The Lages Pintadas aquamarine deposits, ENE Brasil: a new gem province
a new model
Estudosand
Geológicos
v. 14: 54-61
THE LAGES PINTADAS AQUAMARINE DEPOSITS, ENE
BRASIL: A NEW GEM PROVINCE AND A NEW MODEL
A. Bhaskara Rao 1
Claudio de Castro 2
Maria S. Adusumilli 3
Sandra de B. Barreto 4
1 In memorium GRM, Institute of Geosciences, University of Brasília, Brasília, Brasil
2 Dept of Mining Engineering, UFPE,Recife, Brasil.
3 GMP, Institute of Geosciences, University of Brasília, Brasília, Brasil
4 Dept of Geology, Federal University of Pernambuco, Recife, Brasil, [email protected]
RESUMO O domínio pegmatitico granítico da Provincia Borborema é um dos mais ricos
em mineralização das províncias pegmatíticas brasileiras. Uma província com gemas de água
marinha e outra com gemas de turmalina foram definidas por Baskara Rao et al, (1998), ao sul
da cidade de Parelhas no ENE do Brazil. Estas províncias são caracterizadas pelos seus
condicionamentos geológicos, litológicos e estruturais, respectivamente, em um sinforme
desenvolvido por biotita xistos e um antiforme desenvolvido em quartzitos micaceous bandados. Os pegmatitos mineralizados foram abandonados e somente agora estão retomando sua
exploração, ainda de forma esporádica, com a localização de bolsões de gemas, o que resulta
em oscilação na produção de gemas de água marinha e turmalina na região. A região de Lages
Pintadas, NW de Santa Cruz, possui pegmatitos encaixados em migmatitos-ganissicos de
idade Arqueana, complexo migmatitico-gneissico (TTG ?) com cristais excelentes e grandes
fragmentos de água marinha com coloração azul. Elas são excelentes para fins de lapidação e
propiciam bom retorno financeiro. As condições geológicas, posicionamento estrutural e
controle litológico e a mineralização associada a veios de pegmatito/quartzo favorecem a
definição de um novo modelo e de uma nova província.
Palavras-chave: Província Borborema, pegmatitos, Lages Pintadas, água marinha, turmalina.
ABSTRACT Granitic pegmatite domain of the Borborema Structural Province is one of
the most mineralized of the Brazilian pegmatite provinces. One aquamarine gem province and
one tourmaline gem province have been defined by Baskara Rao et al, (1998), south of the
town of Parelhas in ENE Brazil. They are characterized their geological, lithological, and
structural conditioning, respectively in the biotite schist dominated synform, and banded
micaceous quartzite antiform. The mineralized pegmatites where abandoned and now are
beginning to be reworked, sporadically, with the occurrence of gem pockets which result in an
osci1lating gem production of aquamarines and tourmalines in the region. The Lages Pintadas area, NW of Santa Cruz, has pegmatites hosted in Archean migmatitic-gneissic complex
(TTG ?) with excellent crystals and large fragments of aquamarines with pleasant water blue
color and hue. They are excellent for lapidary purposes and fetch good to high returns. The
geological conditioning, structural positioning, lithological controls and pegmatite/quartz
vein type mineralization permit the denomination of a new model and also a new province.
Keywords: Borborema Province, pegmatites, Lages Pintadas, aquamarine, tourmaline.
Estudos Geológicos v. 14, 2004
54
IniciaisEstudosGeologicosOk.p65
54
29/6/2005, 09:00 Bagaço dez
A. Bhaskara Rao et al
INTRODUCTION
The first Aquamarine Gem Pegmatite
(AGP) province in ENE Brazil is defined
from Acari, in the State of Rio Grande do
Norte to São José do Sabugi in the State of
Paraíba, including seven recognized aquamarine gem deposits. They are conditioned to a regional synform constituted of
Neoproterozoic biotite schists with intense structural deformation due to squeezing.
Small granitic stocks and granitic pegmatite fi1lings intrude them. The pegmatites are
K- feldspar and quartz rich, poorly zoned
and poorly mineralized, often with two generations of beryl. The first generation is
opaque with occasional nucleus of aquamarine, the other is characterized by gem
crystals embedded in quartz veins (Baskara Rao et al, 1998).
Samples of aquamarine from gem pockets of the Lages Pintadas area, State of
Rio Grande do Norte, were obtained where large quantities of opaque beryl continues to be recovered. Besides being
geographica1ly disconnected from the
first AGP province, the geological conditioning is also different. In this study we
present and discuss this new mode of
occurrence and propose the denomination of a new province.
