AGATE AS AN INDICATOR OF IMPACT STRUCTURES - AN EXAMPLE FROM SAAKSJARVI, FINLAND

Mineralogical, petrographical and chemical determinations were made :f or 743 agate (banded variety of chalcedonic quartz) nodules (diameters from 5 mm to 5 cm) formed during postimpact, low-temperature hydrothe rmal activity as vesicle fillings in the melt rocks of the Saaksjarvi meteorite impact structure (diameter 5 km) in southwest Finland. Other hydrothermal vesicle fillings in the impact melt rocks include chlori te, mordenite, smectite and kaolinite. The agates were classified into two types, whose mineralogical properties and chemical compositions f all within the range of volcanic agates (basaltic and rhyolitic host r ocks). The relatively high age (similar to 510 Ma) of the Saaksjarvi i mpact melt rocks, however, is reflected by the presence of recrystalli zation textures, which are rare in younger volcanic agates. The Saaksj arvi structure was initially located after following the fortuitous di scovery of agate ''path-finders'' in the glacial overburden. It is rec ommended that wherever volcanic type agates are found as float in Prec ambrian shield areas devoid of younger volcanic rocks, the possible pr esence of impact (or volcanic) craters in the vicinity should be consi dered.