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 to intrusive, 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 of granite, but components such as Li 2 0, Rb 2 0, B2 0 3 , F and rarely Cs 2 0 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
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 pollucite and amblygonite, but their marked feature is the arrangement of their minerals in a zonal sequence from the contact inwards. An example of a complex, zoned pegmatite from Zimbabwe is gi ven in Fig. 9.1 and Table 9.1. Contacts between different zones may be sharp or gradational. fnner zones may cut across or replace outer zones, but not vice versa, so that inside the wall zones at Bikita no two 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
NW
0
Feldspar Beryl
~
Lepidolite
o~~ ~ 00 1
G
QI
Cobble zone Quartz Spodumene
0
Petalite
~ SF,
Spodumene-feldspa r
,
0 I
30 m 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)
Hanging wall greenstone Border zone
Selvage of fine-grained albite, quartz, muscovite
Wall zones
Mica band. Coarse muscovite, some quartz. Hanging wall feldspar zone. Large microcline crystals
Intermediate zones
Petalite-feldspar zone Spodumene zone (a) massive (b) mixed spodumene, quartz, plagioclase and lepidolite Pollueite zone. Massive pollucite with 40% quartz Feldspar-quartz zone. Virtually devOId of lithium minerals 'All mix' zone. Microline. lepidolite, qual1z
Core J:ones
Massive lepidolite (a) high grade core, nearly pure lepidolite (b) lepidolite-quartz subzone Lepidolite-quartz shell
Intermediate zones
'Cobble' zone. Rounded masses of lepidolite in an albite matrix Feldspathic lepidolite zone
Wall zone
Beryl zone. Albite, lepidolite, beryl Footwall feldspar. Albite, museovite, 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 zones in iepidolite-greisen, hut these had been mined out by 1950. Mining ceased in 1986 but the company will continue production for many years using stockpile material to produce petalitc concentrates (Russell i988c). Broadly speaking there is a worldwide similarity of zonal sequences in pegmatites, an important point which. among many others characteristic of complex pegmatites, must be explall1cd by any proposed genetic theory. Useful diSCUSSIOns of internal zoning, terminology used, etc., are to be found in Cameron et at. (1949), Cerny (1982a), Jahns (1955, 1982) and Norton (1983). Pegmatites have been classified in numerous fashions using as many as nine major criteria (Cerny
1982a). A simple classification into four pegmatite formations which has exploration implications is that of Ginsburg et al. (1979) and Cerny (1989). 1 Pegmatite formation of shallow depths (miaroiitic pegmatites, 1.5-3.5 km); pegmatite pods in the upper parts of epizonal granites. Cavities may supply piezoelectric rock-quartz, optical fluorite, gemstones. 2 Pegmatite formation of intermediate depths (rare mctal pegmatites, 3.5-7 km); filling fractures in and around possible parent granites or quite isolated from intrusive granites; cmplaced in low pressure (Abukumu Series), amphibolite facies metamorphites. Mainly or wholly magmatic differentiates. 3 Pegmatite formation of great depths (m icabearing pegmatites, 7-8 to 10-11 km); only minor rare metal mineralization, emplaced in intermediate pressure (Barrovian Series), upper amphibolite facies metamorphites. Largely the products of anatexis. 4 Pegmatite formation of maximal depth (> 11 km);
PEGMATITIC ENVIRONMENT
in granulite facies terranes, usually no economic mineralization. Commonly grade into migmatites. Reference to texts on metamorphic petrology suggests that the depths given by Ginsburg el al. (1979) err very much on the shallow side. For example, the pegmatites of the Bancroft field, Ontario, belonging to formation 2, are emplaced in pelitic schists carrying curdierite and almandine, which suggests a depth of emplacement of about 20 km or so (Winkler 1979). Pegmatites may occur singly or in swarms forming pegmatite fields, and these in tum may be strung out in a linear fashion to form pegmatite belts, one of the largest being the rare metal pegmatite belt of the Mongolian Altai, about 450 km long and 20-70 km broad, which contains over 20 pegmatite fields. Pegmatites in a particular field may show a regional mineralogical-chemical zonation. When a swarm of pegmatites is associated with a parent granite pluton then the more highly fractionated pegmatites, enriched in rare metals and volatile components, are found at greatcr distances from the pluton. The degree of internal zoning also increases with an increase in rare metals and volatiles. The regional zoning may reflect the fact that melts enriched in Li, P, Band F have considerably lower solidi than H 2 0-only saturated magma and can penetrate further from their source. This regional zoning is of course of great