Formation of Mn-skarn Ores at Thapsana Mines, Paros Island, Attico-Cycladic Metallogenetic Massif, Greece

Mn-skarn ore deposits are relatively infrequent worldwide. A typical example of a Mn-skarn in the Attico-Cycladic Metallogenetic Massif is located at the mining area of Thapsana, Paros Island. The skarn occurs adjusted to the Thapsana biotite-garnet-tourmaline-lepidolite, highly sericitized, leucogranite apophyses of the Paros granitoid and related pegmatites and aplites dated from 11.5 to 7.5 Ma. The Mn-skarns orebodies occur as lenses and NE-trending veins hosted in the Cyclades Blueschist Unit (CBU) marbles and intercalated calcic schists. They comprise two discontinuous paragenetic zones (with widths of ≤ 10 m): A zone contain vesuvianite, Mn-enriched salite to johannsenite-diopside and spessartine with cores enriched in grossular component (Sps_~75Grs_~15) placed close to the Thapsana leucogranite and a zone of grossular (Sps_~85Grs_~5), actinolite to Mn-cummingtonite (with Mn ~ 0.6 apfu) and phlogopite more distal from the leucogranite. The Mn-skarns are crosscut by later WNW- to W-trending veins filled with Ca-K-Mg-bearing pyrolusite, manganite, rhodonite and rhodochrosite, carbonates, hydroxylapatite and johnbaumite.

The Mn-ores occur as massive aggregates or disseminated comprising jacobsite, hausmannite, Mn- enriched magnetite, braunite and hollandite (with crystals of ≤ 4cm). Occasionally jacobsite and hausmanite display oriented intergrowths. The ore paragenesis also includes secondary pyrolusite, manganite, rhodonite, rhodochrosite, cryptomelane, manjiroite, vernadite and supergene Fe-oxides. Geothermo-oxygen-barometry suggests that the main zones of the Mn-skarns at Thapsana have been formed at temperatures ranging from ~ 440º to ~ 510ºC, pressures of 0.11 to 0.12 GPa and logfO₂ values just below the magnetite-hematite buffer. Almost constant average isotopic compositions obtained from the Mn-ores (n = 10) of δ⁴⁴Ca_BSE and δ²⁶Mg_DSM-3 of 0.5 ± 0.05 and -0.6 ± 0.1 and δ¹⁸O and δD of 7.2 ± 0.5 and -92 ± 2 per mil, suggest a magmatic source for the metasomatic ore fluids related to the Thapsana leucogranite which have also interacted and isotopically equilibrated with the host CBU marbles.