Mineralogy

Two types of sphalerite mineralization have been distinguished in Aliva deposit on a basis appearance, texture, paragenesis and geochemical characteristic (García-Fernández et al., Economic Geology, 2000).

Early phase consist of Type I mineralization represented by dark brown sphalerite, galena, and dolomite. This type shows granular, laminated, and botryoidal textures, with alternating layers of brown and yellow sphalerite. It is also present as cavity fillings, as replacement patches, and in hydrothermal dolomitic breccias located close to the dolostone-limestone contacts. All these minerals were deposited in several pulses. Type I sphalerite never form gem quality crystals and it is not suitable for faceting.

Later Type II mineralization is more abundant and is characterized by large crystals of sphalerite (up to 20 cm across), galena and calcite. Type II mineralization is frequently found cementing breccias, filling fractures and cavities. All gem quality sphalerite belongs to Type II mineralization. Its properties are described in detail in the next section of our web (Sphalerite properties).

Generalized paragenetic sequence in the Picos de Europa mining district.
(García-Fernández et al., 2000).

Following description of the main minerals is based on the publication of R. Sanabria Orellana and J. R. García Álvarez (Revista de Minerales, 2005).

Galena. It is widely distributed in all types of mineralization, but less abundant than sphalerite. General proportion between sphalerite and galena is 6.5:1 in the deposit. Galena forms cubic or cuboctahedron crystals and twins on (111) plane. The size of crystals can reach 4-5 cm across.

Calcite. Is a common mineral in all mineralization phases, and also occurs as a later mineral covering sphalerite crystals. Typical crystals have scalenohedral forms, sometimes combined with rhombohedra. The color of calcite is milky or yellowish, translucent and very rarely transparent.

Dolomite. After the dolomitization phase previous to Zn-Pb mineralization, dolomite is also present in all the phases of mineral formation. Dolomite crystals have typical saddle forms, white color and nacreous luster, the size of crystals is up to 2-3 cm.

Fluorite. Is one of the latest minerals that usually cover other crystals and fulfill fractures and cavities. Fluorite crystals are usually violet to colorless, the typical forms are cubes, rarely also with some octahedron an rhombic dodecahedron faces.

Pyrite is the unique sulfide distinguished to the naked eye, apart of sphalerite and galena. It is present in both types of Zn-Pb mineralization and also as a later mineral, but always in small crystals up to several millimeters. The typical forms are cubes and cuboctahedrons.

Other minerals. Microscopic study reveal also the presence of other sulfides: chalcopyrite, tetrahedrite, cinnabar, guadalcazarite and other minerals.

Large sample of transparent toffee-colored sphalerite crystals covered by white dolomite.
Width: 25 cm, weight 12.5 kg (62,500 ct)