Tin-Bearing Sodic Episyenites Associated with the Proterozoic, A-Type Agua Boa Granite, Pitinga...

Gondwana Research, V 5, No. 2, pp. 435-451. 0 2002 International Association for Gondwana Research, Japan. ISSN: 1342-937X G R G ; Z

Tin-Bearing Sodic Episyenites Associated with the Proterozoic, A-Type Agua Boa Granite, Pitinga Mine, Amazonian Craton, Brazil

H.T. Costil) 2*, R. Dall'Agnoll, R.M.K. Borges', O.R.R. Minuzzi3 and J.T. Teixeira4

' Grupo de Pesquisa Petrologia de Rochas Granitijides, Centro de Geocitncias, Universidade Federal do Para' - C.P. 1611 - 66075-900 - Bele'm - P A - Brazil Laboratijrio de Microscopia Eletrdnica de VarreduralMuseu Paraense Emilio Goeldi, Av. Perimetral, 1901,66077-530 - Bele'm - PA, Brazil, E-mail: [email protected] Minerapio Taboca S.A. - Vila Pitinga, 69737-000, Presidente Figueiredo - A M - Brazil Grupo Paranapanema S.A. - Praia do Botafogo, 228,15" andar, Bloco A - Rio de Janeiro, RJ - Brazil

* Corresponding author

(Manuscript received April 26,2001; accepted October 18,2001)

Abstract

This paper reports the first occurrence of tin-mineralized episyenites in the Amazonian craton. The Agua Boa and Madeira plutons in the Pitinga region are stanniferous Proterozoic A-type granites of the rapakivi series featuring metasomatic episyenitization. The biotite granite facies of the Agua Boa pluton is metasomatically altered to sodic episyenite and minor potassic episyenite and micaceous episyenite. The sodic episyenites formed by: (a) albitization of K-feldspar, (b) vug formation by dissolution of magmatic quartz, (c) vug filling by albite, chlorite, lithian muscovite, cassiterite & fluorite & K-feldspar and (d) deposition of late quartz+ cassiterite in remaining cavities. In the potassic episyenites, vugs produced by quartz dissolution are filled by secondary K-feldspar, which also replaces magmatic phases. Micaceous episyenites formed by zinnwaldite replacement of secondary albite in the sodic episyenites. In contrast to the sodic episyenites, the potassic and micaceous episyenites contain only traces of cassiterite.

Relative to unaltered biotite granite, the sodic episyenites are strongly depleted in SiO,, KzO, LREE and Sr, moderately depleted in HREE, and enriched in Na,O, Al,O,, Rb and Sn. The potassic episyenites resemble the sodic episyenite in SiO, and Al,O, but exhibit higher values of K,O, Ba, Y and Rb; their Na,O, REE, and Sn concentrations match those of the biotite granite host.

Metasomatic episyenitization is attributed to: (1) a high temperature gradient and fluid-rock ratio during subsolidus cooling of the Agua Boa pluton, (2) quartz dissolution in the biotite granite facies by reaction with an high-temperature, silica-undersaturated fluid of probable magmatic origin, (3) hydrothermal cavity filling by a low-temperature, silica- saturated fluid of meteoric, or mixed magmatic/meteoric character.

Significant tin values in metasomatic sodic episyenites near the Pitinga mine suggest that exploration of similar rocks in other A-type granites may be worthwhile.

Key words: Amazonian craton, Pitinga, tin granites, episyenite, alkali metasomatism.

Introduction

Episyenites are defined in European literature as products of subsolidus dequartzification and modification of magmatic feldspar (Recio et al., 1997). These peculiar metasomatic rocks (Leroy, 1984; Cathelineau, 1986; Charoy and Pollard, 1989; Petersson and Eliasson, 1997) are commonly associated with uranium (Leroy, 1984; Maruejol, 1989) or tin-tungsten mineralization (Cheilletz

and Giuliani, 1982; Charoy and Pollard, 1989) but unmineralized episyenites are also known (Petersson and Eliasson, 1997; Recio et al., 1997).

Almost all of the major tin deposits discovered in the Amazonian craton during the last thirty years are associated with Paleoproterozoic or Mesoproterozoic granites emplaced in extensional settings. These granites are generally described as anorogenic, or post-tectonic, and display within-plate, A-type geochemical signatures

436 H.'T. COST1 ET AL. ~

(Horbe et al., 1991; Dall'Agnol et al., 1993, 1994; Bettencourt et al., 1995) and many attributes of Proterozoic rapakivi granites (Ram6 and Haapala, 1995; Bettencourt et al., 1995; Dall'Agnol et al., 1999).

In the Amazonian Craton, tin mineralization is associated with the most evolved leucogranites of the rapakivi series. These F-rich granites and country rocks are enriched in Sn and related elements by late- and post- magmatic fluids (Horbe et al., 1991; Bettencourt et al., 1987, 1995; Dall'Agnol et al., 1993, 1994) that produce cassiterite-bearing greisens and quartz veins. At the Pitinga mine, in the north-central craton (Fig. l), two distinct types of tin mineralization are recognized: (1) Sn associated with the Madeira pluton (Costi et al., 2000) and (2) fault-confined Sn greisens in the Agua Boa pluton (Daoud and Antonietto, 1985; Borges et al., 1996; Costi et al., 1997).

Mineralized episyenites have rarely been described in A-type granites. Haapala (1995) discussed the mineralization associated with the rapaltivi granite series. The studied tin-mineralized episyenites can represent a distinct mineralization style within the rapakivi association. This paper describes Sn-mineralized episyenitic rocks in the Agua Boa batholith of the Pitinga region with emphasis on petrographic and geochemical changes of the metasomatic transition from biotite granite

to episyenite. We will relate these processes to the broader topic of tin mineralization in the Amazonian Craton.

Geological Setting

The Pitinga region is situated ca. 300 km north of Manaus, in the state of Amazonas (Fig. 1). Igneous rocks dominate this region of the southern Guyana shield, where the oldest rocks are the calc-alkaline Agua Branca granitoids, with 207Pb/20GPb-zir~on evaporation age of 1960+21 to 1938f37 Ma (see Costi et al., 2000 for references). Rhyolites, rhyodacites, and pyroclastic rocks of the Iricoumk Group cover large portions of this region. In the Pitinga area (Fig. l), these volcanic rocks have a 2"7Pb/206Pb-zir~on evaporation age of 1888-t 3 Ma (Costi et al., 2000). The volcanic sequence in Pitinga area is intruded by the Agua Boa and Madeira plutons. Lenharo (1998) obtained a U-Pb age of 1798+ 10 Ma for the early facies of the Agua Boa pluton; Costi et al. (2000) obtained a 2"7Pb/206Pb-zir~on evaporation age of 1824-t 2 Ma for the early facies of the Madeira pluton.

The Madeira stock (Fig. 1) exhibits four facies (Costi, 2000; Costi et al., 2000): (1) porphyritic amphibole-biotite syenogranite with rapakivi texture, (2) uniformly medium- grained biotite leucogranite, (3) porphyritic hypersolvus alkali feldspar granite and (4) albite granite. Two subfacies

- ~ ~~~ ~~~

Albite granite + porphyritic hypersolvus granite

Porphyritic topaz granite

Biotite alkali feldspar granite

Porphyritic biotite granite

Amphibole biotite syenogranite

+ t t * . + . * * . * +

+ + + .

Study area (Fig. 2)

+ * * * . * + * * *

+ * + * . * * + *

* . . . * * + +

Fig. 1. Geological map of the Pitinga region showing the location of the studied area.

