Origin of the As anomaly in Quaternary sediments of Southern Tuscany (Italy): New insights from geological, geochemical and fluid inclusion data from the Pecora River and Bruna River Valleys

Abstract

Quaternary sediments along a belt that extends from inland to bottom sediments of the Tyrrhenian Sea in southern Tuscany, Italy, are characterized by high concentrations of As (up to >1000 mg/kg). Earlier studies ascribed this anomaly to anthropogenic activities, e.g., past metallurgical processes of materials mined from pyrite and polymetallic deposits. Such activities, however, can only explain local anomalies. Alternatively, the extensive distribution of these high As contents in the Quaternary sediments is referred to a natural dispersion of eroded materials from ore deposits present in the area. This study provides fluid inclusion data in detrital quartz, as well as mineralogical and geochemical data of Quaternary sediments, from two contiguous catchments draining their waters toward the Tyrrhenian Sea. Results constrain the nature of the mineralized crust eroded and deposited in the coastal continental valleys and at the bottom of the Tyrrhenian Sea. Fluid inclusion data suggest that the As anomaly is related to the erosion of pyrite and polymetallic ore deposits, and that the original depth of the ore deposits ranged between about 500 and 2000 m. This shows that the erosion of such ores left an extensive legacy in southern Tuscany coastal valleys and in the Tyrrhenian sea.