rl57

The Carndinn Mine ralo gi st Vol. 36, pp. t 157-l I 58 (l 998)

THEVOLTA GRANDE PEGMATITES, , BRMIL: AN EXAMPLEOF RARE.ELEMENTGRANITIC PEGMATITES EXCEPTIONALLYENRICHED IN LITHIUMAND RUBIDIUM: DISCUSSION

FERNANDO R.M. PIRES ero A. RAPHAEL CABRALT

Departatnento dz Geolngin, Universida^de Federal do Rio dz Janeiro, Av. Brigadeiro Trompowski s/n, 21949-900 Rio de Janeiro (M, Bra:il

INrnorucrroN the inclusion of cassiterite grains in the latler. Further textural evidence demonstrated that replacement of In spite of the well-doc 'mented chemical composi- tantalite by microlite did occur (Peixoto & Guimaraes tions of minerals of the Volta Grande Sn-Ta-Li 1953). Whether tantalite altered to microlite or pegmatitesprovided by Lagache & Qu6m6neur(1997), uranmicrolite was attributed to the local availability of the proposed explanation for the recorded distribution uranium in the hydrothermal solution (Belezkij 1956, of Rb and Cs using experimentally determined partition Guimardes & Belezkij 1956). coeffrcients should be examined with care, since the A similar paragenetic sequencein two stageswas paragenetic relations amongst minerals have not been proposed by Heinrich (1964): i) magmatic, with quartz, considered.Furthermore, Lagache & Qudmdneur(1997) muscovite, microcline, spodumene,sodic plagioclase, have disregarded all the previous investigations casiterite and tantalite as main phases, and ii) hydro- that have been carried out for decades bv Brazilian thermal, characteized by lepidolite and microlite, and geologists. barian microlite (which is intergrown with microlite and replaces it). He considered that several of the pegmatite Pnruous Wonr bodies were the result of two or more periods of injec- tibn of pegmatite-forming magma. The calcic-sodic The pegmatitesof the Rio da Mortes tin district stageadvanced by Guimar6es& Guedes(1944) was not (Roltr 1948)were frst investigatedby Guimardes& recogaized by him; howevero Francesconi (1972) char- Guedes(1944, 1946),who recordedthe following min- acteized the existence of the calcic-sodic stageoand eralogicaldistribution in outcropsofthe pegmatitealong attributed to a final alkaline stage (K, Li, Rb, Cs) the its strike from Rio das Mortes toward FazendaVolta formation of lepidolite, bityite, microlite and zircon Grande,after which the pegmatitebodies were collec- (now metamict). tively named:80 m of cm-sizedcrystals of spodumene Although Heinrich (1964) stated that nearly all andfeldspars, 300 m ofcoarse-grainedcassiteriteo mus- pegmatites are either unzoned or indistinctly zoned, the coviteand quartz, followed by 30 m of mm-sizedcrys- distribution of spodumene, lepidolite and muscovite tals of lepidolite.Relics of spodumeneenclosed in recorded by Guimar6es & Guedes(1944, 1946) is sug- quartzand microcline, or in a micaceousaggregate, were gestive of zoning of the bodies. At the Volta Grande foundin thespodumene-poor zones. These authors pro- mine, a well-defined lepidolite - spodumeneboundary posedtwo stagesof pegmatiteformation: l) calcic- can be traced along orebodies A and E (Pires & Pires sodicstage, marked by crystallizationof oligoclaseoto 1992). Internal zoning in the pegmatites has been which cassiteriteand spodumene are probably related, reported by Qu6m6neur (1987). and2) a potassicstageo responsible for theformation of microclineand muscovite, both alongwith quartz,at the DrscussroN expenseof oligoclaseand spodumene,respectively. Spodumenemay be replacedby quartz with formation The first point to which attention should be drawn of lepidolite(Guimar6es 1948, 1950). concerns the statement by Lagache & Qu6m6neur The parageneticrelations amongst ore (opaque)min- (1997) that "these pegmatites were previously investi- eralswere establishedby Guimar6es(1950). He con- gated by Heinrich (19@)" . One could get the erroneous cluded that tantalite was contemporaheouswith impression that before their present worko there only cassiterite;uranmicrolite formed later, as indicatedby existed that of Heirich. This is far from being true,

