Noronha, F.

English

Noronha F.

Dória M. A.

Ramos R.

Ramos J. M. F.

University of Szeged

Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 FLUIDS ASSOCIATED WITH Pb-Ag MINERALISATIONS OF FACUCA DEPOSIT (NORTHERN PORTUGAL). N O R O N H A , F. ' , DÓRIA, M. A. ' , R A M O S , R.1, R A M O S J. M. F.2 1 GIMEF, Centro de Geología Univ. Porto, Fac. Ciencias, Praya Gomes Teixeira, 4099-002 Porto, Portugal. 2 Instituto Geológico e Mineiro, Rua da Amieira, 4465-021 S. Mamede de Infesta Portugal. E-mail: fmnoronh@fc.up.p t The old mine of Facuca is located immediately SE of Vila Cova (Vila Real - Trás-os-Montes) not to far f rom an old iron (magnetite) mine of Vila Cova (Maráo Mountain) . In the Facuca deposit (also known by Cando mine), during the first half of the XX century a brecciated quartz vein was exploited for lead and silver, bearing macroscopically visible sulfide minerals, essentially arsenopyrite, pyrite, galena and sphalerite (Neiva, 1955; Priem, 1962). Geology The Variscan belt is characterised by several geotectectonic zones roughly E - W trending with specific and peculiar paleogeographic, tectonic, metamorphic and magmatic characteristics. Tectonic characteristics of the European Variscides are those of a classical obduction-collision belt and it is described as a stacking of large-scale thrust crustal nappes, between 380 and 340 Ma. The inner zone of the orogen is represented by the "Central Iberian Zone" (CIZ) in the Iberian Peninsula (Dias, Ribeiro 1995). In the N W of the Iberian Peninsula, three main phases of deformation D I , D2 and D3 are usually considered responsible for the structuration of this part of the Variscan belt, the last one being intra-Westphalian in age. At the higher structural levels the D1 structures are well preserved. At lower levels, the D1 structures were transposed by D2 giving rise to the regional schistosity (S2). In the metamorphic domains a peak of regional metamorphism, of low-pressure type (T 650 to 700°C at P< 5 kb) was reached during or just after D2 being Lower-Carboniferous in age. The latest stage of ductile deformation of the basement corresponds to D3, intra-Westphalian in age. After a later essentially brittle phase D4 is responsible for two conjugated fracture systems striking N N E - S S W and NNW-SSE. The Facuca area belongs to the autochthonous units of CIZ. The area comprises essentially of metasedimentary rocks and Variscan granites of different types. The mineralized vein is situated between rocks of the Lower Ordovician. Nearby a thrust occurs, striking N130°E and dipping 30° SW, separating this formation f rom the "Complexo Xisto-Grauváquico" (Desejosa Formation) ante-Ordovician in age. The main foliation (SI ) strikes f rom N120°E to N140°E dipping 30 to 40° NE. The effect of the thermal metamorphism, due to the emplacement of the two-mica granite, is not distinguishable f rom the regional metamorphism (Pereira, 1989). The peak of regional metamorphism, of low-pressure type (T 650 to 700°C at P< 5kb) was reached during or just after D2 being lower - Carboniferous in age. The structure of the vein is generally complex, which results f rom a poliphasic infilling of quartz. However the vein is filled by massive milky quartz (QI), with a brecciated zone near the footwall. This outer zone is characterised by a breccia cemented with quartz. The vein has a N130°E strike and a NE dip of 50° to 80°; the thickness amounts to about 40-60cm. The vein has a visible extension of 300m. Mineralogy The ore minerals are essentially arsenopyrite, pyrite, galena and sphalerite being quartz the main gangue mineral. The mineralogical and geochemical electron microprobe studies, allowed the further identification of chalcopyrite, pyrrhotite, violarite, freibergite, stephanite, pyrargyrite as hypogene mineral assemblage and anglesite, cerussite, scorodite, covellite, bornite, hematite and limonite as secondary alteration products. The mineralogical study helps to establish a chronological depositional sequence composing four main stages: a first stage (Ni, Co, As, Fe) characterized by deposition of pyrrhotite, pentlandite, arsenopyrite I and pyrite I; a second stage (As, Fe, Zn, Cu) with arsenopyrite II, sphalerite and chalcopyrite I; a third stage (Zn, Pb, Ag) contains