Geochemistry and economic potencial of columbite-group minerals from granitic pegmatites of the Pampean Ranges, Argentina

GALLISKI M. A. CERNÝ P.

St. Chatarines

Congreso; Mineralogical Association of Canada Annual Meeting; 2004

Mineralogical Association of Canada

Columbite-group minerals (CGM) were studied from 30 rare-element granitic pegmatitesrepresentative of 7 (out of a total of 8) LCT-family pegmatite fields that comprise the Pampeanpegmatite province of Argentina. The province extends ~900 km in the N-S direction, and islocated in upper-greenschist- to lower-amphibolite-facies rocks of an Abukuma-typemetamorphic belt, dominantly composed of metagreywackes and metapelites. The pegmatitesbelong to the beryl (beryl-columbite, beryl-columbite-phosphate), complex (spodumene, rarelypetalite or lepidolite) and albite types. They are are related mainly to late-orogenic, Cambrianto Silurian, calc-akaline, S-(+I)-type granites, and in part to post-orogenic, Devonian to UpperCarboniferous, high-K, calc-alkaline monzogranites. Over 400 EMP analyses of the CGMswere performed. The most primitive ferrocolumbite characterizes the beryl-type pegmatites,particularly the post-orogenic sequence; in the late-orogenic suite, they show incipientenrichment in both Mn and Ta. In contrast, manganocolumbite to manganotantalite are typicalof the petalite- and lepidolite-subtype pegmatites. CGMs from spodumene-subtype pegmatitesare highly variable across the province, generally with extensive ranges of Ta/(Ta+Nb) andlocally with near-extreme enrichment in Mn; however, in some cases the CGM compositionsare virtually restricted to ferrocolumbite on one hand, or to manganotantalite on the other.Pegmatites of the albite type cluster at intermediate values of both Mn/(Mn+Fe) andTa/(Ta+Nb). With very few exceptions, individual and collective concentrations of Mg, Ca, Zn,Bi, Sb, As, Y, U, Sn, Zr, Ti and W are below 10% (at.); in individual pegmatite categories,these trace elements show variations restricted to +10x up to -10x their average contents inCGMs from beryl-columbite pegmatites. Eighty sets of unit-cell parameters on natural andheated CGMs were refined. They cluster at moderately disordered and moderately orderedvalues, with some intermediate data and none that would correspond to extreme order ordisorder. Disorder is positively correlated with trace-element content; however, all heat-treatedsamples developed a well-ordered structure. Geochemical indicators suggest that the petalite-,lepidolite-, and some of the spodumene-subtype pegmatites are most promising sources of Ta> Nb mineral concentrates; however, none of these pegmatites are sufficiently rich in theCGMs. Thus it is possible that some pegmatites of the spodumene and other categories maybe more enriched in Ta because of a higher content of Ta-poorer CGMs. This can be,however, established only by quantitative exploration of individual localities.