Abstract

Electron microprobe and petrographic data on tourmaline-rich assemblages, in addition to available data from fluid inclusions and geochronological studies, are used to model the early stage of hydrothermal metamorphism in the Larderello geothermal field, Italy. During this early stage different hydrothermal events are characterized by deposition of tourmaline, biotite, calcic plagioclase, K feldspar, quartz, and pyrrhotite at lithostatic pressures of 0.75 to 1 kbarsand temperatures in the range 400 degrees to 600 degrees C. Pressures exceeding hydrostatic values and inferred temperatures of 450 degrees to 460 degrees C at the bottom of well San Pompeo 2 suggest a possible present-day deposition of tourmaline in the deepest levels of the central and hottest part of the field.Hydrothermal tourmalines appear to have crystallized at temperatures in excess of 370 degrees C. They show significant replacement of Al by Fe (super +3) in the Z site, substitutions of Al for Si in the tetrahedral site, and slight vacancies in the X and Y sites produced by limited cation- and proton-deficient substitutions.In the hydrothermal tourmaline nearest to the inferred granitic contact and in the tourmaline of a leucogranitic dike crossed by well VCll, a negative correlation existing between Mg and Fe (super +2) reveals significant reciprocal substitutions of these elements in the Y site.Tschermak's and aluminobuergerite substitutions and an increase in the molecular fractions of schorl and elbaite in the schorl-dravite and schorl-elbaite solid solutions were observed from rim to core of the VCll magmatic tourmalines. Rims and intermediate growth zones of magmatic tourmalines and most of the hydrothermal tourmalines have compositions falling in the field of metamorphic tourmalines. This behavior reveals the dominant control of the host rock on fluid chemistry.Some growth zones and most cores have compositions that plot in the field of tourmalines from granitoids. A gradual schorl or ferridravite enrichment from magmatic to hydrothermal tourmalines with increasing distance from the granite contact is also present. In a microprobe traverse from the base to the tip of an acicular tourmaline, four peaks of Fe (super +3) replacing Al can be ascribed to four distinct hydrothermal events generated by episodic ascent from below of magmatic fluids that became more and more oxidized with increasing distance from the granite contact, owing to an easier hydrogen leakage in the shallower and more fractured levels.The studied mineral assemblages and available fluid inclusion data reveal close analogies between the deepest and hottest part of the Larderello geothermal system and quartz-tour-maline and K silicate alteration zones of porphyry tin and copper deposits. The mineralizing fluids are inferred to have been of magmatic origin and were characterized by T = 400 degrees to 600 degrees C, pressure varying between hydrostatic (0.2-0.35 kbars) and lithostatic (0.75-1 kbars), commonly near lithostatic values, high f (sub O 2 ) as inferred by the presence of hematite and anhydrite among fluid inclusion daughter minerals, and very high salinities (up to 50-60 equiv wt % NaCl).

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