SECONDARY MINERAL ASSEMBLAGES WITHIN EPICLASTITES OF WESTERN LOGUDORO, SARDINIA, ITALY

Inside the Upper Oligocene-Burdigalian volcanoclastic succession of Logudoro region (Northern Sardinia, Italy) the presence of an epiclastic unit of wide lateral continuity was recently evidenced. This research accounts for geological and mineralogical studies performed in the western sector of this unit. In this area the deposit, ranging in thickness from 15 up to 30 meters, consists of ash-rich lacustrine sediments. Among the epiclastic component accretionary lapilli often occur, moreover remnants of fossil plants are common. Quartz, feldspars, scarce biotite and volcanic glass are present as primary phases whereas, as far as secondary minerals are concerned, the epiclastic unit shows two assemblages: 1) kaolinite + opal-CT ± smectite; 2) smectite + clinoptilolite ± opal-CT. The glassy fraction, rhyolitic to rhyodacitic in composition, decreases moving toward the normal faults which affect the unit. Conversely, the second assemblage shows striking relations to the fault zones toward which progressively grow in importance. The linkage of the kaolinite + opal-CT ± smectite assemblage with fault zones is even closer. In a wall of a small open pit a vertical, abrupt and sharp passage from the first assemblage (bottom) to the second one (top) was observed. This variation is accompanied by a change in colour from white to grey-greenish, and by a strong decrease in the degree of lithification from coherent to loosely. A fault-controlled fluid circulation can produce the observed lateral variations in mineral assemblage, but cannot account also for the abrupt, sharp vertical changes. To generate such a spatial mineral distribution, changes in fluid composition are required, essentially in terms of pH values. A drop of the only temperature is not able to product such abrupt vertical assemblage variation. To account for this horizontal boundary between kaolinite + opal-CT ± smectite level and smectite + clinoptilolite ± opal-CT level an interaction surface between hydrothermal fluids and percolating waters could be invoked.