atinite, chalcopyrite, ram m elsbergite, laggisite, cassiterite (S ta g e II); and galena, argentiteacanthite, p olyb asite, electrum , argentopyrite (S ta g e III). A supergene stage (S tage IV ) fo l­ lo w s the h y p ogen e stages (T om bros & St. Seym our, 1998). E lectrum precipitation in S tages I and III w a s controlled by reaction (7):

0.5H2O(l) + 0.7Au(HS)-2(aq) + 0.3Ag(HS)-2(aq) = Au0.7Ag0.3(s) + 2HS"(aq) + H+(aq) + 0.2502(g) (7) 5 F L U ID IN C L U S IO N S T U D IE S M icrotherm om etric m easurem ents w ere m ade in the Laboratory o f H igh Tem perature P ro cesses at the D epartm ent o f Earth and Planetary S cien ces o f M cG ill U n iversity. T em ­ peratures w ere m easured w ith an alum elchrom el th erm ocouple, and the readings w ere calibrated w ith synthetic in clu sion s. M easure­ m ents are accurate to w ith in ± 1oC. M icrother­ m om etric data w ere reduced u sin g the F L IN C O R softw are (B row n, 1989). A t room tem perature, fluid in clu sion s w ere cla ssified into three typ es b ased on the num ber and proportion o f phases: (i) L -V in clu sio n s predom inate, and co n sist o f aqueous liquid + vapor (< 25 vol. %). T hey contain no solids, w h ereas form ation o f gas hydrates w a s ob ­ served during freezin g and th ese h o m o g en ize to liquid upon heating, (ii) V -L in clu sio n s w ith ir­ regular shapes and con sistin g o f aqueous liquid + vapour (up to 80 vol. %). The V -L in clu sio n s are primary, infrequently ob served and h o ­ m o g e n ize to the vapor phase upon heating, and (iii) L -L -V in clu sio n s co n sist o f aqueous liquid + carbon d ioxid e liquid + vapor. Form ation o f gas hydrates w a s ob served in som e o f the L -L V in clu sio n s during freezing. L -L -V primary fluid in clu sio n s (~ 5 % o f in ­ clu sion population) occur in m ilk y quartz v ein lets cross cutting garnet cores from the skarn, and in m ilk y quartz and calcite from P anorm os B ay. V -L primary in clu sio n s occur in sch eelite and m ilk y quartz v ein lets, sm ok y and m ilk y quartz and calcite from P anorm os B a y m ineralization. L -V primary, pseudosecondary and secondary in clu sion s are located in all three m ineralizations. T he tem peratures o f last m eltin g o f ic e (Tmice) and clathrates (Tm_dathrate) o f the L -V , V -L and L -L -V in clu sion s, in sch eelite and a sso c i­ ated m ilk y quartz vary from -10.4° to -1 0 .2 oC and 0 .7 o to 5 .7 oC, respectively. T hese v alu es correspond to salin ities o f 10.4 to 14.8 wt% N aC l equivalent, u sin g the equations o f B row n 6 Lam b (1 9 8 9 ) and D arling (1 9 9 1 ). The h o ­ m ogen ization tem peratures (Th) o f th ese in clu ­ sions vary from 3 1 5 o to 4 0 0 oC. T he Tm.ice and Tm-ckthrate v a lu es for fluid in clu sio n s in sm oky quartz range from -6 .6 o to -5 .7 oC and 6 .7 o to 7 .7 oC, resp ectively, corresponding to salinities

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o f 4 .5 to 6.8 wt% N aC l equivalent, w h ereas the Th v a lu es range from 2 8 1 o to 320°C . A t P anorm os B ay, the Tm_ice and Tm-clathrate v a lu es o f fluid in clu sio n s in m ilk y v ein and clear quartz range from -4.4° to -2.3°C , 7.7° to 8 .5 oC, and -4 .1 o to -2 .0 oC, 8 .8 o to 9 .6 oC corre­ sponding to salinities o f 1.1 to 7.9 and 0.8 to 5.6 wt% N aC l equivalent. T he Th valu es o f in ­ clu sio n s range from 2 4 5 o to 2 9 2 oC and 198o to 2 5 3 oC, respectively. A t A p igan ia B a y the Tm-ice v a lu es o f fluid in clu sio n s in m ilk y v ein and clear quartz range from -4 .3 o to -0 .4 oC, and 3 .6 o to -0.1 oC, resp ectively, corresponding to salin ities o f 3.0 to 6.8 and 0 .2 to 5.5 wt% N aC l equivalent. The Th v a lu es range from 173o to 2 3 5 oC and 125o to 168oC, respectively. 6

D IS C U S S IO N

B a sed on results derived from reactions 1 to 7, it is lik ely that precipitated the m in eraliza­ tion s ore d ep osition on T inos Island w ere co n ­ trolled by pH, C O 2-efferv escen ce, d ep letion o f H 2S, and ch an ges in the oxidation state o f the ore fluid. E fferv escen ce o f C O 2 cau ses a sharp increase in pH, w h ich in turn d estab ilizes the tungsten and preciou s m etal-bearing co m ­ p lex es. T his su ggests that the d ep osition o f sch eelite, A u -A g tellurides and n ative precious m etals and electrum w a s due to phase separa­ tion related to C O 2-efferv escen ce. T he escape o f v o la tiles neutralized the pH o f the m in eraliz­ in g fluid from 3.3 at 4 0 0 oC to 4 .6 at 3 0 0 oC, 5.7 at 2 5 0 oC and 6.5 at 2 0 0 oC for the Panorm os B a y ores and from 6.9 at 2 0 0 oC to 7.6 at 150oC at A p ig a n ia B ay (T om bros, 2 0 0 1 ). In co n clu ­ sion, W , A u -A g -T e and A u -A g ore d ep osition w a s predom inantly controlled by ga seo u s phase separation, w h ich resulted in pH neutralization. ACKNOW LEDGM ENTS W e thank the E uropean Social Fund (E SF), O perational Program for E ducational and V o c a ­ tional Training II (E P E A E K II), and particu­ larly the Program Pythagoras II, for funding the ab ove w ork as a grant to K StS and p ost­ doctoral fello w sh ip to S. Tom bros.

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"Digging Deeper” C.J. Andrew et al (editors)