Giovanni Grieco

Plinian pumice fall deposit of the Campanian Ignimbrite eruption (Phlegraean Fields, Italy)

Abstract A plinian pumice fall deposit associated with the Campanian Ignimbrite eruption (36 ka, Phlegraean Fields caldera, Italy) occurs at the base of the distal grey ignimbrite in 15 localities spread over an area exceeding 1500 km2 between Benevento and the Sorrentina peninsula. In the thickest stratigraphic section at Voscone (130 cm), 45 km east of the Phlegraean caldera centre (Pozzuoli), the deposit consists of two units: the lower fall unit (LFU) is well sorted, exhibits reverse size grading and is composed of equidimensional light-grey pumice clasts with very subordinate accidental lithics; the upper fall unit (UFU) is from well to poorly sorted, crudely stratified, richer in lithics and composed of both equidimensional and prolate pumice clasts. The two fall units show slightly different dispersal axis: N90° for the LFU and N95° for the UFU. Volumes calculated with the method of Pyle (1989) are about 8 km3 for the LFU and 7 km3 for the UFU. The maximum height of the eruptive columns are estimated, using the model of the maximum lithic clasts dispersal, at 44 km for the LFU and 40 km for the UFU, classifying both fall units as ultraplinian in character. Reverse size grading within the LFU suggests an increase of the height of the column and magma discharge rate with time. Moderate sorting and crude stratification of the UFU are consistent with short-period oscillation of the column, possibly associated with repeated partial column collapses. Sharp increases in lithic content at the transition to UFU and within the UFU suggest that changes in the eruptive behaviour was produced by a dramatic increase in conduit/vent erosion. The phase of column instability preceded the emplacement of widely dispersed pyroclastic flow. The ultraplinian nature of the fall fits well with the wide dispersal of the Campanian Ignimbrite with an estimated aspect ratio of 3–4×10−4 (LARI).

Ferritchromite and chromian-chlorite formation in mélange-hosted Kalkan chromitite (Southern Urals, Russia)

Abstract Spinel is often used as a magmatic indicator of crystallization processes, without considering the effects of metamorphic alteration on spinel geochemical features. Serpentinized mélanges in the southern Urals host different kinds of disseminated to massive chromitite mineralization. In mélange environments, intense metamorphic alteration above 300 °C leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chromian-chlorite. Based on textural and chemical analyses, mélange-hosted Kalkan chromitites exhibit a hydration and oxidation reaction that can explain the formation of ferritchromite and chromian-chlorite from chromite and serpentine: Textural analyses fit well with the proposed reaction and show that it usually proceeds very close to completion. The degree of alteration of chromite into ferritchromite is controlled by the initial chromite to serpentine ratio. In chromitites, high ratios prevent complete transformation of chromite into ferritchromite. The most likely environment for such reaction is a prograde metamorphic event post-dating serpentinization of the Kalkan ophiolite, possibly related to emplacement within an accretionary wedge.

Chromite alteration processes within Vourinos ophiolite

The renewed interest in chromite ore deposits is directly related to the increase in Cr price ruled by international market trends. Chromite, an accessory mineral in peridotites, is considered to be a petrogenetic indicator because its composition reflects the degree of partial melting that the mantle experienced while producing the chromium spinel-bearing rock (Burkhard in Geochim Cosmochim Acta 57:1297–1306, 1993). However, the understanding of chromite alteration and metamorphic modification is still controversial (e.g. Evans and Frost in Geochim Cosmochim Acta 39:959–972, 1975; Burkhard in Geochim Cosmochim Acta 57:1297–1306, 1993; Oze et al. in Am J Sci 304:67–101, 2004). Metamorphic alteration leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chlorite. In this study, we investigate the Vourinos complex chromitites (from the mines of Rizo, Aetoraches, Xerolivado and Potamia) with respect to textural and chemical analyses in order to highlight the most important trend of alteration related to chromite transformation. The present study has been partially funded by the Aliakmon project in collaboration between the Public Power Corporation of Greece and Institute of Geology and Mineral Exploration of Kozani.

Phreatic sulphide bearing quartz breccias between crystalline basement and Collio formation (Southern Alps, Italy)

In this work a report is given of the geological, petrographic and ore mineralogical features of several sulphide-bearing quartz breccia (SQB) bodies outcropping in a structurally complex sector of the Orobic Alps (Southern Alpine domain) and close to the uranium-rich mining area of Novazza. Although little studied, these breccia bodies were previously attributed to different genetic processes and/or geological domains (e.g., Crystalline Basement, Basal Conglomerate formation). The features of the SQB bodies suggest a hydrothermal origin likely related to the large-scale hydrothermal process causing the formation of the nearby uranium ore deposit at Novazza.

The effect of Alpine metamorphism on an oceanic Cu-Fe sulfide ore: the Herin deposit, Western Alps, Italy

Herin mine (Champdepraz, Aosta, Italy), located in Aosta Valley, approximately between 1600 and 1800 m a.s.l., was exploited for at least 250 years for its Cu-Fe sulfide ore. The deposit host rocks belong to the metaophiolitic Zermatt-Saas unit, the eclogitic lower portion of the Piedmont Western Alpine Nappe. The ore mineral association mainly comprises pyrite and chalcopyrite, along with other sulfides such as pyrrhotite, sphalerite, cubanite and oxides (magnetite, rutile, ilmenite). The deposit consists in lenticular massive bodies and thin layers hosted in various greenschist-facies metamorphosed lithotypes. New data on geometric features, mineralogy, mineral chemistry, petrography, minerography were collected and compared with the existing models for massive-sulfide mineral deposits. We suggest a hydrothermal-volcanogenic primary origin of the mineralization with primary characters largely obliterated by subsequent metamorphic history. On the basis of previous works and our results, we identified two parameters as driving criteria for a comprehension of the multistage process that led to the present configuration of the ore: (a) textural characters of pyrite and (b) distribution of selected trace elements (Co, Ni, As) in sulfides. Spot analyses and atomic maps obtained by electron microprobe provided an integration of these two sets of data. Trace elements, in fact, show a zoned distribution, in particular in pyrite, that can be related to specific textural styles. We selected Co as a useful trace element, due to its high concentration and wide range in pyrite (270-22200 ppm). Besides we determined a critical concentration value for cobalt of 3160 ppm, as discriminating between two generations of pyrite. This led to outline a series of dissolution and crystallization events, that describes the metamorphic history of the sulfide ore.