Gold deposition in the western Abitibi greenstone belt and its relation to regional metamorphism.

Abstract

Native metals contain primary fluid inclusions that provide information on the P-T-X conditions associated with the deposition of native gold, silver, copper and platinum. Insights gained from gold-hosted inclusions include that some mesothermal gold deposits formed at pressures &le;0.4 kbars. The chemical and isotopic compositions of inclusion fluids extracted from intergrown Pamour Mine gold, quartz and sphalerite also indicate they precipitated from one fluid. This observation validates using gangue-hosted fluid inclusions to determine P-T-X characteristics of the gold-bearing fluid. Veins in the Pamour Mine can be divided into shallow-dipping (<45&deg;) and steep-dipping (>45&deg;) types because native gold is associated with different vein structures, width, and gangue minerals in each. Gold in steep-dipping veins occurs in sheeted sections where concentrations in the thinnest part of a sheet are 5 to 1000 times greater than in wider areas. Increased H₂O content and CO₂:CH₄ ratios of quartz-hosted inclusion fluids that accompany gold indicate it was deposited by phase separation. The predominantly water-rich inclusions also suggest that phase separation was initiated by fluid turbulence at constrictions in hydrothermal conduits. This mechanism may account for the great vertical extent of mesothermal gold deposits. Gold along the hanging wall of shallow-dipping veins and a related decrease in CO₂:CH₄ ratios of quartz-hosted fluids also indicate that gold was deposited when fluids reacted with country rocks. <super>4</super>OAr/<super>39</super>Ar ages obtained from muscovite in gold-bearing veins of the Dome, Bell Creek and Pamour Mines indicate that most Porcupine camp gold deposits formed between 2550 and 2560 Ma. <super> 40</super>Ar/<super>39</super>Ar ages of 2550 to 2570 Ma, and 2613 +/- 19 Ma from the western Abitibi greenstone belt and Quetico-Opatica Subprovince suggest the Porcupine camp was affected by two periods of metamorphism. The younger of these may be related to formation of the gold deposits. Post-mineralization emplacement of Matachewan and Hearst diabase dikes reset country rock ages within two-dike widths of a dike but did not perturb the timing of gold deposition in 95% of the Abitibi greenstone belt. Combining a battery of modern geochemical techniques with careful field observations has resulted in a deeper understanding of gold deposit formation throughout the world.