Fvdgdb

9 / The pegmatitic environment
Some general points
Pegmatites are very coarse-grained igneous or metamorphic
rocks, generally of granitic composition.
Those of granulite and some amphibolite facies
terranes are frequently indistinguishable mineralogically
from the migmatitic leucosomes associated
with them, but those developed at higher structural
levels and often spatially related tointrusive, late
tectonic granite plutons, are often marked by minerals
with volatile components (OH, F, B) and a
wide range of accessory minerals containing rare
lithophile clements. These include Be, Li, Sn, W, Rb,
Cs, Nb, Ta, REE and U, for which pegmatites are
mined (rare element or (better) rare metal pegmatites).
Chemically, the bulk composition of most
pegmatites is close to that ofgranite, but components
such as Li20, Rb20, B20 3, F and rarely Cs20
may range up to or just over 1%.
Pegmatite bodies vary greatly in size and shape.
They range from pegmatitic schlieren and patches in
parent granites, through thick dykes many kilometres
long and wholly divorced in space from any
possible parent intrusion, to pegmatitic granite
NW
plutons many kilometres squared in area.They form
simple to complicated fracture-filling bodies in
competent country rocks, or ellipsoidal, lenticular,
turnip-shaped or amoeboid forms in incompetent
hosts. Pegmatites are often classed as simple or
complex. Simple pegmatites have simple mineralogy
and no well developed internal zoning while
complex pegmatites may have a complex mineralogy
with many rare minerals, such as polluciteand
amblygonite, but their marked feature is the arrangement
of their minerals in a zonal sequence
from the contact inwards. An example ofa complex,
zoned pegmatite from Zimbabwe is given in Fig. 9.1
and Table 9.1. Contacts between different zones
may be sharp or gradational. Inner zones may cut
across or replace outer zones, but not vice versa, so
that inside the wall zones at Bikita notwo cross-cuts
expose the same zonal sequence. The crystals in
complex pegmatites can be very large and at Bikita,
for example, the spodumene crystals are commonly
3 m long. The Bikita Pegmatite, which is about
2360-2650 Ma old-it is notoriously difficult to
obtain concordant radiometric ages for zoned pegmatites
(Clark 1982)-is emplaced in the Archaean
SE
D Feldspar ~o~~ Cobble zone 0Petalite
Beryl IQ QQI Quartz ~ Spodumene-feldspa r , SF,
~ Lepidolite G Spodumene 0 30 m
I I
Fig. 9.1 Section through the Bikita Pegmatite showing the generalized zonal structure and the important minerals of
each zone. Cleavelandite is a lamellar variety of white albite. (After Symons 1961.)
121
122 CHAPTER 9
Table 9.1 Zoning in the Bikita Pegmatite, Zimbabwe. (From Symons 1961)
Hangingwall greenstone
Border zone
Wall zones
Intermediate zones
Core zones
Intermediate zones
Wall zone
Selvage of fine-grained albite, quartz, muscovite
Mica band. Coarse muscovite, some quartz.
Hanging wall feldspar zone. Large microcline
crystals
Petalite-feldspar zone
Spodumene zone
(a) massive
(b) mixed spodumene, quartz, plagioclase and
lepidolite
Pollueite zone. Massive pollucitewith 40% quartz
Feldspar-quartz zone. Virtually devoid of lithium
minerals
'All mix' zone. Mieroline, lepidolite, qual1z
Massive lepidolite
(a) high grade core, nearly pure lepidolite
(b) lepidolite-quartz subzone
Lepidolite-quartz shell
'Cobble' zone. Rounded masses of lepidolite in an
albite matrix
Fe1dspathie lepidolite zone
Beryl zone. Albite, lepidolite, beryl
Footwall feldspar.Albite, muscovite, quartz
Footwall greenstone
Fort Victoria Greenstone Belt. It is one of the
world's largest Li-Cs-Be deposits; the main pegmatite
is about 2 km long and 45-60 m thick, and the
minerals of major economic importance were petalite,
lepidolite, spodumene, pollucite, beryl, eucryptite
and amblygonite. Cassiterite, tantalite and
microlite were disseminated through quartz-rich...