Gonduiana Rasearch, V 5, No. 2,2002

TIN-BEARING SODIC EPISYENITES, AMAZON CFMTON, BRAZIL 437

are recognized in the albite granite. The core subfacies of the stock is gray, medium-grained granite consisting of quartz, albite and K-feldspar and cryolite (up to 6 modal%), polylithionite and other dark iron-rich micas, riebecltitekaegirine, zircon, cassiterite, pyrochlore group, f thoritefopaques; catapleiite, ghentelvite, thomsenolite, sphalerite, and galena occur in trace amounts. The border subfacies is red, quartz-rich granite lacking cryolite, iron- rich micas, riebeckitekaegirine, and pyrochlore minerals; accessories include fluorite, chlorite, haematite, and columbite. The core is peralkaline, the border is moderately to strongly peraluminous and both subfacies are strongly enriched in Sn, Zr, Rb, Nb, Ta, Y, F, Th, Li, U and Th relative to average granite (Costi et al., 2000).

The Agua Boa pluton is also composed of four facies (Fig. 1). The early border facies is composed of amphibole- biotite syenogranite with even-grained, porphyritic and pyterlitic textural variations with localized zones of rapaltivi texture. The border facies is intruded by porphyritic biotite granite along the eastern and southern margin (Fig. 1) of the pluton (Daoud and Antonietto, 1985). This porphyritic granite may be a marginal variant of even-grained biotite alkali feldspar granite facies that comprises most of the intrusion (Lenharo et al., 1997). Topaz granite, restricted to the central area of the pluton, is the latest and most evolved facies of the Agua Boa pluton (Lenharo et al., 1997).

Field Aspects of Episyenites and Associated Rocks

Episyenite occurs in the western sector of the Agua Boa pluton (Figs. 1,2) beneath 30-40 m of saprolitic cover, where it was intercepted by drilling in biotite alkali feldspar granite (Costi et al., 1997). Geologic relationships of this episyenite association compare with those reported by Cheilletz and Giuliani (1982) except that the Agua Boa pluton and episyenite lack foliation and ductile deformation. The episyenite (Fig. 3) occurs in a SW-dipping (60°), fault-controlled, lenticular zone enclosed by granite that is cut by numerous Sn-barren greisen lenses and dykes of granophyre and diabase. It is likely that the episyenite zone extends up-dip through DH-7, where saprolite contains altered dark micas and high Sn values; the zone may also extend down-dip below the bottom of DH-3 (Fig. 3 ) . Lenticular geometry for the episyenite zone is inferred from the fact that DH-9, DH-10, DH-12, and DH-13 did not encounter episyenite (Fig. 2). DH-9 did intersect several high Sn intervals indicative of episyenite proximity but DH-4 and DH-11 did encounter Sn-rich rocks. In spite of high tin grade, the small volume of the episyenite, indicated by figures 2 and 3, diminishes the economic potential of the deposit at present.

Granophyric granite

I I I - I I I I I I I I I

I (LDH-~ I

I I I I I I

I I I I tDH-3 tDH-12

_I - - - - _ I _ - - - - 1 - - - L4700 L4600 L4500

. 5 0 m .

Fig. 2. Sketch map of the Queixada area, showing the location of drilling lines, drill holes and simplified lithological relationships.

Macroscopic Description of Drill Hole-4 Core

DH-4, which provided the samples for this study, is represented in figure 4. Fresh biotite granite is encountered below saprolite in the depth interval 40.0-66.4 m. This reddish or sometimes yellowish rock exhibits an unfoliated fabric with coarse- to medium-grained seriate texture and is cut by numerous thin (530 cm) veins of medium- to fine-grained greisenized granite. At a depth of 66.4 m, the biotite granite is cut by unfoliated, medium- to fine- grained reddish granophyre; this contact is sharp and marked by a 5 cm thick zone of leucocratic microgranite. Greisenization occurs along fractures, throughout the granophyric interval (Fig. 4), to a depth of 74.5 m where a transition from medium-grained, mottled red granophyre to episyenite begins. This zone records a progressive decrease in grain size, color intensification to brick red, and constant quartz content.

Unfoliated episyenite encountered in DH-4 (Fig. 4) exhibits a chemically zoned depth sequence (Fig. 6) comprising of (1) 1.6 m of silica-rich (quartz-bearing) sodic episyenite, (2) 9.4 m of true sodic episyenite (quartz rare or absent), (3) 1.2 m of dark green altered micaceous episyenite and (4) 0.3 m of red potassic episyenite. Below the episyenite zone, DH-4 encountered medium-grained reddish biotite granite similar to that of the roof of the

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438 H.T. COST1 ET AL.

sw NE DH-3

DH-4 DH-11 A

Micaceous altered episyenite Granophyric granite L Topographic surface

Episyenite 0 Biotite granite \ Drill hole

Diabase @ Greisenized rocks \ Fractures

Fig. 3. Cross sections along the drilling lines depicted on figure 2. Vertical scale = horizontal scale.

main hydrothermal zone; shearing and greisening are prominent at depths of 90 and 98 m (Fig. 4).

(1) mesoperthites exhibit internal ‘chessboard’ replacement, (2) ‘mobilized’ albite is more abundant, ( 3 ) dark mica is altered to chlorite and white mica and (4) secondary topaz

Petrography

Coarse- to medium-grained biotite granite

The coarse- to medium-grained alkali feldspar biotite granite above the main hydrothermal zone contains abundant quartz and ‘disturbed’ mesoperthite, sparse oligoclase, and dark mica arranged in seriate-granular texture. The potassic host of the mesoperthite forms euhedra and subhedra up to 10 mm that enclose abundant rods, strings, veins and patches of albite. Late ‘mobilized’ albite commonly forms ‘swapped rims’ (Smith, 1974; Dall’Agnol et al., 1993) along perthite grain boundaries. Quartz forms 2-5 mm solitary euhedral or subhedral grains and larger glomerocrysts. The rare subhedral oligoclase grains are altered to sericitek fluorite. Dark mica (zinnwaldite?) forms clusters of brown intensely pleochroic grains enclosing fluorite and zircon. Secondary white mica (probably lithian muscovite, according the nomenclature of Rieder et al., 1998) and fluorite are important minor phases and are always associated with the dark brown mica clusters. Near the contact with the medium- to fine-grained granite, the coarse-to medium granite exhibits the following hydrothermal effects:

may appear.

Medium- to fine-grained red granophyre

In this granite, seriate K-feldspar ranges from medium- grained euhedra to fine-grained anhedra. Quartz occurs as medium-grained embayed subhedra, fine-grained anhedra, and granophyric intergrowths. Plagioclase forms subhedral grains replaced by fine-grained sericite, muscovite, fluorite and topaz. Primary micas are replaced by lithian muscovite, fluorite, and iron oxideskchlorite. Zircon and opaque phases are associated with the altered micas. At the episyenite contact, K-feldspar is partially replaced by chessboard albite and ‘mobilized’ albite is coarser and more abundant than elsewhere, and quartz is stable. Plagioclase and mica are cut and partially replaced by veinlets of Fe-oxides and fluorite.

Medium-grained reddish granite

The granite below the main hydrothermal zone is similar to coarse- to medium-grained granite described above but is finer grained and more intensely altered. Primary micas are almost completely replaced by lithian muscovite+fluorit& iron oxide& chlorite and feldspars are partially greisenized.

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TIN-BEARING SODIC EPISYENITES, AMAZON CRATON, BRAZIL 439

(0

;c= c ci,

.-

granophyrlc

greisenized biotite granite

x x granite

u biotite granite

f thin section geochemical

3 sample

j fractum

-

70 x x x x x x x : x x x x x x x : x x x x x x x : x x x x x x x : x x x x x x x : x x x x x x x : x x x x x x x : x x x x x x x : x x x x x x x :

Fig. 4. DH-4 drill hole graphic logs showing the granite types, the episyenitic zone, and sample locations. The depths are measured along the core axis.

