Geochemical and petrogenetic features of mineralization in rare-element granitic pegmatites in the light of current research

Abstract

Tremendous expansion of high-technology applications of rare metals and specialty minerals requires periodic reassessment of their geological sources, keeping in mind the long-range technological potential of all hi-tech materials available from a given type of deposit. Granitic pegmatites are a classic example of diversified sources (Li, Rb, Cs, Be, Ga, Sc, Y, REE, Sn, Nb, Ta, U, Th, Zr, Hf; optical quartz and fluorite, high-purity feldpar, petatite and refractory spodumene, ceramic amblygonite,e tc.). Such rare-element pegmatites can be subdivided into three principal families: LCT pegmatites with Li, Rb, Cs Be, Ga, Sn, Ta > Nb (B, P, F); NYF pegmatites with Nb > Ta, Ti, Y, REE, Zr, Th, and U(F); pegmatites with a mixed geochemical signature. The less evolved members of the LCT family produce mainly only, Be, Nb > Ta and Sn, but exotic types with Be, Ta > Nb, F, B(P) with Li, Rb and Cs and are also known. So far, no differences ini mineralization styles have been observed between the spodumene, petalite and amblygonite subtypes of the complex type, except the mineralogy of their substantial Li contents. However, extensive divergence in K/Rb/Cs fractionation is observed, violating the classic criteria for, for example, pollucite potential, and economic Ga concentration is recognized. The lepidolite subtype is commonly enriched in Y and (H)REE. Albitespodumene pegmatites have largely simple Li (Sn, Nb ≷ Ta, Be) mineralization but a suprisingly broad range of alkali fractionation levels. The not so common albite type seems to be typically enriched in Sn (±Nb>Ta, Ti). The NYF family is much less abundant. A general subdivision of these pegmatites is not yet available, except for the recognition of sporadic LREE-enriched, and much more widespread Y, HREE-enriched subtypes. Scandium is an element recognized recently as much more abundant than previously thought. Internal evolution of individual pegmatites is better understood today in terms of London's model of crystallization from highly hydrous but homogeneous melts. Experimental evidence does not support the Jahns and Burnham proposal of early separation of a fluid phase. Peraluminous fertile (S-)granites generating LCT pegmatites are derived by partial melting of upper/middle crustal rocks undergoing their first anatexis, and the pegmatites commonly show regional zoning. Metaluminous granites yeilding poorly zoned groups of NYF pegmatites are largely of the A type, generated by second melting of short-lived, depleted lower-crustal protoliths. However, a juvenile igneous or fluid component may be involved. Mixed NYF > LCT granite-pegmatite suites may originate in several ways. Peraluminous LCT sequences were traditionally regarded as orogenic, in contrast to anorogenic NYF systems. Tectonic correlation with geochemistry is now considered secondary to the control by source lithologies. It is the understanding of the association of pegmatite types with their geological setting and petrogenetic affiliation which assists in the search for selected hi-tech materials.