Vena carbonatica della Sassa, La Sassa, Montecatini Val di Cecina, Provincia di Pisa, Toscana, Italia
Hydrothermal carbonate vein, up to 30 m thick and more than 300 m long, outcropping near the village of La Sassa (or Sassa). Related to hydrothermal activity is also the strong silicization affecting various rock types in this area. The hydrothermal vein is clearly visible along the road connecting Gabella to La Sassa, ca. 700 m from the village. Mineralized rocks, discharged onto the slope below the road during its construction, are still found near the cemetry and in the nearbay agricultural field.
The La Sassa carbonate vein mainly consists of spatic calcite in which are widespread cavities lined with pluricentimetric scalenohedral crystals, and more rarely, spheroidal quartz aggregates often showing a fibrous-radiating structure and a strong UV fluorescence.
Field work, undertaken at the end of the 1980s by the company RIMIN in all the area affected by hydrothermal activity, indicated the presence of gold up to ca. 100 ppb.
According to Dallegno and coauthors, the fluorescing quartz is genetically linked to an early stage of hydrothermal metamorphism of the Larderello geothermal field, in whose peripheral zone the La Sassa vein system is located. Magmatic fluids with high concentrations of electrolytes (NaCl, KCl) and silica migrated upward to land surfaces, along extensional faults. At La Sassa, some siliceous sinter outcrops have been individualised and described. They are due to intense subaereal hot springs activity, which are spatially and genetically linked to Carlin-type gold deposits. Fluorescing quartz originates in outcrops located in the most tectonized area, with the strongest (hot spring) hydrothermal activity. In today rocks which correspond to fossil siliceous sinter deposits, some remnants of glowing quartz are found. Several different generations of quartz, both fluorescing and non-fluorescing, are known. Within the bright lemon-yellow fluorescing quartz, the authors detected the presence of electrolytes and lithic fragments of tourmaline (dravite-schorl series) as solid inclusions within the radiating fibers of quartz, and a rare artichoke quartz which is typical of an environment where conditions forced a rapid growth of the crystalline aggregate. The luminescence centres in quartz are attributed to the presence of hydrocarbons (both aliphatic and polyciclic-aromatic), (UO2)2+, REE, crystalline lattice defects as Si and O vacancy, Al3+/Metal+/2+. By ICP-MS analyses on some quartz samples, a relatively high concentration of incompatible elements (Li, Be, Ga, Rb, Cs), calcophile elements (Sb, As, Te, Ag), as well as a high content of Al and Ti, has been measured.
Most of the La Sassa quartz shows bright photoluminescence under UV light (SW, MW, and LW). The most typical form consists of macroscopic spheroidal aggregates of closely packed needle-like crystalline fibres (star quartz), probably resulting from crystallisation of previous deposited silica gel in hot springs system, in carbonate matrix. Bright yellow/yellow-green fluorescence is observed in honey-yellow fibrous-radiating aggregates, yellow-green-bluish fluoresence in colourless to white fibrous-radiating aggregates. Other luminescent quartz forms include massive aggregates with saccharoidal texture, white to pale grey colour and yellow-orange fluorescence, and aggregates of euhedral crystals (also in the form of "artichoke quartz") with bright yellow-green fluorescence. These quartz forms occur in a mature silica-carbonate sinter and/or within a system of surficial veins, sometimes also included in a metasomatized and/or silicized ophiolite protolith (listvinite-birbirite). Star quartz in listvinite-birbirite shows a white-blue-greeinsh fluoresence and the host rock (in which are present Fe carbonates, non-fluorecent quartz, Cr-bearing mica and chromite relics) a red-brown emission due to the presence of Fe3+ and Cr3+ luminescence centres. Also some carbonate levels (calcite-aragonite levels in banded thermogenic sediments) exhibit spectacular photoluminescence colours due to the presence of traces of organic substances, REE, etc.
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La Sassa carbonate vein, La Sassa, Montecatini Val di Cecina, Pisa Province, Tuscany, Italy