Geology and 40 Ar/ 39 Ar geochronology of the Beaver Dam gold deposit, Meguma Terrane, Nova Scotia, Canada; evidence for mineralization at 370 Ma

Abstract

The Beaver Dam gold deposit, Nova Scotia, Canada, is a lode gold deposit hosted by lower Paleozoic metaturbidites of the Meguma Group; the host rocks were deformed and metamorphosed (greenschist facies) during the Acadian orogeny (dated by the 40 Ar/ 39 Ar method at 400 + or - 10 Ma). The deposit area is located about 2 km east of the River Lake intrusion, a two-mica granodiorite-monzogranite complex of ca. 35 km 2 , and is hosted by intercalated psammites and pelites which occupy the southern, overturned limb (dip 75 degrees N) of the easterly trending Beaver Dam anticline (F 1 ). The pelitic lithologies around the River Lake intrusion contain coarse-grained hornfels (andalusite-garnet-cordierite-staurolite) that overgrow the regional cleavage (S 1 ).The Beaver Dam deposit consists of three subparallel ore zones which contain voluminous amounts of quartz veins of dominantly bedding-concordant nature (lesser discordant vein types occur). The wall rocks contain a greenschist facies mineral assemblage, but in addition subrounded clots (oikocrysts) of coarse quartz-biotite + or - ilmenite + or - calcite + or - sphene + or - chlorite are present which are variably deformed and flattened with high aspect ratios; these may represent variably retrograded andalusite of contact metamorphic origin. Although a single fabric is present in most of the metasedimentary rocks, in the hinge zones of folds an overprint shear cleavage (S 2 ) is observed; this is the same fabric which has flattened the oikocrysts. The veins record two distinct paragenetic stages with an early quartz-calcic plagioclase-biotite-tourmaline (+ or - garnet + or - apatite + or - amphibole) assemblage succeeded by a quartz-calcite-chlorite-muscovite-albite (+ or - anatase) stage. Sulfides (including Bi-Te-Ag sulfides) and gold are predominantly late stage paragenetically. The simple paragenesis is consistent with stable isotope and fluid inclusion data which collectively suggest vein formation during a single hydrothermal event.The relative timing of vein formation is indicated from the following relationships: (1) microveinlets of quartz + calcite crosscut and retrograde the hornfels mineralogy related to the River Lake intrusion, (2) S 2 crenulates S 1 and is axial planar to the folded quartz veins, and (3) the fabric that flattens the oikocrysts is S 2 . Absolute timing of vein formation is inferred to be ca. 370 Ma based on 40 Ar/ 39 Ar age spectra for (1) biotite and muscovite from the River Lake intrusion; (2) muscovite from a pegmatite spatially related to the River Lake intrusion that crosscuts hornfelsic metasediments; and (3) hydrothermal muscovite, biotite, and amphibole from a variety of vein types. The ages are not considered to reflect updating related to the River Lake intrusion but instead to indicate that the vein-forming event postdated regional metamorphism by some 30 m.y. These results would, therefore, invalidate genetic models that related vein formation to regional metamorphism and deformation associated with the 400 + or - 10 Ma Acadian orogeny. Instead, the data support models that relate the mineralization with later thermal and tectonic activity associated with generation of the large granitoid batholiths and late shear zones developed within southern Nova Scotia.