Vein mineral assemblage in partially serpentinized peridotite xenolithsfrom Hyblean Plateau (south-eastern Sicily, Italy)

Abstract

Periodico di Mineralogia (2011), 80, 2, 247-266 - DOI:10.2451/2011PM0019 Vein mineral assemblage in partially serpentinized peridotite xenoliths from Hyblean Plateau (south-eastern Sicily, Italy) Fabio C. Manuella Dipartimento di Scienze Geologiche, Universita di Catania, C.so Italia 57, I-95129 Catania - Italy fmanuella@alice.it Abstract A mineralogical study was performed on seven selected peridotite xenoliths, found in diatremic tuff-breccia deposits from Valle Guffari (Hyblean Plateau, Sicily, Italy), in order to investigate evidence for serpentinization. Petrographic examinations of spinel-harzburgites revealed the presence of the pervasive network of composite serpentine veins, suggesting a variable degree of alteration of 50-80%. Veins were characterized by X-ray Powder Diffraction (XRPD), which allowed the identification of chrysotile 2 M c1 and lizardite 1 T . No peaks referable to antigorite polymorphs were detected. Microprobe WDS (wavelength dispersion system) data evidenced a different iron content between chrysotile [average composition (Mg2.58Fe 2+ 0.27Al0.01)Σ2.86Si2.06O5(OH)4, Fe/(Fe+Mg) = 0.09] and lizardite [average composition (Mg2.49Fe 2+ 0.37Al0.01)Σ2.87 Si2.05O5(OH)4, Fe/(Fe+Mg) = 0.12]. Chlorine was detected in both polytypes, with an average concentration of 0.09 wt% in chrysotile and 0.05 wt% in lizardite. A series of cogenetic secondary minerals are enclosed in serpentine veins, described hereunder: a) Sulphides, predominantly S-poor phases (atomic sulphur/metal ratio 1), such as millerite [mean composition (Ni0.95Fe0.03)Σ0.98S] and polydymite-violarite solid solution [mean composition (Fe1.38Ni1.69)Σ3.07S4]. b) Interstitial dendritic aggregates of Na-rich sylvite, whose molar ratio of NaCl/(NaCl+KCl) ( X NaCl) varies from 0.03 to 0.21. c) Widespread veinlets of (Ca, Na, S)-rich phosphates (CaO 50.30-52.70 wt%, Na2O 1.50-3.10 wt%, P2O5 31.76-34.60 wt%, SO3 7.40-12.60 wt%), whose chemical composition is similar to (Na, S)-rich apatite-(CaOH)- M (CaO 48.70-54.30 wt%, Na2O 0.10-3.90 wt%, P2O5 32.01-40.82 wt%, SO3 0.40-11.40 wt%). d) Aragonite veins. e) Fe-rich saponit. f) Chalcedony. The studied secondary mineral assemblage hints that serpentinization...