The Candoglia Marble and the “Veneranda Fabbrica del Duomo di Milano”: A Renowned Georesource to Be Potentially Designed as Global Heritage Stone

Marbles from Alpine area have been widely employed to build and decorate masterpieces and buildings which often represent the cultural heritage of an area (statuary, historic buildings and sculptures). Candoglia marble, object of the present research, is one of the most famous and appreciated marbles from Alpine area; it has been quarried since Roman times in the Verbano-Cusio-Ossola (VCO; Piemonte – NW Italy) extractive area. Candoglia Marble outcrops are present as lenses within the high-grade paragneisses of the Ivrea Zone, a visible section of deep continental crust characterised by amphibolite- to granulite-facies metamorphism (Palaeozoic period). Candoglia calcitic marble (80–85% CaCO3 and the 15–20% other minerals) shows a characteristic pink to gray colour and a coarse-grained texture (>3 mm): frequent centimetre-thick dark-greenish silicate layers (mainly represented by diopside and tremolite) characterize the texture of the marble. It has been largely used in local rural constructions and historical buildings, but its most famous application has been (and still is) for the “Duomo di Milano” construction (fourteenth century). The Veneranda Fabbrica del Duomo di Milano carried out the anthropogenic activities dealing with the Candoglia marble exploitation; it has to be highlighted that the company have managed the Marble exploitation during the last seven centuries and that the quarry itself is a tangible sign of the development of extraction and heritage in the VCO area. Candoglia marble can be recognized as a significant example of a “Global Heritage Stone Resource”: its exploitation from quarry to building (the Duomo di Milano) well represents the close correlation between stone and cultural heritage, between georesources and humankind development

Sulfur and metal fertilization of the lower continental crust

Mantle-derived melts and metasomatic fluids are considered to be important in the transport and distribution of trace elements in the subcontinental lithospheric mantle. However, the mechanisms that facilitate sulfur and metal transfer from the upper mantle into the lower continental crust are poorly constrained. This study addresses this knowledge gap by examining a series of sulfide- and hydrous mineral-rich alkaline mafic-ultramafic pipes that intruded the lower continental crust of the Ivrea-Verbano Zone in the Italian Western Alps. The pipes are relatively small (< 300 m diameter) and primarily composed of a matrix of subhedral to anhedral amphibole (pargasite), phlogopite and orthopyroxene that enclose sub-centimeter-sized grains of olivine. The 1 to 5 m wide rim portions of the pipes locally contain significant blebby and disseminated Fe-Ni-Cu-PGE sulfide mineralization. Stratigraphic relationships, mineral chemistry, geochemical modelling and phase equilibria suggest that the pipes represent open-ended conduits within a large magmatic plumbing system. The earliest formed pipe rocks were olivine-rich cumulates that reacted with hydrous melts to produce orthopyroxene, amphibole and phlogopite. Sulfides precipitated as immiscible liquid droplets that were retained within a matrix of silicate crystals and scavenged metals from the percolating hydrous melt, associated with partial melting of a metasomatized continental lithospheric mantle. New high-precision chemical abrasion TIMS U-Pb dating of zircons from one of the pipes indicates that these pipes were emplaced at 249.1 ± 0.2 Ma, following partial melting of lithospheric mantle pods that were metasomatized during the Eo-Variscan oceanic to continental subduction (~420-310 Ma). The thermal energy required to generate partial melting of the metasomatized mantle was most likely derived from crustal extension, lithospheric decompression and subsequent asthenospheric rise during the orogenic collapse of the Variscan belt (< 300 Ma). Unlike previous models, outcomes from this study suggest a significant temporal gap between the occurrence of mantle metasomatism, subsequent partial melting and emplacement of the pipes. We argue that this multi-stage process is a very effective mechanism to fertilize the commonly dry and refractory lower continental crust in metals and volatiles. During the four-dimensional evolution of the thermo-tectonic architecture of any given terrain, metals and volatiles stored in the lower continental crust may become available as sources for subsequent ore-forming processes, thus enhancing the prospectivity of continental block margins for a wide range of mineral systems.