POST-MIOCENE AND BRONZE-AGE SUPERGENE Cu-Pb ARSENATE - HUMATE-OXALATE - CARBONATE MINERALIZATION AT MEGA LIVADI, SERIFOS, GREECE

摘要

As a result of the Late Miocene skarn-type Fe mineralization at Mega Livadi on the Isle of Serifos, Greece, arsenical copper mineralization of hydrothermal origin formed in calcareous roof-rocks of a granodioritic intrusion. The hypogene mineralization, with pyrite, As-rich tennantite-group minerals and arsenopyrite, underwent strong alteration during the Pliocene and Quaternary by descending meteoric waters. We subdivide the geogenic and anthropogenic processes responsible for this alteration into five mineralizing stages, differing from each other by their element budget, Eh and pH. The supergene stages I to III comprise a variegated assemblage of oxides, hydroxides and arsenates: stage I (goethite, mixture of fine-grained Fe-As-Sb-Cu minerals, pharmacosiderite), stage II (Sb-bearing beudantite, REE-bearing beudantite), stage III (arseniosiderite). The anthropogenic stages IV and V are characterized by a set of hitherto unknown K-Cu humates and oxalates (stage IV) and the common Cu carbonates malachite and azurite (stage V). Chemical weathering induced by the (sub)tropical climatic conditions during the Late Miocene and Pliocene was responsible for the supergene associations of stages I through III at Mega Livadi. In the course of this alteration, the content of Fe decreased and As increased. Anomalously high contents of LREE in beudantite are used as a "minerostrati-graphic" tracer to correlate the mineralization of stage II with a phase of pervasive formation of laterite in the Aegean region during the Pliocene. After a hiatus, an anthropogenic episode of mineralization was triggered by ancient mining activities during the early Bronze Age, around 3325 to 2890 BC, when miners began exploiting the soft arsenical Fe and Cu ore at shallow depth. This mineralization evolved under more temperate climatic conditions as a result of ventilation during mining. Bronze-age K-Cu humate-oxalate aggregates and Cu carbonates precipitated from pedogenetic fluids as a function of variable redox conditions and a pH value fluctuating around neutral. This sequence of per descensum mineralization with arsenates, humates and carbonates at Serifos, Greece, is of importance in three ways. It offers an insight into the most recent weathering processes of base-metal mineralization in the Aegean Sea region. It gives an overview of mining activities across Europe from Cyprus, the "Cradle of Cu mining", to Great Britain during the Bronze Age. Also, the physicochemical results obtained from the study of this multistage alteration may be employed to explain the compositional variation under (sub)tropical through temperate climatic conditions in tailings derived from beneficiation of As-Cu ores elsewhere.