The Serranillo mine is located within metamorphic slates at one end of the metamorphic aureole of the Logrosán batholith. Metamorphic aureole is the area of contact between the molten magma subject to high pressure and temperatures that slowly rises from the interior of the crust and the surrounding sedimentary rocks, which are transformed into metamorphic rocks (e.g. slates and mottled slates).
The main mineralisation consists of a group of lodes of quartz with cassiterite, together with arsenopyrite, chalcopyrite, tourmaline, molybdenite, and accessory pyrite. The lodes face in several different directions and orientations; in the Serranillo gallery horizontal lodes can be observed but in the main trench they are vertical.
The cassiterite crystals appearing in the Serranillo mine are similar to those of the Cerro de San Cristóbal; they show macles (symmetrical or regular groups of two crystals) in “tin spike” form, of which some examples of great mineralogical interest have been found.
Owing to the alteration of the underground water, the presence of chalcopyrite (copper and iron sulphide) in the inner lodes of the Serranillo gallery leads to the appearance of malachite and azurite (copper carbonates), which cover some inner sections of this mining gallery with their greenish and bluish tones.
Inside the Serranillo gallery a Stockwork type deposit can be seen in which the ore is found in a dense network of numerous larger and smaller lodes of quartz with cassiterite spread through the slates (primary deposit).
The hillsides of the Cerro del Serranillo contain colluvium (deposits of clayey materials with sharp edges transported by the stream waters) with a high cassiterite content (secondary or sedimentary deposit), which in the past were also exploited with washing pans.
