HYDROGEOCHEMICAL EXPLORATION FOR GOLD IN THE OSILO AREA, SARDINIA, ITALY

A study to test the use of hydrogeochemical methods for gold prospecti ng was carried out in the Osilo area, northern Sardinia. The study are a, covering about 30 km(2), is characterised by Tertiary andesitic roc ks. Gold concentrations up to several ppm, associated with abundant py rite, arsenopyrite, stibnite, tetrahedrite and electrum, and subordina te galena, sphalerite and chalcopyrite, are present in quartz veins as sociated with a polyphase, incipient and pervasive alteration of the a ndesitic rocks. Forty-eight water samples(17 streams, 29 springs and 2 boreholes) were analysed for Au and a wide range of major and trace e lements, both in solution (< 0.4 mu m) and in suspension. Background v alues for dissolved Au were below the detection limit of the methods u sed (between 0.3 and 0.5 ng L(-1) Au). Gold concentrations in solution up to 3 ng L(-1) were found in waters draining the mineralised vein s ystem. The observed dispersion of Au in surface waters was restricted to about 500 m from the auriferous veins. Dissolved Au anomalies do no t vary significantly in water samples, taken monthly over a one year p eriod, suggesting that the dispersion of Au is unaffected by seasonal conditions in the Osilo area. For samples where Au was detected both i n solution and in suspension, the Au content of the suspended matter w as usually lower than that in solution. The best indicators of Au mine ralisation, apart from Au itself both in solution and in suspension, w ere As and Sb which showed a dispersion clearly related to the known a uriferous veins.