Hydrogen and oxygen isotope studies of alteration in the Timna Igneous Complex

Alteration processes have resulted in significant modification of the mineralogy and stable isotope composition of the Precambrian igneous rocks of the Timna Igneous Complex (TIC). Petrographic studies show that two general patterns of mineralogical alteration can be recognized: chlorite-biotite-sericite alteration in intermediate to acidic rocks (diorites to granites); chlorite-serpentine-clay alteration in gabbroic rocks. The least altered igneous rocks have δD values around -70 to -80‰ and δ¹⁸O values around 7‰ (gabbros and diorites) and 9‰ (monzonites, syenites, and granites). Alteration first leads to D-enrichment in all rocks to δD values of about -50‰ and a corresponding slight increase in δ¹⁸O values of about 1-2‰. These trends are shown to be consistent with hydrothermal alteration at low to medium water/rock ratios with warm basinal brine (<200 °C), as suggested by a previous study. Veins in the gabbro and highly altered samples from the center of the "Deshe" depression show higher δ¹⁸O values (>13‰) indicating alteration at high water/rock ratios. The "Deshe" values are close to the "Kaolinite" and "Supergene/Hypogene" lines on a δ¹⁸O vs. δD plot and suggest that low-temperature alteration and weathering has superimposed itself on the earlier hydrothermal alteration.