1999Apatite fission track dating of fluorite ore veins from Santa Catarina state

Apatite ?ssion track dating of ˉuorite ore veins from Santa Catarina state,Brazil:a complex hydrothermal evolution

A.R.Jelinek a,*,A.C.Bastos Neto a ,M.L.V.Lelarge b ,E.Soliani Jr a

a

Centro de Estudos em Petrologia e Geoqu??m ica,Universidade Federal do Rio Grande do Sul,Brazil

b

Instituto de Geocie ?ncias,Universidade Federal do Rio Grande do Sul,Brazil

Abstract

The ˉuorite of Santa Catarina that occurs in veins cutting Precambrian granitic rocks of coastal Brazil has been di cult to date by Rb/Sr,K/Ar and Sm/Nd methods.New ?ssion track dating of apatite in granites next to the veins yields ages of 144±76Ma,which are related to the opening of the South Atlantic Ocean.Four groups of ?ssion track ages were identi?ed:the ca 145Ma group is a hydrothermal event that preceded ˉuorite mineralization;the second group of ages,131±107Ma,records the ?rst hydrothermal mineralizing event;the third group,98±93Ma,represents the second hydrothermal mineralizing event;and the fourth group,89±76Ma,dates the last hydrothermal mineralizing event.As shown by previous studies,the temperatures of these events varied from 1708to 708C,but the last hydrothermal event occurred during a gradual cooling.The smaller lengths of the con?ned ?ssion tracks from the fourth event support this interpretation.These results are based on sixteen carefully selected samples from four veins ranging from 1to 4m in thickness.The ages of these samples were established using the standard methods of ?ssion track dating.Our study clearly demonstrates the value of apatite ?ssion track dating for deposits whose mineralization occurred over a long time span at a wide range of temperatures.#1999Elsevier Science Ltd.All rights reserved.

1.Introduction

Most of the present ˉuorite production of Brazil originates from the Santa Catarina Fluorite District (SCFD)in southern Brazil (Fig.1).Total ˉuorite reserves in the SCFD are estimated at about 5.5Mt of ore of a grade of 40%CaF 2.

These low temperature ˉuorite hydrothermal vein deposits are hosted by ``Brasiliano''granites (about 0.6Ga).These veins also cut the diabase of the Serra Geral Formation and thus are clearly less than 133Ma (Renne et al.,1992).These veins have been related to the South Atlantic opening but with two di erent interpretations;one interpretation genetically links them with alkaline magmatism (Willig,1973;Horbach and Marimon,1980;Morgental,1984)and the other

more recent idea links them to the leaching of ˉuorine from granites by hydrothermal activity connected to geothermal anomalies and regional uplift (Savi,1980;Dardenne and Savi,1984;Dardenne and Touray,1988).

It is now recognized that ˉuorite deposition is re-lated to three successive hydrothermal events of which the last one,with distinct physical±chemical con-ditions,could be very late.Wall-rock alteration data (Bastos Neto et al.,1997)attests to the complex hy-drothermal evolution of ˉuorite in the SCFD,but also demonstrates the existence of hydrothermal events prior to ˉuorite deposition as well as post-ˉuorite de-position from supergene processes that a ected the hydrothermally alternated wall-rock of the veins.

Unfortunately,neither model provides su cient geo-logical constraints to estimate the age of the ˉuorite veins.For example,ages of ca 133Ma by K/Ar method (Sonoki and Garda,1988)and ages of 138±90Ma by apatite ?ssion track (Baitelli,1992)were

Journal of South American Earth Sciences 12(1999)367±377

0895-9811/99/$-see front matter #1999Elsevier Science Ltd.All rights reserved.PII:S 0895-9811(99)00028-

*Corresponding author.Tel.:+55-051-316-6369;fax:+55-051-319-1811.

E-mail address:jelinek@if.ufrgs.br (A.R.Jelinek).

obtained for the Anita

polis magmatism,but the mag-matism at Lages has been dated by Scheibe (1986)at ca 70Ma using K/Ar.These conˉicting ages may be the result of the anomalous evolution of the passive continental margin (Asmus,1984)between

Floriano

polis and Rio de Janeiro lineaments (Fig.1),which has known tectonic reactivations and is still somewhat seismically active.Some local hydrothermal activity also occurs even today.Thus,any age between 133Ma and a very late age could be assigned to the hydrothermal ore veins.In addition,the complex evol-ution of the ˉuorite deposits explains the di culties in using K/Ar (Teixeira,1969;Santos and Bonhomme,1993)and Rb/Sr methods (Tassinari and Flores,