Episyenites and related rocks

Albite, the dominant mineral in the sodic episyenites, occurs as ‘chessboard’ replacements of the K-feldspar perthite host and as clear secondary, twinned euhedra associated with chlorite and muscovite. Both forms of albite are medium-grained, but the former is comparatively larger. Textural evidence indicates that secondary phases fill cavities generated by quartz dissolution. Partially oxidized chlorite is the main dark mineral in the episyenites and is associated with yellowish lithian muscovite whose abundance increases with chlorite oxidization. Minor phases include reddish brown cassiterite anhedra, fluorite, subhedral zircon, opaques, and cloudy K-feldspar that occur in irregular aggregates of largely secondary origin. A later generation of hydrothermal quartz fills cavities and replaces chlorite, muscovite, and even cassiterite. Near the top of the episyenite body, the silica content increases due to the presence of hydrothermal quartz and relict magmatic quartz. The latter appear as corroded solitary grains and clusters enclosed by secondary muscovite.

The micaceous altered episyenite near the footwall of the main hydrothermal zone is a metasomatic derivative of sodic episyenite. Chessboard albite and outlines of secondary albite crystals replaced by brownish-green mica and chlorite can be discerned. In view of the calculated Li content of this mica (Table 2) and the high Li content of this rock (Table 3), this mica is probably zinnwaldite. Fine-grained fluorite is abundant, and sphalerite, zircon, and opaques are common throughout this zone; quartz and cassiterite are absent. Cloudy K-feldspar (adularia?) is present near the contact with the potassic episyenite.

Potassic episyenite is the product of hydrothermal addition of K-feldspar, probably as adularia. The latter takes the form of fine-grained anhedral grains and medium- to coarse-grained replacements of perthite- hosted ‘chessboard’ albite. Additional phases include cloudy albite, quartz, muscovite, chlorite, and dark mica (zinnwaldite?); both of the latter are partially oxidized. Cassiterite is rare.

Mineral Chemistry

Phyllosilicates and feldspars of episyenites and related rocks were analyzed with the Cameca SX-50 electron microprobe at the Instituto de Geocihcias, Universidade de Brasilia, Brazil, using natural and synthetic standards with accelerating voltage = 15 kv beam current = 25 nA, and counting time = 10 s.

Feldspars

Table 1 shows the chemical compositions and structural formulae of the alkali feldspars. Albite and K-feldspar of

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Table 1. Average electron microprobe analyses of feldspars from the Agua Boa episyenites.

Albite

Sample F4QX/83,83 F4QX/79,31 F4QX/76,32 F4QX/82,52 Rock KEPS NaEpS NaEpS MNaEpS

n = 9 n =20 n = 25 n = 12

SiO, 68.53 68.62 68.69 68.44 A1203 19.54 19.54 19.55 19.66 Fe 0 0.05 0.07 0.06 0.01 MnO 0.00 0.01 0.02 0.00 BaO 0.06 0.04 0.00 0.00 CaO 0.03 0.04 0.00 0.03 Na,O 11.82 11.79 11.99 11.99 YO 0.11 0.11 0.22 0.14

Total 100.15 100.21 100.53 100.26 Rb,O n.a. n.a. 0.00 0.00

Structural formulae based on 32 oxygen atoms

Si 11.967 11.971 11.959 11.943 A1 4.022 4.017 4.012 4.043 Fe 0.007 0.014 0.009 0.001 Mn 0.000 0.001 0.003 0.000 Na 4.002 3.988 4.048 4.055 Ca 0.006 0.006 0.000 0.005 K 0.023 0.024 0.050 0.031 Rb 0.000 0.000 0.000 0.000 Ba 0.005 0.003 0.000 0.000

Ab 99.3 99.3 98.8 99.1 An 0.1 0.2 0.0 0.1 Or 0.6 0.6 1.2 0.8

End-member feldspar, mole%

Alkali feldspar

F4QX/83,83 F4QX/79,31 F4QX/76,32 KEPS NaEpS NaEpS

n = 22 n = 4 n = 17

64.26 18.14 0.10 0.01 0.04 0.00 0.24

16.54 ma.

99.32

11.989 3.990 0.016 0.001 0.088 0.000 3.937 0.000 0.002

2.2 0.0

97.8

64.32 18.10 0.24 0.02 0.09 0.00 0.23

16.41 n.a.

99.40

11.990 3.978 0.038 0.003 0.083 0.000 3.903 0.000 0.005

2.1 0.0

97.9

64.65 18.27 0.12 0.01 0.05 0.01 0.29

16.37 0.03

99.79

11.992 3.994 0.018 0.001 0.106 0.001 3.874 0.003 0.004

2.7 0.0

97.3

KEpS = potassic episyenite, NaEpS = sodic episyenite, MNaEpS = micaceous altered episyenite, n.a. = not analyzed, n = number of analyses.

all textural types in the episyenite suite are Ca-poor, compositionally uniform, and approximate Ab,, and Or,7-Or98 respectively. The main textural feature of the K-feldspar is their cloudy appearance and absence of twinning and perthitic intergrowths that, coupled with their composition, suggest that they formed as hydrothermal adularia.

Micas

Table 2 shows the average chemical compositions and structural formulae of the analyzed micas. The Li contents were calculated using the regression equations of Tindle and Webb (1990) for the trioctahedral micas and Tischendorf et al. (1997) for the dioctahedral micas.

The compositions of trioctahedral and dioctahedral micas in the episyenite suite are shown in the Li-R+2-R+3 diagram of Foster (1960) and Li-M'2-Al of Monier and Robert (1986) in figures 5a and 5b. Trioctahedral micas (field 1) plot along the polylithionite-siderophyllite join and dioctahedral micas (field 2) are distributed along the muscovite-zinnwaldite join. A detailed account of the crystal chemistry of these micas will be presented elsewhere (Borges et al., in preparation).

Chlorites

Aver age c h e m ic a1 c o m p o s i t i o ns and c a1 c u 1 ate d structural formulae of chlorites in the episyenite suite are presented in table 2. Due to their Fe-rich character, all the analyzed grains are daphnite according to the classification of Hey (1954).

Whole-Rock Geochemistry

The chemical analyses listed in table 3 were performed by Geosol Lab., in Brazil. Major and most trace elements were analyzed by XRE The exceptions are: Na,O, Li, Cu, Zn and Pb, determined by AA method; Fez+ was analyzed by titration; F was measured by specific ion electrode; REE concentrations were obtained by ICP

The various facies of the Agua Boa pluton (Horbe et al., 1991; Lenharo et al., 1997) display traits of metaluminous and peraluminous A-type granite (Collins et al., 1982; Whalen et al., 1987; Dall'Agnol et al., 1994; King et al., 1997). Biotite granite and granophyric granite (Table 3) exhibit high silica (-76 wt. %), (WNa > l), low Al,O,, TiO, and CaO, and extremely low MgO and

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Table 2. Average electron microprobe analyses of phyllosilicates from the Agua Boa episyenites.

