Mass changes during hydrothermal alteration/mineralization at the Sar-Cheshmeh porphyry copper deposit, Southeastern Iran

The Sar-Cheshmeh porphyry copper deposit is associated with a Miocene dioritic to granodioritic pluton that intruded Eocene volcano-sedimentary and related rocks. Copper mineralization was accompanied mainly by phyllic, and to a lesser extent potassic, alteration. These hydrothermal alteration episodes were studied in terms of mass transfer and element mobility. Isocon plots illustrate these changes quantitatively. Al, Ti, and Nd were relatively immobile, and mass was essentially conserved during alteration. At all stages in the evolution of the hydrothermal system, the volume change was inappreciable. In the potassic alteration zone, enrichment of K and depletion of Mg, Ca, and Fe took place. These changes attended replacement of plagioclase and amphibole by Na- and/ or K-feldspar and biotite, respectively. Potassic alteration was associated with a major addition of Cu, as evident from the occurrence of disseminated chalcopyrite and bornite in this zone. Phyllic alteration was accompanied by depletion of Na, K, Mg, Rb, and Ba and enrichment of Si and Cu. Losses of Na, K, and Fe reflect sericitization of alkali feldspar and destruction of ferromagnesian minerals. The addition of Si is consistent with widespread silicification, which is a major feature of phyllic alteration, as well as the addition of Cu mobilized from the potassic zone.