galena, freibergite, stephanite, pyrargyrite, pyrite II and chalcopyrite II. Secondary alteration products as Pb-sulphates, Cu- sulfides, Fe-arsenates and anglesite characterize the late stage. As previously mentioned quartz is the main gangue mineral. Usually occurs as a milky quartz variety (QI) generally as large anhedral crystals with highly undulous extinction. More rarely idiomorphic crystals occur and that happens when they crystallized in small geodes. It occurs associated with almost all the observed primary minerals, sometimes infilling their fractures suggesting that the quartz is the first mineral of the vein and after continued to precipitate until the later stages of mineralization. Fluid inclusion studies The fluid inclusion studies were carried out in two types of quartz, a clear quartz associated with arsenopyrite II (QII) and subhedral to euhedral quartz spatially associated with galena (QUI). No microthermometric analyses have been made on the fluid inclusions in the earliest milky deformed quartz (QI) due to their small size. Microthermometric and microspectroscopic 149 Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 R a m a n s tudies r evea led the p r e s e n c e of th ree f lu id types : (1 ) a q u e o u s - c a r b o n i c H 2 0 - C 0 2 - C H 4 - N 2 - N a C l ; (2) a q u e o u s wi th l o w sal ini ty H j O - N a C l ( L w O and (3) a q u e o u s with high sal ini ty H 2 0 - N a C l - C a C l 2 ( L w 2 and L w 3 ) . T h e a q u e o u s - c a r b o n i c f lu ids h a v e b e e n obse rved in p r i m a r y and p s e u d o s e c o n d a r y f lu id i nc lus ions ( L c - w a n d L w - c t y p e ) in QI I and QUI . All the f luid inc lus ions h a v e t w o phases at r o o m t empera tu r e with a d e g r e e of f i l l ing ( F l w ) , v a r y i n g f r o m 0 . 5 0 to 0 .70 . T h e s e f luid inc lus ions h a v e a m e l t i n g t empe ra tu r e of C 0 2 ( T m C 0 2 ) r ang ing f r o m - 6 3 . 2 to - 6 1 . 1 ° a l r eady s u g g e s t t he p r e s e n c e of volat i le c o m p o u n d s o the r than p u r e C 0 2 ( C H 4 and N 2 ) that w a s c o n f i r m e d by R a m a n m i c r o p r o b e ana lys i s . C 0 2 is the d o m i n a n t species in the vola t i le phase rang ing f r o m 59 .7 to 9 5 . 0 m o l %. CH4 is in the 5 . 0 - 4 0 . 3 m o l % r a n g e . N 2 c o n t e n t r a n g e s f r o m 10.5 to 4 .7mol % . H o m o g e n i z a t i o n t empe ra tu r e of C 0 2 ( T h C 0 2 ) to the v a p o r p h a s e in the r a n g e o f - 1 7 . 2 to З . Г С . T h e T m c l is in the range 10 to 14°C. To t a l h o m o g e n i z a t i o n t empera tu re s (Th to the l iqu id p h a s e ) r a n g e f r o m 2 4 0 to 3 6 0 ° C in ( L c - w ) and 160 to 320°C in ( L w - c ) c o r r e s p o n d i n g to the m i n i m u m t r app ing t e m p e r a t u r e of the f lu ids . T h e L w ! f lu ids occu r as p l a n e s of f lu id inc lus ions (FIPs ) and /or as c lus te r s in QI I . T h e F l w var ies f r o m 0 . 7 0 t o 0 . 9 5 . Me l t i ng t empera tu re of ice ( T m i ) r a n g e s f r o m - 5 . 5 to - 0 . Г С (ca lcu la ted sal ini ty b e t w e e n 0 . 1 8 to 8 .55 w t % N a C l eq. ) ( B o d n a r , 1993) and h o m o g e n i z a t i o n (Th) o c c u r s in the l iquid phase b e t w e e n 102 to 200°C. T h e LW2 f lu ids occu r as c lus te r s in Q U I , the Tmi r a n g e s f r o m -32 .5 to -21 .2°C c o r r e s p o n d i n g to an h igh sa l in i ty v a r y i n g f r o m 2 1 . 6 and 25 .7 w t % C a C l 2 eq . (Go lds t e in & Reyno lds , 1994) . T h o c c u r s in the l iquid p h a s e in the r a n g e 102 to 145°C. T h e L w 3 f lu ids occur in P I F in QUI , the T m i ranges f r o m -47 .1 to -37 .8°C, c o r r e s p o n d i n g to the h ighes t sa l in i ty ( b e t w e e n 2 7 . 