Hydrothermal alteration of basic island-arc volcanic rocks north and south of Mytilini Town,Lesvos Island,Greece

The mobilities of major and trace elements have been investigated in hydrothermally altered volcanic rocks of Pliocene age on the island of Lesvos. One type of hydrothermal assemblage, montmorillonite-K-feldspar-goethite, was recognized in outcrops north of Mytilini town. The alteration occurs a few metres under the surface of unaltered volcanic rocks of high-K andesite and high-K basaltic andesite composition. The unaltered latite-andesite consists of plagioclase, clinopyroxene, sanidine and magnetite with quartz, calcite, pumpellyite and montmorillonite as subordinate secondary minerals. The main control over major oxide distribution appears to be the extent of conversion of plagioclase to K-feldspar. Three distinct groups of elements were revealed to be positively correlated:

• 1 Si, Mg, Ca, Na, Mn, Sr and Ba, which are leached during the alteration process. This group of elements reflects their association with plagioclase feldspar and clinopyroxene.
• 2 Fe, K, Ti, P, Nb, Zr, Y, Rb, Ni, Cr, V. This group includes elements which are enriched relative to the unaltered volcanic rock, reflecting their association with K-feldspar, iron hydroxides and Ti and P-bearing minerals.
• 3 Ti, P, Nd, La, Ce. From this group of elements it is indicated that Nd, La and Ce are associated with Ti- and P-bearing minerals.

There is no evidence of Cu or Zn addition to the system by hydrothermal fluids since there are no correlations between these base metals and elements enriched in the K-feldspar, montmorillonite-rich zone. This potassic rich zone may be connected with hidden epithermal sulphide mineralization with gold.