EPISODIC FLUID INFILTRATION AND GENESIS OF THE PROTEROZOIC MACLELLAN AU-AG DEPOSIT, LYNN LAKE GREENSTONE-BELT, MANITOBA

The stratiform MacLellan Au-Ag deposit occurs within a sequence of amp hibolite-grade metavolcanic and metasedimentary rocks of the Lower Pro terozoic Lynn Lake greenstone belt in the Trans-Hudson orogen of Manit oba, Canada. The deposit has previously been interpreted as syngenetic with remobilization of Au and Ag into veins during deformation. Sedim entary, syngenetic sulfides occur within the sequence but contain low concentrations of Au and Ag. Field, textural, mineralogical and geoche mical data indicate that the MacLellan deposit is epigenetic and has f ormed as a result of multiple, temporally distinct stages of hydrother mal fluid infiltration over a protracted time period. The host sequenc e has been metamorphosed to lower to middle amphibolite facies as indi cated by the presence of garnet, staurolite and kyanite and is dominat ed by chlorite-hornblende (metavolcanic) and biotite-plagioclase (meta pyroclastic?) schists. An early phase of Au-Ag mineralization is repre sented by deformed, pre-peak metamorphic quartz-biotite-sulfide veins. A second phase of Au mineralization is associated with syn-metamorphi c, arsenopyrite-quartz replacement veins. In both of the above veins s ets, most of the Au is probably in iron sulfides and arsenopyrite. The host rocks to the arsenopyrite-quartz veins contain lineated silliman ite and abundant, coarse staurolite which are interpreted to represent a localized fluid-induced elevation of the metamorphic grade in the v icinity of the veins and temperatures in excess of 510 degrees C. Post -metamorphic, late-kinematic quartz-amphibole veins and associated amp hibole alteration are abundant but are not temporally associated with precious metal mineralization. Late-stage disseminated and vein sulfid e mineralization is common within the quartz-amphibole veins and schis ts and contains Au and Ag. Large quartz +/- sulfide veins, which are a ssociated with late fault zones, are also mineralized where they cut e arlier phases of mineralization. Shear zones occur within the deposit but do not appear to be genetically related to any vein or mineralizin g event. They post-date metamorphism and are characterized by chlorite and muscovite alteration of metamorphic assemblages, but pre-date the quartz-amphibole vein stage. The stratiform nature of the MacLellan d eposit does not reflect syngenetic-exhalative deposition. Rather, the Au-Ag mineralization in the deposits is epigenetic and the quasi-confo rmable nature of the ore bodies reflects veins and replacement zones, whose geometry was controlled by the metamorphic fabric within the hos t rocks and/or subsequent transposition of veins parallel to the regio nal fabric. The timing (syn-middle amphibolite metamorphism) and high temperatures of the arsenopyrite stage of mineralization rule out gree nschist-amphibolite dehydration as a fluid source for this stage and r equire a deep-crustal or magmatic fluid source.