importance to the exploration geologist (Trueman & Cerny 1982). Parental (fertile) granites of rare metal pegmatites are found in compressional regimes of orogenic chains within back-arc type sedimentary-volcanic sequences or their analogues in flysch-filled cratonic margins or rifts. They are forceful intrusions that post-date the peak of regional metamorphism and granite emplacement, and most are leucocratic alkali granites that display heterogeneity in ranging from biotite granite at depth to two-mica and muscovite-garnet facies in their cupolas. In contrast to their surrounding haloes of mineralized pegmatites, fertile granites are typically barren. Rare earth element abundances are low and disturbed, and stable and radiogenic isotopic ratios also show disturbances. The above and much more useful information on parental granites can be found in Cerny & Meintzer (1988). Most pegmatites, whether igneous or metamorphic in origin, have similar bulk compositions and these correspond closely to low temperature melts near the minima of Ab-An-Or-Q-H 2 0 systems (Cerny 1982b). This is to be expected for melts
123
developed by extreme magmatic differentiation or anatexis. Many rare metal pegmatites appear to be of magmatic origin; their unusual composition appears Lo be a consequence uf retrograde boiling and the manner in which elements are partitioned between crystals, melt and volatile phases during the cooling of the magma. Bonding factors such as ionic size and charge largely prevent many constituents, originally present in minor or trace concentrations in the magma, from being incorporated into precipitating crystals. They thus become more highly concentrated in the residual melt which, in the case of granites, is also enriched in water, since quartz and feldspars are anhydrous minerals. The list of residual elements is long but includes Li, Be, B, C, P, F, Nb, Ta, Sn and W. Tin and tungsten are exploited more commonly in hydrothermal deposits, but there is no doubt of their orthomagmatic origin in pegmatite occurrences since there is never any evidence uf hydrothermal activiLy (GouanviL: & Gagny 1983). A steady increase in water content, as anhydrous phases are precipitated, will also mark the crystallization of pegmatitic melts and a point may be reached where retrograde boiling produces a water-rich phase. Na, K, Si and some other residual elements listed above will fractionate from the melt into the water-rich phase within which atoms can diffuse much more rapidly than in a condensed silicate melt. Consequently, rates of crystal growth are greatly enhanced and this may have an important bearing on the growth of giant crystals in pegmatites. London (1984) has shown how different pressures give rise to the crystallization of different phases, e.g. whether spodumene or petalite is the primary lithium alumino-silicate phase. Broadly speaking, three hypotheses have been put forward to account for the development of internal zoning. The first is that of fractional crystallization under non-equilibrium conditions leading to a steady change in the composition of the melt with time. The second is of deposition along open channels from solutions of changing composition. The third is a two stage model: (a) crystallization of a simple pegmatite, with (b) partial or complete replacement of the pegmatite as hot aqueous solutions pass through it. At the present time, most workers prefer the first or third hypothesis as the majority of the evidence, such as complete enclosure in many pegmatites of the interior zones, does not favour the existence of open channels during crystallization. As the third theory encounters difficulties in explaining the worldwide similarity of
124 CHAPTER 9
zonal sequences, the first theory finds most favour. An early statement of this preferred theory was given by Jahns & Burnham (1969) who, on the basis of field observations and experimental data, postulated that the internal evolution of zoned granitic pegmatites resulted from the crystallization of watersaturated melts that evolved to produce systems with a melt and a separate aqueous fluid. A useful discussion of this theory appears in Thomas et al. (1988) who, following a comprehensive study of fluid inclusions from the Tanco Pegmatite, southeastern Manitoba, were able to confirm the JahnsBurnham theory. They traced the thermal evolution of this pegmatite from the initial intrusion of a mixture of an alumino-silicate melt plus an H 2 0CO 2 -dissolved salt fluid at ~ 720째C to final crystallization of the quartz zone at - 262째C. Initial crystallization was from the wall rock inwards with the wall zone crystallization commencing at about 600째C and the intermediate zone temperatures being around 475째C. Useful and more general discussions may be found in London (1987, 1990) and London et al. (1989).
