Geochronology and characteristics of Ni-Cu-(PGE) mineralization at Rožany, Lusatian Granitoid Complex, Czech Republic
Abstrakt
The Ni-Cu-(PGE) mineralization at Rožany is located in the northern part of the Bohemian Massif and it is hosted by gabbroic dykes (dolerites) cross-cutting the granitic Lusatian Granitoid Complex. The intrusion of dolerite is newly well constrained by in-situ LA-ICP-MS U-Pb zircon age of 349 +- 3 Ma (2σ), which is much younger than previous K-Ar and Pb-Pb data.
The barren dolerites are characterized by high enrichments of some incompatible elements (e.g., LREE, Zr, Sr) and radiogenic Os isotopic composition suggesting their derivation from enriched (metasomatized) mantle source, possibly due to subduction processes. The mineralization itself is represented by pyrrhotite, chalcopyrite and pentlandite; the latter is commonly replaced by violarite most likely due to late-stage hydrothermal alteration.
The origin and formation of at least some platinum-group minerals was probably closely associated with late-stage hydrothermal processes, causing remobilization of platinum-group elements from the primary base-metal sulphides. The Ni-rich ores exhibit the highest I-PGE (Os, Ir, Ru) concentrations whereas Pd and Pt tend to be enriched in Cu-rich ores.
Observed variations in Re-Os isotopic compositions of the massive and disseminated ore types from the Rožany expressed by highly variable initial (349 Ma) γOs values of +50 to +134 in part indicates important, but variable amounts of incorporated crustal material. However, very similar 187Re/188Os values found in all rock types can be best explained by post-crystallization Re loss associated with late-stage hydrothermal alteration.
When compared to other Ni-Cu-(PGE) mineralizations in the Bohemian Massif, the total PGE contents found in Rožany are much lower than that reported from the Ransko Massif, but similar to those in the Svitavy ultramafic Complex.