An unusual occurrence of an exploitable K-feldspar deposit hosted in the Ordovician Porphyroids (Southern Sardinia) Geology, mineralogy, geochemical features and economic potential

摘要

An atypical potassium feldspar deposit of real economic potential has been found at Arcu de su Bentu, Southern Sardinia, Italy. The massive mineralization occurs in the Middle Ordovician Porphyroids' metavolcanic complex, which consists of dominant metarhyolite and minor metarhyodacite lavas and/or ignimbritic flows, erupted in a sub-aerial environment. During Variscan orogenesis, the mineralized area was affected by low-grade (anchizone to greenschist facies) metamorphism. The deposit generally shows uniform macroscopic textural features over the entire outcrop, the significant presence of K-feldspar (sanidine) and quartz megacrystals, the latter being generally smaller than the former. Mafic minerals do not occur in significant proportions although minor amounts of biotite and muscovite occur locally. Besides the uncommon lithological type, the typical feature of this deposit is the unusual coarseness (10 to 20 cm in diameter) of the potassium feldspar crystals. The favourable geomorphological conditions of the area, high K_2O contents (6 to 8 wt.%), high proportion of K-feldspar in the mineralized rock (35 to 51 vol.%), and large reserves (estimated at 4,368,000 t in just one restricted part of the deposit), are all favourable conditions for future economic exploitation. In spite of this, mineralogical and chemical impurities may adversely affect the possible use of K-feldspar from Arm de su Bentu in some conventional applications (e.g., manufacture of high-quality products such as ceramics). To assess the real future economic development of the deposit, market-specific beneficiation is required, although it is important to note that the bulk raw materials, even if they have a lower market value, could be still be targeted to specific industrial sectors (e.g., red gres).