PALYGORSKITE FROM FRACTURE-ZONES IN THE EYE-DASHWA LAKES GRANITIC PLUTON, ATIKOKAN, ONTARIO

The mode of occurrence, X-ray, chemical, uranium series and stable iso tope characteristics of palygorskite found in fracture zones to a dept h of 25 m in the Eye-Dashwa Lakes granitic pluton, located near Atikok an, Ontario, are documented. The palygorskite formed as a product of a lteration of epidote according to the reaction: epidote + chlorite + w ater --> palygorskite + goethite +/- smectite-group mineral. The palyg orskite has d values of 10.48 to 2.28 angstrom, is orthorhombic (Pbmn) , and is compositionally uniform, with cation values of Si 4.07 to 4.1 5, Mg 0.62 to 0.68 and Al 1.05 to 1.17, based on 11 atoms of oxygen, a nd minor Fe. Ca and K contents. There are significant abundances of Sr (557 mug/g), U (10 mug/g) and Th (20 mug/g), inherited from the paren t epidote. U is preferentially incorporated into palygorskite, whereas Th is incorporated into goethite. It is fractionated in light rare-ea rth elements (SIGMALREE = 1974; La(N)/Yb(N) = 142) inherited from the parental epidote (SIGMALREE = 2844; La(N)/Yb(N) = 56). Most of the ana lyzed samples of palygorskite from greater depths have U-234/U-238 rat ios close to unity, indicating secular equilibrium, whereas palygorski te samples located closer to the surface have Th-230/U-234 ratios grea ter than one, suggesting geochemical disturbance in recent times (<0.3 5 Ma). Stable isotope characteristics (deltaD between -75 and -28 part s per thousand deltaO-18 between 18.4 and 23.4 parts per thousand) ind icate that palygorskite formed in equilibrium with modern groundwaters at temperatures less-than-or-equal-to 25-degrees-C, but in some cases , it experienced selective depletion in D accompanied by loss of U.