Mont'Orfano (Montorfano), Mergozzo, Verbano-Cusio-Ossola Province, Piedmont, Italyi

Regional Level Types
Mont'Orfano (Montorfano)	Mountain
Mergozzo	Commune
Verbano-Cusio-Ossola Province	Province
Piedmont	Region
Italy	Country

Mont'Orfano (Montorfano), Ossola Valley, Verbano-Cusio-Ossola Province, Piedmont, Italy

Mont’Orfano (or Montorfano) is a strikingly prominent landmark 794 meters high, rising steeply from the surrounding alluvial plain of River Toce. Because of its isolated position at the confluence of Strona and Ossola valleys, this mountain earned the name of “Mont'Orfano” (i.e. the orphan mount).

It is formed by one of the five late Variscan plutons that intruded the Serie dei Laghi crystalline basement and mainly consists of a white medium-grained granite. The country rocks (medium-grade metapelites of the Scisti dei Laghi formation) are exposed only in small outcrops on the northwestern flank of the mount. Here, in the vicinity of Brusco village, the so-called "green granite" of Mergozzo occurs as a narrow band near the contact with the metapelites. This is clearly the result of episyenitisation, i.e. subsolidus, low-temperature transformation of a preexisting intrusive rock, because, despite its igneous appearance, it is composed of albite, chlorite, quartz, sericite, titanite, and carbonates.

The mineral assemblage of white granite is: albite, quartz, K-feldspar, biotite, apatite, zircon, and rare allanite. Green hornblende is only present in a restricted zone in the northwestern part of the granite stock (Gallitelli, 1938; Sassi & Sesana, 1986). In the amphibole-bearing granite, allanite is abundant in large, automorphic crystals.
At the Donna quarry (Cava Donna; also known as Cava Montorfano), in the easternmost part of the Montorfano stock, the granite is full of enclaves of a greyish heterogranular facies, in turn containing mafic fine-grained enclaves. The mineral assemblage of the grey granite is the same as that of the white granite: the minerals are present in two distinct generations. The margin towards the white granite is sharp, without any particular contact textures or compositional variations.
Microgranular, compact, dark enclaves are widespread in the Montorfano stock, whilst they are rare in the Mottarone-Baveno pluton. The maximum concentration is within the heterogranular facies of Cava Donna; their size normally ranges from a few millimetres to some decimeters (rarely up to 1-2 m). The enclaves show an intersertal texture of biotite, hornblende and plagioclase; new biotite is formed from hornblende. K feldspar and quartz, introduced by granitisation, are present as integranular material and patches.
A small distinct granite body, crowded with schistose xenoliths, occurs in the northern part of the stock. It is composed of white medium- to fine-grained rocks, sometimes slightly porphyritic in texture, with pale pink K-feldspar phenocrysts; it does not contain plagioclase clots of early crystallisation. The size of the xenoliths ranges from microscopic to more than 1 m across. They preserve the original schistosity and are oriented at random in the enclosing granite; their mineral assemblage includes sillimanite, andalusite, spinel, corundum, and cordierite (Boriani, Burlini et al., 1988).
A dyke of granite porphyry cuts across the northern contact of the granite and penetrates into the country rocks.
Acidic differentiates are represented by many aplitic dykes and very few pegmatitic pockets.

White granite quarries are located on the southern foothills of the mount. Nowadays, this white granite is utilized in buildings and urban décor, while in the past it was mostly used as building stone although in a lesser extent than the famous pink granite of Baveno (which is quarried just a couple of kilometres to the south-east). Its use started in the 16th century and continued up to now: in Milan numerous columns of this granite can be observed in the cloister of San Pietro in Gessate monastery (16th century), Sant’Angelo church, cloisters of San Vittore monastery, courtyard of Ospedale Maggiore (17th century), etc. In the 19th century, 146 monolithic columns (up to 14.5 m in length and 4.5 m in diametre) were used for the reconstruction of the patriarchal basilica of San Paolo fuori le Mura in Rome and transported to Rome by lake, rivers and sea.

The green granite, occurring on the northwestern slope of Mont'Orfano, was surely exploited since 1700 as evidenced by buildings in Milan (e.g. Palazzo Mellerio), but its quarrying ceased at the end of the 20th century. The major problem for the exploitation of this rock is the presence of iron carbonates, that are easily weathered to give yellow spots and decrease the resistance to polishing.

Yellowish hexagonal crystals occurring in a crust, consisting of chlorite, stilbite, chabazite, fluorite, and pyrite, or perched on quartz crystals were classified as parisite, on the basis of chemical and physical investigations (Tacconi, 1905). The material of the original description is no more available for modern investigations. According to Gramaccioli (1975), it should be attributable to synchysite-(Ce). In Palache et al. (1951) the presence of parisite at Mont'Orfano is erroneously reported from veinlets in riebeckite-granite. Mont'Orfano granite does not contain riebeckite and, obviously, the name riebeckite does not appear in the original literature.

The following minerals were found at the abandoned Piovetta granite quarry: allanite-(Ce), bismutite, ferberite, rozenite, and wulfenite (Savia et al., 2024).

Other Regions, Features and Areas containing this locality

References