LOCATION
Santa Cruz is located at 80 km from the
city of Natal, following the highway to Currais Novos. Lages Pintadas (36° 7' W, 6° 9'
S) is a small town located 16 km NW of Santa Cruz with good access. Within a radius of
10 km, numerous pegmatite fillings occur as
dykes, veins or smal1 pods. Four have been
catalogued as having produced or produ-
cing aquamarines. All pegmatites have been
worked in the past and are now reworked.
The following pegmatites were visited by
the authors:
§
Tapera: Small scale mining activity stil1
contributing to the recovery of some
gems.
§
Bom Destino: Abandoned, and occasionally reworked. Reports show that
it produced good quality colored
gems.
§
Ramal: Abandoned, and occasionally
reworked.
GEOLOGICAL CONDITIONING
The geological setting and the structural aspects of the locality are summarized
in figure 01, from the geological map of the
State of Rio Grande do Norte in the scale of
1:500.000. (DNPM & UFRN 1998).
Geologically the area is situated in the
Archean migmatitic-gneissic complex, dominated by a variety of porphyritic (Ab-K
feldspar) and banded migmatites with dark
bands and/or stringers of mafics constituted of homblende and biotite. They are sutured by a foliation with variation between
N-S and NNE-SSW due to the elongated
monzonite-granodiorite intrusion showing
also an NE-SW trend. Intense folding is
observed, characterized by anticlines, isoclines, local ruptures and overturned folds,
and evidenced by the horizontal foliation of
the gneisses.
The region has suffered intense weathering resulting in altered gneisses and regolith soils showing scanty vegetation with
small elevations, with the exception of intrusive bodies.
55
Estudos Geológicos v. 14, 2004
IniciaisEstudosGeologicosOk.p65
55
29/6/2005, 09:00 Bagaço dez
The Lages Pintadas aquamarine deposits, ENE Brasil: a new gem province and a new model
Figure 1 - Geologic context of the Aquamarine deposits of Lages Pintadas - Rio Grande do Norte
LOCAL GEOLOGY
The local geological aspects are presented in table 1.
Table 1- Local Geological Sequence.
Estudos Geológicos v. 14, 2004
56
IniciaisEstudosGeologicosOk.p65
56
29/6/2005, 09:00 Bagaço dez
A. Bhaskara Rao et al
The general structural pattern is restricted to the foliation in NNE-SSW trend, varying to N-S due to the intrusions, during a
deformational event characterized by recumbent, isoclinal folds and thrusts faults imprinting schistosity and banding in granitoids and
in gneissic-migmatitic complex. Some transcurrent shear zones were also mapped.
Four aquamarine occurrences in the
Archean and one in the porphyritic granitoid
suite are represented in the geological map
of the State of Rio Grand do Norte. They are
controlled by the NNE-SSW foliation.
Previously considered as flebitic migmatite by Adusumilli & Bhaskara Rao, (1968)
these bodies are now classified as granodioritic-monzonitic banded and porphyritic
gneisses (or TfG gneissic complex), of Archean age. They show the following characteristics:
§
porphyritic gneiss, with porphyroblasts of white albite, and occasionally red K-feldspar, with bands and/
or stringers of biotite and hornblende, with medium to fine grained matrix
of extremely compact and hard nature, submitted to intense deformations,
resulting in the fracturing of feldspar
porphyroblasts.
Sizes: Length 30, breadth 10m, and
depth to 15 m.
Host rock: Granodiorite- monzonite
gneiss.
Topographically the terrain is denuded, not similar to the biotite schist degradation, but with some small and accessible
elevations, as a resultant of resistant regional gneiss.
MINERALOGY
Similar to the other aquamarine gem
pegmatites (AGP), the ones studied in this
region are also poor in their mineralogical
content and variation. They commonly present the following mineralogy:
Quartz occurrs as independent veins,
or as part of pegmatites. These crystals are
characterized by its colorless, smoky and rose
varieties. The association of rose quartz in
the pegmatites or in the quartz veins, seems
to characterize them as mineral guide for aquamarine deposits. The smoky variety hosts
independent crystals of aquamarine gems.
K-feldspar occur as coarse to fine bands or veins of light-red microcline dominating
OCCURRENCE
The aquamarine occurs associated
with veins and dykes of granitic pegmatites
and quartz veins. They are usually thin with
few centimeters to metric dimensions, outcropping in a terrain degraded showing low
relief of the resistant litho- types.
The mined areas are both old, and
always targets for reworking. They are small
and could be generalized as follows:
MINERALIZED ROCKS
The mineralized rocks are granitic pegmatites and quartz veins.
Pegmatites: composed dominantly by
K-feldspar and quartz. Commonly they are
not zoned, but show bands of K-feldspar
and quartz veins, intercalated, giving the
aspect of a pegmatite vein. Their grain size
is very variable, depending on the thickness
of the bands and veins, and occasiona1
aplitic nature.