Some economic aspects Pegmatitic deposits of spodumene, petalite, lepidolite and other Li minerals are exploited throughout the world for use in glass, ceramics, fluxes in aluminium reduction cells and the manufacture of numerous lithium compounds. These deposits often yield by-product Be, Rb, Cs, Nb, Ta and Sn. They may be internally zoned, or unzoned as are the highly productive lithium pegmatites of King's Mountain, North Carolina. The largest known pegmatitic lithium resources are in ZaIre in two laccoliths each about 5 km long and 0.4 km wide. Reserves have been put at 300 Mt and Ta, Nb, Zr and 1'1 values have been reported (Harben & Bates 1984). Political insecurity, however, is a major drawback as far as overseas investors are concerned and these projects are in abeyance. Another big resource, mainly in granite, is the Echassieres deposit in Fran<.:c, which contains some 50 Mt grading 0.71 % Li, 0.022% Nb, 0.13% Sn and 0.023% Ta. The reader should note that lithium has already been produced from brines in the USA, and a brine producer in Chile is now in production (Crozier 1986). In May 1989 the Japanese Industrial Research Institute announced the development of an efficient method of removing lithium from sea
water. These new sources will present an economic challenge to the hard rock producers and the market price may suffer, as by 1984 lithium production had already exceeded demand. The traditional source of beryllium (beryl), has now been overtaken by bertrandite, which occurs on a commercial scale in hydrothermal deposits (Farr 1984). The main source of beryl is pegmatites, with the USSR (2 kt p.a.) being the world's largest producer, followed by Brazil with less than 400 t p.a. Pegmatites are also important as a source of tantalum, but it must be noted that the largest reserves-about 7.25 Gt oera-are in slags (running about 12% Ta205) produced during the smelting of tin ores in Thailand. Significant reserves are present in the Greenbushes Pegmatite, Western Australia (Hatcher & Bolitho 1982), and in the Taneo Pegmatite at Bernie Lake, Manitoba (Cerny 1982c) which up to 1982 supplied about 20% of the market, but then closed down owing to the weak tantalum price. It reopened in 1988. The backbone of the Greenbushes operation is tin production and most of the tin mined in Thailand is won from pegmatites, not veins (Manning 1986). London (1986) pointed out that many economically important rare metal pegmatites are marked by the development of holmquistite in their immediate wall rocks and that a search for this mineral during pegmatite prospecting is a valuable exploration tool. Pegmatites are also important producers of feldspar and sheet mica.
Uraniferous pegmatites Pegmatites and pegmatitic granite have been exploited in a number of localities. Among the more important deposits are those of Bancroft, Ontario and the enormous Rossing Deposit in Namibia. Bancroft Field, Ontario
This area is part of the south-western extremity of the Grenville Province of the Canadian Shield. Granitic and related pegmatites of this province yield ages of 1100-900 Ma (Lumbers 1979), coincident with the waning stages of the high grade Grenvillian metamorphism. The Bancroft Field lies in the Central Metasedimentary Belt, which consists of a metasedimentary-metavolcanic sequence developed around a number of complex granitoid and syenitoid gneiss bodies having domal and periclinal shapes. The pegmatites occur within these gneisses, in the adjacent metasedimentary and metavolcanic
PEGMATITIC ENVIRONMENT
sequence and in large metagabbro bodies (Ayres & Cerny 1982). Most of the pegmatites are conformable with the metamorphic fabric of their host rocks and internal zoning is but poorly developed, except in the less common fracture-filling bodies that cut across the regional structures. The common presence of pyroxene, hornblende and biotite relates the
pegmatitic compositions to the upper amphibolite facies grade of the enclosing rocks. Rare metal mineralization consists of a variety of U, Th, Nb- Ta, REE, Y, Ti, Zr and Be minerals. One school of thought (references in Ayres & Cerny 1982) relates the origin of these pegmatites to igneous differentiation, the other to some form of ultra_ _
N
+ o I
50 m
125
Pegmatite of ore grade Non-ore pegmatite
~ ~
Scapolitized amphibolite
D
Oligoclase-quartzbiotite-gneiss
n
~
Pelitic gneiss
I
Fig. 9.2 Geology of the First Level, Bircroft Mine, Ontario. (From the work of the mine geologists and the author.)