Trioctahedral micas Dioctahedral micas Chloritcs

Sample F4QW83.83 F4QW82.52 F4Qw54.72 F4Qw83.83 F4QW76.32 F4QW79.31 F4QW83.83 F4QW76.32 F4QW79.31 Rock KEpS MNaEpS BtGran KEPS NaEpS NaEpS KEPS NaEpS NaEpS

n=12 n=12 n = l l n=10 n=12 n = 4 n=9 n=10 n = 7 SiO, TiO, *1,03

MgO CaO MnO FcO* ZnO Na,O

Rb,O

F Li,O** Li,O+ Minus F=O Total

YO cs,o

Si N A l "'A1 Ti

Mn FefZ Zn Li Sum [Y] Ca c s Na K Rb Sum [XI F

Mg

41.22 0.54

20.84 0.20 0.01 0.67

20.14 0.07 0.12 9.95 0.64 0.03 4.06 2.28

1.71 99.05

6.081 1.919 1.705 0.060 0.043 0.084 2.486 0.007 1.351 5.736 0.001 0.002 0.035 1.874 0.061 1.972 1.895

39.79 0.37

21.16 0.17 0.00 0.70

21.42 0.06 0.10 9.71 0.83 0.07 4.10 1.87

1.72 98.62

5.968 2.032 1.708 0.042 0.039 0.088 2.687 0.006 1.126 5.696 0.000 0.004 0.029 1.859 0.080 1.972 1.939

38.27 48.16 46.60 1.15 0.16 0.27

20.68 26.76 26.31 0.27 0.23 0.24 0.02 0.01 0.01 0.55 0.35 0.30

22.40 8.21 9.26 0.22 0.04 0.05 0.17 0.11 0.09 9.65 10.59 10.71 1.16 0.70 0.68 0.07 0.05 0.05 4.18 2.43 2.34 1.43

1.29 1.22 1.76 1.02 0.99

98.45 98.06 97.14 Cations normalized to 22 oxygens

5.838 6.571 6.483 2.162 1.425 1.517 1.556 2.882 2.797 0.132 0.016 0.028 0.062 0.047 0.050 0.072 0.041 0.035 2.861 0.940 1.080 0.024 0.004 0.005 0.875 0.707 0.682 5.583 4.638 4.678 0.003 0.001 0.001 0.004 0.004 0.003 0.046 0.030 0.024 1.879 1.845 1.900 0.114 0.061 0.061 2.049 1.941 1.989 2.016 1.049 1.031

46.93 0.22

26.32 0.27 0.02 0.35 9.96 0.02 0.09

10.70 0.69 0.04 2.21

1.13 0.93

98.02

6.484 1.516 2.773 0.023 0.056 0.042 1.151 0.002 0.627 4.674 0.003 0.002 0.024 1.886 0.061 1.976 0.965

22.02 0.05

20.01 0.10 0.03 1.39

43.19 0.06 n.a.

0.27 n.a. n.a.

0.22

22.11 0.03

20.35 0.14 0.02 1.32

43.93 0.08 n.a.

0.05 n.a. n.a.

0.14

21.26 0.04

19.62 0.10 0.02 1.19

44.56 n.a. n.a.

0.04 n.a. n.a. n.a.

0.09 0.06 87.25 88.12 86.83

Cations normalized to 28 oxygens 5.245 5.218 5.144 2.756 2.782 2.856 2.853 2.879 2.736 0.007 0.006 0.009 0.036 0.050 0.037 0.278 0.263 0.244 8.625 8.668 9.014 0.011 0.013

11.811 11.881 12.040 0.008 0.006 0.003

0.082 0.015 0.010

0.1 69 0.103

*Total Fe as FeO, **Li,O = (0.287 x SiO,) - 9.552 (Tindle and Webb, 1990), +Li,O = 0.3935 x F1.326 (Tischendorf et al., 1997), KEpS = potassic episyenite, NaEpS = sodic episyenite, MNaEpS = micaceous altered episyenite, BtGran = biotite granite, n.a. = not analysed, n = number of analysis.

P,O, values. They are enriched in F, Sn and Rb and depleted in Ba and Sr, a signature of tin-specialized granites in the Amazonian craton. High Nb, Ga, Th and Y values and low Zr and Li contents (Table 3) are characteristic of anorogenic granites of the Amazonian craton (Dall'Agnol et al., 1993). The chemical composition of the granite textural facies is uniform throughout DH-4 except for the reddish granophyre transition zone above the episyenites (Table 3, interval 73.98-74.14; Fig. 5) which is enriched in K,O and depleted in Na,O.

The episyenites and associated hydrothermal rocks show a sharp geochemical contrast to the host granites. Sodic episyenites are depleted in SiO, and K,O, enriched in Na,O, Al,O, and FeO,. CaO, MnO, TiO,, P,O, and F

contents resemble those found in the granite host. Potassic episyenites are distinguished from sodic episyenites by extreme K,O enrichment, but their Na,O content is similar to that of host granite. The micaceous altered episyenites are strongly depleted in SiO, and lower in Na,O than host granite; their K,O values are intermediate between those of the granite and potassic episyenite.

Figure 6 shows simple wt.% ratios of altered samples to the assumed parent rock (biotite granite, DH-4 interval 54.80-55.00, Table 3 ) and indicates that extreme compositional variation of SiO,, A1,0,, Na,O and KzO occurred in the hydrothermal zone. These data suggest that episyenites could have been derived from a granite parent rock by 3 distinct metasomatic processes (Fig. 7):

Gondwana Research, V; 5, No. 2,2002


Table 3. Chemical composition of Agua Boa granite and associated episyenites.

Biotite Granophyric granite Silica-rich sodic episyenite Sodic episyenite granite ______

Interval 54.80 70.41 71.41 71.85 72.05 72.22 72.41 73.41 73.98 74.50 75.49 75.66 76.12 76.63 76.79 77.41 55.00 71.85 71.85 72.22 72.20 72.41 73.41 74.50 74.14 75.49 76.12 75.79 76.63 77.41 76.91 78.01 _ _ _ ~

SiO, TiO, A1203 Fez03 FeO MnO MgO CaO Na,O KZO P A H,O+ L.O.I. F O=F Total Ba Cr c s c u Ga Li Nb Pb Rb S sc Sn Sr Th U w Y Zn Zr La Ce Nd Sm Eu Gd DY Ho Er Yb Lu CREE

75.90 0.05

11.80 0.18 1.40 0.08

<0.10 0.73 3.60 4.40

0.32 0.57 0.80 0.34

99.49 96 28 62

9 53

114 89 55

1422 50

< l o 71 21 68 1 5 26

201 84

178 37.100 97.220 32.580

6.987 0.261 4.860 6.568 1.466 4.713 5.959 0.780

198.49

<0.05

(La/Yb), 4.202 Eu../Eu* 0.130

76.80 0.07

11.60 0.63 0.70 0.01 0.38 0.65 3.30 4.80

<0.05 nd

0.64 0.54 0.23

99.89 80 nd 30 nd nd

107 57 31

916 nd nd 66 32 75 <5 29

133 69

198

nd nd nd nd nd nd nd nd nd nd nd

76.90 0.07

11.80 0.69 0.42

<0.05 <0.10

0.80 3.00 5.20

<0.05 nd

0.66 0.61 0.26

99.89 102 nd

9 nd nd 43

208 52

962 nd nd

220 39

221 15 15

162 42

343 nd nd nd nd nd nd nd nd nd nd nd

76.60 0.06

11.60 1.20 0.42

<0.05 <0.10

0.64 3.10 5.30

<0.05

76.80 0.09

11.40 1.20 0.42 0.06

<0.10 0.78 2.80 5.00

<0.05 nd 0.54

0.71 0.61 0.44 0.55 0.18 0.23

99.89 100.02

92 99 nd 19 16 11 nd 10 nd 41 43 33 49 58 30 41

991 1109 nd 60 nd <10 70 70 36 35 85 108 <5 <5 31 31

115 188 42 48

186 201

nd 62.840 nd 158.50 nd 50.470 nd 8.930 nd 0.355 nd 5.825 nd 7.054 nd 1.528 nd 4.702 nd 5.004 nd 0.663