3 and 29 .7 w t % C a C l 2 eq . ) wi th Т е a r o u n d -70°C. T h occu r s in the l iquid phase in the r a n g e 82 to 100°C. Discussion and conclusions In o ther s tudied e x a m p l e s f r o m N o r t h e r n Por tugal the ea r ly f luids , cons ide r ed as " m e t a m o r p h i c f l u i d s " (in a n t e - D 3 q u a r t z ve ins ) , a re a q u e o u s and s o m e t i m e s con ta in c la thra tes of C H 4 and/or C 0 2 and are a s s u m e d as r e su l t i ng f r o m d e h y d r a t i o n p r o c e s s e s ( G u e d e s et al. , 2002) . D u e to the f lu id / rock in teract ion those f lu ids w e r e m o d i f i e d and a c q u i r e d a m o r e c o m p l e x c o m p o s i t i o n r icher in C 0 2 and /o r C H 4 in the volat i le phase . E x a m p l e s of these f l u ids a r e t h o s e t r a p p e d wi th in m i l k y q u a r t z vein matr ix that cou ld s u p p o r t the ear l ie r su l f ides (a r senopyr i t e ) and are re la ted to the t h e r m a l p e a k i n d u c e d by the p e r a l u m i n o u s two-mica g ran i t e e m p l a c e m e n t (311 M a ) . T h e latter f lu ids e v o l v e t o w a r d s d o m i n a n t l y a q u e o u s f lu ids wi th l o w sal in i t ies and p robab ly of me teo r i c or ig in . T h e s e c o n d i t i o n s ind ica te a c o n s i d e r a b l e b a s e m e n t up l i f t a n d / o r p r e s s u r e f luc tua t ions at the end of the D 3 phase (305 to 300 M a ) (Dor ia et al., 1995; N o r o n h a et al. , 2 0 0 0 ) M o s t of the f lu ids that migra ted wi th in the h igher c rus t at the end of the V a r i s c a n o r o g e n y w e r e " m e t a m o r p h i c w a t e r s " s.l. that w e r e mixed with sur f ic ia l wa te r s a long with a d e c r e a s e in P and T cond i t ions . T h e ma in d r i v i n g f o r c e s a f f e c t i n g th i s migra t ion were the s t ructural d i scon t inu i t i e s (act ing as a d ra in ing zones ) , the e m p l a c e m e n t of syn t ec ton i c and t hen pos t -tec tonic grani tes and the upl i f t and genera l d e c o m p r e s s i o n of the Var i scan units . T h e younges t f lu ids c o r r e s p o n d to a q u e o u s high sa l ine and low t e m p e r a t u r e f l u ids ( G u e d e s e t al. 1999) . T h e r e s e m b l a n c e b e t w e e n these f lu ids and o the r r e spons ib l e for s imilar ba r r en and mine ra l i s ed ve ins a s soc i a t ed wi th p o s t - V a r i s c a n s t ruc tu re s o n a E u r o p e a n scale (Cana l s et al . , 1992) , indica tes that at least par t of the f lu id h is tory o f t he ve in is p o s t - V a r i s c a n . T h e d a t i n g o f these p h e n o m e n a is ques t i onab l e d u e to the non-ex i s t ence of P e r m o - C r e t a c e o u s f o r m a t i o n s in the a rea . W e assume, in the e x a m p l e of the F a c u c a vein, that the a q u e o u s - c a r b o n i c f lu ids a s soc i a t ed with ear l ie r s u l f i d e s r e su l t ed f r o m the in teract ion f lu ids /hos t rocks and the aqueous f lu ids related to ga lena r ep resen t supe r f i c i a l f l u ids and b r ines . Acknowledgements T h i s work is in tegra ted in the C e n t r o de Geo log ia act ivi t ies , with f inanc ia l suppor t f r o m F C T . References BODNAR, R. J. (1993) : G e o c h i m . C o s m o c h i m . Acta , 57 , 6 8 3 - 6 8 4 . DIAS, R. , RLBIERO, A. (1995) : T e c t o n o p h y s i c s 246 , 113-128 . D6RIA, A. , NORONHA, F., BOIRON, M . C. & CATHELINEAU, M . (1995) : T e r r a Abs t r ac t s , 7 , 312 . (Abs t r ac t S u p p l e m e n t n ° l to T e r r a N o v a , E U G 8, S t r a sbourg ) . GOLDSTEIN, R. H. & REYNOLDS, T . J. (1994) : Sys t ema t i c s of f luid inc lus ions in d i a g e n e t i c mine ra l s . 199 pp. S E P M - S o c i e t y f o r S e d i m e n t a r y G e o l o g y . Tu l sa , Ok la . ( S E P M Shor t C o u r s e 31) GUEDES A., BANKS D. & NORONHA F . (1999) : E u r o p e a n Cur ren t R e s e a r c h on F l u i d I n c l u s i o n s - X V E C R O F I . Terra Nostra. Abst rac t s : pp. 135-136. GUEDES A., NORONHA, F. , BOIRON M . C . , BANKS, D.A. (2002) : C h e m i c a l G e o l o g y , 190 , 2 7 3 - 2 7 9 NEIVA, J. M . C. (1955) : M e m . e N o t . C o i m b r a , 30, 21 -32 . NORONHA, F. , CATHELINEAU M . , BOIRON M . С . , BANKS, D. A. , DORIA A. , RIBEIRO M . A . , NOGUEIRA P. , GUEDES A. ( 2 0 0 0 ) : J . G e o c h e m . Exp l . 71 , 2 0 9 - 2 2 4 . PRIEM, H. N. A. (1962) : Thes i s Un ive r s i ty of A m s t e r d a m , 160 pp. CANALS, A. , CARDELLACH, E . , RYE, D. M . & AYORA, C . (1992) . E c o n o m i c G e o l o g y 87 : pp . 142-153 . 150 

Fluids associated with Pb-Ag mineralisations of facuca deposit (Northern Portugal)

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Fluids associated with Pb-Ag mineralisations of facuca deposit (Northern Portugal)

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