Since the San Giovanni lead-silver-zinc mine (1554-1985) near Gonnesa (SW Sardinia, 2D and 3D pano) has been shut-down, the miners gladly guide you in their elevator almost 500 meters down into the mountain to hot, humid and sulfurous galleries or 50 m up at 200 m above sea level to the cool fresh Santa Barbara Cave, named after the patron saint of the miners. The geodic cave without natural opening was discovered during excavation of an aeration shaft for the mine in 1952 and remained closed for the public until quite recently. It is very special in that it shows large barium sulfate (barite) and calcium carbonate (aragonite, calcite) crystals that precipitated while the cave was still filled with water. As concerns the origin of the cave there are different possible scenarios.
The metal sulphide ore bodies of galena and sphalerite in the mine formed when hot acid hydrothermal brine rose toward the surface along faults within the 500 million years old Cambrian limestone, probably during the late Paleozoic or possibly the younger Tertiary mountain building phases and long after the Cambrian limestone had been deposited, buried, folded, uplifted and fractured.
The miners bring you with an underground train to the elevator that reaches the gallery leading to the staircase into the cave. In the excavated wall of the staircase one observes the original limestone that has been dissolved and then covered by layers of calcium carbonate crystals, alternating with iron-oxide layers and finally covered by large barium sulfate and younger calcium carbonate crystals. Within the cave, the most recent precipitates are stalagmites and stalactites of calcium carbonate.
As concerns the sequence of events leading to the formation of the Santa Barbara cave, one can imagine that it started by the gradual intrusion of oxygen with groundwater along a fault, coming in contact with a metal sulphide ore body, followed by the oxidation of sulfide into sulfuric acid. The acid keeps the metals in solution and dissolves the Cambrian limestone, creating the cave. If the carbon dioxide that is produced during this reaction can escape, crystal veneers of calcite are precipitated on the wall that alternate with insoluble oxides of iron from the ore body. The barium and sulfate from the oxidized ore body react to form the large brown colored barite crystals presently lining the cave. Only after the groundwater level fell and the Santa Barbara cave was exposed to the air, the stalactites and stalagmites formed due to the escape of carbon dioxide from the dripping ground water into the air in the cave and the consequent saturation with respect to- and precipitation of the aragonite and calcite in the form of stalagmites and stalactites.
The processes may have started more than 20 million years ago before the ice ages during the Oligocene when sea level was as high as the current level of the Santa Barbara cave, although a later origin with a lower sea level is also possible if later-Tertiary tectonic uplift of the Cambrian limestone is taken into account. These and other matters as concern the genesis of the Santa Barbara cave shall remain subject to discussion for a while and give ample reason to continue research into this extraordinary natural phenomenon.