Quartz veins: Thin to medium sized,
colorless, milky, smoky and rose quartz veins,
are recorded. The smoky quartz vein is coarse-grained, fractured and has light grey to
shady smoky in color. A1ong the fractures,
excellent hexagonal prisms of aquamarine
crystals wit sizes up to 2 cm are obtained.
Their transparency, color and hue are of good
quality. They show basal partings, occasionally causing horizontal color zoning. Some
crystals also show simple vertical color zoning, with the nucleus lighter in hue probably due to depletion in Fe.
57
Estudos Geológicos v. 14, 2004
IniciaisEstudosGeologicosOk.p65
57
29/6/2005, 09:00 Bagaço dez
The Lages Pintadas aquamarine deposits, ENE Brasil: a new gem province and a new model
the pegmatites. These bands host thick to
giant prismatic crysta1s of opaque, grassy
green beryl.
Beryl crystals are opaque green and
represent an economic product from these
pegmatites. It can grow up to100 x 30 cm,
but also occurs as smal1 prismatic crystals
in the feldspar zone of the pegmatites, and
normal1y have a coating of Mn-oxides and
hydroxides. Some small opaque crystals
show a substitution resulting in the gem
variety, along the nucleus. This is usually
restricted to the Si-rich part of the pegmatite, or exclusively to quartz veins.
The main accessory minerals are tourmalines, mainly elbaites, well developed
crystals up to centimetric in size, biotite
flakes, and muscovite are also recorded. The
tourmaline crystals are fractured, and are
found more frequently in the feldspar zone.
GENETIC ASPECTS
Earlier studies of the on aquamarine
deposits in the region are summarized below, and they are basically in consonance
with genetic considerations for these occurrences. The works of Barton (1986), Ricca & Berger (1996), and Evensen et al. (1999)
were used to formulate the following ideas
to explain the evolution of the gem beryl in
ENE Brazil.
1 – The opaque beryl has crystallized
at high temperature magmatic-pegmatitic
phase.
Evensen et al. (1999) show that beryl
bearing pegmatites are commonly peraluminous in composition. This mineral assemblage beryl + chrysoberyl + quartz may be
relevant to the magmatic portion of their
crystallization. But, in natural systems, the
back reaction may occur:
AQUAMARINES
The gems obtained in this region find
market easily principal1y due to their color
and sizes. The characteristics of the aquamarines are shown in Table 2.
3BeAl2O4 + 2 KAlSi3O8 + 6 SiO2 =
Chrysoberyl Potash feldpar Quartz
Be2Al2Si6OI8 + 2 KAl3Si3O10(OH)2 (1)
Beryl
Muscovite
Table 2 – Characteristics of the Aquamarines.
Estudos Geológicos v. 14, 2004
58
IniciaisEstudosGeologicosOk.p65
58
29/6/2005, 09:00 Bagaço dez
A. Bhaskara Rao et al
This leaves beryl as a predominant mineral. The assemblage chrysoberyl + beryl +
phenakite is not expected in quartz saturated
systems, viz. granitic pegmatites, which contain all important “magmatic” Be-phases.
2 – Baskara Rao et al. (1999) consider
the evolution of the second phase, resulting
in the aquamarines, in the following manner.
§
§
§
Active hydrothermal fluids, under high
pressure traversed the first phase opaque beryls. Along the channel structure, some chemical elements were destabilized, mobilized and ionic alkaline elements were left (Baskara Rao et al., 1997);
Fluid inclusions, as “bottles” (Barton,
1986) were also left as trails;
Magneto-hydrodynamic motivated
complex physical mechanisms, such
as the formation of knotted and linked
structure for the fluid flow, must have
resulted in the interaction and linking
of two elliptical ring movements (Ric-
§
§
ca & Berger ,1996);
This movement of the fluids resulting
in the mobility of the ions and their
migration along the nucleus might
have contributed to the generation of
the clear gem variety;
The isolated presence of gem beryl in
quartz vein is probably conditioned to
the enrichment of silica in the fluid system, intensifying (?) the process of flow
mechanisms that permit the growth of
gem variety in the SiO2 rich environment.
(Baskara Rao et al., 1999) promoting ideal
physical conditions, as detailed above.
EXPLORATION PARAMETERS
A comparative study of both the
AGP’s of the region is shown with similarities and differences (Table 3), including exploration guides.
A follow-up with ground penetration
radar (GPR) should be very useful in detecting gem pockets, leading to the exploration
work economically feasible for exploitation.