1992)

Fig.1.Main physiographical elements of the southeastern Brazil continental margin,modi?ed after Asmus (1984)and Melo et al.(1985)and the

location of the Santa Catarina Fluorite District (SCFD).1:Coastal deposits;2:Tertiary basins;3:Serra Geral Formation;4:Parana

Basin sedi-mentary rocks;5:Ribeira fold belt;6:alkaline massives;7:Isopachs (km);8:lineaments;A:Ponta Grossa dome alkaline province;B:Serra do

Mar alkaline province;C:Lages alkaline complex;D:Anita

polis alkaline massive;Pg:Ponta Grossa dome;T:Torres syncline.A.R.Jelinek et al./Journal of South American Earth Sciences 12(1999)367±377

368

on hydrothermally altered wall rock in an attempt to date the ore veins;the ages obtained by these methods range from between 450and 30Ma.Earlier appli-cation of the Sm/Nd method on ˉuorite gave values of 150245Ma (Tassinari and Flores,1992),values too imprecise to be helpful.Because of this wide impreci-sion,we have used the apatite ?ssion track method as an alternative,which is capable of dating successive thermal events,of ``excluding''thermal events with temperatures incompatible with the ˉuorite deposition conditions between 1708and 1008C (both over and below),and of dating a later hydrothermal event that could possibly be relatively recent.To our knowledge,this is the ?rst time ?ssion track dating has

been

Fig.2.Geological map of the Segunda Linha Torrens vein system.1:alluvial deposits;2:Serra Geral Formation;3:Rio Bonito Formation;4:Rio do Sul Formation;5:sub-volcanic acidic rocks;6:granitic basement;7:ˉuorite vein;8:nonmineralized fault;9:diabase dyke;10:quartz dyke;11:working mine;12:inactive mine.

A.R.Jelinek et al./Journal of South American Earth Sciences 12(1999)367±377369

employed to date a hydrothermal ore vein deposit with a complex history.

2.Geology of the Santa Catarina ˉuorite district 2.1.Pedras Grandes granitic suite

Most ˉuorite ore veins are hosted by the Pedras Grandes granitic suite which is composed predomi-nantly of monzogranites.These monzogranites have a medium to coarse grained inequigranular texture.Potassic feldspar forms the dominant phase of the rock,followed by plagioclase,quartz and biotite.Titanate,epidote,opaque minerals,apatite and zircon form its magmatic accessory phases.The volumetri-cally large secondary assemblage consists of epidote,sericite,chlorite,opaque minerals,ˉuorite and carbon-ate.

Prior to ˉuorite deposition,the Pedras Grandes granite was a ected by earlier hydrothermal activities recognized in the district and related to the cooling of the Southern Santa Catarina granitic batholith.These hydrothermal activities are responsible for potassic (biotite+microcline),white mica,and propylitic altera-tions that occur throughout the district.During these processes,a signi?cant amount of ˉuorine was removed from the silicates and redeposited as acces-sory secondary ˉuorite in the granites.Close to the ore veins the propylitic alteration was more intense;for example,a chlorite±quartz±titanate±ˉuorite paragen-esis has entirely replaced primary and secondary bio-tite,formed during potassic alteration.The propylitized rocks have been partially silici?ed and are cross-cut by two generations of quartz-rich micro?s-sures.

2.2.Fluorite ore veins:tectonic control and geochemistry

The ˉuorite deposits in the district occur in veins that are controlled by well de?ned lineaments,chieˉy the Canela Grande which is about 150km long and 2km wide and extends from the southern part of the

district to the north of the Anita

polis alkaline intru-sion (Fig.1).The Canela Grande lineament is chieˉy de?ned by many closely spaced N±S to NNE±SSW faults (Fig.2).In the Segunda Linha Torrens veins system (Fig.2),the ˉuorite veins are located along a main structure,the Segunda Linha Torrens fault,and

in secondary structures as the Cocal,Aqua

rio and Fumac a ore veins.

