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 Ontario. The pods range from less than a centimeter to meters across and consist predominantly of albite (Ab(95-99)) and minor amounts of hornblende, biotite and actinolite. The hornblende contains up to 2.5 wt.% F and crystallized prior to biotite and actinolite. The pods are typically surrounded by a zone, less than about 20 cm thick, of very coarse-grained GL gabbro. In these zones, olivine is strongly zoned, plagioclase is rimmed by albite-oligoclase, and the relative proportion of the minerals varies significantly. The abundances of Zr, Hf, Nb, Th, U and REE in the albite pods are high relative to GL gabbro, and interelement ratios for the two rock types are equivalent. The data indicate that albite pods represent pockets of fluid-enriched residual magma. The albite probably formed by a two-stage process. In step one, hornblende and plagioclase crystallized from the residual magma. In step two, a hydrous fluid separated from the residual magma and interacted with the plagioclase to form albite. This model is consistent with textural evidence, the high F content of hornblende, and the fractionation trends exhibited by the alkalis in the albite pods. Comparison with experimental data indicates that the albite formed at a temperature below about 600 degrees C. The sulfides consist of chalcopyrite and bornite, and minor amounts of sphalerite, pyrite and galena. They are invariably intergrown with biotite +/- actinolite and were deposited after the formation of albite. The close spatial association of biotite, actinolite, sulfides, palladium minerals and albite implies that they formed from a single fluid, but at different times as the temperature decreased and the composition of the fluid evolved. The fluid probably was derived from the highly evolved magma that formed the albite pods.