The 1.5 km-large hydrothermal system of Balya is characterized by three alteration styles which from the outer halo towards the center are: (i) propylitic alteration with the hydrothermal mineral assemblage of calcitedaphnite-albite-epidote-quartz-pyrite; (ii) argillic/phyllic alteration with the hydrothermal mineral assemblage of sericite/muscovite-kaolinite-rutile-quartz + pyrite; (iii) advanced argillic alteration with the hydrothermal mineral assemblage of alunite-jarosite-kaolinite-quartz-sericite __+ pyrite. Hornblende andesite is the protolith of the hydrothermal alteration system. Enrichment in Si, Sb and Rb, and depletion in Na, Ca, Mg, Fe, Mn, P, Ba, Sr, and Zn distinguishes the argillic/phyllic and advanced alteration types from propylitic alteration and the unaltered hornblende andesite protolith. REE distribution patterns indicate an essentially immobile behaviour of REEs during the alteration cycle. K-Ar age data for unaltered and hydrothermally altered rocks define a synchronous age of 25.3 __+ 1.2 Ma for both igneous and hydrothermal activity.
The Balya lead-zinc-silver deposit is located about 50 km NW of the city of Balikesir on the Biga peninsula in NW Turkey (Fig. 1). The Balya Mine was the main producer of Pb and Ag in Turkey during the period 1880--1935. Its mining history goes back to antique times when the mine probably supplied the first known lead products to nearby Troy. Although the Balya deposit is historically a substantial Pb-Zn-Ag producer and still has large ore reserves, very little is published on its petrology and geochemistry. Some geological and mining information is given by Kovenko (1940), Aygen (1956), Gjelsvik (1958) and Akyol (1982). Our reconnaissance study describes some alteration features of the hydrothermal system, based on field observations and petrographic and geochemical work.