A STUDY OF SHEAR ZONES, MESOTHERMAL GOLD, AND P-T-t-FLUID EVOLUTION IN THE FLIN FLON DOMAIN
Fedorowich, John Samuel
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Shear zones exposed in the Flin Flon area record five episodes of development, reflecting deformation at various crustal levels during sequential collisions between the Archean Rae-Hearne and Archean Superior plates throughout the Trans-Hudson Orogeny (-2.0 to 1.7 Ga). The sequence encompasses a transition from island-arc supracrustal development (-1.9 Ga) to tectonic mountain building under brittle-ductile conditions (>1.86 Ga), two episodes of ductile L-S shear zones on the prograde metamorphic path (1.86-1.81 Ga), and two episodes of retrograde displacement at 1.79-1.77 and 1.69 Ga, respectively, during suturing of the Archean cratons with Paleoproterozoic assemblages. Metamorphism evolved in two stages. Contact aureoles to granitoid plutons (1.86-1.84 Ga)and related intrusive breccias record hornblende hornfels facies,with hornblende 40Ar/39Ar ages indicative of cooling through 500° Cat -1.84 Ga, whereas pervasive peak metamorphic recrystallization occurred on a regional scale -20 Ma later, at greenschist to amphibolite conditions. The subsequent tectono-thermal history for the Flin Flon Domain can been divided into three general periods, based on 40Ar/39Ar thermochronology: thrust-stacking and plutonism with cooling rates of 5°C/Ma between 1860-1820 Ma; regional peak metamorphic heating at about 1820-1790 Ma, and transpressional unroofingat 2-4°C/Ma between 1790-1690 Ma. Gold mineralization is concentrated at dilational jogs where metamorphic fluid discharge was focussed coeval with retrograde transpression and shear zone reativation at 1.79-1.77 Ga. Ore fluids were H20-C02-NaCIin composition, with modal salinity of 0.64 g/cm", pHof -4-5, f02of -31, 8180 5.2-6.3°/00, and 8D -30 to -86°/00, consistent with advecting metamorphic fluids, along the retrograde portion of a clockwise P-T-t path. The lodes formed at temperatures of 320-470° C and at a pressure of 1.2 to 2.4 kbars, corresponding to 4.3-8.6 km, assuming lithostatic fluid pressure. A new method of REE and trace element analysis by laser sampling ICP-MS was developed, and applied to international whole-rock reference materials, and gabbroic bodies in the Flin Flon Domain. The REE patterns of the gabbros are similar to proximal basaltic flows, indicating possible comagmatic origin,endorsed by 1.89 Ga zircon 207Pbfo6Pb ages obtained for the gabbroic rocks, essentially the same as the age of the volcanic rocks.