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Abstract:

The Copper Lake area of mainland Nova Scotia is one of several vein-controlled mineralized (Cu-Au-Co) systems associated with widespread carbonate and iron-oxide alteration proximal to the east-trending Cobequid-Chedabucto Fault System. Although this mineralization has been known for decades, its metallogenic affinity remains poorly defined, and in recent years an IOCG (iron oxide-copper-gold) model has been suggested. In order to determine the age of mineralization and provide an important time constraint for developing a metallogenic model, direct dating of the mineralization and associated alteration was undertaken. At Copper Lake, mineralization occurs in a set of sulphide-carbonate fissure veins hosted by fine-grained metasedimentary rocks of the Middle Devonian Guysborough Group. Dating of the sulphide-alteration (pyrite) and phyllic-alteration (muscovite) stages of the ore system utilized the Re-Os and 40Ar/39Ar methods, respectively. The two different chronometers yield ages of about 320 Ma and provide an absolute age for the mineralization. As part of this study additional geochronological data were obtained for detrital zircon (U-Pb age of 1634 ± 11.2 Ma) from the host sedimentary rocks, as well as timing of thermal events at ca. 370–380 Ma, 350 Ma and < 300 Ma based on whole rock 40Ar/39Ar and chemical Th-Pb dating of host rocks and monazite. The Th-Pb dating of monazite indicates that rare-earth element mobility accompanied mineralization. Collectively, the data indicate that the area experienced multiple thermal events, but hydrothermal activity related to mineralization is constrained to about 320 Ma and is tentatively interpreted to relate to structural focusing of fluids that may have been driven by a mid-crustal level mafic heat source. The mineralizing event coincides with regional Alleghanian deformation in this part of the Appalachian orogen and thus reflects larger-scale tectonothermal processes.