VARIATIONS AND CONTROLS ON THE COPPER/ARSENIC RATIOS IN WATERS AT THE FURTEI GOLD MINE, SARDINIA, ITALY

Waters draining the area of the epithermal high sulfidation gold deposit at Furtei, Sardinia (Italy) show the typical trend of increasing "heavy metal" load with decreasing pH. In the primary (hypogene) assemblage, pyrite and enargite/luzonite are the most abundant sulfides. Therefore, the copper and arsenic contents of waters interacting with highly mineralized rocks could be controlled by enargite. However, in such waters, Cu/As molar ratios are different from those of enargite. In surface, oxygenated waters, they are much higher. Hence, copper is more mobile than arsenic in these conditions. This is in agreement with 1) the composition of secondary minerals found at the mine, the Cu-bearing minerals being in general more soluble than the As-bearing ones; 2) laboratory experiments suggesting that under some conditions acidic solutions remove more easily copper than arsenic from enargite. On the other hand, in subsurface, reduced waters, Cu/As ratios are lower, probably because reducing conditions do not allow the formation of iron oxy-hydroxy-sulfates that may act as adsorbent media for arsenic species.