A metallogenic survey of alkalic rocks of Mt. Somma-Vesuvius volcano

Somma-Vesuvius is an alkaline volcano whose products (pumice, scoria and la va) have alkaline (Na2O + K2O) contents between 6 and 16 wt%, Mg number <50 , SiO2 59-47 wt% and MgO 0-7.8 wt% (more than 50% of the samples have a con tent <2 wt%). Immobile-element ratios (Th/Yb, Ta/Yb, Ce/Yb) indicate a shos honitic character, while the K2O content (4-10 wt%) is characteristic of ul trapotassic rocks. The behavior of selected metals is discussed by grouping them on the basis of the stratigraphic sequence and differentiating the vo lcanic activity between plinian and interplinian (Rolandi et al., 1998; Ayu so et al., 1998). This allows observation of the variation within each form ation from 25.000y. BP to the last historic eruptive cycle (1631-1944 AD). The main processes to explain the wide distribution of the data presented a re fractional crystallization of a mantle-derived magma, magma mixing, and contamination with heterogeneous lower and/or upper crust. Variation diagra ms distinguish different behavior for groups of metals: Ag (0.01-0.2 ppm), Mo (1-8.8 ppm), W (1.3-13 ppm), Pb (16-250 ppm). Sb (0.2-2.6 ppm), Sc (0.2- 61 ppm), Li (15-140 ppm) and Be (1-31 ppm) increase with increasing differe ntiation and tend to correlate with the incompatible trace elements (Th, Hf , etc). Cu (10-380 ppm), Au (2-143 ppb), Co (0.7-35.1 ppm) and Fe (1.3-6.2w t%) decrease towards advanced stage of differentiation. Iron also identifie s three magmatic groups. The ratio Fe3+/Fe2+ ranges between 0.2 and 1.8, an d Fe2O3/(Fe2O3+FeO) ranges between 0.2 and 0.8, giving rise to an oxidized environment; exceptions are in the samples belonging to the interplinian fo rmations: I, II, medieval and 1631-1994 AD. Fluorine ranges between 0.1 and 0.4 wt% for the complete Mt. Somma-Vesuvius activity, except for the Ottav iano and Avellino plinian (0.8 wt%) events. Chlorine has a wider range, fro m 0.1 wt% to 1.6 wt%. Mt Somma-Vesuvius has some features similar to those of mineralized alkalin e magmatic systems which coincide with the transition between subduction-re lated compression and extension-related to continental rifting. We infer th at a prospective time for the formation of mineralization at Mt Somma-Vesuv ius was during the 1631-1944 eruptive period.