Four tectonic phases were recognizable (Table 1)as responsible for the opening of the vein structures,with alternating compressional and tensional regimes.During the tectonic phases 1,2and 3,six ˉuorite gen-

T a b l e 1S y n t h e t i c p r e s e n t a t i o n o f m i n i n g d a t a w i t h a b r i e f d e s c r i p t i o n o f h y d r o t h e r m a l a s s o c i a t i o n s i n a l t e r e d w a l l -r o c k s o f ˉu o r i t e v e i n f r o m S a n t a C a t a r i n a F l u o r i t e D i s t r i c t (S C F D )a

O r e

T e c t o n i c p h a s e s

G e n e r a t i o n s P a r a g e n e s i s

T e x t u r e s

L a /Y b L a /S m W a l l -r o c k a l t e r a t i o n b

C o m p r e s s i o n N E ±S W (1)I C h a l c e d o n y ,g r e e n a n d v i o l e t ˉu o r i t e ,s i d e r i t e M a s s i v e

4.61

2.83S i l i c i ?c a t i o n C /S

I I

G r e e n a n d v i o l e t ˉu o r i t e ,c h a l c e d o n y ,s i d e r i t e

0.921.19

E x t e n s i o n N W ±S E (2)

I I I G r e e n a n d v i o l e t ˉu o r i t e ,c h a l c e d o n y B a n d e d ,``c o c a r d e ''a n d c o l l a p s e b r e c c i a m a r k e d v e r t i c a l z o n a t i o n 0.580.80I /S R 3I ±L C M s e r i e

I V

Y e l l o w ˉu o r i t e ,c h a l c e d o n y

0.36

0.47

C o m p r e s s i o n N E ±S W (3)V G r e e n ˉu o r i t e M a s s i v e

1.331.87I /S R 1I ±H C M s e r i e

V I

Y e l l o w ˉu o r i t e

0.36

0.84

E x t e n s i o n N W ±S E (4)

V I I W h i t e ˉu o r i t e ,c h a l c e d o n y ,b a r i t e B a n d e d ,``c o c a r d e ''a n d c o l l a p s e b r e c c i a m a r k e d v e r t i c a l z o n a t i o n 0.721.19I /S R 0I ±L C M s e r i e

V I I I Y e l l o w ˉu o r i t e ,c h a l c e d o n y ,b a r y t e ,p y r i t e ,I /S 0.470.92I X

W h i t e ˉu o r i t e ,c h a l c e d o n y ,p y r i t e ,b a r i t e ,s m e c t i t e 0.42

0.59

a

F o u r t e c t o n i c p h a s e s w e r e r e c o g n i z e d ,r e s p o n s i b l e f o r t h e o p e n i n g o f t h e v e i n s t r u c t u r e s ,a l t e r n a t i n g c o m p r e s s i o n a n d t e n s i o n r e g i m e s .T h e d e p o s i t i o n o f s i x ˉu o r i t e g e n e r a t i o n s I a n d I I ;I I I a n d I V ;V a n d V I h a p p e n e d d u r i n g t e c t o n i c p h a s e s 1,2a n d 3,r e s p e c t i v e l y .D u r i n g t e c t o n i c p h a s e 4,g e n e r a t i o n s V I I ,V I I I a n d I X w e r e d e p o s i t e d .b I /S :i n t e r l a y e r e d i l l i t e /s m e c t i t e ;C /S :i n t e r l a y e r e d c h l o r i t e /s m e c t i t e ;I ±L C M :i l l i t e ±l o w c h a r g e m o n t m o r i l l o n i t e ;I ±H C M :i l l i t e ±h i g h c h a r g e m o n t m o r i l l o n i t e ;R (R e i c h w e i t e ):o r d e r f a c t o r .

A.R.Jelinek et al./Journal of South American Earth Sciences 12(1999)367±377

370

erations were deposited(I and II;III and IV;V and VI;respectively).During tectonic phase4,ˉuorite gen-erations VII,VIII,and IX were deposited(Table1). These nine generations ofˉuorite have been studied for their rare earth elements(REE)andˉuid inclusions (Dardenne and Savi,1984;Bastos Neto et al.,1991). Three successive hydrothermal cycles have been ident-i?ed,each with di erent REE patterns(La/Sm and La/Yb).These three hydrothermal cycles formedˉuor-ite vein generations I±IV,V±VI,and VII±IX,respect-ively.Fluorite veins associated with the third hydrothermal cycle(generations VII±IX)have positive Eu anomalies and signi?cant dispersal of their REE patterns.Almost constant ratios of heavy REE(for example,Yb/Tb near5)of the di erent nine gener-ations ofˉuorite suggest a common source of theˉu-orine for all generations.The homogenization temperatures of the primary aqueous phase inˉuorite range from1708to1008C.Each hydrothermal cycle has a progressively lower homogenization temperature. The homogenization temperature of the primaryˉuor-iteˉuid inclusions is assigned to a relatively shallower deposition in respect to older generations(Bastos Neto et al.,1991).

