Hydrothermal Manganese mineralization in a Triassic back-arc rift-related volcaniclastic succession of the Cycladic Blueschist Unit, Greece

The Cycladic Blueschist Unit (CBU) of the Aegean (Greece) contains several occurrences of metamorphosed Manganese mineralization within a Triassic volcaniclastic sequence. The latter includes quartz-mica schists intercalated with bimodal metavolcanics and blue-grey marble layers. SHRIMP U-Pb zircon dating has documented the Triassic age (~242 Ma) of the volcanic rocks. Herein we revisit the Mn metallogenic system of the CBU through an extensive study of Mn mineralization at Varnavas area, Northern Attica, and a similar occurrence at central Andros Island (Mparades hill). Manganese mineralogy at both localities is manifested in a typical high-P metamorphic silicate assemblage dominated by piemontite, spessartine garnet, and minor pyroxmangite (rhodonite). At Andros, Mn-rich subdomains contain brecciated braunite micronodules. The preservation of similar nodular form is documented from Varnavas, comprising dominant todorokite, lesser hollandite, pyrolusite, and minor Mn-bearing hematite. The contrasting Mn oxide mineralogy at the two sites is tentatively interpreted as the result of locally incomplete reduction of precursor Mn(IV) phases during metamorphism. Common geochemical characteristics of the Mn-rich rocks include low transition metal concentrations; positive-sloping, PAAS-normalized REE spidergrams; positive Ce anomalies of variable magnitude across individual samples; and high As, Ba, Pb. The geochemical variability recorded is ascribed to the varying mixing of a hydrothermal-sourced, hydrogenous metalliferous component that precipitated penecontemporaneously with the deposition of the host tuffs. The primary Mn precipitates are thought to have been in the form of tetravalent Mn assemblages, which may locally be partially preserved through metamorphism, as appears to be the case in the Varnavas occurrence. All these reveal the interplay between the felsic/intermediate back-arc volcanism and associated hydrothermal activity and the Mn mineralization within the rift setting of the CBU domain.