The Balmuccia peridotite massif is one of the subcontinental mantle peridotite bodies intruded in the mafic-ultramafic Ivrea-Verbano complex in the Western Alps, Northern Italy. The Balmuccia body consists of dominant foliated lherzolites crosscut by several generations of pyroxenite dikes, approximately belonging to an earlier “Cr-diopside suite” and a later “Al-augite suite”. These dike suites are interpreted as mineral segregations from melts which percolated into the lherzolite, refertilized it and caused focused dunitization of the host peridotite. Our study is focused on some thin, cm- to dm-thick pale grey-violet dikes presumably belonging to the late-stage Al-augite suite and intersecting the lherzolite mass at its eastern margin, near the intrusive contact with the Ivrea-Verbano complex. These dikes often show a spotted texture due to the segregation of abundant coarse vitreous black spinel phenocrysts. The mineral assemblage is both Al- and sulphide-rich. The silicate-oxide assemblage includes dominant clinopyroxene (diopside-augite) and blue-green, transparent Al-rich spinel, with minor Ti-rich Na-K pargasite amphibole, Al-bearing orthopyroxene (enstatite), accessory Ti-rich phologopite and plagioclase and rare olivine (intergrown with spinel droplets). Abundant Fe-Ni-Cu sulphides (pentlandite, phyrrotite, chalcopyrite and bornite) are widely disseminated across the dikes both as mm-sized polyphase nodules interstitial to the silicate-spinel matrix and as tiny, locally extremely abundant drop-like inclusions in spinel. Assemblages containing bornite exclusively occur as inclusions in spinels, which host a redox-dependant equilibrium pentlandite-chalcopyrite-bornite association. The richness in Ni sulphides copes with the scarcity of olivine in this association and is one of the interesting features of these dikes. Another curious feature involves the margins of the dikes. The contacts with the host lherzolite are sharp but not tectonic and are characterized by lateral passage from dike-related undeformed to lherzolite-related, variably foliated assemblages (olivine, ortho- and clino-pyroxenes, fine-grained dark brown spinel and rare amphibole). A visual inspection of the host lherzolite/grey dike contact zones showed a peculiar, progressive change in colour of the spinel from dark brown, in “distal” position, to green transparent near the dike margin. That was the most visible evidence of the interaction between lherzolite and intruding dikes, as an actual reaction zone is not easy to recognize texturally. Analytical transects across these mm- to cm-sized “contact zones” by means of microprobe showed that there may be a cryptic reaction zone. The lherzolite phases do record some changes in major element composition, and in particular spinel, pyroxenes and amphibole show variations in Chromium concentration, among others. Across the reaction zones lherzolite spinel vary their Cr content from about 19 wt% Cr2O3 to <1%. They also decrease their Fe2+ content whereas Al and Mg are enriched. Clinopyroxene shows a variable “response” to this reaction zone as it shows a tendency to Cr depletion coupled with Ti enrichment and fluctuations in alkalis. Accessory amphibole crystals in lherzolite show a trend of Cr depletion and Ti enrichment as well as a variability in Al and Na contents towards the dike margin. Cr is rather low in lherzolite orthopyroxene, however it shows a tendency to Cr and Mg depletion (and Fe enrichment) towards the dike contact. Olivine tends to be more forsteritic in a distal position. Ongoing and future analytical investigations are aimed to characterize and verify the meaning and the extent of this Cr-depleting Al-enriching and dike-lherzolite metasomatic interaction. Of particular interest is the fate of Cr, one of the economic elements enriched (after mobilization from a source) in relation to metasomatic processes, e.g., dunitization, affecting peridotites.

Al-Spinel-sulphide bearing dikes and the fate of chromium in marginal reaction zones at Balmuccia / M. Moroni, G. Trezzi, G. Grieco. - In: EPITOME. - ISSN 1972-1552. - 4:(2011), pp. 337-338. ((Intervento presentato al 8. convegno Geoitalia tenutosi a Torino nel 2011.

