Cerchiara Mine, Borghetto di Vara, La Spezia Province, Liguria, Italyi

Regional Level Types
Cerchiara Mine	Mine (Abandoned)
Borghetto di Vara	Commune
La Spezia Province	Province
Liguria	Administrative Region
Italy	- not defined -

This area [which is situated in the municipal territories of Borghetto Vara and Pignone, near Faggiona (Pignone)] is certainly known since 1600, when hematite, associated with Mn minerals, was mined by locals to extract iron. The deliberate mining of Mn minerals started in 1790 and ended after World War II in 1945.

It is a manganese and iron ore deposit hosted in Jurassic ophiolitic metacherts consisting of braunite with minor quartz layers interbedded with hematite and quartz bands formed at the condition of prehnite–pumpellyite facies (Lucchetti et al.,1988; Cabella et al., 1998). The ore was produced by precipitation from submarine hydrothermal vents and reworked by turbiditic resedimentation (Cabella et al., 1998). According to Lucchetti et al. (1988), the two very common assemblages of ores are braunite+quartz and hematite+quartz, and mineral assemblages are characterized by Na-clinopyroxene, alkali and sodic-calcic amphiboles, andradite, piemontite, pectolite, ganophyllite, and carbonates. In particular, the compositional variety of Na-clinopyroxene is distinct, which varies from pure aegirine to aegirine–augite with variable Mg, Mn, and Ca contents. The presence of hematite and braunite as the main constituents of the orebody suggests highly oxidizing conditions. The mineralisation at the Cerchiara mine experienced hydrothermal metasomatism induced by diffuse interaction with alkali-rich fluids, which produced peculiar mineral assemblages enriched in Na, K and Li and also provided appreciable amounts of Ba, Sr, Ca and Cu (e.g. Kolitsch et al., 2018). Aluminosugilite (IMA 2018-142, formerly "Al-rich, Fe-poor manganoan sugilite" of Cabella et al., 1990) is one of the representative (Na,K,Li)-rich minerals formed under alkali-rich and oxidized conditions (Nagashima et al., 2020).

Furthermore, some minerals (aragonite, serpentine subgroup minerals, etc.) are not related to the manganese mineralisation, although they occur inside the territory of the former mining concession. In fact, they have been found along the left bank of the Redarena stream, near the eastern border of the mining concession, in small lenses of serpentinite embedded in sandstones, evidenced by stream erosion (Di Lisi, 2011).

The Cerchiara mine is the type locality for aluminosugilite, an Al analogue of sugilite belonging to the milarite group. It occurs as aggregates of small prismatic and/or granular crystals; it is pinkish-purple with a pale purple to white streak and a vitreous lustre (Nagashima et al., 2020). This pinkish-purple mineral was originally described as "sugilite" (Lucchetti et al.,1988) in the form of big zoned purple veins of some centimetres thick, sometimes interleaved with light-blue richterite veins and braunite. The mineral was later reported as "Al-rich, Fe-poor manganoan sugilite" by Cabella et al. (1990). According to their study, it occurs in very compact aggregates filling veins in the metacherts, and its aggregate commonly contains fibrous pectolite. Chemical heterogeneity of this "sugilite" due to Mn³⁺– Al substitution was reported in their study. The Mn₂O₃ and Al₂O₃ contents range between 1.5 wt% – 6.5 wt% and 5.8 wt% – 9.7 wt%, respectively. They confirmed the significant amount of lithium by means of atomic absorption determination; however, the difficulty to remove very fine inclusions of fibrous pectolite did not allow them to provide a reliable composition. Several other occurrences followed in the last 30 years in the abandoned dumps and in the bed of the Redarena stream, where what is now known as aluminosugilite may form prismatic euhedral purple crystals up to 3 mm in length. In the specimens studied by Nagashima et al. (2020), collected in 1997, aluminosugilite appeared closely associated with quartz and K feldspar, but close association with pectolite was not observed. Also, these authors observed in some crystals the chemical heterogeneity due to Al-Fe³⁺ and/or Al-Mn³⁺ substitutions. The Fe₂O₃ and Mn₂O₃ attain up to 5.60 wt% and 4.43 wt%, respectively. Even in such Fe³⁺- and/or Mn³⁺-enriched parts, aluminium ions are still dominant among the octahedral cations.

Notes on the mineral list:

— At Cerchiara the presence of sugilite proper (Fe³⁺-dominant) or possible "manganisugilite" (Mn³⁺-dominant) remains doubtful. In addition to the pinkish-purple aluminosugilite, orange sugilite-like crystals have rarely been found. Analytical data on this orange "sugilite" is presently lacking.

— The following 10 mineral species occur in the concession but not strictly in the Mn-Fe mine: almandine, dravite, graphite, ilmenite, millerite, pentlandite, pyrope, rutile, sphalerite, and titanite.

Other Regions, Features and Areas containing this locality

References