The systematic determination of87Sr/86Sr has accu-rately identi?ed theˉuorine of the ore veins as having been derived from accessoryˉuorite in the enclosing granites(Sallet et al.,1996;Rocha,1997).Thisˉuorine is believed to have been leached by meteoric water involved in convective hydrothermal calls based on the D/H isotopic composition of water from theˉuorite ˉuid inclusions in the ore veins(Bastos Neto et al., 1996).

2.3.Wall-rock alteration

Wall-rock alterations were promoted by the hydro-thermal mineralizing events.The host rock has been pervasively alternated by the F-bearing hydrothermal solutions.This alteration is as wide as8m and gradu-ally increases in intensity towards the ore veins.A greenish,?ne-grained mass of illite/smectite,chlorite/ smectite,smectite,quartz,andˉuorite has replaced most primary minerals and propylitic minerals such as epidote,chlorite,and secondary titanate.Primary F-apatite and much of the zircon were not a ected by these low temperature alterations,even very close to the ore veins.

Four successiveˉuorite-rich micro?ssures and gener-ations of breccia associated with these low temperature hydrothermal alterations have been distinguished. Their chronological relations and?llings may be dis-tinguished at intermediate distances from the ore vein; however,close to the ore vein these phases are di cult to distinguish.The paragenesis of these micro?ssures and breccias are:(1)quartz+ˉuorite+chlorite/smectite;(2)illite-low charge montmorillonite+ˉuorite+pyrite;(3)illite-high charge montmorillo-nite+ˉuorite+pyrite;(4)R0montmorillonite+ˉuorite+barite+quartz+pyrite.Microchemistry and X-di ractometry of secondary paragenesis(associated with microfracturing or pervasive alterations)indicate thatˉuorite deposition was related to three hydrother-mal episodes(Bastos Neto et al.,1997).

In summary,four hydrothermal alteration processes occurred in the district prior toˉuorite ore vein depo-sition.They were(1)potassic,(2)white mica,(3)pro-pylitic alterations,all of which occur throughout the district and are related to the cooling of the granitic batholith,and(4)silici?cation recognized only in the wall-rock of the ore veins.The low-temperatureˉuor-ite ore veins were deposited by three successive hydro-thermal events inferred from REE andˉuid inclusion studies.The hydrothermal alterations of the wall-rock hosting of theˉuorite veins agree with this complex hydrothermal evolution.

3.Apatite Fission Tracks Analysis(AFTA)

3.1.Sampling

Several geological factors were considered in select-ing the area within theˉuorite district to be sampled and the veins within it.These factors include mini-mizing the e ects of possible di erential uplift within the district(which might complicate AFTA age in-terpretation),making certain that the relative ages of each vein sampled are well established,and making certain that the sampled veins represent all four tec-tonic phases.We minimized the impact of di erential uplift within the base by sampling only one tectonic block.

With these factors in mind,we sampled the Segunda Linha Torrens vein system.Within this system the Segunda Linha Torrens,Cocal,Fumac a and Aqua rio veins were selected(Fig.2).All of these veins are re-lated to the Segunda Linha Torrens fault,which is a large fault older thanˉuorite mineralization(Bastos Neto,1990).The earliestˉuorite deposition(gener-ation I,see Table1)took place within this fault on a ?rst phase of reactivation;laterˉuorite deposition also took place during subsequent phases of reactivation. Samples from the Segunda Linha Torrens vein all come from Mina2and from its``southern lens''where micro?ssures and associated alteration on the wall-rock were previously studied by Bastos Neto et al. (1997).The Cocal vein,which is related to a dextral strike slip fault,was sampled because it is believed that this fault was formed and opened during tectonic phase1(Bastos Neto et al.,1992)and only represents generation II.The Fumac a and the Aqua rioˉuorite

A.R.Jelinek et al./Journal of South American Earth Sciences12(1999)367±377371

T a b l e 2A p a t i t e ?s s i o n t r a c k a g e s a n d c o n ?n e d l e n g t h d a t a f r o m P e d r a s G r a n d e s g r a n i t e o f t h e S e g u n d a L i n h a T o r r e n s v e i n s y s t e m a

S a m p l e (V e i n /L o c a t i o n )

A l t i t u d e /d e p t h (m )n r s

(N S )(?105c m à2)

r i

(N i )(?105c m à2)

r d

(N d )(?104c m à2)z 962W /963T 2s (M a )P (w 2)N t M e a n t r a c k l e n g t h 21s (m m )S D (m m )