I E-mail ad.dress: arcabral @rio.com.br

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not only in respectof the Volta Grande pegmatites,but and demonstrated that the proportion of muscovite also of the regional geological setting. Lagache & diminishes and that of polylithionite increases with Qu6m6neuralluded to the existenceof a greenstonebelt increasing F content. This increase in IIF fugacity is in the area without reference to Almeida (1976), who consistentwith the common occurrenceof lepidolite as first recognized that suite of rocks, later referred to as a late-stage mineral in Li-bearing ganitic pegmatites, the Greenstone Belt @ires 1977). T\e im- where the presenceof a F-bearing aqueous fluid medium portant geological map and field relations defined by favors subsolidusreaction ofspodrmene and K-feldspar Ebert (1956, 1957a, b) were not consideredat all. (l\4unoz 1971). The high activity of F and H2O would Upon examination of the previous results, some facts also destabilizetantalite in favor of microlite in this late seem established, such as the initial replacement of (subsolidus) stage. oligoclase by microcline and the late formation of lepi- A concluding remark concerning the presence of dolite at the expense of spodumene.Such paragenetic holmquistite at Volta Grande is necessary. First iden- relations amongst pegmatite-forming minerals, as well tified by Leonardos (1973), the holmquistite was as the suite of alteration minerals in the wall rock suggestedto have been formed by replacementofhorn- (wherein epidote, fluorite, anthophyllite and titanite are blende of the amphibolite wallrock (Qu6m6neur & relatively common), were not defined by Lagache & Lagache 1994). Its occurrence near such highly Qu6m6neur (1997). As a consequence,the proposed fractionated, Li-rich pegmatites as at Volta Grande can evolution of the concentrationsof Rb and Cs in the min- be used as a guide to exploration for Li-rich rare- erals using experimentally determined partition coeffi- element-enrichedpegmatites (London 1986). cients loses its strength inasmuch as the paragenetic evolution has not been taken into consideration. Lagache & Qudmdneur (1997)have documented a positive linear correlation between F and Rb in micas ReceivedJanuary 25, 1998.

The CanadianMineralogist Vol.36, pp. 1160-1162 (1998)

THEVOLTA GRANDE PEGMATITES, MINAS GERAIS, : AN EXAMPLEOF RARE.ELEMENTGRANITIC PEGMATITES EXCEPTIONALLYENRICHED lN LITHIUMAND RUBIDIUM: REPLY

MARTINE LAGACFIE' lzboratoire dz G4ologie,Ecole Normale Supdrieure,UM 1316 du CNRS,24, ruz Lfumond, F- 75231Paris Cedex05, Franr:e

JoELeuEr,cNEURr Departamento dz Geologid" IGC, Universidnde Fedzral de Minas Gerais, Av. Antonio Carlos 6627, 31270 , Brasil

The Discussion of Pires and Cabral focusses on tlrree descriptions. These references were to a large extent themesoto which we intend to reply in sequence. cited in the 1987 article. In view of the length of our articleo we restricted our coverage of the previous lit- Bmllocnepnv eratureto a citation to Heinrich (1964). We selectedthis article becausethe author wrote a synthesis of the ear- We could, indeed, have written a section to review lier work on Volta Grande and becauseit was published the historical aspectsofthe discovery ofthe deposit and in an intemational journal. In addition, we did not use, the early writings of geologists who provided the flrst in our article, any ideas or findings attributable to the

' E-mail addresses:[email protected], jokarfu n @igc.ufmg.br

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