The effect of metasomatism on the Cr-PGE mineralization in the Finero Complex, Ivrea Zone, Southern Alps

The magmatic metasomatism that was responsible for producing chromitite–dunite bodies in the unusual phlogopite peridotite of the Finero Complex in Permian to Triassic times also influenced the Cr-platinum group elements (PGE) mineralization. At least the end stages of this metasomatism are recorded in compositional zoning of chromite grains in the podiform chromitite. Metasomatic melt, with or without vapor, reacted with chromite to produce core-to-rim Cr enrichment of extant chromite grains and was concurrent with pyroxene crystallization. Under conditions of lower melt/rock ratio, metasomatism resulted in core-to-rim Al enrichment in chromite and crystallization of amphibole between chromite and clinopyroxene. This early, high-temperature metasomatism is unrelated to the later and pervasive K-metasomatism that crystallized phlogopite and was associated with the intrusion of clinopyroxenite dikes that cut the peridotite. Much later, serpentinization of olivine locally depleted chromite in Al and enriched it in Fe and formed minor amounts of magnetite.The PGE, which are present mainly as laurite inclusions in chromite, were remobilized by the early metasomatism. This resulted in substantial variation in the PGE contents of chromitites and imposed a characteristic PGE pattern in which chondrite-normalized Os, Ir, Ru and Rh contents are high but Pt and Pd contents are low. The slopes of PGE chondrite-normalized concentration patterns are systematically related to absolute PGE abundance and to rock mode. Chromitites with low modal orthopyroxene, clinopyroxene, and amphibole exhibit negative PGE slopes and contain relatively high PGE concentrations, whereas chromitites rich in these silicate minerals have positive slopes and low PGE contents.     

Laurite and zircon from the Finero chromitites (Italy): New insights into evolution of the subcontinental mantle

Chromitites enclosed within metasomatised Finero phlogopite peridotite (FPP) contain accessory platinum-group minerals, base metal (BM) sulfides, baddeleyite, zircon, zirconolite, uraninite and thorianite. To provide new insights into mantle-crustal interaction in the Finero lithosphere this study evaluates (1) the mineral chemistry and Os-isotope composition of laurite, (2) the crystal morphology, internal structure, in-situ U-Pb, trace-element and Hf-isotope data of zircon from two chromitite localities at Alpe Polunia and Rio Creves. The osmium isotope results reveal a resticted range of ‘unradiogenic’ ¹⁸⁷Os/¹⁸⁸Os values for laurite at Alpe Polunia (0.1247–0.1251, mean 0.1249 ± 0.0001). Re-Os model ages (T_RD) of laurite reflect an Early Paleozoic partial melting event (ca 450 Ma or older), presumably before the Variscan orogeny. The Os isotopic composition of laurite/chromitite probably preserves their mantle signature and was not affected by later metasomatic processes. U-Pb and Hf-isotope data show that the Finero chromitites have distinct zircon populations with peculiar morphology, internal cathodoluminescence textures, trace-element composition and an overall U-Pb age span from ∼310 Ma to 190 Ma. Three age peaks at Rio Creves (220 ± 4 Ma, 234.2 ± 4.5 Ma and 277.5 ± 3.2 Ma) are consistent with a prolonged formation and multistage zircon growth, in contrast to the common assumption of a single metasomatic event during chromitite formation. The trace-element signatures of zircons are comparable with those of mantle-derived zircons from alkaline ultramafic rocks, supporting the carbonatitic nature of the metasomatism. Hf-isotope compositions of the Finero zircons, with εHf values ranging mainly from −3 to +1, are consistent with crustal input during metasomatism and could indicate that the parental melts/fluids were derived from a relatively old source; the minimum estimates for Hf model ages are 0.8–1.0 Ga. Our findings imply that mantle rocks and metasomatic events at Finero have a far more complex geological history than is commonly assumed.