G r o u p 1:P r e -m i n e r a l i z a t i o n h y d r o t h e r m a l e v e n t S u r f a c e 23040.525(21)0.475(19)36.697(22711)38428W

146.6239.9F a i l 58.3221.753.92S L T D1à230.7

15

0.426(64)

0.520(78)36.697(22711)38428W 143.829.9

F a i l

25

8.7320.52

2.59

G r o u p 2:F i r s t h y d r o t h e r m a l m i n e r a l i z i n g e v e n t S L T D3à18040.300(12)0.125(5)0.9365(1765)113582264130.726.8P a s s 278.9120.733.82S L T D2à180160.562(90)0.700(112)0.9365(1765)113582264121.328.0F a i l 208.3620.753.88C o c a l D22550.400(20)0.240(12)36.697(22711)38428W

120.4244.0P a s s 148.3920.662.46C o c a l D4à175410.714(293)0.512(210)0.9365(1765)113582264116.423.9F a i l 2610.6820.794.05F u m a c a 45.380.425(34)0.725(58)36.697(22711)38428W

109.9213.9F a i l 178.6620.793.27A q u a r i o D3à226.4220.313(69)2.368(521)36.697(22711)38428W

107.925.3F a i l 438.0320.422.74A q u a

r i o D1157.3

35

1.160

(409)

1.448

(496)0.9365(1765)113582264

107.023.9

F a i l

8

10.2821.02

2.87

G r o u p 3:S e c o n d h y d r o t h e r m a l m i n e r a l i z i n g e v e n t

C o c a l D1à175310.451(140)0.677(210)0.9365(1765)11358226498.323.9F a i l 578.7820.503.79C o c a l D1?

à175300.630(189)0.690(207)36.697(22711)354213W

92.924.6F a i l ±±±C o c a l D325290.565(164)1.296(376)0.9365(1765)11358226497.224.0F a i l 2510.6120.753.77A q u a

r i o D4à31.5

24

0.833

(200)

1.296

(253)0.9365(1765)113582264

96.324.5

F a i l

17

10.7021.04

4.28

G r o u p 4:T h i r d h y d r o t h e r m a l m i n e r a l i z i n g e v e n t

A q u a r i o D212110.318(35)0.600(66)36.697(22711)38428W

89.028.2F a i l 146.2420.802.99S L T D4à180230.630(145)1.156(266)0.9365(1765)11358226488.024.0F a i l 139.8820.843.03C o c a l D5à108.5300.486(146)0.580(174)0.9365(1765)11358226485.823.5F a i l 139.9521.023.68C o c a l D6

à7539

0.638(249)

0.456(178)0.9365

(1765)11358226476.527.9

P a s s

23

7.1420.53

2.55

a

n =n u m b e r o f c r y s t a l s c o u n t e d ;r s =s p o n t a n e o u s t r a c k d e n s i t y o f a s a m p l e ;N S n u m b e r o f t r a c k s c o u n t e d t o d e t e r m i n e d r s ;r i i n d u c e d t r a c k d e n s i t y o f a s a m p l e m e a s u r e d i n a K a p t o n e x t e r -n a l d e t e c t o r ;N i =n u m b e r o f t r a c k s c o u n t e d t o d e t e r m i n e d r i ;r d =i n d u c e d t r a c k d e n s i t y o f a g l a s s d o s i m e t e r S R M 962o r 963m e a s u r e d i n a K a p t o n e x t e r n a l d e t e c t o r ;N d =n u m b e r o f t r a c k s c o u n t e d t o d e t e r m i n e d r d ;T =a p a t i t e ?s s i o n t r a c k a g e w i t h a 1s t a n d a r d e r r o r ;P (w 2)=p r o b a b i l i t y o f o b t a i n i n g t h e o b s e r v e d w 2v a l u e f o r n d e g r e e s o f f r e e d o m w h e r e n =n u m b e r o f c r y s t a l s à1;z 962W /963=c a l i b r a t i o n f a c t o r u s e d f o r a p a t i t e ?s s i o n t r a c k a g e c a l i b r a t i o n f o r N B S d o s i m e t e r g l a s s 962a n d 963d e t e r m i n e d b y m u l t i p l e a n a l y s i s o f D u r a n g o s t a n d a r d .W o r k e r s :A .J e l i n e k ;?M .L .L e l a r g e .T h e 2p /4p g e o m e t r y c o r r e c t i o n f a c t o r i s 0.51a n d 0.50f o r t h e d i e r e n t o b s e r v a t o r s r e s p e c t i v e l y ;N t =n u m b e r o f c o n ?n e d t r a c k s m e a s u r e d ;S D =s t a n d a r d d e v i a t i o n o f t h e d i s t r i b u t i o n ;s u r f a c e c o u n t e d :10.000m m 2.S L T =S e g u n d a L i n h a T o r r e n s .

