Granite Geochemistry

Canadian Rare-Element Pegmatites

A.G. Tindle and F. W. Breaks #

# Ontario Geological Survey, Precambiran Geoscience Centre, 933 Ramsey Lake Road, Sudbury, Ontario P3E 6B5, Canada

Work is currently in progress in characterizing two rare-element pegmatite fields located in the Canadian Shield of N.W. Ontario. The areas are of particular interest because the pegmatites are spatially linked to fertile (and probably parental) granite plutons. To date most emphasis has been on the Separation Rapids pegmatites (north of Kenora) where rare-element mineralization is present in beryl- and petalite-bearing pegmatites and occurs mainly in the form of columbite-tantalite and cassiterite mineralization. Associated tantalum minerals include microlite ((Na,Ca)2Ta2O6(O,OH,F)), wodginite (MnSnTa2O8) and ferrowodginite (FeSnTa2O8) - the latter two minerals being first occurrences for Ontario. The second area is located much further north, beyond the town of Red Lake along the shores of Pakeagama Lake. Here manganotantalite mineralization is most commonly encountered within a zoned petalite (now spodumene), beryl, fluorapatite-rich suite of pegmatites. The pegmatites also contain high concentrations of Cs and Rb, in the form of potassium feldspar, lepidolite and pollucite.

Globally, petalite (LiAlSi4O10) pegmatites are quite rare, found only in 2% of all lithium-rich rare-element pegmatites. They are, however, economically important because petalite is an important feed-stock in the ceramics and glassware industry and because they host the world's major resources of tantalum and cesium. Elsewhere in the world, petalite-bearing pegmatites include the "giant" rare-element pegmatites of Tanco, Manitoba and Bikita, Zimbabwe. In the Separation Rapids area, the Big Whopper and Big Mack pegmatites together probably represent the world's second largest petalite deposit. There is also significant potential for economic Ta, Cs and Rb deposits to be found.

The project involves a detailed mineralogical and geochemical study of the pegmatites to establish paragenetic sequences and will be integrated with additional studies on associated fertile granites. This work will give an insight into the relative roles of source lithology, magmatic fractionation and tectonic setting in the origin of these rare but economically important assemblages. This project will test the hypothesis that various types of rare-element pegmatites and their associated fertile granites can be correlated with syn-, late- and post-orogenic events within the Superior craton.

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Radioelement distribution in the Tertiary Lundy granite (Bristol Channel, U.K.)

Richard S. Thorpe, Andrew G. Tindle and Olwen Williams-Thorpe

The radioelement distribution and content of the Lundy granite, a coarse-grained megacrystic granite of Tertiary age, has been measured using a portable gamma-ray spectrometer. Results indicate a systematic variation of K, Th and U (with a few notable exceptions) that follows a partially concentric distribution to lower concentrations inland. The plateau region of the island (particularly the southern half) is relatively depleted in all radioelements. Over the island, measurements of K vary from 1.3-4.9 wt.%, Th varies from 5.0-20.3 ppm and U varies from 2.0-12.5 ppm.

A petrographic, microprobe and autoradiography examination of the granite indicates that the radioelements mainly reside in discrete major and accessory minerals, of which K-feldspar (K), biotite (K), monazite (Th), xenotime (U), tungsteniferous columbite (U) and uraninite (U) are the most important. Uraninite is rare, being preserved only in fresh samples which come mainly from quarries. Mass balance modelling indicates that up to 70 percent of uranium could reside in uraninite and where this has been leached by secondary processes such as hydrothermal alteration or weathering then the present radioelement content no longer reflects the original rock composition. Fission track evidence is presented to show the pathways along which uranium has been mobilized from the granite. Secondary sites of radioelements include fractures cross-cutting all major minerals (but especially quartz), grain boundaries, the altered cores of plagioclase feldspar and occasionally the yellowy brown mixed chlorite/smectite replacement product after biotite. Biotite itself may exhibit secondary tracks along cleavage traces.

Combined effects of crystal fractionation (primary variation) and secondary alteration best explain the distribution of radioelements, with K controlled by fractionation of the major phases K-feldspar and biotite, Th by fractionation of the accessory mineral monazite (± xenotime and uraninite) and U contents by uraninite and tungsteniferous columbite. Secondary processes have removed much of the uraninite leaving behind indeterminate Fe-U material along fractures and residual U (and Th) enrichment within altered major minerals. There is some evidence to suggest that late radioelement-bearing fluids precipitated monazite and uraniferous zircon along fractures during the waning stages of magmatic activity.

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