| Abstract | The basement rocks along the P2 fault are extensively altered, particularly where they host the McArthur River Zone 2 uranium ore body. Two generations of tourmaline occur along the P2: i) early,
euhedral-subhedral, coarse-grain (>0.5 mm), 1-2 cm wide veins and isolated grains of dravite (Mgtourmaline), and ii) later fine-grain (<0.2 mm), radial magnesiofoitite (alkali-deficient dravite) forms veinlets (< 2
mm), overgrowths on earlier
dravite, and grains disseminated within fine-grain illite. Fe-clinochlore, coarse-grain illite, rutile and hematite are ubiquitous along the P2 and occur as pervasive replacement minerals or veins, and post-date dravite crystallization. To ensure
minerals were free of inclusions, individual grains were first inspected with BSE-SEM at highest magnification and elemental peaks were then carefully monitored during trace element analysis (LA-ICPMS). Dravite
[(square0.4Na0.6)(square0.2Mg1.9Fe0.5Ca0.2Ti0.2)(Al5.9Fe0.1)(Si5.7 Al0.3O18)(BO3)3(OH3.8F0.2)] contains 1.24 (± 0.09, 1&) wt% TiO2, 89 - 280 ppm Zn, 51 - 630 ppm Cr, 190 - 1500 ppm V, and atomic F/Cl ratios range 98 - 11000. Magnesiofoitite
[(square0.7K0.1Na0.2)(square0.4Fe0.1Mg2.0Al0.5)Al6(Al0.1Si5.9O18)(BO3)3(F0.02O H3.98)] contains 65 - 260 ppm V, 2.9 - 110 ppm Cr, 0.2 - 3.7 ppm U, and 0.2 - 34 ppm Th, and ranges 3.2 - 80 atomic F/Cl. Dravite and magnesiofoitite contain low Li (< 12
ppm) and high Ni (1 - 28, 13 - 250 ppm, respectively); however, they also show contrasting trace element behaviours: dravite is enriched in LREE relative to HREE ([Ce]N/[Ce]*N) > 1, and has a positive Eu anomaly, whereas, magnesiofoitite is enriched
in HREE relative to LREE ([Ce]N/[Ce]*N) < 1, and has a negative Eu anomaly. Chlorite [(Fe1.9Mg2.6Al1.4)(Si2.7 Al1.3O10)(OH)8] contains significant Li (40 - 669 ppm), and Mn (803 - 4083 ppm); illite [(K0.9) (Al1.8Mg0.1Fe0.1)(Si3.2Al 0.8O10)(OH)2]
contains significant B (17 - 250 ppm), Li (<4.9 - 144 ppm), Ti (36 - 14500 ppm), Rb (343 - 692 ppm), U (<0.01 - 0.6 ppm), Sn (1.2 - 148 ppm), and Ba (78 - 1670 ppm); and both minerals show atomic F/Cl ratios > 10. High F/Cl, U, Th and B, and a
negative Eu anomaly in alteration phases suggests a contribution of pegmatite to the fluid. |