A.R.Jelinek et al./Journal of South American Earth Sciences 12(1999)367±377

372

veins are secondary tensional structures of the Segunda Linha Torrens vein system,related to the tec-tonic phase1.Onlyˉuorite generations II,VII,VIII and IX are present at the Fumac a ore vein.At the Aqua rio vein,several generations are present but gen-eration II predominates.Most samples were collected from underground mines but?ve samples came from the drill core.None of these samples are more than 2m from theˉuorite ore vein.In addition,a granite control sample was collected more than1km from the ore veins.All samples were carried out on the Pedras Grandes granitic suite.

3.2.Analytical procedure

Sample analysis was done at the Fission Track Analysis Laboratory of the IG-UFRGS.The granite samples were crushed and ground to a grain size of 80±125mm and the apatites were separated with the aid of heavy liquids and a Frantz magnetic separator. The apatites were mounted in epoxy resin,polished to expose internal surfaces,etched,and covered with Kapton1?ssion track detector(external detector method,cf.Gleadow,1981).The etching conditions were80s in1M HNO3at208C for apatite and8min in boiling in NaOCl(14%)+NaCl(12%)for Kapton1.The observations were made in transmitted light using a100?immersion oil objective at a total magni?cation of1250?.Track counting and track length measurements were performed with an Olympus biological microscope,a drawing tube and a digitizing table linked to a microcomputer.All samples were irradiated in a24B facility of the Instituto de Pesquisas Energe ticas e Nucleares(IPEN-CNEN)reac-tor at Sa o Paulo(Brazil).The spatialˉux variation of this reactor was studied for?ssion track dating by Bigazzi et al.(1988)and Osorio et al.(1993)and no substantial neutronˉux variation was detected.The ˉux gradient was monitored using SRM-963and SRM-962glass dosimeters interspersed among samples.All?ssion track ages were calculated using the zeta calibration approach(Hurford and Green, 1983).The Durango apatite age standard was used to determine zeta calibration factors for the two dosi-meter glasses,and zeta values are given in Table2. The w2test was applied to evaluate the spread in the single grain ages(Galbraith,1981).The?ssion track ages and errors of samples that passed the w2test were calculated using the conventional technique of Green (1981),and ages and error estimates for those analyses that failed the w2test were calculated using the mean ratios.Lengths of con?ned?ssion tracks were measured following the recommendations of Laslett et al.(1982).Track lengths were measured using con-?ned fossil?ssion track.Only horizontal tracks were measured in grains with polished surfaces parallel to a prismatic surface.The analytical results were sum-marized in Table 2.All apatites analyzed in this study were F-rich,as shown by microprobe analysis.

4.Results and discussion

Our results allow us to distinguish an older patient AFT age group(Table2,pre-mineralization hydro-thermal event).It is de?ned by ages of147240Ma and144210Ma and includes the control sample and one sample from Segunda Linha Torrens ore vein.Their standard deviations correspond to an ana-lytical indetermination uncertainty related to scarcity of the few number of grains and tracks.The mean track length of these samples is about8±9m m,in-dicative of a thermal event,with a component of small con?ned track length in spite of the reduced number of track counts(Fig.3a).It is important to note that the older age(144Ma)was only obtained on the Segunda Linha Torrens ore vein,which is con-trolled by an older structure.Hydrothermal alteration temperature of Segunda Linha Torrens wall-rock, including post-propylitic alteration(the propylitic alteration temperature is around2508C,i.e.,higher than the retention temperature of apatite?ssion tracks)and prior toˉuorite deposition,would prob-ably correspond to this age.This event is correlated to the pre-rifting thermal dome on the continental margin(Asmus,1984).

A Rb/Sr(whole rock)age of18527Ma was obtained by Tassinari and Flores(1992)on alternated granitic wall-rocks from the Fumac a ore vein and attributed to the hydrothermal mineralizing event. Based on both the dating of the apatites and measure-ments of con?ned track length,this age might correlate with the beginning of the thermal event associated with the pre-rift stage.The?ssion track ages registered a thermal event at145Ma due to the lower retention temperature ofˉuorine-apatite?ssion tracks,expected to be completely annealed at temperatures of H1108C (Gallagher et al.,1994)for time-scales of1±100Ma. In spite of this temperature being compatible with ˉuorite deposition,the ore vein formation cannot be correlated with the end of this thermal event because, as mentioned by previous authors(Savi,1980; Dardenne and Savi,1984;Morgental,1984),ˉuorite ore veins cross-cut the Serra Geral Formation NW±SE diabase dykes.Consequently,ˉuorite ages are equal or younger than133Ma.

