U-PB & STABLE ISOTOPE STUDY OF ZIRCONS: FLUID SOURCES, TIMESCALES, & CRUSTAL IMPLICATIONS FROM THE PORCUPINE GOLD CAMP IN THE ABITIBI GRANITE GREENSTONE BELT, SUPERIOR PROVINCE

The Abitibi granite-greenstone belt was accreted during the Kenoran orogeny (2750-2670 Ma), with associated intrusions having crystallized from primitive magmas with mantle-like zircon d18O signatures of ~5.8‰ (King et al. 1998). The Porcupine gold camp, a world-class quartz-carbonate vein deposit proximal to the Porcupine Destor Deformation Zone, was generated during late to post-Kenoran deformation and metamorphism. In an attempt to constrain mineralization and determine fluid sources at Hoyle Pond and Pamour mines (separated by 2 km), d18O and U-Pb isotopic analysis was conducted on unpolished zircon crystals using SIMS U-Pb depth-profiling with UCLA's Cameca ims 1270 ion microprobe. Hoyle Pond zircons have TIMS U-Pb crystallization ages of 2684 ± 2 Ma and 2688 ± 2 Ma for a quartz-albite porphyry and a quartz-porphyritic sericite schist, respectively (Ayer et al. 2005). From concordant zircon analyses of the same suite of rocks, we obtained intercept ages of 2640 ± 6 Ma (porphyry; MSWD: 4.0) and 2628 ± 6 Ma (schist; MSWD: 3.3). Zircons in a mineralized syn-tectonic metagreywacke from Pamour yielded two age populations at 2675 ± 2.5 Ma (MSWD: 1.3) and 2641 ± 2.5 (MSWD: 0.9). Bimodality of the ages at Pamour is likely from patchy, thin (<5 µm) zircon rims with short-lived growth at relatively low temperatures. The absence of core ages in the SIMS data at Hoyle Pond suggests thicker rims; zircon imaging has revealed complex rim and core geometries. O-isotope analysis on the same rims yielded a Hoyle Pond d18O-value of 5.33 ± 0.4‰ (n: 19) and a slightly heavier signature of 6.03 ± 0.4‰ (n: 18) at Pamour where rims may have undergone isotopic exchange with the higher 18O/16O metagreywacke host rock during growth. Combined data suggests rim growth at ca. 2637 Ma, which is consistent with the lower limits of the camp's Au mineralization and is slightly younger then regional retrograde metamorphism (2677-2643 Ma). Further, rim d18O-values are 3-5‰ lighter than similar Superior Province metamorphic zircon overgrowths (Moser et al. 2008). Thus, rims likely grew from hydrothermal fluids with mantle-like magmatic d18O-values during Au mineralization. These results reveal that lower to mid-crustal plutonism ~30 m.y. after Abitibi accretion probably supplied the necessary fluid source for genesis of the world's largest Archean lode gold camp.