- 305.87 - 8.476 - 0.141

76.50 0.07

11.50 1.30 0.42

<0.05 <0.10

0.64 2.80 5.20

<0.05 nd

0.82 0.67 0.29

99.64 84 nd 16 nd nd 31 52 26

892 nd nd 50 31 64 <5 38 95 41


76.40 0.05

11.80 0.99 0.56

<0.05 <0.10

0.69 3.30 5.20

<0.05 nd

0.68 0.48 0.20

99.95 86 nd 14 nd nd 41 66 30

953 nd nd 84 35 72 <5 35

145 38


76.30 0.07

11.90 0.83 0.28 0.01

<0.10 0.75 3.10 5.40

<0.05

75.70 0.08

12.00 0.48 0.98 0.06

<0.10 0.58 2.80 5.80

<0.05 nd 0.23

0.67 0.52 0.64 0.58 0.27 0.24

99.68 99.57 98 68 nd 20

9 19 nd 8 nd 41 58 138 61 80 33 77

954 1264 nd 60 nd <lo 34 65 34 45 66 58 <5 <5 23 16

126 180 42 66

189 153 nd 30.490 nd 74.960 nd 30.620 nd 7.073 nd 0.281 nd 5.748 nd 7.855 nd 1.715 nd 5.343 nd 6.273 nd 0.757

- 171.11 - 3.280 - 0.131

70.00 0.25

15.20 1.40 2.40 0.08

<0.10 0.74 7.20 0.89

<0.05 nd

1.13 0.54 0.23

99.60 22 nd 5

nd nd 49

158 30

530 nd nd

840 65

175 57 48

216 50


68.40 0.24

16.90 1.70 1.30 0.04 <0.10

0.71 8.10 0.93

<0.05 nd

0.97 0.53 0.22

99.60 15 nd 9

nd nd 42

134 33

479 nd nd

2800 81

105 39 37

111 37


66.90 0.06

17.10 1.00 0.84 0.06

10.10 1.50 9.20 0.57

<0.05 0.72 0.80 0.96 0.40

99.31 5

24 14 33 73 31 68 34

386 80

< l o 6000

100 59 <5 31 75 30

162 1.243 2.001 0.521 0.120 0.025 0.140 0.188 0.036 0.100 0.092 0.019

4.48 9.124 0.588

64.30 0.12

18.70 1.80 1.50 0.07 0.13 0.54 8.40 1.60

<0.05 nd

1.11 0.56 0.23

98.60 52 nd 47 nd nd 95

127 30

688 nd nd

10000 83 61 12 33 44 40


64.00 0.07

18.70 1.90 0.84 0.04

<0.10 0.46 9.00 1.60

<0.05

62.70 0.09

19.40 1.70 1.70 0.08

<0.10 0.46 8.80 1.90

<0.05 nd 0.58

0.87 0.89 0.45 0.39 0.19 0.16

97.74 98.53 43 68 nd 24

7 19 nd 7 nd 94 69 120 81 97 37 34

658 853 nd 90 nd <lo

17000 12200 104 91 51 39 <5 <5 34 16 37 68 37 39

206 191 nd 9.347 nd 20.640 nd 6.811 nd 1.338 nd 0.128 nd 1.568 nd 4.153 nd 1.042 nd 3.869 nd 4.795 nd 0.647

- 54.33 - 1.316 - 0.270

65.60 <0.05 19.10 1.60 0.42 0.03

<0.10 0.46 9.40 1.20

<0.05 nd

0.82 0.39 0.16

98.83 45 nd 12 nd nd 3

59 114 608

nd nd

8600 96 44 <5 26 17 52


1s (granite-+sodic episyenite), l p (granite+potassic episyenite) and 2 (sodic episyenite+micaceous altered episyenite). Process 1s evidently required SiO, and K 2 0 depletion (Figs. 7a, c) and increase of Al,O, (Fig. 7b); process l p must have involved similar SiO, loss coupled

with gains in A1,0,, Na,O, and K,O (Figs. 7a, b, c). Similarly, transformation of sodic episyenite to micaceous episyenite (process 2) requires significant SiO, and Na20 loss (Fig. 7a) and gains in K,O (Fig. 7c), F (Fig. 7d) and Fe (not shown); figure 7b suggests that during this process,

Gondwana Research, K 5, No. 2,2002


Table 3. Contd.

Sodic episyenite Micaceous altered Potassic Biotite granite episyenite epis yenite

Interval 78.01 78.09 78.38 79.41 80.41 81.06 81.58 81.80 82.31 82.41 83.08 83.52- 83.75 83.88 85.41 97.51 78.38 78.25 79.41 80.41 81.06 81.58 82.31 81.91 83.08 82.57 83.52 83.88 83.88 85.41 90.56 97.58

SiO, TiO, A403 Fe20, FeO MnO MgO CaO Na,O K,O p20, H,O+ L.O.I. F O=F Total Ba Cr CS c u Ga Li Nb Pb Rb S sc Sn Sr Th U w Y Zn Zr La Ce Nd Sm Eu Gd DY Ho Er Yb Lu CREE (LaA’nln EuJEu*

66.50 66.20 <0.05 10.05 19.40 19.80 1.40 <0.10 0.42 1.40 0.03 0.07

<0.10 <0.10 0.34 0.32 8.60 8.50 1.70 1.80

<0.05 <0.05 nd 0.74

1.02 0.90 0.33 0.28 0.14 0.12

99.60 99.89

43 65 nd 26 <5 6 nd 54 nd 75 80 30 25 18 65 221

833 860 nd 690 nd < l o

2300 3135 82 79 22 13 10 <5 18 115 42 62 78 84

263 265


- 47.04 - 1.504 - 0.224

62.80 0.09

19.50 1.80 1.80 0.07

<0.10 0.67 8.30 1.70

10.05 nd

1.29 0.50 0.21

98.31

46 nd 16 nd nd 74 67 37

876 nd nd

11000 86 54 22 32 38 72

200


62.00 0.12

19.10 1.80 2.40 0.09

co.10 0.89 8.50 1.60

<0.05 nd

1.33 0.67 0.28

98.22

29 nd 18 nd nd 35 83

131 773 nd nd

12000 85 52 <5 30 45 72


61.70 0.17

19.60 2.20 3.00 0.10

<0.10 0.83 7.90 1.80

10.05 nd

1.61 0.54 0.23

99.22

42 nd <5 nd nd 62 84

146 923 nd nd

2300 75 59 22 10 36

144 229


64.00 61.60 59.20 0.07 0.20 0.26

19.40 19.60 19.80 2.50 3.30 2.90 0.99 2.40 3.90 0.05 0.12 0.17

<0.10 <0.10 <0.10 0.93 0.73 0.82 8.70 8.70 8.30 1.60 1.30 1.60

<0.05 <0.05 10.05 nd

1.35 0.47 0.20

99.86

32 nd <5 nd nd 51 39

175 815

nd nd

156 76 45 15

<15 30

123 226


nd 1.20 1.43 1.58 0.56 0.87 0.23 0.37

99.71 100.23

39 5 nd 27 <5 11 nd 14 nd 108 61 62 85 111 92 199

658 881 nd 310 nd < l o

144 334 70 67 65 92 18 <5 17 <15 34 74

129 174 212 173


- 22.58 - 3.649 - 0.319

48.10 48.90 56.60 0.29 0.26 0.19

20.10 19.70 19.90 7.30 7.90 4.00 8.90 7.60 4.80 0.52 0.53 0.29

0.80 0.78 1.10 2.70 3.30 5.80 6.30 6.00 4.00

~ 0 . 0 5 <0.05 <0.05 nd 1.45 nd

2.20 1.99 1.62 2.30 1.70 1.40 0.97 0.71 0.59

98.67 99.55 99.11

94 95 53 nd 30 nd

260 244 130 nd 19 nd nd 120 nd

1187 1284 745 127 167 80 103 420 135

2377 2802 1645 nd 2200 nd nd 15 nd 36 588 166 47 56 59 17 17 44 <5 <5 <5 15 <15 <15 42 213 118