Al-Spinel-sulphide bearing dikes and the fate of chromium in marginal reaction zones at Balmuccia

M. Moroni
Primo
;
G. Grieco
Ultimo
2011

Abstract

The Balmuccia peridotite massif is one of the subcontinental mantle peridotite bodies intruded in the mafic-ultramafic Ivrea-Verbano complex in the Western Alps, Northern Italy. The Balmuccia body consists of dominant foliated lherzolites crosscut by several generations of pyroxenite dikes, approximately belonging to an earlier “Cr-diopside suite” and a later “Al-augite suite”. These dike suites are interpreted as mineral segregations from melts which percolated into the lherzolite, refertilized it and caused focused dunitization of the host peridotite. Our study is focused on some thin, cm- to dm-thick pale grey-violet dikes presumably belonging to the late-stage Al-augite suite and intersecting the lherzolite mass at its eastern margin, near the intrusive contact with the Ivrea-Verbano complex. These dikes often show a spotted texture due to the segregation of abundant coarse vitreous black spinel phenocrysts. The mineral assemblage is both Al- and sulphide-rich. The silicate-oxide assemblage includes dominant clinopyroxene (diopside-augite) and blue-green, transparent Al-rich spinel, with minor Ti-rich Na-K pargasite amphibole, Al-bearing orthopyroxene (enstatite), accessory Ti-rich phologopite and plagioclase and rare olivine (intergrown with spinel droplets). Abundant Fe-Ni-Cu sulphides (pentlandite, phyrrotite, chalcopyrite and bornite) are widely disseminated across the dikes both as mm-sized polyphase nodules interstitial to the silicate-spinel matrix and as tiny, locally extremely abundant drop-like inclusions in spinel. Assemblages containing bornite exclusively occur as inclusions in spinels, which host a redox-dependant equilibrium pentlandite-chalcopyrite-bornite association. The richness in Ni sulphides copes with the scarcity of olivine in this association and is one of the interesting features of these dikes. Another curious feature involves the margins of the dikes. The contacts with the host lherzolite are sharp but not tectonic and are characterized by lateral passage from dike-related undeformed to lherzolite-related, variably foliated assemblages (olivine, ortho- and clino-pyroxenes, fine-grained dark brown spinel and rare amphibole). A visual inspection of the host lherzolite/grey dike contact zones showed a peculiar, progressive change in colour of the spinel from dark brown, in “distal” position, to green transparent near the dike margin. That was the most visible evidence of the interaction between lherzolite and intruding dikes, as an actual reaction zone is not easy to recognize texturally. Analytical transects across these mm- to cm-sized “contact zones” by means of microprobe showed that there may be a cryptic reaction zone. The lherzolite phases do record some changes in major element composition, and in particular spinel, pyroxenes and amphibole show variations in Chromium concentration, among others. Across the reaction zones lherzolite spinel vary their Cr content from about 19 wt% Cr2O3 to <1%. They also decrease their Fe2+ content whereas Al and Mg are enriched. Clinopyroxene shows a variable “response” to this reaction zone as it shows a tendency to Cr depletion coupled with Ti enrichment and fluctuations in alkalis. Accessory amphibole crystals in lherzolite show a trend of Cr depletion and Ti enrichment as well as a variability in Al and Na contents towards the dike margin. Cr is rather low in lherzolite orthopyroxene, however it shows a tendency to Cr and Mg depletion (and Fe enrichment) towards the dike contact. Olivine tends to be more forsteritic in a distal position. Ongoing and future analytical investigations are aimed to characterize and verify the meaning and the extent of this Cr-depleting Al-enriching and dike-lherzolite metasomatic interaction. Of particular interest is the fate of Cr, one of the economic elements enriched (after mobilization from a source) in relation to metasomatic processes, e.g., dunitization, affecting peridotites.
Settore GEO/09 - Georisorse Miner.Appl.Mineral.-Petrogr.per l'amb.e i Beni Cul
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