Preserved primary textures of the Pyhaesalmi Zn-Cu VMS deposit, Finland: Evidences of subsea-floor replacement and Svecofennian post-depositional sulfide mobilization

摘要

Pyhaesalmi is a Paleoproterozoic volcanogenic massive sulfide deposit located in the Main Sulfide Belt of Central Finland. Recent studies on the ore mineralogy of the highly deformed "Deep Orebody" in the Pyhaesalmi Zn-Cu VMS deposit reveal the preservation of many textures developed during the early crystallization history. These primary features include fine grained ore mineral assemblages, spotty carbonate segregations, hydraulic breccias, crustification textures and disseminated sulfides in the local relicts of the subaqueous pyroclastic host rocks. These textures, together with the abundant altered felsic inclusions, provide very good examples of hydrothermal infiltration and replacement processes occurring in a highly permeable tuff unit under the sea-floor during the Paleoproterozoic (c. 1.93―1.91 Ga) at this Primitive Arc Complex (PAC) of the Fennoscandian Shield. Polyphase tectonic deformation is recorded in the sulfide assemblages; syn-metamorphic mobilization has re-crystallized important amounts of chalcopyrite and sphalerite into cataclastic fractures of pyrite porphyro-blasts during D_2―D_3 (thrust - compressional deformation stages) at the peak of metamorphism. The latest phase is of a shearing nature (D_4), which enabled the remobilization of pyrrhotite, chalcopyrite, plus minor amounts of galena, fahlore minerals, arsenopyrite, anhydrite, quartz and precious metals into fractures within felsic porphyry inclusions and out of the system in more competent country rocks. Crackle and sulfide matrix breccias are common features associated with these structures.