Lithium rich clays in the Miocene volcano-sedimentary formations of Karlovassi Basin, Samos Island, E. Aegean, Greece

The Karlovassi basin at Samos Island, East Aegean, Greece, hosts Li-rich volcanosedimentary rocks of Upper Miocene age. The parent materials were deposited and altered in an alkaline lake. The bulk sedimentary rocks contain up to 1200 ppm Li. The Li-occurrences are typical clay-type ones, with the main Li-mineral being trioctahedral, smectite, having characteristics of hectorite with minor stevensite. Most Li-rich outcrops contain mixtures of trioctahedral and dioctahedral smectites, and few Li-rich sedimentary rocks host Li in swinefordite-type smectite (dioctahedral smectite with trioctahedral domains) and dioctahedral illite. At least two generations of smectite have been recognized according to the smectite chemical characteristics. Except for the trioctahedral smectite, the alkaline environment of the lake is evidenced by the presence of Mg‑carbonates, namely dolomite and magnesite, borates (colemanite and ulexite), sulphates (gypsum, celestine), alkaline zeolites (HEU-type and analcime) authigenic illite and authigenic K-feldspar. The relative abundances of the authigenic phases, which are distributed in alteration zones, depend on local variations of alkalinity, salinity, pH and α SiO2 of lake water. The source of Li to form the occurrences is hydrothermal, linked to a magma chamber, located underneath or at the E -SE margins of the basin, with the hydrothermal fluids circulating along NW-SE trending fault lines crosscutting the basin. Surface runoff was vital for the supply of Mg 2+ and controlling alkalinity.