Manganese metallogenesis in the Hellenic arc: Case studies from a Triassic rift-related volcaniclastic succession of the Cycladic Blueschist Unit, Greece

Abstract

The Triassic volcaniclastic sequence in the Cycladic Blueschist Unit (CBU) of Greece is known to contain several occurrences of metamorphosed Mn mineralization. Despite previous literature reports, a comprehensive and holistic review of the origin of these occurrences is hitherto lacking. Here we revisit the Mn metallogenic system of the CBU through a comprehensive study of Mn mineralisation at Varnavas area, Northern Attica, and its correlation with a similar occurrence at Mparades Hill, Andros island. Manganese mineralogy at both localities is manifested in a typical high-P metamorphic silicate assemblage dominated by piemontite, spessartine garnet, and minor pyroxmangite (rhodonite). Whereas at Andros Mn-rich subdomains contain brecciated braunite micronodules, preservation of tetravalent Mn oxides in a similar nodular form is documented from Varnavas, comprising dominant todorokite along with lesser hollandite, pyrolusite, and minor Mn-bearing hematite. The contrasting Mn oxide mineralogy and comparable textural characteristics at the two sites are tentatively interpreted as the result of locally incomplete reduction of precursor Mn(IV) phases during metamorphism. Bulk Mn concentrations of the studied materials generally do not exceed the value of 10 wt% Mn, unless small-scale subsampling of Mn-rich domains is undertaken. Key geochemical characteristics of the Mn-rich rocks include low transition metal concentrations (sum of Co + Ni + Cu + Zn between 0.02 and 0.06 wt%); positive-sloping, PAAS-normalised REE spidergrams at a maximum Ndsn/Ybsn ratio of 0.3; positive Ce anomalies, albeit of variable magnitude across individual samples; and high As (up to 1930 ppm) and Ba (up to 2767 ppm) contents. A combination of structural, geochemical and mineralogical evidence from the two localities along with published results from similar occurrences elsewhere (e.g., Kythnos) suggest that Mn-oxide accumulation in the CBU is genetically linked to hydrothermal venting in a back-arc rift setting during the Triassic. The geochemical variability recorded is attributed primarily to the variable mixing of a hydrothermal-sourced, hydrogenous metalliferous component that precipitated broadly contemporaneously with the deposition of the host tuffs. Low transition metal contents coupled with substantial enrichments in elements such as Ba, As, Pb and REY, attest to the felsic/intermediate character of back-arc magmatism/volcanism and associated hydrothermal activity. 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. The fully oxic and hydrogenous character of the precursor Mn(IV) oxy-hydroxides is supported by the consistently positive Ce anomalies observed in practically all samples from both sites. Although the Mn concentrations and distribution of the studied assemblages are sub-economic at best, we consider possible that their geographically widespread occurrence may still hold untapped potential for future discovery of commercially viable Mn ores sensu stricto, both in the Hellenic arc and in other regions of similar geotectonic and metallogenic activity.