LIDDICOATITE AND ASSOCIATED SPECIES FROM THE MCCOMBE SPODUMENE-SUBTYPE RARE-ELEMENT GRANITIC PEGMATITE, NORTHWESTERN ONTARIO, CANADA

摘要

Liddicoatite, a rare species of tourmaline, has been found in the McCombe pegmatite, of complex type and spodumene subtype, an example of the LCT (lithium, cesium, tantalum) family. It is one of a group of beryl- and complex-type pegmatites exposed along the Uchi–English River subprovincial boundary-zone, Ontario, in the Superior Province of the Canadian Shield. The mineral assemblage containing zoned "fluor-elbaite"–liddicoatite crystals was the last product of primary, near-solidus crystallization of the pegmatite and is unique not only to this subprovincial boundary, but also throughout the vast number of rare-element pegmatites distributed across Ontario. The most calcic composition of liddicoatite analyzed here corresponds to (Ca_0.5Na_0.40.1) (Li_1.5Al_1.2Mn_0.2Fe_0.1) Al₆(Si₆O₁₈) (BO₃)₃ (OH_3.4F_0.6). Other calcium-enriched species found in the McCombe pegmatite include microlite (Ca_1.2Na_0.60.2)(Ta_1.6Nb_0.3Sn_0.1)O₆(F_0.7OH_0.3), Ca-enriched almandine (Alm_44.6Sps_26.4Grs_26.2), Ca-enriched spessartine (Sps_69.3Alm_19.8Grs_10.9), and fluorapatite. Evidence from progressively evolved units of the pegmatite and mineral zonation shows that contamination of the pegmatite-forming melt occurred, and that fluids from the pegmatite migrated out into the mafic metavolcanic host-rocks, creating a Li-, Cs-, and B-enriched metasomatic aureole. The evolution of the pegmatite-forming melt resulted in increased activity of fluorine, and the essentially in situ fractionation of the contaminated melt led to the crystallization of the rare-element-enriched minerals. Patterns of zonation seen in back-scattered-electron images show that primary crystallization of columbite–tantalite, garnet and tourmaline species involved multiple episodes of growth and dissolution in a dynamic system. Liddicoatite was able to form, not because of any one of these processes, but because they all acted together to concentrate both Li and Ca, a sequence of events that is apparently very rare in nature.