Alfenza Mine, Crodo, Verbano-Cusio-Ossola Province, Piedmont, Italyi

Regional Level Types
Alfenza Mine	Mine
Crodo	Commune
Verbano-Cusio-Ossola Province	Province
Piedmont	Region
Italy	Country

Alfenza Mine, Ossola Valley, Verbano-Cusio-Ossola Province, Piedmont, Italy

Old gold mine on the right bank of Rio Alfenza, about 1200 m from its confluence with the Toce River and 700 m NW of Crodo.

The Au-bearing ores are hosted in quartz dykes intruding NNW-SSE oriented fractures in garnet-biotite micaschists, partially covered by tillitic deposits.

Due to the presence of auriferous pyrite, the dykes were intensively mined during the XVIII century (the peak in the production of gold was recorded in the period 1771-74: 23.32 kg in 1771, approximately 19.5 kg in 1772, a little less than 22 kg in 1773, about 15.5 in 1774). After an alternance between exploitation and stop periods, the mine was definitively abandoned in 1941.

The ore mineralogy mainly consists of pyrite with subordinate arsenopyrite, bismuthinite and cosalite, scarsce chalcopyrite (bisphenoidal crystals up to 42 mm were found at level 4) and sporadic bornite. Gangue consists of compact or cavernous quartz with calcite, dolomite, and siderite. Bismuthinite is more abundant at level 1, while cosalite at level 4 (which was exploited from 1939 to 1941 only).

Interesting bismuthinite alteration products have been recently identified:
- cannonite, as straw-coloured crystalline aggregate (Orlandi, 1999) and radiating acicular aggregates of colourless elongated crystals (Capitani et al., 2014);
- a new phase consisting of white hemispherical ("hortensia-like") aggregates of pseudohexagonal platelets (up to 20 microns in size) of approximate composition Bi2O2(SO4) with cell parameters, as determined by automatic diffraction tomography, a = 22.0(4), b = 16.7(3), c = 15.9(3) Å, β = 102.9(5)°, and space group Pc or P2/c (Capitani et al., 2014);
- a second new phase, detected only by TEM, distinguishable by its random orientation on the TEM grid (i.e., absence of preferential parting), the higher resistance under the electron beam, and different cell parameters and structure, whereas the composition is similar (Bi/S ~ 2.2/1) to the previous phase, apart for the presence of tellurium up to ~6 cation %. The unit cell is hexagonal, space group P6̄2c, a = 9.5(2) and c = 15.4(3) Å. A structure model, obtained ab initio from electron diffraction data, shows that this mineral has a porous structure and looks like a "bismuth-zeolite" with one dimensional porosity (diameter of the channel ~7 Å) (Capitani et al., 2014).

Beryl, as colourless acicular crystals in association with quartz and pyrite, has been recently found (Guastoni et al., 2014).

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References