558 900 189 125 119 162

nd 14.610 nd nd 33.770 nd nd 14.010 nd nd 3.879 nd nd 0.172 nd nd 3.526 nd nd 5.487 nd nd 1.228 nd nd 3.965 nd nd 4.510 nd nd 0.609 nd

0.13 0.15 <0.10

- 85.77 - 2.186 - 0.140

63.10 62.20 <0.05 0.06 19.20 18.80 1.80 0.88 0.42 1.50 0.04 0.08

<0.10 <0.10 0.84 0.80 4.10 3.70 9.00 9.70

<0.05 <0.05 nd 0.47

0.84 0.96 0.70 0.87 0.29 0.37

99.75 99.65

188 210 nd 13 16 42 nd 18 nd 72

114 162 52 53 35 36

1508 1739 nd 80 nd < l o

114 285 52 50 72 74 <5 <5 31 19

187 208 59 57

204 186


- 167.20 - 2.738

76.20 0.06

12.30 0.84 0.42

<0.05 <0.10

0.60 3.60 4.90

<0.05 nd

0.63 0.46 0.19

99.82 82 nd 14 nd nd 70 57 41

873 nd nd 16 31 75 <5 32

137 51

198


76.50 75.80 0.05 0.05

12.00 12.20 1.10 <0.10 0.28 1.40 0.01 0.06

<0.10 <0.10 0.65 0.58 3.60 3.30 4.70 4.90

<0.05 <0.05 nd 0.56

0.64 0.56 0.58 0.73 0.24 0.31

99.87 99.83

64 83 nd 23 23 31 nd 7 nd 48 77 126 63 74 37 34

937 1292 nd 50 nd < l o 30 84 21 16 89 64 <5 <5 28 27

168 192 45 51

216 152


- 250.17 - 4.959 - 0.115 -

Interval - sample of inclined drill core (see Fig. 41, nd - not determined.

A1,0, remained constant and Na,O/K,O declined significantly. Micaceous altered episyenites (Fig. 7d) are the only metasomatic rocks to register F addition.

Rb/Sr ratios are generally high (> 10) in tin-specialized Proterozoic granites of the Amazonian craton (Horbe et

al., 1991; Dall’Agnol et al., 1993; Bettencourtet al., 1995). Data from the Pitinga episyenites (Table 3, Figs. 8a, b) show this ratio is markedly lower in the sodic episyenite relative to host granite due to replacement of K-feldspar by albite in the sodic episyenite; no such replacement

Gondwana Research, I? 5, No. 2,2002


Li Li

Mu

Al

M Ann

Ann Cel Sid Phe R+2 M+2 Phe Cel Sid

Fig. 5. Distribution of trioctahedral and dioctahedral micas: (5a) Li - R.'l (Fez' + Mg + Mn) - R'? ("'A1 + Ti) plot of Foster (1960) and (5b) Li - A1 (total Al) - M + 2 (Fe" + Zn + Mg + Mn) plot of Monier and Robert (1986). Ann - annite, Cel- celadonite, MU - muscovite, Phe - phengite, Pol - polylithionite, Sid - siderophyllite, Tri - trilithionite, Zin - zinnwaldite, 1 - trioctahedral micas field, 2 - dioctahedral micas field.

$5 -

70 -

75 -

80 -

85- (m)

+++++ + + + + +

x x x x x x x x x x x x x x x x x x x x x x x x x

x x x x x x x x x x

$ ~ ~ & [ ~ d i u m - s r a i n e d silica-rich sodic episyenlte potasslc episyenite + thin section

sodic episyenite continuous intervals o micaceous altered episyenlte C selected samples 0

x medlum-to fine-grained x granophyrlc granite

re;;;;; granophyric

Na204~ I. J K 2 0 4" I P

Fig. 6. Sketch of part of the drill hole DH-4, showing the petrographic and geochemical features of the main hydrothermal zone and adjacent country rocks. The contact between the granophyric granite and the biotite granite is marked by a fine-grained leucogranite (see text). The geochemical profiles show the downhole-normalized variation of SiO,, K,O, Na,O, A1,03, Sn and F, according to the results exposed on table 3 . The different oxides were normalized against the biotite granite sample (54.80-55.00), considering weight % ratios. For Sn and F the plots are presented in log values of the ratio samples/normalizing granite. The open circles and black dots in the profiles indicate the top intervals of, respectively, continuous intervals and selected analyzed samples.

Condwana Research, V. 5, No. 2,2002


- - + biotite granlte d - 0 Salk eplsyenlte

- + potasslc eplsyenlte - -

0 mlcaceous altered eplsyenlte 0 -

9 -

8 -

7 -

3 8 -

0 5 5 -

2 4 - 3 -

2

1-

C

* \

.

.. \

- \ /+ O @@,6

I 00: I I I I

9 -

8 -

7 -

3 8 -

0 5 5 -

2 4 - 3 -

2

1-

C 7

* \

- \ /+ ..

O @@,6 I 00:

I I I I

Fig. 7. Binary geochemical plots for granites and episyenites. See text for explanation of the paths lp, Is and 2.

0"

1 1 0 0 B / o I

+ 1 0' I I I I I I I 2 3 4 5 8 7 8 8 10 11

Na,O (wt x)

Fig. 8. Binary plots for granites and episyenites. See text for explanation of paths Is, l p and 2. The dashed arrow in figure 8b shows the evolution of the different facies of the Madeira pluton. Continuous arrows indicate the evolution trend of granites and associated episyenites of the Agua Boa batholith.

Gondwana Reseauch, V. 5, No. 2, 2002


low=

s! 100: 'C U C 0 c B g l o _

1

occurred in the potassic episyenite so it retains the Rb/Sr ratio of the host granite. Micaceous altered episyenites exhibit higher Rb/Sr ratios than host granite due to the increase in secondary mica that replaces albite and other phases.

Petrographic observations indicate a positive correlation between the presence of cassiterite and episyenitization. Figure 8c shows a dramatic increase in Sn and Na,O in most sodic episyenite samples relative to host rock that is not shared by the potassic and micaceous altered episyenites. The low-Sn sodic episyenite samples occur near the bottom of the hydrothermal zone (Table 3, Fig. 6) and show elevated values of Zn, Pb, S, and Nb and a low concentration of mafic minerals; lithian muscovite replaces feldspar in these samples and quartz and is associated with pyrite. Figure 8d shows that micaceous altered episyenites have high F contents relative to the other episyenite facies. The capacity of the various altered

I I I I I I I I I I - - b j I + biotite granite - - - - 0 sodic episyenite - - + potassic episyenite -

0 micaceous altered episyenite - - - - - - - - - - - - - -

= Fig. 9. Chondrite-normalized (Evensen et al., 1978) - - - - REE plots for Agua Boa granjtes and episyenites. (a) Granites of the Agua Boa batholith. (b) Episyenitic rocks of the Agua Boa

- - batholith. Also shown are the fields of the

I I I I I I I I I I I (1989, C and P) and Petersson and Eliasson

- - - - - -

episyenites studied by Charoy and Pollard

facies to retain hydrothermal F is evidently related to mica content; this explains the low F-content in the mica-poor episyenite facies and the absence of a strong Sn:F correlation.

