Zircon Alteration in Wall Rock of Pamour and Hoyle Pond Au Deposits,Abitibi Greenstone Belt: Constraints on Timescales of Fluid Flow from Depth-Profiling Techniques

摘要

Quartz-carbonate-Au vein deposits in the Timmins-Porcupine gold camp, Abitibi greenstone belt, have equivocal protracted parageneses and formed during late to post-Kenoran (2750-2670 Ma) deformation and metamorphism. Secondary ion mass spectrometry (SIMS) U-Pb, δ~(18)O, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) rare earth element (REE) depth-profiling techniques were employed on unpolished wall-rock zircons to resolve temporal constraints on mineralization and hydrothermal fluid sources at Hoyle Pond and Pamour Mines. Depth-profiling techniques successfully uncovered <3-μm alteration domains in zircon rims based on Th/U, REE, and isotopic signatures, and rim ~(207)Pb/~(206)Pb ages are significantly younger than host-rock ages. Zircon alteration is from a complex interplay of recrystallization and localized dissolution-reprecipitation of the crystals during hydrothermal fluid flow. Alteration of Hoyle Pond and Pamour zircons at 2660 Ma correlates to intensely mineralized and deformed quartz-carbonate-Au shear veins, and this age represents peak fluid infiltration at the mines, coinciding with peak regional metamorphism Subsequent zircon alteration Correlating to thin, shallow-dipping and less altered or mineralized vein networks occurred at ca. 2640 Ma, and this age likely represents a late hydrothermal fluid pulse at the end of regional retrograde metamorphism. Protracted hydrothermal alteration along discrete zones within the camp continued d into the Proterozoic. Zircon alteration occurs as light REE-enriched "rims," and Th/U and ~(18)O values of alitera tion domains suggest that the source had a limited crustal recycling component. Whereas the underutilized depth-profiling technique has resolved –200 m.y. of continuous crustal evolution and fluid flow in the southern Abitibi, further investigation of the southern Abitibi is required to clarify the genetic relationship between ore mineralization and zircon alteration.