The mine of Piazza, near Deiva (La Spezia), was exploited for copper-rich mineralization until the beginning of XXth century. The Piazza Cu deposit occurs within the ophiolite of the Bracco Unit (Internal Ligurides), that represents a remnant of the Mesozoic Tethyan oceanic lithosphere. The ophiolite of the Piazza area consists of pillow basalts, coarse-grained to pegmatoid gabbros and serpentinite intruded by polyphase dike injections (dolerite and plagiogranite), and of breccias. These rocks suffered prehnite-pumpellyite to greenschist facies metamorphism related to the Apennine orogeny. The Apennine tectonics mainly produced large-scale open folds, brittle-ductile to brittle structures such as S-C structures, faults and fractures that did not obliterate the primary features of the ophiolitic rocks. Both intrusive and extrusive magmatic structures are still well recognizable in mafic and ultramafic rocks. Greenschist- to amphibolite facies mineral assemblages, associated with both ductile and brittle structures, have been observed in gabbros and interpreted to be relics of the pre-orogenic oceanic history. In the Piazza mine, galleries distributed in five levels are only partially accessible, but they still reveal the main features of the ore deposit. The original mineralization was included in a complex framework of discordant, steep conjugate E-W to NE-SW trending veins. The vein system was emplaced along fractures and brittle shear zones cutting the pegmatoid gabbro, and along the primary intrusive (or tectonically reactivated) contacts between basaltic dikes and gabbro. According to mining reports, single vein thickness varied between 0.5 and 1.8 m, to build up a mineralized stockwork up to 4-5 m thick. Mineralized zones with a stockwork texture and an average 20 cm thickness are still preserved on the walls and ceiling of the mine galleries. The ore is dominated by copper sulphides, chalcopyrite and bornite altered in secondary Cu-sulphides (covellite, chalcocite), with accessory pinkish pyrite, sphalerite and very fine-grained Ag-bearing sulphosalts. Ore minerals occur as irregular aggregates and disseminated blebs within the brecciated host rock, filling fractures and open cavities together with euhedral zeolites, albite, and rare quartz. The host gabbro shows a pervasive hydrothermal alteration mostly consisting of polyphase chloritization of partially serpentinized pyroxenes and of replacement of zoned plagioclase by clay minerals. The host dolerite shows a weak hydrothemal alteration, with minor chloritization of mafic minerals (pyroxenes and amphiboles) and sericitization of plagioclase. The mineralized veins appear to be the last mineral-forming event in these rocks. This mineralization might be compared to the better known Libiola deposit (Cu-Fe sulphides in pillow lavas) near Sestri Levante. However, the relationships between the Piazza veins and host rocks, their textures and the nature of the vein fillings suggest that this mineralization may not be entirely of “oceanic” origin, but it may be the product of partial metal recycling during late-stage hydrothermal activity of the Apennine orogeny. This setting may be comparable to what was observed by other Authors in the zeolite-bearing copper veins at the nearby zone of Campegli (SP).

The Piazza copper mine (Deiva, La Spezia, Italy) : preliminary assessment of the relationships between Cu-bearing vein systems and a gabbro-dike complex in the Bracco Unit, Internal Ligurides / M. Moroni, P. Tartarotti, A. Franklin. - In: EPITOME. - ISSN 1972-1552. - 2:(2007), pp. 462-462.

The Piazza copper mine (Deiva, La Spezia, Italy) : preliminary assessment of the relationships between Cu-bearing vein systems and a gabbro-dike complex in the Bracco Unit, Internal Ligurides

M. Moroni
Primo
;
P. Tartarotti
Secondo
;
2007

Abstract

The mine of Piazza, near Deiva (La Spezia), was exploited for copper-rich mineralization until the beginning of XXth century. The Piazza Cu deposit occurs within the ophiolite of the Bracco Unit (Internal Ligurides), that represents a remnant of the Mesozoic Tethyan oceanic lithosphere. The ophiolite of the Piazza area consists of pillow basalts, coarse-grained to pegmatoid gabbros and serpentinite intruded by polyphase dike injections (dolerite and plagiogranite), and of breccias. These rocks suffered prehnite-pumpellyite to greenschist facies metamorphism related to the Apennine orogeny. The Apennine tectonics mainly produced large-scale open folds, brittle-ductile to brittle structures such as S-C structures, faults and fractures that did not obliterate the primary features of the ophiolitic rocks. Both intrusive and extrusive magmatic structures are still well recognizable in mafic and ultramafic rocks. Greenschist- to amphibolite facies mineral assemblages, associated with both ductile and brittle structures, have been observed in gabbros and interpreted to be relics of the pre-orogenic oceanic history. In the Piazza mine, galleries distributed in five levels are only partially accessible, but they still reveal the main features of the ore deposit. The original mineralization was included in a complex framework of discordant, steep conjugate E-W to NE-SW trending veins. The vein system was emplaced along fractures and brittle shear zones cutting the pegmatoid gabbro, and along the primary intrusive (or tectonically reactivated) contacts between basaltic dikes and gabbro. According to mining reports, single vein thickness varied between 0.5 and 1.8 m, to build up a mineralized stockwork up to 4-5 m thick. Mineralized zones with a stockwork texture and an average 20 cm thickness are still preserved on the walls and ceiling of the mine galleries. The ore is dominated by copper sulphides, chalcopyrite and bornite altered in secondary Cu-sulphides (covellite, chalcocite), with accessory pinkish pyrite, sphalerite and very fine-grained Ag-bearing sulphosalts. Ore minerals occur as irregular aggregates and disseminated blebs within the brecciated host rock, filling fractures and open cavities together with euhedral zeolites, albite, and rare quartz. The host gabbro shows a pervasive hydrothermal alteration mostly consisting of polyphase chloritization of partially serpentinized pyroxenes and of replacement of zoned plagioclase by clay minerals. The host dolerite shows a weak hydrothemal alteration, with minor chloritization of mafic minerals (pyroxenes and amphiboles) and sericitization of plagioclase. The mineralized veins appear to be the last mineral-forming event in these rocks. This mineralization might be compared to the better known Libiola deposit (Cu-Fe sulphides in pillow lavas) near Sestri Levante. However, the relationships between the Piazza veins and host rocks, their textures and the nature of the vein fillings suggest that this mineralization may not be entirely of “oceanic” origin, but it may be the product of partial metal recycling during late-stage hydrothermal activity of the Apennine orogeny. This setting may be comparable to what was observed by other Authors in the zeolite-bearing copper veins at the nearby zone of Campegli (SP).
copper deposits ; ophiolite ; zeolite
Settore GEO/03 - Geologia Strutturale
Settore GEO/09 - Georisorse Miner.Appl.Mineral.-Petrogr.per l'amb.e i Beni Cul
2007
Article (author)
File in questo prodotto:
Non ci sono file associati a questo prodotto.
Pubblicazioni consigliate

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/33834
Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
social impact