GEOCHEMICAL FEATURES OF THE KAOLIN DEPOSITS FROM SARDINIA (ITALY) ANDGENETIC-IMPLICATIONS

Seventy six samples of altered and fresh volcanic (mainly acidic) rock s from four major kaolin deposits of Sardinia have been studied for ma jor- and trace-element geochemistry. Kaolinization generally led to Al 2O3 increase and release of alkalis, alkaline earths, most of Fe and S iO2. Among the trace elements those of lower valency were mobile, wher eas elements of higher valency and Pb were released only at an advance d stage of alteration. Besides kaolinite other alteration products are cristobalite, alunite, dickite, illite, montmorillonite and quartz. I n particular, taking into account the cristobalite --> quartz relation , temperature of kaolinization has been estimated by 100-degrees-C in most deposits, but at Serrenti-Furtei it was higher, as only quartz ex ists. pH of the solutions, that were of low salinity and initially in equilibrium with quartz, except at Serrenti-Furtei where they were qua rtz-saturated, ranged around 4.4 because of the kaolinite-illite relat ion, and was likely controlled by H2CO3. With evolution of kaolinizati on the solutions became progressively alkaline and quartz undersaturat ed. The alteration fluids had generally low H2S/H2SO4 ratio, thus allo wing deposition of alunite, except at Serrenti-Furtei where the lower oxygen fugacity stabilized pyrite instead of alunite. In most deposits alunitization predated kaolinization, thus explaining why sometimes k aolinite is Sr- and Ba-rich. The geochemical features of the Sardinian kaolins are independent of the parentrock-type, and dependent, on the contrary, on the physical-chemical conditions of formation. This rule s out what is claimed by some authors about the possibility of disting uishing among deposits derived by alteration of various parent rocks u sing trace-element abundances, at least for the lithotypes involved in this study.