AMETHYST FROM THE BORBOREMA PEGMATITIC PROVINCE IN NORTHEAST BRAZIL: GENESIS AND PROVENANCE CONFIRMED BY FLUID INCLUSION STUDIES.

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

AMETHYST FROM THE BORBOREMA PEGMATITIC PROVINCE IN NORTHEAST BRAZIL: GENESIS AND PROVENANCE CONFIRMED BY FLUID INCLUSION STUDIES.

BEURLEN. H.', DA SILVA, M. R. R.', SILVA, D.2, SOARES, D. R.1

1 Universidade Federal de Pernambuco, Depto. Engenharia de Minas -UFPE-DEMINAS-, R. Academico Helio Ramos s.n., 50740-530 Recife-PE, Brazil.

2 Universidade Estadual de Campinas-UNICAMP, Campinas-SP, Brazil.

Sponsored by CNPq grant 470199/01-6.

E-mail: beurlen@npd.ufpe.br

Some large amethyst druses and individual crystals, up to 40 cm long, were obtained from mineral dealers with the information to have been collected in the Alto dos Mamoes and Tanquinhos pegmatites, in the Borborema Pegmatitic Province (BPP) in the municipality of Equador, State of Rio Grande do Norte (RN). All crystals exhibit a "phantom quartz"

habit with large, more or less irregular and/or corroded cores of smoky quartz, coaxially overgrown by a zone of milky quartz followed by hyaline quartz - these two zones may be recurrent- , and an outermost light purple amethyst zone up to 1 cm thick. In a few samples caulinised feldspar and muscovite inclusions at the limit of the milky and hyaline quartz are the only macroscopic features supporting the supposed pegmatitic provenance. The occurrence of amethyst in pegmatites of the BPP so far is not referred at all in the classical literature on this province (DA SILVA et al 1995 and references therein). Only a very recent reference in the Gemologic Map of the State of Rio Grande do Norte (MORAES 1999) refers two sites of amethyst occurrences related to pegmatites of the Lajes Pintadas pegmatite field of the BPP, far Northeast from the Equador- Parelhas/RN field. There is, however, no description of these two occurrences.

Aiming to check the supposed source of the amethyst crystals under examination, a fluid inclusion (FI) study was carried out and the results compared with FI in other quartz crystals from Alto dos Mam5es, Boqueirao and Capoeiras pegmatites, in the same pegmatite field. The study evidenced clearly that the complete sequence of four FI types and two additional subtypes observed in normal pegmatite quartz crystals (BEURLEN et al. 2001) is also observed in the studied amethyst crystals as shown in Fig. 1 and summarized below. In both cases the innermost, smoky cores of the crystals include type A, primary aqueous-carbonic fluids (liquid water, liquid carbon dioxide and vapor, AL + CL + V) with low salinity ranging between 2 and 4 wt% NaCl eq (the salinity in aqueous-carbonic FI was calculated from measured final clathrate melting temperatures - Tiri^, - in this case between 7.5 and 8.8° C - using the equations of DIAMOND 1992) and high total homogenization temperatures ( Thom, around 300°C) and type B, pseudo-secondary inclusions composed by liquid water and carbon dioxide vapor and accidentally included solids (AL + C v± S) with Thom = 220 - 290°C and low salinity (2-10 wt%

NaCl eq - Tmciat4-9 °C). Successive growth zones of milky and colorless quartz, around the cores of both, the quartz and amethyst crystals, contain primary aqueous FI of type Ca (Thom 200-240°C, salinity 3 - 6.5 wt%NaCleq., Tmice 2 - 4°C), type Cb (Thora = 160-220°C, salinity = 15-25 wt% NaCleq.) and type Cc (Thom = 130-180°C, salinity = 1-4 wt% NaCU,,.), in this order from the core outwards, all with frequent accidentally included solids (muscovite, albite, K-feldspar, carbonate etc.).

The salinity of the of the aqueous FI was obtained from measured final ice melting temperatures -Tmi c e- using the equation of BODNAR (1993). In addition to this similarity based on microthermometric data, Raman spectrometric analyses of the carbonic phase of type A FI revealed an almost pure carbon dioxide composition with only a few molar percent N2 and complete lack of methane in both cases, reinforcing a pegmatitic origin of the amethist crystals.

