Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003

FLUIDS AND CEMENTS IN THE LOWER DEVONIAN LA VID GROUP, CANTABRIAN MOUNTAINS (NW-SPAIN)

S C H N E I D E R . J.1. BAKKER, R. J.2, B E C H S T Ä D T , T.1

1 Geologisch-Paläontologisches Institut, University Heidelberg, INF 234, 69120 Heidelberg, Germany. 2 Institut für Geowissenschaften, Mineralogie & Petrologie, Montanuniversität , 8700 Leoben, Austria. E-mail: jo.schneider@urz.uni-heidelberg.de

Cements in the different carbonate units f rom the Lower Devonian La Vid Group record various fluid events , which penetrated the rocks during the burial history of that succession. After sedimentation this succession was buried by about 2 7 0 0 m of sediments until the orogenic movements of Variscan and later Alpine age set in where these deposi ts were faulted and tilted. Several fluids were active during these processes and precipitated different kinds of cements . Fluid inclusions in these cements provide substantial information of the chemical and physical condit ions of these processes. The succession is composed at the base by transitional sandstones, which merge into dolostone, and different types of l imestones. T h e top of the carbonates is constituted by cinoidal grainstones interlayering with shales, which build up the upper part of the La Vid Group .

Cement Sequence Petrographic studies reveal several cement generations, which are put in a relative temporal diagenetic sequence, based on

geometrical relations. During basin stage a first calcite cement (Cal 1) crystallised at the inner rim of open cavit ies, fossil f ragments and vein walls. This was fol lowed by precipitation of minor amounts of barite (Bar), restricted to veins in only a small limestone unit of the succession. Subsequently, ferroan carbonate cements exert a major impact on the carbonates : siderite, ferroan saddle dolomite (iS-Dol) and ankerite precipitated in the remaining open spaces in veins and fossil f ragments , and replaced mimetically aragonite and high-Mg shells. The second saddle dolomite (S-Dol) without a s ignif icant Fe component has a cloudy appearance and occurs only in irregular veins and on fault planes of the transitional sandstones and dolostones. After a brecciation event, a clearer type of saddle dolomite (cS-Dol) and minor amounts of quartz (Qtz) precipitated in veins and open cavities of dolostones. However , there are no obvious genetic relations to subsequent cement generations. A late tectonic event is indicated by the crosscutting of fractures filled with f ibrous calcites (Cal 2 and Cal 3), which is associated with layer-parallel shortening stylolites and brecciated f ragments of former ly mineralised cements . Significantly later a blocky calcite occurs together with celestite (Cel) and kaolinite in reactivated thrust faults and their vicinity. Goethite and hematite occur as oxidation/weathering products of the iron-rich carbonatic cements , e.g. iS-Dol.

Fluid Inclusions and Organic Matter Fluid inclusions have been characterised by microthermometry, Raman spectroscopy and UV-Fluorescence in Cal 1, Bar,

iS-Dol, cS-Dol, Qtz and Cal 3 as well as in Cel. Inclusions in the other cements were either absent or too small for optical analysis.

Stage 1 : Burial diagenesis

Cal 1 contains trails of secondary petroleum inclusions, which have a blue f luorescence. Homogenisa t ion temperatures of the petroleum and CH4-rich phases in these inclusions vary f rom 32 to 63 °C (disappearance of vapour bubble) . These trails can not be traced in the overgrowing iS-Dol. The barite contains many primary petroleum inclusions, which also have a blue f luorescence with an average wavelength of 452 nm. Homogenisat ion behaviour is similar to Cal 1 and occurs around 45 °C. Locally, primary petroleum inclusions are also found in iS-Dol, which have also a blue f luorescence (average wavelength of 441 nm). Primary aqueous inclusion def ining growth zones are also found in iS-Dol. Total homogenisa t ion of these inclusions occurs around 114 °C in the liquid phase.

The iS-dol in dolostones may also contain solid inclusions of organic matter. Mic ro -Raman spectra of these inclusions are characteristic for first and second order graphite peaks, indicating a thermal maturity be low 1.4% RQ (Spötl et al., 1998). The surrounding host rock contains grey bituminite in cavities of microfossils and irregular shaped cavities, which can be categorised after Jacob (1989) as epi-impsonite. Bituminite reflection values can be t ransformed after Baker and Pawlewicz (1994) to temperatures of 154°C. The preserved f luorescence wavelength of pet roleum in iS-Dol corresponds with this temperature and can be correlated with an API gravity of 43° after Allan & Wiggins (1993) classifying the petroleum as condensate. The thermal overprint for the petroleum inclusions in barite is less, as indicated by a longer wavelength. This oil has a lower maturity (c.f. Hagemann & Hollerbach 1986).

172

Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003

Stage 2 : Syn-Variscan

Qtz and cS-Dol h a v e s imi la r p r i m a r y f luid inc lus ions that o c c u r in g r o w t h z o n e s as i l lustrated by c a t h o d o l u m i n e s c e n c e . T w o types of a q u e o u s inc lus ions a re present :

(1) l o w salinity. T h e inc lus ions h a v e a s ize up to 2 0 / im with m a x i m a l l y 10 vo l .% CH4 vapour bubb le . H o m o g e n i s a t i o n t empera tu re s va ry b e t w e e n 7 3 to 2 0 0 °C (in the liquid phase) . T h e re la t ive low dens i ty of the v a p o u r b u b b l e is indicated by m e l t i n g of a c la th ra te phase b e f o r e ice.

