ORIGIN OF ALBITE PODS IN THE GEORDIE LAKE GABBRO, PORT COLDWELL ALKALINE COMPLEX, NORTHWESTERN ONTARIO - EVIDENCE FOR LATE-STAGE HYDROTHERMAL CU-PD MINERALIZATION

Disseminated sulfides and palladium minerals are spatially associated with albite pods in the Geordie Lake (GL) gabbro, located in the north -central part of the Port Coldwell alkaline complex, northwestern Onta rio. The pods range from less than a centimeter to meters across and c onsist predominantly of albite (Ab(95-99)) and minor amounts of hornbl ende, biotite and actinolite. The hornblende contains up to 2.5 wt.% F and crystallized prior to biotite and actinolite. The pods are typica lly surrounded by a zone, less than about 20 cm thick, of very coarse- grained GL gabbro. In these zones, olivine is strongly zoned, plagiocl ase is rimmed by albite-oligoclase, and the relative proportion of the minerals varies significantly. The abundances of Zr, Hf, Nb, Th, U an d REE in the albite pods are high relative to GL gabbro, and interelem ent ratios for the two rock types are equivalent. The data indicate th at albite pods represent pockets of fluid-enriched residual magma. The albite probably formed by a two-stage process. In step one, hornblend e and plagioclase crystallized from the residual magma. In step two, a hydrous fluid separated from the residual magma and interacted with t he plagioclase to form albite. This model is consistent with textural evidence, the high F content of hornblende, and the fractionation tren ds exhibited by the alkalis in the albite pods. Comparison with experi mental data indicates that the albite formed at a temperature below ab out 600 degrees C. The sulfides consist of chalcopyrite and bornite, a nd minor amounts of sphalerite, pyrite and galena. They are invariably intergrown with biotite +/- actinolite and were deposited after the f ormation of albite. The close spatial association of biotite, actinoli te, sulfides, palladium minerals and albite implies that they formed f rom a single fluid, but at different times as the temperature decrease d and the composition of the fluid evolved. The fluid probably was der ived from the highly evolved magma that formed the albite pods.