Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Re-Os geochemistry and geochronology of the Ransko gabbro-peridotite massif, Bohemian Massif, Czech Republic
Abstract
The Ransko gabbro-peridotite massif is a strongly differentiated intrusive complex which hosts low grade Ni-Cu ores. These are mainly developed in troctolites close to the contact of olivine-rich rocks with gabbros, and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase-rich peridotites. Gabbro, troctolite, peridotite and Ni-Cu ore were analyzed for Re-Os concentrations and Re-Os isotopic ratios. Barren gabbro and troctolite have very low Os concentrations (0.02 to 0.1 ppb), high Re contents (0.6 to 2.4 ppb) and corresponding high Re/Os ratios. Ni-Cu mineralized rocks and massive ores have high Re (from 1.67 to 121.2 ppb) and Os (0.81 to 206.8 ppb) and low Re/Os ratios 187 188 (0.5 az 5.1). The Os/ Os ratios show variable contribution of mantle- and crustal-derived 187Os in Ni-Cu mineralization. Four selected samples with similar Os model ages suggest closed Re-Os system after emplacement of Ni-Cu mineralization. These samples yield a Re-Os age of 505 ± 52 Ma confirming Cambrian-Ordovician age ofthe Ransko gabbro-peridotite massif.