Chondrite-normalized REE patterns for the Agua Boa granites and episyenites are presented in figure 9. The granite patterns (Fig. 9a) exhibit typical A-type features (Whalen et al., 1987; Horbe et al., 1991; Dall'Agnol et al., 1993,1994; Bettencourt et al., 1995): (1) high, nearly equal La-Ce values relative to other LREE (average Lan/Smn= 3 . 3 , (2) deep negative Eu anomalies and ( 3 ) a flat HREE pattern. Potassic episyenite and micaceous altered episyenite retain the REE traits of their parent rock (Fig. 9b) but the sodic episyenites are depleted in all REE, particularly LREE, and display a subdued negative Eu anomaly. The silica-rich episyenite is strongly depleted in total REE, but is otherwise similar to the sodic episyenites.

t .lI I I I I 1 I I I I 1 1 I

La Ce Nd Sm Eu Gd Dy Ho Er Yb Lu

___



Figure 1 0 presents REE patterns obtained by normalizing altered samples by their host rock (biotite granite sample 54.80-55.00, Table 3 ) . The resulting distribution (Fig. 10) suggests that REE were mobilized during episyenitization in the Pitinga area. Significant LREE losses occurred during the generation of the sodic episyenites; less severe losses are recorded in the potassic episyenites. ‘Positive Eu anomalies’ in sodic episyenite (Fig. 10) indicate that Eu, presumably Eu2+, was significantly less mobile than other REE during the hydrothermal process. Petrographic observations suggest that the micaceous episyenites were produced by two stages of metasomatic activity. Evidently, initial REE losses related to the destruction of their primary mineral hosts were nearly negated by REE capture during growth of secondary mica. This scenario is supported by positive correlation between F and CREE in the micaceous episyenites.

X Madeira albite granite (core facies)

0 rnicaceous altered episyenite

+ potasslc eplsyenlte

Osodic episyenite .- i l l E 8

.1

- - -

La Ce Nd Sm Eu Gd Dy Ho Er Yb Lu

Fig. 10. Parent-rock normalized REE plot for episyenitic rocks of the Agua Boa batholith and albite granite of theMadeira pluton. The contents were normalized against the Agua Boa biotite granite (sample 54.80-55.00).

Discussion

Comparison of episyenites in dijjerent geological settings

The episyenite literature (Demange, 1975; Leroy, 1984; Chauris, 1985; Cathelineau, 1986; Charoy and Pollard, 1989; Recio et al., 1997; Peterson and Eliasson, 1997) shows that the episyenites are petrographically and geochemically inhomogeneous. We compare these reported occurrences with the Pitinga episyenites utilizing the Q-F diagram of La Roche (1964) in figure 11. Ca, mainly in fluorite, is neglected on account of low

concentration (Charoy and Pollard, 1989). Granite compositions plot on, or close to, the Q-F join in figure 11, sodic episyenites occur near the Ab vertex, and potassic episyenites lie in the region of low, positive Q-F values. Micaceous altered episyenites plot in the potassic episyenite field. Most of the sodic episyenites described in this study exhibit significantly lower Q values than those reported by Charoy and Pollard (1989), Demange (1975), and Chauris (1985). However, the high Q values of the Emuford episyenites, and perhaps the other high Q sodic episyenites, result from post-episyenitization quartz veining (Charoy and Pollard, 1989). Lacking secondary quartz enrichment, it is likely that the Emuford episyenites would plot in the compositional field (Fig. 11) of Agua Boa sodic episyenites. Both rocks are Sn-rich, but higher Sn values of the Emuford rocks relate to higher Ca and F (Charoy and Pollard, 1989; Table 4) suggesting a relationship between cassiterite and fluorite precipitation at Emuford; this relationship is not observed in the Agua Boa episyenites.

The ultrasodic character of the Agua Boa sodic episyenites (Fig. 11) contrasts markedly with that of episyenites reported by Leroy (1984), Recio et al. (1997), and Peterson and Eliasson (1997) which more closely approximate potassic episyenite and micaceous altered episyenite compositions of the Agua Boa suite. Leroy (1984) noted intense muscovite replacement of feldspar.

The sodic or potassic nature of episyenite suites is generally related to parent rock composition or geological setting and extreme compositional types are not associated in the field. In this study we show that a metasomatic episode can produce a compositional spectrum between extremely sodic and potassic episyenites from the same parent rock. Cathelineau (1986) stressed the need to classify episyenites on the basis of chemical composition and we propose that episyenites (corrected for secondary quartz) be classified as sodic, moderately sodic or potassic using field boundaries shown in figure 11.

Paragenesis

The Charoy and Pollard (1989) model for episyenitization at Emuford appears to be valid for Agua Boa sodic episyenites. The Emuford trend (Charoy and Pollard, 1989; Fig. 10a) resembles the IIA trend of Cathelineau (1986) with the dequartzification and albitization stages of the paragenetic sequence reversed. This model involves: (1) initial albitization of granite K- feldspar, (2) dissolution of quartz, (3) overgrowth of vuggy albite and local crystallization of muscovite and/or K-feldspar, followed by (4) cassiterite precipitation and deposition of quartz5fluorite in cavities. Differences between Emuford and Agua Boa episyenites include

Gondwana Research, V. 5, No. 2, 2002


Granite Episyenite Greisen

Chauris (1985) * Peterson & Eliasson (1997) 0

Reoioet a1 (1997) CB 0

Charoy& Pollard (1989) 7

Leroy (1984) @ @

rnicaceous altered episyenite

moderately sodic episyenite

poiassic episyenite

F Fig. 11. Q-F diagram of La Roche (1964) adapted from Cathelineau (1986), showing the distribution of granites and episyenites of the Agua Boa

batholith compared to similar rocks of other localities. [Q = Si/3 - (K + Na), F = K - Na (elements in millications)].

precipitation of albite associated with chlorite, lithian muscovite and fluorite during the third stage. At Agua Boa, cassiterite precipitation begins in stage 3 and extends into stage 4. Quartz filling of dissolution vugs (stage 4) occurred in the upper hydrothermal zone in Agua Boa episyenites; the combination of vug quartz and relic quartz is responsible for high silica content in the silica-rich episyenites (Fig. 11). Petrographic and geochemical evidence indicate that micaceous altered episyenite derives from sodic episyenite through zinnwaldite replacement of albite and quartz which produces the muscovitization trend in figure 11.

The initial stage in the evolution of Agua Boa potassic episyenites is dequartzification (trend 1A of Cathelineau, 1986). This was followed by K-enrichment [trends lB, and I(IV), Cathelineau, 19861 related either to appearance of hydrothermal K-feldspar or muscovitization of feldspars; these processes produce the feldspathic and micaceous episyenites described by Leroy (1984). At Agua Boa, partial muscovitization of albite is observed in sodic episyenites, but not in potassic episyenites. Moderately sodic episyenites (Fig. 11) described by Cathelineau (1986), Recio et al. (1997), and Peterson and Eliasson (1997) are lacking in the Agua Boa pluton.

Comparison between magmatic and metasomatic albite-rich rocks

Rb/Sr ratios in the tin-mineralized granitic plutons in the Madeira stock (Costi, 2000; Costi et al., 2000), increase with the evolved nature of the granitic facies (Fig. 8a). Accordingly, a strong positive correlation between Rb/Sr and Na,O, Sn and F is recorded (Figs. 8c, d). However, in sodic episyenites of Agua Boa, Rb/Sr correlates negatively with Na,O and Sn (Figs. 8b, c) and there is no correlation between F and Sn (Fig. 8d) or Na,O. In addition, HREE concentrations in the Madeira granite greatly exceed that of the episyenites (Fig. 10).