()
I ):> "'0
-I
m
::JJ to
Uraniferous pegmatitic Slranite Migmatitic pelitic schist/gneiss
l22J c=J IJCO
v
n
"
(j
l}
Metaconglomerate Marble Biotite-amphiboleschist Pyroxene-hornblende gneiss
~
Pyroxene-
L::.::.::J garnet-gneiss Fig. 9.3 Cross section of the Rossing uranium deposit. (After Berning el ai, 1976.)
o I
100 m
PEGMATITIC ENVIRONMENT
metamorphic activity such as anataxis (Evans 1966, Kash et af. 1981, and references in Ayres & Cerny 1982), The large amounts of uranium and other rare metals would be derived from the country rocks, a view held by many Russian workers for certain pegmatites in the USSR, e.g. Shmakin (1983). FJwler & Doig (1983), on the basis of stable isotope data have, however, suggested a mantle source for these Grenvillian pegmatites and their incompatible elements. Average uranium grades in the Bancroft Field were a little above 0.1 % U 308 with each of the five main mines working a r.umber of orebodies usually located in swarms of dominantly granitic pegmatite bodies. The Bicroft Mine was one of the largest operations (Fig. 9.2). The pegmatite dykes of this mine area occur in a 5 km long zone only about a Q'J3rter of which was developed (Hewitt 1967), The orebodies occurred at LIe footwall or hanging wall contacts but sometimes occupied the entire pegmatite, The excellent vertical continuity of the orebodies contrasted markedly with their rather tortuous p:anform (Bryce et at. 1958) and two of the largest orebodies were about 90 m long by 3 m wide but extended vertically for over 400 ill, The pegmatites are very variable in their lithology, frequently carry aegirine-augite, show no signs of forcible intrusion, have a metamorphic fabric, contain non-rotated enclaves and show a tendency for their lithology to be governed by changes in the nature of their host rocks. The principal uranium minerals are uraninite and uranothorite and the mineralization is best developed where the pegmatites cross a zone containing graphitic pelitic gneiss. Evans (1962) suggested that this association may indicate that the uHimate source of much of the uranium is to be fClUnd in these altered black shales, The Rossing uranium deposit, Namibia
This is the world's largest uranium producer. The operati on is a large tonnage, low grade one-15-16 Mt of ore p.a" grading 0,031 % U 308, being produced from an open pit and underground workings (Anon. 1982), The uranium mineralization occurs within a migmatite zone (Fig. 9,3), characterized by largely concordant relationships between uraniferous, pegmatitic granites and the country rocks. These are metasediments of similar age (c. 900 Ma)
127
to the Grenvillian rocks of the Bancroft area, although the pegmatitic granites have been dated at 950-550 Ma (Jacob et al. 1986), The metasediments are very similar to those of the Bancroft area and, as in that area, occupy the ground between and around granite-gneiss domes and periclines (Berning et af. 1976, 1986), The grade of regional metamorphism, upper amphibolite facies, is identical. with cordierite in the pelitic gneisses of both areas indicating low pressure, Abukuma type metamorphism, The pegmatitic granites have a very low colour index and are termed alaskites, About 55% of the uranium is in uranimtc and 40% in secondary uranium minerals, Economic uranium mineralization is conc'entrated where the pegmatites are emplaced in certain metasedimentary zones, including pelitic gneisses, in a manner reminiscent of the Bicroft Mine mineralization, The arid climate of the Namib Desert played an important role in the formation of this deposit because, while giving rise to the enrichment of the primary mineralization wlth secondary minerals released by weathering, it prevented the leaching of this secondary mineralization by meteoric water. Berning et at. (1976) favoured an initial distribution of uranium within the metasedimentary sequence and its later concentration in anatectic melts of alaskitic composition, which show only minor evidence of movement from their zone of generation. The host megastructure has been interpreted as an aborted and closed aulacogen that developed 700-500 Ma ago (Burke & Dewey 1973), but 1000-500 Ma is a more probable time span. As an example of a large tonnage, low grade, disseminated deposit, Rossing could have been included in Chapter 14 and some authors have unfortunately referred to it as a porphyry uranium deposit, but there is little justification for this description. In its geological environment, host rock type, lack of hydrothermal alteration, etc., it bears no resemblance to porphyry copper deposits, As an appropriate ending to this chapter I refer the interested reader to the excellent volume edited by Cerny (1982d) which provides a comprehensive survey of granitic pegmatites and their economic importance and to the 30 papers in the Jahns Memorial Issue of the American Mineralogist, 71, 233-651.