Genesis of a new type of mangan skarn associated with peraluminous granitoids in Greece

Abstract

Two world-first examples of mangan oxide skarns were investigated in this study, namely the Panorama skarn of Drama and the Thapsana skarn of Paros Island, in the Rhodope and Attico-Cycladic Massifs, respectively. Transitional calcic-to-mangan exoskarn at Panorama is exposed in the garnet-epidote zone (Grt-Ep), proximal to the Panorama (micro-)granite. In Paros Island, mangan skarn is related to rhodonite (Rdn ± Ves), johannsenite-spessartine (Jhn-Sps) and spessartine-cummingtonite (Sps-Cum) zones, adjacent to marbles and/or gneisses and the Thapsana leucogranite. A two-stage manganese-oxide assemblage is present in the Thapsana skarn and comprises jacobsite, hausmannite, braunite with minor magnetite (stage I) followed by hollandite, cryptomelane and pyrite (stage II) with gangue rhodochrosite, calcite, and ankerite.Multiple isotopic evidence (i.e., δ18O, δD, δ44Ca, δ26Mg, 87Sr/86Sr, U/Pb) support the granitic source of the Mn-rich metasomatic fluids in both case studies. The Thapsana mangan skarn formed at a pressure of ∼110 MPa and a temperature range of ∼305° to 565 °C, from initially acidic (pH = 3.5), saline (up to ∼48.0 wt% NaCl equivalent), Mn chloride-bearing, skarn-forming fluids with elevated log[αMn2+/(αH+)2] and log[αMn3+/(αH+)3]. Fluid inclusions results obtained from the Jhn-Sps zone suggest phase separation of the metasomatic fluids at ∼480 °C and ∼120 MPa and ∼400 °C and ∼100 MPa. Thermodynamic modeling suggests that the manganoan skarn assemblages formed due to simple cooling and fluid-rock interaction, increase in pH and changes in the redox state of the metasomatic fluids in concert with successive deposition of Mn2+ and Mn3+ assemblages. This led to a paragenesis that comprises early anhydrous manganoan silicates, followed by more complex assemblages dominated by hydrous manganoan silicates and Mn3+ oxides.In this study we propose a metallogenic model in which mangan skarns are formed proximally to their parental leucogranites, primarily from anatectic reworking of crustal Mn-rich sources (e.g., gneisses and marbles) that delivered peraluminous melts with increased primary endowments in manganese. Exsolution of chloride magmatic fluids atypically enriched in incompatible manganese, deposited manganoan silicates and oxides with declining temperature in the skarn environment. However, in situ reworking of manganese upon fluid-rock interaction during skarnification cannot be discounted as an additional contributing source for manganese. The results of this study encourage us to propose the establishment of a new mangan-oxide mineralized skarn class as a distinct candidate for skarn mineralization.