Acid Mine Drainage and PTE distribution in a volcanic sulfur mine: the Thiorichia Mine, Milos Island, Greece

Acid Mine Drainage is a major environmental concern in sulfur-rich ore deposits and is widely studied in different geological contexts and for a variety of sulfide ore deposits. Milos volcanic sulfur mine represents in this picture a type of ore deposit whose acid mine drainage concern is little studied. Milos Island is an active volcano of the Hellenic volcanic arc, with a main caldera collapse structure and a widespread secondary activity. The island hosts seven active mines that exploit bentonite (with the biggest mine in Europe), perlite and pozzolan deposits and several abandoned sulfur, kaolin, barite and manganese mines. Such intense mining activity in a 160 km2 highly touristic island increases the importance of environmental studies for the sustainability of the island economy. Sulfur mining at Milos started in V century BC, but the first modern mining concession dates back to 1862 and led to the opening of the Thiorichia Mine on the Eastern shore of the island. The mining site comprised several tunnels, sulfur purification facilities and other buildings for workers accommodation and administration. Sulfur purification was gained firstly with the Calcaroni method and later with Gill four chambers furnace. Extraction declined in the second half of last century and the mine was definitely closed in 1981. During the field survey three different earthen materials were sampled: sands from the adjacent beach, stream sediments downstream from the purification plant and dumped material, close to the purification plant and uphill on the road leading to the mine. Beach samples are coarser (sandy gravels) due to wave erosion while both sediments and wastes fall in the range of sands and gravely sands. Acid Mine Drainage potential was assessed with ABA procedure and results show that beach sands have no acid potential due to complete leaching of sulfur while sediments and wastes have variable acid potential closely related to the degree of sulfur oxidation. Whole rock ICP analyses show that the highest environmental hazard for PTE is related to the high mercury content of stream sediments and wastes. XRD-powder diffraction analyses show that quartz represents the overwhelming mineral phase in all the samples analysed. Minor phases as alunite-like minerals, micas, elemental sulfur, opal and kaolinite-like minerals have also been detected. The present work shows that Acid Mine drainage and PTE concern studies in sulfur deposits are particularly difficult due to the complete decoupling between the acid potential production, strictly related to the elemental sulfur content of the materials, and the potential release of PTE in the environment, probably associated to secondary clay and alunite-like minerals.