Major- and trace-element mobility in altered volcanic rocks near Stypsi, Lesbos, Greece and genesis of a kaolin deposit

Abstract

Major- and trace-element mobilities have been investigated in hydrothermally altered zones of Miocene volcanic rocks on the island Lesbos. Three types of alteration have been recognized, and are characterized by the mineralogical composition: (a) zone of illite and smectite; (a-b) transitional zone of kaolinite, illite, smectite; and (b) zone of kaolinite. The alteration zones are encountered in the Lower Lava Unit (latite—andesite) and are related to a channel (fault) which allowed the circulation of hydrothermal fluids. The zonation of hydrothermal alteration is attributed to the step-wise character of the kaolinitization process. The following relationships between mineral paragenesis of the different alteration zones and element behaviour are apparent. 1. (1) K and Rb enrichment relative to fresh volcanic rock occurs in zones a and a-b, while K and Rb are leached during destruction of illite in the kaolinite zone (b). 2. (2) Zones a and a-b are characterized by depletion of Fe, Mg, Ca, Na, P, Sr, Zn, Cu, Ni and Ba when compared with the unaltered volcanic rock. 3. (3) Relative to the volcanic rock, the kaolinitic zone (b) is characterized by depletion of Fe, Mg, Ca, Na, K, P, and Ti and enrichment of Al. Of the trace elements Nb, Zr, Ce, Nd and La are enriched while Sr, Rb, Zn, Cu, Ni, Cr, V, Ba and Sc are depleted. 4. (4) There is no evidence of Cu, Ni, Cr, Zn or Mo addition to the system by the hydrothermal fluids. Recognition of the hydrothermal origin of Stypsi Kaolin is based on the following criteria: (a) mineralogical zonation; (b) association of pyrite with hydrothermally altered zones; (c) Hg-geochemical anomalies of the alteration zones, which are indicative of postvolcanic geothermal activity; (d) comparison of the analytical data derived from the present study with relevant data on kaolins of known hydrothermal origin.