(2) h igh sal ini ty. T h e f o r m a t i o n of salt hydra tes du r ing f r e e z i n g e x p e r i m e n t s indica tes the p r e s e n c e of N a C l and M g C l 2

in the e n t r a p p e d a q u e o u s so lu t ion .

Stage 3: Post-Variscan

T h e Ce l con ta ins a h igh a m o u n t of p r i m a r y and secondary f lu id inc lus ions , wh ich m a i n l y h a v e a low sal in i ty a q u e o u s charac te r , wi th a va ry ing filling d e g r e e ( f r o m a l l -vapour to al l - l iquid) . H o m o g e n i s a t i o n t empera tu re s of l iquid-r ich inc lus ions va ry b e t w e e n 197 and 2 1 1 °C. Mel t ing t e m p e r a t u r e s of ice are abou t -0 .1 °C. F u r t h e r m o r e , Cel m a y con ta in s o m e p e t r o l e u m inc lus ions wi th a C H 4 v a p o u r bubb le , and sol id inc lus ions , like anhydr i t e and o rgan ic mat ter . M i c r o - R a m a n inves t iga t ions of t he o rgan ic mat te r ind ica te g raphi t i sa t ion at l o w tempera tu res , be low 2% Ro.

T h e s e h igh h o m o g e n i s a t i o n t empera tu re s in Ce l , wh ich o c c u r s cogene t i c with Ca l 3, are in cont ras t to t he a l l - l iquid a q u e o u s inc lus ions in this c e m e n t ind ica t ing t e m p e r a t u r e s b e l o w 5 0 ° C (Golds te in & R e y n o l d s , 1994). T h e kaol in i te mine ra l i s a t ion c o r r e s p o n d to these l o w t e m p e r a t u r e s cond i t i ons

Conclusions T h r e e m a j o r f luid e v e n t s had a m a j o r impac t o n the L o w e r D e v o n i a n L a Vid G r o u p . A f t e r an initial ca lc i te p rec ip i t a t ion ,

Fe -en r i ched fluids coex i s t en t with p e t r o l e u m f lu ids w e r e assoc ia ted with an ear ly do lomi t i sa t ion . T h e F e - s o u r c e o r ig ina ted p r o b a b l y in the unde r ly ing S a n P e d r o F m . (Si lur ian) . Dur ing the V a r i s c a n o r o g e n y ( inc lud ing brecc ia t ion) , d o l o m i t i s a t i o n con t inued wi thout the F e - c o m p o n e n t in the fluid sys tem, and c lear sadd le d o l o m i t e and quar tz w e r e prec ip i ta ted ou t of bo th h ighly sa l ine and l o w sa l ine f luids . In a pos t -Var i scan s tage, ce les t i te and ca lc i te prec ip i ta ted ou t of a low sal in i ty a q u e o u s f lu id at low t e m p e r a t u r e - p r e s s u r e cond i t ions .

References ALLAN, J. R. , WIGGINS, W . D . (1993) : D o l o m i t e reservoi rs ; geochemica l t e chn iques fo r eva lua t ing or ig in and d is t r ibu t ion . -

A A P G C o n t i n u i n g E d u c a t i o n Cour se N o t e Ser ies , 36 , 129 pp. BARKER, M . , PAWLEWICZ, C . (1994) : An empi r i ca l de te rmina t ion of the m i n i m u m n u m b e r of m e a s u r e m e n t s to e s t i m a t e the

m e a n r a n d o m vi tr ini te r e f l e c t a n c e of d i s s e m i n a t e d o rgan ic mat ter . - A A P G and S E P M Annua l M e e t i n g Abs t r ac t s , 1994, S .231 .

GOLDSTEIN, R. H , REYNOLDS, T . J. (1994) : Sys t ema t i c s of f luid inc lus ions in d iagene t i c minera ls . - S E P M Shor t C o u r s e , 31 , 198 pp .

HAGEMANN H . W. , HOLLERBACH, A. (1985) : T h e fluorescence b e h a v i o u r of c r u d e oi ls with respec t to their thermal ma tu ra t i on and deg rada t ion . - Org . G e o c h e m . , 1 0 (1-3) , S .473 -480 .

JACOB, H . (1989) : C las s i f i ca t ion , s t ruc ture , genes i s and pract ical i m p o r t a n c e of natural sol id oil b i t u m e n ( " m i g r a b i t u m e n " ) . -Int. J. Coa l Geol . , 11, 6 5 - 7 9 .

SPÖTL, C . , HOUSEKNECHT, D. W. , JAQUES, R. C. (1998) : K e r o g e n ma tu ra t i on and inc ip ient g raphi t iza t ion of h y d r o c a r b o n sou rce rocks in the A r k o m a Bas in , O k l a h o m a and Arkansas ; a c o m b i n e d pe t rog raph ic and R a m a n spec t rome t r i c s tudy. -O r g a n i c G e o c h e m i s t r y , 2 8 (9-10) , 5 3 5 - 5 4 2 .

173