GEOCHEMISTRY OF WEATHERED HORIZONS WITHIN THE SOUTH MOUNTAIN BATHOLITH, NOVA SCOTIA, CANADA: A WINDOW INTO THE EARLY STAGES OF METAL MIGRATION INTO THE ENVIRONMENT

The process of weathering is critical to the release of metals into the environment. In Devonian granitoid rocks of the South Mountain Batholith of Nova Scotia, Canada, weathering processes at different times in the geologic past have brought about chemical and physical modification of the granitoids at a number of localities, and resulted in the concomitant release of metals into the environment. Relatively intense weathering events occurred at three different times in the past, and a record of their geochemical effects is recorded in the rocks. Drill core through Carboniferous sediments into the granitoid, reveal the existence of an intense weathering event which resulted in the formation of a saprolite composed almost exclusively of clay minerals and quartz; little remains of the original metal contents of the granitoid. In pre-Jurassic times, a weathering event resulted in the formation of a saprolite with a dominant clay-quartz mineralogy, preserved today beneath Jurassic sediments. Unlike the pre-Carboniferous weathering event, the nature of the weathering during this time preserved relatively intact biotite, indicating that most of the metals released during this event were the alkali metals. Of most significance to the release of metals into the environment today, is the weathering event of pre-Pleistocene age, identified at a number of surface localities throughout the batholith. Glacial erosion has exposed the deeper levels of these horizons, where mineralogical and chemical changes are incipient, in spite of the much-reduced physical integrity of the rock. Comparative analysis of fresh and weathered samples from surface outcrops as well as from sub-Jurassic drill core, suggest that in addition to other elements, U, and hence Rn, as well as Hg, are mobilized during the weathering process. Given that the mineralogical evidence indicates disequilibrium, the minerals in these physically weakened, permeable materials are primed for continued reaction in the moist, oxidizing conditions common in Nova Scotia: conditions of particular significance in the continued release and migration of elements such as U and Hg, whose mobilities depends upon whether the environment is reducing or oxidizing.