Paper No. 61-6
Presentation Time: 2:15 PM
FURTHER RE-OS ARSENOPYRITE GEOCHRONOLOGY FROM SELECTED MEGUMA AU DEPOSITS, MEGUMA TERRANE, NOVA SCOTIA: POSSIBLE EVIDENCE FOR A PROTRACTED GOLD-FORMING SYSTEM
The Meguma terrane is dominated by two rock types, regionally deformed and metamorphosed Cambro-Ordovician metasedimentary rocks and ca. 380-370 Ma meta- to peraluminous granites. The metasedimentary rocks contain numerous orogenic-type, vein-hosted gold deposits which occur throughout the sandstone-dominated Goldenville Group rather than the overlying siltstone- and mudrock-dominated Halifax Group. These mineralized veins are dominated by quartz-carbonate-sulfide assemblages and occupy structures consistent with emplacement during late-stage fold tightening of the regional, northeast-trending, upright folds formed during Neoacadian deformation at ca. 410-385 Ma. Our previous work suggests vein formation, hence gold mineralization, spanned 30-40 Ma, as constrained from field observations and radiometric dating. The former indicates veins post-date cleavage formation given that cleaved wall-rock fragments occur in some veins and that rarely veins post-date hornfels related to 380 Ma granites. Existing absolute age dates indicates two events at 408 Ma (Re-Os Aspy, 40Ar/39Ar wr) and 380-362 Ma (Re-Os Aspy; 40Ar/39Ar am, ms, bi, wr). Here we report new Re-Os geochronological data generated from arsenopyrite in gold-rich veins for two deposits sampled, both of which lie in the same stratigraphic-structural position in the lower part of the Goldenville Group. The Re-Os analysis of arsenopyrite from three veins from the Beaver Dam deposit, which is dominated by bedding-concordant type veins, yielded isochron ages of ca. 461 Ma, 459 Ma and 440 Ma. In contrast, Re-Os analysis of one vein-hosted arsenopyrite sample from the Touquoy deposit of the Moose River gold district, where mineralization occurs in a vein-poor sequence of carbonate-rich metasiltstone rocks, yielded two isochron ages of ca. 380 Ma and 438 Ma; one other sample from the same deposit provided Re-Os model ages from ca. 400 Ma to 440 Ma. In addition, obvious zoning of the arsenopyrite is noted from both SEM and LA ICP-MS trace element mapping, which also indicates multiple generations for this mineral. This new data suggests that in the Meguma terrane several stages of gold mineralization exist, the first pre-dating the Neoacadian deformation and metamorphism of the host rocks.