The other ages are distributed in a great time inter-val(131±76Ma).This high span observed on?ssion track ages obtained on apatite hosted in granites, assigned to hydrothermal events,apparently also a ected the Anita polis alkaline rocks,according to data reported by Baitelli(1992).

A.R.Jelinek et al./Journal of South American Earth Sciences12(1999)367±377373

The ore vein geochemistry and wall-rock alteration data demonstrate that the ˉuorite was deposited by three successive hydrothermal events with own characteristics.Thus,ages in the range 131±76Ma should be related to these three hydrothermal epi-sodes,which are associated with the opening of the South Atlantic Ocean.The distribution of the veins studied here,their apparent ages,and the age of events in the evolution of the continental margin lead us to interpret the apparent ages in the interval 131±76Ma as forming groups 2,3and 4(Table 2),dis-cussed below.

The second group of apparent ages falls in the range 13127to 10724Ma (Table 2,?rst hydro-thermal mineralizing event)and the mean con?ned ?ssion track length range from 8to 11m m.This age range corresponds approximately to the rift stage (130±115Ma)of the South Atlantic opening (Asmus,1984).In comparison with the two younger hydro-thermal mineralizing events (Table 2)this event shows a larger range of ages.The geochemistry of ˉuorites and wall-rock alteration data (Table 1)agree that ˉuorites deposited during tectonic phases 1and 2display a single geochemical evolution,so the for-mer hydrothermal mineralizing event could have been relatively longer.The apatite ?ssion track analysis and the con?ned ?ssion track length measurements allow us to con?rm the time interval of this event.The distribution of the apatite apparent ages for single crystals and the con?ned track lengths of the

sample Aqua

rio 3illustrate this case (see Fig.3b).Thermal anomalies linked to the rifting process are responsible for hydrothermal circulations.The mag-matism at the Floriano

polis dome Dlying on the E±W zone separating stretched continental crust (at the north)and new oceanic crust (at the south)Dmay explain the prolongation of hydrothermalism until after the rift stage end (115Ma).It is also noted that individual ˉuorite vein ages agree with the tectonic model and ˉuorite generation distribution:the older ages correspond to an older NNE±SSW structure (STL),the only one where generation I is abundant;the next ages were obtained from the Cocal ore vein (samples Cocal 2and 4)where the ?rst ˉuorite depo-sition corresponds to generation II.

The third group includes samples with ages of 9824to 9325Ma (Table 2,second ˉuorite hydro-thermal mineralizing event),which are the same within one standard deviation.The distributions of apatite con?ned ?ssion track lengths,ranging from 8to 11m m,are bimodal (Fig.3c);these distributions are typical of more than one thermal event.These in-termediate mineralization ages have not been obtained by previous https://www.360docs.net/doc/206652713.html,ing the complex model described by Bastos Neto (1990),these ages would correspond to the third ore vein

opening-?lling

Fig.3.Summary of ?ssion track data observed in SCFD (Santa Catarina Fluorite District)in four samples.The distribution of apparent ?ssion track ages are broader and illustrate the mixed ages.For samples of ?gures a and b the mean track length distribution represents various degrees of annealing.Samples of ?gures c and d present a bimodal distribution typical of a partial annealing that erases the older tracks of samples.

