Magmatic and hydrothermal ammonium in granites of the Tuscan magmatic province, Italy

The granites of the Tuscan magmatic province have total ammonium contents that vary from 0 to 636 ppm. In the three largest intrusions of Elba, Giglio, and Montecristo, the granites have not been affected by hydrothermal alteration and have low ammonium contents, ranging up to 50 ppm. There are detectable differences between the separate intrusions, and also within the biggest intrusion. Granitic samples from the smaller outcrops and boreholes on the Tuscan mainland are nearly all hydrothermally altered, and these rocks show much higher ammonium contents, ranging up to 636 ppm. In the hydrothermally altered rocks, the ammonium contents vary considerably over short distances, and where they have been most closely sampled (in the S2 borehole of Castel di Pietra), there is a clear correlation between the ammonium content and the intensity of sulphide mineralization. The major conclusion of the study is that the amount of ammonium present in the granites is determined much more by post-magmatic hydrothermal alteration than by magmatic variation. The mean ammonium contents of the fresh and hydrothermally altered granites are 17 and 95 ppm, respectively.