The importance of primary t in mineralization associated with the Agua Boa episyenites

Haapala (1988,1995) emphasized the metallogenic importance of rapakivi granites and distinguished two major types of ore deposits: (1) greisen-, vein-, and skarn- type Sn (W-Be-Zn-Cu-Pb) deposits and (2) Fe oxide-Cu (U-Au-Ag) deposits. The Sn-polymetallic deposits of Rondbnia and Pitinga in Brazil are clearly greisen- and vein-type deposits but Sn-bearing sodic episyenites, hosted by evolved rapakivi granites, appear to be a new

Condwana Research, V. 5, No. 2,2002


mineralization class in the rapakivi association. Sn values in the Agua Boa sodic episyenites significantly exceed those of the Madeira albite granite (Fig. 8d) whose saprolite is exploited for tin; this suggests the episyenites are an attractive target for lode Sn that has been overlooked by explorationists. I t is likely that Sn-rich episyenites are more common in the Amazonian craton and other Proterozoic A-type provinces than reported in the literature.

Mechanisms and fluids involved in episyenitization

Subsolidus metasomatic events in Agua Boa granite require a significant temperature gradient and a high fluid:rock ratio (Cathelineau, 1986; Charoy and Pollard, 1989). Petrographic features of the episyenites suggest reactions involving two fluid regimes as implied by Recio et al. (1997). The initial stage of quartz dissolution (vug creation) and ensuing albitization and cassiterite precipitation requires a high fluid:rock ratio and a high temperature, silica-undersaturated fluid. Subsequently, at lower fluid:rock ratio, a cooler, silica-saturated fluid produced precipitation of quartz and other vug-filling phases. The high temperature fluid was probably a deuteric fluid related to the final stages of granite crystallization. The lower temperature fluid may have been a meteoric or partly meteoric fluid (Recio et al., 1997). It is also possible that the lower temperature fluid was simply derived from that the initial high temperature fluid by cooling and silica enrichment.

Conclusions

(1) The Agua Boa pluton is a Proterozoic A-type granite of the rapakivi series that was locally altered to episyenite at structural discontinuities by hydrothermal metasomatism. These episyenites are mostly enriched in Na,O but are locally altered to micaceous episyenite and potassic episyenite. Hydrothermal albite is the principal mineral in the sodic episyenites and occurs in two habits: (a) chessboard albite formed by replacement of alkali feldspar in the parental granite, (b) albite subhedra filling vugs produced by early dequartzification. Primary quartz is absent but hydrothermal quartz appears as a late vug- filling phase. Chlorite occurs as disseminations and in vugs with late lithian muscovite, fluorite, and cassiterite. Cassiterite content in sodic episyenite is significant. In the potassic episyenites, K-feldspar (adularia) is the major mineral phase whereas micaceous altered episyenites are characterized by abundant zinnwaldite. Cassiterite is rare in both of the latter rock types.

(2) Relative to the parent biotite granite, sodic episyenite is strongly depleted in SO,, K,O, LREE and Sr

and enriched in Na,O, Al,O,, Rb and Sn. There is no clear evidence of F enrichment in these rocks and the HREE are moderately depleted. Potassic episyenite exhibits SiO, and Al,O, values similar to the sodic episyenite but higher concentrations of K,O, Rb, Ba, and Y; REE and Na,O of these rocks approximate parent rock values. The micaceous altered episyenites are similar in Al,O, but lower in SiO, than sodic and potassic episyenites and their alkali and REE contents are intermediate between those of sodic and potassic episyenites. F, Li, Rb, Nb and Zn concentrations in the micaceous episyenites exceed those of associated rocks. Sn concentrations in the potassic and micaceous episyenites approximate the low values of their parent rock.

(3) Plots of episyenite compositions on the Q-F diagram (La Roche, 1964; modified by Cathelineau, 1986) distinguishes three major groups: (a) sodic episyenites, (b) moderately sodic episyenites, (c) potassic episyenites. Moderately sodic episyenites do not occur in the Agua Boa metasomatic suite. The studied Agua Boa metasomatic rocks are dominated by sodic episyenites similar to those described by Charoy and Pollard (1989) at Emuford. The Agua Boa potassic episyenites resemble rocks described by Recio et al. (1997) and Peterson and Eliasson (1997). Mica-rich episyenites of this study correspond to episyenites micac6es and episyenites feldsphatiques of Leroy (1984).

(4) The paragenetic evolution of the Agua Boa sodic episyenites is similar to that of the Emuford episyenites (Charoy and Pollard, 1989). The principal steps are: (a) albitization of the igneous K-feldspar, (b) vug formation by quartz dissolution, (c) filling of vugs by albite, chlorite, lithian muscovite, cassiteritekfluoritet adularia and (d) deposition of late quartzt cassiterite in the remaining cavities. The episyenites near the top of the main hydrothermal zone contain more quartz than episyenites in the lower part. Most of this quartz occurs as vug fillings but some quartz is interpreted as partially resorbed grains inherited from parent rock. In the case of potassic episyenites, the initial hydrothermal stage is one of vug formation by dequartzification; these vugs were subsequently filled with adularia which also replaced other late phases. Micaceous episyenites were derived from sodic episyenites through zinnwaldite replacement of albite and quartz. Partial replacement of albite by muscovite is widespread in the lower part of the main hydrothermal zone.

(5) Comparison of peralkaline albite granite of the Madeira pluton with metasomatic sodic episyenites of the Agua Boa pluton indicates: (a) both rocks are Sn- mineralized, but the sodic episyenites have much higher Sn contents, (b) the albite granite exhibits positive



correlation between Rb/Sr and Na,O, Sn, and F but these correlations are lacking in sodic episyenite, (c) geochemical contrasts between these albite-rich rocks is due to their different origins and the important role played by F in the petrogenesis of the albite granites, (d) metasomatic episyenites can be geochemically distinguished from similar-looking albite-rich granitoids in the Proterozoic anorogenic provinces of the Amazonian craton and similar provinces elsewhere.

(6) Sodic episyenites of the type as described in this study are a newly recognized Sn source which should be present in similar environments elsewhere.

(7) Episyenitization in the Agua Boa pluton required a steep thermal gradient and high fluid:rock ratio during subsolidus cooling. It is likely that two fluids participated in the metasomatic process; namely, an early high temperature silica-undersaturated deuteric fluid capable of dequartzification and a cooler late silica-saturated fluid possibly bearing a meteoric component.

Acknowledgments

Financial support from CNPQ (HTC: 141927/96-8; RMKB: 141869/98-4; RD: 301170/80-0,400038/99 and 463196/200-7) and UFPA (PROINT98/00/01) is acknowledged. The authors acknowledge additional financial support from the Paranapanema Group for the chemical analysis and permission to sample drill core. We thank the microprobe laboratory staff at the Federal University of Brasilia for help during analytical sessions. An early version of the manuscript benefited from the critical comments by B. Charoy, which is gratefully acknowledged. The reviewers, J.S. Bettencourt, C.C.G. Tassinari and M.K. Pandit, are also thanked. A thorough revision of the final manuscript by G. L. Lowell was greatly appreciated and helped to improve the paper, but the responsibility for all remaining shortcomings rests with the authors.

This paper is a contribution to PRONEX 103/98 (CNPq Proc. 66.2103/1998) and IGCP 426 Projects.

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