In the case of the amethyst crystals the zone of hyaline quartz with Cc type FI is surrounded by an outermost zone of alternating smoky quartz and violet colored amethyst with Cc type FI and coexisting primary aqueous-carbonic type D FI with very variable H20 / C 02 ratios. This is the first case where primary type D inclusions where observed. Elsewhere they where only found as secondary inclusions. This coexistence of type Cc and type D FI in the amethyst zone is suggestive of fluid immiscibility and would allow to constrain the trapping conditions at this stage, the stage of amethyst formation, at approx. 2.5kb and 300°C. The estimate of trapping conditions for the type A and B FI were constrained in previous works at 580°C and 3.8kb (BEURLEN et al. 2001) considering minimum pressure stability conditions of primary spodumene in these pegmatites and maximal pressure conditions of the coexistence of quartz and euclase (according to BARTON 1986) and type Cb fluid inclusions. Summarizing, these results indicate that the amethyst crystals were really formed in the pegmatites, in a late, typically hydrothermal stage.

References

BARTON, M. D. (1986): Amer. Mineral., 71, 277-300.

BEURLEN, H . , D A SILVA, M . R . R . , CASTRO, C . ( 2 0 0 1 ) : C h e m i c a l G e o l o g y , 1 7 3 , 1 0 7 - 1 2 3 . BODNAR, R. J. (1993): Geochemica C o s m o c h e m i c a Acta, 57, 683-684.

DA SILVA, M . R . R „ HOLL, R „ BEURLEN, H . ( 1 9 9 5 ) : J. S o u t h A m . E a r t h Sci., 8 , 3 5 5 - 3 6 4 DIAMOND, L. W . (1992): Geochemica C o s m o c h e m i c a Acta, 57, 683-684.

25

28

24 20 16

I 12

S 8

? Mamoes Pegmatite?, amethyst crystal with quartz core

1

o type A FI, core

• type B FI, zone 2

» type Ca FI, zone 3

& type Cc FI, zone 4

•#• type Cc FI, amethyst rim

p q i r z q :

100 120 140 160 180 200 220 240 260 280 300 320 340 360 T h o mt o, ° C

Mamôes Pegmatite, zoned quartz with euclase inclusions 28

24

j

О n

Z 16

12

с

«Л го

• V V ' " !

• type В and Са FI, Qz-core д type Cb FI, Qz-zone 2

» type Cb FI, Qz-zone 3 + type Cc FI, Qz-rim

* type Cb FI, f о type A FI, I

» type Cc FI, I

• type A FI, Mn-Ta

m

r \

100 120 140 160 180 200 220 240 260 280 300 320 340 360

i i a

Boqueirao Pegmatite, zoned quartz crystals

—

Sample BO-OI .i.J...

o type B FI, core o type Ca FI, zone 2 .,....,.. • type Cb FI. zone 3D

> type Cb FI, ione 33

• l - i - i - i - j - . ! . . i*-\ WI»Co.FI.<mi i. J....L..SampleBO-14 .L.J....

: . type Ca FI, jone 2

: : type C c F I . rim. .

U i 1 i - i - L - i -

Н Ы

100 120 140 160 180 200 220 240 260 280 300 320 340 360

ThonV e t.°C

Capoeiras pegmatite, zoned quartz crystals

. . . : t r r r

> i

: Ш : L t : i : . ! : : n :

W ^m

- ш - 4 4 4 - н Ч -

type A FI, CA11 type В FI, CA11 typeCiFI, CA11 type Cb FI. CA11 type Cb FI.CA1IB type A FI. CA12A type Cl FI. CA12A type Cb FI, CA12A type Cc FI. CA12A type A FI, CA12B typeC» FI.CAI2B type Cb FI, CA12B

100 120 140 160 180 200 220 240 260 280 300 320 340 360

T h o n\ o t °c T h o r r \o t° C

Fig. 1: Salinity versus total homogenization (Thon>0t ° C ) plots for the amethyst crystal ( u p p e r left)in c o m p a r i s o n with normal zoned quartz crystals from different pegmatites of the B P P : o b s e r v e that all diagrams present the s a m e pattern with the evolution tr^m type A to type C c F I .