A.R.Jelinek et al./Journal of South American Earth Sciences 12(1999)367±377

374

phase,and correspond to the second hydrothermal mineralizing event(see Table1).Bastos Neto et al. (1991)correlated this tectonic opening of vein struc-tures(tectonic phase3)to a disturbance of tensional stress linked to reactivations of ENE±WSW structures at the SCFD area in response to detachment(``de col-lements''formation)during the stretching of the Sa o Paulo``plateau''.However,the present?ssion track age analysis rejects this interpretation because it is not possible to consider this tectonic reactivation after100Ma,when South America and Africa plates were completely separated(Nu rnburg and Mu ller, 1991).A group of apparent?ssion track ages obtained by Lelarge(1993)in the Ponta Grossa Dome(see Fig.1)falls in the range between113and 100Ma and can be interpreted as the formation of a ``Proto-Serra do Mar''in this area.After Bastos Neto(1990),the``Proto-Serra do Mar''was formed by lateral conduction(Alvarez et al.,1984)just in front of the zone a ected by the crustal stretching re-sponsible for the formation of the Sa o Paulo``pla-teau''.Thus,this``Proto-Serra do Mar''would have to be limited to regions north of the Floriano polis lineament.Subsequently,the coastal area from the southern SCFD to the Ponta Grossa Dome was a ected by the Serra do Mar uplift.The third AFTA group can be correlated with the beginning of the process responsible for di erential vertical movements of adjacent blocks and opening of fractures,leading to signi?cant isothermal adjustments which were able to start convective hydrothermal activity and,conse-quently,ˉuorite ore vein formation.

The last group of ages range from8928to7625 Ma(Table2,third hydrothermal mineralizing event). This event can be correlated with a very important reactivation on the continental margin responsible for the paroxysm of the subsidence on the Santos Basin and the synchronical uplift on the coastal area (Pereira,1992).After the ages obtained by Lelarge (1993),the uplift of Serra do Mar was already indeed implanted at86Ma.In SCFD,aligned with the Santos basin depocenter,ENE±WSW structures that controlled the gradual uplift to the north were reacti-vated.Thus,our AFTA data agree with geochemical data and wall-rock alteration in the sense thatˉuorite generations VII,VIII and IX were deposited at shal-lower levels than the older generations.

Two apatite?ssion track ages determined by Baitelli(1992)for Lages Alkaline Massive of84and 78Ma can be associated with this reactivation of the southern continental margin.The apatite appar-ent?ssion track ages of this group are in agreement with the86Ma(K/Ar)age obtained by Santos (1994)on clays associated with theˉuorite mineraliz-ations of SCFD;and also with the88.4Ma K/Ar whole rock age in a diabase dike of the Serra Geral Formation cut byˉuorite mineralization in the Segunda Linha Torrens veins,Santa Catarina Mine (Teixeira,1969).

The mean con?ned?ssion track lengths of the samples of the last group fall in the range6to10m m, and the distributions of apparent single crystals ages are bimodal(Fig.3d).The application of the method of apatite?ssion track analysis(AFTA)to these samples illustrate a thermal event erasing the?ssion tracks.In this case,it seems probable that the gra-dual cooling of the last hydrothermal event,whose ?nal stage is characterized by barite deposition, occurred at lower temperatures(708C)with respect to the annealing?ssion tracks.The duration of this deposition was su ciently long to promote the partial annealing of?ssion tracks related to previous events.

5.Conclusions

Sixteen apparent ages obtained by the?ssion track technique on apatites associated withˉuorite veins in the Santa Catarina Fluorite District are arranged into four groups,which can be related to hydrothermal events,and are supported by geochemical data.The age groups present values between146and76Ma. The?rst group,with ages around145Ma,represents a hydrothermal event older than theˉuorite mineral-ization.The other ages are distributed in a great time interval(131±76Ma).As with AFTA data from the literature on alkaline regional rocks(con?rmed by K/ Ar data),we have found a high span in?ssion track ages on apatites hosted in https://www.360docs.net/doc/206652713.html,ing geochem-ical and structural data on the ore veins,correlated with the continental margin evolution,we classify these ages into three groups(groups2±4).The second group of ages,131±107Ma,records the?rst hydro-thermal mineralizing event.The third group of ages, 98±93Ma,represents the second hydrothermal miner-alizing event.The fourth group of ages,89±76Ma, dates the last hydrothermal mineralizing event.

The results display a distribution pattern of age groups that are clearly connected to evolution of the continental margin,and are capable of promoting the convective hydrothermal cycles.To our knowl-edge,the application of AFTD toˉuorite ore veins from the Santa Catarine Fluorite District is the?rst time?ssion track dating has been employed to date a hydrothermal ore vein deposit with a complex his-tory.

Acknowledgements

The authors acknowledge the Conselho Nacional de

A.R.Jelinek et al./Journal of South American Earth Sciences12(1999)367±377375

Desenvolvimento Cient??co e Technolo gico for?nan-cial support,wish to thank Drs.Paul Potter and Vitor Pereira for reading the manuscript and making valu-able suggestions,and Dr.Luiz Paulo Geraldo for the sample irradiations.Bastos Neto and Soliani Jr. acknowledge the Ministe rio da Cie ncia e Technologia for?nancial support through projeto PRONEX/IG/ UFGRS.

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