HIGH AU, AG, MO, PB, V-CONTENT AND W-CONTENT OF FUMAROLIC DEPOSITS AT MERAPI VOLCANO, CENTRAL JAVA']JAVA, INDONESIA

Spectacular blue sublimates and red-green incrustations were deposited in late 1991 around high temperature (800-degrees-C) fumaroles at Merapi volcano, central Java. The blue sublimates cover an area 10 m radius around fumarole vents, and comprise a thin coating of Mo oxide (''Mo blue'') on a substrate of mainly alpha-cristobalite-alunogen-anhydrite. They contain up to 3 wt.% Mo, 1.64% Pb (as anglesite), as well as As, Zn, W, Bi, Tl and Cs (1000s of ppm), B, Cd, Ti, Sn and Rb (100s of ppm), and Sb, In, Au, Ag and Te (10s of ppm). Red-green incrustations centimetres from the vent are mineralogically dominated by Pb, Na, K, Al sulphates formed at less-than-or-equal-to 400-degrees-C, and are enriched in similar elements to the blue sublimate, except that Mo is lower (1440 ppm), and Au (101 ppm) and Ag (62 ppm) are higher; in addition, V is also high (4800 ppm). Comparison with previous studies at Merapi indicate that fumarolic deposits show a strong inheritance of metallic elements found in the volcanic gas. The appearance of blue sublimates at Merapi corresponds to increased shallow seismicity, SO2 flux, and increased surface temperature of the fumaroles. These changes are related to ascent of new magma and degassing at depths of 2 km or less below the summit of the volcano. Metal enrichment due to high temperature fumarolic activity has implications for geochemical exploration in volcanic environments, since it may form ''false'' anomalies with respect to key elements such as Au, unrelated to hydrothermal fluids with the potential for mineralization.