| Abstract |
A powerful and convenient methodology (cheap and easy to perform) has been successfully adopted for studying the mineralogy and the trace elements geochemistry of a silica-carbonate sinter deposit, linked to the Larderello geothermal field, Italy. The study carried out by means of luminescence spectroscopy added to microscope mineralogical analysis, proved to be useful as a complementary tool to investigate geothermal systems, both fossil and active, providing indications about the physicalchemical characteristics of the reservoir. The casestudy site is located at La Sassa (Pisa Province, Tuscany). The area shows: (i) ‘classical’ hydrothermal mineral deposits along a network of fractures, and (ii) three Carlin-type gold mineralizations described in Italian ore deposit geology literature (La Sassa, Frassine and Campigliola). Minerals which formed in hydrothermal systems are commonly doped with ‘foreign’ trace-elements, as a replacement for ‘basecomponent’ elements in their crystalline lattice. Such a replacement, and lattice defects due to particular growth mechanisms in hydrothermal environment, often make involved minerals capable to fluoresce under ultraviolet light. A calibrated methodology, coupling adequate laser UV source and a highresolution spectrometer, allows to determine the trace-elements content of hydrothermal minerals in the studied area and to establish the related environmental conditions. This analysis technique can be useful among the preliminary investigations, both on the outcrops and on boreholes cuttings. The studied silica-carbonate sinter deposit close to La Sassa formed where an ancient geyser was active. Silica mostly corresponds to (a) macro-crystalline á- quartz; (b) crypto- or micro-crystalline quartz (chalcedony); or rarely, (c) opal. The crystalline lattice of quartz includes ‘foreign’ elements like (UO2)2+, Tb3+, Eu3+, Dy3+, and quantitatively subordinate Ce3+, and Sm3+, which activate a typical fluorescence, glowing different colors in the greenyellow to orange-yellow regions of the spectra, under different ultraviolet wavelengths. La Sassa hydrothermal minerals show: (i) elements replacement in both silica phases and hosting calcite; (ii) lattice growth defects; (iii) enrichment in REEs, (UO2)2+, and polycyclic aromatic hydrocarbons (PAHs). These minerals are linked to early-stage evolution of the Larderello geothermal field. Similar mineral deposits are likely to be found in similar geological and geothermal contexts worldwide. Luminescence spectroscopy and UVF microscopy proved to be an expeditious tools in geothermal research, allowing to identify hydrothermal minerals and their fluid-metal sources parameters, linked to the various ore forming fluid processes. Combined with geochemistry, luminescence spectroscopy and UVF microscopy can identify trace elements, even when their content is below the threshold of sensitivity limits of the most common microprobe analyses. |