Formation of Carboniferous Pb-Zn and barite mineralization from basin-derived fluids, Nova Scotia, Canada

Abstract

Lead-zinc and barite deposits in central Nova Scotia generally occur at the Tournaisian-Visean (Mississippian) clastic carbonate-evaporite disconformity overlying Cambrian to Devonian metasedimentary and granitic rocks. The deposits are located at the southern margin of the > 10-km-deep Carboniferous Maritimes (Fundy-Magdalen) basin.Fluid inclusion homogenization temperatures of ore-stage to postore minerals range from 230 degrees to 130 degrees C and fluid salinity is > or = 24 equiv wt percent NaCl. A mineralization age of ca. 300 to 330 Ma is suggested by: (1) fission-track dates on zircon from clastic host rock at Gays River, (2) K-Ar dates on clays from altered clastic rock in close proximity to mineralization, and (3) Rb-Sr data on illite from samples collected over a 10-m section of Tournaisian sandstone (far away from mineralization). Lead and strontium isotope data indicate that Tournaisian rock of the Horton Group could have been a source (at ca. 300 Ma) of lead and radiogenic strontium in the ore-stage minerals at the larger deposits. Visean carbonate carbon and sulfate were the dominant sources of carbon and sulfur, respectively. Hydrogen and oxygen isotope data suggest that the mineralizing fluids were basinal brines. Nonbiogenic sulfate reduction at the deposits is the likely sulfide precipitation mechanism.The ore-forming fluids may have originated at a >5-km depth under Visean evaporites in the Magdalen basin and migrated rapidly through Tournaisian clastic rocks and the immediately overlying Pembroke breccia to the basin margins. A pulse of regional tectonism may have set off at least one massive, hydrofracturing, compaction-driven, fluid expulsion event by about 300 Ma.