Table 3: Comparison of the Two Aquamarine Gem Provinces in ENE Brazil
59
Estudos Geológicos v. 14, 2004
IniciaisEstudosGeologicosOk.p65
59
29/6/2005, 09:00 Bagaço dez
The Lages Pintadas aquamarine deposits, ENE Brasil: a new gem province and a new model
NEW AQUAMARINE GEM
PEGMATITE MODEL
The present study reveals new aspects charactering different geological condition and thus confirms the Lages Pintadas as a new model.
This is another aquamarine gem pegmatite province, totally distinct in, geographic
placement and structural positioning that,
characterize it as another AGP Province. Thus,
the Lages Pintadas is a new AGP in ENE Brazil, and the established genetic and exploration parameters could be useful for the location
of other occurrences in the region.
CONCLUSION
The important conclusions of this
study are:
§
The aquamarines obtained from this Archean migmatite-gneissic ambient hosted pegmatites are more colored than
those located in the Neoproterozoic biotite schists. They also have better sizes,
§
§
ACKNOWLEDGEMENTS
Thanks are due to Rao’s Ideas Bank,
who arranged a sponsor for our field travels
towards R & D works; and to Mr. José Oliveira de Nóbrega, whose information on the
localities proved valuable and who also provided some gem samples.
Estudos Geológicos v. 14, 2004
60
IniciaisEstudosGeologicosOk.p65
§
and together with their transparency and
uniform hue, become more attractive.
The host rock is highly weathered favoring a better and an easy recovery
of the gem stones without much damage during mining.
These conditions associated with
good quality imply in higher prices for
the gems, and assured market and
economic returns.
Thus, the program of the government
of the State of Rio Grande do Norte
(REN, 1998), and the Ministry of National integration should give more attention to the gem projects in order to
stimulate the gem industry.
60
29/6/2005, 09:00 Bagaço dez
A. Bhaskara Rao et al
REFERENCES
Adusumilli, M.S. & Bhaskara Rao, A., 1968.
Considerações sobre os migmatitos do
Nordeste. I. Classificação, Nomenclature e exemplos – Jor. De Mineral, Vol.VI, p.61-81.
Adusumilli, M.S.& Bhaskara Rao. A, 1991.
Small Scale Mining in Brazilian
Pegmatites: Mineralogy and Economic Importance. In: Small Scale Mining- An
Overview.
Arem, J.E., 1987. Color Encyclopaedia ofGemstones. Van Nostrand Reinhold, New York.
A.K. Ghose-Editor. Oxford Publ. Co. New
Delhi. 1994. p. 301-324.
Barton, M.D., 1986. Phase equilibria and thermodynamic properties of mineraIs in the
BeO-Al2O3-SiO2-H2O (BASH) system, with
petrologic applications. Amer. Mineral,
Vol. 71, p.277-300.
Bhaskara Rao, A, Adusumilli, M.S., & Castro,
C., 1996. Diagnostic Characteristics of the
Aquamarine deposit of Canoas, Acari, RN
State, Brazil. 39 Congr. Brasil. Geol., Salvador. ResumosErpandidos, p.197-200.
Bhaskara Rao, A., Adusumilli, M.S., & Castro, C., 1997. Evolution and exploration
parameters of gem beryls from Varzea
do Serrote Pegmatite, RN State, Brazil.
XVII Simp. Geol. Nordeste, Soc. Brasil.
Geol., Fortaleza. ResumosExpandidos,
Bol.15, p.414-418.
Bhaskara Rao, A., Adusumilli, M.S., & Castro,
C., 1998. Geological and Exploration Chracteristics of an Aquamarine Gem Pegmatite
Sub-Province in ENE Brazil. IMA 98, Toronto, Abstracts, p. AI7. Poster 18pp.
Bhaskara Rao, A., Adusumilli, M.S. & Castro, C., 1999. Genetic and exploration
aspects of Serrote do Meio Aquamarine Pegmatites near Parelhas, ENE Brazil. (Communicated).
Brisbin, W. C., 1986. Mechanics of pegmatite
intrusion. Amer.Mineral, Vol.71, p.644-651.
DNPM & UFRN., 1998. Mapa Geologico do
Estado do Rio Grande do Norte.DNPM,
Recife.
Evensen, J.M., London, D. & Wendlandt,
R.F., 1999. Solubility and stability of
beryl in granitic melts. Amer. Mineral,
Vol.84, p.733-745.
REN., 1998. Programas do Desenvolvimento
do Estado do Rio Grande do Norte na
Area de Geologia e Mineração. Governo do
Estado, Natal.
Ricca, R.L., & Berger, M.A., 1996. Topological Ideas and Fluid Mechanics. Physics
Today, Vol.49 (12), p. 28-34.
61
Estudos Geológicos v. 14, 2004
IniciaisEstudosGeologicosOk.p65
61
29/6/2005, 09:00 Bagaço dez